THE EQUILIBRIUM MOLECULAR-STRUCTURES OF CROSS-CONJUGATED ALKENES WITHPARTICULAR REFERENCE TO [3]-DENDRALENE AND [4]-DENDRALENE FROM AB-INITIO STUDIES OF VARIOUS CONFORMERS

A study of the equilibrium structures and relative energies for the co nformers of 3-methylenepenta-1,4-diene (MPD) and bis(3,4-methylene)-1, 5-hexadiene (MHD) have been obtained by SCF and MP2 correlated methods ; a double zeta + polarisation basis set (DZP) was used. Both planar a nd non-planar forms were evaluated for a variety of point groups. The force constants for each conformer were obtained, in order to determin e whether the minima were saddle points rather than genuine minima owi ng to the constraints of molecular symmetry. For both molecules, all t he non-planar forms studied were of lower energy than the best planar- forms. Indeed the planar structures are all saddle-points between the interconverting pairs of non-planar forms, as shown by one or more neg ative vibration frequencies for the planar molecules. The relative ene rgies and structures of the different conformers of MPD and MHD are al most identical between the SCF and MP2 series, the MP2 calculations me rely leading to slight extensions of the C = C bonds, minor changes in dihedral angles, and small changes otherwise. Because of the H ... H interactions, all the planar forms showed considerable in-plane angle bending, and small but consistent H-C bond compressions were also obta ined in the most hindered cases. The lowest energy conformer of MPD is a gauche-trans conformer, with symmetry C-1, but a second conformer o f trans-trans type and C-2 symmetry lies only 6 kJ mol(-1) above, The C-2 non-planar conformer of MHD containing (basically) trans-butadiene units is the lowest energy by a significant amount, but several trans -cis and cis-cis conformers were all identified as potential species f or gas phase mixtures. (C) 1997 Elsevier Science B.V.