LATTICE-DYNAMICS OF ACETYLENE

A simple force constant model is described and applied to the vibrations of crystalline acetylene, C2H2 and C2D2 in its low temperature orthorhombic phase. The model uses ten (six intermolecular and four intramolecular) adjustable parameters to calculate the frequencies of seven lattice modes and fourteen internal mode components, and good agreement with spectroscopic data is obtained. Intermolecular force constants decrease sharply with increasing separation and are well represented by exponential functions. Intramolecular force constants show only minor changes from their gas phase values.