Reaction dynamics of S(P) with 1,3-butadiene and isoprene: crossed-beam scattering, low-temperature flow experiments, and high-level electronic structure calculations.

Sulfur atoms serve as key players in diverse chemical processes, from astrochemistry at very low temperature to combustion at high temperature. Building upon our prior findings, showing cyclization to thiophenes following the reaction of ground-state sulfur atoms with dienes, we here extend this investigation to include many additional reaction products, guided by detailed theoretical predictions. The outcomes highlight the complex formation of products during intersystem crossing (ISC) to the singlet surfaces.

Site-Specific Photochemistry along a Protonated Peptide Scaffold.

We present a detailed study of the time-dependent photophysics and photochemistry of a known conformation of the two protonated pentapeptides Leu-enkephalin (Tyrosine-Glycine-Glycine-Phenylalanine-Leucine, YGGFL) and its chromophore-swapped analogue FGGYL, carried out under cryo-cooled conditions in the gas phase. Using ultraviolet-infrared (UV-IR) double resonance, we record excited state IR spectra as a function of time delay between UV and IR pulses. We identify unique Tyr OH stretch transitions due to the S state and the vibrationally excited triplet state(s) formed by intersystem crossing, T(v).

Sulfur (P) Reaction with Conjugated Dienes Gives Cyclization to Thiophenes under Single Collision Conditions.

We combine crossed-beam velocity map imaging with high-level /transition state theory modeling of the reaction of S(P) with 1,3-butadiene and isoprene under single collision conditions. For the butadiene reaction, we detect both H and H loss from the initial adduct, and from reaction with isoprene, we see both H loss and methyl loss. Theoretical calculations confirm these arise following intersystem crossing to the singlet surface forming long-lived intermediates.

Molecular Cage Reports on Its Contents: Spectroscopic Signatures of Cryo-Cooled K- and Ba-Benzocryptand Complexes.

UV photofragment spectroscopy and IR-UV double resonance methods are used to determine the structure and spectroscopic responses of a three-dimensional [2.2.2]-benzocryptand cage to the incorporation of a single K or Ba imbedded inside it (labeled as K-BzCrypt, Ba-BzCrypt).

Quantum resonances and roaming dynamics in formaldehyde photodissociation.

The unimolecular dissociation of formaldehyde is studied excitation to the à band at several excitation energies from just below the ground state radical dissociation threshold to 5000 cm above it. CO product rotational distributions, photofragment excitation spectroscopy and state-correlated slice imaging results are combined with quasi-classical trajectory calculations to reveal manifestations of quantum effects in this complex dissociation process involving interactions among radical, molecular, and roaming pathways. Evidence of nodal structure at the tight transition state to molecular products is investigated and correlations between the CO rotational and H vibrational distributions are used to suggest the transition state modes that are responsible.

Orbiting resonances in formaldehyde reveal coupling of roaming, radical, and molecular channels.

The roaming chemical reaction mechanism involves near-dissociation of an energized molecule to radicals that leads instead to intramolecular reaction after reorientation at long range. Surprisingly, no clear quantum signatures of roaming have been observed to date, despite the quantum nature of the roaming event. We found evidence of quantum dynamics in the photodissociation of formaldehyde near the roaming threshold.

Resolving Isomers of Star-Branched Poly(Ethylene Glycols) by IMS-MS Using Multiply Charged Ions.

Ion mobility spectrometry (IMS) mass spectrometry (MS) centers on the ability to separate gaseous structures by size, charge, shape, and followed by mass-to-charge (/). For oligomeric structures, improved separation is hypothesized to be related to the ability to extend structures through repulsive forces between cations electrostatically bonded to the oligomers. Here we show the ability to separate differently branched multiply charged ions of star-branched poly(ethylene glycol) oligomers (up to 2000 Da) regardless of whether formed by electrospray ionization (ESI) charged solution droplets or from charged solid particles produced directly from a surface by matrix-assisted ionization.

Comparison of gaseous ubiquitin ion structures obtained from a solid and solution matrix using ion mobility spectrometry/mass spectrometry.

Rationale: Examining surface protein conformations, and especially achieving this with spatial resolution, is an important goal. The recently discovered ionization processes offer spatial-resolution measurements similar to matrix-assisted laser desorption/ionization (MALDI) and produce charge states similar to electrospray ionization (ESI) extending higher-mass protein applications directly from surfaces on high-performance mass spectrometers. Studying a well-interrogated protein by ion mobility spectrometry-mass spectrometry (IMS-MS) to access effects on structures using a solid vs.

New mass spectrometry concepts for characterization of synthetic polymers and additives.

Rationale: New ionization processes have been developed for biological mass spectrometry (MS) in which the matrix lifts the nonvolatile analyte into the gas phase as ions without any additional energy input. We rationalized that additional fundamental knowledge is needed to assess analytical utility for the field of synthetic polymers and additives.

