vibnspectr1998melamines

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2517740
Peter
Peter.Larkin2@bms.com
9/6/2012 5:18 PM
Peter
Bristol-Myers Squibb
,
New Brunswick, NJ 08903
United States
09242031
Elsevier
Vibrational spectroscopy
17 (1) p.53-72
Vibrational analysis of some important group frequencies of melamine derivatives containing methoxymethyl,
and carbamate substituents: mechanical coupling of substituent vibrations with triazine ring modes
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Ž .Vibrational Spectroscopy 17 1998 53–72
Vibrational analysis of some important group frequencies of
melamine derivatives containing methoxymethyl, and carbamate
substituents: mechanical coupling of substituent vibrations with
triazine ring modes
P.J. Larkin )
, M.P. Makowski 1
, N.B. Colthup 2
, L.A. Flood
CYTEC Industries, Research and DeÕelopment, Stamford, CT, USA
Received 10 December 1997; revised 10 April 1998; accepted 21 April 1998
Abstract
This study uses ab initio force field calculations for the vibrational analysis of the IR and Raman spectra of 3 different
Žstructural types of melamine derivatives. These compounds include: tris- and hexa-methoxymethyl melamines TMMM and
. Ž .HMMM , and s-triazine substituted with 3 methyl carbamate groups TMCT . Detailed assignments are made for selected
IR and Raman active vibrations of TMMM, HMMM, and TMCT and important group frequencies are identified to aid in
future investigations of melamine-based cross-linkers. This study systematically identifies the various mechanical interac-
tions of vibrations involving the substituent groups and the triazine ring of three general types of melamine derivatives to
better understand the vibrational origin of bands which provide good group frequencies. Standing vibrational waves are used
to dramatically simplify the description of the complex vibrational modes involving the triazine ring and the relative phasing
Ž y1.of the substituents to the triazine ring. The NH and CH stretching vibrations observed above 2800 cm of the substituent
groups on melamine derivatives are shown to be mechanically independent of the triazine ring modes; however, extensive
mechanical coupling of the substituent groups with the triazine ring modes occurs for many of the vibrations typically found
y1 Ž . Ž .below 1700 cm . These substituent vibrations include: 1 the aliphatic CH and CH bends, wags, twists, and rocks; 22 3
Ž . Ž .the symmetric and asymmetric C–O–C stretches; 3 the carbamate CNH stretchrbend and stretchropen; 4 the carbonyl
Ž .rock; and 5 the resonance-stiffened exogenous C–N stretching vibration. Furthermore, we demonstrate that the FT-IR and
Raman spectra of TMCT are dependent upon the orientation of the carbamate substituent relative to the triazine ring. q 1998
Elsevier Science B.V. All rights reserved.
Keywords: Vibrational analysis; Melamine derivatives; Triazine ring
)
Corresponding author.
1
Present address: PPG Industries, 4325 RoseAnna Dr., Allison
Park, PA 15101, USA.
2
Retired. Home address: Apt. 704, 71 Strawberry Hill Ave.,
Stamford, CT 06902, USA.
1. Introduction
Ž .Melamine derivatives such as methoxymethyl
melamine resins are widely used as cross-linking
w xagents in thermosetting coatings 1–3 . Recent devel-
0924-2031r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
Ž .PII: S0924-2031 98 00015-0

( )P.J. Larkin et al.rVibrational Spectroscopy 17 1998 53–7254
opment of new melamine derivatives has increased
the variety of cross-linking agents available for coat-
ing formulations and the technology is now suitable
for water-borne, solvent-based, or even powder coat-
w xing systems 4,5 . Melamine-based cross-linkers are
used with alkyds, polyesters, acrylics, and epoxy-
based backbone polymers to provide durable, attrac-
tive coatings that protect metal and other substrates
w xfrom environmental stresses 1–3 . Typical applica-
tions include coatings found in automobiles, metal
cans, household appliances, farm equipments, and
wood furnitures.
Systematic research to improve the material prop-
erties of coatings is limited by an incomplete under-
standing of the final network structure of various
coatings. The chemical structure, morphology, orien-
tation and dynamics of the polymer backbone all
have important effects on ultimate material proper-
w xties 6 . The ability to analyze the network structure
of cured melamine resin coatings can be advanced
by the application of new analytical technologies.
The variety of chemical species typically found in
cured melamine resin coatings, coupled with their
generally poor solubility as a result of cross-linking,
limits the use of analytical techniques such as liquid
Ž .chromatography LC , LC–MS and liquid phase
w xNMR 7–10 . FT-IR and Raman spectroscopies offer
flexible and versatile techniques for characterizing
cured coating formulations and can contribute to a
more complete understanding of the relationship be-
tween structure on the molecular level and the
w xmacroscopic behavior of the coating 11,12 . Typical
applications of FT-IR and Raman spectroscopy to
study coating formulations include identification of
chemical species present as a result of curing, weath-
ering, and UV irradiation; however, a more funda-
mental understanding of the vibrational spectroscopy
of melamine derivatives is essential to fully utilize
these techniques in coating and resins research.
w xRecently, Meier et al. 13 utilized Raman spec-
troscopy and a class II force field to assign a few of
the fundamentals and present the form of some of
the normal modes of methylol melamines and bridged
methylol melamine compounds. In their work, the
mechanical coupling of substituent vibrations with
triazine ring modes is briefly mentioned; however,
only a superficial description of the ring and sub-
stituent vibrations was presented. Previous studies
have discussed band assignments for melamine and
whexa-methyoxymethyl melamine, respectively 14–
x17 . Although fundamental studies have provided the
w xform of the normal modes of melamine 14,16 ,
works characterizing the vibrational spectra of sub-
stituted melamine derivatives have described only a
w xfew of the forms of the normal modes 13,15,17 .
This study examines the IR and Raman spectra of
three different structural types of melamine deriva-
tives and uses ab initio Cartesian coordinate force
field calculations to assist in the full interpretation of
important group frequencies. Derivatives include the
Žtris– and hexa-methoxymethyl melamines TMMM
.and HMMM , and s-triazine substituted with three
Ž .methyl carbamate groups TMCT . The structures of
TMMM, HMMM, and TMCT are shown in Fig. 1.
In this study, we are most interested in deriving
the form of the normal modes for three structural
types of melamine-based derivatives to ensure a full
understanding of the vibrational spectroscopy of these
compounds and to identify the strengths and weak-
nesses of IR and Raman spectroscopy for the study
of coating formulations. Previous works focused only
on Raman spectroscopy and ignored IR spectroscopy
w x13,15,17 . We find that IR spectroscopy generally
provides a more useful structural probe of melamine
derivatives than Raman spectroscopy does. The IR
spectra provide a greater selection of intense and
moderately intense bands that are good group fre-
quencies than the Raman spectra. The main advan-
tage of the Raman spectra is the presence of nar-
rower band widths which, in certain applications, can
be advantageous.
The critical role that mechanical coupling of the
substituent vibrations with the triazine ring modes
play in the vibrational spectra of melamine deriva-
tives is carefully examined in this work. Standing
vibrational waves are used to dramatically simplify
the description of the complex vibrations involving
the triazine ring and the relative phasing of the
substituent vibrations to the triazine ring vibrations.
The use of the standing vibrational waves also facili-
tates comparison of how various ring modes mechan-
ically couple with different substituent groups. In
this study, we show that the NH and CH stretching
Ž y1 .vibrations observed above 2800 cm of the sub-
stituent groups on melamine derivatives are mechan-
ically independent of the triazine ring modes. How-

