Isomerization and fragmentation reactions of gaseous dimethyl phenylarsane radical cations and methyl phenylarsenium cations. A study by tandem mass spectrometry and density functional theory calculations

The unimolecular reactions of the radical cation of dimethyl phenylarsane, C6H5As(CH3)2, 1*+ and of the methyl phenylarsenium cation, C6H5As+CH3, 2+, in the gas phase were investigated using deuterium labeling and methods of tandem mass spectrometry. Additionally, the rearrangement and fragmentation processes were analyzed by density functional theory (DFT) calculations at the level UBHLYP/6- 311+G(2d,p)//UBHLYP/5-31+G(d). The molecular ion 1*+ decomposes by loss of a .CH3 radical from the As atom without any rearrangement, in contrast to the behavior of the phenylarsane radical cation. In particular, no positional exchange of the H atoms of the CH3 group and at the phenyl ring is observed. The results of DFT calculations show that a rearrangement of 1*+ by reductive elimination of As and shift of the CH3 group is indeed obstructed by a large activation barrier. The MIKE spectrum of 2+ shows that this arsenium cation fragments by losses of H2 and AsH. The fragmentation of the trideuteromethyl derivative 2-d3+ proves that all H atoms of the neutral fragments originate specifically from the methyl ligand. Identical fragmentation behavior is observed for metastable m-tolyl arsenium cation, m-CH3C6H4As+H, 2tol+. The loss of AsH generates ions C7H7+ which requires rearrangement in 2+ and bond formation between the phenyl and methyl ligands prior to fragmentation. The DFT calculations confirm that the precursor of this fragmentation is the benzyl methylarsenium cation 2bzl+, and that 2bzl+ is also the precursor ion fo the elimination of H2. The analysis of the pathways for rearrangements of 2+ to the key intermediate 2bzl+ by DFT calculations show that the preferred route corresponds to a 1,2-H shift of a H atom from the CH3 ligand to the As atom and a shift of the phenyl group in the reverse direction. The expected rearrangement by a reductive elimination of the As atom, which is observed for the phenylarsenium cation and for halogeno phenyl arsenium cations, requires much more activation enthalpy.     

Reaction dynamics of phenyl radicals (C6H5) with propylene (CH3CHCH2) and its deuterated isotopologues

The reactions between phenyl radicals (C6H5) and propylene (CH3CHCH2) together with its D6- and two D3-isotopologues were studied under single collision conditions using the crossed molecular beams technique. The chemical dynamics inferred from the center-of-mass translational and angular distributions suggests that the reactions are indirect and initiated by an addition of the phenyl radical to the alpha-carbon atom (C1 carbon atom) of the propylene molecule at the =CH2 unit to form a radical intermediate (CH3CHCH2C6H5) on the doublet surface. Investigations with D6-propylene specified that only a deuterium atom was emitted; the phenyl group was found to stay intact. Studies with 1,1,2-D3- and 3,3,3-D3-propylene indicated that the initial collision complexes CH3CDCD2C6H5 (from 1,1,2-D3-propylene) and CD3CHCH2C6H5 (from 3,3,3-D3-propylene) eject both a hydrogen atom via rather loose exit transition states to form the D3-isotopomers of cis/trans-1-phenylpropene (CH3CHCHC6H5) (80-90%) and 3-phenylpropene (H2CCHCH2C6H5) (10-20%), respectively. Implications of these findings for the formation of polycyclic aromatic hydrocarbons (PAHs) and their precursors in combustion flames are discussed.     

Dissociation mechanisms of neutral methyl stearate and its hydrogen atom adduct formed from the respective positive ions by electron transfer

The mechanism of dissociation of neutral methyl stearate and its hydrogen atom adduct was investigated by charge inversion mass spectrometry using an alkali metal target. Migrations of functional groups in fatty acid ester ions are often observed during the dissociation of the cations in collisionally activated dissociation (CAD). In the charge inversion spectrum, the main dissociation channels of methyl stearate molecule are the loss of a CH3 radical or a H atom. To identify the source of the CH3 radical and the H atom, the charge inversion spectra of partially deuterated methyl stearate (C17H35COOCD3) were measured. The loss of CH3 occurred through elimination from the methoxy methyl group and that of H occurred through elimination from the hydrocarbon chain of the fatty acid group. In the protonated ester, a simultaneous loss of CH3 (from the methoxy methyl group) and a H atom or a H2 molecule was observed. The charge inversion process gave the dissociation fragments with almost no migration of atoms. Only a few peaks that were structure sensitive were observed in the higher mass region in the charge inversion spectra; these peaks were associated with dissociations of energy-selected neutral species, unlike the case of CAD spectra in which they result from dissociation of ions. Charge inversion mass spectrometry with alkali metal targets provided direct information on the dissociation mechanism of methyl stearate and its hydrogen atom adduct without any migration of functional groups.     

