Are water-aromatic complexes always stabilized due to pi-H interactions? LMP2 study

摘要

Eight complexes of various aromatic molecules with water have been studied theoretically at the local Moller-Plesset 2nd order theory (LMP2)/aug-cc-pVTZ(-f)//LMP2/6-31+G* level of theory. Two types of complexes can be formed, depending on the electronic structure of aromatic molecules. Donor hydrocarbons form A-type complexes, while aromatics bearing electron-withdrawing substituents form B-type complexes. A-type complexes are stabilized due to pi-H interactions with the OH bond pointing to the aromatic molecule plane, while B-type complexes have geometry with the oxygen atom pointing to the aromatic molecule plane stabilized by the interaction of highest occupied molecular orbital (HOMO) of water molecule with pi* orbitals of the aromatics. It has been found that a (-HOMO-lowest unoccupied molecular orbital (LUMO)/2 value of aromatic molecule, which can be called "molecular electronegativity," is useful to predict the type of complex formed by aromatic molecule and water. Aromatic hydrocarbons with "molecular electronegativity" of < 0.15 tend to form A-type complexes, while aromatic molecules with "molecular electronegativity" of < 0.15 a.u. form B-type complexes. The binding energy of water-aromatic complexes undergoes a minimum in the area of switching from A-type to B type complexes, which can be rationalize in terms of frontier orbital interactions. (c) 2005 Wiley Periodicals, Inc.