Abstract
The far-infrared and low-frequency Raman spectra of dipropionamide have been measured, and the lattice vibrational frequencies have been assigned on the basis of the infrared dichroism and the single crystal laser-Raman polarization measurement. The normal coordinate treatment has been made for optically active intra- and intermolecular vibrations of four molecules in the Bravais unit cell. The lattice vibrational frequencies are explained well by the force field including the hydrogen bond stretching force constant, which reflects the weakness of the bifurcated hydrogen bond, and non-bonded atom—atom repulsion constants. In order to reproduce the observed band splittings of intramolecular vibrations, it is necessary to introduce appropriate coupling constants between the equivalent internal symmetry coordinates of neighbouring molecules. The origin of these coupling constants were discussed in relation to the dipole—dipole interaction.
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