An AB initio study of the cis- and trans -conformers of 1,3-butadiene, acrolein and glyoxal: evidence for a stabilizing interaction in cis-acrolein

Abstract

The cis- and trans-conformers of 1,3-butadiene, acrolein and glyoxal have been studied using the 4-31G and (7, 3) basis sets with full geometry optimization. The changes in geometry in going from the cis- to the trans-structures indicate that the terminal carbon and oxygen atoms in cis-acrolein are closer together than would otherwise be expected from the butadiene and glyoxal data, and that the hydrogens of the CH 2 group are also affected. These results, together with a comparison of the changes in force constants for key structural elements, suggest that some sort of attractive interaction is present in cis-acrolein. Δ E T, Δ E K, Δ V ee, Δ V nn and Δ V en are reported for the cis- trans isomerizations, the special geometrical features of the cis-conformer being reflected in Δ V nn for the acrolein reaction. The positive Δ E T values for the disproportionation reactions of both conformers 2 acrolein → 1,3-butadiene + glyoxal show that the acrolein framework provides the most stable combination of the H 2C, CH- and O structural elements, while the bigger value for the cis-conformers affords additional evidence that the interaction in cis-acrolein is stabilizing in nature. However, in going from the trans- to the cis-structure neither the change in f C-H,C-H for the C-H that would be involved in an H- bond, nor the change in charge on the atoms in the C-H ⋯ O unit. based on Mulliken population analyses, are in accord with what would be expected for a hydrogen-bonding type of interaction. Instead the stabilization may be a manifestation of the different pattern of charge distribution along the polyene chain of acrolein, i.e. with the ends oppositely charged, compared to butadiene and glyoxal, in which the ends carry the same charge.