Carbocations homoaromatic

For some homoaromatic carbocations the NICS values and chemical shifts have been calculated.105,106 IGLO-HF and GIAO-MP2 calculated 13C NMR chemical shifts for bishomoaromatic 7-norbornenyl 90 and 7-norbornadienyl cation 91 have also been reported.107... 

Kleinpeter and Koch have recently introduced spatial magnetic properties as a measure of homoaromaticity. These are calculated as through-space NMR shieldings (TSNMRS) and visualized as iso-chemical shielding surfaces (ICSF). The structures, the intramolecular flexibility, and the and C NMR chemical shifts are calculated on the MP2/6-311 + G level of theory. TSNMRS of benzene and the cyclopropenylium cation as prototype aromatic molecules published earlier are used as references for homoaromaticity in a number of homoaromatic carbocations. The existence and the degree of homoaromaticity are deduced from the TSNMRSs by comparing the experimental and computed H and NMR NMR chemical shifts. 

Hypercarbon chemistry is not restricted only for carbocations and their borane analogs but a few of examples of silicon- and germanium-containing bridged and homoaromatic compounds are also known. These will be discussed briefly in this section. 

Secondary and tertiary carbocations derived from cage hydrocarbons 89 Carbenium ions with formal tt systems 98 Two-electron aromatic and homoaromatic ions 101 Six-electron aromatic ions 108 Phenyl-substituted carbocations 110 Solution reactivity 116... 

The carbocation in [2-hydroxyhomotropylium][SbCl0] [374] (mpt. 89-90°C [737] Fig. 11.30a) is found to be homoaromatic, whereas that in the related [1-ethoxyhomotropylium] [SbCl0] (mpt. 103-104°C [483] Fig. 11.30hl is not. Cation-anion interactions are confined to ionic and van der Waals interactions. Both of these salts are unstable in air. 

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