Instant Notes: Analytical Chemistry

electrons, and this determines the value of s, and hence n. The exact position of

each resonance frequency is referred to as its chemical shift, which is character-

istic of the chemical nature of the particular nucleus.

Chemical shift is conventionally measured relative to the frequency of a refer-

ence compound, using a dimensionless parameter, d, defined as the ratio of their

chemical shift difference divided by the operating frequency of the spectrometer

and multiplied by 10^6 to give more convenient numerical values, i.e.

d= · 10^6 ppm (5)

where nrefandnspectrometerare the resonance and operating frequencies of the

reference compound and the spectrometer respectively.

It should be noted that:

● d values are assigned units of ppm (parts per million) because the ratio is

multiplied by 10^6 ;

● by definition, the chemical shift of the reference compound is assigned a

value of zero and, conventionally, dvalues are presented as a scale that

increases from right to left (Fig. 4);

● the greater the shielding of the nucleus (larger s), the smaller the value of d

and the further to the right, or upfield, the resonance signal appears;

● the less the shielding of the nucleus (smaller s), the larger the value of dand

the further to the left, or downfield, the resonance signal appears;

● because field and frequency are directly proportional, it follows that upfield,

or high fieldresonance signals correspond to lower frequenciesthan down-

field, or low fieldsignals, and vice versa;

● dvalues are independent of the operating frequency of the spectrometer,

enabling chemical shifts in ppm from spectra recorded on instruments with

different operating frequencies to be compared (Fig. 4);

● for recording proton and carbon-13 spectra in nonaqueous solvents, the

reference compound is normally tetramethylsilane(TMS, (CH 3 ) 4 Si) which

gives a single, high field (low frequency) resonance signal for the twelve

(n- nref)




nspectrometer

252 Section E – Spectrometric techniques

660

11

1100
11

1000

10

900

9

800

8

700

7

100 MHz

B 0

600

6

500

5

400

4

300

3

200

2

100

1

0

0

600

10

540

9

480

8

420

7

360

6

300

5

240

4

180

3

120

2

60

1

0

0

Low

field

High

field

n

d (ppm)

n

d (ppm)

60 MHz

B 0

Fig. 4. Proton chemical shift scales in d/ppm at spectrometer frequencies of 60 MHz and 100 MHz.