Combined infrared and ab initio study of the H2HN2+ complex

Abstract Combined spectroscopic and ab initio studies of the H 2 HN 2 + proton-bound complex are presented. Infrared spectra of mass-selected H 2 HN 2 + complexes in the 2500–4200 cm −1 range display a number of vibrational bands, assigned as due either to the HH stretch vibration, or tentatively to the NH stretch in combination with intermolecular stretch and bend motions. Due to subpicosecond energy redistribution, almost all rotational structure is obscured by lifetime broadening. The ab initio calculations conducted at the QCISD(T)/6-311G(2df,2pd) level indicate that the complex is composed of largely undistorted H 2 and HN 2 + subunits, and has a T-shaped minimum energy geometry with an H 2 ···HN 2 + intermolecular bond length of approximately 1.44 A. Both spectral and theoretical results show that combining the H 2 and HN 2 + molecules drastically reduces the frequency of the NH stretch vibration, although the H 2 stretch frequency is only modestly affected.