Reliability of estimation of recombination energies of molecular radical cations by charge exchange and a test of the exoergicity rule

Charge exchange occurring in a collision cell of a mass spectrometer was used previously to find the very long-lived excited electronic states of molecular cations generated by electron ionization. The underlying criterion in the method, called the exoergicity rule, was that only electronically exoergic and resonant processes have large cross sections at low collision energy. The validity of this rule has been checked in this work by estimating the recombination energies of some molecular ions based on the presence/absence of charge-exchange signals from gases with different ionization energies, and comparing these estimates with the literature values. It is concluded that a recombination energy can be estimated to within an upper limit uncertainty of 0.1 eV with this technique, which validates the exoergicity rule proposed previously.     

Crossed beams study of the reaction 1CH2+C2H2-->C3H3+H

The reaction of electronically excited singlet methylene (1CH2) with acetylene (C2H2) was studied using the method of crossed molecular beams at a mean collision energy of 3.0 kcal/mol. The angular and velocity distributions of the propargyl radical (C3H3) products were measured using single photon ionization (9.6 eV) at the advanced light source. The measured distributions indicate that the mechanism involves formation of a long-lived C3H4 complex followed by simple C-H bond fission producing C3H3+H. This work, which is the first crossed beams study of a reaction involving an electronically excited polyatomic molecule, demonstrates the feasibility of crossed molecular beam studies of reactions involving 1CH2.     

Ion-molecule reactions of [C,H(3), S](+) ions and evidence for long-lived triplet CH(3)S(+)

Gas-phase [C, H(3), S](+) ions obtained by electron impact from (CH(3))(2)S at 14 eV undergo two distinct low-pressure ion-molecule reactions with the parent neutral: proton transfer and charge exchange. The kinetics of these reactions studied by Fourier transform ion cyclotron resonance (FT-ICR) techniques clearly suggests the [C, H(3), S](+) species to be a mixture of isomeric ions. While proton transfer is consistent with reagent ions displaying the CH(2)SH(+) connectivity, the observed charge exchange strongly argues for the presence of thiomethoxy cations, CH(3)S(+), predicted to be stable only in the triplet state. Charge exchange reactions are also observed in the reaction of these same [C, H(3), S](+) ions with benzene, toluene and phenetole. For these substrates, the CH(2)SH(+) ions can promote proton transfer and electrophilic methylene insertion in the aromatic ring with elimination of H(2)S. The results obtained for the different substrates suggest that the fraction of long-lived fraction of thiomethoxy cations obtained at 14 eV by electron ionization of dimethyl sulfide amounts to ~(22 -/+ 4)% of the [C, H(3), S](+) fragments.     

丙烯酸分子的激发态超快预解离动力学

利用飞秒泵浦-探测技术结合飞行时间质谱(TOF-MS),研究了丙烯酸分子被200nm泵浦光激发到第二电子激发态(S2)后的超快预解离动力学.采集了母体离子和碎片离子的时间分辨质谱信号,并利用动力学方程对时间分辨离子质谱信号进行拟合和分析,揭示了预解离通道的存在.布居在S2激发态的分子通过快速的内转换弛豫到第一电子激发态(S1),时间常数为210fs,随后再经内转换从S1态弛豫到基态(S0)的高振动态,时间常数为1.49ps.分子最终在基态高振动态势能面上发生C-C键和C-O键的断裂,分别解离生成H2C=CH和HOCO、H2C=CHCO和OH中性碎片,对应的预解离时间常数分别约为4和3ps.碎片离子的产生有两个途径,分别来自于母体离子的解离和基态高振动态势能面上中性碎片的电离.     

Charge exchange mass spectrometry at high energy

It is shown that charge exchange mass spectra using various reagent ions can be determined on a commercial double focusing mass spectrometer. The experiment relies upon the presence of a collision region held at a potential above ground with the product ions being selected by their unique kinetic energies. Cyclohexene has been studied in detail and the results are in agreement with previous thermochemical and field ionization kinetics data. Agreement with reported charge exchange spectra of n-propanol, taken at lower energies, is good. The present results, including both the variation of spectra with reagent gas and the small range of kinetic energies present in the charge exchange beam, indicate that a resonant or near resonant process is involved. Thus, the internal energy of the nascent ion is defined within quite narrow limits, just as is the case for charge exchange at lower energy.     

Internal energy effects in collision induced dissociation spectra

The effect of the precursor ion internal energy on the branching ratios obtained from collision induced dissociation fragmentation patterns was examined for [NH₃]⁺ and [C₂H₄N]⁺. The ion internal energy was changed by varying both the chemical ionization reagent gas and the ion source pressure. Effects observed in the collision induced dissociation fragmentation patterns as a function of the ion source pressure are explained by the reaction exothermicities and by collisional deactivation of internally excited ions (at high pressure).     

