Abinitio study of rotational isomerism in acrolein

Abstract

Analytic gradient (force) methods at the STO-3G, 3-21G, and 6-31G* basis set levels have been used to optimize the geometry of acrolein completely at each critical point (minima, maximum) in the torsional potential energy curves for rotation about the single C—C bond (dihedral angle θ). The STO-3G and 6-31G* optimizations predict the planar trans conformation (θ = 180°) to be more stable than the cis conformation (θ = 0°) by 1.87 and 6.97 kJ/mol, respectively. The 3-21G optimizations, in disagreement with experiment, place the planar cis structure below the trans by 4.5 J/mol. The predicted relative energy (ΔE) and position for the transition state (TS) for rotation from the trans conformer are ΔE = 22.35, 37.14, and 34.41 kJ/mol and θ = 91.8, 91.6, and 91.0° for the STO-3G, 3-21G, and 6-31G* optimizations, respectively. The computed and experimental geometries, relative energies, dipole moments, and coefficients for the torsional potential expansion are compared.