Infrared and Raman spectra, conformational stability, ab initio calculations of structure, and vibrational assignment of ethynylmethyl cyclobutane

The infrared spectra (3200-300 cm(-1)) of the gas and solid and the Raman spectra (3200-30 cm(-1)) of the liquid with quantitative depolarization values and the solid have been recorded of ethynylmethyl cyclobutane (cyclobutylmethyl acetylene), c-C4H7CH2C drop CH Both the equatorial and the axial conformers have been identified in the fluid phases and both the gauche and trans conformations of the methyl acetylenic group have been identified for each ring conformer. Variable temperature ( - 105 to - 150 degreesC) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data the enthalpy differences have been determined to the 112 +/- 11 cm(-1) (1.34 +/- 0.13 kJ/mol) between the most stable equatorial-trans (Et) conformer and the equatorial-gauche (Eg) conformer which is the second most stable conformer and 327 +/- 35 cm(-1) (3.91 +/- 0.42 kJ/mol) with the axial-gauche (Ag) conformer which is the least stable conformer. The enthalpy difference between the axial-trans (At) and the equatorial-gauche(Eg) is 56 +/- 6 cm(-1). At ambient temperature there is approximately 33% of the Et conformer, 38% of the Eg form, 15% of the At and 14% of the Ag conformer. For the polycrystalline solid the Eg conformer is the only form present which probably results form the packing in the crystal. A complete vibrational assignment is proposed for both equatorial conformers and additionally a few of the fundamentals of the At and Ag conformers have been assigned. The vibrational assignments are supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G(d) calculations. Complete equilibrium geometries have been determined for all four possible rotamers by ab initio calculations employing the 6-31G(d) and 6-311 + + G(d,p) basis sets at levels of restricted Hartree-Fock (RHF) and /or Moller-Plesset (MP2) with full electron correlation by the perturbation method to second order. The results are discussed and compared to those obtained for some similar molecules. (C) 2002 Elsevier Science B.V. All rights reserved.