PAH formation under single collision conditions: reaction of phenyl radical and 1,3-butadiene to form 1,4-dihydronaphthalene

The crossed beam reactions of the phenyl radical (C(6)H(5), X(2)A(1)) with 1,3-butadiene (C(4)H(6), X(1)A(g)) and D6-1,3-butadiene (C(4)D(6), X(1)A(g)) as well as of the D5-phenyl radical (C(6)D(5), X(2)A(1)) with 2,3-D2-1,3-butadiene and 1,1,4,4-D4-1,3-butadiene were carried out under single collision conditions at collision energies of about 55 kJ mol(-1). Experimentally, the bicyclic 1,4-dihydronaphthalene molecule was identified as a major product of this reaction (58 ± 15%) with the 1-phenyl-1,3-butadiene contributing 34 ± 10%. The reaction is initiated by a barrierless addition of the phenyl radical to the terminal carbon atom of the 1,3-butadiene (C1/C4) to form a bound intermediate; the latter underwent hydrogen elimination from the terminal CH(2) group of the 1,3-butadiene molecule leading to 1-phenyl-trans-1,3-butadiene through a submerged barrier. The dominant product, 1,4-dihydronaphthalene, is formed via an isomerization of the adduct by ring closure and emission of the hydrogen atom from the phenyl moiety at the bridging carbon atom through a tight exit transition state located about 31 kJ mol(-1) above the separated products. The hydrogen atom was found to leave the decomposing complex almost parallel to the total angular momentum vector and perpendicularly to the rotation plane of the decomposing intermediate. The defacto barrierless formation of the 1,4-dihydronaphthalene molecule involving a single collision between a phenyl radical and 1,3-butadiene represents an important step in the formation of polycyclic aromatic hydrocarbons (PAHs) and their partially hydrogenated counterparts in combustion and interstellar chemistry.     

Gas-phase halonium metathesis and its competitors. Skeletal rearrangements of cationic adducts of saturated ketones

The methyl cation and CF(3)(+) attack saturated, acyclic ketones to make vibrationally excited adduct ions. Despite their high internal energies and short lifetimes, these adducts undergo deep-seated rearrangements that parallel slower processes in solution. Observed pathways include alkene and alkane expulsions, in addition to (in the case of CF(3)(+)) the precedented loss of CF(2)O + HF. For the vast majority of ketones, the principal charged products are the CF(3)(+) adducts of lighter carbonyl compounds, ions that are not easily prepared by other avenues. Evidence for ion structures comes from collisionally activated unimolecular decomposition and bimolecular ion-molecule reactions. Typical examples are di-n-propyl and diisopropyl ketones (both of which produce CH(3)CH=OCF(3)(+) as the principal ion-molecule reaction product) and pentamethylacetone (which produces (CH(3))(2)C=OCF(3)(+) as virtually the sole ion-molecule reaction product). Isotopic labeling experiments account for mechanisms, and DFT calculations provide a qualitative explanation for the relative abundances of products from unimolecular decompositions of the chemically activated CF(3)(+) adduct ions that are initially formed.     

C-C versus C-H bond activation of alkynes by early second-row transition metal atoms

The reactions of Y (a2D), Zr (a3F), Nb (a6D), Mo (a7S), and electronically excited-state Mo* (a5S) with propyne (methylacetylene) and 2-butyne (1,2-dimethylacetylene) were investigated using crossed molecular beams. For all of the metals studied, reactions with propyne led to H2 elimination, forming MC3H2. For Y + propyne, C-C bond cleavage forming YCCH + CH3 also was observed, with an energetic threshold in good agreement with an earlier determination of D0(Y-CCH). For Y + 2-butyne, three reactive channels were observed: YC4H4 + H2, YC3H3 + CH3, and YC3H2 + CH4. The C-C bond cleavage products accounted for 21 and 27% of the total products at Ecoll = 69 and 116 kJ/mol, respectively. For Zr and Nb reactions with 2-butyne, competition between H2 and CH4 elimination was observed, with C-C bond cleavage accounting for 12 and 4% of the total product signal at Ecoll = 71 kJ/mol, respectively. For reactions of Mo and Mo* with 2-butyne, only H2 elimination was observed. The similarity between reactions involving two isomeric species, propyne and allene, suggests that H atom migration is facile in these systems.     

