High Resolution Crossed Molecular Beams Study of the H+HD→H2+D Reaction

The H+H₂ reaction is the simplest chemical reaction system and has long been the prototype model in the study of reaction dynamics. Here we report a high resolution experimental investigation of the state-to-state reaction dynamics in the H+HD→H₂+D reaction by using the crossed molecular beams method and velocity map ion imaging technique at the collision energy of 1.17 eV. D atom products in this reaction were probed by the near threshold 1+1'(vacuum ultraviolet+ultraviolet) laser ionization scheme. The ion image with both high angular and energy resolution were acquired. State-to-state differential cross sections was accurately derived. Fast forward scattering oscillations, relating with interference effects in the scattering process, were clearly observed for H₂ products at H₂(v'=0,j'=1) and H₂(v'=0,j'=3) rovibrational levels. This study further demonstrates the importance of measuring high-resolution differential cross sections in the study of state-to-state reaction dynamics in the gas phase.     

Reproducibility in the unfolding process of protein induced by an external electric field

The dynamics of proteins are crucial for their function. However, commonly used techniques for studying protein structures are limited in monitoring time-resolved dynamics at high resolution. Combining electric fields with existing techniques to study gas-phase proteins, such as single particle imaging using free-electron lasers and gas-phase small angle X-ray scattering, has the potential to open up a new era in time-resolved studies of gas-phase protein dynamics. Using molecular dynamics simulations, we identify well-defined unfolding pathways of a protein, induced by experimentally achievable external electric fields. Our simulations show that strong electric fields in conjunction with short-pulsed X-ray sources such as free-electron lasers can be a new path for imaging dynamics of gas-phase proteins at high spatial and temporal resolution.

Controlled unfolding of proteins can reveal structural properties and give insights of the proteins'' dynamics. We show the feasibility of unfolding proteins in the gas phase using electric fields, with a well-defined path at high field strengths.     

Imaging Reaction Dynamics of Y+SO2

The reaction dynamics of yttrium atoms with sulfur dioxide molecules at a high collision energy of 36 kcal/mol was studied using time-sliced velocity map ion imaging, crossed molecular beam and laser-ablation method. The product YO was detected via multiphoton ionization at various wavelengths in the region of 482\begin{document}$ - $\end{document}615 nm. The slice images of YO show a broad velocity distribution and forward-backward peaking angular distribution. The forward scattering signal is stronger than its backward distribution. This indicates that the reaction proceeds via an intermediate complex and the lifetime of the intermediate state is less than one rotational period. The formation of complex suggests that electron transfer occurs in the oxidation reaction.     

Imaging the [1+1] Two-Photon Dissociation Dynamics of Br2+ in a Cold Ion Beam

The [1+1] two-photon dissociation dynamics of mass-selected ⁷⁹Br₂⁺ has been studied in acold ion beam using a cryogenic cylindrical ion trap velocity map imaging spectrometer. The quartet 1⁴Σ^-_u,3/2 state of ⁷⁹Br₂⁺ is employed as an intermediate state to initiate resonance enhanced two-photon excitation to high-lying dissociative states in the 4.0-5.0 eV energy region above the ground rovibronic state. Total kinetic energy release (TKER) and the twodimensional recoiling velocity distributions of fragmented ⁷⁹Br⁺ ions are measured using the technique of DC-slice velocity map imaging. Branching ratios for individual state-resolved product channels are determined from the TKER spectra. The measured photofragment angular distributions indicate that the dissociation of ⁷⁹Br₂⁺ occurs in dissociative Ω=3/2 state via ΔΩ=0 parallel transition from the 1⁴Σ^-_u,3/2 intermediate state. Due to the considerable spin-orbit coupling effects in the excited states of ⁷⁹Br₂⁺, higher-lying dissociative quartet states are likely responsible for the observed photodissociation processes.     

Reactive collisions in quadrupole cells. Part I. Reaction of [CH3NH2]+˙ with the isomeric butenes and pentenes

The reactions of mass-selected [CH₃NH₂]⁺˙ ions with the isomeric butenes and pentenes were studied at low collision energies in the radiofrequency-only quadrupole collision cell of a hybrid BEqQ tandem mass spectrometer. Characteristic iminium ions arising by addition of the methylamine to the olefin followed by fragmentation are observed for but-1-ene pent-1-ene and 3-methylbut-1-ene. However, for but-2-ene pent-2-ene 2-methylpropene 2-methylbut-1-ene and 2-methylbut-2-ene the major reaction channel of [CH₃NH₂]⁺˙ is charge exchange to form the olefinic molecular ion. The isomeric olefins are characterized to a considerable extent by the characteristic ion–molecule reactions that these molecular ions undergo with the neutral olefin.     

