Valence bound anions

Figure 79. Potential energy surfaces for CH3NO2 and (CH3N02) anions. The surface representing the dipole-bound anion lies ( 0.012 eV) below that of CH3NO2 and the valence-bound anion lies 0.26 eV below the neutral.

The discussion above describes the events of electron-molecule interaction in the gas phase. However, in condensed aqueous media, the nature of these processes is significantly altered. For example, dipole-bound states are not likely to be present as they are suppressed by the surrounding medium, and this is confirmed from the photoelectron spectroscopy of the hydrated uracil, thymine, cytosine, and adenine (Schiedt et al. 1998 Eustis et al. 2007) as the photoelectron spectra are blue-shifted with the increase of the number of hydrated water molecules and show valence-bound anion formation. For a visual inspection of this phenomenon, we plotted the LUMO and SOMO surfaces of guanine in neutral and anion radical states, using the B3LYP/6-311++G(2d,p) method in gas phase, and in aqueous media using polarized continuum model (PCM). The LUMO and SOMO siufaces in the gas phase and in the solvated phase of guanine in neutral and in anion radical states are shown in O Fig. 34-10. From O Fig. 34-10, it is inferred that LUMO and SOMO represent a dipole-bound state (Li et al. 2002) in the gas phase (see O Fig. 34-lOa, c), which is destabilized imder the influence of the full solvation and becomes the valence bound state (see O Fig. 34-lOb, d). 

The equation can also be illustrated in Figure 9.1. When a semiconductor such as Ti02 absorbs photons, the valence band electrons are excited to the conduction band. For this to occur, the energy of a photon must match or exceed the band-gap energy of the semiconductor. This excitation results in the formation of an electronic vacancy or positive hole at the valence band edge. A positive hole is a highly localized electron vacancy in the lattice of the irradiated Ti02 particle. This hole can initiate further interfacial electron transfer with the surface bound anions. 

Haranczyk M, Gutowski M (2005). Valence and dipole-bound anions of the most stable tautomers of guanine. J Am Chem Soc 127 699-706. 

It is generally agreed that nitrogen and the rare gases will not form bound valence-state anions because of their closed-shell configurations. The best adiabatic electron affinity for the elements for which Ea is a slightly positive value due to the polarization attractions is 0+. This is consistent with the precise definition of adiabatic electron affinity. It is more accurate than the statements less than zero, does not exist, or is unstable. It was once believed that the Group IIA and KB elements did not possess bound anion states. Subsequently, small positive Ea were measured for Ca, Sr, and Ba. 

Since the dipole bound anions are formed by double-photon absorption, the valence states could also be formed, in which case the onsets and initial peaks results from the photodetachment of the isolated ground-state or excited-state anions. These can be a measure of the Ea. The onsets that are lower than the... 

Nitrobenzene, j zra-dinitrobenzene (/zDNB) and me/a-dinitrobenzene (mDNB) anions have both diffuse multipole-bound and valence-anion states. For nitrobenzene and m-dinitrobenzene, the diffuse states are believed to correspond to dipole-bound anions whereas />DNB anions are considered to be primarily described as quadrupole-bound states. Nitrobenzene anions (NB ) in both valence and dipole-bound states were examined using RET spectroscopy. R<3ra-dinitrobenzene (zero dipole moment and a large quadrupole moment) and me <3-dinitrobenzene (large dipole moment and a small quadrupole moment) were also studied using R T 106 20 shows the /-dependence of the reaction rates for... 

Readers who wish to learn more about how molecular EAs (and to a lesser extent, IPs) have been studied theoretically are directed to this author s web site http //simons.hec. utah.edu as well as to a series [38] of his reviews and chapters. The species that this group have examined include dipole-bound anions, zwitterion ions, conventional valence anions, multiply charged anions as well as a wide variety of metastable anions. 

Left) The valence ai orbitals and expanded viewof thedipole-bound anion state (DBS) of uracil on the molecular frame. [Right) Potential energies of the neutral, dipole-bound state and valence cr anion state as a function of NrH separation (Reprinted with permission from Burrow et al. [2006]. (2006) American Institute of Physics)... 

B3LYP/6-311++G(2d,p)-calculated lowest unoccupied molecular orbital (LUMO) of neutral guanine in gas phase (a) and in solution (b). The singly occupied molecular orbital (SOMO) of guanine anion radical in gas phase (c) and in solution (d). The dipole-bound anion in (a, c) becomes a Ti -valence anion in (b, d). The effect of full solvation was considered through the use of polarized continuum model (PCM) considering the optimized gas-phase geometries... 

As a result of these competing interactions, the distinction between a valence anion and a dipole-bound anion is sometimes ambiguous. For example, the acetonitrile (CH3CN) molecule has a calculated dipole moment of 3.94 debye, well above the threshold value, and an experimental VDE of 0.012 These values might suggest that electron binding will be... 

Even in cases where it seems safe to classify M as a dipole-bound anion, the balance of long-range charge-dipole interactions and short-range valence interactions means that there is no clear correlation between the magnitude of the neutral molecule s dipole moment (call it hq) and the VDE of its anion. For example, whereas fiQ = 3.94 debye for CH3CN (computed at the MP2 level),and the experimental VDE of this anion is a mere 0.012 the... 

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