IGLO method

The IGLO method starts with the MO-LCAO-SCF calculation which produces the conventional (canonical) molecular orbitals [fa). These satisfy Hartree-Fock equations written in the form 

In the next step the set of canonical molecular orbitals is transformed to the set of localised molecular orbitals according to the criterion of Boys [21]. This means that the following functional is maximised 

When the magnetic perturbation is on, each (localised) molecular orbital can be written in the form of an expansion 

In these equations the first-order Fock operator is given as 

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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. 

Stabilization of the systems due to homoconjugation is discussed. 13C and nB NMR chemical shifts of the compounds were also calculated using the individual gauge for localized orbitals (IGLO) method <2000JOC5956>. 

The IGLO method, an important breakthrough for the calculation of magnetic properties, was developed in the mid-1980s by Kutzelnigg et al.2 After some... 

IGLO method, 125-126 Infrared spectroscopy, for TV-acyloxy-iV-alkoxyamides structure, 51, 52/— 54/, 55-56... 

NMR chemical shifts are also used to demonstrate the effects of homoaromatic interactions and they may be obtained from theoretical calculations. An example of this approach is the calculations of Svensson et al. (1991) on the 1,4-bishomotropylium ion, [45, R=H], They used the IGLO method of Schindler and Kutzelnigg (1982) to calculate the chemical shifts. 

Identiflability of structure, 32 319-320 IFMSI, see Interfacial metal-support interaction IGLO method (individual gauge for localized orbitals)... 

The structures of the intriguing dications 156 and 158 were also computed by DFT calculations. The C NMR chemical shifts were also calculated using both GIAO and IGLO methods. Both dications 156 and 158 can also be characterized as 4c/2e a-bishomoaromatic rectangular cyclobutane dications as well as frozen Woodward Hoffmann transition state analogs. 

Distinct evidence for the equilibration of bicyclobutonium with a minor isomer, bisected cyclopropylcarbinyl cation, comes from the ultra-low temperature CPMAS studies of Myhre, Webb and Yannoni25. They have observed a major isomer, the bicyclobutonium ion, with a l3C chemical shift of 15 ppm for the pentacoordinated carbon, and a minor bisected cyclopropylcarbinyl cation, whose cationic center s chemical shift was found to be at 235 ppm. The NMR chemical shifts of the cation are also comparable with those calculated by the IGLO method at that temperature26 27. The energies of these cations were shown to be nearly the same (AAH° = 0.05 kcalmol1). 

The conformational dependence of the 33S isotropic a in dimethyldisulphide, (CH3)2S2, has been studied by HF calculations using the IGLO method.121 (CH3)2S2 is a good model for the disulphide linkage in polypeptides and proteins. 33S a has been calculated as a function of the dihedral angle C-S-S-C. Predicted chemical shifts vary between —84.4 ppm (< C-S-S-C = 0°) and —42.3 ppm (Calculated values refer to CS2 and cannot be converted to the SO4 scale, since the absolute shielding of the SQj anion has not been reported... 

Observed and Calculated [IGLO Method (130)] Components of the Anisotropic Shielding Tensors of Phosphorus Oxides... 

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. 

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. 

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