Instant Notes: Analytical Chemistry

A consequence of the small difference between the energy levels is that their

populations are almost equal at room temperature. When the nuclei are irradi-

ated with radiation of frequency n, both upward (absorption) and downward

(emission) transitions occur, and the system is said to be in resonance.

However, initially there is a small excess of a few nuclei per million in the lower

energy level, which results in a net absorptionof energy at the resonance

frequency. As only this small excess of nuclei is detectable, NMR spectrometry

is basically a much less sensitive technique than ultraviolet/visible (electronic)

and infrared (vibrational) spectrometry (Topics E8 to E11). Following the net

absorption of energy during resonance, the equilibrium ground state popula-

tions are re-established by the excited nuclei relaxingto the lower energy level,

the process normally taking only a few seconds in liquids and solutions.

Computer control and data processing enables sensitivity to be enhanced

considerably by using pulsed techniques and accumulating scans.

For the nuclei of each element, the magnetic moment, m, is directly propor-

tional to the spin angular momentum, I, the proportionality constant, g, being

known as the magnetogyricor gyromagnetic ratio, i.e.

m= gI or g= m/I

The magnitude of the resonance frequency, n,and hence of DE, is directly

proportional to the strength of the applied magnetic field, Bo, being related by

the equation

n= (g/ 2 p) · Bo (2)

Figure 2illustrates the proportionality and Table 1lists some values of g, nand

Bofor a number of nuclei along with their natural isotopic abundances.

Table 1. Natural abundance, magnetogyric ratio and resonance frequencies for protons,

carbon-13, fluorine-19 and phosphorus-31

Nucleus Natural Magnetogyric Resonance frequency, n(MHz)

abundance ratio, g at applied field, Bo (Tesla)

%10^7 T-^1 s-^1 2.3 7.1 11.7

(^1) H 99.985 26.75 100 300 500
(^13) C 1.108 6.73 25.1 75.4 125.7
(^19) F 100 25.18 94.1 282.3 470.5
(^31) P 100 10.84 40.5 121.4 202.4
The resonance frequency of the nuclei of one element is sufficiently different
from those of others to enable their spectra to be recorded independently, as
shown in Figure 1 for hydrogen (protons), carbon-13, phosphorus-31 and
fluorine-19, each of which provides useful information on the identity and struc-
ture of organic compounds. Small variations of the resonance frequency for the
nuclei of a particular element due to different chemical environments within a
molecule enable structural groups and features to be identified; for example, for
proton spectra it is possible to distinguish between acidic, saturated and unsatu-
rated molecular groups, etc., as indicated in Figure 1. Note that the proton has the
largest nuclear magnetogyric ratio and a very high natural abundance making it
the nucleus that can be detected with the highest sensitivity. Both proton and
carbon-13 spectra have been the most widely studied, and it is in the analysis of
organic compounds that NMR spectrometry has had the greatest impact.
250 Section E – Spectrometric techniques