

Michael,

Optimizing with PCFF in the trans position gives an energy of -46
kcal/mol and a C-N-C-C dihedral of 179 deg.  Simply changing that
dihedral to 0 deg and reoptimizing gives an energy of -44
kcal/mol and a dihedral of 21 deg.  If the dihedral is
constrained to zero degrees with a 1000 kcal/mol quadratic
restraint the potential energy is still -44 kcal/mol.  The phenyl
rings must be free to rotate relative to the peptide bond.

Paul Soper

Answer 6: