Methanols, hydrogen bonding cooperativity

R. Wugt Larsen and M. A. Suhm, Cooperative organic hydrogen bonds The librational modes of cyclic methanol clusters. J. Chem. Phys. 125, 154314 (2006). 

So far, ab initio quantum mechanical techniques were applied to the methanol/water dimer and the methanol/water/water trimer. It is weU known that the methanol/water dimer can adopt two possible configurations depending on whether water (WdM) or methanol (MdW) acts as the hydrogen-bond donor. Because there is no large energetic difference between the two dimers, it was not easy to select the more stable dimer. Nevertheless, it was recently established that the dimer in which the water molecule is the proton donor (WdM) is more stable. - However, one should point out that a dimer or a trimer cannot represent a real patch of a dilute condensed phase because they cannot represent, for instance, the cooperative effect of many molecules. To achieve this goal, one must consider a much larger cluster. 

M. Masella, and J. P. Flament, Relation between cooperative effects in cyclic water, methanol/ water, and methanol trimers and hydrogen bonds in methanol/water, ethanol/water, and dimethylether/water heterodimers, J. Chem. Phys. 108, 7141-7151 (1998). 

Cooperative Effect. - The ab initio study of Parra and Zeng194 focused on the cooperative effect in mixed dimers and trimers of methanol and trifluoro-methanol. Cooperativity is the enhancement of of the dipole moment in a HB complex compared to the sum of the dipole moments of the constituents. The authors interpreted the value of pb for the C-O and O-H bonds in four dimers and four trimers in terms of bond strength. A weakening of the O-H bond is seen to be favourable for hydrogen bonding. Another indication of cooperativity is noticed in the strengthening of the HB in the trimer as opposed to the dimer. Their AIM observations provide a consistent picture in support of the cooperative effect. 

A review has illustrated the importance of atomic-level DFT studies in elucidation of the function of hydrogen bonds in organocatalytic reactions through influence on the mechanism of substrate activation and orientation, and the stabilization of transition states and intermediates. Examples discussed include stereoselective catalysis by bifunctional thioureas, solvent catalysis by fluorinated alcohols in epoxidation by hydrogen peroxide, and intra-molecular cooperative hydrogen bonding in trans-a,a -(dimethyl-l,3-dioxolane-4,5-diyl)bis(diphenyl methanol) (TADDOL) (7)-type catalysts. ... 

Mo O, Yanez M, Elguero J (1994) Cooperative effects in the cyclic trimer of methanol. An ab initio molecular orbital study. J Mol Sttuct Theochem 314(l-2) 73-81 Suhai S (1994) Cooperativity and eleetron correlation effects on hydrogen-bonding in infinite systems. Int J Quantum Chem 52(2) 395-412. doi 10.1002/qua.560520213... 

---