Discovery of long-lived excited electronic states of vinylchloride,vinylbromide, vinyliodide,and acrylonitrile cations

Charge exchange technique has been used to detect the presence of long-lived excited electronic states of some monosubstituted ethene cations. The technique is based on the criterion that charge exchange between polyatomic species is efficient only when the energy of reaction is close to zero or negative (DeltaE < or = 0), or the exoergicity rule. The A2A' states of vinylchloride, vinylbromide, vinyliodide, and acrylonitrile cations have been found to have long lifetimes (tens of microsecond or longer) while all the excited electronic states of vinylfluoride cation have been found to undergo rapid dissociation or nonradiative relaxation to the ground state. The long-lived states found are those displaying well-resolved vibrational structures in the photoelectron spectra. In particular, these are the states generated by removal of an electron from the in-plane nonbonding p orbitals of halogens or the in-plane pi orbital of the triple bond C[triple bound]N. The present findings are very similar to those for the monosubstituted benzene cations reported previously.     

A fast flow tube study of gas phase H/D exchange of multiply protonated ubiquitin

An electrospray ionization (ESI)/fast-flow technique has been applied to the study of gas phase hydrogen/deuterium (H/D) exchange kinetics. Multiply charged ubiquitin ions [ubiquitin + nH](n)(+), in charge states n = 7-13, were reacted with ND(3). The behavior of ND(3) as exchange reagent is different from that of the previously studied reagents, D(2)O and CH(3)OD. Contrary to those, the maximum number of exchanged hydrogen atoms and the overall exchange rate were observed to increase with increasing charge state of the ubiquitin ions. The results are reagent-dependent because the exchange mechanisms are different for the different reagents. This observation is in agreement with a recent conclusion by Beauchamp and co-workers that contrary to the assumption often expressed in earlier studies, H/D exchange kinetics may not directly reflect ion structures. The results for all three reagents are, however, consistent with observations of previous ion mobility experiments that with increasing charge state the conformers change from more compact, partially folded structures to elongated nearly linear ones. H/D exchange of (ubiquitin + 13H)(13+) with ND(3) leads to two separated ion populations reflecting the possible existence of two conformers with different exchange rates. The ions (ubiquitin + 8H)(8+) and (ubiquitin + 11H)(11+) represent a partially folded structure and an unfolded structure, respectively, and were studied in greater detail. The relative abundances of ions were measured in steps of 0.5 m/z (mass-to-charge ratio), as a function of the ND(3) flow rate. The experimental results were simulated by computer fitted curves based on a recently developed algorithm. The algorithm allows the extraction of sets of grouped rate constants. Eight rate constant groups were deduced for each of the two ions. These rate constants correspond to 32 and 44 H/D exchanges for the 8+ and 11+ charged ions, respectively. The results indicate higher individual rates for most of the exchanged atoms in the 11+ ion compared to the 8+ ion.     

Structure and fragmentation of dimethyl methylphosphonate and trimethyl phosphite

The structures and fragmentation pathways of two isomeric organophosphorus esters, dimethyl methylphosphonate (DMMP) and trimethyl phosphite (TMP) have been determined. The long-lived, low-energy molecular ions of DMMP were found to undergo a keto-to-enol isomerization prior to collision-induced dissociation. This isomerization was established through the comparison of the collision spectra from DMMP, TMP, isotopically labeled DMMP and a model precursor ion. Electron ionization and charge exchange reactions were used to study the isomerization as a function of the internal energy of the molecular ion. The structure of the TMP molecular ion retained the structure of the neutral molecule. The daughter ion spectra of the isomeric fragment ions from DMMP and TMP were used to infer the fragment ion structures. Negative ions of DMMP and TMP were also studied, and their collision spectra were found to be indistinguishable.     

Reaction between electronically excited species: O(1D2)+NO2→NO(A)+O2

The high-intensity laser permits the study of reactions between electronically excited species. The laser irradiation of NO₂ generated NO₂ by one-photon excitation and O(¹D) by two-photon dissociation. These two species react with each other, producing electronically excited NO(A). The product energy distribution was nearly statistical, indicating the possible presence of a long-lived collision complex.     

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.     

