Understanding electronic properties of water: a theoretical approach to the calculation of the adiabatic band gap of liquid water

Tese de doutoramento em Química (Química-Física), apresentada à Universidade de Lisboa através da Faculdade de Ciências, 2008Electronic properties of condensed phase water are not very well understood. They are, however, of fundamental importance for a molecular level understanding of the properties of water as well as the role of water, both as participant and medium where chemical reactions occur. It is generally accepted that bulk water can be described as a lone pair amorphous insulator or a large gap lone-pair amorphous semiconductor and the condensed phase ionization potential de_ned as a vertical quantity. A usually accepted value is 8.7 _ 0.5 eV. However, a de_nition of condensed phase properties such as the condensed phase ionization potential is not straightforward and ultimately dependent our understanding of the elementary mechanisms for ionization/excitation in condensed phase water. A longstanding issue is the concearns the fact that hydrated electrons are produced by photoabsorption at _6.5 eV. It has recently been pointed out that the ability of water to reorganize about charged (and neutral) molecules, as well as the reactive nature of electronically excited water molecules, should be taken into account in explaining observed aqueous anion thermochemical and photochemical properties. Moreover, it has been shown that an adiabatic route for accessing the conduction band of liquid water can be de_ned and that the bottom of the conduction band is characterized by the reorganization of the water molecules around the H3O+ cation and OH radical as well as by the presence of a delocalized or quasi-free electron. A _6.9 eV value was proposed by J.V. Coe [Int. Rev. Phys. Chem. 20, 33, 2001], based on known aqueous anion thermochemical and photochemical properties,The present work presents a theoretical estimate for two key reference quantities required for the quantitative estimate of the adiabatic band gap of pure liquidwater as de_ned by J.V. Coe: the hydration energy of the hydroxyl radical andthe electron a_nity of liquid water. The hydration of the hydroxyl radical wasinvestigated by microsolvation modeling and statistical mechanics Monte Carlosimulation. The electron electronic density of states and a_nity of liquid water wasinvestigated trough a sequential Quantum Mechanical / Monte Carlo methodology.Finally, as the adiabatic approach for a de_nition of a band gap for pure liquidwater is based on macroscopic quantities, a molecular level veri_cation of the adiabatic picture was also undertaken