Loss Of Hydrogen Atom Research Articles

Formation of Covalently Bonded Polycyclic Aromatic Hydrocarbons in the Interstellar Medium

Photo-/ion-induced ionization and dissociation processes are commonly observed for polycyclic aromatic hydrocarbon (PAH) molecules. This work performs theoretical studies of PAHs and their fragments. Molecular dynamics simulations in combination with static quantum chemical calculations reveal that following a single hydrogen atom loss, the fragments, PAH-H, are extremely reactive. They catch a neighbor molecule within picoseconds to form a covalently bonded large molecule regardless of orientations/angles and temperatures. We calculate the infrared spectra of the covalently bonded molecules, which indicate that such species could be the carrier of unidentified infrared emission bands. It also implies that regular PAHs might be less abundant in space than what is expected.

Frontier Orbitals of Dehydrogenated Tetrahydrocurcumin in Water Solvent: A Theoretical Study

We studied two frontier orbitals - the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)- of tetrahydrocurumin (THC) using density-functional theory (DFT) in water solvent. These orbitals were observed in THC molecule without one hydrogen atom (dehydrogenated THC). The loss of hydrogen atom is due to the transfer of the atom from THC molecule toward reactive oxygen species (ROS) (Hydrogen atom transfer –HAT- mechanism). We began our investigation by optimizing dehydrogenated THC at three X-H sites. Then, water solvent was added by using polarized continuum model (PCM) method. This study observed that dehydrogenated THC at two O-H sites has wider gap of HOMO-LUMO compare to C-H site.

Photoinduced polycyclic aromatic hydrocarbon dehydrogenation

The physical and chemical conditions in photodissociation regions (PDRs) are largely determined by the influence of far ultraviolet radiation. Far-UV photons can efficiently dissociate molecular hydrogen, a process that must be balanced at the HI/H2 interface of the PDR. Given that reactions involving hydrogen atoms in the gas phase are highly inefficient under interstellar conditions, H2 formation models mostly rely on catalytic reactions on the surface of dust grains. Additionally, molecular hydrogen formation in polycyclic aromatic hydrocarbons (PAHs) through the Eley–Rideal mechanism has been considered as well, although it has been found to have low efficiency in PDR fronts. In a previous work, we have described the possibility of efficient H2 release from medium to large sized PAHs upon photodissociation, with the exact branching between H-/H2-loss reactions being molecule dependent. Here, we investigate the astrophysical relevance of this process, by using a model for the photofragmentation of PAHs under interstellar conditions. We focus on three PAHs cations (coronene, ovalene, and circumcoronene), which represent three possibilities in the branching of atomic and molecular hydrogen losses. We find that, for ovalene (H2-loss dominated) the rate coefficient for H2 formation reaches values of the same order as H2 formation in dust grains. This result suggests that this hitherto disregarded mechanism can account, at least partly, for the high level of molecular hydrogen formation in dense PDRs.

Atomic oxygen and hydrogen loss coefficient on functionalized polyethylene terephthalate, polystyrene, and polytetrafluoroethylene polymers

Coefficients for atomic loss of hydrogen and oxygen atoms on polyethylene terephthalate (PET), polystyrene (PS), and polytetrafluoroethylene (PTFE) are determined. Measurements are performed in a flowing afterglow with different atom densities between 6 and 60 × 1020 m−3 and in the pressure range between 8 and 200 Pa. The loss coefficients do not depend significantly on the atom density or pressure. For H atoms they are (2.3 ± 0.4) × 10−3, (1.5 ± 0.4) × 10−3, and (8.0 ± 3.0) × 10−4 for PET, PS, and PTFE, respectively, whereas for O‐atoms they are (2.4 ± 1.2) × 10−3, (2.2 ± 1.2) × 10−3, and (1.1 ± 0.6) × 10−3 for PET, PS, and PTFE, respectively. These values are valid after saturation of surfaces with particular atoms.

