Loss Of Hydrogen Atom Research Articles

Gas-phase preparation of silylacetylene (SiH3CCH) through a counterintuitive ethynyl radical (C2H) insertion.

Elementary reaction mechanisms constitute a fundamental infrastructure for chemical processes as a whole. However, while these mechanisms are well understood for second-period elements, involving those of the third period and beyond can introduce unorthodox reactivity. Combining crossed molecular beam experiments with electronic structure calculations and molecular dynamics simulations, we provide compelling evidence on an exotic insertion of an unsaturated sigma doublet radical into a silicon-hydrogen bond as observed in the barrierless gas-phase reaction of the D1-ethynyl radical (C2D) with silane (SiH4). This pathway, which leads to the D1-silylacetylene (SiH3CCD) product via atomic hydrogen loss, challenges the prerequisite and fundamental concept that two reactive electrons and an empty orbital are required for the open shell, unsaturated radical reactant to insert into a single bond.

Investigation of the mechanism of [M-H]+ ion formation in photoionized N-alkyl-substituted thieno[3,4-c]-pyrrole-4,6-dione derivatives during higher order MSn high-resolution mass spectrometry.

The mechanism underlying dopant-assisted atmospheric pressure photoionization's (APPI) formation of ions is unclear and still under debate for many chemical classes. In this study, we reexamined the gas-phase reaction mechanisms responsible for the generation of [M-H]+ precursor ions, resulting from the loss of a single hydrogen atom, in a series of N-alkyl-substituted thieno[3,4-c]-pyrrole-4,6-dione (TPD) derivatives. Atmospheric pressure photoionization combined with higher order MS/MSn using high-resolution mass spectrometry (APPI-HR-CID-MSn) and electronic structure calculations using density functional theory were used to determine the chemical structure of observed [M-H]+ ions. As a result, the higher order MSn (n = 3) experiments revealed a reversed Diels-Alder fragmentation mechanism, leading to a common fragment ion at m/z 322 from the studied [M1-5-H]+ ion species. In addition, the calculation for two chemical structure models (N-alkyl-TPD1 and N-alkyl-TPD5) showed that the fragment structure, resulting from the removal of the hydrogen atom connected to the third carbon atom of the N-alkyl side chain, has a more stable cyclic form compared with the linear one. The proposed chemical structure of the N-alkyl TPD ion species, following the loss of a single hydrogen atom, was revealed during APPI-HR-CID-MSn (n= 3) experiments on the [M-H]+ species. Hydrogen radical (H•) abstraction from the alkyl side chain (e.g., hexyl, heptyl, octyl, 2-ethylhexyl, and nonyl) triggered a rearrangement in the radical cation structure of the N-alkyl-TPD derivatives, initiating cyclization and forming a six-membered ring that connects the oxygen atom to the third carbon atom in the alkyl chain. In addition, theoretical calculations supported the APPI-HR-CID-MSn (n = 3) experiments by demonstrating that the proposed chemical structure, resulting from the intramolecular cyclization of the N-alkyl-TPD ion species, was stable in the presence of chlorobenzene. These findings will aid the structural determination and elucidation of molecules with similar core structures.

<i>Ab initio</i> molecular dynamics study on dissociation process of 2-thiouracil and its tautomers under low-energy electron interactions

