Influence of through-space and through-bond interactions on the ionization behavior of pyridyl and diazinyl radicals: A computational study

Understanding the structure and properties of heterocyclic radicals and their cations is essential for elucidating reaction mechanisms, as these species act as versatile reactive intermediates in fields ranging from synthetic organic chemistry to atmospheric, combustion and interstellar chemistry. In this work, we investigated the role of orbital interactions, namely through-space (TS) and through-bond (TB) interactions, between the nitrogen lone pair and the radical electron in the ionization of pyridyl radicals and diazinyl radicals, as well as in the formation of the corresponding pyridyl cations and diazinyl cations. The photoelectron spectra of six dehydrodiazine radicals ( 2a-c , 3a-b and 4a ) were simulated through the calculation of Franck-Condon (FC) factors using density functional theory (DFT) at the B3LYP/aug-cc-pVQZ level of theory. Additionally, the adiabatic ionization energies (AIEs) of all three pyridyl radicals and six diazinyl radicals leading to their singlet and triplet cations were computed using DFT as well as the G4, CBS-QB3 and W1BD composite methods. Our investigation revealed that the presence of strong TS and TB interactions in the radicals leads to lower adiabatic ionization energies because the corresponding cations are also strongly stabilized through these interactions. In contrast, higher AIE values are observed for radicals in which TS and TB interactions are minimal, resulting in cations that are not effectively stabilized by orbital interactions. Interestingly, the singlet-triplet energy gap (ΔE S-T ) of the resulting pyridyl and diazinyl cations depends directly on the extent of TS interaction, where stronger TS interaction in the radical leads to larger ΔE S-T gaps.