Ring-Opening Dynamics of the Cyclopropyl Radical and Cation: the Transition State Nature of the Cyclopropyl Cation

Abstract

We provide compelling experimental and theoretical evidence for the transition state nature of the cyclopropyl cation. Synchrotron photoionization spectroscopy employing coincidence techniques together with a novel simulation based on high-accuracy ab initio calculations reveal that the cation is unstable via its allowed disrotatory ring-opening path. The ring strains of the cation and the radical are similar, but both ring opening paths for the radical are forbidden when the full electronic symmetries are considered. These findings are discussed in light of the early predictions by Longuet-Higgins alongside Woodward and Hoffman; we also propose a simple phase space explanation for the appearance of the cyclopropyl photoionization spectrum. The results of this work allow the refinement of the cyclopropane C–H bond dissociation energy, in addition to the cyclopropyl radical and cation cyclization energies, via the Active Thermochemical Tables approach.

Sub 20 cm−1 computational prediction of the CH bond energy – a case of systematic error in computational thermochemistry

“…Indeed, after modifying the most recent developmental version of the TN, ATcT TN ver. 1.130, 115 to create an updated version 1.130a of the ATcT TN, the resulting ATcT value for the bond dissociation energy of CH became D 0 (CH) = 27 961.0 ± 8.0 cm −1 , a shift downwards by about 10 cm −1 . Noting that this shift is still contained within the combined uncertainties of the original ATcT benchmark (27 971.0 ± 7.3 cm −1 70 ) and the amended ATcT value, the correction of the systematic error in previous computations reduces the difference between the extended HEAT value and the amended ATcT value to 7 cm −1 , which is well within the desired target accuracy of the extended HEAT result.…”

“…(Note that the latter determination was included in the subsequent ATcT TN ver. 1.130, 115 further tightening the uncertainty of the ATcT value to AE0.6 cm À1 , without changing the value itself. )…”

Carbon-Centered Radical with Leaving Group-Mediated Ring Opening of Cyclopropenes via the Rearrangement of Cyclopropyl to the Allyl Radical: A General Access to Multisubstituted 1,3-Dienes

“…43−45 Recently, Baraban and co-workers provided a refined high value of 124 ± 0.74 kJ/mol for the allyl−cyclopropyl radical energy difference (cyclization energy) via the Active Thermochemical Tables approach. 46 Combined with the energy barrier value of only 20.67 kcal/mol, 47 the rearrangement of the cyclopropyl radical to the allyl radical could be an energetically favorable process (Scheme 1b). Based on the rearrangement process, the Waser group first reported a ringopening reaction of cyclopropenes after the addition of an azide radical to the strained double bond coupled with the release of dinitrogen, giving the iminyl radical intermediate and finally affording alkenyl nitriles 44 or quinolines 45 (Scheme 1c).…”

“…Recently, Baraban and co-workers provided a refined high value of 124 ± 0.74 kJ/mol for the allyl–cyclopropyl radical energy difference (cyclization energy) via the Active Thermochemical Tables approach . Combined with the energy barrier value of only 20.67 kcal/mol, the rearrangement of the cyclopropyl radical to the allyl radical could be an energetically favorable process (Scheme b).…”

Hyperfine-Resolved Rotational Spectroscopy of Phenyl Radical

“… a Q rot is calculated by direct summation using rigid rotor energies and rotational constants from refs and − . b Dipole moments from refs , , , and . c Figure of merit ( μ σ / Q rot ( 2 K ) 2 n normalH for laboratory cavity detection and μ 2 σ / Q rot ( 7 K ) 2 n normalH for astronomical detection in TMC-1) relative to phenyl. d Assumed equal to phenyl. …”