904 CHEMICAL COMMUNICATIONS [24]Annulene: Dependence of Nuclear Magnetic Resonance Spectrum on Temperature By I. C. CALDER and F. SONDHEIMER ( University Chemical Laboratory, Lensfield Road, Cambridge) IN 1964, our group reported1 that the nuclear magnetic resonance (n.m.r.) spectra of [14]- annulene (major conformer) and of [ 18lannulene were temperature-dependent. At - 60°, both substances showed n.m.r. spectra typical of aromatic systems,2 the outer protons appearing at low field (T 2.4 and 0.72, respectively), and the inner protons at high field (T 10.0 and 12-99, respectively). At higher temperatures, the n.m.r. spectra of both these annulenes consisted of singlets (T 4-42 and 4-55, respectively), these bands being due to the interchange of the protons at such a rate that an average value is observed.The n.m.r. spectra of [lSIannulene and [24]- annulene at room temperature had been reported to consist of singlets at T 3 ~ 2 7 ~ and 3 ~ 1 6 , ~ respec- tively. These bands presumably again represent average values, and it was of interest to investi- gate the effect on the spectra of cooling the solutions. In the case of [16]annulene, it has been shown very recently that the n.m.r. spectrum is indeed temperature-dependent, consisting a t - 110" of inner proton bands a t T -0.43, and outer proton bands at T 4 ~ 6 0 . ~ [24]Annulene [for which structures (I) or (11) appear to be the most likelyI6j6 has now been prepared again by the previously described method,%' and the temperature-dependence of the n.m.r. spectrum has been studied (see Figure).At 40°, the spectrum, determined in perdeutero- tetrahydrofuran, consists of a singlet at T 2.75.* Cooling the solution results in progressive broaden- ing of this band, until a t ca. -20" it can no longer be observed. At lower temperatures, two new signals appear, the spectrum a t -80" exhibiting bands a t ca. T -2.9 to - 1.2 and at 5.27 in a ratio of ca. 35:65. The ratio shows the low-field band to be due to the inner protons, and the high-field one to the outer protons. The degree of accuracy of the integration is insufficientNUMBER 24, 1966 905 to allow a distinction to be made between structure (I) (inner : outer proton ratio, 37-5 : 62-5) and structure (11) (ratio, 33.3 : 66.7). H H The room temperature n.m.r. spectrum of [24]annulene evidently again represents an average, due to rotation of the carbon-carbon bonds (conformational isomerism).In addition, movement of the n-bonds (valence isomerism) may be involved] as has been postulated for cyclo-octatetraenes and [ 161ann~lene.~ It is remarkable that in the low-temperature n.m.r. spectra of the 4n n-electron systems, [ 16lannulene and [24]annulene, the inner protons appear at low field and the outer protons at high field. This is a reversal of the behaviour of the (4% + 2) systems, [14]annulene and [18]annulene. A similar reversal between 4n and (4% + 2) systems in the dehydro-annulene series has already been observed,l* and a theoretical explana- tion based on quantum-mechanical considerations has been advanced.u - 3 - 2 - 1 0 I 2 3 4 5 6 I ' I I I I I I I 1 - 20' Outer Protons -527 Inner Protons J ca.- 2.9 to - 1.2 I I I I I I I I I I - 3 - 2 - 1 0 I 2 3 4 5 6 f- FIGURE N.m.r. spectrum of [24]annulene, in perdeuterotetra- hydrofuran solution (100 Mc./sec. , tetramethylsilane used as internal standard). (Received, October ZOth, 1966; Corn. 794.) 1Y. Gaoni, A. Melera, F. Sondheimer, and R. Wolovsky, Proc. Chem. Soc., 1964, 397. 2 See L. M. Jackman, F. Sondheimer, Y. Amiel, D. A. Ben-Efraim, Y . Gaoni, R. Wolovsky, and A. A. Bothner-By, 3F. Sondheimer, Pure Appl. Chem., 1963, 7 , 363. J . Amer. Chem. SOC., 1962, 84, 4307. G. Schroder and J. F. M. Oth, Tetrahedron Letters, 1966, 4083. F. Sondheimer, R. Wolovsky, and Y . Amiel, J . Amer. Chem. SOC., 1962,84, 274. H. C. Longuet-Higgins and L. Salem, Proc. Roy. SOC., 1960, A , 257, 445. F. Sondheimer and R. Wolovsky, J . Amer. Chem. SOC., 1962, 84, 260. In deuterochloroform solution, the 40' singlet appears a t 7 2-78. The previously reported value (ref. 2) of T 3.16 See F. A. L. Anet, J . Amer. Chem. SOC., 1962,84, 671; F. A. L. Anet, A. J. R. Bourn, and Y . S. Lin, ibid., 1964, lo F. Sondheimer, I. C. Calder, J. A. Elix, Y . Gaoni, P. J. Garratt, K. Grohmann, G. Di Maio, J. Mayer, M. V. l1 J. A. Pople and K. G. Untch, J . Amer. Chem. Soc., 1966, 88, 4811; H. C. Longuet-Higgins, Chem. SOC. Special must have been in error, due to a mistake in the calibration. 86, 3576; D. E. Gwynn, G. AT. Whitesides, and J. D. Roberts, ibid., 1965, 87, 2862. Sargent, and R. Wolovsky, Chem. SOC. Special Publ., No. 21, in the press. Publ., No. 21, in the press.