Molecular decompositions of acetaldehyde and formamide: theoretical studies using Hartree-Fock, Moller-Plesset and density functional theories

Optimized geometries and vibrational frequencies for all reactants, transition states and products of the unimolecular decompositions of acetaldehyde (CH3CHO) to CO + CH4 and to H-2 + CH2CO, and of formamide (NH2CHO) to CO + NH3 and to H-2 + HNCO were investigated at the HF, MP2 and B3LYP levels of theory with the 6-31G(d,p) basis set. Total energies for all species were calculated with the 6-311G(d,p) basis set at the MP4 level at both the HF and MP2 optimized geometries, with B3LYP at the B3LYP optimized geometries, and with Gaussian-2 theory. The effects of basis sets and correlation corrections on barrier heights, reaction enthalpies, and molecular structures are discussed. Most notable are the similarities in barrier heights for any level of theory which incorporates electron correlation. Inclusion of electron correlation has significant effects on energetics, and for some geometrical parameters, including a change in point group symmetry for the transition state in the decomposition of NH2CHO to CO + NH3. Very good agreement with experiment, both for structures and energetics, is given by B3LYP theory.