Vibrational assignment and force constants of S8 from a normal-coordinate treatment

Normal-coordinate calculations for S8 were revised by use of new far-infrared spectral data that locate the low infrared-active fundamentals at 191 (e1) and 243 (b2) cm−1 (solution state). Digital computer methods were used. Agreement of calculated with observed frequencies could not be obtained with the basic Urey-Bradley field alone, but was obtained when an extra quadratic cross-term between adjacent SS bond stretchings was included. The vibrational assignment based on the calculations is consistent with all well-established features of the observed spectra. Thermodynamic comparisons with vapor-pressure data for rhombic sulfur suggest that low fundamental frequencies of the S8 molecule are lower in the vapor state than in condensed states.