Cluster phase chemistry: gas-phase reactions of anionic sodium salts of dicarboxylic acid clusters with water molecules

A homologous series of anionic gas-phase clusters of dicarboxylic acids (oxalic acid, malonic acid, succinic acid, glutaric acid, and adipic acid) generated via electrospray ionization (ESI) are investigated using collision-induced dissociation (CID). Sodiated clusters with the composition (Na(+))(2)(n+1)(dicarboxylate(2-)(n+1) for singly charged anionic clusters, where n = 1-4, are observed as major gas-phase species. Isolation of the clusters followed by CID results mainly in sequential loss of disodium dicarboxylate moieties for the clusters of succinic acid, glutaric acid, and adipic acid (C4-C6). However, all oxalate (C2) and malonate (C3) clusters and dimers (n = 1) of succinate (C4) and glutarate (C5) exhibit more complex chemistry initiated by collision of the activated cluster with water molecules. For example, with water addition, malonate clusters dissociate to yield sodium acetate, carbon dioxide, and sodium hydroxide. More generally, water molecules serve as proton donors for reacting dicarboxylate anions in the cluster and introduce energetically favorable dissociation pathways not otherwise available. Density functional theory (DFT) calculations of the binding energy of the cluster correlate well with the cluster phase reactions of oxalate and malonate clusters. Clusters of larger dicarboxylate ions (C4-C6) are more weakly bound, facilitating the sequential loss of disodium dicarboxylate moieties. The more strongly bound small dicarboxylate anions (oxalate and malonate) preferentially react with water molecules rather than dissociate to lose disodium dicarboxylate monomers when collisionally activated. Implications of these results for the atmospheric aerosol chemistry of dicarboxylic acids are discussed.     

Comparative study of mono- and dinuclear complexes of late 3d-metal chlorides with N,N-dimethylformamide in the gas phase

Mono- and binuclear complexes of N,N-dimethylformamide (DMF) with chlorides of the divalent, late 3d metals M = Co, Ni, Cu, and Zn are investigated by means of electrospray ionization (ESI). Specifically, ESI leads to monocations of the type [(DMF)(n)MCl](+) and [(DMF)(n)M(2)Cl(3)](+), of which the species with n = 2 and 3 were selected for in-depth studies. The latter include collision-induced dissociation experiments, gas-phase infrared spectroscopy, and calculations using density functional theory. The mononuclear complexes [(DMF)(n)MCl](+) almost exclusively lose neutral DMF upon collisional activation with the notable exception of the copper complex, for which also a reduction from Cu(II) to Cu(I) concomitant with the release of atomic chlorine is observed. For the dinuclear clusters, there exists a competition between loss of a DMF ligand and cluster degradation via loss of neutral MCl(2) with decreasing cluster stability from cobalt to zinc. For the specific case of [(DMF)(n)ZnCl](+) and [(DMF)(n)Zn(2)Cl(3)](+), ion-mobility mass spectrometry indicates the existence of two isomeric cluster ions in the case of [(DMF)(2)Zn(2)Cl(3)](+) which corroborates parallel theoretical predictions.     

Electrospray Lonization Fourier Transform Ion Cyclotron Resonance Mass Spectrometric Study on Sodium Azide Cluster Ions

Sodium azide has rarely been studied in gas phase or in the form of cluster ions and as a model of solid energetic substances and inorganic azide salt was ionized by electrospray ionization (ESI) and studied by high resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) systematically. This paper highlights the effects of experimental conditions on the formation of salt cluster and the collision activation dissociation pathways of cluster ions to develop a microscopic understanding of inorganic azide salt clusters.     

Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometric Study on Sodium Azide Cluster Ions

Introduction Mass spectrometry (MS) has been an important technique for studying size-dependent cluster properties with unparalleled sensitivity, specificity, and versatil-ity.1,2 Especially Fourier transform ion cyclotron reso-nance (FT-ICR) MS offers the benefits of ultra-high mass resolving power, superior mass accuracy, and the ability of affording useful structure information with tandem MS. The introduction of electrospray ionization (ESI) as a gentle mode of ion generation for MS…     

电喷雾电离质谱在化学中应用新进展

电喷雾电离质谱(ESI-MS)是本世纪发展的非常重要的质谱,具有无碎片的特点,可分析检测非挥发性的、极性的、热不稳定的化合物。评述了ESI-MS在富勒烯化学、无机配合物、簇合物、有机化学反应,金属有机化合物及超分子化学中的应用进展。     

