Stable carbocations. 220. Carbon-13 NMR spectroscopic study of potential tris- and bishomocyclopropenyl cations

Structure and NMR Spectra of the 2-Norbornyl Carbocation: Prediction of 1J(13C13C) for the Bridged, Pentacoordinate Carbon Atom

“…The calculated NMR chemical shifts of the nonclassical form are in good agreement with the experimentally measured NMR chemical shifts of the 2-norbornyl cation at −158 °C in SbF 5 /SO 2 ClF/SO 2 F 2 solution − The absence of highly deshielded (downfield) carbon atoms, a characteristic of the trivalent sp 2 carbon atoms, as occurs for example in tert -butyl cation and cyclopentyl cation, also supports the bridged nature of the 2-norbornyl cation. 1 GIAO-MBPT(2) NMR Chemical Shifts (δ, in ppm) of the Nonclassical 2-Norbornyl Cation a chemical shiftexpt b chemical shiftexpt b C 6 22.9221.2 H 8 , H 9 1.221.37 C 5 24.8120.4 H 10 , H 11 2.483.17 C 4 40.4437.7 H 1 , H 2 6.346.75 C 1 , C 2 121.79124.5 H 5 , H 6 2.072.13 C 3 , C 7 40.9736.3 H 3 , H 4 2.032.13H 7 2.952.82 a The GIAO-MBPT(2) NMR chemical shifts are calculated at the MBPT(2)/DZP optimized geometry employing a DZP basis on C atoms and a DZ basis on hydrogen atoms.…”

“…This shows that in the gas phase there is no energy barrier at the MBPT(2)/DZP level between the nonclassical and the classical forms, making the stable structure exclusively nonclas- 7 Observation of the C 1 and C 2 carbon 13 C chemical shifts of about 125 and C 6 chemical shift of about 20-30 supports a symmetrical bridged structure for the 2-norbornyl cation since five (or higher coordinate) carbons usually show shielded (upfield) 13 C chemical shifts. [26][27][28] The absence of highly deshielded (downfield) carbon atoms, a characteristic of the trivalent sp 2 carbon atoms, as occurs for example in tert-butyl cation and cyclopentyl cation, 29 also supports the bridged nature of the 2-norbornyl cation.…”

Carbocations in Superacid Systems

“…78 The X-ray crystal structure, the optimized geometry 1035 (MP2FC/6-31G*), and calculated 13 C NMR shift data 78 agree well. 222,1043 They gave instead the cyclopentenyl cation. (3.144)].…”

“…1056 Apparently, despite the strain imposed by the planar cyclobutane moiety in the polycyclic cage structure, the strong Coulombic repulsion that could lead to geometric distortion and, consequently, cancellation of bishomoaromaticity is successfully counterbalanced by the rigid molecular framework thus contributing to stabilization. Attempts to prepare methyl-and aryl-substituted trishomocyclopropenyl cations were, however, unsuccessful, 1043 The ethano-bridged analog 611 was also prepared from the 8-chloroticyclo [3.2.1.0 2,4 ]octane precursor. 222 Masamune et al 1012 were first able to prepare the ion 610 by the ionization of cis-3-chlorobicyclo[3.1.0]hexane in the superacidic media.…”

Three-Membered-Ring-Based Molecular Architectures

“…144 Interestingly, the cation derived from 134 was predicted by computations to be relatively stable. 145 Apart from several preparations of trishomocyclopropenyl cations of type 210 in superacidic media, 146 no experimental studies directed toward the tetracyclic hydrocarbons 135 and 209 have been published up to now. DFT computations at the B3LYP level of theory predict an enhanced stability for the aromatic planar dication 211 (C 6 H 6 2+ ) with essentially normal lengths of the cyclopropane-type carbon-carbon bonds.…”