Deuterium isotope effects and mechanism of the gas phase reaction [C3H7]+→[C3H5]+ + H2

The deuterium kinetic isotope effect and the deuterium isotope effect upon kinetic energy release have been calculated for the loss of H₂ from the [C₃H₇]⁺ ion. The calculations are based on the transition state structure suggested recently from ab initio calculations on the reaction pathway. The results obtained are in good agreement with experimental data.     

C3H2环丙烯基自由基与O(3P)反应机理的理论研究

本文用量子化学密度泛函方法对C3H2 (环丙烯基自由基)与O(3P)反应的机理进行了理论研究。在B3LYP/6-311++G**计算水平上优化了各驻点（过渡态,中间体,产物）的几何结构,在QCISD(T)/6-311++G**水平下计算了各物质的单点能量,在两种水平下计算了298K和600K时的能量。计算结果表明：C3H2 + O(3P) 反应可以生成P1 (C2H +HCO),P2 (C2H2 + CO) 和P3 (HC3O+H)三种产物。生成P1反应通道的能垒最低,即P1为主要产物,与实验的结果一致。产物P1可以通过路径：R→ IM1→ IM2→ P1获得。本文详细地讨论了C3H2 + O(3P) 的反应机理,并从理论上对实验结果进行了验证。研究结果有助于深入理解C3H2 + O(3P)反应机理以及C3H2在大气中的燃烧过程。     

Theoretical studies of the reactions of O(3P) with halogenated methyl (I)

The reaction of o(³P) with CH₂C1 radical has been studied usingab initio molecular orbital theory. G2 (MP.2) method is used to calculate the geometrical parameters, vibrational frequencies and energies of various stationary points on the potential energy surface. The reaction mechanism is revealed. The addition of o(³P) with CH₂Cl leads to the formation of an energy rich intermediate OCH₂Cl^* which can subsequently undergo decomposition or isomerization to the final products. The calculated heat of reaction for each channel is in agreement with the experimental value. The production of H+CHClO and C1+CH₂O are predicted to be the major channels. The overall rate constants are calculated using transition state theory on the basis ofab initio data. The rate constant is pressure independent and exhibits negative temperature dependence at lower temperatures, in accordance with the experimental results.     

The calculated structures of the C4H8NO+ cations resulting from the unimolecular gas-phase dissociation (CH3)2NCOCH2X·+ → C4H8NO+ + X· (X  CL,NO2)

The structures and rearrangements of various cyclic and acyclic C₄H₈NO⁺ cations resulting from the title reaction have been investigated by means of semiempirical (MNDO ) and ab initio (4-31G ) calculations.     

Structures,Energies, and Fundamental Vibrations of the Sulfonium Ions H3Sn+ (n = 1–4)

Using ab initio MO calculations at the MP2/6‐311G(2df,2pd) level of theory the most stable structures of the following seven ions were determined: H₃S⁺ (C_3v), H₂S–SH⁺ (C_s), H₂S–S–SH⁺ (C₁), HS–S(H)–SH⁺ (C₁), H₂S–S–S–SH⁺ (C₁), HS–S(H)–S–SH⁺ (C₁) and S(SH)₃⁺ (C₃). In the case of the isomeric H₃S₃⁺ cations the species protonated at the terminal sulfur atom is most stable while in the case of the H₃S₄⁺ ions the protonation at the β sulfur atom is energetically preferred. However, the energy differences between isomeric cations are rather small. At the same level of theory the wavenumbers of the harmonic fundamental vibrations were calculated and compared to the available experimental data leading to a support for the existing assignments in certain cases but in some cases to revisions. The reaction enthalpies and Gibbs free energies of the proton transfer reactions H₂S_n + H₂S_n+1 → H₃S_n⁺ + HS_n+1^– were calculated by the G2 method. For n = 1–3 the enthalpies are found in the range 639–731 kJ mol^–1.     

An ab initio molecular orbital study of the deprotonation of amines

The geometries of the amines NH₂X and amido anions NHX^?, where X = H, CH₃, NH₂, OH, F, C₂H, CHO, and CN have been optimized using ab initio molecular orbital calculations with a 4-31G basis set. The profiles to rotation about the N? X bonds in CH₃NH^?, NH₂NH^?, and HONH^? are very similar to those for the isoprotic and isoelectronic neutral compounds CH₃OH, NH₂OH, and HOOH. The amines with unsaturated bonds adjacent to the nitrogen atoms undergo considerable skeletal rearrangement on deprotonation such that most of the negative charge of the anion is on the substituent. The computed order of acidity for the amines NH₂X is X = CN > HCO > F ≈ C₂H > OH > NH₂ > CH₃ > H and for the reaction NHX^? + H⁺ → NH₂X the computed energies vary over the range 373–438 kcal/mol.     