A structural investigation of [C6H6]2+ and [C6H6]+˙ ions involved in charge exchange reactions

Mass-analysed ion kinetic energy spectra for collisional activation (CA) of [C₆H₆]⁺˙ formed via electron capture by [C₆H₆]²⁺ ions in collision with neutral benzene molecules have been compared for the C₆H₆ isomers benzene, 1,5-hexadiyne and 2,4-hexadiyne. Comparisons of fragment abundance and total CA fragment yields were also made for [C₆H₆]⁺˙ ions generated by electron ionization (EI). CA conditions of ion velocity and collision gas pressure were identical in these comparisons. In general the fragment abundance patterns for the ions formed by charge exchange were very similar to those for singly charged benzene ions generated by EI. However, significant variations in CA fragment yield (the ratio of the total CA fragment ion abundance to the abundance of the incident unfragmented ions) were observed. It is not clear from the results whether these variations reflect structurally different ions or ions of different internal energies. The CA spectra of [C₆H₆]⁺˙ ions derived from charge exchange reactions between the benzene dication and the target gases He, Ne, Ar, Kr and Xe have also been recorded and, once again, very similar fragment abundance patterns were observed along with large variations in total CA fragment yields. Charge exchange efficiency measurements are reported for reactions between the benzene dication and the targets He, Ne, Ar, Kr, Xe and C₆H₆ (benzene) and also for the doubly charged ions derived from the linear C₆H₆ isomers. In the latter case Xe and benzene targets were used. The energetics and efficiency measurements for the former reactions suggest that for targets such as He and Ne the processes probably involve excited states of the doubly charged ions. The efficiencies measured for the latter reactions were distinctly different for the three C₆H₆ isomers and may indicate a strong dependence of charge exchange cross-section on doubly charged ion structure.     

FT-ICRMS distinguishes the mechanism of the charge state reduction for multiply charged protein cations admixed with redox reagents in ESI

Recently, we reported on a phenomenon in which multiply charged protein cations produced by electrospray ionization could be reduced to lower and narrower charge state distributions when admixed with reducing reagents 1,4-benzoquinone or quinhydrone. Circular dichroism spectra of the proteins indicated that secondary and tertiary structural changes upon addition of these reducing reagents were negligible, thus eliminating conformational effects as playing a role in the charge reduction mechanism. Furthermore, the extent of charge state reduction did not correspond with gas-phase basicities of the redox reagents, suggesting that solution-phase, and not gas-phase, behavior dominates the observed charge state reduction. The relatively low resolution of the triple quadrupole employed did not make it possible to distinguish isotopic distributions of the multiply charged cations in order to determine whether the observed phenomenon was the result of proton-transfer reactions between the multiply charged cations and the reducing reagent or because of electron transfer from the reducing reagent to the protein cations. Here, high-resolution ESI-Fourier transform ion cyclotron resonance mass spectrometry of several peptide amides in the presence of a redox reagent show isotopic distributions that are consistent only with the proton-transfer mechanism.     

Free-electron model for the inelastic collision of fast electrons with benzene

Using a free-electron wave function the cross section for the excitation of the first excited state and that for the ionization of benzene in the inelastic collision with fast electrons have been estimated. In the ionization process the cross section has been found to be maximum when the secondary electron moves away with a kinetic energy of about 3 eV. For incident-electron energy above 20 eV the cross section for ionization is larger than that for excitation, while below it the reverse is the case.     

Declustering and fragmentation of protein ions from an electrospray ion source

A triple quadrupole mass spectrometer with a high pressure collision cell has been used to explore the declustering and fragmentation processes that may occur in the vacuum interface region of an electrospray or ionspray ion source. Using apomyoglobin as a model protein compound, collisional processes in Q2 were used to elucidate possible mechanisms which could occur in the orifice-skimmer region to affect the observed charge state distribution. The results indicate that charge loss or gain through collisional loss of a proton or electron does not occur; rather, higher collision energy results in better declustering of lower charge state ions, and fragmentation of higher charge state ions. The net result is an apparent shift toward lower charge state as the collision energy in the free jet region is increased. In addition, the data suggest that a mixture of heavily clustered monomers and possibly dimers and multimers are present in the expansion from ion source into vacuum, and it is this mixture which is acted on by the declustering field to produce the observed mass spectrum. The presence of these “superclusters” needs to be considered in any theory of ion desorption and transport processes in the source and interface region.     

