NMR—the chemical shift—IV (original) (raw)
Related papers
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.
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.
NMR chemical shifts. Substituted acetylenes
The Journal of organic chemistry, 2004
The MPW1PW91/6-311+G(2d,p) and MP2/6-311+G(2d,p) GIAO nuclear shieldings for a series of monosubstituted acetylenes have been calculated using the MP2/6-311G(2d,p) geometries. Axially symmetric substituents such as fluorine may lead to large changes in the isotropic shielding but have little effect on the tensor component (zz) about the C[triple bond]C bond axis. On the other hand, substituents such as vinyl and aldehyde groups lead to essentially no difference in the isotropic shielding but are calculated to give a large zz paramagnetic shift to the terminal carbon of the acetylene group, without having much effect on the inner carbon. The tensor components of the chemical shifts for trimethylsilylacetylene, methoxyacetylene, and propiolaldehyde have been measured and are in reasonable agreement with the calculations. The downfield shift at the terminal carbon of propiolaldehyde along with a small upfield shift at the adjacent carbon has been found to result from the coupling of th...
NMR Chemical Shifts. 3. A Comparison of Acetylene, Allene, and the Higher Cumulenes
The Journal of Organic Chemistry, 1999
The 13 C NMR shieldings for acetylene, allene, and some higher cumulenes have been calculated at the MPW1PW91/6-311+G(2d,p) level using both the GIAO and IGAIM procedures. The calculations satisfactorily reproduce the tensor components of the shielding for both acetylene and allene, including the remarkably large difference in the components about the long axes of these molecules. A linear molecule such as acetylene has no paramagnetic shielding about the symmetry axis. The strong diamagnetic shielding about this axis for acetylene results in part from the energetically and spatially degenerate π-orbitals. With allene, the π-orbitals are energetically degenerate, but are not spatially degenerate, and as a result they cannot be mixed and cannot give a shielding diamagnetic term. Instead, in the presence of a magnetic field, they mix with virtual orbitals, leading to a downfield paramagnetic term. The shieldings for all the compounds are analyzed on an MO basis.
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.
NMR chemical shift substituent effects: 2-α-monosubstitutedN,N-diethylacetamides
Organic Magnetic Resonance, 1980
H NMR chemical shifts for some a-hetero-substituted N,N-diethylacetamides were recorded. The resonance assignments for the syn-and anti-methylene and -methyl protons have been made unambiguously through their aromatic solvent induced shifts and are opposed to the literature assignments for the N-methylene protons. An empirical relationship between the Charton polar (a,) and steric (v) parameters and the a-methylene proton resonances was found. The N-methylene proton chemical shifts also showed a qualitative dependence on the a-substituent electronegativity, while the N-ethyl methyl proton chemical shifts were related to the a-substituent steric effects. The Paulsen and Todt anisotropic model and the more populated rotamers proposed seem to explain the results very well.
The Journal of Physical Chemistry A, 2013
The interpretation of nuclear magnetic resonance (NMR) parameters is essential to understanding experimental observations at the molecular and supramolecular levels and to designing new and more efficient molecular probes. In many aromatic natural compounds, unusual 13 C NMR chemical shifts have been reported for out-of-plane methoxy groups bonded to the aromatic ring (∼62 ppm as compared to the typical value of ∼56 ppm for an aromatic methoxy group). Here, we analyzed this phenomenon for a series of aromatic natural compounds using Density Functional Theory (DFT) calculations. First, we checked the methodology used to optimize the structure and calculate the NMR chemical shifts in aromatic compounds. The conformational effects of the methoxy group on the 13 C NMR chemical shift then were interpreted by the Natural Bond Orbital (NBO) and Natural Chemical Shift (NCS) approaches, and by excitation analysis of the chemical shifts, breaking down the total nuclear shielding tensor into the contributions from the different occupied orbitals and their magnetic interactions with virtual orbitals. We discovered that the atypical 13 C NMR chemical shifts observed are not directly related to a different conjugation of the lone pair of electrons of the methoxy oxygen with the aromatic ring, as has been suggested. Our analysis indicates that rotation of the methoxy group induces changes in the virtual molecular orbital space, which, in turn, correlate with the predominant part of the contribution of the paramagnetic deshielding connected with the magnetic interactions of the BD CMet−H →BD* CMet−OMet orbitals, resulting in the experimentally observed deshielding of the 13 C NMR resonance of the out-of-plane methoxy group.
Steric effects on carbon-13 NMR shifts: carbon-hydrogen bond polarization contributions
Magnetic Resonance in Chemistry, 1998
The shielding observed in the chemical shifts of carbon atoms subjected to steric compression was originally attributed to carbonÈhydrogen bond polarization. There is increasing evidence that this is not universally the case and theoretical studies reveal that changes in dihedral angles may be responsible for many steric e †ects. Hydrogen shifts, bond lengths and charge distributions were used as probes for these e †ects in methylnorbornanes and similar tetracyclic norbornyl hydrocarbons. CarbonÈhydrogen bond polarization can make a signiÐcant contribution to shielding and can be distinguished from e †ects caused by changes in dihedral angles due to steric congestion.
Molecular Physics, 2000
The 'Gauge Including Atomic Orbitals' (CIAO) approach is used to investigate the question of intramolecular rotation. Ab initio CIAO calculations of NMR chemical shielding tensors carried out with GAUSSIAN 94 within the SCF-Hartree-Fock approximation are described. In order to compare the calculated chemical shifts with experimental ones, it is important to use consistent nuclear shieldings for NMR reference compounds like TMS. The influence of rotating functional groups X=CH3, CHO, NO2, NH2, CONH2, COOH or C6H5 on the shielding tensors in seven vinylic derivatives H2C=CH-X is studied; the molecules are propene, acrolein, nitroethylene, ethyleneamine, acrylamide, acrylic acid and styrene. We observe a marked dependence of nuclear shielding and chemical shift on the torsional movement. Different Boltzmann averages over the conformational states are considered and compared for gas phase, liquid and solid state NMR. Their applicability to model cases for rigid or freely rotating molecules and for fixed molecules (e.g. polymers or proteins) with rapidly rotating groups is discussed and simple calculation models are presented. On the basis of this work it can be concluded that intramolecular rotation clearly affects the observed averages. Effects of up to 2 ppm have been observed for isotropic chemical shifts, and up to 17 ppm difference have been observed for individual tensor components, for example, of the carboxylic I3C atom in acrylic acid. The variation of the shielding tensor on a nucleus in a fixed molecular backbone resulting from an attached rotating group furthermore leads to a new relaxation mechanism by chemical shift anisotropy.