The IR spectra of nonionized glycine and its deuterated derivatives isolated in the low-temperature argon matrices have been measured, and for the first time the infrared spectral characteristics of the three most stable conformers have been determined and assigned. Correlated level ab initio and density functional theory (DFT) calculations of IR frequencies and intensities with extended basis sets were performed and their results were employed to separate the bands of the glycine conformers in the experimental spectra and to assist the assignment. The intramolecular interconversion, conformer III w conformer I, which is observed in the matrices at temperatures higher than 13 K, was found to cause a significant decrease of the band intensities of conformer III in the spectra. This phenomenon was used to distinguish the vibrational bands of this conformer from the bands of the other conformers. The reliability of the Møller-Plesset second-order perturbation theory (MP2) method and the DFT method with the three-parameter density functional, Becke3LYP, in the prediction of the IR spectra of the nonionized glycine conformers was examined. We found that the DFT/Becke3LYP method with aug-cc-pVDZ basis set yields vibrational frequencies of the glycine conformers very similar to the MP2 results. Both DFT and MP2 results are in the excellent agreement with the experimental data.

The relaxation of the higher-energy cis conformer of formic acid to the lower-energy trans form by a tunneling mechanism has been investigated in low-temperature rare gas matrices. In the temperature range 8 -60 K, the tunneling takes place dominantly from the vibrational ground state of the cis form and the temperature dependence of the tunneling rate constant is influenced by the interactions with the environment. The temperature-dependent tunneling rates for HCOOH and DCOOH in solid Ar, Kr, and Xe are measured including data for molecules in different local environments within each host. It was found that the medium and the local environment has a significant influence on the tunneling rate. In reaction kinetics, tunneling of atoms is often negligible compared with over-barrier transitions. At very low temperatures, however, the population of energy states above the barrier becomes exceedingly small and tunneling becomes comparatively more important.1 In a condensed environment, phonons participate in a tunneling reaction and the environment should have some effect on tunneling reactions.2,3 However, in several previous experiments it was found that the tunneling rate constant was unaffected by the change of solvent. 1,4 -6 In this work, we have studied the conversion of cis formic acid ͑HCOOH͒ to trans formic acid in solid rare gases ͑Ar, Kr, Xe͒. This reaction is dominated by tunneling from the vibrational ground state at temperatures below 60 K. The results show that the tunneling rate depends strongly on the environment.The samples were made by mixing vapors of formic acid ͑FA͒ ͑KEBO lab, Ͼ99%͒ or its isotopomers ͑IT Isotope 95%-98% deuteration͒ with rare gases ͑Rg͒ Ar ͑AGA, 99.9999%͒, Kr ͑Air Liquid, 99.95%͒, Xe ͑AGA, 99.997%͒ in the gas phase in a proportion FA/RgϷ1/1000. The gas mixture was deposited on a CsI substrate at 15 K ͑Ar͒, 25 K ͑Kr͒ or 35 K ͑Xe͒ yielding highly monomeric matrices with respect to FA. Thickness of the sample was typically about 100 m. After deposition, the samples were cooled to ϳ8 K which was the lower limit for the cryostat ͑APD DE 202 A͒. The spectra were measured with a FTIR spectrometer ͑Nico-let 60 SX͒ with a resolution of 1 or 0.25 cm Ϫ1 . FA has energy minima in two planar forms differing by orientation of the hydroxyl group as shown in Fig. 1. The interconversion of the conformers involves mainly the torsional motion of the hydroxyl group. In the gas phase, cis-FA is 1365Ϯ30 cm Ϫ1 higher in energy than trans-FA. 7 The barrier from trans to cis has been calculated to be ϳ4200 cm Ϫ1. 8 In this work, cis-FA was prepared by exciting the vibrational transitions of trans-FA in rare-gas matrices with narrowband infrared radiation of an optical parametric oscillator ͑Sunlite, Continuum, FWHM ϳ0.1 cm Ϫ1 ͒. The excitation energy flows into the torsional coordinate inducing the conformer conversion. 9 The IR spectra of cis and trans FA differ significantly from each other making it possible to distinguish them easily in rare-gas matrices.9 FA is trapped in several sites correspo...