Measuring internal energy deposition in collisional activation using hydrated ion nanocalorimetry to obtain peptide dissociation energies and entropies

The internal energy deposited in both on- and off-resonance collisional activation in Fourier transform ion cyclotron resonance mass spectrometry is measured with ion nanocalorimetry and is used to obtain information about the dissociation energy and entropy of a protonated peptide. Activation of Na⁺(H₂O)₃₀ results in sequential loss of water molecules, and the internal energy of the activated ion can be obtained from the abundances of the product ions. Information about internal energy deposition in on-resonance collisional activation of protonated peptides is inferred from dissociation data obtained under identical conditions for hydrated ions that have similar m/z and degrees-of-freedom. From experimental internal energy deposition curves and Rice-Ramsperger-Kassel-Marcus (RRKM) theory, dissociation data as a function of collision energy for protonated leucine enkephalin, which has a comparable m/z and degrees-of-freedom as Na⁺(H₂O)₃₀, are modeled. The threshold dissociation energies and entropies are correlated for data acquired at a single time point, resulting in a relatively wide range of threshold dissociation energies (1.1 to 1.7 eV) that can fit these data. However, this range of values could be significantly reduced by fitting data acquired at different dissociation times. By measuring the internal energy of an activated ion, the number of fitting parameters necessary to obtain information about the dissociation parameters by modeling these data is reduced and could result in improved accuracy for such methods.     

Internal energy distributions deposited in doubly and singly charged tungsten hexacarbonyl ions generated by charge stripping,electron impact,and charge exchange

The distribution Pε of internal energies deposited in W(CO)₆ ^+?. ions upon charge stripping (that is, electron detachment to yield the doubly charged ion in the course of a single kiloelec-tronvolt energy collision) was estimated by a thermochemical method from the measured relative abundances of the doubly charged fragment ions produced. The thermochemical information needed to estimate P/ge was obtained by measuring the threshold translational energy losses associated with charge stripping of the singly charged fragment ions, W(CO) _n ⁺ (n = 0-5). The P(/ge) curve falls exponentially with increasing internal energy. The average energy transferred to W(CO)₆ ^+? upon a 7.8-keV collision with O₂ is 19 eV, yielding W(CO)₆ ^2? ions with an average of 4 eV of internal energy. In its general appearance, the P(ε) distribution associated with charge stripping is similar to the curves obtained from simple collisional activation of either W(CO) ₆ ^+?. or W(CO)₆ ^2+? in kiloelectronvolt energy gaseous collisions. Given that charge stripping occurs by way of an electronic excitation process, this similarity in the energy deposition function is taken to indicate that electronic excitation is also the major mechanism for simple collisional activation in this system at zero scattering angle in the kiloelectronvolt energy regime. The internal energy distribution associated with a related charge-stripping process, charge inversion from the metal carbonyl anions to yield the corresponding cations, was also recorded. This reaction shows a large (～7 eV) average internal energy deposition with a distribution that indicates near-zero probability of formation of unexcited ions. These data are tentatively interpreted in terms of vibrationalelectron detachment. The internal energy distribution associated with an exothermic process, charge exchange [W(CO)₆ ^2+? + O₂ → W(CO) ₊ ^6?+O₂ ^+?], was also characterized. Unexpectedly strong coupling of translational to internal energy is observed, and there is a large probability of depositing internal energies in excess of 10 eV, even though the exothermicity is only 3 eV. Finally, the internal energy distributions associated with the formation of doubly charged W(CO)₆ ^2+? ions by electron ionization have been measured. Unlike the distribution for charge stripping, but like that for singly charged ions generated by electron impact, this distribution shows considerable structure, presumably due to Franck-Condon factors.     

Desorption due to charge exchange in low-energy collisions of organofluorine ions at solid surfaces

Collisions of organofluorine ions at a metal surface result in efficient emission of adsorbate species as gas-phase ions. The experiments are done at 120° scattering angle in a hybrid (BQ) mass spectrometer; the primary ions, mass-selected by a magnetic sector (B), are allowed to collide with a target at a selected kinetic energy in the tens of eV range and the emitted ions are mass-analyzed using a quadrupole mass filter (Q). It is proposed that the impinging ions undergo neutralization accompanied by desorption of hydrocarbon ions and that the amount of internal energy deposited in the desorbed ions is strongly dependent on the collision energy and affects their degree of fragmentation. Competing processes include reflection and fragmentation of the colliding particle, along with such ion/adsorbate reactions as hydrogen atom abstraction by the fluorinated ion. Small even-electron ions, such as [CHF₂]⁺ and [C₂H₂F]⁺ are more effective in promoting chemical sputtering of the surface adsorbate as compared to larger ions (e.g. [C₃F₅]⁺) and odd-electron ions (e.g. [C₂F₄]⁺˙ and [C₂HF₂]⁺˙). At low energies some odd-electron fluorinated ions undergo collision without any secondary ions being emitted from the surface. In these cases the parent ions are apparently neutralized, but without sufficient energy transfer to cause hydrocarbon ion desorption. Non-fluorinated organic ions yield fragment ions and ion/surface reaction products under the condition of these experiments, but do not cause significant desorption of hydrocarbon ions.     

