Ab initio and density functional theory study on the geometries, energies,and vibrational spectra of the conformations of 3-methyl-3-penten-2-one

The stable s-trans and s-cis conformations of (E)-3-methyl-3-penten-2-one h ave been studied using both the Hartree-Fock (HF) and Becke's three-paramet er (B3LYP) density functional theory (DFT) with a large Gaussian 6-31G* bas is set. We have compared equilibrium geometries, thermal properties of the two conformations, and discussed the potential energy curve of the internal rotation of the molecule. The vibrational frequencies, intensities, and po tential energy distributions of the two conformations have been calculated and then compared with experimental IR and Raman spectra. The calculated re sults show that: (i) there are limited differences for bond angles between the skeleton of the s-trans and that of the s-cis conformation; (ii) the to tal energy of the s-trans conformation is lower by 0.99 kcal/mole than that of the s-cis one, the energy barrier from the s-trans to the s-cis conform ation is 9.07 kcal/mole, and the two conformations can simultaneously exist ; and (iii) the calculated spectra are in good agreement with experimental data. (C) 1999 Elsevier Science B.V. All rights reserved.