Linear structured water dimer

It is worthwhile analyzing whether the same conclusion holds when the water dimer structure is considered. A series of calculations have been performed for a linear structured water dimer, the values are listed in Table II. 

The results presented in this work show that in the linear structured water dimer the partitioned energy terms calculated for the proton donor and acceptor molecules are significantly different (except the kinetic energy). The electron structure of the proton donor molecule was found more compact than that of the acceptor subsystem, when compared their (partitioned) total energy EM values. This result is in an excellent agreement with our pre-vious results obtained on the separated molecular orbital energies [17]. 

Figure 5.38 displays the optimized structure and primary npt— ooh donor-acceptor interaction of the complex (5.70a) in which PtH2 serves as the Lewis-base donor. The qualitative similarity to the water dimer structure (Fig. 5.5(b)) is immediately apparent, including the short Pt H distance (2.47 A, more than 0.5 A inside van der Waals contact), the roughly linear Pt H—O angle (161°), and the characteristic elongation of the H-bonded versus free O—H bond of the water monomer (by 0.01 A). The leading nPt a0H donor-acceptor interaction in Fig. 5.38(b) is... 

Results obtained for a linear water dimer structure... 

Following the analysis of the characteristic features determined for the proton donor and acceptor molecules in a linear water dimer, we performed the same study for some water hexamer clusters. The structures considered are representative of those published in a previous paper [18]. 

In this work the separated representation was used to determine the energetic quantities for each contributing monomer in some water hexamer systems, too. On one hand, the results obtained for the monomers in these structures did not suggest to identify the same proton donor or acceptor nature as it was found for the linear water dimer. On the other hand, in three of the water hexamer structures that were investigated, some monomers showed a specific nature. These results confirm previous results [18] that there are extra interaction abilities in these hexamer structures (prism, w6t and w6q). 

The PES of the water dimer system is characterised by three main stationary points the "quasi-linear" structure, representing the global minimum (Fig. 11), and the cyclic and bifurcated configurations (saddle points). 

Hydrogen fluoride dimer, like with the water dimer, is one of the most frequently studied complexes. Both extensive theoretical and experimental data are available for analysis. The PES of the hydrogen fluoride dimer (see Figs. 15 and 16), if compared with that of water, is simpler two stationary points exist the quasi-linear" structure (see Fig. 15) and the cyclic structure (see Fig. 16). 

Figure 21.1 Four representative structures (a) water monomer (b) acetic acid monomer (c) water linear dimer (d) the most stable acetic acid water dimer (see text in Section 3.1).

For the bifurcated water dimer, 2, the calculated energy was about half that of the linear dimer. No examples have been observed in hydrate crystal structures, except as components of three-center bonds, see Part IV, Chapter 22. 

The ubiquitous occurrence of water and its importance as a solvent medium have motivated a great deal of research into the fundamental nature of the interaction between water molecules by theoretical as well as experimental means. Some of the more recent work has been summarized in a review article. Prior to experimental elucidation of the geometry of the water dimer in the gas phase or to the ability of calculations to provide an unambiguous resolution to this question, a number of different candidate structures were considered. In addition to the standard linear arrangment wherein the bridging hydrogen lies near the O O axis in Fig. 2.8, cyclic and bifurcated structures were considered as illustrated in Fig. 2.9. 

While experimental measurements of the water dimer in the gas phase had yielded an unambiguous linear structure, the results for were less clear . Cyclic and bifurcated... 

The energetic preference for placement of a deuterium at a bridging position in the water dimer carries over nearly unchanged in the trimer. This complex forms a triangular structure, almost equilateral. Each H-bond is distorted from linearity by the constraints of the structure, with 6 (0"0H) angles close to 20°. Nonetheless, it is again the vibrational inodes which distort this bridging atom from the H-bond axis that can be traced as the source... 

When two molecules interact, there is often a small amount of electron flow from one to the other. For example, in the equilibrium geometry of the linear water dimer HO— H... OH2, the water molecule that is the proton acceptor has transferred about 0.05e to the proton donor water (9,10). The attractive energy associated with this charge transfer is the charge transfer energy and can be thought of as a mixing of an ionic resonance structure... 

In Table 2 we present the binding energies and structures for the linear water dimers obtained with the different water models and the different water model combinations. 

Several ab initio and DFT calculations have been made on the water dimer [50-64, 233, 235]. The strength of the H-bonding in water dimer is 5.5 + 0.7 kcal/mol [235]. The water trimer can exist in cyclic arrangement as well as in open chain conformation with linear H-bonds. In the case of the tetramer, previous theoretical calculations have predicted four H-bonded cyclic structures with S4 symmetry corresponding to the global minimum [235]. This observation has also been supported by the IR spectra of various (H20)4 complexes with benzene and VRT spectra of (H20)4 and (D20)4[238, 239]. 

The gas-phase structure of the water dimer is presented in Fig. 18.10 [24]. The three atoms of the acceptor molecule and the O atom of the donor are lying in a symmetry plane, and the O- H-0 fragment appears to be linear. Both H-O-H valence angles are close to 104°. The dissociation energy at zero K is 15 kJ moP [25,26,27]. 

The presence of water in every living system has made the study of water-water interactions of great importance. The simplest relevant system one can consider is the water dimer. It has in fact been the subject of calculations at various levels conventional ab initio theory and is almost surely the most studied and best understood hydrogen-bonded system. Experiment and theory conclude the equilibrium structure to be the C, trans-linear structure depicted in Figure 2. 

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