ANALYSIS OF TORSIONAL SPECTRA OF MOLECULES WITH 2 INTERNAL C3V ROTORS .16. INFRARED AND RAMAN-SPECTRA, VIBRATIONAL ASSIGNMENT, METHYL TORSIONAL POTENTIAL FUNCTION, AND GAS-PHASE THERMODYNAMIC FUNCTIONS OF 2,3-DIMETHYLBUTA-1,3-DIENE

The vibrational spectra of gaseous, liquid, and solid 2,3-dimethylbuta-1,3-diene have been investigated between 4000 and 50 cm-1, and a complete vibrational assignment is proposed. No spectral evidence was found for the presence of any high-energy conformer. Two series of bands in the far-infrared spectrum of the gaseous phase have been assigned to transitions involving the methyl torsional modes; the higher frequency series near 400 cm-1 is due to difference bands of the Raman active methyl torsion and a higher frequency mode. The methyl torsional potential function has been calculated, leading to a value of 4.27 kcal/ mol for the effective barrier height to internal rotation, and the reasons for this relatively large barrier have been discussed. Values for the gas phase thermodynamic functions have been calculated over a range of temperatures by using these data. © 1979 American Chemical Society.