Hydrogen sulfide radicals in the ground state, SH(X), and hydrogen disulfide molecules, H2S, are both detected in the interstellar medium, but the returned SH(X)/H2S abundance ratios imply a depletion of the former relative to that predicted by current models (which assume that photon absorption by H2S at energies below the ionization limit results in H + SH photoproducts). Here we report that translational spectroscopy measurements of the H atoms and S(1D) atoms formed by photolysis of jet-cooled H2S molecules at many wavelengths in the range 122 ≤ λ ≤155 nm offer a rationale for this apparent depletion; the quantum yield for forming SH(X) products, Γ, decreases from unity (at the longest excitation wavelengths) to zero at short wavelengths. Convoluting the wavelength dependences of Γ, the H2S parent absorption and the interstellar radiation field implies that only ~26% of photoexcitation events result in SH(X) products. The findings suggest a need to revise the relevant astrochemical models.

. (2015). Ultraviolet photodissociation action spectroscopy of gas-phase protonated quinoline and isoquinoline cations. Physical Chemistry Chemical Physics, 17 (39), 25882-25890. Ultraviolet photodissociation action spectroscopy of gas-phase protonated quinoline and isoquinoline cations AbstractThe gas-phase photodissociation action spectroscopy of protonated quinoline and isoquinoline cations (quinolineH+ and isoquinolineH+) is investigated at ambient temperature. Both isomers exhibit vibronic detail and wavelength-dependent photoproduct partitioning across two broad bands in the ultraviolet. Photodissociation action spectra are reported spanning 370-285 nm and 250-220 nm and analysed with the aid of electronic structure calculations: TD-DFT (CAM-B3LYP/aug-cc-pVDZ) is used for spectra simulations and CBS-QB3 for dissociation enthalpies. It is shown that the action spectra are afforded predominantly by two-photon excitation. The first band is attributed to both the S1 ← S0 and S2 ← S0 electronic transitions in quinolineH+, with a S1 ← S0 electronic origin assigned at 27 900 cm−1. For isoquinolineH+ the S1 ← S0 transition is observed with an assigned electronic origin at 27 500 cm−1. A separate higher energy band is observed for both species, corresponding to the S3 ← S0 transition, with origins assigned at 42 100 cm−1 and 42 500 cm−1 for quinolineH+ and isoquinolineH+, respectively. FranckCondon absorption simulations provide an explanation for some vibrational structure observed in both bands allowing several normal mode assignments. The nature of the electronic transitions is discussed and it is shown that the excited states active in the reported spectra should be of ππ* character with some degree of charge transfer from the homocycle to the heterocycle. AbstractThe gas-phase photodissociation action spectroscopy of protonated quinoline and isoquinoline cations (quinolineH + and isoquinolineH + ) is investigated at ambient temperature. Both isomers exhibit vibronic detail and wavelength-dependent photoproduct partitioning across two broad bands in the ultraviolet.Photodissociation action spectra are reported spanning 370 -285 nm and 250 -220 nm and analysed with the aid of electronic structure calculations: TD-DFT (CAM-B3LYP/aug-cc-pVDZ) is used for spectra simulations and CBS-QB3 for dissociation enthalpies. It is shown that the action spectra are a↵orded predominantly by two-photon excitation. The first band is attributed to both the S 1 S 0 and S 2 S 0 electronic transitions in quinolineH + , with a S 1 S 0 electronic origin assigned at 27 900 cm 1 . For isoquinolineH + the S 1 S 0 transition is observed with an assigned electronic origin at 27 500 cm 1 .A separate higher energy band is observed for both species, corresponding to the S 3 S 0 transition, with origins assigned at 42 100 cm 1 and 42 500 cm 1 for quinolineH + and isoquinolineH + , respectively.Franck-Condon absorption simulations provide an explanation for some vibrational structure observed in both bands allowing several normal mode assig...

