Relativistic Heavy-Neighbor-Atom Effects on NMR Shifts: Concepts and Trends Across the Periodic Table (original) (raw)
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Recent Advances in Theoretical and Physical Aspects of NMR Chemical Shifts
Kimika, 2015
In the first part of this review, theoretical aspects of nuclear magnetic shielding include (a) general theory, for example, newly developed approaches in relativistic theory of nuclear shielding, the relation between the spin-rotation tensor and shielding in relativistic theory, ab initio methods for treating open shell systems and a complete theory of chemical shifts in paramagnetic systems, the link between the definitions of the elusive concepts aromaticity and anti-aromaticity and the magnetic properties: the magnetizability tensor and the nuclear magnetic shielding tensor via delocalized electron currents and electron current maps, (b) ab initio and DFT calculations, both relativistic and non-relativistic, for various nuclei in various molecular systems using various levels of theoretical treatment. Physical aspects include (a) anisotropy of the shielding tensor, usually from solid state measurements, and calculations to support these, (b) shielding surfaces and rovibrational averaging, paying special attention to the sensitive relationship between shielding and bond angles or torsion angles that makes shielding such a powerful tool for structural/conformational determination in macromolecules, (c) chemical shifts that arise from isotopic substitution of NMR nucleus or neighboring nuclei, (d) intermolecular effects on nuclear shielding, and (e) absolute shielding scales.
The importance of relativistic effects on the NMR parameters in heavy-atom (HA) compounds, particularly the SO-HALA (Spin−Orbit Heavy Atom on the Light Atom) effect on NMR chemical shifts, has been known for about 40 years. Yet, a general correlation between the electronic structure and SO-HALA effect has been missing. By analyzing 1 H NMR chemical shifts of the sixth-period hydrides (Cs−At), we discovered general electronic-structure principles and mechanisms that dictate the size and sign of the SO-HALA NMR chemical shifts. In brief, partially occupied HA valence shells induce relativistic shielding at the light atom (LA) nuclei, while empty HA valence shells induce relativistic deshielding. In particular, the LA nucleus is relativistically shielded in 5d 2 −5d 8 and 6p 4 HA hydrides and deshielded in 4f 0 , 5d 0 , 6s 0 , and 6p 0 HA hydrides. This general and intuitive concept explains periodic trends in the 1 H NMR chemical shifts along the sixth-period hydrides (Cs−At) studied in this work. We present substantial evidence that the introduced principles have a general validity across the periodic table and can be extended to nonhydride LAs. The decades-old question of why compounds with occupied frontier π molecular orbitals (MOs) cause SO-HALA shielding at the LA nuclei, while the frontier σ MOs cause deshielding is answered. We further derive connection between the SO-HALA NMR chemical shifts and Spin−Orbit-induced Electron Deformation Density (SO-EDD), a property that can be obtained easily from differential electron densities and can be represented graphically. SO-EDD provides an intuitive understanding of the SO-HALA effect in terms of the depletion/ concentration of the electron density at LA nuclei caused by spin−orbit coupling due to HA in the presence of a magnetic field. Using an analogy between the SO-EDD concept and arguments from classic NMR theory, the complex question of the SO-HALA NMR chemical shifts becomes easily understandable for a wide chemical audience.
Direct magnetic-field dependence of NMR chemical shift†
Physical Chemistry Chemical Physics, 2020
Nuclear shielding and chemical shift are considered independent of the magnetic-field strength. Ramsey proposed on theoretical grounds in 1970 that this may not be valid for heavy nuclei. Here we present experimental evidence for the direct field dependence of shielding, using 59 Co shielding in Co(acac) 3 [tris(acetylacetonate)cobalt(III)] as an example. We carry out NMR experiments in four field strengths for this low-spin diamagnetic Co(III) complex, which features a very large and negative nuclear shielding constant of the central Co nucleus. This is due to a magnetically accessible, low-energy e g ' t 2g orbital excitation of the d 6 system. The experiments result in temperature-dependent magnetic-field dependence of À5.7 to À5.2 ppb T À2 of the 59 Co shielding constant, arising from the direct modification of the electron cloud of the complex by the field. First-principles multiconfigurational non-linear response theory calculations verify the sign and order of magnitude of the experimental results.
