Abstract
The interaction of low-energy scattering electrons/positrons with molecular targets characterized by a “supercritical” permanent dipole moment (≳2.0 D) presents special physical characteristics that affect their spatial distributions, around the nuclear network of the molecular partners, both above and below the energy thresholds. Such special states are described as either dipole scattering states (DSS) above thresholds or as dipole bound states (DBS) below thresholds. The details of their respective behaviour will be presented and discussed in this work in the case of the purinic DNA bases of adenine and guanine. The behavior of the additional electron, in particular, will be discussed in detail by providing new computational results that will be related to the findings from recent experiments on the same DNA bases, confirming the transient electron’s behaviour surmised by them.
Highlights
An interesting finding by Fermi and Teller [1,2], which was rediscovered once again in the 60’s [3,4], dealt with the special properties of a stationary electric dipole of supporting bound states only if the dipole moment exceeds a critical value of about 0.639 a.u
Their spatial shapes play an important role in the physics that leads to stable anions’ final formation since the spatial proximity of such nearthreshold electrons increases overlap effects with the final states and favours anions’ restructuring. They are further affected by environmental dynamics and by the operating conditions of the impinging electrons: from the solutestate of biosystems to the low-temperature, low-density characteristics of the interstellar space anions [14] the role of dipole bound states (DBS) states has been shown to provide pathways for structural rearrangements and energy redistributions after a molecule interacts with low-energy scattering electrons
The computational studies related to such possible anions indicate them to be described by diffuse orbitals located outside the molecular structures, just as we found here to be the case for the near-threshold dipole scattering states (DSS) scattering electrons for the same nucleic bases (NBs) systems [51]
Summary
An interesting finding by Fermi and Teller [1,2], which was rediscovered once again in the 60’s [3,4], dealt with the special properties of a stationary electric dipole of supporting bound states only if the dipole moment exceeds a critical value of about 0.639 a.u. The presence of free electrons in their environment has been considered important since they are known to be produced via radiative ionization of the surrounding water molecules [10,11] and capable of driving further changes in the biological species interacting with them [12] Since such molecular systems exhibit in the gas-phase large permanent dipoles but often have large and positive electron affinity (EA) values [8] indicated that they can in principle form two different types of bound molecular anions (radical states for closed shell targets): one with a diffuse extra electron trapped by the dipole potential of equation (1) to the molecular target, and another more strongly bound with the extra electron closer to the molecular nuclear framework.
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