Threshold Photoelectron Spectrum of Cyclobutadiene: Comparison with Time-Dependent Wavepacket Simulations

Abstract

The C 4 H 4 isomer cyclobutadiene (CBD) is the prime model for antiaromaticity and thus a molecule of considerable interest in chemistry. Because it is highly reactive, it can only be studied under isolated conditions. Its electronic structure is characterized by a pseudo-Jahn−Teller effect in the neutral and a E ⊗ β Jahn−Teller effect in the cation. As a result, recording photoelectron spectra as well as describing them theoretically has been challenging. Here we present the photoion mass-selected threshold photoelectron spectrum of cyclobutadiene together with a simulation based on time-dependent wavepacket dynamics that includes vibronic coupling in the ion, taking into account eight vibrational modes in the cation. Excellent agreement between theory and experiment is found, and the ionization energy is revised to 8.06 ± 0.02 eV.

Continuous Pyrolysis Microreactors: Hot Sources with Little Cooling? New Insights Utilizing Cation Velocity Map Imaging and Threshold Photoelectron Spectroscopy

“…Small, nitrogen-containing reactive species, such as H 2 CN, H 2 CNH, and NH 2 have been studied, delivering great spectral detail up to partial rotational resolution (K-stacks) in favorable cases. In addition, new insights could be obtained about the Jahn–Teller effect in cyclobutadiene cations . Vibrationally resolved reference spectra for methylpropargyl and methylallyl radicals help their direct detection in flames .…”

“…In addition, new insights could be obtained about the Jahn−Teller effect in cyclobutadiene cations. 34 Vibrationally resolved reference spectra for methylpropargyl and methylallyl 35 radicals help their direct detection in flames. 36 Furthermore, larger hydrocarbon radicals such as benzyl, 37 picolyl, 38 xylyl, 39 indenyl, 40 and tropyl, 41 along with exotic species such as methylbismuth, 42 iminoborane, 43 and lignin pyrolysis intermediates 44,45 have been successfully synthesized in situ and characterized utilizing ms-TPE spectra as well as Franck−Condon (FC) simulations.…”

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

“…11,12 These effects manifest themselves in the chemical properties of different cyclopropyl derivatives, from their relatively slow reaction rates upon nucleophilic attack to their tendency to open upon various activation processes; understanding these effects based on the underlying bond characteristics is important for chemical applications. 7,11,13−16 Characterizations of the fundamental origins of the unique properties of these small cyclic systems 17 are of value to modern regio-and stereo-selective syntheses utilizing such rings. 18 Despite this longstanding fundamental interest, few direct observations of cyclopropyl exist, no doubt due to the difficulty in forming the bare strained ring while avoiding its opening to allyl.…”

“…While not universally valid due to the then unknown nature of three-dimensional bonding and hybridization, Baeyer’s prediction of large ring strain in three-membered rings specifically is borne out by the strong destabilization of their cyclic geometries. Aside from their instability, an additional consequence of the highly bent structures of three-membered rings is the high s character in the C–H bonds, giving rise to unusually strong, short (close to alkene C–H) bonds, − and the high p character in the C–C bonds, forming “quasi π” bonds. , These effects manifest themselves in the chemical properties of different cyclopropyl derivatives, from their relatively slow reaction rates upon nucleophilic attack to their tendency to open upon various activation processes; understanding these effects based on the underlying bond characteristics is important for chemical applications. ,,− Characterizations of the fundamental origins of the unique properties of these small cyclic systems are of value to modern regio- and stereo-selective syntheses utilizing such rings …”

Threshold Photoelectron Spectrum of m-Benzyne

“…Biradicals have fascinated chemists for a long time, due to their unusual binding properties. − New experimental tools and computational approaches as well as the realization that a biradical character might lead to useful electronic properties in molecules led to a recent surge of interest in this class of compounds. − In particular, the three isomers of benzyne, C 6 H 4 , have attracted considerable interest with regard to their structure, spin multiplicity, and reactivity and the properties of their positive and negative ions. , Recently, o -benzyne has been identified as a molecule of considerable importance in astrochemistry, because its reaction with allyl leads to indene, one of the few unambiguously identified polycylic hydrocarbons (PAHs) in interstellar space. It is also involved in PAH formation under combustion conditions. − Comparable importance is anticipated for the meta and para isomers.…”