Metastabile Übergänge des Ions C2H3O+ im Massenspektrometer

The metastable transitions of C₂H₃O⁺ ions generated from ten different compounds have been investigated. The intensity ratios of the metastable peaks have been found to be independent of the structure of the initial compound. They are dependent however on the internal energy of the ions. The variation of the intensity ratios with the molecular size is in agreement with the degrees-of-freedom-effect. C₂H₃O⁺ ions decompose from the same structure (mixture of structures) in all cases.

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Mit 2 Abbildungen     

Structures of gas phase C5H8 radical cations: A collisional ionization study

Collisional ionization (charge stripping) and charge exchange ionization spectrometry were utilized to determine structures of fourteen cyclic and acyclic C₅H₈ radical cations, including ionized 1,2- 1,3-, 1,4- and 2,3-pentadienes (-PD), isoprene, 1- and 3-methylcyclobutenes (1- and 3-MCB), 3-methyl-1,2-butadiene (3-M-1,2-BD), methenecyclobutane (MECB), cyclopentene, 3-methyl-1-butyne (3-MB), 1- and 2-pentynes and vinylcyclopropane (VCP). The pressure of the charge exchange reagent gas in the ion source was adjusted to generate ions of different energy contents. The structures of the C₅H₈ ions are energy dependent, and their isomerization reactions can be monitored as a function of the amount of internal energy deposited by charge exchange. 1,3-PD, isoprene and cyclopentene radical cations are identified as stable ion structures. 1-MCB, 3-M-1,2-BD and 3-MB radical cations isomerize to isoprene ions, whereas ionized VCP, 3-MCB, 1,2-PD, 2,3-PD, 1,4-PD, 1-pentyne and 2-pentyne ultimately isomerize to the [1,3-PD]⁺˙. Thermodynamic arguments are invoked to corroborate these isomerization reactions. The critical energies of the isomerizations are also estimated.     

Internal energy effects on metastable characteristics. The structure of [C3H7O]+ ions

The effect of changes in the internal energy distribution of the fragmenting ion on the ratio of metastable ion intensities for two competing fragmentation reactions has been investigated both theoretically and experimentally. Model calculations have shown that if the competing reactions have significantly different activation energies the metastable intensity ratio does depend on the internal energy distribution although large changes are necessary before the ratio changes by more than a factor of two. Experimentally the metastable characteristics of [C₃H₇O]⁺ ions of nominal structures [CH₃CH₂O⁺?CH₂] (I), [(CH₃)₂C?O⁺H] (II), [CH₃CH₂CH?O⁺H] (III) and [CH₃O⁺?CHCH₃] (IV) have been examined. For each structure the metastable characteristics are found to be distinctive and independent of changes in the internal energy distribution of the fragmenting ion where these changes result from altering the precursor of the [C₃H₇O]⁺ ions. It is suggested that these internal energy changes can be estimated from the fraction of [C₃H₇O]⁺ ions which fragment in the ion-source. It is concluded that structures I to IV represent stable and distinct ionic structures.     

Effect of internal excitation on the collision-induced dissociation and reactivity of Co 2 +

The kinetic energy dependence of collision-induced dissociation (CID) of dicobalt ion (Co ₂ ⁺ ) with He, Ar, and Xe has been investigated using guided ion-beam mass spectrometry. The change in efficiency of CID as the target gas is changed is in general agreement with previous CID studies of other systems: the cross section with Ar is 0.5 that with Xe, and no product ions are found with He. By varying the conditions under which the reactant ions are formed, the degree of internal excitation of the dicobalt ions is changed. The internal energies can be characterized by a Maxwell-Boltzmann distribution. We find that CID and reactions with O₂ and CO are very sensitive to Co ₂ ⁺ internal energy. The bond-dissociation energy derived from this work is D^o(Co ₂ ⁺ )=2.75±0.10 eV (63.4±2.3 kcal/mol). The Co ₂ ⁺ results are compared with a previous study of Fe ₂ ⁺ .     

