Influence of Hydrogen Bonding in the Paramagnetic NMR Shieldings of Nitronylnitroxide Derivative Molecules

Abstract

We apply our recently developed methodology for first-principles computations of paramagnetic NMR shieldings and explore the shieldings in a selected set of radicals that form core units in molecular magnets. The influence on these parameters of hydrogen bonding, that corresponds to the crystal environment, is the prime objective of the study. Nitronylnitroxide radicals with the hydroxyphenyl group in the ortho, meta, and para positions, as well as p-methoxyphenyl derivatives, are chosen for this purpose. The strong NO‚‚‚HO hydrogen bonding in the real crystal structure has been simulated by adding water molecules in the twelve molecular complexes investigated. Comparison of calculated and experimental data is made by taking the special features of the solid state and solution environments into account. The observed change in the shielding constant due to hydrogen bonding is explained by the spin delocalization picture; the dominating contribution is the temperature-dependent contact shielding containing the isotropic hyperfine coupling constant.

Four-Component Relativistic Density Functional Theory Calculations of NMR Shielding Tensors for Paramagnetic Systems

“…Due to these approximations, they were not able to obtain the isotropic pseudocontact shift. Therefore the method could be applied only to systems containing light elements 14 with large gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), because the SO effect otherwise can be big even for a relatively light system. 15 A new chapter in the development of computational approaches for pNMR came with the work of Moon and Patchkovskii, 16 where the authors presented a modern theory of the pNMR shift (recently an alternative theory was proposed by Van Den Heuvel and Soncini 17,18 ).…”

“…Due to these approximations, they were not able to obtain the isotropic pseudocontact shift. Therefore the method could be applied only to systems containing light elements with large gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), because the SO effect otherwise can be big even for a relatively light system …”

Advanced Magnetic Resonance Techniques for the Structural Characterization of Aminoxyl Radicals and Their Inorganic–Organic Nanocomposite Systems

“…1 H and 13 C liquid and solid‐state NMR spectra have been measured for various TEMPO derivatives, and the large Fermi contact shifts detected have been quantitatively confirmed by theoretical spin density calculations . A typical result is shown in Figure , which allows a rationalization of the isotropic shift trends in terms of the spin polarization pathway governed by simply concepts based on Pauli's and Hund's rules.…”

Paramagnetic Perturbation of the 19F NMR Chemical Shift in Fluorinated Cysteine by O2: A Theoretical Study

“…Unfortunately, tools for first-principles calculations and analysis of chemical shifts observed in paramagnetic NMR experiments are still in their infancy of development and can, at the moment, provide only limited support to the interpretations of experimental results. − Unlike the theory of the NMR spin Hamiltonian parameters in closed-shell molecules, the first-principles theory and the corresponding computational approaches, in the case of open shells, still remain largely unexplored due to the difficulties arising from the necessity to treat the electronic and nuclear Zeeman effects simultaneously . In recent years, many efforts have been made in this field to develop a computationally tractable yet accurate methodology for predictions of nuclear shielding constants or chemical shifts in open-shell molecules.…”

“…Rinkevicius et al reported the first fully first-principles calculation of nuclear shielding tensors in paramagnetic molecules and demonstrated the accuracy and applicability of the density functional theory (DFT) methods in paramagnetic NMR calculations. This methodology was used later on to study the influence of hydrogen bonds on nuclear shielding constants of 13 C and 1 H nuclei in paramagnetic nitronylnitroxide radicals . Recently, Moon and Patchkovskii extended the theory and derived a general formula for doublet open-shell systems with no thermally accessible electronically excited states.…”