Conformational Stability from Variable Temperature FT-IR Spectra of Krypton Solutions, r₀ Structural Parameters, Vibrational Assignment, and ab Initio Calculations of 4-Fluoro-1-butene

Abstract

Variable temperature (-115 to -155 degrees C) studies of the infrared spectra (3200-400 cm-1) of 4-fluoro-1-butene, CH2=CHCH2CH2F, dissolved in liquid krypton have been carried out. The infrared spectra of the gas and solid as well as the Raman spectra of the gas, liquid, and solid have also been recorded from 3200 to 100 cm-1. From these data, an enthalpy difference of 72 +/- 5 cm-1 (0.86 +/- 0.06 kJ x mol-1) has been determined between the most stable skew-gauche II conformer (the first designation refers to the position of the CH2F group relative to the double bond, and the second designation refers to the relative positions of the fluorine atom to the C-C(=C) bond) and the second most stable skew-trans form. The third most stable conformer is the skew-gauche I with an enthalpy difference of 100 +/- 7 cm-1 (1.20 +/- 0.08 kJ x mol-1) to the most stable form. Larger enthalpy values of 251 +/- 12 cm-1 (3.00 +/- 0.14 kJ x mol-1) and 268 +/- 17 cm-1 (3.21 +/- 0.20 kJ x mol-1) were obtained for the cis-trans and cis-gauche conformers, respectively. From these data and the relative statistical weights of one for the cis-trans conformer and two for all other forms, the following conformer percentages are calculated at 298 K: 36.4 +/- 0.9% skew-gauche II, 25.7 +/- 0.1% skew-trans, 22.5 +/- 0.2% skew-gauche I, 10.0 +/- 0.6% cis-gauche, and 5.4 +/- 0.2% cis-trans. The potential surface describing the conformational interchange has been analyzed and the corresponding two-dimensional Fourier coefficients were obtained. Nearly complete vibrational assignments for the three most stable conformers are proposed and some fundamentals for the cis-trans and the cis-gauche conformers have been identified. The structural parameters, dipole moments, conformational stability, vibrational frequencies, infrared, and Raman intensities have been predicted from ab initio calculations and compared to the experimental values when applicable. The adjusted r0 structural parameters have been determined by combining the ab initio predicted parameters with previously reported rotational constants from the microwave data. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.

Microwave Spectrum and Conformational Properties of 4-Isocyano-1-butene (H2C═CHCH2CH2N≡C)

“…The structural and conformational properties of this class of compound, which are analogous to that of H 2 CCHCH 2 CH 2 NC, have received considerable attention in the past. One example is 4-fluoro-1-butene (H 2 CCHCH 2 CH 2 F), which has been studied several times. − A conformer corresponding to III has been found to be the lowest energy form. − Two additional forms with slightly higher energies were found in the gas-phase in a MW work, while all five forms were identified in a far-infrared study of a krypton solution . A gas electron diffraction (GED) and molecular-mechanics investigation of H 2 CCHCH 2 CH 2 Cl and H 2 CCHCH 2 CH 2 Br again showed that forms analogous to III are the most stable ones in these two cases and that there are minor contributions from other conformers .…”

“…11−13 Two additional forms with slightly higher energies were found in the gas-phase in a MW work, 12 while all five forms were identified in a far-infrared study of a krypton solution. 13 A gas electron diffraction (GED) and molecular-mechanics investigation of H 2 C CHCH 2 CH 2 Cl and H 2 CCHCH 2 CH 2 Br again showed that forms analogous to III are the most stable ones in these two cases and that there are minor contributions from other conformers. 14 The preferred form of H 2 CCHCH 2 CH 2 OH corresponds to II, and it is stabilized by an intramolecular hydrogen bond between the hydroxyl group and the π- electrons of the double bond.…”