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Abstract: Nitriles and nitrile chemistry are found in a variety of astronomical environments and have been utilized in many areas of astronomy and planetary science. Cyanogen (NCCN) has been observed in Titan’s atmosphere though it is not detected in comets or interstellar space. Nitriles formed in the atmosphere of Titan are expected to precipitate down onto the satellite’s surface where cosmic radiations can convert them into other molecules. In view of the significance of the HCN tetramers diaminomaleonitrile (DAMN) and diaminofumaronitrile (DAFN) in reactions leading to the formation of adenine, the possibility of their formation from CN and H radicals, which are known to be abundant in the nitrile rich astronomical environments, has been explored on the basis of radical–radical and radical–molecule interaction schemes using quantum chemical methods. Global and local descriptors such as chemical potential, molecular hardness and softness, electrophilicity and condensed Fukui functions were used to understand the mechanistic aspects of chemical reaction and region-selectivity of the CN and H radical additions. The presence and nature of weak bonds in the reaction complexes and transition states formed during the reaction process have been ascertained using Bader’s quantum theory of atoms in molecules (QTAIM). The reaction leading to the formation of DAFN is found to be exothermic with a very large net energy gain but involves a potential barrier of about 4.75kcal/mol. The present reaction scheme shall provide a simpler route to the formation of 4-amino-1H-imidazole-5-carbonitrile (AICN) and adenine. This reaction involves the formation of a neutral intermediate 2-imino-malononitrile for which no structural or spectroscopic information is available. A full vibrational analysis has, therefore, been attempted for 2-imino-malononitrile in the harmonic and anharmonic approximations and spectroscopic parameters such as rotational constants, rotation–vibration coupling constants, and centrifugal distortion constants are being reported. [Copyright &y& Elsevier]

Abstract: A systematic study has been conducted on the geometrical and electronic structure, heliomeric conformations of a series of 8-oxa[5]helicenes based on density functional theory (DFT) computations. A complete vibrational analysis has also been attempted for one of the 8-oxa[5]helicenes (molecule 5) on the basis of experimental infrared spectra in the far and mid infrared regions (60–3100cm−1) and density functional theory computations using B3LYP/6-31G** method characteristic bands of the molecule identified. The approximate mean plane angle between the terminal rings A and E in the presently studied molecules are found to have values between 48.64° and 59.46°. This angle is much larger than the corresponding angle between the terminal rings in benzo[c]phenanthrene (∼27°) and partially reduced benzo[c]phenanthrene (34.6–46.0°) and indicates that the presence of oxygen-containing six-membered ring provides a greater helicity to the molecules. Detailed quantum chemical study on molecule 4 shows the existence of two enantiomeric forms M- and P- of almost equal energy separated by a potential barrier of 15.55kcal/mol. It is expected that similar 8-oxa[5]helicines (molecules 3, 5 and 6) may also exist in two enantiomeric forms. [Copyright &y& Elsevier]

Abstract: Experimental methods of infrared, Raman and electronic absorption spectroscopy and DFT calculations using B3LYP functionals and 6-31G** and 6-311++G** basis sets have been used to understand the structural and spectral characteristics of 2-pyranones, 6-phenyl-4-methylsulfanyl-2-oxo-2H-pyran and 6-phenyl-4-methylsulfanyl-2-oxo-2H-pyran-3-carbonitrile in the electronic ground (S0) and first excited (S1) states. Information about the size, shape, charge density distribution and site of chemical reactivity of the molecules has been obtained by mapping electron density isosurface with electrostatic potential surfaces (ESP). Based on TD-DFT calculations using 6-31+G**5D basis set, an assignment of absorption peaks in the UV–VIS region has been suggested. The S1 state is found to be a 1(π,π*) state. A complete vibrational analysis has been attempted on the basis of experimental infrared and Raman spectra and calculated frequency and intensity of the vibrational bands and potential energy distribution over the internal coordinates. Characteristic vibrational bands of the 2-pyranone ring and methylsulfanyl and carbonyl groups have been identified. [Copyright &y& Elsevier]

