Heavy‐Atom Tunneling in Bicyclo[4.1.0]hepta‐2,4,6‐trienes

Abstract

Heavy‐atom tunneling limits the lifetime and observability of bicyclo[4.1.0]hepta‐2,4,6‐triene, a key intermediate in the rearrangement of phenylcarbene. Bicyclo[4.1.0]hepta‐2,4,6‐triene had been proposed as the primary intermediate of the rearrangement of phenylcarbene, but despite many efforts evaded its characterization even in cryogenic matrices. By introducing fluorine substituents into the ortho‐positions of the phenyl ring of phenylcarbene, the highly strained cyclopropene 1,5‐difluorobicyclo[4.1.0]hepta‐2,4,6‐triene becomes stable enough to be characterized in argon matrices. However, even at 3 K this cyclopropene is only metastable and rearranges via heavy‐atom tunneling to the corresponding cycloheptatetraene. Calculations suggest that fluorination is necessary to slow down the tunneling rearrangement of the bicycloheptatriene. The parent bicycloheptatriene rapidly rearranges via heavy‐atom tunneling and therefore cannot be detected under matrix isolation conditions.

Norcaradiene–Cycloheptatriene Equilibrium: A Heavy-Atom Quantum Tunneling Case

“…Our revDSD/CBS energies exhibit discrepancies compared to the calculations conducted at the B3LYP(D3)/def2-TZVP level of theory . The reaction energy derived with the DFT method including ZPE Har corrections is −15.1 kcal mol –1 , while our revDSD/CBS calculations in argon (SMD) reveal an energy difference of −10.71 kcal mol –1 , incorporating ZPE Anh corrections.…”

“…Our results show noticeable discrepancies among experimental data and misconceptions on thermochemical analysis. The study provides compelling computational evidence that this isomerization is ultimately controlled by QMT, preventing the alleged detection of norcaradiene at low temperatures, similar to the case found for fluorinated bicycloheptatriene . Our analysis accounts for an accurate reinterpretation of data collected so far and expands the repertoire of molecular isomerizations where tunneling should reasonably be involved.…”

“…From our viewpoint, at temperatures closer to absolute zero, no reliable data can be obtained by using the methodology employed in this work. The comparison of our CVT/SCT calculations with the experimental data conducted by Sander and coworkers is shown in Table . In general, our CVT/SCT rate constants are 1 order of magnitude higher than those obtained experimentally.…”

“…Surprised by this unexpected deviation between our theoretical results and those experimentally reported, further pursuits were taken to validate our calculations, since computing rate constants at such low temperatures for systems of this size is an extremely challenging task. To this end, we have followed the same methodology previously reported to study the isomerization between the fluorinated derivatives bicyclo[4.1.0]hepta-2,4,6-triene ( 14 ) and the allenic compound cyclohepta-1,2,4,6-tetraene ( 15 ) (Scheme ), which has recently been investigated using state-of-the-art experimental techniques. Accordingly, the same level employed for computing the isomerization between 1 and 2 was applied to its simulation in an argon matrix (SMD) to try to reproduce the experimental conditions.…”

“…In some instances, QMT prevents the detection of molecules due to its extremely rapid decay. This phenomenon, known as “tunneling instability”, has recently been observed in systems distinct from those mentioned so far, such as hexazine, pentazine, and diazo compounds. , However, this effect has likewise been observed for molecules similar to norcaradiene, such as bicyclo[4.1.0]hepta-2,4,6-triene ( 11 ) . The detection of this intermediate proved to be unattainable due to its swift conversion into its isomer 12 , driven by heavy-atom tunneling.…”