When biomolecules interact with high-energy particles and rays, they are directly ionized or dissociated, then a large number of low-energy electrons are formed as secondary particles. These low-energy electrons will attach to biomolecules, and trigger off the secondary dissociation, forming free radicals and ions with high reactivity, which can damage the structure and function of the biomolecule and cause irreversible radiation damage to the biomolecule. It is important to study the low-energy dissociative electron attachment (DEA) process of biomolecules for understanding radiation damage to biological organisms. Currently, the theoretical studies of DEA have mainly focused on the bound states of negative ions and the types of resonances in the dissociation process. The dissociation process is well described by quantum computational method, but the diversity and complexity of dissociation channels present in the dissociation process of 2-thiouracil molecule also pose a great computational challenge to these methods. In addition, the quantum computational methods are not ideal for dealing with the discrete states of chemical bonds and the problem of continuity coupling of electrons. The dissociation dynamics of biomolecules mainly results from ionization and electron attachment. <i>Ab initio</i> molecular dynamics simulation can reasonably describe these processes. In light of these considerations, <i>ab initio</i> molecular dynamics simulation is used in this work to study dynamic variation process in DEA. The low-energy electron dissociative attachment to 2-thiouracil in the gas phase is studied by using the Born-Oppenheimer molecular dynamics model combined with density functional theory. It is found that an important dehydrogenation phenomenon of 2-thiouracil and its tautomers occurs in the DEA process, and that the N—H and C—H bond are broken at specific locations. Due to the loss of hydrogen atoms at the N and C sites, the closed-shell dehydrogenated negative ion (TU-H)<sup>–</sup> forms, which is the most important negative ion fragments in the dissociation process. The potential energy curves, the bond dissociation energy and the electron affinity energy of the broken bond show that the N—H bond is the most likely to break, indicating the formation of the negative ion (TU-H)<sup>–</sup> mainly comes from the breaking of N—H bond. The theoretical calculations in this work are in good agreement with the available experimental results, indicating that the chosen calculation method is fully reliable. ​The BOMD simulations can not only dynamically recover the process of dissociative attachment of low-energy electrons to 2-thiouracil, but also more importantly provide an insight into the mechanisms of dehydrogenation and dissociation channels of 2-thiouracil molecules in DEA process.

Elucidating the chemical dynamics of the elementary reactions of the 1-propynyl radical (CH3CC; X2A1) with 2-methylpropene ((CH3)2CCH2; X1A1).

Exploiting the crossed molecular beam technique, we studied the reaction of the 1-propynyl radical (CH3CC; X2A1) with 2-methylpropene (isobutylene; (CH3)2CCH2; X1A1) at a collision energy of 38 ± 3 kJ mol-1. The experimental results along with ab initio and statistical calculations revealed that the reaction has no entrance barrier and proceeds via indirect scattering dynamics involving C7H11 intermediates with lifetimes longer than their rotation period(s). The reaction is initiated by the addition of the 1-propynyl radical with its radical center to the π-electron density at the C1 and/or C2 position in 2-methylpropene. Further, the C7H11 intermediate formed from the C1 addition either emits atomic hydrogen or undergoes isomerization via [1,2-H] shift from the CH3 or CH2 group prior to atomic hydrogen loss preferentially leading to 1,2,4-trimethylvinylacetylene (2-methylhex-2-en-4-yne) as the dominant product. The molecular structures of the collisional complexes promote hydrogen atom loss channels. RRKM results show that hydrogen elimination channels dominate in this reaction, with a branching ratio exceeding 70%. Since the reaction of the 1-propynyl radical with 2-methylpropene has no entrance barrier, is exoergic, and all transition states involved are located below the energy of the separated reactants, bimolecular collisions are feasible to form trimethylsubstituted 1,3-enyne (p1) via a single collision event even at temperatures as low as 10 K prevailing in cold molecular clouds such as G+0.693. The formation of trimethylsubstituted vinylacetylene could serve as the starting point of fundamental molecular mass growth processes leading to di- and trimethylsubstituted naphthalenes via the HAVA mechanism.

One Collision-Two Substituents: Gas-Phase Preparation of Xylenes under Single-Collision Conditions.

