Volume 238, 2022

Unimolecular isomerisation of 1,5-hexadiyne observed by threshold photoelectron photoion coincidence spectroscopy

Abstract

The unimolecular isomerisation of the prompt propargyl + propargyl “head-to-head” adduct, 1,5-hexadiyne, to fulvene and benzene by the 3,4-dimethylenecyclobut-1-ene (DMCB) intermediate (all C6H6) was studied in the high-pressure limit by threshold photoelectron (TPE) spectroscopy. TPE spectra (TPES) were recorded with photoelectron photoion coincidence spectroscopy using synchrotron vacuum ultraviolet radiation. Reference TPES, obtained using pure compounds or judiciously extracted from the pyrolysis data, served as basis functions for pyrolysis quantification. From these spectra, we measured a revised fulvene ionisation energy of 8.401 ± 0.005 eV. Temperature-dependent pyrolysis spectra were decomposed using these basis functions. The basis function coefficients were converted to product yields relying on assumed integral threshold photoionisation cross sections obtained by three, partially mutually exclusive sets of assumptions. Thus, the product yields of DMCB, fulvene, and benzene have been established, as well as their uncertainty. The derived mole fractions are consistent with modeling based on the C6H6 potential and RRKM master equation model of Miller and Klippenstein [J. Phys. Chem. A, 2003, 107, 7783]. Although our results are fully consistent with the parallel isomerisation pathways to benzene and fulvene found by Miller and Klippenstein, we observe the onset of fulvene at a lower temperature than that of the onset of benzene, in agreement with the master equation model but in contrast to the previous experiments of Stein et al. [Proc. Combust. Inst., 1990, 23, 85]. This work promotes the use of photoion mass-selected threshold photoelectron spectroscopy as a rapid, sensitive, isomer-selective, and quantitative detection tool among the panoply of established analytical techniques.

Graphical abstract: Unimolecular isomerisation of 1,5-hexadiyne observed by threshold photoelectron photoion coincidence spectroscopy

Associated articles

Supplementary files

Article information

Article type
Paper
Submitted
01 Feb 2022
Accepted
11 Mar 2022
First published
30 Mar 2022

Faraday Discuss., 2022,238, 645-664

Author version available

Unimolecular isomerisation of 1,5-hexadiyne observed by threshold photoelectron photoion coincidence spectroscopy

J. D. Savee, B. Sztáray, P. Hemberger, J. Zádor, A. Bodi and D. L. Osborn, Faraday Discuss., 2022, 238, 645 DOI: 10.1039/D2FD00028H

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