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Predicting accurate NMR chemical shieldings relies upon cancellation ofdifferent types of error in the ab initio methodology used to calculate theshielding tensor of the analyte of interest and the reference. Often theintrinsic error in computed shieldings due to basis sets, approximations in theHamiltonian, description of the wave function, and dynamic effects, is nearlyidentical between the analyte and reference, yet if the electronic structure orsensitivity to local environment differs dramatically, this cannot be taken forgranted. Detailed prior work has examined the octahedral trivalent cation$\text{Al}(\text{H}_{2}\text{O})_{6}^{3+}$ , accounting for ab initio intrinsicerrors. However, the fact that this analyte is used as a reference for thechemically distinct tetrahedral anion $\text{Al}(\text{OH})_{4}^{-}$ inspiresthe study of how these errors cancel in an attempt to understand the limits ofpredictive capability for accurately determining $^{27}\text{Al }$ shielding in$\text{Al}(\text{OH})_{4}^{-}$. In this work, we estimate the absoluteshielding of $^{27}\text{Al }$ nucleus in $\text{Al}(\text{OH})_{4}^{-}$ at thecoupled cluster level (515.1 $\pm$ 5.3 ppm). Shielding sensitivity to thechoice of method approximation and atomic basis sets treatment has beenevaluated. Solvent and thermal effects are assessed through ensemble averagingtechniques using ab-initio molecular dynamics. The contribution of each type ofintrinsic error is assessed for $\text{Al}(\text{H}_{2}\text{O})_{6}^{3+}$ and$\text{Al}(\text{OH})_{4}^{-}$ ions, revealing significant differences thatfundamentally hamper the ability to accurately calculate the $^{27}\text{Al }$chemical shift of $\text{Al}(\text{OH})_{4}^{-}$ from first principles.

journal of chemical physics online/the journal of chemical physics/journal of chemical physics

$^{27}\text{Al }$ 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