The fundamental reaction pathways to the simplest dialkylsubstituted aromatics-xylenes (C6 H4 (CH3 )2 )-in high-temperature combustion flames and in low-temperature extraterrestrial environments are still unknown, but critical to understand the chemistry and molecular mass growth processes in these extreme environments. Exploiting crossed molecular beam experiments augmented by state-of-the-art electronic structure and statistical calculations, this study uncovers a previously elusive, facile gas-phase synthesis of xylenes through an isomer-selective reaction of 1-propynyl (methylethynyl, CH3 CC) with 2-methyl-1,3-butadiene (isoprene, C5 H8 ). The reaction dynamics are driven by a barrierless addition of the radical to the diene moiety of 2-methyl-1,3-butadiene followed by extensive isomerization (hydrogen shifts, cyclization) prior to unimolecular decomposition accompanied by aromatization via atomic hydrogen loss. This overall exoergic reaction affords a preparation of xylenes not only in high-temperature environments such as in combustion flames and around circumstellar envelopes of carbon-rich Asymptotic Giant Branch (AGB) stars, but also in low-temperature cold molecular clouds (10 K) and in hydrocarbon-rich atmospheres of planets and their moons such as Triton and Titan. Our study established a hitherto unknown gas-phase route to xylenes and potentially more complex, disubstituted benzenes via a single collision event highlighting the significance of an alkyl-substituted ethynyl-mediated preparation of aromatic molecules in our Universe.

One Collision—Two Substituents: Gas‐Phase Preparation of Xylenes under Single‐Collision Conditions

AbstractThe fundamental reaction pathways to the simplest dialkylsubstituted aromatics—xylenes (C6H4(CH3)2)—in high‐temperature combustion flames and in low‐temperature extraterrestrial environments are still unknown, but critical to understand the chemistry and molecular mass growth processes in these extreme environments. Exploiting crossed molecular beam experiments augmented by state‐of‐the‐art electronic structure and statistical calculations, this study uncovers a previously elusive, facile gas‐phase synthesis of xylenes through an isomer‐selective reaction of 1‐propynyl (methylethynyl, CH3CC) with 2‐methyl‐1,3‐butadiene (isoprene, C5H8). The reaction dynamics are driven by a barrierless addition of the radical to the diene moiety of 2‐methyl‐1,3‐butadiene followed by extensive isomerization (hydrogen shifts, cyclization) prior to unimolecular decomposition accompanied by aromatization via atomic hydrogen loss. This overall exoergic reaction affords a preparation of xylenes not only in high‐temperature environments such as in combustion flames and around circumstellar envelopes of carbon‐rich Asymptotic Giant Branch (AGB) stars, but also in low‐temperature cold molecular clouds (10 K) and in hydrocarbon‐rich atmospheres of planets and their moons such as Triton and Titan. Our study established a hitherto unknown gas‐phase route to xylenes and potentially more complex, disubstituted benzenes via a single collision event highlighting the significance of an alkyl‐substituted ethynyl‐mediated preparation of aromatic molecules in our Universe.

Benzimidazole-diamide (bida) Pincer Chromium Complexes: Structures and Reactivity.

