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Rate constants and product ions were determined for a series of anions reacting with singlet molecular oxygen O2 (a 1Δg) at thermal energy using an electrospray ionization-selected ion flow tube. The 20 naturally occurring amino acids were used to produce corresponding deprotonated anions; only [Cys-H]− and [Pro-H]− were found to be reactive with O2 (a 1Δg), generating OSCH2CH(NH2)CO2– + HO and C5H6NO2– + H2O2, respectively. The reaction of O2 (a 1Δg) with [Cys-H]− has a rate constant more than ten times larger than the reaction of O2 (a 1Δg) with [Pro-H]−. Furthermore, reactions of O2 (a 1Δg) with carboxylic acid and thiol anions were carried out to elucidate the reactivity of the sulfur-containing functional groups. Potential energy surfaces and overall reaction exothermicities were calculated for representative reactions using density functional theory. Reactions in which attack occurs at the sulfur produce HCSO– as an ionic product. Reactions of several carboxylic acid anions likely proceed through a ...

Rate constants and product ions were determined for a series of anions reacting with singlet molecular oxygen O

The reactivity of microsolvated fluoride ions, F−(CH3OH)0–2, with methyl, ethyl, n-propyl, and t-butyl bromide is evaluated over a broad range of temperatures. Significant decreases in reactivity are observed as either solvation or temperature increases. Increasing solvation increases sensitivity to the reaction barrier as revealed by a larger temperature dependence. These reactions are dominated by an SN2 mechanism for the methyl bromide reaction, while the SN2 and E2 mechanisms compete for the reactions with ethyl and n-propyl bromide reactions. The elimination mechanism, with some association, dominates the t-butyl bromide reactions. In all cases the unsolvated bromide ion is the primary ionic product. Branching ratios are discussed in both qualitative and quantitative terms for all reactions at 300 K.

The kinetics of eighteen positive ions with chlorine azide (ClN3) have been studied using a selected ion flow tube (SIFT). These measurements allowed for the estimation of both the ionization energy, >930 kJ mol−1 (>9.6 eV), and the proton affinity, 713 ± 41 kJ mol−1, of chlorine azide. Reaction rate constants have been determined. Product ions have been identified, and quantified when synthetic complications allowed. In addition, general reaction trends have been observed. A reaction coordinate diagram for the novel reaction of N+ with ClN3 is discussed.

Recently, it was discovered that specific aluminum clusters (e.g., Al13–) that demonstrate enhanced resistance to reactivity with oxygen may do so not only because of a closed electronic jellium shell as originally supposed but also because of a forbidden spin-flip in the transition state of the reaction. Herein, we discuss an experiment using a multiple-species laminar flow reaction vessel coupled to a singlet oxygen generator. The present results suggest that all clusters react with singlet oxygen. Additionally, we observe Al9–, a cluster previously unidentified as having any notable stability, as being resistant to reaction with triplet oxygen. Furthermore, we discuss a means of estimating rate constants in a multiple-species flow tube where the products and reactants do not allow the use of traditional methods.

Room-temperature ionic liquids exert vanishingly small vapor pressures under ambient conditions. Under reduced pressure, certain ionic liquids have demonstrated volatility, and they are thought to vaporize as intact cation-anion ion pairs. However, ion pair vapors are difficult to detect because their concentration is extremely low under these conditions. In this Letter, we report the products of reacting ions such as NO(+), NH4(+), NO3(-), and O2(-) with vaporized aprotic ionic liquids in their intact ion pair form. Ion pair fragmentation to the cation or anion as well as ion exchange and ion addition processes are observed by selected-ion flow tube mass spectrometry. Free energies of the reactions involving 1-ethyl-3-methylimidazolium bis-trifluoromethylsulfonylimide determined by ab initio quantum mechanical calculations indicate that ion exchange or ion addition are energetically more favorable than charge-transfer processes, whereas charge-transfer processes can be important in reactions involving 1-butyl-3-methylimidazolium dicyanamide.

The competition between substitution (S N 2) and elimination (E2) has been studied for the reactions of methyl, ethyl, isopropyl, and tert- butyl iodide with Cl − , CN − , and HS − in the gas phase. Previous studies have shown a dominance of the S N 2 mechanism for sulfur anions and for some cyanide–alkyl iodide reactions. Although our results support this conclusion for the reactions studied, they reveal that competition between the S N 2 and E2 pathways exists for the isopropyl reactions. Steric and electronic effects, upon alkyl group substitution, produce looser and less stable S N 2 transition states, however, they can favor the E2 process. These opposing effects on barrier heights produce E2/S N 2 competition as steric hindrance increases around the α-carbon, however the relative differences in intrinsic barrier heights lead to significantly different branching ratios. This interpretation is discussed in terms of reaction efficiencies, kinetic isotope effects, linear basicity–reactivity relationships, electrostatic models, and transition state looseness parameters.