Reaction dynamics on the formation of styrene: a crossed molecular beam study of the reaction of phenyl radicals with ethylene

The reaction dynamics of phenyl radicals (C6H5) with ethylene (C2H4) and D4-ethylene (C2D4) were investigated at two collision energies of 83.6 and 105.3 kJ mol-1 utilizing a crossed molecular beam setup. The experiments suggested that the reaction followed indirect scattering dynamics via complex formation and was initiated by an addition of the phenyl radical to the carbon-carbon double bond of the ethylene molecule forming a C6H5CH2CH2 radical intermediate. Under single collision conditions, this short-lived transient species was found to undergo unimolecular decomposition via atomic hydrogen loss through a tight exit transitions state to synthesize the styrene molecule (C6H5C2H3). Experiments with D4-ethylene verified that in the corresponding reaction with ethylene the hydrogen atom was truly emitted from the ethylene unit but not from the phenyl moiety. The overall reaction to form styrene plus atomic hydrogen from the reactants was found to be exoergic by 25 +/- 12 kJ mol(-1). This study provides solid evidence that in combustion flames the styrene molecule, a crucial precursor to form polycyclic aromatic hydrocarbons (PAHs), can be formed within a single neutral-neutral collision, a long-standing theoretical prediction which has remained to be confirmed by laboratory experiments under well-defined single collision conditions for the last 50 years.     

Trends in the reactions of gaseous phenyl pnictogen radical cations C6H5EH2*+ (E = N, P, As)

In context of an analysis of the effect of the central atom E of gaseous radical cations of phenyl pnictogens C(6)H(5)EH(2), E = N (1), P (2), and As (3), the mass spectrometric reactions of phenyl phosphane 2 have been re-investigated by D-labeling and by using methods of tandem mass spectrometry. The 70 eV mass spectrum of 2 shows the base peak for ion [M-2H](*+) and significant peaks for ions [M-H](+), [M-(2C,3H)](+), [M-PH] (*+), and [M-(C,P,2H)](+). Metastable 2(*+) fragments exclusively by loss of H(2), and the investigation of deuterated 2-d(2) shows that excessive H/D migrations occur before fragmentation. Other significant fragment ions in the mass spectrum of 2 arise by losses of C(2)H(2,) P, or HCP from the ion [M-H](+). This mass spectrometric behavior puts the radical cation 2(*+) in between the fragmentation reactions of aniline radical cation 1(*+) (loss of H and subsequent losses of C(2)H(2,) or HCN) and phenyl arsane radical cation 3(*+) (elimination of H(2) and loss of As from ion [M-H](+)). The fragmentation mechanisms of the radical cations 1(*+) -3(*+) and of related ions were analyzed by calculations of the enthalpy of relevant species at the stationary points of the minimum enthalpy reaction pathways using the DFT hybrid functionals UBHLYP/6-311+G(2d,p)//UBHLYP/6-311+G(d). The results show that, in contrast to ionized aniline 1(*+), the reactions of the derivatives 2(*+) and 3(*+) of the heavier main group elements P and As are characterized by an easy elimination of H(2)via a reductive elimination of group C(6)H(5)-E (E = P, As) and by a special stability of bicyclic isomers of 2(*+) and 3(*+). Thus, while 1(*+) rearranges by ring expansion and formation an 7-aza-tropylium cation by loss of H., the increased stability of bicyclic intermediates in the rearrangement of 2(*+) and in particular of 3(*+) results in separate rearrangement pathways. The origin of these effects is the more extended and diffuse nature of the 3p and 4p AO of P and As.     