Serendipitous discovery of bida (i.e., N1-Ar-N2-((1-Ar-1-benzo[d]imidazol-2-yl)methyl)benzene-1,2-diamide; Ar = 2,6-iPr-C6H3), a potentially redox noninnocent, hemilabile pincer ligand with a methylene group that may facilitate proton/H atom reactivity, prompted its investigation. Chromium was chosen for study due to its multiple stable oxidation states. Disodium salt (bida)Na2(THF)n was prepared by thermal rearrangement of (dadi)Na2(THF)4 (i.e., (N,N'-di-2-(2,6-diisopropylphenylamine)phenylglyoxaldiimine)-Na2(THF)4). Salt metathesis of (bida)Na2(THF)n (generated in situ) with CrCl3(THF)3 or Cl3V═NAr (Ar = 2,6-iPr2C6H3) afforded (bida)CrCl(THF) (1-THF) and (bida)ClV═NAr, respectively. Substitutions provided (bida)CrCl(PMe2Ph) (1-PMe2Ph) and (bida)CrR(THF) (2-R, where R = Me, CH2CMe2Ph (Nph)). Oxidation of 1-THF with ArN3 (Ar = 2,6-iPr2C6H3) or AdN3 (Ad = 1-adamantyl) generated (bida)ClCr═NAr (3═NAr) and (bida)ClCr═NAd (3═NAd) and subsequent alkylation converted these to (bida)R'Cr═NR (R' = Me, R = Ad, Ar, 5═NR; R' = CH2CMe2Ph (Nph), R = Ad, Ar, 6═NR). In contrast, the addition of AdN3 to 2-Nph gave the insertion product (bida)Cr(κ2-N,N-ArN3Nph) (7). Addition of N-chlorosuccinimide to 1-THF produced (bia)CrCl2(THF) (8), where bia is the pincer derived via hydrogen atom loss from bida methylene. A similar HAT afforded (bia)ClCr(CNAr')2 (9, Ar' = 2,6-Me2C6H3) when 3═NAd was exposed to Ar'NC. An empirical equation of charge was applied to each bida species, whose metric parameters are unchanging despite formal oxidation state conversions from Cr(III) to Cr(V). Calculations and Mulliken spin density assessments reveal several situations in which antiferromagnetic (AF) coupling and admixtures of integer ground states (GSs) describe a complicated electronic structure.

On the aromaticity and stability of benzynes in the ground and lowest-lying triplet excited states.

In this work, we have revisited the aromaticity of benzyne isomers at the unrestricted density functional theory level (UDFT) using the energetic, magnetic, and delocalization criteria. In addition, this last criterion has also been analyzed employing complete active space (CASSCF) calculations. The results show conservation of aromaticity in these monocycles. Additionally it is observed that this trend is maintained in polycyclic aromatic hydrocarbon derivatives such as biradical didehydrophenanthrenes. Do these results imply a violation of Baird's rule? The answer is No, because this conservation in aromaticity is due to the loss of hydrogen atoms affects only the electronic σ skeleton and exerts a minor influence on the π cloud. Additionally, we have analyzed the relative stability of benzyne isomers and their relationship with experimental ΔES-T values. According to the literature, the stability of the benzynes in the singlet state is due to an effective interaction between the electrons of the biradical centers; however, this effect is completely reversed in the triplet state, which explains why the para isomer has the lowest ΔES-T gap.

Gas-phase formation route for trans-HC(O)SH and its isomers under interstellar conditions: a state-of-the-art quantum-chemical study

ABSTRACT Sulphur is an important and ubiquitous element of the interstellar medium (ISM). Despite its importance, its chemistry still needs to be elucidated, with one of the main issues being the missing sulphur problem. In this work, small molecular species, already detected in the ISM (SH, OH, H2CS, H2CO, H2S, H2O, HCS/HSC, and HCO), were combined to set five different gas-phase reactions for the formation of isomers belonging to the CH2SO family, with one of its member, namely trans-HC(O)SH, already identified as well. Through a state-of-the-art computational study, it has been found that, thermochemically, only one of the reactions considered is open in the ISM conditions: H2CS + OH can produce cis/trans-HC(S)OH and cis/trans-HC(O)SH via hydrogen-atom loss. Kinetically, the favoured product is trans-HC(S)OH followed by trans-HC(O)SH. In view of the recent detection of this latter, our study suggests that trans-HC(S)OH is a good candidate for astronomical observations. Since this species has never been studied experimentally, as a first step towards its laboratory characterization, accurate estimates of the rotational constants have been provided.