Mass spectrometric study of the negative and positive secondary ions emitted from ethanol microdroplets by MeV-energy heavy ion impact

Abstract We have investigated the negative and positive secondary ions emitted from ethanol droplets by 4.0-MeV C3+ impact to reveal the characteristic features of the reaction processes induced by fast heavy ions at the liquid ethanol surface. Analysis of the secondary ions was performed by time-of-flight mass spectrometry for microdroplet targets in a high vacuum environment. Fragment ions, deprotonated cluster ions, and trace amounts of the reaction product ions are observed in the negative secondary ions. The main fragment anions are C2HmO− (m = 1, 3, and 5) and C2H− generated by loss of hydrogen and oxygen atoms. The reaction product anions include deprotonated glycols, larger alcohols, and their dehydrated and dehydrogenated forms generated by secondary reactions between fragments and radicals. Furthermore, C3Hm− (m = 0–2) and C4Hm− (m = 0 and 1) are observed, which could be produced through a plasma state generated in the heavy ion track. Deprotonated ethanol cluster ions, [(EtOH)n − H]−, are observed up to about n = 25. [(EtOH)n − H]− have smaller kinetic energies than the protonated cluster ions (EtOH)nH+. This probably represents the effect of the positive Coulomb potential transiently formed in the ion track. We also discuss the size distributions and structures of the water- and CH2OH-radical-attached ethanol cluster ions.

Combined Experimental and Computational Investigation of the Elementary Reaction of Ground State Atomic Carbon (C; 3P j) with Pyridine (C5H5N; X1A1) via Ring Expansion and Ring Degradation Pathways.

We explored the elementary reaction of atomic carbon (C; 3P j) with pyridine (C5H5N; X1A1) at a collision energy of 34 ± 4 kJ mol-1 utilizing the crossed molecular beams technique. Forward-convolution fitting of the data was combined with high-level electronic structure calculations and statistical (RRKM) calculations on the triplet C6H5N potential energy surface (PES). These investigations reveal that the reaction dynamics are indirect and dominated by large range reactive impact parameters leading via barrier-less addition to the nitrogen atom and to two chemically nonequivalent "aromatic" carbon-carbon bonds forming three distinct collision complexes. At least two reaction pathways through atomic hydrogen loss were identified on the triplet surface. These channels involve multiple isomerization steps of the initial collision complexes via ring-opening and ring expansion forming an acyclic 1-ethynyl-3-isocyanoallyl radical (P1; 2A″) and a hitherto unreported seven-membered 1-aza-2-dehydrocyclohepta-2,4,6-trien-4-yl radical isomer (P3; 2A), respectively. For RRKM calculations at zero collision energy, representing conditions in cold molecular clouds, the ring expansion product P3 is formed nearly exclusively for the atomic hydrogen loss channel, but based on these computations, the molecular fragmentation channel forming acetylene (C2H2) plus 3-cyano-2-propen-1-ylidene (P6; 3A″) accounts for nearly all of the degradation products of the reaction of atomic carbon with pyridine, proposing a destruction pathway of interstellar pyridine, which may account for the absence in the detection of pyridine in the interstellar medium. These results are also discussed in light of the isoelectronic carbon-benzene (C6H6; X1A1) system with important implications to the rapid degradation of nitrogen-bearing polycyclic aromatic hydrocarbons (NPAHs) in the interstellar medium compared to mass growth processes of PAH counterparts through ring expansion.

Unimolecular reaction energies for polycyclic aromatic hydrocarbon ions.

Imaging photoelectron photoion coincidence spectroscopy was employed to explore the unimolecular dissociation of the ionized polycyclic aromatic hydrocarbons (PAHs) acenaphthylene, fluorene, cyclopenta[d,e,f]phenanthrene, pyrene, perylene, fluoranthene, dibenzo[a,e]pyrene, dibenzo[a,l]pyrene, coronene and corannulene. The primary reaction is always hydrogen atom loss, with the smaller species also exhibiting loss of C2H2 to varying extents. Combined with previous work on smaller PAH ions, trends in the reaction energies (E0) for loss of H from sp2-C and sp3-C centres, along with hydrocarbon molecule loss were found as a function of the number of carbon atoms in the ionized PAHs ranging in size from naphthalene to coronene. In the case of molecules which possessed at least one sp3-C centre, the activation energy for the loss of an H atom from this site was 2.34 eV, with the exception of cyclopenta[d,e,f]phenanthrene (CPP) ions, for which the E0 was 3.44 ± 0.86 eV due to steric constraints. The hydrogen loss from PAH cations and from their H-loss fragments exhibits two trends, depending on the number of unpaired electrons. For the loss of the first hydrogen atom, the energy is consistently ca. 4.40 eV, while the threshold to lose the second hydrogen atom is much lower at ca. 3.16 eV. The only exception was for the dibenzo[a,l]pyrene cation, which has a unique structure due to steric constraints, resulting in a low H loss reaction energy of 2.85 eV. If C2H2 is lost directly from the precursor cation, the energy required for this dissociation is 4.16 eV. No other fragmentation channels were observed over a large enough sample set for trends to be extrapolated, though data on CH3 and C4H2 loss obtained in previous studies is included for completeness. The dissociation reactions were also studied by collision induced dissociation after ionization by atmospheric pressure chemical ionization. When modeled with a simple temperature-based theory for the post-collision internal energy distribution, there was reasonable agreement between the two sets of data.

