Intramolecular fluorine migration via four-member cyclic transition states

Gaseous CF(3)(+) interchanges F(+) for O with simple carbonyl compounds. CF(3)(+) reacts with propionaldehyde in the gas phase to produce (CH(3))(2)CF(+) via two competing pathways. Starting with 1-(13)C-propionaldehyde, the major pathway (80%) produces (CH(3))(2)CF(+) with the carbon label in one of the methyl groups. The minor pathway (20%) produces (CH(3))(2)CF(+) with the carbon label in the central position. The relative proportions of these two pathways are measured by (19)F NMR analysis of the neutral CH(3)CF=CH(2) produced by deprotonation of (CH(3))(2)CF(+) at <10(-)(3) Torr in an electron bombardment flow (EBFlow) reactor. Formation of alkene in which carbon is directly bonded to fluorine means that (in the minor product, at least) an F(+) for O transposition occurs via adduct formation followed by 1,3-atom transfer and then isomerization of CH(3)CH(2)CHF(+) to the more stable (CH(3))(2)CF(+). Use of CF(4) as a chemical ionization (CI) reagent gas leads to CF(3)(+) adduct ions for a variety of ketones, in addition to isoelectronic transposition of F(+) for O. Metastable ion decompositions of the adduct ions yield the metathesis products. Decompositions of fluorocycloalkyl cations formed in this manner give evidence for the same kinds of rearrangements as take place in CH(3)CH(2)CHF(+). Density functional calculations confirm that F(+) for O metathesis takes place via addition of CF(3)(+) to the carbonyl oxygen followed by transposition via a four-member cyclic transition state. A computational survey of the effects of different substituents in a series of aldehydes and acyclic ketones reveals no systematic variation of the energy of the transition state as a function of thermochemistry, but the Hammond postulate does appear to be obeyed in terms of progress along the reaction coordinate. Bond lengths corresponding to the central barrier correlate with overall thermochemistry of the F(+) for O interchange, but in a sense opposite to what might have been expected: the transition state becomes more product-like as the metathesis becomes increasingly exothermic. This reversal of the naive interpretation of the Hammond postulate is accounted for by the relative positions of the potential energy wells that precede and follow the central barrier.     

Aldehyde and ketone adducts of the gaseous trifluoromethyl cation

IR spectra of CF(3)(+) adduct ions are reported for the first time using infrared multiple photon dissociation (IRMPD). Carbonyl stretches of ketone conjugate acids shift less than do those of ions containing C═OCF(3)(+) groups. Carbonyl absorptions shift in proportion to the empirical softness of the corresponding neutrals. Anharmonic DFT fits band positions of C═OCF(3)(+) stretches for aliphatic ions, but unscaled harmonic calculations match O-CF(3) stretches more closely. Calculated O-CF(3) bond orders increase with gas phase CF(3)(+) affinities of the corresponding neutral carbonyl.     

Primary branching ratios for the low-temperature reaction of state-prepared N2+ with CH4, C2H2, and C2H4

Product branching ratios (BRs) are reported for ion-molecule reactions of state-prepared nitrogen cation (N(2)(+)) with methane (CH(4)), acetylene (C(2)H(2)). and ethylene (C(2)H(4)) at low temperature using a modified ion imaging apparatus. These reactions are performed in a supersonic nozzle expansion characterized by a rotational temperature of 40 ± 5K. For the N(2)(+) + CH(4) reaction, a BR of 0.83:0.17 is obtained for the dissociative charge-transfer (CT) reaction that gives rise to the formation of CH(3)(+) and CH(2)(+) product ions, respectively. The N(2)(+) + C(2)H(2) ion-molecule reaction proceeds through a nondissociative CT process that results in the sole formation of C(2)H(2)(+) product ions. The reaction of N(2)(+) with C(2)H(4) leads to the formation of C(2)H(3)(+) and C(2)H(2)(+) product ions with a BR of 0.74:0.26, respectively. The reported BR for the N(2)(+) + C(2)H(4) reaction is supportive of a nonresonant dissociative CT mechanism similar to the one that accompanies the N(2)(+) + CH(4) reaction. No dependence of the branching ratios on N(2)(+) rotational level was observed. In addition to providing direct insight into the dynamics of the state-prepared N(2)(+) ion-molecule reactions with the target neutral hydrocarbon molecules, the reported low-temperature BRs are also important for accurate modeling of the nitrogen-dominated upper atmosphere of Saturn's moon, Titan.     

