Azines internal bond angles

In Figure 2 the bond lengths and internal bond angles are given for some of the simple azines. Gas-phase electron diffraction, microwave spectroscopy, or the two techniques in combination, provided the results on compounds which were sufficiently volatile but with insufficient tendency to crystallize at accessible temperatures X-ray diffraction provided the remainder. 

Clearly, there is a distinct difference between the kinetic results for the benzologs of azines and azoles. The former definitely show a sizable steric effect in N-methylation reactions when the site of quaternization is a peri position, while any effect for the latter is insignificant. Perhaps this simply reflects the smaller internal bond angles for five- over six-membered rings. 

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See also in sourсe #XX -- [ ]

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