NMR paramagnetic relaxation enhancement: Measurement of an axial/equatorial T-1 ratio for S=1 in the zero-field splitting limit

An experimental test of one of the central predictions of the theory of par amagnetic enhancement of NMR relaxation rates in solution (the NMR-PRE) for spins S > 1/2 is reported. For S greater than or equal to 1, zero-field sp litting (zfs) interactions are present that, when larger than the electroni c Zeeman interaction, act to align the spatial quantization of the electron spin motion along the molecule-fixed principal axis system of the zfs tens or. When the zfs energy is comparable to or greater than the Zeeman energy, the NMR-PRE has been predicted theoretically to be a function of the angul ar variables that specify the orientation of the electron-nuclear interspin vector in the molecular coordinate frame, such that the paramagnetic relax ation enhancement is larger for nuclear spins on the molecular z-axis than for nuclear spins in the x-y plane. The theoretically predicted range for r ho, the ratio of axial/equatorial NMR T-1 relaxation rates, is 1 less than or equal to rho less than or equal to 4, the value of unity corresponding t o the Zeeman limit (H-Zeem much greater than H-zfs); in the zfs limit, rho is predicted to reach its maximum value, which is significantly greater tha n unity. The ratio rho has been determined experimentally for the first tim e for the axial (H2O) and equatorial (CH3) protons of an S = 1 complex, Ni( II)(acac)(2)(H2O)(2), under conditions that approximate the zfs limit, as w as demonstrated from a measurement of the magnetic field dispersion profile of the H2O proton T-1's. The measured axial/equatorial T-1(-1) ratio, rho( exp) = 2.2 +/- 0.3, was significantly greater than unity as expected theore tically. The measured T-1(-1) ratio was in agreement with the results of sp in dynamics simulations carried out by the method of Abernathy and Sharp (J . Chem. Phys. 1997, 106, 9032).