IR and UV induced photorotamerization of 2-nitroethanol in matrices. Ab initio optimization of conformers and potential energy calculations

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Abstract
Conformer interconversion of 2-nitroethanol was studied in rare gases Ar, Kr, and Xe as well as in N2 matrices. The initial conformer Gg′ is transformed into conformer Tt both by UV (selectively with wavelengths of about 280 nm) and by IR irradiation. Frequencies in the νOH region were found to be the most effective in initiating the IR induced photorotamerization. The process is reversible, i.e., irradiation at the νOH band frequencies of the product conformer Tt produces the conformer Gg′. The IR induced process obeys the first order rate law in all the matrices studied, and the rate in different hosts decreases in the order Xe≫Kr≳Ar≳N2. Upon prolonged broadband IR irradiation the process proceeds to an equilibrium; the final Tt/Gg′ conformer ratio (determined from the νOH bands) was the same, ∼0.5, in all matrices, while upon UV irradiation in Ar the final ratio was ∼1.1. The observation that in a Xe matrix at deposition temperatures above 20 K the rotamerization is almost totally inhibited, is explained in terms of the formation of guest–host interactions of van der Waals type. The photorotamerization mechanism is discussed. Our conformer assignment is based on ab initio (STO-3G and 3-21G) calculations; only five conformers were found. The calculations indicate a strong intramolecular interaction for species Gg′. However, the ΔνOH between the Gg′ and Tt species is only about 40 cm−1. The related nitroalcohols 2-nitro-1-propanol and 3-nitro-1-propanol also show relatively small ΔνOH’s in matrices. Semirigid STO-3G and 3-21G torsional potential energy surfaces were calculated and used to discuss the torsional barriers.