Reaction of phenyl radical with propylene as a possible source of indene and other polycyclic aromatic hydrocarbons: an ab initio/RRKM-ME study

Ab initio G3(MP2,CC)//B3LYP/6-311G** calculations have been performed to investigate the potential energy surface (PES) and mechanism of the reaction of phenyl radical with propylene followed by kinetic RRKM-ME calculations of rate constants and product branching ratios at various temperatures and pressures. The reaction can proceed either by direct hydrogen abstraction producing benzene and three C(3)H(5) radicals [1-propenyl (CH(3)CHCH), 2-propenyl (CH(3)CCH(2)), and allyl (CH(2)CHCH(2))] or by addition of phenyl to the CH or CH(2) units of propylene followed by rearrangements on the C(9)H(11) PES producing nine different products after H or CH(3) losses. The H abstraction channels are found to be kinetically preferable at temperatures relevant to combustion and to contribute 55-75% to the total product yield in the 1000-2000 K temperature range, with the allyl radical being the major product (~45%). The relative contributions of phenyl addition channels are calculated to be ~35% at 1000 K, decreasing to ~15% at 2000 K, with styrene + CH(3) and 3-phenylpropene + H being the major products. Collisional stabilization of C(6)H(5) + C(3)H(6) addition complexes is computed to be significant only at temperatures up to 1000-1200 K, depending on the pressure, and maximizes at low temperatures of 300-700 K reaching up to 90% of the total product yield. At T > 1200 K collisional stabilization becomes negligible, whereas the dissociation products, styrene plus methyl and 3-phenylpropene + H, account for up to 45% of the total product yield. The production of bicyclic aromatic species including indane C(9)H(10) is found to be negligible at all studied conditions indicating that the phenyl addition to propylene cannot be a source of polycyclic aromatic hydrocarbons (PAH) on the C(9)H(11) PES. Alternatively, the formation of a PAH molecule, indene C(9)H(8), can be accomplished through secondary reactions after activation of a major product of the C(6)H(5) + C(3)H(6) addition reaction, 3-phenylpropene, by direct hydrogen abstraction by small radicals, such as H, OH, CH(3), etc. It is shown that at typical combustion temperatures 77-90% of C(9)H(9) radicals formed by H-abstraction from 3-phenylpropene undergo a closure of a cyclopentene ring via low barriers and then lose a hydrogen atom producing indene. This results in 7.0-14.5% yield of indene relative to the initial C(6)H(5) + C(3)H(6) reactants within the 1000-2000 K temperature range.     

Reactions of diacetylene radical cation with ethylene

Ion-molecule reactions and energy-resolved mass spectrometry have been used to determine the structures of the products formed in the reaction of diacetylene radical cation with ethylene in a flowing afterglow-triple quadrupole instrument. The structure of the adduct ion, C(6)H(6)(.+), has been determined to be that of singly ionized benzene. The reaction thus presents a first example of the ability of diacetylene radical cation to undergo an aromatic ring forming reaction. The other products formed in the reaction are m/z 52, C(4)H(4)(.+), and m/z 39, C(3)H(3)(+). Isotopic labeling studies show that C(4)H(4)(.+) and C(3)H(3)(+) are formed with nearly statistical hydrogen incorporation, indicating a complex mechanism that scrambles all protons.     

Kinetics and mechanisms of chlorine dioxide oxidation of tryptophan

The reactions of aqueous ClO2 (*) and tryptophan (Trp) are investigated by stopped-flow kinetics, and the products are identified by high-performance liquid chromatography (HPLC) coupled with electrospray ionization mass spectrometry and by ion chromatography. The rates of ClO2 (*) loss increase from pH 3 to 5, are essentially constant from pH 5 to 7, and increase from pH 7 to 10. The reactions are first-order in Trp with variable order in ClO2 (*). Below pH 5.0, the reactions are second- or mixed-order in [ClO2 (*)], depending on the chlorite concentration. Above pH 5.0, the reactions are first-order in [ClO2 (*)] in the absence of added chlorite. At pH 7.0, the Trp reaction with ClO2 (*) is first-order in each reactant with a second-order rate constant of 3.4 x 10(4) M(-1) s(-1) at 25.0 degrees C. In the proposed mechanism, the initial reaction is a one-electron oxidation to form a tryptophyl radical cation and chlorite ion. The radical cation deprotonates to form a neutral tryptophyl radical that combines rapidly with a second ClO 2 (*) to give an observable, short-lived adduct ( k obs = 48 s(-1)) with proposed C(H)-OClO bonding. This adduct decays to give HOCl in a three-electron oxidation. The overall reaction consumes two ClO2 (*) per Trp and forms ClO2- and HOCl. This corresponds to a four-electron oxidation. Decay of the tryptophyl-OClO adduct at pH 6.4 gives five initial products that are observed after 2 min and are separated by HPLC with elution times that vary from 4 to 17 min (with an eluent of 6.3% CH 3OH and 0.1% CH 3COOH). Each of these products is characterized by mass spectrometry and UV-vis spectroscopy. One initial product with a molecular weight of 236 decays within 47 min to yield the most stable product, N-formylkynurenine (NFK), which also has a molecular weight of 236. Other products also are observed and examined.     

