The role of excited Rydberg States in electron transfer dissociation

Ab initio electronic structure methods are used to estimate the cross sections for electron transfer from donor anions having electron binding energies ranging from 0.001 to 0.6 eV to each of three sites in a model disulfide-linked molecular cation. The three sites are (1) the S-S sigma(*) orbital to which electron attachment is rendered exothermic by Coulomb stabilization from the nearby positive site, (2) the ground Rydberg orbital of the -NH(3)(+) site, and (3) excited Rydberg orbitals of the same -NH(3)(+) site. It is found that attachment to the ground Rydberg orbital has a somewhat higher cross section than attachment to either the sigma orbital or the excited Rydberg orbital. However, it is through attachment either to the sigma(*) orbital or to certain excited Rydberg orbitals that cleavage of the S-S bond is most likely to occur. Attachment to the sigma(*) orbital causes prompt cleavage because the sigma energy surface is repulsive (except at very long range). Attachment to the ground or excited Rydberg state causes the S-S bond to rupture only once a through-bond electron transfer from the Rydberg orbital to the S-S sigma(*) orbital takes place. For the ground Rydberg state, this transfer requires surmounting an approximately 0.4 eV barrier that renders the S-S bond cleavage rate slow. However, for the excited Rydberg state, the intramolecular electron transfer has a much smaller barrier and is prompt.     

Dipole and Coulomb forces in electron capture dissociation and electron transfer dissociation mass spectroscopy

Ab initio electronic structure calculations were performed on a doubly charged polypeptide model H(+)-Lys(Ala)(19)-CO-CH(NH(2))-CH(2)-SS-CH(2)-(NH(2))CH-CO-(Ala)(19)-Lys-H(+) consisting of a C-terminal protonated Lys followed by a 19-Ala α-helix with a 20th Ala-like unit whose side chain is linked by a disulfide bond to a corresponding Ala-like unit connected to a second 19-Ala α-helix terminated by a second C-terminal-protonated Lys. The Coulomb potentials arising from the two charged Lys residues and dipole potentials arising from the two oppositely directed 72 D dipoles of the α-helices act to stabilize the SS bond's σ* orbital. The Coulomb potentials provide stabilization of 1 eV, while the two large dipoles generate an additional 4 eV. Such stabilization allows the SS σ* orbital to attach an electron and thereby generate disulfide bond cleavage products. Although calculations are performed only on SS bond cleavage, discussion of N-C(α) bond cleavage caused by electron attachment to amide π* orbitals is also presented. The magnitudes of the stabilization energies as well as the fact that they arise from Coulomb and dipole potentials are supported by results on a small model system consisting of a H(3)C-SS-CH(3) molecule with positive and negative fractional point charges to its left and right designed to represent (i) two positive charges ca. 32 ? distant (i.e., the two charged Lys sites of the peptide model) and (ii) two 72 D dipoles (i.e., the two α-helices). Earlier workers suggested that internal dipole forces in polypeptides could act to guide incoming free electrons (i.e., in electron capture dissociation (ECD)) toward the positive end of the dipole and thus affect the branching ratios for cleaving various bonds. Those workers argued that, because of the huge mass difference between an anion donor and a free electron, internal dipole forces would have a far smaller influence over the trajectory of a donor (i.e., in electron transfer dissociation (ETD)). The present findings suggest that, in addition to their effects on guiding electron or donor trajectories, dipole potentials (in combination with Coulomb potentials) also alter the energies of SS σ* and amide π* orbitals, which then affects the ability of these orbitals to bind an electron. Thus, both by trajectory-guiding and by orbital energy stabilization, Coulomb and dipole potentials can have significant influences on the branching ratios of ECD and ETC in which disulfide or N-C(α) bonds are cleaved.     

