Gas-phase structure, rotational barrier and vibrational properties of trimethylsilyl trifluoroacetate CF₃C(O)OSi(CH₃)₃: An experimental and computational study

The molecular structure of trimethylsilyl trifluoroacetate. CF₃C(O)OSi(CH₃)₃, has been determined in the gas 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 that only one conformer, with C_s symmetry [dihedral angle φ(CCOSi) = 180 degrees, and all groups staggered], is observed in the gas phase. Theoretical data indicate that both this anti conformer and a gauche conformer, created by rotating about the C(O)-O bond, are possible, although the preferred conformation is the staggered anti one. The torsional energies for different values of the CCOSi and COSiC dihedral angles have been calculated using the RHF, MP2 and B3LYP methods with the 6-311++G(d,p) basis set. For rotation around the CC-OSi bond, a sixfold decomposition of the rotational barrier has been performed in terms of a Fourier-type expansion, enabling us to analyze the nature of the potential function, showing that the coefficients related to electrostatic interactions and steric effects are the dominant terms. The preference for the anti conformation was studied using the total-energy scheme, comparison of dipole moments, and the natural bond orbital partition scheme. The infrared spectra for the liquid and gas phases and the Raman spectrum for the liquid phase have also been recorded and the observed bands assigned to the vibrational 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 us to estimate the measured frequencies with a final root-mean-square deviation of 9.7 cm^-1.