Cluster ions of diquat and paraquat in electrospray ionization mass spectra and their collision-induced dissociation spectra

Cluster ions such as [Cat+X+nM](+) (n = 0-4); [Cat-H+nM](+) (n = 1-3); and [2(Cat-H)+X+nM](+) (n = 0-2), where Cat, X, and M are the dication, anion, and neutral salt (CatX(2)), respectively, are observed in electrospray ionization (ESI) mass spectrometry of relatively concentrated solutions of diquat and paraquat. Collision-induced dissociation (CID) reactions of the clusters were observed by tandem mass spectrometry (MS/MS), including deprotonation to form [Cat-H](+), one-electron reduction of the dication to form Cat(+.), demethylation of the paraquat cation to form [Cat-CH(3)](+), and loss of neutral salt to produce smaller clusters. The difference in acidity and reduction power between diquat and paraquat, evaluated by thermodynamical estimates, can rationalize the different fractional yields of even-electron ([Cat-H](+) and its clusters) and odd-electron (mostly Cat(+)) ions in ESI mass spectra of these pesticides. The [Cat+n. Solv](2+) doubly charged cluster ions, where n 相似文献   

Negative ion electrospray ionization mass spectral study of dicarboxylic acids in the presence of halide ions

The negative ion electrospray ionization (ESI) mass spectra of a series of dicarboxylic acids, a pair of isomeric (cis/trans) dicarboxylic acids and two pairs of isomeric (positional) substituted benzoic acids, including a pair of hydroxybenzoic acids, were recorded in the presence of halide ions (F(-), Cl(-), Br(-) and I(-)). The ESI mass spectra contained [M--H](-) and [M+X](-) ions, and formation of these ions is found to be characteristic of both the analyte and the halide ion used. The analytes showed a greater tendency to form adduct ions with Cl(-) under ESI conditions compared with the other halide ions used. The isomeric compounds yielded distinct spectra by which the isomers could be easily distinguished. The collision-induced dissociation mass spectra of [M+X](-) ions reflected the gas-phase basicities of both the halide ion and [M--H](-) ion of the analyte. However, the relative ordering of gas-phase basicities of all analyte [M--H](-) and halide ions could not account for the dominance of chloride ion adducts in ESI mass spectra of the analytes mixed with equimolar quantities of the four halides.     

Cluster phase chemistry: collisions of vibrationally excited cationic dicarboxylic acid clusters with water molecules initiate dissociation of cluster components

A homologous series of cationic gas-phase clusters of dicarboxylic acids (oxalic acid, malonic acid, succinic acid, glutaric acid, and adipic acid) generated via electrospray ionization (ESI) are investigated using collision-induced dissociation (CID). Singly charged cationic clusters with the composition (Na(+))(2n+1)(dicarboxylate(2-))(n), where n = 1-5, are observed as major gas-phase species. Significant abundances of singly charged sodiated hydrogen dicarboxylate clusters with the composition (Na(+))(2n)(dicarboxylate(2-))(n)(H+), where n = 1-6, are observed with oxalic acid, malonic acid, and succinic acid. Isolation of the clusters followed by CID results mainly in sequential loss of disodium dicarboxylate moieties for the clusters of succinic acid, glutaric acid, and adipic acid. However, the dimer of sodiated hydrogen succinate, all malonate clusters, and all oxalate clusters, with the exception of the dimer, exhibit complex chemical reactions initiated by the collision of vibrationally excited clusters with water molecules. Generally, water molecules serve as proton donors for reacting dicarboxylate anions in the cluster, initiating dissociation pathways such as the decomposition of the malonate ion to yield an acetate ion and CO(2). The reactivity of several mixed dicarboxylate clusters is also reported. For example, malonate anion is shown to be more reactive than oxalate anion for decarboxylation when both are present in a cluster. The energetics of several representative cluster phase reactions are evaluated using computational modeling. The present results for cationic clusters are compared and contrasted to earlier studies of anionic sodiated dicarboxylic acid clusters.     