Coulomb-assisted dissociative electron attachment: application to a model peptide

The fragmentation of positively charged gas-phase samples of peptides is used to infer the primary structure of such molecules. In electron capture dissociation (ECD) experiments, very low-energy electrons attach to the sample and rupture bonds to effect the fragmentation. It turns out that ECD fragmentation tends to produce cleavage of very specific types of bonds. In earlier works by this group, it has been suggested that the presence of positive charges produces stabilizing Coulomb potentials that allow low-energy electrons to exothermically attach to sigma orbitals of certain bonds and thus to cleave those bonds. In the present effort, the stabilizing effects of Coulomb potentials due to proximal positive charges are examined for a small model peptide molecule that contains a wide range of bond types. Direct attachment of an electron to the sigma orbitals of eight different bonds as well as indirect sigma bond cleavage, in which an electron first binds to a carbonyl C=O pi orbital, are examined using ab initio methods. It is found that direct attachment to and subsequent cleavage of any of the eight sigma bonds is not likely except for highly positively charged samples. It is also found that attachment to a C=O pi orbital followed by cleavage of the nitrogen-to-alpha-carbon bond is the most likely outcome. Interestingly, this bond cleavage is the one that is seen most commonly in ECD experiments. So, the results presented here seem to offer good insight into one aspect of the ECD process, and they provide a means by which one can estimate (on the basis of a simple Coulomb energy formula) which bonds may be susceptible to cleavage by low-energy electron attachment.     

Negative-ion electron capture dissociation: radical-driven fragmentation of charge-increased gaseous peptide anions

The generation of gaseous polyanions with a Coulomb barrier has attracted attention as exemplified by previous studies of fullerene dianions. However, this phenomenon has not been reported for biological anions. By contrast, electron attachment to multiply charged peptide and protein cations has seen a surge of interest due to the high utility for tandem mass spectrometry (MS/MS). Electron capture dissociation (ECD) and electron transfer dissociation (ETD) involve radical-driven fragmentation of charge-reduced peptide/protein cations to yield N-C(α) backbone bond cleavage, resulting in predictable c'/z(?)-type product ions without loss of labile post-translational modifications (PTMs). However, acidic peptides, e.g., with biologically important PTMs such as phosphorylation and sulfonation, are difficult to multiply charge in positive ion mode and show improved ionization in negative-ion mode. We found that peptide anions ([M - nH](n-), n ≥ 1) can capture electrons within a rather narrow energy range (~3.5-6.5 eV), resulting in charge-increased radical intermediates that undergo dissociation analogous to that in ECD/ETD. Gas-phase zwitterionic structures appear to play an important role in this novel MS/MS technique, negative-ion electron capture dissociation (niECD).     

The effect of fixed charge modifications on electron capture dissociation

Electron capture dissociation (ECD) studies of two modified amyloid beta peptides (20-29 and 25-35) were performed to investigate the role of H* radicals in the ECD of peptide ions and the free-radical cascade (FRC) mechanism. 2,4,6-Trimethylpyridinium (TMP) was used as the fixed charge tag, which is postulated to both trap the originally formed radical upon electron capture and inhibit the H* generation. It was found that both the number and locations of the fixed charge groups influenced the backbone and side-chain cleavages of these peptides in ECD. In general, the frequency and extent of backbone cleavages decreased and those of side-chain cleavages increased with the addition of fixed charge tags. A singly labeled peptide with the tag group farther away from the protonated site experienced a smaller abundance decrease in backbone cleavage fragments than the one with the tag group closer to the protonated site. Despite the nonprotonated nature of all charge carriers in doubly labeled peptide ions, several c and z* ions were still observed in their ECD spectra. Thus, although H* transfer may be important for the NC(alpha) bond cleavage, there also exist other pathways, which would require a radical migration via H* abstraction through space or via an amide superbase mechanism. Finally, internal fragment ions were observed in the ECD of these linear peptides, indicating that the important role of the FRC in backbone cleavages is not limited to the ECD of cyclic peptides.     

