Walter Niemczura

The conformational preference and barrier to internal rotation of an equatorial 3,5-dichlorophenyl group by the J method. Derivatives of cyclohexane, 1,3-dithiane, 1,3-dioxane, and 1,3-dioxolane

We report the preparation and the analysis of the phenyl ring proton magnetic resonance spectra of 3,5-dichlorophenylcyclohexane and of the 2-(3,5-dichlorophenyl) derivatives of 1,3-dioxane, 1,3-dithiane, and 1,3-dioxolane. With the exception of the dioxolanes these compounds exist predominantly as the equatorial isomers. The J method is used to show that the phenyl moiety prefers the conformation in which the α C—H bond lies in the phenyl plane. The predominantly twofold barriers to rotation about the carbon–carbon bond between the two ring systems are 2.0 ± 0.3, 0.4 ± 0.2, 2.2 ± 0.3, 0.85 ± 0.3 kcal/mol for these compounds, in the order given above. The low value for the barrier in the 1,3-dioxane derivative agrees reasonably well with molecular mechanics calculations and with the results of calorimetric and X-ray studies on equatorial 2-phenyl-1,3-dioxane.

A nuclear magnetic resonance determination of the barrier to internal rotation in phenylethane

Abstract The long-range nuclear spin-spin coupling constant between the methylene protons and the ring protons at the para position in 3,5-dibromo-phenylethane in benzene solution is consistent with a two-fold barrier of 1.2 ± 0.1 kcal/mole to rotation about the sp 2 -sp 3 carbon-carbon bond, in agreement with thermodynamic data on phenylethane and with deductions based on hyperfine interactions in related radical anions. The low-energy conformation has a plane of symmetry, the methyl group being situated out of the aromatic plane, in disagreement with Raman depolarization data interpretations and with deductions based on proton chemical shifts.

Electron-coupled through-space or fragment spin-spin coupling between 19F nuclei

The indirect spin-spin coupling constants over four formal bonds between fluorine nuclei in 2-fluoro-5-nitrobenzotrifluoride, 2,5-difluorobenzotrifluoride, pentafluorobenzoyl fluoride, and 2-fluoro-6-chlorobenzoyl fluoride, are +12.94 ± 0.02, +13.0 ± 0.1, +38.8 ± 0.1, and +30.6 ± 0.1 Hz, respectively. These “through-space” or “fragment” interactions are compared with previous sign and magnitude measurements and are in agreement with two semiempirical theories.

THE INTERNAL BARRIERS TO ROTATION ABOUT THE CARBON-CARBON BOND IN 3,5-DICHLOROBENZYL ALCOHOL AND SELENOL BY THE J METHOD

TED SCHAEFER, WERNER DANCHURA, WALTER NIEMCZURA, and WILLIAM J. E. PARR. Can. J. Chen~. 56, 1721 (1978). The J method, depending on a comparison between observed spin-spin coupling constants over six bonds between protons on a side chain andparcr ring protons and those calculated by a hindered rotor treatment, is applied to the determination of the twofold barrier to internal rotation about the carbon-carbon bonds in 3,5-dichlorobenzyl alcohol and selenol. In the alcohol, the C-0 bond prefers the benzene plane by 0.3 + 0.2 kcal/mol whereas, in the selenol, the C-Se bond prefers a plane perpendicular to the benzene ring by 3.8 + 0.7 kcallmol. Con~parison with the thiol suggests that a major component of the barrier arises from repulsive interactions, increasing as the size of the X H (X = 0 , S, Se) group increases.