Proton Magnetic Resonance Studies of Rotational Isomerism in Halotoluene Derivatives. IX. Rotational Barriers and Conformational Energy Differences in α,α,α′,α′,α″,α″, 2,4,6-Nonachloromesitylene

Abstract

The activation parameters for the rotation of the dichloromethyl groups in the two conformations of a,a,a',af,a",a'',2,4,6-nonachloromesitylene are reported for solutions in toluene-d8 and in methylene chloride. In addition, free energies of activation are given for solutions i n bromochloromethane, trichloroethylene, and in carbon disulfide. The free energies of activation are lower in the toluene solution than in the other solutions. The entropies of activation are near zero, perhaps slightly negative. The symmetrical conformation is more stable than the unsymmetrical one in all the solvents.Les parametres d'activation pour la rotation des groupes dichloromCthyles dans les deux conformations de l'a,a,a',a',a",a",2,4,6-nonachloromesitylene ont t t C rapportks pour des solutions dans le toluene-d8 et le dichloromethane. D e plus, les energies libres d'activation ont 8te presentees pour les solutions dans le bromo chloromCthane, trichloroethylene, et disulfure de carbone. Les energies libres d'activation sont plus basses dans le toluene que pour les autres solvants; les entropies d'activation sont proches de zCro, voire mCme 16gkrement negatives. La conformation symetrique est plus stable que celle qui est non symttrique dans tous les solvants.[Traduit par le journal]Can.

The proton magnetic resonance determination of the hindered rotation in α,α,α′,α′,3,4,6-heptachloro-o-xylene

“…This assignment was confirmed with the steric crowding method of Peeling et al (6). The magnitude of the crowding was estimated from a scale diagram of the molecule using standard covalent atomic radii and bond angles, and a maximum of 5" bond angle distortion (6).…”

“…The magnitude of the crowding was estimated from a scale diagram of the molecule using standard covalent atomic radii and bond angles, and a maximum of 5" bond angle distortion (6). HA (1) and HB (2) are most crowded and thus their resonances are farthest down field; H, (2) and HA (2) are least crowded so are to highest field; while HB (1) and Hx (1) occur a t intermediate field.…”

“…Proton magnetic resonance ('H nmr) studies of the hindered rotation of the dichloromethyl group in a series of substituted toluenes (1-3), substituted xylenes (4,5), and a substituted mesitylene (6) have been examined by Schaefer et al In this paper we present the results of the study of the hindered rotation in a,a,a1,a',3,4,6-heptachloro-o-xylene (HCOX) as a 5 m o l z solution in methylcyclohexane. Mark and Pattison (7) have previously studied the hindered rotation in a series of o-bis(dich1oromethyl) benzenes, including HCOX in tetradecane, using the variation of the line width of the coalescing resonances as a function of temperature (7).…”

Internal barriers to rotation in 2,6-difluoroisopropyl and 2,6-difluoroethylbenzenes. Overlap of dynamic nuclear magnetic resonance (DNMR) and the J method

“…a negligible rate of exchange. Above coalescence, which occurs very near 156K, it was deemed Variable temperature pre-exchange lifetimes correct to use a constant limiting internal chemical Pre-exchange lifetimes were extracted from the shift with a reference linewidth given by C6F6 and exchange-broadened proton-decoupled 19F nmr only one unknown parameter, the pre-exchange spectra of 1 (X = F) by methods used in this lifetime, T. In this way a temperature range of 60 K laboratory for many years (21)(22)(23)(24)(25)(26)(27). The many could be achieved, rather smaller than desired.…”

“…possibilities of error in such extractions (6), partic-Viscosity and/or solubility problems precluded ularly for two-site exchange between components measurements below 140 K, while at temperatures Observed spectra were compared with calculated lineshapes from a Versatic plotter, whose scale was ca. 25 x 60cm and had been tested and adjusted for uniformity. Particular attention was paid to comparisons at 0.25, 0.50, and 0.75 of the maximum peak heights.…”