In search for an optimal methodology to calculate the valence electron affinities of temporary anions

Abstract

Recently, we have proposed an approach for finding the valence anion ground state, based on the stabilization exerted by a polar solvent; the methodology used standard DFT methods and relatively inexpensive basis sets and yielded correct electron affinity (EA) values by gradually decreasing the dielectric constant of the medium. In order to address the overall performance of the new methodology, to find the best conditions for stabilizing the valence state and to evaluate its scope and limitations, we gathered a pool of 60 molecules, 25 of them bearing the conventional valence state as the ground anion and 35 for which the lowest anion state found holds the extra electron in a diffuse orbital around the molecule (non valence state). The results obtained by testing this representative set suggest a very good performance for most species having an experimental EA less negative than -3.0 eV; the correlation at the B3LYP/6-311+G(2df,p) level being y = 1.01x + 0.06, with a correlation index of 0.985. As an alternative, the time dependent DFT (TD-DFT) approach was also tested with both B3LYP and PBE0 functionals. The methodology we proposed shows a comparable or better accuracy with respect to TD-DFT, although the TD-DFT approach with the PBE0 functional is suggested as a suitable estimate for species with the most negative EAs (ca.-2.5 to -3.5 eV), for which stabilization strategies can hardly reach the valence state. As an application, a pool of 8 compounds of key biological interest with EAs which remain unknown or unclear were predicted using the new methodology.

In search of the best DFT functional for dealing with organic anionic species

“…Direct EA computation The procedure followed has been previously described. 23,27,28 The EAs were computed as the energy difference between the anionic and the neutral species. In the case of vertical EAs (VEA) the energy of the anion was calculated at the frozen geometry of the neutral, (eq.…”

“…28 From the latter set, five limiting cases were included, compounds 56-60, whose V states could not be found by extrapolation. Since the standard ETS derived data point to V anions and, as previously denmostrated, 23,27 only these anions can be correlated with the experimental EAs, Figure S-0 included in the SI. The species of both groups which afforded N anions after the corresponding procedure were not considered for the correlations and further statistical analysis for benchmarking the functionals.…”

“…26 Latter on, two practical methods for solving the problem of the EA of N anions were published. 27,28,29 One alternative to overcome this problem is the use of solvents of different polarities (ε) for getting the V anion and then extrapolating the energies of the "solvated" V anions to the limiting gas phase value of ε=1. The other approach uses the TDDFT approximation for correcting the energy of the anion affording good results even for species of EAs lower than -2.5 eV up to a limit value of -3.5 eV.…”

“…The other approach uses the TDDFT approximation for correcting the energy of the anion affording good results even for species of EAs lower than -2.5 eV up to a limit value of -3.5 eV. 27 In these studies only B3LYP, PBE0 and PW91 functionals were evaluated being B3LYP the best. When changing from gas phase (EA) to solution, the key task is the computation of the reduction potential ‫ܧ(‬ ோௗ ).…”

Should negative electron affinities be used for evaluating the chemical hardness?

“…13,15,[26][27][28] In addition, the working equation must be closely correlated to the physical properties associated with hard (soft) species. [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] A negative electron affinity indicates that the ''outermost'' electron is described by a resonance embedded in the continuous spectrum of the neutral species. A survey of the physical and chemical properties of the hardness was recently performed by one of the authors.…”