Ion cyclotron resonance studies of some reactions of C+ ions

Product distributions and rate constants for the reaction of ground state C⁺ ions with O₂, NO, HCl, CO₂, H₂S, H₂O, HCN, NH₃, CH₄, H₂CO, CH₃OH, and CH₃NH₂ have been measured. Rate constants were obtained using ion cyclotron resonance trapped ion methods at JPL, and product distributions were obtained using a tandem (Dempster-ICR) mass spectrometer at the University of Utah. Rapid carbon isotope exchange has also been observed in C⁺-CO collisions.     

Introduction of a 20 kHz Nd:YVO4 laser into a hybrid quadrupole time-of-flight mass spectrometer for MALDI-MS imaging

A commercial hybrid quadrupole time–of–flight mass spectrometer has been modified for high-speed matrix-assisted laser desorption ionisation (MALDI) imaging using a short-pulse optical technology Nd:YVO₄ laser. The laser operating in frequency-tripled mode (λ = 355 nm) is capable of delivering 1.5-ns pulses of energy at up to 8 μJ at 5–10 kHz and 3 μJ at 20 kHz. Experiments to improve beam homogeneity and reduce laser speckle by mechanical vibration of the fibre-optic laser delivery system are reported along with data from trial and tissue imaging experiments using the modified instrument. The laser appeared to yield best results for MALDI-MS imaging experiments when operating at repetition rates 5–10 kHz. Combining this with raster imaging allowed images of rat brain sections to be recorded in 37 min. Similarly, images of the distribution of peptides in “on-tissue” digest experiments from tumour tissues were recorded in 1 h and 30 min rather than the 8-h acquisition time previously used. A brief investigation of targeted protein analysis/imaging by multiple reaction monitoring experiments “on-tissue” is reported. A total of 26 transitions were recorded over a 3-s cycle time and images of abundant proteins were successfully recorded.     

Collinear Velocity-map Photoelectron Imaging Spectrometer for Cluster Anions

We describe a collinear velocity-map photoelectron imaging spectrometer, which combines a Wiley-McLaren time-of-flight mass analyzer with a dual-valve laser vaporization source for investigating size-selected cluster and reaction intermediate anions. To generate the reaction anions conveniently, two pulsed valves and a reaction channel are employed instead of premixing carrier gas. The collinear photoelectron imaging spectrometer adopts modified velocity-map electrostatic lens, and provides kinetic energy resolution better than 3%. The performance of the instrument is demonstrated on the photodetachment of Si₄^- at 532 and 355 nm, and Si₃C^- at 532 nm, respectively. In both cases, photoelectron spectra and anisotropy parameters are obtained from the images. For Si₄^-, the spectra show two well-resolved vibrational progressions which correspond to the ground state and the first excited state of the neutral Si₄ with peak spacing of 330 and 312 cm^-1, respectively. Preliminary results suggest that the apparatus is a powerful tool for characterizing the electronic structure and photodetachment dynamics of cluster anions.     

Delayed predissociation and collision-induced processes of the ammonia di-cation NH32+

Reactions of NH₃²⁺ di-cations, observed a few microseconds after formation by electron ionisation of ammonia, have been studied in a double-focusing mass spectrometer using techniques of ion translational energy spectroscopy. Fragmentation reactions occuring under unimolecular conditions correspond to predissociation processes, but lack of knowledge about states of NH₃²⁺ precludes any definitive interpretation. A partial tentative interpretation, using arguments by analogy with isoelectronic neutral species, is proposed. Almost all collision-induced fragmentations required electron transfer forming NH₃⁺ as an intermediate step, so the non-dissociative electron transfer processes were also studied. The most unexpected collision-induced fragmentation reaction was that which formed H₃⁺ fragment ions.     

Combined electrospray ionization source with a velocity map imaging spectrometer for studying large gas phase molecular ions

We present a new compact and versatile experimental set-up that has been designed to perform electron and ion imaging experiments on large multiply charged gas phase molecular and cluster species. It combines an electrospray ionization source, a quadrupole mass filter guiding ion optics and a velocity map imaging spectrometer. Characterization of the spectrometer has been performed on atomic ions. Results obtained on molecular species (stilbene 420 dianions) demonstrate the possibility offered by this experimental set-up.     

