Electron attachment to chlorouracil: a comparison between 6-ClU and 5-ClU

Low energy electron impact to the isomers 6-chlorouracil (6-ClU) and 5-chlorouracil (5-ClU) yields a variety of negative ion fragments with surprisingly high cross sections. These ions are dominantly formed via sharply structured resonance features at energies below the threshold for electronic excitation and result from dissociative electron attachment (DEA). The most dominant DEA channel is formation of (M-HCl)-, i.e., ejection of a neutral HCl molecule with the negative charge remaining on the ring. The reaction cross section is 9 x 10(-18) m2 and 5 x 10(-18) m2 for 6-Cl and 5-ClU, respectively, and thus about two orders of magnitude higher than the geometrical cross section of the molecule. Further reactions also operative via low energy resonances (<2.5 eV) are Cl- abstraction, dehydrogenation [formation of (M-H)-, M=ClU], and DEA processes associated with a ring opening. Most of the ion yield curves exhibit remarkably sharp structures which have not been observed before in DEA to a polyatomic system. Although some possibilities on their origin are discussed, their interpretation remains a challenge for theory and further experiments. While electron attachment to both 6-ClU and 5-ClU generates fragments of the same stoichiometric composition, their ion yields and also their relative intensities show some very pronounced differences which can be explained by the different structure but also the different energetic situation in the two isomers.     

Transition metal complexes of 5-chlorouracil

Summary Complexes of 5-chlorouracil (5-ClU) (1) with 3d metal ions were characterized by elemental analysis, various spectroscopic methods (i.r., u.v. spectroscopy) and magnetic susceptibility measurements. The spectral evidence suggest that 5-ClU behaves as bidentate ligand in Ni^II, Cu^II, Zn^II, and Cd^II compounds, coordinating through its one carbonyl oxygen and one nitrogen whereas with Mn^II and Co^II it coordinates through the carbonyl oxygen only. The insolubility of the new complexes in organic solvents suggests that these are polymeric except for the Co^II complex which is soluble in pyridine. There is probable OH bridging in the Mn^II and Cu^II complexes and the 5-ClU may bridge in the rest.     

Magnetic properties after irradiation of 1:4 complexes consisting of CoX2, X=NCS⁻, Cl⁻, and NCO⁻, and phenylpyridyldiazomethane in dilute frozen solutions: axial ligand effect in heterospin single-molecule magnets

The solutions of 1:4 complexes of Co(X)(2)(D1py)(4), X = Cl(-), and NCO(-) and D1py = phenylpyridyldiazomethane, were photolyzed under cryogenic conditions, and their magnetic properties were investigated by direct current (DC) and alternating current (AC) magneto/susceptometries. After irradiation, the resulting cobalt-carbene complexes, Co(X)(2)(C1py)(4), exhibited the behaviors of heterospin single-molecule magnets (SMMs) strongly depending on the axial ligands. In Co(X)(2)(C1py)(4): X = Cl(-) and NCO(-), the effective activation barriers, U(eff), for the reorientation of the magnetic moment and the resonant quantum tunneling time, τ(Q), characteristic to SMM properties were estimated to be 91 and 130 K, and 4 × 10(3) and 2 × 10(5) s, respectively. The τ(Q) of Co(NCS)(2)(C1py)(4) with U(eff) = 89 K was found to be 6 × 10(2) s. In Co(X)(2)(C1py)(4): X = Cl(-) and NCO(-), temperature-dependent hysteresis loops were also observed below the blocking temperature (T(B) = 3.2 and 4.8 K, respectively) and the coercive forces, H(c), of 7.0 and 20 kOe at 1.9 K, respectively, were obtained. In a series of 1:4 complexes of Co(X)(2)(C1py)(4), X = NCS(-), Cl(-), and NCO(-), the axial ligands strongly affected the heterospin SMM properties, and the NCO(-) ion having the large magnitude of the ligand-field splitting in a spectrochemical series, gave the largest U(eff) and H(c) and the longest τ(Q).     

