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Three model systems with strong interactions bordering intra- and intermolecular were studied by experimental X-ray charge density analysis: hydrogen bonding, halogen interactions and π-stacking. The studies revealed that in all three cases the interaction involves multicentric two-electron covalent bonding. Nature of the strong hydrogen bond was studied on the well-known protonated dimer of water molecules, the Zundel cation. Our study shows that both O-H bonds in the central O-H-O fragment have a maximum electron density of about 1 e Å-3, and the negative value of the Laplacian, consistent with 2-electron/3-centre bonding ; O-H bonds have an order of 0.5. Recent studies showed that nature and strength of hydrogen and halogen bonds are similar. We studied two systems with very short and strong halogen bond N-Br···N. Pure N-bromosuccinimide was used as a standard, to evaluate charge density of an single N-Br bond. In its co-crystal with 3, 5-dimethylpyridine, the Br atom is displaced toward the acceptor by almost 0.4 Å and the maximum electron density in the contact Br···N is 0.4 e Å-3, indicating a significant covalent contribution. The strongest halogen bond, a symmetrical N-Br-N fragment, was studied in crystals of bromonium salt, bis(3-methylpyridine)bromonium perchlorate and it involves a two-electron/three-centre covalent bond N-Br-N. Our studies of π-interactions of semiquinone radicals involved three types of stacks: closely interacting radical dimers ('pancake bonding') [3], trimers of partially charged radicals and stacks of equidistant radicals. X-ray charge density supported by quantum chemical modelling revealed considerable covalent contribution (i.e. spin coupling and mixing of SOMO orbitals) in all of them. Therefore, π-interactions between the radicals can be regarded two-electron multicentric covalent bonding.

The crystal structure of [4-damp])2[Cl4Q]3 (4-damp = 4-dimethylamino-N-methylpyridinium, Cl4Q = tetrachloroquinone) salt is built up from slipped columnar stacks of quinoid rings composed of closely bound trimers with the intra-trimer separation distance of 2.84 Å and total charge of -2 whereas the inter-trimer distance is 3.59 Å. The individual rings exhibit partial negative charges that are distributed unevenly among the three Cl4Qs in the trimer. The strong interactions within a trimer (Cl4Q)32- have a partially covalent character with two-electrons / multi-centered bonding, that is extended over three rings, plausibly termed as 'pancake bonding'. The electron pairing within this multicentre bond leads to the fact that the crystals are diamagnetic and act as insulators. The studies of the structure and nature of bonding are based on X-ray charge density analysis and density functional theory.

Charge density was studied for three model systems with strong interactions bordering intra- and intermolecular: hydrogen bonding, halogen interactions and π-stacking. Our studies revealed that in all three cases the interaction involves multicentric two-electron covalent bonding. Nature of the strong hydrogen bond was studied on the well-known Zundel cation. It has long been known that the strongest hydrogen bonds are in fact weak covalent 2-electron/3-centre bonds of order 0.5 ; our study indicated that the proton donor and acceptor can't be distinguished. Both O-H bonds have a maximum electron density of ca. 1 e Å-3, and the negative Laplacian indicates a dominantly covalent interaction. It is quite well known that hydrogen and halogen bonding are similar, and recent studies showed that they are also similarly strong. In a halogen bond D-X···A the covalent bond D-X is elongated and the halogen is displaced towards the acceptor. Therefore we studied three systems with varying strenght of the halogen bond. Pure N-bromosuccinimide was used as a standard, to evaluate charge density of an unperturbed N-Br bond. In a co-crystal of N-bromosuccinimide with 3, 5-dimethylpyridine, the Br atom is displaced towards the acceptor by almost 0.4 Å and the maximum electron density in the "intermolecular" Br···N contact is 0.4 e Å-3, indicating a significant covalent contribution. The strongest halogen bond was studied on a bromonium ion, whose central N-Br-N fragment is in fact a two-electron/three-centre covalent bond. π-stacking of planar radicals involves spin coupling and mixing of SOMO orbitals, so it also has a considerable covalent character. Our studies of semiquinone radicals involved three types of contact: closely interacting radical dimers (known as 'pancake bonding'), trimers of partially charged rings and stacks of equidistant radicals. X-ray charge density and quantum chemical modelling showed significant covalent contribution in all of them. Therefore, 'pancake bonding' can be regarded two-electron multicentric covalent bonding. Apparently, covalent bond can also be unlocalised.

