Photodetachment and photoreactions of substituted naphthalene anions in a tandem ion mobility spectrometer

Abstract

Substituted naphthalene anions (deprotonated 2-naphthol and 6-hydroxy-2-naphthoic acid) are spectroscopically probed in a tandem drift tube ion mobility spectrometer (IMS). Target anions are selected according to their drift speed through nitrogen buffer gas in the first IMS stage before being exposed to a pulse of tunable light that induces either photodissociation or electron photodetachment, which is conveniently monitored by scavenging the detached electrons with trace SF 6 in the buffer gas. The photodetachment action spectrum of the 2-naphtholate anion exhibits a band system spanning 380-460 nm with a prominent series of peaks spaced by 440 cm −1 , commencing at 458.5 nm, and a set of weaker peaks near the electron detachment threshold corresponding to transitions to dipole-bound states. The two deprotomers of 6-hydroxy-2-naphthoic acid are separated and spectroscopically probed independently. The molecular anion formed from deprotonation of the hydroxy group possesses a photodetachment action spectrum similar to that of the 2-naphtholate anion with an onset at 470 nm and a maximum at 420 nm. Near threshold, photoreaction with SF 6 is observed with displacement of an OH group by an F atom. In contrast, the anion formed from deprotonation of the carboxylic acid group features a photodissociation action spectrum, recorded on the CO 2 loss channel, lying to much shorter wavelength with an onset at 360 nm and maximum photoresponse at 325 nm. † Electronic Supplementary Information (ESI) available: Synthesis procedure for 6-hydroxy-2-methylnaphthoate, calculated vibrational frequencies for the naphtholate anion, photodetachment action spectra of 2-naphtholate anion, 1-naphtholate anion, the two deprotomers of 6-hydroxy-2-naphthoic acid, and deprotonated 6hydroxy-2-methylnaphthoate. See

Autodetachment dynamics of 2-naphthoxide and implications for astrophysical anion abundance

“…2b), which was assigned to the principal Franck-Condon active mode for excitation of the S 1 (pp*) state (calculated n 32 (a 0 ) = 437 cm À1 ) through Franck-Condon-Herzberg-Teller simulations. 21 It is also worth noting that this vibrational structure is consistent with the principal Franck-Condon modes for direct photodetachment characterised by an earlier cryogenic photoelectron spectroscopy experiment, 22 involving n 32 (a 0 ) = 422 cm À1 (neutral), which is an in-plane CCC bending mode and has most displacement localised on the oxygen-containing benzyl ring. In the present photoelectron spectra, the vibrational structure over the prompt detachment band in the photoelectron spectra is presumably associated with prompt autodetachment through electron shake-off by the n 32 mode of the anion excited state.…”

“…Other experiments which subjected PAHs to interstellar conditions found extensive hydrogenation, oxygenation, and hydroxylation, ultimately leading to more complex organics. 20 In a recent contribution, one of the present authors reported a photodetachment study on 2-Np À thermalised to T E 300 K, 21 providing an action spectrum of the S 1 (pp*) band as well as identifying several other weak spectral features spanning the electron detachment threshold, which were attributed to a progression of dipole-bound states. Two recent investigations using cryogenic photoelectron and photodetachment techniques have reported near-threshold spectra of 2-Np À , and also identified dipole-bound states.…”

“…The low-eKE vibrational structure with spacing of E35 meV (282 cm À1 ) could potentially correspond to the n 33 (a 0 ) = 313 cm À1 mode calculated for the S 1 (pp*) state. 21 This mode is the lowest frequency a 0 -symmetry mode involving an in-plane bend. However, it is difficult to reconcile the observation of similar low-eKE structure in the photoelectron spectra for photon energies hn o 2.7 eV (e.g.…”

“…2b), which has been reproduced from ref. 21. Photoelectron spectra taken at other photon energies are given in the ESI.…”

“…We also note that our earlier study using ion mobility spectrometry coupled with laser spectroscopy on carboxylate substituted versions of 2-Np À infer triplet formation through the observation of fluorine-substitution photoreactions following excitation of the S 1 (pp*) band but not the S 2 (pp*) band. 21 Those experiments indicate that the excited state in the carboxylate substituted versions of 2-Np À must survive for nanoseconds based on the buffer gas pressure and the rate for ion-molecule collisions leading to products. No such photoreactions were observed for 2-Np À .…”