AN ANALYSIS OF THE BONDING PROPERTIES OF BENZ[ALPHA]AZULENE BY X-RAY,NMR, AND COMPUTATIONAL STUDIES

The X-ray crystal structures of 9-phenylbenz[a]azulene (4) and the cor responding non-benzannelated form, 4-phenylazulene (5), have been dete rmined (cf. Fig. 2). In contrast to 5, the skeleton of which shows nea rly 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 substructu re in 4 by the [a] benzannelation. The alternating bond lengths of 4 a nd of its parent structure 3 are also reflected in the corresponding v ariations of the (3)J(H,H) and (1)J(C-13,C-13) 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 valu es (cf. Tables 6-8).