Activation of the C-I and C-OH bonds of 2-iodoethanol by gas phase silver cluster cations yields subvalent silver-iodide and -hydroxide cluster cations

The gas phase ion-molecule reactions of silver cluster cations (Ag(n)(+)) and silver hydride cluster cations (Ag(m)H(+)) with 2-iodoethanol have been examined using multistage mass spectrometry experiments in a quadrupole ion trap mass spectrometer. These clusters exhibit size selective reactivity: Ag(2)H(+), Ag(3)(+), and Ag(4)H(+) undergo sequential ligand addition only, while Ag(5)(+) and Ag(6)H(+) also promote both C-I and C-OH bond activation of 2-iodoethanol. Collision induced dissociation (CID) of Ag(5)HIO(+), the product of C-I and C-OH bond activation by Ag(5)(+), yielded Ag(4)OH(+), Ag(4)I(+) and Ag(3)(+), consistent with a structure containing AgI and AgOH moieties. Ag(6)H(+) promotes both C-I and C-OH bond activation of 2-iodoethanol to yield the metathesis product Ag(6)I(+) as well as Ag(6)H(2)IO(+). The metathesis product Ag(6)I(+) also promotes C-I and C-OH bond activation.DFT calculations were carried out to gain insights into the reaction of Ag(5)(+) with ICH(2)CH(2)OH by calculating possible structures and their energies for the following species: (i) initial adducts of Ag(5)(+) and ICH(2)CH(2)OH, (ii) the subsequent Ag(5)HIO(+) product, (iii) CID products of Ag(5)HIO(+). Potential adducts were probed by allowing ICH(2)CH(2)OH to bind in different ways (monodentate through I, monodentate through OH, bidentate) at different sites for two isomers of Ag(5)(+): the global minimum "bowtie" structure, 1, and the higher energy trigonal bipyramidal isomer, 2. The following structural trends emerged: (i) ICH(2)CH(2)OH binds in a monodentate fashion to the silver core with little distortion, (ii) ICH(2)CH(2)OH binds to 1 in a bidentate fashion with some distortion to the silver core, and (iii) ICH(2)CH(2)OH binds to 2 and results in a significant distortion or rearrangement of the silver core. The DFT calculated minimum energy structure of Ag(5)HIO(+) consists of an OH ligated to the face of a distorted trigonal bipyramid with I located at a vertex, while those for both Ag(4)X(+) (X = OH, I) involve AgX bound to a Ag(3)(+) core. The calculations also predict the following: (i) the ion-molecule reaction of Ag(5)(+) and ICH(2)CH(2)OH to yield Ag(5)HIO(+) is exothermic by 34.3 kcal mol(-1), consistent with the fact that this reaction readily occurs under the near thermal experimental conditions, (ii) the lowest energy products for fragmentation of Ag(5)HIO(+) arise from loss of AgI, consistent with this being the major pathway in the CID experiments.     

Intracluster ion-molecule reactions of Ti+ with C2H5OH and CF3CH2OH clusters: influence of fluorine substituents on chemical reactivity

A laser ablation-molecular beam/reflectron time-of-flight mass spectrometric technique was used to investigate the ion-molecule reactions that proceed within Ti+(ROH)n (R = C2H5, CF3CH2) heterocluster ions. The mass spectra exhibit a major sequence of cluster ions with the formula Ti+(OR)m(ROH)n (m = 1, 2), which is attributed to sequential insertions of Ti+ into the O-H bond of C2H5OH or CF3CH2OH molecules within the heteroclusters, followed by H eliminations. The TiO+ and TiOH+ ions produced from the reactions of Ti+ with C2H5OH are interpreted as arising from insertion of Ti+ into the C-O bond, followed by C2H5 and C2H6 eliminations, respectively. When Ti+ reacted with CF3CH2OH, by contrast, considerable contributions from TiFOH+, TiF2+, and TiF2OH+ ions were observed in the mass spectrum of the reaction products, indicating that F and OH abstractions are the dominant product channels. Ab initio calculations of the complex of Ti+ with 2,2,2-trifluoroethanol show that the minimum energy structure is that in which Ti+ is attached to the O atom and one of the three F atoms of 2,2,2-trifluoroethanol, forming a five-membered ring. Isotope-labeling experiments additionally show that the chemical reactivity of heterocluster ions is greatly influenced by the presence of fluorine substituents and cluster size. The reaction energetics and formation mechanisms of the observed heterocluster ions are discussed.     

