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I. Introduction 603 II. Spin Catalysis in Three Spin Systems 604 A. Spin Wave Functions and Spin States of Triads 604 B. Spin Catalysis: How It Functions 605 C. Two Mechanisms of Spin Catalysis 606 III. Spin Catalysis by Radicals 606 A. Radicals as the Spin Catalysts 606 B. Biradicals and Triradicals 608 IV. Paramagnetic Ions as the Spin Catalysts 608 V. Cis−Trans Isomerization 610 VI. Dynamics of Spin Catalytic Processes 610 A. Static Model and Quantum Beats 611 B. Dynamic Model 611 VII. Conclusion 611 VIII. Acknowledgments 612 IX. References 612

We report an investigation of the nuclear spin-lattice relaxation of H 2 and H 2 @C 60 1 as a function of solvent and temperature. These studies explore and compare the nature of the interactions of a guest H 2 molecule confined transiently within the walls of a solvent cavity and a guest H 2 molecule encapsulated within the walls of the C 60 cavity.The relaxation time (T 1 ) of H 2 has been extensively studied in the gas phase and in liquid hydrogen at low temperatures. 6 The values of T 1 are 10-20 times smaller for H 2 @C 60 than for H 2 even though the ratios of T 1 for H 2 and H 2 @C 60 are similar in all the solvents.The temperature dependences of T 1 for H 2 and H 2 @C 60 were investigated in detail for toluene-d 8 ( Figure 1) and for benzene-d 6 , 1,1,2,2-tetrachloroethane-d 2 , 1,2-dichlorobenzene-d 4 , and chloroformd 1 . Striking features of the data are the occurrence of a maximum for T 1 at ∼240 K for both the H 2 and H 2 @C 60 in toluene-d 8 and a ratio of T 1 values which is nearly independent of temperature. A maximum of the value of T 1 with temperature is also found in 1,1,2,2-tetrachloroethane-d 2 and chloroform-d 1 . For benzene-d 6 and 1,2-dichlorobenzene-d 4 in the available range of temperatures, only a decrease of T 1 with increasing temperature was observed.This kind of dependence of T 1 on temperature is uncommon, although a maximum of T 1 has previously been observed for small molecules such as H 2 O, 7 HCl, and HBr in solution, 8,9 and it is consistent with two relaxation mechanisms with different temperature dependences dominating in turn below and above 240 K for both H 2 and H 2 @C 60 . Since the value of T 1 for both H 2 and H 2 @C 60 does not significantly change in going from benzene-h 6 to benzened 6 (Table 1), the dominating interactions determining H 2 and H 2 @C 60 nuclear relaxation must be intramolecular. Furthermore, the intramolecular dipole-dipole interaction and spin-rotation interaction are known 2 to be responsible for the relaxation of gaseous H 2 and their magnitude has been measured for H 2 in molecular beams. 10 Therefore it is likely that the relaxation of H 2 in solution also depends on the competition between intramolecular dipole-dipole interaction and spin-rotation interaction.The contribution to 1/T 1 (in extreme narrowing conditions) from intramolecular dipolar and spin-rotation interaction may be estimated by eq 1 2 and eq 2, 11,12 respectively:where γ H is the magnetogyric ratio for the proton, r is the equilibrium internuclear distance of H 2 (0.74 Å), C is the spinrotation coupling constant (7.16 × 10 5 rad s -1 ), 10 I is the moment of inertia of H 2 (4.6 × 10 -48 kg m 2 ), and k B is the Boltzmann constant. The correlation times τ dip and τ sr are measures of the timedependent fluctuations in the orientation and angular velocity of H 2 , respectively. Both correlation times are expected to be functions of viscosity and temperature which depend on the details of the motion of H 2 molecules and the surrounding medium. 11 Qualitatively, the dipole-...