Conformations of triplet carbonyl compounds: Formaldehyde, acetaldehyde, propionaldehyde and acetone

Abstract

A conformational study on the lowest triplet states of formaldehyde, acetaldehyde, propionaldehyde and acetone has been done using a minimal basis set, within the unrestricted Hartree—Fock framework. For the C 3H 6O species, the energy hypersurfaces ( E θ 1, θ 2, θ 3) were generated, where energy is a function of the methyl rotations (θ 1, θ 2) and CO out-of-plane bending for acetone, and a function of methyl rotation (θ 1), C 2H 5C rotation (θ 2) and CHO out-of-plane deformation (θ 3) for propionaldehyde. The analysis of the hypersurface equations revealed the location and relative energies of the critical points (minima, first and second order saddle points as well as maxima): the barriers to inversion at the carbonyl group were 2.7 kcal mol −1 for acetone and 4.2 kcal mol −1 for propionaldehyde. Partial geometry optimization reduced these barriers to 2.5 and 2.4 kcal mol −1 respectively. For comparison, both the pyramidal minimum and planar saddle point for the inversion of triplet formaldehyde and acetaldehyde were totally optimized; the resultant barriers were 2.0 kcal mol −1 and 2.3 kcal mol −1, respectively. The barrier to rotation about the bond to the α-carbon was 1.1 kcal mol −1 for pyramidal acetone, 1.0 for acetaldehyde and ranged from 0.8 to 1.8 kcal mol −1 for the various propionaldehyde conformers.