UNDERSTANDING NMR CHEMICAL SHIFTS
The NMR chemical shift serves as a paradigm for molecular electronic properties. We consider the factors that determine the general magnitudes of the shifts, the state of the art in theoretical calculations, the nature of the shielding tensor, and the multidimensional shielding surface that describes the variation of the shielding with nuclear positions. We also examine the nature of the intermolecular shielding surface as a general example of a supermolecule property surface. The observed chemical shift in the zero-pressure limit is determined not only by the value of the shielding at the equilibrium geometry, but the dynamic average over the multidimensional shielding surface during rotation and vibration of the molecule. In the gas, solution, or adsorbed phase it is an average of the intermolecular shielding surface over all the configurations of the molecule with its neighbors. The temperature dependence of the chemical shift in the isolated molecule, the changes upon isotopic substitution, the changes with environment, are well characterized experimentally so that quantum mechanical descriptions of electronic structure and theories related to dynamics averaging of any electronic property can be subjected to stringent test.
The significant role of relativistic effects in altering the NMR chemical shifts of light nuclei in heavy-element compounds has been recognized for a long time; however, full understanding of this phenomenon in relation to the electronic structure has not been achieved. In this study, the recently observed qualitative differences between the platinum and gold compounds in the magnitude and the sign of spin−orbit-induced (SO) nuclear magnetic shielding at the vicinal light atom (13C, 15N), σSO(LA), are explained by the contractions of 6s and 6p atomic orbitals in Au complexes, originating in the larger Au nuclear charge and stronger scalar relativistic effects in gold complexes. This leads to the chemical activation of metal 6s and 6p atomic orbitals in Au complexes and their larger participation in bonding with the ligand, which modulates the propagation of metal-induced SO effects on the NMR signal of the LA via the Spin−Orbit/Fermi Contact (SO/FC) mechanism. The magnitude of the σSO(LA) in these square-planar complexes can be understood on the basis of a balance between various metal-based 5d → 5d* and 6p → 6p* orbital magnetic couplings. The large and positive σSO(LA) in platinum complexes is dominated by the shielding platinum-based 5d → 5d* magnetic couplings, whereas small or negative σSO(LA) in gold complexes is related to the deshielding contribution of the gold-based 6p → 6p* magnetic couplings. Further, it is demonstrated that σSO(LA) correlates quantitatively with the extent of M−LA electron sharing that is the covalence of the M−LA bond (characterized by the QTAIM delocalization index, DI). The present findings will contribute to further understanding of of the origin and propagation of the relativistic effects influencing the experimental NMR parameters in heavy-element systems.
Chemical Physics Letters, 1998
The internuclear distance dependence of the electronic spin-orbit coupling contribution to nuclear magnetic resonance shielding constants -which causes the internal heavy-atom chemical shift of the proton shielding -is investigated using quadratic response theory. Calculations using a complete active space self-consistent field wavefunction on hydrogen iodide at different internuclear distances show a strong dependence of the spin-orbit coupling correction to the 1 H shielding constant on internuclear separation. These results, combined with comparative calculations on HCl and HBr, indicate that the conformational dependence of the shielding constant is qualitatively changed when relativistic spin-orbit coupling corrections are important. This change is observed for HI and HBr, but not for HCl. q 1998 Elsevier Science B.V. All rights reserved.
Tetrahedron, 1970
Abdmet-Tk caIcuIation of chemical shift values by the metbod used in Part I has been extended to a derivation of the shielding et&t of the ketonic CO group. Tbe calculations include consideration of both the magnetic anisotropy screening and tbe electric Fold scwning effect in this substituent. Using the complete series of mowketoandtostanes, tbe values of tbe anisotropies of tbe C==O doubk bond and of K, a parameter descriptive of tbe electric screening effect, have been derived for various, coincidental locations of the magnetic and electric dipoles along tbe >c-O bond. As with tbe C=C double bond, our results necessitate a substantial modification of the conventional pictute of the shielding 'cone' around the CO group.
