Hydrogen bonding water trimer

FIGURE 41. (a) Hydrogen-bonded water trimer (shown in violet), (b) Hydrogen bonding situation describing the water trimer (shown in violet) and its peripheral hydrogen bonding sites. Color code. Mo, yellow O, red Cu, blue H, white. (Reprinted with permission from ref. 52.)... 

Early theoretical calculations on hydrogen-bonded water polymers predicted cooperativity. Even when it was possible to carry out calculations only at relatively low approximation levels, comparison of the relative values for the water dimers, trimers, tetramers and higher polymers were very informative. They predicted the nonadditivity of hydrogen-bond energies and the phenomenon of cooperativity, discussed in Chapter 1 and elsewhere throughout this text [103-105]. One such calculation involved successive polymers, the results of which are shown in Thble 4.5. 

The triple helix is another type of superstructure of DNA [91]. A-T-T (A-T-U) and G-C-C base-trimers are formed by combination of Watson-Crick and Hoogsteen-type hydrogen bonding. Base-trimers are formed in the DNA-mimetic system at the air-water interface between the Watson-Crick-type monolayer and... 

Fig. 2. Perspective view of the V(OH2)6 (OH2)i2 ion exhibiting D2 symmetry. The dashed lines represent the hydrogen bonds within the cyclic water trimers in the second coordination sphere. Reproduced from Ref. (3) by permission of the Royal Society of Chemistry.

Water and methanol can form strong hydrogen bonds in solutions. These kinds of solvent-solvent interactions have a pronounced effect in the NIR spectral region. For example, in pure methanol solutions it is possible to have dimers, trimers, and other intermolecular hydrogen-bonded species in equilibrium. Equilibrium concentrations of these species are very sensitive to impurity concentrations and temperature changes. 

Perhaps the most exciting development in the theory of hydrogen bonding has concerned the analysis of the stability of water polymers. The three structures for the trimer shown in Fig. 3 differ in the way in which the central water molecule of the trimer takes part in the hydrogen bonding, and it is possible to examine theoretically the stabilization energy per H-bond in each case. There is a non-... 

Figure 3. Water trimers showing three possible arrangements where two water molecules are hydrogen bonded to a third water molecule I is asymmetric while II and III are symmetric.

Early quantum mechanical calculations [105] on the water trimer showed clearly that the sequential hydrogen bonding, 1, was energetically favored over the alternative double donor, 2, or double acceptor, 3, hydrogen bonding. 

Hydrogen bonding must have an effect on the electron density distribution of a molecule. In principle, this should be observed in the deformation density distributions discussed in Chapter 3. There are, in fact, two methods available. One is purely theoretical, in which the calculated deformation density for a hydrogen-bond dimer or trimer is compared with that of the isolated molecule. The other method compares the experimental deformation density of a hydrogen-bonded molecule in a crystal structure with the theoretical deformation density of the isolated molecule. Formamide has been studied by both methods [298, 380], and there appear to be significant differences in the results which are not well accounted for. Theoretical difference (dimer vs. monomer) deformation density maps have been calculated for the water dimer and the formaldehyde-water complex [312]. When those for the water dimer are decomposed into the components described in Chapter 4, a small increase in the charges on the atoms in the O-H -O bond due to the charge-transfer component is predicted [312]. 

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