IGLO theory

Recently, ab initio shielding calculations based on well-established theories, IGLO (individual gauge for localized orbitals)9, GIAO (gauge including atomic orbital)10 and LORG... 

Schleyer and coworkers have shown35 that in order to use effectively the IGLO/NMR approach to structure elucidation, it is necessary to employ high-level optimized geometries (e.g. at the correlated MP2/6-31G or DFT levels of theory) in order to obtain best agreement between computed and experimental chemical shifts. Early GIAO-SCF... 

Over the years, several computational methods have been developed. The variational theory can be used either without using experimental data other than the fundamental constants (i.e., ab initio methods) or by using empirical data to reduce the needed amount of numerical work (i.e., semiempirical data methods). There are various levels of sophistication in both ab initio [HF(IGLO), DFT GIAO-MP2, GIAO-CCSD(T)] and semiempirical methods. In the ab initio methods, various kinds of basic sets can be employed, while in the semiempirical methods, different choices of empirical parameters and parametric functions exist. The reader is referred to reviews of the subject.18,77... 

Quantum chemical calculations can provide a direct answer to this question and show whether Pauling s arguments are correct. Accordingly, we will discuss the silylium cation problem by focusing on the contribution that Quantum Chemistry can provide in this case. First, we will describe the quantum chemical methods needed for this purpose. Accordingly, Section 2 of this work is devoted to a discussion of the NMR/ab initio/IGLO method and its extension to density functional theory (DFT), namely the NMR/DFT/IGLO method. 

Figure 6. Selected geometrical parameters and NMR chemical shifts of neutral R3SiX compounds and neutral pentacoordinated complexes (S)R3SiX calculated at the HF/6-3lG(d) and IGLO-HF/[7s6p2d/5s4pld/3slp]//HF/6-3lG(d) level of theory. Bond lengths in A, angles in deg, NMR chemical shifts relative to TMS. Values in parentheses refer to experimental results. [41]...

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