The high resolution rotational spectrum of methyl chloride. The spin—rotation and nuclear magnetic shielding tensors

A high resolution study of the rotational spectrum of CH₃³⁵Cl in the vibrational ground state has been performed using a molecular beam spectrometer. By fitting the measured transitions to an appropriate molecular hamiltonian the values of the following molecular parameters have been obtained: B = 13 292.8763 MHz; D_J = 18.089 kHz; D_JK = 198.76 kHz; H_KJ = 9 Hz; eqQ = ?74.750 MHz; C_⊥ = ?2.7 kHz and C_∥ = ?7 kHz. These data are used to determine the paramagnetic contribution to the chlorine nuclear magnetic shielding tensor. The diamagnetic contribution is also calculated using a semi-empirical method. Comparisons of the average shieldings are made for related molecules.     

Switchable Guest Molecular Dynamics in a Perovskite‐Like Coordination Polymer toward Sensitive Thermoresponsive Dielectric Materials

A new perovskite‐like coordination polymer [(CH₃)₂NH₂][Cd(N₃)₃] is reported which undergoes a reversible ferroelastic phase transition. This transition is due to varied modes of motion of the [(CH₃)₂NH₂]⁺ guest accompanied by a synergistic deformation of the [Cd(N₃)₃]^? framework. The unusual two‐staged switchable dielectric relaxation reveals the molecular dynamics of the polar cation guest, which are well controlled by the variable confined space of the host framework. As the material switches from the ferroelastic phase to the paraelastic phase, a remarkable increase of the rotational energy barrier is detected. As a result, upon heating at low temperature, this compound shows a notable change from a low to a high dielectric state in the ferroelastic phase. This thermoresponsive host–guest system may serve as a model compound for the development of sensitive thermoresponsive dielectric materials and may be key to understanding and modulating molecular/ionic dynamics of guest molecules in confined space.     

A Surface Femtosecond Two-Photon Photoemission Spectrometer for Excited Electron Dynamics and Time-Dependent Photochemical Kinetics

A surface femtosecond two-photon photoemission (2PPE) spectrometer devoted to the study of ultrafast excited electron dynamics and photochemical kinetics on metal and metal oxide surfaces has been constructed. Low energy photoelectrons are measured using a hemispheri-cal electron energy analyzer with an imaging detector that allows us to detect the energy and the angular distributions of the photoelectrons simultaneously. A Mach-Zehnder interferom-eter was built for the time-resolved 2PPE (TR-2PPE) measurement to study ultrafast surface excited electron dynamics, which was demonstrated on the Cu(111) surface. A scheme for measuring time-dependent 2PPE (TD-2PPE) spectra has also been developed for studies of surface photochemistry. This technique has been applied to a preliminary study on the photochemical kinetics on ethanol/TiO₂(110). We have also shown that the ultrafast dy-namics of photoinduced surface excited resonances can be investigated in a reliable way by combining the TR-2PPE and TD-2PPE techniques.     

Observation of Interference Effects in (e, 2e) Electron Momentum Spectroscopy of SF6 (cited: 1)

Two-dimensional electron density map (2D map) of binding energy and relative azimuthal angle (i.e., momentum) for the outer-valence molecular orbitals of SF₆ has been measured by a highly sensitive electron momentum spectrometer with noncoplanar symmetric geometry at the impact energy of 1.2 keV plus binding energy. The experimental electron momentum profiles for the relevant molecular orbitals have been extracted from the 2D map and interpreted on the basis of the quantitative calculations using the density functional theory with B3LYP hybrid functional. For the outermost F2p nonbonding orbitals of SF₆, the interference patterns are clearly observed in the ratios of the electron momentum profiles of molecular orbitals to that of atomic F2p orbital.     

