Diastereomeric discrimination in the lifetimes of Norrish Type II triplet 1,4-biradicals and stereocontrolled partitioning of their reactivity (Yang cyclization versus Type II fragmentation)

The stereochemistry at C2 and C3 carbons controls the partitioning of triplet 1,4-biradicals of ketones 2 among various pathways. Differences in the major reaction pathways, for example, cyclization (syn) and fragmentation (anti), adopted by the diastereomeric 1,4-radicals of ketones 2 have permitted unprecedented diastereomeric discrimination in their lifetimes to be observed by nanosecond laser flash photolysis. From quantum yield measurements and transient lifetime data, the absolute rate constants for cyclization and fragmentation of a pair of diastereomeric triplet 1,4-biradicals have been determined for the first time.     

Atmospheric pressure photoionization. II. Dual source ionization

In this paper we describe results based on the combination of atmospheric pressure photoionization (APPI) with atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI). The main purpose of combining more than one ionizer is to extend the range of compounds that can be simultaneously analyzed. Three modes of operation are presented; use of either ionizer, simultaneous use of two ionizers, and rapid switching between ionizers during a single chromatographic run. The dual ionizer configurations only minimally affect the performance of either ionizer relative to the standard single-ionizer sources. However, it is observed that the operation of both ionizers together does not typically give the sum signal from either source operating alone. For APCI/APPI the signal can range from less than that of either source alone to the sum of the two individual sources. For ESI/APPI, we observed large suppressions of the ESI multiply-charged signal of proteins when the APPI source was on. These behaviors are presumed to be due to the interaction of the initially formed ions by both sources and attests to the importance of ion-molecule reactions that occur during and after the primary ionization events. We give examples of compounds that are preferentially ionized by either APPI, APCI or ESI and present thermochemical arguments based on molecular structure and functionality to explain this behavior. The dual source is also shown to be able to operate in negative ion mode opening up the potential to conduct wide ranging chemical analyses.     

Early events in radiation chemistry and in photoionization

Real-time studies of aliphatic and aromatic hydrocarbons by pulse radiolysis and laser photoionization reveal the chemistry of the ionic species in the condensed phase. The occurrence of radical cation reactions with solvent molecules provides the core mechanism capable of explaining a wide range of observations in photoionization and radiation chemistry. The study of products and transients in photoionization of aromatic solutes in hydrocarbon and alcohol solvents illustrates several details of this “high-energy” chemistry. A reaction pathway involving ion-molecule reaction of excited ions is indicated for a series of polycyclic aromatic hydrocarbons photoionized using intense excimer laser (248 and 308 nm) pulses in hydrocarbon and alcohol solutions. We have found that condensed-phase ion-molecule reactions in radiolysis are ubiquitous and we speculate on their overall role in hydrocarbon radiolysis.     

Transient species and its properties of melatonin

Reactive oxygen species (ROS) are generated dur- ing radiation, respiratory burst, normal metabolic processes and so on. There are enzymatic and non-enzymatic antioxidants such as superoxide dis- mutase (SOD), glutathione peroxidase (GSH-Px), vi- tamin E (VE) and carotenoids that can either inhibit or repair the ROS-induced damage. ROS is essential to maintain physiological homeostasis. However, exces- sive ROS give rise to oxidative damage to proteins, lipids and DNA which related t…     

Photochemical properties of a new kind of anti-cancer drug: N-glycoside compound

Due to the nontoxicity and efficient anti-cancer activity, more and more attention has been paid to N-glycoside compounds. Laser photolysis of N-(α-D-glucopyranoside) salicyloyl hydrazine (NGSH) has been performed for the first time. The research results show that NGSH has high photosensitivity and is vulnerable to be photo-ionized via a monophotonic process with a quantum yield of 0.02, generating NGSH · and hydrated electrons. Under the aerobic condition of cells, the hydrated electrons are very easy to combine with oxygen to generate 1O2 and O2-, both of which are powerful oxidants that can kill the cancer cells. In addition, NGSH · can be changed into neutral radicals by deprotonation with a pKa value of 4.02 and its decay constant was determined to be 2.55×109dm3·mol-1·s-1. NGSH also can be oxidized by SO4-. with a rate constant of 1.76×109 dm3·mol-1.s-1, which further confirms the results of photoionization. All of these results suggest that this new N-glycoside compound might be useful for cancer treatment.     

