Electronically excited states of closed-shell azabenzene and purine anion structures: Is 2 D enough for dipole-bound excited states?

Abstract

Deprotonated azabenzene anions require dipole moments in their corresponding neutral radicals of more than 3.5 D in order to exhibit dipole-bound excited states (DBXSs). This is notably larger than the typical 2.0–2.5 D associated with such behavior. Similar computational analysis on deprotonated purine derivatives also conducted herein only requires the more traditional 2.5 D dipole moment, implying that the single six-membered azabenzene rings have additional factors at play in binding diffuse electrons. The present study also shows that the use of coupled cluster singles and doubles with a double-zeta correlation consistent basis set and additional diffuse functions originating from the center-of-charge for all aspects of the computations decreases the error in predicting DBXSs to less than 0.006 eV at worst and likely less than 0.003 eV for most cases. These results can influence the modeling of molecular spectra beyond fundamental chemical curiosity with application to astrochemistry, solar energy harvesting, and combustion chemistry among others.