Formation of cationic peptide radicals by gas-phase redox reactions with trivalent chromium, manganese, iron, and cobalt complexes

The collision-induced dissociation (CID) of a series of gas-phase complexes [M(III)(salen)(P)](+) [where M = Cr, Mn, Fe, and Co; P = hexapeptides YGGFLR, WGGFLR, and GGGFLR; and salen = N,N'-ethylenebis(salicylideneaminato)] has been examined with respect to the ability of the complexes to form the corresponding cationic peptide radical ions, P(+)(*), by homolytic cleavage of the metal peptide bond. This is the first example of the use of gas-phase ternary metal peptide complexes to produce the corresponding cationic peptide radical for a metal other than copper(II). The fragmentation reactions competing with radical formation are highly dependent on the metal ion used. In addition, examination of modified complexes in which the periphery of the salen was substituted allowed evaluation of electronic effects on the CID process, presumably without significant change in the geometry surrounding the metal. This substitution demonstrates that the ligand can be used to tune the dissociation chemistry to favor radical formation and suppress unwanted further fragmentation of the peptide radical that is typically observed immediately following its dissociation from the complex.     

Exploring the effect of metal ions and counteranions on the structure and magnetic properties of five dodecanuclear Co(II) and Ni(II) clusters

We present the synthesis, characterization of the structures, and magnetic properties of five isostructural dodecanuclear coordination clusters of Ni(II) and Co(II): [Co(12)(bm)(12)(NO(3))(O(2)CMe)(6)(EtOH)(6)](NO(3))(5) (1), [Ni(12)(bm)(12)(NO(3))(O(2)CMe)(6)(H(2)O)(3)(EtOH)(3)](NO(3))(5)·2H(2)O (2), mixed-metal composition (Ni/Co 1:1) [Co(6)Ni(6)(bm)(12)(NO(3))(O(2)CMe)(6)(NO(3))(5) (3), and [M(12)(bm)(12)(NO(3))(O(2)CMe)(6)(EtOH)(6)](ClO(4))(5) (M=Co (4), Ni (5)), in which Hbm=(1H-benzimidazol-2-yl)methanol. They consist of analogous structural cores that are constructed by three cubanes (M(4)O(4)) that surround the templating nitrate and bridging auxiliary acetate and the directing ligands bm. They have different magnetic behaviors. Whereas there is the absence of the out-of-phase ac susceptibility (χ') for the Ni(II)-based compounds 2 and 5, the Co(II)-containing compounds 1, 3, and 4 have prominent χ' signals that exhibit frequency dependence, which indicates slow magnetic relaxation behavior above 1.8 K. In particular, the larger perchlorate counterions in 4 further change the overall correlation interaction between clusters, thus leading to an enhanced blocking temperature for the less-symmetrical 4 (pseudo-C(3)) relative to 1 and 3 (true C(3)). Interestingly, electrospray ionization mass spectrometry (ESI-MS) indicates that the three dodecanuclear clusters of 1-3 retain their compositions in solution. The mixed-metal cluster cores of 3 are formed based on the nature of the interchangeability between metal centers in solution.     

Assessment of the various ionization methods in the analysis of metal salen complexes by mass spectrometry

Metal salen complexes are one of the most frequently used catalysts in enantioselective organic synthesis. In the present work, we compare a series of ionization methods that can be used for the mass spectral analysis of two types of metalosalens: ionic complexes (abbreviated as Com⁺X^?) and neutral complexes (NCom). These methods include electron ionization and field desorption (FD) which can be applied to pure samples and atmospheric pressure ionization techniques: electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) which are suitable for solutions. We found that FD is a method of choice for recording molecular ions of the complexes containing even loosely bonded ligands. The results obtained using atmospheric pressure ionization methods show that the results depend mainly on the structure of metal salen complex and the ionization method. In ESI spectra, Com⁺ ions were observed, while in APCI and APPI spectra both Com⁺ and [Com + H]⁺ ions are observed in the ratio depending on the structure of the metal salen complex and the solvent used in the analysis. For complexes with tetrafluoroborate counterion, an elimination of BF₃ took place, and ions corresponding to complexes with fluoride counterion were observed. Experiments comparing the relative sensitivity of ESI, APCI and APPI (with and without a dopant) methods showed that for the majority of the studied complexes ESI is the most sensitive one; however, the sensitivity of APCI is usually less than two times lower and for some compounds is even higher than the sensitivity of ESI. Both methods show very high linearity of the calibration curve in a range of about 3 orders of magnitude of the sample concentration. Copyright © 2014 John Wiley & Sons, Ltd.     

