Predicting anisotropic thermal displacements for hydrogens from solid-state NMR: a study on hydrogen bonding in polymorphs of palmitic acid

Abstract

The hydrogen-bonding environments at the COOH moiety in eight polycrystalline polymorphs of palmitic acid are explored using solid-state NMR. Although most phases have no previously reported crystal structure, measured 13C chemical shift tensors for COOH moieties, combined with DFT modeling establish that all phases crystallize with a cyclic dimer (R22(8)) hydrogen bonding arrangement. Phases A2, Bm and Em have localized OH hydrogens while phase C has a dynamically disordered OH hydrogen. The phase designated As is a mix of five forms, including 27.4% of Bm and four novel phases not fully characterized here due to insufficient sample mass. For phases A2, Bm, Em, and C the anisotropic uncertainties in the COOH hydrogen atom positions are established using a Monte Carlo sampling scheme. Sampled points are retained or rejected at the ±1σ level based upon agreement of DFT computed 13COOH tensors with experimental values. The collection of retained hydrogen positions bear a remarkable resemblance to the anisotropic displacement parameters (i.e. thermal ellipsoids) from diffraction studies. We posit that this similarity is no mere coincidence and that the two are fundamentally related. The volumes of NMR-derived anisotropic displacement ellipsoids for phases with localized OH hydrogens are 4.1 times smaller than those derived from single crystal X-ray diffraction and 1.8 times smaller than the volume of benchmark single crystal neutron diffraction values.

NMR Crystallography: Evaluation of Hydrogen Positions in Hydromagnesite by 13C{1H} REDOR Solid‐State NMR and Density Functional Theory Calculation of Chemical Shielding Tensors

“…[19] XRD suffers,h owever, from being insensitive to low atomic number (Z) nuclei, such as hydrogen;h ence,i ns ome cases,t he structural models produced by XRD are not sufficiently accurate for understanding how the resulting chemistry is directed by coordination. [20] Creative efforts in the emerging field of NMR crystallography [21][22][23][24][25] have pushed the use of NMR as ac omplementary tool for the elucidation of precise atomic coordinates,especially protons,int he unit cell.NMR crystallography,c ombines NMR, XRD,a nd computational chemistry, [26][27][28][29][30][31][32][33] to resolve atomic positions within crystals.Astrength of this scheme is using NMR chemical shifts and dipolar couplings between nuclei to validate ab initio quantum calculations for refining atom positions that are invisible to XRD. [34][35][36] Generally,i sotropic chemical shifts (d iso )a re used to identify spin-1/2 (nuclear spin, I = 1/2) NMR active species in structures,b ecause they are the most readily observed in NMR spectra.…”

“…[19] XRD suffers,h owever, from being insensitive to low atomic number (Z) nuclei, such as hydrogen;h ence,i ns ome cases,t he structural models produced by XRD are not sufficiently accurate for understanding how the resulting chemistry is directed by coordination. [20] Creative efforts in the emerging field of NMR crystallography [21][22][23][24][25] have pushed the use of NMR as ac omplementary tool for the elucidation of precise atomic coordinates,especially protons,int he unit cell.…”

Resolving alternative organic crystal structures using density functional theory and NMR chemical shifts

“…Given the importance of correctly positioned hydrogen atoms for describing hydrogen bonding arrangements, independent experimental evidence from a technique that is sensitive to hydrogen atom positioning would be highly desirable. Nuclear magnetic resonance (NMR) spectroscopy is highly sensitive to the local environment about the nuclei studied (for example, effective anisotropic displacement parameters derived from NMR chemical shis have been estimated to be consistently smaller than those associated with X-ray diffraction 7,8 ). "NMR crystallography" 9 is a rapidly developing eld that has recently been recognised by the IUCr in the form of a commission on NMR crystallography.…”

Solid‐State 13C NMR Evidence for Long Multicenter Intradimer Bonding in Zwitterion‐like Structures

“…[22][23][24][25][26][27] More recent work has also demonstrated that NMR shifts can reliably predict crystal structures [28][29][30][31] and provide estimates of uncertainties in individual atomicp ositions. [32,33] To understand the nature and strength of the intra-dimer bonding in b and d-[TTF···TCNE] the 13 Cs hift tensors, reported here for the first time, are compared to previously reported benchmark experimental data for TTF, [TTF] 2 2 + + , [19] TCNE, [TCNE] 2 2À ,and [TCNE] 2À . [13] Previously,atheoretical study has evaluated the validity of the proposedl ong multi-center bonds in the a, b and d phases of [TTF···TCNE].…”