Ketalization of phosphonium ions by 1,4-dioxane: selective detection of the chemical warfare agent simulant DMMP in mixtures using ion/molecule reactions

Phosphonium ions CH(3)P(O)OCH(3)(+) (93 Th) and CH(3)OP(O)OCH(3)(+) (109 Th) react with 1,4-dioxane to form unique cyclic ketalization products, 1,3,2-dioxaphospholanium ions. By contrast, a variety of other types of ions having multiple bonds, including the acylium ions CH(3)CO(+) (43 Th), CH(3)OCO(+) (59 Th), (CH(3))(2)NCO(+) (72 Th), and PhCO(+) (105 Th), the iminium ion H(2)C[double bond]NHC(2)H(5)(+) (58 Th) and the carbosulfonium ion H(2)C[double bond]SC(2)H(5)(+) (75 Th) do not react with 1,4-dioxane under the same conditions. The characteristic ketalization reaction can also be observed when CH(3)P(OH)(OCH(3))(2)(+), viz. protonated dimethyl methylphosphonate (DMMP), collides with 1,4-dioxane, as a result of fragmentation to yield the reactive phosphonium ion CH(3)P(O)OCH(3)(+) (93 Th). This novel ion/molecule reaction is highly selective to phosphonium ions and can be applied to identify DMMP selectively in the presence of ketone, ester, and amide compounds using a neutral gain MS/MS scan. This method of DMMP analysis can be applied to aqueous solutions using electrospray ionization; it shows a detection limit in the low ppb range and a linear response over the range 10 to 500 ppb.     

On the parallel mechanism of the dissociation of energy-selected P(CH3)3+ ions

Energy selected trimethyl phosphine ions were prepared by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. This ion dissociates via H, CH(3), and CH(4) loss, the latter two involving hydrogen transfer steps. The ion time-of-flight distribution and the breakdown diagram are analyzed in terms of the statistical RRKM theory, which includes tunneling. Ab initio and DFT calculations provide the vibrational frequencies required for the RRKM modeling. CH(3) loss could produce both the P(CH(3))(2)(+) by a simple bond dissociation step, and the more stable HP(CH(2))CH(3)(+) ion by a hydrogen transfer step. Quantum chemical calculations are extensively used to uncover the reaction scheme, and they strongly suggest that the latter product is exclusively formed via an isomerization step in the energy range of the experiment. The data analysis, which includes modeling with the trimethyl phosphine thermal energy distribution, provides accurate onset energies for both H (E(0K) = 1024.1 +/- 3.5 kJ/mol) and CH(3) (E(0K) = 1024.8 +/- 3.5 kJ/mol) loss reactions. From this analysis, we conclude that the Delta(f)H(298K) degrees [HP(CH(2))(CH(3))(+)] = 783 +/- 8 kJ/mol and Delta(f)H(298K) degrees [P(CH(2))(CH(3))(2)(+)] = 711 +/- 8 kJ/mol.     

Structure and unimolecular chemistry of protonated sulfur betaines, (CH3)2S(+)(CH2)(n)CO2H (n = 1 and 2)

