DFT (U)B3LYP/cc-pVTZ and (U)M06-2X/cc-pVTZ and multireference CASSCF/cc-pVTZ level of theories have been used to investigate the electronic structure of isomeric dehydrooxazole (1b-d, 3 isomers), dehydrothiazole (2b-d, 3 isomers) and dehydroimidazole (3a-d, 4 isomers) radicals. The ground state electronic structure of each radical isomer has been confirmed by predicting their doublet excited state structure and calculating the adiabatic energy difference. The stability order of the individual isomeric radicals has been estimated through the comparison of absolute energies. A hypothetical isodesmic reaction has been utilized to calculate radical stabilization energies (RSEs). The results show N-dehydro imidazole 3a as the thermodynamically most stable radical. In order to understand the structural and stability aspects of the radicals and interactions between the radical electron and the electron lone pairs, we have analysed spin densities and hybridisation changes and also performed MCSCF calculations. The reason for the higher stability of 3a has been attributed to the attainment of a π-character and subsequent delocalization, whereas, all other carbon centred radicals are found to be localized σ-radicals. Furthermore, the kinetic stability of the radicals has been investigated through unimolecular decomposition channels. All the studies showed a weak to strong coupling between the N3 and the radical centre depending on the location of the radical centre. NBO analysis suggests that through space coupling between N3 and σ* of the radical centre leads to a stabilising effect when the radical centre is adjacent to N3, whereas such interactions are absent when the radical centre is away from it. However, only a weak coupling is observed between the radical centre and X1 (O/S/NH). Particularly the interaction strength has the following trend: S < O < N-H. Indeed S, O and N-H show a stabilising effect through bond interactions with the antibonding orbitals of the alternate bonds on either side.

Each of the three isomeric pyridyl radicals (2-, 3-, and 4-dehydropyridines) contains a lone pair and an unpaired electron. As a result, a potential two centered-three electron interaction between the radical electron and the lone pair through-space (TS) and/or through-bond (TB) can exist that may influence the stability of the radicals. Due to the change in geometrical positions relative to each other, the strength of interaction can be varied. In this study, we investigated the structural and stability aspects of pyridyl radical isomers with a major emphasis on the interaction of a nitrogen lone pair with the radical center. In order to obtain evidence for such interactions, protonated and N-oxide analogues of the corresponding isomeric pyridyl radicals have been considered in such a way to understand the consequences due to unavailability of the lone pair. Similarly, electron attachment and detachment energies at the radical center (vertical detachment energy, VDE, of corresponding anions and vertical ionization energy, VIE, of radical isomers) have been calculated to find out the interaction trend upon modification at the radical center. Different levels of theory including (U)B3LYP/cc-pVTZ, (U)M06/cc-pVTZ, CBS-QB3, single-point energy calculations at (U)CCSD(T)/cc-pVTZ, and multireference CASSCF/cc-pVTZ methods have been employed in this regard. A closer inspection of geometries, relative stability order, spin density, electrostatic potential, molecular orbitals, NBO analysis, and vibrational analysis have showed a strong and stabilizing TS interaction between the radical center and the lone pair in the case of the 2-pyridyl radical. On the other hand, the 4-pyridyl radical showed stabilizing interactions only via TB coupling, whereas the TS interaction is nonexistent. Despite the presence of both interactions in the case of the 3-pyridyl radical, their overall influence is less effective toward stability.

The introduction of defect-pairs into centrosymmetric rutile TiO2 produces a new form of collective nonlinear electrical polarization.