Electronic Spectroscopy of cis- and trans-meta-Vinylbenzyl Radicals

Abstract

Resonance-stabilized radical (RSR) isomers of C 9 H 9 persist in complex energetic environments such as flames and plasmas derived from aromatic precursors. Of the myriad possible C 9 H 9 RSRs, only 1-indanyl (the global minimum) and 1-phenylallyl have been conclusively identified spectroscopically. Electronic spectra near 600 nm, close to several DIBs, were recently reported by the group of Maier for C 9 H 9 products of a heptadiyne discharge. They were tentatively ascribed to isomers of vinylbenzyl on the basis of computed transition energies and ionization potentials, the best agreement with the latter property (ca. 7.3 eV) provided by the meta isomer. To further examine this conjecture, we have undertaken surveys for the electronic spectra of o-,m-, and p-vinylbenzyl radicals in discharges of vinyltoluenes, using resonant two-color ionization and fluorescence spectroscopy. In a jet-cooled discharge of m-vinyltoluene, we have detected cisand transm-vinylbenzyl radicals near 525 nm. We observe adiabatic ionization energies (ca. 7.15 eV) for both conformers that are comfortably bracketed by B3LYP (7.11 eV) and CBS-QB3 (7.22 eV) calculations, from which we conclude that the carrier of the 600 nm spectrum remains unidentified. Optical-optical holeburning spectroscopy has been used to untangle cis and trans features of m-vinylbenzyl. There is very little to distinguish the two conformers from calculations, thermochemically or spectroscopically: they are similarly stable, their ground and excited state equilibrium geometries are planar, and their electronic transition energies, AIEs, predicted rotational contours, and excited state vibrational frequencies are highly similar. The most significant point of difference is a large (for cis) and relatively small (for trans) increase in the vinyl torsion force constant upon excitation, strongly suggesting identification of the cis conformer from an origin dispersed fluorescence spectrum, acceptance of which identification allows several other ground-state assignments to fall into place. A considerable breakdown in mirror symmetry between excitation and emission spectra for a modes is tentatively attributed to interference between Franck-Condon and Herzberg-Teller contributions to the transition moment, the low symmetry of the molecule (C s ) placing little restriction on such an interaction.

Optical Identification of the Resonance-Stabilized para-Ethynylbenzyl Radical

“…Within this context, we have embarked on optical surveys for new transient species using resonant two-color two-photon ionization (R2C2PI) and laser-induced fluorescence (LIF). We recently reported electronic spectra of previously unobserved resonance-stabilized isomers of C 7 H 7 (3-ethynylcyclopentenyl), and much more latterly, C 9 H 9 ( cis - and trans - meta -vinylbenzyl) . The present study concerns isomers with the molecular formula C 9 H 7 , which appears regularly in the mass spectra of hydrocarbon flames, − and is highly conspicuous in the soot formation study by Johansson et al The C 9 H 7 potential energy hypersurface has not been well explored spectroscopically, with most gas-phase studies having taken place only in the last 15 years.…”

“…To the best of our knowledge, no gas-phase investigation of ethynylbenzyl has been reported, which is surprising in view of the existing vast spectroscopic literature on substituted benzyl systems . More generally, beyond a handful of largely preliminary studies of the isoelectronic cyanobenzyl radicals − and our own very recent report on m -vinylbenzyl, apparently no attention has been devoted to benzyl radicals bearing a ring substituent that extends π-conjugation.…”