Intermediate motions and dipolar couplings as studied by Lee–Goldburg cross-polarization NMR: Hartmann–Hahn matching profiles

Abstract

In this article, we evaluate the use of simple Lee-Goldburg cross-polarization (LG-CP) NMR experiments for obtaining quantitative information of molecular motion in the intermediate regime. In particular, we introduce the measurement of Hartmann-Hahn matching profiles for the assessment of heteronuclear dipolar couplings as well as dynamics as a reliable and robust alternative to the more common analysis of build-up curves. We have carried out dynamic spin dynamics simulations in order to test the method's sensitivity to intermediate motion and address its limitations concerning possible experimental imperfections. We further demonstrate the successful use of simple theoretical concepts, most prominently Anderson-Weiss (AW) theory, to analyze the data. We further propose an alternative way to estimate activation energies of molecular motions, based upon the acquisition of only two LG-CP spectra per temperature at different temperatures. As experimental tests, molecular jumps in imidazole methyl sulfonate, trimethylsulfoxonium iodide, and bisphenol A polycarbonate were investigated with the new method.

MAS NMR Investigation of Molecular Order in an Ionic Liquid Crystal

“…As the CH dipolar couplings in CAGE-oct are relatively weak, a recoupled version of the original DIPSHIFT experiment was utilized, where the phase accumulated due to the evolution under the dipolar coupling during t 1 is amplified by a REDOR-like train of π pulses . We further chose a variant of the DIPSHIFT experiment reported by Cobo et al where the evolution time under heteronuclear dipolar coupling is not constant (in contrast to frequently used constant time experiments), and a potential T 2 decay indicative of intermediate time-scale motions is possible . The pulse sequence for the T 2 -recDIPSHIFT experiment is shown in Figure S10a.…”

“…A complement to probing 13 C– 1 H dipolar coupling is the measurement of 1 H– 1 H homonuclear dipolar couplings, and the related order parameter S HH . − 1 H– 1 H RDCs can be quantified by 1 H double-quantum (DQ) MAS spectroscopy, − from which the value of the dipolar coupling constant can be extracted either by fitting the intensity of the build-up behavior with varying DQ excitation time (τ DQ ) or by analyzing the spinning sideband pattern in a 2D spectrum recorded with fixed τ DQ . Brown et al utilized 1 H– 1 H residual dipolar couplings to study rotation of the aromatic core in columnar hexabenzocoronene LCs, as well as differential mobility along the alkyl chains in a triphenylene-based mesogen. , 1 H– 1 H RDCs have also been used to investigate chain dynamics in polymer systems, ,− and for the study of lipid membranes with guest molecules …”

Effects of solvent concentration and composition on protein dynamics: 13 C MAS NMR studies of elastin in glycerol–water mixtures

“…Specifically, when molecular dynamics is absent or slow, i.e., when the correlation time is longer than 10 −4 s, 13 C CP MAS NMR spectra feature well resolved lines. Anisotropic motions with a characteristic time scale in the range 10 −4 -10 −6 s lead to a line broadening and a loss of signal intensity due to a reduced CP efficiency [48] and an interference of the stochastic time dependence of molecular dynamics with the coherent averaging associated with the applied dipolar decoupling and sample spinning techniques [49][50][51]. Finally, highly restricted reorientations with correlation times shorter than 10 −6 s result in narrow lines as motional averaging is completed on the experimental time scales, while less restricted reorientations on this time scale can cause a merging or disappearance of lines [35].…”