INELASTIC NEUTRON-SCATTERING STUDY OF THE PROTON-TRANSFER DYNAMICS INPOLYGLYCINE-I AT 20-K

Inelastic neutron-scattering (INS) spectra of three isotopic derivativ es of polyglycine I (-COCH2NH-)(n), (-COCD2NH-)(n), and (-COCH2ND-)(n) at 20 K are presented from 30 to 4000 cm(-1). The band frequencies ar e compared to those observed in the infrared and Raman. Assignments in terms of group vibrations are proposed. These mostly resemble previou s assignment schemes, except for the amide bands. The INS intensities reveal that the proton dynamics for the (N)H proton are totally differ ent from those proposed previously. They are independent of the molecu lar frame and the valence bond approach is not consistent with observa tion. A phenomenological approach is proposed in terms of localized mo des. The calculated intensities reveal that the (N)H stretching mode h as two components at similar to 1377 and 1553 cm(-1). This is a dramat ic change compared to all former assignments at similar to 3280 cm(-1) based on infrared and Raman data. These proton-dynamics are associate d with a weakening of the N-H bond due to the ionic character of the h ydrogen bond (N-delta-...H+Odelta'-) and proton transfer. The infrared and Raman spectra are re-examined and a new assignment scheme is prop osed for the amide bands; the amide A and B bands are re-assigned to t he overtones of the stretching modes. A symmetric double-minimum poten tial for the proton is consistent with all the observations.