We describe a particularly stable isolated divacancy [VCCV] in diamond, in which the two vacant sites are separated by two bonded carbons. Its structure, vibrational properties, and stability are described by using Density Functional Theory (DFT). We validated the calculated CC stretching frequencies for eleven molecules. The isolated divacancy [VCCV] is found to be very stable and is separated by a high barrier of 5 eV from the divacancy of two missing adjacent carbons, V2. The predicted characteristic CC stretching frequency of the isolated divacancy [VCCV] is 1607 cm−1, close to Raman bands at 1620−1630 cm−1 observed in various nanodiamonds, offering an alternative interpretation for the assignment of this vibrational band. We conclude that the alternative interpretation based on the CC stretching mode of an interstitial defect can be ruled out. Our accurate and validated vibrational calculations offer further evidence that the vibrational structure of the 3H optical center in diamonds should be assigned to the interstitial defect because it provides excellent agreement with the observed vibrational frequencies of three isotopologues.

We show that the reaction path connecting the tethered bi(anthracene-9,10-dimethylene) and its photodimer proceeds stepwise via a diradicaloid transition state where one σ-bond is made before a second. The newly found transition state (TS) has a smaller molecular volume than either the reactant or the product giving an atomistic explanation to the recently found pressure catalyzed barrier lowering and rate enhancement. The density functional methods used include long-range contributions as required in a system where the dispersion interactions are significant. We discuss this transformation in the context of the diamond-to-graphite transition owing to the similarity of σ-bond breakage into a delocalized π-system. We also comment on the controversy surrounding the equilibrium geometries of photoisomerized cyclophanes, concluding that D2h symmetry in the photoisomer of the title molecule is a transition structure connecting a pair of degenerate ground state D2 geometries.

Diffusion of monovacancies in diamond creates various trapped multivacancy clusters. The simplest diffusion process leads to a divacancy, V2. The formation of V2 is a critical step in the formation of larger vacancy clusters. We explored the relaxed potential energy surfaces in the formation of V2 by ab initio density functional theory (DFT) obtaining structures, relative energies, diffusion barriers and reaction paths. The constricted environment of a diamond lattice leads to unexpected chemical bonding situations with unusually elongated carbon-carbon bonds that have multicenter character. Divacancies separated by one carbon are not stable. Even though the divacancy is the most stable final product, a novel isolated divacancy with two vacant sites separated by two bonded carbons [V-C=C-V] can be transformed into a divacancy only via a very large (>4.3 eV) barrier and therefore it should be a defect observable in irradiated diamonds. Trapping of a vacancy by another by forming a stable divacancy trap affects the development of NV-defects in nitrogen implanted diamonds that are subject to active current interest in a wide range of applications.

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Point defects and pores in diamond affect its optical and electrical properties. We generated and evaluated a large number of vacancy V(n) clusters representing nanosized voids in diamonds for n up to 65. Our generational algorithm spawns the new generation n + 1 from the list of the most stable structures in the previous generation n. With energy as the only criterion, we generate a large structural diversity that allows their unbiased analysis. Since π-electron delocalization is important for carbon, we used quantum mechanical tight-binding density functional theory (TBDFT). Adamantane-like globular shapes are preferred for n up to ∼22. Beginning around n≈ 35, the most stable structures show overall oblate shapes with some irregularities. These novel structures have not been seen before because hitherto only highly regular structures were considered. We see local graphitization in these relaxed structures providing an atomistic justification for the widely used "slit pore" model. The preference for structures with minimum number of cut bonds diminishes as n increases. There are no particularly stable "magic" sizes for vacancy clusters larger than n = 22 indicating that these larger voids can easily incorporate small vacancies and vacancy clusters. Radial distribution analysis shows that unusual contact or bond distances in the 1.6 to 2.8 Å range appear in the vicinity of the internal surfaces of the vacancy clusters. Extremely long C-C bonds emerge as a result of structural relaxation of the dangling bonds in the vicinity of the vacancy clusters that cannot be simply described by ordinary sp(2)/sp(3) hybridization.