The fixed charge zwitterionic sulfur betaines dimethylsulfonioacetate (DMSA) (CH(3))(2)S(+)CH(2)CO(2)(-) and dimethylsulfoniopropionate (DMSP) (CH(3))(2)S(+)(CH(2))(2)CO(2)(-) have been synthesized and the structures of their protonated salts (CH(3))(2)S(+)CH(2)CO(2)H···Cl(-) [DMSA.HCl] and (CH(3))(2)S(+)(CH(2))(2)CO(2)H···Pcr(-) [DMSP.HPcr] (where Pcr = picrate) have been characterized using X-ray crystallography. The unimolecular chemistry of the [M+H](+) of these betaines was studied using two techniques; collision-induced dissociation (CID) and electron-induced dissociation (EID) in a hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer. Results from the CID study show a richer series of fragmentation reactions for the shorter chain betaine and contrasting main fragmentation pathways. Thus while (CH(3))(2)S(+)(CH(2))(2)CO(2)H fragments via a neighbouring group reaction to generate (CH(3))(2)S(+)H and the neutral lactone as the most abundant fragmentation channel, (CH(3))(2)S(+)CH(2)CO(2)H fragments via a 1,2 elimination reaction to generate CH(3)S(+)=CH(2) as the most abundant fragment ion. To gain insights into these fragmentation reactions, DFT calculations were carried out at the B3LYP/6-311++G(2d,p) level of theory. For (CH(3))(2)S(+)CH(2)CO(2)H, the lowest energy pathway yields CH(3)S(+)=CH(2)via a six-membered transition state. The two fragment ions observed in CID of (CH(3))(2)S(+)(CH(2))(2)CO(2)H are shown to share the same transition state and ion-molecule complex forming either (CH(3))(2)S(+)H or (CH(2))(2)CO(2)H(+). Finally, EID shows a rich and relatively similar fragmentation channels for both protonated betaines, with radical cleavages being observed, including loss of ˙CH(3).     

Mass spectral characterization of tetracyclines by electrospray ionization,H/D exchange,and multiple stage mass spectrometry

Electrospray ionization (ESI) and collisionally induced dissociation (CID) mass spectra were obtained for five tetracyclines and the corresponding compounds in which the labile hydrogens were replaced by deuterium by either gas phase or liquid phase exchange. The number of labile hydrogens, x, could easily be determined from a comparison of ESI spectra obtained with N2 and with ND3 as the nebulizer gas. CID mass spectra were obtained for [M + H]+ and [M - H]- ions and the exchanged analogs, [M(Dx) + D]+ and [M(Dx) - D]- , and produced by ESI using a Sciex API-III(plus) and a Finnigan LCQ ion trap mass spectrometer. Compositions of product ions and mechanisms of decomposition were determined by comparison of the MS(N) spectra of the un-deuterated and deuterated species. Protonated tetracyclines dissociate initially by loss of H2O (D2O) and NH3 (ND3) if there is a tertiary OH at C-6. The loss of H2O (D2O) is the lower energy process. Tetracyclines without the tertiary OH at C-6 lose only NH3 (ND3) initially. MSN experiments showed easily understandable losses of HDO, HN(CH3)2, CH3 - N=CH2, and CO from fragment ions. The major fragment ions do not come from cleavage reactions of the species protonated at the most basic site. Deprotonated tetracyclines had similar CID spectra, with less fragmentation than those observed for the protonated tetracyclines. The lowest energy decomposition paths for the deprotonated tetracyclines are the competitive loss of NH3 (ND3) or HNCO (DNCO). Product ions appear to be formed by charge remote decompositions of species de-protonated at the C-10 phenol.     

Metastable decomposition and hydrogen migration of ethane dication produced in an intense femtosecond near-infrared laser field

We investigated a formation channel of triatomic molecular hydrogen ions from ethane dication induced by irradiation of intense laser fields (800 nm, 100 fs, ～1 × 10(14) W∕cm(2)) by using time of flight mass spectrometry. Hydrogen ion and molecular hydrogen ion (H,D)(n)(+) (n = 1-3) ejected from ethane dications, produced by double ionization of three types of samples, CH(3)CH(3), CD(3)CD(3), and CH(3)CD(3), were measured. All fragments were found to comprise components with a kinetic energy of ～3.5 eV originating from a two-body Coulomb explosion of ethane dications. Based on the signal intensities and the anisotropy of the ejection direction with respect to the laser polarization direction, the branching ratios, H(+):D(+) = 66:34, H(2)(+):HD(+):D(2)(+) = 63:6:31, and H(3)(+):H(2)D(+):HD(2)(+):D(3)(+) = 26:31:34:9 for the decomposition of C(2)H(3)D(3)(2+), were determined. The ratio of hydrogen molecules, H(2):HD:D(2) = 31:48:21, was also estimated from the signal intensities of the counter ion C(2)(H,D)(4)(2+). The similarity in the extent of H∕D mixture in (H,D)(3)(+) with that of (H,D)(2) suggests that these two dissociation channels have a common precursor with the C(2)H(4)(2+)...H(2) complex structure, as proposed theoretically in the case of H(3)(+) ejection from allene dication [A. M. Mebel and A. D. Bandrauk, J. Chem. Phys. 129, 224311 (2008)]. In contrast, the (H,D)(2)(+) ejection path with a lower extent of H∕D mixture and a large anisotropy is expected to proceed essentially via a different path with a much rapid decomposition rate. For the Coulomb explosion path of C-C bond breaking, the yield ratios of two channels, CH(3)CD(3)(2+)→ CH(3)(+) + CD(3)(+) and CH(2)D(+) + CHD(2)(+), were 81:19 and 92:8 for the perpendicular and parallel directions, respectively. This indicates that the process occurs at a rapid rate, which is comparable to hydrogen migration through the C-C bond, resulting in smaller anisotropy for the latter channel that needs H∕D exchange.     

