No abstract

Alkane molecules are held together in the crystal state by purportedly weak homonuclear R-H···H-R dihydrogen interactions. In an apparent contradiction, the high melting points and vaporization enthalpies of polyhedranes in condensed phases require quite strong intermolecular interactions. Two questions arise: 'How strong can a weak C-H···H-C bond be?' and 'How do the size and topology of the carbon skeleton affect these bonding interactions?' A systematic computational study of intermolecular interactions in dimers of n-alkanes and polyhedranes, such as tetrahedrane, cubane, octahedrane or dodecahedrane, showed that attractive C-H···H-C interactions are stronger than usually thought. We identified factors that account for the strength of these interactions, including the tertiary nature of the carbon atoms and their low pyramidality. An alkane with a bowl shape was designed in the search for stronger dihydrogen intermolecular bonding, and a dissociation energy as high as 12 kJ mol⁻¹ is predicted by our calculations.

To understand the dispersion stabilization of hydrocarbons in solids and of encumbered molecules, wherein CH···HC interactions act as sticky fingers, we developed here a valence bond (VB) model and applied it to analyze the H···H interactions in dimers of H2 and alkanes. The VB analysis revealed two distinct mechanisms of "dispersion." In the dimers of small molecules like H-H···H-H and H3CH···HCH3, the stabilization arises primarily due to the increased importance of the VB structures which possess charge alternation, e.g., C(+)H(-)···H(+)C(-) and C(-)H(+)···H(-)C(+), and hence bring about electrostatic stabilization that holds the dimer. This is consistent with the classical mechanism of oscillating dipoles as the source of dispersion interactions. However, in larger alkanes, this mechanism is insufficient to glue the two molecules together. Here, the "dispersion" interaction comes about through perturbational mixing of VB structures, which reorganize the bonding electrons of the two interacting CH bonds via recoupling of these electrons to H···H, C···C, and C···H "bonds." Finally, an attempt is made to create a bridge from VB to molecular orbital (MO) and local pair natural-orbital coupled electron pair approximation (LPNO-CEPA/1) analyses of the interactions, which bring about CH···HC binding.

The binuclear complexes of d8 transition metal ions of type [L2M(μ-XR n )2ML2] (n = 0 to 2) show a wide variety of bending angles between the coordination planes of the two metal atoms. A theoretical and structural analysis of those compounds with unsubstituted bridges, [L2M(μ-X)2ML2], tries to unravel the factors that determine the degree of bending of such compounds. A rationale is given for the structures of 139 crystallographically independent molecules. A driving force for bending of the molecules is the attractive metal···metal interaction that results from donor−acceptor interactions between the d z 2 and p z orbitals of the two metal atoms and is modulated by the nature of (a) the metal atom, (b) the terminal ligands, and (c) the bridging atoms. In all cases the energy difference between the planar and the bent form is within the 10 kcal/mol range, and steric effects seem to be important in preventing bending only for the bulkiest terminal ligands. For the studied bridging atoms, the stability of the bent form increases in the order O < S > Se > Te and F < Cl < Br < I, and for different metal atoms it increases in the order Ni(II) < Pd(II) < Pt(II) < Rh(I) < Ir(I).