Heteroatom-vacancy centres in molecular nanodiamonds: a computational study of organic molecules possessing triplet...

Small molecules possessing a triplet ground state are fundamentally intriguing but also in high demand for applications such as quantum sensing and quantum computing. Such molecules are rare, and most examples involve extended π-systems. Topology and shape of the spin density will be very different for molecules where the triplet state arises from σ-overlap. Drawing inspiration from NV^- (anionic nitrogen-vacancy) centres in a diamond crystal, which possess triplet ground states that are robust due to the distortion-preventing crystal lattice, we investigate hetero-atom substituted diamondoids (molecular nanodiamonds) as molecular mimics for NV^- centres. It is found that even in these small systems, distortions that stabilize singlet states are energetically costly, and the triplet states are more stable than the singlets. The stabilization of the triplet over the singlet is 13, 16, and 18 kcal mol^-1, in anionic C_3v-C₃₃H₃₆N^- and in the charge-neutral molecules C_3v-C₃₃H₃₆O and C_3v-C₃₃H₃₆S, respectively, using CAM-B3LYP-D3(BJ)/Def2-QZVPP. Comparable numbers are obtained with other density functional theory (DFT) methods, including double-hybrids. Wavefunction-based approaches on the other hand disagree in their predictions: While the MP2 method applied with the DLPNO approximation predicts a preference for the singlet, density matrix renormalization group (DMRG) calculations qualitatively agree with DFT in their prediction of a triplet ground state, although by a small margin, for C_3v-C₃₃H₃₆N^- and C_3v-C₃₃H₃₆O, but not for C_3v-C₃₃H₃₆S. Weighing the evidence, we conclude, with reasonable confidence for C_3v-C₃₃H₃₆N^- and C_3v-C₃₃H₃₆O and lesser confidence for C_3v-C₃₃H₃₆S, that the ground state for the molecular nanodiamonds studied is a triplet state.