Transacetalization with gaseous carboxonium and carbosulfonium ions

Primary carboxonium (H2C=O+-R) and carbosulfonium (H2C=S+-R) ions (R = CH3, C2H5, Ph) and the prototype five-membered cyclic carboxonium ion are found to react in the gas phase with cyclic acetals and ketals by transacetalization to form the respective O-alkyl-1,3-dioxolanium and S-alkyl-1,3-oxathiolanium ions. The reaction, which competes mainly with proton transfer and hydride abstraction, initiates by O-alkylation and proceeds by ring opening and recyclization via intramolecular displacement of the carbonyl compound previously protected in its ketal form. As indicated by product ion mass spectra, and confirmed by competitive reactions, carbosulfonium ions are, by transacetalization, much more reactive than carboxonium ions. For acyclic secondary and tertiary carboxonium ions bearing acidic alpha-hydrogens, little or no transacetalization occurs and proton transfer dominates. This structurally related reactivity distinguishes primary from both secondary and tertiary ions, as exemplified for the two structural isomers H2C=O+-C2H5 and CH3C(H)=O+-CH3. The prototype five- and six-membered cyclic carboxonium ions react mainly by proton transfer and adduct formation, but the five-membered ring ion also reacts by transacetalization to a medium extent. Upon CID, the transacetalization products of the primary ions often dissociate by loss of formaldehyde, and a +44 u neutral gain/-30 u neutral loss MS3 scan is shown to efficiently detect reactive carboxonium and carbosulfonium ions. Transacetalization with either carboxonium or carbosulfonium ions provides a route to 1,3-oxathiolanes and analogs alkylated selectively either at the sulfur or oxygen atom.     

Mono and double polar [4 + 2(+)] Diels-Alder cycloaddition of acylium ions with O-heterodienes

Gas-phase reactions of acylium ions with alpha,beta-unsaturated carbonyl compounds were investigated using pentaquadrupole multiple-stage mass spectrometry. With acrolein and metacrolein, CH(3)-C(+)(double bond)O, CH(2)(double bond)CH-C(+)(double bond)O, C(6)H(5)-C(+)(double bond)O, and (CH(3))(2)N-C(+)(double bond)O react to variable extents by mono and double polar [4 + 2(+)] Diels-Alder cycloaddition. With ethyl vinyl ketone, CH(3)-C(+)(double bond)O reacts exclusively by proton transfer and C(6)H(5)-C(+)(double bond)O forms only the mono cycloadduct whereas CH(2)(double bond)CH-C(+)(double bond)O and (CH(3))(2)N-C(+)(double bond)O reacts to great extents by mono and double cycloaddition. The positively charged acylium ions are activated O-heterodienophiles, and mono cycloaddition occurs readily across their C(+)(double bond)O bonds to form resonance-stabilized 1,3-dioxinylium ions which, upon collisional activation, dissociate predominantly by retro-addition. The mono cycloadducts are also dienophiles activated by resonance-stabilized and chemically inert 1,3-dioxonium ion groups, hence they undergo a second cycloaddition across their polarized C(double bond)C ring double bonds. (18)O labeling and characteristic dissociations displayed by the double cycloadducts indicate the site and regioselectivity of double cycloaddition, which are corroborated by Becke3LYP/6-311++G(d,p) calculations. Most double cycloadducts dissociate by the loss of a RCO(2)COR(1) molecule and by a pathway that reforms the acylium ion directly. The double cycloadduct of the thioacylium ion (CH(3))(2)N-C(+)(double bond)S with acrolein dissociates to (CH(3))(2)N-C(+)(double bond)O in a sulfur-by-oxygen replacement process intermediated by the cyclic monoadduct. The double cycloaddition can be viewed as a charge-remote type of polar [4 + 2(+)] Diels-Alder cycloaddition reaction.     