Structure investigation of sertraline drug and its iodine product using mass spectrometry, thermal analyses and MO-calculations

Sertraline (C(17)H(17)Cl(2)N) as an antidepressant drug was investigated using thermal analysis (TA) measurements (TG/DTG and DTA) in comparison with electron impact (EI) mass spectral (MS) fragmentation at 70eV. Semi-empirical MO-calculations, using PM3 procedure, has been carried out on neutral molecule and positively charged species. These calculations included bond length, bond order, bond strain, partial charge distribution and heats of formation (DeltaH(f)). Also, in the present work sertraline-iodine product was prepared and its structure was investigated using elemental analyses, IR, (1)H NMR, (13)C NMR, MS and TA. It was also subjected to molecular orbital calculations (MOC) in order to confirm its fragmentation behavior by both MS and TA in comparison with the sertraline parent drug. In MS of sertraline the initial rupture occurred was CH(3)NH(2)(+) fragment ion via H-rearrangement while in sertraline-iodine product the initial rupture was due to the loss of I(+) and/or HI(+) fragment ions followed by CH(2)NH(+) fragment ion loss. In thermal analyses (TA) the initial rupture in sertraline is due to the loss of C(6)H(3)Cl(2) followed by the loss of CH(3)-NH forming tetraline molecule which thermally decomposed to give C(4)H(8), C(6)H(6) or the loss of H(2) forming naphthalene molecule which thermally sublimated. In sertraline-iodine product as a daughter the initial thermal rupture is due to successive loss of HI and CH(3)NH followed by the loss of C(6)H(5)HI and HCl. Sertraline biological activity increases with the introduction of iodine into its skeleton. The activities of the drug and its daughter are mainly depend upon their fragmentation to give their metabolites in vivo systems, which are very similar to the identified fragments in both MS and TA. The importance of the present work is also due to the decision of the possible mechanism of fragmentation of the drug and its daughter and its confirmation by MOC.     

Dissociative photoionization of methyl chloride studied with threshold photoelectron-photoion coincidence velocity imaging

Utilizing threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging, dissociation of state-selected CH(3)Cl(+) ions was investigated in the excitation energy range of 11.0-18.5 eV. TPEPICO time-of-flight mass spectra and three-dimensional time-sliced velocity images of CH(3)(+) dissociated from CH(3)Cl(+)(A(2)A(1) and B(2)E) ions were recorded. CH(3)(+) was kept as the most dominant fragment ion in the present energy range, while the branching ratio of CH(2)Cl(+) fragment was very low. For dissociation of CH(3)Cl(+)(A(2)A(1)) ions, a series of homocentric rings was clearly observed in the CH(3)(+) image, which was assigned as the excitation of umbrella vibration of CH(3)(+) ions. Moreover, a dependence of anisotropic parameters on the vibrational states of CH(3)(+)(1(1)A') provided a direct experimental evidence of a shallow potential well along the C-Cl bond rupture. For CH(3)Cl(+)(B(2)E) ions, total kinetic energy released distribution for CH(3)(+) fragmentation showed a near Maxwell-Boltzmann profile, indicating that the Cl-loss pathway from the B(2)E state was statistical predissociation. With the aid of calculated Cl-loss potential energy curves of CH(3)Cl(+), CH(3)(+) formation from CH(3)Cl(+)(A(2)A(1)) ions was a rapid direct fragmentation, while CH(3)Cl(+)(B(2)E) ions statistically dissociated to CH(3)(+) + Cl via internal conversion to the high vibrational states of X(2)E.     