Absolute photoionization cross sections of two cyclic ketones: cyclopentanone and cyclohexanone

Absolute photoionization cross sections for cyclopentanone and cyclohexanone, as well as partial ionization cross sections for the dissociative ionized fragments, are presented in this investigation. Experiments are performed via a multiplexed photoionization mass spectrometer utilizing vacuum ultraviolet (VUV) synchrotron radiation supplied by the Advanced Light Source of Lawrence Berkeley National Laboratory. These results allow the quantification of these species that is relevant to investigate the kinetics and combustion reactions of potential biofuels. The CBS‐QB3 calculated values for the adiabatic ionization energies agree well with the experimental values, and the identification of possible dissociative fragments is discussed for both systems. Copyright © 2017 John Wiley & Sons, Ltd.     

Direct UVC photodegradation of imipramine in aqueous solutions: a mechanistic study

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Higher-order processes in X-ray photoionization of atoms

There are several fourth-generation X-ray light source projects now underway around the world and it is anticipated that by the end of the decade, one or more of these X-ray free-electron lasers will be operational. In this contribution, we describe recent measurements and future plans to study both multielectron and multiphoton atomic photoionization. Although such higher-order processes are rare with present third-generation sources, they will be commonplace in experimental work with the new sources. The topics we discuss here are double K-shell ionization and two-photon X-ray photoionization.     

Investigation on the absolute and relative photoionization cross sections of 3 potential propargylic fuels

Absolute photoionization cross sections for 2 potential propargylic fuels (propargylamine and dipropargyl ether) along with the partial ionization cross sections for their dissociative fragments are measured and presented for the first time via synchrotron photoionization mass spectrometry. The experimental setup consists of a multiplexed orthogonal time‐of‐flight mass spectrometer and is located at the Advanced Light Source facility of the Lawrence Berkeley National Laboratory in Berkeley, California. Data for a third propargylic compound (propargyl alcohol) were taken; however, because of its low signal, due to its weakly bound cation, only the dissociative ionization fragment from the H‐loss channel is observed and presented. Suggested pathways leading to formation of dissociative photoionization fragments along with CBS‐QB3 calculated adiabatic ionization energies and appearance energies for the dissociative fragments are also presented.     

Laser Access to Quercetin Radicals and Their Repair by Co-antioxidants

We have demonstrated the feasibility and ease of producing quercetin radicals by photoionization with a pulsed 355 nm laser. A conversion efficiency into radicals of 0.4 is routinely achieved throughout the pH range investigated (pH 2–9), and the radical generation is completed within a few ns. No precursor other than the parent compound is needed, and the ionization by-products do not interfere with the further fate of the radicals. With this generation method, we have characterized the quercetin radicals and studied the kinetics of their repairs by co-antioxidants such as ascorbate and 4-aminophenol. Bell-shaped pH dependences of the observed rate constants reflect opposite trends in the availability of the reacting protonation forms of radical and co-antioxidant and even at their maxima mask the much higher true rate constants. Kinetic isotope effects identify the repairs as proton-coupled electron transfers. An examination of which co-antioxidants are capable of repairing the quercetin radicals and which are not confines the bond dissociation energies of quercetin and its monoanion experimentally to 75–77 kcal mol⁻¹ and 72–75 kcal mol⁻¹, a much narrower interval in the case of the former than previously estimated by theoretical calculations.     

Bulky Substituent Effect on Reactivity of Localized Singlet Cyclopentane‐1,3‐diyls with π‐Single Bonding (C‐π‐C) Character

Thorough investigation of key intermediates, such as long‐lived singlet diradicals, is essential to understand the homolytic bond cleavage reactions. In this study, we evaluate the effect of bulky substituents at the meta‐position of the phenyl ring on the bond formation process in singlet 2,2‐diethoxy‐1,3‐diarylcyclopentane‐1,3‐diyls. The bulky groups have significant influence on the diradical lifetime, as such, when the triisopropylphenyl group was used, the lifetime was 45 times longer than that of the parent diradical in benzene at 293 K.     

