Molecular Structure and Internal Rotation of CF<sub>3</sub> Group of Methyl Trifluoroacetate: Gas Electron Diffraction, Microwave Spectroscopy, and Quantum Chemical Calculation Studies

The molecular structure of methyl trifluoroacetate (CF<sub>3</sub>COOCH<sub>3</sub>) has been determined by gas electron diffraction (GED), microwave spectroscopy (MW), and quantum chemical calculations (QC). QC study provides the optimized geometries and force constants of the molecule. They were used to estimate the structural model for GED study and to calculate the vibrational corrections for GED and MW data. In addition, potential energy curves for the internal rotations of CF<sub>3</sub> and CH<sub>3</sub> groups have been calculated for <i>anti</i> (dihedral angle of α­(CCOC) is 180°) and <i>syn</i> (α­(CCOC) = 0°) conformers of methyl trifluoroacetate. Both the GED and MW data revealed the existence of the <i>anti</i> conformer. Molecular constants determined by MW are <i>A</i><sub>0</sub> = 3613.4(3) MHz, <i>B</i><sub>0</sub> = 1521.146(8) MHz, <i>C</i><sub>0</sub> = 1332.264(9) MHz, Δ<sub>J</sub> = 0.09(2) kHz, and Δ<sub>JK</sub> = 0.23(6) kHz. The GED data were well-reproduced by the analysis in which a large-amplitude motion of the CF<sub>3</sub> group was taken into account. The barrier of the internal rotation of the CF<sub>3</sub> group was determined to be <i>V</i><sub>3</sub> = 2.3(4) kJ mol<sup>–1</sup>, where <i>V</i><sub>3</sub> is the potential coefficient of the assumed potential function, <i>V</i>(ϕ) = (<i>V</i><sub>3</sub>/2)­(1 – cos 3ϕ), and ϕ is a rotational angle for the CF<sub>3</sub> group. The values of geometrical parameters (<i>r</i><sub>e</sub> structure) of the <i>anti</i> conformer of CF<sub>3</sub>COOCH<sub>3</sub> are <i>r</i>((O)­C–O) = 1.326(6) Å, <i>r</i>(O–CH<sub>3</sub>) = 1.421(4) Å, <i>r</i>(C–H<sub>in‑plane</sub>) = 1.083(14) Å, <i>r</i>(C–H<sub>out‑of‑plane</sub>) = 1.087(14) Å, <i>r</i>(CO) = 1.190(7) Å, <i>r</i>(C–C) = 1.533(4) Å, <i>r</i>(C–F<sub>in‑plane</sub>) = 1.319(4) Å, <i>r</i>(C–F<sub>out‑of‑plane</sub>) = 1.320(6) Å, ∠COC = 116.3(5)°, ∠OCH<sub>in‑plane</sub> = 105.2° (fixed), ∠OCH<sub>out‑of‑plane</sub> = 110.0° (fixed), ∠OCC = 123.7° (fixed), ∠O–CC = 111.2(5)°, ∠OCO = 125.2(5)°, ∠CCF = 110.1(3)°, and OCCF (out-of-plane dihedral angles) = ± 121.5(1)°. Numbers in parentheses are three times the standard deviations of the data fit.