( )P.J. Larkin et al.rVibrational Spectroscopy 17 1998 53–72 55
Ž . Ž . ŽFig. 1. The structures of TMMM tris-methoxymethyl melamine , HMMM hexa-methoxymethyl melamine , and TMCT tris-methyl
.carbamoyl triazine .
ever, extensive mechanical coupling of the sub-
stituent groups with the triazine ring modes occurs
below 1700 cmy1
for melamine derivatives, includ-
Ž .ing: 1 the aliphatic CH and CH bends, wags,2 3
Ž .twists, and rocks; 2 the symmetric and asymmetric
Ž .C–O–C stretches; 3 the carbamate CNH
Ž .stretchrbend and stretchropen; and 4 the carbonyl
rock. The resonance-stiffened exogenous C–N
stretching vibration also interacts with many of the
triazine ring modes. This study systematically identi-
fies the various mechanical interactions of the sub-
stituent groups with the triazine ring modes in the
context of the three general types of melamine
derivatives presented here. Furthermore, we demon-
strate that the FT-IR and Raman spectra of TMCT
are dependent upon the orientation of the carbamate
substituent relative to the triazine ring.
2. Experimental
2.1. Computational details
All computations were performed using a Silicon
Graphics Power Indigo2 XZ graphics workstation
with 256 Mbytes of main memory and employing an
MIPS R8000 processor. All software programs were
Žprovided by Molecular Simulations MSI, San Diego,
.CA . Initial molecular models for TMMM, HMMM,
and TMCT were constructed using the Insightw
II
Builder Module version. 3.0. To obtain reasonable
geometries and Hessian matrices as input for the ab
initio calculations, the structures were first optimized
using the Discoverw
version 95 molecular mechan-
Ž .ics MM package. The MSI CFF91 force field and
VA09A optimization algorithm were used with the
electrostatic potentials included. The MM optimized
structures were then submitted directly as input to
the Turbomole ab initio package for further HFrSCF
optimization and force calculations using 3-21G ba-
sis set. Explicit symmetry operators were employed
for both SCF geometry optimizations and subsequent
force calculations to reduce computational cost and
provide more accurate representations of the equilib-
rium structures and normal modes. TMMM and
TMCT both possess C symmetry. HMMM has 23h
local conformational minima, 1 has D symmetry3
and 1 has C symmetry. We chose to examine the1
structure with the higher D symmetry.3
2.2. Choice of basis sets
In order to make a choice of the appropriate basis
set to utilize in the present study, we have used
STO-3G, 3-21G, 6-31G, and 6-31G) basis sets to
calculate the IR spectrum of s-triazine presented in
Fig. 2. Here, the calculated wave numbers have been
scaled by 0.86 for STO-3G and 0.9 for the others.
The ordinates are proportional to the calculated IR
intensities. These are compared with our spectrum of
gas-phase s-triazine presented in a bar graph format,
where the vertical coordinate is the integrated ab-

Fig. 2. The experimental gas-phase spectrum of s-triazine and the calculated gas spectra calculated with STO-3G, 3-21G, 6-31G and
6-31G). The vertical coordinate of the experimental data is presented as integrated absorbance, while the calculated intensities are relative.
The calculated wave numbers have been scaled by 0.86 for STO-3G and 0.9 for the others.
sorbance. Although computational methods cannot
be expected to reproduce experimental data exactly,
we found that the 6-31G and 3-21G basis sets were
best for predicting both frequencies and intensities.
The differences between the observed and the calcu-
lated frequencies and intensities indicate that the
force constants are slightly miscalculated for s-tri-
azine. However, the calculated bands can be easily
associated with specific experimental bands in the
figure.
In the present study, we are most concerned with
the actual form of the normal modes. Fig. 3 illus-
trates the Cartesian displacement vectors for 1 com-
ponent of the degenerate 1557 cmy1
s-triazine band
using 3-21G and 6-31G basis sets. We observe only
very minor differences between the Cartesian dis-
placement vectors as a function of the basis set.
Furthermore, for each of the other vibrations of
s-triazine, the Cartesian displacement vectors for the
2 basis sets were equally similar. We conclude that
the 3-21G basis set is sufficiently accurate to derive
the forms of the normal modes of more complex
melamine derivatives such as TMMM, HMMM, and
TMCT.
Fig. 3. The Cartesian displacement vectors for 1 component of the
degenerate 1557 cmy1
s-triazine band using 3-21G and 6-31G
basis sets.

2.3. Experimental details
2.3.1. Materials
The methoxymethyl melamine resins used in this
study are products of CYTEC Industries. HMMM
was purified by distillation and TMMM was purified
by repeated crystallization. Both samples were char-
acterized by LC–MS. Other melamine derivatives
Žused in this study included: TMCT, TBCT tris–butyl
. Žcarbamoyl triazine and NMTBCT N-methyl substi-
.tuted TBCT . Although these melamine derivatives
are generally considered safe, they should be handled
with the usual safety precautions.
2.4. Infrared and Raman measurements
All IR spectra were measured using a Digilab
FTS-60A spectrometer at 4 cmy1
resolution with 16
scans. The FT-Raman spectra were measured using a
BioRad FT-Raman accessory attached to a Digilab
FTS-6000 spectrometer with 1064 nm Nd:YAG exci-
tation. The FT-Raman spectra were collected at 4
cmy1
resolution with 100 scans.
3. Results and discussion
3.1. IR and Raman spectra of TMMM, HMMM and
TMCT
Figs. 4 and 5 show the FT-IR and FT-Raman
spectra of TMMM and HMMM in the solid state.
Tables 1 and 2 list selected IR and Raman bands, as
well as vibrational descriptions for the assigned bands
of TMMM and HMMM, respectively. The vibra-
tional descriptions in the tables list the largest com-
ponent vibration first and the lesser components
after. For example, the strong TMMM band at 1515
cmy1
is assigned as a triazine quadrant stretchqNH
bendqCH open. Also listed in both tables are the2
Ž .scaled calculated wave numbers calculated=0.9
for the gas-state molecules obtained from the Carte-
sian coordinate force field calculations, and the ratios
of the observed to the calculated wave numbers
Ž .which lead to a working scaling factor ca. 0.9 . The
experimental wave numbers are for the condensed
hydrogen-bonded state for TMMM and TMCT,
whereas the calculated wave numbers are for the gas
state with no hydrogen bonding. For TMMM and
TMCT, modes involving NH vibrations have a
greater wave number variation between the calcu-
Ž . Ž .lated gas and the observed solid . The calculated
IR band intensities are only roughly accurate but are
useful as a guide. Both TMMM and HMMM sam-
ples are highly purified and represent model com-
pounds of commercially available methylated
melamine–formaldehyde resins.
Fig. 6 shows the FT-IR and FT-Raman spectra of
TMCT. Table 3 lists selected IR and Raman bands,
the vibrational descriptions for the assigned bands,
the scaled calculated wave numbers and the calcu-
lated IR intensities. The TMCT sample represents a
model compound for a separate class of melamine
derivatives.
The band assignments listed in Tables 1–3 were
made by first matching the bands with strong calcu-
lated IR intensities to the strong observed bands and
noting the average scaling factors. Next, weaker
bands were assigned using similar scaling factors.
The exact observed-to-calculated wave number ratios
vary somewhat from band to band as seen in the
tables. However, when the same type of MO calcula-
tion is done for a set of related compounds, the ratios
of the observed-to-calculated wave numbers are
nearly the same for vibrations of similar form. This
is one of the reasons we have investigated the three
different melamine derivatives together.
3.2. NH and CH stretching Õibrations
In general, both NH and CH stretching frequen-
cies are considerably higher than either C–O or C–N
stretching frequencies and are expected to be me-
chanically independent of the rest of the molecules
w xrepresented here 18 . Consistent with this, the NH
and CH stretching vibrations of melamine deriva-
tives do not mechanically couple with any triazine
ring vibrational modes. The vibrations of the CH
stretching region of the triazine–N–CH –O–CH2 3
group of TMMM and HMMM are illustrated in Fig.
7. Table 4 lists the CH stretching bands and the
vibrational descriptions for the assigned bands.
Previous work has established that the individual
CH bonds of methyl or methylene groups are not
identical when these groups are adjacent to an oxy-

