Instant Notes: Analytical Chemistry

Section E – Spectrometric techniques

E13 NUCLEAR MAGNETIC

RESONANCE SPECTROMETRY:

INTERPRETATION OF

PROTON AND CARBON-13

SPECTRA

Examples of chemical shift charts for protons and carbon-13 nuclei are given in

Figure 1. They give a general indication of the ranges in d/ppm within which the

resonances for different types of structure or functional groups will occur. More

detailed tabulated numerical data can be used to establish the presence or

absence of specific molecular structural features or groups. The chemical shift

range for protons that can be involved in hydrogen bonding is particularly wide,

as the variation in degree of shielding of these protons may be highly concentra-

tion-dependent. Factors affecting chemical shifts are described in Topic E12.

Peak areas Peak areas are measured by electronic integrationof the resonance signals in a
spectrum. For proton spectra, the total area, or integral, of a multiplet is directly
proportional to the number of protons in the group. Integrals are recorded as a

Chemical shift
data

Key Notes

The interpretation of proton and carbon-13 spectra is facilitated by

reference to published chemical shift data in the form of charts and tables

of numerical values for a range of nuclear environments, principally in

organic structures.

The relative areas of resonance peaks in a proton spectrum are directly

proportional to the numbers of nuclei responsible for each signal, which

enables the presence of specific groups of nuclei to be confirmed.

These are the most widely studied NMR spectra, the proton being the

nucleus with the highest sensitivity. Information from chemical shifts,

spin–spin coupling and peak areas enables the structural features of

organic compounds to be recognized and their identities established.

Due largely to the very low natural isotopic abundance of carbon-13,

spectra can be recorded only by a pulsed Fourier transform spectrometer.

Chemical shifts are much greater than in proton spectra and are of

particular value in establishing the skeletal structures of organic

molecules.

Related topic Nuclear magnetic resonance spectrometry: principles and

instrumentation (E12)

Chemical shift data

Peak areas

Proton spectra

Carbon-13 spectra