Electron Correlation or Basis Set Quality: How to Obtain Converged and Accurate NMR Shieldings for the Third-Row Elements?

Abstract

The quality of theoretical NMR shieldings calculated at the quantum-chemical level depends on various theoretical aspects, of which the basis set type and size are among the most important factors. Nevertheless, not much information is available on the basis set effect on theoretical shieldings of the NMR-active nuclei of the third row. Here, we report on the importance of proper basis set selection to obtain accurate and reliable NMR shielding parameters for nuclei from the third row of the periodic table. All calculations were performed on a set of eleven compounds containing the elements Na, Mg, Al, Si, P, S, or Cl. NMR shielding tensors were calculated using the SCF-HF, DFT-B3LYP, and CCSD(T) methods, combined with the Dunning valence aug-cc-pVXZ, core-valence aug-cc-pCVXZ, Jensen polarized-convergent aug-pcSseg-n and Karlsruhe x2c-Def2 basis set families. We also estimated the complete basis set limit (CBS) values of the NMR parameters. Widely scattered nuclear shieldings were observed for the Dunning polarized-valence basis set, which provides irregular convergence. We show that the use of Dunning core-valence or Jensen basis sets effectively reduces the scatter of theoretical NMR results and leads to their exponential-like convergence to CBS. We also assessed the effect of vibrational, temperature, and relativistic corrections on the predicted shieldings. For systems with single bonds, all corrections are relatively small, amounting to less than 4% of the CCSD(T)/CBS value. Vibrational and temperature corrections were less reliable for H3PO and HSiCH due to the high anharmonicity of the molecules. An abnormally high relativistic correction was observed for phosphorus in PN, reaching ~20% of the CCSD(T)/CBS value, while the correction was less than 7% for other tested molecules.

Getaway from the Geometry Factor Error in the Molecular Property Calculations: Efficient pecG-n (n = 1, 2) Basis Sets for the Geometry Optimization of Molecules Containing Light p Elements

“…Being based on different principles, they demonstrate different basis set behavior. In particular, the basis set convergence of electron correlation methods such as CCSD is the inverse polynomial in the highest angular momentum functions included in the basis set, while the convergence of the DFT methodology is exponential. , The DFT methods perform better with the polarization consistent basis sets, optimized partially by means of the molecular calculations for the polarization shells, to provide the best description of the distortions of the electron density in molecules. At the same time, coupled cluster correlation methods such as CCSD are better used with the correlation consistent basis sets, which are optimized toward the best recovering of the correlation energy of valence electrons by means of atomic calculations.…”

Theory and computation of nuclear shielding

“…The use of series of specially designed basis sets, for example Dunning- [45][46][47][48] or Jensen 49 -type, enabling a regular and smooth convergence of energy and energy related parameters is often used in benchmark calculations on small molecules. Using Hartree-Fock, DFT, MP2 and CCSD(T) methods it was observed 50 that high quality aug-cc-pVXZ Dunning's type basis set family produce scattered, instead of converging nuclear shielding parameters of thirdrow row elements. For example, such scattered patterns of nuclear shieldings for regularly improved basis sets were observed in case of compounds containing 31 P or 33 S nuclei.…”

Are we still chasing molecules that were never there? The role of quantum chemical simulations of NMR parameters in structural reassignment of natural products

“…Just like in the geometry optimization calculations, in most cases, both SSCC and σ are most commonly computed employing DFT or post-HF level of theory. Both parameters are very sensitive to electron correlation effects, and DFT and post-HF approaches take into account these effects (Rzepiela et al, 2022). Several benchmark studies have been conducted to compare different levels of theory in the reproduction of experimental data (Bally and Rablen, 2011;Flaig et al, 2014;Oliveira et al, 2021;Schattenberg and Kaupp, 2021).…”