Associates / Fellows
Research Associate Professor of Chemistry
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+,
1. Formation of six coordinate BH6+ and seven
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.
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.
G.; Olah, G. A.; Prakash, G. K. S. "GIAO –MP2/SCF/DFT Calculated
NMR Chemical Shift Relationships in Isostructural Onium Dications Containing
Hypercoordinate Boron, Carbon, Aluminum and Silicon Atoms", J. Phys.
Chem. A, 2004, 108, 8456.
J. F.; Rasul, G.; Seidl, P. R.; Prakash, G. K. S., Olah, G. A., "Structures
and Stabilities of B2H2n2+ Dications (n
= 1 - 4)", J. Phys. Chem. A, 2003, 107,
G.; Dias, J. F.; Seidl, P. R.; Prakash, G. K. S., Olah, G. A., "Structures,
Nature of Bondings and Charge Distribution in Hydridoborane Dications BHn2+ (n = 1 - 8)", J. Phys. Chem. A, 2003, 107,
G.; Prakash, G. K. S., Olah, G. A., "Complexes of Guanidinium Ion (NH2)3C+ with the Super Lewis Acidic XH4+ (X = B and Al): Comparison
with XH3 and Protonated and Methylated Guanidinium Dications", Inorg. Chem., 2003, 42, 8059
G. A.; Prakash, G. K. S.; Rasul, G.; "Superelectrophilic Protio Methyl-
and Protio Dimethylmethyleniminium Dications" J. Org. Chem., 2002, 67, 8547
G.; Prakash, G. K. S.; Olah, G. A.; "DFT Investigation of Tri(amino)amine
N(NH2)32+ and Tri(azido)amine N(N3)32+ Dications and Related Mixed Amino(azido)ammonium Ions (N3)xN(NH2)4-x+ (x = 0 - 4) " Inorg. Chem., 2002, 41,
G. A.; Prakash, G. K. S.; Rasul, G.; "Ab Initio Study of the Structures
and Stabilities of the Dimer of Ethyl Cation, (C2H5+)2 and Related C4H102+ Isomers" J.
Org. Chem., 2001, 66, 9011.
G. A.; Prakash, G. K. S.; Rasul, G.; "Gitonic and Distonic Alkanonium
Dications (Diprotonated Alkane Dications CnH2n+42+,
n = 1 - 4)" J. Org. Chem., 2001, 66,
G. A.; Prakash, G. K. S.; Rasul, G.; "N62+ and
N42+ Dications and their N12 and N10 Azido Derivatives: DFT/GIAO-MP2 Theoretical Studies" J. Am. Chem.
Soc., 2001, 123, 3308.
G. A.; Rasul, G. "From Kekule's Tetravalent Methane to Five, Six and
Seven Coordinate Protonated Methanes", Acc. Chem. Res. 1997, 30, 245.