Nitrites, rotational barriers

The barriers to rotation of esters deserve mention here, especially in comparison to amide barriers. The H NMR spectra of some nitrites (45) were measured in 1957 (83). The temperature had to be lowered to - 58°C at 30 MHz to see the separate signals of propyl nitrite. The barriers to rotation were ca. 10 kcal/mol. This result may be rationalized by considering the lesser electron-donating ability of the alkoxy relative to the dialkylamino group. The dipolar canonical form (46) of nitrite esters is not as stable as that of nitrosamines. 

Hindered rotation occurs on the NMR time scale for numerous other systems with partial double bonds, including carbamates, thioamides, enamines, nitrosamines, alkyl nitrites, diazoketones, aminoboranes, and aromatic aldehydes. Formal double bonds can exhibit free rotation when alternative resonance structures suggest partial single bonding. The calicene 5-5, for example, has a barrier to rotation about the central bond of only 20 kcal mol . ... 

Similarly, 13,14-bis(2-pyridyl)pentaphene (29) (Fig. 8) was synthesized from condensation of 26 with benzyne, generated in situ from anthranilic acid and isoamyl nitrite [36]. Variable-temperature NMR studies of the mono- (30) and dimethylated pyridinium salts (31) indicated rapid rates of racemization on the NMR time scales below 79°C with the monomethylated 30 having an activation barrier of 16.6 kcal/ mol at 30°C. A S5uichronous rotation with the pyridyl rings remaining parallel to each other was proposed as the molecular movement for racemization. 

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