Determination of copper in coal fly ash in the presence of excess titanium by dynamic reaction cell inductively coupled plasma mass spectrometry

Inductively coupled plasma quadrupole mass spectrometry (ICP-QMS) was used for the accurate determination of copper in coal fly ash samples in the presence of excess titanium, using the reaction of Cu(+) ions with NH(3) in the cell. The method eliminated the effect of polyatomic isobaric interferences at m/z 63 and 65 caused by the formation of (47)Ti(16)O(+), (49)Ti(16)O(+) and (47)Ti(18)O(+) on (63)Cu(+) and (65)Cu(+) by detecting Cu(+) as the product cluster ion Cu(NH(3))(2)(+). As the signal of (63)Cu(NH(3))(2)(+) overlapped with that of (97)Mo(+) which existed in the samples, (65)Cu(NH(3))(2)(+) was detected at m/z 99. The effect of the operating conditions of DRC system was studied in order to obtain the best signal to noise ratio for Cu(NH(3))(2)(+) at m/z 99. The formation of Cu(NH(3))(2)(+) was through the clustering reaction Cu(+)+2NH(3)-->Cu(NH(3))(2)(+) which resulted in the separation of analyte from the interfering oxide. The detection limit for Cu(NH(3))(2)(+) was 0.015 ng mL(-1) as Cu. The method was applied to the determination of copper in NIST SRM 1633a and 1633b coal fly ash reference materials. The precision between sample replicates was better than 2.0% and the analysis results were in good agreement with the certified values.     

Ion-molecule reactions of several ions with ethylene oxide and propenal in a selected ion flow tube

The selected ion flow tube (SIFT) technique has been used to investigate the ion-molecule reactions of several ions with the neutral molecules ethylene oxide, CH(2)OCH(2)-c, and propenal, CH(2)CHCHO. Both molecules have been identified in hot-core star forming regions [] and have significance to astrochemical models of the interstellar (ISM) and circumstellar medium (CSM). Moreover, the molecules contain functional groups, such as the epoxide group (ethylene oxide) and an aldehyde group, which are part of a conjugated pi-electron system (propenal) whose reactivities have not been studied in detail in gas-phase ion-molecule reactions. The larger recombination energy ions, Ar(+) and N(2)(+), were reacted with the neutrals to give insight into general fragmentation tendencies. These reactions proceeded via dissociative charge-transfer yielding major fragmentation products of CH(3)(+) and HCO(+) for ethylene oxide and CH(2)CH(+) and HCO(+) for propenal. The amino acids glycine and alanine are of particular interest to astrobiology, especially if they can be synthesized in the gas phase. In an attempt to synthesize amino acid precursors, ethylene oxide and propenal were reacted with NH(n)(+) (n = 1-4) and HCNH(+). As might be expected from the proton detachment energies, NH(+), NH(2)(+), and HCNH(+) reacted via proton transfer. NH(3)(+) reacted with each molecule via H-atom abstraction to produce NH(4)(+), and NH(4)(+) reacted via a ternary association. All binary reactions proceeded near the gas kinetic rate. Several associated molecule switching reactions were performed and implications of these reactions to the structures of the association products are discussed Ikeda et al. and Hollis et al.     

N[bond]C(alpha) bond dissociation energies and kinetics in amide and peptide radicals. Is the dissociation a non-ergodic process?

