Spectroscopic and Computational Studies on the Rearrangement of Ionized [1.1.1]Propellane and Some of its Valence Isomers: The Key Role of Vibronic Coupling

Abstract

, generated by radiolytic oxidation of the parent compound in argon and Freon matrices at low temperatures, undergoes a spontaneous rearrangement to form the distonic 1,1-dimethyleneallene (or 2-vinylideneallyl) radical cation 3•+ consisting of an allyl radical substituted at the 2-position by a vinyl cation. In similar matrix studies, it is found that the isomeric dimethylenecyclopropane radical cation 2•+ also rearranges to 3 •+ . The unusual molecular and electronic structure of 3•+ has been established by the results of ESR, UV-vis, and IR spectroscopic measurements in conjunction with detailed theoretical calculations. Also of particular interest is an NIR photoinduced reaction by which 3 •+ is cleanly converted to the vinylidenecyclopropane radical cation 4•+ , a process that can be represented in terms of a single electron transfer from the allyl radical to the vinyl cation followed by allyl cation cyclization. The specificity of this photochemical reaction provides additional strong chemical evidence for the structure of 3•+ . Theoretical calculations reveal the decisive role of vibronic coupling in shaping the potential energy surfaces on which the observed ring-opening reactions take place. Thus vibronic interaction in 1•+ mixes the 2 A 1 ′ ground state, characterized by its "non-bonding" 3a 1 ′ SOMO, with the 2 E′′ first excited state resulting in the destabilization of a lateral C-C bond and the initial formation of the methylenebicyclobutyl radical cation 5•+ . The further rearrangement of 5 •+ to 3 •+ occurs via 2 •+ and proceeds through two additional lateral C-C bond cleavages characterized by transition states of extremely low energy, thereby explaining the absence of identifiable intermediates along the reaction pathway. In these consecutive ring-opening rearrangements, the "non-bonding" bridgehead C-C bond in 1•+ is conserved and ultimately transformed into a normal bond characterized by a shorter C-C bond length. This work provides strong support for the Heilbronner-Wiberg interpretation of the vibrational structure in the photoelectron spectrum of 1 in terms of vibronic coupling.

Tracing the Fingerprint of Chemical Bonds within the Electron Densities of Hydrocarbons: A Comparative Analysis of the Optimized and the Promolecule Densities

“…Interestingly, this subset includes C−C bonds that have been classified as “exotic” bonds previously, and a large number of studies have been conducted in the last decades to elucidate the detailed nature of these bonds. The members are the naked “formally” triple C−C bond in ortho ‐benzyne ( 73 ), the central single C−C bond in the highly strained bicyclo‐1,1,0‐butane ( 77 ), the strongly deformed central double bond in the strained bicyclo‐2,2,0‐hex‐1,4‐ene ( 81 ), and finally the central inverted C−C bond in 988 . The computed % ρ (%▿ 2 ρ ) are 34, 12, and 35 (72, 121, and 82) for 73 , 77 , and 81 , respectively.…”

Rationalizing the diverse reactivity of [1.1.1]propellane through σ–π-delocalization

“…The geometry of 1 has been determined experimentally by low-temperature X-ray diffraction 40 and gas-phase electron diffraction; 41 from the latter, the C1-C3 distance was found to be 1.594Å which, being similar to the C-C bond length in ethane (1.524Å), 42 is suggestive of a C1-C3 bonding interaction. The nature of this bond has been much debated over the past 30 years, with theoretical [43][44][45][46][47][48][49][50][51][52][53][54][55] and experimental 30,41,43,[56][57][58][59] studies offering contrasting evidence. For example, despite a closed-shell singlet ground state and a calculated bond energy of $65 kcal mol À1 , 43 photoelectron spectroscopy reveals the HOMO (C1-C3 s MO) to be nonbonding due to poor overlap of atomic orbitals between the 'inverted' carbon atoms.…”

Die Ästhetik der [1.1.1]Propellane

“…Umfangreiche Reaktivitätsstudien zeigen die ungewöhnliche Reaktivität dieser so einfach erscheinenden Kohlenwasserstoffe. Etliche Übersichtsartikel erschienen zu diesem Thema,6 dennoch gibt es immer wieder neue Ergebnisse zur Chemie der Kohlenstoffpropellane 7…”

“…Die radikalähnliche Reaktivität steht im Einklang mit quantenchemischen Rechnungen, wonach die schweren [1.1.1]Propellane einen zwar kleinen, aber nicht zu vernachlässigenden biradikalischen Charakter aufweisen 2,7…”