UV photodissociation dynamics of deprotonated 2'-deoxyadenosine 5'-monophosphate [5'-dAMP-H]-

The UV photodissociation dynamics of deprotonated 2'-deoxyadenosine 5'-monophosphate ([5'-dAMP-H](-)) has been studied using a unique technique based on the coincident detection of the ion and the neutral fragments. The observed fragment ions are m/z 79 (PO(3)(-)), 97 (H(2)PO(4)(-)), 134 ([A-H](-)), 177 ([dAMP-H-A-H(2)O](-)), and 195 ([dAMP-H-A](-)), where "A" refers to a neutral adenine molecule. The relative abundances are comparable to that found in previous studies on [5'-dAMP-H](-) employing different excitation processes, i.e., collisions and UV photons. The fragmentation times of the major channels have been measured, and are all found to be on the microsecond time scale. The fragmentation mechanisms for all channels have been characterized using velocity correlation plots of the ion and neutral fragment(s). The findings show that none of the dissociation channels of [5'-dAMP-H](-) is UV specific and all proceed via statistical fragmentation on the ground state after internal conversion, a result similar to fragmentations induced by collisions.     

Computational studies of σ-type weak interactions between NCO/NCS radicals and XY(X = H,Cl;Y = F,Cl,and Br)

The triatomic radicals NCO and NCS are of interest in atmospheric chemistry,and both the ends of these radicals can potentially serve as electron donors during the formation of σ-type hydrogen/halogen bonds with electron acceptors XY(X = H,Cl;Y = F,Cl,and Br).The geometries of the weakly bonded systems NCO/NCS···XY were determined at the MP2/aug-cc-pVDZ level of calculation.The results obtained indicate that the geometries in which the hydrogen/halogen atom is bonded at the N atom are more stable than those where it is bonded at the O/S atom,and that it is the molecular electrostatic potential(MEP)-not the electronegativity-that determines the stability of the hydrogen/halogen bond.For the same electron donor(N or O/S) in the triatomic radical and the same X atom in XY,the bond strength decreases in the order Y = F > Cl > Br.In the hydrogen/halogen bond formation process for all of the complexes studied in this work,transfer of spin electron density from the electron donor to the electron acceptor is negligible,but spin density rearranges within the triatomic radicals,being transferred to the terminal atom not interacting with XY.     

Photoelectron spectroscopic studies of 5-halouracil anions

The parent negative ions of 5-chlorouracil, UCl(-) and 5-fluorouracil, UF(-) have been studied using anion photoelectron spectroscopy in order to investigate the electrophilic properties of their corresponding neutral halouracils. The vertical detachment energies (VDE) of these anions and the adiabatic electron affinities (EA) of their neutral molecular counterparts are reported. These results are in good agreement with the results of previously published theoretical calculations. The VDE values for both UCl(-) and UF(-) and the EA values for their neutral molecular counterparts are much greater than the corresponding values for both anionic and neutral forms of canonical uracil and thymine. These results are consistent with the observation that DNA is more sensitive to radiation damage when thymine is replaced by halouracil. While we also attempted to prepare the parent anion of 5-bromouracil, UBr(-), we did not observe it, the mass spectrum exhibiting only Br(-) fragments, i.e., 5-bromouracil apparently underwent dissociative electron attachment. This observation is consistent with a previous assessment, suggesting that 5-bromouracil is the best radio-sensitizer among these three halo-nucleobases.     

Lifetime of C2Cl4(-) ions produced by nondissociative electron attachment to C2Cl4

The lifetimes of long-lived C2Cl4(-) ions formed by Rydberg electron transfer in K(np)/C2Cl4 collisions are investigated using a Penning ion trap. Measurements at high n, n > or = 30, show that low-energy electron attachment to C2Cl4 leads to the production of C2Cl4(-) ions with a broad range of lifetimes that extends up to at least 1 ms. This is attributed to capture by molecules in different initial vibrational states. At low n, internal-to-translational energy transfer in postattachment interactions between the product K+ and C2Cl4(-) ions becomes important and leads to a substantial increase in ion lifetimes.     

Size-selective formation of C78 fullerene from a three-dimensional polyyne precursor

Multicyclic cagelike cyclophanes 2 a and 2 b containing cyclobutene rings have been prepared as precursors of three-dimensional polyynes C78H18 (1a) and C78H12Cl6 (1b), respectively. Laser irradiation of 2a and 2b induced expulsion of the aromatic fragment, indane, to give the three-dimensional polyyne anions C78H18- and C78H12Cl6-, respectively. Whereas the former anion lost only four hydrogen atoms to form C78H14-, complete loss of all hydrogen and chlorine atoms was observed from the latter anion, to yield a C78- ion that has a fullerene structure which was proven by its characteristic fragmentation pattern.     

