MOLECULAR-STRUCTURE AND VIBRATIONAL IR-SPECTRUM OF KETENE - COMPARISON OF CONVENTIONAL AB-INITIO POST-HARTREE-FOCK AND DENSITY-FUNCTIONAL THEORY CALCULATIONS

The results of comparative quantum-mechanical studies based on conventional ab initio post-Hartree-Fock calculations, with electron correlations incorporated by second-order Moller-Plesset perturbation theory (MP2(full)) and density functional theory with the Becke3-LYP functionals (DFT(B3-LYP)) using three basis sets 6-31G(d,p), 6-311G(d,p) and 6-311G(3df,2p), are reported for the molecular parameters (equilibrium geometry, rotational constants, dipole moment) and vibrational IR spectrum (harmonic wavenumbers, absolute intensities) of ketene. The calculations show: (a) when using a defined basis set, the MP2 calculations slightly better reproduce the experimental data than the DFT(B3-LYP) calculations; (b) for proper reproduction of the experimental data, the 6-31G(d,p) basis set is good enough for calculations carried out at the MP2(full) or DFT(B3-LYP) levels. The molecular parameters computed by MP2 with the use of the extended basis set 6-311G(3df,2p) reproduce the corresponding experimental data very well. Both methods using the 6-311G(3df,2p) basis set give good predictions of the experimental spectrum of ketene. The calculations also reproduce correctly the shifts of the vibrational band positions of ketene on isotopic substitution.