Methanol-acetonitrile complexes trapped in argon and nitrogen matrices: Infrared induced isomerization and theoretical calculations

Infrared induced isomerization of methanol-acetonitrile mixed aggregates tr apped in argon and nitrogen matrices has been studied using an optical para metric oscillator for monochromatic irradiations at the OH and CH stretchin g frequencies of complexed methanol. Several forms of the 1:1 complex have been characterized by most of their vibrational modes; two (Ar) or three (N -2) hydrogen-bonded forms are observed after matrix deposition; upon irradi ation they are converted into another H-bonded and one non-H-bonded species . This unbonded species is highly unstable, with a lifetime of the order of 24 min in argon at 7.2 K. In the presence of an excess of acetonitrile, la rger aggregates are identified, with OH stretching frequencies weakly redsh ifted with respect to those of the 1:1 complex. They are also infrared phot osensitive, giving rise to non-hydrogen-bonded structures. The possible str uctures of the heterodimer have been calculated theoretically. The deepest minimum of the potential energy surface is the quasilinear H-bonded form, w ith vibrational properties which agree with the matrix data obtained after sample deposition. As for the photolysis product, its vibrational spectrum is compatible with a secondary minimum with a pseudocyclic structure in whi ch the OH group is perturbed on the oxygen atom. (C) 1999 American Institut e of Physics. [S0021-9606(99)00520-6].