Role of internal motions and molecular geometry on the NMR relaxation of hydrocarbons

Abstract

The role of internal motions and molecular geometry on H NMR relaxation rates in liquid-state hydrocarbons is investigated using MD (molecular dynamics) simulations of the autocorrelation functions for intramolecular and intermolecularH-H dipole-dipole interactions. The effects of molecular geometry and internal motions on the functional form of the autocorrelation functions are studied by comparing symmetric molecules such as neopentane and benzene to corresponding straight-chain alkanes n-pentane and n-hexane, respectively. Comparison of rigid versus flexible molecules shows that internal motions cause the intramolecular and intermolecular correlation-times to get significantly shorter, and the corresponding relaxation rates to get significantly smaller, especially for longer-chain n-alkanes. Site-by-site simulations of H's across the chains indicate significant variations in correlation times and relaxation rates across the molecule, and comparison with measurements reveals insights into cross-relaxation effects. Furthermore, the simulations reveal new insights into the relative strength of intramolecular versus intermolecular relaxation as a function of internal motions, as a function of molecular geometry, and on a site-by-site basis across the chain.

Molecular dynamics simulations of $^1$H NMR relaxation in Gd$^{3+}$--aqua

“…4(a), the residual between the G(t) data and the ILT fit is not dominated by Gaussian noise. As such, the regularization parameter is fixed to α = 10 −1 in accordance with previous studies [49][50][51][52][53]. As shown in Fig.…”

“…[50][51][52][53] and the supplementary material in Refs. [49,80]. We briefly highlight the essential details here.…”

“…G(t) is available only at discrete time intervals, and the inversion is a rather ill-posed problem. In our approach, we use Tikhonov regularization [49,80], with the vector P being one for which…”

“…More specifically, the translational correlation time τ T ≃ 40 ps is plotted in Fig. 6, where the relation τ D = 9/4 τ T and the origin of the factor 9/4 is explained in [49,73]. The width of the main peak in P (τ ) can therefore be accounted for by the sharp peak at τ in = 26 ps from inner-sphere water, plus a broad peak at τ T = 40 ps from second/outer-sphere water.…”

“…MD simulation were previously used for like-spins, e.g. 1 H-1 H dipole-dipole pairs, such as liquid-state alkanes, aromatics, and water [47][48][49][50], as well as methane over a large range of densities [51]. In all these cases, good agreement was found between simulated and measured 1 H NMR relaxation and diffusion, without any adjustable parameters in the interpretation of the simulations.…”