LIQUID STRUCTURE, INFRARED AND ISOTROPIC ANISOTROPIC RAMAN NONCOINCIDENCE OF THE AMIDE-I BAND, AND LOW-WAVE-NUMBER VIBRATIONAL-SPECTRA OF LIQUID FORMAMIDE - MOLECULAR-DYNAMICS AND AB-INITIO MOLECULAR-ORBITAL STUDIES/

The relationship between the liquid structure of formamide and wavenum ber differences among its infrared (IR), isotropic Raman, and anisotro pic Raman bands in the amide I region is analyzed theoretically. The f ollowing two methods are employed: (1) ab initio molecular orbital (MO ) calculations on a few different cluster species of formamide molecul es and (2) calculations of the LR and Raman spectra in the amide I reg ion on the basis of the transition dipole coupling mechanism and the L iquid structures derived from molecular dynamics simulations. It is sh own that intermolecular interactions other than those involved in a on e-dimensional hydrogen-bonded chain are required to reproduce the obse rved wavenumber difference between the amide I IR and isotropic Raman bands. This wavenumber difference originates from the difference in th e vibrational patterns of the modes giving rise to these two bands. In the Raman noncoincidence, i.e., the wavenumber difference between the isotropic and anisotropic Raman bands, disorder in hydrogen-bonded ch ains in the liquid state plays an important role. Ab initio MO calcula tions of the low-wavenumber IR and Raman spectra of the cluster specie s of formamide are also performed. Existence of a large concentration of cyclic hexamers in the liquid state is unlikely because the low-wav enumber IR spectrum calculated for this cluster species does not accou nt for the observed spectrum.