Fig. 4. The solid-state FT-IR and FT–Raman spectra of TMMM. For the IR spectrum, TMMM was prepared as a methylene chloride cast
film on KRS5. Both the IR and Raman spectra were measured at 4 cmy1
resolution.
w xgen or nitrogen heteroatom 19–22 . In the O–CH3
Ž .group, the CH bond trans to the attached O–C H2
bond has a higher force constant than the other 2 CH
bonds in the methoxy group. As shown in Fig. 7a,
the out-of-phase CH stretch symmetric to the COC3
plane involves mainly the stretch of the CH bond
Ž .which is trans to the O–C H bond. Similarly, the2
in-phase CH symmetric stretch shown in Fig. 7e3
involves mainly the other 2 CH stretches, while the
amplitude of the trans CH stretch is quite small. For
Fig. 5. The solid-state FT-IR and FT–Raman spectra of HMMM. For the IR spectrum, HMMM was prepared as a methylene chloride cast
film on KRS5. Both the IR and Raman spectra were measured at 4 cmy1
resolution.

Table 1
Ž .Selected band assignments for TMMM Tris-methyoxy methylol melamine
y1Ž .Calculated Description of vibration Observed Observed cm Calculated Ratio Observedr
y1 y1Ž . Ž . Ž .cm cm Raman solid relative IR Calculated
Ž . Ž .=0.9 gas IR solid intensity
1629 Exo C–N i-ph. str. 1608 w 0 0.888
1564 Exo C–N contr.qCH bendqquad. str.qNH bend 1590–65 s 3 0.9082
1556 CH bendqexo C–N str. 1552 mw 0 0.8992
1534 Quad. str.qNH bendqCH open 1515 vs 9 0.8892
1483 Semi str.qexo C–N contr.qCH i-ph. bendqCH bend 1464 ms 1465 m sh 5 0.8883 2
1481 CH i-ph. bendqCH wag, opp. direction 1448 vs 0 0.8803 2
1470 Semi str.qexo C–N contr.qCH i-ph. bend 1434 m 5 0.8783
1414 CH wagqi-ph. CH bendqquad. str. 1372 s 1369 mw 7 0.8732 3
1198 CH rk. sym. to COC plane 1176 m 1 0.8843
1151 CH rk. asym. to COC plane 1136 w 1 0.8883
1138 N–C–O–C str.–contr.–str.qNH bendqsemi str. 1098 ms 1104 w 2 0.869
1106 C–O–C o-ph. str.qsemi str. 1071 s 5 0.870
952 C–O–C i-ph. str.qsext. i-pl. bendqCH rk. 923 s 0 0.8722
927 Ring N i-ph. radial vibration 976 vs 0 0.948
919 C–O–C i-ph. str.qsemi str. 905 m 900 s 1 0.881
885 Sext. o-pl. bend 817 m 3 0.831
575 NH i-ph. o-pl. wag 600 mw 4 0.939
The first column lists the calculated wave numbers scaled by 0.9 for comparison. The last column provides the actual scaling factor obtained
by taking the ratio of the observed wave number relative to the calculated wave number.
Ž . Ž . ŽAbbreviations used: contr. contract ; exo C–N the C–N bond directly attached to the ring ; i-ph., o-ph. in-phase and out-of-phase,
. Ž . Ž . Ž . Ž . Ž .respectively ; i-pl., o-pl. in-plane and out-of-plane, respectively ; quad. quadrant ; rk. rock ; semi semi-circle ; sext. sextant ; str.
Ž . Ž .stretch ; sym., asym. symmetric and asymmetric, respectively .
TMMM and HMMM, the out-of-phase CH stretch3
occurs at 2988–2980 cmy1
and the in-phase CH3
symmetric stretch occurs at 2824–2821 cmy1
. For
Ž .the C 5O –O–CH substituent groups in TMCT,3
these O–CH bands are observed at 2965–29553
cmy1
and 2860–2850 cmy1
. The 2824 cmy1
band
is particularly useful to monitor the methoxy group
in methylated melamine–formaldehyde resins, since
it is isolated from other CH stretching vibrations.
The methylene in- and out-of-phase stretching for
TMMM and HMMM mechanically couple with the
CH stretches of the methoxy group as shown in Fig.
7b,c,d. Two of these modes involve the out-of-phase
CH stretches of both the methylene and methyl
Žgroups and differ only in relative phase see Fig.
.7b,c . The in-phase methylene stretch shown in Fig.
7d involves mostly the methylene stretch with only a
very small component from the methoxy group.
The three NH stretching vibrations of a tri-sub-
Žstituted melamine derivative such as TMMM or
.TMCT are shown in Fig. 8. The NH bonds are those
shown with displacement vectors. The NH stretching
and contracting vibrations have been labeled with an
‘S’ and ‘C’, respectively, for clarity. The Raman
active symmetric in-phase NH stretching mode is
labeled ‘a’ and the two degenerate IR active out-of-
phase NH stretching modes are labeled ‘b’ and ‘c’.
The NH stretching vectors for the two degenerate
modes differ in both amplitude and phase. Using a
standing wave description, nodal lines separate
molecular segments which vibrate out-of-phase with
w xrespect to each other 18 . A nodal line is used to
Ž .divide the ring into 2 halves see Fig. 8b,c , such that
the individual NH vibrations in each half differ in
phase. The nodal lines for the two degenerate out-
of-phase NH stretching modes are perpendicular to
each other. Where the nodal line bisects the exoge-
Ž .nous C–NH group see Fig. 8b , that NH group does
not vibrate. Where the nodal line bisects the ring
C–N bonds, all three NH groups vibrate as shown in
Fig. 8c. The substituent phase relationship shown in
Fig. 8a,b,c are also observed for other substituent
vibrations. The Raman active totally symmetric vi-
brations of the substituent groups can interact with