Electron capture detector response and dissociative electron attachment cross sections in chloroalkanes and chloroalkenes

Electron capture detectors (ECDs) are widely used in gas chromatography to detect electronegative compounds. In this work, we examine the connections between the ECD response and the cross sections for dissociative electron attachment (DEA) determined from low energy electron beam studies in the chloroalkane family, stressing in particular the role of temporary anion state energies. We show that attachment rate coefficients computed from these cross sections are well correlated with ECD response, and that the latter decreases exponentially with increasing energies of the lowest anion states. ECD measurements are also carried out in monochloroalkanes substituted with unsaturated ethenyl and phenyl moieties, and the response is shown to depend strongly on the mixing between the unsaturated pi* and the C-Cl sigma* temporary anions as exhibited by the vertical attachment energies (VAEs) of these states. The results show good correlations between the chloroalkene and phenyl chloride ECD responses and the VAEs for the mixed states.     

Charge remote fragmentation in electron capture and electron transfer dissociation

Secondary fragmentations of three synthetic peptides (human αA crystallin peptide 1-11, the deamidated form of human βB2 crystallin peptide 4-14, and amyloid β peptide 25-35) were studied in both electron capture dissociation (ECD) and electron-transfer dissociation (ETD) mode. In ECD, in addition to c and z· ion formations, charge remote fragmentations (CRF) of z· ions were abundant, resulting in internal fragment formation or partial/entire side-chain losses from amino acids, sometimes several residues away from the backbone cleavage site, and to some extent multiple side-chain losses. The internal fragments were observed in peptides with basic residues located in the middle of the sequences, which was different from most tryptic peptides with basic residues located at the C-terminus. These secondary cleavages were initiated by hydrogen abstraction at the α-, β-, or γ-position of the amino acid side chain. In comparison, ETD generates fewer CRF fragments than ECD. This secondary cleavage study will facilitate ECD/ETD spectra interpretation, and help de novo sequencing and database searching.     

Divalent metal ion-peptide interactions probed by electron capture dissociation of trications

Electron capture dissociation (ECD) of the peptide Substance P (SubP) complexed with divalent metals has been investigated. ECD of [SubP + H + M]3+ (M2+ = Mg2+ -Ba2+ and Mn2+ -Zn2+) allowed observation of a larger number of product ions than previous investigations of doubly charged metal-containing peptides. ECD of Mg-Ba, Mn, Fe, and Zn-containing complexes resulted in product ions with and without the metal from cleavage of backbone amine bonds (c' and z* -type ions). By contrast, ECD of Co and Ni-containing complexes yielded major bond cleavages within the C-terminal methionine residue (likely to be the metal ion binding site). Cu-containing complexes displayed yet another behavior: amide bond cleavage (b and y'-type ions). We believe some results can be rationalized both within the hot hydrogen atom mechanism and mechanisms involving electron capture into excited states, such as the recently proposed amide superbase mechanism. However, some behavior, including formation of (cn 'M - H)+ ions for Ca-Ba, is best explained within the latter mechanisms with initial electron capture at the metal. In addition, the ECD behavior appears to correlate with the metal second ionization energy (IE2). Co and Ni (displaying sequestered fragmentation) have IE2s of 17.1 and 18.2 eV, respectively, whereas IE2s for Mg-Ba, Mn, and Fe (yielding random cleavage) are 10.0 to 16.2 eV. This behavior is difficult to explain within the hot hydrogen atom mechanism because hydrogen transfer should not be influenced by IE2s. However, the drastically different fragmentation patterns for Co, Ni, and Cu compared to the other metals can also be explained by their higher propensity for nitrogen (as opposed to oxygen) binding. Nevertheless, these results imply that directed fragmentation can be accomplished via careful selection of the cationizing agent.     

