Errors in the Calculation of 27Al Nuclear Magnetic Resonance Chemical Shifts

Abstract

Computational chemistry is an important tool for signal assignment of 27Al nuclear magnetic resonance spectra in order to elucidate the species of aluminum(III) in aqueous solutions. The accuracy of the popular theoretical models for computing the 27Al chemical shifts was evaluated by comparing the calculated and experimental chemical shifts in more than one hundred aluminum(III) complexes. In order to differentiate the error due to the chemical shielding tensor calculation from that due to the inadequacy of the molecular geometry prediction, single-crystal X-ray diffraction determined structures were used to build the isolated molecule models for calculating the chemical shifts. The results were compared with those obtained using the calculated geometries at the B3LYP/6-31G(d) level. The isotropic chemical shielding constants computed at different levels have strong linear correlations even though the absolute values differ in tens of ppm. The root-mean-square difference between the experimental chemical shifts and the calculated values is approximately 5 ppm for the calculations based on the X-ray structures, but more than 10 ppm for the calculations based on the computed geometries. The result indicates that the popular theoretical models are adequate in calculating the chemical shifts while an accurate molecular geometry is more critical.

Multinuclear Magnetic Resonance Spectroscopy and Density Function Theory Calculations for the Identification of the Equilibrium Species in THF Solutions of Organometallic Complexes Suitable As Electrolyte Solutions for Rechargeable Mg Batteries

“…Increasing the basis set with the B3LYP functional does not significantly change the chemical shifts. However, slightly better agreement with experiment is obtained with the mPW1PW91 functional with a larger basis set. , We note that calculating 27 Al chemical shifts is challenging for modern DFT methods, although the errors herein are somewhat larger than what is typically observed for 27 Al species. We also computed the 1 J ( 27 Al, 13 C) coupling constants for AlPh 4 – .…”

“…However, slightly better agreement with experiment is obtained with the mPW1PW91 functional with a larger basis set. 17,18 We note that calculating 27 Al chemical shifts is challenging for modern DFT methods, 19 although the errors . The computed value using mPWPW91/ 6-31+G(d,p) is 74.9 Hz (while the measured value from our experiments was 96.0 Hz), comparable to the computed value for Al(t-Bu) 3 of 76.3 Hz.…”

Biomass-mediated ZSM-5 zeolite synthesis: when self-assembly allows to cross the Si/Al lower limit

“…S24 † ) whatever C 5 or C 6 sugar nature. 45 One may therefore deduce, from those NMR data, that stronger interactions occur between organic molecules (BSST) and aluminates rather than silicates (step 3). Those evolving surfactant-like BSST molecules may interfere in the zeolite crystallisation process.…”

Al 27 NMR chemical shift of Al(OH)4− calculated from first principles: Assessment of error cancellation in chemically distinct reference and target systems

“…The calculation of 27 Al NMR shielding in the aluminate anion has been studied previously using HF, MP2 and DFT levels of theory. [8][9][10][11] In this work, the performance of a given QM method to yield reliable NMR data was assessed by comparing calculated chemical shifts of 27 11,12 Despite these studies, a critical assessment of NMR spectroscopy data from the perspective of absolute shielding computations is still necessary to advance our understanding of error cancellations. This assessment will allow theoretical chemical shifts to be confidently compared with experimental data, and will demonstrate the potential for chemical models to predict, and account for, the significant interactions that influence experimental NMR observables.…”