A theoretical study of the effects governing the internal rotation process in allyl derivatives

Total molecular energy as a function of internal rotation around the C(sp(2))-C(sp(3)) bond, referred to as the C-C bond, in the molecules CH2=CHCY3, CH2=CHCH2Y, with Y = H, F, Cl, CH3, and CH2=CHCHY3, with Y = F, Cl, was calculated at the ab initio MO level, with the 6-31G* basis set. The molecular geometry was fully relaxed at fixed rotational angles phi. The potential energy profiles V(phi) were obtained with a truncated Fourier series and the V-n terms (n = 1-3) are derived and discussed. The C-C bond distances change on rotation and the highest values of the increment Delta d are found in the rotational maxima. For the molecules with the CY3 top the changes in Delta d are linearly related to the changes in total molecular energy Delta E-TOT, but with different slopes in the different molecules. The slopes seem to depend on the absolute length of the C-C bond, and while the Delta d behaviour within a molecule seems mostly dictated by hyperconjugative effects, the absolute length d is most probably determined by through-space effects. The elongation Delta d is due, in the majority of the molecules examined, to a decrease in attractive interactions (barrier of attractive type: in the rotational maxim., although in 3,3,3-trichloropropene and cis-1,3,3,3-tetrachloropropene the barrier is mostly of the repulsive type (increase in repulsive interactions). This kind of correlation is less statisfactory for tops of lower symmetry: in the molecules with the CH2Y top it would appear that the gauche-skew barrier is of the repulsive type, and the trans-skew barrier of the attractive type. The rotational behaviour of the total energy was also analyzed according to a model of factorization in energy components ascribed to different effects, identified in the first instance as hyperconjugative and through-space effects. The hyperconjugative effect of the groups examined, Delta E-x, calculated with the Natural Bond Orbital (NBO) approach, is similar to that found for the corresponding benzyl derivatives, yet slightly stronger. The rotational pattern of Delta E-x mirrors that of Delta E-TOT, even though it seems necessary to take into account the presence of other effects to explain peculiar differences between these patterns. The through-space effect was further factorized in the contribution of van der Waals, Delta E-vdW, and electrostatic, Delta E-el., interactions. For the calculation of Delta E-d from atomic charges different approaches were employed (Mulliken NBO, CHELPG), yet none afforded a Delta E-el. contribution quantitatively in agreement with the other factors. A qualitative description of the through-space effect of the groups considered is nevertheless attempted in order to suggest a combination of effects exerted by the susbtituents examined on the vinyl moiety, stemming from the theoretical treatment of these molecules.