Hydrogen bonding water dimer

This is not associated with a particular method, like HF or Cl, but rather is a basis set problem. Consider what happens when we compare the energy of the hydrogen-bonded water dimer with that of two noninteracting water molecules. Here is the result of an MP2(fc)/6-31G calculation both structures were geometry-optimized, and the energies are corrected for ZPE ... 

The hydrogen-bonded water dimer is without any doubt the most used system to study intermolecular interactions, be it from the QM [34,72] QM/MM [13,26,31,32,40,52,108], or MM [25,42,45,48,50,72] perspective. In the past we have also used it to show that the DRF model indeed gives static and response potentials that are as good as, e.g., SCF calculations [74,137], Of course, if this is the case, it allows for arbitrary separation of the total system into different subsystems, which can then be arbitrarily described at the QM or MM level e.g., for a simple system like the water dimer, one may treat both monomers at the QM level, one monomer at QM and the other at MM, or both monomers at MM. Hence, we may go from the computationally expensive fully QM to QM/MM and to MM, without significant loss of accuracy. Alternatively, we can do MD simulations at the MM level, take snapshots from them and submit these to QM/MM (or QM) calculations to obtain UV-Vis spectra, excitation energies, NLO properties, etc., for the solute in solvent, i.e., sequential MD. 

In order to show that this interchangeability is indeed feasible, we focus on the hydrogen-bonded water dimer (see Figure 3-2). We treated it first completely with... 

Figure 3-2. The hydrogen-bonded water dimer. The x-axis coincides with O-H-O...

Table 4.3. Ab-initio molecular orbital calculations on the hydrogen-bonded water dimer. Variation of geometry with level of approximation [290]...

In 6-O-o-D-glucopyranosyl-D-glucitol [BAVCAC], there are three infinite chains linked by the minor components of three-center bonds. In 1-O-a-D-gluco-pyranosyl-D-mannositol dihydrate [BAGZEO], a hydrogen-bonded water dimer links the hydroxyls on 0(2)H and 0(5)H of the same mannositol residue. 

Dyke TR, Muenter JS (1974) Microwave spectrum and structure of hydrogen-bonded water dimer. J Chem Phys 60 2929- 2930... 

Suhai128 investigated water dimer and an infinite chain of hydrogen-bonded water molecules by means of the DFT and post-Hartree-Fock calculations. For the infinite system, the DFT(BLYP), MP2, and MP4 binding energies were within 0.2 kcal/mol, whereas the corresponding interatomic distances were within 0.04 A. A similar agreement was reported for water dimer. 

Upon dilution in solvents which may associate via hydrogen bonds (water, methanol, dioxane) the situation is more complex. I.R. and Raman spectroscopy indicate the formation of various monomer-solvent complexes (4, 6). The corresponding absorption bands are in the same range as the characteristic bands for open dimers and oligomers and the latter cannot therefore be determined quantitatively. However, the viscosity of carboxylic acids was found to rise upon addition of water or methanol (4, 7) suggesting that these solvents bind together "oligomers". The persis-... 

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. 

Changes in covalent nonhydrogen bond length as a consequence of hydrogen-bond formation is predicted theoretically and observed experimentally in particular examples. Apart from the hydrogen-bonded water polymers discussed in the preceding chapter, the simple carboxylic acids provide one of the few examples where both monomers and hydrogen-bonded dimers can be studied by the same... 

CI2, Bt2,12 and dihalogen compounds XX react with electron donors like amines or ethers to form complexes where the dihalogen molecules act as electron acceptors. The structure of some such complexes will be discussed in Section 18.6. Similarly the hydrogen halides HX react with electron donors to form hydrogen-bonded complexes, and the structures of some of them will be described in Section 18.7. The chapter ends with descriptions of the structures of the hydrogen-bonded H2O dimer in the gas phase, of solid ice and liquid water, and a brief account of the poly water episode. 

However, when the sodium complex was crystallized from a solvent of medium polarity such as acetone, a head-to-head dimer structure was obtained This is also true for the free acid of 5-bromolasalocid crystallized from CgHj /CH Clj in which the antibiotic molecules are connected to each other by a hydrogen-bonded water molecule and by an additional hydrogen bond from the carboxy group of one ligand to the carbonyl group of the other. 

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