Photo-induced isomerization reactions lie at the heart of many chemical processes in nature. The mechanisms of such reactions are determined by a delicate interplay of coupled electronic and nuclear dynamics occurring on the femtosecond scale, followed by the slower redistribution of energy into different vibrational degrees of freedom. Here we apply time-resolved photoelectron spectroscopy with a seeded extreme ultraviolet free-electron laser to trace the ultrafast ring opening of gas-phase thiophenone molecules following ultraviolet photoexcitation. When combined with ab initio electronic-structure and molecular-dynamics calculations of the excitedand ground-state molecules, the results provide insights into both the electronic and nuclear dynamics of this fundamental class of reactions. The initial ring opening and non-adiabatic coupling to the electronic ground state is shown to be driven by ballistic S-C bond extension and to be complete within 350 femtoseconds. Theory and experiment also enable visualization of the rich ground-state dynamics -involving formation of, and interconversion between, ring-opened isomers and the cyclic structure, and fragmentation over much longer timescales.

. (2013). UV photodissociation action spectroscopy of haloanilinium ions in a linear quadrupole ion trap mass spectrometer. Journal of the American Society for Mass Spectrometry, 24 (6), 932-940. UV photodissociation action spectroscopy of haloanilinium ions in a linear quadrupole ion trap mass spectrometer Abstract UV-vis photodissociation action spectroscopy is becoming increasingly prevalent because of advances in, and commercial availability of, ion trapping technologies and tunable laser sources. This study outlines in detail an instrumental arrangement, combining a commercial ion-trap mass spectrometer and tunable nanosecond pulsed laser source, for performing fully automated photodissociation action spectroscopy on gas-phase ions. The components of the instrumentation are outlined, including the optical and electronic interfacing, in addition to the control software for automating the experiment and performing online analysis of the spectra. To demonstrate the utility of this ensemble, the photodissociation action spectra of 4-chloroanilinium, 4-bromoanilinium, and 4-iodoanilinium cations are presented and discussed. Multiple photoproducts are detected in each case and the photoproduct yields are followed as a function of laser wavelength. It is shown that the wavelength-dependent partitioning of the halide loss, H loss, and NH3 loss channels can be broadly rationalized in terms of the relative carbon-halide bond dissociation energies and processes of energy redistribution. The photodissociation action spectrum of (phenyl)Ag2 + is compared with a literature spectrum as a further benchmark. AbstractUV-Vis photodissociation action spectroscopy is becoming increasingly prevalent due to advances in, and commercial availability of, ion trapping technologies and tunable laser sources. This study outlines in detail an instrumental arrangement, combining a commercial ion-trap mass spectrometer and tunable nanosecond pulsed laser source, for performing fully automated photodissociation action spectroscopy on gas-phase ions. The components of the instrumentation are outlined, including the optical and electronic interfacing, in addition to the control software for automating the experiment and performing online analysis of the spectra. To demonstrate the utility of this ensemble, the photodissociation action spectra of 4-chloroanilinium, 4-bromoanilinium and 4-iodoanilinium cations are presented and discussed. Multiple photoproducts are detected in each case and the photoproduct yields are followed as a function of laser wavelength. It is shown that the wavelength-dependent partitioning of the halide loss, H loss and NH 3 loss channels can be broadly rationalized in terms of the relative carbon-halide bond dissociation energies and processes of energy redistribution. The photodissociation action spectrum of (phenyl)Ag 2 + is compared to a literature spectrum as a further benchmark.3

Excited states formed by electron promotion to an antibonding σ* orbital are now recognized as key to understanding the photofragmentation dynamics of a broad range of heteroatom containing small molecules: alcohols, thiols, amines, and many of their aromatic analogues. Such excited states may be populated by direct photoexcitation, or indirectly by nonadiabatic transfer of population from some other optically excited state (e.g., a ππ* state). This Perspective explores the extent to which the fast-growing literature pertaining to such (n/π)σ*-state mediated bond fissions can inform and enhance our mechanistic understanding of photoinduced ring-opening in heterocyclic molecules.

