Intramolecular dihydrogen bonds

This is a very good illustration, supporting the idea that there is no clear border between van der Waals interactions and weak dihydrogen bonds. 

One of the important criteria for dihydrogen bonding is the loss of charge of hydrogen atoms participating in the interaction. This tendency is observed clearly in calculations of atomic charges. The charges on atoms H(6), H(7), H(8 ), and H(12 ) in the dimer strnctnre have been calcnlated as 0.254, 0.258,-0.022, and 0.01 e, respectively, versns 0.260, 0.260, 0.003, and 0.033 for the same atoms in a monomer structure. 

For geometrical reasons, multiple intramolecular C-H B-H contacts can be expected in the aminoboron hydride shown in Structure 5.5 and also in relative compounds. In fact, these compounds are very stable. It is probable that the intramolecular dihydrogen bonds could act against their own disproportion [9]. 

On the one hand, x-ray crystallography data could support this idea because the heterocycles are coplanar relative to the B-H bonds. Moreover, in the case of the compound in Strncture 5.5, this conformation remains in solution, according to NOE NMR measnrements. On the other hand, the solid-state CH- -HB distances are too long ( 2.65 A), and the HF/6-31G calculations carried ont for the componnd have shown, that where the heterocyclic rings are oriented orthogonally, the conformation is more stable. Thns, this question is still open. It is quite probable that the interaction between protonic hydrogens on the a-carbons and hydridic BH hydrogens is electrostatically attractive. 

---

TABLE 4.3. Strength of Intramolecular Dihydrogen Bonding in Iridium Hydride Complexes (Structure 4.3) as a Function of the Nature of the frans-Ligand Y... 

INTRAMOLECULAR DIHYDROGEN BONDS IN SOLID AMINO ACIDS... 

INTRAMOLECULAR DIHYDROGEN BONDS IN SOLID AMINO ACIDS C-H BONDS AS WEAK PROTON ACCEPTORS... 

The formation of an intramolecular dihydrogen bond should be accompanied geometrically by the creation of a ring of size four to six, with the five-membered cycle preferred. 

Figure 5.4 Geometries of glycine, alanine, and proUne optimized at a B3LYP/6-311G level. The solid lines show intramolecular dihydrogen bonds. (Reproduced with permission from ref. 6.)...

Figure 5.11 Linear relationship between the interaction energy and the H- -H distances found for the intramolecular dihydrogen bonds O-H- -H-B shown in Structure 5.6. (Reproduced with permission from ref. 16.)...

INTRAMOLECULAR DIHYDROGEN BONDS IN METAL HYDRIDE COMPLEXES... 

Intramolecular Dihydrogen Bonds in Metal Hydride Clusters... 

Few examples are known for this type of intramolecular dihydrogen bond. One of them is the trimetallic osmium cluster shown in Structure 5.15 [28]. This compound, described well by various methods, has revealed a number of problems connected with characterizations of dihydrogen-bonded complexes that deserve separate discussion. The hydrogen atoms localized in the x-ray molecular structure of complex 1 of Figure 5.17 provide a formulation of interaction N-H -... 

CONNECTION BETWEEN INTRAMOLECULAR DIHYDROGEN BONDING AND DEHYDROGENATION REACTIONS... 

In previous sections we have shown clearly that intramolecular dihydrogen bonds X-H- H-Y, with X and Y representing various chemical elements, can exist in both the solid state and in solution. In addition, the bonds can be a critical factor in the control of molecular conformational states or effects on rapid and reversible hydride-proton exchanges related to the process shown in Scheme 5.1, or the well-known H-D isotope exchanges in similar subsystems [23]. Such bonds could also play an important role in the stabilization of transition states, appearing as a reaction coordinate in many transformations. This is particularly... 

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.

The geometry of intramolecular dihydrogen bonds is governed by the molecular geometry. 

Intramolecular dihydrogen bonds play an important role in molecular systems that lose H2 readily, serving as organizing interactions and factors, and stabilizing transition states in these reactions. 

Stabilization of transition states by intramolecular dihydrogen bonding explains the high degree of fluxionality of polyhydride transition metal complexes. 

---