Cluster ions derived from sodium and potassium tetrafluoroborate and their collision induced dissociation in an ion trap mass spectrometer

Electrospray ionization was used to produce distributions of gas-phase cluster ions from solutions of sodium and potassium tetrafluoroborate. The majority of the cluster species followed the trend (MBF(4))(n)M(+), where M=Na and K. The values of n, for both salts, ranged from 1-15. Collision induced dissociation (MS/MS and MS(n)) in an ion trap mass spectrometer was used to determine the dissociation pathways for the cluster ions. The (NaBF(4))(n)Na(+) cluster ions fragmented via two pathways: (a) the loss of one or multiple neutral BF(3) molecules and (b) the loss of one or more NaBF(4) units. Of the two, the product ions corresponding to the loss of BF(3) units were more prominent. Unlike the Na salt, the (KBF(4))(n)K(+) cluster ions decomposed primarily by the loss of one or multiple KBF(4) units. Similar differences in dissociation behavior were observed when the heated transfer capillary, normally used to desolvate ions, was used to investigate cluster ion stability to thermal degradation and dissociation. The dissociation profiles (decrease in ion abundance with increasing activation amplitude) for several (NaF)(n)Na(+) and (KF)(n)K(+) cluster ions were measured and compared to probe the influence of the relative stability of the alkali fluorides (NaF and KF) on the dissociation behavior exhibited by the tetrafluoroborate cluster distributions. We found that the (NaF)(n)Na(+) cluster ions required higher activation amplitudes to induce fragmentation than the corresponding (KF)(n)K(+) species, indicative of stronger ionic bonding and higher gas-phase stability for the former. This in turn indicates that the reaction pathway involving only the loss of one or multiple units of BF(3), favored for the (NaBF(4))(n)Na(+) cluster series, but not for the analogous (KBF(4))(n)K(+) series, may be due to the high gas-phase stability of NaF, and relatively lower stability of KF, towards dissociation.     

Gas-phase synthesis and reactivity of binuclear gold hydride cations, (R3PAu)2H+ (R = Me and Ph)

Electrospray ionization of a mixture of the two gold phosphine chlorides, R3PAuCl (R = Ph and Me), silver nitrate and the amino acid N,N-dimethylglycine (DMG) yields a range of gold containing cluster ions including: (R3P)Au(PR'3)+; (R3PAu)(R'3PAu)Cl+ and (R3PAu)(R'3PAu)(DMG-H)+ (where R = R' = Ph; R = R' = Me; R = Me and R' = Ph). Collision induced dissociation (CID) of the (R3PAu)(R'3PAu)(DMG-H)+ precursor ions yielded the hitherto unknown gold hydride dimers (R3PAu)(R'3PAu)H+. The gas-phase chemistry of these dimers was studied using ion-molecule reactions, collision induced dissociation, electronic excitation dissociation (EED) and DFT calculations on the (H3PAu)2H+ model system. A novel phosphine ligand migration was found to occur prior to fragmentation under CID conditions and this was supported by DFT calculations, which revealed a transition state with a bridging phosphine ligand.     

Solvation of copper(II) sulfate in binary water/N,N-dimethylformamide mixtures: from the solution to the gas phase

The solvation of copper(II) sulfate in binary mixtures of water and N,N-dimethylformamide (DMF) is studied by a combined approach using electrochemical studies in solution and a mass spectrometric assay of the solvated ions formed from these solutions upon electrospray ionization (ESI). In the condensed phase, the limiting transference numbers (t(+/-)(o)) and the apparent ion association constants (K(A)'s) of CuSO(4) have been determined in water/DMF solutions at 20 degrees C. The t(+)(o) values decrease with increasing DMF content, demonstrating a gradual solvation of Cu(2+) by DMF molecules. The association constants indicate that aggregation becomes more pronounced as the DMF content increases. In order to achieve complementary insight, the intrinsic interactions among the ions and solvent molecules are investigated in gas-phase experiments of the CuSO(4)/water/DMF system using ESI mass spectrometry. Under the conditions used, the dications [Cu(DMF)(n)](2+) (n = 3-6), [Cu(2)(DMF)(n)SO(4)](2+) (n = 2-7), and [Cu(3)(DMF)(n)(SO(4))(2)](2+) (n = 2-7), and the monocations [Cu(OH)(DMF)(n)](+), [Cu(DMF)(n)(HSO(4))](+) (both, n = 1-3), and [Cu(DMF)(n)](+) (n = 1, 2), are formed as the leading copper-containing cations. Likewise, polynuclear copper clusters observed in the anion ESI spectra support partial aggregation occurring in solution. The gas-phase studies clearly support the conclusions that (i) DMF is a highly preferred ligand for CuII in comparison to water and that (ii) DMF supports ion association for which the mass spectrometric data suggest the formation of polynuclear copper clusters.     