Photodissociation of TEMPO-modified peptides: new approaches to radical-directed dissociation of biomolecules AbstractRadical-directed dissociation of gas phase ions is emerging as a powerful and complementary alternative to traditional tandem mass spectrometric techniques for biomolecular structural analysis. Previous studies have identified that coupling of 2-[(2,2,6,6-tetramethylpiperidin-1-oxyl)methyl]benzoic acid (TEMPO-Bz) to the N-terminus of a peptide introduces a labile oxygen-carbon bond that can be selectively activated upon collisional activation to produce a radical ion. Here we demonstrate that structurally-defined peptide radical ions can also be generated upon UV laser photodissociation of the same TEMPO-Bz derivatives in a linear ion-trap mass spectrometer. When subjected to further mass spectrometric analyses, the radical ions formed by a single laser pulse undergo identical dissociations as those formed by collisional activation of the same precursor ion, and can thus be used to derive molecular structure. Mapping the initial radical formation process as a function of photon energy by photodissociation action spectroscopy reveals that photoproduct formation is selective but occurs only in modest yield across the wavelength range (300-220 nm), with the photoproduct yield maximised between 235 and 225 nm. Based on the analysis of a set of model compounds, structural modifications to the TEMPO-Bz derivative are suggested to optimise radical photoproduct yield. Future development of such probes offers the advantage of increased sensitivity and selectivity for radicaldirected dissociation. AbstractRadical-directed dissociation of gas phase ions is emerging as a powerful and complementary alternative to traditional tandem mass spectrometric techniques for biomolecular structural analysis. Previous studies have identified that coupling of 2-[(2,2,6,6-tetramethylpiperidin-1-oxyl)methyl]benzoic acid (TEMPO-Bz) to the N-terminus of a peptide introduces a labile oxygen-carbon bond that can be selectively activated upon collisional activation to produce a radical ion. Here we demonstrate that structurally-defined peptide radical ions can also be generated upon UV laser photodissociation of the same TEMPO-Bz derivatives in a linear ion-trap mass spectrometer. When subjected to further mass spectrometric analyses, the radical ions formed by a single laser pulse undergo identical dissociations as those formed by collisional activation of the same precursor ion, and can thus be used to derive molecular structure. Mapping the initial radical formation process as a function of photon energy by photodissociation action spectroscopy reveals that photoproduct formation is selective but occurs only in modest yield across the wavelength range (300 -220 nm), with the photoproduct yield maximised between 235 and 225 nm. Based on the analysis of a set of model compounds, structural modifications to the TEMPO-Bz derivative are suggested to optimise radical photoproduct yield. Future development of such probes offers t...

Abstract. A cationic degradation product, formed in solution from retinal Schiff base (RSB), is examined in the gas phase using ion mobility spectrometry, photoisomerization action spectroscopy, and collision induced dissociation (CID). The degradation product is found to be N-n-butyl-2-(β-ionylidene)-4-methylpyridinium (BIP) produced through 6π electrocyclization of RSB followed by protonation and loss of dihydrogen. Ion mobility measurements show that BIP exists as trans and cis isomers that can be interconverted through buffer gas collisions and by exposure to light, with a maximum response at λ = 420 nm.

The dicarbon molecule (C2) is found in flames, comets, stars, and the diffuse interstellar medium. In comets, it is responsible for the green color of the coma, but it is not found in the tail. It has long been held to photodissociate in sunlight with a lifetime precluding observation in the tail, but the mechanism was not known. Here we directly observe photodissociation of C2. From the speed of the recoiling carbon atoms, a bond dissociation energy of 602.804(29) kJ·mol−1 is determined, with an uncertainty comparable to its more experimentally accessible N2 and O2 counterparts. The value is within 0.03 kJ·mol−1 of high-level quantum theory. This work shows that, to break the quadruple bond of C2 using sunlight, the molecule must absorb two photons and undergo two “forbidden” transitions.