Endocyclic bond angles

Figure 8. The effect of ring conformation on endocyclic bond angles in 7 C1-C2-C3 (a), C2-C3-C4...

Fig. 2. Endocyclic bond and torsional angles in heterocycles.33 (Within rings endocyclic bond angles. Outside rings torsional angles subtended by ring bonds.)...

All bond angles in adamantane are nearly 109.5 ° (see above). Theory170) predicts, therefore, that all carbon bonding orbitals should be nearly pure sp3 hydrids. The situation in norbomane is quite different. Endocyclic bond angles of this molecule are considerably less than 109.5 ° 17 l. The p character of the endocyclic carbon orbitals should be greater than that of the exocydic orbitals170). 

Prototropic tautomerism is observed in the pyrrolopyridines. As a consequence, the endocyclic bond angle centered on the five-membered ring nitrogen closes while the endocyclic bond angle... 

The expansion of endocyclic bond angles to 120° in the diequatorial situation (56 ) requires considerable energy. The analogous situation for an OPC angle has been evaluated by CNDO/2 calculations58 as shown in... 

The endocyclic P-C bond lengths cover the wide range between 1.808(6) A in 122 and 1.933(9) A in 124. The standard value for a P-C single bond amounts to 1.85 A. The endocyclic bond angles at the carbon atom range from 69.4(3)° in compound 124 to 77.1(3)° in 5e, thereby correlating with the P-P distances. Endocyclic CPP angles are... 

X-ray crystal structures of both 97 (all-tra/is isomer) and 99 trans is,trans isomer) have been obtained [39], and they confirm the relief of angle strain at the acetylenic carbon atoms that the phosphorus atoms provide in these small-ring pericyclynes (actual geometries shown in Fig. 9-28 average phosphorus atom endocyclic bond angles = 96° in 97 and 91° in 99 average acetylene carbon bond angles = 174° in 97 and 163° in 99). 

---