To understand the intermolecular interactions between chalcogen centers (O, S, Se, Te), quantum chemical calculations on pairs of model systems were carried out. For the oxygen derivatives, one of the components of the supermolecules consists of dimethyl ether, while the second component is either dimethyl ether (1) or ethynyl methyl ether (2) or methyl cyanate (3). The model calculations were also extended to the sulfur (4-6), selenium (7-9), and tellurium congeners (10-12). The MP2/SDB-cc-pVTZ, 6-311G level of theory was used to derive the geometrical parameters and the global energies of the model systems. A detailed analysis based on symmetry adapted perturbation theory (SAPT) reveals that induction and dispersion forces contribute to the bonding in each case. For 1-3 the electrostatic energy also contributes to the intermolecular bonding, but not for 4-12. The NBO analysis reveals that the interaction in the dimers 1-3 is mainly due to weak hydrogen bonding between methyl groups and chalcogen centers. Similar hydrogen bonding is also found in the case of 4 and to a lesser extent in 5 and 7. For the aggregates with heavier centers the chalcogen-chalcogen interaction dominates, and hydrogen bonding only plays a minor role. Electron-withdrawing groups on the chalcogen centers increase the interaction energy and reduce the intermolecular distance dramatically. The one-electron picture of an interaction between the lone pair of the donor and the chalcogen carbon sigma orbital allows a qualitatively correct reproduction of the observed trend.

To understand the intermolecular interactions between chalcogen centers (O, S, Se, Te), quantum chemical calculations on model systems were carried out. These model systems were pairs of monomers of the composition (CH3)2X1 (X1 = O, S, Se, Te) as the donors and CH3X2Z (with X2 = O, S, Se, Te and Z = Me, CN) as the acceptors. The variation of X1, X2, and Z leads to 32 pairs with 8 homonuclear cases (X1 = X2 = O, S, Se, Te) and 24 heteronuclear cases (X1 not equal X2). The MP2/SDB-cc-pVTZ, 6-311G* level of theory was used to derive the geometrical parameters and the interaction energies of the model systems. The pairs with Z = CN (17-32) show a considerably higher interaction energy than the pairs with CH3 groups only (1-16). Natural bond orbital (NBO) analysis revealed that the interaction of the dimers 1, 2, 5, 6, 9, 10, 13, 14, 17, 21, 25, and 29 is mainly due to weak hydrogen bonding between methyl groups and chalcogen centers. These systems all contain hard chalcogen atoms as acceptors. For all other systems, the chalcogen-chalcogen interaction dominates. The one-electron picture of an interaction between the lone pair of the donor chalcogen atom and the chalcogen-carbon antibonding sigma* orbital serves as a model to qualitatively rationalize trends found in many of these systems. However, it has to be applied with some amount of skepticism. A detailed analysis based on symmetry-adapted perturbation theory (SAPT) reveals that induction and dispersion forces dominate and contribute to the bonding in each case. Hydrogen-bonded compounds involve bonding electrostatic contributions. Compounds dominated by chalcogen-chalcogen interactions exhibit bonding due to electrostatic interactions only if one of the chalcogen atoms involved is sulfur or oxygen.

This review considers noncovalent bonds between divalent chalcogen centers. In the first part we present X-ray data taken from the solid state structures of dimethyl- and diphenyl-dichalcogenides as well as oligoalkynes kept by alkyl-sulfur, -selenium, and -tellurium groups. Furthermore, we analyzed the solid state structures of medium sized (12-24 ring size) selenium coronands and medium to large rings with alkyne and alkene units between two chalcogen centers. The crystal structures of the cyclic structures revealed columnar stacks with close contacts between neighboring rings via noncovalent interactions between the chalcogen centers. To get larger space within the cavities, rings with diyne units between the chalcogen centers were used. These molecules showed channel-like structures in the solid state. The flexibility of the rings permits inclusion of guest molecules such as five-membered heterocycles and aromatic six-membered rings. In the second part we discuss the results of quantum chemical calculations. To treat properly the noncovalent bonding between chalcogens, we use diffuse augmented split valence basis sets in combination with electron correlation methods. Our model substances were 16 dimers consisting of two Me-X-Me (X = O, S, Se, Te) pairs and dimers of Me-X-Me/Me-X-CN (X = O, S, Se, Te) pairs. The calculations show the anticipated increase of the interaction energy from (Me-O-Me) (-2.15 kcal/mol) to (Me-O-Me/Me-Te-CN) (-6.59 kcal/mol). An analysis by the NBO method reveals that in the case of the chalcogen centers O and S the hydrogen bridges between the molecules dominate. However, in the case of Se and Te the major bonding between the pairs originates from dispersion forces between the chalcogen centers. It varies between -1.7 and -4.0 kcal/mol.

