# Ambiguities in Assigning Single‐Crystal NMR Data to Individual Atoms in the Crystal Structure: A Case Study of Hambergite, Be2BO3OH, by 9Be and 11B NMR Spectroscopy, XRD Measurements and DFT Calculations

## TL;DR

This paper uses NMR and DFT calculations to resolve ambiguities in assigning atomic resonances in the crystal structure of hambergite.

## Contribution

A new approach combining NMR, XRD, and DFT calculations is presented to resolve resonance ambiguities in crystal structures.

## Key findings

- NMR interaction tensors for 9Be and 11B in hambergite were determined using single-crystal NMR.
- DFT calculations and XRD confirmed the assignment of NMR resonances to specific atomic positions.
- Quadrupolar coupling constants and isotropic chemical shifts were precisely measured for 9Be and 11B.

## Abstract

The NMR interaction tensors of 9Be and 11B of hambergite, Be
BO
OH, were derived from single‐crystal NMR experiments. In the orthorhombic crystal structure of hambergite (which we redetermined by single‐crystal XRD, confirming the results of previous studies), both beryllium and boron atoms occupy Wyckoff position
8c, with atoms pairwise related by inversion symmetry. This leads to four magnetically independent 9Be and 11B atoms per site, which are observable in the NMR spectra. Unequivocal assignment of these resonances to atomic positions in the unit cell is generally impossible, as an analysis of the symmetry relations shows. For the hambergite system, this assignment ambiguity could be resolved with the help of DFT calculations using the VASP code, with the resulting eigenvectors compared with the experimental ones. Examination of 9Be–1H dipolar coupling effects, which could be detected in some of the 9Be spectra, in combination with XRD experiments to confirm the goniometer axis orientation, provided further spatial information and confirmed the assignment. The thus determined numerical values for the quadrupolar coupling constants
χ and isotropic chemical shifts
δiso are as follows: for 9Be[1]
222.6±0.6 kHz and 1.6 ppm, for 9Be[2]
−121.2±0.4 kHz and 1.4 ppm and for 11B[1]
2.648±0.004 MHz and 18.1 ppm.

The fundamental question of how to assign NMR resonances to specific atoms in the unit cell is addressed by using the 9Be and 11B spectra of the natural mineral hambergite as a model system.