Some thoughts on electrospray ionization mechanisms

Electrospray ionization (ESI) mechanisms are highly complex, due to a series of physical and chemical phenomena taking place on a complex system, as a solution is. In fact, even if the solution of an analyte in a protic medium can be considered at first sight to be a two-component system, the presence of solvent dissociation equilibria and the possible interactions solvent-solvent dissociation products, solvent dissociation products-analyte make this system highly complex, also for the presence of possible ionic compounds (for example, Na(+), K(+)) which strongly affect the above equilibria. A high number of research articles have been published, mainly devoted to charged droplet production and to gas-phase ion generation. They all show the high complexity of the processes affecting electrospray measurements related to either the chemical equilibria present in the condensed phase and to electrolysis processes at the emitter tip or to the processes occurring in the sprayed droplets. As a result, the chemical composition inside the small droplets from which the analyte ions are generated can be significantly different from those in sprayed solution. In this review, after a short survey of the proposed ESI mechanisms, some experiments are described. They were performed to examine if ion mobility in solution, before the formation of the sprayed charged droplets, can affect the ESI results. The data, obtained by studying both inorganic and organic analytes, indicate that the ESI spectra are dependent on the analyte dimension and charge state which, as a consequence, affect their ion mobility in solution.     

Structure and dissociation characteristics of metal chloride anion clusters containing redox-active metal ions studied by laser desorption and electrospray ionization mass spectrometry and ab initio calculations

Copper chloride anion clusters with both copper oxidation states can be made by laser desorption of CuCl(2) crystals. We have used this method to study the dissociation characteristics of such cluster ions. The stability and the structure of the observed complexes were probed by ab initio calculations. These calculations show that many of these complexes are bridged structures. Thus, for the Cu(2)Cl(4) dimer anion, formally [ClCu-Cl-CuCl(2)](-) , with putative mixed copper oxidation states, the two copper ions become equivalent through bridging. Such bridging does not occur when redox inactive metal ions are present as in [ClCu-Cl-CaCl(2)](-) . By observing the dissociation characteristics of a variety of metal chloride cluster anions produced from binary mixtures, the following Cl(-) affinity order is obtained: FeCl(3) > CuCl > CaCl(2) > FeCl(2) > AgCl ≈ CuCl(2) ≈ ZnCl(2) > LiCl. Ab initio calculations on the Cl(-) affinity of selected chlorides confirm this order as do Cl(-) affinity estimates from the experimentally known vertical electron detachment energies of the superhalogens CaCl(3)(-) and LiCl(2)(-) . An equimolar mixture of CuCl(2) and FeCl(3) produces an intense cluster ion, which, from (65)Cu labeling experiments, is best described as FeCl(4)(-)···Cu(+)···(-)Cl(4) Fe, a Cu(+) bound superhalogen FeCl(4)(-) dimer. The Cu(+) ion can be replaced by the redox inactive alkali cations and by Ag(+) but these metal ion bound FeCl(4)(-) dimers show an entirely different fragmentation behavior which is attributed to the absence of bridging. Electrospray ionization (ESI) of CuCl(2) produces an extended series of (CuCl(2))(n) Cl(-) anions (n = 1-11) and so in ESI very limited reduction of Cu(2+) takes place. The (CuCl(2))(n) Cl(-) anions show an abundant dissociation via loss of neutral Cu(2)Cl(4) which according to our ab initio calculations is 9 kcal/mol more stable than two CuCl(2).     

Assigning the ESI mass spectra of organometallic and coordination compounds

Electrospray ionization mass spectrometry (ESI‐MS) is a useful technique for solving organometallic and coordination chemistry characterization problems that are difficult to address using traditional methods. However, assigning the ESI mass spectra of such compounds can be challenging, and the considerations involved in doing so are quite different from assigning the mass spectra of purely organic samples. This is a tutorial article for organometallic/coordination chemists using ESI‐MS to analyze pure compounds or reaction mixtures. The fundamentals of assigning ESI mass spectra are discussed within the context of organometallic and coordination systems. The types of ions commonly observed by ESI‐MS are categorized and described. Finally, a step‐by‐step guide for the assignment of organometallic and coordination chemistry ESI mass spectra is provided along with two case studies.     

