Dipole bound state

In Tables 20-5 and 20-6, we see that the largest disagreement among calculated values is for the base guanine, which has theoretical AEA values from —0.7 to 0 eV. The values near zero are a result of basis sets with diffuse functions which mix valence states with diffuse dipole bound states and do not represent good... 

Solvation of DNA bases/base pairs is of fundamental importance to biological processes as they take place in aqueous media. The effect of hydration on neutral bases or base pairs has been addressed using quantum chemical methods [106-112] as well as molecular dynamics (MD) simulations [113, 114], It is known that unlike the gas phase, dipole bound anions do not exist in condensed environments because such diffuse states are destabilized in the aqueous phase [115]. The drastic change in the nature of excess electron binding in the presence of water molecules with uracil has been observed experimentally by Bowen and co-workers [95b] using negative electron photoelectron spectroscopy (PES). They observed that even with a single water molecule the dipole bound state of uracil anion in gas phase... 

ECD, TCT, and AMB studies. In the case of the PES data there are low-energy bands that can be attributed to excited valence states, one of which is a dipole bound state. The new question becomes, Why are the excited states not stabilized to the ground state The answer could be that the pressure is not sufficiently high [43 16],... 

Nitromethane makes a nice transition from small molecules to large organic molecules. The Ea of N02 was determined using the ECD (above 425 K) from the activation energy for dissociative thermal electron attachment to CH3N02. The Ea of CH3N02 was determined from data in the lower-temperature region. The data were presented in Chapter 5 as an example of the determination of a Qan much smaller than 1. The ECD Ea of 0.50 0.02 eV for nitromethane is supported by AMB and TCT values. The PES value is 0.26 0.08 eV and could exist for an excited state. A dipole bound state is observed at 0.012 eV [1-13]. 

In the C—N dimension anion curves are drawn to four different dissociation limits the two complementary limits of N02(—) and CH3 and two limits leading to excited states of N02(—) and CH3. Dipole bound states could also be drawn in these dimensions. The ground-state curves and dipole bound curves are 2). The lower excited-state curve A is M( 1) since the EDEA and VEa are negative but the Ea is positive. More than three data points from PES, ECD, El, or anion absorption and emission spectra define the ground state and the first excited valence state. The B state is D(0) but could lead to molecular anion formation via the C, D,... 

For cytosine two DBEA are reported 0.230 eV and 0.085 eV. Interestingly, the dipole bound states are observed in the PES of the dihydrates due to two-photon absorption. The first photon removes the waters and then a second photon detaches the electron from the dipole bound state. These spectra for cytosine are shown in Figure 12.3. The spectrum of the bare cytosine is shown in the inset offset by 1.5 eV. The peak at 0.230 eV is assigned to the dipole bound anion of keto-cytosine, whereas the peak at 0.085 eV is assigned to the enol form of the anion. The dipole moment of the enol form is about 4 Debye, while that for the keto form is about 6.5 Debye. The two peak intensities are quite different in the spectrum for the monomer. In the spectrum for the dihydrate the intensities are about the same. The double-photon process explains this difference. The absorption of the initial photon by the dihydrate leaves equal concentrations of two forms that are then photode-tached. In the spectrum of the bare anion the distribution reflects the equilibrium concentrations. The extra structure in the spectrum of the dihydrate can be attributed to excited states of the anion and offers a different interpretation of the onset. 

Figure 12.15 Three-dimensional spin densities for the planar guanine anion calculated using AMI. The spin density is localized on the hydrogen bonding sites and is similar to a dipole bound state.

A brief discussion of dipole-and quadrupole-bound states was recently included in a review of neutral/neutral charge transfer reactions.In summary, although there exists some experimental and theoretical evidence for quadrupole-bound states, their existence is not as certain as the case dipole-bound states. ... 

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... 

Even more so than for the case of radiative association, very little experimental information exists for radiative attachment, and the reliability of the simple phase-space treatment has not been tested. One possible problem is that the formation of the negative ion complex may require so-called doorway states known as dipole-bound states, in a similar manner to the Rydberg mechanism for dissociative recombination of positive molecular ions. Such states have been seen in the spectra of negative ions, although a detailed calculation employing them for radiative attachment has not been attempted. 

Petrie S, Herbst E. (1997) Some interstellar reactions involving electrons and neutral species Attachment and isomerization. Astrophys. J. 491 210-215. Giithe F, Tulej M, Pachkov MV, Maier JP. (2001) Photodetachment Spectrum of EC3H2H The Role of Dipole Bound States for Electron Attachment in Interstellar Clouds. Astrophys. J. 555 466-471. 

Rotational analysis of the autodetachment spectra of CH2CN and CD2CN . No structural parameters were deduced for the ground state of the ion since there is evidence of a large-amplitude out-of-plane hydrogen motion. The small inertial defect for the excited dipole-bound state indicates that it is planar. 

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)... 

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). 

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