GIAO-MP2 method

Quantum chemical calculations predict the primary ethylcation, C H/" to have /(-hydrido-bridgcd structure 5 which is 6-8 kcal/mol more stable than the Kekule line-bond structure for the primary cation 4. The ethylcation is not stable enough to be observable directly in superacid media. The NMR chemical shifts were calculated for both isomers 4 and 5 using the GIAO-MP2 method for CCSD optimized structures.23... 

A series of hypercoordinated square-pyramidal carbocations were optimized at the MP2/6-31G(d) level and the 13C NMR chemical shifts of the carbocations were calculated using IGLO-HF and GIAO-MP2 methods.86... 

The 13C, 15N and 170 NMR chemical shifts of some substituted methyl cations and the corresponding protonated dications were calculated by the GIAO-MP2 method for MP2/6-31G(d) optimized geometries.129... 

The calculated 13C and 15N NMR chemical shifts, using the IGLO-HF and GIAO-MP2 methods, however, deviate substantially from the experimental results. This is in contrast with the previously reported results on protonated guanidines 113... 

It shows l3C absorptions (-135 °C) at <5I3C 234.18 (C+), 21.21 (C2), 38.15 (C3, C4), 51.67 (C5), 43.9 (C6, C7). Compared to dicyclopropylcarbinyl cation, the l3C signal for the cationic center is shielded by 20 ppm, indicating cyclopropyl group participation in the stabilization of the cation. The chemical shifts were compared with the values obtained from GIAO-SCF and GIAO-MP2 methods. 

Biihl, M., Thiel, W., Fleischer, U., and Kutzelnigg, W., Ab initio computation of Se NMR chemical shifts with the IGLO-SCF, the GIAO-SCF, and the GIAO-MP2 methods, J. Phys. Chem. 99, 4000-4007 (1995). 

Using the GIAO-MP2 method (tzp/dz basis set), Rasul et aU " calculated the B NMR chemical shifts of hypercoordinate boron atoms in selected boron compounds and NMR chemical shifts of the corresponding isoelectronic and isostructural carbocations for systems not yet observed under long-lived superacidic conditions B NMR and NMR chemical shift values for the methonium ion CH5+ (5) and the corresponding neutral boron analog BHs (15) were shown to reproduce well the experimental values. Calculations were also performed for the six-coordinate carbocation (diprotonated methane, 18) seven-coordinate carbocation (triprotonated methane, 27) the... 

Equations (6.30) to (6.47) summarize the theoretical framework for the GIAO-MP2 method. 

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