Vibrational spectra of indene .4. Calibration, assignment, and ideal-gas thermodynamics

The vibrational spectra of indene have been examined in the vapor and the liquid state using infrared and Raman spectroscopy. Wavenumbers of the liquid spectra are measured to 0.1 cm(-1) to re-examine the potential of indene as a calibration molecule. An assignment for its normal modes of vibration is made considering vapor band shapes, polarization ratios and calculated frequencies from a scaling procedure of the indene AM1 force field. An AMI analysis of the cyclopentadiene vibrations is performed to obtain the scale factors for the constituent five-membered ring. The root-mean standard deviation of the calculated relative to the observed frequencies is 11 cm(-1) for the planar modes and 16 cm(-1) for the nonplanar modes. The potential energy distribution for the nonplanar modes from a refinement calculation is discussed. Ideal-gas thermodynamic functions are evaluated using the vapor phase vibrational wavenumbers and the previously reported moments-of-inertia. The spectroscopic ideal-gas entropy exceeds that determined from calorimetry by approximate to 0.95 R (R = 8.31451 J K-1 mol(-1)) from 300 to 500 K, indicating that the crystal most probably contains some residual disorder at 0 K.