Dissociations of aminoketyl radicals and cation radicals derived from beta-alanine N-methylamide, N-acetyl-1,2-diaminoethane, N(alpha)-acetyl lysine amide, and N(alpha)-glycyl glycine amide are investigated by combined density functional theory and M?ller-Plesset perturbational calculations with the goal of elucidating the mechanism of electron capture dissociation (ECD) of larger peptide and protein ions. The activation energies for dissociations of N[bond]C bonds in aminoketyl radicals decrease in the series N[bond]CH(3) > N-CH(2)CH(2)NH(2) > N[bond]CH(2)CONH(2) approximately N[bond]CH(CONH(2))(CH(2))(4)NH(2). Transition state theory rate constants for dissociations of N[bond]C(alpha) bonds in aminoketyl radicals and cation-radicals indicate an extremely facile reaction that occurs with unimolecular rate constants >10(5) s(-1) in species thermalized at 298 K in the gas phase. In neutral aminoketyl radicals the N[bond]C(alpha) bond cleavage results in fast dissociation. In contrast, N[bond]C(alpha) bond cleavage in aminoketyl cation-radicals results in isomerization to ion-molecule complexes that are held together by strong hydrogen bonds. The facile N[bond]C(alpha) bond dissociation in thermalized ions indicates that it is unnecessary to invoke the hypothesis of non-ergodic behavior for ECD intermediates.     

Hydrogen bonds in 1,4-dioxane/ammonia binary clusters

With synchrotron radiation, we have studied the photoionization and dissociation of 1,4-dioxane/ammonia clusters in a supersonic expansion. The observed major product ions are the 1,4-dioxane cation M(+) and protonated cluster ions M(NH(3))(n)H(+) (where M=1,4-dioxane), and the intensities of the unprotonated cluster ions M(NH(3))(n) (+) are much lower. Fully optimized geometries and energies of the neutral cluster M(NH(3))(2) and related cluster ions have been obtained using the ab initio molecular orbital method and density functional theory. The potential energy surface of the excited state of M(NH(3))(2) (+) was also calculated. With these results, the mechanisms of different photoionization-dissociation channels have been suggested. The most probable channel is electron ejection from the highest occupied molecular orbital, followed by the dissociation into M(+) and (NH(3))(2). For another main channel, after removing an electron from the second highest occupied molecular orbital, the intracluster proton transfer process takes place to form the stable unprotonated cluster ion M(NH(3))H(+)-NH(2), which usually leads to the dissociated protonated cluster ion M(NH(3))H(+) and a radical NH(2).     

Formation of melamium adducts by pyrolysis of thiourea or melamine/NH4 Cl mixtures

Pyrolysis of prominent precursor compounds for the synthesis of carbon nitride type materials (e.g., melamine, thiourea) have been studied in detail. Molecular adducts containing monoprotonated melamium C(6)N(11)H(10)(+) and melaminium HC(3)N(3)(NH(2))(3)(+) ions, respectively, have been identified as intermediates. The adduct C(6)N(11)H(10)Cl·0.5NH(4)Cl was obtained by the reaction of melamine C(3)N(3)(NH(2))(3) with NH(4)Cl at 450 °C. During the pyrolysis of thiourea, guanidinium thiocyanate was initially formed and subsequently the melamium thiocyanate melamine adduct C(6)N(11)H(10)SCN·2C(3)N(3)(NH(2))(3) was isolated at 300 °C. A second melaminium thiocyanate melamine adduct with the formula HC(3)N(3)(NH(2))(3)SCN·2C(3)N(3)(NH(2))(3) represents an intermediary reaction product that is best accessible at low pressures. The crystal structures of the compounds were solved by single-crystal XRD. Unequivocal proton localization at the C(6)N(11)H(10)(+) ion was established. A typical intramolecular and interannular hydrogen bridge and other characteristic hydrogen-bonding motifs were identified. Additionally, the adducts were investigated by solid-state NMR spectroscopy. Our study provides detailed insight into the thermal condensation of thiourea by identifying and characterizing key intermediates involved in the condensation process leading to carbon nitride type materials. Furthermore, factors promoting the formation of melamium adduct phases over melem are discussed.     

