Dibenzo position shift

As might be anticipated from the behaviour of the parent heterocycles, C-2 of indole, benzo[i]furan and benzo[i]thiophene (Table 13) is shifted to lower field than C-3. However, the shifts for C-2 (O, 144.8 Se, 128.8 S, 126.1 NH, 124.7 Te, 120.8) and C-7a (O, 155.0 Se, 141.3 S, 139.6 NH, 135.7 Te, 133.0) in the benzo[i] heterocycles vary irregularly (80OMR(l3)3l9), and the sequence is different to that observed for C-2 in the parent heterocycles, namely 0>Se>Te>S>NH. Also noteworthy is the upheld position of C-7, especially in indole and benzofuran, relative to the other benzenoid carbons at positions 4, 5 and 6. A similar situation pertains in the dibenzo heterocycles (Table 14), where not only are C-1 and C-8 shifted upheld in carbazole and dibenzofuran relative to the corresponding carbons in dibenzothiophene and fluorene, but similar, though smaller, shifts can be discerned for C-3 and C-6 in the former compounds. These carbon atoms are of course ortho and para to the heteroatom and the shifts reflect its mesomeric properties. Little variation in the carbon-hydrogen coupling constants is observed for these dibenzo compounds with V(qh) = 158-165 and V(c,h) = 6-8 Hz. 

The visible absorption maxima for a series of chlorinated dibenzo-p-dioxins in TFMS acid are tabulated in Table I. The visible absorption spectra of these compounds vary with the position and number of chlorine atoms. In general, a bathochromic shift was observed as more chlorine atoms were attached to the dibenzo-p-dioxin nucleus. 

Benzopyrylium salts (chromylium salts) are colored and possess long-wave UV/VIS maxima at 385 nm. A bathochromic shift is observed in the presence of a 2-phenyl substituent, that is, for 2-phenyl-l-benzopyrylium ions (flavylium ions). H NMR data (cf pyrylium ion, p. 298) show the influence of the positively charged oxygen, which reduces the electron density on C-2 and C-4 5 = 9.75 (2-H), 8.40 (3-H), and 8.75 (4-H) (CF3COOD). This effect is even more pronounced with two benzannulations thus, the dibenzo[b,e]pyrylium ion (xanthyHum ion) shows shifts at 5 = 10.18 (9-H) and 165.1 (C-9) (CF3COOD). 

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