The reactivity of ClN(3) with 17 negative ions has been investigated at 300 K. The electron affinity (EA) of ClN(3) was bracketed to be between that of NO(2) and N(3), giving EA(ClN(3)) = 2.48 +/- 0.20 eV, in agreement with an electronic structure calculation. Reaction rate constants and product ion branching ratios were measured. In nearly all cases the major product of the reaction was chloride ions. Charge transfer, N(3)(-) production, and O atom incorporation is also observed. DFT calculations of stable complexes and transition states are presented for two typical ions. Mechanistic details are discussed in terms of reaction coordinate diagrams.

Direct comparisons of the reactivity and mechanistic pathways for anionic systems in the gas phase and in solution are presented. Rate constants and kinetic isotope effects for the reactions of methyl, ethyl, isopropyl, and tert-butyl iodide with cyanide ion in the gas phase, as well as for the reactions of methyl and ethyl iodide with cyanide ion in several solvents, are reported. In addition to measuring the perdeutero kinetic isotope effect (KIE) for each reaction, the secondary alpha- and beta-deuterium KIEs were determined for the ethyl iodide reaction. Comparisons of experimental results with computational transition states, KIEs, and branching fractions are explored to determine how solvent affects these reactions. The KIEs show that the transition state does not change significantly when the solvent is changed from dimethyl sulfoxide/methanol (a protic solvent) to dimethyl sulfoxide (a strongly polar aprotic solvent) to tetrahydrofuran (a slightly polar aprotic solvent) in the ethyl iodide-cyanide ion S(N)2 reaction in solution, as the "Solvation Rule for S(N)2 Reactions" predicts. However, the Solvation Rule fails the ultimate test of predicting gas phase results, where significantly smaller (more inverse) KIEs indicate the existence of a tighter transition state. This result is primarily attributed to the greater electrostatic forces between the partial negative charges on the iodide and cyanide ions and the partial positive charge on the alpha carbon in the gas phase transition state. Nevertheless, in evaluating the competition between S(N)2 and E2 processes, the mechanistic results for the solution and gas phase reactions are strikingly similar. The reaction of cyanide ion with ethyl iodide occurs exclusively by an S(N)2 mechanism in solution and primarily by an S(N)2 mechanism in the gas phase; only approximately 1% of the gas phase reaction is ascribed to an elimination process.

The 351.1 nm photoelectron spectrum of the peroxyacetate anion, (CH3C(O)OO−) was measured. Analysis of the spectrum shows that the observed spectral features arise almost exclusively from transitions between the trans-conformer of the anion and the X˜22A″ and Ã2A′ states of the corresponding radical. The electron affinity of trans-CH3C(O)OO is 2.381 ± 0.007 eV and the term energy splitting of the Ã2A′ state is 0.691 ± 0.009 eV, in excellent agreement with two prior values [Zalyubovsky et al. J. Phys. Chem. A 107, 7704 (2003); Hu et al. J. Phys. Chem. 124, 114305/1 (2006); Hu et al. J. Phys. Chem. 110, 2629 (2006)]. The gas-phase acidity of trans-peroxyacetic acid was bracketed between the acidity of acetic acid and tert-butylthiol at Δa G298( trans-CH3C(O)OOH) = 1439 ± 14 kJ mol−1 and Δa H298( trans-CH3C(O) OOH) = 1467 ± 14 kJ mol−1. The acidity of cis-CH3C(O)OOH was found by adding a calculated energy correction to the acidity of the trans-conformer; Δa G298[ cis-CH3C(O)OOH] = 1461 ± 14 kJ mol−1 and Δa H298[ cis-CH3C(O)OOH] = 1490 ± 14 kJ mol−1. The O–H bond dissociation energies for both conformers were determined using a negative ion thermodynamic cycle to be D0[ trans-CH3C(O)OOH] = 381 ± 14 kJ mol−1 and D0[ cis-CH3C(O)OOH] = 403±14 kJ mol−1. The atmospheric implications of these results and relations to the thermochemistry of peroxyacetyl nitrate are discussed briefly.

