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

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 SF6 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 gives rise to 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 the threshold, the photoreaction with SF6 is observed with displacement of an OH group by an F atom. In contrast, the anion formed from deprotonation of the carboxylic acid group gives rise to a photodissociation action spectrum, recorded on the CO2 loss channel, lying at much shorter wavelengths with an onset at 360 nm and maximum photoresponse at 325 nm

Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation

A recent study of soft X-ray absorption in native and hydrogenated coronene cations, C$_{24}$H$_{12+m}^+$ $m=0-7$, led to the conclusion that additional hydrogen atoms protect (interstellar) Polycyclic Aromatic Hydrocarbon (PAH) molecules from fragmentation [Reitsma et al., Phys. Rev. Lett. 113, 053002 (2014)]. The present experiment with collisions between fast (30-200 eV) He atoms and pyrene (C$_{16}$H$_{10+m}^+$, $m=0$, 6, and 16) and simulations without reference to the excitation method suggests the opposite. We find that the absolute carbon-backbone fragmentation cross section does not decrease but increases with the degree of hydrogenation for pyrene molecules.Comment: 10 pages, 5 figure

Mechanosensitivity during lower extremity neurodynamic testing is diminished in individuals with Type 2 Diabetes Mellitus and peripheral neuropathy: a cross sectional study

<p>Abstract</p> <p>Background</p> <p>Type 2 Diabetes Mellitus (T2DM) and diabetic symmetrical polyneuropathy (DSP) impact multiple modalities of sensation including light touch, temperature, position sense and vibration perception. No study to date has examined the mechanosensitivity of peripheral nerves during limb movement in this population. The objective was to determine the unique effects T2DM and DSP have on nerve mechanosensitivity in the lower extremity.</p> <p>Methods</p> <p>This cross-sectional study included 43 people with T2DM. Straight leg raise neurodynamic tests were performed with ankle plantar flexion (PF/SLR) and dorsiflexion (DF/SLR). Hip flexion range of motion (ROM), lower extremity muscle activity and symptom profile, intensity and location were measured at rest, first onset of symptoms (P1) and maximally tolerated symptoms (P2).</p> <p>Results</p> <p>The addition of ankle dorsiflexion during SLR testing reduced the hip flexion ROM by 4.3° ± 6.5° at P1 and by 5.4° ± 4.9° at P2. Individuals in the T2DM group with signs of severe DSP (n = 9) had no difference in hip flexion ROM between PF/SLR and DF/SLR at P1 (1.4° ± 4.2°; paired t-test p = 0.34) or P2 (0.9° ± 2.5°; paired t-test p = 0.31). Movement induced muscle activity was absent during SLR with the exception of the tibialis anterior during DF/SLR testing. Increases in symptom intensity during SLR testing were similar for both PF/SLR and DF/SLR. The addition of ankle dorsiflexion induced more frequent posterior leg symptoms when taken to P2.</p> <p>Conclusions</p> <p>Consistent with previous recommendations in the literature, P1 is an appropriate test end point for SLR neurodynamic testing in people with T2DM. However, our findings suggest that people with T2DM and severe DSP have limited responses to SLR neurodynamic testing, and thus may be at risk for harm from nerve overstretch and the information gathered will be of limited clinical value.</p

Ion mobility action spectroscopy of flavin dianions reveals deprotomer-dependent photochemistry

The intrinsic optical properties and photochemistry of flavin adenine dinucleotide (FAD) dianions are investigated using a combination of tandem ion mobility spectrometry and action spectroscopy. Two principal isomers are observed, the more stable form being deprotonated on the isoalloxazine group and a phosphate (N-3,PO4 deprotomer), and the other on the two phosphates (PO4,PO4 deprotomer). Ion mobility data and electronic action spectra suggest that photo-induced proton transfer occurs from the isoalloxazine group to a phosphate group, converting the PO4,PO4 deprotomer to the N-3,PO4 deprotomer. Comparisons of the isomer selective action spectra of FAD dianions and flavin monoanions with solution spectra and gas-phase photodissociation action spectra suggests that solvation shifts the electronic absorption of the deprotonated isoalloxazine group to higher energy. This is interpreted as evidence for significant charge transfer in the lowest optical transition of deprotonated isoalloxazine. Overall, this work demonstrates that the site of deprotonation of flavin anions strongly affects their electronic absorptions and photochemistry

Collision- and photon-induced dynamics of complex molecular ions in the gas phase

