Overview of information that can be obtained from mass spectra, C-13 and proton nmr spectra

1. Mass Spectroscopy &
NMR Spectroscopy
Big Picture

2. What information is contained
in each type of spectroscopy?
O Mass Spectroscopy
O Molecular weight from molecular ion peak
O Molecular formula from molecular
weight, M+1, M+2 peaks
O Functional group and hydrocarbon
branches from fragmentation patters

3. What information is contained
in each type of spectroscopy?
O 13C NMR
O Number of signals = types of chemically
nonequivalent carbons
O Chemical shift
O Alkane, alkene, aromatic carbons
O Presence (or absence) of bonded electron
withdrawing group (electronegative atom)

4. What information is contained
in each type of spectroscopy?
O 1H (proton) NMR
O Number of signals = types of chemically
nonequivalent hydrogens
O Chemical shift
O Alkane, alkene, aromatic hydrogens
O Presence (or absence) of nearby electron
withdrawing group (electronegative atom)
O Integration – number of hydrogens which give
rise to each signal
O Splitting patterns – number of hydrogens on
adjacent carbons

5. Mass Spectra: Information
from Molecular Ions
O Check M+
O Odd number – contains odd number N
O Even number – contains combination of
C, H, and possibly O
O Check M, M+2 peaks
O Two peaks of equal intensity Br present
O Two peaks, M+2 about 1/3 of M Cl
present

6. Mass Spectra: Molecular
Formula Determination
O Rule of 13 can be used for hydrocarbons (see
class notes). Before applying rule of 13
subtract out
O If N, Br or Cl present (previous slide), subtract
mass of lower molecular weight isotope
(halogens) from M peak value
O If IR is available, check for presence of O
functional groups and subtract O or O2
(carboxylic acid)
O Given relative abundance of M, M+1
peaks, use 13C abundance of 1.10% to
determine number of C’s in molecule (see
class notes)

7. Mass Spectra: Fragmentation Patterns
O Hydrocarbons will fragment at branches and favor
more stable fragment (radical) patterns
CH3-CH2-CH2-CH2-CH2-CH3 CH3CH2
CH3CH2CH2
CH3CH2CH2CH2

8. Mass Spectra: Fragmentation Patterns
H3C CH2 CH3
HC CH2
CH3
CH3CH2CH2
And (CH3)2CH
M-15 (CH3)

9. Mass Spectra: Fragmentation Patterns
CH3
H3C
C CH3 CH3CH2
CH2
H3C
(CH3)3C
M-15 (CH3)

10. 13C NMR
O How many different signals?
O Peak at 77 ppm is CDCl3 solvent
O # signals = how many chemically different
carbons
O Chemical shift
O High C=O (150-220 ppm)
O Mid alkene, aromatic, nitrile
O Low
O 15-60 ppm sp3 C
O 20-90 ppm sp3 C bonded to electronegative
atom

11. 1 H (Proton) NMR
O How many different signals? Gives how
many types of chemically nonequivalent
H’s

12. 1 H (Proton) NMR Chemical Shift
O High (9-12 ppm) aldehyde or carboxylic acid H
O Mid
O Aromatic H 6-8.5 ppm
O Phenol H 4.5-7.5 ppm
O Alkene H 4-6.5 ppm
O Low
O H on sp3 C 0.9 – 1.8 ppm
O H on sp3 C next to C=O or C=C (alkene or
aromatic), alkyne H 2-3 ppm
O H on sp3 C bonded to halogen, N or O 3.0-4.2
ppm
O NH or OH 1-6 ppm

13. 1 H (Proton) NMR
O Integration (area under peaks)
O Gives relative number of H giving rise to
each signal
O When done manually, results may be a bit
off
O Splitting patterns
O Split into n+1 peaks, n= number of H on
adjacent carbons
O More than 4 peaks may be hard to interpret
correctly