Instant Notes: Analytical Chemistry

identical protons or the four identical carbon-13 nuclei. The protons and the

carbons are all highly shielded due to the relatively electropositive silicon

atom, consequently the TMS signal is observed at a higher field (lower

frequency) than those of most other organic compounds;

● two or three drops of TMS, which is an inert, low-boiling liquid

miscible with most organic solvents, are added to each sample. For

aqueous samples, deuterated sodium 3-trimethylsilylpropanoate (TSP,

(CH 3 ) 3 SiCD 2 CD 2 COONa) is used.

Four principal factors affect the chemical shifts of nuclei in organic

compounds.

● Diamagnetic shielding, which opposes Bo. The degree of shielding (positive

s) is related to the electron density around the nucleus and is therefore deter-

mined by the electronegativity of neighboring elements in the structure. This

is illustrated in Table 2, which shows how the chemical shift for the protons

of a methyl group in a series of compounds moves progressively downfield

(increasing d) as the electronegativity of the adjacent group increases and the

protons become more deshielded. This inductive effectis reinforced when

there is more than one electronegative group adjacent to a proton group

(Table 3), but falls off rapidly with distance in saturated structures, i.e. with

the number of intervening bonds (Table 4). In unsaturated structures, the

effects travel further and are less easy to predict.

● Diamagnetic anisotropy, which may oppose or augment Bo. An anisotropic

property varies with direction in three-dimensional space, the effect in NMR

spectrometry being associated with the circulation of p-electrons in unsatu-

rated structures. This circulation, induced by the applied magnetic field, Bo,

creates conical-shaped shielding (+) and deshielding (-) zones in the space

surrounding the group or structure. Figure 5shows the zones associated with

an alkene double bond, a carbonyl group, an aromatic ring and an alkyne

E12 – Nuclear magnetic resonance spectrometry: principles and instrumentation 253

Table 2. Variation of the chemical shift of methyl protons and carbons with the

electronegativity of the adjacent group

CH 3 X Electronegativity of X dH dC

CH 3 Si(CH 3 ) 3 1.90 0 0

CH 3 H 2.20 0.23 -2.3

CH 3 COCH 3 2.60 2.08 29.2

CH 3 I 2.65 2.16 -20.7

CH 3 Br 2.95 2.68 10.0

CH 3 NH 2 3.05 2.50 28.3

CH 3 Cl 3.15 3.05 25.1

CH 3 OH 3.50 3.40 49.3

CH 3 F 3.90 4.26 75.4

Table 3. Variation of the chemical shift of methyl, methylene and methine protons with

the number of adjacent electronegative groups

CHnX X=CH 3 Cl C 6 H 5 OCH 3

CH 3 X 0.9 3.05 2.35 3.25

CH 2 X 2 – 5.30 4.00 4.50

CHX 3 – 7.25 5.55 5.00