In this thesis, I report experiments probing collision- and photon-induced molecular dynamics in the gas phase. Excited molecules formed in such interactions may relax by emitting electrons or photons, isomerization or fragmentation. For complex molecular systems, these processes typically occur on timescales exceeding picoseconds following statistical redistribution of the excitation energy across the internal degrees of freedom. However, energy transfer to molecules through ion/atom impact may in some cases lead to prompt atom knockout in Rutherford-type scattering processes on much faster timescales. Another example of such a non-statistical process is photon-induced excited-state proton transfer, a structural rearrangement occurring on the femtosecond timescale. In this work, I investigate the competition between statistical and non-statistical fragmentation processes for a range of molecules colliding with He at center-of-mass energies in the sub-keV range. I show that heavy atom knockout is an important process for systems containing aromatic rings such as Polycyclic Aromatic Hydrocarbons (PAHs) or porphyrins, while statistical fragmentation processes dominate for less stable and/or smaller systems such as adenine or hydrogenated PAHs. Furthermore, I present the first measurements of the threshold energies for prompt single atom knockout from isolated molecules. The experimental results are interpreted with the aid of Molecular Dynamics (MD) simulations which allow us to extract the energy deposited into the system during a collision, knockout cross sections, fragmentation pathways and the structures of the fragments. The results presented in this work may be important for understanding the response of complex molecules to energetic processes in e.g. astrophysical environments. Furthermore, I present the results of photodissociation and luminescence experiments probing flavin mono-anions in the gas phase. These are compared against calculations and previously measured spectra in solution. The discrepancies between the present results and the theoretical values suggest that more consideration of the vibronic structure is needed to model the photoabsorption and emission in flavins. Finally, I present the results of photoisomerisation experiments of flavin di-anions where two different isomers have been found and I discuss the proton transfer mechanisms which govern the structural changes.At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 1: Manuscript.</p

Collision- and photon-induced dynamics of complex molecular ions in the gas phase

In this thesis, I report experiments probing collision- and photon-induced molecular dynamics in the gas phase. Excited molecules formed in such interactions may relax by emitting electrons or photons, isomerization or fragmentation. For complex molecular systems, these processes typically occur on timescales exceeding picoseconds following statistical redistribution of the excitation energy across the internal degrees of freedom. However, energy transfer to molecules through ion/atom impact may in some cases lead to prompt atom knockout in Rutherford-type scattering processes on much faster timescales. Another example of such a non-statistical process is photon-induced excited-state proton transfer, a structural rearrangement occurring on the femtosecond timescale. In this work, I investigate the competition between statistical and non-statistical fragmentation processes for a range of molecules colliding with He at center-of-mass energies in the sub-keV range. I show that heavy atom knockout is an important process for systems containing aromatic rings such as Polycyclic Aromatic Hydrocarbons (PAHs) or porphyrins, while statistical fragmentation processes dominate for less stable and/or smaller systems such as adenine or hydrogenated PAHs. Furthermore, I present the first measurements of the threshold energies for prompt single atom knockout from isolated molecules. The experimental results are interpreted with the aid of Molecular Dynamics (MD) simulations which allow us to extract the energy deposited into the system during a collision, knockout cross sections, fragmentation pathways and the structures of the fragments. The results presented in this work may be important for understanding the response of complex molecules to energetic processes in e.g. astrophysical environments. Furthermore, I present the results of photodissociation and luminescence experiments probing flavin mono-anions in the gas phase. These are compared against calculations and previously measured spectra in solution. The discrepancies between the present results and the theoretical values suggest that more consideration of the vibronic structure is needed to model the photoabsorption and emission in flavins. Finally, I present the results of photoisomerisation experiments of flavin di-anions where two different isomers have been found and I discuss the proton transfer mechanisms which govern the structural changes.At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 1: Manuscript.</p

Non-statistical fragmentation in photo-activated flavin mononucleotide anions

The spectroscopy and photo-induced dissociation of flavin mononucleotide anions in vacuo are investigated over the 300-500 nm wavelength range. Comparison of the dependence of fragment ion yields as a function of deposited photon energy with calculated dissociation energies and collision-induced dissociation measurements performed under single-collision conditions suggests that a substantial fraction of photo-activated ions decompose through non-statistical fragmentation pathways. Among these pathways is the dominant photo-induced fragmentation channel, the loss of a fragment identified as formylmethylflavin. The fragment ion specific action spectra reveal electronic transition energies close to those for flavins in solution and previously published gas-phase measurements, although the photo-fragment yield upon excitation of the S 2 ← S 0 transition appears to be suppressed

Selective Generation of the Radical Cation Isomers [CH3CN] •+ and [CH2CNH] •+ via VUV Photoionization of Different Neutral Precursors and Their Reactivity with C2H4

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