Ion-molecule reactions of several ions with ethylene oxide and propenal in a selected ion flow tube

The selected ion flow tube (SIFT) technique has been used to investigate the ion-molecule reactions of several ions with the neutral molecules ethylene oxide, CH(2)OCH(2)-c, and propenal, CH(2)CHCHO. Both molecules have been identified in hot-core star forming regions [] and have significance to astrochemical models of the interstellar (ISM) and circumstellar medium (CSM). Moreover, the molecules contain functional groups, such as the epoxide group (ethylene oxide) and an aldehyde group, which are part of a conjugated pi-electron system (propenal) whose reactivities have not been studied in detail in gas-phase ion-molecule reactions. The larger recombination energy ions, Ar(+) and N(2)(+), were reacted with the neutrals to give insight into general fragmentation tendencies. These reactions proceeded via dissociative charge-transfer yielding major fragmentation products of CH(3)(+) and HCO(+) for ethylene oxide and CH(2)CH(+) and HCO(+) for propenal. The amino acids glycine and alanine are of particular interest to astrobiology, especially if they can be synthesized in the gas phase. In an attempt to synthesize amino acid precursors, ethylene oxide and propenal were reacted with NH(n)(+) (n = 1-4) and HCNH(+). As might be expected from the proton detachment energies, NH(+), NH(2)(+), and HCNH(+) reacted via proton transfer. NH(3)(+) reacted with each molecule via H-atom abstraction to produce NH(4)(+), and NH(4)(+) reacted via a ternary association. All binary reactions proceeded near the gas kinetic rate. Several associated molecule switching reactions were performed and implications of these reactions to the structures of the association products are discussed Ikeda et al. and Hollis et al.     

Surface-aligned ion-molecule reaction on the surface of a molecular crystal CD3+ + CD3I --> C2D5+ + DI

An ion-molecule reaction has been observed from a condensed molecular crystal of CD(3)I using the time-of-flight electron-stimulated desorption ion angular distribution technique. The CD(3)I multilayer is produced by growth on an ordered substrate. The reaction occurs between CD(3)(+) ions produced by electron-stimulated desorption and neighbor CD(3)I molecules in the topmost layer of the molecular crystal of CD(3)I, forming product C(2)D(5)(+) ions whose desorption dynamics have been measured. The normal momentum of the product ion is close to that of the reactant ion, suggesting that the reaction is dominated by a two-body collision, i.e., the momentum of the reactant CD(3)(+) ion governs the momentum of the product C(2)D(5)(+) ion. The ion-molecule reaction is of high cross section since the C(2)D(5)(+) yield is comparable to the CD(3)(+) yield. It is found that the yield and directionality of the emission of the C(2)D(5)(+) product ion is governed by the molecular order that is characteristic of the molecular crystal of CD(3)I. Destroying or modifying this order by using a spacer layer of H(2)O diminishes the C(2)D(5)(+) product ion yield relative to the reactant CD(3)(+) yield and broadens the ion emission directions.     

Products, rate constants and mechanisms of gas-phase reactions of CX(3)(+), CX(2)(+), CX(+) (X = F and/or Cl) and Cl(+) with 1,1,1- and 1,1,2-trichlorotrifluoroethane.

