Probing the pyrolysis of methyl formate in the dilute gas phase by synchrotron radiation and theory

The thermal dissociation of the atmospheric constituent methyl formate was probed by coupling pyrolysis with imaging photoelectron photoion coincidence spectroscopy (iPEPICO) using synchrotron VUV radiation at the Swiss Light Source (SLS). iPEPICO allows threshold photoelectron spectra to be obtained for pyrolysis products, distinguishing isomers and separating ionic and neutral dissociation pathways. In this work, the pyrolysis products of dilute methyl formate, CH₃OC(O)H, were elucidated to be CH₃OH + CO, 2 CH₂O and CH₄ + CO₂ as in part distinct from the dissociation of the radical cation (CH₃OH^+• + CO and CH₂OH⁺ + HCO). Density functional theory, CCSD(T), and CBS-QB3 calculations were used to describe the experimentally observed reaction mechanisms, and the thermal decomposition kinetics and the competition between the reaction channels are addressed in a statistical model. One result of the theoretical model is that CH₂O formation was predicted to come directly from methyl formate at temperatures below 1200 K, while above 1800 K, it is formed primarily from the thermal decomposition of methanol.     

The gas phase aldose‐ketone isomerization mechanism: Direct interconversion of the model hydroxycarbonyls 2‐hydroxypropanal and hydroxyacetone

We report a novel mechanism for the interconversion of 2‐hydroxypropanal with its more‐stable ketone isomer hydroxyacetone. Reaction proceeds via concerted transfer of two H atoms, requires a barrier of only ～40 kcal mol^?1, bypasses the enediol intermediate, and is general for α‐hydroxy carbonyls. A similar isomerization mechanism is shown to persist for β, γ, and δ‐hydroxy carbonyls; these compounds are skeletal forms of the monosaccharides and this work, therefore, discloses the gas‐phase mechanism for aldose‐ketose isomerization. As an example, the isomerization of glyceraldehyde to dihydroxyacetone is shown to proceed via this mechanism with a barrier of 31 kcal mol^?1. Rate coefficients and thermochemical properties are reported for the isomerization of 2‐hydroxypropanal and hydroxyacetone for use in detailed kinetic models. Additionally, RRKM theory k (E ) values for this reaction suggest that it may transpire in the troposphere following solar excitation.     

Ab initio studies on the pyrolysis mechanism of 2-haloacetic acids in the gas phase

The dehydrohalogenation mechanism of 2-haloacetic acids (XCH2CO2H, X=F, Cl and Br) has been studied theoretically by HF/3-21G and AM1 methods. The results indicate that these reactions are most probably proceeded in terms of a polar five-membered cyclic transition state in the gas phase. Their microscopic processes are beleived to be a stepwise reaction and the rate-determining step is the first one. By comparing the energy barriers of different 2-haloacetic acids, it can be realized that 2-fluoroacetic acid is easier to react than 2-chloroacetic and 2-bromoacetic acids.     

Spectroscopic and Computational Study of a Nonheme Iron Nitrosyl Center in a Biosynthetic Model of Nitric Oxide Reductase

A major barrier to understanding the mechanism of nitric oxide reductases (NORs) is the lack of a selective probe of NO binding to the nonheme Fe_B center. By replacing the heme in a biosynthetic model of NORs, which structurally and functionally mimics NORs, with isostructural ZnPP, the electronic structure and functional properties of the Fe_B nitrosyl complex was probed. This approach allowed observation of the first S=3/2 nonheme {FeNO}⁷ complex in a protein‐based model system of NOR. Detailed spectroscopic and computational studies show that the electronic state of the {FeNO}⁷ complex is best described as a high spin ferrous iron (S=2) antiferromagnetically coupled to an NO radical (S= 1/2) [Fe²⁺‐NO^.]. The radical nature of the Fe_B‐bound NO would facilitate N? N bond formation by radical coupling with the heme‐bound NO. This finding, therefore, supports the proposed trans mechanism of NO reduction by NORs.     

Density Functional Theory Study of Reaction Equilibria in Signal Amplification by Reversible Exchange

An in-depth theoretical analysis of key chemical equilibria in Signal Amplification by Reversible Exchange (SABRE) is provided, employing density functional theory calculations to characterize the likely reaction network. For all reactions in the network, the potential energy surface is probed to identify minimum energy pathways. Energy barriers and transition states are calculated, and harmonic transition state theory is applied to calculate exchange rates that approximate experimental values. The reaction network energy surface can be modulated by chemical potentials that account for the dependence on concentration, temperature, and partial pressure of molecular constituents (hydrogen, methanol, pyridine) supplied to the experiment under equilibrium conditions. We show that, under typical experimental conditions, the Gibbs free energies of the two key states involved in pyridine-hydrogen exchange at the common Ir-IMes catalyst system in methanol are essentially the same, i. e., nearly optimal for SABRE. We also show that a methanol-containing intermediate is plausible as a transient species in the process.     

