The Water Dimer - A Worked Example

The water dimer is probably the most intensively studied intermolecular hydrogen bonded system of all. Hence, ample theoretical and experimental data is available for this system, 

50 A recent discussion of the water dimer in the DFT domain has been given by Guo et al., 1997. 

Basis Set HF MP2 CCSD(T) SVWN BLYP SLYP BVWN B3LYP 

51 We also noted already that BLYP results show a better agreement with directly observed, anharmonic frequencies. This is also the case here the BLYP deviations from the fundamental experimental frequencies (Vs = 3657 cm-1, Vas = 3756 cnV1) are 2 and 0 cm4, respectively. 

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Ben-Naim (1980) has recently reviewed the work in this field. The results obtained so far are not very satisfying because there does not seem to be a single method that can provide direct information on the properties of the hydrophobic interactions between two simple solutes in water at a realistic interparticle distance. Most studies have been concerned with the volume difference between a dimer 2A and a monomer A, for example, ethane and methane. However, such comparisons are not realistic because there is a covalent bond between the two monomers in the dimer, and also two hydrogen atoms are missing in the dimer. The volume of these two hydrogens seems to account for the resulting volume difference. 

Xantheas and co-workers [159,160] have incorporated polarization in a model scheme and have used that to provide a clear basis for the enhancement of water s dipole in ice. A model potential with polarization has been reported for the formaldehyde dimer [161]. It is an example of a carefully crafted potential, which is system-specific because of its application to pure liquid formaldehyde, but which has terms associated with properties and interaction elements as in the above models. As well, some of the earliest rigid-body DQMC work, which was by Sandler et al. [162] on the nitrogen-water cluster, used a potential expressed in terms of interaction elements derived from ab initio calculations with adjustment (morphing). Stone and co-workers have developed interaction potentials for HF clusters [163], water [164], and the CO dimer [165], which involve monomer electrical properties and terms derived from intermolecular perturbation theory treatment. SAPT has been used for constructing potentials that have enabled simulations of molecules in supercritical carbon dioxide [166]. There are, therefore, quite a number of models being put forth wherein electrical analysis and/or properties of the constituents play an essential role, and some where electrical analysis is used to understand property changes as well as the interaction energetics. 

A final example of aqueous media used in the hydrocyanation of butadiene is provided by Waddan at ICI [23]. In this chemistry, copper nitrate salts in aqueous media (among many others) are used for the oxidative dihydrocyanation of butadiene to dicyanobutenes (Eq. 9). Good conversions of butadiene are reported in nonaqueous media but no examples are actually provided in which water is added as a solvent. Moreover, because of problems with alkene and HCN dimerization and the risk of explosion hazards, these reactions appear to work best when conducted stepwise (i.e., HCN addition to catalyst followed by oxidation followed, in turn, by butadiene addition), leading one to wonder about the productivity of these systems. 

In Chap. 5, Oriol Vendrell et al. give a similar example the infrared spectrum of the protonated dimer or Zundel cation, H5O+. Using a time-dependent description with the MCTDH approach including all 15 internal degrees of freedom, they have been able to completely characterize the infrared spectra in the gas phase for this system as well as all its analogues where the hydrogen atoms have been replaced by deuterium atoms [153,154]. They have also performed time-dependent simulations of the dynamics of a proton between the two water molecules [213]. As explained by S. S. Xantheas, the scope of this work exceeds the simple description of this small cluster alone since hydrogen cations are ubiquitous in nature [214]. The... 

The bicyclic guanidinium tetramer 43 was first reported as a possible binder for helical oligonucleotides. Initial studies with 43 and sulfate anions showed that the tetramer formed double-helical dimers around sulfate counterions [69]. Despite the initial proposal of using this receptor for membrane transport of oligonucleotides, all subsequent work on 43 has been conducted on helical peptides. For example, the binding of 43 with several synthetic peptides caused an increase in the helicity and helical stability of the peptides in 10% water/methanol [70]. The peptide containing four Asp derivatives showed the... 

Sequence-selective binding of peptides and small proteins is of considerable interest. We saw that some cyclodextrin dimers could selectively doubly bind peptides in water with appropriately placed hydrophobic side chains. This built on our earlier collaborative work on the selective binding of peptides by simple cydodextrin. We then showed that we could break up a protein dimer and a protein tetramer with appropriate cydodextrin dimers in water, since such protein aggregation ordinarily involved hydrophobic side chains that our dimers could bind to. In perhaps the most striking example, our cydodextrin dimers and trimers were able to inhibit the protein aggregation involved in the formation of Alzheimer s plaques. ... 

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