Predicting the NMR Spectra of Paramagnetic Molecules by DFT: Application to Organic Free Radicals and Transition‐Metal Complexes

Abstract

Nuclear shieldings, including the Fermi contact and pseudocontact terms, have been calculated with DFT methods in a variety of open-shell molecules (nitroxides, aryloxyl and various transition-metal complexes), thereby predicting (1)H and (13)C chemical shifts. In general, when experimental data are reliable a good agreement with experimental values is observed, thus demonstrating the predictive power of DFT also in this context. However, the general accuracy is lower than that for closed-shell species. A few inconsistencies in literature values are reconciled by reassigning some shifts. Structural, magnetic, and dynamic parameters have also been put into the Solomon-Bloembergen equations to predict signal line shapes, in particular those of signals that are difficult to locate or are undetectable. Guidelines are provided to predict the order of magnitude of relaxation rates. It is shown that DFT-predicted paramagnetic shifts can greatly assist in obtaining and understanding the NMR spectra of paramagnetic molecules, which generally require different experimental strategies and exhibit problems in detection and assignment.

High resolution NMR of free radicals: 13C magic angle spinning of two solid organic free radicals derived from 4,5-dihydro-1H-imidazol-3-oxide-1-oxyl and theoretical calculation of their NMR properties

“…This level is comparable to that used by Rastelli and Bagno [GIAO/cc-pVTZ//B3LYP/6-31G(d,p)] in their recent study on the use of DFT calculations to predict the NMR spectra of paramagnetic molecules, including free radicals. 13 These theoretical calculations correspond to isolated molecules, so consequently the optimized geometries could not agree with the structures found in the corresponding crystals. However, as the studied compounds have rather rigid geometries, we are confident that the calculated geometries are close to the experimental ones.…”

“…Samples were carefully packed in a 4-mm diameter cylindrical zirconia rotors with Kel-F end-caps. Ramped cross polarization 30,31 from 1 H was used to create transverse 13 C magnetization with contact time of 0.5 ms. The 1 H 90° pulse length was of duration 3.2 µs.…”

“…Rastrelli and Bagno 13 have calculated several types of paramagnetic molecules, including the four radicals depicted in Scheme 2. In most cases, they averaged the calculated absolute shieldings since the observed experimental chemical shifts, due to conformational freedom, correspond to average signals.…”

“…2 The purpose of the present paper is twofold. On one hand to record in the solid state the 13 C CPMAS NMR spectra of two nitronylnitroxide radicals 3 and 4, and on the other, to evaluate the utility of unrestricted DFT calculations to obtain absolute shieldings of these radicals and those of 4,4,5,5-tetramethyl-2-methyl-imidazoline-1-oxyl-3-oxide 1 and of 4,4,5,5-tetramethyl-2-(4'-hydroxyphenyl)-2-imidazoline-1-oxyl-3-oxide 2. Scheme 1.…”

“…10,11 Another important contribution to pyrazolyl-nitronylnitroxides is that of Catala, Feher, Amabilino, Wurst and Veciana who prepared compound 4. 2 Note that we published in 1995 a 13 C NMR study in solution of paramagnetic Co(II) complexes. 12 The present paper is one of the rare publications where experimental and theoretically calculated NMR chemical shifts of radicals are reported.…”