Canonical statistical adiabatic channel model calculations of the H + CH3 → CH4 recombination reaction on an ab initio potential energy surface. The role of the transitional modes

The canonical formalism of the statistical adiabatic channel model is used to calculate limiting high pressure rate constants for the H + CH₃ → CH₄ recombination reaction on a recently reported analytic potential energy surface based on ab initio calculations. An effective adiabatic channel potential which incorporates the G_?? matrix element of the twofold degenerate H₃C? H transitional bending mode, quartic anharmonicity, and state selected mode coupling effects is implemented. The rate constants calculated over the temperature range 200–1000 K are in very good agreement with recent canonical variational transition state theory calculations performed on the same surface. The comparison with experimental results is also discussed.     

MNDO study of fragmentations in mass spectrometry. Part I. The geometry and electron structure of [C3H6O] + ˙ systems and [C2H3O]+ ions

MNDO calculations of [C₃H₆O]^{+ ˙} predict the parallel existence of both structures of radical cations of acetone (1) and propen-2-ol (2) in electron ionization spectra. The calculated heats of formation of 1^{+ ˙} (ΔH_f^MNDO = 783.2 kj mol^?1) and of 2^{+ ˙} (ΔH_f^MNDO = 649.8 kJ mol^?1) are in very good agreement with the experimental results. A comparison with the results of ab initio calculations (3–21 G and 6–31 G) and experimental data for the individual structures of the main fragment [C₂H₃O]⁺ demonstrates a sufficient accuracy of MNDO results, suggesting the possibility of applying the method also in other cationic systems, especially in larger ones.     

Experimental and Theoretical Study of Hydrogen Atom Abstraction from Ethylene by Stoichiometric Zirconium Oxide Clusters

The reactions of cationic zirconium oxide clusters （ZrxOy^＋） with ethylene （C2H4） were investigated by using a time-of-flight mass spectrometer coupled with a laser ablation/supersonic expansion cluster source. Some hydrogen containing products （ZrO2）xH^＋（x=-1-4） were observed after the reaction. The density functional theory calculations indicate that apart from the common oxygen transfer reaction channel, the hydrogen abstraction channel can also occur in （ZrO2）x^＋＋C2H4, which supports that the observed （ZrO2）xH^＋ may be due to （ZrO2）x^＋＋C2H4→（ZrO2）xH^＋＋C2H3. The rate constants of different reaction channels were also calculated by Rice-Rarnsberger-Kassel-Marcus theory.     

Ab initio SCF Calculation of the Fluoronium Ion: Geometry,electronic structure and vibrational constants

An ab initio SCF calculation of 42 points of the energy hypersurface of the fluoronium ion is presented using a contracted F(5s/3p), H(2s) gaussian basis set. In its equilibrium structure a bond length of 1.812 a.u. and a HFH bond angle of 127.2° are predicted. The calculated vibrational frequencies for H₂F⁺, HDF⁺, and D₂F⁺ are in good agreement with the experimental data.     

Neutralization-reionization mass spectrometry (NRMS). Structural information from vertical neutralization and reionization efficiencies

The neutralization-reionization mass spectra of alkane radical ions indicate significant differences between the structures and geometries of alkane molecules and their molecular ions, confirming recent ab initio predictions. Ionic isomers that are indistinguishable by collisionally-activated dissociation because of easy interconversion can be characterized by neutralization-reionization if the corresponding neutrals show different reactivities, as is demonstrated for the [C₂H₅]⁺/C₂H₅˙ system and for [C₂H₄O₂]⁺˙ isomers. For identification of mixtures of more than one neutral species, the relative efficiency for reionizing each neutral must be determined; e.g. the O₂ reionization efficiency of ˙CH₂OH radicals is ～4 times greater than that of CH₃O˙. This information and reference reionization spectra of CH₃O˙ and ˙CH₂OH show that metastable or collisionally activated methyl acetate cations lose CH₃O˙, not ˙CH₂OH as previously reported; the newly-formed CH₃O˙ undergoes partial (～20%) isomerization to ˙CH₂OH in the ～10^?6s before reionization. Similar results are obtained for [B(OCH₃)₃]⁺˙.     

