These spectra were measured at -60 OC. D5 and D6 were too short-lived to obtain 13C spectra. UV Spectra. The spectrum of D5 in Figure 1 was obtained by irradiating a solution of 5 in hexane in a quartz Dewar-UV cell at -45 OC; the spectrum was recorded at that temperature. In a similar manner, A,, for D4 was at 270 nm, and for D11 at 272 nm. D6 was too unstable at -45 OC to obtain a UV spectrum.Endoperoxides 18 and 19. Brief irradiation (30 min) of an undegassed solution of 13 in benzene-de in an NMR tube as described in the general photolysis procedure gave endoperoxide 18: 'H NMR (C8D6) 6 1.79 (s, 3 H), 1.99 (s, 3 H), 2.07 ( 8 , 6 H), 6.71-7.06 (m. 6 H): mass mectrum, m l e (relative intensity) 266 the solvent peak); mass spectrum, m l e (relative intensity) 266 (4), 234 (loo), 219 (32), 85 (22), 40 (16).Irradiation of 1,4,9-Trimethylanthracene (15). Samples of 15 were irradiated in several solvents (benzenede, pyridine-d5, and chloroform-d) according to the general photolysis procedure. The singlet at 6 8.20 (aromatic proton at ClO) in the spectrum of 15 disappeared, and new singlets appeared at 6 4.27 and 4.45 (in CDC13), area ratio 5 1 , ascribed to the bridgehead protons in 20 and 21, respectively. In addition, the major product 20 showed three methyl singlets approximately 0.5 ppm upfield from those in 15 (in CDC13, at 6 2.25,2.39, and 2.55) whereas the minor isomer 21 showed a singlet at 6 2.50 (ratio of 6 2.39j2.50 = 5:l; the other methyl peaks for 21 were coincidental with those of 20 at 6 2.25 and 2.55; in pyridine-d5, two of the three methyl signals for 21 were nicely resolved). The photoproducts were stable even afterWe have synthesized 86 semicarbazones of variously substituted benzaldehyde and acetophenone and of several other aldehydes and ketones for comparison. The IR data in Nujol mull revealed numerous abnormally high frequencies for the carbonyl of semicarbazones of up to ca. 1760 cm-' (cf. 1690 cm-' as a normal value for semicarbazones carbonyls and 1640-1650 cm-' for the amide and urea carbonyls). These high C=O frequencies shift to normal values in DMSO solution. The origin of the high C=O frequencies could not be ascribed to the stereoelectronic effects of the substituents on the aromatic ring. It appeared that the high C=O frequencies are caused by the solid-state structure. The X-ray structural determination of two semicarbazones indicated a network of H-bonds in which each C=O is involved in a bifurcated H-bond with NH2 and NH from two different molecules. The solution and solid-state 13C NMR studies of semicarbazones are also described.