The embedded ion method: A new approach to the electrostatic description of crystal lattice effects in chemical shielding calculations

Abstract

ABSTRACT:The nuclear magnetic shielding anisotropy of NMR active nuclei is highly sensitive to the nuclear electronic environment. Hence, measurements of the nuclear magnetic shielding anisotropy represent a powerful tool in the elucidation of molecular structure for a wide variety of materials. Quantum mechanical ab initio nuclear magnetic shielding calculations effectively complement the experimental NMR data by revealing additional structural information. The accuracy and capacity of these calculations has been improved considerably in recent years. However, the inherent problem of the limitation in the size of the systems that may be studied due to the relatively demanding computational requirements largely remains. Accordingly, ab initio shielding calculations have been performed predominantly on isolated molecules, neglecting the molecular environment. This approach is sufficient for neutral nonpolar systems, but leads to serious errors in the shielding calculations on polar and ionic systems. Conducting ab initio shielding calculations on clusters of molecules (i.e., including the nearest neighbor interactions) has improved the accuracy of the calculations in many cases. Other methods of simulating crystal lattice effects in shielding calculations that have been developed include the electrostatic representation of the crystal lattice using point charge arrays, full ab initio methods, ab initio methods under periodic boundary conditions, and hybrid ab initio/molecular dynamics methods. The embedded ion method (EIM) discussed here follows the electrostatic approach. The method mimics the intermolecular and interionic interactions experienced 347by a subject molecule or cluster in a given crystal in quantum mechanical shielding calculations with a large finite, periodic, and self-consistent array of point charges. The point charge arrays in the EIM are generated using the Ewald summation method and embed the molecule or ion of interest for which the ab initio shielding calculations are performed. The accuracy with which the EIM reproduces experimental nuclear magnetic shift tensor principal values, the sensitivity of the EIM to the parameters defining the point charge arrays, as well as the strengths and limitations of the EIM in comparison with other methods that include crystal lattice effects in chemical shielding calculations, are presented.

Chemical shift computations on a crystallographic basis: some reflections and comments

“…5 Cluster models have also been combined with classical point-charge arrays to simulate the long-range electrostatic effects of crystal lattices. 6,7 It is clearly very attractive to formally and efficiently exploit the translational repetition inherent in crystals, and in the last decade such a method for the prediction of magnetic resonance parameters has been developed. 8 Perhaps unsurprisingly, the method has its origins in the solid-state physics community, and makes extensive use of theoretical techniques developed to describe the behaviour of electrons in crystals.…”

Enhanced NMR Discrimination of Pharmaceutically Relevant Molecular Crystal Forms through Fragment-Based Ab Initio Chemical Shift Predictions

“…Before the widespread use of GIPAW DFT, molecular crystal chemical shift calculations often mimicked the crystalline environment using an individual molecule surrounded by a field of point charges 56,57 or by a few key neighboring molecules. Recently, there has been renewed interest in modernized versions of these sorts of methods.…”

Spin–orbit effects on the 119Sn magnetic-shielding tensor in solids: a ZORA/DFT investigation

“…37 Predicted isotropic 119 Sn chemical shifts in tin-containing solids calculated with plane-wave DFT techniques employing relativistic pseudopotentials at the ZORA level (without the SO component) are in good agreement with isotropic chemical shifts, although the calculated values do deviate by up to 200 ppm from experiment in some cases. 29,31,[46][47][48][49][50][51][52][53][54][55] The effect of hybrid functionals on computed magnetic shielding is also assessed. 16 In this study, we present calculations of the principal components of 119 Sn magneticshielding tensors for tin sites with different oxidation states and different co-ordination environments.…”