Resonant dissociative electron capture by the simplest amino acids and dipeptides

The formation of ions from amino acids (glycine and alanine) and dipeptides (glycylglycine, alanylalanine, and glycylalanine) under the resonant electron capture conditions was studied by negative ion resonant electron capture mass spectrometry. The isobaric ions were found, their effective yield curves were experimentally separated, and the elemental composition was determined. The thermochemical aspect of ion formation was considered, and probable dissociative channels of fragmentation ion formation and their structures were established on the basis of this aspect. Bond cleavage reactions only and H-shift processes were revealed. The rearrangements occur presumably through the stage of formation of intramolecular hydrogen bonds. The cross-sections of formation of ions [M − H]⁻ were measured in the energy range 1.1–1.3 eV. The metastable decay channels of ions [M − H]⁻ and [M − COOH]⁻ were found in the energy range 4.5–7.5 eV for dipeptides, which enabled establishing the genetic relationship between the parental and daughter ions and revealing hidden fragmentation pathways.     

Ion–molecule reactions and dissociation of glycols in a quadrupole ion trap mass spectrometer: Evidence of intramolecular interactions

Polyethylene glycols react with CH₃OCH₂⁺ ions from dimethyl ether to form [M + 13]⁺ products. The [M + 13]⁺ ions are stabilized by intramolecular interactions involving the internal ether oxygen atoms and the terminal methylene group. Collisionally activated dissociation (CAD), including MSⁿ and deuterium labeling experiments show that fragmentation reactions involving intramolecular cyclization are predominant. Scrambling of hydrogen and deuterium atoms in the ion-molecule reaction products is not indicated. The CAD spectra of the [M + 13]⁺ ions provide unambiguous assignment of the glycol size.     

Decompositions of metastable [C6H12O]+ ions with the oxygen on the third carbon: Ring-size preferences in [CnH2nO]+˙ ions

The isomerizations preceding the metastable decompositions in the mass spectrometer of a number of [C₆H₁₂O]⁺˙ ions with the oxygen on the third carbon are characterized utilizing deuterium labeling. Hydrogens are transferred in these ions by three-, five- and six-membered ring rearrangements, with propensities determined by features of the individual reactions. Three-membered ring hydrogen transfers between α and β-carbons are preferred to all five-membered ring hydrogen transfers. However, six-membered ring hydrogen transfers take place to the apparent exclusion of three-membered ring hydrogen transfers to enol carbons when the products are of comparable stability. The low-energy [C₆H₁₂O]⁺˙ isomerizations characterized are predictable from the behavior of their lower homologs. It is concluded that the determinants of these reactions are the same as those of other highly reactive organic intermediates.     

Neutralization-Reionization of alkenylammonium cations: An experimental and ab initio study of intramolecular N-H … C=C interactions in cations and hypervalent ammonium radicals

A series of isomeric hexenylammonium and hexenyldimethylammonium cations were neutralized by collisional electron transfer in the gas phase in an attempt to generate hypervalent ammonium radicals. The radicals dissociated completely on the 4.8–5.4 µs time scale. Radicals in which the hexene double bond was in the 3-, 4-, and 5-positions dissociated by competitive N-H and N=C bond cleavages. Allylic 2-hexen-1-ylammonium and 2-hexen-1-yldimethylammonium radicals underwent predominant cleavages of allylic N-C bonds. Deuterium labeling experiments revealed no intramolecular hydrogen transfer from the hypervalent ammonium group to the hexene double bond. Ab initio and density functional theory calculations showed that alkenylammonium and alkenylmethyloxonium ions preferred hydrogen bonded structures in the gas phase. The stabilization through intramolecular H bonding in 3-buten-1-ylammonium and 3-buten-1-yl methyloxonium ions was calculated by B3LYP/6-311G(2d,p) at 26 and 18 kJ mol^?1, respectively. No intramolecular hydrogen bonding was found for the allylammonium ion. The hypervalent 3-buten-1-yl-methyloxonium radical was calculated to be unbound and predicted to dissociate exothermically by O-H bond cleavage. This dissociation may provide kinetic energy for the hydrogen atom to overcome a small energy barrier for exothermic addition to the double bond. The 3-butten-1-ylammonium and allylammonium radicals were found to be bound and preferred gauche conformations without intramolecular hydrogen bonding. Vertical neutralization of alkenylammonium ions was accompanied by small Franck-Condon effects. The failure to detect stable or metastable hypervalent alkenylammonium radicals was ascribed to the low activation barriers to exothermic dissociations by N-H and N-C bond cleavages.     

Urea and thiourea based anion receptors in solution and on polymer supports

Herein we report the development of a new series of surface bound anion sensors exploiting the urea or thiourea motif capable of binding anions through hydrogen bonding interactions. The use of high resolution magic angle spinning ¹H NMR allows the direct comparison of the anion binding properties of these receptors in solution versus those tethered to polymer resins. Some intramolecular hydrogen bonding and solvent effects were observed at the solution:surface interface however in general the anion binding properties of the polymer bound urea and thiourea receptors were maintained.     

