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

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.

Figures (6)

• Figure 1: TOF mass spectra at 21.2 eV with (a) pyrolysis off and (b) pyrolysis on. The spectra were normalized to the He+ signal at m/z 4. Asterisks indicate water as impurity.
• Figure 2: (a) C1s NEXAFS spectra of the precursor propargyl bromide without and with pyrolysis. (b) C1s NEXAFS spectrum of the propargyl radical obtained from subtraction.
• Figure 3: High resolution NEXAFS spectrum of the C1s to SOMO transition (band A).
• Figure 4: Comparison between experimental NEXAFS spectrum (black line) and computed (a) fc-CVS-EOM-CCSD/aug-cc-pVTZ and (b) CVS-ADC(2)-x/aug-cc-pVTZ NEXAFS spectra of the propargyl radical using UHF reference orbitals. The sticks were convoluted with a Lorentzian lineshape function with a FWHM of 200 meV. The vertical red dashed lines are the fc-CVS-EOM-IP-CCSD ionization potentials to singlet ionic states and the dotted ones to triplet ionic states.
• Figure 5: Comparison between the high-resolution experimental NEXAFS spectrum (black line) and vibrational structure of the (a, b) first, (c, d) second and (e,f) sum of both first and second CVS-ADC(2)-x electronic transitions at 0 K, with PES obtained from the Vertical Gradient (left) and Adiabatic Shift (right) approximations. Vertical lines show the vibronic stick transitions from the vibrational ground state in the initial electronic state obtained using the time-independent formalism. avilaferrer2012verticaladiabatic The labels correspond to the vibrational state in the final electronic states, represented as $X^v$, where $X$ is the normal mode index and $v$ is the number of quanta. All VG results have been shifted by +175 meV and all AS ones by +318 meV to align with the first experimental peak. The spectra have been scaled such that the intensity of the 0-0 band of the first electronic transition overlaps with experiment.