Experimental structure of CF3CO2CH2CF3 from electron diffraction data, infrared spectra and quantum chemical calculation of vibrational properties

M. E. DEFONSI LESTARD; M. E. TUTTOLOMONDO; E. L. VARETTI; D. A. WANN; H. E.ROBERTSON; D. W. H. RANKIN; A. BEN ALTABEF

Opatija, Croatia

Congreso; XXIX European Congress on Molecular Spectroscopy; 2008

Experimental structure of CF3CO2CH2CF3 from electron diffraction data, infrared spectra and quantum chemical calculation of vibrational properties   M. E. Defonsi Lestard a , M. E. Tuttolomondo a , E. L. Varetti b , D. A. Wann c, H. E. Robertsonc ,  D. W. H. Rankin c and A. Ben Altabef a   a Instituto de Química Física, Facultad de Bioquímica, Química y Farmacia, Universidad   Nacional de Tucumán, San Lorenzo 456, 4000 Tucumán, R.  Argentina.   E-mail: altabef@fbqf.unt.edu.ar b CEQUINOR, Departamento de Química, Facultad de Ciencias Exactas, Universidad   Nacional de La Plata, C. Correo 962, 1900 La Plata, R. Argentina c School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, UK EH9 3JJ.   E-mail: d.w.h.rankin@ed.ac.uk     The molecular structure of 2,2,2-trifluoroethyl trifluoroacetate, CF3CO2CH2CF3, was  determined in the gas phase from electron-diffraction data supplemented by ab initio (HF, MP2) and DFT calculations using 6-31G(d), 6-311++G(d,p) and 6-311G(3df,3pd) basis sets1. The experimental and theoretical data indicate that, although both trans and gauche conformers are possible by rotation about the O-CH2 bond, the preferred conformation is trans. To explain this fact, the total energy of each conformation and the natural bond orbital (NBO) partition schemes were considered. In addition, the total potential energy was deconvoluted using a Fourier-type expansion with six terms. The infrared spectra of the substance in the gaseous, liquid and solid states as well as the Raman spectrum of the liquid were also obtained. These spectra show the presence of both trans and gauche conformers by the components of some fundamental mode bands. The harmonic vibrational frequencies and the force field were calculated2 using the above-mentioned techniques. The force constants were adjusted by scaling to achieve a final root-mean-square deviation of 10 cm-1 between calculated and experimental frequencies.                       Figure 1. Molecular structure, including numbering         scheme, for (a) gauche conformer, and (b) trans conformer of CF3CO2CH2CF3. Figure 2. Infrared spectra of CF3CO2CH2CF3. a) calculated with B3LYP/6-311++G** for the mixture of conformers; b) solid (-160ºC); c)liquid (room T); d) gas.           [1] Gaussian 03, Revision B.02, Gaussian, Inc., Pittsburgh PA, 2003. [2]  P. Pulay, G. Fogarasi, G. Pongor, J. E. Boggs, A. Vargha, J. Am. Chem. Soc., 105 (1983) 7037