Kinetic and thermodynamic studies of the nonisothermal decomposition of anhydrous copper(II) formate in different gas atmospheres

The thermal decomposition of anhydrous (orthorhombic) copper(II) formate was studied by programmed rising-temperature methods (TG, DTG, DTA and DSC) to about 250 °C in flowing gas atmospheres of nitrogen (inert), hydrogen (reducing) and air (oxidizing). The degradation reaction, anion breakdown, proceeded to completion in two distinct, but partially overlapping, rate processes and apparent Arrhenius parameters, calculated by the Ozawa nonisothermal kinetic method, agreed satisfactorily with the literature results. It was concluded that the two consecutive processes, contributing to the overall reaction, involved stepwise cation reduction: Cu²⁺→Cu⁺→Cu⁰, with copper(I) formate as intermediate. This mechanism is similar to that proposed in previous studies of the decompositions of copper(II) oxalate, malonate, maleate, fumarate, mellitate and squarate. For all of these reactants, the Cu⁺ salt has been identified as an intermediate, exhibiting a (slightly) lower relative reactivity than the corresponding Cu²⁺ salt. For copper(II) formate the response curves in the three different gaseous atmospheres were generally similar, showing that neither oxidizing nor reducing conditions caused a marked change in reactivity. The temperature of reaction initiation in H₂ was slightly diminished and the temperature of the second stage of reaction in O₂ was raised appreciably. It is believed that electron transfer contributed to the control of reactivity and that the gases present appreciably influence the rates of the contributory reactions occurring.     

Photochemistry of molecules relevant to the atmosphere: photodissociation of nitric acid in the gas phase.

This Minireview gives an account of the photochemical decay of nitric acid HNO3 in the gas phase, which has been well investigated under bulk and molecular-beam conditions. Due to the importance of this molecule in atmospheric chemistry, attention was paid to the irradiation regions around 300 and 200 nm, where solar photolysis of HNO3 is expected to be particularly efficient. While the low-energy region is characterized by the products OH and NO2, the high-energy region gives rise to a variety of photochemical decay pathways, dominated by channels which lead to the products HONO + O in different electronic states.     

Photoionization study of L-valine in the gas phase by vacuum ultraviolet synchrotron radiation

The photoionization and photodissociation of L-valine are studied by tunable synchrotron vacuum ultraviolet photoionization mass spectrometry at the photon energy of 13 eV. The ionization energy of L-valine and the appearance energies of major fragments are measured by the photoionization efficiency spectrum in the photon energy range of 8-11 eV. Possible formation pathways of the major fragments, NH(2)CHC(OH)(2)(+) (m/z=75), NH(2)(CH(3))(2)(CH)(2)(+) (m/z=72) and NH(2)CHCO(+) (m/z=57), are discussed in detail with the theoretical calculations at the B3LYP/6-31++G (d, p) level. Hydrogen migration is considered as the key way for the formation of NH(2)CHC(OH)(2)(+) (m/z=75) and NH(2)CHCO(+) (m/z=57). Furthermore, other fragments, NH(2)CHCOOH(+) (m/z=74), (CH(3))(2)(CH)(2)(+) (m/z=56), C(4)H(7)(+) (m/z=55), NH(2)CHOH(+) (m/z=46), NH(2)CH(2)(+) (m/z=30) and m/z=18, species are also briefly described.     

Kinetics and mechanism of 2-pyridylacetic acid pyrolysis in the gas phase: A joint experimental and theoretical study

A combination of the experimental and theoretical study was carried out on the reaction mechanism associated with the pyrolysis of 2-pyridylacetic acid in the gas phase. Methylpyridine and carbon dioxide were analyzed as the products, using a static system over the pressure range of 18–55 torr and the temperature of 541.2–583.4 K. The experimental kinetic data show that the pyrolysis process is homogeneous, unimolecular and proceeds through a concerted mechanism. Theoretical studies at the B3LYP level using the 6-31G^* basis set confirmed an asynchronous concerted mechanism for the reaction. Computed kinetic and activation parameters are in good agreement with the experimental one.     

Homogeneous pyrolysis kinetics of ethyl 3-hydroxy-3-methylbutanoate in the gas phase

The pyrolysis kinetics of ethyl 3-hydroxy-3-methylbutanoate have been examined over the temperature range of 286–330°C and pressure range of 29–108 Torr. In a seasoned vessel and in the presence of the free radical inhibitor cyclohexene or toluene the reaction is homogeneous, unimolecular and obeys a first-order rate law. The elimination products are mainly acetone and ethyl acetate, and very small amounts of ethyl 3-butenoate, acetic acid, ethylene and H₂O. The rate coefficient is expressed by the following equation: log k₁(s^–1)=(12.39±0.46)–(174.5±5.2) kJ mol^–1 (2.303RT)^–1. The mechanism appears to proceed via a six-membered cyclic transition state, where polarization of the (CH₃)C(OH)^+...-CH₂COOCH₂CH₃ bond is rate determining.     

