Instant Notes: Analytical Chemistry

strength of bonds and their interaction. For analytical purposes, it is sufficient to

note that there are 3N - 6 fundamental vibrations for a molecule with Natoms

(or 3N - 5 if the molecule is linear). If the molecule has a center of symmetry

(e.g. CO 2 , ethyne, CH∫CH or benzene, C 6 H 6 ) then those bands that are IR active

are notR active, and vice versa.

Besides the fundamental vibrations, it is important to note that some IR

absorptions may correspond to combinations of vibrationsand also to over-

tones.

For example, for HCl, which has a fundamental stretching band around

2800 cm-^1 , there is a first overtone at just below 5600 cm-^1 in the NIR. Carbon

dioxide, whose fundamentals are listed above, has overtones of n 1 at 6980 and

11 500 cm-^1 in the NIR and several combination vibrations such as (n 2 +n 3 ) at

2076 cm-^1.

In complex molecules, the structure is dominated by functional groups. For

example, a large number of compounds contain the carbonyl group, C=O.

These include aldehydes, ketones, acids, esters, amides and quinines. Almost

every organic compound has C-H bonds, although they may differ consider-

ably in their behavior. Alcohols, acids and phenols contain the -OH group.

These groups and also aromatic rings and other larger structural units may be

considered as giving rise to characteristic group frequencies.

There are three main types of functional group that give rise to absorptions in IR

(and R) spectra which are highly characteristic.

(i) The stretching of bonds between a heavier atom and hydrogen, H-X-.

Relating these bonds to the harmonic oscillator model, the reduced mass

depends chiefly on the mass of the H atom, because this has such a low

mass compared to all the other atoms, and the force constant is high

because the bonds connecting the hydrogen and other atoms are strong.

This accounts for the H-X-stretching vibrations, which almost all occur in

the region between 2000 and 4000 cm-^1. The X-atoms and the structure to

which they are attached will determine the exact frequency, as shown in

Table 1. It should be noted that hydrogen bonding, which occurs with

electronegative atoms such as oxygen or nitrogen, but notwith carbon, will

make the H-X- bond both weaker and more variable. Consequently,

whereas the free OH stretch of gaseous alcohols has a sharp absorption at

3600 cm-^1 , liquid alcohols show a very broad band nearer 3200 cm-^1.

(ii) Doubleand triple bonds; aromatic systems. Because these are bonds with

high bond energies, they are less affected than single bonds by the struc-

tures to which they are attached. It is worth noting that the higher the bond

order, the higher the IR absorption frequency or wavenumber, as shown for

C-C and C-N bonds as shown in Table 2.

(iii) The bending vibrations of organic molecules also give characteristic group

frequencies. As a general ‘rule of thumb’, it is usually found that the

bending vibrations occur at the lowest frequency or wavenumber, the

symmetric stretch next and the antisymmetric stretch at the highest value.

Figure3 and Table 3 give a selection of the most useful group frequencies in

the mid-IR and NIR regions respectively.

Raman spectra have similar group frequency correlations, but two features are

of special interest. The R spectrum of water is much less intense than the IR

Group
frequencies

236 Section E – Spectrometric techniques