Przemyslaw Kolek

LIF excitation spectra of o- and m-cyanoanilines

LIF excitation spectra of 2-cyanoaniline (2-CA) and 3-cyanoaniline (3-CA) in their hydrogen (–NH2) and deuterium (–ND2) forms are presented and discussed. Analyses of harmonic motions for the three mono cyanoanilines are presented, using the results of ab initio (HF/6-31++G** and CIS/6-31++G**) quantum mechanical calculations for ground and excited electronic states, as well as IR and Raman frequencies. Previous assignments of IR and Raman spectra of the cyanoanilines are compared with the ab initio calculations. Potential energy changes upon electronic excitation, to the lowest singlet state, for the cyanoanilines are interpreted in terms of harmonic frequencies, displacements of normal modes and Dushinsky rotation matrices.

New laboratory data on a molecular band at 4429 Å

New laboratory data are presented for the previously reported molecular absorption band at 4429 8 observed in a benzene plasma matching the strongest diffuse interstellar band (DIB) at 4428.98. Gas-phase absorption spectra are presented for rotational temperatures of� 15 and 200 K. The observations indicate that it is unlikely that the laboratory band and the 4429 8 DIB are related. Eleven isomers of C5H5 (+) and C6H5 (+) , both neutral and cationic, were considered as possible carriers of the laboratory band in view of the observed rotational profiles and deuterium isotope shifts. The experimental data and theoretical calculations (CASPT3, MRCI) indicate that the HCCHCHCHCH radical, a planar but nonlinear chain with one hydrogen on each carbon, is the most probable candidate causing the 4429 8 laboratory absorption. Subject headingg ISM: lines and bands — ISM: molecules — line: identification — methods: laboratory — molecular data

Theoretical Modeling of Deuteration-Induced Shifts of the 0–0 Bands in Absorption Spectra of Selected Aromatic Amines: The Role of the Double-Well Potential

The harmonic approximation fails for inversion of the NH2 group in the ground state of aromatic amines as this vibration is characterized by a symmetric double-well potential with relatively small energy barrier. In such cases, the standard harmonic vibrational analysis is inapplicable: the inversion frequency calculated for the bottom of the potential well is strongly overestimated, while it attains imaginary values for the planar conformation of the molecule. The model calculations are discussed taking explicitly into account the presence of the double-well potential. The study is initially focused on reproduction of the deuteration-induced shifts of the 0-0 absorption band for anthranilic acid. The (incorrect) harmonic frequency of the NH2 inversion is replaced by a better one, obtained from numerical calculations employing a simple, quartic-quadratic model for the double-well potential, which is parametrized using just the harmonic frequency of the inversion and the height of the energy barrier. This operation brings theoretical results to qualitative agreement with experiment. A still better match is achieved with a modified version of the model that accounts for mixing of the NH2 inversion mode with other normal modes while retaining the initial simplicity of one-dimensional approach. The corrected results show surprisingly good accuracy, with deviations of the calculated shifts from the experimental values reduced to less than 5 cm(-1). In order to test the performance of the model for systems with higher energy barrier for the NH2 inversion, we have measured the LIF excitation spectra of three different amminobenzonitriles. Partial assignment of the 0-0 bands has been achieved based on their relative intensities for samples with different isotopic exchange ratios. Calculated shifts are in excellent agreement with experimental values for the identified bands. Theoretical predictions are used to complete the assignment of the 0-0 bands in the spectra of the studied amminobenzonitriles.

Quantitatively Adequate Calculations of the H-Chelate Ring Distortion upon the S0 → S1(ππ*) Excitation in Internally H-Bonded o-Anthranilic Acid: CC2 Coupled-Cluster versus TDDFT

The S0 → S1(π → π*) excitation in o-aminobenzoic acid causes strengthening of the N-H···O intramolecular hydrogen bond. The interplay of the hydrogen bond shortening, the hydrogen atom dislocation along the hydrogen bond, and the skeletal relaxation is investigated. These effects often cause the appearance of dual fluorescence from the π-conjugated internally H-bonded molecules, which is traditionally interpreted as the evidence of the excited-state intramolecular proton transfer process: ESPIT. Hence, their quantitative modeling is an important but demanding task for computational photochemistry. Extensive calculations using CC2 method (the perturbative approximation to CCSD coupled-cluster) and TDDFT(B3LYP) were performed with the series of (aug)-cc-pVXZ(X = D,T,Q) basis sets. CC2 predicts remarkable shortening of the O···H distance by 0.273 Å accompanied by the skeleton relaxation that involves considerable distortions of valence angles of the amino group (up to 7.3°) and within the benzene ring (up to 5°). Additionally, moderate changes (<0.046 Å) of the bond alternation in the π-electronic system and the hydrogen atom dislocation along the hydrogen bond (0.043 Å) are predicted. The CC2 method yields 90% of the magnitude of the experimentally based geometry changes, estimated in the earlier studies via Franck-Condon fit to the LIF spectra, while the TDDFT results approach only 65% of the experimental values.