Infrared spectrum and ab initio calculations of the CH3CNH+-H2 ionic complex

Abstract

The infrared photodissociation spectrum of the CH3CNH+-H2 complex has been recorded in the vicinity of the H2 (ν1) and N-H (ν2) stretch fundamentals. The complexation-induced frequency redshifts (Δν1≈64, Δν2=183 cm−1) and the rotational structure of the ν1 band are consistent with a T-shaped proton-bound CH3CNH+-H2 geometry with a vibrationally averaged intermolecular bond length of 1.9 Å. Ab initio calculations at the MP2 level of theory predict a binding energy of 1100 cm−1 for the T-shaped complex, an intermolecular separation of 1.77 Å, and frequency shifts of Δν1=88 cm−1 and Δν2=215 cm−1, in agreement with the experimental data. Whereas the CH3 internal rotation is nearly free (barrier Vb<0.1 cm−1), H2 rotation via a transition state with a linear -H⋯H-H bond is severely hindered (Vb≈1050 cm−1). The lifetime of the ν1 state (>130 ps) greatly exceeds that of the lower lying ν2 state (<30 ps), indicating that the relaxation dynamics are mode-selective and not statistical.