Isomer Selectivity in Low-Energy Electron Attachment to Nitroimidazoles.

Low-energy electrons effectively decompose the isomers 2-nitroimidazole and 4(5)-nitroimidazole by dissociative electron attachment (DEA) into a variety of fragment anions and radicals. The present study shows that a distinct selectivity for the two isomers occurs in the DEA reactions. Several new decay channels are observed for 2-nitroimidazole, including a dominant one leading to the loss of molecular H2 O by attachment of a low-energy electron. In contrast, the loss of a single hydrogen atom is a much more efficient reaction in DEA to 4(5)-nitroimidazole. Quantum chemical calculations were carried out to explain the pronounced isomer effect found in the DEA experiment. Although the free energies of the reactions are similar for the different isomers, the very different natures of the dipole-bound states and valence-bound anions lead to preference for or hindrance of a particular dissociation channel. Nitroimidazolic compounds are considered as radiosensitizing compounds in tumor radiation therapy. The enhanced formation of fragments, including the highly reactive hydroxyl radical, in DEA to 2-nitroimidazole suggests that it may be a more efficient radiosensitizing agent than 4(5)-nitroimidazoles.

Accurate and sensitive determination of molar fractions of 13C-Labeled intracellular metabolites in cell cultures grown in the presence of isotopically-labeled glucose

This work describes a methodology based on multiple linear regression and GC-MS for the determination of molar fractions of isotopically-labeled intracellular metabolites in cell cultures. Novel aspects of this work are: i) the calculation of theoretical isotopic distributions of the different isotopologues from an experimentally measured value of % 13C enrichment of the labeled precursor ii) the calculation of the contribution of lack of mass resolution of the mass spectrometer and different fragmentation mechanism such as the loss or gain of hydrogen atoms in the EI source to measure the purity of the selected cluster for each metabolite and iii) the validation of the methodology not only by the analysis of gravimetrically prepared mixtures of isotopologues but also by the comparison of the obtained molar fractions with experimental values obtained by GC-Combustion-IRMS based on 13C/12C isotope ratio measurements. The method is able to measure molar fractions for twenty-eight intracellular metabolites derived from glucose metabolism in cell cultures grown in the presence of 13C-labeled Glucose. The validation strategies demonstrate a satisfactory accuracy and precision of the proposed procedure. Also, our results show that the minimum value of 13C incorporation that can be accurately quantified is significantly influenced by the calculation of the spectral purity of the measured cluster and the number of 13C atoms of the labeled precursor. The proposed procedure was able to accurately quantify gravimetrically prepared mixtures of natural and labeled glucose molar fractions of 0.07% and mixtures of natural and labeled glycine at molar fractions down to 0.7%. The method was applied to initial studies of glucose metabolism of different prostate cancer cell lines.

Computed Regioselectivity and Conjectured Biological Activity of Ene Reactions of Singlet Oxygen with the Natural Product Hyperforin.

Hyperforin is a constituent of St. John's wort and coexists with the singlet oxygen sensitizer hypericin. Density functional theory, molecular mechanics and Connolly surface calculations show that accessibility in the singlet oxygen "ene" reaction favors the hyperforin "southwest" and "southeast" prenyl (2-methyl-2-butenyl) groups over the northern prenyl groups. While the southern part of hyperforin is initially more susceptible to oxidation, up to 4 "ene" reactions of singlet oxygen can take place. Computational results assist in predicting the fate of adjacent hydroperoxides in hyperforin, where the loss of hydrogen atoms may lead to the formation of a hydrotrioxide and a carbonyl instead of a Russell reaction.

