Infrared photochemistry of hydrogen-bonded complexes trapped in low-temperature matrices

Abstract

A number of base–hydrogen iodide complexes have been reported to photodissociate under infrared irradiation in low-temperature matrices; in each case the hydrogen-bonded complex could be regenerated by warming the matrix a few degrees. Results for these complexes, ranging from the weakly hydrogen-bonded complex with cyanomethane to the strongly hydrogen-bonded N, N-dimethylacetamide–hydrogen iodide complex, are discussed, particularly in relation to the role of the matrix and the nature of the dissociation product. The barrier to regeneration of the hydrogen-bonded complex is found to increase with the strength of the base, showing that the process is not simply an artefact of the matrix environment. It is suggested that the metastable species generated by the photodissociation of the hydrogen-bonded complex is a ‘reverse’ complex BIH. The small dipole moment and large polarizability of hydrogen iodide, compared with the other hydrogen halides, makes such a structure comparatively favourable, thus rationalising the observation that the photodissociation only occurs for hydrogen iodide complexes. Photodissociation only occurs for hydrogen iodide complexes. Photodissociation occurs most readily for those complexes where excitation of the HI stretching mode can effect the process, and is much slower for complexes where excitation in the CH stretching region is necessary to provide sufficient energy to dissociate the hydrogen-bonded complex.