Dissociation of energy selected Sn(CH3)4(+), Sn(CH3)3Cl+, and Sn(CH3)3Br+ ions: evidence for isolated excited state dynamics

The dissociation dynamics of Sn(CH(3))(4)(+), Sn(CH(3))(3)Cl(+), and Sn(CH(3))(3)Br(+) were investigated by threshold photoelectron photoion spectrometry using an electron imaging apparatus (iPEPICO) at the Swiss Light Source. The tetramethyltin ion was found to dissociate via Sn(CH(3))(4)(+) → Sn(CH(3))(3)(+) + CH(3) → Sn(CH(3))(2)(+) + 2CH(3), while the trimethyltin halide ions dissociated via methyl loss at low energies, and a competitive halogen loss at somewhat higher energies. The 0 K methyl loss onset for the three ions was found to be 9.410 ± 0.020 eV, 10.058 ± 0.020 eV, and 9.961 ± 0.020 eV, respectively. Statistical theory could not reproduce the observed onsets for the halogen loss steps in the halotrimethyltin ions. The halide loss signal as a function energy mimicked the excited state threshold photoelectron spectrum, from which we conclude that the halide loss from these ions takes place on an isolated excited state potential energy surface, which we describe by time dependent density functional calculations. The sequential loss of a second methyl group in the Sn(CH(3))(4)(+) ion, observed at about 3 eV higher energies than the first one, is also partially non-statistical. The derived product energy distribution resulting from the loss of the first methyl group is two-component with about 50% being statistical and the remainder associated with high translational energy products that peak at 2 eV. Time dependent DFT calculations show that a dissociative ?B state lies in the vicinity of the experimental measurements. We thus propose that 50% of the Sn(CH(3))(4)(+) ions produced in this energy range internally convert to the ?X state, on which they dissociate statistically, while the remainder dissociate directly from the repulsive ?B state leading to high kinetic energy products.     

Photoionization study of L-valine in the gas phase by vacuum ultraviolet synchrotron radiation

The photoionization and photodissociation of L-valine are studied by tunable synchrotron vacuum ultraviolet photoionization mass spectrometry at the photon energy of 13 eV. The ionization energy of L-valine and the appearance energies of major fragments are measured by the photoionization efficiency spectrum in the photon energy range of 8-11 eV. Possible formation pathways of the major fragments, NH(2)CHC(OH)(2)(+) (m/z=75), NH(2)(CH(3))(2)(CH)(2)(+) (m/z=72) and NH(2)CHCO(+) (m/z=57), are discussed in detail with the theoretical calculations at the B3LYP/6-31++G (d, p) level. Hydrogen migration is considered as the key way for the formation of NH(2)CHC(OH)(2)(+) (m/z=75) and NH(2)CHCO(+) (m/z=57). Furthermore, other fragments, NH(2)CHCOOH(+) (m/z=74), (CH(3))(2)(CH)(2)(+) (m/z=56), C(4)H(7)(+) (m/z=55), NH(2)CHOH(+) (m/z=46), NH(2)CH(2)(+) (m/z=30) and m/z=18, species are also briefly described.     

UV photodissociation of ethylamine cation: a combined experimental and theoretical investigation

Direct current (DC) slice imaging of state-selected ions is combined with high-level ab initio calculations to give insight into reaction pathways, dynamics, and energetics for ethylamine cation photodissociation at 233 nm. These reaction pathways are of interest for understanding the rich chemistry of Titan's ionosphere recently revealed by the Cassini mission. The result for the H-loss product has a bimodal translational energy distribution, indicating two distinct H-loss pathways: these are assigned to triplet CH(3)CH(2)NH(+) product ions and the singlet CH(3)CHNH(2)(+) species. The distribution shows a modest fraction of energy available in translation and is consistent with barrierless dissociation from the ground state. HCNH(+) formation is observed as the dominant channel and exhibits a bimodal translational energy distribution with the faster component depicting a significant angular anisotropy. This suggests a direct excited-state decay pathway for this portion of the distribution. We have also observed the H + H(2) loss product as a minor secondary dissociation channel, which correlates well with the formation of CH(2)CNH(2)(+) ion with an exit barrier.     