Exploring the Gas Phase Synthesis of the Elusive Class of Boronyls and the Mechanism of Boronyl Radical Reactions under Single Collision Conditions.

Until recently, the chemistry of boronyl (BO), a diatomic radical isolectronic with the cyano (CN) species, has remained unknown. The boronyl group is characterized by a boron-oxygen multiple bond, and because of the inherent electron deficiency of the boron atom, boronyls (RBO) are highly reactive and typically only exist in their cyclotrimeric form (RBO)3. Due to their invaluable role as reactants, the isolation of the monomers in gas phase experiments has been extensively sought after by the organic synthesis and physical organic chemistry communities but never achieved. Besides the interests from a physical organic and synthetic point of view, boronyls also play a role as reaction intermediates in boron-assisted rocket propulsion systems. In this Account, we review recent experimental work in which gas phase organo boronyl monomers (RBO) are formed via bimolecular reactions of the boronyl radical (BO) with C2-C6 unsaturated hydrocarbons. The investigated hydrocarbons are widely exploited as fuels, and their reactions with boronyl radicals under single collision conditions lead to the formation of organo boronyls. Our studies also elucidate the mechanisms of their formation reactions thus furnishing a comprehension at the molecular level of this reaction class. The variety of the employed hydrocarbon substrates has allowed us to systematically classify the chemical behavior of the boronyl radicals. With the exception of the case of the dimethylacetylene reaction, the boron monoxide radical versus atomic hydrogen exchange mechanisms were always open leading to the formation of highly unsaturated organo boronyl monomers (RBO), which could be easily identified because they cannot trimerize under single collision conditions. Besides the hydrogen displacement pathway, methylacetylene, dimethylacetylene, and propylene, carrying one or two methyl groups, were also found to eliminate a methyl group. In all systems, the reactions were barrierless, indirect, and initiated by addition of the boron monoxide radical to the π electron density of the hydrocarbon molecule, with the radical center located at the boron atom of the BO radical, thus leading to doublet radical intermediates. These intermediates either decompose via hydrogen or methyl loss or isomerize prior to their decomposition via atomic hydrogen or migration of the BO moiety. A consistent trend suggests that all exit transition states are rather tight with those involved in the hydrogen atom loss depicting exit barriers of typically 25 to 35 kJ mol-1, whereas the methyl loss pathways are associated with tighter exit transition states located about 30-50 kJ mol-1 above the separated products. Further, the overall energetics suggest that those bimolecular reactions are exoergic by 40-90 kJ mol-1. These findings confirm that this reaction class leads to the formation of highly unsaturated organo boronyl molecules.

Formation of the 2,3-Dimethyl-1-silacycloprop-2-enylidene Molecule via the Crossed Beam Reaction of the Silylidyne Radical (SiH; X(2)Π) with Dimethylacetylene (CH3CCCH3; X(1)A1g).

We carried out crossed molecular beam experiments and electronic structure calculations to unravel the chemical dynamics of the reaction of the silylidyne(-d1) radical (SiH/SiD; X(2)Π) with dimethylacetylene (CH3CCCH3; X(1)A1g). The chemical dynamics were indirect and initiated by the barrierless addition of the silylidyne radical to both carbon atoms of dimethylacetylene forming a cyclic collision complex 2,3-dimethyl-1-silacyclopropenyl. This complex underwent unimolecular decomposition by atomic hydrogen loss from the silicon atom via a loose exit transition state to form the novel 2,3-dimethyl-1-silacycloprop-2-enylidene isomer in an overall exoergic reaction (experimentally: -29 ± 21 kJ mol(-1); computationally: -10 ± 8 kJ mol(-1)). An evaluation of the scattering dynamics of silylidyne with alkynes indicates that in each system, the silylidyne radical adds barrierlessly to one or to both carbon atoms of the acetylene moiety, yielding an acyclic or a cyclic collision complex, which can also be accessed via cyclization of the acyclic structures. The cyclic intermediate portrays the central decomposing complex, which fragments via hydrogen loss almost perpendicularly to the rotational plane of the decomposing complex exclusively from the silylidyne moiety via a loose exit transition state in overall weakly exoergic reaction leading to ((di)methyl-substituted) 1-silacycloprop-2-enylidenes (-1 to -13 kJ mol(-1) computationally; -12 ± 11 to -29 ± 21 kJ mol(-1) experimentally). Most strikingly, the reaction dynamics of the silylidyne radical with alkynes are very different from those of C1-C4 alkanes and C2-C4 alkenes, which do not react with the silylidyne radical at the collision energies under our crossed molecular beam apparatus, due to either excessive entrance barriers to reaction (alkanes) or overall highly endoergic reaction processes (alkenes). Nevertheless, molecules carrying carbon-carbon double bonds could react, if the carbon-carbon double bond is either consecutive like in allene (H2CCCH2) or in conjugation with another carbon-carbon double bond (conjugated dienes) as found, for instance, in 1,3-butadiene (H2CCHCHCH2).

