Metastable ion studies. VII—loss of water from the molecular ion of cyclopentanol

Metastable peaks for the loss of water from the molecular ion of cyclopentanol and six ²H labelled analogues have been studied. The results, together with some thermochemical date and a detailed examination of metastable peak shapes, have allowed the identification of the two mechanisms for water loss, both of which take place from the α-cleaved molecular ion. Daughter ion structures are proposed.     

Elimination of water from the carboxyl group of GlyGlyH+

The elimination of water from the carboxyl group of protonated diglycine has been investigated by density functional theory calculations. The resulting structure is identical to the b(2) ion formed in the mass spectrometric fragmentation of protonated peptides (therefore named "b2" in this study). The most stable geometry of the fragment ion ("b2") is an O-protonated diketopiperazine. However, its formation is kinetically disfavored as it requires a free energy of 58.2 kcal/mol. The experimentally observed N-protonated oxazolone is 3.0 kcal/mol less stable. The lowest energy pathway for the formation of the "b2" ion requires a free energy of 37.5 kcal/mol and involves the proton transfer from the amide oxygen of protonated diglycine to the hydroxyl oxygen. Fragmentation initiated by proton transfer from the terminal nitrogen has also a comparable free energy of activation (39.4 kcal/mol). Proton transfer initiating the fragmentation, from the highly basic terminal nitrogen or amide oxygen to the less basic hydroxyl oxygen is feasible at energies reached in usual mass spectrometric experiments. Amide N-protonated diglycine structures are precursors of mainly y(1) ions rather than "b2" ions. In the lowest energy fragmentation channels, proton transfer to the hydroxylic oxygen, bond breaking and formation of an oxazolone ring occur concertedly but asynchronously. Proton transfer to hydroxyl oxygen and cleavage of the corresponding C-O bond take place at the early stages of the fragmentation step, while ring closure to form an oxazolone geometry occurs at the later stages of the transition. The experimentally observed low kinetic energy release is expected to be due to the existence of a strongly hydrogen bonded protonated oxazolone-water complex in the exit channel. Whereas the threshold energy for "b2" ion formation (37.1 kcal/mol) is lower than for the y(1) ion (38.4 kcal/mol), the former requires a tight transition state with an activation entropy, DeltaS++ = -1.2 cal/mol.K and the latter has a loose transition state with DeltaS++ = +8.8 cal/mol.K. This leads to y(1) being the major fragment ion over a wide energy range.     

Expulsion of the CHO radical from the molecular ion of homochromones

For some homochromones, it was confirmed by labelling C-9 with deuterium that the CHO radical expelled from the molecular ion contains either the methine hydrogen or one of the methylene hydrogens of the cyclopropane ring. The origin of the CO group in the expelled CHO radical was also inferred, and a plausible mechanistic interpretation is presented.     

Quantum mechanical/molecular mechanical simulations of the Tl(III) ion in water

Classical molecular dynamics (MD) and combined quantum mechanical/molecular mechanical (QM/MM) MD simulations have been performed to investigate the structural and dynamical properties of the Tl(III) ion in water. A six-coordinate hydration structure with a maximum probability of the Tl-O distance at 2.21 A was observed, which is in good agreement with X-ray data. The librational and vibrational spectra of water molecules in the first hydration shell are blue-shifted compared with those of pure liquid water, and the Tl-O stretching force constant was evaluated as 148 Nm(-1). Both structural and dynamical properties show a distortion of the first solvation shell structure. The second shell ligands' mean residence time was determined as 12.8 ps. The Tl(III) ion can be classified as "structure forming" ion; the calculated hydration energy of -986 +/- 9 kcal mol agrees well with the experimental value of -986 kcal mol.     