Semiquinones are a class of stable organic radicals with a great potential for design of novel organic electronics and multifunctional materials. Since the radicals are planar, they typically form π-stacks, which stabilise crystal packing and determine magnetic and electrical properties of the crystals. Two types of stacks π-have been described: 1) Peierls-distorted with alternating short (< 3.1 Å) and long (> 3.35 Å) interplanar distances (i.e. comprising dimers of radicals with paired spins), which are diamagnetic and isolators and 2) stacks of equidistant radicals which are 1D antiferromagnetic and semiconductors. Despite its importance, π-stacking of planar organic radicals has been little studied from the fundamental point of view. Here we present a detailed study of a series of salts with different semiquinones: tetrachloro- (Cl4Q), tetrabromo (Br4Q) and 5, 6-dichloro-2, 3- dicyanosemiquinone (DDQ). We have correlated geometry of the π-interaction with electrical conductivity and magnetic properties, and used crystal engineering to optimise them. Electronegativity (i.e. electron-withdrawing) of the substituents plays a double role: i) it stabilises the radical by enhancing delocalisation of π electrons and ii) lowers the energy of the HOMO orbital, thus increasing band gap related to electronic transport. We have described a novel type of stacking with charge transfer between semiquinone radicals and neutral quinone molecules. Studies of X-ray charge density revealed fine details of electronic structure of the radical and the π- interactions of semiquinones.

Fine details of stacking interactions in three different types of π-stacked tetrachlorosemiquinone radical anions (Cl4Q) were studied by a combination of X-ray charge density, quantum chemical computation and atoms-in- molecules (AIM) analysis: 1) stacks of pancake- bonded radical dimers in triclinic polymorph of N- MePy·Cl4Q (N-MePy = N-methylpyridinium cation), 2) stacks of trimers of partially charged semiquinones in [4-damp])2[Cl4Q]3 (4-damp = 4- dimethylamino-N-methylpyridinium) and 3) stacks of equidistant radicals in orthorhombic polymorph of N-MePy·Cl4Q. For the first time, we provide experimental evidence (based on X-ray charge density) of two-electron multicentric covalent bonding (pancake bonding) between the radicals. Typical pancake-bonded radical dimers in 1) are characterised by short interplanar distance (2.86 Å) and multiple bonding (3, -1) critical points between the rings with maximum electron density exceeding 0.095 e Å-3 ; in addition, a (3, +3) critical point (local minimum of electron density) was also found, indicating a cage-like electronic structure [1]. The covalent contribution to total interaction in a dimer was calculated to be -9.4 kcal mol-1. Between the dimers, interplanar separation is 3.60 Å and only negligible electron density is found. In trimers there are two electrons shared between three closely interacting rings (interplanar separations are ca. 2.84 Å), and (3, -1) bonding critical points are found with maximum electron density of 0.077 e Å-3 ; there are also two (3, +3) local minima [2]. The calculated covalent contribution is -6.8 kcal mol- 1. Maximum electron density between the rings in a stack of equidistant radicals (interplanar separation of 3.17 Å) is much lower, 0.050 e Å-3, and there is no local minimum of electron density [1]. However, the HOMO orbitals extend between the rings, and the calculated covalent contribution is -2.9 kcal mol-1. This compound is a 1D semiconductor [3, 4], and its semiconductivity is explained by pancake bonding extending along the stack.