Pyridines, ring anisotropy

Figure 2.28 shows the anisotropy of the rotation of the pyridine ring in nicotine (46). The main axis passes through C-3 and C-6 C-6 relaxes correspondingly more rapidly (3.5 s) than the three other CH atoms (5.5 s) of the pyridine ring in nicotine, as can be seen from the times at which the appropriate signals pass through zero. 

These are expressed in terms of scalar products between the unit axis system vectors on sites 1 and 2 (on different molecules) and the unit vector 6. from site 1 to 2. The S functions that can have nonzero coefficients in the intermolecular potential depend on the symmetry of the site. This table includes the first few terms that would appear in the expansion of the atom-atom potential for linear molecules. The second set illustrate the types of additional functions that can occur for sites with other than symmetry. These additional terms happen to be those required to describe the anisotropy of the repulsion between the N atom in pyridine (with Zj in the direction of the conventional lone pair on the nitrogen and yj perpendicular to the ring) and the H atom in methanol (with Z2 along the O—H bond and X2 in the COH plane, with C in the direction of positive X2). The important S functions reflect the different symmetries of the two molecules.Note that coefficients of S functions with values of k of opposite sign are always related so that purely real combinations of S functions appear in the intermolecular potential. 

The NMR spectra of pyridine, phosphabenzene, arsabenzene, and stibabenzene are quite similar in appearance (11,12). The y protons appear at lowest field partially as a result of remoteness from the heteroatom. The effect of the magnetic anisotropy of the heteroatom should decrease for the )3 and y protons. The chemical shift differences for the y protons in pyridine and arsabenzene are almost negligible. The low-field chemical shift value was interpreted in terms of the presence of ring current in the heteroatomic derivatives. 

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