Acyclic distonic acylium ions: Dual free radical and acylium ion reactivity in a single molecule

Three gaseous acyclic distonic acylium ions: *CH2-CH2-C+=O, *CH2-CH2-CH2-C+=O, and *CH2=C(CH2)-C+=O, are found to display dual free radical and acylium ion reactivity; with appropriate neutrals, they react selectively either as free radicals with inert charge sites, or (and more pronouncedly) as acylium ions with inert radical sites. The free radical reactivity of the ions is demonstrated via the Kenttamaa reaction: CH3S* abstraction with the spin trap dimethyl disulfide; their ion reactivity by two reactions most characteristic of acylium ions: transacetalization with 2-methyl-1,3-dioxolane and the gas-phase Meerwein reaction, that is, expansion of the three-membered epoxide ring of epichlorohydrin to the five-membered 1,3-dioxolanylium ion ring. In "one-pot" reactions with gaseous mixtures of epichlorohydrin and dimethyl disulfide, the ions react selectively at either site, but more readily at the acylium charge site, to form the two mono-derivatized ions. Further reaction at either the remaining free radical or acylium charge site forms a single bi-derivatized ion as the final product. Becke3LYP/6-31G(d) calculations predict the reactions at the acylium charge sites of the three distonic ions to be highly exothermic, and both the "hot" transacetalization and epoxide ring expansion products of *CH2-CH2-CH2-C+=O to dissociate rapidly by H2C=CH2 loss in overall exothermic processes. The calculations also predict highly spatially separate odd spin and charge sites for the novel cyclic distonic ketal ions formed by the reactions at the acylium charge sites.     

Gas-phase reactions of organic radicals and diradicals with ions

Reactions of polyatomic organic radicals with gas phase ions have been studied at thermal energy using a flowing afterglow-selected ion flow tube (FA-SIFT) instrument. A supersonic pyrolysis nozzle produces allyl radical (CH2CHCH2) and ortho-benzyne diradical (o-C6H4) for reaction with ions. We have observed: [CH2CHCH2 + H3O+ --> C3H6+ + H2O], [CH2CHCH2 + HO- --> no ion products], [o-C6H4 + H3O+ --> C6H5+ + H2O], and [o-C6H4 + HO- --> C6H3- + H2O]. The proton transfer reactions with H3O+ occur at nearly every collision (kII approximately with 10(-9) cm3 s(-1)). The exothermic proton abstraction for o-C6H4 + HO- is unexpectedly slow (kII approximately with 10(-10) cm3 s(-1)). This has been rationalized by competing associative detachment: o-C6H4 + HO- --> C6H5O + e-. The allyl + HO- reaction proceeds presumably via similar detachment pathways.     

Ion-molecule reactions of [C,H(3), S](+) ions and evidence for long-lived triplet CH(3)S(+)

Gas-phase [C, H(3), S](+) ions obtained by electron impact from (CH(3))(2)S at 14 eV undergo two distinct low-pressure ion-molecule reactions with the parent neutral: proton transfer and charge exchange. The kinetics of these reactions studied by Fourier transform ion cyclotron resonance (FT-ICR) techniques clearly suggests the [C, H(3), S](+) species to be a mixture of isomeric ions. While proton transfer is consistent with reagent ions displaying the CH(2)SH(+) connectivity, the observed charge exchange strongly argues for the presence of thiomethoxy cations, CH(3)S(+), predicted to be stable only in the triplet state. Charge exchange reactions are also observed in the reaction of these same [C, H(3), S](+) ions with benzene, toluene and phenetole. For these substrates, the CH(2)SH(+) ions can promote proton transfer and electrophilic methylene insertion in the aromatic ring with elimination of H(2)S. The results obtained for the different substrates suggest that the fraction of long-lived fraction of thiomethoxy cations obtained at 14 eV by electron ionization of dimethyl sulfide amounts to ~(22 -/+ 4)% of the [C, H(3), S](+) fragments.     

