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

Abstract

Abstract:The internal rotational potential for benzal fluoride is computed at various levels of molecular orbital theory, including correlation-gradient, MP2 (frozen core) methods. The perturbations of the potential caused by solvents are calculated with the Onsager model (ellipsoidal cavity with I = 6 in the multipole expansion) as well as with the self-consistent isodensitypolarizable continuum model (SCI-PCM). Analysis of the 'H and "F nuclear magnetic resonance spectra in cyclohexane-'I,2 and acetone-d6 solutions yields long-range spin-spin coupling constants from which the expectation values of sin2+ can be derived. These expectation values can be compared with those calculated from the theoretical internal rotational potential. Reasonable agreement is found for potentials obtained from MP216-3 IG* approaches in both solvent models. Long-range coupling constants between I9F and nuclei are also reported and provide very rough checks of the (sin2+) values. For the isolated molecule an additivity scheme based on the potential for benzyl fluoride reproduces much of the potential for benzal fluoride except for a deviation caused by the rather larger relative magnitude of the fourfold component in the latter. The minimum in the potential for benzal fluoride occurs for a torsional angle, +, of 90". corresponding to aconformation in which the C-H bond of the side chain lies in a plane perpendicular to the phenyl plane and is rationalized on the basis of electrostatic forces. The conformations of minimum energy for the benzyl and benzal fluorides and chlorides are compared and contrasted.The magnitudes of the internal potentials of the fluorides are only a little larger than thermal energies at 300 K and can become smaller than the latter in solution.

Conformational Equilibrium of Cinchonidine in C6D12 Solution. Alternative NMR/DFT Approach

“…For an internal rotor as light as a vinyl group and for so low rotational barriers (Figure ), this is only a crude approximation. Consequently, in the case in hand, one may expect that rovibrational effects modify values of the involved NMR parameters. − These effects can also be responsible for the previously observed deviations of δ­(H11), δ­(C11), and δ­(C12) from the regression lines and for apparently enhanced contribution of /2 and/3 orientations of the vinyl group.…”

A Computational Study of Charge Delocalization and Ring Fluoro Substituent Effects in 4-Fluoromethylphenoxides

“…As stated above, determining the extent of charge delocalization from the phenoxide anion to the fluoromethyl group will be accomplished by removing the oxy anion, leaving benzyl fluoride. The examination of the solvent and ring substituent effects on these systems has the additional benefit of allowing for a comparison with an important series of papers by Schaefer and co-workers on the benzyl fluoride system. − On the basis of calculations at the HF/6-31G(d)//HF/6-31G(d) and MP2/6-31G(d)//MP2/6-31G(d) levels and NMR studies, they have concluded that the preferred conformation in isolation for benzyl fluoride has the C−F bond oriented in the plane of the aromatic ring and the energetic maximum along the C(Ar)−CH 2 F rotational profile exists when the C−F bond is perpendicular to the ring. As will be shown below, the opposite energetic ordering is predicted when the basis set is augmented with diffuse functions.…”