Molecular structure, conformation, vibrational and electronic spectra of methyl trans crotonate

Abstract

Rotational isomerism in methyl trans crotonate (MC) and its properties in the ground (S 0) and first excited (S 1) 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 S 0 and S 1 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C and C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 S 1 state easier. Fully optimized geometries of the two stable conformers in the S 0 and S 1 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 S 0 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.