N(1)-C(5')-linked dimer hydrates of 5-substituted uracils produced by anodic oxidation in aqueous solution

Electrochemical dimerization reactivity has been studied for 5-substituted uracils (5XU) including thymine (1a: X = Me) and 5-halouracil derivatives (1b: X = F; 1c: X = Cl; 1d: X = Br; 1e: X = I). Upon galvanostatic electrolysis of Ar-saturated aqueous solution 1a underwent anodic oxidation to produce N(1)-C(5')- and N(1)-C(6')-linked dimer hydrates, 1-(6'-hydroxy-5',6'-dihydrothymin-5'-yl)thymine (5a) and 1-(5'-hydroxy-5',6'-dihydrothymin-6'-yl)thymine (6a), as the major products. These N-C-linked dimerizations were accompanied by the formation of novel stereoisomeric C(5)-C(5')-linked dimers (meso isomer: 13a[meso]; racemic isomer: 13a[rac]) with a condensed tetrahydrofuran ring skeleton. Similar electrolyses of 5-fluorouracil (1b) and 5-chlorouracil (1c) also afforded the corresponding N(1)-C(5')-linked dimer hydrates, 1-(5'-fluoro-6'-hydroxy-5',6'-dihydrouracil-5'-yl)-5-fluorouracil (5b) and 1-(5'-chloro-6'-hydroxy-5',6'-dihydrouracil-5'-yl)-5-chlorouracil (5c), respectively, while resulting in neither N(1)-C(6')-linked dimer analogues nor C(5)-C(5')-linked dimers, unlike the reactivity of 1a. In contrast to 1a-c, no dimeric products were obtained from 5-bromouracil (1d) and 5-iodouracil (1e). The present electrochemical method was applicable to the cross-dimerization into N(1)-C(5')-linked heterodimer hydrates composed of binary 5-substituted uracils that occurred in competition with the formation of homodimer hydrates. A mechanism of the N(1)-C(5')-linked dimerization of 1a-c has been proposed, by which allyl-type radical intermediates with limiting mesomeric forms of N(1)-centered and C(5)-centered pyrimidine radicals (2a-c [N(1)]/2a-c [C(5)]) are generated via anodic one-electron oxidation and subsequent deprotonation at N(1) and undergo a head-to-tail coupling.     

NCO/NCS自由基与XY（X=H,Cl;Y=F,Cl,Br）间σ-型弱作用的理论研究

NCO和NCS是大气化学中非常引人关注的自由基,它们均有三个原子并且两个端基原子均可作为电子给体形成σ-型氢/卤键.本文在MP2/aug-cc-pVDZ水平上研究了NCO/NCS...XY（X=H,Cl;Y=F,Cl,Br）体系中的弱化学键.计算结果表明,氢/卤原子与N原子相连形成的复合物比与O/S原子相连形成的复合物稳定;氢/卤键的稳定性由分子静电势决定,而非原子电负性;对相同的电子给体B（B=N,O/S）和相同的卤原子来说,化学键的强度按Y=F,Cl,Br的顺序逐渐减弱.在氢/卤键形成过程中,自旋电子密度在电子给体和电子受体间的转移较少,但它在自由基内部发生重排,就本文研究的所有复合物而言,自旋电子密度均转移向XY分子的相反位置.     

NCO–, a Key Fragment Upon Dissociative Electron Attachment and Electron Transfer to Pyrimidine Bases: Site Selectivity for a Slow Decay Process

We report gas phase studies on NCO^– fragment formation from the nucleobases thymine and uracil and their N-site methylated derivatives upon dissociative electron attachment (DEA) and through electron transfer in potassium collisions. For comparison, the NCO^– production in metastable decay of the nucleobases after deprotonation in matrix assisted laser desorption/ionization (MALDI) is also reported. We show that the delayed fragmentation of the dehydrogenated closed-shell anion into NCO^– upon DEA proceeds few microseconds after the electron attachment process, indicating a rather slow unimolecular decomposition. Utilizing partially methylated thymine, we demonstrate that the remarkable site selectivity of the initial hydrogen loss as a function of the electron energy is preserved in the prompt as well as the metastable NCO^– formation in DEA. Site selectivity in the NCO^– yield is also pronounced after deprotonation in MALDI, though distinctly different from that observed in DEA. This is discussed in terms of the different electronic states subjected to metastable decay in these experiments. In potassium collisions with 1- and 3-methylthymine and 1- and 3-methyluracil, the dominant fragment is the NCO^– ion and the branching ratios as a function of the collision energy show evidence of extraordinary site-selectivity in the reactions yielding its formation.

