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 C, symmetry, whereas 3-FA exists as a mixture of two planar conformers of C, 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 (r(a) distances and angle(alpha), angles with 3 sigma uncertainties) were derived for 3,5-DFA: r(C-C)(av)=1.391(2)angstrom, r(C-ph-O)=1.359(13)angstrom, r(C-Me-O)=1.427(19)angstrom, r(C-F)(av)= 1.350(6),angstrom, angle C-C-C=116.0-123.6 degrees, angle C-O-C=118.7(12)degrees, angle C2-Cl-O=114.9(10)degrees, angle C6-Cl-O=124.9(10)degrees, angle(C-C-F)(av)= 118.4(17)degrees, and for 3-FA, syn conformer: r(C-C)(av)=1.393(3)angstrom, r(C-ph-O)=1.364(13)angstrom, r(C-Me-O)=1.423(14)angstrom, r(C-F)=1.348(9)angstrom, angle C-C-C=117.7-123.1 degrees, angle C-O-C=118.4(11)degrees, angle C2-C1-0=124.7(17)degrees, angle C6-CI-O=115.1(17)degrees, angle C2-C3-F=118.0(24)degrees. 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 degrees(T), C-p(degrees),(T), H degrees(T)-H degrees(0), for anisole, 3-FA, and 3,5-DFA were obtained on the basis of B3LYP calculations. Enthalpies of formations, Delta H-f(298)degrees, were calculated using a Gaussian-3X (G3X) method and the method of isodesmic reactions. Calculated values of C-p(degrees)(T) and Delta H-f degrees(298) for anisole are in good agreement with experimental data. (c) 2006 Elsevier B.V. All rights reserved.