$^{27}\text{Al }$ NMR chemical shift of $\text{Al}(\text{OH})_{4}^{-}$ from first principles. Assessment of error cancellation in NMR chemical shift computations in chemically distinct reference and targeted systems

TL;DR

This study evaluates the accuracy of first-principles calculations of $^{27}$Al NMR chemical shifts in $ ext{Al}( ext{OH})_{4}^{-}$, highlighting the challenges in error cancellation when using different reference systems and the impact of various computational factors.

Contribution

The paper investigates the limits of predictive accuracy for $^{27}$Al NMR shifts in $ ext{Al}( ext{OH})_{4}^{-}$ by analyzing error sources and their cancellation in chemically distinct systems.

Findings

Significant differences in intrinsic errors between analyte and reference hinder accurate predictions.

Solvent and thermal effects influence shielding calculations and are assessed via ensemble averaging.

Intrinsic error contributions vary notably between $ ext{Al}( ext{H}_{2} ext{O})_{6}^{3+}$ and $ ext{Al}( ext{OH})_{4}^{-}$, affecting predictive reliability.

Abstract

Predicting accurate NMR chemical shieldings relies upon cancellation of different types of error in the ab initio methodology used to calculate the shielding tensor of the analyte of interest and the reference. Often the intrinsic error in computed shieldings due to basis sets, approximations in the Hamiltonian, description of the wave function, and dynamic effects, is nearly identical between the analyte and reference, yet if the electronic structure or sensitivity to local environment differs dramatically, this cannot be taken for granted. Detailed prior work has examined the octahedral trivalent cation $\text{Al}(\text{H}_{2}\text{O})_{6}^{3+}$ , accounting for ab initio intrinsic errors. However, the fact that this analyte is used as a reference for the chemically distinct tetrahedral anion $\text{Al}(\text{OH})_{4}^{-}$ inspires the study of how these errors cancel in an attempt to understand the limits of predictive capability for accurately determining $^{27}\text{Al }$ shielding in $\text{Al}(\text{OH})_{4}^{-}$. In this work, we estimate the absolute shielding of $^{27}\text{Al }$ nucleus in $\text{Al}(\text{OH})_{4}^{-}$ at the coupled cluster level (515.1 $\pm$ 5.3 ppm). Shielding sensitivity to the choice of method approximation and atomic basis sets treatment has been evaluated. Solvent and thermal effects are assessed through ensemble averaging techniques using ab-initio molecular dynamics. The contribution of each type of intrinsic error is assessed for $\text{Al}(\text{H}_{2}\text{O})_{6}^{3+}$ and $\text{Al}(\text{OH})_{4}^{-}$ ions, revealing significant differences that fundamentally hamper the ability to accurately calculate the $^{27}\text{Al }$ chemical shift of $\text{Al}(\text{OH})_{4}^{-}$ from first principles.