Elimination of surfactants and small dyes from water with silica-supported dendritic amphiphiles

Silica-supported branched polyethylenimine(Sil@PEI) is a conventional adsorbent and shows a limited affinity to anionic surfactants and small dyes(K = 106?107 L/mol). If the PEI is alkylated with cetyl groups(C16), the K of the resulting adsorbents(Sil@PEI@C16-x, where x is the fraction of PEI units being alkylated) is significantly improved. Optimization shows that Sil@PEI@C16-0.15 can best reduce aqueous surfactants to a residue around 10?10 mol/L; while Sil@PEI@C16-0.6 can reduce even small aqueous dyes to a residue below 10?10 mol/L, nearly 105-fold lower than that by Sil@PEI. The adsorbents are well recyclable. It is believed that in the case of dyes, the dense cetyl shell can isolate the PEI from the bulky water and thus suppress the competitive binding by water; while in the case of surfactants, the semiclosed cetyl shell can simultaneously meet electrostatic complement and hydrophobic complement to the surfactants.     

The structure of the benzene molecular ion

The kinetic energy released by the [C₆H₆]^+. ion generated (i) by direct ionization of benzene and (ii) by charge-exchange of [C₆H₆]⁺⁺ suggests that the reactive form of the benzene molecular ion has an acyclic structure.     

Moment functions of the hydrogen molecular ion

The moment functions of H⁺₂ are computed for internuclear distances up to 5a₀ from the dipole osciliator strengths.     

Energy-dependent fragmentation of the p-ethyltoluene molecular ion

The p-ethyltoluene molecular ion fragments by loss of either the methyl group attached to the ring or the β-methyl of the ethyl group. Using specific isotopic labeling and charge exchange techniques the relative importance of the losses of the two methyl groups has been studied as a function of internal energy from metastable ions to ions of 7 eV internal energy. The ratio of loss of the β-methyl to loss of the ring-methyl increases from 3.4 for metastable ions to 6.7 for ions of 7 eV internal energy. This variation is interpreted in terms of an energy-dependent competition between fragmentation and hydrogen migration in a dimethylcy-cloheptatriene intermediate. In variable energy collision-induced dissociation studies the ratio of loss of the β-methyl to loss of the ring-methyl decreases with increasing collision energy. It is speculated that this different behavior is related to the differences in preparation of the molecular ions. Those subjected to collisional activation are prepared initially as ground-state ethyltoluene ions and are vibrationally excited upon collision and may fragment largely from the original structure. By contrast, in the charge exchange and electron impact experiments the molecular ions with sufficient energy to fragment are probably formed initially in electronically excited states and the rearrangement to the cycloheptatriene structure may be more facile in these excited states or during the internal conversion processes leading to vibrationally excited ground-state ions.     

Collision-induced dissociation of the phenylsilane molecular ion

The high-energy collision-induced dissociation of the phenylsilane molecular ion generated by electron ionization has been investigated using tandem mass spectrometry (MS/MS). It was observed that the dissociation of the molecular ion (M(+*)) occurs mainly via [M-H](+), [M-2H](+*), and [M-3H](+), followed by two consecutive losses of C(2)H(2). The structures of the precursors for the [M-CH(3)](+), [M-SiH](+), and [M-SiH(2)](+*) ions are proposed. The data suggest that the molecular ion undergoes rearrangements to several isomers prior to dissociation, including the ion containing a five-membered carbon ring. Reaction mechanisms are proposed for the dissociations via the isomeric molecular ions.     

The problem of the azatropylium ion: Hydrogen elimination from the molecular ion of γ-picoline

Comparison of the mass spectrum of γ-picoline with that of analogues, specifically deuterated in the methyl group, the ring or both, has shown that the molecular ion loses the α,β and methyl hydrogen atoms (ω-hydrogen') in the ratio 1.43: 1: 1. 13. The same trend is found for the elimination of HCN from the molecular ion. Moreover, a value of 1.52 is found for the isotope effect, expressing the favoured loss of H over D, irrespective of its position.     

Chronopotentiometric studies of the tetrabutylammonium ion transfer from water to 1,2-dichloroethane

Electrochemically reduced graphite in contact with tetralkylammonium ions/DMF may react with electrophiles either as a nucleophile or as a reducing reagent. The reaction at the interface or at the immediately accessible active centres of the crystal causes some modification of the structure, which is studied here. This method allows the formation of polybutylated or poly-carboxylated graphite and may lead to other modified crystals of great interest for building modified electrodes.     

