High-Resolution Photoelectron Spectroscopy of Vibrationally Excited Vinoxide Anions

Abstract

High-resolution photoelectron spectra of vibrationally pre-excited vinoxide anions (CH 2 CHO − ) are reported using the recently developed IR-cryo-SEVI technique. This method is combined with a newly developed implementation of vibrational perturbation theory that can readily identify relevant anharmonic couplings among nearly degenerate vibrational states. IR-cryo-SEVI spectra are obtained by resonant infrared excitation of vinoxide anions via the fundamental C−O (ν 4 , 1566 cm −1 ) or isolated C−H (ν 3 , 2540 cm −1 ) stretching vibrations prior to photodetachment. Excitation of the ν 4 mode leads to a well-resolved photoelectron spectrum that is in excellent agreement with a harmonic Franck−Condon simulation. Excitation of the higher-energy ν 3 mode results in a more complicated spectrum that requires consideration of the calculated anharmonic resonances in both the anion and the neutral. From this analysis, information about the zeroth-order states that contribute to the nominal ν 3 wave function in the anion is obtained. In the neutral, we observe anharmonic splitting of the ν 3 fundamental into a polyad feature with peaks at 2737(22), 2 835(18), and 2910(12) cm −1 , for which only the center frequency has been previously reported. Overall, 9 of the 12 fundamental frequencies of the vinoxy radical are extracted from the IR-cryo-SEVI and ground-state cryo-SEVI spectra, most of which are consistent with previous measurements. However, we provide a new estimate of the ν 5 (CH 2 scissoring) fundamental frequency at 1395(11) cm −1 and attribute the discrepancy with previously reported values to a Fermi resonance with the 2ν 11 overtone (CH 2 wagging).

Spectroscopy of Radicals, Clusters, and Transition States Using Slow Electron Velocity-Map Imaging of Cryogenically Cooled Anions

“…The electron affinity of vinoxy is 1.825 eV, which is convenient for photodetachment with a dye laser, and the SEVI spectrum exhibits some vibrational structure but is relatively sparse. Quantum chemistry calculations on the vinoxide anion at the B2PLYP-D3/aug-cc-pVTZ level of theory including anharmonicity find two strong infrared transitions, the ν 4 CO stretch at 1560 cm –1 and the ν 3 carbonyl CH stretch at 2512 cm –1 . These two modes were thus targeted for our IR cryo-SEVI experiment, with the calculated frequencies taken as starting points for our search.…”

“…This new experimental configuration, IR cryo-SEVI, has been carried out on a diatomic anion, OH − , 51 and, more recently, on the polyatomic vinoxide anion CH 2 CHO − . 52 The OH − anion served as a first demonstration of the technique; it is a special case in the sense that the cryo-SEVI spectrum is fully rotationally resolved 51 and the gas phase infrared spectrum is well characterized. 131,132 The vinoxide results illustrate the potential and complexity of IR cryo-SEVI, most notably the observation of effects associated with anharmonic coupling between vibrational modes in the anion and neutral as well as the dependence of these effects on the vibrational frequency of the pre-excited mode.…”

“…Quantum chemistry calculations on the vinoxide anion at the B2PLYP-D3/aug-cc-pVTZ level of theory including anharmonicity find two strong infrared transitions, the ν 4 CO stretch at 1560 cm −1 and the ν 3 carbonyl CH stretch at 2512 cm −1 . 52 These two modes were thus targeted for our IR cryo-SEVI experiment, 52 with the calculated frequencies taken as starting points for our search. Figure 14a,b shows photoelectron images of vinoxide with the IR laser off (a) and tuned to the ν 4 transition at 1570 cm −1 (b).…”

“…Cryo-SEVI experiments in our laboratory and elsewhere were reviewed in 2018 . This Feature Article focuses on results obtained since then, with particular focus on the cryo-SEVI spectra of NO 3 – , the hydrated metal oxide clusters TiO 3 H 2 – and ZrO 2 H 2 – , , and the transition state precursor anions H 2 CC – and F – (NH 3 ). , In addition, a new experimental direction is discussed in which anions are pre-excited with an infrared (IR) laser prior to photodetachment. , This IR cryo-SEVI experiment probes the effect of vibrational excitation on the anion photoelectron spectrum and also provides a new means to measure the vibrational spectrum of a gas phase negative ion.…”

“…13,50 In addition, a new experimental direction is discussed in which anions are pre-excited with an infrared (IR) laser prior to photodetachment. 51,52 This IR cryo-SEVI experiment probes the effect of vibrational excitation on the anion photoelectron spectrum and also provides a new means to measure the vibrational spectrum of a gas phase negative ion.…”