Reactions of gaseous acylium ions with 1,3-dienes: further evidence for polar [4 + 2+] Diels-Alder cycloaddition

A novel reaction of acylium and thioacylium ions, polar [4 + 2(+)] Diels-Alder cycloaddition with 1,3-dienes and O-heterodienes, has been systematically investigated in the gas phase (Eberlin MN, Cooks RG. J. Am. Chem. Soc. 1993; 115: 9226). This polar cycloaddition, yet without precedent in solution, likely forms cyclic 2,5-dihydropyrylium ions. Here we report the reactions of gaseous acylium ions [(CH(3))(2)N-C(+)=O, Ph-C(+)=O, (CH(3))(2)N-C(+)=S, CH(3)-C(+)=O, CH(3)CH(2)-C(+)=O, and CH(2)=CH-C(+)=O] with several 1-oxy-substituted 1,3-dienes of the general formula RO-CH=CH-C(R(1))=CH(2), which were performed to collect further evidence for cycloaddition. In reactions with 1-methoxy and 1-(trimethylsilyloxy)-1,3-butadiene, adducts are formed to a great extent, but upon collision activation they mainly undergo structurally unspecific retro-addition dissociation. In reactions with Danishefsky's diene (trans-1-methoxy-3-(trimethylsilyloxy)-1,3-butadiene), adducts are also formed to great extents, but retro-addition is no longer their major dissociation; the ions dissociate instead mainly to a common fragment, the methoxyacryl cation of m/z 85. This fragment ion is most likely formed with the intermediacy of the acyclic adduct, which isomerizes prior to dissociation by a trimethylsilyl cation shift. Theoretical calculations predict that meta cycloadducts bearing 1-methoxy and 1-trimethylsilyloxy substituents are unstable, undergoing barrierless ring opening induced by the charge-stabilizing effect of the 1-oxy substituents. In contrast, for the reactions with 1-acetoxy-1,3-butadiene, both the experimental results and theoretical calculations point to the formation of intrinsically stable cycloadducts, but the intact cycloadducts are either not observed or observed in low abundances. Both the isomeric ortho and meta cycloadducts are likely formed, but the nascent ions dissociate to great extents owing to excess internal energy. The ortho cycloadducts dissociate by ketene loss; the meta cycloadducts undergo intramolecular proton transfer to the acetoxy group followed by dissociation by acetic acid loss to yield aromatic pyrylium ions. Either or both of these dissociations, ketene and/or acetic acid loss, dominate over the otherwise favored retro-Diels-Alder alternative. The pyrylium ion products therefore constitute compelling evidence for polar [4 + 2(+)] cycloaddition since their formation can only be rationalized with the intermediacy of cyclic adducts.     

Competition of proton and electron transfers in gas-phase reactions of hydrogen-containing dications CHX2+ (X = F, Cl, Br, I) with atoms, nonpolar and polar molecules

The competition between proton and electron transfer in reactions of mass-selected dications CHX2+ (X = F, Cl, Br, and I) with rare gas atoms (Ne, Ar, Kr, and Xe) and selected molecular reagents (N2, O2, CO, H2O, and HCl) is studied in the gas phase. In the ion-molecule reactions of CHX2+ dications with atoms and nonpolar molecules, it is the energy balance of electron transfer that acts as the decisive factor: when the exothermicity of electron transfer exceeds 2 eV, this process predominates at the expense of bond-forming proton transfer. In marked contrast, the reactions between these triatomic dications and polar molecules are governed for the benefit of the thermochemically more favored products resulting from proton transfer.     

CH3CH+* formation from some C3H6O+* isomers according to theory

Dissociations to alkane ions in gas phase ion chemistry are rare and poorly characterized. Therefore, the pathways to CH3CH3+* + CO from *CH2CH2O+=CH2 and some of its isomers are investigated by theory. The pathway found for this reaction is *CH2CH2O+=CH2 --> CH3CH2O+=CH* --> [CH3CH2- -H- -CO]+* --> CH3CH+* + CO. The crucial intermediate in this pathway is the stable hydrogen-bonded ion-neutral complex [CH3CH2- -H- -CO]+*, a species held together by a strong hydrogen bond. CH3CH3+* + CO rather than CH3CH3 + CO+* is formed from *CH2CH2O+=CH2 and other C3H6O+* ions because the former pair is much more stable than the latter. The photoionization appearance energies of CH3CH3+* from CH3CH2CHO+* and from CH3CH2CO2H+* demonstrate that the onsets of these reactions are at to just above their thermochemical thresholds, consistent with the intermediacy of ion-neutral complexes. We also found transition states for interconversion of CH3CH2CHO+* and CH3CH2O+=CH* and transformation of CH3CH2C:=OH+* to CH3CH2CHO+*; the latter reaction occurs by a 1,2-H-shift from O to C.     

