Hydrogen migrations in mass spectrometry. V—loss of olefin from n-propyl esters following chemical ionization

The sources of the migrating hydrogen in the elimination of propylene from the protonated and ethylated n-propyl acetate and n-propyl benzoate molecules have been determined by studying the CH₄ and H₂ chemical ionization mass spectra of esters specifically deuterated in the propyl group. It is shown that the migrating hydrogen originates from C-1 ( ～ 27%), C-2 ( ～ 23%) and C-3 ( ～ 50%) of the propyl group, independent of ester and mode of ionization. It is argued that the observed reaction involves specific competing H-migration reactions from each propyl position to the ether oxygen in a keto-protonated (ethylated) ester molecule.     

Hydrogen migrations in mass spectrometry. II—single and double hydrogen migrations in the electron impact fragmentation of n-propyl benzoate

The sources of the migrant hydrogen atom(s) in reactions (a) and (b) in the electron impact mass spectrum of n-propyl benzoate have been investigated: (a) [C₆H₅CO₂C₃H₇]⁺ →[C₆H₅CO₂H]⁺ + C₃H₆; (b) [C₆H₅CO₂C₃H₇]⁺ → [C₆H₅CO₂H₂]⁺ + C₃H₅^sdot;. Deuterium labelling of the propyl group showed that, for reaction (a) at 70 eV ionizing energy 3 ± 1% of the hydrogen originates from C-1 of the propyl group, 86 ± 4% from C-2 and 11 ± 3% from C-3. The specificity of the transfer from C-2 increases as the internal energy of the fragmenting ions decreases, indicating that the results cannot be rationalized in terms of H/D interchanges between positions in the propyl group, but rather that the reaction involves specific, competing, H transfer reactions from each propyl position, in contrast to the high site specificity characteristic of the McLafferty rearrangement. Reaction (b) involves, almost exclusively, transfer of one hydrogen from C-2 and one from C-3 with only very minor participation of C-1 hydrogens. The [C₆H₅COOH]⁺ ion produced in reaction (a) fragments further to [C₆H₅CO]⁺ + OH. and the labelling results indicate some interchange of the carboxylic hydrogen with (ortho) ring hydrogens for those ions fragmenting in the first drift region. The extent of interchange is less than that observed for fragmentation of the same ion produced by direct ionization of benzoic acid or by reaction (a) in ethyl benzoate.     

A DFT study of proton transfers for the reaction of phenol and hydroxyl radical leading to dihydroxybenzene and H2O in the water cluster

Reactions of phenol and hydroxyl radical were studied under the aqueous environment to investigate the antioxidant characters of phenolic compounds. M06‐2X/6‐311 + G(d,p) calculations were carried out, where proton transfers via water molecules were examined carefully. Stepwise paths from phenol + OH^• + (H₂O)_n (n = 3, 7, and 12) to the phenoxyl radical (Ph O^•) via dihydroxycyclohexadienyl radicals (ipso, ortho, meta, and para OH‐adducts) were obtained. In those paths, the water dimer was computed to participate in the bond interchange along hydrogen bonds. The concerted path corresponding to the hydrogen atom transfer (HAT, apparently Ph OH + OH^• → Ph O^• + H₂O) was found. In the path, the hydroxyl radical located on the ipso carbon undergoes the charge transfer to prompt the proton (not hydrogen) transfer. While the present new mechanism is similar to the sequential proton loss electron transfer (SPLET) one, the former is of the concerted character. Tautomerization reactions of ortho or para (OH)C₆H₅=O + (H₂O)_n → C₆H₄(OH)₂(H₂O)_n were traced with n = 2, 3, 4, and 14. The n = 3 (and n = 14) model of ortho and para was calculated to be most likely by the strain‐less hydrogen‐bond circuit.     

