Infrared, Raman and far infrared spectra, ab initio calculations, and internal rotation of 3-fluoro-3-methyl-1-butyne

Abstract

The infrared (3500–60 cm⁻¹) spectrum of the gas and Raman (3500–20 cm⁻¹) spectra of liquid and solid 3-fluoro-3-methyl-1-butyne, HCCCF(CH₃)₂, have been recorded. Ab initio calculations of energies, geometrical structures, vibrational frequencies, infrared intensities, Raman activities and the potential energy function for the methyl torsions have been carried out to assist in the interpretation of the spectra. The in-phase fundamental torsional mode is observed at 259 cm⁻¹ with a series of sequence peaks falling to lower frequencies. By utilizing a coupled two-top rotor model, five of these bands have been assigned and their frequencies fit by varying the effective three-fold barrier value and the V^′₃₃ term while utilizing the predicted V₃₃ and V₆ terms from the B3LYP/6-31G(d) and MP2(full)/6-31G(d) calculations. The three-fold torsional barriers of 1324 ± 14 cm⁻¹ (3.79 ± 0.04 kcal mol⁻¹) and 1338 ± 27 cm⁻¹ (3.83 ± 0.08 kcal mol⁻¹), respectively, have been obtained from their fit. Attempts to obtain additional terms, v^″₃₃ (sin 3τ₀cos 3τ₁) and v₆′ (sin 6τ_0,1), by using a weighting factor for the optimized minimum, maximum and saddle points as additional constraints in the least-squares fit was made, but this resulted in a significantly lower barrier of 1260 ± 14 cm⁻¹ (3.50 ± 0.04 kcal mol⁻¹). These experimental results are compared to the predicted values from the ab initio calculations where the MP2 predicted value of 1505 cm⁻¹ is expected to be the best value based on the experimentally determined barriers for some similar molecules. A complete vibrational assignment is proposed based on band contours, relative intensities, and ab initio predicted frequencies. Several fundamentals are significantly shifted in the condensed phases compared to values in the vapor state. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.