Close contacts between sulfur, selenium, and tellurium centers are used to construct nanotubes in the solid state which are able to host other molecules.

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A carbon nanotube, if it could be cut sideways at the end like a pipe, would yield beltlike structures. These subunits of carbon nanotubes, in their simplest manifestation as the [6] n cyclacenes, are composed of conjugated six-membered rings that are annelated, that is, made of a series of consecutively fused ring structures. These seemingly simple “one-benzene-thick” slices are of considerable interest as modelsfor electronic structure and spectroscopic properties, for examplefor carbon nanotubes. In the late 1980s Stoddart and co-workers, soon to be followed by other groups, embarked on the synthesis of [6] n cyclacenes. The necessary curved shape of the belt was achieved with boat-shaped precursors for Diels−Alder reactions, namely, 7-oxanorbornene derivatives. The preferred endo-addition in Diels−Alder reactions assured an efficient synthesis of beltlike systems containing 12 to 18 six-membered rings. However, the removal of the auxiliary oxygen centers to achieve [6] n cyclacenes as fully conjugated systems has failed so far: they remain an inaccessible target despite some 25 years of synthetic effort. Concurrently, theoretical studies revealed that [6] n cyclacenes as linearly annelated systems show small energy gaps between triplet and singlet states; they can thus be expected to be unstable species. Angularly annelated systems, on the other hand, were predicted to have large singlet−triplet splittings. Nakamura and co-workers and our laboratory have found ways around the obstacles to create cyclacenes. Nakamura and co-workers prepared the first angular annelated cyclacene by selective reduction of the north and south poles of C60. We used a cyclic system that is fully conjugated and also adopts a boat conformation: cyclooctatetraene. By following this principle, we synthesized two types of linearly annelated cyclacenes: metal-stabilized four-membered rings alternating with eight-membered rings ([4.8] n cyclacenes), and six-membered rings alternating with eight-membered rings ([6.8] n cyclacenes). These new scaffolds, if extended along the molecular axis, would represent as-yet unknown forms of carbon nanotubes and are thus inviting synthetic targets. Calculations show that the incorporation of heteroatoms (S and NH) into these scaffolds is promising, encouraging the synthetic pursuit of thia- and azacyclacenes.

It is shown that tubular structures arise in the solid state through close chalcogen-chalcogen (X...X) contacts. As examples a variety of cyclic systems containing sulfur and selenium centers is presented. Common to all of them are close contacts between the chalcogen centers of neighboring stacks giving rise to a zigzag or ladder-type arrangement. In the case of cyclic systems of ring size 24-33 the resulting tubes are able to include hydrocarbons as guest molecules. For 2,7-ditelluraocta-3,5-diyne (17) the close contacts between the tellurium centers of the acyclic C(2) symmetric building blocks generate a helical arrangement in which n-hexane was included. The favored X...X contacts can be traced back to np(X)-sigma*(X[bond]C) interactions.

Cyclobutane-1,2,3,4-tetrone has been both predicted and found to have a triplet ground state, in which a b(2g) σ MO and an a(2u) π MO are each singly occupied. The nearly identical energies of these two orbitals of (CO)(4) can be attributed to the fact that both of these MOs are formed from a bonding combination of C-O π* orbitals in four CO molecules. The intrinsically stronger bonding between neighboring carbons in the b(2g) σ MO compared to the a(2u) π MO is balanced by the fact that the non-nearest-neighbor, C-C interactions in (CO)(4) are antibonding in b(2g), but bonding in a(2u). Crossing between an antibonding, b(1g) combination of carbon lone-pair orbitals in four CO molecules and the b(2g) and a(2u) bonding combinations of π* MOs is responsible for the occupation of the b(2g) and a(2u) MOs in (CO)(4). A similar orbital crossing occurs on going from two CO molecules to (CO)(2), and this crossing is responsible for the triplet ground state that is predicted for (CO)(2). However, such an orbital crossing does not occur on formation of (CO)(2n+1) from 2n + 1 CO molecules, which is why (CO)(3) and (CO)(5) are both calculated to have singlet ground states. Orbital crossings, involving an antibonding, b(1), combination of lone-pair MOs, occur in forming all (CO)(2n) molecules from 2n CO molecules. Nevertheless, (CO)(6) is predicted to have a singlet ground state, in which the b(2u) σ MO is doubly occupied and the a(2u) π MO is left empty. The main reason for the difference between the ground states of (CO)(4) and (CO)(6) is that interactions between 2p AOs on non-nearest-neighbor carbons, which stabilize the a(2u) π MO in (CO)(4), are much weaker in (CO)(6), due to the much larger distances between non-nearest-neighbor carbons in (CO)(6) than in (CO)(4).