Mechanism of the oxidation of aromatic sulfides catalysed by a water soluble iron porphyrin

The oxygen atom transfer-electron transfer (ET) mechanistic dichotomy has been investigated in the oxidation of a number of aryl sulfides by H2O2 in acidic (pH 3) aqueous medium catalysed by the water soluble iron(III) porphyrin 5,10,15,20-tetraphenyl-21H,23H-porphine-p,p',p",p"'-tetrasulfonic acid iron(III) chloride (FeTPPSCl). Under these reaction conditions, the iron-oxo complex porphyrin radical cation, P+. Fe(IV)=O, should be the active oxidant. When the oxidation of a series of para-X substituted phenyl alkyl sulfides (X = OCH3, CH3, H, Br, CN) was studied the corresponding sulfoxides were the only observed product and the reaction yields as well as the reactivity were little influenced by the nature of X as well as by the bulkiness of the alkyl group. Labelling experiments using H(2)18O or H(2)18O2 clearly indicated that the oxygen atom in the sulfoxides comes exclusively from the oxidant. Moreover, no fragmentation products were observed in the oxidation of a benzyl phenyl sulfide whose radical cation is expected to undergo cleavage of the beta C-H and C-S bonds. These results would seem to suggest a direct oxygen atom transfer from the iron-oxo complex to the sulfide. However, competitive experiments between thioanisole (E degree = 1.49 V vs. NHE in H2O) and N,N-dimethylaniline (E degree = 0.97 V vs. NHE in H2O) resulted in exclusive N-demethylation, whereas the oxidation of N-methylphenothiazine (10, E degree = 0.95 V vs. NHE in CH3CN) and N,N-dimethyl-4-methylthioaniline (11, E degree = 0.65 V vs. NHE in H2O) produced the corresponding sulfoxide with complete oxygen incorporation from the oxidant. Since an ET mechanism must certainly hold in the reactions of 10 and 11, the oxygen incorporation experiments indicate that the intermediate radical cation, once formed, has to react with PFe(IV)=O (the reduced form of the iron-oxo complex which is formed by the ET step) in a fast oxygen rebound. Thus, an ET step followed by a fast oxygen rebound is also suggested for the other sulfides investigated in this work.     

Possible molecular hydrogen formation mediated by the radical cations of anthracene and pyrene

Hydrogen molecules cannot be formed readily by the association of gaseous hydrogen atoms. Possible H(2) formation mediated by the radical cations of typical polycyclic aromatic hydrocarbons (PAHs), anthracene and pyrene, was studied at the B3LYP/6-31G** level of theory. We presumed that H(2) is formed by way of two elementary reactions: the addition of an H atom to a PAH molecular cation, and the H abstraction from the resulting monohydro-PAH cation (i.e., arenium ion) by a second H atom to yield H(2). The first reaction takes place without any activation energy. The second reaction is also predicted to proceed along almost barrierless pathways, although it is far from being a typical ion-molecule reaction. There is a possibility that these reactions might constitute one of the mechanisms for H(2) formation in extremely cold interstellar space. Deuterium enrichment in PAH cations is possibly accompanied by such H(2) formation because deuteration lowers the energies of polyatomic PAH cations appreciably.     

