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(11)B and (13)C NMR spectra of so-far experimentally unknown carbon-rich cationic closo-carboranes C(3)B(n-3)H(n)(+) (n = 5, 6, 7, 10, 12) have been calculated at the GIAO-MP2 level and subsequently analyzed to reveal the nature of bonding in these potentially weakly coordinating cations. All previous rules derived for understanding (11)B NMR spectra of borane derivatives can be applied to realistically account for the corresponding shieldings. The correlated wavefunction for n = 5 and, to a lesser extent, for n = 10 seems to be decisive when trying to compute realistic shielding tensors, which is in agreement with the corresponding known dicarbaboranes. For other cluster dimensions noncorrelated wavefunctions also work well, in particular in relation to the corresponding dicarbaboranes. All these observations strongly support the fact that experimental availability of these unique clusters is possible.

Performance of various density functionals for use in geometry and energetics calculation of thiocarbonyl systems is evaluated. The three biologically significant systems such as 2-methyl-3-thiosemicarbazide, 1-acetyl-2-thiohydantoin and bornane-2-thione are selected for the geometry analysis. Energy of tautomerization of thioacetone and enthalpy of formation of 2-thiohydantoin are considered for analyzing the performance in energetics. Minnesota functionals such as M05-2X, M06-2X and range separated hybrid functional wB97 give best results in both geometry and energetics calculations. Studies on basis set effect showed reliable performance by 6-31 + G* and 6-31++G** over higher basis sets. Feasibility of tautomerization in the aforementioned thiones has been investigated using the best performing density functionals. The proposed thione–thiol tautomerization step during PPC-DC catalyzed decarboxylation reaction has also been investigated and the computed high energy barrier shows less feasibility of the reaction.

The unexplored carbon rich cationic closo carboranes, C3Bn−3Hn+1 (n=5, 6, 7, 10, 12) are investigated theoretically. The position isomers were calculated at the B3LYP/6-31G* level, and the charge distribution in the cluster is estimated by NBO analysis. The criterion of ring-cap orbital overlap compatibility along with the number of BC, CC, and BB bonds help in explaining the stability order in each category. The most stable isomer is the one with maximum ring-cap orbital overlap and largest number of BC bonds. The order of relative stability among the trigonal bipyramid is 1c>1b>1a′, where the stability is proportional to the number of CH caps over the small three-membered ring. The C3B3H6+ isomer with the one allyl C3 group (2b) is more favorable than the one with a cyclopropenyl group (2a). Among the C3B4H7+ isomers the stability order is 3e>3d>3c>3b>3a, which mostly depends on the ring-cap orbital overlap. In the bicapped square antiprism (4) where there is large number of isomers, the order follows the rule of ring cap compatibility and the number of BC bonds. The order of 5e>5d>5c>5b>5a obtained from the calculations is in perfect agreement with the above sited rules. Equations (1)–(5) devised for estimating the stability of isomers of C3Bn−3Hn+ indicate an increase in stability with cage size. The mono-positive charge of the isomers is distributed throughout the cage, making them suitable candidates as weakly electrophillic cations. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1542–1551, 2001

An ab initio study of ionic and ion pair displacement reactions involving allylic systems has been carried out at the RHF/6-31+G* level. The geometries and natural charges show the absence of conjugative stabilization in the ionic transition states, thus differing from traditional explanations. The high reactivity of allyl halides is explained by electrostatic polarization of the double bond. Substituent effects were also studied; in general, electron-withdrawing groups lower the barriers of the ionic SN2 reactions but increase the barriers of the ion pair reactions. The allylic reactions are compared with related benzylic systems. Hammett correlations give ρ of opposite sign for the ionic and ion pair displacement reactions, in agreement with some experimental results.