MAPPING THE CONFORMATION SPACE OF α-PROLINE BY MATRIX-ISOLATION IR SPECTROSCOPY COMBINED WITH NIR LASER INDUCED CONFORMATIONAL CHANGE AND STATE-OF-THE-ART AB INITIO COMPUTATIONS

The conformers of $\alpha$-proline were studied by matrix-isolation IR spectroscopy. In order to group the vibrational bands to different conformers, conformational changes were induced by NIR laser irradiations, by exciting the OH stretching overtone of a selected conformer. For identification, the single conformer spectra were compared to state-of-the-art ab initio computations. Infrared spectra and anharmonic thermodynamic corrections have been computed by means of second order vibrational perturbation theory (VPT2), with GVPT2 and HDCPT2 models, respectively. A hybrid force-field was developed by addition of the cubic and semi-diagonal quartic B3LYP-D3/SNSD force constants to the (harmonic quadratic) B2PLYP-D3/maug-cc-pVTZ results. As the result of the spectrum analysis, seven conformers could be identified. Four of these conformers were present in the as-deposited matrix, two of them were generated by the NIR laser irradiation, and found to be stable in Ar matrix. The seventh conformer could also be produced upon the NIR laser irradiation, and it decays by H-atom tunneling to a lower energy form on the sub-second and minute timescales in Ar and N$_{2}$ matrices, respectively

Polyaromatic Hydrocarbons with an Imperfect Aromatic System as Catalysts of Interstellar H2_{2} Formation

Although H$_{2}$ is the simplest and the most abundant molecule in the Universe, its formation in the interstellar medium, especially in the photodissociation regions is far from being fully understood. According to suggestions, the formation of H$_{2}$ is catalyzed by polyaromatic hydrocarbons (PAHs) on the surface of interstellar grains. In the present study, we have investigated the catalytic effect of small PAHs with an imperfect aromatic system. Quantum chemical computations were performed for the H-atom-abstraction and H-atom-addition reactions of benzene, cyclopentadiene, cycloheptatriene, indene, and 1H-phenalene. Heights of reaction barriers and tunneling reaction rate constants were computed with density functional theory using the MPWB1K functional. For each molecule, the reaction path and the \warn{rate constants} were determined at 50 K using ring-polymer instanton theory, and the temperature dependence of the \warn{rate constants} was investigated for cyclopentadiene and cycloheptatriene. The computational results reveal that defects in the aromatic system compared to benzene can increase the rate of the catalytic H$_{2}$ formation at 50 K

TAUTOMERS OF CYTOSINE AND THEIR EXCITED ELECTRONIC STATES: A MATRIX ISOLATION SPECTROSCOPIC AND QUANTUM CHEMICAL STUDY

Author Institution: Laboratory of Molecular Spectroscopy, Institute of Chemistry, Eotvos Lorand University, Pf. 32, Budapest, H-1518, Hungary; Laboratory of Theoretical Chemistry, Institute of Chemistry, Eotvos Lorand University, Pf. 32, Budapest, H-1518, HungaryWe have measured the IR and UV spectra of cytosine in a low-temperature argon matrix. An attempt was made to determine the tautomeric ratios existing in the matrix, making use of the matrix-isolation IR spectrum and computed IR intensities of the tautomers in a least squares fitting procedure. The mole fractions are about 0.22 for oxo(-amino) form, 0.26 and 0.44 for the two rotamers, respectively, of the hydroxy(-amino) form and 0.08 for the (oxo-)imino tautomer. These ratios were then used to simulate the matrix-isolation UV spectrum as a composite of the individual spectra, the latter calculated \emph{ab initio} at high levels of electron correlation theory. The agreement between simulated and experimental UV spectra seems satisfactory. This indicates that, in contrast to the solid state and solution spectra described up to now by the oxo(-amino) form alone, the reproduction of the matrix-isolation UV spectrum needs at least the hydroxy(-amino) and oxo(-amino) forms, and probably also the (oxo-)imino form