Making the invisible visible: improved electrospray ion formation of metalloporphyrins/-phthalocyanines by attachment of the formate anion (HCOO(-)).

A protocol is developed for the coordination of the formate anion (HCOO(-)) to neutral metalloporphyrins (Pors) and -phthalocyanines (Pcs) containing divalent metals as a means to improve their ion formation in electrospray ionization (ESI). This method is particularly useful when the oxidation of the neutral metallomacrocycle fails. While focusing on Zn(II)Pors and Zn(II)Pcs, we show that formate is also readily attached to Mn(II), Mg(II) and Co(II)Pcs. However, for the Co(II)Pc secondary reactions can be observed. Upon collision-induced dissociation (CID), Zn(II)Por/Pc·formate supramolecular complexes can undergo the loss of CO2 in combination with transfer of a hydride anion (H(-)) to the zinc metal center. Further dissociation leads to electron transfer and hydrogen atom loss, generating a route to the radical anion of the Zn(II)Por/Pc without the need for electrochemical reduction, although the Zn(II)Por/Pc may have a too low electron affinity to allow electron transfer directly from the formate anion. In addition to single Por molecules, multi Por arrays were successfully analyzed by this method. In this case, multiple addition of formate occurs, giving rise to multiply charged species. In these multi Por arrays, complexation of the formate anion occurs by two surrounding Por units (sandwich). Therefore, the maximum attainment of formate anions in these arrays corresponds to the number of such sandwich complexes rather than the number of porphyrin moieties. The same bonding motif leads to dimers of the composition [(Zn(II)Por/Pc)2·HCOO](-). In these, the formate anion can act as a structural probe, allowing the distinction of isomeric ions with the formate bridging two macrocycles or being attached to a dimer of directly connected macrocycles.

Combined Experimental and Theoretical Study on the Formation of the Elusive 2-Methyl-1-silacycloprop-2-enylidene Molecule under Single Collision Conditions via Reactions of the Silylidyne Radical (SiH; X(2)Π) with Allene (H2CCCH2; X(1)A1) and D4-Allene (D2CCCD2; X(1)A1).

The crossed molecular beam reactions of the ground-state silylidyne radical (SiH; X(2)Π) with allene (H2CCCH2; X(1)A1) and D4-allene (D2CCCD2; X(1)A1) were carried out at collision energies of 30 kJ mol(-1). Electronic structure calculations propose that the reaction of silylidyne with allene has no entrance barrier and is initiated by silylidyne addition to the π electron density of allene either to one carbon atom (C1/C2) or to both carbon atoms simultaneously via indirect (complex forming) reaction dynamics. The initially formed addition complexes isomerize via two distinct reaction pathways, both leading eventually to a cyclic SiC3H5 intermediate. The latter decomposes through a loose exit transition state via an atomic hydrogen loss perpendicularly to the plane of the decomposing complex (sideways scattering) in an overall exoergic reaction (experimentally: -19 ± 13 kJ mol(-1); computationally: -5 ± 3 kJ mol(-1)). This hydrogen loss yields the hitherto elusive 2-methyl-1-silacycloprop-2-enylidene molecule (c-SiC3H4), which can be derived from the closed-shell cyclopropenylidene molecule (c-C3H2) by replacing a hydrogen atom with a methyl group and the carbene carbon atom by the isovalent silicon atom. The synthesis of the 2-methyl-1-silacycloprop-2-enylidene molecule in the bimolecular gas-phase reaction of silylidyne with allene enriches our understanding toward the formation of organosilicon species in the gas phase of the interstellar medium in particular via exoergic reactions of no entrance barrier. This facile route to 2-methyl-1-silacycloprop-2-enylidene via a silylidyne radical reaction with allene opens up a versatile approach to form hitherto poorly characterized silicon-bearing species in extraterrestrial environments; this reaction class might represent the missing link, leading from silicon-bearing radicals via organosilicon chemistry eventually to silicon-carbon-rich interstellar grains even in cold molecular clouds where temperatures are as low as 10 K.

