The NMR ab initio IGLO Method

For experimentalists, NMR spectroscopy is one of the most important tools to investigate a newly synthesized compound. Nowadays, NMR measurements are standard in synthetic chemistry and, therefore, it is common that the first experimental information on an unknown chemical compound is obtained by recording its NMR spectrum. By using measured chemical shifts and coupling constants, and comparing these magnetic properties of the molecule under investigation with those of suitable reference molecules, it is in most cases possible to provide useful structural information on the new compound. 

However, if a newly synthesized compound contains unusual structural features, for which no useful reference data are available, it will be difficult to derive structural information from the measured NMR spectrum. In such a situation, theory can provide the missing link between measured NMR chemical 

NMR chemical shifts can be measured with high accuracy and, therefore, the reliability of IGLO chemical shifts enables the determination of molecular geometries. For example, it was found by Ottosson and Cremer [45] that the experimental shift value of 111 ppm [9] corresponds to a Si-C(benzene) bond length of 2.29 A. Such a distance is indicative of covalent bonding and, therefore, documents the loss of silylium ion character in the benzene complex. 

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. 

Although the geometry of a molecule is determined by 3K-6 internal coordinates, only a few of these coordinates are really critical in a specific case. For instance, any ab initio calculation can provide a reasonable estimate of C-H bond lengths or C-C-H bond angles of a hydrocarbon and it is well-known that these parameters do not change very much when the molecule is transferred from gas to solution phase. The same is true for many other geometrical parameters of a molecule, and there are just a few that critically depend on the molecular 

---

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. 

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. 

---