The gas-phase ion chemistry of 1,1,1- and 1,1,2-trichlorotrifluoroethane was investigated with an ion trap mass spectrometer. Following electron ionization both compounds (M) fragment to [M - Cl](+), CX(3)(+), CX(2)(+), CX(+) (X = F and/or Cl) and Cl(+). The reactivity of each of these fragments towards their neutral precursors was studied to obtain product and kinetic data. Whereas [M - Cl](+), CCl(3)(+) and CCl(2)F(+) cations are unreactive under the experimental conditions used, all other species react via halide abstraction to give [M - Cl](+) and, to a far lesser extent, [M - F](+). In addition, CX(2)(+) ions form CClX(2)(+) in a process which formally amounts to chlorine atom abstraction, but more likely involves chloride ion abstraction followed by charge transfer. CX(+) ions also form minor amounts of CX(3)(+) product ions, possibly via chloride abstraction followed by or concerted with dihalocarbene elimination from the (incipient) [M - Cl](+) ion. Trivalent carbenium ions are less reactive than divalent species, which in turn are less reactive than the monovalent ions (reaction efficiencies are given in parentheses): CF(3)(+)(0.70) < CF(2)(+)(0.78) < CF(+)(0.96). More interestingly, within each family of ions reactivity increases with the number of fluorine substituents (e.g. CF(2)(+) > CFCl(+) > CCl(2)(+) and CF(+) > CCl(+)), i.e. reactivity increases with the ion thermochemical stability, as measured by available standard free enthalpies of formation. Evaluation of the energetics involved shows that reactions are largely driven by the stability of the neutrals more than of the ions. Finally, the products observed in the reaction of Cl(+) are attributed to ionization of the neutral via charge transfer and fragmentation.     

Gas-phase ion-molecule reactions of metal-carbide cations MCn+ (M=Y and La; n=2, 4, and 6) with benzene and cyclohexane investigated by FTICR mass spectrometry and DFT calculations

Yttrium- and lanthanum-carbide cluster cations YC(n)(+) and LaC(n)(+) (n = 2, 4, and 6) are generated by laser ablation of carbonaceous material containing Y(2)O(3) or La(2)O(3). YC(2)(+), YC(4)(+), LaC(2)(+), LaC(4)(+), and LaC(6)(+) are selected to undergo gas-phase ion-molecule reactions with benzene and cyclohexane. The FTICR mass spectrometry study shows that the reactions of YC(2)(+) and LaC(2)(+) with benzene produce three main series of cluster ions. They are in the form of M(C(6)H(4))(C(6)H(6))(n)(+), M(C(8)H(4))(C(6)H(6))(n)(+), and M(C(8)H(6))(C(6)H(6))(m)(+) (M = Y and La; n = 0-3; m = 0-2). For YC(4)(+), LaC(4)(+), and LaC(6)(+), benzene addition products in the form of MC(n)(C(6)H(6))(m)(+) (M = Y and La; n = 4, 6; m = 1, 2) are observed. In the reaction with cyclohexane, all the metal-carbide cluster ions are observed to form metal-benzene complexes M(C(6)H(6))(n)(+) (M = Y and La; n= 1-3). Collision-induced-dissociation experiments were performed on the major reaction product ions, and the different levels of energy required for the fragmentation suggest that both covalent bonding and weak electrostatic interaction exist in these organometallic complexes. Several major product ions were calculated using DFT theory, and their ground-state geometries and energies were obtained.     

Reaction of methacrolein with the hydroxyl radical in air: incorporation of secondary O2 addition into the MACR + OH master equation

Methacrolein (MACR) plays an important role in atmospheric chemistry within the planetary boundary layer, as it is one of the major oxidation products of isoprene and has a short lifetime toward the hydroxyl radical (OH). In this study, quantum chemical techniques and statistical reaction rate theory have been used to simulate the addition of OH to MACR at conditions representative of the troposphere. In this chemically activated reaction, the time scales for product formation versus collisional deactivation of the vibrationally excited adduct are explicitly considered. Furthermore, the subsequent addition of O(2) is also incorporated within a single master equation, so as to investigate doubly activated peroxyl radical formation. The major reaction product of OH addition to MACR is the HOCH(2)C(?)(CH(3))CHO radical formed via addition to the outer (β) carbon. This radical is predominantly in the Z isomer although around a third of the population is quenched as the higher-energy E isomer. Calculated rate constants agree well with experiment when using M06-2X/aug-cc-pVTZ barrier heights, but are somewhat overpredicted using G3SX energies. The overall rate constant is controlled by competition between dissociation of the MACR···OH van der Waals complex back to reactants and isomerization on to MACR-OH adducts, which takes place on a time scale of several nanoseconds, but collisional deactivation of the MACR-OH adducts occurs on a time scale that is around an order of magnitude longer. When O(2) addition is included in the master equation, we observe that the MACR-OH adducts are removed by reaction with O(2) on a similar time scale to collisional deactivation. Around 50% of the subsequent peroxyl radical population is formed with some identifiable excess vibrational energy above singly activated [MACR-OH-O(2)]*, with around 20% provided with an additional 20 kcal mol(-1) (>40 kcal mol(-1) relative to quenched MACR-OH-O(2)) that can go into further unimolecular reaction. This double activation process is expected to lead to some prompt unimolecular decomposition of excited [MACR-OH-O(2)]** peroxyl radicals to yield products including hydroxyacetone and methylglyoxal, regenerating the initiating OH radical in the process.     

