Raman and infrared spectra, conformational stability, normal coordinate analysis andab initio calculations of 3-chloro-1-butene

Abstract

The Raman and infrared spectra (3300-30 cm −1 ) of gaseous and solid 3-chloro-1-butene, H 2 C CHCCl (CH 3 )H, were recorded. Additionally, the Raman spectrum (3300-30 cm −1 ) of the liquid was recorded. All three conformers were observed in the fluid phases and the conformer with the hydrogen atom eclipsing the double bond (HE form) was identified as the most predominant. From variable-temperature measurements in liquified xenon, the enthalpy differences between the HE form and the two less stable conformers, i.e. the methyl group eclipsing the double bond (ME form) and the chlorine atom eclipsing the double bond (ClE form), were determined to be 75 ± 8 cm −1 (214 ± 23 cal mol −1 ) and 197 ± 37 cm −1 (563 ± 106 cal mol −1 ), respectively. Nearly complete vibrational assignments are proposed for all three conformers, which are consistent with the predicted wavenumbers utilizing the force constants from ab initio MP2/6-31G(d) calculations. Both the infrared intensities and the Raman activities and depolarization values were obtained from the ab initio calculations. Complete equilibrium geometries were determined by ab initio calculations employing the 6-31G(d) and 6-311YYG(d,p) basis sets with full electron correlation by the Möller-Plesset (MP2) perturbation method to second order. The structural parameters obtained with the larger basis set are comparable to those obtained from a previously reported electron diffraction study. The results are discussed and the theoretical values are compared with the experimental values when appropriate.

Conformational Effects on Optical Rotation. 3-Substituted 1-Butenes

“…An early IR study showed that it exists in at least two forms, and the ED study found three conformers, with the major form being 76% ± 10% at 20 °C, with the proportion of this form decreasing to 62% ± 10% at 180 °C . As part of a more recent vibrational spectroscopic study, the MP2/6-311++G**-calculated energy differences for the conformers were obtained, and are in agreement with other calculated relative energies (see below). However, this study also reported variable-temperature infrared studies using xenon solutions of 1 .…”

Conformational Analysis, Barriers to Internal Rotation, Vibrational Assignment, and ab Initio Calculations of 3-Fluoro-1-butene

“…As opposed to the case of 3-chloro-1-butene, where the calculated ab initio energy differences between the conformers were grossly overestimated compared to the experimental enthalpy values, the predicted ab initio energy differences (Table ) for 3-fluoro-1-butene have more realistic values. However, this is achieved only with the relatively large basis set (MP2/6-311+G(d,p)).…”

“…e Frequencies are taken from the infrared spectrum of the gas, except the ones in parentheses, which are taken from the infrared spectrum of the liquid or sample dissolved in liquid xenon. As opposed to the case of 3-chloro-1-butene, 21 where the calculated ab initio energy differences between the conformers were grossly overestimated compared to the experimental enthalpy values, the predicted ab initio energy differences (Table 2) for 3-fluoro-1-butene have more realistic values. However, this is achieved only with the relatively large basis set (MP2/ 6-311+G(d,p)).…”

“…This can be explained with a favorable interaction between the fluorine atom and the methylene hydrogen (H9) which eclipse each other in the ME/HE transition state. In contrast, in 3-chloro-1-butene 21 the ME/HE transition state has the highest energy due to the larger size of the chlorine atom eclipsing the methylene hydrogen.…”

“…This, in fact, is also the case with the FE rotamer, where the CF stretch is observed at a lower frequency (927 cm -1 ) than the CH 3 rock (1061 cm -1 ) in the spectrum of the xenon solution. The CH 2 twist and rock have approximately the same locations as those of 3-chloro-1-butene and produce very strong bands at 993 and 975 cm -1 in the infrared spectra of the condensed phases. The doublet at about 900 cm -1 in the same spectrum is due to an almost pure CH 2 wag (98%).…”