Conformational Studies of Cyclopropylmethyl Isothiocyanate from Temperature-Dependent FT-IR Spectra of Rare Gas Solutions and Ab Initio Calculations

Variable temperature (−55 to −150 °C) studies of the infrared spectra (3200−100 cm^-1) of cyclopropylmethyl isothiocyanate, c-C₃H₅CH₂NCS, dissolved in liquefied rare gases (Xe and Kr), have been carried out. The infrared spectra of the gas and solid, as well as the Raman spectrum of the liquid, have also been recorded from 3200 to 100 cm^-1. By analyzing six conformer pairs in xenon solutions, a standard enthalpy difference of 228 ± 23 cm^-1 (2.73 ± 0.27 kJ·mol^-1) was obtained with the gauche−cis (the first designation indicates the orientation of the CNCS group with respect to the three-membered ring, the second designation indicates the relative orientation of the NCS group with respect to the bridging C−C bond) rotamer the more stable form, and it is also the only form present in polycrystalline solid. Given statistical weights of 2:1 for the gauche−cis and cis−trans forms (the only stable conformers predicted); the abundance of cis−trans conformer present at ambient temperature is 14 ± 2%. The potential surface describing the conformational interchange has been analyzed, and the corresponding two-dimensional Fourier coefficients were obtained. From MP2 ab initio calculations utilizing various basis sets with diffuse functions, the gauche−cis conformer is predicted to be more stable by 159−302 cm^-1, which is consistent with the experimental results. However, without diffuse functions, the conformational energy differences are nearly zero even with large basis sets. For calculations with density functional theory by the B3LYP method, basis sets without diffuse functions also predict smaller energy differences between the conformers, although not nearly as small as the MP2 results. A complete vibrational assignment for the gauche−cis conformer is proposed, and several fundamentals for the cis−trans conformer have been identified. The structural parameters, dipole moments, conformational stability, vibrational frequencies, and infrared and Raman intensities have been predicted from ab initio calculations and compared to the experimental values when applicable; the r₀ structural parameters are also estimated. The energies for the linear CNCS moiety were calculated. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.