Exploring ground and low-lying excited states for diquat, paraquat, and dipyridyl isomers

In this work, we present a computational investigation on diquat, paraquat, and six dipyridyl isomers (2,2'-dipyridyl, 2,3'-dipyridyl, 2,4'-dipyridyl, 3,3'-dipyridyl, 3,4'-dipyridyl, and 4,4'-dipyridyl). Ground state properties such as equilibrium structures, relative energetics, transition states for cis-trans interconversion, and vibrational frequencies were determined for all the isomers at the MP2 level of theory with the cc-pVTZ basis set in the gas-phase; the MP2/cc-pVTZ and MP2/6-311+G(d,p) levels of theory in water were employed for diquat and paraquat. The trans structures are the most stable ones among the compounds that present such isomeric forms, with relative energies of 6.24 kcal/mol, 0.61 kcal/mol, and 0.12 kcal/mol lower than the cis counterparts in 2,2'-dipyridyl, 2,3'-dipyridyl, and 3,3'-dipyridyl, respectively. The transition state lies at 7.65 kcal/mol above the trans form in the case of 2,2'-dipyridyl, 3.68 kcal/mol for 2,3'-dipyridyl, and 2.02 kcal/mol for 3,3'-dipyridyl, indicating that the interconversion is feasible in the cases of 2,3'-dipyridyl and 3,3'-dipyridyl and unlikely to occur in 2,2'-dipyridyl at room temperature. Vertical excitation energies and respective generalized oscillator strengths (GOS) were determined using time-dependent density functional theory (CAM-B3LYP/cc-pVTZ and PBE0/cc-pVTZ) and EOM-CCSD/cc-pVTZ. In terms of excitation (and independent of computational method), all the isomers except 4,4'-dipyridyl, presented excited electronic states that are both bright (with GOS > 0.1) and energetically accessible at UV-vis wavelengths. Two bright states were found for diquat and paraquat in the UV region. Therefore, we expect that photoinduced degradation of both compounds can benefit from the utilization of techniques combining more than one source of radiation.