Reaction dynamics of phenyl radicals (C6H5, X2A') with methylacetylene (CH3CCH(X1A1)), allene (H2CCCH2(X1A1)), and their D4-isotopomers

Crossed molecular beam experiments were utilized to untangle the reaction dynamics to form 1-phenylmethylacetylene [CH(3)CCC(6)H(5)] and 1-phenylallene [C(6)H(5)HCCCH(2)] in the reactions of phenyl radicals with methylacetylene and allene, respectively, over a range of collision energies from 91.4 to 161.1 kJ mol(-1). Both reactions proceed via indirect scattering dynamics and are initiated by an addition of the phenyl radical to the terminal carbon atom of the methylacetylene and allene reactants to form short-lived doublet C(9)H(9) collision complexes CH(3)CCHC(6)H(5) and C(6)H(5)H(2)CCCH(2). Studies with isotopically labeled reactants and the information on the energetics of the reactions depict that the energy randomization in the decomposing intermediates is incomplete. The collision complexes undergo atomic hydrogen losses via tight exit transition states leading to 1-phenylmethylacetylene [CH(3)CCC(6)H(5)] and 1-phenylallene [C(6)H(5)HCCCH(2)]. The possible role of both C(9)H(8) isomers as precursors to PAHs in combustion flames and in the chemistry of circumstellar envelopes of dying carbon stars is discussed.     

Ion/molecule reactions performed in a miniature cylindrical ion trap mass spectrometer

A recently constructed miniature mass spectrometer, based on a cylindrical ion trap (CIT) mass analyzer, is used to perform ion/molecule reactions in order to improve selectivity for in situ analysis of explosives and chemical warfare agent simulants. Six different reactions are explored, including several of the Eberlin reaction type (M. N. Eberlin and R. G. Cooks, Org. Mass Spectrom., 1993, 28, 679-687) as well as novel gas-phase Meerwein reactions. The reactions include (1) Eberlin transacetalization of the benzoyl, 2,2-dimethyloximinium, and 2,2-dimethylthiooximinium cations with 2,2-dimethyl-1,3-dioxolane to form 2-phenyl-1,3-dioxolanylium cations, 2,2-dimethylamine-1,3-dioxolanylium cations and the 2,2-dimethylamin-1,3-oxathiolanylium cations, respectively; (2) Eberlin reaction of the phosphonium ion CH3P(O)OCH3+, formed from the chemical warfare agent simulant dimethyl methylphosphonate (DMMP), with 1,4-dioxane to yield the 1,3,2-dioxaphospholanium ion, a new characteristic reaction for phosphate ester detection; (3) the novel Meerwein reaction of the ion CH3P(O)OCH3+ with propylene sulfide forming 1,3,2-oxathionylphospholanium ion; (4) the Meerwein reaction of the benzoyl cation with propylene oxide and propylene sulfide to form 4-methyl-2-phenyl-1,3-dioxolane and its thio analog, respectively; (5) ketalization of the benzoyl cation with ethylene glycol to form the 2-phenyl-1,3-dioxolanylium cation; (6) addition/NO2 elimination involving benzonitrile radical cation in reaction with nitrobenzene to form an arylated nitrile, a diagnostic reaction for explosives detection and (7) simple methanol addition to the C7H7+ ion, formed by NO2 loss from the molecular ion of p-nitrotoluene to form an intact adduct. Evidence is provided that these reactions occur to give the products described and their potential analytical utility is discussed.     

The "missing link": the gas-phase generation of platinum-methylidyne clusters Pt(n)CH+ (n=1, 2) and their reactions with hydrocarbons and ammonia

Electrospray ionization (ESI) of tetrameric platinum(II) acetate, [Pt(4)(CH(3)COO)(8)], in methanol generates the formal platinum(III) dimeric cation [Pt(2)(CH(3)COO)(3)(CH(2)COO)(MeOH)(2)](+), which, upon harsher ionization conditions, sequentially loses the two methanol ligands, CO(2), and CH(2)COO to form the platinum(II) dimer [Pt(2)(CH(3)COO)(2)(CH(3))](+). Next, intramolecular sequential double hydrogen-atom transfer from the methyl group concomitant with the elimination of two acetic acid molecules produces Pt(2)CH(+) from which, upon even harsher conditions, PtCH(+) is eventually generated. This degradation sequence is supported by collision-induced dissociation (CID) experiments, extensive isotope-labeling studies, and DFT calculations. Both PtCH(+) and Pt(2)CH(+) react under thermal conditions with the hydrocarbons C(2)H(n) (n=2, 4, 6) and C(3)H(n) (n=6, 8). While, in ion-molecule reactions of PtCH(+) with C(2) hydrocarbons, the relative rates decrease with increasing n, the opposite trend holds true for Pt(2)CH(+). The Pt(2)CH(+) cluster only sluggishly reacts with C(2)H(2), but with C(2)H(4) and C(2)H(6) dihydrogen loss dominates. The reactions with the latter two substrates were preceded by a complete exchange of all of the hydrogen atoms present in the adduct complex. The PtCH(+) ion is much less selective. In the reactions with C(2)H(2) and C(2)H(4), elimination of H(2) occurs; however, CH(4) formation prevails in the decomposition of the adduct complex that is formed with C(2)H(6). In the reaction with C(2)H(2), in addition to H(2) loss, C(3)H(3)(+) is produced, and this process formally corresponds to the transfer of the cationic methylidyne unit CH(+) to C(2)H(2), accompanied by the release of neutral Pt. In the ion-molecule reactions with the C(3) hydrocarbons C(3)H(6) and C(3)H(8), dihydrogen loss occurs with high selectivity for Pt(2)CH(+), but in the reactions of these substrates with PtCH(+) several reaction routes compete. Finally, in the ion-molecule reactions with ammonia, both platinum complexes give rise to proton transfer to produce NH(4)(+); however, only the encounter complex generated with PtCH(+) undergoes efficient dehydrogenation of the substrate, and the rather minor formation of CNH(4)(+) indicates that C-N bond coupling is inefficient.     

