The theory of vibronic coupling is developed for infrared vibrational transitions. It is shown that the lowest order nonadiabatic Bornndash;Oppenheimer correction terms contain an important adiabatic component which may be used to describe infrared transition intensity for imaginary Hermitian operators, such as the momentum and angular momentum operators. This previously unrecognized source ofadiabaticinfrared intensity forms a complement to the traditional Herzbergndash;Teller vibronic coupling expressions, which are active for the position operator, and resolves the paradox of vanishing electronic intensity for momentum operators in the Bornndash;Oppenheimer approximation. Expressions for infrared absorption and vibrational circular dichroism are derived that utilize only ground electronic state wave functions; LCAO wave functions are used in these expressions to provide a more detailed description of these new momentum intensity contributions.
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