Asthagiri D, Chapman Walter G, Hirasaki George J, Singer Philip M
Rice University, Department of Chemical and Biomolecular Engineering, 6100 Main Street, Houston, Texas 77005, United States.
J Phys Chem B. 2020 Nov 25;124(47):10802-10810. doi: 10.1021/acs.jpcb.0c08078. Epub 2020 Nov 13.
The intramolecular H NMR dipole-dipole relaxation of molecular fluids has traditionally been interpreted within the Bloembergen-Purcell-Pound (BPP) theory of NMR intramolecular relaxation. The BPP theory draws upon Debye's theory for describing the rotational diffusion of the H-H pair and predicts a monoexponential decay of the H-H dipole-dipole autocorrelation function between distinct spin pairs. Using molecular dynamics (MD) simulations, we show that for both -heptane and water this is not the case. In particular, the autocorrelation function of individual H-H intramolecular pairs itself evinces a rich stretched-exponential behavior, implying a distribution in rotational correlation times. However, for the high-symmetry molecule neopentane, the individual H-H intramolecular pairs do conform to the BPP description, suggesting an important role of molecular symmetry in aiding agreement with the BPP model. The intermolecular autocorrelation functions for -heptane, water, and neopentane also do not admit a monoexponential behavior of individual H-H intermolecular pairs at distinct initial separations. We suggest expanding the autocorrelation function in terms of modes, provisionally termed molecular modes, that do have an exponential relaxation behavior. With care, the resulting Fredholm integral equation of the first kind can be inverted to recover the probability distribution of the molecular modes. The advantages and limitations of this approach are noted.
分子流体的分子内氢核磁共振偶极-偶极弛豫传统上是在核磁共振分子内弛豫的布洛姆伯根-珀塞尔-庞德（BPP）理论框架内进行解释的。BPP理论借鉴德拜理论来描述氢-氢对的旋转扩散，并预测不同自旋对之间氢-氢偶极-偶极自相关函数呈单指数衰减。通过分子动力学（MD）模拟，我们发现对于正庚烷和水而言并非如此。具体来说，单个分子内氢-氢对的自相关函数本身呈现出丰富的拉伸指数行为，这意味着旋转相关时间存在分布。然而，对于高对称性分子新戊烷，单个分子内氢-氢对确实符合BPP描述，这表明分子对称性在与BPP模型达成一致方面起着重要作用。正庚烷、水和新戊烷的分子间自相关函数也不允许不同初始间距下单个分子间氢-氢对呈现单指数行为。我们建议根据模式（暂且称为分子模式）展开自相关函数，这些模式确实具有指数弛豫行为。谨慎地说，由此得到的第一类弗雷德霍姆积分方程可以求逆以恢复分子模式的概率分布。文中指出了这种方法的优缺点。
