Dinesen T R, Bryant R G
Department of Chemistry, University of Virginia, Charlottesville 22901, USA.
J Magn Reson. 1998 May;132(1):19-24. doi: 10.1006/jmre.1998.1373.
Measurements of 7Li nuclear spin-lattice relaxation times are made at applied magnetic field strengths from 0.25 mT to 7.05 T, in order to determine directly the form of the frequency-dependent spectral densities that modulate relaxation. This magnetic resonance dispersion (MRD) technique provides detailed information regarding molecular dynamics down to the picosecond time scale. 7Li MRD measurements made on aqueous lithium ion in the presence of small concentrations of nitroxide free radicals give direct evidence supporting the formation of a coordination complex. The dipole-dipole electron-nuclear coupling is modulated by both translational and rotational diffusive motions, and both of these contributions are resolved. However, scalar coupling arising from the presence of paramagnetic electron spin density at the nucleus dominates the nuclear relaxation. Changes in the pH and free radical moiety are compared with dynamical variables, geometric constraints, and formation constants obtained from a model of nuclear relaxation. The calculated bimolecular formation constants are on the order of 2 x 10(-3) M-1, and the relative accuracy of this parameter is tested.
为了直接确定调制弛豫的频率相关谱密度的形式，在0.25 mT至7.05 T的外加磁场强度下测量了7Li核自旋 - 晶格弛豫时间。这种磁共振色散（MRD）技术提供了有关皮秒时间尺度下分子动力学的详细信息。在存在低浓度氮氧自由基的情况下对水合锂离子进行的7Li MRD测量给出了支持形成配位络合物的直接证据。偶极 - 偶极电子 - 核耦合由平移和旋转扩散运动调制，并且这两种贡献都得到了解析。然而，由原子核处顺磁电子自旋密度的存在引起的标量耦合主导了核弛豫。将pH值和自由基部分的变化与从核弛豫模型获得的动力学变量、几何约束和形成常数进行比较。计算得到的双分子形成常数约为2×10⁻³ M⁻¹，并对该参数的相对精度进行了测试。