A novel discharge source of hydronium ions for proton transfer reaction ionization: design, characterization, and performance

A novel ion source based on direct current (d.c.) discharge has been developed for proton transfer reaction ionization operated at relatively high ion drift tube pressure. The shape and geometry of the ion source are designed to maximize overall ion intensity and to minimize interference from sample air. The initial performance of the technique, including speciation and intensity of reagent ions, their stability, and the impact of artifact signals, is evaluated by means of a proton transfer reaction time-of-flight mass spectrometer (PTR-TOFMS) newly built in our laboratory. Intensities of the hydronium (H(3)O(+)) ions are typically (5-7) x 10(5) counts for a 1-min integration time with a duty cycle of approximately 1%. The fluctuations of the ion signals over a period of hours are within 4%. Although the formation of artifact ions from sample air (NO(+) and O(2) (+)), which react with volatile organic compounds (VOCs) and subsequently cause fragmentation, is observed as background signals in addition to hydronium and mono- and di-hydrate H(3)O(+) ions, intensities of both NO(+) and O(2) (+) ions are only approximately 0.5% of those of H(3)O(+) ions. Using our PTR-TOFMS system at a drift tube pressure of approximately 5 Torr, the detection sensitivities are significantly improved and the detection limits for propene, acetaldehyde, acetone, isoprene, benzene, toluene, and p-xylene are estimated to be at the sub-ppbv level for 1-min integration.     

Size-restricted proton transfer within toluene-methanol cluster ions

To understand the interaction between toluene and methanol, the chemical reactivity of [(C6H5CH3)(CH3OH) n=1-7](+) cluster ions has been investigated via tandem quadrupole mass spectrometry and through calculations. Collision Induced Dissociation (CID) experiments show that the dissociated intracluster proton transfer reaction from the toluene cation to methanol clusters, forming protonated methanol clusters, only occurs for n = 2-4. For n = 5-7, CID spectra reveal that these larger clusters have to sequentially lose methanol monomers until they reach n = 4 to initiate the deprotonation of the toluene cation. Metastable decay data indicate that for n = 3 and n = 4 (CH3OH)3H(+) is the preferred fragment ion. The calculational results reveal that both the gross proton affinity of the methanol subcluster and the structure of the cluster itself play an important role in driving this proton transfer reaction. When n = 3, the cooperative effect of the methanols in the subcluster provides the most important contribution to allow the intracluster proton transfer reaction to occur with little or no energy barrier. As n >or= 4, the methanol subcluster is able to form ring structures to stabilize the cluster structures so that direct proton transfer is not a favored process. The preferred reaction product, the (CH3OH)3H(+) cluster ion, indicates that this size-restricted reaction is driven by both the proton affinity and the enhanced stability of the resulting product.     

Reaction dynamics of OH+(3Sigma-)+C2H2 studied with crossed beams and density functional theory calculations

The reactions between OH+(3Sigma-) and C2H2 have been studied using crossed ion and molecular beams and density functional theory calculations. Both charge transfer and proton transfer channels are observed. Products formed by carbon-carbon bond cleavage analogous to those formed in the isoelectronic O(3P)+C2H2 reaction, e.g., 3CH2 + HCO+, are not observed. The center of mass flux distributions of both product ions at three different energies are highly asymmetric, with maxima close to the velocity and direction of the precursor acetylene beam, characteristic of direct reactions. The internal energy distributions of the charge transfer products are independent of collision energy and are peaked at the reaction exothermicity, inconsistent with either the existence of favorable Franck-Condon factors or energy resonance. In proton transfer, almost the entire reaction exothermicity is transformed into product internal excitation, consistent with mixed energy release in which the proton is transferred with both the breaking and forming bonds extended. Most of the incremental translational energy in the two higher-energy experiments appears in product translational energy, providing an example of induced repulsive energy release.     

