Theoretical spectroscopy of trans-HNNH+ and isotopomers.

The six-dimensional potential energy surface of the electronic ground state of trans-HNNH⁺ (X ²A_g) is mapped at the RCCSD(T)/aug-cc-pV5Z level of theory. This potential energy surface is incorporated later into perturbative and variational treatments to solve the nuclear motion and to derive a set of spectroscopic data for trans-HNNH⁺, trans-HNND⁺, and trans-DNND⁺. Our vibrational spectra are compared with those deduced from the earlier photoelectron spectra by Frost et al. [J. Chem. Phys. 64, 4719 (1976)], for which a good agreement between the theoretical and experimental results is found. Our calculations reveal the presence of strong anharmonic resonances between the vibrational levels of these cations even at low energies, thus complicating even more their assignment by vibrational quantum numbers. These resonances should participate in the transfer of intensities between the active modes during the direct photoionization of the neutral molecule and the combination modes and overtones of the inactive modes belonging to the totally symmetric irreducible representation. [ABSTRACT FROM AUTHOR]