The barrier to internal rotation in benzyl fluoride investigated using the N.M.R. spectra of samples dissolved in liquid crystalline solvents

Abstract

The proton and fluorine spectra of samples of benzyl fluoride dissolved in two nematic solvents (ZL11132 and I52) have been analysed to yield sets of dipolar couplings/~j between the interacting nuclei. These were compared with values calculated by a theoretical model which allows for the dependence of orientational order on the angle that the C-F bond makes with the plane of the phenyl ring. The observed and calculated/~ are in excellent agreement, and the potential for rotation about the C-CH2F bond is found to have a minimum when the C-F bond is in the plane at 90 ~ to the phenyl ring, but the barrier height changes from 0-3 __+0.02kJmo1-1 in 152 to 1-5 _0.08kJmo1-1 for ZLI 1132 as solvent.

Influence of Fluorination on the Conformational Properties and Hydrogen‐Bond Acidity of Benzyl Alcohol Derivatives

“…When X is an alkyl group or a halide, the C-C-C-X dihedral angle was shown to be 90°, with the C–X bond in a plane orthogonal to the benzene ring. [ 20 ] This perpendicular conformation minimises steric repulsive effects between the -CH 2 X group and the phenyl ring. If the X group contains a triple bond (ethynyl or cyanide), the dihedral angle is near 0°, with the C≡C bond lying in the plane of the phenyl ring.…”

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

“…Note that the minimum in the sinusoidal potential well occurs at + = 90°, that is, conformer 2 is the most stable in both solutions; in the liquid crystalline solvents, 2 is also most stable (7). For a vanishing barrier the expectation values are 0.5, SO that 2 immediately follows as the stable conformer from the fact that (sin2 +) is larger than 0.5; for a rigid conformer, 2, (sin2 +) would be unity, 0.f course.…”

“…3 * 0.02 kJ/mol in one such solvent and a barrier of 1.5 * 0.08 kJ/mol in another, more polar, liquid crystal environment. In both instances, at 300 K the stable conformer is 2 (7).…”

“…More recently (1), 'J(C,F), 6~( H , H ) , and 6~(~,~) in benzyl fluoride, in a number of solvents, suggest strongly that, if the barrier is twofold, then 2 is stabilized by about 2 kJ/mol in passing from CS2 to acetone-d6 solution; in CS2 solution the internal potential is rather flat and may be completely so at 3 0 0 K. The latest investigation, for solutions in liquid crystalline solvents (7), finds a twofold barrier of 0 . 3 * 0.02 kJ/mol in one such solvent and a barrier of 1.5 * 0.08 kJ/mol in another, more polar, liquid crystal environment.…”

The Conformation and Internal Rotation Barrier in Benzyl Fluoride from Its Microwave Spectrum

“…The low-resolution microwave spectrum of benzyl fluo-A study of benzyl fluoride dipolar NMR coupling con-ride reveals only a single a-type R-branch series of bands stants in liquid crystalline solvents reported a 90Њ equilibrium with spacings corresponding to B / C Å 2623.1 (.5) MHz torsional angle with internal rotation barriers of 25(2) and (12). No conformational information can be derived from 125(7) cm 01 in two different solvents (4). Also, analyses that.…”