Density Functional Theory Calculations on Hydrogen-Bonded Tropolone−(H2O)2Clusters

Abstract

The geometries, harmonic vibrational frequencies, and binding energies of tropolone−H2O and tropolone−(H2O)2 complexes have been investigated through the use of the B3LYP density functional approach. The geometries of the different complexes were optimized using 6-31G(d) and 6-31+G(d) basis sets. The harmonic frequencies were evaluated using the latter basis. The binding energies for the most stable clusters were calculated at the B3LYP/6-311+G(3df,2p) level. We have located nine different local minima which meet the general criteria posed by the experimental infrared spectra of possessing at least one free OH oscillator and only two hydrogen-bonded oscillators, which act as proton donors. However, only the two most stable complexes present frequency gaps between the free OH and the hydrogen-bonded OH stretching modes in agreement with the experimental values. These two species, Tw2ring and Tw2ext, are very near in energy to one another, although they present very different bonding characteristics since in the former the intramolecular hydrogen bond of the tropolone moiety is broken, while this is not the case for the latter. The binding energies of both complexes (10.0 and 9.9 kcal/mol, respectively) are sizably larger than those predicted for tropolone−H2O complexes. This indicates that significant cooperative effects appear when a second water molecule is added to the system. These cooperative effects are reflected in the strengths of the intra- and intermolecular hydrogen bonds and in the shiftings of the vibrational modes affected by the hydrogen bonding. Although using the available experimental information, one cannot favor one isomer over the other, according to our results they could be discriminated in terms of the shifting of the O−H stretching and bending modes of the tropolone moiety. For Tw2ext the stretching mode should appear slightly blue-shifted, while for Tw2ring it should appear red-shifted. Similar differences in the out-of-plane bending mode should also be observed. Both complexes have also quite different rotational constants, so nonnegligible dissimilarities should be observed in their microwave spectra.