Instant Notes: Analytical Chemistry

Principles Nuclear magnetic resonance (NMR) transitions can be observed in the radio-
frequency region of the electromagnetic spectrum (Fig. 1). For elements whose
nuclei have spin angular momentumand a magnetic moment, or dipole, the
application of an external magnetic fieldcreates two or more quantized energy
levels (Fig. 2). (Note: in the absence of the external field, the energy levels are
degenerate, and no spectroscopic transitions can be observed.)
The energy difference, DE, between these levels is extremely small, and corre-
sponds to radiofrequency energy (Fig. 1), the relation being expressed by the
Planck equation

DE = hn (1)

where his the Planck constant and nis the corresponding radiofrequency.

E12 – Nuclear magnetic resonance spectrometry: principles and instrumentation 249

1022

1020

1018

1016

1014

1012

1010

108

106

g-rays

X-rays

Ultraviolet

Visible

Infrared

Mircowave

Radiofrequency

Mössbauer

Electronic

Vibrational

Rotational

NMR

n (Hz)

n (MHz)

600

500

400

300

200

100

10

8

6

4

2

0

Aldehydic

Aromatic

Olefinic

Acetylenic

Aliphatic

4 kHz

d (ppm)

(^1) H
(^19) F
(^31) P
(^13) C
Fig. 1. The electromagnetic spectrum including the NMR region and the frequency range for proton NMR.
No applied field Increasing
applied
field, Bo
Energy
DE=hv
E 1
Fig. 2. Quantized nuclear spin energy levels created by the application of an external
magnetic field to nuclei with a half-integral spin quantum number.