Infrared photodissociation of the CH3CN:HI hydrogen bonded 1:1 complex trapped in a nitrogen matrix

Abstract

Infrared spectra are reported of a weakly hydrogen-bonded 1:1 complex of acetonitrile with hydrogen (deuterium) iodide in nitrogen matrices. Two forms of the complex have been found. The hydrogen bonded form (C), characterized by HI stretching frequency at 2142 cm−1, is dissociated by infrared irradiation and also at low temperature (below 16 K) in the dark. The process may be reversed by warming above 16 K. The nonhydrogen bonded form (U) which results from the photodissociation of the hydrogen bonded one has its HI stretching frequency very close to that of monomer HI in N2 at 2237 cm−1. The data are better interpreted as occurring from a reversible conversion C⇄U, in which the stable form C is preferentially photodissociated because of larger absorption coefficients but is thermally regenerated. The activation energy for the U→C conversion is found of the order of 1.5 kJ mol−1. This interpretation however implies the existence of an unusually efficient dark process, which suggests that the C→U photo-conversion occurs in several steps requiring low energy photons and involving correlated reorientation of both HI and N2 molecules.