Determination of the rate constants for the NCO(X2Pi) + Cl(2P) and Cl(2P) + ClNCO(X1A') reactions at 293 and 345 K

The rate constant for the reaction of the isocyanato radical, NCO(X2Pi) with chlorine atoms, Cl(2P), has been measured at 293 +/- 2 and 345 +/- 3 K to be (6.9 +/- 3.8) x 10(-11) and (4.0 +/- 2.2) x 10(-11) cm3 molecules(-1) s,(-1) respectively, where the uncertainties include both random and systematic errors. The measurements were carried out at pressures of 1.3-6.2 Torr with either Ar or CF4 as the bath gas and were independent of both pressure and nature of the third body. Equal concentrations of NCO and Cl atoms were created by 248 nm photolysis of ClNCO. The reaction was monitored by following the temporal dependence of NCO(X2Pi) using time-resolved infrared absorption spectroscopy on rotational transitions of the NCO(10(1)1) <-- (00(1)0) combination band. The reaction rate constant was determined by using a simple chemical model and minimizing the sum of the residuals between the experimental and computer generated temporal NCO concentration profiles. The reaction Cl + ClNCO --> Cl2 + NCO was found to contribute to the observed NCO. The rate constant for this reaction was found to be (2.4 +/- 1.6) x 10(-13) and (1.9 +/- 1.2) x 10(-13) cm3 molecules(-1) s,(-1) at 293 and 345 K, respectively, where the uncertainties include both random and systematic error.     

Primary and secondary kinetic isotope effects in proton (H(+)/D(+)) and chloronium ion ((35)Cl(+)/(37)Cl(+)) affinities.

The Cooks' kinetic method and tandem-in-space pentaquadrupole QqQqQ mass spectrometry were used to measure primary and secondary kinetic isotope effects (KIEs) in H(+) and Cl(+) (X(+)) affinity for a series of A/A(') isotopomeric pairs. Gaseous, isotopomeric, and loosely bound dimers [A...X(+)...A(')] were formed in combinations in which X = H(+), D(+), (35)Cl(+) or (37)Cl(+) and A/A(') = acetonitrile/acetonitrile - d(3), acetonitrile/acetonitrile-(15)N, acetonitrile-d(3)/acetonitrile-(15)N, acetone/acetone-d(6), acetone/acetone-(18)O, acetone-d(6)/acetone-(18)O, pyridine/pyridine-d(5), pyridine/pyridine-(15)N, pyridine-d(5)/pyridine-(15)N, or 3-((35)Cl)chloropyridine/3-((37)Cl)chloropyridine. Under nearly the same experimental conditions, the dimers were mass-selected and then dissociated by low-energy collisions with argon, yielding AX(+) and A(')X(+) as the fragment ions. KIEs were measured from the changes in ion affinities of the neutrals (DeltaX(+)) as estimated by the AX(+)/A(')X(+) abundance ratios. Using [A...H(+)(D(+))...A(')] and [A...(35)Cl(+)((37)Cl(+))...A(')] dimers and by comparing their extent of dissociation under nearly identical collision-induced dissociation conditions, the kinetic method was also applied, for the first time, to measure primary KIEs of the central ion as well as their influence on secondary KIEs. Becke3LYP/6-311++G(2df,2p) calculations were found to provide Delta(DeltaZPE)s for the competitive dissociation reactions that accurately predict the nature (normal or inverse) of the measured KIEs.     

