Ab initio studies of methylated 1,3-butadienes

To understand the effect of methyl substitution on substituted butadiene potential surfaces, the structures for the stable conformations and the potential energy functions about the central C-C single bond are determined for 18 different methylated 1,3-butadienes from the 6–31G Hartree-Fock (HF) calculations with geometry optimization at each of the torsional angles. They are (E)-penta-1,3-diene ((E)-PD), (Z)-penta-1,3-diene ((Z)-PD), 2-methylbuta-1, 3-diene (MB), (E, E)-hexa-2,4-diene ((E, E)-HD), (E, Z)-hexa-2,4-diene ((E, Z)-HD), (Z, Z)-hexa-2,4-diene ((Z, Z)-HD), 4-methylpenta-1,3-diene (4-MPD), (E)-2-methylpenta-1, 3-diene ((E)-2-MPD), (E)-3-methylpenta-1,3-diene ((E)-3-MPD), (Z)-2-methylpenta-1,3-diene ((Z)-2-MPD), (Z)-3-methylpenta-1,3-diene ((Z)-3-MPD), 2,3-dimethylbuta-1,3-diene (DMB), 2,4-dimethylpenta-1, 3-diene (2,4-DMPD), (Z, Z)-3-methylhexa-2,4-diene ((Z, Z)-3-MHD), (Z)-2,3-dimethylpenta-1, 3-diene ((Z)-2,3-DMPD), (E, E)-3,4-dimethylhexa-2,4-diene ((E,E)-3,4-DMHD), (E, Z)-3,4-dimethylhexa-2,4-diene ((E, Z)-3,4-DMHD) and (Z, Z)-3, 4-dimethylhexa-2,4-diene ((Z, Z)-3,4-DMHD). The methyl substitutions at the 2(3) and terminal Z positions of butadiene are found to have significant effects on the shape of the potential function due to the introduction of strong steric repulsions between the two ethylenic (or methylated ethylenic) units. Although the terminal E methyl group in the monomethyl- and dimethyl-butadienes has little effect on the shape of the potential function, sizable effects of the (E)-methyl group were observed in the trimethyl- and tetramethylbutadienes. While the most stable conformation is s-trans for most (11) of the dienes, the major form for all the dienes (6) with both Z-4- and 2-positioned (or (Z)-1- and 3-positioned) methyl groups is predicted to have a gauche or “orthogonal” structure with a C  C-C  C torsional angle (τ) between 50° and 90°. The present study confirms the earlier suggestion that the most stable conformer of (Z)-2-MPD has a gauche structure. For 2,4-DMPD the ab initio results agree with a nuclear magnetic resonance (NMR) analysis in that only one conformer should be populated; the calculated structure is also very close to that from NMR experiments. This is in contrast with the results of previous molecular mechanics calculations which suggested that non-planar s-cis and s-trans forms of 2,4-DMPD were both present with an energy difference of only about 0.3 kcal mol−1. For (E, E)-DMHD the two stable conformations (s-trans and gauche) were calculated to have similar energies, but only one of them (s-trans) has been suggested experimentally. The ab initio conformations for (E, Z)-DMHD and (Z, Z)-DMHD are similar to those from experiment. The calculations also give results that are of help in interpreting certain reactions. An energy function is proposed for analyzing the ab initio potential energy, and it is suggested that such an energy function is useful for estimating the increase of the torsional angle dependent steric interactions induced by methyl substitutions. The length of the central C-C single bond as a function of the torsional angle is examined for each of the methylated butadienes.