213 nm Ultraviolet Photodissociation on Peptide Anions: Radical-Directed Fragmentation Patterns.

Characterization of acidic peptides and proteins is greatly hindered due to lack of suitable analytical techniques. Here we present the implementation of 213 nm ultraviolet photodissociation (UVPD) in high-resolution quadrupole-Orbitrap mass spectrometer in negative polarity for peptide anions. Radical-driven backbone fragmentation provides 22 distinctive fragment ion types, achieving the complete sequence coverage for all reported peptides. Hydrogen-deficient radical anion not only promotes the cleavage of Cα-C bond but also stimulates the breaking of N-Cα and C-N bonds. Radical-directed loss of small molecules and specific side chain of amino acids are detected in these experiments. Radical containing side chain of amino acids (Tyr, Ser, Thr, and Asp) may possibly support the N-Cα backbone fragmentation. Proline comprising peptides exhibit the unusual fragment ions similar to reported earlier. Interestingly, basic amino acids such as Arg and Lys also stimulated the formation of abundant b and y ions of the related peptide anions. Loss of hydrogen atom from the charge-reduced radical anion and fragment ions are rationalized by time-dependent density functional theory (TDDFT) calculation, locating the potential energy surface (PES) of ππ* and repulsive πσ* excited states of a model amide system.

H-atom loss and migration in hydrogen-rich peptide cation radicals: The role of chemical environment

Hydrogen-atom loss and intramolecular hydrogen-atom migration in peptide radical and cation radical were investigated by quantum chemical calculations for models that include zero to three hydrogen bonds with the ammonium groups. Five different DFT functionals have been checked to study the geometry relaxation and fragmentation processes after electron attachment to peptide cations. The structural changes associated with the geometry relaxation are shown to be dependent on the level of calculation. Furthermore, comparison with CCSD(T) calculations indicates that an adequate description of the electronic state of the reduced protonated peptide is not a sufficient prerequisite to provide accurate energy barrier. Calculating fragmentation reactions of hydrogen-rich peptide cation radicals is pointed out to be a challenging task as none of the DFT functionals are fully effective to describe the whole process. M06-2X is an adequate choice if it provides the correct ground state, otherwise LC-BLYP should be chosen.The electron attachment site and the electron affinity value are highly sensitive to the number of hydrogen bonds with the ammonium group. An ammonium cation with zero, one, and, to a smaller extent, two hydrogen bonds easily captures an incoming electron and subsequently undergoes an H-atom loss process rather than a proton coupled electron transfer. This suggests an explanation for the low amount of c and z fragments observed upon ECD for small protonated peptides, where the ammonium groups cannot be highly intramolecularly “solvated”.

Hypervalent radical formation probed by electron transfer dissociation of zwitterionic tryptophan and tryptophan-containing dipeptides complexed with Ca2+ and 18-crown-6 in the gas phase.

The relationship between peptide structure and electron transfer dissociation (ETD) is important for structural analysis by mass spectrometry. In the present study, the formation, structure and reactivity of the reaction intermediate in the ETD process were examined using a quadrupole ion trap mass spectrometer equipped with an electrospray ionization source. ETD product ions of zwitterionic tryptophan (Trp) and Trp-containing dipeptides (Trp-Gly and Gly-Trp) were detected without reionization using non-covalent analyte complexes with Ca(2+) and 18-crown-6 (18C6). In the collision-induced dissociation, NH3 loss was the main dissociation pathway, and loss related to the dissociation of the carboxyl group was not observed. This indicated that Trp and its dipeptides on Ca(2+) (18C6) adopted a zwitterionic structure with an NH3 (+) group and bonded to Ca(2+) (18C6) through the COO(-) group. Hydrogen atom loss observed in the ETD spectra indicated that intermolecular electron transfer from a molecular anion to the NH3 (+) group formed a hypervalent ammonium radical, R-NH3 , as a reaction intermediate, which was unstable and dissociated rapidly through N-H bond cleavage. In addition, N-Cα bond cleavage forming the z1 ion was observed in the ETD spectra of Trp-GlyCa(2+) (18C6) and Gly-TrpCa(2+) (18C6). This dissociation was induced by transfer of a hydrogen atom in the cluster formed via an N-H bond cleavage of the hypervalent ammonium radical and was in competition with the hydrogen atom loss. The results showed that a hypervalent radical intermediate, forming a delocalized hydrogen atom, contributes to the backbone cleavages of peptides in ETD.