Gas phase reactions of protonated 1,3-diphenylpropyne and some isomeric [C15H13]+ ions

Metastable (3-phenyl-2-propynyl)benzenium ions, generated by electron impact induced fragmentation from the appropriately substituted 1,4-dihydrobenzoic acid, react by loss of ˙CH₃ and C₆H₆. The study of deuterated derivatives reveals that hydrogen/deuterium exchanges involving all hydrogen and deuterium atoms precede the fragmentations. The results suggest a skeletal rearrangement by electrophilic ring-closure reactions giving rise to protonated phenylindene and protonated 9,10-methano-9,10-dihydroanthracene prior to the elimination of C₆H₆ and ˙CH₃, respectively. A study of isomeric [C₁₅H₁₃]⁺ ions by collision-induced decomposition and by deuterium labelling shows that these ions interconvert by hydrogen migrations and skeletal rearrangements.     

Anti-inhibition and Promotion of Radiolytic Hydrogen Formation in Water by Hydroxonium Ions

The ability of hydroxonium ions to enhance the yield of radiolytic hydrogen in aqueous potassium nitrate and potassium chloride solutions is shown. The proposed explanation of the effect is based on the concept of a presolvated electron as a hydrogen precursor. The interception of electrons by H⁺ _aq ions yielding the weakly bound transient species (H⁺ _aq...e^–) retards the hydration of electrons, thus providing a possibility of their longer involvement in hydrogen formation reactions and, hence, enhancement of the yield of hydrogen. The revealed effect is similar to the phenomenon known in positronium chemistry as hydrogen anti-inhibition occurring in a nonpolar liquid containing an electron scavenger, when a second solute that, unlike the first solute, weakly binds electrons is added.     

Mass spectrometry of heterocyclic compounds VIII–electron-impact-induced fragmentation of 1,2-diphenyl-pyrazolidine-3,5-dione and some 4-substituted derivatives

The mass spectra of 1,2-diphenyl-pyrazolidine-3,5-dione and twenty-one 4-substituted derivatives are reported. Their fragmentation patterns have been studied by deuterium labelling, exact mass measurements, metastable studies by the defocusing technique and low energy spectra. Hydrogen rearrangements from the 4-position of the heterocycle and/or from the ß-position of the 4-substituent groups, lead to the main primary fragment ions [C₁₂H₁₁N₂]⁺ (m/e 183) as shown by the metastables. The 4,4-d₂ derivative shows an appreciable isotope effect even for molecular ions decomposing in the ion source. By comparison with the metastable abundances of competitive reactions, the molecular ions (m/e 252) of the 4-unsubstituted compound appear to be structurally different from the corresponding m/e 252 fragment ions formed from 4-derivatives by the loss of 4-substituent with H rearrangement. If only vinylic or aromatic hydrogen atoms are present, primary cleavage of the heterocyclic ring occurs with loss of OH·, C₃O₂ and C₃HO₂. Important rearrangements leading to elimination of C₆H₆N and C₆H₇N are typical for unsaturated substituents on position four having allylic hydrogen atoms. Fragment ions, identical to molecular ions of some compounds discussed here, are obtained by electron-impact and/or thermal decompostion of some complex compounds containing more than one 1,2-diphenyl-pyrazolidine-3,5-dione system. The [C₆H₅N₂]⁺ (m/e 105) and [C₆H₅]⁺ (m/e 77) ions are common fragments of all the title compounds. Any hydrogen scrambling reactions between phenyl and heterocycle or 4-substituent groups can be excluded.     

Ion–molecule reactions between organometallic ions and crown ethers in the gas phase

Ion–molecule reactions of the metal-containing ions LM⁺ (L = (acac)₂, acac, C₆H₆, C₅H₅; M = In, Ga, Co, Fe, Ni, Cr, Mn, Pd, Rh, Tl, La, Pr, Yb, Nd) with crown ethers in the gas phase were studied. Two major reactions were observed: adduct formation and substitution of a metal atom ligand by a crown ether. The relative abundances of the two reactions depends on the ease with which the metal atom may be reduced. Ligand substitution can involve hydrogen rearrangements with loss of acetylacetone or cyclopentadiene for crown ethers having mobile H atom(s). The use of ion–molecule reactions in the structural characterization of crown ethers and transition metalcontaining ions is discussed.     