Selected Ion Flow Tube Study of the Gas-Phase Reactions of CF(+), CF(2)(+), CF(3)(+), and C(2)F(4)(+) with C(2)H(4), C(2)H(3)F, CH(2)CF(2), and C(2)HF(3)

We study how the degree of fluorine substitution for hydrogen atoms in ethene affects its reactivity in the gas phase. The reactions of a series of small fluorocarbon cations (CF(+), CF(2)(+), CF(3)(+), and C(2)F(4)(+)) with ethene (C(2)H(4)), monofluoroethene (C(2)H(3)F), 1,1-difluoroethene (CH(2)CF(2)), and trifluoroethene (C(2)HF(3)) have been studied in a selected ion flow tube. Rate coefficients and product cations with their branching ratios were determined at 298 K. Because the recombination energy of CF(2)(+) exceeds the ionization energy of all four substituted ethenes, the reactions of this ion produce predominantly the products of nondissociative charge transfer. With their lower recombination energies, charge transfer in the reactions of CF(+), CF(3)(+), and C(2)F(4)(+) is always endothermic, so products can only be produced by reactions in which bonds form and break within a complex. The trends observed in the results of the reactions of CF(+) and CF(3)(+) may partially be explained by the changing value of the dipole moment of the three fluoroethenes, where the cation preferentially attacks the more nucleophilic part of the molecule. Reactions of CF(3)(+) and C(2)F(4)(+) are significantly slower than those of CF(+) and CF(2)(+), with adducts being formed with the former cations. The reactions of C(2)F(4)(+) with the four neutral titled molecules are complex, giving a range of products. All can be characterized by a common first step in the mechanism in which a four-carbon chain intermediate is formed. Thereafter, arrow-pushing mechanisms as used by organic chemists can explain a number of the different products. Using the stationary electron convention, an upper limit for Δ(f)H°(298)(C(3)F(2)H(3)(+), with structure CF(2)═CH-CH(2)(+)) of 628 kJ mol(-1) and a lower limit for Δ(f)H°(298)(C(2)F(2)H(+), with structure CF(2)═CH(+)) of 845 kJ mol(-1) are determined.     

Dimethyl ether chemical ionization of arylalkylamines

The gas-phase reactions of dimethyl ether (DME) ions with a number of biologically active arylalkylamines of the general formula R(1)R(2)C(6)H(3)CHR(3)(CH(2))(n)NR(4)R(5), where R(1) = H or OH, R(2) = H, F, NO(2), OH or OCH(3), R(3) = H or OH, R(4) and R(5) = H or CH(3), have been studied by means of chemical ionization mass spectrometry. Under the experimental conditions used, the most abundant DME ion is the methoxymethyl cation (CH(3)OCH(2)(+), m/z 45). The unimolecular metastable decompositions of the [M + 45](+), [M + 13](+) and [M + 15](+) adducts formed have been interpreted in terms of the initial site of reaction with the amines and the presence of different functional groups in the molecule. This has permitted establishment of general fragmentation patterns for the adducts, and their correlation with structural features of the molecules. The main site of reaction of the ion CH(3)OCH(2)(+) with the amines seems to be the amino group, particularly if the amine is primary, although a competition with attack on the aromatic ring and especially on the benzylic hydroxy group is observed. In a few cases the reaction mechanisms have been elucidated through the use of deuterated amines obtained by H/D exchange with D(2)O.     