Photochemical Hydrogen Evolution at Metal Centers Probed with Hydrated Aluminium Cations,Al+(H2O)n,n=1–10

Hydrated aluminium cations have been investigated as a photochemical model system with up to ten water molecules by UV action spectroscopy in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Intense photodissociation was observed starting at 4.5 eV for two to eight water molecules with loss of atomic hydrogen, molecular hydrogen and water molecules. Quantum chemical calculations for n=2 reveal that solvation shifts the intense 3s–3p excitations of Al⁺ into the investigated photon energy range below 5.5 eV. During the photochemical relaxation, internal conversion from S₁ to T₂ takes place, and photochemical hydrogen formation starts on the T₂ surface, which passes through a conical intersection, changing to T₁. On this triplet surface, the electron that was excited to the Al 3p orbital is transferred to a coordinated water molecule, which dissociates into a hydroxide ion and a hydrogen atom. If the system remains in the triplet state, this hydrogen radical is lost directly. If the system returns to singlet multiplicity, the reaction may be reversed, with recombination with the hydroxide moiety and electron transfer back to aluminium, resulting in water evaporation. Alternatively, the hydrogen radical can attack the intact water molecule, forming molecular hydrogen and aluminium dihydroxide. Photodissociation is observed for up to n=8. Clusters with n=9 or 10 occur exclusively as HAlOH⁺(H₂O)_n-1 and are transparent in the investigated energy range. For n=4–8, a mixture of Al⁺(H₂O)_n and HAlOH⁺(H₂O)_n-1 is present in the experiment.     

Bis(dimethyl sulfoxide‐S)tetrakis(μ‐p‐hydroxybenzoato‐O:O′)dirhodium(II)–tetrakis(μ‐butyrato‐O:O′)bis(dimethyl sulfoxide‐S)dirhodium(II) cocrystal ethanol disolvate

The title structure, [Rh₂(C₇H₅O₃)₄(C₂H₆OS)₂]·[Rh₂(C₄H₇­O₂)₄(C₂H₆OS)₂]·2C₂H₆O, contains two discrete neutral Rh–Rh dimers cocrystallized as the ethanol disolvate. Each dimer is situated on an inversion center. The butyrate chain displays disorder in one C‐atom position. In each dimer, the di­methyl sulfoxide ligand (dmso) is bound via S, as expected. The ethanol is a hydrogen‐bond acceptor for one p‐hydroxy­benzoate hydroxyl group and acts as a hydrogen‐bond donor to the dmso O atom of a neighboring p‐hydroxy­benzoate dirhodium complex. A third hydrogen bond is formed from the other p‐hydroxy­benzoate hydroxyl group to the dmso O atom of a butyrate–dirhodium complex.     

catena‐Poly­[[(benzoato‐κO)silver(I)]‐μ‐2‐amino­pyrimidine‐κ2N1:N3]

The title complex, [Ag(C₇H₅O₂)(C₄H₅N₃)]_n, is a polymer based on a mononuclear silver(I)‐centered fragment. The Ag^I atom is trigonally coordinated by two N atoms from two 2‐­amino­pyrimidine ligands and one O atom from one benzoate anion, giving zigzag polymeric chains with an [–Ag—N—C—N–]_n backbone running along the a axis. It is proposed that intermolecular hydrogen bonding drives the formation of the chain polymer.     