A combined experimental and computational study on the reaction dynamics of the 1-propynyl (CH3CC, X2A1) – propylene (CH3CHCH2, X1A′) system: formation of 1,3-dimethylvinylacetylene (CH3CCCHCHCH3, X1A′) under single collision conditions

The reaction of the 1-propynyl radical (CH3CC; X2A1) with propylene (CH3CHCH2; X1A′) was studied in a crossed molecular beam machine at a collision energy of 37 ± 1 kJ mol−1. Experimental data combined with high-level electronic structure (CCSD(T)-F12/cc-pVTZ-F12//ωB97X-D/6-311G(d,p)) and RRKM calculations reveal the reaction mechanism. The overall barrierless and exoergic reaction involves indirect reaction dynamics and commences preferentially with addition of 1-propynyl with its radical centre to the carbon–carbon double bond at the terminal carbon atom of propylene. This work focuses on molecular mass growth process (hydrogen loss channels) although theory suggests methyl loss as a prevalent channel. In these processes, the C6H9 collision complexes either emit atomic hydrogen or undergo isomerisation followed by atomic hydrogen loss to preferentially yield the cis/trans isomers of 1,3-dimethylvinylacetylene (2-hexen-4-yne) as the primary product. Analysis of reaction dynamics of 1-propynyl and ethynyl radicals with propylene along with their fractional abundance in deep space suggests formation of methyl- and dimethyl derivatives of vinylacetylene in cold molecular clouds. Once formed they may engage in fundamental molecular mass growth processes via the barrierless Hydrogen ion Vinylacetylene Addition mechanism that leads to the formation of methyl- and dimethylnaphthalenes thus providing a versatile route to methyl-substituted PAHs in interstellar medium.

Comprehensive Kinetics on the C7H7 Potential Energy Surface under Combustion Conditions.

The automated kinetics workflow code, KinBot, was used to explore and characterize the regions of the C7H7 potential energy surface that are relevant to combustion environments and especially soot inception. We first explored the lowest-energy region, which includes the benzyl, fulvenallene + H, and cyclopentadienyl + acetylene entry points. We then expanded the model to include two higher-energy entry points, vinylpropargyl + acetylene and vinylacetylene + propargyl. The automated search was able to uncover the pathways from the literature. In addition, three important new routes were discovered: a lower-energy pathway connecting benzyl with vinylcyclopentadienyl, a decomposition mechanism from benzyl that results in side-chain hydrogen atom loss to produce fulvenallene + H, and shorter and lower energy routes to the dimethylene-cyclopentenyl intermediates. We systematically reduced the extended model to a chemically relevant domain composed of 63 wells, 10 bimolecular products, 87 barriers, and 1 barrierless channel and constructed a master equation using the CCSD(T)-F12a/cc-pVTZ//ωB97X-D/6-311++G(d,p) level of theory to provide rate coefficients for chemical modeling. Our calculated rate coefficients show excellent agreement with measured ones. We also simulated concentration profiles and calculated branching fractions from the important entry points to provide an interpretation of this important chemical landscape.

Isomer-Dependent Threshold Photoelectron Spectroscopy and Dissociative Photoionization Mechanism of Anisaldehyde.

We studied the threshold photoionization and dissociative ionization of para-, meta-, and ortho-anisaldehyde by photoelectron photoion coincidence spectroscopy in the 8.20-19.00 eV photon energy range. Vertical ionization energies by equation of motion-ionization potential-coupled cluster singles and doubles (EOM-IP-CCSD) calculations reproduce the photoelectron spectral features in all three isomers. The dissociative photoionization (DPI) pathways of para- and meta-anisaldehyde are similar and differ markedly from those of ortho-anisaldehyde. In the para and meta isomers, the lowest-energy DPI channel corresponds to hydrogen atom loss to form the C8H7O2+ fragment at m/z 135, which undergoes sequential dissociation processes at higher energies, such as carbon monoxide loss to C7H7O+ (m/z 107) and further, sequential CH3, CH2O, and CH2CO losses to produce C6H4O+ (m/z 92), C6H5+ (m/z 77), and C5H5+ (m/z 65), respectively. Carbon monoxide loss from the parent ions, yielding C7H8O+ (m/z 108), is a subordinate dissociation channel parallel to H atom loss. At higher energies, it also gives rise to sequential formaldehyde (CH2O) loss to produce C6H6+ (m/z 78). In the ortho-anisaldehyde cation, the vicinity of the aldehyde and methoxy groups opens up low-energy hydrogen-transfer processes, which allow for seven fragmentation channels to compete effectively with the H- and CO-loss channels. Thus, the fragmentation mechanism changes considerably, thanks to the steric interaction of the substituents. Functional group interactions, in particular H transfer pathways, must therefore be considered when predicting the isomer-specific unimolecular decomposition mechanism of cationic and neutral species, as well as mass spectra for isomers.