Origins of the Enantio‐ and N/O Selectivity in the Primary‐Amine‐Catalyzed Hydroxyamination of 1,3‐Dicarbonyl Compounds with In‐Situ‐Formed Nitrosocarbonyl Compounds: A Theoretical Study

Chemoselective control over N/O selectivity is an intriguing issue in nitroso chemistry. Recently, we reported an unprecedented asymmetric α‐amination reaction of β‐ketocarbonyl compounds that proceeded through the catalytic coupling of enamine carbonyl groups with in‐situ‐generated carbonyl nitroso moieties. This process was facilitated by a simple chiral primary and tertiary diamine that was derived from tert‐leucine. This reaction featured high chemoselectivity and excellent enantioselectivity for a broad range of substrates. Herein, a computational study was performed to elucidate the origins of the enantioselectivity and N/O regioselectivity. We found that a bidentate hydrogen‐bonding interaction between the tertiary N⁺? H and nitrosocarbonyl groups accounted for the high N selectivity, whilst the enantioselectivity was determined by Si‐facial attack on the (E)‐ and (Z)‐enamines in a Curtin–Hammett‐type manner. The bidentate hydrogen‐bonding interaction with the nitrosocarbonyl moieties reinforced the facial selectivity in this process.     

Acyl Group Migration in Pyranosides as Studied by Experimental and Computational Methods

Acyl group migration affects the synthesis, isolation, manipulation and purification of all acylated organic compounds containing free hydroxyl groups, in particular carbohydrates. While several isolated studies on the migration phenomenon in different buffers have been reported, comprehensive insights into the overall migration process in different monosaccharides under similar conditions have been lacking. Here, we have studied the acyl migration in different monosaccharides using five different acyl groups by a combination of experimental, kinetic and theoretical tools. The results show that the anomeric configuration in the monosaccharide has a major influence on the migration rate, together with the relative configurations of the other hydroxyl groups and the nature of the migrating acyl group. Full mechanistic model, based on computations, demonstrates that the acyl migration proceeds through an anionic stepwise mechanism with linear dependence on the [OH⁻] and the pK_a of the hydroxyl group toward which the acyl group is migrating.     

Structures and fragmentation of [Cu(uracil-H)(uracil)]+ in the gas phase

Complexes of copper (II) ions and uracil were studied using tandem mass spectrometry (Fourier transform ion cyclotron resonance, FTICR, mass spectrometry) including extensive isotopic labeling as well as theoretical calculations. Positive ion electrospray mass spectra of aqueous solutions of CuCl(2) and uracil show that the [Cu(Ura-H)(Ura)](+) ion is the most abundant ion even at low concentrations of uracil. Sustained off-resonance irradiation collision-induced dissociation (SORI-CID) experiments show that the lowest energy decomposition pathway for [Cu(Ura-H)(Ura)](+) , surprisingly, is not the loss of uracil, but the loss of HNCO followed by HCN as the most abundant secondary fragmentation product. MS(n) studies identified primary, secondary and tertiary fragmentation products. Extensive isotopic labeling studies, as well as computational studies allowed for a detailed fragmentation scheme for the [Cu(Ura-H)(Ura)](+) ion, beginning with the lowest energy structure.     

Conformations of methoxy(hydroxy)-substituted benzenes and benzaldehydes in the gas phase

The electronic structure ofo-methoxy(hydroxy)-substituted benzenes has been investigated by the combined use of photoelectron spectroscopy and molecular orbital calculations by means of the semi-empirical AM1 method. Absorption bands in the PE spectra have been assigned, the dependence of someπ-orbital energies on the molecular conformation has been found, and the estimation of the mean torsion angle of the OCH₃-group with the plane of the benzene ring has been carried out. The results obtained show that the gas-phase conformations of the heavy-atom framework of guaiacol and vanillin molecules are planar. For veratrole and veratraldehyde, nonplanar conformations with mean torsion angles ϕ ≈ 80° and 55°, respectively, have been observed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1757–1760, October, 1993.     

Theoretical study of the thermolysis reaction of β‐hydroxynitriles in the gas phase

The gas‐phase thermal decomposition of 3‐hydroxypropionitrile, 3‐hydroxybutyronitrile, and 3‐hydroxy‐3‐methylbutyronitrile has been studied at the MP2/6‐31G(d) level of theory at 683.15 K and 0.06 atm. Results based both in energy and structure data seem to indicate a favorable route of decomposition via a six‐membered cyclic transition state (similar to those suggested for thermal decomposition of other related compounds, such as β‐hydroxyketones, β‐hydroxyalkenes, and β‐hydroxyalkynes) rather than a four‐membered cyclic transition state or even a quasiheterolytic pathway. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

烯烃及α,β-不饱和醛、酮在气相中的无催化环氧化反应

概述了碳碳双键在气相中无催化环氧化反应研究方法、反应机理、测定了2－甲基丙烯醛与过氧化乙酰基在393K和413K时环氧化反应的速率,实验结果表明，不同结构烯烃的环氧化反应速率主要取决于双键所连取代基的性质，取代基的给电子能力越强环氧化反应速率越快。