Far infrared spectrum, conformational stability, barriers to internal rotation, vibrational assignment, and ab initio calculations of 2-chloro-3-fluoropropene

Abstract

The far infrared spectrum (375 to 30 cm⁻¹) of gaseous 2-chloro-3-fluoropropene, CH₂=C(CH₂F)CI, has been recorded at a resolution of 0.10 cm⁻¹. The fundamental asymmetric torsional mode is observed at 117.5 cm⁻¹ with ten excited states falling to low frequency for thes-cis (fluorine atom eclipsing the double bond) conformer. For the higher energy gauche conformer, the asymmetric torsion is estimated to be at 94 cm⁻¹. From these data the asymmetric torsional potential function has been calculated. The potential function coefficients are calculated to be in cm⁻¹):V ₁=803±21,V ₂=−94±21,V ₃= 1025±10,V ₄=95±10, andV ₆=2±1, with an enthalpy difference between the more stables-cis and gauche conformera of 550±100 cm⁻¹ (1.57±0.29 kcal/mol). This function gives values of 1227±50cm⁻¹(3.51±0.14kcal/mol), 1266±200 cm⁻¹ (3.62±0.57 kcal/mol), and 665±100 cm⁻¹ (1.90±0.29 kcal/mol), for thes-cis to gauche, gauche to gauche, and gauche tos-cis barriers, respectively. From the relative intensities of the Raman lines of the gas at 652 cm⁻¹ (gauche) and 731 cm⁻¹ (s-cis) as a function temperature, the enthalpy difference is found to be 565±96 cm⁻¹ (1.62±0.27 kcal/mol). However, the more polar gauche conformer remains in the crystalline solid. The Raman spectrum of the gas has been recorded from 3500 to 70 cm⁻¹ and, utilizing these data and the previously reported infrared data, a complete vibrational analysis is proposed for both conformers. The conformational stability, barriers to internal rotation, fundamental vibrational frequencies, and structural parameters that have been determined experimentally are compared to those obtained from ab initio Hartree-Fock gradient calculations employing both the 3–21 G^* and 6–31G^* basis sets and to the corresponding quantities for some similar molecules.