Electron-Transfer Reactions of Re(CO)(5): Atom-Transfer-Concerted Mechanism vs Bimetallic Intermediate Formation

Flash photochemically generated Re(CO)(5) reacts with halide complexes, Cu(Me(4)[14]-1,3,8,10-tetraeneN(4))X(+), Cu(Me(2)pyo[14]trieneN(4))X(+), and Ni(Me(2)pyo[14]trieneN(4))X(+) (X = Cl, Br, I) and ion pairs, [Co(bipy)(3)(3+), X(-)]. The rate constants for the electron transfers have values, k approximately 10(9) M(-1) s(-1), close to expectations for processes with diffusion-controlled rates. Reaction intermediates, probably bimetallic species, were detected in electron-transfer reactions of Re(CO)(5) with Cu(Me(6)[14]dieneN(4))X(+), (X = Cl, Br, I). In the absence of the halides X(-), the electron-transfer reactions between Re(CO)(5) and these complexes are slow, k < 10(6) M(-1) s(-1). The results are discussed in terms of inner-sphere pathways, namely an atom-transfer-concerted mechanism. The mediation of bimetallic intermediates in the electron transfer is also considered.     

Electrochemical reduction of 5-halo-5-nitro-1,3-dioxanes and 2-halo-2-nitro-1,3-propanediols

The character of the first stage of reduction of 5-X-5-nitro-1,3-dioxanes 1—10 and 2-X-2-nitro-1,3-propanediols 11 and 12 is independent of the nature of halogen (X = Br, Cl) and substituents in position 2 of the dioxane cycle. The transfer of two electrons to a molecule of compound 1—12 is accompanied by the anionoid elimination of halogen and formation of the anion of nitronic acid. The high mobility of halogen is mainly due to the acceptor nitro group capable of further transformations in the -position to halogen. The direction of further reduction involving the electron transfer to electrochemically active groups in the aromatic fragment of the molecule is determined by the nature of these groups. Chloro-, bromo-, and iodophenyl-substituted derivatives 4, 5, and 8—10 are reduced as typical halobenzenes. In the case of nitrophenyl-substituted compounds 3 and 7, the dioxane cycle opens to form dianions of p- and m-nitrobenzaldehydes along with the reduction of the nitroso group through the stages of formation of the radical anion and radical anion of the nitroso group. The radical anions of the nitro and nitroso derivatives were identified by ESR.     

Collision-induced gas-phase reactions of perhalogenated closo-dodecaborate clusters--a comparative study

The gas phase reactivity of perhalogenated closo-dodecaborate clusters [B(12)X(12)](2-) (X = F, Cl, Br, I) with N-tetraalkylated ammonium counter ions was investigated by electrospray ionization ion trap mass spectrometry (ESI-IT-MS). Collisions with the background gases introduced a broad variety of gas phase reactions. This study represents the first experimental approach to a new class of boron-rich boron clusters that are not accessible in the condensed phase. The anionic ion pair [B(12)X(12) + N(C(n)H(2n+1))(4)](-) is generally found as the ion of highest mass. Its reaction sequence starts with an alkyl transfer from the ammonium ion to the dodecaborate cluster. Subsequently, the alkylated intermediate [B(12)X(12) + C(n)H(2n+1)](-) decomposes to give very reactive ions of the general formula [B(12)X(11)](-). These ions possess a free boron vertex and immediately bind to the residual gases N(2) and H(2)O in the ion trap by formation of the corresponding adducts [B(12)X(11) + N(2)](-) and [B(12)X(11) + H(2)O](-). Subsequent fragmentations of the water adduct repetitively substitute halogen atoms by hydroxyl groups. The fragmentation process of the free anion [B(12)X(12)](2-) depends on the applied excitation energy and on the halogen substituent X. A radical dehalogenation of the B(12) unit is observed for X = I, whereas for X = Cl or F the loss of small molecules (mainly BX(3)) dominates. The different reaction behavior is explained by the different electron affinity of the halogens and the strength of the boron-halogen-bonds. Surprisingly, isolation of the fragment ion [B(12)I(9)](-) in the ion trap yields the highly stable [B(24)I(18)](2-) dianion. This observation suggests a reaction between two negative ions in the gas phase.     