The 351.1 nm photoelectron spectrum of the peroxyformate anion has been measured. The photoelectron spectrum displays vibronic features in both the 2A'' ground and 2A' first excited states of the corresponding radical. Franck-Condon simulations of the spectrum show that the ion is formed exclusively in the trans-conformation. The electron affinity (EA) of the peroxyformyl radical was determined to be 2.493 +/- 0.006 eV, while the term energy splitting was found to be 0.783-0.020+0.060 eV. Extended progressions in the C-OO (973 +/- 20 cm-1) and O-O (1098 +/- 20 cm-1) stretching modes are observed in the ground state of the radical. The fundamental frequency of the in-plane OCO bend was found to be 574 +/- 35 cm-1. The gas-phase acidity of peroxyformic acid has been determined using an ion-molecule bracketing technique. On the basis of the size of the trans- to cis- isomerization barrier, the measured acidity was assigned to the higher energy trans-conformer of the acid. The gas-phase acidity of the lower energy cis-conformer of peroxyformic acid was found from the measured acidity for the trans-form and a calculated energy correction: DeltaaG298(cis-peroxyformic acid) = 346.8 +/- 3.3 kcal mol-1 and DeltaaH298(cis-peroxyformic acid) = 354.4 +/- 3.3 kcal mol-1. Using a negative ion EA/acidity thermochemical cycle, the O-H bond dissociation energy (D0) values of the trans- and cis-conformers of peroxyformic acid to form the trans-radical were determined to be 94.0 +/- 3.3 and 97.1 +/- 3.3 kcal mol-1, respectively. The heat of formation (DeltafH298) of the trans-peroxyformyl radical was found to be -22.8 +/- 3.5 kcal mol-1.

The reactivity of O(2)(a (1)Delta(g)) was studied with a series of anions, including (-)CH(2)CN, (-)CH(2)NO(2), (-)CH(2)C(O)H, CH(3)C(O)CH(2)(-), C(2)H(5)O(-), (CH(3))(2)CHO(-), CF(3)CH(2)O(-), CF(3)(-), HC(2)(-), HCCO(-), HC(O)O(-), CH(3)C(O)O(-), CH(3)OC(O)CH(2)(-), and HS(-). Reaction rate constants and product ion branching ratios were measured. All of the carbanions react through a common pathway to produce their major products. O(2)(a) adds across a bond at the site of the negative charge, resulting in the cleavage of this bond and the O=O bond. Oxyanions react through a hydride transfer to produce their major products. Proton transfer within these product ion-dipole complexes can occur, where the final branching ratios reflect the basicity of the resulting anions. Several of these anions (CF(3)(-), HC(2)(-), CH(3)OC(O)CH(2)(-)) were also found to undergo several sequential reactions within a single encounter. These three basic types of mechanisms are supported by calculations; a potential energy diagram for each type of reaction has been calculated. Additionally, six of these reactions had been qualitatively studied before; our results are in agreement with previous data.

We have measured the gas-phase acidities of nine compounds: formic acid, acetic acid, 1,3-propanedithiol, 2-methyl-2-propanethiol, 3-methyl-1-butanethiol, 2-propanethiol, 1-propanethiol, ethanethiol, and methanethiol, with acidities ranging from 338.6 to 351.1 kcal mol −1 using proton transfer kinetics and the resulting equilibrium constants. These acids were anchored to the well-known acidity of hydrogen sulfide; the measured acidities are in good agreement with previous experimental values, but error bars are significantly reduced. The gas-phase acidity of 3-methyl-1-butanethiol was determined to be 347.1 (5) kcal mol −1 ; there were no previous measurements of this value. Entropies of deprotonation were calculated and enthalpies of deprotonation were determined.

Reaction rate constants and deuterium kinetic isotope effects for the reactions of BrO(-) with RCl (R = methyl, ethyl, isopropyl, and tert-butyl) were measured using a tandem flowing afterglow-selected ion flow tube instrument. These results provide qualitative insight into the competition between two classical organic mechanisms, nucleophilic substitution (S(N)2) and base-induced elimination (E2). As the extent of substitution in the neutral reactants increases, the kinetic isotope effects become increasingly more normal, consistent with the gradual onset of the E2 channel. These results are in excellent agreement with previously reported trends for the analogous reactions of ClO(-) with RCl. [Villano et al. J. Am. Chem. Soc. 2006, 128, 728.] However, the reactions of BrO(-) and ClO(-) with methyl chloride, ethyl chloride, and isopropyl chloride were found to occur by an additional reaction pathway, which has not previously been reported. This reaction likely proceeds initially through a traditional S(N)2 transition state, followed by an elimination step in the S(N)2 product ion-dipole complex. Furthermore, the controversial alpha-nucleophilic character of these two anions and of the HO(2)(-) anion is examined. No enhanced reactivity is displayed. These results suggest that the alpha-effect is not due to an intrinsic property of the anion but instead due to a solvent effect.

