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Abstract: Structural and spectral characteristics of ethyl 2-cyano-3-N,N-dimethyl amino acrylate have been studied by methods of X-ray crystallography, infrared spectroscopy and quantum chemistry. The compound crystallizes in monoclinic space group P21/n with unit cell parameters a =4.26(1)Å, b =11.16(1)Å, c =19.63(3)Å and β =95.5(1)°. The X-ray based three-dimensional structure analysis has been carried out by direct methods and fully refined. Density functional theory calculations for potential energy curves, optimized geometries and vibrational spectra have been carried out using 6-31G and 6-31G** basis sets and B3LYP functionals. These suggest the possibility of existence of two structural isomers for the molecule—a more stable s-cis and a less stable s-trans isomer, having enthalpy difference of 2.85kcal/mol. The optimized molecular geometry is in agreement with experimental geometry from X-ray analysis and suggests a preferential s-cis conformation for the molecule in the solid state. Based on experimental and theoretical studies, it may be concluded that the molecule has an almost planar conformation with the cyanide group also lying in the molecular plane; the deviation from planarity does not exceed 3°. The structure is stabilized by the presence of intra-molecular and inter-molecular interactions. [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]

Rotational isomerism in methyl trans crotonate (MC) and its properties in the ground (S0) and first excited (S1) electronic states have been studied by spectroscopic and quantum chemical techniques such as Density Functional Theory (DFT) with B3LYP functionals and RHF using extended basis sets like 6-31G, 6-31G*, 6-31G** and 6-311+G**. Electron correlational corrections at Moller–Plesset MP2 level have been included in RHF calculations. The molecule was treated as a two-rotor system with the additional possibility of hindered rotation of the methyl group about C–O bond. Conformations in which the carbonyl and methoxy groups are in cis position are found to be most stable with the methyl group having staggered conformation relative to the carbonyl group. Plot of potential energy curves in the S0 and S1 states by RHF /6-31G based calculations indicates in each case the presence of two stable rotameric conformations, Cc and Tc separated by 0.645 and 1.653 kcal/mol, respectively. The former, in which the C&z.dbnd6;C and C&z.dbnd6;O bonds are in cis positions, is more stable than the latter, in which these bonds are in trans position. Electronic excitation is found to substantially reduce rotational barrier between the two conformers, making their inter-conversion in the S1 state easier. Fully optimized geometries of the two stable conformers in the S0 and S1 states are being reported by RHF/ 6-31G** and RHF/6-311+G** basis sets. Based on suitably scaled RHF/ 6-31G** and DFT/6-311G** calculations, assignments have been provided to the fundamental vibrational bands of both the Cc and Tc conformers in terms of frequency, form and intensity of vibrations and potential energy distribution across the symmetry coordinates in the S0 state. A complete interpretation of the electronic spectra of Cc and Tc conformers of MC in terms of nature, energy and intensity of electronic transitions has been provided on the basis of configuration interaction level calculations. [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]

A systematic quantum mechanical study of the possible conformations, their relative stabilities, vibrational and electronic spectra and thermodynamic parameters of methyl methacrylate (MMA) has been reported for the electronic ground (S0) and first excited (S1) states using Density Functional Theory (DFT) with B3LYP functionals and RHF methods in extended basis sets 6-31G, 6-31G**, 6-311G** and 6-311+G** have been conducted. The molecule was considered as a two rotor system having internal rotation about C–C and C–O bonds with the possibility of hindered rotation of the methyl group. Rotation about C–O bond indicates that conformations having the carbonyl and methoxy groups in cis positions are the most stable, while the methyl group has staggered conformation relative to the carbonyl group. Plots of the potential energy curves for rotation about C–C bonds in the S0 and S1 states show two energy minima corresponding to the Tc and Cc conformations, separated by 0.365 and 1.104 kcal/mol, respectively, and indicate that the former is more stable than later in both the electronic states. Electronic excitation is found to substantially reduce rotation barrier between the two conformers, which may make their inter-conversion in the S1 state easier. Fully optimized geometries of the two stable conformers in the S0 and S1 states are being reported; the later being reported for the first time. Based on suitably scaled RHF/6-31G** and DFT/6-311G** results, a complete assignment is provided to the fundamental vibrational bands of both the Tc and Cc conformers in terms of frequency, form and intensity of vibrations and potential energy distribution across the symmetry coordinates in the S0 state. A complete interpretation of the electronic spectra of the Tc and Cc conformers of MMA has been provided by MP2/6-31G** calculations in terms of the nature, energy and intensity of electronic transitions. [Copyright &y& Elsevier]