气相自由基反应动力学的光电离质谱研究

气相自由基能与各种气体发生快速反应,在大气化学、燃烧化学和星际化学等重要的化学过程中起着关键性的催化作用。许多实验方法(例如荧光法和吸收法)已经用于研究气相自由基反应动力学过程,并取得许多重要的成果,但这些技术局限于探测小分子自由基反应。流动管反应器和闪光光解结合光电离质谱的实验技术以其通用性、多重探测性、选择性和灵敏性等多种优势,成为研究气相自由基反应的主要实验方法。本文介绍利用高通量、高分辨、连续可调的同步辐射光电离质谱开展多种自由基反应研究所取得的一些独创性的成果。另外,该技术具有独有的时间分辨、能量分辨和异构体分辨的能力,能够广泛用于大气化学、燃烧化学和星际化学中一些重要的气相自由基(如烷氧自由基、烃类自由基等)反应宏观动力学过程的研究。     

Photoionization studies on various quinones by an infrared laser desorption/tunable VUV photoionization TOF mass spectrometry

Photoionization and dissociative photoionization characters of six quinones, including 1,2-naphthoquinone (1,2-NQ), 1,4-naphthoquinone (1,4-NQ), 9,10-phenanthroquinone (PQ), 9,10-anthraquinone (AQ), benz[a]- anthracene-7,12-dione (BAD) and 1,2-acenaphthylenedione (AND) have been studied with an infrared laser desorption/tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry (IR LD/VUV PIMS) technique. Mass spectra of these compounds are obtained at different VUV photon energies. Consecutive losses of two carbon monoxide (CO) groups are found to be the main fragmentation pathways for all the quinones. Detailed dissociation processes are discussed with the help of ab initio B3LYP calculations. Ionization energies (IEs) of these quinones and appearance energies (AEs) of major fragments are obtained by measuring the photoionization efficiency (PIE) spectra. The experimental results are in good agreement with the theoretical data.     

Experimental and theoretical study on the photoionization of styrene

This study employed a vacuum ultraviolet synchrotron radiation source and reflectron time-of-flight mass spectrometry (TOF-MS) to investigate the photoionization and dissociation of styrene. By analyzing the photoionization mass spectrum and efficiency curve alongside G3B3 theoretical calculations, we determined the ionization energy of the molecular ion, appearance energy of fragment ions, and relevant dissociation pathways. The major ion peaks observed in the photoionization mass spectra of styrene correspond to C₈H₈⁺, C₈H₇⁺ and C₆H₆⁺. The ionization energy of styrene is measured as 8.46 ± 0.03 eV, whereas the appearance energies of C₈H₇⁺ and C₆H₆⁺ are found to be 12.42 ± 0.03 and 12.22 ± 0.03 eV, respectively, in agreement with theoretical values. The main channel for the photodissociation of styrene molecular ions is the formation of benzene ions, whereas the dissociation channel that loses hydrogen atoms is the secondary channel. Based on the experimental results and empirical formulas, the required dissociation energies (E_d) of C₈H₇⁺, C₈H₆⁺ and C₆H₆⁺ are calculated to be (3.96 ± 0.06), (4.00 ± 0.06) and (3.76 ± 0.06) eV, respectively. Combined with related thermochemical parameters, the standard enthalpies of formations of C₈H₈⁺, C₈H₇⁺, C₈H₆⁺ and C₆H₆⁺ are determined to be 964.2, 1346.3, 1350.2 and 1327.0 kJ/mol, respectively. Based on the theoretical study, the kinetic factors controlling the styrene dissociation reaction process are determined by using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. This provides a reference for further research on the atmospheric photooxidation reaction mechanism of styrene in atmospheric and interstellar environments.     

Photophysical and photochemical processes of riboflavin (vitamin B2) by means of the transient absorption spectra in aqueous solution

Using time-resolved techniques of 337 and 248 nm laser flash photolysis, the photo physical and photochemical processes of riboflavin (RF, vitamin B₂) were studied in detail in aqueous solution. The excited triplet state of riboflavin (³RF^*) was produced with 337 nm laser, while under 248 nm irradiation, both³RF^* and hydrated electron (e_aq) formed from photoionization could be detected. Photobiological implications have been inferred on the basis of reactivity of³RF^* including energy transfer, electron transfer and hydrogen abstraction. The RF^·+ was generated by oxidation of SO₄ ^·-radical with the aim of confirming the results of photolysis.