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

Abstract

). The 'H and "F nuclear magnetic resonance spectra of 3,5-dichlorobenzyl fluoride, dissolved in solvents of varying polarity, are analyzed to yield precise values of the long-range coupling constants between ring protons and the nuclei in the fluoromethyl group. The conformational properties of these coupling constants, dependent on the angle by which the C-H and C-F bonds of the side chain twist out of the aromatic plane, are used to determine the solvent dependence of the predominantly twofold barrier to rotation about the ~s p~--~s p~ bond. In all solvents, the conformer in which the C-F bond lies in the aromatic plane represents the minimum in the internal rotational potential. The magnitude of the twofold potential changes from 3.2(2) kJ/mol in the nonpolar medium, C6D,,, to 0.6(2) kJ/mol in the polar solvent, CD3CN. Intermediate magnitudes occur for solvents of intermediate dielectric constant. Comparisons are made with the parent molecule, benzyl fluoride, for which the perpendicular conformer is most stable in the same solvents. Such comparisons indicate that for the free molecules, the in-plane conformer of the dichloro derivative is also more stable than the same conformer of benzyl fluoride. This indication is in qualitative agreement with molecular orbital predictions.TED SCHAEFER, JAMES R. MANSFIELD, FRANK E. HRUSKA et RUDY SEBASTIAN. Can. J. Chem. 70, 1750 (1992). On a analysk les spectres en rksonance magnktique nuclkaire du 'H et du I9F du fluorure de 3,5-dichlorobenzyle dissous dans des solvants de polaritks diffkrentes pour obtenir des valeurs prkcises pour les constantes de couplage ?i longue distance entre les protons du cycles et les noyaux du groupe fluoromkthyle. On utilise les propriktks conformationnelles de ces constantes de couplage, qui varient avec l'angle de torsion qui existe entre les liaisons C-H et C-F de la chaine laterale et le plan du noyau aromatique, pour determiner l'influence du solvant sur la barrikre principalement binaire B la rotation autour de la liaison ~s p~-~s p~.Dans tous les solvants, le conformkre dans lequel la liaison C-F se trouve dans le plan du noyau aromatique correspond ?i un minimum dans le potentiel de la rotation interne. L'amplitude des potentiels binaires varie de 3,2(2) kJ/mol dans un solvant non-polaire, le C6D,,, B 0,6(2) kJ/mol dans un solvant polaire, CD3CN. Avec des solvants de constante diklectrique intermkdiaire, on observe des amplitudes intermkdiaires. On a effectuC des comparaisons avec la molCcule parente, le fluorure de benzyle, dans laquelle le conformkre perpendiculaire est le plus stable dans les m&mes solvants. Ces comparaisons indiquent que, pour les molCcules libres, le conformkre dans le plan du dkrivk dichlork est aussi plus stable que le m&me conformkre du fluorure de benzyle. Cette indication est en accord qualitatif avec les prkdictions faites B l'aide de calculs d'orbitales molkculaires.[Traduit par la rkdaction]

Experimental and theoretical assessments of the substituent and medium dependence of the internal rotational potentials in benzyl fluoride. 3,5-Difluorobenzyl fluoride and 4-fluorobenzyl fluoride

“…Turning to the 3,5-dichloro derivative, 4, the substituents shift the phase of the potential so that its minimum occurs at + = 0" (12), but the polar solvent again tetlds to stabilize the perpendicular conformer and therefore lowers the height of the potential for the range of solvents used for 4 (12); two examples are given in (2) "2.3 mol% in CS? containing also 10 mol% of C,D,,, 0.5 mol% of TMS, and 0.25 mol% of C,F,.…”

“…"2.8 mol% in acetone-d6 containing also 0.5 mol% of TMS and 0.25 mol% of C6F6 (I). '5.0 mol% and as for 1 (12). "These are values for "(F, CH2F).…”

“…The long-range coupling constants between ring protons and the protons or ' 9~ nuclei in the side chain show that the conformer analogous to A is now most stable for 3,5-dichlorobenzyl fluoride in solvents of various polarities (12). Just as for the parent compound, however, the polar solvents act to favor a phase shift of the potential minimum towards the perpendicular B.…”

Theoretical and experimental approaches to the effects of solvation on the small internal rotational potential of benzal fluoride

“…On the other hand, electronegative substituents in the meta position could cause further polarization of the bonds o~h o to the CHF, group and, on the electrostatic model, stabilize 1 relative to 2. Such polarization apparently stabilizes the in-plane conformers of the 3,5-dichloro and 35-difluoro derivatives of benzyl fluoride (6,26).…”

“…An example (23-25) is 6~(~, H) for the methyl protons in toluene, which can be written as 6~ = 6~9 0 sin2+, where 6~9 0 corresponds to + = 90" (see 1 for the definition of +). Another is 6~(~, F) in benzyl fluoride and some of its derivatives (6,26), as is 5~( C , F). The measured value of such a coupling constant is then determined by "J90 and (sin2+), the expectation value of sin2+, which, in turn, depends on the potential governing how the bond in question samples the various angles + at a given temperature.…”