It has recently been shown that cycl[3.3.3]azine and heptazine (1,3,4,6,7,9,9b-heptaazaphenalene) as well as related azaphenalenes exhibit inverted singlet and triplet states, that is, the energy of the lowest singlet excited...

Ab initio computational methods are employed to explore whether hydrated electrons can be produced by the photodetachment of the excess hydrogen atom of the heptazinyl radical (HzH) in finite-size HzH•••(H 2 O) n clusters. The HzH radical is an intermediate species in the photocatalytic oxidation of water with the heptazine (Hz) chromophore. Hz (heptaazaphenalene) is the monomer of the ubiquitous polymeric wateroxidation photocatalyst graphitic carbon nitride (g-C 3 N 4 ). The energy profiles of minimum-energy excited-state reaction paths for proton-coupled electron transfer from HzH to water molecules were computed for the HzH•••H 2 O and HzH•••(H 2 O) 4 complexes with the CASPT2 method. The results reveal that the photodetachment of the excess H-atom from the HzH radical is a barrierless reaction in these hydrogen-bonded complexes, resulting in the formation of H 3 O and H 3 O(H 2 O) 3 radicals, respectively, which are finite-size models of the hydrated electron. The computational results suggest that the photocatalytic formation of hydrated electrons from water with visible light could be possible in principle.

In the present work, the mechanisms of the reduction of the CO2 molecule with hydrated electrons to the hydroxyl-formyl (HOCO) radical were studied with ab initio computational methods. Hydrated hydronium radicals, H3O­(H2O) n (n = 0,3,6), are considered as finite-size models of the hydrated electron in liquid water. The investigation of cluster models allows the application of high-accuracy electronic-structure methods, which are not computationally feasible in condensed-phase simulations. Reaction paths and potential-energy (PE) profiles of the proton-coupled electron-transfer reaction from hydrated H3O radicals to the CO2 molecule were explored on the ground-state PE surface. The computationally efficient unrestricted second-order Møller–Plesset method is employed, and its accuracy has been carefully benchmarked in comparison with complete-active-space self-consistent-field and multi-reference second-order perturbation calculations. The results provide insights into the interplay of electron transfer from the diffuse Rydberg-type unpaired electron of H3O to the CO2 molecule, the contraction of the electron cloud by the re-hybridization of the carbon atom of CO2, and proton transfer from the nearest water molecule to the CO2 – anion, followed by Grotthus-type proton rearrangements to form stable clusters. Starting from local energy minima of hydrogen-bonded CO2–H3O­(H2O) n complexes, the reaction to form HOCO–(H2O) n+1 complexes is exothermic by about 1.3 eV (125 kJ/mol). The reaction is barrier controlled with a barrier of the order of a few tenths of an electron volt, depending on size and conformation of the water cluster. This barrier is at least an order of magnitude lower than the barrier of the reaction of CO2 with any closed-shell partner molecule. The HOCO radicals can recombine by H-atom transfer (disproportionation), resulting in formic acid or a dihydroxycarbene product, as well as by the formation of a C–C bond, resulting in oxalic acid. The strong exothermicity of these radical–radical recombination reactions likely results in the fragmentation of the closed-shell products formic acid and oxalic acid, which explains the strong specificity for CO formation observed in recent experiments of Hamers and co-workers.

The photocatalytic conversion of carbon dioxide to liquid fuels with electrons taken from water with solar photons is one of the grand goals of renewable energy research. Polymeric carbon nitrides recently emerged as metal-free materials with promising functionalities for hydrogen evolution from water as well as the activation of carbon dioxide. Molecular heptazine (Hz), the building block of polymeric carbon nitrides, is one the strongest known organic photo-oxidants and has been shown to be able to photo-oxidize water with near-visible light, resulting in reduced (hydrogenated) heptazine (HzH) and OH radicals. In the present work, we explored with ab initio computational methods whether the HzH chromophore is able to reduce carbon dioxide to the hydroxy-formyl (HOCO) radical in hydrogen-bonded HzH-CO 2 complexes by the absorption of a photon. In remarkable contrast to the high barrier for carbon dioxide activation in the electronic ground state, the excited-state proton-coupled electron transfer (PCET) reaction is nearly barrierless, but requires the diabatic passage of three conical intersections. The possibility of barrierless carbon dioxide activation by excited-state PCET has so far not been taken into consideration in the interpretation of photocatalytic carbon dioxide reduction on carbon nitride materials.