ISSN:
0749-1581
Keywords:
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
13C NMR spin-lattice relaxation times (T1) have been measured for a series of meta- and para-substituted phenylbicyclo[2.2.2]octanes in CDCl3 solution. The non-equivalence of NT1 values (N = number of attached hydrogens) for the various protonated carbons in these molecules indicates that motion is highly anisotropic. Analysis of the T1 data using the Woessner formalism for axially symmetric ellipsoids yields a set of diffusion coefficients for overall and internal motion. In para-substituted derivatives, motion about the symmetry axis is 3-12 times faster than motion about the perpendicular axes, depending on the substituent. Both the size (inertia) and the interacting properties of the substituent are important in determining the motional anisotropy, as indicated by the following observed order: H 〈 NH2 〈 CH3 〈 NO2 〈 CH2OH 〈 Br 〈 NH3+ 〈 CO2H. The change in anisotropy is a result of the anchoring effect of the substituent (via either its inertia or its hydrogen-bonding ability), which restricts motion about axes perpendicular to the long axis of the molecule. In meta derivatives, a similar model of motional anisotropy is observed, although the absolute magnitudes of diffusion coefficients are smaller than for the para series. In both series, the bicyclooctane ring undergoes fast internal motion relative to the phenyl ring. This reflects the fact that the bicyclooctane ring is able to rotate essentially in its own volume, with minimum disturbance to neighbouring solvent molecules. On the other hand, the planar phenyl ring must displace solvent molecules as it rotates.
Additional Material:
6 Tab.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/mrc.1260240910
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