Calculations on the van der Waals spectrum of Ar–tetrazine

Abstract

The van der Waals bound states of Ar–tetrazine are calculated by a method which treats the tetrazine fragment as a rigid rotor, but which is otherwise exact within the Born–Oppenheimer approximation. The results are used to obtain frequencies and intensities for transitions between van der Waals states in the S1←S0 electronic spectrum of the complex. Selection rules for such transitions are derived using permutation–inversion symmetry arguments. The normal rigid molecule selection rules are relaxed due to coupling between the low frequency van der Waals vibrations and the overall rotation of the complex, leading to the possibility of observing single excitation of the nontotally symmetric van der Waals bending modes. Transition intensities for van der Waals stretching excitation are predicted much smaller than experiment and various possible explanations for this disagreement are discussed.