Sequence analysis of peptide:oligonucleotide heteroconjugates by electron capture dissociation and electron transfer dissociation

Mass spectrometry analysis of protein-nucleic acid cross-links is challenging due to the dramatically different chemical properties of the two components. Identifying specific sites of attachment between proteins and nucleic acids requires methods that enable sequencing of both the peptide and oligonucleotide component of the heteroconjugate cross-link. While collision-induced dissociation (CID) has previously been used for sequencing such heteroconjugates, CID generates fragmentation along the phosphodiester backbone of the oligonucleotide preferentially. The result is a reduction in peptide fragmentation within the heteroconjugate. In this work, we have examined the effectiveness of electron capture dissociation (ECD) and electron-transfer dissociation (ETD) for sequencing heteroconjugates. Both methods were found to yield preferential fragmentation of the peptide component of a peptide:oligonucleotide heteroconjugate, with minimal differences in sequence coverage between these two electron-induced dissociation methods. Sequence coverage was found to increase with increasing charge state of the heteroconjugate, but decreases with increasing size of the oligonucleotide component. To overcome potential intermolecular interactions between the two components of the heteroconjugate, supplemental activation with ETD was explored. The addition of a supplemental activation step was found to increase peptide sequence coverage over ETD alone, suggesting that electrostatic interactions between the peptide and oligonucleotide components are one limiting factor in sequence coverage by these two approaches. These results show that ECD/ETD methods can be used for the tandem mass spectrometry sequencing of peptide:oligonucleotide heteroconjugates, and these methods are complementary to existing CID methods already used for sequencing of protein-nucleic acid cross-links.     

Identification and localization of the fatty acid modification in ghrelin by electron capture dissociation

Electron capture dissociation (ECD) has been demonstrated to be an effective fragmentation technique for characterizing the site and structure of the fatty acid modification in ghrelin, a 28-residue growth-hormone-releasing peptide that has an unusual ester-linked n-octanoyl (C8:0) modification at Ser-3. ECD cleaves 21 of 23 possible backbone amine bonds, with the product ions (c and z· ions) covering a greater amino acid sequence than those obtained by collisionally activated dissociation (CAD). Consistent with the ECD nonergodic mechanism, the ester-linked octanoyl group is retained on all backbone cleavage product ions, allowing for direct localization of this labile modification. In addition, ECD also induces the ester bond cleavage to cause the loss of octanoic acid from the ghrelin molecular ion; the elimination process is initiated by the capture of an electron at the protonated ester group, which is followed by the radical-site-initiated reaction known as -cleavage. The chemical composition of the attached fatty acid can be directly obtained from the accurate Fourier transform ion cyclotron resonance (FTICR) mass measurement of the ester bond cleavage product ions.     

Study of the dissociation of a charge-reduced phosphopeptide formed by electron transfer from an alkali metal target

Doubly protonated phosphopeptide (YGGMHRQET(p)VDC) ions obtained by electrospray ionization were collided with Xe and Cs targets to give singly and doubly charged positive ions via collision-induced dissociation (CID). The resulting ions were analyzed and detected by using an electrostatic analyzer (ESA). Whereas doubly charged fragment ions resulting from collisionally activated dissociation (CAD) were dominant in the CID spectrum with the Xe target, singly charged fragment ions resulting from electron transfer dissociation (ETD) were dominant in the CID spectrum with the Cs target. The most intense peak resulting from ETD was estimated to be associated with the charge-reduced ion with H2 lost from the precursor. Five c-type fragment ions with amino acid residues detached consecutively from the C-terminal were clearly observed without a loss of the phosphate group. These ions must be formed by N--Calpha bond cleavage, in a manner similar to the cases of electron capture dissociation (ECD) and ETD from negative ions. Although the accuracy in m/z of the CID spectra was about +/-1 Th because of the mass analysis using the ESA, it is supposed from the m/z values of the c-type ions that these ions were accompanied by the loss of a hydrogen atom. Four z-type (or y--NH3, or y--H2O) ions analogously detached consecutively from the N-terminal were also observed. The fragmentation processes took place within the time scale of 4.5 micros in the high-energy collision. The present results demonstrated that high-energy ETD with the alkali metal target allowed determination of the position of phosphorylation and the amino acid sequence of post-translational peptides.     