Table 2
Ž .Selected band assignments for HMMM hexa-methoxy methylol melamine
1595 Exo C–N i-ph. str. 1630 w 0 0.920
1523 Quad. str.qCH o-ph. bendqCH bend 1585 w sh 1 0.9373 2
1503 Quad. str.qexo C–N contr.qCH open 1554 vs 10 0.9312
1472 Semi str.qexo C–N contr.qCH i-ph. bend 1490 ms 3 0.9113
1424 CH wag 1452 s 0 0.9182
1411 Quad. str.qexo C–N contr.qCH wag 1453–39 m 7 0.9222
1323 CH twistqexo C–N i-ph. str. 1403-1391 m 0 0.9502
1316 Semi str.qexo C–N contr.qCH twist 1386 s 3 0.9482
1279 Semi str.qCH twist 1345 mw 1344 w 1 0.9472
1231 Quad. str.qCNC o-ph. str.qCH twist 1305 mw 1306 w 3 0.9542
1168 Semi str.qCH rk. 1192 mw 1193 w 1 0.9183
1081 C–O–C o-ph. str. 1081 ms 1091 w 6 0.900
Ž .1030 Semi str.qexo C–N str.qN CH i-ph. str. 1019 m 1018 w 3 0.8912 2
Ž .978 N CH i-ph. str.qCH rk. 946 ms 0 0.8732 2 2
947 Ring N i-ph. radial vibration 980 s 0 0.932
936 C–O–C i-ph. str.qsext. o-pl. bend 911 m 3 0.876
929 C–O–C i-ph. str. 908 ms 0 0.877
878 Sext. o-pl. bend 813 mw 1 0.834
. Ž . Ž . Ž . Ž . Ž . Ž .respectively ; o-pl. out-of-plane ; quad. quadrant ; rk. rock ; semi semi-circle ; sext. sextant ; str. stretch .
A symmetric ring modes but not the doubly degen-1
erate modes such as the quadrant or semi-circle
stretches. Conversely, the out-of-phase vibrations of
the substituents can interact with these doubly de-
generate modes, but not with ring modes of A1
symmetry.
Fig. 6. The solid-state FT-IR and FT-Raman spectra of TMCT. For the IR spectrum, TMCT was prepared as a chloroform cast film on
KRS5. Both the IR and Raman spectra were measured at 4 cmy1
resolution.

Table 3
Ž .Selected band assignments for the coplanar form of TMCT tris-methoxy carbamoyl triazine
1765 C5O o-ph. str. 1755 s 3 0.895
1761 C5O i-ph. str. 1767 m 0 0.903
1596 Exo C–N i-ph. str.qCNH str. bend 1622 w 0 0.915
1583 Quad. str.qNH bend 1608 m 2 0.914
1480 Exo C–N str.qNH bendqCH i-ph. open 1517 m 0 0.9233
1479 Quad. str.qexo C–N contr.qNH bend 1492 vs 10 0.901
1462 Semi str.qexo C–N contr.qCH i-ph. bend 1440 m 3 0.8873
1453 Exo C–N str.qNH bendqCH i-ph. bend 1458 s 0 0.9033
1371 Semi str.qexo C–N contr.qNH bend 1310 m 1 0.860
1203 N–C–O o-ph. str.qNH bendqsemi. str. 1198 vs 9 0.896
1180 CH rk. sym. to COC pl. 1146 mw 1 0.8743
1124 Semi str.qNH bend 1016 mw 1 0.813
948 Ring N i-ph. radial vibration 987 vs 0 0.937
w Ž . x941 Sext. o-pl. bendqNH wag N H moves with ring N 822 mw 4 0.787
w Ž . x815 Sext. o-pl. bendqNH wag N H moves with ring C 769 mw 2 0.849
. Ž . Ž . Ž . Ž . Ž . Ž . Ž .respectively ; o-pl. out-of-plane ; pl. plane ; quad. quadrant ; rk. rock ; semi semi-circle ; sext. sextant ; str. stretch ; sym.
Ž .symmetric .
3.3. Methyl and methylene bending Õibrations
Unlike the CH stretching region, the various
methyl and methylene bending vibrations mechani-
cally couple with various triazine ring modes. Table
5 lists the spectral regions that involve mixing of the
aliphatic CH and CH vibrations with the triazine2 3
ring modes. For many of the IR bands observed in
this region, the intensities are derived from the com-
ponents involving a triazine ring mode and not the
aliphatic CH and CH vibrations. In the case of2 3
Ž .TMMM, moderately strong bands near 1448 R vs
y1 Ž .and 1176 cm IR m are derived from mechani-
cally independent vibrations involving methyl and
Fig. 7. The CH stretching vibrations of the triazine–N–CH –O–CH group. The amplitude of the calculated vectors have been multiplied2 3
by 5 for clarity.

Table 4
CH stretching bands of the triazine–N–CH –O–CH group2 3
y1Ž .Observed band cm Description: CH stretching bands in triazine–N–CH –O–CH2 3
Ž .2988–2980 IR m, R ms: mainly CH stretch in CH , trans to O–C H bonds3 2
2954–2939 IR s: CH and CH , o-ph. stretch in same direction, asym. to C–O–C pl.2 3
2954–2939 IR w, R s: CH and CH , o-ph. stretch in opposite direction, asym. to C–O–C pl.2 3
2904–2896 IR m, R s: mainly CH i-ph. stretch2
2824–2821 IR m, R ms: mainly CH i-ph. stretch3
Ž . Ž . Ž .Abbreviations used: asym. asymmetric ; i-ph., o-ph. in-phase and out-of-phase, respectively ; pl. plane .
methylene bend, and wag vibrations. For HMMM,
y1 Ž .only the CH wag at 1452 cm R s is mechani-2
cally independent.
TMCT also has some important bands in this
region that are mechanically independent from the
triazine ring modes. This includes the 1146 cmy1
Ž .band IR mw which results from the CH rock. A3
y1 Ž .strong band observed at 1198 cm IR vs results
mostly from the N–C–O out-of-phase stretch and is
only weakly coupled to the triazine ring semi-circle
stretch. The remaining bands derive from aliphatic
substituent vibrations which are mechanically cou-
pled with the triazine ring modes.
3.4. Bands inÕolÕing the C–O–C stretch
We assign the C–O–C out-of-phase stretching
modes for TMMM and HMMM to the bands in the
y1 Ž .1081–1071 cm region IR ms, R w . This falls
within the well-known CH –O–CH ether group2 2
w xfrequency region 18 . Our calculated wave numbers
Ž . y1
scaled by 0.9 are 1106–1081 cm as seen in
Tables 1 and 2.
The in-phase C–O–C stretching mode in TMMM
and HMMM is assigned by us to the bands in the
y1 Ž .911–900 cm region IR m, R ms . Our calculated
Ž . y1
wave numbers scaled by 0.9 are 936–919 cm as
seen in Tables 1 and 2. The earlier work by Scheep-
w xers et al. 15 on hexa-methoxymethyl melamine
Ž . y1
HMMM also assigns a 910 cm Raman band to
the C–O–C in-phase stretching mode. In a later,
w xmore extensive work, Meier et al. 13 interpreted
this 910 cmy1
band to result instead from the pure
CH rock vibration in the N–CH –O group. There2 2
are two arguments against the later assignment. In
HMMM, the intensity of the band in the 910 region
is medium–strong in the Raman spectra and medium
in the IR spectrum. This is consistent with the inten-
sities expected for a C–O–C in-phase stretching
band. For example, in dimethyl ether, the in-phase
Fig. 8. The three NH stretching vibrations of TMMM. The symmetric in-phase NH stretching mode is labeled ‘a’ and the 2 degenerate
out-of-phase NH stretching modes are labeled ‘b’ and ‘c’. The NH bonds are those with the displacement vectors which have been
Ž .multiplied by 5 for clarity. The displacement vectors for the 2 degenerate modes b and c differ both in phase and amplitude. For b and c,
Ž .we have divided the ring with a nodal line where the 2 halves differ in phase i.e., stretching and contracting of the NH group.