Practical one-pot transformation of electron-rich aromatics into aromatic nitriles with molecular iodine and aq NH3 using Vilsmeier–Haack reaction

Various electron-rich aromatics could be efficiently transformed into the corresponding aromatic nitriles in good to moderate yields by treatment with DMF and POCl₃, followed by the reaction with molecular iodine or 1,3-diiodo-5,5-dimethylhydantoin (DIH) in aq NH₃. Some of less reactive aromatics, such as anisole, 1,2-dimethoxybenzene, 1,4-dimethoxybenzene, and mesityrene, could be also transformed into the corresponding aromatic nitriles in good to moderate yields using N-methylformanilide and O(POCl₂)₂, followed by the reaction with molecular iodine in aq NH₃. Moreover, propiophenone derivatives could be successfully transformed into the corresponding β-chlorocinnamonitriles by the reaction with DMF and POCl₃, followed by the reaction with molecular iodine and aq NH₃. These reactions are novel metal-free one-pot methods for the preparation of aromatic nitriles from electron-rich aromatics and β-chlorocinnamonitriles from propiophenones.     

Ion-Velocity Map Imaging Study of Photodissociation Dynamics of Acetaldehyde

The photodissociation dynamics of acetaldehyde in the radical channel CH3＋HCO has been reinvestigated using time-sliced velocity map imaging technique in the photolysis wavelength range of 275-321 nm. The CH3 fragments have been probed via （2＋1） resonance-enhanced multiphoton ionization. Images are measured for CH3 formed in the ground and excited states （v2=0 and 1） of the umbrella vibrational mode. For acetaldehyde dissociation on T1 state after intersystem crossing from S1 state, the products are formed with high translational energy release and low internal excitation. The rotational and vibrational energy of both fragments increases with increasing photodissociation energy. The triplet barrier height is estimated at 3.8814-0.006 eV above the ground state of acetaldehyde.     

Time-resolved velocity map imaging of methyl elimination from photoexcited anisole

To date, H-atom elimination from heteroaromatic molecules following UV excitation has been extensively studied, with the focus on key biological molecules such as chromophores of DNA bases and amino acids. Extending these studies to look at elimination of other non-hydride photoproducts is essential in creating a more complete picture of the photochemistry of these biomolecules in the gas-phase. To this effect, CH(3) elimination in anisole has been studied using time-resolved velocity map imaging (TR-VMI) for the first time, providing both time and energy information on the dynamics following photoexcitation at 200 nm. The extra dimension of energy afforded by these measurements has enabled us to address the role of πσ* states in the excited state dynamics of anisole as compared to the hydride counterpart (phenol), providing strong evidence to suggest that only CH(3) fragments eliminated with high kinetic energy are due to direct dissociation involving a (1)πσ* state. These measurements also suggest that indirect mechanisms such as statistical unimolecular decay could be contributing to the dynamics at much longer times.     

Exploring ultrafast H-atom elimination versus photofragmentation pathways in pyrazole following 200 nm excitation

The role of ultraviolet photoresistance in many biomolecules (e.g., DNA bases and amino acids) has been extensively researched in recent years. This behavior has largely been attributed to the participation of dissociative (1)πσ* states localized along X-H (X ═ N, O) bonds, which facilitate an efficient means for rapid nonradiative relaxation back to the electronic ground state via conical intersections or ultrafast H-atom elimination. One such species known to exhibit this characteristic photochemistry is the UV chromophore imidazole, a subunit in the biomolecules adenine and histidine. However, the (1)πσ* driven photochemistry of its structural isomer pyrazole has received much less attention, both experimentally and theoretically. Here, we probe the ultrafast excited state dynamics occurring in pyrazole following photoexcitation at 200 nm (6.2 eV) using two experimental methodologies. The first uses time-resolved velocity map ion imaging to investigate the ultrafast H-atom elimination dynamics following direct excitation to the lowest energy (1)πσ* state (1(1)A" ← X(1)A'). These results yield a bimodal distribution of eliminated H-atoms, situated at low and high kinetic energies, the latter of which we attribute to (1)πσ* mediated N-H fission. The time constants extracted for the low and high energy features are ~120 and <50 fs, respectively. We also investigate the role of ring deformation relaxation pathways from the first optically bright (1)ππ* state (2(1)A' ← X(1)A'), by performing time-resolved ion yield measurements. These results are modeled using a (1)ππ* → ring deformation → photofragmentation mechanism (a model based on comparison with theoretical calculations on the structural isomer imidazole) and all photofragments possess appearance time constants of <160 fs. A comparison between time-resolved velocity map ion imaging and time-resolved ion yield measurements suggest that (1)πσ* driven N-H fission gives rise to the dominant kinetic photoproducts, re-enforcing the important role (1)πσ* states have in the excited state dynamics of biological chromophores and related aromatic heterocycles.