Ab initio studies on conformation, vibrational and electronic spectra of methyl methacrylate

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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 (S 0) and first excited (S 1) 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 S 0 and S 1 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 S 1 state easier. Fully optimized geometries of the two stable conformers in the S 0 and S 1 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 S 0 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.