PHYSICAL CHEMISTRY IN BRIEF

CHAP. 12: BASIC TERMS OF CHEMICAL PHYSICS [CONTENTS] 435

and it occurs in atoms with a nonzero spin of the nucleus, particularly in hydrogen. Magnetic
field acting on the nucleus is, however, influenced by the magnetic moment of the surrounding
electrons. The accurate value of the resonance frequency is thus different for different func-
tional groups containing hydrogen (e.g. OH, CH 3 , CH 2 ). The relative change in the resonance
frequency

δ=

ν−νref

νref

(12.54)

is called thechemical shift, andνref is the resonance frequency of a reference substance,
usually tetramethylsilane Si (CH 3 ) 4 , commonly referred to as TMS.

Example

What is the resonance frequency shift of a substance whose chemical shift isδ= 1ppm if the

spectrometer works at a frequency of 500 MHz?

Solution

The chemical shift of 1 ppm corresponds to a relative frequency shift by 10−^6 , and we have

ν−νref=δ νref= 10−^6 × 500 × 106 Hz= 500Hz.

The resonance frequency thus has to be measured very accurately.