IR spectra of C2H5(+)-N2 isomers: evidence for dative chemical bonding in the isolated ethanediazonium ion

The potential energy surface (PES) of C(2)H(5)(+)-N(2) is characterized in detail by infrared photodissociation (IRPD) spectroscopy of mass-selected ions in a quadrupole tandem mass spectrometer and ab initio calculations at the MP2/6-311G(2df,2pd) level. The PES features three nonequivalent minima. Two local minima, 1-N(2)(H) and 1-N(2)(C), are adduct complexes with binding energies of D(0) = 18 and 12 kJ/mol, in which the N(2) ligand is weakly bonded by electrostatic forces to either the acidic proton or the electrophilic carbon atom of the nonclassical C(2)H(5)(+) ion (1), respectively. The global minimum 3 is the ethanediazonium ion, featuring a weak dative bond of D(0) = 38 kJ/mol. This interaction strength is sufficient to switch the C(2)H(5)(+) structure from nonclassical to classical. The 1-N(2)(C) isomer corresponds to the entrance channel complex for addition of N(2) to 1 yielding the product 3. This reaction involves a small barrier of 7 kJ/mol as a result of the rearrangement of the C(2)H(5)(+) ion. The partly rotationally resolved IRPD spectrum of C(2)H(5)(+)-N(2) recorded in the C-H stretch range is dominated by four bands assigned to 3 and one weak transition attributed to 1-N(2)(H). The abundance ratio of 1-N(2)(H) and 3 estimated from the IRPD spectrum as ～1% is consistent with the calculated free energy difference of 12 kJ/mol. As the ethanediazonium ion escaped previous mass spectrometric detection, the currently accepted value for the ethyl cation affinity of N(2) is revised from -ΔH(0) = 15.5 ± 1.5 to ～42 kJ/mol. The first experimental identification and characterization of 3 provides a sensitive probe of the electrophilic character and fluxionality of the ethyl cation. Comparison of 3 with related alkanediazonium ions reveals the drastic effect of the size of the alkyl chain on their chemical reactivity, which is relevant in the context of hydrocarbon plasma chemistry of planetary atmospheres and the interstellar medium, as well as alkylation reactions of (bio)organic molecules (e.g., carcinogenesis and mutagenesis of DNA material).     

Gas-phase transuranium chemistry: reactions of actinide ions with alcohols and thiols.

The laser ablation with prompt reaction and detection method was employed to provide a survey of some gas-phase reactions of actinide (M = U, Np, Pu and Am) and lanthanide (M = Tb and Tm) ions, M(+) and MO(1,2)(+), with alcohols, thiols and ethers. Particular attention was given the changing behavior in progressing across the actinide series beyond uranium. With alcohols, ROH, major products included hydroxides and alkoxides, M(OH)(1,2)(+), M(OR)(1,2)(+), MO(OH)(+) and MO(OR)(+); these products are presumed to have resulted from RO&bond;H and R&bond;OH bond cleavage by ablated M(+) and MO(+). The abundance distributions for these elementary products reflected the decrease in stabilities of high oxidation states between U and Am. Other alcohol reaction products included electrostatically bonded adducts, such as HO&bond;Np(+)ellipsisC(3)H(7)OH, sigma-bonded organometallics, such as HO&bond;Pu(+)&bond;C(2)H(5), and pi-bonded organometallics, such as Np(+)&bond;eta(3)-?C(3)H(5)?. In view of the inability of actinide and lanthanide ions to dehydrogenate alkanes, the exhibition of dehydrogenation of the alkyl chain of alcohols, as in HO-Pu(+)-C(3)H(5)O from propanol, suggests a non-insertion mechanism involving complexation of the reactant ion to the alcohol. Whereas O abstraction products from ROH were obfuscated by directly ablated MO(1,2)(+), S abstraction from thiols, RSH, was manifested by the appearance of MS(+), MS(2)(+) and MOS(+). In analogy with OH abstraction from alcohols to produce metal hydroxides, SH abstraction from thiols resulted in hydrosulfides, including Am(SH)(+) and Np(SH)(2)(+). In addition to several other reaction pathways with the thiol reagents, products presumed to be thiolates included Am(C(3)H(7)S)(+) and NpO(C(3)H(7)S) from propanethiol. A primary product of reaction with dimethyl ether were methoxides resulting from C--O bond cleavage, including Am(OCH(3))(+) and Np(OCH(3))(2)(+). With methyl vinyl ether, more complex pathways were exhibited, most of which corresponded to the elimination of stable organic molecules. An ancillary result was the discovery of several small oxide clusters, Am(2)O(n)(+), Np(2)O(n)(+) and AmNpO(n)(+). The compositions and abundance distributions of these clusters reflected the propensity of Np to exist in higher oxidation states than Am; the dominant binary clusters were Am(2)O(2)(+) and Np(2)O(3)(+).     

