Our research interests involve the combined application of theory and experiment in structural-mechanistic studies. Our research work is centered on the generation and investigation of electron deficient carbocations by NMR spectroscopy, ab initio, DFT, IGLO and GIAO calculations. This include the structures of some of the most important carbocation intermediates, effects of heteroatoms on the stability of carbocations and their role in the electrophilic substitution reactions.
Our work is also concerned with the possible role of onium dications (superelectrophiles) in superacid catalyzed reactions. Some electrophiles, such as carboxonium ions, nitronium ion and related systems are capable of further interaction with Bronsted or Lewis superacids can be greatly activated in superacidic systems. Such further interactions produce superelectrophiles, which are highly reactive, energetic, high lying intermediates of dipositive nature. They are the de facto reactive intermediates (such as NO2H2+, CH3COH2+ etc.) of many electrophilic reactions in superacidic systems.
We also engaged in the computational study of the higher coordinate main group hydriodo cations. Such studies are very useful to identify new and exciting hypercoordinate carbonium (such as CH73+), boronium, alonium, sulfonium and ammonium ions. The real significance of these ions is that they led to the realization of electrophilic substitution and related transformations of saturated hydrocarbons (alkanes) and in general of electrophilic activation of C-H and C-C single bonds. CH5+ is considered the parent of nonclassical carbocations containing a five coordinate carbon atom. The cation preferred a Cs symmetrical structure with a two-electron three-center (2e-3c) bond as originally suggested by Olah et al in 1969. Olah also showed that the parent six coordinate carbocation, diprotonated methane (CH62+), has two 2e-3c bonding interactions in its minimum-energy structure. We later showed that even the parent seven coordinate carbocation, triprotonated methane (CH73+) is an energy minimum and has three 2e-3c bonding interactions in its minimum-energy structure. These results indicate the general importance of 3c-2e interactions in protonated alkanes. Schmidbaur and his associates have prepared a variety of highly interesting higher coordinate boron, carbon, nitrogen, oxygen, phosphorus and sulfur gold complexes and determined their X-ray structures.
Boron and carbon are consecutive first-row elements. Hexacoordinate dipositively charged carbonium dications (CH62+) are isoelectronic with the corresponding monopositively charged hexavalent boron ions (BH6+). We have calculated the intriguing structures of tetracoordinate BH4+ and hexacoordinate boronium ion, BH6+ (Fig. 1). Structure of BH4+ is a planar C2v symmetrical with a 2e-3c bond. Structure of BH6+ is a C2v symmetrical with two 3c-2e bonds. The structures of BH4+ and BH6+ were found to be isostructural with their isoelectronic carbon analogs CH42+ and CH62+, respectively.
Fig. 1. Formation of six coordinate BH6+ and seven coordinate BH72+.
We have suggested that BH6+ could be made by the complexation of BH4+ and H2. DePuy et al were indeed able to prepare and observe the BH4+ and BH6+ experimentally in the gas phase by reacting BH2+ and H2 and BH4+ and H2, respectively. Several interesting structures of the protonated borane-Lewis base complexes H4BX+ (X = NH3, PH3, H2O, H2S, CO, N2, HF, HCl, CO2 and CS2) were also computed. All of the monocations H4BX+ are B-H protonated involving hypercoordinate boron with a 2e-3c bond and can be considered as onium-boronium ions. Interestingly formations of these cations by protonation of H3BX were shown to be highly exothermic.
Fig. 2. Calculated structures of N42+, N62+ and (N3)3N2+
We also engaged in the computational study of the polynitrogen ions such as N42+, N62+ and (N3)3N2+ dications (Fig. 2). These are very important highly energetic dications.