Magnetic Resonance in Chemistry

Methods for the first-principles calculation of the parameters of the electron paramagnetic resonance are reviewed. A full potential Kohn-Sham DFT-based approach for the calculation of hyperfine A- and Zeeman g-tensors in systems with periodic boundary conditions is developed. The robustness of the full potential scheme is demonstrated by considering a muonium defect in IIIA-VA zinc-blende semiconductors as well as the g-tensor of S2 anion radical in KCl host crystal lattice.

This review summarizes the results of systematic computational studies devoted to the calculation of EPR and PNMR parameters of aminoxyl radicals using methods rooted in the DFT and the N07D basis set.

We review the calculation of NMR J coupling in solid materials using the planewave pseudopotential formalism of Density Functional Theory. The methodology is briefly summarised and an account of recent applications is given. We discuss various aspects of the calculations which should be taken into account when comparing results with solid-state NMR experiments including; anisotropy and orientation of the J tensors, the reduced coupling constant, and the relation between J and crystal structure.

Calculations of absolute shieldings and NICS(1) values of 84 compounds (benzenes, silabenzenes, pyridines and phosphabenzenes) allowed to discuss the absolute shieldings in function of Brown σ_p⁺ substituent constant and to predict 71 new chemical shifts. The NICS(1) analysis of the aromaticity lead to the unexpected result for the silabenzenes, that it is almost insensitive to substituent effects. In these set of compounds, it can be concluded that aromaticity and its sensitivity to substituent effects are orthogonal.

Comparison between experimental and calculated DFT ¹⁹⁵Pt chemical shifts for [PtX_6−nY_n]²⁻ complexes (X = Cl, Br, F, I) shows that discrepancies increase [PtCl_6−nBr_n]²⁻ < [PtBr_6−nI_n]²⁻ < [PtCl_6−nI_n]²⁻ < [PtF_6−nBr_n]²⁻≈ [PtF_6−nCl_n]²⁻, and are correlated with the electronegativity difference Δχ between the halide ions (X⁻ and Y⁻) bound to Pt(IV). The computed ¹⁹⁵Pt sensitivity to Pt-X bond displacement, ∂ (δ¹⁹⁵Pt)/∂ (ΔPt-X) is large depending on the halide ion, decreasing from 24 800, 18 300, 15 700 to 12 000 ppm/Å for [PtF₆]²⁻, [PtCl₆]²⁻, [PtBr₆]²⁻ and [PtI₆]²⁻ respectively.

A combined theoretical and experimental study of the stereochemical behavior of ³¹P¹H spin–spin coupling constants has been performed in the series of trivinylphosphine and related trivinylphosphine oxide, sulfide and selenide. All ³¹P¹H spin–spin coupling constants involving phosphorus and either of the vinyl protons are found to demonstrate a marked stereochemical dependences with respect to the geometry of the coupling pathway and internal rotation of the vinyl group around the PC bond.

The influence of computational parameters in Car-Parrinello molecular dynamics simulations on computed NMR chemical shifts can be significant. We gauge the impact of the choice of the exchange-correlation functional, hard versus soft pseudopotentials, and basis set convergence on 1H NMR chemical shifts in liquid water and give hints where to focus when setting up a molecular dynamics run.

DFT methods are assessed for their capability to reproduce⁹³Nb chemical shifts of a number of inorganic and organometallic Nb complexes. An inadvertent error in the reported δ(⁹³Nb) values of some cyclopentadiene borane complexes is corrected.

Computations of ¹J(²⁷Al,¹⁷O), ¹J(³¹P,¹⁷O), and ²J(³¹P,²⁷Al) corroborate experimental results and provide potentially useful correlations between these coupling constants and the local structure in small molecules, grossite, and an aluminophosphate model system.

The NMR nuclear shielding tensors for the series LaX₃, with X = F, Cl, Br and I, have been computed using two-component relativistic density functional theory based on the zeroth-order regular approximation (ZORA). A detailed analysis of the inverse halogen dependence (IHD) of the La shielding was performed via decomposition of the shielding tensor elements into contributions from localized and delocalized molecular orbitals.

New insights into the calculation of NMR/NQR parameters using projected augmented wave and gauge including projected augmented wave approaches are proposed. Selected examples related to silicophosphates, explosives and cyclopropene derivatives illustrate these topics.

GIPAW computations of ¹³C NMR spectra have been used to assess the validity of two crystal structure determinations for terbutaline sulfate Form B, and one structure is shown to be defective. Two– dimensional NMR experiments, in combination with the computations, were used to assign signals to crystallographic atomic positions and probe hydrogen– bonding motifs.

Using recently acquired ultrahigh-field ²⁹Si NMR data for the pure silica zeolite ITQ-4, we report the results of calculations of ²⁹Si chemical shielding tensors using plane-wave density functional theory methods for periodic crystalline solids (as implemented in CASTEP) and compare to calculations on extracted molecular clusters performed with Gaussian. We also report the completion of the NMR crystallography of the zeolite ITQ-4 by exploring a number of structure refinement approaches involving quantum-chemical calculations.

The 4,5-dihydro-1,3,2-dithiazolyl radical g tensor is calculated by hybrid density functional (HDF) and multi-reference configuration interaction-sum over states (MRCI-SOS) methods. The tensor components are in very good agreement (1000 ppm) with those determined experimentally by electron paramagnetic resonance (EPR) spectroscopy. Although more costly, the MRCI technique, that includes the in-plane and out-of-plane interactions in a unbiased way, produced g tensor components that are more accurate than those obtained by the HDF method.

The ¹H NMR spectra of paramagnetic Ru(III) complexes are characterized by sharper line widths than most other transition metals, and are therefore easier to obtain. Contact shifts are still ‘unpredictable’, however. DFT calculations of magnetic properties (contact and pseudocontact shifts, relaxation rates) allow one to make such predictions in a useful manner.

We discuss the ability to reproduce NMR parameters in the case of phosphates materials through electronic structure calculation within density functional theory linear response, using the gauge-including projector-augmented wave. We emphasise on the crucial points, i.e. electronic structure and geometry, that should be considered to perform such calculations. Experimental and theoretical comparison of ³¹P, ¹H and ²³Na NMR data on a series of sodium phosphate systems are presented to illustrate the review.

Adequate analyses of canonical molecular orbitals (CMOs) can provide rather detailed information on the importance of different σ-Fermi contact (FC) coupling pathways. Their relative importance for transmitting the σ-FC contribution to a given spin–spin coupling is estimated by resorting to the expression of the FC term given by the polarization propagator formalism. Significant differences between the FC terms of ²J_C2H3 and ²J_C3H2 are taken as probes to gauge the differences in σ-ring currents for the five- and six-membered rings.

Molecular dynamics simulations are combined with DFT GIPAW-NMR calculations to devise novel procedures to process and interpret the solid-state NMR data acquired on binary borosilicate glasses. Important information on the structure such as bond angles distribution can then be estimated.

EFG values measured by ²³Na NMR experiments were reproduced with a level of accuracy comparable to the one achievable in diamagnetic compounds and allowed the assignment of observed ²³Na NMR signals. This work demonstrates the ability of a periodic PAW-based approach for the calculation of EFG parameters in paramagnetic compounds.