IGLO method , chemical shift calculation

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. 

In this study we have presented the first simulations of solid state NMR spectra of polymers that are based on ab initio quantum chemistry. By combining statistical conformational models, empirical force-field optimization of geometries, and the Individual Gauges for Localized Molecular Orbitals (IGLO) method for the calculation of the chemical shift, it is possible to reproduce the shape and the width of the experimental solid state NMR spectra. 

For strained or unsaturated cations such as the cyclobutenyl cation (6), or the cyclohexadienyl cation (7) (i.e., protonated benzene), no good agreement between IGLO (or GIAO)-SCF and experimental chemical shifts could initially be obtained. In these cases, electron correlation effects need to be included in the chemical shift calculations, in addition to the geometry optimizations which have to be performed at correlated levels as well. This necessity could be demonstrated with the advent of electron-correlated methods such as GIAO-MP2. 

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>. 

---