Jet-Cooled Spectroscopy of the α-Methylbenzyl Radical: Probing the State-Dependent Effects of Methyl Rocking Against a Radical Site

Abstract

The state-dependent spectroscopy of α-methylbenzyl radical (α-MeBz) has been studied under jet-cooled conditions. Two-color resonant two-photon ionization (2C-R2PI), laser-induced fluorescence, and dispersed fluorescence spectra were obtained for the D0-D1 electronic transition of this prototypical resonance-stabilized radical in which the methyl group is immediately adjacent to the primary radical site. Extensive Franck-Condon activity in hindered rotor levels was observed in the excitation spectrum, reflecting a reorientation of the methyl group upon electronic excitation. Dispersed fluorescence spectra from the set of internal rotor levels are combined with the excitation spectrum to obtain a global fit of the barrier heights and angular change of the methyl group in both D0 and D1 states. The best-fit methyl rotor potential in the ground electronic state (D0) is a flat-topped 3-fold potential (V3" = 151 cm(-1), V6" = 34 cm(-1)) while the D1 state has a lower barrier (V3' = 72 cm(-1), V6' = 15 cm(-1)) with Δφ = ± π/3, π, consistent with a reorientation of the methyl group upon electronic excitation. The ground state results are compared with calculations carried out at the DFT B3LYP level of theory using the 6-311+G(d,p) basis set, and a variety of excited state calculations are carried out to compare against experiment. The preferred geometry of the methyl rotor in the ground state is anti, which switches to syn in the D1 state and in the cation. The calculations uncover a subtle combination of effects that contribute to the shift in orientation and change in barrier in the excited state relative to ground state. Steric interaction favors the anti conformation, while hyperconjugation is greater in the syn orientation. The presence of a second excited state close by D1 is postulated to influence the methyl rotor properties. A resonant ion-dip infrared (RIDIR) spectrum in the alkyl and aromatic CH stretch regions was also recorded, probing in a complementary way the state-dependent conformation of α-MeBz. Using a scheme in which infrared depletion occurs between excitation and ionization steps of the 2C-R2PI process, analogous infrared spectra in D1 were also obtained, probing the response of the CH stretch fundamentals to electronic excitation. A reduced-dimension Wilson G-matrix model was implemented to simulate and interpret the observed infrared results. Finally, photoionization efficiency scans were carried out to determine the adiabatic ionization threshold of α-MeBz (IP = 6.835 ± 0.002 eV) and provide thresholds for ionization out of specific internal rotor levels, which report on the methyl rotor barrier in the cation state.

Gas-Phase Optical Detection of 3-Ethynylcyclopentenyl: A Resonance-Stabilized C7H7 Radical with an Embedded 1-Vinylpropargyl Chromophore

“…DF spectra for the four lowest bands in the LIF spectrum are shown in Figure . Because the discharge nozzle constricts the jet expansion, we adopted a large nozzle-laser distance (5 cm, or x / D = 100) to guarantee collision-free measurements. , Our signal-to-noise ratio and spectral resolution (∼25 cm –1 ) under these conditions are modest owing to the diminished radical number density in the laser interaction region, although it was possible to measure portions (bottom of figure) of the origin DF spectrum at higher resolution (15 cm –1 ).…”

Computationally Supported Neutron Scattering Study of Natural and Synthetic Amorphous Carbons

“…At the same time, the spin-dependent dynamics is practically a virgin land. Open-shell electronic configuration of radical molecules requires a different, much more complicated approach so that the information concerning the correct solving of vibrational dynamic problem of the species is rather scarce. , Because of this circumstance, in the current study, the open-shell UHF approach, implemented in the AM1 semiempirical tool CLUSTER-Z1, was suggested to solve the spin-dependent vibrational dynamic problem of highly radicalized graphene oxyhydride molecules I–IV, whereas molecules V–VIII are considered in the framework of the DMol 3 approach.…”

Interconversion of Methyltropyl and Xylyl Radicals: A Pathway Unavailable to the Benzyl–Tropyl Rearrangement

“…Although the electronic spectra of the three xylyl radicals have been the subject of several previous studies, only very recently was the electronic spectrum of α-methylbenzyl radical reported, and nothing is known about the optical spectroscopy of the methyltropyl radical. With respect to the xylyl radicals, excitation spectra to the first excited state (Ã ← X) have previously been reported in the visible region by Fukushima and Obi and Miller and co-workers .…”