Valiya Parambathu Arjun, Pinheiro Dos Santos Thiago J, Chapman Walter G, Hirasaki George J, Asthagiri Dilipkumar N, Singer Philip M
Department of Chemical and Biomolecular Engineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States.
Department of Chemical and Biomolecular Engineering, University of Delaware, 150 Academy Street, Newark, Delaware 19716, United States.
J Phys Chem B. 2024 Aug 22;128(33):8017-8028. doi: 10.1021/acs.jpcb.4c02631. Epub 2024 Aug 8.
The Bloembergen, Purcell, and Pound (BPP) theory of nuclear magnetic resonance (NMR) relaxation in fluids dating back to 1948 continues to be the linchpin in interpreting NMR relaxation data in applications ranging from characterizing fluids in porous media to medical imaging (MRI). The BPP theory is founded on assuming molecules are hard spheres with H-H dipole pairs reorienting randomly; assumptions that are severe in light of modern understanding of liquids. Nevertheless, it is intriguing to this day that the BPP theory was consistent with the original experimental data for glycerol, a hydrogen-bonding molecular fluid for which the hard-sphere-rigid-dipole assumption is inapplicable. To better understand this incongruity, atomistic molecular simulations are used to compute H NMR relaxation dispersion (i.e., frequency dependence) in two contrasting cases: glycerol, and a (non hydrogen-bonding) viscosity standard. At high viscosities, simulations predict distinct functional forms of for glycerol compared to the viscosity standard, in agreement with modern measurements, yet both in contrast to BPP theory. The cause of these departures from BPP theory is elucidated, without assuming any relaxation models and without any free parameters, by decomposing the simulated response into dynamic molecular modes for both intramolecular and intermolecular interactions. The decomposition into dynamic molecular modes provides an alternative framework to understand the physics of NMR relaxation for viscous fluids.
布洛姆伯根、珀塞尔和庞德（BPP）于1948年提出的流体中核磁共振（NMR）弛豫理论，至今仍是解释NMR弛豫数据的关键，其应用范围涵盖从表征多孔介质中的流体到医学成像（MRI）。BPP理论基于这样的假设：分子是硬球体，氢 - 氢偶极对随机重新定向；鉴于现代对液体的理解，这些假设过于严苛。然而，时至今日，令人好奇的是，BPP理论与甘油的原始实验数据一致，而甘油是一种氢键分子流体，硬球体 - 刚性偶极假设对其并不适用。为了更好地理解这种不一致性，采用原子分子模拟来计算两种对比情况下的氢核磁共振弛豫色散（即频率依赖性）：甘油和一种（非氢键型）粘度标准物。在高粘度下，模拟预测甘油的弛豫色散与粘度标准物相比具有不同的函数形式，这与现代测量结果一致，但两者均与BPP理论不同。通过将模拟的弛豫响应分解为分子内和分子间相互作用的动态分子模式，在不假设任何弛豫模型且无任何自由参数的情况下，阐明了这些与BPP理论偏差的原因。分解为动态分子模式为理解粘性流体中NMR弛豫的物理原理提供了一个替代框架。