Hydrogen rearrangement to and from radical z fragments in electron capture dissociation of peptides

Hydrogen rearrangement is an important process in radical chemistry. A high degree of H. rearrangement to and from z. ionic fragments (combined occurrence frequency 47% compared with that of z.) is confirmed in analysis of 15,000 tandem mass spectra of tryptic peptides obtained with electron capture dissociation (ECD), including previously unreported double H. losses. Consistent with the radical character of H. abstraction, the residue determining the formation rate of z' = z. + H. species is found to be the N-terminal residue in z. species. The size of the complementary c(m)' fragment turned out to be another important factor, with z' species dominating over z. ions for m < or = 6. The H. atom was found to be abstracted from the side chains as well as from alpha-carbon groups of residues composing the c' species, with Gln and His in the c' fragment promoting H. donation and Asp and Ala opposing it. Ab initio calculations of formation energies of .A radicals (A is an amino acid) confirmed that the main driving force for H. abstraction by z. is the process exothermicity. No valid correlation was found between the NC(alpha) bond strength and the frequency of this bond cleavage, indicating that other factors than thermochemistry are responsible for directing the site of ECD cleavage. Understanding hydrogen attachment to and loss from ECD fragments should facilitate automatic interpretation ECD mass spectra in protein identification and characterization, including de novo sequencing.     

Toward a general mechanism of electron capture dissociation

The effects of positive charge on the properties of ammonium and amide radicals were investigated by ab initio and density functional theory calculations with the goal of elucidating the energetics of electron capture dissociation (ECD) of multiply charged peptide ions. The electronic properties of the amide group in N-methylacetamide (NMA) are greatly affected by the presence of a remote charge in the form of a point charge, methylammonium, or guanidinium cations. The common effect of the remote charge is an increase of the electron affinity of the amide group, resulting in exothermic electron capture. The N-Calpha bond dissociation and transition state energies in charge-stabilized NMA anions are 20-50 kJ mol(-1) greater than in the hydrogen atom adduct. The zwitterions formed by electron capture have proton affinities that were calculated as 1030-1350 kJ mol(-1), and are sufficiently basic for the amide carbonyl to exothermically abstract a proton from the ammonium, guanidinium and imidazolium groups in protonated lysine, arginine, and histidine residues, respectively. A new mechanism is proposed for ECD of multiply charged peptide and protein cations in which the electron enters a charge-stabilized electronic state delocalized over the amide group, which is a superbase that abstracts a proton from a sterically proximate amino acid residue to form a labile aminoketyl radical that dissociates by N-Calpha bond cleavage. This mechanism explains the low selectivity of N-Calpha bond dissociations induced by electron capture, and is applicable to dissociations of peptide ions in which the charge carriers are metal ions or quaternary ammonium groups. The new amide superbase and the previously proposed mechanisms of ECD can be uniformly viewed as being triggered by intramolecular proton transfer in charge-reduced amide cation-radicals. In contrast, remote charge affects N-H bond dissociation in weakly bound ground electronic states of hypervalent ammonium radicals, as represented by methylammonium, CH3NH3*, but has a negligible effect on the N-H bond dissociation in the strongly bound excited electronic states. This refutes previous speculations that loss of "hot hydrogen" can occur from an excited state of an ammonium radical.     

Does Electron Capture Dissociation Cleave Protein Disulfide Bonds?

Peptide and protein characterization by mass spectrometry (MS) relies on their dissociation in the gas phase into specific fragments whose mass values can be aligned as ‘mass ladders’ to provide sequence information and to localize possible posttranslational modifications. The most common dissociation method involves slow heating of even-electron (M+n H)ⁿ⁺ ions from electrospray ionization by energetic collisions with inert gas, and cleavage of amide backbone bonds. More recently, dissociation methods based on electron capture or transfer were found to provide far more extensive sequence coverage through unselective cleavage of backbone N–Cα bonds. As another important feature of electron capture dissociation (ECD) and electron transfer dissociation (ETD), their unique unimolecular radical ion chemistry generally preserves labile posttranslational modifications such as glycosylation and phosphorylation. Moreover, it was postulated that disulfide bond cleavage is preferred over backbone cleavage, and that capture of a single electron can break both a backbone and a disulfide bond, or even two disulfide bonds between two peptide chains. However, the proposal of preferential disulfide bond cleavage in ECD or ETD has recently been debated. The experimental data presented here reveal that the mechanism of protein disulfide bond cleavage is much more intricate than previously anticipated.     

