Liquid Structure, Infrared and Isotropic/Anisotropic Raman Noncoincidence of the Amide I Band, and Low-Wavenumber Vibrational Spectra of Liquid Formamide: Molecular Dynamics and ab Initio Molecular Orbital Studies

摘要

The relationship between the liquid structure of formamide and wavenumber differences among its infrared (IR), isotropic Raman, and anisotropic Raman bands in the amide I region is analyzed theoretically. The following two methods are employed: (1) ab initio molecular orbital (MO) calculations on a few different cluster species of formamide molecules and (2) calculations of the IR and Raman spectra in the amide I region on the basis of the transition dipole coupling mechanism and the liquid structures derived from molecular dynamics simulations. It is shown that intermolecular interactions other than those involved in a one-dimensional hydrogen-bonded chain are required to reproduce the observed wavenumber difference between the amide I IR and isotropic Raman bands. This wavenumber difference originates from the difference in the 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 chains in the liquid state plays an important role. Ab initio MO calculations of the low-wavenumber IR and Raman spectra of the cluster species of formamide are also performed. Existence of a large concentration of cyclic hexamers in the liquid state is unlikely because the low-wavenumber IR spectrum calculated for this cluster species does not account for the observed spectrum.