NMR shielding tensors

The methods listed thus far can be used for the reliable prediction of NMR chemical shifts for small organic compounds in the gas phase, which are often reasonably close to the liquid-phase results. Heavy elements, such as transition metals and lanthanides, present a much more dilficult problem. Mass defect and spin-coupling terms have been found to be significant for the description of the NMR shielding tensors for these elements. Since NMR is a nuclear effect, core potentials should not be used. 

Since HF calculations have a tendency to underestimate the N—N and the C—N bond lengths in triazoles [98JPC(A)620, 98JPC(A) 10348], the structural parameters should be computed at least at the DFT or MP2 levels. This is particularly true if electron-donating substituents are attached to the ring. Nitrogen NMR shielding tensors were computed for a set of methylated triazoles and tetrazoles but will be discussed in the context of tetrazoles (cf. Section IV,B). 

Schreckenbach, G., Ziegler, T., 1997a, Calculation of NMR Shielding Tensors Based on Density Functional Theory and a Scalar Relativistic Pauli-Type Hamiltonian. Application to Transition Metal Complexes , Int. J. 

A more complete list of early applications of QM/MM methods to enzymatic reactions can be found elsewhere [18, 35, 83, 84], Gao [85] has reviewed QM/MM studies of a variety of solution phenomena. QM/MM methods have also been used to study the spectra of small molecules in different solvents [86] and electrochemical properties of photosynthetic reaction centers within a protein environment [87-89], An approach has also been developed for calculation of NMR shielding tensors by use of a QM/ MM method [90]. 

Magnetic Balance and Explicit Diamagnetic Expressions for NMR Shielding Tensors... 

We apply this technique to study the effect of the spin-orbit coupling on an NMR shielding tensor and the shielding polarizability of the xenon atom. The shielding polarizabilities are defined as the second derivatives of nuclear shielding constants with respect to an electric field E... 

The calculation of NMR shielding tensors based on DFT and the GIAO approach has been implemented into the Amsterdam Density Functional code ADF (27,25-27). The non-relativistic as well as scalar relativistic (Pauli) implementations are the work of Schreckenbach and Ziegler (5-7) whereas the spin-orbit (Pauli) and ZORA NMR approaches were implemented by Wolff et al. (9,10). For the mathematical and technical details, the reader is referred to the literature. 

Optimized Geometries. As the elements of the NMR shielding tensor for a nucleus represent local properties, the most important geometrical parameter influencing the value of the 19F shielding constant can be expected to be the length of the bond which attaches the fluorine to the benzene ring. The optimized values of the carbon-fluorine... 

Let us now have a look at the possibility to modelize NMR shielding tensors using the same theoretical frame. In an appropriate PAS, the nine-component shielding tensor oap which describes the very small magnetic fields arising from electronic motions around the nucleus can be diagonalized leading to (49) ... 

Table VIII. Absolute O NMR shielding tensors (values in ppm) for the free water molecule (AEE = 56.1 eV) and a crystalline tungstate K2W04 (AEE = t ) assuming that <(a0/r)3> = 5.0 + q(O). Parametrization for K1WOd was r t(W) =...

NMR shielding tensors calculated using Hartree-Fock method the 6-... 

Y. Ruiz-Morales, G. Schreckenbach, and T. Ziegler, J. Phys. Chem., 100, 3359 (1996). Theoretical Study of 13C and 170 NMR Shielding Tensors in Transition Metal Carbonyls Based on Density Functional Theory and Gauge-Including Atomic Orbitals. 

The form of the free energy functional G appearing in the Polarizable Continuum Model is discussed in refs [35-37], Recently, Mennucci and Cammi have extended their integral equation formalism model for medium effects on shielding to the NMR shielding tensor for solutions in liquid crystals [38,39],... 

B. Mennucci and R. Cammi, Ab initio model to predict NMR shielding tensors for solutes in liquid crystals, Int. J. Quantum Chem., 93 (2003) 121-130. 

NMR shielding tensors were calculated using the Gauge-Independent Atomic Orbital (GIAO) method as implemented in Gaussian 98 (44-46). The basis sets and level of theory are the same as used in the geometry optimizations and frequency calculations mentioned above. 

Given the huge volume of work on the calculation of molecular properties in MO theory, it is surprising that there is so little effort in this direction within the VB area. The fact that the orbitals used in VB approaches are, as a mle, well-localised, suggests that these approaches would be very suitable for the calculation and interpretation of local properties, such as NMR shielding tensors and nuclear spin-spin coupling constants. 

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