Isomerization and fragmentation of aliphatic ether radical cations: Interconversion of distonic ions and cyclopropane intermediates

The reactions of propyl ether radical cations close to threshold are initiated by (reversible) formation of γ-disitonic isomers, R$ \mathop {\rm O}\limits^ + $ (H)CH₂CH₂CH₂^·. The three methylene groups in these ions lose their positional identity by ring closure/ring opening via [cyclopropane + alcohol]^+· intermediates. Extensive hydrogen exchange occurs within the C₃-chain. When R is not methyl the γ-distonic isomer undergoes further intramolecular hydrogen atom transfer reactions that lead to formation of α- and β-distonic ions. The α-distonic isomers expel ethyl and propyl radicals by C? O bond cleavage.     

Ion formation by [Li]+ ion attachment in high electric fields

An improved method for ionization by [Li]⁺ ion attachment in high electric fields is described in which gaseous molecules collide with a tungsten wire covered with a lithium salt. The simple mass spectra of a number of compounds which exhibit only [nM + Li]+ ions demonstrate the particular advantage of this ionization method. So far, additional peaks resulting from reactions of the molecules with the salt on the emitter surface have been found only in the mass spectra of acids.     

THE MASS SPECTRA OF PHOSPHONYL COMPOUNDS PART 31. DIMETHYL AND DIETHYL ALKANE- AND SUBSTITUTED-METHANE PHOSPHONATES

Abstract

The electron impact mass spectra of 28 related phosphonates have been determined. Ethyl, iso-propyl and tert-butyl groups which are bound directly to phosphorus, fragment to the corresponding alkenes; similar iso-propyl and tert-butyl groups in the dimethyl esters also fragment to ethylene and propene respectively, i.e. the P-alkyl group rearranges with transfer of the elements of a methyl group to the phosphorus ion. The diethyl alkenephosphonates undergo double hydrogen rearrangements of an ethoxy group to give dihydroxyphosphonium ions. The di- and trihydroxyphosphonium ions have a characteristic fragmentation which involves loss of water. This characteristic has been used as evidence for the rearrangement of a phosphacylium ion to a dihydroxyphosphonium ion. Some other unusual rearrangements involving the combination of the groups bound to phosphorus and oxygen have been observed.     

Bond-forming reactions occurring in azines upon electron impact

Skeletal rearrangement processes of the general type \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm ABC}^{\mathop {\rm + }\limits_{\rm .} } {\rm } \to {\rm AC}^{\mathop {\rm + }\limits_{\rm .} } {\rm + B} $\end{document} in which HCN is eliminated and also bond-forming reactions involving intramolecular aromatic substitution occur in aromatic azines upon electron impact. In addition, hydrogen rearrangements can lead to abundant benzonitrile and benzalimine molecular ions and their substituted analogues. The abundant [M? 1] ions are predominantly formed by loss of the methine hydrogen preceded by complete scrambling of the aromatic and methine hydrogens. The similarities between the processes occurring in azines and those in anils, both in their normal bond cleavages and in some bond-forming reactions are emphasized.     

Electron impact induced ortho effects in 2-nitro-substituted aromatic sulphides containing 2-pyridyl, 2-(5-methylthio-1,3,4-thiadiazolyl) and/or 2-(4-methyl-1,3,4-thiadiazolinyl-5-thione) moieties

The most significant mass spectral features of 14 title compounds are discussed with the aid of deuterium labelling experiments. The decomposition patterns of these compounds are strongly affected by several competing ortho effects, due to the interaction of the nitro function(s) with neighbouring electron-poor N-heterocycles. Very intense polycyclic ions are produced via addition-elimination reactions by loss of simple radicals (H˙, OH˙, NO₂˙) from the molecular ion, followed by the ejection of neutral molecules (HNO₂,CH₃SCN or CH₃NCS). In addition, primary or secondary intramolecular oxygen transfers, preceded or not by hydrogen migration, from the nitro group to the imino carbon via spirocyclic intermediates, are generally observed. Minor skeletal rearrangements, triggered by single or multiple intramolecular oxygen transfer to the bridgehead sulphur atom, followed by SO or SO₂ ejection, are also noticed.     

Polytopal form and isomerism

A thesis is developed that accurate structural data for molecules in the solid state can be utilized to derive direct information about the geometric parameters for solution reaction mechanisms, This is specifically illustrated for intramolecular rearrangements but the basic approach should be applicable to bimolecular reactions. A dihedral angle criterion is employed to quantitatively assess shape parameters for polyhedra found in coordination compounds and cluster molecules. These data are expressed in reaction coordinate form whereby a real structure is related to two idealized polyhedra (a reaction path) or to three or more idealized polyhedra (a reaction cycle or chain). It is demonstrated through an analysis of structural data for five coordinate complexes that the Berry type of rearrangement is the lowest energy physical pathway for rearrangements in ML₅ molecules or ions. Solvents may alter the relative energies of ground and excited state forms but should not significantly alter the physical character of the rearrangement process unless the solvent strongly interacts with the molecules. This feature is discussed with respect to polytopal polymorphism in clusters, e.g., B₈H₈^2?.     

