Abstract
The excitation of an infrared-inactive mode (νm) in a polyatomic molecule through the electric dipole transition is discussed taking into account the high-order derivatives of the dipole moment with respect to the normal coordinates. It is revealed that this excitation requires one of the specific degenerate modes (νt) to be thermally excited in advance of infrared absorption. The third-order dipole-derivatives responsible for the hot band transition of “νm + νt − νt” are identified. The author also deals with examples of hot band transitions in benzene and recommends reassignment of weak absorption features detected in previous infrared spectra of gaseous polyatomic molecules.
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