Yong-Hui Tian

Is There a Lower Limit to the CC Bonding Distances in Neutral Radical π-Dimers? The Case of Phenalenyl Derivatives

Two-electron multicenter (2e/mc) bonding of phenalenyl (PHYL) pi-dimers was found to be significantly affected by the electron density on the bonding active sites. The computational analysis shows that, upon appropriate beta-substitutions, the newly introduced dimers have the shortest and strongest covalent bonding interactions seen in any neutral pi-dimer. The unusual strengthening of the bonding was attributed to the reduced lone pair bond weakening effect, LPBWE, upon substitutions with electron-withdrawing groups.

Charge shift bonding concept in radical π-dimers.

We show that pancake bonding in radical π-dimers display features of charge shift (CS) bonding. While the CS bonding concept has been developed to interpret the unusual aspects of σ-bonds around centers with a large number of lone pairs, such as F(2) and HOOH, we find a similar role played by the nonbonding or slightly bonding π-electron pairs in π-stacking radical dimers. Arguments and computational evidence indicate that the CS bonding concept developed by Shaik and Hiberty et al. captures essential features of the intermolecular bonding in radical π-dimers in which the overlap of the two radical centered singly occupied molecular orbitals (SOMOs) play a crucial role. By using the tetracyanoethylene anion dimer, [TCNE](2)(2-), as a model, we show that compared to CAS(2,2) calculations, significant binding contributions are recovered in the calculations simply by including selected intrapair excitations of the SOMO-SOMO bonding orbitals and the nonbonding π-orbitals. This observation is the basis for the analogy of chemical bonding between pancake bonded radical π-dimers and other charge shift bonded molecules, such as F(2). By extending the CS bonding concept to a new class of molecules, we find a novel application of the lone pair bond weakening effect (LPBWE) in which the doubly occupied π-orbitals play the role of lone pairs.

Pancake π–π Bonding Goes Double: Unexpected 4e/All-Sites Bonding in Boron- and Nitrogen-Doped Phenalenyls

Chemical bonding interactions are the main driving force for the formation of molecules and materials from atoms. The two-electron/multicenter pancake π-π bonding found in phenalenyl (PLY, 1) radical π-dimers is intriguing due to its unconventional nature of covalent bonding for molecular aggregations and its propensity to induce unique optical, electronic, and magnetic properties. By using high-level quantum chemistry calculations, we show that the B- or N-doped PLYs (2 and 4), usually considered closed-shell and therefore trifling, can be rendered open-shell singlet by proper edge substitutions (3 and 5). The resulting two unpaired valence electrons on each molecular unit contribute to the formation of a genuine pancake-shaped 4e/all-sites double π-π bonding upon intermolecular π-dimerization, in contrast to the 2e/half-sites single π-π bonding in the parent PLY π-dimers. The unusual double π-π bonding motif discovered in these PLY analogues may broaden the landscape of, and find new applications for, intermolecular covalent bonding interactions.

On the Anisotropy of van der Waals Atomic Radii of O, S, Se, F, Cl, Br, and I

The Cambridge Structural Database (CSD) was used to obtain flattening factors to describe the overall anisotropy of nonbonding van der Waals (vdW) contacts between several main group elements. The method for obtaining the flattening factors is based on a novel minimization process. Results show that the vdW contact distances are significantly dependent on the environment and the orientations of the surrounding covalently bonded atoms: head-on vdW contacts are generally shorter than sideways contacts in overall agreement with earlier results by Nyburg and Faerman (Acta Crystallogr., Sect. B: Struct. Sci. 1985, 41, 274-279). With the exception of Se, we find flattening factors that are somewhat smaller than those found earlier. High-level ab initio quantum chemical calculations using Ar and Ne as a probe also confirm the flattening effect and its dependency on the environment. A dozen popular long-range corrected and dispersion supplemented density functionals are compared with the CCSD(T) data. While several of them perform quite poorly, four DFT-D methods, especially B3LYP-GD3BJ, provided vdW flattening similar to those found by the CCSD(T) theory and experiment.

Non-Transition-Metal Catalytic System for N2 Reduction to NH3: A Density Functional Theory Study of Al-Doped Graphene

The prevalent catalysts for natural and artificial N2 fixation are known to hinge upon transition-metal (TM) elements. Herein, we demonstrate by density functional theory that Al-doped graphene is a potential non-TM catalyst to convert N2 to NH3 in the presence of relatively mild proton/electron sources. In the integrated structure of the catalyst, the Al atom serves as a binding site and catalytic center while the graphene framework serves as an electron buffer during the successive proton/electron additions to N2 and its various downstream NxHy intermediates. The initial hydrogenation of N2 can readily take place via an internal H-transfer process with the assistance of a Li+ ion as an additive. In view of the recurrence of H transfer in the first step of N2 reduction observed in biological nitrogenases and other synthetic catalysts, this finding highlights the significance of heteroatom-assisted H transfer in the design of synthetic catalysts for N2 fixation.

Bimolecular hydrogen transfer in phenalene by a stepwise ene-like reaction mechanism

For the hydrogen transfer of phenalene, a bimolecular ene-like mechanism is proposed, which is preferable over the hypothesized unimolecular rearrangement in the literature. Unique SOMO-SOMO pi-bonding of phenalenyl reduces the barriers of pericyclic reactions significantly. Pi-bonding between radicals is being recognized as a novel type of bonding interaction. This paper adds to the use of this interaction by pointing out its effect on reaction barriers.

Why is there no in-plane H-atom transfer from aryloxy radicals? A theoretical and experimental investigation.

A combined experimental and theoretical study of the mechanisms and energies associated with intramolecular H-atom transfers from methyl groups with varying numbers of phenyl substituents to oxygen atoms of aryloxy radicals is reported. It is shown that the transfers within the six aryloxy radicals investigated would have high activation energies and, in all but one case, are endothermic. A detailed analysis of the calculated reaction coordinates indicates proton-coupled electron transfers as the favored mechanisms.