Modeling Small Structural and Environmental Differences in Solids with ¹⁵N NMR Chemical Shift Tensors

Abstract

The ability to theoretically predict accurate NMR chemical shifts in solids is increasingly important due to the role such shifts play in selecting among proposed model structures. Herein, two theoretical methods are evaluated for their ability to assign 15N shifts from guanosine dihydrate to one of the two independent molecules present in the lattice. The NMR data consist of 15N shift tensors from 10 resonances. Analysis using periodic boundary or fragment methods consider a benchmark dataset to estimate errors and predict uncertainties of 5.6 and 6.2 ppm, respectively. Despite this high accuracy, only one of the five sites were confidently assigned to a specific molecule of the asymmetric unit. This limitation is not due to negligible differences in experimental data, as most sites exhibit differences of >6.0 ppm between pairs of resonances representing a given position. Instead, the theoretical methods are insufficiently accurate to make assignments at most positions.

Fast and Accurate Electric Field Gradient Calculations in Molecular Solids With Density Functional Theory

“…NMR parameters derived from the CS and EFG tensors have been used extensively in molecular structure refinement (Olsen et al, 2003;Witter et al, 2006;Harris et al, 2010;Apperley et al, 2012;Harper et al, 2013;Kalakewich et al, 2015;Yang et al, 2016;Soss et al, 2017;Chalek et al, 2021;Wang et al, 2021). Recently, plane-wave EFG tensor predictions involving 14 N, 17 O, and 35 Cl nuclei were used to obtain optimized damping parameters for crystal geometry refinement using Grimme's DFT-D2 scheme (Holmes and Schurko, 2018).…”

“…SSNMR investigations are often coupled with X-ray diffraction and first-principals calculations to form the interdisciplinary field of NMR crystallography. The success of NMR crystallography has been greatly facilitated by the availability of accurate computational methods for predicting NMR parameters which typically employ density functional theory (DFT) (Gervais et al, 2005;Wu, 2008;Kong et al, 2013;Yang et al, 2016;Kong et al, 2017;Soss et al, 2017;Holmes and Schurko, 2018;Yamada et al, 2020;Chalek et al, 2021;Wang et al, 2021).…”

NMR-guided refinement of crystal structures using 15N chemical shift tensors

“…Overall, it has been reported that 15 N shift tensors exhibit a variation more than six times larger than 13 C sites. 18 A focus on 15 N has the further advantage of being relevant to protein structural refinement. Proteins are target structures of high interest because their experimental crystal structures are often much lower in resolution than comparable studies on small molecules.…”

Predicting 51V nuclear magnetic resonance observables in molecular crystals

“…[40] These efforts have proven particularly effective in improving the accuracy of fragment-based 15 N CS tensor calculations. [41,42] In the context of 51 V, a previous benchmark study identified electrostatic embedding as a crucial ingredient for accurate two-body fragment CS tensor calculations. [29] Specifically, two-body fragment calculations employing self-consistent point-charge embedding improved the accuracy of predicted 51 V isotropic chemical shifts by 20% relative to traditional planewave methods.…”

“…Self‐consistent charge embedding models have been developed to improve the accuracy of predicted NMR parameters in situations where higher order contributions strongly influence the NMR observable [40] . These efforts have proven particularly effective in improving the accuracy of fragment‐based 15 N CS tensor calculations [41,42] . In the context of 51 V, a previous benchmark study identified electrostatic embedding as a crucial ingredient for accurate two‐body fragment CS tensor calculations [29] .…”