The Water Dimer

The interpretation of the microwave spectra recorded in the molecular beam electric resonance experiments shows that the dipole moment increases on complex 

Stretching vibration of HF monomer Stretching vibration of proton donor Stretching vibration of proton acceptor Intermolecular frequencies harmonic frequency harmonic frequency is 4139 cm 1 (H20)2 

Thereby the monomers A and B are arranged in precisely the same geometrical positions which they occupy in the complex. The experimental data on the increase in dipole moments are in good agreement with the calculated data44 (Table 8) provided the large amplitude motions are taken into account appropriately (see 

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Leforestier C, Braiy L B, Liu K, Eirod M J and Saykaiiy R J 1997 Fuiiy ooupied 6-dimensionai oaiouiations of the water dimer VRT states with a spiit Wigner pseudo-speotrai approaoh J. Chem. Phys. 106 8527-44... 

The intennolecular forces between water molecules are strongly non-additive. It is not realistic to expect any pair potential to reproduce the properties of both the water dimer and the larger clusters, let alone liquid water. There has therefore been a great deal of work on developing potential models with explicit pairwise-additive and nonadditive parts [44, 50, 51]. It appears that, when this is done, the energy of the larger clusters and ice has a nonadditive contribution of about 30%. 

Compare your results to the experimental values of -34.0 2 kcal mol for the lithium reaction and -3.6 .5 kcal mol" for the water dimer reaction. Use the same model chemistry as in Example 8.2 B3LYP/6-311+G(2df,2p) // B3LYP/6-31G(d). 

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,... 

Table 12-1. Deviation in the computed Rq 0 distance of the water dimer [A] from the experimental Re value of 2.952 A.

Table 12-4. Computed interaction energy (AE) and counterpoise-corrected interaction energies (AECP) of the water dimer [kcal/mol]. The BSSE is given in parentheses.

Table 12-6. Harmonic frequency shifts [cm 11 of the donor O-H stretching mode and elongation of the O-H bond [A] in the water dimer computed at several levels of theory (aug-cc-pVTZ basis set). 

Table 12-7. Selected computed properties for the water dimer (taken from Tuma, Boese, and Handy, 1999). 

Halkier, A., Koch, H., Jprgensen, P., Christiansen, O., Nielsen, I. M. B., Helgaker, T., 1997, A Systematic Ab Initio Study of the Water Dimer in Hierarchies of Basis Sets and Correlation Models , Theor. Chem. Acc., 97, 150. 

Hobza, P., Bludsky, O. Suhai, S., 1999, Reliable Theoretical Treatment of Molecular Clusters Counterpoise-Corrected Potential Energy Surface and Anharmonic Vibrational Frequencies of the Water Dimer , Phys. Chem. Chem. Phys., 1, 3073. 

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