Normal vibrations of crystalline polyglycine I

A valence force field has been refined for crystalline polyglycine I using its known antiparallel chain pleated‐sheet structure and without replacing the CH 2 group by a point mass. Polyglycine I and four of its isotopic derivatives were used in the refinement. The calculated frequencies are in good agreement with the observed, except for the amide I modes. It is shown that this is a consequence of the fact that no reasonable force field predicts a large D 10 term of the Miyazawa perturbation treatment. The amide I splittings can, however, be satisfactorily accounted for by introducing a direct interaction force constant between adjacent CO groups in neighboring chains. This can reasonably arise from transition dipole coupling and corresponds to the heretofore neglected D 11 term.