Molecular structure, conformation, potential to internal rotation, and ideal gas thermodynamic properties of 3-fluoroanisole and 3,5-difluoroanisole as studied by gas-phase electron diffraction and quantum chemical calculations

3-Fluoroanisole (3-FA) and 3,5-difluoroanisole (3,5-DFA) have been studied by gas-phase electron diffraction, ab initio (HF and MP2), and density functional theory (B3LYP) calculations. Both molecules have a planar heavy atom skeleton. 3,5-DFA exists as a single conformer of Cs symmetry, whereas 3-FA exists as a mixture of two planar conformers of Cs symmetry with the syn form (the O–CH3 bond is oriented toward the fluorine atom) being 0.1–0.2 kcal/mol lower in energy than the anti form (the O–CH3 bond is oriented away from the fluorine atom). From the experimental scattering intensities the following geometric parameters (ra distances and ∠α angles with 3σ uncertainties) were derived for 3,5-DFA: r(C–C)av=1.391(2) Å, r(CPh–O)=1.359(13) Å, r(CMe–O)=1.427(19) Å, r(C–F)av=1.350(6) Å, ∠C–C–C=116.0–123.6°, ∠C–O–C=118.7(12)°, ∠C2–C1–O=114.9(10)°, ∠C6–C1–O=124.9(10)°, ∠(C–C–F)av=118.4(17)°, and for 3-FA, syn conformer: r(C–C)av=1.393(3) Å, r(CPh–O)=1.364(13) Å, r(CMe–O)=1.423(14) Å, r(C–F)=1.348(9) Å, ∠C–C–C=117.7–123.1°, ∠C–O–C=118.4(11)°, ∠C2–C1–O=124.7(17)°, ∠C6–C1–O=115.1(17)°, ∠C2–C3–F=118.0(24)°. The mole fractions of the syn and anti conformers were found to be 0.55(17) and 0.45, respectively, in good agreement with the theoretical prediction. Ideal gas thermodynamic functions, S°(T), C p ° ( T ) , H°(T)−H°(0), for anisole, 3-FA, and 3,5-DFA were obtained on the basis of B3LYP calculations. Enthalpies of formations, Δf H 298 ° , were calculated using a Gaussian-3X (G3X) method and the method of isodesmic reactions. Calculated values of C p ° ( T ) and Δf H 298 ° for anisole are in good agreement with experimental data.