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

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 C₃H₆O species, the energy hypersurfaces (E θ₁, θ₂, θ₃) were generated, where energy is a function of the methyl rotations (θ₁, θ₂) and C---O out-of-plane bending for acetone, and a function of methyl rotation (θ₁), C₂H₅---C rotation (θ₂) and CHO out-of-plane deformation (θ₃) 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.