NMR chemical shift computation

Buhl, M. 1998, NMR Chemical Shift Computation Structural Applications , in Encyclopedia of Computational Chemistry, Schleyer, P. v. R. (Editor-in-Chief), Wiley, Chichester. 

This indicates that the deviations are due to systematic errors, for example deficiencies of the applied methods and basis sets. DFT-based methods, such as GIAO/DFT calculations are known to overestimate paramagnetic contributions to the chemical shielding. This results, for critical cases with small HOMO/LUMO separations, in overly deshielded competed chemical shifts. Notorious examples for these deficiencies are 29Si or 13C NMR chemical shift computations of silylenes, silylium ions or dienyl cation .(5/-54) Taking into account the deficiencies of the applied method, and bearing in mind very reasonable correlations shown in Figures 4 and 5, the computational results do support the structural characterization of the synthesized vinyl cations. 

Basis Sets Correlation Consistent Sets Benchmark Studies on Small Molecules Coupled-cluster Theory Gradient Theory M0ller-Plesset Perturbation Theory NMR Chemical Shift Computation Ab Initio Spin Contamination Symmetry in Chemistry. 

Basis Sets Correlation Consistent Sets Carbocation Force Fields Coupled-cluster Theory Enthalpies of Hydrogenation G2 Theory Heats of Formation Hyperconjugation NMR Chemical Shift Computation Structural Applications NMR Data Correlation with Chemical Structure Proton Affinities. 

The definitive method for determining static structures is X-ray diffraction. Indeed, the 1976 Nobel Prize in Chemistry was awarded to Professor William N. Lipscomb for his work in determining structures of the boron hydrides by diffraction methods. However, it must be remembered that packing forces and solvation effects may change the preferred structure between solid state and solution. Another technique, which combines theory and experiment, has established a reliability on a par with X-ray diffraction for confirming structures. It is called the ab /n/n o/IGLO/NMR method (see NMR Chemical Shift Computation Structural Applications for an extensive discussion of calculated NMR chemical shifts) and combines calculated chemical shifts for a number of possible structures with the experimentally measured chemical shifts in solution. 

Density Functional Applications MNDO NMR Chemical Shift Computation Structural Applications Reaction Path Following Topological Methods in Chemical Structure and Bonding. 

Conformational Sampling Distance Geometry Theory, Algorithms, and Chemical Applications Macromolecular Structure Calculation and Refinement by Simulated Annealing Methods and Applications NMR Chemical Shift Computation Structural Applications NMR Refinement. 

Nuclear magnetic resonance (NMR) is one of the major experimental tools in structural chemistry and biochemistry. The prediction of NMR shifts from ab initio calculations has been demonstrated for isolated molecules (see NMR Chemical Shift Computation Ab Initio), but the development of a practical ab initio approach for the calculation on NMR shifts in solids has been accomplished only quite recently. Based on DFT-LDA and a pseudopotential plane wave approach, these authors have presented an approach which promises to be useful in the investigation of NMR shifts in crystalline solids as well as in amorphous materials and liquids. As a demonstration of this approach, Mauri et al. have calculated the H NMR shifts of LiH and HF in the state of isolated molecules and in a crystal. In the case of LiH the results show very little change between the free molecule (a = 26.6 ppm) and the crystal (cr = 26.3 ppm). However, a significant change is found for the crystal at high pressures (65 GPa), where the chemical shift increases to 31.2 ppm. A quite different picture is obtained for the HF molecule, where the theory predicts a shift of 28.4 ppm in remarkable agreement with the experimental value of 28.4 ppm. For the HF crystal, a shift of... 

Another important second-order property, which is considerably improved by MP correlation effects is the NMR chemical shift (see NMR Chemical Shift Computation Ab Initio and NMR Chemical Shift Computation Structural Applications). Calculations by Gauss have shown that for C shifts accurate values are already obtained at the GIAO-MP2 level. Further improvements are obtained by GIAO-MP3 and GIAO-MP4(SDQ) calculations. In the case of molecules with multiple bonds such as N2, the inclusion of T effects... 

Basis Sets Correlation Consistent Sets Complete Active Space Self-consistent Field (CASSCF) Second-order Perturbation Theory (CASPT2) Configuration Interaction Coupled-cluster Theory Density Functional Theory (DFT), Hartree-Fock (HF), and the Self-consistent Field G2 Theory Geometry Optimization 1 Gradient Theory Inter-molecular Interactions by Perturbation Theory Molecular Magnetic Properties NMR Chemical Shift Computation Ab Initio NMR Chemical Shift Computation Structural Applications Self-consistent Reaction Field Methods Spin Contamination. 

Atoms in Molecules Electron Transfer Calculations Electronic Wavefunctions Analysis Hyperconjugation Intermolecular Interactions by Perturbation Theory Localized MO SCF Methods Natural Orbitals NMR Chemical Shift Computation Ab Initio Rotational Barriers Barrier Origins Valence Bond Curve Crossing Models. 

After the introduction of IGLO, molecules of chemical interest became accessible to ab initio calculations. Early applications included the ab initio derivation of an increment system for hydrocarbons, a definite confirmation of the non-classical structure of the 2-norbomylcation, the assignment of the shift tensors of cyclopropane, bicyclobutane, and [1.1.1]-propellane, and the prediction of the F shift in SF2 prior to its measurement. These applications have been reviewed (see NMR Chemical Shift Computation Structural Applications). 

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