The reactivities of mono- and dihalocarbene anions (CHCl − , CHBr − , CF 2 − , CCl 2 − , and CBrCl − ) were studied using a tandem flowing afterglow-selected ion flow tube instrument. Reaction rate constants and product branching ratios are reported for the reactions of these carbene anions with six neutral reagents (CS 2 , COS, CO 2 , O 2 , CO, and N 2 O). These anions were found to demonstrate diverse chemistry as illustrated by formation of multiple product ions and by the observed reaction trends. The reactions of CHCl − and CHBr − occur with similar efficiencies and reactivity patterns. Substitution of a Cl atom for an H atom to form CCl 2 − and CBrCl − decreases the rate constants; these two anions react with similar efficiencies and reactivity trends. The CF 2 − anion displays remarkably different reactivity; these differences are discussed in terms of its lower electron binding energy and the effect of the electronegative fluorine substituents. The results presented here are compared to the reactivity of the CH 2 − anion, which has previously been reported.

The current uncertainty in many reaction rate constants causes difficulties in providing satisfactory models of interstellar chemistry. Here we present new measurements of the rate constants and product branching ratios for the gas phase reactions of C+ with NH3, CH4, O2, H2O, and C2H2, using the flowing afterglow-selected ion flow tube (FASIFT) technique. Results were obtained using two instruments that were separately calibrated and optimized; in addition, low ionization energies were used to ensure formation of ground-state C+, the purities of the neutral reactants were verified, and mass discrimination was minimized.

The gas-phase acidity of thioacetic acid, determined using ion–molecule equilibrium reactions, was found to be ΔGacid = 329.0 ± 1.5 kcal mol−1. The gas-phase acidity of thiobenzoic acid was determined by ion–molecule bracketting reactions to be ΔGacid = 323 ± 5 kcal mol−1. Gas-phase acidities of these compounds, as well as of thioformic acid, were calculated; our calculations and experiments are in good agreement. The calculated value of the enthalpy of deprotonation of thioformic acid is consistent with previously published computational results.

The 351.1 nm photoelectron spectra of the N-methyl-5-pyrazolide anion and the N-methyl-5-imidazolide anion are reported. The photoelectron spectra of both isomers display extended vibrational progressions in the X2A' ground states of the corresponding radicals that are well reproduced by Franck-Condon simulations, based on the results of B3LYP/6-311++G(d,p) calculations. The electron affinities of the N-methyl-5-pyrazolyl radical and the N-methyl-5-imidazolyl radical are 2.054 +/- 0.006 eV and 1.987 +/- 0.008 eV, respectively. Broad vibronic features of the A(2)A' ' states are also observed in the spectra. The gas-phase acidities of N-methylpyrazole and N-methylimidazole are determined from measurements of proton-transfer rate constants using a flowing afterglow-selected ion flow tube instrument. The acidity of N-methylpyrazole is measured to be Delta(acid)G(298) = 376.9 +/- 0.7 kcal mol(-1) and Delta(acid)H(298) = 384.0 +/- 0.7 kcal mol(-1), whereas the acidity of N-methylimidazole is determined to be Delta(acid)G(298) = 380.2 +/- 1.0 kcal mol(-1) and Delta(acid)H(298)= 388.1 +/- 1.0 kcal mol(-1). The gas-phase acidities are combined with the electron affinities in a negative ion thermochemical cycle to determine the C5-H bond dissociation energies, D(0)(C5-H, N-methylpyrazole) = 116.4 +/- 0.7 kcal mol(-1) and D(0)(C5-H, N-methylimidazole) = 119.0 +/- 1.0 kcal mol(-1). The bond strengths reported here are consistent with previously reported bond strengths of pyrazole and imidazole; however, the error bars are significantly reduced.

