Ab initio calculation of electronic absorption spectra and ionization potentials of C3H3 radicals

Abstract

C3H3 isomers play a major role in combustion chemistry and are of fundamental interest in understanding photodissociation processes of hydrocarbons. Nevertheless, their electronic and ionization spectra are largely unknown. This study presents the first extended investigation of the electronically excited states and ionization potentials of the four most stable isomers, namely propargyl, 1-propynyl, cycloprop-2-enyl, and cycloprop-1-enyl. Electronic spectra are studied by ab initio calculations of the vertical excitation energies at the multi-reference configuration interaction (MRCI) level of theory. Vertical and adiabatic ionization energies are determined by coupled-cluster (RCCSD(T)) and multi-reference perturbation theory (CASPT2). The obtained results are suitable to aid the experimental detection of the different C3H3 isomers. Most of all, the calculations show that the assignment of an experimentally observed absorption at 242 nm to the propargyl radical was erroneous. Therefore, some recent experiments, based on this assignment, also need to be re-interpreted because this absorption is due to a yet unknown species.

The simulated photoelectron spectrum of 1-propynide

“…Using energies calculated at the 6-311 +G ‫ءء‬ / CCSD͑T͒ level of theory based on geometries opti-mized employing 6-311G ‫ء‬ / B3LYP wave functions, Mebel et al 25 predicted the 2 A 1 state to lie 2.3 kcal mol −1 ͑805 cm −1 ͒ below the 2 E state. A recent detailed study by Eisfeld,26 employing aug-cc-pVTZ/CCSD͑T͒ optimized geometries and Davidson-corrected aug-cc-pVTZ/MR-CISD energies confirmed that the 1-propynyl radical displays a X 2 A 1 ground state, with the minimum energy structure possessing a C 3v point group symmetry. The à 2 E state was again found to be very low lying, with the vertical excitation energy predicted to be 3262 cm −1 .…”

“…20 Initial computational studies of these isomeric species employing various Hartree-Fock methods [21][22][23] all predicted a 2 E ground state for the 1-propynyl radical, as did a more recent study by Vereecken et al 24 using density functional theory at the 6-31G ‫ء‬ / B3LYP level. However, subsequent computational studies [25][26][27] employing more rigorous treatments of electron correlation predicted a nondegenerate ground state. Using energies calculated at the 6-311 +G ‫ءء‬ / CCSD͑T͒ level of theory based on geometries opti-mized employing 6-311G ‫ء‬ / B3LYP wave functions, Mebel et al 25 predicted the 2 A 1 state to lie 2.3 kcal mol −1 ͑805 cm −1 ͒ below the 2 E state.…”

“…The à 2 E state was again found to be very low lying, with the vertical excitation energy predicted to be 3262 cm −1 . 26 Furthermore, a more recent treatment predicted an adiabatic transition energy from the X 2 A 1 ground state to the à 2 E ͑C s distorted͒ minimum, neglecting zero-point energy effects, of only 1706 cm −1 . 27 The presence of not only the 2 E x -2 E y minimum energy crossing point ͑MECP͒ associated with the à 2 E state, but a low-lying 2 A 1 -2 E x seam of conical intersection as well, 27 suggests that nonadiabatic vibronic coupling involving the 2 A 1 and 2 E states will be significant.…”

Photodissociation of the propargyl and propynyl (C3D3) radicals at 248 and 193 nm

“…Its electron affinity [20][21][22][23][24] ͑EA͒ and ionization potential 23,[25][26][27][28] have been determined. UV absorption studies [29][30][31] have probed the excited states of the radical, particularly an absorption band around 240 nm, assigned to the B 2 B 1 ← X 2 B 1 transition by Fahr et al 30 Theoretical work has focused on the structure and thermochemistry of propargyl [12][13][14][15]28,32,33 as well as its role as an intermediate in small hydrocarbon reactions. 15,[34][35][36][37] The photodissociation of propargyl was first observed by Jackson et al 38 as secondary photodissociation from the photolysis of allene.…”

Theoretical Studies on the Halogen Bond between Propargyl Radical and Dihalogen Molecules

“…[25][26][27] Theoretical studies of propargyl have focused on its structure and energetics, the topology of its ground state potential energy surface with respect to both isomerization and dissociation, and its low-lying excited states. [28][29][30] Experimental studies 31 have been carried out on the π-type halogen-bonded complexes B…XY (X, Y＝F, Cl, Br), in which B is nonaromatic π-electron donors, ethane, ethyne, cyclopropane, 1,3-butadiene, allene and methylenecyclopropane. Until now, there is no experimental study on the π-type halogen bond between propargyl radical and XY (X, Y＝F, Cl, Br).…”