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RESEARCH PRODUCT

On the hexagonal ice-like model of structured water: Theoretical analysis of the low-lying excited states

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Abstract The basic molecular unit ( BMU ) of the hexameric ice-like model has been identified as the negatively charged radical species [H 19 O 10 ] − . On the basis of high-level ab initio computations, by means of using the CASPT2 methodology, the low-lying excited states of BMU and its π-stacked dimer are analysed. It is concluded that the BMU constitutes certainly a versatile unit having two spectroscopic features in the near infrared region, two in the visible range and two in the near ultraviolet zone. The main absorption feature is found however for the π-stacked dimer formed by two BMUs , the H 38 O 20 system, which has a pronounced stabilization in the ground state with respect to the isolated BMUs . Most of the states of the π-stacked dimer, including the ground state, have a marked biradical character. In all the cases, the one- and two-electron promotions are located on the central O–H–O subunits of the fused hexagons. A total of twelve singlet → singlet electronic transitions have been computed for the H 38 O 20 system, seven in the visible range and five in the near ultraviolet region. In the latter, the electronic transition 1 1 A g  → 2 1 B 1u of π character is predicted at 4.58 eV (271 nm), with a computed oscillator strength of 0.29, in agreement with the experimental band maximum recorded at ∼270 nm (4.59 eV). The ability of water to build π-stacked aggregates under certain conditions is reinforced as the key feature to understand at the molecular level the intricacies of structured water.