Raman, infrared and far infrared spectra, ab initio calculations, r0 structural parameters, and internal rotation of 3-methyl-1-butyne

The Raman (3500-20 cm−1) spectra of the gas and liquid and the infrared (3500-60 cm−1) spectra of the gas and solid of 3-methyl-1-butyne, HCCCH(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 273.2 cm−1 with a series of sequence peaks proceeding to lower frequencies along with a large number of 'hot bands' associated with the fundamental and some of the sequence bands. The out-of-phase torsional mode was observed in the Raman spectrum of the liquid at 226 cm−1. The two-quantum transitions of the in-phase torsion were observed in the Raman spectrum of the gas which made it possible to confidently assign the 2←1 transition for this mode. By utilizing a coupled two-top rotor model, eight of these bands have been assigned and their frequencies fit by varying the effective three-fold barrier value along with the V33, V33′ and V6 terms. The three-fold torsional barrier of 1485±14 cm−1 (4.25±0.04 kcal/mol) has been obtained from their fit as well as V33=85±20, V33′=−287±13, and V6=−28±5 cm−1. The barrier and the same coefficients have been predicted from ab initio and density functional theory calculations with those obtained from the MP2(full)/6-31G(d) calculation in good agreement with the experimental values except for the very small V6 term. 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 adjusted r0 structural parameters have been obtained by combining ab initio MP2(full)/6-311+G(d,p) predicted values and previously reported rotational constants from a microwave study. The values for the determined heavy atom parameters are: r(C1C2)=1.209(3), r(C2-C3)=1.467(5), r(C3-C4,5)=1.535(5) Å, ∠C2C3C4,5=110.6(5) and ∠C4C3C5=111.2(5)° with a large difference between this C2-C3 distance and the previously reported value of 1.495(11) Å. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.