Gas-phase proton transfer reactions involving multiply charged cytochrome c ions and water under thermal conditions

Investigations of gas-phase proton transfer reactions have been performed on protein molecular ions generated by electrospray ionization (ESI). Their reactions were studied in a heated capillary inlet/reactor prior to expansion into a quadrupole mass spectrometer. Results from investigations involving protonated horse heart cytochrome c and H, O suggest that Coulombit effects can lower reaction barriers as well as aid in entropically driven reactions. For example, the charge state distribution observed by a quadrupole mass spectrometer for multiply protonated cytochrome c without the addition of any reactive gas ranges from 9+ to 19+ , with the [M + 15H]¹⁵⁺ ion being the most intense peak. With the addition of H₂O (proton affinity approximately 170.3±2 kcal/mol) to the capillary reactor at 120°C, the charge state distribution shifts to a lower charge, ranging from 13+ to less than 9+. Under the same conditions with argon (proton affinity approximately 100 kcal/mol) as the reactive gas, no shift in the charge state distribution is observed. The results demonstrate that proton transfer to water can occur for highly protonated molecular ions, a process that would be expected to be highly endothermic for singly protonated molecules (for which Coulombic destabilization is not significant). The results imply that the charge state distribution from ESI is somewhat dependent upon the mechanism and speed of the droplet evaporation/ion desolvation process, which may vary substantially with the ESI/mass spectrometry interface design.     

Kinetics and mechanism of addition of benzylamines to benzylidene-1,3-indandiones in acetonitrile

Nucleophilic addition reactions of benzylamines (BA; XC6H4CH2NH2) to benzylidene-1,3-indandiones (BID; YC6H4CH=C(C=O)2C6H4) have been studied in acetonitrile at 25.0 degrees C. The rate is first-order with respect to BA and BID, and no base catalysis is observed. The structure-reactivity behaviors (k2, rhoX, betaX, and betaY) are intermediate between the two series of addition reactions of BA to beta-nitrostyrene (NS) and benzylidenemalononitrile (BMN) in acetonitrile. The normal kinetic isotope effects, kH/kD > 1.0, involving deuterated BAs (XC6H4CH2ND2) are smaller than those for the reactions of NS and BMN suggesting a somewhat looser bond formation in the transition state. The reaction is predicted to proceed in a single step with concurrent C(alpha)-N bond formation and proton transfer to C(beta). A hydrogen-bonded, four-center type cyclic transition state is proposed.     

Survey on the phase transitions and their effect on the ion-exchange and on the proton-conduction properties of a flexible and robust Zr phosphonate coordination polymer

The flexible zirconium tetraphosphonate coordination polymer with formula Zr(O(3)PCH(2))(2)N-C(6)H(10)-N(O(3)CH(2)P)(2)X(2-x)H(2+x)·nH(2)O (X = H, Li, Na, K, 0 < x < 1, 4 < n < 7.5) (1) possesses an open framework structure with 1D cavities decorated with polar and acids P═O and P-OH groups. 1 has been fully protonated by adding HCl and then subjected to several acid-base ion-exchange reactions with alkaline metals hydroxides. 1 is a very robust coordination polymer because it can be regenerated in H- form using strong acid solutions and ri-exchanged several times without hydrolysis and loss of crystallinity. The flexibility of 1 has been also studied by means of TDXD (temperature dependent X-ray diffraction) evidencing remarkable phase transformations that lead to a different disposition of the water molecules. These transformations also influence the accessibility of the cations on the P-OH groups placed inside the channels and thus the ion-exchange properties. The dependence of the proton conductivity properties on these phase transitions has been also investigated and discussed.     

