Threshold Photoelectron Spectroscopy of the Methyl Radical Isotopomers, CH₃, CH₂D, CHD₂ and CD₃: Synergy between VUV Synchrotron Radiation Experiments and Explicitly Correlated Coupled Cluster Calculations

Abstract

Threshold photoelectron spectra (TPES) of the isotopomers of the methyl radical (CH(3), CH(2)D, CHD(2), and CD(3)) have been recorded in the 9.5-10.5 eV VUV photon energy range using third generation synchrotron radiation to investigate the vibrational spectroscopy of the corresponding cations at a 7-11 meV resolution. A threshold photoelectron-photoion coincidence (TPEPICO) spectrometer based on velocity map imaging and Wiley-McLaren time-of-flight has been used to simultaneously record the TPES of several radical species produced in a Ar-seeded beam by dc flash-pyrolysis of nitromethane (CH(x)D(y)NO(2), x + y = 3). Vibrational bands belonging to the symmetric stretching and out-of-plane bending modes have been observed and P, Q, and R branches have been identified in the analysis of the rotational profiles. Vibrational configuration interaction (VCI), in conjunction with near-equilibrium potential energy surfaces calculated by the explicitly correlated coupled cluster method CCSD(T*)-F12a, is used to calculate vibrational frequencies for the four radical isotopomers and the corresponding cations. Agreement with data from high-resolution IR spectroscopy is very good and a large number of predictions is made. In particular, the calculated wavenumbers for the out-of-plane bending vibrations, nu(2)(CH(3)(+)) = 1404 cm(-1), nu(4)(CH(2)D(+)) = 1308 cm(-1), nu(4)(CHD(2)(+)) = 1205 cm(-1), and nu(2)(CD(3)(+)) = 1090 cm(-1), should be accurate to ca. 2 cm(-1). Additionally, computed Franck-Condon factors are used to estimate the importance of autoionization relative to direct ionization. The chosen models globally account for the observed transitions, but in contrast to PES spectroscopy, evidence for rotational and vibrational autoionization is found. It is shown that state-selected methyl cations can be produced by TPEPICO spectroscopy for ion-molecule reaction studies, which are very important for the understanding of the planetary ionosphere chemistry.

Threshold Photoelectron Spectroscopy of Cyclopropenylidene, Chlorocyclopropenylidene, and Their Deuterated Isotopomeres

“…The rather high temperature was also observed in former continuous beam studies on radicals produced by pyrolysis. 18,20,49 Interestingly, the unassigned low-energy band (9.02 eV) is more pronounced in our spectrum compared to the PES from Clauberg et al [13][14][15] When we follow the assignments of the previous work, the IE ad is 9.17 eV with an estimated error of 15 meV.…”

“…In previous experiments on CH 3 we found a rotational temperature of about 400 K. 20 This fairly high temperature contributes to the peak width. The experimental resolution is about 7 meV (60 cm -1 ), calculated by the square root of the sum of the photon resolution squared and the electron resolution squared.…”

“…Such a high temperature seems highly unlikely, compared to vibrational temperatures of 400-500 K that were recently determined for the methyl radical. 20 In addition, the shape of the peak is not well represented. We therefore suggest that the peak at 9.02 eV results from a contribution of the propargylene isomer, HCCCH.…”

“…We showed previously that this approach is well suited to characterize the photoionization of reactive intermediates. [17][18][19][20] As a second intermediate, we studied the c-C 3 HCl carbene, chlorocyclopropenylidene. Chlorinated carbenes are supposedly also important in combustion modeling and in astrophysics.…”

Photoionisation of the tropyl radical

“…It is assigned to the 1 A 1 ’ (v + =0) ← 2 E 2 ” (v” = 0) transition and corresponds to the adiabatic ionisation energy of the molecule. As the radical vibrational temperature is typically around 500 K in a continuous beam experiment [31] a contribution from hot and sequence bands cannot be excluded and could be responsible for the signal between 6.1 eV and 6.2 eV. The small peak at around 6.12 eV may correspond to a bending-mode hot band.…”