Qualitative Study of Substituent Effects on NMR 15 N and 17 O Chemical Shifts
The Journal of Physical Chemistry A, 2009
A qualitative approach to analyze the electronic origin of substituent effects on the paramagnetic part of chemical shifts is described and applied to few model systems, where its potentiality can be appreciated. The formulation of this approach is based on the following grounds. The influence of different inter-or intramolecular interactions on a second-order property can be qualitatively predicted if it can be known how they affect the main virtual excitations entering into that second-order property. A set of consistent approximations are introduced in order to analyze the behavior of occupied and virtual orbitals that define some experimental trends of magnetic shielding constants. This approach is applied first to study the electronic origin of methyl-substituent effects on both 15 N and 17 O chemical shifts, and afterward it is applied to a couple of examples of long-range substituent effects originated in charge transfer interactions such as the conjugative effect in aromatic compounds and σ-hyperconjugative interactions in saturated multicyclic compounds.
Journal of Chemical Theory and Computation
We apply approximate relativistic methods to calculate the magnetic property tensors, i.e., the g-tensor, zero-field splitting (ZFS) tensor (D), and hyperfine coupling (HFC) tensors, for the purpose of constructing paramagnetic nuclear magnetic resonance (pNMR) shielding tensors. The chemical shift and shielding anisotropy are calculated by applying a modern implementation of the classic Kurland-McGarvey theory (J. Magn. Reson. 1970, 2, 286), which formulates the shielding tensor in terms of the g-and HFC tensors obtained for the ground multiplet, in the case of higher than doublet multiplicity defined by the ZFS interaction. The g-and ZFS tensors are calculated by ab initio complete active space self-consistent field and N-electron valence-state perturbation theory methods, with spin-orbit (SO) effects treated via quasidegenerate perturbation theory. Results obtained with the scalar relativistic (SR) Douglas-Kroll-Hess Hamiltonian used for the g-and ZFS tensor calculations are compared with nonrelativistically based computations. The HFC tensors computed using the fully relativistic four-component matrix Dirac-Kohn-Sham approach are contrasted against perturbationally SO-corrected nonrelativistic results, in the density-functional 1 theory framework. These approximations are applied on paramagnetic metallocenes (MCp 2) (M = Ni, Cr, V, Mn, Co, Rh, Ir), a Co(II) pyrazolylborate complex, and a Cr(III) complex. SR effects are found to be small for g and D in these systems. The HFCs are found to be more influenced by relativistic effects for the 3d systems. However, for some of the 3d complexes, nonrelativistic calculations give a reasonable agreement with the experimental chemical shift and shielding anisotropy. The influence of scalar relativity is strong for the 5d IrCp 2 system. This mixed ab initio/DFT technique, with a fully relativistic method used for the critical HFC tensor, should be useful for the treatment of both electron correlation and relativistic effects at a reasonable computational cost, to compute the pNMR shielding tensors in transition-metal systems.
1H and13C NMR chemical shifts and spin-spin coupling constants intrans- andcis-decalins
Magnetic Resonance in Chemistry, 2005
The NMR parameters characterizing the spectra of trans- and cis-decalins were determined from theoretical calculations and experimental spectra. The calculated values of the shielding constants are in good agreement with the measured chemical shifts, with a small but noticeable difference in accuracy for the bridgehead atoms. Of all the spin-spin coupling constants, only most of (1)J(C,C) and (1)J(C,H) values could be extracted from the spectra, and the corresponding computed values are in good agreement with experiment. It appears that the applied density functional theory (DFT) approach overestimates slightly the J(C,C) coupling and underestimates the differences between one-bond (1)J(C,H) coupling constants. For all these constants [J(C,C), J(C,H) and J(H,H)] through one to three bonds, which could not be obtained experimentally, the predicted values are in good agreement with the general rules relating spin-spin coupling to the number and spatial arrangement of the intervening bonds.