Table 5
Regions involving mixing of aliphatic CH and CH vibrations2 3
with triazine ring modes
y1Ž .Region cm Description: CH and CH vibrations2 3
that mix with triazine ring modes
1588–1515 CH bend2
;1500 CH o-ph. bend sym. to C–O–C pl.3
;1485 CH o-ph. bend asym. to C–O–C pl.3
1464–1372 CH i-ph. bend3
1464–1254 CH wag2
1345–1195 CH twist2
1176–1043 CH rk. sym. to C–O–C pl.3
1192–1136 CH rk. asym. to C–O–C pl.3
ŽAbbreviations used: i-ph., o-ph. in-phase and out-of-phase, re-
. Ž . Ž . Žspectively ; pl. plane ; rk. rock ; sym., asym. symmetric and
.asymmetric, respectively .
C–O–C stretching band intensity at 924 cmy1
is
strong in the Raman spectrum and medium in the IR
w xspectrum 23,24 . On the other hand, the X–CH –X2
pure CH rock band intensity is expected to be quite2
weak in the Raman and IR spectra. For example, in
CH Cl , the CH rock band at 896 cmy1
is very2 2 2
w xweak in both the IR and Raman spectra 24,25 . In
our other argument against this assignment, we note
Žthat in butoxymethylol melamine where an O–C H4 9
.group is substituted for the O–CH group , the 9103
y1 w xcm IR band observed in HMMM disappears 26 .
It is replaced by weaker, lower frequency bands,
where the attached C–C bonds interact with the C–O
bonds. In support of this, we note that in di-n-butyl
ether, a Raman band is observed at 840 cmy1
which
involves the in-phase stretch of all of the C–O and
w xC–C bonds 23 . The N–CH –O group in alkylated2
melamine–formaldehyde derivatives is unchanged by
the substitution of the butoxy for the methoxy groups,
and the isolated CH rock vibration should not be2
markedly different from that in HMMM.
3.5. Quadrant stretch Õibrations: melamine deriÕa-
tiÕes
Standing vibrational waves can be used to dramat-
ically simplify the descriptions of the complex vibra-
w xtions of aromatic and heteroaromatic rings 16,18,27 .
Unsubstituted 6-member rings have 12 elementary
vibrations which include 6-ring stretches, 3 in-plane
ring bends and 3 out-of-plane ring bends. These are
described using nodal lines to define the sextant,
quadrant, semi-circle and whole ring vibrations
w x16,27 . The two triazine ring quadrant stretching
vibrations are each characterized by two perpendicu-
lar nodal lines separating the ring into 4 quadrants.
Each nodal line separates quadrants vibrating in op-
posite phases, such as stretching and contracting. In
these melamine derivatives, it is not necessary for
the two nodal lines to pass through the symmetric
elements of the ring.
Typically, in the quadrant stretch, the triazine
carbon and nitrogen atoms move in-plane in approxi-
mately opposite directions, resulting in strong, char-
acteristic bands at 1600–1500 cmy1
. The two quad-
rant stretch vibrations are often degenerate or nearly
degenerate for melamine derivatives, where the three
substituents are identical.
The highest frequency vibration for the triazine
ring of melamine derivatives not involving either CH
or NH stretches is the in-phase stretch of the C–N
bonds exogenous to the triazine ring. These C–N
bonds involve the triazine ring carbon atom attached
to the external nitrogen atom. This vibration results
in a weak Raman band between 1600–1635 cmy1
indicating a high force constant for this bond, which
is stiffened by resonance with the triazine ring bonds.
An important consequence of this is the interaction
of the exogenous C–N stretching vibrations with
ring stretching vibrations such as the quadrant stretch.
Typically, at frequencies above 1400 cmy1
, the ex-
ogenous C–N group stretches out-of-phase with the
attached part of the ring during the quadrant ring
stretching vibration. The quadrant stretching vibra-
tion also mechanically couples with vibrations in-
volving the external N substituent groups.
Fig. 9 shows the quadrant ring stretches for
TMMM and HMMM observed above 1500 cmy1
.
Each of these bands correspond to degenerate or
nearly degenerate quadrant stretching vibrations me-
chanically coupled to vibrations involving the N–
CH –O–CH group. Because of phasing complexi-2 3
Ž .ties of the three or six attached groups, only 1 of
the attached –CH –O–CH groups is shown. How-2 3
ever, the relative phasing and amplitudes of the
remaining groups can be worked out as in Fig. 8. For
example, when determining the relative phases for
TMMM substituents, the same 2 modes used in Fig.
8 are employed, resulting in 2 sets of modes with

Fig. 9. The quadrant ring stretching vibrations for TMMM and HMMM observed above 1500 cmy1
. Each of the observed IR bands
corresponds to degenerate or nearly degenerate components of the triazine ring quadrant stretch mechanically coupled to vibrations
Ž .involving the N–CH –O–CH groups. Because of phasing complexities of the three or 6 attached groups, only 1 of the attached2 3
–CH –O–CH groups are shown on the right hand side of the figure. The relative phasing and amplitudes of the remaining groups can be2 3
worked out as in Fig. 8. The calculated amplitude vectors have been multiplied by 10 for clarity.
different phases as well as 1 mode in which all of the
substituents have identical phases.
Table 6 summarizes the bands involving the quad-
rant stretching vibrations above 1490 cmy1
for
TMMM, HMMM, and TMCT. In TMMM, strong
mechanical interaction involving the NH and CH2
bending vibrations with the triazine quadrant stretch
vibration results in bands at 1590–1565 and 1515
cmy1
. The bands at 1590–1565 cmy1
also involve
the exogenous C–N stretch. In HMMM, the triazine
quadrant stretch is coupled with the CH and CH2 3
bending vibrations, resulting in bands at 1585 and
1554 cmy1
. The 1554 cmy1
band also involves the
contraction of the exogenous C–N group. The two
sets of bands involving the degenerate quadrant
stretching modes of TMMM and HMMM results in a
difference of almost 50 cmy1
compared to only 30
cmy1
, respectively. The larger separation between
the two sets of quadrant stretching modes in TMMM
is a result of the mechanical coupling with the NH
bending vibration.
The CH and CH bend, wag, and twist vibra-2 3
tions are somewhat higher than typical of a hydro-
carbon. This is a result of coupling with the exoge-
nous C–N stretch, as well as the local environment
of the methylene group between 2 electronegative