Methods: Different mass spectrometers (Thermo Orbitrap (Q-)Exactive (Focus); Waters SYNAPT G2(S)) were employed.

Imaging the infrared multiphoton excitation and dissociation of propargyl chloride.

Infrared multiphoton excitation is combined with UV excitation and state-resolved probes of Cl(P), Cl*(P), and HCl to study the photochemistry of propargyl chloride. The results show evidence both of infrared multiphoton dissociation on the ground electronic state and infrared multiphoton excitation followed by UV dissociation. The results are interpreted with the aid of a full characterization of the stationary points on the ground state using ab initio methods, as well as our recent experimental and theoretical characterization of the UV photochemistry of the molecule.

Mixed transitions in the UV photodissociation of propargyl chloride revealed by slice imaging and multireference ab initio calculations.

Resonance-enhanced multiphoton ionization (REMPI) and DC slice imaging were used to detect photoproducts Cl (2P3/2), spin-orbit excited Cl* (2P1/2), and C3H3 in the photodissociation of propargyl chloride at 212 and 236 nm. Cl and Cl* translational energy distributions peak at high recoil energy, suggesting impulsive energy release in the C-Cl coordinate. Near 236 nm, photofragment angular distributions show rapidly changing anisotropy across the main peak for both Cl and Cl*, indicating excitations arising from in-plane or out-of-plane π systems.

Unprecedented Ionization Processes in Mass Spectrometry Provide Missing Link between ESI and MALDI.

In the field of mass spectrometry, producing intact, highly-charged protein ions from surfaces is a conundrum with significant potential payoff in application areas ranging from biomedical to clinical research. Here, we report on the ability to form intact, highly-charged protein ions on high vacuum time-of-flight mass spectrometers in the linear and reflectron modes achievable using experimental conditions that allow effective matrix removal from both the sample surfaces and from the charged clusters formed by the laser ablation event. The charge states are the highest reported on high vacuum mass spectrometers, yet they remain at only around a third of the highest charge obtained using laser ablation with a suitable matrix at atmospheric pressure.

Matrix-Assisted Ionization on a Portable Mass Spectrometer: Analysis Directly from Biological and Synthetic Materials.

Matrix-assisted ionization (MAI)-mass spectrometry (MS) eliminates the need for high voltage, a heat source, lasers, and compressed gases in the ionization process and uses minimal solvents in sample preparation, thus making MAI ideal for field-portable mass spectrometers. The broad applicability of MAI is demonstrated by simple, rapid, and robust positive and negative detection mode analyses of low and high mass compounds including some pesticides, dyes, drugs, lipids, and proteins (186 Da to 8.5 kDa) from various materials including urine, biological tissue sections, paper, and plant material on a low pumping capacity, single-quadrupole mass spectrometer.

Myelin-targeted, texaphyrin-based multimodal imaging agent for magnetic resonance and optical imaging.

Reliable methods of imaging myelin are essential to investigate the causes of demyelination and to study drugs that promote remyelination. Myelin-specific compounds can be developed into imaging probes to detect myelin with various imaging techniques. The development of multimodal myelin-specific imaging probes enables the use of orthogonal imaging techniques to accurately visualize myelin content and validate experimental results.

Rapid high mass resolution mass spectrometry using matrix-assisted ionization.

Matrix-assisted ionization (MAI) is demonstrated to be a robust and sensitive analytical method capable of analyzing proteins such as cholera toxin B-subunit and pertussis toxin mutant from conditions containing relatively high amounts of inorganic salts, buffers, and preservatives without the need for prior sample clean-up or concentration. By circumventing some of the sample preparation steps, MAI simplifies and accelerates the analytical workflow for biological samples in complex media. The benefits of multiply charged ions characteristic of electrospray ionization (ESI) and the robustness of matrix-assisted laser desorption/ionization (MALDI) can be obtained from a single method, making it well suited for analysis of proteins and other biomolecules at ultra-high resolution as demonstrated on an Orbitrap Fusion where protein subunits were resolved for which MALDI-time-of-flight failed.

Use of Ion Mobility Spectrometry-Mass Spectrometry to Elucidate Architectural Dispersity within Star Polymers.

The power of ion mobility spectrometry-mass spectrometry (IMS-MS) as an analytical technology for differentiating macromolecular architecture is demonstrated. The presence of architectural dispersity within a sample is probed by sequentially measuring both the drift time and the mass-to-charge ratio for every component within a polymer sample. The utility of this technology is demonstrated by investigating three poly(ethylene glycol) (PEG) architectures with closely related average molecular weights of about 9000 Da: a linear PEG, an unevenly branched miktoarm star PEG, and evenly branched homoarm star PEGs.