Unimolecular reactions of ionized methyl allyl ether

The fragmentation of CH₂?CHCH₂OCH₃^+· cation-radicals has been investigated by means of ²H- and ¹³C-labelling experiments and by analysis of collision-induced dissociation spectra. Metastable C₄H₈O^+· species decompose via one of three main channels which involve loss of (a) a hydrogen atom, (b) a methyl radical or (c) a formaldehyde molecule. Extensive, but not complete, exchange of the hydrogen and deuterium atoms in specifically labelled C₄H₈-_nD_nO^+· analogues precedes each of the three fragmentation pathways. The role of distonic ions in the rearrangement steps which bring about hydrogen exchange is discussed. The influence of isotope effects on the relative rates of the major reactions and the associated kinetic energy releases is examined. Only loss of a hydrogen atom is subject to a substantial isotope effect. Elimination of a methyl radical releases a large amount of kinetic energy, as is shown by the broad and dish-topped appearance of the corresponding metastable peak (T_1/2 ≈ 42 kJ mol^?1). The carbon atom of the original methoxy group is specifically expelled in this process. Both the large T_1/2 value and the unusual site selectivity are atypical of methyl and other alkyl radical losses from ionized alkenyl methyl ethers. The carbon atom of the methoxy group also participates specifically in formaldehyde elimination, but the two hydrogen atoms are not always selected from the three contained in the initial methoxy group. The implications of these labelling results for the synchronicity of concert of formaldehyde loss, which can be formu lated as a pericyclic process, is analysed.     

The adsorption of carbon monoxide on a platinum electrode

The adsorption of CO from 0.5 M H₂SO₄ solution on platinum has been studied using CO labelled with C-14. The adsorption of CO on Pt occurs in the potential range of hydrogen adsorption as well as in the double layer region. In the whole potential range the rate of adsorption follows first order kinetics. From the surface concentrations and charges for oxidation of adsorbed species it follows that the product of chemisorption consists at least of two kinds of species. One of them is the COOH radical probably formed by the reaction of CO with water.     

The mechanism of ethylene pyrolysis at small conversions

Kinetic modelling is used in conjunction with measurements of product yields to develop a mechanism for the pyrolysis of ethylene at 896K and ethylene pressures ranging from approximately 3 to 78 kPa. An induction period was observed for all products except H₂, and was followed by a steady rate, which was of second-order for all products except 1,3-C₄H₆, the most abundant product. The mechanism quantitatively accounts for the yields of H₂, CH₄, C₂H₆, C₃H₆, 1-C₄H₈ and 1,3-C₄H₆. The reaction is initiated by disproportionation of C₂H₄ and the product 1,3-C₄H₆ results from decomposition of the C₄H₇ radical, formed by addition of C₂H₃ to C₂H₄. The other organic products that were measured are formed as a result of reactions involving the C₂H₅ radical. The hydrogen is produced by abstraction from C₂H₄ by atomic hydrogen and its rate is controlled by the reaction C₂H₅ → C₂H₄ + H which is nearly equilibrated. The main termination reaction is recombination of C₂H₅. The auto-acceleration which is evident particularly in the yields of H₂, CH₄, C₂ H₆, and C₃H₆ is accounted for by the decomposition of 1-C₄H₈. © 1996 John Wiley & Sons Inc.     

Mechanism of the condensation reaction between 1-aryl- and heteroaryl-1,4-dihydro-3(2H)-isoquinolinones and aldehydes

4-Benzylidene-1-phenyl-1,4-dihydro-3(2H)-isoquinolinone, the intermediary product of the carbonyl condensation reaction between 1-phenyl-1,4-dihydro-3(2H)-isoquinolinone and benzaldehyde, rearranges in the presence of an equivalent quantity of sodium hydride into 4-benzyl-1-phenyl-3(2H)-isoquinolinone. As the possibility of the migration of the hydrogen at C-1 in the form of a proton or a hydrogen atom (radical reaction) was excluded, the mechanism of the rearrangement could be depicted as an intermolecular hydride anion migration. In case of the 1-(4-pyridyl)- and 1-(3-pyridyl)-1,4-dihydro-3(2H)-isoquinolinones, however, the rearrangement can be carried out also in polyphosphoric acid and in this case a proton loss-proton gain mechanism was proved.     

