Abstract
Motional averaging has been proven to be significant in predicting the chemical shifts in ab initio solid-state NMR calculations, and the applicability of motional averaging with molecular dynamics has been shown to depend on the accuracy of the molecular mechanical force field. The performance of a fully automatically generated tailor-made force field (TMFF) for the dynamic aspects of NMR crystallography is evaluated and compared with existing benchmarks, including static dispersion-corrected density functional theory calculations and the COMPASS force field. The crystal structure of free base cocaine is used as an example. The results reveal that, even though the TMFF outperforms the COMPASS force field for representing the energies and conformations of predicted structures, it does not give significant improvement in the accuracy of NMR calculations. Further studies should direct more attention to anisotropic chemical shifts and development of the method of solid-state NMR calculations.
Highlights
In silico molecular modelling methods, such as electronic structure methods and classical mechanics, have played a significant role in the elucidation of the structural and dynamic properties of molecular crystals over the past few decades (Beran, 2016; Gavezzotti, 2012; Abramov, 2016)
The tailor-made force field (TMFF) of free base cocaine employed for further molecular dynamics (MD) simulations was parameterized in the DREIDING format (Mayo et al, 1990) with van der Waals interactions described by the Lennard–Jones 9-6 (LJ 9-6) form
An overlay of the experimental structure and structure 1 from the crystal structure prediction (CSP) is shown in Fig. 3, demonstrating the good agreement between experiment and prediction; the non-hydrogen root-mean-square Cartesian displacement (RMSCD) between the experimental structure and structure 1 is 0.0556 A
Summary
In silico molecular modelling methods, such as electronic structure methods and classical mechanics, have played a significant role in the elucidation of the structural and dynamic properties of molecular crystals over the past few decades (Beran, 2016; Gavezzotti, 2012; Abramov, 2016). SS-NMR spectroscopy shows the robustness necessary to handle the vast majority of samples, and the resolution is extremely high so that small differences in the electronic environment of atoms can be identified (Apperley et al, 2012). It can interpret dynamic aspects such as disorder in molecular crystals (Martineau et al, 2014; Apperley et al, 2012).
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