Abstract
Highly accurate chemical-shift predictions in molecular solids are behind the success and rapid development of NMR crystallography. However, unusually large errors of predicted hydrogen and carbon chemical shifts are sometimes reported. An understanding of these deviations is crucial for the reliability of NMR crystallography. Here, recently reported large deviations of predicted hydrogen and carbon chemical shifts of a series of solid pyridinium fumarates are thoroughly analyzed. The influence of the geometry optimization protocol and of the computational level of NMR calculations on the accuracy of predicted chemical shifts is investigated. Periodic calculations with GGA, meta-GGA and hybrid functionals are employed. Furthermore, molecular corrections at the coupled-cluster singles-and-doubles (CCSD) level are calculated. The effect of nuclear delocalization on the structure and NMR shielding is also investigated. The geometry optimization with a computationally demanding hybrid functional leads to a substantial improvement in proton chemical-shift predictions.
Highlights
In the past two decades, the progress of experimental and computational solid-stateNMR (SS-NMR) methods has led to the rapid development of NMR crystallography, which combines theory and experiment to obtain otherwise inaccessible insights into the structure and dynamics of solids [1]
The most remarkable success has been achieved by the gauge-including projector-augmented wave (GIPAW) procedure, which was developed for the prediction of magnetic-resonance parameters in crystalline materials [10]
We have recently investigated the factors contributing to the accuracy of the chemicalshift predictions of hydrogen nuclei in molecular solids and observed that the GGAcalculated proton chemical shifts deviated up to 1.5 ppm from the experiment, with the largest deviation observed for a hydrogen atom attached to sulfur, which has been explained by the neglect of relativistic effects in the calculations [21]
Summary
In the past two decades, the progress of experimental and computational solid-state. NMR (SS-NMR) methods has led to the rapid development of NMR crystallography, which combines theory and experiment to obtain otherwise inaccessible insights into the structure and dynamics of solids [1]. For instance, have recently reported an NMR crystallography study of a series of pyridinium fumarates and observed differences of up to 6 ppm between the experimental and calculated carbon chemical shifts [18]. In the above-mentioned study of pyridinium fumarates, large deviations (up to 1.9 ppm) of the GIPAW predictions were observed [18] These exceptions to the usually good accuracy of the predictions of SS-NMR parameters are worrying, because they undermine the credibility of the standard NMR crystallography approaches. We investigate the influence of the geometry optimization protocol and the computational level of NMR calculations on the accuracy of the predicted proton and carbon chemical shifts.
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