Ab initio computations on simple carbonyl compounds

Abstract

Assignments of vibrational spectra for formaldehyde, acetaldehyde and acetone are briefly reviewed and ab initio Gaussian calculations reported for both the Hartree–Fock and the Density Functional Theory method, B3LYP, using ten basis sets up to 6-311++G(3df,3pd). Using estimated harmonic values the difference between the calculated and measured carbonyl stretching band is reported. These differences decrease with number of basis functions up to 6-311++G(2d,2p) but increase at the highest basis set, 6-311++G(3df,3pd). The electronic energy for the ten basis sets used is reported for both the HF and B3LYP methods and decreases with number of basis functions in a parallel manner up to 6-311++G(2d,2p) but, unlike the carbonyl wavenumber differences, shows a further reduction at the highest basis set used in line with the variation principle. The calculated wavenumber and intensity values of the normal harmonic modes of formaldehyde, acetaldehyde and acetone are reported and compared with published experimental values using the HF and B3LYP methods with the 6-311++G(2d,2p) basis set. The respective RMS error for wavenumber values is 8.8 and 1.4% for formaldehyde, 8.1 and 1.1% for acetaldehyde and 7.4 and 1.1% for acetone illustrating the greater accuracy obtained by the existence of electron correlation effects in the second method. The calculated bond lengths, bond angles and dipole moments are compared with published measured values; best agreement occurs using the B3LYP method with the two highest basis sets used.