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Direct evidence for σ-bond fluxionality in a phenalenyl σ-dimer was successfully obtained by a detailed investigation of the solution-state dynamics of 2,5,8-trimethylphenalenyl (TMPLY) using both experimental and theoretical approaches. TMPLY formed three diamagnetic dimers, namely, the σ-dimer (RR/SS), σ-dimer (RS), and π-dimer, which were fully characterized by 1H NMR spectroscopy and electronic absorption measurements. The experimental findings gave the first quantitative insights into the essential preference of these competitive and unusual dimerization modes. The spectroscopic analyses suggested that the σ-dimer (RR/SS) is the most stable in terms of energy, whereas the others are metastable; the energy differences between these three isomers are less than 1 kcal mol-1. Furthermore, the intriguing dynamics of the TMPLY dimers in the solution state were fully revealed by means of 1H-1H exchange spectroscopy (EXSY) measurements and variable-temperature 1H NMR studies. Surprisingly, the σ-dimer (RR/SS) demonstrated a sixfold σ-bond shift between the six sets of α-carbon pairs. This unusual σ-bond fluxionality is ascribed to the presence of a direct interconversion pathway between the σ-dimer (RR/SS) and the π-dimer, which was unambiguously corroborated by the EXSY measurements. The proposed mechanism of the sixfold σ-bond shift based on the experimental findings was well-supported by theoretical calculations. [ABSTRACT FROM AUTHOR]

Phenalenyl and a wide variety of its derivatives form stable radicals, which often associate in various aggregates with interesting properties that include magnetism and high electrical conductivity. The two main modes of aggregation involve π-stacking pancake multicenter bond formation and σ-bond formation. We explore the energetics of the various σ-and π-dimers for six phenalenyl derivatives with both computational and experimental methods. A modern density functional theory (M05-2X) is used to survey the potential energy surface revealing the mechanism of the aggregation. In order to enrich experimental data, the triphenyl and trimethyl derivatives are newly prepared and their aggregation behaviors are investigated by various analytical methods including ESR, ¹H NMR, UV--vis, and single-crystal X-ray diffraction. The agreement between computations and experiments are very good forming the basis of describing trends in this series. We find that π-dimer formation can proceed via an asynchronous concerted path from the monomers or in a stepwise process via σ-dimers. The strength of the π-stacking pancake interaction depends strongly on substituents and covers a wide range both in terms of binding energies and contact distances. The spin densities in the π-stacking dimers reflect these trends and display a wide range of diradicaloid characters. Many σ-dimer configurations compete some of which are separated by small barriers leading to fluxional structures between σ-bonded configurations or σ- and π-bonded configurations. [ABSTRACT FROM AUTHOR]

We demonstrate that the electrocyclic (EC) ring-closure of cethrene in solution proceeds in a conrotatory mode both thermally and photochemically. The facile photochemical EC process promises that cethrene can serve as an efficient chiroptical switch operated solely by light. As for the thermally activated EC reaction, a low reaction barrier and a solvation effect on the EC rate indicate that the C2-symmetric pathway predicted by DFT calculations might not be the correct mechanism. Instead, we argue that the molecular symmetry decreases along the reaction coordinate as a consequence of the low-energy singlet excited state in this diradicaloid molecule, which might lead to a lower activation energy in accord with that determined through kinetic studies. Cethrene, therefore, represents a thought-provoking molecular chameleon of the Woodward-Hoffmann rules that puts our chemical concepts and intuition to test. [ABSTRACT FROM AUTHOR]