An experimental and chemical kinetic modeling study of the role of potassium in the moist oxidation of CO

The effect of KCl on moist CO oxidation in a laminar flow quartz reactor was investigated. Experiments were conducted in the absence of O2 (gasification), as well as under reducing and fuel-lean conditions, in the temperature range 873–1573 K, and the results were interpreted in terms of a chemical kinetic model. The impact of reactions on the quartz surface of the reactor was carefully examined. Under the conditions of interest, KCl reacts with SiO2 to form potassium silicates, releasing HCl to the gas phase. This reaction alters the condition of the quartz surface, making it more active in catalyzing radical recombination, even in the presence of water vapor. Under gasification and reducing conditions, loss of hydrogen atoms on the wall, enhanced by the exposure to KCl, strongly inhibits CO oxidation, with gas-phase inhibition playing a minor role. Under fuel-lean conditions, the state of the surface does not affect the CO oxidation and the observed inhibition can be attributed to gas-phase reactions. The most important reaction for the inhibition is the chain terminating step KO2 + OH ⇆ KOH + O2. Assuming that this reaction proceeds without a barrier, the rate constant is controlled by a long-range dipole–dipole interaction. We calculate a capture rate constant of 2.5E15 T−0.163 cm3 mol−1 s−1. This value, which is significantly higher than earlier estimates used in modeling, allows a satisfactory prediction of the inhibiting effect of KCl on CO oxidation under oxidizing conditions.

Mathematical and Kinetic Study of Horiuti-Polanyi Reaction Mechanism for the Methylcyclohexane Dehydrogenation Over Various Pt/Al2O3 Catalysts

The Horiuti-Polanyi (HP) mechanism has been studied for the methylcyclohexane dehydrogenation reaction over a Pt/AlO catalyst. Three HP mechanistic schemes such as competitive, non-competitive, and combined-competitive-non-competitive were used in the kinetic modeling of the experimental data. The loss of first hydrogen atom was regarded as the rate-determining step and the mathematical expressions were developed and fitted against the data. A FORTRAN routine was employed where 4th order Runge-Kutta method was used to solve the related differential equation and Marquardt algorithm was used for the parametric estimation. A kinetically and statistically best-fit HP kinetic equation was obtained with 87.96 kJ/mol of activation energy. The best-fit kinetic model was tested with five additional Pt-containing AlO catalysts and yet again found in good agreement with the experimental data.

Sustainable production of lignin-derived porous carbons for high-voltage electrochemical capacitors

• CuCl 2 activation mechanism is elucidated. • Lignin-derived porous carbon with high specific surface area. • Lignin-derived porous carbon with high yields up to 49.4%. • High-voltage electrochemical capacitors are fabricated. The high energy storage cost is a crucial factor limiting the wide application of electrochemical capacitors. Herein, we proposed a comprehensive strategy to reduce the cost of electrochemical capacitors with aqueous electrolytes, i.e. , reducing the cost of electrode materials and increasing the energy density of electrochemical capacitors. Low-cost lignin-derived porous carbon electrode materials with high specific surface area and hierarchical porous structure were prepared by CuCl 2 activation. The CuCl 2 activation agent was recycled and thus can be reused. What is more, we proposed that the mechanism of CuCl 2 activation is a solid-phase reaction in which the chloride ions in copper chloride deprive the hydrogen atoms in lignin, resulting in the loss of hydrogen atoms and the formation of pores. High energy density C//C symmetric electrochemical capacitors, Zn//C and Pb//C asymmetric electrochemical capacitors based on lignin-derived porous carbon were fabricated using sulfate electrolytes which endow high working voltage window. This work provides a comprehensive strategy for designing electrochemical capacitors with high energy densities and low cost of energy storage, which is expected to promote the industrial application of electrochemical capacitors with aqueous electrolytes.