The structure of the 1H‐imidazol‐3‐ium lawsonate salt aided by ab initio gas‐phase calculations

For the new organic salt 1H‐imidazol‐3‐ium 1,4‐dioxo‐1,4‐dihydronaphthalen‐2‐olate, C₃H₅N₂⁺·C₁₀H₅O₃⁻, ab initio calculations of the gas‐phase structures of the lawsonate and imidazolium ions were performed to help in the interpretation of the structural features observed. Three different types of hydrogen bond are responsible for the three‐dimensional packing of the salt.     

Theoretical study of “protonated pyruvate”: A methylhydroxycarbene—carbon dioxide complex—implications for the decarboxylation of pyruvic acid

Two conformers of protonated pyruvate, CH₃C⁺(OH)COO, with the OH group either trans or cis to the methyl group and the carboxylate group in the C? C? C plane have been studied using the ab initio SCF/3-21G method, as well as by some semiempirical AM1 calculations. Both ab initio SCF and AM1 curves for the potential energy as a function of the C? COO distance exhibit a minimum corresponding to a complex of methylhydroxycarbene, CH₃COH, associated with carbon dioxide, but only the AM1 curves predict an inner minimum corresponding to a covalently bonded protonated pyruvate molecule with a C? COO distance of 1.6–1.7 Å. The two models also disagree on the dissociation pathway for pyruvic acid, with the AM1 calculations predicting formation of acetyl and HOCO radicals while the ab initio method predicts dissociation into methylhydroxycarbene and carbon dioxide following an initial intramolecular proton transfer. The weakly bound complexes of methylhydroxycarbene and carbon dioxide have been studied in some detail using ab initio SCF and MP2 methods in conjunction with 6-311G** basis sets, obtaining equilibrium geometries and vibrational frequencies. In addition, the lactone-type isomer of protonated pyruvate, which contains a C? C? O ring, was also studied. The conclusions of these calculations are consistent with those from earlier work using the smaller 3-21G basis set. The most stable complex is predicted to occur between trans-methylhydroxycarbene and carbon dioxide where substantial stabilization is provided by an OH ? OC hydrogen bond. © 1993 John Wiley & Sons, Inc.     

Ion cyclotron resonance studies of some reactions of C+ ions

Product distributions and rate constants for the reaction of ground state C⁺ ions with O₂, NO, HCl, CO₂, H₂S, H₂O, HCN, NH₃, CH₄, H₂CO, CH₃OH, and CH₃NH₂ have been measured. Rate constants were obtained using ion cyclotron resonance trapped ion methods at JPL, and product distributions were obtained using a tandem (Dempster-ICR) mass spectrometer at the University of Utah. Rapid carbon isotope exchange has also been observed in C⁺-CO collisions.     

Quantum chemical studies on electrophilic addition

The geometries of the 2-chloroethyl and ethylenechloronium cations, two possible intermediates in the electrophilic addition of chlorine to ethylene, have been fully optimized using ab initio molecular orbital calculations employing the split valence shell 4-31G basis set.These geometries were then used to compute more accurate wave functions using Dunning's double-zeta basis set. The bridged chloronium ion was found to be more stable by 9.35 kcal/mole, the opposite order of stability from the C₂H₄F⁺ ions. Interconversion of the two C₂H₄Cl⁺ cations was computed to have a barrier of 6.25 kcal/mole.The activation energy for this chlorination reaction, using the ethylenechloronium cation and a chlorine anion at infinite separation as the model for the activated complex, was computed to be 128.7 kcal/mole, showing that this is not a feasible gas phase reaction.     

Floating spherical Gaussian orbital (FSGO) studies with a model potential: Application to two-valence-electron systems

A Gaussian based model potential is used within FSGO formalism to study a series of two-valence-electron diatomics (Li₂, Na₂, K₂, LiH, NaH, KH, MgH⁺, CaH⁺, LiNa, LiK and NaK) and triatomic ions (H₂Li⁺, H₂Na⁺, Li₂Na⁺, Na₂Li⁺, Li ₃ ⁺ , Na ₃ ⁺ , Li₂H⁺ and Na₂H⁺). Results for calculated equilibrium geometries, force constants, and energy changes for certain chemical reactions are compared to the corresponding quantities from available all-electronab initio studies and experimental results. The predicted results are generally satisfactory.Aided by grants to the University of North Carolina from the National Institute of Health and the National Science Foundation.     

