The X-ray absorption spectrum of the propargyl radical, C$_3$H$_3^{\cdot}$

TL;DR

This study combines experimental and computational methods to analyze the X-ray absorption spectrum of the propargyl radical, revealing detailed electronic transitions and vibrational features relevant to combustion and astrochemistry.

Contribution

It provides the first detailed NEXAFS spectrum of the propargyl radical, linking experimental data with ab initio calculations to elucidate electronic and vibrational structure.

Findings

Identification of a prominent absorption band at 282.2 eV

Assignment of transitions to specific molecular orbitals

Observation of vibrational progression in the spectrum

Abstract

We report a combined experimental and computational study of the near-edge X-ray absorption fine structure (NEXAFS) spectrum of the propargyl radical, C$_3$H$_3^{\cdot}$. As a central intermediate in the formation of polycyclic aromatic hydrocarbons, the propargyl radical is a species of considerable relevance in combustion and astrochemistry and was here generated by pyrolysis from propargyl bromide. The NEXAFS spectrum shows a pronounced band at 282.2 eV corresponding to transitions from carbon 1s orbitals to singly occupied molecular orbitals. Ab initio calculations show that two transitions to the lowest lying states 1 $^2$A$_1$ and 2 $^2$A$_1$, which take place from the C1s orbital of the two terminal carbon atoms, contribute to this band. In addition, a 420 meV spacing of the first band is visible and is assigned to a vibrational progression in the symmetric CH$_2$ stretch. Transitions at higher energies are also described reasonably well by theory. The fragmentation pattern was investigated at the different resonant transitions and shows the cleavage of one as well as both C--C bonds.