Conformational analysis and structural determinations from ab initio calculations for chloroacetaldehyde

Abstract

Structural parameters for both the more stable s-cis (chlorine atom s-cis to the aldehyde hydrogen atom) and high energy s-trans conformers of chloroacetaldehyde, ClCH 2CHO, have been obtained from ab initio Hartree-Fock gradient calculations employing the 3-21G ∗ and 6-31G ∗ basis sets with electron correlation. The results are compared to the r a values obtained for the s-cis rotamer from an electron diffraction study as well as with some r o values determined from some previously reported rotational constants for both the s-cis and s-trans conformers. Potential surface calculations have been carried out to determine the barrier to internal rotation and the conformational stability. At the 6-31G ∗ basis set level the s-cis to s-trans barrier is 1203 cm −1 (3.44 kcal mol −1) with the s-cis conformer being more stable by 509 cm −1 (1.46 kcal mol −1) than the s-trans form. There is a small “hump” of 12 cm −1 (34 cal mol −1) at the s-cis position which is below the first torsional energy level for this conformer. Infrared band contour simulation of the carbon-chlorine stretching mode for fluoroacetyl chloride predicts an s-trans to gauche equilibrium which is inconsistent with the ab initio results where the high energy conformer is predicted to have the s-cis structure. All of these results are compared with the corresponding data for some similar molecules.