Abstract: A systematic study has been conducted on the conformation, electronic structure and vibrational spectra of benzo[c]phenanthrene and some of its partially reduced derivatives by experimental infrared spectroscopic and quantum chemical techniques. Electrostatic potential surfaces have been mapped over the electron density isosurfaces to obtain information about the size, shape, charge density distribution and chemical reactivity of the molecules. Possibility of hydrogen–hydrogen bonding has been explored in all the molecules. Partial reduction of the aromatic rings in benzo[c]phenanthrene leads to considerable molecular distortion with the approximate mean angle between the terminal rings increasing from 27.3° to 46.0°. The distortion is unequally distributed near the aromatic and saturated rings; the latter absorbs most of strain due to flexibility of the rings. A complete vibrational analysis of the experimental infrared spectra has been reported on the basis of frequency and intensity of the vibrational bands and potential energy distribution over the internal coordinates and characteristic bands have been identified. [Copyright &y& Elsevier]

Abstract: The paper reports main results of a comprehensive study of the vibrational spectrum of ketene computed using second-order perturbation theory treatment based on quartic, cubic and semidiagonal quartic force constants. Two different models – a homogeneous model using the same density functionals and basis functions for the harmonic calculations and anharmonic corrections and a hybrid model in which the two parts of the calculation are conducted using different density functionals and basis sets – have been employed in the present calculations. Different DFT and CCSD methods and DZ and TZ extended basis sets involving diffuse and polarization functions have been used to calculate optimized and vibrationally averaged geometrical parameters, the harmonic and anharmonic vibrational frequencies and the spectroscopic constants such as anharmonicity constants, rotational constants, rotation–vibration coupling constants, Nielsen''s centrifugal distortion constants and Coriolis coupling constants. Homogeneous model is found to be superior to the hybrid model in several respects. Difficulties in the hybrid model may arise due to one of the following reasons: (a) the basic requirement that the geometry optimization and frequency calculations must be done at the same level of theory to have valid frequencies is not met in the hybrid model; (b) the molecular structure gets reoptimized at the low level for anharmonic corrections; (c) in addition, the perturbation could also diverge for the above reasons, particularly for the very low, very anharmonic terms where the harmonic approximation is not close enough to make the perturbation work. [Copyright &y& Elsevier]

Abstract: Ab initio quantum chemical and experimental spectroscopic studies in the infrared (4000–60cm−1) and UV spectral regions are being reported on 4-chloro-2,6-dimethylsulfanyl pyrimidine-5-carbonitrile and 4-chloro-2-methylsulfanyl-6-(2-thienyl) pyrimidine-5-carbonitrile. Optimized geometries, electronic charge distribution, dipole moments and three-dimensional molecular electrostatic potential surfaces have been obtained. These have been used to understand the structure and spectral characteristics of the two compounds. A complete assignment of vibrational spectra on the basis of DFT/6-311G** and electronic spectra on the basis of TD-DFT/6-31+G* 5D calculations have been attempted for the two molecules. [Copyright &y& Elsevier]

In order to understand conformational isomerism in methacryloyl bromide (MABR) in the ground (S0) and the first excited (S1) electronic states and to interpret the vibrational and electronic spectra of its conformers in the S0 state, quantum mechanical calculations using Density Functional Theory (DFT) and RHF methods with extended basis sets 6-31G, 6-31G** and 6-311+G(d,p) have been conducted. In RHF calculations, electron correlation effects have been included at the Møller–Plesset MP2 level. It is inferred that in both the electronic states the molecule may exist in two isomeric forms—s-trans and s-cis; the former being more stable than the later by about 1.629 kcal mol−1 in the S0 state and by about 2.218 kcal mol−1 in the S1 state. Electronic transition tends to increase the s-trans/s-cis and s-cis/s-trans, rotational barriers from 7.059 kcal mol−1 (2468.1 cm−1) and 5.428 kcal mol−1 (1897.8 cm−1) in S0 state to 23.594 kcal mol−1 (8249.4 cm−1) and 21.376 kcal mol−1 (7473.9 cm−1) in the S1 state. Completely optimized geometries of the two conformers in S0 state reveal that while there is no significant difference in their bond lengths, some of the bond angles associated with COBr group are appreciably different. Electronic excitation tends to change both the bond lengths and bond angles. Based on suitably scaled DFT and RHF results obtained from the use of 6-31G** and 6-311+G(d,p) basis sets, a complete assignment is provided to the fundamental vibrational bands of both the s-trans and s-cis conformers in terms of frequency, form and intensity of vibrations and potential distribution across the symmetry coordinates in the S0 state and a comparison has been made with experimental assignments. A theoretical prediction of the electronic transitions in the near UV-region in the two conformers and their tentative assignment has been provided on the basis of CI level calculations using 6-31G basis set. [Copyright &y& Elsevier]