Infrared induced isomerizations of water polymers trapped in nitrogen matrix

Abstract

In a first step mid-infrared spectra of water (H 2O and D 2O) doped rare gas (Ne, Ar) or nitrogen matrices have been reexamined with the purpose of analyzing the aggregation processes occurring on annealing. In rare gases there is a smooth evolution, the concentration in polymers increasing steadily whatever their size; in nitrogen dimers and probably trimers nearly totally vanish so that the spectra of large aggregates in the absorption domains of these small polymers are better observed. In a second step selective irradiation has been performed at absorption frequencies of free and bonded OH or OD oscillators. Significant spectral changes have been observed only in nitrogen matrices for the partially deuterated dimer and for polymers larger than the heptamer. For H/D mixed dimers, two isomerization processes are identified: a donor/acceptor exchange between the two partners and, for HDO acting as donor, a proton donor/deuteron donor exchange between the OH and OD bonds. For larger polymers of natural water an isomerization process is observed for one species characterized by absorptions at 3185 and 3133 cm −1 which is converted into several metastable species distinguishable by their behaviour upon temperature increase or large band infrared irradiation. For H/D mixed polymers proton donor/deuteron donor exchanges for the HDO partners are clearly identified after irradiation in the free OH and OD and in the bonded OH regions, leading to complicated coupling/decoupling effects within aggregates involving several equivalent OH(D) oscillators.