High level ab initio and density functional theory studies on methanol–water dimers and cyclic methanol(water)2 trimer

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Abstract
The methanol-water dimers and the potential energy surface of the cyclic methanol(water)2 trimer have been studied through the use of high-level ab initio calculations and density functional methods. The geometries have been optimized at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The harmonic vibrational frequencies were obtained at the latter level. The final energies of the different local minima were calculated in the framework of the G2 and G2(MP2) theories. These values were compared with those obtained using the B3LYP/6-311+G(3df,2p) approach. At all the levels of theory considered the most stable conformer of methanol-water heterodimers corresponds to that in which water behaves as a hydrogen bond donor, in agreement with the most recent experimental evidences [P. A. Stockman et al., J. Chem. Phys. 107, 3782 (1997)]. The energy differences between the different conformers of the cyclic methanol(water)2 trimer are rather small, as well as the energy barriers connecting them. The global minimum corresponds to a conformer with the methyl group on one side of the O-O-O plane and the two free OH groups of the water molecules on the other side. Other stationary points associated with a systematic flipping of the methyl group and the free OH groups have been also located. These stationary points, which are transition states or saddle points of higher order, are very close in energy to the global minimum, indicating that the potential energy surface of the methanol(water)2 trimer is very flat and very similar to that reported before for water and methanol trimers. The calculated enthalpies of association are slightly smaller than those estimated for methanol trimers. The most stable trimer has three different O–H donor stretching frequencies, showing that the HB in which the methanol moiety behaves as a HB donor is the weakest one. Cooperative effects are significant. They are reflected in larger frequency shifts, greater lengthening of the O–H bonds, and shorter O⋯O distances than in (water)2 and methanol-water dimers. The estimated additive interaction energy is also significantly large.