AgI-Ag2O-V2O5 glasses as ion-to-electron transducers for the construction of all-solid-state microelectrodes

A method for the fabrication of ion-selective all-solid-state microelectrodes is presented. The ion-to-electron transduction process takes place into the transducer material. In this approach, AgI-Ag₂O-V₂O₅ glasses, which exhibit ionic and electrical conductivity are applied as ion-to-electron transducers of polymeric membrane microelectrodes. All-solid-state electrodes based on potassium-sensitive poly(vinyl chloride) membranes, deposited directly on the surface of glass composites, exhibited theoretical responses. Their selectivity and durability were comparable to planar microelectrodes containing an internal electrolyte immobilized in the intermediate hydrogel layer. The only disadvantage of the proposed structures was their limited reproducibility. Moreover, it was found that the unmodified AgI-Ag₂O-V₂O₅ glasses can be applied as ion-sensitive membrane of solid-state microelectrodes for the determination of Ag⁺ and I⁻ ions.     

Mass spectrometric fragmentation of isomeric 2-alkyl-substituted 1,3-indandiones and 3-alkylidenephthalides: a seven-step consecutive isomerization of regular and distonic molecular radical cations

The electron impact-induced fragmentation of 2,2-dimethyl- and 2-ethyl-1,3-indandione, 1 and 2, and their isomers, 3-isopropylidene- and 3-propylidenephthalide, 3 and 4, respectively, was studied in detail by mass-analysed ion kinetic energy (MIKE) and collision-induced dissociation (CID-MIKE) spectrometry, including ²H and ¹³C. labelled analogues of 1 and 2. In all regimes of internal energy, the molecular ions 1^+. ? 4^+. interconvert by up to seven consecutive, reversible isomerization steps prior to the main fragmentation processes, viz. loss of CH₃^. and C₂H₄. 1,3-Indandione and 3-methylenephthalide ions with identical alkylidene moieties (i.e. 1^+.?3^+. and 2^+.?4^+.) equilibrate rapidly and completely prior to fragmentation, whereas these pairs of isomers interconvert only slowly via a five-step rearrangement of the indandione ions 1^+.?2^+.. Distinct from the behaviour of simpler ionized carbonyl species, a 1,2-C shift of a (formally) neutral carbonyl group is found to occur along with that of a protonated one. Also distinct from simpler cases, methyl loss does not take place from the ionized enol intermediates formed within the interconversion 1^+.?2^+. of the diketone ions but rather from the n-propylidenephthalide ions 4^+.. This follows from CID-MIKE spectrometry of the [M ? CH₃]⁺ ions of 1–4 and two reference C₁₀H₇O₂⁺ (m/z 159) ions of authentic structures (protonated 2-methylene-1,3-indandione and protonated 1,4-naphthoquinone). The characteristic CID fragmentation of the C₁₀H₇O₂⁺ ions is rationalized. Finally, the multistep isomerization of ionized 1,3-indandiones apparently also extends to higher homologues [e.g. 5^+. from 2-ethyl-2-methyl-1,3-indandione (5) and 6^+. from 2,2-diethyl-1,3-indandione (6)]: the ionized phthaloyl group of 1,3-indandione radical cations 1^+., 2^+., 5^+. and 6^+., originally attached with its two acyl functionalities to the same carbon of the aliphatic chain, performs, in fact, a ‘multi-step migration’.     

A combined theoretical and experimental approach to the unimolecular loss of molecular hydrogen from protonated formaldehyde: Determination of the average internal energy of metastable [CH2OH]+ ions

Ab initio quantum chemical calculations (MP2/4–31G**) were performed for the dihydrogen elimination reaction from protonated formaldehyde. The energy difference between reactants and products and the activation energies were found to be in good agreement with the corresponding experimental quantities. Theoretical rate vs. energy curves were computed for a series of isotopic variants of the reaction using the Rice–Ramsperger–Kassel–Marcus (RRKM) method. The vibrational frequencies used in these calculations were taken from the 4–31G** geometry-optimized transition state and reactant structures. Quantum mechanical tunnelling was introduced to explain the existence of metastable CH₂OH ions, and a negative kinetic shift of about 0.1 eV was found. The intramolecular kinetic isotope effect for loss of HH/HD and DH/DD was calculated and compared with the experimental data. The result is consistent with the assumption that the average internal energy of metastable [CH₂OH]⁺ ions is very close to the critical energy for H₂ loss.     