Bis{μ‐(E)‐2‐[(2‐pyridylamino)(2‐pyridylimino)methyl]benzato}bis[acetatozinc(II)] tetrahydrate

In the C₂‐symmetric dinuclear title complex, [Zn₂(C₁₈H₁₃N₄O₂)₂(C₂H₃O₂)₂]·4H₂O, each Zn^II ion is five‐coordinated in a distorted trigonal bipyramidal fashion by one carboxylate O atom from one benzoate ligand, one imine N atom and two pyridyl N atoms from a second benzoate ligand, and one O atom from an acetate anion. The two Zn atoms are bridged by the two benzoate ligands, forming a dinuclear structure with a 14‐membered macrocycle. Adjacent dinuclear units are further connected by extensive hydrogen bonds involving the solvent water molecules, giving a three‐dimensional hydrogen‐bonded framework. The framework can be regarded as an example of the four‐connected node network of the PtS topology.     

Mechanistic Studies on the Gas‐Phase Dehydrogenation of Alkanes at Cyclometalated Platinum Complexes

In the ion/molecule reactions of the cyclometalated platinum complexes [Pt(L? H)]⁺ (L=2,2′‐bipyridine (bipy), 2‐phenylpyridine (phpy), and 7,8‐benzoquinoline (bq)) with linear and branched alkanes C_nH_2n+2 (n=2–4), the main reaction channels correspond to the eliminations of dihydrogen and the respective alkenes in varying ratios. For all three couples [Pt(L? H)]⁺/C₂H₆, loss of C₂H₄ dominates clearly over H₂ elimination; however, the mechanisms significantly differs for the reactions of the “rollover”‐cyclometalated bipy complex and the classically cyclometalated phpy and bq complexes. While double hydrogen‐atom transfer from C₂H₆ to [Pt(bipy? H)]⁺, followed by ring rotation, gives rise to the formation of [Pt(H)(bipy)]⁺, for the phpy and bq complexes [Pt(L? H)]⁺, the cyclometalated motif is conserved; rather, according to DFT calculations, formation of [Pt(L? H)(H₂)]⁺ as the ionic product accounts for C₂H₄ liberation. In the latter process, [Pt(L? H)(H₂)(C₂H₄)]⁺ (that carries H₂ trans to the nitrogen atom of the heterocyclic ligand) serves, according to DFT calculation, as a precursor from which, due to the electronic peculiarities of the cyclometalated ligand, C₂H₄ rather than H₂ is ejected. For both product‐ion types, [Pt(H)(bipy)]⁺ and [Pt(L? H)(H₂)]⁺ (L=phpy, bq), H₂ loss to close a catalytic dehydrogenation cycle is feasible. In the reactions of [Pt(bipy? H)]⁺ with the higher alkanes C_nH_2n+2 (n=3, 4), H₂ elimination dominates over alkene formation; most probably, this observation is a consequence of the generation of allyl complexes, such as [Pt(C₃H₅)(bipy)]⁺. In the reactions of [Pt(L? H)]⁺ (L=phpy, bq) with propane and n‐butane, the losses of the alkenes and dihydrogen are of comparable intensities. While in the reactions of “rollover”‐cyclometalated [Pt(bipy? H)]⁺ with C_nH_2n+2 (n=2–4) less than 15 % of the generated product ions are formed by C? C bond‐cleavage processes, this value is about 60 % for the reaction with neo‐pentane. The result that C? C bond cleavage gains in importance for this substrate is a consequence of the fact that 1,2‐elimination of two hydrogen atoms is no option; this observation may suggest that in the reactions with the smaller alkanes, 1,1‐ and 1,3‐elimination pathways are only of minor importance.     