Table 6
Quadrant stretching bands for TMMM, HMMM, and TMCT
y1Ž .Melamine derivative Observed band cm Description of vibration
a
TMMM 1588–1566 , 1515 Quad. str.qCH bendqNH bend2
HMMM 1585 sh, 1554 Quad. str.qCH bend2
TMCT coplanar 1607, 1494 Quad. str.qCNH bendrstr.
TMCT non-coplanar 1559 Quad. str. and mechanically independent CNH bendrstr.
a
Band multiplicity due to hydrogen bonding differences.
Ž . Ž .Abbreviations used: quad. quadrant ; str. stretch .
groups. As shown in Fig. 9 for TMMM and HMMM,
the CH bend interacts in two different phases with2
the quadrant stretching modes.
Mechanical coupling of the carbamate substituent
vibration with the quadrant stretch has dramatic ef-
fects on the vibrational spectra of TMCT. Surpris-
ingly, neither the highly characteristic 1550 cmy1
band of the carbamate group, which involves the NH
bendrC–N stretching mode, is observed, nor is the
typical intense melamine ring quadrant stretch band.
As shown in Fig. 10, the quadrant stretch mechani-
cally couples with the carbamate CNH bend–stretch,
resulting in 2 bands at 1608 and 1492 cmy1
. This
interaction results in a difference of almost 100
cmy1
, which is twice that observed for TMMM as a
result of strong mechanical coupling. Fig. 10 shows
that in TMCT, the NH bond rotates either clockwise
or counter-clockwise relative to the neighboring vi-
brating triazine ring atoms. These two vibrations are
coupled because not only do they exert forces on
each other, but also because their frequencies are
similar. This mechanical interaction requires that the
carbamate substituent is coplanar with the triazine
ring. If the carbamate group is not coplanar, bands
will be observed in the usual region near 1559 cmy1
,
which enables us to make a clear distinction between
the coplanar and non-coplanar forms of TMCT. The
coplanar and non-coplanar orientation of the carba-
mate group relative to the triazine ring in TMCT is a
function of the solvent used to crystallize this
melamine derivative. Care was taken that the copla-
nar form of TMCT was dominant in the IR and
Raman spectra shown in this work.
Fig. 11 shows the IR spectra from 1800–1400
y1 Ž .cm of TBCT tris–butyl carbamoyl triazine and
N-methyl TBCT which further demonstrate the effect
of the mechanical interaction of the CNH bend–
stretch of the carbamate group with the triazine
Ž . y1
Fig. 10. The quadrant ring stretching vibrations =10 for the coplanar form of TMCT observed above 1490 cm . In TMCT, strong
interaction occurs between the triazine quadrant stretch and the carbamate C–N stretchrN–H bending vibration resulting in bands at 1607
and 1494 cmy1
.

Ž . Ž .Fig. 11. The IR spectra of TBCT tris-butyl carbamoyl triazine and TNMBCT tris-N-methyl butyl carbamoyl triazine between 1800–1400
cmy1
. Substitution of the carbamoyl NH group with a methyl group results in a quadrant stretch band at 1566 cmy1
rather than 1607 and
1492 cmy1
.
quadrant stretch. TBCT differs from TMCT only in
substitution of a butyl group for a methyl group on
the carbamate substituent. The IR spectrum of the
coplanar form of TBCT is similar to TMCT and
shows bands at 1755 cmy1
from the carbonyl C5O
stretch, and at 1607 and 1482 cmy1
from the quad-
rant stretch interacting with the carbamate CNH
bend–stretch. The N-methylated TBCT molecule no
longer has an NH group but rather an N–CH group3
in the carbamate substituents. Thus, there is no longer
a CNH bend–stretch vibration to mechanically cou-
ple with the triazine quadrant stretch in the coplanar
form. The IR spectrum of the N-methyl-substituted
TBCT shows a band at 1566 cmy1
which involves
the quadrant stretch for both the coplanar and non-
coplanar form, and no bands at 1607 and 1482
cmy1
. Two carbonyl bands observed at 1755 and
1719 cmy1
are from the C5O stretches of the
coplanar and non-coplanar form of the N-methyl
carbamate groups, respectively.
Orientation of the carbamate group coplanar with
the triazine ring in TMCT results in carbonyl bands
y1 Ž .at 1780–1745 cm see Fig. 6 Multiple carbonyl
bands in this region can result due to variation in
hydrogen bonding involving the carbonyl and NH
groups of TMCT. The non-coplanar orientation of
the carbamate group relative to the triazine ring
discussed previously results in a carbonyl band at
1722–1719 cmy1
. The coplanar form has a higher
C5O stretching frequency due to the mesomeric
electron-withdrawing effects of the triazine ring on
the external N–C5O nitrogen atom, which competes
with the electron-withdrawing carbonyl group of this
same nitrogen. This effect is reduced in the non-
coplanar form, so that the carbonyl group can with-
draw more electrons from the nitrogen, lowering the
carbonyl frequency to that more typical of secondary
carbamates.
The Raman spectra of TMCT have prominent
bands from the non-coplanar orientation of the sub-
stituents relative to the triazine ring. The Raman
intensity of the non-coplanar carbonyl band is much
more intense than the coplanar form. Precipitating
TMCT from different solvents results in different
populations of the coplanarrnon-coplanar form
which we characterized using IR spectroscopy. This