The distonic ions HO+CHCH2C˙H2 and CH3C(O+H)CH2C˙H2

The distonic ions HO⁺?CHCH₂C^˙H₂ (1) and CH₃C(?O⁺H)CH₂C^˙H₂ (2) were directly generated, their decompositions characterized and their appearance energies determined by photoionization. Heats of formation derived from the appearance energies were 757 kJ mol^?1 for 1 and 692 kJ mol^?1 for 2. Deuterium labeling demonstrates that both ions decompose at low energies in the same ways as their isomers with the same skeletal structures, consistent with proposals that 1 and 2 are intermediates in the decompositions of those systems. Surprisingly, the values of the translational energy releases accompanying the formation of CH₃CO⁺ and C₂H₅CO⁺ from 2 appear to be inversely proportional to the available excess energy. The 1,2-H-shift RC(?O⁺H)CH₂C˙H₂ → RC(?O₊H)C˙HCH₃ is compared to the corresponding, non-occurring 1,2-H-shift in alkyl free radicals.     

Mass spectra of aliphatic dicarboxylic acids and their dimethyl esters: Cyclic structures for the [M − H2O]+˙ ions from the diacids and [M − MeOH]+˙ ions from the dimethyl esters

Methyl 2-oxocycIoalkane carboxylate structures are proposed lor the [M ? MeOH]^+˙ ions from dimethyl adipate, pimelate, suberate and azelate. This proposal is based on a comparison of the metastable ion mass spectra and the kinetic energy releases for the major fragmentation reaction of these species with the same data for the molecular ions of authentic cyclic β-keto esters. The mass spectra of α,α,α′,α′-d₄-pimelic acid and its dimethyl ester indicate that the α-hydrogens are involved only to a minor extent in the formation of [M ? ROH]^+˙ and [M ? 2ROH]^+˙ ions, while these α-hydrogens are involved almost exclusively in the loss of ROH from the [M ? RO˙]⁺ ions (R = H or CH₃). The molecules XCO(CH₂)₇COOMe (X = OH, Cl) form abundant ions in their mass spectra with the same structure as the [M ? 2MeOH]^+˙ ions from dimethyl azelate.     

Benzyl vs. tropylium ions in the decomposition of some alkylnitrobenzenes

The [NO₂C₇H₆]⁺ ions generated from m-alkylnitrobenzenes have been shown to be different in their decomposition from those generated from p-alkylnitrobenzenes, even when the alkyl group is methyl and the departing fragment a hydrogen radical. Thus, in these cases even molecular ions of relatively high internal energy do not reversibly ring-expand to cycloheptatriene structures. In addition, the [NO₂C₇H₆]⁺ ions, assumed to be benzylic, do not ring-expand to nitrotropylium ions at internal energies sufficient to cause subsequent loss of NO or NO₂ from the p- and m-isomers, respectively.     

On the defect center electron energy loss structures from MgO surfaces

Electron energy loss spectroscopy (EELS) was applied to surfaces of (1) clean MgO(100), (2) ultrathin (0.5 Å average thickness) Cu layers deposited on MgO(100) by an electron beam evaporation technique, and (3) a carbon-contaminated MgO(100). The surface-defect-related energy loss peak was ascribed to the presence of surface states arising from the V_s⁻ centers rather than from the F_s⁺ centers. The copper deposit is supposed to be trapped by the magnesium ion vacancies and bonded to the oxygen ligands as ions. The new electronic structures caused by the Cu deposit aie explained in terms of Cu impurity levels.     

Loss of methyl from [H2CC(OH)?CH3]+˙ ions prepared by electron impact ionization of unstable 2-hydroxypropene

Unstable 2-hydroxpropene was prepared by retro-Diels-Alder decomposition of 5-exo-methyl-5-norbornenol at 800°C/2 × 10^?6 Torr. The ionization energy of 2-hydroxypropene was measured as 8.67±0.05 eV. Formation of [C₂H₃O]⁺ and [CH₃]⁺ ions originating from different parts of the parent ion was examined by means of ¹³C and deuterium labelling. Threshold-energy [H₂C?C(OH)? CH₃]^+˙ ions decompose to CH₃CO⁺+CH₃^˙ with appearance energy AE(CH₃CO⁺) = 11.03 ± 0.03 eV. Higher energy ions also form CH₂?C?OH⁺ + CH₃ with appearance energy AE(CH₂?C?OH⁺) = 12.2–12.3 eV. The fragmentation competes with hydrogen migration between C(1) and C(3) in the parent ion. [C₂H₃O]⁺ ions containing the original methyl group and [CH₃]⁺ ions incorporating the former methylene and the hydroxyl hydrogen atom are formed preferentially, compared with their corresponding counterparts. This behaviour is due to rate-determining isomerization [H₂C?C(OH)? CH₃]^+˙ →[CH₃COCH₃]^+˙, followed by asymmetrical fragmentation of the latter ions. Effects of internal energy and isotope substitution are discussed.     

