Hydrogen bonding complex, elongation

Table 5.1. Properties of binary B - -HA hydrogen-bonded complexes (see Fig. 5.1), showing the H-bond energy A b-h, H-bond length Rb-e, van der Waals contact distance f B---H(vdW), cind covalent-bond elongation A/sAn (relative to isolated monomer)...

Figure 15. Chemical reaction dynamics of methanol oxidation to formaldehyde one of the C-H bonds of the methyl group becomes elongated (top left) and eventually breaks (top right). The adsorbed hydroxymethyl group stabilized by forming a hydrogen bonded complex to a water molecule (bottom left) and dissociates rapidly into adsorbed formaldehyde and a hydronium ion (bottom right) which further stabilized by undergoing structural diffusion steps to form Zundel ions H50+.123,125 Reprinted from Chemical Physics, Vol. 319, C. Hartnig and E. Spohr, The role of water in the initial steps of methanol oxidation on Pt(l 1 1), p. 108, Copyright (2005), with permission from Elsevier.

Table 2. Elongation of HX bonds in hydrogen-bonded complexes...

The network structure of unfilled and filled elastomers was probed by the analysis of the quadrupolar splitting in H solid echo spectra of uniaxially strained samples (55) (see Fig. 6). The local chain order at a given elongation is larger by a factor of 1.5-2 in the filled system. A decrease of local chain mobility in the absorption layer is observed under stress. The same method was applied to investigate molecular dynamic in thermoplastic elastomer based on hydrogen bonding complexes (61,62). 

A hydrogen-bond adduct between a Lewis base and a hydrogen-bond donor could also be considered as a frozen deprotonation the Lewis base donates electron density into the o orbital of the R-H bond and thus typically weakens and elongates the bond, but not to the point of an actual cleavage of the R-H bond. The hydrogen-bonded complex 6 is then a stable, thermodynamic minimum (Fig. 3). If, however, a (stronger) Lewis base donates sufficient electron density into the o orbital to actually cleave the R-H bond, deprotonation occurs and the product 7 consists of two fragments, R and H-LB In the latter case, 6 is not a stable minimum, but resembles or equals the transition state of the deprotonation 7. 

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