Conformational stability, structural parameters, vibrational spectra, and ab initio calculations of isopropyl- and tertiary-butylisothiocyanate

Variable temperature (−60 to −100 °C) studies of the infrared spectra (3200–400 cm−1) of isopropylisothiocyanate, (CH3)2CHNCS, dissolved in liquefied xenon, have been carried out. Additionally the infrared spectra of the gas and solid have been recorded for both isopropylisothiocyanate and tertiary-butylisothiocyanate, (CH3)3CNCS, from 3200 to 100 cm−1. The analyses of these spectral data for the isopropyl molecule and the Raman spectrum of the liquid indicated one stable conformer (trans) in the annealed solid but in the fluid phases most of the molecules have energies above the barriers of the two predicted bound vibrational states i.e., trans and gauche forms. The MP2(full) ab initio calculations, employing a variety of basis sets with and without diffuse functions, have been used to predict the conformational stabilities with the trans conformer, the most stable form, for isopropylisothiocyanate and the second most stable form is predicted to be either the skew or gauche conformer depending in some cases on whether diffuse functions are used. However, even when the energy values indicate the skew form more stable than the gauche rotamer, one imaginary frequency indicates the skew form is a first-order saddle point. These results should be contrasted with the microwave data where the experimental B + C value was previously interpreted to indicate the skew conformer as the most stable form. For t-butylisothiocyanate the staggered conformer is the more stable form with the eclipsed conformer a transition state with a barrier of ∼50 cm−1 so there is nearly free internal rotation of the NCS moiety. For both molecules, the structural parameters, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and complete vibrational assignments are proposed. The r0 structural parameters are estimated by combing the MP2(full)/6-311+G(d,p) predicted values for the CH parameters with some adjustments to the heavy atom distances. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.