Double transacetalization of diacylium ions

A novel gas-phase reaction of diacylium ions of the O=C=X(+)=C=O type (X = N, CH) is reported: double transacetalization with cyclic acetals or ketals. The reaction is exothermic and highly efficient, and forms members of a new class of highly charged-delocalized ions: cyclic ionic diketals. Pentaquadrupole double- and triple-stage mass spectrometric (MS(2) and MS(3)) experiments reveal the high double transacetalization reactivity of O=C=N(+)=C=O and O=C=CH(+)=C=O, whereas the synthesis of differently substituted cyclic ionic diketals is performed in MS(3) experiments via sequential mono- and double transacetalization of O=C=N(+)=C=O and O=C=CH(+)=C=O with different acetals. With cyclic acetals, the acylium-thioacylium ion O=C=N(+)=C=S reacts promptly and selectively by mono-transacetalization at its acylium site, but the free thiacylium site of its cyclic ionic ketal is nearly unreactive by double transacetalization. Therefore, only the acylium site of O=C=N(+)=C=S can be efficiently protected by transacetalization. Low-energy MS(3) collision-induced dissociation of the cyclic ionic diketals of O=C=N(+)=C=O and O=C=CH(+)=C=O sequentially frees each of the protected acylium site to form the mono-derivatized ion, and then the fully deprotected diacylium ion.     

Reaction dynamics study of O- + C2H2 with crossed beams and density-functional theory calculations

The reactions between O(-) and C(2)H(2) have been studied using the crossed-beam technique and density-functional theory (DFT) calculations in the collision energy range from 0.35 to 1.5 eV (34-145 kJmol). Both proton transfer and C-O bond formation are observed. The proton transfer channel forming C(2)H(-) is the dominant pathway. The center-of-mass flux distributions of the C(2)H(-) product ions are highly asymmetric, with maxima close to the velocity and direction of the precursor acetylene beam, characteristic of direct reactions. The reaction quantitatively transforms the entire reaction exothermicity into internal excitation of the products, consistent with mixed energy release in which the proton is transferred in a configuration in which both the breaking and the forming bonds are extended. The C-O bond formation channel producing HC(2)O(-) displays a distinctive kinematic picture in which the product distribution switches from predominantly forward scattering with a weak backward peak to sideways scattering as the collision energy increases. At low collision energies, the reaction occurs through an intermediate that lives a significant fraction of a rotational period. The asymmetry in the distribution leads to a lifetime estimate of 600 fs, in reasonable agreement with DFT calculations showing that hydrogen-atom migration is rate limiting. At higher collision energies, the sideways-scattered products arise from repulsive energy release from a bent transition state.     

Gas-phase uranyl-nitrile complex ions

Electrospray ionization was used to generate doubly charged complex ions composed of the uranyl ion and nitrile ligands. The complexes, with general formula [UO2(RCN)n]2+, n = 0-5 (where R=CH3-, CH3CH2-, or C6H5-), were isolated in an ion-trap mass spectrometer to probe intrinsic reactions with H2O. For these complexes, two general reaction pathways were observed: (a) the direct addition of one or more H2O ligands to the doubly charged complexes and (b) charge-reduction reactions. For the latter, the reactions produced uranyl hydroxide, [UO2OH], complexes via collisions with gas-phase H2O molecules and the elimination of protonated nitrile ligands.     

FeCl3: an efficient catalyst for reactions of electron-rich arenes with imines or aziridines

Iron(III) chloride was discovered highly effective as catalyst in the Friedel-Crafts reactions of electron-rich arenes with imines or aziridines. It was found that reactions of imines were highly substrate-dependent, which generated mono- or double-addition products, while arenes reacted with aziridines regioselectively leading to the formation of desired ring-opening products in 2 min with moderate to good yields.