ALIGNMENT EFFECTS IN HIGH-FIELD PROTON NMR-SPECTRA OF THE HYDROGENATED FULLERENES C60H2 AND C60H4 - EVIDENCE FOR RESIDUAL ANISOTROPIC DIPOLE-DIPOLE COUPLINGS

500 MHz and especially 750 MHz H-1 NMR spectra of the hydrogenated ful lerenes C60H2 and C60H4 reveal much unexpected fine structure that app arently results from the partial. alignment of the molecules in the ma gnetic field. Molecules with an anisotropic magnetic susceptibility ar e known to be partially aligned by a magnetic field. The effect is muc h more pronounced at 750 MHz because the order parameters describing t he molecular orientation are proportional to the square of the magneti c field. At 750 MHz, a splitting of 0.30 Hz results from a residual an isotropic dipole-dipole interaction between the two magnetically and c hemically equivalent protons in C60H2. Similarly, a splitting of 0.74 Hz is observed for D-2h-C60H4 Neither of these splittings has been obs erved previously at lower field strengths. Some of the C60H4 isomers w ith two symmetry-equivalent K-C-C-H groups clearly give two highly ove rlapped AB,quartets at 750 MHz (separation <1 Hi) because of two diffe rent H-1-H-1 dipole-dipole coupling constants (or only one non-zero di polar coupling constant) for the two H-C-C-H groups. Some of the signa ls from different isomers apparently overlap even at 750 MHz. At least two factors need to be considered; in analyzing the unequal peak heig hts of the two AB quartets from a given isomer: (1) anisotropic relaxa tion and the resulting line width variations resulting from anisotropi c motion and (2) line width variations resulting fi om cross correlati on between H-1-H-1 dipole-dipole relaxation and 1H chemical shift anis otropy relaxation or between H-1 CSA relaxation of one spin and 1H CSA relaxation of another spin. More detailed experiments under carefully controlled conditions are required for a full analysis, particularly for the e isomer 5. As noted previously, the choice of lock solvent cl early becomes much more important at very high field strengths.