An Analysis of the Bonding Properties of Benz[a]azulene by X‐Ray, NMR, and Computational Studies

The X‐ray crystal structures of 9‐phenylbenz[ a ]azulene ( 4 ) and the corresponding non‐benzannelated form, 4‐phenylazulene ( 5 ), have been determined ( cf. Fig.2 ). In contrast to 5 , the skeleton of which shows nearly equal C,C bond lengths ( cf. Table 1 ), the seven‐membered ring of 4 exhibits clearly alternating C,C bond lengths ( cf. Table 1 ). This is in agreement with a strong accentuation of the heptafulvene substructure in 4 by the [ a ] benzannelation. The alternating bond lengths of 4 and of its parent structure 3 are also reflected in the corresponding variations of the 3 J (H,H) and 1 J ( 13 C, 13 C) values of these benz[ a ]azulenes ( cf. Tables 4 and 5 ). Computations on the MP2/6‐31G * level as well as on the BP86/6‐31G * level for azulene ( 6 ), benz[ a ]azulene ( 3 ), and heptafulvene ( 7 ) are in good agreement with the experimental values ( cf. Tables 6–8 ).