Derivatization of protonated peptides via gas phase ion—molecule reactions with acetone

The protonated [M + H]+ ions of glycine, simple glycine containing peptides, and other simple di- and tripeptides react with acetone in the gas phase to yield [M + H + (CH3)2CO]+ adduct ion, some of which fragment via water loss to give [M + H + (CH3)2CO - H2O]+ Schiff's base adducts. Formation of the [M + H + (CH3)2CO]+ adduct ions is dependent on the difference in proton affinities between the peptide M and acetone, while formation of the [M + H + (CH3)2CO - H2O]+ Schiff's base adducts is dependent on the ability of the peptide to act as an intramolecular proton "shuttle." The structure and mechanisms for the formation of these Schiff's base adducts have been examined via the use of collision-induced dissociation tandem mass spectrometry (CID MS/MS), isotopic labeling [using (CD3)2CO] and by comparison with the reactions of Schiff's base adducts formed in solution. CID MS/MS of these adducts yield primarily N-terminally directed a- and b-type "sequence" ions. Potential structures of the b1 ion, not usually observed in the product ion spectra of protonated peptide ions, were examined using ab initio calculations. A cyclic 5 membered pyrrolinone, formed by a neighboring group participation reaction from an enamine precursor, was predicted to be the primary product.     

Gas-phase ion-molecule reactions in organophosphorus esters

Self-condensation ion-molecule reactions of trimethyl phosphite, triethyl phosphite, dimethyl phosphonate, trimethyl phosphate and 2, 2-dichlorovinyl dimethyl phosphate (dichlorvos) were investigated by ion trap mass spectrometry and Fourier transform ion cyclotron resonance mass spectrometry. Reaction paths for the main processes observed were elucidated by parent ion selection and for reaction times up to 500 ms. In parallel, high-resolution measurements were performed in order to determine the composition of the principal ions. Among the compounds under examination, trimethyl phosphite and triethyl phosphite mainly give [M + H](+) and [M + (RO)(2)P](+) (R = CH(3), C(2)H(5)) adduct ions, whereas trimethyl phosphate and dimethyl phosphonate display [2M + H](+) ions, as the only abundant products, formed by reaction of [M + H](+) and M. 2,2-Dichlorovinyl dimethyl phosphate mostly shows fragmentation processes. The reaction patterns of the compounds examined were related to their different structural features. Gas-phase basicities of the phosphoryl compounds were also determined or re-examined. Copyright 1999 John Wiley & Sons, Ltd.     

Reactive processes in gas phase Na(+)-iso-C3H7Cl collisions: experimental guided-ion-beam and ab initio studies of the reactions on the ground singlet potential surface of the system up to 12.00 eV

Reactive processes, taking place when sodium ions collide with neutral iso-C(3)H(7)Cl molecules in the 0.02-12.00 eV range of energies in the center of mass frame, have been studied using an octopole radiofrequency guided-ion-beam apparatus developed in our laboratory. A dehydrohalogenation reaction channel leading to Na(C(3)H(6))(+) formation has been observed up to 1.00 eV while another process producing NaHCl(+) continues up to 4.00 eV. Furthermore, C(3)H(7)(+) formation resulting from decomposition of the reactants, ion-molecule adducts, has also been observed as well as its decomposition into C(2)H(3)(+) on increasing collision energy. Cross-section energy dependences for all these reactions have been obtained in absolute units. The ab initio electronic structure calculations have been done at the MP2 level for the colliding system ground singlet potential surface, giving information on the reactive surface main topological features. From the surface reactants side to the products' one, different potential wells and barriers have been characterized and their connectivity along the reaction evolution has been established using the intrinsic-reaction-coordinate method, thus interpreting the dynamical evolution of the reactants' collision complex to products. Experimental results demonstrate that NaHCl(+) can be produced via different channels. Reaction rate constants at 308.2 K for both dehydrohalogenation reactions have been calculated from measured excitation functions. It has been also confirmed that the reactants adduct decomposition giving C(3)H(7)(+) and NaCl takes place on the same potential surface. A qualitative interpretation of the experimental results in terms of ab initio calculations is also given.     

