Structures and excitation energies of ozone–water clusters O3(H2O)n (n = 1–4)

Hybrid density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been carried out for ozone–water clusters O3(H2O)n (n = 1–4) in order to obtain hydration effects on the absorption spectrum of ozone. The first water molecule in n = 1 is bound to the ozone molecule by an oxygen orientation form in which the oxygen atom of H2O orients the central oxygen atom of O3. In n = 2, the water dimer is bound to O3 and then the cyclic structure is formed as the most stable structure. For n = 3 (or n = 4), the cyclic water trimer (or tetramer) is bound by a hydrogen bond to the ozone molecule. The TD-DFT calculations of O3(H2O)n (n = 0–4) show that the first and second excitation energies of O3 are blue-shifted by the interaction with the water clusters. The magnitude of the spectral shift is largest in n = 2, and the shifts of the excitation energies are +0.07 eV for S1 and +0.13 eV for S2 states. In addition to the spectral shifts (S1 and S2 states), it is suggested that a charge-transfer band is appeared as a low-lying excited state above the S1 and S2 states. The origin of the spectrum shifts was discussed on the basis of theoretical results.