H-atom and O-atom methods: from balancing redox reactions to determining the number of transferred electrons

AbstractDefining and balancing redox reaction requires both chemical knowledge and mathematical skills. The prevalent approach is to use the concept of oxidation number to determine the number of transferred electrons. However, the task of calculating oxidation numbers is often challenging. In this article, the H-atom method and O-atom method are developed for balancing redox equations. These two methods are based on the definition of redox reaction, which is the gain and loss of hydrogen or oxygen atoms. They complement current practices and provide an alternate path to balance redox equations. The advantage of these methods is that calculation of oxidation number is not required. Atoms are balanced instead. By following standard operating procedures, H-atom, O-atom, and H2O molecule act as artificial devices to balance both inorganic and organic equations in molecular forms. By using the H-atom and O-atom methods, the number of transferred electrons can be determined by the number of transferred H-atoms or O-atoms, which are demonstrated as electron-counting concepts for balancing redox reactions. In addition, the relationships among the number of transferred H-atom, the number of transferred O-atom, the number of transferred electrons, and the change of oxidation numbers are established.

Gas-Phase Study of the Elementary Reaction of the D1-Ethynyl Radical (C2D; X2Σ+) with Propylene (C3H6; X1A′) under Single-Collision Conditions

The bimolecular gas-phase reactions of the D1-ethynyl radical (C2D; X2Σ+) with propylene (C3H6; X1A') and partially substituted D3-3,3,3-propylene (C2H3CD3; X1A') were studied under single collision conditions utilizing the crossed molecular beams technique. Combining our laboratory data with electronic structure and statistical calculations, the D1-ethynyl radical is found to add without barrier to the C1 and C2 carbons of the propylene reactant, resulting in doublet C5H6D intermediate(s) with lifetime(s) longer than their rotational period(s). These intermediates undergo isomerization and unimolecular decomposition via atomic hydrogen loss through tight exit transition states forming predominantly cis/trans-3-penten-1-yne ((HCC)CH═CH(CH3)) and, to a minor amount, 3-methyl-3-buten-1-yne ((HCC)C(CH3)═CH2) via overall exoergic reactions. Although the title reaction does not lead to the cyclopentadiene molecule (c-C5H6, X1A1), high-temperature environments can convert the identified acyclic C5H6 isomers through hydrogen atom assisted isomerization to cyclopentadiene (c-C5H6, X1A1). Since both the ethynyl radical and propylene reactants have been observed in cold interstellar environments such as TMC-1 and the reaction is exoergic and all barriers lie below the energy of the separated reactants, these C5H6 product isomers are predicted to form in those low-temperature regions.

UV Photolysis Study of Para-Aminobenzoic Acid Using Parahydrogen Matrix Isolated Spectroscopy

Many sunscreen chemical agents are designed to absorb UVB radiation (and in some cases UVA) to protect the skin from sunlight, but UV absorption is often accompanied by photodissociation of the chemical agent, which may reduce its UV absorption capacity. Therefore, it is important to understand the photochemical processes of sunscreen agents. In this study, the photolysis of para-aminobenzoic acid (PABA), one of the original sunscreen chemical agents, at three different UV ranges (UVA: 355 nm, UVB: >280 nm, and UVC: 266 nm and 213 nm) was investigated using parahydrogen (pH2) matrix isolation Fourier-Transform Infrared (FTIR) Spectroscopy. PABA was found to be stable under UVA (355 nm) irradiation, while it dissociated into 4-aminylbenzoic acid (the PABA radical) through the loss of an amino hydrogen atom under UVB (>280 nm) and UVC (266 nm and 213 nm) irradiation. The radical production supports a proposed mechanism of carcinogenic PABA-thymine adduct formation. The infrared spectrum of the PABA radical was analyzed by referring to quantum chemical calculations, and two conformers were found in solid pH2. The PABA radicals were stable in solid pH2 for hours after irradiation. The trans-hydrocarboxyl (HOCO) radical was also observed as a minor secondary photoproduct of PABA following 213 nm irradiation. This work shows that pH2 matrix isolation spectroscopy is effective for photochemical studies of sunscreen agents.