A kinetic and spectroscopic study of the CH3I-Cl and ICH2I-Cl adducts

Laser-induced fluorescence from the CH3I-Cl and ICH2I-Cl adducts formed in association reactions between chlorine atoms and CH3I and CH2I2 has been observed for the first time. The LIF excitation and dispersed fluorescence spectra have been measured in the range 345-375 nm and 380-480 nm, respectively, at 204 and 296 K. The excitation spectra exhibit vibrational fine structure, and a semiquantitative analysis of the spectra yields a similar binding energy for both adducts of approximately 60 kJ mol(-1). The adduct fluorescence is efficiently quenched by N2 and exhibits a zero-pressure lifetime of approximately 25-30 ns. Using LIF excited from the CH3I-Cl and ICH2I-Cl adducts, the kinetics of the reactions of atomic chlorine with methyl iodide and diiodomethane have been investigated, the results showing that both reactions proceed via two independent channels, an association reaction to form the adduct and a bimolecular abstraction reaction. At T approximately 200 K, the association reaction is predominant, and CH3I-Cl formation is irreversible, with rate coefficients for adduct formation found to be pressure-dependent and in reasonable agreement with the literature. At approximately 200 K, removal of the adduct is dominated by reaction with radical species (CH3 and ClSO) and by self-reaction, which proceed at close to the gas kinetic limit. At 296 K, CH3I-Cl formation is reversible, and the equilibrium constant, K(p) = (70.9 +/- 27.4) x 10(3) atm(-1), was determined, which is in excellent agreement with the literature, and the adduct does not significantly react with CH3I. The uncertainty is at the 95% confidence level (2sigma) and includes systematic errors. At approximately 200 K, the ICH2I-Cl adduct is again stabilized, with pressure-dependent rate coefficients reaching the high pressure limit at lower pressures than for the Cl + CH3I reaction. At room temperature, the ICH2I-Cl adduct is removed via an additional unimolecular decomposition channel, which dominates over the reversible decomposition channel to reform Cl + CH2I2. Neither adduct was observed to undergo significant reaction with molecular oxygen at approximately 200 or 296 K, with an upper limit rate coefficient determined as k < 10(-16) cm(3) molecule(-1) s(-1).     

C_（60）的胺加成反应和电喷雾电离质谱（ESI－MS）检测

报道了Ｃ６０与１，３－丙二胺和Ｎ，Ｎ－二甲基－１，３－丙二胺的加合反应，反应产物未经预先离子化处理直接用ＥＳＬ－ＭＳ进行检测。由于反应产物从甲苯溶液中析出，避免了生成多胺基加合物，产物以单加成物为主。当加合反应在空气氛下进行时，有加合氧的产物Ｃ６０Ｏｎ（ＮＨ２—ＣＨ２ＣＨ２ＣＨ２ＮＲ２）ｍ（Ｒ＝Ｈ，ＣＨ３）存在。实验发现：Ｎ，Ｎ－二甲基－１，３－丙二胺比１，３－丙二胺更容易与Ｃ６０发生多胺基加成和氧加成反应。通过控制反应条件可制备Ｃ６０二胺的单加成产物。     

Unimolecular reactions in the CF3CH2Cl ↔ CF2ClCH2F system: isomerization by interchange of Cl and F atoms

The recombination of CF(2)Cl and CH(2)F radicals was used to prepare CF(2)ClCH(2)F* molecules with 93 ± 2 kcal mol(-1) of vibrational energy in a room temperature bath gas. The observed unimolecular reactions in order of relative importance were: (1) 1,2-ClH elimination to give CF(2)═CHF, (2) isomerization to CF(3)CH(2)Cl by the interchange of F and Cl atoms and (3) 1,2-FH elimination to give E- and Z-CFCl═CHF. Since the isomerization reaction is 12 kcal mol(-1) exothermic, the CF(3)CH(2)Cl* molecules have 105 kcal mol(-1) of internal energy and they can eliminate HF to give CF(2)═CHCl, decompose by rupture of the C-Cl bond, or isomerize back to CF(2)ClCH(2)F. These data, which provide experimental rate constants, are combined with previously published results for chemically activated CF(3)CH(2)Cl* formed by the recombination of CF(3) and CH(2)Cl radicals to provide a comprehensive view of the CF(3)CH(2)Cl* ? CF(2)ClCH(2)F* unimolecular reaction system. The experimental rate constants are matched to calculated statistical rate constants to assign threshold energies for the observed reactions. The models for the molecules and transition states needed for the rate constant calculations were obtained from electronic structures calculated from density functional theory. The previously proposed explanation for the formation of CF(2)═CHF in thermal and infrared multiphoton excitation studies of CF(3)CH(2)Cl, which was 2,2-HCl elimination from CF(3)CH(2)Cl followed by migration of the F atom in CF(3)CH, should be replaced by the Cl/F interchange reaction followed by a conventional 1,2-ClH elimination from CF(2)ClCH(2)F. The unimolecular reactions are augmented by free-radical chemistry initiated by reactions of Cl and F atoms in the thermal decomposition of CF(3)CH(2)Cl and CF(2)ClCH(2)F.     

