Potential models and local mode vibrational eigenvalue calculations for acetylene

Abstract

Two potential models for acetylene are developed and tested by comparison between variational calculations for the stretching vibrational term values and available spectroscopic data. The first model based on local bond potentials with harmonic interbond coupling gives root mean square deviations of 6 cm−1 for C2H2 and 3 cm−1 for C2D2. The second model is more ambitious, being designed to reproduce the dissociation characteristics of the molecules, and the calculated root mean square deviations from the experimental vibrational term values are larger, 32 cm−1 for C2H2 and 24 cm−1 for C2D2. The eigenvalue spectrum of C2H2 is shown to differ from that of C2D2 in showingmarked local mode features and this difference in behaviour is underlined by means of a correlation diagram. Finally it is shown how the known normal mode frequencies and anharmonic constants may be introduced into a simple model in order to predict the excited term values of C2H2, again with a root mean square deviation of 6 cm−1.