Experimental and theoretical estimates of the internal rotational barrier of benzyl fluoride in the vapor phase

Abstract

The twofold barrier to rotation about the [Formula: see text] bond in benzyl fluoride is deduced from the long-range 1H,1H; 1H,19F; and 13C,19F nuclear spin–spin coupling constants in solution. The barrier changes from 3.2(2) kJ/mol in the polar solvent, acetonitrile-d3, to 0.7(2) kJ/mol in the nonpolar environment provided by cyclohexane-d12. In all solutions the conformer of greatest stability has the C—F bond in a plane perpendicular to that of the phenyl group. Extrapolation of the barrier to the vapor phase, using a simple reaction field model, indicates that the most stable conformer for the free (unclustered) molecule is now that with the C—F bond in the phenyl plane and that the barrier to internal rotation is 1.1(7) kJ/mol. Molecular orbital calculations with the basis sets STO-3G, 4-21G, 4-31G, 6-31G, and 6-31G* all predict the latter conformer as that of lowest energy. However, they disagree significantly among themselves as to the height of the internal barrier. The complete geometries are given for both conformers, as computed with the 6-31G basis, and the side-chain geometries are tabulated for the planar and perpendicular conformers, as given by all the bases. Keywords: benzyl fluoride, internal rotational potential; 13C,19F spin–spin coupling constants in benzyl fluoride; benzyl fluoride, molecular orbital computations.

The solvent dependence of the small internal barrier to rotation in 3,5-dichlorobenzyl fluoride

“…For benzyl fluoride, 1, this relative stability was estimated as 1.1 5 0.7 kJ/mol (2). Qualitatively, the relative stability agrees with the STO-3G MO data (2).…”

“…Again, the temperature dependence of the ' 9~ chemical shift in acetone solution suggests 3 as the conformer of lowest energy (3). An extrapolation of the twofold barrier magnitudes, based on a dielectric model, yields 4 as the most stable conformer for the free molecule, perhaps by about 1 kJ/mol (2). This result agrees qualitatively with ab initio molecular orbital computations, various bases yielding 4 as the stable conformer but with large variations in the barrier height (2).…”

“…The internal barrier to rotation in benzyl fluoride, 1, is small and solvent dependent (1,2). In solution, long-range spin-spin coupling constants, between nuclei in the side chain and those in the phenyl group, imply an apparent twofold barrier to rotation about the ~s p~--~s p~ bond that ranges from 3.2 k 0.2 kJ/mol in the polar solvent, CD3CN, to 0.7 5 0.2 kJ/mol in the nonpolar solvent, C6DI2 (2).…”

“…In solution, long-range spin-spin coupling constants, between nuclei in the side chain and those in the phenyl group, imply an apparent twofold barrier to rotation about the ~s p~--~s p~ bond that ranges from 3.2 k 0.2 kJ/mol in the polar solvent, CD3CN, to 0.7 5 0.2 kJ/mol in the nonpolar solvent, C6DI2 (2). In both instances, the conformer corresponding to the minimum in the internal rotational potential is 3.…”

“…An extrapolation of the twofold barrier magnitudes, based on a dielectric model, yields 4 as the most stable conformer for the free molecule, perhaps by about 1 kJ/mol (2). This result agrees qualitatively with ab initio molecular orbital computations, various bases yielding 4 as the stable conformer but with large variations in the barrier height (2). Electron diffraction patterns of the vapor imply a conformer of minimum energy near a cp of 60" and a barrier height near 1.7 kJ/mol (4).…”