Stable heterocyclic stannylene: The metal, ligand‐centered reactivity, and effective catalytic hydroboration of aldehydes

AbstractA stable O,N‐heterocyclic stannylene 1 bearing novel tetradentate 2,4‐di‐tert‐butyl‐6‐((2‐(1‐((pyridin‐2‐ylmethyl)imino)ethyl)phenyl)amino)phenolate ligand (RAPH2) was synthesized and characterized. Complex 1 is monomeric in crystal and solution due to two additional intramolecular N → Sn donor–acceptor interactions. The diverse reactivity of compound 1 towards various iron carbonyl compounds was studied. Oxidative addition of [Fe2(μ‐S2)(CO)6]2 to the tin center of 1 resulted in a new six‐coordinate Sn(IV) compound 2 containing two Sn–S bonds. The reaction of 1 with Fe2(CO)9 led to the formation of a stable tin(II) adduct 3 – RAPSn‐Fe(CO)4. The interaction between 1 and [(η5‐C5H5)Fe(CO)2]2 resulted in a transformation of the ligand skeleton. During this reaction, the tin center is oxidized to the tetravalent state, and the donor ═N–CH2–Py group is converted into an anionic one with the loss of the methylene hydrogen atom. The molecular structures of all obtained compounds were determined by single‐crystal X‐ray diffraction. The novel stannylene 1 demonstrates striking activity in benzaldehydes' hydroboration reactions down to 0.02 mole percent of the catalyst loading under mild conditions.

Thermodynamic Properties: Enthalpy, Entropy, Heat Capacity, and Bond Energies of Fluorinated Carboxylic Acids.

Fluorinated carboxylic acids and their radicals are becoming more prevalent in environmental waters and soils as they have been produced and used for numerous commercial applications. Understanding the thermochemical properties of fluorinated carboxylic acids will provide insights into the stability and reaction paths of these molecules in the environment, in body fluids, and in biological and biochemical processes. Structures and thermodynamic properties for over 50 species related to fluorinated carboxylic acids with two and three carbons are determined with density functional computational calculations B3LYP, M06-2X, and MN15 and higher ab initio levels CBS-QB3, CBS-APNO, and G4 of theory. The lowest energy structures, moments of inertia, vibrational frequencies, and internal rotor potentials of each target species are determined. Standard enthalpies of formation, ΔfH298°, from CBS-APNO calculations show the smallest standard deviation among methods used in this work. ΔfH298° values are determined via several series of isodesmic and/or isogyric reactions. Enthalpies of formation are determined for fluorinated acetic and propionic acids and their respective radicals corresponding to the loss of hydrogen and fluorine atoms. Heat capacities as a function of temperature, Cp(T), and entropy at 298 K, S298°, are determined. Thermochemical properties for the fluorinated carbon groups used in group additivity are also developed. Bond dissociation energies (BDEs) for the carbon-hydrogen, carbon-fluorine, and oxygen-hydrogen (C-H, C-F, and O-H BDEs) in the acids are reported. The C-H, C-F, and O-H bond energies of the fluorinated carboxylic acids are in the range of 89-104, 101-125, and 109-113 kcal mol-1, respectively. General trends show that the O-H bond energies on the acid group increase with the increase in the fluorine substitution. The strong carbon fluorine bonds in a fluorinated acid support the higher stability of the perfluorinated acids in the environment.