Gas-phase structure and new vibrational study of methyl trifluoroacetate (CF₃C(O)OCH₃)

The molecular structure of methyl trifluoroacetate (CF₃C(O)OCH₃) has been determined in the gas phase from electron-diffraction data supplemented by ab initio (MP2) and DFT calculations using different basis sets. Experimental data revealed an anti conformation with a dihedral angle θ(CCOC) = 180 degrees. Quantum mechanical calculations indicate the possible existence of two conformers, differing by a rotation about the C(O)-O bond. The global minimum represents a C_s-symmetric structure in which the CF₃ group has the anti orientation with respect to the CH₃ group, but there is another potential minimum, much higher in energy, representing a C_s-symmetric structure with a cis conformation. The preference for the anti conformation was studied using the total energy scheme and the natural bond orbital (NBO) partition scheme. Additionally, the total potential energy has been deconvoluted using a six-fold decomposition in terms of a Fourier-type expansion, showing that the electrostatic and steric contributions are dominant in stabilizing the anti conformer. Infrared spectra of CF₃C(O)OCH₃ were obtained for the gaseous and liquid phases, while the Raman spectrum was recorded for the liquid phase. Harmonic vibrational frequencies and a scaled force field have been calculated, leading to a final root mean-square deviation of 9 cm^-1 when comparing experimental and calculated frequencies.