Structures and stabilities of [C3H3O]+ ions in the gas phase: A molecular orbital study

The relative energies of 11 [C₃H₃O]⁺ ions are calculated by different molecular orbital methods (MINDO/3, MNDO, ab initio with 3-21G and 4-31G* basis set and configuration interaction). The four most stable structures are: a ([CH₂?CH? CO]⁺), b c ([CH?C? CHOH]⁺) and d ([CH₂?C?COH]⁺); their relative energies at the CI/4-31G*//3-21G level are 0, 117, 171 and 218 kJ mol^?1, respectively. The isomerizations c→[CH?CH? CHO]⁺→[CH₂?C? CHO]⁺→ a and dissociations into [C₂H₃]⁺+CO and [HCO]⁺+C₂H₂ are explored. The calculated potential energy profile reveals that the energy-determining step is the 1,3-H migration c→[CH?CH? CHO]⁺. This explains the value of unity of the branching ratio and the spread of kinetic energy released for the two dissociation channels.     

Anilinium mono­hydrogen dl‐malate

Crystals of the title salt, [(C₆H₅NH₃)]⁺·[(HOOC(CH₂)CH(OH)COO)]⁻ or C₆H₈N⁺·C₄H₅O₅⁻, are built up from protonated anilinium residues and monodissociated dl ‐malate ions. The NH₃⁺ group of the anilinium cation is ordered at room temperature. Rotation of the NH₃⁺ group along the C(aromatic)—Nsp³ bond (often observed at room temperature in other anilinium salts) is prevented by N—H⋯O hydrogen bonds between the NH₃⁺ group and the malate anions. The anions are connected by four O—H⋯O hydrogen bonds into two‐dimensional sheets parallel to the (001) plane. The charged moieties, i.e. the anilinium cations and the sheets of hydrogen‐bonded malate anions, form two‐dimensional layers in which the phenyl rings of the anilinium residues lie perpendicular to the malate‐ion sheets. The conformation of the monodissociated malate ion in the crystal is compared with that obtained from ab initio molecular‐orbital calculations.     

Evaluation of AM1-calculated radical cation ion-neutral complexes

AM1 semiempirical molecular orbital calculations are reported for 20 ion-neutral complexes, including hydrogen-bonded complexes, presumably involved in the gas-phase unimolecular decomposition of simple organic radical cations. The systems investigated are [C₂H₄O₂]^˙+, [C₂H₅NO]^˙+, [C₂H₆O]^˙+, [C₂H₆O₂]^˙+, [C₃H₆O]^˙+, [C₃H₆O₂]^˙+, [C₃H₈O]^˙+, and [C₃H₈O₂]^˙+. The AM1 results are compared with ab initio molecular orbital calculations at different levels of theory up to MP3/6-31G(d, p)//SCF/6-31G(d) + ZPVE and the available experimental data. AM1 fails to predict some local minima and the equilibrium geometries calculated for several complexes are found to be qualitatively different from those predicted by the ab initio calculations. However, reasonable agreement is generally found for the stabilization energies of the complexes toward dissociation into their loosely bound components. © John Wiley & Sons, Inc.     

Electron impact ionization of ethylene–methanol heteroclusters: Stable configurations and mechanisms in intracluster ion–molecule reactions

Reactions that proceed within mixed ethylene–methanol cluster ions were studied using an electron impact time-of-flight mass spectrometer. The ion abundance ratio, [(C₂H₄)_n(CH₃OH)_mH⁺]/[(C₂H₄)_n(CH₃OH)_m⁺], shows a propensity to increase as the ethylene/methanol mixing ratio increases, indicating that the proton is preferentially bound to a methanol molecule in the heterocluster ions. The results from isotope-labelling experiments indicate that the effective formation of a protonated heterocluster is responsible for ethylene molecules in the clusters. The observed (C₂H₄)_n(CH₃OH)_m⁺ and (C₂H₄)_n(CH₃OH)_m–1CH₃O⁺ ions are interpreted as a consequence of the ion–neutral complex and intracluster ion–molecule reaction, respectively. Experimental evidence for the stable configurations of heterocluster species is found from the distinct abundance distributions of these ions and also from the observation of fragment peaks in the mass spectra. Investigations on the relative cluster ion distribution under various conditions suggest that (C₂H₄)_n(CH₃OH)_mH⁺ ions with n + m ≤ 3 have particularly stable structures. The result is understood on the basis of ion–molecule condensation reactions, leading to the formation of fragment ions, $ {\rm CH}_2=\!=\mathop {\rm O}\limits^ + {\rm CH}_3 $ and (CH₃OH)H₃O⁺, and the effective stabilization by a polar molecule. The reaction energies of proposed mechanisms are presented for (C₂H₄)_n(CH₃OH)_mH⁺(n + m ≤ 3) using semi-empirical molecular orbital calculations.