Ab initio IGLO method

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. 

The NMR/ab initio/IGLO method can provide this information provided reliable NMR chemical shifts are available from experiment. In the following, we... 

Arshadi M, Johnels D, Edlund U, Ottosson C-H, Crtaner D (1996) Solvated silylium cations structure determination by NMR spectroscopy and the NMR/Ab initio/IGLO method. J Am Chem Soc 118 5120-5131... 

In Sections 5, 6 and 7, three different approaches to the problem of silylium ions in solution are described. First, the typical gas phase versus solution phase ab initio (DFT) description of silyl compounds and silylium ions is given (Section 5). In Section 6, the NMR/ab initio/IGLO and NMR/DFT/IGLO methods are used to investigate solvation of silylium ions in different solvents. This work demonstrates how complex the solvation process of a silylium ion can be and, therefore, there is a need to generate silylium ions under well-defined situations in solution which simplify investigations. Out of this necessity, the idea of intramolecular solvation of silylium ions was born, which is discussed in Section 7. 

The dependence of calculated NMR chemical shifts on the molecular geometry is the basis of the NMR/ab initio/IGLO and NMR/DFT/IGLO methods. Schleyer and co-workers [33] and, independently, Cremer and co-workers [37,38] found in the case of 13C NMR chemical shifts that once an accurate geometry of a molecule was calculated, then in almost all cases measured and calculated NMR shifts did agree. 

Magnetic susceptibility tensors were calculated by the ab initio IGLO (individual gauge for localized molecular orbitals) method with a large basis set. The calculations showed... 

W. Kutzelnigg, U. Fleischer, M. Schindler, The IGLO-method Ab initio Calculation and Interpretation of NMR Chemical Shifts and Magnetic Susceptibilities, in NMR Basic Principles and Progress, Vol. 23, p. 165 ff., Springer, Berlin 1990. 

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... 

---