Infrared spectrum and ab initio calculations of the CH3CNH+-H-2 ionic complex

The infrared photodissociation spectrum of the CH3CNH+-H-2 complex has been recorded in the vicinity of the H-2 (nu(1)) and N-H (nu(2)) stretch fundam entals. The complexation-induced frequency redshifts (Delta nu(1)approximat e to 64, Delta nu(2)=183 cm(-1)) and the rotational structure of the nu(1) band are consistent with a T-shaped proton-bound CH3CNH+-H-2 geometry with a vibrationally averaged intermolecular bond length of 1.9 Angstrom. Ab ini tio calculations at the MP2 level of theory predict a binding energy of 110 0 cm(-1) for the T-shaped complex, an intermolecular separation of 1.77 Ang strom, and frequency shifts of Delta nu(1)=88 cm(-1) and Delta nu(2)=215 cm (-1), in agreement with the experimental data. Whereas the CH3 internal rot ation is nearly free (barrier V-b< 0.1 cm(-1)), H-2 rotation via a transiti on state with a linear -H ... H-H bond is severely hindered (V(b)approximat e to 1050 cm(-1)). The lifetime of the nu(1) state (> 130 ps) greatly excee ds that of the lower lying nu(2) state (< 30 ps), indicating that the relax ation dynamics are mode-selective and not statistical. (C) 1999 American In stitute of Physics.