The ortho-benzyne cation is not planar

Abstract

A recent review on the photoionisation of the CH isomer ortho-benzyne suggests that bands reported in earlier photoelectron spectra might be due to side products or contaminations, while computations raise doubts, whether the cation has a planar geometry. We therefore reinvestigate the photoionisation of ortho-benzyne, generated by pyrolysis from benzocyclobutenedione, by photoion mass-selected threshold photoelectron (ms-TPE) spectroscopy using synchrotron radiation. The experiments are accompanied by a theoretical study that investigates the structure of the ortho-benzyne cation systematically as a function of the computational method, up to CASPT2(11,14) ab initio computations. Our study leads to a re-evaluation of the ionisation energy of ortho-benzyne. It reveals that the ortho-benzyne cation has indeed a twisted C geometry rather than a C structure. A vertical ionisation energy IE of 9.77 eV and an adiabatic ionisation energy of IE = 9.56 eV are computed for ortho-benzyne. A Franck-Condon simulation of the photoelectron spectrum based on the CASPT2 results and including three electronic states of the cation is in agreement with the experiment and yields IE = 9.51 eV (+50 meV/-100 meV). Since this value is in contrast with previous work, the ionisation energy has to be revised based on our study. Computational methods based on density functional theory give a reasonable description of the cationic ground state, but fail for the corresponding excited electronic states that are indispensible for a proper assignment of the photoelectron spectrum.

Pentadiynylidene and Its Methyl-Substituted Derivates: Threshold Photoelectron Spectroscopy of R1-C5-R2 Triplet Carbon Chains

“…For a CCSD­(T) calculation of the cation with the geometry being obtained at the ωB97xD level of theory, the diagnostic, which is a measure for the multireference character of a wave function, has a value of 0.16. This indicates that a single-reference method is unable to predict the equilibrium geometry correctly. − Further informations are given in the SI. Therefore, we optimized the geometry of 9 + by CASPT2­(10,10)/cc-pVDZ.…”

“…This indicates that a single-reference method is unable to predict the equilibrium geometry correctly. [71][72][73] Further informations are given in the SI. Therefore, we optimized the geometry of 9 + by CASPT2(10,10)/cc-pVDZ.…”

Direct observation ofo-benzyne formation in photochemical hexadehydro-Diels–Alder (hν-HDDA) reactions

“…The formation, characterization, and application of benzynes has fascinated chemists for more than 90 years due to their Kekulé-type diradical structure, special role in combustion and astrochemistry, and great potential in synthetic chemistry. [48][49][50][51][52][53] o-Benzyne was rst proposed as an intermediate in 1927 by Bachmann and Clarke, 54 and IR, [55][56][57][58][59][60][61] microwave, [62][63][64][65][66] NMR, 67,68 and photoelectron [69][70][71][72][73][74] spectra of o-benzynes have been reported in the last half century. A recent report, 75 however, explains that the NMR data 68 of an o-benzyne generated inside of a hemicarcerand do not belong to an o-benzyne inside the molecular cage, but instead, the signals result from a species formed from a reaction of the o-benzyne with the molecular cage.…”

Photoelectron Photoion Coincidence Spectroscopy Provides Mechanistic Insights in Fuel Synthesis and Conversion

“…While the neutral is planar, it has recently been shown that the cation has a twisted C 2 geometry. 200 Benzyne and allyl were in situ-produced from benzocyclobutenedione (1% in argon) and allyliodide (16% in argon), respectively, in a Chen-type pyrolysis reactor. The mass spectra show clean formation of m/z 116, if both reactants are present.…”