Crossed-beams studies of the dynamics of the H-atom abstraction reaction, O(3P) + CH4 → OH + CH3, at hyperthermal collision energies

The H-atom abstraction reaction, O((3)P) + CH(4) → OH + CH(3), has been studied at a hyperthermal collision energy of 64 kcal mol(-1) by two crossed-molecular-beams techniques. The OH products were detected with a rotatable mass spectrometer employing electron-impact ionization, and the CH(3) products were detected with the combination of resonance-enhanced multiphoton ionization (REMPI) and time-sliced ion velocity-map imaging. The OH products are mainly formed through a stripping mechanism, in which the reagent O atom approaches the CH(4) molecule at large impact parameters and the OH product is scattered in the forward direction: roughly the same direction as the reagent O atoms. Most of the available energy is partitioned into product translation. The dominance of the stripping mechanism is a unique feature of such H-atom abstraction reactions at hyperthermal collision energies. In the hyperthermal reaction of O((3)P) with CH(4), the H-atom abstraction reaction pathway accounts for 70% of the reactive collisions, while the H-atom elimination pathway to produce OCH(3) + H accounts for the other 30%.     

The electronic origin of the dual fluorescence in donor-acceptor substituted benzene derivatives

The origin of the dual fluorescence of DMABN (dimethylaminobenzonitrile) and other benzene derivatives is explained by a charge transfer model based on the properties of the benzene anion radical. It is shown that, in general, three low-lying electronically excited states are expected for these molecules, two of which are of charge transfer (CT) character, whereas the third is a locally excited (LE) state. Dual fluorescence may arise from any two of these states, as each has a different geometry at which it attains a minimum. The Jahn-Teller induced distortion of the benzene anion radical ground state helps to classify the CT states as having quinoid (Q) and antiquinoid (AQ) forms. The intramolecular charge transfer (ICT) state is formed by the transfer of an electron from a covalently linked donor group to an anti-bonding orbital of the pi-electron system of benzene. The change in charge distribution of the molecule in the CT states leads to the most significant geometry change undergone by the molecule which is the distortion of the benzene ring to a Q or AQ structure. As the dipole moment is larger in the perpendicular geometry than in the planar one, this geometry is preferred in polar solvents, supporting the twisted intramolecular charge transfer (TICT) model. However, in many cases the planar conformation of CT excited states is lower in energy than that of the LE state, and dual fluorescence can be observed also from planar structures.     

Charge exchange in collisions of beryllium with its ion

Close-coupling calculations of the resonance and near resonance charge exchange in ion-atom collisions of Be at low and intermediate energies are presented. Accurate ab initio calculations are carried out of the Born-Oppenheimer potentials and the non-adiabatic couplings that are due to the finite nuclear masses and drive the near resonance charge exchange. We show that the near resonance charge exchange cross section follows Wigner's threshold law of inelastic processes for energies below 10(-8) eV and that the zero temperature rate constant for it is 4.5 × 10(-10) cm(3) s(-1). At collision energies much larger than the isotope shift of the ionization potentials of the atoms, we show that the near resonance charge exchange process is equivalent to the resonance charge exchange with cross sections having a logarithmic dependence. We also investigate the perturbation to the charge exchange process due to the non-adiabatic interaction to an electronic excited state. We show that the influence is negligible at low temperatures and still small at intermediate energies despite the presence of resonances.     

Experimental studies of charge transfer reactions between argon and the third row metals calcium through copper in the inductively coupled plasma

The effect of charge transfer reactions on analyte excitation and ionization in the inductively coupled plasma was studied by two independent techniques. In one technique, pulsed lasers were used to either deplete the ground state of neutral analyte atoms or enhance the population of selected states of the singly charged ion. In both cases the perturbed species were collision partners with argon in potential charge transfer reactions. The effects of charge transfer collisions could be detected in the form of changes in emission from product species. In the second technique, a simple correlation method was used to detect the link via charge transfer of neutral atom ground states and highly excited ionic levels. In the presence of charge transfer collisions, the populations of such linked levels show strong positive correlations. The two techniques were used to study the effects of charge transfer reactions on the third row elements Ca–Cu. With the exception of Cr and Mn, all of the elements studied showed positive evidence of excitation and ionization by charge transfer collision with argon.     

Nuclear motion captured by the slow electron velocity imaging technique in the tunnelling predissociation of the S1 methylamine

Predissociation dynamics of methylamines (CH(3)NH(2) and CH(3)ND(2)) on the first electronically excited states are studied using the slow-electron velocity imaging method to unravel the multi-dimensional nature of the N-H(D) chemical bond dissociation reaction which occurs via tunnelling. The nearly free internal rotation around the C-N bond axis is found to be strongly coupled to the reaction pathway, revealing nuclear motions actively involved in the tunnelling process on the S(1) potential energy surfaces. The vibrational state-resolved energy and angular distributions of photoelectron, ejected from the ionization mediated by the metastable intermediate S(1) state provide a unique way for mapping the predissociative potential energy surfaces.