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

Abstract Variable temperature (−55 to −155 °C) studies of the infrared spectra (400–3500 cm −1 ) of ethylisocyanate, CH 3 CH 2 NCO, dissolved in liquid xenon and krypton have been recorded. Additionally the infrared spectra of the gas and solid have been re-investigated. These spectroscopic data indicate two conformers in the fluid states which are the cis and trans forms with a large proportion of molecules in the gas phase at ambient temperature in the excited states of the NCO torsional mode which has a very low barrier to conformational interchange. Variable temperature (−110 to −155 °C) studies of krypton solutions were carried out and by using two conformer pairs, an enthalpy difference of 100 ± 4 cm −1 (1.20 ± 0.05 kJ/mol) was obtained with the cis conformer the more stable form. To aid in the analyses of the vibrational and rotational spectra, ab initio calculations have been carried out by the perturbation method to the second order (MP2) with full electron correlation using a variety of basis sets up to 6-311+G(2df,2pd) and cc-PVQZ. With the basis sets 6-311+G(2d,2p) and larger, the barrier at the cis position ranged from a low value of 11 cm −1 to a high value of 31 cm −1 with a value of 19 cm −1 from the largest basis set of cc-PVQZ. Thus, the gauche well is probably so shallow that it does not contain a bound vibrational state. This results in the cis conformer as the most stable form which is consistent with the experimental rotational and vibrational data. The predicted energy difference from these calculations between the cis conformer and the transitional-state skew form is ∼100 cm −1 which is consistent with the assigned microwave lines for four excited states of the NCO torsion. Density functional theory calculation by the B3LYP method with many of the same basis sets provided little information. By utilizing the previously reported microwave rotational constants with the structural parameters predicted by the ab initio MP2(full)/6-311+G(d,p) calculations, adjusted r 0 structural parameters have been obtained for the cis form. The determined heavy atom parameters are: r (C N) = 1.211(5), r (C O) = 1.167(5), r (C N) = 1.448(5), r (C C) = 1.516(5) A for the distances and angles of ∠CCN = 112.6(5), ∠CNC = 137.5(5), ∠NCO = 172.9(5)°. The centrifugal distortion constants, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and compared to experimental quantities when available. These results are compared to the corresponding quantities of some similar molecules.