NMR-SPECTROSCOPIC INVESTIGATIONS OF POTENTIALLY PLANARIZED NONAFULVENES

H-1- and C-13-NMR spectra of nonafulvene 1e and nonafulvenes 2 and 3 have been assigned, high-resolution H-1-NMR spectra of 2 (600 MHz, Fig.3) and of 3 (400 MHz, Fig. 2) have been analyzed, and the data are compared with those of other nonafulvenes (Tables 1-6). Generally speaking, according to their spectroscopic behavior, four classes of nonafulvenes (A-D) may be distinguished (Fig. 1). The investigation shows that compounds 1e and 3 belong to class A, being characterized by H-1-chemical shifts around 6 ppm, strongly alternating (3)J(H,H) and C-13 chemical shifts in the range of 123 to 130 ppm, thus existing in the olefinic form with a non-planar nine-membered ring. On the other hand, 2 is the first nonafulvene of class D, being characterized by H-1 chemical shifts in the aromatic range, large (3)J(H,H) values of the same size, and C-13 chemical shifts around 110 ppm. Since NMR parameters are virtually not influenced by temperature (-50 degrees to 50 degrees) or solvents, it is concluded that 2 exclusively exists in the dipolar structure 2(+/-) with a planarized nine-membered ring. According to Fig. 4, these classes (and their spectroscopic data) are linked by 10, 10-bis(dimethylamino)nonafulvene (1c; and its temperature-dependent NMR parameters): for 1c, a temperature-dependent equilibrium 1c reversible arrow 1c(+/-) had earlier been established.