Instant Notes: Analytical Chemistry

254 Section E – Spectrometric techniques

Table 4. Variation of the chemical shift of methylene

protons with distance (number of bonds) from an

electronegative group

-(CH 2 )nBr dH

• CH 2 Br 3.30


• CH 2 -CH 2 Br 1.69


• CH 2 -CH 2 -CH 2 Br 1.25

H

H H

H

H

C

H

H

C

+

+

CH 3

H

C O

+

+

• –

H

H H

+

+

––

• –

H

C

C

H +

+

• –

Alkene Carbonyl

Aromatic ring Alkyne

Magnetic field,

B^0

Fig. 5. Shielding and deshielding effects in unsaturated groups and structures due to

diamagnetic anisotropy.

triple bond. The magnitude of the effect varies with the orientation of the

molecule relative to the applied field, and the illustrations show the

maximum effect in each case, where the molecular axis is parallel or perpen-

dicular to the field. In reality, the molecules in a liquid or solution are

constantly tumbling due to thermal motion, so that only a net effect is

observed, being the average over all possible orientations.