NMR relaxation by the antisymmetric component of the shielding tensor: a longer transverse than longitudinal relaxation time (original) (raw)
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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.
Journal of the American Chemical Society, 2002
Based on multifield NMR relaxation measurements and quantum chemistry calculations, a strategy aiming at the determination of the chemical shielding tensor (CST) in the liquid state is described. Brownian motions in the liquid state restrict the direct observation of CST to a third of its trace (isotropic shift), and even if CST can be probed indirectly through some spin relaxation rates (specific longitudinal relaxation rates, dipolar chemical shift anisotropy (CSA) cross-correlation rates), an insufficient number of experimental parameters prevents its complete determination. This lack of information can be compensated by using quantum chemical calculations so as to obtain the molecular CST orientation even if a relatively modest level of computation is used. As relaxation parameters involve a dynamic part, a prerequisite is the determination of the molecular anisotropic reorientation which can be obtained independently from dipolar cross-relaxation rates. A polycyclic molecule exhibiting a well-characterized anisotropic reorientation serves as an example for such a study, and some (but not all) carbon-13 chemical shielding tensors can be accurately determined. A comparison with solid-state NMR data and numerous chemical quantum calculations are presented.
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 Journal of Chemical Physics, 2010
Relaxation processes induced by the antisymmetric part of the chemical shift anisotropy tensor ͑henceforth called anti-CSA͒ are usually neglected in NMR relaxation studies. It is shown here that anti-CSA components contribute to longitudinal relaxation rates of the indole 15 N nucleus in tryptophan in solution at different magnetic fields and temperatures. To determine the parameters of several models for rotational diffusion and internal dynamics, we measured the longitudinal relaxation rates R 1 =1/ T 1 of 15 N, the 15 N-1 H dipole-dipole ͑DD͒ cross-relaxation rates ͑Overhauser effects͒, and the cross-correlated CSA/DD relaxation rates involving the second-rank symmetric part of the CSA tensor of 15 N at four magnetic fields B 0 = 9.4, 14.1, 18.8, and 22.3 T ͑400, 600, 800, and 950 MHz for protons͒ over a temperature range of 270Ͻ T Ͻ 310 K. A good agreement between experimental and theoretical rates can only be obtained if the CSA tensor is assumed to comprise first-rank antisymmetric ͑anti-CSA͒ components. The magnitude of the hitherto neglected antisymmetric components is of the order of 10% of the CSA.
Ab initio study of the NMR shielding constants and spin-spin coupling constants in cyclopropene
Theoretical Chemistry Accounts, 1993
Ab initio calculations of parameters which characterize the NMR spectrum are presented for the cyclopropene molecule. The London orbitals CHF (or GIAO-CHF, Gauge-Independent Atomic Orbital Coupled Hartree-Fock) results for the shielding constants are in good agreement with the experimental data, accurately determined, and with otherab initio values. The calculations of the NMR spin-spin coupling constants have been performed using the Multiconfiguration Time-Dependent Hartree-Fock (MC TDHF) approach. Different basis sets and MC SCF wavefunctions were used to estimate the accuracy of the results. Good agreement is obtained with the coupling constants estimated using the available experimental data.
The Journal of Chemical Physics, 2002
This work investigates the nature of second order effects resulting in solid state nuclear magnetic resonance ͑NMR͒, from cross-correlations between the quadrupolar and shielding couplings. Using an average Hamiltonian theory, it is shown that these effects can bring the nonsecular terms of the shielding interaction into the realm of conventional detection. Such terms include the antisymmetric components of the chemical shift tensor, which, although postulated to exist, have hitherto eluded direct experimental measurement. As numerical calculations supported these analytical derivations, an experimental study was undertaken to measure such components from the single-crystal rotation plot of a half-integer quadrupolar nucleus. A custom-made probehead was built, a data analysis procedure developed, and together these were used to analyze the satellite transition spectra arising from a 59 Co single crystal of cobalt͑III͒tris͑acetylacetonate͒. The results of repetitive studies on such sample are reported.
Physical chemistry chemical physics : PCCP, 2015
In this study, stereoelectronic interactions were considered to explain the experimental difference in the magnitude of the known heavy-atom effect on the (13)C NMR chemical shifts in cis- and trans-1,2-dihaloethene isomers (halo = F, Cl, Br or I). The experimental values were compared to the calculated values with various DFT functionals using both the nonrelativistic approach (NR) and the relativistic approximations SR-ZORA (SR) and SO-ZORA (SO). NBO and NLMO contributions to the (13)C NMR shielding tensors were determined to assess which stereoelectronic interactions have a more important effect on the shielding tensor in each principal axis system (PAS) coordinate. These analyses associated with the orbital rotation model and the HOMO-LUMO energy gap enable rationalization of trends between cis and trans isomers from fluorine to iodine derivatives. Both paramagnetic and SO shielding terms were responsible for the observed trends. It was possible to conclude that the steric inter...
Journal of biomolecular NMR, 1999
A method is presented that makes it possible to estimate both the orientation and the magnitude of the chemical shift anisotropy (CSA) tensor in molecules with a pair of spin 1/2 nuclei, typically (13)C-(1)H or (15) N-(1)H. The method relies on the fact that the longitudinal cross-correlation rate as well as a linear combination of the autorelaxation rates of longitudinal heterospin magnetization, longitudinal two-spin order and longitudinal proton magnetization are proportional to the spectral density at the Larmor frequency of the heterospin. Therefore the ratio between the cross-correlation rate and the above linear combination is independent of the dynamics. From the field dependence of the ratio both the magnitude and the orientation of the CSA tensor can be estimated. The method is applicable to molecules in all motional regimes and is not limited to molecules in extreme narrowing or slow tumbling, nor is it sensitive to chemical exchange broadening. It is tested on the 22 ami...