The role of conformation on electron capture dissociation of ubiquitin

Effects of protein conformation on electron capture dissociation (ECD) were investigated using high-field asymmetric waveform ion mobility spectrometry (FAIMS) and Fourier-transform ion cyclotron resonance mass spectrometry. Under the conditions of these experiments, the electron capture efficiency of ubiquitin 6+ formed from three different solution compositions differs significantly, ranging from 51 +/- 7% for ions formed from an acidified water/methanol solution to 88 +/- 2% for ions formed from a buffered aqueous solution. This result clearly indicates that these protein ions retain a memory of their solution-phase structure and that conformational differences can be probed in an ECD experiment. Multiple conformers for the 7+ and 8+ charge states of ubiquitin were separated using FAIMS. ECD spectra of conformer selected ions of the same charge states differ both in electron capture efficiency and in the fragment ion intensities. Conformers of a given charge state that have smaller collisional cross sections can have either a larger or smaller electron capture efficiency. A greater electron capture efficiency was observed for ubiquitin 6+ that has the same collisional cross section as one ubiquitin 7+ conformer, despite the lower charge state. These results indicate that the shape of the molecule can have a greater effect on electron capture efficiency than either collisional cross section or charge state alone. The cleavage locations of different conformers of a given charge state were the same indicating that the presence of different conformers in the gas phase is not due to difference in where charges are located, but rather reflect conformational differences most likely originating from solution. Small neutral losses observed from the singly- and doubly-reduced ubiquitin 6+ do not show a temperature dependence to their formation, consistent with these ions being formed by nonergodic processes.     

Side-chain fragmentation of alkylated cysteine residues in electron capture dissociation mass spectrometry

The recent development of novel fragmentation processes based on either electron capture directly or transfer from an anion show great potential for solving problems in proteomics that are intractable by the more widely employed thermal-based fragmentation processes such as collision induced dissociation. The dominant fragmentation occurring upon electron capture dissociation of peptides is cleavage of N-C alpha bonds in the peptide backbone to form c and z* ions. In the case of disulfide-linked peptides, it has also been shown that electron capture on one of the cystine sulfur atoms is favored, resulting in cleavage of the disulfide bond. In this study, we report that electron capture on the sulfur of alkylated cysteine residues is also a dominant process, causing cysteine side-chain loss from z* ions.     

Electron capture dissociation of gaseous multiply charged ions by Fourier-transform ion cyclotron resonance

Fourier-transform ion cyclotron resonance instrumentation is uniquely applicable to an unusual new ion chemistry, electron capture dissociation (ECD). This causes nonergodic dissociation of far larger molecules (42 kDa) than previously observed (<1 kDa), with the resulting unimolecular ion chemistry also unique because it involves radical site reactions for similarly larger ions. ECD is highly complementary to the well known energetic methods for multiply charged ion dissociation, providing much more extensive protein sequence information, including the direct identification of N- versus C-terminal fragment ions. Because ECD only excites the molecule near the cleavage site, accompanying rearrangements are minimized. Counterintuitively, cleavage of backbone covalent bonds of protein ions is favored over that of noncovalent bonds; larger (>10 kDa) ions give far more extensive ECD if they are first thermally activated. This high specificity for covalent bond cleavage also makes ECD promising for studying the secondary and tertiary structure of gaseous protein ions caused by noncovalent bonding.     

