Conformational stability from variable temperature infrared spectra of krypton and xenon solutions, ab initio calculations, vibrational assignment and r0 structural parameters of vinylcyclopropane

The infrared spectra (3200-400 cm−1) of krypton and xenon solutions of vinylcyclopropane c-C3H5CHCH2, at variable temperatures (−55 to −150 °C) have been recorded. Additionally, the infrared spectra (3200-250 cm−1) of the gas and solid have been recorded. From a comparison of the spectra of the fluid phases with that of the solid, all of the fundamental vibrations of the trans conformer have been identified and assigned as well as many of those for the gauche form. By utilizing five pairs of fundamentals for these two conformers in the xenon solutions, an enthalpy difference of 415±42 cm−1 (4.96±0.50 kJ/mol) has been obtained with the trans conformer the more stable form. There is 21±3% of the gauche conformer present at ambient temperature. Results from ab initio calculations show the cis rotamer is a transition state. Equilibrium geometries and total energies of both conformers have been determined from ab initio calculations with full electron correlation by the perturbation method to second order as well as by hybrid density functional theory calculations with the B3LYP method using a number of basis sets. The MP2 calculations predict the trans conformer to be more stable than the gauche form ranging from 358 to 440 cm−1 whereas the B3LYP calculations predict significantly larger values between 571 and 616 cm−1. By adjusting ab initio MP2(full)/6-311+G(d,p) predicted structural parameters to fit previously reported rotational constants for the trans conformer, adjusted r0 parameters have been obtained. Comparisons of these parameters to those previously obtained from the microwave and electron diffraction studies are made. Many of the spectroscopic and theoretical results are compared to the corresponding properties for some similar molecules.