Probing the pyrolysis of ethyl formate in the dilute gas phase by synchrotron radiation and theory

Abstract

The thermal decomposition of the atmospheric constituent ethyl formate was studied by coupling flash pyrolysis with imaging photoelectron photoion coincidence (iPEPICO) spectroscopy using synchrotron vacuum ultraviolet (VUV) radiation at the Swiss Light Source (SLS). iPEPICO allows photoion mass-selected threshold photoelectron spectra (ms-TPES) to be obtained for pyrolysis products. By threshold photoionization and ion imaging, parent ions of neutral pyrolysis products and dissociative photoionization products could be distinguished, and multiple spectral carriers could be identified in several ms-TPES. The TPES and mass-selected TPES for ethyl formate are reported for the first time and appear to correspond to ionization of the lowest energy conformer having a cis (eclipsed) configuration of the O=C(H)-O-C(H 2 )-CH 3 and trans (staggered) configuration of the O=C(H)-O-C(H 2 )-CH 3 dihedral angles. We observed the following ethyl formate pyrolysis products: CH 3 CH 2 OH, CH 3 CHO, C 2 H 6 , C 2 H 4 , HC(O) OH, CH 2 O, CO 2 , and CO, with HC(O)OH and C 2 H 4 pyrolyzing further, forming CO + H 2 O and C 2 H 2 + H 2 . The reaction paths and energetics leading to these products, together with the products of two homolytic bond cleavage reactions, CH 3 CH 2 O + CHO and CH 3 CH 2 + HC(O)O, were studied computationally at the M06-2X-GD3/ aug-cc-pVTZ and SVECV-f12 levels of theory, complemented by further theoretical methods for comparison. The calculated reaction pathways were used to derive Arrhenius parameters for each reaction. The reaction rate constants and branching ratios are discussed in terms of the residence time and newly suggest carbon monoxide as a competitive primary fragmentation product at high temperatures.

The Unimolecular Chemistry of Methyl Chloroformate Ions and Neutrals: A Story of Near-Threshold Decomposition

“…However, the abundance of the R2 products is predicted to be negligible at low temperatures. The experimentally derived branching ratio of 0.1 for R2 is only consistent with the HPL kinetic model at temperatures well above 1361 K. In line with previous work, , we observe more pronounced minor channels in the mechanism than predicted at the reaction temperature, which the model predicts to be the case for significantly higher reaction temperatures than measured in the experiment.…”

“…8 The ionization energy (IE) and He(I) photoelectron spectrum for MCF have been reported by Zeng et al 9 In this study, we use a low-pressure pyrolysis microreactor coupled to an imaging photoelectron photoion coincidence (iPEPICO) spectrometer to determine the pyrolysis products for MCF, as we have done previously for methyl and ethyl formate. 10,11 We have explored the dissociative photoionization of MCF at room temperature and report the threshold photoelectron spectrum (TPES). The experimental breakdown of the ions is explored computationally.…”

On the fate of protonated chloroformates in the gas phase: a competition between forming HCl and chloroformic acid

“…Our attempts to study these molecules using imaging photoelectron photoion coincidence spectroscopy at the Swiss Light Source resulted only in the photoionization of degradation products of the chloroformates, other than methyl chloroformate and phenyl chloroformate. 16,21 In the present contribution, we successfully generated the protonated chloroformates by chemical ionization of the corresponding neutrals in the EI source of a double focusing magnetic sector mass spectrometer.…”