Ion chemistry of OV(OCH3)3 in the gas phase: molecular cations and anions and their primary fragmentations

Trimethyl vanadate(V), OV(OCH(3))(3) (1), is examined by various mass spectrometric means. Photoionization experiments yield an ionization energy of IE(OV(OCH(3))(3)) = 9.54 +/- 0.05 eV for the neutral molecule. The primary fragmentation of the molecular cation 1(+), i.e., loss of neutral formaldehyde, can occur via two independent routes of hydrogen migrations to afford the formal V(IV) compounds HOV(OCH(3))(2)(+) and OV(OCH(3))(CH(3)OH)(+), respectively. These two pathways are associated with low-lying activation barriers of almost identical height. At elevated energies, direct V-O bond cleavage of 1(+) allows for expulsion of a methoxy radical concomitant with the generation of the cationic fragment OV(OCH(3))(2)(+), a formal V(V) compound. Trimethyl vanadate can also form a molecular anion, 1(-), whose most abundant dissociation channel involves loss of a methyl radical, thereby leading to the formal V(V) compound OV(OCH(3))(2)O(-). Various mass spectrometric experiments and extensive theoretical studies provide detailed insight into the ion structures and the relative energetics of the primary dissociation reactions of the molecular cations and anions of 1.     

A multicoincidence study of fragmentation dynamics in collision of γ-aminobutyric acid with low-energy ions

Fragmentation of the γ-aminobutyric acid molecule (GABA, NH(2)(CH(2))(3)COOH) following collisions with slow O(6+) ions (v≈0.3?a.u.) was studied in the gas phase by a combined experimental and theoretical approach. In the experiments, a multicoincidence detection method was used to deduce the charge state of the GABA molecule before fragmentation. This is essential to unambiguously unravel the different fragmentation pathways. It was found that the molecular cations resulting from the collisions hardly survive the interaction and that the main dissociation channels correspond to formation of NH(2)CH(2)(+), HCNH(+), CH(2)CH(2)(+), and COOH(+) fragments. State-of-the-art quantum chemistry calculations allow different fragmentation mechanisms to be proposed from analysis of the relevant minima and transition states on the computed potential-energy surface. For example, the weak contribution at [M-18](+), where M is the mass of the parent ion, can be interpreted as resulting from H(2)O loss that follows molecular folding of the long carbon chain of the amino acid.     

Fragmentation reactions of alkylphenylammonium ions

The fragmentation reactions of a variety of alkylphenylammonium ions, C(6)H(5)NH(3 -n)R(n)(+) (n >/= 1, R = CH(3), C(2)H(5), i-C(3)H(7), n-C(4)H(9)) were studied by energy-resolved mass spectrometry. Ionization was by fast atom bombardment (FAB) or electrospray ionization. Energy-resolved fragmentation data were obtained by low-energy collision-induced dissociation (CID) in the quadrupole cell of a hybrid sector/quadrupole instrument following FAB ionization and by cone-voltage CID in the interface region of the electrospray/quadrupole instrument. A comparison of the two methods of obtaining energy-resolved data showed that very similar results are obtained by the two methods. The fragmentation reactions of the alkylphenylammonium ions are rationalized in terms of competitive formation of an [R(+)-NC(6)H(5)H(3-n)R(n-1)] complex or a [C(6)H(5)H(3-n)R(n-1)N(+.)-(.)R] complex. The former complex fragments by internal proton transfer to yield C(6)H(5)H(3 -n)R(n -1)NH(+) and [R -H] whereas the latter complex fragments to form C(6)H(5)H(3 -n)R(n -1)N(+) and an alkyl radical. Alkane elimination, which is very prominent for tetraalkylammonium ions, most likely involves sequential elimination of an alkyl radical and either an H atom or an alkyl radical for the phenyl-substituted ammonium ions. Copyright 1999 John Wiley & Sons, Ltd.     

