Infrared-Induced Isomerization of Ethanol Dimers Trapped in Argon and Nitrogen Matrices: Monochromatic Irradiation Experiments and DFT Calculations

Abstract

Selective vibrational excitations of OH stretching modes of ethanol dimers trapped in solid argon or nitrogen were carried out in the range 3550−3500 cm-1. This proved an efficient means to interconvert these dimers and thus to characterize several conformers by their most specific vibrational modes. The structures, energies, and vibrational properties of four minima of the potential energy surface have been investigated by the density functional method. These minima correspond to an open chain dimer where both proton donor (PD) and proton acceptor (PA) subunits are either anti- or gauche-ethanol, with well-differentiated vibrational spectra in the domains 1400−1240 and 1100−1020 cm-1. The comparison between calculated and observed spectra in these domains enables the anti or gauche character of both PD and PA to be distinguished. In argon matrix four dimeric species were identified after deposition. All of them are sensitive to irradiation at the νOH frequency of their PD moiety, with simple conversion schemes for two of them. On one hand, the most abundant one, characterized by νOH bands at 3660.1 and 3527.2 cm-1, is found to be anti−anti. Upon irradiation at 3527 cm-1, it is converted into an unstable gauche−gauche form, not present before irradiation, which spontaneously reconverts into its precursor in the dark. On the other hand, another conformer with νOH frequencies at 3665.4−3536.2 cm-1, also anti−anti, is converted upon irradiation at 3536 cm-1 into a new form whose identification is rendered difficult by the small number of bands assigned. In nitrogen not less than seven νOH bands in the range 3520−3470 cm-1 are assignable to dimers. Irradiations at various νOH PD frequencies are very efficient to interconvert these species whose structures are discussed on the same grounds as in the case of argon matrix. However the greater complexity of the conversion processes precludes a full assignment of the spectra except for the most abundant species for which the data are complete enough to conclude an anti−gauche structure.