Chemical shift computational methods

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. 

Since a vast majority of NMR experiments are performed in solution, incorporation of solvent within the computation is a reasonable expectation. One might anticipate inclusion of solvent using a continuum method as described in Section 1.4.2. In a consistent manner, the molecular geometry should be optimized with the solvent field, and the chemical shifts computed with this geometry and with the solvent field. As will be demonstrated below, optimization in the solvent field turns out to oftentimes be unnecessary and the gas-phase geometry will suffice. 

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. 

Integrals of Electron Repulsion Molecular Magnetic Properties Mpller-Plesset Perturbation Theory NMR Chemical Shift Computation Ab Initio Nonadiabatic Derivative Couplings Normal Modes Reaction Path Following Spectroscopy Computational Methods Time-dependent Multi-configurational Hartree Method Transition Structure Optimization Techniques. 

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. 

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. 

Since about 1990, powerful post-Hartree-Fock approaches for the inclusion of electron correlation in chemical shift calculations have been developed and applied in main group chemistry (see NMR Chemical Shift Computation Ab Initio and NMR Chemical Shift Computation Structural Applications). Unfortunately, these correlated methods are computationally too demanding at present to be applied to transition metal complexes and clusters of chemically relevant size. In particular, the least expensive post-CHF method available, the MP2-GIAO approach, is expected to fail for systems with significant nondynamical correlation effects. 

Chemometrics Multivariate View on Chemical Problems Combinatorial Chemistry Factual Information Databases Fuzzy Methods in Chemistry Infrared Data Correlations with Chemical Structure Infrared Spectra Interpretation by the Characteristic Frequency Approach Inorganic Chemistry Databases Inorganic Compound Representation NMR Chemical Shift Computation Ab Initio NMR Chemical Shift Computation Structural Applications NMR Data Correlation with Chemical Structure Online Databases in Chemistry Spectroscopy Computational Methods Standard Exchange Formats for Spectral Data Structure and Substructure Searching Structure Determination by Computer-based Spectrum Interpretation Structure Generators Synthesis Design. 

One way to overcome the limitation of the cluster method is to take into account the crystal periodicity in the chemical shift computation through the use of software that includes the periodic boundary conditions [122]. 

Semiempirical methods are parameterized to reproduce various results. Most often, geometry and energy (usually the heat of formation) are used. Some researchers have extended this by including dipole moments, heats of reaction, and ionization potentials in the parameterization set. A few methods have been parameterized to reproduce a specific property, such as electronic spectra or NMR chemical shifts. Semiempirical calculations can be used to compute properties other than those in the parameterization set. 

There is one semiempirical program, called HyperNMR, that computes NMR chemical shifts. This program goes one step further than other semiempiricals by defining different parameters for the various hybridizations, such as sp carbon vs. sp carbon. This method is called the typed neglect of differential overlap method (TNDO/1 and TNDO/2). As with any semiempirical method, the results are better for species with functional groups similar to those in the set of molecules used to parameterize the method. 

In general, the computation of absolute chemical shifts is a very difficult task. Computing shifts relative to a standard, such as TMS, can be done more accurately. With some of the more approximate methods, it is sometimes more reliable to compare the shifts relative to the other shifts in the compound, rather than relative to a standard compound. It is always advisable to verify at least one representative compound against the experimental spectra when choosing a method. The following rules of thumb can be drawn from a review of the literature ... 

LORG (localized orbital-local origin) technique for removing dependence on the coordinate system when computing NMR chemical shifts LSDA (local spin-density approximation) approximation used in more approximate DFT methods for open-shell systems LSER (linear solvent energy relationships) method for computing solvation energy... 

TNDO (typed neglect of differential overlap) a semiempirical method for computing NMR chemical shifts... 

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