Ab initio evaluation of the thermodynamic and electrochemical properties of alkyl halides and radicals and their mechanistic implications for atom transfer radical polymerization

High-level ab initio molecular orbital calculations are used to study the thermodynamics and electrochemistry relevant to the mechanism of atom transfer radical polymerization (ATRP). Homolytic bond dissociation energies (BDEs) and standard reduction potentials (SRPs) are reported for a series of alkyl halides (R-X; R = CH 2CN, CH(CH 3)CN, C(CH 3) 2CN, CH 2COOC 2H 5, CH(CH 3)COOCH 3, C(CH 3) 2COOCH 3, C(CH 3) 2COOC 2H 5, CH 2Ph, CH(CH 3)Ph, CH(CH 3)Cl, CH(CH 3)OCOCH 3, CH(Ph)COOCH 3, SO 2Ph, Ph; X = Cl, Br, I) both in the gas phase and in two common organic solvents, acetonitrile and dimethylformamide. The SRPs of the corresponding alkyl radicals, R (*), are also examined. The computational results are in a very good agreement with the experimental data. For all alkyl halides examined, it is found that, in the solution phase, one-electron reduction results in the fragmentation of the R-X bond to the corresponding alkyl radical and halide anion; hence it may be concluded that a hypothetical outer-sphere electron transfer (OSET) in ATRP should occur via concerted dissociative electron transfer rather than a two-step process with radical anion intermediates. Both the homolytic and heterolytic reactions are favored by electron-withdrawing substituents and/or those that stabilize the product alkyl radical, which explains why monomers such as acrylonitrile and styrene require less active ATRP catalysts than vinyl chloride and vinyl acetate. The rate constant of the hypothetical OSET reaction between bromoacetonitrile and Cu (I)/TPMA complex was estimated using Marcus theory for the electron-transfer processes. The estimated rate constant k OSET = approximately 10 (-11) M (-1) s (-1) is significantly smaller than the experimentally measured activation rate constant ( k ISET = approximately 82 M (-1) s (-1) at 25 degrees C in acetonitrile) for the concerted atom transfer mechanism (inner-sphere electron transfer, ISET), implying that the ISET mechanism is preferred. For monomers bearing electron-withdrawing groups, the one-electron reduction of the propagating alkyl radical to the carbanion is thermodynamically and kinetically favored over the one-electron reduction of the corresponding alkyl halide unless the monomer bears strong radical-stabilizing groups. Thus, for monomers such as acrylates, catalysts favoring ISET over OSET are required in order to avoid chain-breaking side reactions.     

Dissociative electron attachment to dinitrogen pentoxide, N2O5

Electron attachment was studied in gaseous dinitrogen pentoxide, N(2)O(5), for incident electron energies between a few meV and 10 eV. No stable parent anion N(2)O(5) (-) was observed but several anionic fragments (NO(3) (-), NO(2) (-), NO(-), O(-), and O(2) (-)) were detected using quadrupole mass spectrometry. Many of these dissociative pathways were found to be coupled and provide detailed information on the dynamics of N(2)O(5) fragmentation. Estimates of the cross sections for production of each of the anionic fragments were made and suggest that electron attachment to N(2)O(5) is amongst the most efficient attachment reactions recorded for nonhalogenated polyatomic systems.     

Intramolecular electron transfer in heterosubstituted benzene derivatives as probed by dissociative electron attachment

The electron transmission and dissociative electron attachment spectra of the 1-chloroalkyl benzene derivatives, C(6)H(5)(CH(2))(3)Cl and C(6)H(5)(CH(2))(4)Cl, and of the sulfur and silicon derivatives, C(6)H(5)SCH(2)Cl, C(6)H(5)Si(CH(3))(2)CH(2)Cl and C(6)H(5)CH(2)Si(CH(3))(2)CH(2)Cl, are presented for the first time. The relative Cl(-) fragment anion currents generated by electron attachment to the benzene pi* LUMO are measured in the series C(6)H(5)(CH(2))(n)Cl, with n = 1-4, and in the heteroatomic compounds. The Cl(-) yield reflects the rate of intramolecular electron transfer between the pi-system and the remote chlorine atom, which in turn depends on the extent of through-bond coupling between the localized pi* and sigma*(Cl-C) orbitals. In compounds C(6)H(5)(CH(2))(n)Cl the Cl(-) current rapidly decreases with increasing length of the saturated chain. This decrease is significantly attenuated when a carbon atom of the alkyl skeleton is replaced with a third-row heteroatom. This greater ability to promote through-bond coupling between the pi* and sigma*(Cl-C) orbitals is attributed to the sizably lower energy of the empty sigma*(S-C) and sigma*(Si-C) orbitals with respect to the sigma*(C-C) orbitals. In the sulfur derivative the increase of the Cl(-) current is larger than in the silicon analogue. In this case, however, other negative fragments are observed, due to dissociation of the S-C bonds.     

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.