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 1 H NMR spectra of the hydrogenated fullerenes C 60 H 2 and C 60 H 4 reveal much unexpected fine structure that apparently results from the partial alignment of the molecules in the magnetic field. Molecules with an anisotropic magnetic susceptibility are known to be partially aligned by a magnetic field. The 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. At 750 MHz, a splitting of 0.30 Hz results from a residual anisotropic dipole−dipole interaction between the two magnetically and chemically equivalent protons in C 60 H 2 . Similarly, a splitting of 0.74 Hz is observed for D 2 h -C 60 H 4 . Neither of these splittings has been observed previously at lower field strengths. Some of the C 60 H 4 isomers with two symmetry-equivalent H−C−C−H groups clearly 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 non-zero dipolar coupling constant) for the two H−C−C−H groups. Some of the signals from different isomers apparently overlap even at 750 MHz. At least two factors need to be considered in analyzing the unequal peak heights of the two AB quartets from a given isomer: (1) anisotropic relaxation and the resulting line width variations resulting from anisotropic motion and (2) line width variations resulting from cross correlation between 1 H− 1 H dipole−dipole relaxation and 1 H chemical shift anisotropy relaxation or between 1 H CSA relaxation of one spin and 1 H 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 clearly becomes much more important at very high field strengths.