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Ab Initio/IGLO/GIAO-MP2 Study of Hypercoordinate Square-Pyramidal Carbocations

1998; American Chemical Society; Volume: 102; Issue: 15 Linguagem: Inglês

10.1021/jp972470u

1520-5215

G. K. Surya Prakash, Golam Rasul, George A. Olah,

Mass Spectrometry Techniques and Applications

The structures of hypercoordinate square-pyramidal carbocations were calculated at the correlated MP2/6-31G* level. The 13C NMR chemical shifts of the cations were also calculated using IGLO and GIAO-MP2 methods. The IGLO calculated data show only a reasonable correlation with the experimental 13C NMR chemical shifts. The correlated GIAO-MP2 calculated 13C NMR shifts, however, showed significant improvements over the SCF IGLO calculated chemical shifts. It was also found that among the C8H9+ isomers, the bishomo square-pyramidal cation 7 is only 0.2 kcal/mol more stable than the trishomocyclopropenium-type ion 8 at the MP4(SDQ)/6-31G*//MP2/6-31G* + ZPE level. Almost a calculated 1:2 equilibrium mixture of ions 7 and 8 seems to best represent the experimental NMR spectrum of C8H9+ at −80 °C. The structures and 13C NMR chemical shifts for the elusive (CH)5+ 1 and its monomethyl-substituted analogues 2 and 3 were also computed.