Dihydrogen bonds interaction energies

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.)...

The result of an energy decomposition analysis performed for this complex was also unusual. In contrast to numerous dihydrogen-bonded systems with significant predominance of the electrostatic interaction, the dihydrogen-bonded dimer (LiH H20)2 has shown the charge transfer contribution to exceed the electrostatic energy —125.30 versus —81.40 kcal/mol, respectively. 

TABLE 6.3. BSSE-Corrected Interaction Energies and H- -H Distances for Li and Na Dihydrogen-Bonded Complexes Formed with Various Proton-Donor Molecules ... 

It is well known that rare-gas compounds such as HArF or HKrF exhibit the ion-pair character (HAr)+F . Therefore, they are of great interest as proton donors in dihydrogen bonding [15], The equilibrium structures of systems BeH2 HArF and BeH2-HKrF and of isolated components have been optimized at the MP2/6-311-H-G(2d,2p) level. The calculations led to dihydrogen-bonded complexes with short H- H distances 1.312 and 1.590 A for H-Be-H- HArF and H-Be-H- HKrF, respectively. The interaction energies have been determined as —5.28 and —1.70 kcal/mol (see Table 6.7). 

One unusual characteristic of molecular systems 34 to 36 requires additional explanation. Complexes 34 and 35 show H- -H distances that are very close to the sum of the van der Waals radii of H, 2.4 A whereas the distance in system 36 is notably larger. The formulation of this system as dihydrogen-bonded is particularly questionable because the H- -H separation is even longer than the 2.7 A suggested by Alkorta et al. as the sum of the van der Waals radii of H calculated for the system HsC-H- -H-CHs, with an interaction energy of 0.2 kcaFmol [17]. 

TABLE 6.11. Experimental Interaction Energies Determined by IR Spectra for Dihydrogen-Bonded Complexes Eormed by Hydrides of Group 3A Elements ... 

TABLE 6.12. H- -H Distances and Interaction Energies for Dihydrogen-Bonded Systems SI to S7 in Figure 6.12 ... 

The first high-level theoretical evidence for the existence of dihydrogen bonds between CH4 and [NH4]+ came in 1996 [5]. In the sense of bonding geometry, this complex is not linear (Structure 6.5), has Csv symmetry, and exhibits three relatively short H- -H contacts with distances of 2.237, 2.357, and 2.226 A calculated at the RHF/6-31G, RHF/6-311++G, and MP2/6-31G lev-els, respectively. According to the frequency analysis at the RHF/6-31G level, the complex occupies a minimum on the potential energy surface of this system. The BSSE-corrected interaction energies have been calculated as —2.46,... 

Comparing the data in Tables 6.14 and 6.15 shows that dihydrogen bonding to the same proton donor depends strongly on the natme of the group 3A elements The interaction energy increases in the order... 

Table 7.1 lists energies of dihydrogen bonding and the H- H distances that have been calcnlated for transition metal hydride systems in the gas phase. As shown, the transition metal complexes calcnlated have different ligand environments and interact with different proton donors. 

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