HYDROGEN-BONDED DIMERS OF CH3OCH2CH2OH - AB-INITIO STRUCTURES AND MULTIVARIATE-ANALYSIS

Structures and vibrational frequencies of relevant hydrogen-bonded dimers of 2-methoxyethanol are obtained at the 3-21G*//AM1 level (the 3-21G* basis set included polarization functions deliberately added to second period atoms) and the results analyzed by standard multivariate methods. Dimers built from the same pair of isolated monomers belong to the same homologous series. The most stable dimer is obtained from the most stable conformations of both monomers, has a 10-membered intermolecular ring with a boat-chair-boat conformation, and presents a zero dipole moment. In addition, the most stable dimers of the three homologous series have 10-membered rings with relatively symmetric conformations and zero or very low dipole moments. The energies of the monomers in these dimers-monomers with geometries frozen in the dimer-are close to the isolated and relaxed monomers energies. Generally speaking, OH...O and CH...O hydrogen bonds are found to contribute appreciably for the stability of the dimers. In particular, for the second most stable dimer, two of the three CH...O hydrogen bonds are shorter than the intramolecular OH...O bond for the isolated monomer. Multivariate analysis of all the results shows that the O...H-O angle is highly correlated with the H-O bond length and that the COC and OCC angles are also highly correlated for both monomers, hence confirming that each monomer in a dimer keeps essentially the same conformation it has in the isolated monomer. The first and second principal components include all the highly correlated variables and account for 45% of the total variance. 3-D plots of the three most important principal components confirm a strong structural resemblance both for 10-membered ring dimers of homomeric series, i.e., formed by the same monomers, and for the more stable dimers in each homologous series. In addition, among the heteromeric dimers with seven membered rings, the ones whose first conformation is G or G' are less stable and located in a separate group.