Hydrogen fluoride vibrational energy distributions for the reactions of fluorine atoms with cyclanes

The hydrogen fluoride infrared chemiluminescence produced by the reactions of fluorine atoms with cyclopropane, cyclopentane, and cyclohexane have been studied. The emission data were used to determine the vibrational energy distributions for the abstraction of hydrogen from the secondary carbon–hydrogen bonds of these small cyclic hydrocarbons. The fraction of reaction exothermicity going into vibrational excitation of hydrogen fluoride was as follows: c-C₃H₆, 45%; c-C₅H₁₀, 53%; c-C₆H₁₂, 49%. The slightly lower fraction for the cyclopropane system may indicate that its radical reorganization energy is not completely available for excitation of product HF.     

catena‐Poly[[[diaqua[3‐(pyridin‐4‐yl)benzoato‐κ2O,O′]terbium(III)]‐bis[μ‐3‐(pyridin‐4‐yl)benzoato‐κ2O:O′]] monohydrate]

In the title compound, {[Tb(C₁₂H₈NO₂)₃(H₂O)₂]·H₂O}_n, the Tb^III cation is in an eight‐coordinate environment, ligated by six carboxylate O atoms from five 3‐(pyridin‐4‐yl)benzoate (L) ligands and by two O atoms from water molecules. The cations are bridged by the carboxylate O atoms of the L ligands to form a two‐stranded polymeric chain which is assembled into a three‐dimensional supramolecular network through regular interchain O—H...N hydrogen bonding. On excitation at 320 nm, the title compound displays a series of emissions, which were assigned to the characteristic electronic transitions of Tb^III.     

A mass spectrometry study of alkanes in air plasma at atmospheric pressure

The positive APCI-mass spectra in air of linear (n-pentane, n-hexane, n-heptane, n-octane), branched [2,4-dimethylpentane, 2,2-dimethylpentane and 2,2,4-trimethylpentane (i-octane)], and cyclic (cyclohexane) alkanes were analyzed at different mixing ratios and temperatures. The effect of air humidity was also investigated. Complex ion chemistry is observed as a result of the interplay of several different reagent ions, including atmospheric ions O₂^+•, NO⁺, H₃O⁺, and their hydrates, but also alkyl fragment ions derived from the alkanes. Some of these reactions are known from previous selected ion/molecule reaction studies; others are so far unreported. The major ion formed from most alkanes (M) is the species [M − H]⁺, which is accompanied by M^+• only in the case of n-octane. Ionic fragments of C _n H_2n ^+1/+ composition are also observed, particularly with branched alkanes: the relative abundance of such fragments with respect to that of [M − H]⁺ decreases with increasing concentration of M, thus suggesting that they react with M via hydride abstraction. The branched C₇ and C₈ alkanes react with NO⁺ to form a C₄H₁₀NO⁺ ion product, which upon collisional activation dissociates via HNO elimination. The structure of t-Bu⁺(HNO) is proposed for such species, which is reasonably formed from the original NO⁺(M) ion/molecule complex via hydride transfer and olefin elimination. Finally, linear alkanes C₅–C₈ give a product ion corresponding to C₄H₇⁺(M), which we suggest is attributed to addition of [M − H]⁺ to C₄H₈ olefin formed in the charge-transfer-induced fragmentation of M. The results are relevant to applications of nonthermal plasma processes in the fields of air depuration and combustion enhancement.     

Poly­[[di­aqua­cobalt(II)]‐di‐μ‐4‐cyano­benzoato‐κ4O:O′]

The title compound, [Co(C₈H₄NO₂)₂(H₂O)₂]_n, which was obtained by the reaction of CoCl₂·6H₂O with 4‐cyano­benzoic acid, is the first two‐dimensional 4‐cyano­benzoate cobalt(II) polymer. The Co atom lies on a centre of symmetry and its coordination polyhedron is a slightly distorted octahedron, defined by two water and four carboxyl­ate O atoms. The 4‐cyano­benzoate (cba) anion is bridging in a syn‐skew coordination mode, which ensures a two‐dimensional architecture with the building block Co₄(cba)₄.     

