4 in which the influence of the THF ring's orientation on the potential energy of the rotamers
The density for Br2' is weaker than that for Br4', likely due to different rotamers
of the second ring, which would maintain the position of Br4' but reposition Br2' in multiple orientations.
Complete equilibrium geometries have been determined for both rotamers
by ab initio calculations employing a variety of basis sets up to 6-311+G(2d,2p) at levels of restricted Hartree-Fock (RHF) and/or Moller Plesset (MP) to second order.
Two of these are dual-conformation peptide units; the remainder involves alternate rotamers
of side chains either alone or in small clusters.
The relative energies of rotamers
range between 0 to 16 kcal/mol.
Briefly, the lowest energy rotamers
present a syn methylester group on an E side chain with a pseudo H-bond between the hydroxy H and the prenyl [pi] density (Fig.