Singlet open-shell diradical nature and redox properties of conjugated carbonyls: a quantum chemical study

Abstract

In the present work, a set of 90 conjugated carbonyls based on cyclic hydrocarbons with different ring sizes are studied by means of theoretical methods of quantum chemistry in order to analyze their singlet open-shell diradical character and its influence in different molecular properties. In particular, we have investigated the molecular structures, singlet–triplet gaps, ionization energies (IE), electron affinities (EA), first (E 1) and second (E 2) redox potentials and coordination of the reduced species to lithium cation. The diradical character is estimated using the diradical index from the spin-projected unrestricted Hartree–Fock theory (y PUHF). Non-negligible diradical character is accounted for most of the studied molecules, and trends may be devised in relation to their molecular properties. Thus, in those groups where several or all molecules show certain diradical character, this feature accomplishes larger EAs, E 1 and E 2. In this study, 35 carbonyls display E 1 values greater than 3 V, and the largest calculated value is 4.22 V. Regarding the IEs, no clear relationship may be established, although in some cases with large values of y PUHF small IEs are recorded. Finally, the binding energy to Li+ mainly depends on the relative position of the carbonyls, and the lithiation process is favored when the carbonyl moieties are adjacent or close in the molecule. These results highlight the importance of the diradical character on conjugated carbonyls and its effect in molecular properties such as redox potentials, a characteristic that may be exploited both theoretically and experimentally to provide with a deeper insight on the application of these diradicals as organic cathode materials.