Topological analysis of the experimental electron density distribution functions for two polymorphs of paracetamol showed that strong H-bonds are responsible for the higher stability of crystal phase I, weak interactions for the higher density of phase II. This made it possible to finally resolve the contradiction between the relative stabilities and the densities of the two paracetamol polymorphs.

High resolution single crystal X ray study of 1 phenyl o carborane was carried out and the experimental and theoretical (B3LYP/6 311G** calculated) electron density distributions in the title compound were investigated. Character of electron delocalization in 1 phenyl o carborane was examined by analyzing the deformation electron density maps, maps of the Laplacian of the electron density, and maps of the electron localization function. Crystal structure forming X-H...H-X (X = C, B) intermolecular contacts were revealed and ana lyzed. The energies and geometric parameters of these contacts were compared with the results of quantum chemical calculations of the crystal structure and with characteristics of the same type of intramolecular contacts.

The icosahedral carboranes 1-C(6)F(5)-2-Ph-1,2-closo-C(2)B(10)H(10) (1), 1-(4'-F(3)CC(6)H(4))-2-Ph-1,2-closo-C(2)B(10)H(10) (2), 1,2-(4'-F(3)CC(6)H(4))(2)-1,2-closo-C(2)B(10)H(10) (3), 1-(4'-H(3)CC(6)F(4))-2-Ph-1,2-closo-C(2)B(10)H(10) (4), 1-(4'-F(3)CC(6)F(4))-2-Ph-1,2-closo-C(2)B(10)H(10) (5), 1,2-(4'-F(3)CC(6)F(4))(2)-1,2-closo-C(2)B(10)H(10) (6), 1,7-(4'-F(3)CC(6)F(4))(2)-1,7-closo-C(2)B(10)H(10) (7) and 1,12-(4'-F(3)CC(6)F(4))(2)-1,12-closo-C(2)B(10)H(10) (8), with fluorinated aryl substituents on cage carbon atoms, have been prepared in good to high yields and characterised by microanalysis, (1)H, (11)B and (19)F NMR spectroscopies, mass spectrometry, single-crystal X-ray diffraction and (spectro)electrochemistry. By analysis of <δ(11)B>, the weighted average (11)B chemical shift, a ranking order for the ortho carboranes 1-6 is established based on the combined electron-withdrawing properties of the C-substituents, and is in perfect agreement with that established independently by electrochemical study. In a parallel computational study the effects of a wide range of different substituents on the redox properties of carboranes have been probed by comparison of ΔE values, where ΔE is the energy gap between the DFT-optimised [7,9-R(2)-7,9-nido-C(2)B(10)](2-) anion and its DFT-optimised basket-shaped first oxidation product. The overall conclusion from the NMR spectroscopic, electrochemical and computational studies is that strongly electron withdrawing substituents significantly stabilise [7,9-nido-C(2)B(10)](2-) dianions with respect to oxidation, and that the best practical substituent is 4-F(3)CC(6)F(4). Thus attention focussed on the reduction of 1,2-(4'-F(3)CC(6)F(4))(2)-1,2-closo-C(2)B(10)H(10), compound 6. The sequence 6/[6](-)/[6](2-) appears reversible on the cyclic voltammetric timescale but on the longer timescale of macroelectrolysis the radical anion is only partially stable. EPR study of the electrogenerated monoanions from the ortho-carboranes 1-6 confirms the cage-centred nature of the redox processes. In contrast, the reduction of the meta- and para-carboranes 7 and 8, respectively, appears to be centred on the aromatic substituents, a conclusion supported by the results of DFT calculation of the LUMOs of compounds 6-8. Bulk 2-electron reduction of 6 affords a dianion which is remarkably stable to reoxidation, surviving for several hours in the open laboratory in the absence of halogenated solvents.

Using the “Atoms in molecules” topological theory the nature of the Ccarb−Ccarb bond variation in 1‐phenyl‐o‐carborane upon rotation of the phenyl ring was investigated within B3LYP/6‐31G** calculations. Despite the low barrier to the rotation of the phenyl ring, the bond paths linking hydrogen atoms of the phenyl ring and the ones of the carborane cage were found for all conformations. The length of the Ccarb−Ccarb bond increases by 0.031 Å without large energetic inputs due to back‐donation of the π‐density of the phenyl ring to the antibonding Ccarb−Ccarb orbital. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)