Reactions of C2H with 1- and 2-butynes: an ab initio/RRKM study of the reaction mechanism and product branching ratios

Ab initio CCSD(T)/cc-pVTZ(CBS)//B3LYP/6-311G** calculations of the C(6)H(7) potential energy surface are combined with RRKM calculations of reaction rate constants and product branching ratios to investigate the mechanism and product distribution in the C(2)H + 1-butyne/2-butyne reactions. 2-Ethynyl-1,3-butadiene (C(6)H(6)) + H and ethynylallene (C(5)H(4)) + CH(3) are predicted to be the major products of the C(2)H + 1-butyne reaction. The reaction is initiated by barrierless ethynyl additions to the acetylenic C atoms in 1-butyne and the product branching ratios depend on collision energy and the direction of the initial C(2)H attack. The 2-ethynyl-1,3-butadiene + H products are favored by the central C(2)H addition to 1-butyne, whereas ethynylallene + CH(3) are preferred for the terminal C(2)H addition. A relatively minor product favored at higher collision energies is diacetylene + C(2)H(5). Three other acyclic C(6)H(6) isomers, including 1,3-hexadiene-5-yne, 3,4-hexadiene-1-yne, and 1,3-hexadiyne, can be formed as less important products, but the production of the cyclic C(6)H(6) species, fulvene, and dimethylenecyclobut-1-ene (DMCB), is predicted to be negligible. The qualitative disagreement with the recently measured experimental product distribution of C(6)H(6) isomers is attributed to a possible role of the secondary 2-ethynyl-1,3-butadiene + H reaction, which may generate fulvene as a significant product. Also, the photoionization energy curve assigned to DMCB in experiment may originate from vibrationally excited 2-ethynyl-1,3-butadiene molecules. For the C(2)H + 2-butyne reaction, the calculations predict the C(5)H(4) isomer methyldiacetylene + CH(3) to be the dominant product, whereas very minor products include the C(6)H(6) isomers 1,1-ethynylmethylallene and 2-ethynyl-1,3-butadiene.     

Carbon-carbon bond formation by radical addition-fragmentation reactions of O-alkylated enols

