Measurement and calculation of ¹³C chemical shift tensors in α-glucose and α-glucose monohydrate

Abstract

The 13 C chemical shift tensors of two crystalline forms of glucose (a-glucose and a-glucose·H 2 O) were determined from one-dimensional (1D) and two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy experiments. The experimental values determined from 1D and 2D methods are in very good agreement. Quantum chemical calculations were also carried out using the gauge-including projector augmented wave (GIPAW) method for plane-wave density functional theory (DFT) as implemented in the CAmbridge Serial Total Energy Package (CASTEP). The calculated 13 C chemical shifts were found to be in excellent agreement with experimental values for crystal structures that had their hydrogen atoms optimized and after an appropriate calibration was applied to convert calculated chemical shieldings into chemical shifts. The work presented here lays an important foundation for future solid-state NMR and quantum chemical calculation investigations of the various crystalline forms of cellulose.Résumé : On a déterminé les tenseurs des déplacements chimiques du 13 C de deux formes cristallines du glucose (a-glucose et a-glucose·H 2 O) à partir des valeurs obtenues par résonance magnétique nucléaire (RMN) monodimensionnelle (1D) et bidimensionnelle (2D), à l'état solide. Les valeurs expérimentales obtenues par les méthodes 1D et 2D sont en très bon accord. On a aussi effectué des calculs de chimie quantique en faisant appel à une onde améliorée par un opérateur de projection comportant une jauge de la théorie de la fonctionnelle de la densité (« GIPAW-DFT ») avec une onde plane telle, qu'appliquée dans le « CAmbridge Serial Total Energy Package (CASTEP) ». On a trouvé que les valeurs calculées pour les déplacements chimiques du 13 C sont en excellent accord avec les valeurs expérimentales pour des structures cristallines pour lesquelles les atomes d'hydrogène avaient été optimisés et après qu'une calibration appropriée a été appliquée pour convertir les déplacements chimiques calculés en déplacements chimiques. Le présent travail pose une fondation importante pour des études futures de RMN à l'état solide et pour des calculs de chimie quantique de diverses formes cristallines de la cellulose.Mots-clés : cristallographie par RMN, hydrates de carbone, cellulose, calculs de chimie quantique, structure cristalline.[Traduit par la Rédaction]

Powder crystallography of pharmaceutical materials by combined crystal structure prediction and solid-state 1H NMR spectroscopy

“…However, we note that even with the (ambiguous) assignment there are four predicted structures that are in good agreement with the unassigned experimental 13 C shifts ( Figure S3). In contrast, we note that the relative variation in the proton shifts is much larger, and only one structure (ranked number 5 by lattice energy) is in good agreement.…”

“…Figure 12 shows the rmsds between the assigned experimental and calculated 1 H chemical shifts for the set of the 50 lowest energy predicted structures of flufenamic acid (spanning just over 8 kJ mol -1 from the global lattice energy minimum). Here, unassigned shifts are again insufficient to identify the correct crystal structure ( Figure S5) and it is seen even more clearly than for flutamide that neither unassigned or assigned 13 C shifts discriminate strongly ( Figures S5 and S6); in fact, all of the structures lead to calculated 13 C chemical shifts that are in poor agreement with the data. This is probably because of the relatively limited range in chemical shifts covered by the carbon-13 NMR spectrum.…”

“…Due to a very long 1 H longitudinal relaxation time, it was not possible to record an INADEQUATE NMR spectrum to obtain the 13 C- 13 C correlations. As a result, the carbon-13 chemical shifts were assigned by comparison with the assigned 13 C shifts measured in a solution of CDCl 3 ( Figure S19). Using this traditional method, and the identification of quaternary carbons from the 2D 1 H-13 C HETCOR NMR spectrum ( Figure 6), we could clearly assign all except the C4, C13 and C11 peaks which are sufficiently close in the two spectra as to be ambiguous.…”

“…For small molecules, however, an approach based on the analysis of chemical shifts would be most attractive. There are today many examples of chemical shifts being combined with density functional theory (DFT) calculations for structure validation in organic molecular compounds with respect to known structures [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] . However, there are very few examples of ab initio structure determination from powders by NMR without a structural hypothesis.…”

Recent Advances in Solid-State Nuclear Magnetic Resonance Spectroscopy

“…(8,9,19,83,84) Occasionally, when models are obtained from neutron diffraction, geometry optimisation may not be necessary, as has been demonstrated for porous materials (85) and glucose. (86) Many molecular systems are bound by dispersion interactions and the underestimation by some DFT methods can have a significant effect on the optimisation of structural models. This is particularly important for systems such as molecular solids or metal-organic frameworks (MOFs) and aluminophosphates (AlPOs), where the structure can "fall apart", or the geometry can be substantially altered, during optimisation.…”