Gas-phase structure, rotational barrier and vibrational properties

MARÍA E. DEFONSI LESTARD ; MARÍA E. TUTTOLOMONDO ; EDUARDO L. VARETTI ,; DEREK A. WANN; HEATHER E. ROBERTSON C ; DAVID W.H. RANKIN ; AIDA BEN ALTABEF

JOURNAL OF MOLECULAR STRUCTURE

ELSEVIER SCIENCE BV

Año: 2010 vol. 978 p. 114 - 123

0022-2860

The molecular structure of trimethylsilyl trifluoroacetate, CF3C(O)OSi(CH3)3, has been determined in thegas phase from electron-diffraction data supplemented by ab initio (MP2) and DFT calculations using 6-31G(d), 6-311G(d,p), 6-311++G(d,p) and 6-311++G(3df,3pd) basis sets. Experimental data indicate thatonly one conformer, with Cs symmetry [dihedral angle /(CCOSi) = 180, and all groups staggered], isobserved in the gas phase. Theoretical data indicate that both this anti conformer and a gauche conformer,created by rotating about the C(O)AO bond, are possible, although the preferred conformation is the staggeredanti one. The torsional energies for different values of the CCOSi and COSiC dihedral angles havebeen calculated using the RHF, MP2 and B3LYP methods with the 6-311++G(d,p) basis set. For rotationaround the CCAOSi bond, a sixfold decomposition of the rotational barrier has been performed in termsof a Fourier-type expansion, enabling us to analyze the nature of the potential function, showing that thecoefficients related to electrostatic interactions and steric effects are the dominant terms. The preferencefor the anti conformation was studied using the total-energy scheme, comparison of dipole moments, andthe natural bond orbital partition scheme. The infrared spectra for the liquid and gas phases and theRaman spectrum for the liquid phase have also been recorded and the observed bands assigned to thevibrational normal modes. The experimental vibrational data, along with calculated theoretical force constants,were used to define a scaled quantum mechanical force field for the target system that enabled usto estimate the measured frequencies with a final root-mean-square deviation of 9.7 cm1. 2010 Elsevier B.V. All righ