Water electronic ground state

Processes (I) and (II) account for H20, whereas processes (I), (III), and (IV) describe the fate of e . According to Kaplan et al. [11], process (I) produces water molecules in high vibrational levels of their electronic ground state. The remaining H2O reacts with water to form H+q and OH in process (II). This ion-molecule reaction is known to occur in the gas phase with a rate constant of 8x10 dm mol sec [12], which, when extrapolated to liquid water, sets the lifetime of H20 in this medium at less than 10 " sec. However, Hamill [13] pointed out that H2O initially has the structure of a neutral water molecule so that it may migrate rapidly over distances of a few molecular diameters by resonant electron transfer with a succession of neighboring water molecules. 

With six electrons from oxygen and two electrons from the hydrogens, the electronic ground state of water in this formulation is... 

The material model consists of a large assembly of molecules, each well characterized and interacting according to the theory of noncovalent molecular interactions. Within this framework, no dissociation processes, such as those inherently present in water, nor other covalent processes are considered. This material model may be described at different mathematical levels. We start by considering a full quantum mechanical (QM) description in the Born-Oppenheimer approximation and limited to the electronic ground state. The Hamiltonian in the interaction form may be written as ... 

Figure 3.33 PE profiles of the electronic ground state (circles), the lowest 1 tttt state (squares) and the lowest 17rcr state (triangles) of (a) the phenol-water cluster and (b) the phenol-ammonia cluster as a function of the hydrogen transfer coordinate, calculated with the CASPT2 method [32].

The electronic ground state of the water molecule is Ai.83 In the region 1430-1860 A. (Fig. 18), there is a continuum whose maximum lies at about 1655 A. with / = 0.041.84 There appears to be some indication of weak bands. The second continuum, from 1250-1430 A., is clearly associated with superimposed diffuse bands and has / = 0.05. In both continua, predissociation is evident. Below 1250 A., a number of strong bands appear. In particular, the band at 1240 A. corresponding to the transition88 is of interest since some photochemistry has been done in this region. 

A promising method, developed in recent years, is the use of first principles molecular dynamics as exemplified by the Car-Parrinello technique (8]. In these calculations the interatomic potentials are explicitly derived from the electronic ground-state within the density functional theory in local or non-local approximation. It combines quantum mechanical calculations with molecular dynamics simulations and, therefore, overcomes the limitations of both methods. Actual computers allow only simulations of aqueous solutions of about 60 water molecules for several ps (10 s). This limit is still at least one order of magnitude shorter than the fastest directly measured water exchange rate, k = 3.5 x 10 s for [Eu(H20)8], i.e. one exchange event every (8 x 3.5 x lO s ) = 36 ps [9]. Nevertheless, several publications appeared in the late 1990s on solvated Be [10], K+ [11] and Cu + [12] presenting mainly structural results. 

Ten years ago, femtosecond IR spectroscopy of an excess electron in pure water showed the existence of an ultrashort-hved prehydrated state (61). This IR nonequihbrium electronic configuration is built up in less than 120 fs in H2O and represents a direct precursor of the hydrated electron ground state (equation 6). In the infrared (0.99 eV), the monoexponential relaxation of the signal toward an s-hke ground state of the hydrated electron (240 20 fs) has been analyzed in the framework of a two-state model (61, 65). With a similar model, an indirect estimate of the infrared electron relaxation in the red spectral region gives a deactivation rate of 2 X 10 s (62, 66). The very fast appearance of the infrared electron (efn) is comparable to any nuclear motion, solvent dipole orientation, or thermal motion of water molecules. The relaxation of... 

It is an empirical fact that for most molecules in their electronic ground states the electronic wave function belongs to the (qondegenerate) totally symmetric species. Also the electronic spins are usually all paired in the ground state, and the ground state is a singlet. For water the ground electronic state is Au for benzene it is... 

Now let us take two interacting water molecules. First, let us ask how many minima we can find on the electronic ground-state energy hypersurface. Detailed calculations have shown that there are two such minima (see Fig. 7.9). The global minimum corresponds to the configuration characteristic for the hydrogen bond (cf. p. 863). One of the molecules is a donor, and the other is an acceptor of a proton (Fig. 7.9a). A local minimum of smaller stability appears when one of the water molecules serves as a donor of two protons, while the other serves as an acceptor of them called the bifurcated hydrogen bond," (Fig. 7.9b). 

---