Conformational Stability from Variable Temperature Infrared Spectra of Xenon Solutions, r0 Structural Parameters, and Ab Initio Calculations of Cyclopropylisocyanate

Infrared spectra (4000 to 400 cm−1) of the gas and variable temperature xenon solutions, and the Raman spectrum of the liquid have been recorded for cyclopropylisocyanate. The enthalpy difference has been determined to be 77 ± 8 cm−1 (0.92 ± 0.10 kJ/mol) with the trans form more stable than the cis conformer with 59 ± 2% present at ambient temperature. By utilizing three rotational constants for each conformer, combined with structural parameters predicted from MP2(full)/6-311+G(d,p) calculations, the adjusted r0 parameters have been obtained. Heavy atom structural parameters for the trans [cis] conformers are the following: distances (A) (C−C2,3) = 1.509(3) [1.509(3)], (C2−C3) = 1.523(3) [1.521(3)], (C−N) = 1.412(3) [1.411(3)], (N═C) =1.214(3) [1.212(3)], (C═O) = 1.163(3) [1.164(3)]; angles (°) ∠CCN = 116.7(5) [120.1(5)], ∠CNC = 136.3(5) [137.6(5)]. The centrifugal distortion constants have been predicted from ab initio and DFT calculations and are compared to the experimentally determined values.