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Conformational Studies of Fluoromethylcyclopropane from Temperature-Dependent FT-IR Spectra of Xenon Solutions and Ab Initio Calculations

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journal contribution
posted on 2004-06-24, 00:00 authored by James R. Durig, Zhenhong Yu, Chao Zheng, Gamil A. Guirgis
Variable-temperature (−55 to −100 °C) studies of the infrared spectra (3500 to 400 cm-1) of fluoromethylcyclopropane, c-C3H5CH2F, dissolved in liquefied xenon have been carried out, and the Raman (liquid and solid) and infrared (gas and solid) spectra have been recorded from 3500 to 60 cm-1. By utilizing four conformer pairs, an enthalpy difference of 262 ± 26 cm-1 (3.13 ± 0.31 kJ/mol) was obtained, with the gauche rotamer the more stable conformer and the only form present in the solid. The abundance of cis conformer present at ambient temperature is 12 ± 1%. On the basis of the far-infrared spectral data along with the experimental enthalpy and gauche dihedral angle, the potential function governing conformational interchange has been obtained, and the determined potential constants are V1 = −245 ± 23, V2 = −414 ± 17, V3 = 1263 ± 6, V4 = 272 ± 17, and V5 = 101 ± 2 cm-1, with the cis-to-gauche barrier of 1223 cm-1 (14.63 kJ/mol) and the gauche-to-gauche barrier of 1362 cm-1 (16.29 kJ/mol). From MP2 ab initio calculations with triple-ζ basis sets with diffuse functions, the gauche conformer is predicted to be the more stable rotamer by about 320 cm-1, which is consistent with the experimental results, but without diffuse functions the two conformers are predicted to have nearly the same energy. Similar results are predicted from density functional theory by the B3LYP method. The complete vibrational assignment for the gauche conformer is proposed, and several fundamentals for the cis conformer have been identified. The structural parameters, dipole moments, conformational stability, vibrational frequencies, and infrared and Raman intensities have been predicted from ab initio calculations. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.

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