Page 102 - Tandem Techniques

Page 83

From quantum rules, the precessing nucleus has only two possible orientations. Consequently, if energy
is supplied to the spinning nucleus, by way of electromagnetic radiation of the necessary frequency,
energy can be absorbed and the precessing nucleus changed from one orientation to the other. The
magnetic quantum number for the proton is ± 1/2 and consequently, the frequency (v) at which this
transition can occur can be calculated from the equation:

where (±) is the nuclear magnet moment,
(H) is the external magnetic field strength,

and (h) is planks constant

Now the nuclear magnet moment for the proton is 2.793 Bohr magnetons (1 magnetron = 5.093 x 10-24
erg/gauss) and thus, if it is situated in a field of 15,000 Gauss, the necessary frequency to make the
transition is given by,

In a similar manner, taking a range of transition frequencies, the different field strengths can be
calculated and are shown in Table 2.1.

Table 2.1 NMR Field Strengths and Frequencies for Some Nuclei
Nucleus 20 MHz 60 MHz 100 MHz 250 MHz 750 MHz

1H 4,700 14,000 23,500 58,750 176,250

2H 30,600 91,800 153,000 382,500 1,147,500

13C 18,700 56,00 93,400 233,750 701,250
14N 65,000 195,000 325,000 812,500 2,437,500