Issue 35, 2016

Testing the limits of NMR crystallography: the case of caffeine–citric acid hydrate

Abstract

The crystal structure of a new 1 : 2 caffeine–citric acid hydrate cocrystal is presented. The caffeine molecules are disordered over two positions, with the nature of the disorder confirmed to be static by 13C solid-state NMR. NMR linewidths in statically disordered systems reflect the distribution of local chemical environments, and this study investigates whether the disorder contribution to 13C linewidths can be predicted computationally. The limits of NMR crystallography calculations using density functional theory are tested by investigating how geometry optimisation conditions affect calculated NMR parameters. Careful optimisation is shown to reduce differences between 13C constants of symmetry-related sites to about 0.1 ppm. This is just sufficient to observe a correlation between calculated and experimental linewidths, and also show that systematic errors associated with geometry optimisation do not compromise other applications of “NMR crystallography”. In addition, the unit cell enthalpies calculated after careful optimisations provide insight into why the disordered structure is adopted.

Graphical abstract: Testing the limits of NMR crystallography: the case of caffeine–citric acid hydrate

Supplementary files

Article information

Article type
Paper
Submitted
27 Jun 2016
Accepted
22 Jul 2016
First published
02 Aug 2016
This article is Open Access
Creative Commons BY license

CrystEngComm, 2016,18, 6700-6707

Author version available

Testing the limits of NMR crystallography: the case of caffeine–citric acid hydrate

H. E. Kerr, H. E. Mason, H. A. Sparkes and P. Hodgkinson, CrystEngComm, 2016, 18, 6700 DOI: 10.1039/C6CE01453D

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

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