Polymeric structure of (ethyl­ene­diamine)silver(I) 3‐nitro­benzoate monohydrate

In the ternary title compound, catena‐poly­[[silver(I)‐μ‐ethylenedi­amine‐κ²N:N′] 3‐nitro­benzoate monohydrate], {[Ag(C₂H₈N₂)](C₇H₄NO₄)·H₂O}_n, the Ag atom is bicoordinated in a linear configuration by two different N atoms from two symmetry‐related ethyl­enedi­amine ligands, thus giving linear polymeric chains with an [–Ag—N—C—C—N–]_n backbone running parallel to the a axis. In the crystal packing, these linear chains are interconnected by N—H⃛O and O—H⃛O hydrogen bonds to form layers parallel to the ab plane.     

Molecule/radical reactions prior to ionization under negative chemical ionization conditions

It is shown that alkyl radical species present in CH₄ or iso-C₄H₁₀ plasma can react with substrate molecules to give [M+C_nH_2n] species. These species become evident especially in negative chemical ionization as [M+C_nH_2n]^?˙ and, less obviously, in positive chemical ionization as [M+C_nH_2n+1]⁺ ions which, for example in natural products chemistry, may be mistaken for a series of homologous compounds present in the sample.     

Ion structural studies by ion kinetic energy spectrometry: [C7H7]+, [C7H8]+˙, [C7H7OCH3]+˙

The use of kinetic energy release measurements in the structural characterization of ions formed in the mass spectrometer and in the determination of fragmentation mechanisms is demonstrated. In combination with information on the mode of energy partitioning in some of these reactions this allows the following conclusions: (i) The metastable [C₇H₈]⁸˙ ions formed from toluene, cyclohepatatriene, n-butylbenzene, the three methyl anisoles, methyl tropyl ether and benzyl methyl ether all undergo loss of H˙ from a common structure. (ii) The metastable [C₇H₇]⁺ ions generated from the same sources and from benzyl bromide, benzyl alcohol, p-xylene and ethylbenzene appear to undergo loss of acetylene from both the benzylic and the tropylium structures. (iii) The metastable [C₇H₇OCH₃]⁺˙ ether molecular ions undergo loss of CH₃˙ by two types of mechanism, simple cleavage to give the aryloxy cation (not observed for benzyl methyl ether) and a rearrangement process which appears to lead to protonated tropone as the product. (iv) Loss of formaldehyde from the metastable [C₇H₇OCH₃]⁺˙ molecular ions involves hydrogen transfer via competitive 4- and 5-membered cyclic transition states in the case of the anisoles and in the case of methyl tropyl ether, while for benzyl methyl ether, hydrogen transfer in the nonisomerized molecular ion occurs via a 4-membered cyclic transition state to yield the cycloheptatriene molecular ion.     

Ab initio study of C4H3 potential energy surface and reaction of ground‐state carbon atom with propargyl radical

The potential energy surface for the reaction of the ground‐state carbon atom [C(³P_j)] with the propargyl radical [HCCCH₂(X²B₁)] is investigated using the G2M(RCC,MP2) method. Numerous local minima and transition states for various isomerization and dissociation pathways of doublet C₄H₃ are studied. The results show that C(³P_j) attacks the π system of the propargyl radical at the acetylenic carbon atom and yields the n‐C₄H₃(²A′) isomer i3 after an 1,2‐H atom shift. This intermediate either splits a hydrogen atom and produces singlet diacetylene, [HCCCCH ( p1 )+H] or undergoes (to a minor amount) a 1,2‐H migration to i‐C₄H₃(²A′) i5 , which in turn dissociates to p1 plus an H atom. Alternatively, atomic carbon adds to the triple C?C bond of the propargyl radical to form a three‐member ring C₄H₃ isomer i1 , which ring opens to i3 . Diacetylene is concluded to be a nearly exclusive product of the C(³P_j)+HCCCH₂ reaction. At the internal energy of 10.0 kcal/mol above the reactant level, Rice–Ramsperger–Kassel–Marcus calculations show about 91.7% of HCCCCH comes from fragmentation of i3 and 8.3% from i5 . The other possible minor channels are identified as HCCCC+H₂ and C₂H+HCCH. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1522–1535, 2001