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

Selective vibrational excitations of OH stretching modes of methanol-pyridi ne hydrogen-bonded complexes trapped in solid argon or nitrogen at 7 K were carried out in the range 3400-3280 cm(-1). This proved an efficient way fo r inducing isomerization processes within heterodimers and larger aggregate s. The main photoproducts are non-H-bonded species, with OH and CO frequenc ies, respectively, close to 3665 and 1020 cm(-1). These unbonded species ar e unstable, with lifetimes strongly temperature dependent in the range 7-20 K. The possible structures of the heterodimer have been calculated theoret ically according to a two-step method. The potential energy surface is firs tly explored using a semiempirical method, then the properties of its minim a are calculated in the framework of the density functional method. The dee pest minimum corresponds to the quasilinear N . . . HO hydrogen-bonded stru cture, with vibrational properties in good agreement with the matrix data o btained after deposition at low concentration in both dopants. Several othe r minima were examined in which H bonding is either weak (OH... pi electron s) or nonexistent. The vibrational perturbations are weak in all cases, but the identification of the photoproduct of the stable dimer to the form inv olving the OH... pi interaction can be discarded. The stable forms of the m ixed trimers were also calculated. (Methanol)(2)-pyridine has a cyclic stru cture, the methanol dimer being tied to pyridine through a strong OH . . .N hydrogen bond and a weak CH . . .O interaction. Methanol-(pyridine)(2) is also cyclic, with a OH . . .N bond slightly stronger than in the heterodime r. Comparison with experimental data allows identification of this heterotr imer in nitrogen matrices, in the presence of an excess of pyridine with re spect to methanol. Among its photolysis products, one class in which the OH group is perturbed on the oxygen atom has been identified. (C) 2000 Americ an Institute of Physics. [S0021-9606(00)01542-7].