Drift tube investigations on the reactions of O 2 + with CH4 and of CO 2 + with NO in various buffer gases

The influence of internal excitation on the reactions of O ₂ ⁺ + CH₄ and of CO ₂ ⁺ + NO has been investigated using a slow flow drift tube. The rate coefficients for these reactions obtained as a function of relative kinetic energy in various buffer gases like He, Ne, Ar, and Kr showed higher values under conditions where the internal excitation of the reactant ions was enhanced. For both reactions the lowest reactivity at all kinetic energies was observed to occur in He, indicating that He is the least effective buffer for collisionally inducing internal excitation of molecular ions.     

Site selection spectroscopy of SiO2:Eu3+ gels

Silica gels doped with Eu³⁺ ions were studied at temperatures between 10 K and 300 K by site selection spectroscopy in samples heated up to 200°C. The ⁵D₀ ⁷F₀ transition shows internal structures due to the different environments of the europium ions. Lifetimes, energy levels and homogeneous linewidths are site dependent. In the wet gel the Eu³⁺ ions prefer a liquid-like environment and only when the liquid is removed by heat treatment, the ion is linked more strongly to the silica network.     

Photodissociation and structures of [C6H4]+ ˙ ions generated from cyanobenzene and other precursors

Previous work on the electron impact induced loss of hydrogen cyanide from the radical cations of cyanobenzene has revealed that ring opening is important in the formation of the corresponding [C₆H₄]^{+ ˙} ions. Photodissociation experiments now show that these [C₆H₄]^{+ ˙} ions and those generated from 2-ethynylpyridine, 1,3-hexadiyn-6-nitrile and 1,2-diiodobenzene all photodissociate in the visible region to [C₄H₂]^{+ ˙}. The corresponding photodissociation spectra are all the same and have a maximum at about 370 nm, in agreement with spectra of ions with three conjugated double or triple bonds. Owing to the high reactivity, the low photodissociation rate and, possibly, the internal energy of the ions, the photodissociation kinetics are too complicated to be solved. The experiments nevertheless show that at least a major fraction of the [C₆H₄]^{+ ˙} ions has a ring-opened structure. This conclusion is supported by MNDO calculations, which indicate that the heats of formation of the possible acyclic structures are about 150 kJ mol^?1 lower than those of the o-, m- and p-benzyne structures.     

Quadrupole ion trap mass spectrometry of fullerenes

The fullerenes C₆₀ and C₇₀ can be ionized by desorption from a liquid matrix upon bombardment by Cs⁺ ions of 7 keV kinetic energy. The resulting radical cations, when activated in the ion trap by collisions with Xe target, in the presence of helium, undergo extensive dissociation by loss of multiple C₂ units. Large internal energies are deposited into these molecular ions and the dissociation efficiency is in excess of 60%.     

Effect of swift heavy ion irradiation on the physical properties of CuIn(S0.4Se0.6)2 alloy thin films prepared by solution growth technique

Alloy thin films of CuIn(S_0.4Se_0.6)₂ material were deposited using the solution growth technique. The various deposition parameters such as pH of solution, time, concentration of ions and temperature have been optimized for the device grade thin films. The as-deposited films were annealed in a rapid thermal annealing (RTA) system at 450 °C in air for 5 min and subjected to high-energy Ag ion irradiations. Ag ion irradiation has been performed with an energy of 100 MeV at a fluency of 5×10¹² ions/cm² on the thin film. The changes in optical and electrical properties that occurred before and after post-deposition treatments in CuIn(S_0.4Se_0.6)₂ thin films were studied using X-ray diffraction (XRD) and AFM; increase in crystallinity was observed after annealing and irradiation. In addition, structural damages were observed in irradiated thin films. After annealing and irradiation, the surface roughness was seen to be increased. Decrease in resistivity was observed, which is consistent with the optical energy band gap. The results are explained by considering the high energy deposited due to the electronic energy loss upon irradiation, which modified the properties of the material.