Infrared spectroscopic studies of the conformation in ethyl α-haloacetates in the vapor, liquid and solid phases

Article abstract of DOI:10.1016/j.saa.2010.03.022

Infrared spectra of ethyl α-fluoroacetate, ethyl α-chloroacetate, ethyl α-bromoacetate and ethyl α-iodoacetate have been measured in the solid, liquid and vapor phases in the region 4000-200 cm^-1. Vibrational frequency assignment of the observed bands to the appropriate modes of vibration was made. Calculations at DFT B3LYP/6-311+G** level, Job: conformer distribution, using Spartan program '08, release 132 was made to determine which conformers exist in which molecule. The results indicated that the first compound exists as an equilibrium mixture of cis and trans conformers and the other three compounds exist as equilibrium mixtures of cis and gauche conformers. Enthalpy differences between the conformers have been determined experimentally for each compound and for every phase. The values indicated that the trans of the first compound is more stable in the vapor phase, while the cis is the more stable in both the liquid and solid phases. In the other three compounds the gauche is more stable in the vapor and liquid phases, while the cis conformer is the more stable in the solid phase for each of the second and third compound, except for ethyl α-iodoacetate, the gauche conformer is the more stable over the three phases. Molar energy of activation Ea and the pseudo-thermodynamic parameters of activation ΔH^?, ΔS^? and ΔG^? were determined in the solid phase by applying Arrhenius equation; using bands arising from single conformers. The respective E_a values of these compounds are 5.1 ± 0.4, 6.7 ± 0.1, 7.5 ± 1.3 and 12.0 ± 0.6 kJ mol^-1. Potential energy surface calculations were made at two levels; for ethyl α-fluoroacetate and ethyl α-chloroacetate; the calculations were established at DFT B3LYP/6-311+G** level and for ethyl α-bromoacetate and ethyl α-iodoacetate at DFT B3LYP/6-311G* level. The results showed no potential energy minimum exists for the gauche conformer in ethyl α-fluoroacetate.