Strong dihydrogen bonds

TABLE 11.1. H -H, O-H, and Be-H Distances, Electron Density Parameters, and BSSE-Corrected Bonding Energies of Strong Dihydrogen-Bonded ComplexeSa... 

S.J. Grabowski et al., Strong dihydrogen bonds - ab initio and atoms in molecules study. Chem. Phys. Lett. 386, 44-48 (2004)... 

The dihydrogen molecule is the smallest molecule in existence. It has a strong covalent bond with a dissociation energy of 103 kcal mol [1], In a hydrogenation reaction, this bond has to be broken and two new C-H bonds are formed, one of the simplest forms of chemical reaction. 

The aim of the book is to show (1) a diversity of dihydrogen bonds (2) the nature, geometry, energetics, and dynamics of these bonds and (3) the factors that control the energy of dihydrogen bonds from very weak to very strong. 

In the framework of AIM theory, which well be used extensively to describe dihydrogen bonds, the situation above corresponds to the electron density distribution in the H2 molecule, where the pc parameter takes the very large value 1.857 au and the Laplacian, V pc, is strongly negative ( —34.15 au). These data have been obtained by ab initio calculations at the MP2/6-31G level [4],... 

One of the important advantages of NMR spectroscopy is that it makes possible calculation of the H- H distances in dihydrogen-bonded complexes formed in solution. As we have shown above, the formation of dihydrogen bonds is accompanied by the appearance of H- H contacts that are shorter than a sum of the van der Waals radii of H (< 2.4 A). Snch short H- -H contacts are connected directly with the spin-lattice (Ti) NMR relaxation of target nnclei [32]. The contacts between two nuclei cause strong homonuclear dipolar coupling, DCh-h. which is written as... 

Figures 5.20 illustrates the equilibrium and transition-state structures obtained for these complexes. As shown, the L1H-H20 complex in equilibrium state 1 shows an intramolecular dihydrogen bond with a very short H- H distance of 1.580 A calculated at the MP2/6-311++G(2d,2p) level. The topological analysis of the electron density on the H- H direction has resulted in the small pc and positive V pc values (0.0388 and 0.0453 an, respectively) typical of dihydrogen bonding. In contrast, no dihydrogen bonding was observed in the LiH-H2S molecule (3), where the corresponding hydrogen atoms are strongly remote.

Strong evidence for the existence of intramolecular dihydrogen interactions C-H - H-C, 0-H- H-C, C-H H-B, N-H- H-B, O-H- H-B, and X-H—H-M with X = 0, N and M = metal atom has been obtained experimentally and theoretically for different classes of organic and organometallic compounds. In all cases, these interactions corresponded to the experimental and theoretical criteria, leading to their formulation as dihydrogen bonds. 

The first study of dihydrogen bonding between LiH and the strong proton donor HF has been carried out by Liu and Hoffman [1], The authors have performed RHF and MP2 calculations where dihydrogen bonds have not been localized because of the high exothermicity of molecular hydrogen elimination ... 

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