Growth of polyaromatic molecules via ion-molecule reactions: an experimental and theoretical mechanistic study

The reactivity of naphthyl cations with benzene is investigated in a joint experimental and theoretical approach. Experiments are performed by using guided ion beam tandem mass spectrometers equipped with electron impact or atmospheric pressure chemical ion sources to generate C(10)H(7)(+) with different amounts of internal excitation. Under single collision conditions, C-C coupling reactions leading to hydrocarbon growth are observed. The most abundant ionic products are C(16)H(13)(+), C(16)H(n)(+) (with n=10-12), and C(15)H(10)(+). From pressure-dependent measurements, absolute cross sections of 1.0±0.3 and 2±0.6 A?(2) (at a collision energy of about 0.2 eV in the center of mass frame) are derived for channels leading to the formation of C(16)H(12)(+) and C(15)H(10)(+) ions, respectively. From cross section values a phenomenological total rate constant k=(5.8±1.9)×10(-11) cm(3) s(-1) at an average collision energy of about 0.27 eV can be estimated for the process C(10)H(7)(+)+C(6)H(6)→all products. The energy behavior of the reactive cross sections, as well as further experiments performed using partial isotopic labeling of reagents, support the idea that the reaction proceeds via a long lived association product, presumably the covalently bound protonated phenylnaphthalene, from which lighter species are generated by elimination of neutral fragments (H, H(2), CH(3)). A major signal relevant to the fragmentation of the initial adduct C(16)H(13)(+) belongs to C(15)H(10)(+). Since it is not obvious how CH(3) loss from C(16)H(13)(+) can take place to form the C(15)H(10)(+) radical cation, a theoretical investigation focuses on possible unimolecular transformations apt to produce it. Naphthylium can act as an electrophile and add to the π system of benzene, leading to a barrierless formation of the ionic adduct with an exothermicity of about 53 kcal mol(-1). From this structure, an intramolecular electrophilic addition followed by H shifts and ring opening steps leads to an overall exothermic loss (-7.1 kcal mol(-1) with respect to reagents) of the methyl radical from that part of the system which comes from benzene. Methyl loss can take place also from the "naphthyl" part, though via an endoergic route. Experimental and theoretical results show that an ionic route is viable for the growth of polycyclic aromatic species by association of smaller building blocks (naphthyl and phenyl rings) and this may be of particular relevance for understanding the formation of large molecules in ionized gases.     

Ionization of methane clusters in helium nanodroplets

The electron ionization of helium droplets doped with methane clusters is investigated for the first time using high-resolution mass spectrometry. The dominant ion products ejected into the gas phase are the unprotonated (CH(4))(n)(+) cluster ions along with the protonated ions, CH(5)(+)(CH(4))(n-1). The mass spectra show clear evidence for magic numbers, which are broadly consistent with icosahedral shell closings. However, unusual features were observed, including different magic numbers for CH(5)(+)(CH(4))(n-1) (n=55, 148) when compared to (CH(4))(n)(+) (n=54, 147). Possible interpretations for some of these differences are proposed. Products of the type [C(2)H(x)(CH(4))(n)](+), which result from ion-molecule chemistry, are also observed and these too show clear magic number features. Finally, we report the first observation of (CH(4))(n)(2+) dications from methane clusters. The threshold for dication survival occurs at n≥70 and is in good agreement with a liquid droplet model for fission of multiply charged ions. Furthermore, we present evidence showing that these dications are formed by an unusual two-step mechanism which is initiated by charge transfer to generate a monocation and is then followed by Penning ionization to generate a dication.