Fluoride ion capture from water with a cationic borane

The reaction of the [Li(THF)4]+ salt of dimesityl-1,8-naphthalenediylborate with [Me2NCH2]I affords a borane (1-(Mes2B)-8-(Me2NCH2)-C10H6) which can be converted into a cationic borane [3]+ ([1-(Mes2B)-8-(Me3NCH2)-C10H6]+) by methylation with MeOTf. This cationic borane promptly complexes fluoride to afford the corresponding zwitterionic ammonium/fluoroborate 3-F (1-(Mes2FB)-8-(Me3NCH2)-C10H6). Cation [3]+ fails to react with chloride, bromide, and iodide indicating that fluoride complexation is selective. Structural, spectroscopic, and computational studies carried out on 3-F show the existence of an unusual C-H...F-B hydrogen bond. Remarkably, [3]+ captures fluoride from water under biphasic conditions (H2O/CHCl3) to form 3-F. The high fluoride affinity of [3]+ can be correlated to the Coulombic forces which stabilize the B-F bond against heterolysis.     

Detection of fast molecular beams by ion ejection from a surface

When superthermal beams of non-alkali-containing molecules strike a pre-treated, oxidized Re filament at 1200 K, positive ions (particularly those of alkali contaminants in the filament) are ejected from the surface, rendering the neutral beams detectable. Within the energy range studied (a few eV) the efficiency of ion production increases approximately exponentially with the translational energy of the incident neutral molecule. Maximum ionization efficiencies were in the range 0.1–10%, for compounds with ionization potentials (IP's) ? 10 eV. For those of IP > 10 eV, ionization efficiencies were unobservably low (? 10^?6), as were yields of negative ions (or electrons) for any compound.     

Elimination of interference of aluminum ion by a novel resin prepared from desferrioxamine and polyallylamine beads and its application to the determination of fluoride ion

A novel functional resin for the selective adsorption of aluminum was prepared from desferrioxamine(DF) and polyallylamine(PAA) resin. The primary amino group of DF was conjugated with PAA by glutaraldehyde(GA). The resin was found to be effective for the elimination of interference caused by aluminum ion on the determination of fluoride ion. A new determination system for low levels of fluoride ion in the presence of aluminum ion was developed by the use of DF-PAA resin combined with anion exchange resin loaded with alizarin fluorine blue sulfonate; 3-[NN-di(carboxymethyl)aminomethyl)-1,2-dihydroxyanthraquinone-5-sulfonate (AFBS) - lanthanum complex, which was a functional resin for selective collection of fluoride ion.     

Keto-to-enol isomerization in the molecular ion of dimethylmethylphosphonate

A mechanism for keto-to-enol isomerization in dimethylmethylphosphonate (DMMP) has been proposed based on deuterium-labeling studies, a model compound and thermodynamic data. An electron ionization study of H/D exchange occurring in CD₃-labeled DMMP suggests that rapid keto-to-enol isomerization occurs in the ion source and a reaction mechanism based on sequential 1,4-H migrations rather than by the direct 1,3-H transfer or by sequential 1,2-H migrations is proposed. The examination of the mass-analyzed ion kinetic energy/collision-induced dissociation spectrum of the methylphosphonic acid molecular ion suggests that keto-to-enol isomerization does not occur for this species and that 1,2- and 1,3-H migrations are not favored. Available thermodynamic data were employed to construct a potential energy surface for keto-to-enol isomerization of the DMMP molecular ion. The thermodynamic data show that the energy barrier to isomerization is below the internal energy required for decomposition of the DMMP molecular ion. Additionally, the ΔH_f° for the intermediate and enolic isbmers are shown to be significantly less than the ΔH_f° for the keto form of the DMMP molecular ion.     

Communication: atomic and molecular Rydbergs from water

We report the formation of energetic neutral Rydberg hydrogen atoms and transient Rydberg molecular ions, [(H(2)O)(q+)](?) in ion-impact dissociation of isolated water molecules. The kinetic energy spectra of the neutral Rydberg H atoms are determined from the complete study of (H(?), H(+), O(+)) dissociation channel. This channel of water dissociation is suggested as a possible additional source of the energetic neutrals detected in upper atmospheres of extra solar planets, and of slow electrons which are known to play a major role in radiation induced damage to living cells.