CCL: What it is the correlation between different ionization potentials

[Arvydas Tamulis <tamulis,,mserv.itpa.lt>]

 Sent to CCL by: Arvydas Tamulis [tamulis_+_mserv.itpa.lt]
 Dear Colleagues,
Usually we obtain quantum mechanically (QM) the ionization potentials
 which corresponds well with the experimental data of ionization
 potential
 of He(I) UV photoelectron (PE) measurements.
 For example, my last year obtained ionization potential using
 Koopmans' theorem approximation for guanine is equal to -4.82 eV
 (calculations were performed with PBEPBE/6-31+G(2p,2d) method).
I see that biologists (see example: K. Sugden, B. Martin, Environ
 Health Perspect. 2002 October; 110(Suppl 5): 725728.) are
 using data of oxidation potential versus to the normal hydrogen
 electrode.
 In this article reference [19] the ionization potential for guanine
 is equal to -1.29 V.
 My question is what it is the correlation between values of these two
 experimental measurements: He(I) UV photoelectron (PE) and oxidation
 potential versus to the normal hydrogen electrode? How to transform our
 obtained QM results to the experimentally observed oxidation potential
 versus to the normal hydrogen electrode?
 With best regards,
 Arvydas Tamulis