enabled us to assign the TMCT Raman carbonyl
band at 1720 cmy1
to a non-coplanar carbamate
group and the 1747 and 1765 cmy1
bands to the
coplanar species of the carbamate groups with two
differently hydrogen-bonded forms.
3.6. Semi-circle stretch Õibrations: melamine deriÕa-
tiÕes
The triazine ring semi-circle stretching vibration
is characterized by a single nodal line which divides
the ring into 2 halves. The two segments of the ring
vibrate out-of-phase with respect to each other. If a
nodal line bisects a bond, that bond will not change
in length during the vibration. Similar to the quad-
rant stretch, there are two degenerate or nearly de-
generate components for the semi-circle stretch.
Table 7 summarizes the significant bands involving
the semi-circle stretching vibration between 1490–
1285 cmy1
for TMMM, HMMM, and TMCT, and
an abbreviated description of the vibrations. More
complete descriptions of the vibrations are found in
Tables 1–3. Common to all three melamine deriva-
tives is the mechanical interaction of the exogenous
C–N stretch out-of-phase with the triazine ring
semi-circle stretch. However, this coupling is more
pronounced in the higher frequency band in the
selected pairs of the semi-circle stretching vibrations.
The triazine ring semi-circle stretch also mechani-
cally couples with vibrations involving the external
nitrogen substituent groups.
Fig. 12 shows the semi-circle ring stretches for
TMMM and HMMM observed at 1390 cmy1
and
above. Because of phasing complexities involving
these vibrations, only one of the attached –CH –O–2
CH groups are shown. In TMMM, the exogenous3
C–N contract and the CH in-phase bend strongly3
interact with the triazine semi-circle stretch vibra-
tions, resulting in moderately strong bands at 1464
Ž . y1 Ž .IR, R and 1434 cm IR . The higher frequency
band involving the semi-circle stretch at 1464 cmy1
also involves the CH bend and is more strongly2
coupled to the exogenous C–N vibration than the
1434 cmy1
band. In HMMM, the ring semi-circle
stretch also involves an interaction with the exoge-
nous C–N vibration, resulting in strong IR bands at
1490 and 1386 cmy1
. The 1490 cmy1
band is not
only strongly coupled with the exogenous C–N con-
tract but also with the CH in-phase bend. The lower3
frequency 1386 cmy1
band also involves the CH2
twist, but is only weakly coupled with the exogenous
C–N contract.
Fig. 13 shows the semi-circle ring stretches for
TMCT observed above 1300 cmy1
. The bands ob-
served for the TMCT melamine derivative also in-
clude vibrations involving the methyl carbamate sub-
stituents. The moderately strong IR band at 1440
cmy1
and the weaker 1310 cmy1
band of TMCT
both involve the semi-circle stretching vibration me-
chanically coupled with the exogenous C–N con-
tract, the CH in-phase bend and the NH bend. The3
NH bend and the CH in-phase bend differ in their3
phase relative to the semi-circle stretch for these two
TMCT bands. Mechanical coupling of the exogenous
C–N stretch with the semi-circle ring vibration is
more pronounced in the higher frequency 1440 cmy1
band. Lastly, the TMCT 1310 cmy1
band also in-
volves some C–O stretch of the carbamate group.
3.7. Raman actiÕe triazine ring nitrogen radial in-
phase Õibration
Two particularly intense Raman ring modes found
in melamine are the ring nitrogen radial in-phase and
w xthe carbon radial in-phase vibrations 13,14,16,17 .
The form of these two vibrations results from the
mixing of the whole ring in-phase stretch with the
in-plane ring bend by sextants, in- and out-of-phase
Table 7
Semi-circle stretching bands for TMMM, HMMM, and TMCT
y1Ž .Melamine derivative Observed band cm Description of vibration
TMMM 1464, 1434 Semi str.qexo CN contr.qCH i-ph. bend3
HMMM 1490, 1386 Semi str.qexo CN contr.qCH i-ph. bendrCH twist3 2
TMCT coplanar 1440, 1310 Semi str.qexo CN contr.qCH i-ph. bendrNH bend3
Ž . Ž . Ž . Ž . Ž .Abbreviations used: contr. contract ; exo exogenous ; i-ph. in-phase ; semi semi-circle ; str. stretch .

Ž . y1
Fig. 12. The semi-circle stretching vibrations =10 for TMMM and HMMM observed between 1490 and 1386 cm . The semi-circle
stretching vibration interacts with the CH twist or wag, the CH in-phase bend, and the exogenous C–N contract.2 3
Ž . y1
Fig. 13. The semi-circle stretching vibrations =10 for the coplanar form of TMCT observed between 1440–1300 cm .

Ž .Fig. 14. The ring nitrogen radial in-phase vibration =5 for
melamine derivatives. For TMMM, HMMM, and TMCT, this
vibration does not couple with the substituent groups and is found
at 992–969 cmy1
.
w x16,18 . The carbon radial in-phase vibration makes a
poor Raman group frequency for melamine deriva-
tives, since it is strongly mechanically coupled with
substituent vibrations. In melamine, the 674 cmy1
carbon radial in-phase vibration involves the exoge-
nous C–N groups moving radially in-phase, but does
not involve a significant change in either C–N bond
w xlength or rotation 13 . The related vibration of
TMMM at 674 cmy1
, involves only a slight change
in the exogenous C–N bond lengths and no radial
movement of the triazine ring carbon atoms. Rather,
the exogenous C–N groups rotate sharply, thereby
changing the C–N bond angles with the ring in an
out-of-plane manner. A more accurate description of
the 674 cmy1
band vibration is an N C–N in-plane2
bend plus a C–O–C in-phase stretch.
The ring nitrogen radial in-phase vibration shown
in Fig. 14 makes an excellent Raman group fre-
w xquency in all normal melamine derivatives 13,14 .
In this vibration, the three-ring nitrogens move radi-
ally in-phase, while all the other atoms for both the
Table 8
Observed frequencies for the Raman active triazine ring nitrogen
radial in-phase vibration in melamine derivatives
Melamine derivative Raman: triazine ring N, in-phase
y1Ž .radial vibration 992–969 cm
s-Triazine 992
Melamine 981
TMMM 976
HMMM 980
a
TMCT 987, 970
a
Non-coplanar form of TMCT.
ring and the substituents are nearly motionless. Table
8 shows the observed frequencies for the triazine
ring nitrogen radial in-phase stretch for s-triazine,
melamine, TMMM, HMMM, and TMCT. This Ra-
man active band is found at 992–969 cmy1
and is
relatively insensitive to changes in the substituent
groups.
w xBoth Scheepers et al. 15,17 and Meier et al.
w x13,14 presented the form of the vibrations of the
Raman active 975 and 675 cmy1
bands for a variety
of melamine–formaldehyde resins and found a loss
of band intensity and change in the form of the mode
of the 675 cmy1
band upon methylolation. By fol-
lowing the ratio of the 675, relative to the 975 cmy1
w xRaman bands, Scheepers et al. 17 were able to
characterize the free melamine content in soluble and
cured resins.
3.8. IR actiÕe triazine ring sextant out-of-plane bend
The IR spectra of melamine derivatives are char-
acterized by a medium intense, sharp band at 815"7
cmy1
which involve the well-known triazine ring
w xsextant out-of-plane bend 18,27 . Three nodal planes
separate the ring into 6 sections which move alterna-
Žtively up and down out of the ring plane see Fig.
.15 . The frequency of this band has been correlated
Ž .Fig. 15. The out-of-plane ring sextant bend =5 . For TMMM
and HMMM, this mode is nearly identical in form and occurs at
817 and 813 cmy1
, respectively. In TMCT, significant interaction
occurs between the out-of-plane vibration of the NH–C5O group,
resulting in bands at 822 and 769 cmy1
.