On the distributions of ion/neutral molecule clusters in electrospray and laser spray—A cluster division model for the electrospray process

The clustering of a medium-sized, involatile, neutral molecule, octyl beta-D-glucopyranoside (OG), with Na(+), Ca(2+), and Yb(3+) (M(z+)) ions in electrospray (ESI) was investigated using laser spray (LSI). Extensive distributions of [(M(z+))(i) (OG)(a)](n+)-clusters, extending beyond 50 kDa, were observed. The distributions were highly stable and reproducible and changed only marginally when concentrations of electrolyte or neutral compound were varied by orders of magnitude. Compared with ESI, laser spray yielded superior intensities, particularly of the larger clusters. The cluster distributions demonstrated a range of remarkable features. In particular, the Yb(3+)/OG cluster distribution was unusual. For example, no clusters with 35-52 or with 110-116 OG molecules were observed. The distribution pattern revealed that the clusters were formed as a result of cluster dissociations, such as [(Yb(3+))(3)(OG) ( approximately 110)W](9+) --> [(Yb(3+))(2)(OG)( approximately 90)W](6+) + [(Yb(3+))(1)(OG) ( approximately 20)W](3+), where W represents the water content at the time of dissociation. Based on this study, a cluster division model for electrospray of aqueous solutions of strongly solvated ions is proposed: the Rayleigh droplet disintegration process, which is well-established for the initial stages of electrospray, maintains its general character as it proceeds through a final regime of multiply charged cluster dissociations to the singly and multiply charged ions in mass spectrometry. In the dissociation of multiply charged clusters, the size of each daughter cluster is roughly proportional to the square of the cluster charge. Observed cluster distributions are consistent with a mixture of symmetric and asymmetric cluster dissociations.     

Energetics and dynamics of electron transfer and proton transfer in dissociation of metal(III)(salen)-peptide complexes in the gas phase

Time- and collision energy-resolved surface-induced dissociation (SID) of ternary complexes of Co(III)(salen)+, Fe(III)(salen)+, and Mn(III)(salen)+ with several angiotensin peptide analogues was studied using a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially equipped to perform SID experiments. Time-resolved fragmentation efficiency curves (TFECs) were modeled using an RRKM-based approach developed in our laboratory. The approach utilizes a very flexible analytical expression for the internal energy deposition function that is capable of reproducing both single-collision and multiple-collision activation in the gas phase and excitation by collisions with a surface. The energetics and dynamics of competing dissociation pathways obtained from the modeling provides important insight on the competition between proton transfer, electron transfer, loss of neutral peptide ligand, and other processes that determine gas-phase fragmentation of these model systems. Similar fragmentation behavior was obtained for various Co(III)(salen)-peptide systems of different angiotensin analogues. In contrast, dissociation pathways and relative stabilities of the complexes changed dramatically when cobalt was replaced with trivalent iron or manganese. We demonstrate that the electron-transfer efficiency is correlated with redox properties of the metal(III)(salen) complexes (Co > Fe > Mn), while differences in the types of fragments formed from the complexes reflect differences in the modes of binding between the metal-salen complex and the peptide ligand. RRKM modeling of time- and collision-energy-resolved SID data suggests that the competition between proton transfer and electron transfer during dissociation of Co(III)(salen)-peptide complexes is mainly determined by differences in entropy effects while the energetics of these two pathways are very similar.     

Laser Desorption Ionization Versus Electrospray Ionization Mass Spectrometry: Applications in the Analysis of Cluster Anions

Mass spectra of transition metal carbonyl cluster anions were recorded using laser desorption ionization time-of-flight (LDI-TOF) and electrospray ionization (ESI) techniques. The LDI spectra generally contain peaks corresponding the intact cluster together with extensive CO loss fragments ions whereas the ESI spectra exhibit peaks corresponding the intact cluster together with few (if any) CO loss fragment ions. The parameters of both techniques can be modified to vary the extent of fragmentation. In all cases no fragmentation of the metal core is observed. Overall, ESI is a more informative method for the analysis of these types of cluster anions.