Helix reversal motion in polyisocyanates

The geometric pathway and energetic barriers associated with the motion of helix reversal sites in poly(methyl isocyanate) have been studied using the conjugate peak refinement algorithm in conjunction with the Merck molecular force field. Motion of the reversal along the chain backbone involves relatively small net changes in the torsional angles as a pair of distorted trans and cis bonds reverse their direction of twist. The present study shows that this change occurs by the trans bond passing through its nearby planar low torsional energy conformation to assume an opposite twist. However, the cis bond is prevented from passing through its nearby low torsional energy planar cis conformation by strong nonbonded interactions. It is instead forced to rotate through its trans conformation, thus overcoming two torsional barriers to reach its final cis conformation of opposite twist. The net translation of the reversal down the chain one monomer unit requires crossing three energetic barriers, the highest of which is 17.5 kcal/mol, in reasonable agreement with the 19.6 kcal/mol value determined in recent temperature-dependent NMR studies of poly(2-butylhexyl isocyanate).