to the aromaticity of the triazine ring in melamine
w xderivatives 18,27 . The heteroaromatic triazine ring
of melamine derivatives are characterized by three
delocalized double bonds in the ring, resulting in a
band at 815 cmy1
. This band occurs below 800
cmy1
for melamine derivatives when the triazine
ring has less than three double bonds in the ring and
at least one double bond external to the ring. Typical
examples of the iso form of melamine derivatives
include melamine HCl salts which result from proto-
nation of the ring nitrogen, and oxidized melamine
rings such as ammeline and isocyanurates.
Fig. 15 shows the triazine ring sextant out-of-plane
bend for TMMM, HMMM, and TMCT, and Table 9
shows the observed frequencies. This vibration is
characterized by the three-ring carbon atoms moving
in-phase and out-of-plane in one direction, while the
three-ring nitrogen atoms similarly move in the op-
posite direction with slightly less amplitude. For
TMMM and HMMM, the exogenous nitrogens move
in the same direction as the ring nitrogens but with
less amplitude. This minor mechanical coupling with
the exogenous nitrogen groups makes the sextant
out-of-plane ring bend a good group frequency for
most melamine derivatives.
In TMCT, significant mechanical coupling occurs
between the sextant out-of-plane ring bend and the
carbonyl wag of the carbamate substituent groups,
resulting in 2 bands at 822 and 769 cmy1
. As shown
in Fig. 15, the carbamate carbonyl wag interacts in
two different phases with the triazine ring sextant
out-of-plane bend for TMCT. For the 822 cmy1
band, the triazine ring and carbonyl carbon atoms
move in-phase and out-of-plane in one direction,
while the triazine ring and exogenous nitrogen atoms
move together in the opposite direction. For the 769
cmy1
band, the triazine ring and carbonyl carbon
Table 9
Observed frequencies for the IR active triazine ring sextant out-
of-plane bend in melamine derivatives
Melamine derivative IR active triazine ring sextant
y1Ž .out-of-plane bend 810–825 cm
Melamine 814
TMMM 817
HMMM 813
Ž .TMCT 822 q769
atoms move out-of-phase as the triazine ring and
exogenous nitrogen atoms do.
4. Conclusions
The present study uses ab initio force field calcu-
lations for the vibrational analysis of the IR and
Raman spectra of three different structural types of
melamine derivatives. Detailed assignments are made
for selected IR and Raman active vibrations of
TMMM, HMMM, and TMCT, and the important
characteristic group frequencies are identified. We
find that IR spectroscopy generally provides a more
useful structural probe of melamine derivatives than
Raman spectroscopy does. The IR spectra provide a
greater selection of intense and moderately intense
bands that are good group frequencies compared
with the Raman spectra, whose narrower band widths
may be useful in certain other applications.
The various mechanical interactions of the sub-
stituent and triazine ring vibrations are systemati-
cally examined to understand the vibrational origin
of bands which provide good group frequencies for
melamine derivatives. Vibrational modes with little
or no mechanical coupling between the substituents
and the triazine ring provide the best group frequen-
cies. Consequently, superior group frequencies for
Ž .melamine derivatives include: 1 the NH and CH
Ž y1 .stretching vibrations observed above 2800 cm of
Ž .the substituent groups, 2 the carbonyl C5O stretch
Ž y1 .observed between 1790–1719 cm of substituent
Ž .groups on TMCT, 3 the Raman active CH wag of2
Ž y1 .TMMM and HMMM observed at ca. 1450 cm ,
Ž .4 the Raman active triazine ring nitrogen radial
Ž y1 .in-phase stretch observed at 992–969 cm , and
Ž .5 the IR active triazine ring sextant out-of-plane
Ž y1 .bend observed at 822–809 cm .
Despite the complications mechanical coupling
between the substituent and triazine ring vibrations
introduce, numerous bands exist in the IR and Ra-
man spectra which provide good group frequencies.
Many of these bands involve coupling of substituent
vibrations with the triazine ring quadrant and semi-
circle stretches. These substituent vibrations include:
Ž .1 the aliphatic CH and CH bends, wags, twists,2 3
Ž .and rocks; 2 the symmetric and asymmetric C–O–C

Ž .stretches; 3 the carbamate CNH stretchrbend and
Ž .stretchropen; and 4 the resonance-stiffened exoge-
nous C–N stretches. The IR spectra of melamine
derivatives have intense bands between 1608–1490
cmy1
involving the quadrant stretches with TMMM
having an additional band at 1372 cmy1
involving
the quadrant stretch. The semi-circle stretching vibra-
tions offer poorer group frequencies and modes that
involve chiefly the semi-circle stretch that are typi-
cally found between 1490–1285 cmy1
.
Not surprisingly, many of the characteristic bands
between 1200–900 cmy1
involving the C–O stretch
of the substituent groups are weakly mechanically
coupled with triazine ring modes. For TMMM, this
includes the 1098 and 1071 cmy1
bands which
involve the C–O–C out-of-phase stretchqthe tria-
zine ring semi-circle stretch and the 906 cmy1
band
which involves the C–O–C in-phase stretchqthe
triazine ring semi-circle stretch. Further, the Raman
active 923 cmy1
band involves the C–O–C in-phase
stretchqthe triazine ring sextant in-plane bend. For
the structure of HMMM examined in this study, 2
bands involving the C–O stretch of the
methoxymethyl groups are mechanically independent
of the triazine ring modes. These bands are the
C–O–C in- and out-of-phase stretches at 908 cmy1
Ž . y1 ŽRaman active and 1081 cm IR and Raman
.active , respectively. Our assignment of the 908
cmy1
band agrees with an earlier assignment by
w xScheepers et al. 15 and disagrees with the more
w xrecent work by Meier et al. 13 , as discussed earlier.
The characteristic band of the carbamate group at
1198 cmy1
of TMCT similarly involves the out-of-
phase N–C–O stretchqthe triazine ring semi-circle
stretch.
The IR and Raman spectra of TMCT provide a
unique probe of the orientation of the carbamate
substituent group relative to the triazine ring. The
frequencies of the carbamate carbonyl band and many
of the triazine in-plane vibrations are sensitive to the
conformation of the carbamate carbonyl relative to
the triazine ring group. The coplanar conformation
has carbonyl bands between 1790–1747 cmy1
and
bands involving the triazine quadrant stretch at ca.
1608 and 1492 cmy1
. The non-coplanar conforma-
tion has carbonyl bands between 1722–1712 cmy1
and bands involving the triazine ring modes at 1556,
1422, and 1336 cmy1
. Future work will examine the
depth dependent changes in various cured
TBCTracrylic formulations using IR spectroscopy
and Dynamic Mechanical Analysis.
Acknowledgements
The authors gratefully acknowledge Dr. Prashant
S. Bhandare of Bio-Rad, Digilab Division, for his
help in the FT-Raman measurements of TMMM,
HMMM, and TMCT, and Dr. Dave Gschneidner and
Ž .Roger Rasch of CYTEC Industries for supplying us
the purified TMMM and HMMM. We also acknowl-
edge CYTEC Industries for support and permission
to publish this work.
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