Ions generated from uranyl nitrate solutions by electrospray ionization (ESI) and detected with fourier transform ion-cyclotron resonance (FT-ICR) mass spectrometry

Electrospray ionization (ESI) of uranyl nitrate solutions generates a wide variety of positively and negatively charged ions, including complex adducts of uranyl ions with methoxy, hydroxy, and nitrate ligands. In the positive ion mode, ions detected by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry are sensitive to instrumental tuning parameters such as quadrupole operating frequency and trapping time. Positive ions correspond to oligomeric uranyl nitrate species that can be characterized as having a general formula of [(UO(2))(n)(A)(m)(CH(3)OH)(s)](+) or [(UO(2))(n)(O)(A)(m)(CH(3)OH)(s)](+) with n = 1-4, m = 1-7, s = 0 or 1, and A = OH, NO(3), CH(3)O or a combination of these, although the formation of NO(3)-containing species is preferred. In the negative ion mode, complexes of the form [(UO(2))(NO(3))(m)](-) (m = 1-3) are detected, although the formation of the oxo-containing ions [(UO(2))(O)(n)(NO(3))(m)](-) (n = 1-2, m = 1-2) and the hydroxy-containing ions [(UO(2))(OH)(n)(NO(3))(m)](-) (n = 1-2, m = 0-1) are also observed. The extent of coordinative unsaturation of both positive and negative ions can be determined by ligand association/exchange and H/D exchange experiments using D(2)O and CD(3)OD as neutral reaction partners in the gas-phase. Positive ions are of varying stability and reactivity and may fragment extensively upon collision with D(2)O, CD(3)OD and N(2) in sustained off-resonance irradiation/collision-induced dissociation (SORI-CID) experiments. Electron-transfer reactions, presumably occurring during electrospray ionization but also in SORI-CID, can result in reduction of U(VI) to U(V) and perhaps even U(IV).     

Potential energy surface for activation of methane by Pt(+): a combined guided ion beam and DFT study

A guided-ion beam tandem mass spectrometer is used to study the reactions of Pt(+) with methane, PtCH(2)(+) with H(2) and D(2), and collision-induced dissociation of PtCH(4)(+) and PtCH(2)(+) with Xe. These studies experimentally probe the potential energy surface for the activation of methane by Pt(+). For the reaction of Pt(+) with methane, dehydrogenation to form PtCH(2)(+) + H(2) is exothermic, efficient, and the only process observed at low energies. PtH(+), formed in a simple C-H bond cleavage, dominates the product spectrum at high energies. The observation of a PtH(2)(+) product provides evidence that methane activation proceeds via a (H)(2)PtCH(2)(+) intermediate. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies in eV (kJ/mol) of D(0)(Pt(+)-H) = 2.81 +/- 0.05 (271 +/- 5), D(0)(Pt(+)-2H) = 6.00 +/- 0.12 (579 +/- 12), D(0)(Pt(+)-C) = 5.43 +/- 0.05 (524 +/- 5), D(0)(Pt(+)-CH) = 5.56 +/- 0.10 (536 +/- 10), and D(0)(Pt(+)-CH(3)) = 2.67 +/- 0.08 (258 +/- 8). D(0)(Pt(+)-CH(2)) = 4.80 +/- 0.03 eV (463 +/- 3 kJ/mol) is determined by measuring the forward and reverse reaction rates for Pt(+) + CH(4) right harpoon over left harpoon PtCH(2)(+) + H(2) at thermal energy. We find extensive hydrogen scrambling in the reaction of PtCH(2)(+) with D(2). Collision-induced dissociation (CID) of PtCH(4)(+), identified as the H-Pt(+)-CH(3) intermediate, with Xe reveals a bond energy of 1.77 +/- 0.08 eV (171 +/- 8 kJ/mol) relative to Pt(+) + CH(4). The experimental thermochemistry is favorably compared with density functional theory calculations (B3LYP using several basis sets), which also establish the electronic structures of these species and provide insight into the reaction mechanism. Results for the reaction of Pt(+) with methane are compared with those for the analogous palladium system and the differences in reactivity and mechanism are discussed.