Eur. Phys. J. D (2017) 71: 92
https://doi.org/10.1140/epjd/e2017-70786-5

Regular Article

Transient negative ion spectrum of the cytosine-guanine pair

¹ Departamento de Física, Universidade Federal do Paraná, Caixa Postal 19044, 81531-990 Curitiba, Paraná, Brazil
² Instituto de Física, Universidade de São Paulo, Rua do Matão 1731, 05508-090 São Paulo, São Paulo, Brazil
³ Tecnologia em Luteria, Universidade Federal do Paraná, 81520-260 Curitiba, Paraná, Brazil
⁴ Instituto de Física Gleb Wataghin, UNICAMP, 13083-970 Campinas, São Paulo, Brazil

^a e-mail: ssanchez@fisica.ufpr.br

Received: 14 December 2016
Received in final form: 10 March 2017
Published online: 13 April 2017

Abstract

We employed elastic scattering calculations, performed in the static exchange approximation, to investigate the π^∗ transient anion states of cytosine, guanine and the cytosine-guanine pair. Our results for the isolated monomers, also obtained in the static-exchange plus polarization approximation, are in good agreement with the available calculations and electron transmission data. Virtual orbital analysis for the lower-lying π^∗ anion states, with pure shape resonance character, indicates that electron attachment to the cytosine-guanine pair gives rise to resonances located on either monomer (the orbitals do not delocalize over the pair). The π^∗ shape resonances of the pair localized on the cytosine unit have lower energies in comparison with those of the isolated base, with the opposite trend for the guanine unit. The underlying mechanism would be the net positive charge transfer to the cytosine unit, as the guanine monomer acts as a proton donor in two out of the three hydrogen bonds formed in the pair. Even though the calculations were performed in the static-exchange approximation (due to the size of the system), the conclusions drawn were also corroborated by empirical estimates of the vertical attachment energies. The results for the cytosine-guanine pair are compared to those previously obtained for the formic acid-formamide complex, having two hydrogen bonds with opposite donor/acceptor characters and negligible charge transfer.