Band contours in gases

The resulting set of lines together forms a distinctive band envelope, whieh depends on the symmetry of the vibration concerned because the rotational seleetion rules are different for different vibrational symmetries. In some cases, the individual lines making up the envelope can be resolved (Section 7.3.2). 

Although the details of rotational line positions in vibration bands are beyond the scope of this book, there are some simple rules that enable us to deduce the symmetry of a vibration from the band envelope of the corresponding IR band. Gas-phase Raman spectra are not considered here. In general, when more than one symmetry species of vibration is IR-active, the bands have shapes that differ from one symmetry species to another, and so can be distinguished. More detail is given in the on-line supplement to Chapter 8. 

Asymmetric tops have three different moments of inertia, and up to three different symmetry species of vibration could give rise to IR bands. These correspond to dipole changes along the x, y and z Cartesian axes. If these axes coincide with the three principal inertial axes A, B and C (Section 7.2.1), either because of 

Rotational fine structure in gas-phase IR spectra of linear molecules (a) parallel band of CO2 (the asymmetric stretch) (b) perpendicular hand of HCN (the bend). Adapted, with permission, from [12]. Copyright 1961 lUPAC, Butterworths, London. 

For many molecules, the Cartesian axes do not coincide uniquely with the principal inertial axes, and hybrid band contours arise. For example, for any planar molecule, the C axis is perpendicular to the plane, so any IR-active out-of-plane motions give rise to C-type bands, but in-plane vibrations can give A/B hybrid bands, depending on the relative directions of the A and B axes and the dipole change. 

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