Conformational investigation of gaseous 1,5-hexadiene by electron diffraction and molecular mechanics

Abstract

The molecular structure of gaseous 1,5-hexadiene has been studied by electron diffraction. The following bond lengths ( r g) and bond angles were obtained (CH) mean, 1.087 ± 0.002 A ̊ ; CC, 1.340 ± 0.003 A ̊ ; C(H 2)C(H), 1.508 ± 0.012 A ̊ ; C(H 2)C(H 2), 1.538 ± 0.027 A ̊ ; C(H 2)C(H 2)C(H), 11.5 ± 0.9°; C(H 2)C(H)C(H 2), 124.6 ± 1.0°; C(H)C(H 2)H, 128.9 ± 1.5°; C(H 2)C(H)H, 116.8 ± 2.3°; HC(H 2)H, 112.0 ± 2.6°. The electron diffraction data are compatible with the presence of anti and gauche conformers with respect to the central CC bond, as predicted by molecular mechanics (MM2) calculations, the anti form being the most stable. Each of the CC bonds eclipses one of the CH bonds of the adjacent methylene group, as suggested by a gas-phase overtone spectroscopic investigation. The electron diffraction bond lengths and bond angles are generally in good agreement with the results of the MM2 calculations. The energy difference between the gauche and anti conformers estimated from the electron diffraction analysis (1.0 ± 0.5 kJ mol −1) has the same sign but considerably smaller magnitude than the difference obtained by MM2 calculations. One of the gauche conformers of the free molecule is remarkably similar to the hexadiene ligand of crystalline (1,5-hexadiene)copper(I) trifluoromethanesulfonate. Apparently, the chelating interaction stabilizes the rotational form whose geometry is the easiest to adapt in the complex although it is not the most favorable energetically for the free molecule.