Ab Initio Highly Correlated Conformational Analysis of 1,2-Difluorethane and 1,2-Dichloroethane Mauro L. Franco, †,‡ Dalva E. C. Ferreira, † Hélio F. Dos Santos, § and Wagner B. De Almeida* ,† Laboratório de Química Computacional e Modelagem Molecular (LQC-MM), Departamento de Química, ICEx, UniVersidade Federal de Minas Gerais (UFMG), Campus UniVersitário, Pampulha, Belo Horizonte, MG, 31270-901, Brazil, Diretoria de Ciências Exatas (DCX), Centro UniVersitário do Leste de Minas Gerais, UniLeste-MG, Campus I, Coronel Fabriciano, MG, 35170-056, Brazil, and Núcleo de Estudos em Química Computacional (NEQC), Departamento de Química, ICE, UniVersidade Federal de Juiz de Fora (UFJF), Campus UniVersitário, Martelos, Juiz de Fora, MG, 36036-330, Brazil Received January 14, 2008 Abstract: Temperature-dependent conformational population calculations for anti and gauche forms of 1,2-dichloroethane and 1,2-difluorethane were carried out at a highly correlated level of theory (MP4(SDTQ) and CCSD(T)) employing good quality basis sets (6-311++G(3df,3pd) and aug-cc-pVQZ) for the determination of gas relative conformational energies, making use of the statistical thermodynamics formalism for the evaluation of the thermal energy correction at the MP2/6-311++G(3df,3pd) and MP2/aug-cc-pVTZ levels. In addition to the standard calculation of thermodynamic partition functions, a treatment of the lowest-frequency vibrational mode as hindered rotation and anharmonic correction to vibrational frequencies was also included. We found a good agreement between ab initio calculated conformational population values and experimental gas-phase electron diffraction data for the 1,2-dicloroethane. However, for the 1,2-difluorethane species, a reasonable agreement with the experimental anti/gauche population ratio obtained from the analysis of gas-phase far-infrared (50–370 cm -1 ) and low-frequency Raman (70–300 cm -1 ) spectra was not obtained. The results reported here indicate that, for 1,2-difluorethane, and probably other substituted alkanes where the gauche effect is of relevance, a more appropriated treatment of the low-frequency modes must be pursued in order to reproduce experimental conformational population data. 1. Introduction Conformational analysis is a fascinating subject, first related to organic chemistry, which has attracted the attention of experimentalists and theoreticians for a long time, being also of great importance to almost all areas of chemistry. Over the past years, temperature-dependent nuclear magnetic resonance (NMR) and solvent effect studies have been reported by many researchers in the area of organic chemistry; a recent example is the conformational analysis of succinic acid. 1 In several cases, the conformational process is not simple, with some vibrational modes being associated with small rotational barriers around C-C single bonds. Therefore, the rationalization of the governing factors operating on nonrigid molecules is still not completely clear, even for small systems, such as alkane- substituted molecules. 2 The substituted ethane molecules, such as 1,2-dicloroethane 3–5 and 1,2-difluorethane, 3,6–8 have been the subject of a considerable number of investigations motivated by the interest in its restricted internal rotation. * Corresponding author fax: +55 31 34995700, e-mail: wagner@ netuno.qui.ufmg.br. † UFMG. ‡ Centro Universitário do Leste de Minas Gerais. § UFJF. J. Chem. Theory Comput. 2008, 4, 728–739 728 10.1021/ct800014w CCC: $40.75  2008 American Chemical Society Published on Web 03/22/2008