Werner Danchura

A deuterium NMR study of orientational order in tertiary-butyl bromide

Abstract An analysis of NMR lineshapes and spin-lattice relaxation times for the 2 H nucleus is reported for the liquid and solid phases I, II, and III of (CD 3 ) 3 CBr. The main results are a discontinuous increase of the rotational correlation time on melting (with a concomitant increase of the Arrhenius parameter) and, for phase II, a demonstration of the feasibility of measuring order parameters on the different timescales probed by the absorption profile and the relaxation experiment.

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.

Concerning the barrier to internal rotation in isopropylbenzene in solution

The barrier to rotation about the sp2C-sp3C bond in 3,5-dibromoisopropylbenzene in CS., solution is derived as 2.0 ± 0.2 kcal/mole from the magnitude of the spin-spin coupling constant between the methine proton and the ring proton in the para position. It is assumed that the coupling, equal to -0.25 0.02 Hz, depends on the rotational angle in a simple manner and that the angle dependence can be predicted from a solution of the hindered-rotor problem using a twofold hindering potential. The derived value of the barrier is compared with values obtained from electron spin resonance data, from spin-lattice relaxation times, and from molecular orbital calculations for some related radicals and molecules.

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.