Conformational stability, barrier to internal rotation, structural parameters, ab initio calculations, and vibrational assignment of cyclopropylcarbonyl chloride

Abstract

The fundamental asymmetric torsional mode of cyclopropylcarbonyl chloride, c-C 3H 5-CClO, for the more stable cis conformer (carbonyl group oriented cis to the ring) is observed at about 54 cm −1. The corresponding mode for the high energy trans form is assigned to a Q-branch series with stronger intensity beginning at 87.4 cm −1, decreasing in frequency with successive excited states. From a study of the Raman spectra of the vapor at variable temperatures, it has been determined that the cis is more stable than the trans form by 171 ± 35 cm −1 (489 ± 100 cal mol −1) and that the cis conformer remains in the solid. From a similar study of the spectrum of the liquid, it is found that the trans form is slightly more stable than the cis form by 93 ± 27 cm −1 (266 ± 77 calmol −1). From these data, the potential function for internal rotation of the asymmetric top has been determined and the potential coefficients are: V 1 = 528 ± 14, V 2 = 1769 ± 27 and V 3 = −371 ± 15 cm −1. This potential is consistent with cis to trans and trans to cis barriers of 1934 and 1779 cm −1, respectively, and a conformational energy difference of 155 ± 56 cm −1 (443 ± 160 cal mol −1). The r 0 structural parameters have been obtained for the cis conformer from a combination of ab initio calculated parameters with appropriate off-set values and the fit of the previously reported microwave rotational constants for the two naturally occurring chlorine isotopes. The conformational stability, barriers to internal rotation and fundamental vibrational frequencies, which have been determined from experimental data, are compared with those obtained from ab initio Hartree—Fock gradient calculations with the 3-21G * and 6-31G * basis sets and results obtained for some similar molecules.