An antibiotic linked to peptides and proteins is released by electron capture dissociation fourier transform ion cyclotron resonance mass spectrometry

Desfuroylceftiofur (DFC) is a bioactive beta-lactam antibiotic metabolite that has a free thiol group. Previous experiments have shown release of DFC from plasma extracts after addition of a disulfide reducing agent, suggesting that DFC may be bound to plasma and tissue proteins through disulfide bonds. We have reacted DFC with [Arg(8)]-vasopressin (which has one disulfide bond) and bovine insulin (which has three disulfide bonds) and analyzed the reaction products by use of electron capture dissociation Fourier transform ion cyclotron resonance mass spectrometry (ECD FT-ICR MS), which has previously shown preferential cleavage of disulfide bonds. We observe cleavage of DFC from vasopressin and insulin during ECD, suggesting that DFC is indeed bound to peptides and proteins through disulfide bonds. Specifically, we observed dissociative loss of one, as well as two, DFC species during ECD of [vasopressin + 2(DFC-H) + 2H](2+) from a single electron capture event. Loss of two DFCs could arise from either consecutive or simultaneous loss, but in any case implies a gas phase disulfide exchange step. ECD of [insulin + DFC + 4H](4+) shows preferential dissociative loss of DFC. Combined with HPLC, ECD FT-ICR-MS may be an efficient screening method for detection of drug-biomolecule binding.     

Long-lived electron capture dissociation product ions experience radical migration via hydrogen abstraction

To explore the mechanism of electron capture dissociation (ECD) of linear peptides, a set of 16-mer peptides were synthesized with deuterium labeled on the alpha-carbon position of four glycines. The ECD spectra of these peptides showed that such peptides exhibit a preference for the radical to migrate to the alpha-carbon position on glycine via hydrogen (or deuterium) abstraction before the final cleavage and generation of the detected product ions. The data show c-type fragment ions, ions corresponding to the radical cation of the c-type fragments, c*, and they also show c*-1 peaks in the deuterated peptides only. The presence of the c*-1 peaks is best explained by radical-mediated scrambling of the deuterium atoms in the long-lived, metastable, radical intermediate complex formed by initial electron capture, followed by dissociation of the complex. These data suggest the presence of at least two mechanisms, one slow, one fast. The abundance of H* and -CO losses from the precursor ion changed upon deuterium labeling indicating the presence of a kinetic isotope effect, which suggests that the values reported here represent an underestimation of radical migration and H/D scrambling in the observed fragments.     

Empty level structure and dissociative electron attachment cross section in (bromoalkyl)benzenes

The gas-phase electron transmission (ET) and dissociative electron attachment (DEA) spectra are reported for the series of (bromoalkyl)benzenes C6H5(CH2)nBr (n = 0-3), where the bromine atom is directly bonded to a benzene ring or separated from it by 1-3 CH2 groups, and the dihalo derivative 1-Br-4-Cl-benzene. The relative DEA cross sections (essentially due to the Br- fragment) are reported, and the absolute cross sections are also evaluated. HF/6-31G and B3LYP/6-31G* calculations are employed to evaluate the virtual orbital energies (VOEs) for the optimized geometries of the neutral state molecules. The pi* VOEs, scaled with empirical equations, satisfactorily reproduce the corresponding experimental vertical electron attachment energies (VAEs). According to the calculated localization properties, the LUMO (as well as the singly occupied MO of the lowest lying anion state) of C6H5(CH2)3Br is largely localized on both the benzene ring and the C-Br bond, despite only a small pi*/sigma*C-Br interaction and in contrast to the chlorine analogue where the LUMO is predicted to possess essentially ring pi character. This would imply a less important role of intramolecular electron transfer in the bromo derivative for production of the halogen negative fragment through dissociation of the first resonant state. The VAEs calculated as the anion/neutral energy difference with the 6-31+G* basis set which includes diffuse functions are relatively close to the experimental values but do not parallel their sequence. In addition the SOMO of some compounds is not described as a valence MO with large pi* character but as a diffuse sigma* MO.