title: Evidence of conformational exchange averaging in the thermal rotational spectrum of ethyl cyanoformate.
abstract: The Stark modulated low resolution microwave spectrum of ethyl cyanoformate between 21.5 and 24.0 GHz at 210, 300, and 358 K, which shows the J + 1 <-- J = 8 <-- 7 bands of three species, is compared to simulations based on electronic structure calculations at the MP2/6-311++G theory level. Calculations at this theory level reproduce the relative energies of the syn-anti and syn-gauche conformers, obtained in a previous study, and indicate that the barrier to conformer exchange is approximately 360 cm(-1) higher in energy than the syn-anti minimum. Simulated spectra of the eigenstates of the calculated O-ethyl torsional potential function reproduce the relative intensities and shapes of the lower and higher frequency bands which correspond to transitions of the syn-anti and syn-gauche conformers, respectively, but fail to reproduce the intense center band in the experimental spectra. A model incorporating exchange averaging reproduces the intensity of the center band and its temperature dependence. These simulations indicate that a large fraction of the thermal population at all three temperatures undergoes conformational exchange with an average energy specific rate constant, <k(E)>, of approximately 25 GHz. This model can explain anomalies present in rotational spectra of many other compounds composed of mixtures of conformers.
authors.affiliation: Department of Chemistry, The University of California, One Shields Avenue, Davis, California 95616, USA. nstrue@ucdavis.edu
authors.forename: Nancy S
authors.initials: NS
authors.lastname: True