Convergence of Nuclear Magnetic Shieldings in the Kohn−Sham Limit for Several Small Molecules

Abstract

Convergence patterns and limiting values of isotropic nuclear magnetic shieldings were studied for several small molecules (N2, CO, CO2, NH3, CH4, C2H2, C2H4, C2H6, and C6H6) in the Kohn-Sham limit. Individual results of calculations using dedicated families of Jensen's basis sets (pcS-n and pcJ-n) were fitted toward the complete basis set limit (CBS) using a simple two-parameter formula. Several density functionals were used; calculated vibrational corrections (ZPV) applied; and, for comparison purposes, similar calculations performed using RHF, MP2, SOPPA, SOPPA(CCSD), and CCSD(T) methods and additionally, the aug-cc-pVTZ-J basis set. Finally, the CBS estimated results were critically compared with earlier reported literature data and experimental results. Among 42 studied DFT methods, the KTn and "pure" functionals produced the most accurate heavy atom isotropic nuclear shieldings.

Small changes—Huge influences: NMR chemical shifts of Ni(II) complexes with polar substrates

“…[12] Nevertheless, they also show that the accurate calculation of the chemical shifts is not straight forward and that probably higher levels of theory combined with larger basis sets and inclusion of solvent effects (and relativistic effects) are needed to get an accuracy of less than 5 ppm for all carbons. [38][39][40][41][42][43][44] But even the qualitative description is very helpful to understand the electronic effects resulting from structural variations. Two such effects were analyzed in detail:…”

DFT calculation of structures and NMR chemical shifts of simple models of small diameter zigzag single wall carbon nanotubes (SWCNTs)

“…In our recent work, [46] a very good performance of Jensen's pcSn basis sets, [41] as well as aug-cc-pVTZ-J basis set, [47 -52] primarily designed for calculations of indirect spin-spin coupling constants were observed in predicting nuclear isotropic shieldings in model molecular systems. Due to the size of our systems, we decided to make a compromise between accuracy and speed and selected the pcS-2 basis set and the hybrid B3LYP exchange-correlation density functional for all calculations of isotropic nuclear shieldings.…”

“…Due to the size of our systems, we decided to make a compromise between accuracy and speed and selected the pcS-2 basis set and the hybrid B3LYP exchange-correlation density functional for all calculations of isotropic nuclear shieldings. This level of theory is not sufficient to produce accurate NMR shieldings [46] and therefore we decided to calculate chemical shifts that could benefit from cancellation of systematic errors. For example, the empirical value of benzene carbon and proton shieldings in vacuum is 59.905 and 23.90 ppm (with rovibrational correction included [46,53 -55] ).…”

Theoretical modeling of 13C NMR chemical shifts—How to use the calculation results

“…Many functionals of general utility such as B3LYP (23), PBE1PBE (21), SSCB-D (42), and others (43) or functionals dedicated specially for magnetic response calculations, such as KT2 (44), have been recommended. There is a consensus that it should be a functional able to describe, at least to some degree, the electron correlation effects, which is ensured by hybrid type functionals.…”