Estimation of gas‐phase acidities of a series of dicarboxylic acids by the kinetic method

The gas‐phase acidities (GA) of a series of dicarboxylic acids were estimated by applying the extended kinetic method. Proton‐bound heterodimeric anions [A^?H⁺B^?] of a series of dicarboxylic acids (A) and reference compounds (B) were generated under electrospray ionization conditions, and the dissociation of these cluster ions was examined using a triple‐quadrupole mass spectrometer. The mass‐selected proton‐bound heterodimeric anions were fragmented to yield individual monomer anions as the only product ions, and the abundance ratios of these anions were used to estimate the GA values of the dicarboxylic acids. The experiment was performed over a range of collision energies in order to more accurately determine GA values and to estimate the differences in the entropy of deprotonation of the dicarboxylic acids and the reference compounds. The trends in GA values obtained for the dicarboxylic acids could imply cyclization of the structures via intramolecular hydrogen bonding. The lower homologues show higher GA values than the higher homologues. The GA order for lower homologues is comparable with that of their solution‐phase pKa1 values. Copyright © 2005 John Wiley & Sons, Ltd.     

Fragmentation of collisionally activated hydroxyphenoxide anions in the gas phase

Low-energy collisionally activated dissociation of O-deprotonated dihydroxybenzenes (catechol, resorcinol, hydroquinone) in the gas phase causes both fragmentation to form [C₆H₄O₂]^– ions by loss of the remaining oxygen-bound hydrogen atom and intramolecular hydrogen atom migration from O to C. The rearranged anions then undergo ring-cleavage reactions which are different in each case. Both catechol and hydroquinone produce fragments which are the result of the loss of two carbon atoms and both oxygen atoms but the proposed mechanisms are different. Resorcinol also produces a fragment which derives from the loss of carbon dioxide. For this process a mechanism is proposed which involves a 6-methylpyranone anion intermediate.     

Loss of hydrogen cyanide from monocyanopyridines upon electron impact: Dewar pyridine structures and ring opening—Ring closure reactions revealed by 13C and 15N Labelling

Extensive ¹³C and ¹⁵N labelling has shown that the molecular ions of 2-, 3- and 4-cyanopyridine with lifetimes up to 10^?6 s eliminate hydrogen cyanide originating predominantly from the ring (?65%). Moreover, this hydrogen cyanide loss occurs after an equilibrated positional interchange of the ring carbon atoms at positions interchange of the ring carbon atoms at positions 2, 4 and 6 via Dewar pyridine structures. In molecular ions with lifetimes of 10^?6–10^?5 s skeletal rearrangements have taken place in such a way that both nitrogen atoms have become equivalent prior to the loss of hydrogen cyanide. Arguments are put forward that this equivalence of nitrogen atoms is caused by the intermediacy of ions with a 1,4-dicyanobuta-1,3-diene structure. About 60% of these intermediate ions eliminate hydrogen cyanide in a fast process. The remaining 40% of these ions undergo ring closure again to a pyridine ring in which the carbon atoms of positions 2, 4 and 6 are positionally interchanged rapidly via Dewar pyridine structures followed by ring opening again and eventual loss of hydrogen cyanide. This interpretation of the ¹³C and ¹⁵N labelling results is further corroborated by a study of the loss of hydrogen cyanide from molecular ions of 1,4-dicyanobuta-1,3-diene labelled with ¹³C in both cyano groups.     

Zundel‐like and Eigen‐like Hydrated Protons on a Platinum Surface

The nature of hydrated protons is an important topic in the fundamental study of electrode processes in acidic environment. For example, it is not yet clear whether hydrated protons are formed in the solution or on the electrode surface in the hydrogen evolution reaction on a Pt electrode. Using mass spectrometry and infrared spectroscopy, we show that hydrogen atoms are converted into hydrated protons directly on a Pt(111) surface coadsorbed with hydrogen and water in ultrahigh vacuum. The hydrated protons are preferentially stabilized as multiply hydrated species (H₅O₂⁺ and H₇O₃⁺) rather than as hydronium (H₃O⁺) ions. These surface‐bound hydrated protons may play an important role in the interconversion between adsorbed hydrogen atoms and solvated protons in solution.     

Some rearrangements initiated by methylene triphenylphosphorane

Treatment of 17α-acetyl-Δ⁴-estren-17β-ol with methylene triphenylphosphorane initiates a d-homo rearrangement followed by the intended Wittig reaction. This rearrangement can be explained by the proton abstracting properties of the applied Wittig reagent. These properties are also responsible for the rearrangements observed when using 17α-acetyl-Δ⁴-estren-17β-ol 17-acetate as a substrate. They lead to an intramolecular condensation of the acetyl- and acetoxy-side-chain. Subsequent reactions depend on the conditions used.