IGLO calculations

Thus, DFT-IGLO calculations of the 31P chemical shift without spin-orbital corrections yielded a value of S = 274, whereas additional spin-... 

The dications display NMR spectra, which are very similar to those of allylic carbocations. The terminal carbons of the dienylic and the allylic systems of the carbodications 107 and 108 show between 192 and 198, characteristic of the typical allylic carbocations. The IGLO-calculated chemical shifts closely match with those of the experimentally observed values for these carbodications. 

The trihydroxyyclopropenium ion (175) shows a single absorption in its C NMR spectrum, 128.7, which can be ascribed to the monoprotonated deltic acid from the DFT/IGLO calculations. Interestingly, the cation does not form the diprotonated species (176) even in Magic Acid (//q = -22). 

The four-, five-, and six-membered analogs (178,180, and 182) were also obtained from the diprotonation of squaric acid (3,4-dihydroxy-3-cyclobutene-l,2-dione, 177), tri-O-protonation of croconic acid (4,5-dihydroxy-4-cyclopentene-l,2,3-trione, 179), and tetra-O-protonated rhodizonic acid (5,6-dihydroxy-5-cyclohexene-l,2,3,4-tetraone, 181), respectively. These ions were prepared in either Magic Acid (1 1 FSOsH-SbFs) or fluorosulfuric acid at low temperature and characterized by NMR. Ab initio/IGLO calculations showed that di-O-protonated squaric acid (178) is planar and aromatic, whereas the polyprotonated croconic and rhodizonic acids (180 and 182) have more carboxonium ion character, and no indication was obtained for any significant contributing homoaromatic structures. 

Geometry at calculated [MP4(SDQ)-MP2/6-31G(d)j energy minimum. "Geometry calculated [MP2/6-31 G(d)] for fold angle of 93 ° obtained from best fit of IGLO calculated chemical shifts. 

A11 shifts in ppm relative to TMS. For the numbering of atoms, see structures. The IGLO calculations for 26 have been conducted with the 6-31 G (d) basis set. Experimental <5 Si value. 

BE standard basis set of triple-zeta quality, including polarization functions for all elements, often employed in IGLO calculations... 

IGLO calculation of the 29Si NMR chemical shift [DZ basis (C,H), basis II (Si)//HF/6-31G(d)] for the p-trimethylsilyltoluenium ion 319 and the trimethylsilylbenzenium ion 54 give <529Si = 60 and 80.9 ppm. These results are in line with the expected trend that more charge will be delocalized towards silicon in the trimethylsilylbenzenium ion 54 compared with the p-trim ethyl si lyltolueni um ion 319. The chemical shift of S = 82.1 ppm calculated for the p-triethylsilyltoluenium ion 318 agrees very well with the experimental value of S = 81.8 ppm in toluene. 

An additional argument is that the average IGLO calculated UB NMR chemical shift [37] based on the experimental model, which gives 8 = 81.1, is not close to the experimentally observed region of 8 = 65.7. However, the fully optimized structure shows very good agreement 8 = 67.0 (II7/MP2/6-31G ) and 66.3 (II7/CISD/DZP) [38]. 

Figure 4.3 IGLO-calculated H NMR shifts for the reaction of ethylene with (a) 1,3-butadiene or (b) cyclopentadiene. ...

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