Syntheses and Reactivity Studies of Square-Planar Diamido-Pyridine Complexes Based on Earth-Abundant First-Row Transition Elements.

The new square-planar complexes M[NNN](pyridine) (M = Fe (1), Co(2); NNN = 2,6-bis(2,6-diisopropylphenylamidomethyl)pyridine) were synthesized and fully characterized to investigate small molecule activation on this platform and also associated ligand innocence. The equatorial pyridine solvent moiety could not be removed; a new bis-ligand species Co[NNN.H]2 (3) was synthesized in low yield while attempting to make the base-free derivative. Attempts to prepare the Ni analogue of 1 and 2 instead yielded crystals of a di-imino-pyridine complex Ni[PDI]Cl (4) (PDI = 2,6-bis(2,6-diisopropylphenyliminomethyl)pyridine), following loss of methylene backbone hydrogen atoms. Structural analysis indicates that the PDI ligand is a mono-anionic radical. This susceptibility of the ligand to oxidative dehydrogenation was also shown when the reaction of 2 with 2 equiv of trityl chloride yielded a new complex with an asymmetric imino-amino pyridine ligand Co[NNN']Cl2 (5) (NNN' = 2-(2,6-(diisopropylphenyliminomethyl)-6-(diisopropylphenylamidomethyl)-pyridine) in good yield.

Selective Formation of Indene through the Reaction of Benzyl Radicals with Acetylene.

The combustion of fossil fuels forms polycyclic aromatic hydrocarbons (PAHs) composed of five- and six- membered aromatic rings, such as indene (C9 H8 ), which are carcinogenic, mutagenic, and deleterious to the environment. Indene, the simplest PAH with single five- and six-membered rings, has been predicted theoretically to be formed through the reaction of benzyl radicals with acetylene. Benzyl radicals are found in significant concentrations in combustion flames, owing to their highly stable aromatic and resonantly stabilized free-radical character. We provide compelling experimental evidence that indene is synthesized through the reaction of the benzyl radical (C7 H7 ) with acetylene (C2 H2 ) under combustion-like conditions at 600 K. The mechanism involves an initial addition step followed by cyclization and aromatization through atomic hydrogen loss. This reaction was found to form the indene isomer exclusively, which, in conjunction with the high concentrations of benzyl and acetylene in combustion environments, indicates that this pathway is the predominant route to synthesize the prototypical five- and six-membered PAH.

Comparison of the Thermal and Photochemical Reaction Pathways of Melamine on TiO2

Fourier transform infrared spectroscopy was employed to study the thermal and photochemical reactions of melamine ((H2N)3(C3N3)) on TiO2. Also tested was the adsorption of urea, cyanamide (H2N–C≡N), dicyandiamide ((H2N)2C═N–C≡N), and cyanuric acid ((OH)3(C3N3)) for identifying possible reaction intermediates. It was found that the thermal decomposition of melamine starts with N–H bond scission, possibly forming intermediates such as (H2N)2(C3N3)NH– and (H2N)(C3N3)(NH)2–. Further loss of hydrogen atoms and ring-opening from these intermediates lead to the formation of −NCO (isocyanate) and −N3 (azide) on the surface. The TiO2-mediated photochemical reaction of melamine proceeds via a different mechanism, forming dicyandiamide. These thermal and photochemical reaction pathways of melamine on TiO2 are reported for the first time. They are different from previous studies showing the processes of polymerization, and substitution of NH2 by OH to form cyanuric acid and urea.