Observation of Alignment Effects in High-Field Proton NMR Spectra of Hydrogenated Fullerenes: Evidence for Residual Anisotropic Dipole-Dipole Couplings Lawrence B. Alemany, Alexander Gonzalez, Weimei Luo, and W. E. Billups* Department of Chemistry Rice UniVersity Houston, Texas 77005-1892 M. Robert Willcott Department of Radiology The UniVersity of Texas Medical Branch GalVeston, Texas 77555-0793 Edward Ezell and Elliott Gozansky Department of Human Biological Chemistry & Genetics The UniVersity of Texas Medical Branch GalVeston, Texas 77555-1157 ReceiVed October 7, 1996 1 H NMR (500 MHz and especially 750 MHz) spectra of the hydrogenated fullerenes C 60 H 2 and C 60 H 4 reveal much unex- pected fine structure that apparently results from the partial alignment of the molecules in the magnetic field. 1-3 Molecules with an anisotropic magnetic susceptibility are known to be partially aligned by a magnetic field. Consequently, anisotropic nuclear interactions are incompletely averaged. The alignment effect is much more pronounced at 750 MHz because the order parameters describing the molecular orientation are proportional to the square of the magnetic field. Multiple field (including 750 MHz 1 H), multidimensional NMR has been used recently to obtain dipolar couplings in structural studies of DNA 4 and proteins. 5,6 Our work illustrates alignment effects, with a resulting elimination of C 2 or C s symmetry, in 1D 1 H NMR spectra of very simple spin systems. Such alignment effects will clearly become more commonly observed as 17.6 T (750 MHz 1 H) and higher field strength magnets become more available. The 750 MHz 1 H spectrum of a dilute solution of C 60 H 2 dissolved in a 1:1 solution of C 6 D 6 -CS 2 reveals two overlapping signals (separation 0.30 Hz without resolution enhancement; digital resolution 0.10 Hz) of equal intensity at δ 6.14 (Figure 1). The two protons in C 60 H 2 are chemically and magnetically equivalent and would be expected to give just a singlet. A residual dipole- dipole coupling appears to be the most reasonable explanation of the splitting. The splitting does not appear to be an artifact of poor shimming, as the reference TMS signal is a singlet with a line width at half-height of 0.20 Hz. The TMS singlet also rules out frequency jumping caused by deuterium quadrupolar coupling splitting the C 6 D 6 lock signal as the cause 7 of the pair of 1 H signals. We are aware of only a few published 1 H spectra of solutions clearly exhibiting 1 H- 1 H dipole-dipole splittings in a two- spin system. 1,3,8 For two equivalent protons, a splitting of 0.30 Hz corresponds to a dipolar coupling constant D HH of 0.20 Hz, 3,8,9 which seems reasonable since the 1 H- 12 C- 13 C- 1 H satellites in the 1 H spectrum of coronene at 600 MHz also exhibit D HH ) 0.20 Hz. 1 Because the magnitude of the dipolar coupling constant varies with the square of the field strength, 1-3 a splitting of 0.13 Hz would be predicted at 500 MHz. However, we were unable to detect this splitting. Prior studies of C 60 H 2 at 200- 500 MHz using various solvents 10-13 mention no splitting; indeed, a “sharp singlet” is sometimes reported. 10-12,14 To detect the splitting in the 750 MHz 1 H spectrum of C 60 H 2 , a solvent such as C 6 D 6 that is also partially aligned by the magnetic field resulting in alignment transfer from solvent to solute 8 is required. No splitting of the C 60 H 2 signal is observed in solutions of CDCl 3 or CD 3 COCD 3 . In general, the anisotropy and asymmetry of the magnetic susceptibility are affected by the concentration and solvent. 