Alpha-tert-butoxystyrene [H2C=C(OBut)Ph] reacts with alpha-bromocarbonyl or alpha-bromosulfonyl compounds [R1R2C(Br)EWG; EWG =-C(O)X or -S(O2)X] to bring about replacement of the bromine atom by the phenacyl group and give R1R2C(EWG)CH2C(O)Ph. These reactions take place in refluxing benzene or cyclohexane with dilauroyl peroxide or azobis(isobutyronitrile) as initiator and proceed by a radical-chain mechanism that involves addition of the relatively electrophilic radical R1R2(EWG)C* to the styrene. This is followed by beta-scission of the derived alpha-tert-butoxybenzylic adduct radical to give But*, which then abstracts bromine from the organic halide to complete the chain. Alpha-1-adamantoxystyrene reacts similarly with R1R2C(Br)EWG, at higher temperature in refluxing octane using di-tert-amyl peroxide as initiator, and gives phenacylation products in generally higher yields than are obtained using alpha-tert-butoxystyrene. Simple iodoalkanes, which afford relatively nucleophilic alkyl radicals, can also be successfully phenacylated using alpha-1-adamantoxystyrene. O-Alkyl O-(tert-butyldimethylsilyl) ketene acetals H2C=C(OR)OTBS, in which R is a secondary or tertiary alkyl group, react in an analogous fashion with organic halides of the type R1R2C(Br)EWG to give the carboxymethylation products R1R2C(EWG)CH2CO2Me, after conversion of the first-formed silyl ester to the corresponding methyl ester. The silyl ketene acetals also undergo radical-chain reactions with electron-poor alkenes to bring about alkylation-carboxymethylation of the latter. For example, phenyl vinyl sulfone reacts with H2C=C(OBut)OTBS to afford ButCH2CH(SO2Ph)CH2CO2Me via an initial silyl ester. In a more complex chain reaction, involving rapid ring opening of the cyclopropyldimethylcarbinyl radical, the ketene acetal H2C=C(OCMe2C3H5-cyclo)OTBS reacts with two molecules of N-methyl- or N-phenyl-maleimide to bring about [3 + 2] annulation of one molecule of the maleimide, and then to link the bicyclic moiety thus formed to the second molecule of the maleimide via an alkylation-carboxymethylation reaction.     

Comparison of the Reactivity of the Three Distonic Isomers of the Pyridine Radical Cation Toward Tetrahydrofuran in Solution and in the Gas Phase

The reactivity of the three distonic isomers of the pyridine radical cation toward tetrahydrofuran is compared in solution and in the gas phase. In solution, the distonic ions were generated by UV photolysis at 300 nm from iodo-precursors in acidic 50:50 tetrahydrofuran/water solutions. In the gas phase, the ions were generated by collisionally activated dissociation (CAD) of protonated iodo-precursors in an FT-ICR mass spectrometer, as described in the literature. The same major reaction, hydrogen atom abstraction, was observed in solution and in the gas phase. Attempts to cleave the iodine atom from the 2-iodopyridinium cation in the gas phase and in solution yielded the 2-pyridyl cation in addition to the desired 2-dehydropyridinium cation. In the gas phase, this ion was ejected prior to the examination of the desired ion’s chemical properties. This was not possible in solution. This study suggests that solvation effects are not significant for radical reactions of charged radicals. On the other hand, the even-electron ion studied, the 2-pyridyl cation, shows substantial solvation effects. For example, in solution, the 2-pyridyl cation forms a stable adduct with tetrahydrofuran, whereas in the gas phase, only addition/elimination reactions were observed.      

Absolute rate coefficient determination and reaction mechanism investigation for the reaction of Cl atoms with CH2I2 and the oxidation mechanism of CH2I radicals

The gas-phase reaction of atomic chlorine with diiodomethane was studied over the temperature range 273-363 K with the very low-pressure reactor (VLPR) technique. The reaction takes place in a Knudsen reactor at pressures below 3 mTorr, where the steady-state concentration of both reactants and stable products is continuously measured by electron-impact mass spectrometry. The absolute rate coefficient as a function of temperature was given by k = (4.70 +/- 0.65) x 10-11 exp[-(241 +/- 33)/T] cm3molecule-1s-1, in the low-pressure regime. The quoted uncertainties are given at a 95% level of confidence (2sigma) and include systematic errors. The reaction occurs via two pathways: the abstraction of a hydrogen atom leading to HCl and the abstraction of an iodine atom leading to ICl. The HCl yield was measured to be ca. 55 +/- 10%. The results suggest that the reaction proceeds via the intermediate CH2I2-Cl adduct formation, with a I-Cl bond strength of 51.9 +/- 15 kJ mol-1, calculated at the B3P86/aug-cc-pVTZ-PP level of theory. Furthermore, the oxidation reactions of CHI2 and CH2I radicals were studied by introducing an excess of molecular oxygen in the Knudsen reactor. HCHO and HCOOH were the primary oxidation products indicating that the reactions with O2 proceed via the intermediate peroxy radical formation and the subsequent elimination of either IO radical or I atom. HCHO and HCOOH were also detected by FT-IR, as the reaction products of photolytically generated CH2I radicals with O2 in a static cell, which supports the proposed oxidation mechanism. Since the photolysis of CH2I2 is about 3 orders of magnitude faster than its reactive loss by Cl atoms, the title reaction does not constitute an important tropospheric sink for CH2I2.