The gas-phase reactions of F(-)(DMSO), F(-)(CH(3)CN), and F(-)(C(6)H(6)) with t-butyl halides were investigated. Reaction rate constants, kinetic isotope effects, and product ion branching ratios were measured using the flowing afterglow selected ion flow tube technique (FA-SIFT). Additionally, the structure of F(-)(DMSO) was investigated both computationally and experimentally, and two stable isomers were identified. The reactions generally proceed by elimination mechanisms; however, the reaction of F(-)(C(6)H(6)) with t-butyl chloride occurs by a switching mechanism. These reactions are compared to previous studies of microsolvated reactions of t-butyl halides where the solvent molecules were polar, protic molecules.

[1] The rate constant for the reaction of Si with O 2 (a -1 Δ g ) was measured in a selected ion flow tube (SIFT) between 200 and 500 K. The temperature dependence is T -(2.2±0.8) . The rate constant at 500 K is below the detection limit (

The quenching of vibrationally excited NO(+) by O(2)(a (1)Delta(g)) has been examined using the monitor ion technique and chemical generation of O(2)(a (1)Delta(g)). In contrast to previous results which showed that the rate constant was much larger than for ground state O(2), this study finds that the rate constant for quenching is below the detection limit (

Rate constants for the dissociation of OH(-)(H(2)O) and OH(-)(H(2)O)(2) by transfer of electronic energy from O(2)(a(1)Delta(g)) were measured. Values of 1.8x10(-11) and 2.2x10(-11) cm(3) molecule(-1) s(-1), respectively, at 300 K were derived and temperature dependences were obtained from 300 to 500 K for OH(-)(H(2)O) and from 300 to 400 K for OH(-)(H(2)O)(2). Dissociative excitation transfer with OH(-)(H(2)O) is slightly endothermic and the reaction appears to have a positive temperature dependence, but barely outside the uncertainty range. In contrast, the reaction of OH(-)(H(2)O)(2) is exothermic and appears to have a negative temperature dependence. The rate constants are analyzed in terms of unimolecular rate theory, which suggests that the dissociation is prompt and is not affected by collisions with the helium buffer gas.

The gas-phase reactivity of the CHCl*- anion has been investigated with a series of halomethanes (CCl4, CHCl3, CH2Cl2, and CH3Cl) using a FA-SIFT instrument. Results show that this anion primarily reacts via substitution and by proton transfer. In addition, the reactions of CHCl*- with CHCl3 and CH2Cl2 form minor amounts of Cl2*- and Cl-. The isotopic distribution of these two products is consistent with an insertion-elimination mechanism, where the anion inserts into a C-Cl bond to form an unstable intermediate, which eliminates either Cl2*- or Cl- and Cl*. Neutral and cationic carbenes are known to insert into single bonds; however, this is the first observation of such reactivity for carbene anions.

Rate constants for the dissociation of OH(-)(H(2)O) and OH(-)(H(2)O)(2) by transfer of electronic energy from O(2)(a(1)Delta(g)) were measured. Values of 1.8x10(-11) and 2.2x10(-11) cm(3) molecule(-1) s(-1), respectively, at 300 K were derived and temperature dependences were obtained from 300 to 500 K for OH(-)(H(2)O) and from 300 to 400 K for OH(-)(H(2)O)(2). Dissociative excitation transfer with OH(-)(H(2)O) is slightly endothermic and the reaction appears to have a positive temperature dependence, but barely outside the uncertainty range. In contrast, the reaction of OH(-)(H(2)O)(2) is exothermic and appears to have a negative temperature dependence. The rate constants are analyzed in terms of unimolecular rate theory, which suggests that the dissociation is prompt and is not affected by collisions with the helium buffer gas.

The gas-phase reactions of F−(CH3OH) and F−(C2H5OH) with t-butyl bromide have been investigated to explore the effect of the solvent on the E2 transition state. Kinetic isotope effects (KIEs) were measured using a flowing afterglow-selected ion flow tube (FA-SIFT) mass spectrometer upon deuteration of both the alkyl halide and the alcohol. Kinetic isotope effects are significantly more pronounced than those previously observed for similar reactions of F−(H2O) with t-butyl halides. KIEs for the reaction of F−(CH3OH) with t-butyl bromide are 2.10 upon deuteration of the neutral reagent and 0.74 upon deuteration of the solvent. KIEs for the reaction of F−(C2H5OH) with t-butyl bromide are 3.84 upon deuteration of the neutral reagent and 0.66 upon deuteration of the solvent. The magnitude of these effects is discussed in terms of transition-state looseness. Additionally, deuteration of the neutral regent and deuteration of the solvent do not produce completely separable isotope effects, which is likely due to a crowded transition state. These results are compared to our previous work on SN2 and E2 solvated systems.