2,9 Unexpected fine structure that apparently results from the partial alignment of the molecules in the magnetic field has also been observed with C 60 H 4 isomers. Thus, some of the C 2 or C s C 60 H 4 isomers with two symmetry-equivalent H-C-C-H groups 15 give two highly overlapped AB quartets at 750 MHz (separation <1 Hz) because of two different 1 H- 1 H dipole- dipole coupling constants (or only one nonzero dipolar coupling constant) for the two H-C-C-H groups. 17 Such dipolar coupling is not detected at 500 MHz. The much greater complexity of the entire 750 MHz spectrum compared to the 500 MHz spectrum and the apparent lack of spectral complexity for C 60 H 4 in C 6 D 5 CD 3 at 400 MHz 16 suggest that the complexity is not related to J HH coupling, is field-dependent, and is barely detectable at 500 MHz. In light of the 1/r 3 dependence of (1) Gayathri, C.; Bothner-By, A. A.; van Zijl, P. C. M.; MacLean, C. Chem. Phys. Lett. 1982, 87, 192-196. (2) van Zijl, P. C. M.; Ruessink, B. H.; Bulthuis, J.; Ruessink, B. H. Acc. Chem. Res. 1984, 17, 172-180. (3) Bothner-By, A. A.; Gayathri, C.; van Zijl, P. C. M.; MacLean, C.; Lai, J.-J.; Smith, K. M. Magn. Reson. Chem. 1985, 23, 935-938. (4) Kung, H. C.; Wang, K. Y.; Goljer, I.; Bolton, P. H. J. Magn. Reson., Ser. B 1995, 109, 323-325. (5) Tolman, J. R.; Flanagan, J. M.; Kennedy, M. A.; Prestegard, J. H. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 9279-9283. (6) Tjandra, N.; Grzesiek, S.; Bax, A. J. Am. Chem. Soc. 1996, 118, 6264-6272. (7) (a) van Zijl, P. C. M. J. Magn. Reson. 1987, 75, 335-344. (b) This paper indicates that even at 620 MHz, splitting of the deuterium lock signal of C 6D6 is not evident. (8) Bastiaan, E. W.; MacLean, C. NMR: Basic Princ. Prog. 1991, 25, 17-43. (9) Bastiaan, E. W.; MacLean, C.; van Zijl, P. C. M.; Bothner-By, A. A. Annu. Rep. NMR Spectrosc. 1987, 19, 35-77. (10) Henderson, C. C.; Cahill, P. A. Science 1993, 259, 1885-1887. (11) Ballenweg, S.; Gleiter, R.; Kra ¨tschmer, W. Tetrahedron Lett. 1993, 34, 3737-3740. (12) Becker, L.; Evans, T. P.; Bada, J. L. J. Org. Chem. 1993, 58, 7630- 7631. (13) Avent, A. G.; Darwish, A. D.; Heimbach, D. K.; Kroto, H. W.; Meidine, M. F.; Parsons, J. P.; Remars, C.; Roers, R.; Ohashi, O.; Taylor, R.; Walton, D. R. M. J. Chem. Soc., Perkin Trans. 2 1994, 15-22. (14) Similarly, the splitting of 0.74 Hz observed for D 2h C60H4 (two pairs of hydrogens added at double bonds on opposite sides of C60 15 ) at 750 MHz has not been observed at lower field strengths. 13,16 (15) Hirsch, A. The Chemistry of the Fullerenes; Thieme Medical Publishers: New York, 1994; p 123. (16) Henderson, C. C.; Rohlfing, C. M.; Assink, R. A.; Cahill, P. A. Angew. Chem., Int. Ed. Engl. 1994, 33, 786-788. (17) A reviewer has suggested that these observations might be explained by the presence of unidentified isomers. To eliminate this possibility, the hydrides were synthesized using 3 He@C60. 3 He NMR spectroscopy was used to show that unidentified isomers were not present. 18 Figure 1. The δ 6.128-6.148 region in the 750 MHz spectrum of C 60H2 dissolved in a 1:1 solution of C6D6-CS2. 5047 J. Am. Chem. Soc. 1997, 119, 5047-5048 S0002-7863(96)03493-2 CCC: $14.00 © 1997 American Chemical Society