Investigating discrepancies between experimental solid-state NMR and GIPAW calculation: N[dbnd]C–N 13C and OH⋯O 1H chemical shifts in pyridinium fumarates and their cocrystals

Emily K Corlett,Helen Blade,Leslie P Hughes,Philip J Sidebottom,David Walker,Richard I Walton,Steven P Brown

doi:10.1016/j.ssnmr.2020.101662

Abstract

An NMR crystallography analysis is presented for four solid-state structures of pyridine fumarates and their cocrystals, using crystal structures deposited in the Cambridge Crystallographic Data Centre, CCDC. Experimental one-dimensional one-pulse 1H and 13C cross-polarisation (CP) magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and two-dimensional 14N–1H heteronuclear multiple-quantum coherence MAS NMR spectra are compared with gauge-including projector augmented wave (GIPAW) calculations of the 1H and 13C chemical shifts and the 14N shifts that additionally depend on the quadrupolar interaction. Considering the high ppm (>10 ppm) 1H resonances, while there is good agreement (within 0.4 ppm) between experiment and GIPAW calculation for the hydrogen-bonded NH moieties, the hydrogen-bonded fumaric acid OH resonances are 1.2–1.9 ppm higher in GIPAW calculation as compared to experiment. For the cocrystals of a salt and a salt formed by 2-amino-5-methylpyridinium and 2-amino-6-methylpyridinium ions, a large discrepancy of 4.2 and 5.9 ppm between experiment and GIPAW calculation is observed for the quaternary ring carbon 13C resonance that is directly bonded to two nitrogens (in the ring and in the amino group). By comparison, there is excellent agreement (within 0.2 ppm) for the quaternary ring carbon 13C resonance directly bonded to the ring nitrogen for the salt and cocrystal of a salt formed by 2,6-lutidinium and 2,5-lutidinium, respectively.

Highlights

Figure 3: 1H (600 MHz) one-pulse magic-angle spinning (MAS) (60 kHz) nuclear magnetic resonance (NMR) spectra of 26L:F, 25L:FFA, 26AMP:F-H2 and 25AMP:FFA with stick spectra corresponding to gauge including projector augmented wave (GIPAW) calculated chemical shifts for the geometry optimised crystal structures
An NMR crystallography study has been presented that reports 1H, 13C chemical shifts and 14N shifts for four differently substituted pyridine molecules and fumaric acid that occur as two salts and two cocrystals of a salt in the solid state
The focus of this paper is on two chemical environments for which a greater discrepancy is observed between experiment and GIPAW calculated chemical shifts that goes beyond the typically encountered maximum of 1% of the chemical shift range

Summary

Introduction

The NMR crystallography approach has been increasingly utilised to provide a detailed characterisation of solid systems, whereby solid-state magic-angle spinning (MAS) NMR and density functional theory (DFT) calculations, in particular using the gauge including projector augmented wave (GIPAW) method,[1] are used alongside complementary techniques such as X-ray diffraction (XRD).[2,3,4,5,6] The power of GIPAW has been demonstrated in numerous applications, notably providing a link between crystal structures and NMR parameters, aiding both their development and validation[7,8,9] as well as adding further insight to investigations of intermolecular interactions.[10,11,12,13,14,15]A key consideration is the level of agreement between experiment and calculation. We report the identification of a 1H and a 13C specific chemical environment within the systems listed in Table 1 (this Table states the CSD reference and number as well as the original literature reference for the crystal structure, and the shorthand names used here) whose chemical shifts exhibit larger than expected discrepancies between experiment and GIPAW calculation.

Results

Conclusion