Pressure induced spectra of axially symmetric and slightly asymmetric quadrupolar molecules

Expressions are derived for the transition moments and absorption resulting from the electric moment induced in one molecule of a pair by the field of the quadrupole moments of a neighbor with axial or near axial symmetry. Values of relevant matrix elements of squares and products of direction cosines are included. For axial polar molecules, the infrared absorption has a different frequency dependence than that by permanent dipoles, permitting separation of the effects. For asymmetric rotors with nearly axial inertial symmetry, the absorption transitions with ΔK=0 are associated with one of the two quadrupole moments and transitions with ΔK=±2 with the other, again permitting spectral resolution of the different contributions. These possibilities are analyzed in some detail for the polar molecules CH3F and CHF3 and for the asymmetric rotor C2H4.