Qualitative Study of Substituent Effects on NMR 15N and 17O Chemical Shifts (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.
Magnetic field-induced effects on NMR properties
Journal of Magnetic Resonance, 2017
In principle, all the NMR observables, spin-spin coupling J, nuclear shielding s and quadrupole coupling q, are magnetic field-dependent. The field dependence may be classified into two categories: direct and indirect (apparent) dependence. The former arises from the magnetic field-induced deformation of the molecular electronic cloud, while the latter stems from a slightly anisotropic orientation distribution of molecules, due to the interaction between the anisotropy of the molecular susceptibility tensor and the external magnetic field. Here we use 1,3,5-D3-benzene as a model system to investigate the indirect effect on the one-bond 1 H-13 C and 2 H-13 C spin-spin couplings (J couplings) and the 2 H quadrupole coupling. Experiments carried out at four magnetic fields (4.7, 9.4, 14.1, and 18.8 Tesla) show that the indirect effect is significant already at the magnetic fields commonly used in NMR spectrometers. A joint fit of the data extracted at the different field strengths provides experimental results for the susceptibility anisotropy, 2 H quadrupole coupling constant and the related asymmetry parameter as well as the one-bond CH and CD coupling constants extrapolated to vanishing field strength. The field-induced contributions are found to exceed the commonly assumed error margins of the latter. The data also indicate a primary isotope effect on the one-bond CH coupling constant. There is a tendency to further increase the magnetic field of NMR spectrometers, which leads to more pronounced indirect contributions and eventually significant direct effects as well.
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.
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.
Paramagnetic NMR chemical shift theory : combined ab initio /density-functional theory method
2017
In this thesis, the classic Kurland-McGarvey theory for the nuclear magnetic resonance (NMR) chemical shift is presented in a modern framework for paramagnetic systems containing one or more unpaired electrons. First-principles computations are carried out for the NMR shielding tensors in paramagnetic transition-metal complexes. A combined ab initio/density-functional theory (DFT) approach is applied to obtain the necessary electron paramagnetic resonance (EPR) property tensors, i.e., the g-tensor, zero-field splitting tensor (D) and hyperfine coupling tensors (A). In DFT, both the generalised-gradient approximation and hybrid DFT are applied to calculate A. The complete active space self-consistent field theory (CASSCF) and N -electron valence-state perturbation theory (NEVPT2) are applied to calculate the gand D-tensors. Scalar relativistic effects are included at the second-order Douglas-Kroll-Hess level for the gandD-tensors and, forA, at the fully relativistic four-component ma...
Chemistry - An Asian Journal
The classical textbook explanation of variations of 1 H NMR chemical shifts in benzene substituted with an electrondonating (NH2) and electron-withdrawing (NO2) group in terms of substituent resonance effects was examined by an analysis of molecular orbital contributions to the total shielding. It was found that the -electronic system shows a more pronounced shielding effect on all ring hydrogen atoms, relative to benzene, irrespective of substituent R/R effects. For the latter, this is in contrast to the traditional explanations of downfield shift of nitrobenzene proton resonances, which are found to be determined by the -electronic system and oxygen in-plane lone pairs. In aniline, R effect of NH2 group can be used to explain fully the upfield position of meta-H signals and partly the upfield position of para-H signal, the latter also being influenced by the -system. The position of the lowest frequency signal of ortho-Hs is fully determined by -electrons.
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.
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.