Gas-phase Ion-molecule Reactions Research Articles

Studies of Heterogeneous/Homogeneous Ion-Molecule Reactions by Ambient Ionization Mass Spectrometry

The general concepts, features and applications of ambient ionization mass spectrometry for studying the heterogeneous/homogeneous ion-molecule reactions are reviewed. Important ambient ionization mass spectrometric methods and their applications in organic chemistry are summarized, focusing on SESI (secondary electrospray ionization), DESI (desorption electrospray ionization-mass spectrometry), ESSI (electrosonic spray ionization), EESI (extractive electrospray ionization-mass spectrometry), ELDI (electrospray-assisted laser desorption/ionization), LTP (low-temperature plasma probe) and APTDI (atmospheric-pressure thermal desorption ionization) technologies. At the same time, the advantages of ambient ionization mass spectrometry in studying organic reactions allow chemists to explore new type of organic reactions, probe reaction mechanisms and to intercept/characterize transient reactive intermediates in ambient condition. Keywords: Ambient mass spectrometry, ionization, desorption, transient reactive intermediates, heterogeneous/homogeneous ion-molecule reaction, Gas phase ion-molecule reaction, analytical technique, organic reactions, atom bombardment, atmospheric pressure, intrinsic, Electrospray Ionization, Nebulizing Gas, unsaturated lipid, ozone

Models of Hot Cores with Complex Molecules

AbstractRecent models of hot cores have incorporated previously-uninvestigated chemical pathways that lead to the formation of complex organic molecules (COMs; i.e. species containing six or more atoms). In addition to the gas-phase ion-molecule reactions long thought to dominate the organic chemistry in these regions, these models now include photodissociation-driven grain surface reaction pathways that can also lead to COMs. Here, simple grain surface ice species photodissociate to form small radicals such as OH, CH3, CH2OH, CH3O, HCO, and NH2. These species become mobile at temperatures above 30 K during the warm-up phase of star formation. Radical-radical addition reactions on grain surfaces can then form an array of COMs that are ejected into the gas phase at higher temperatures. Photodissociation experiments on pure and mixed ices also show that these complex molecules can indeed form from simple species. The molecules predicted to form from this type of chemistry reasonably match the organic inventory observed in high mass hot cores such as Sgr B2(N) and Orion-KL. However, the relative abundances of the observed molecules differ from the predicted values, and also differ between sources. Given this disparity, it remains unclear whether grain surface chemistry governed by photodissociation is the dominant mechanism for the formation of COMs, or whether other unexplored gas-phase reaction pathways could also contribute significantly to their formation. The influence that the physical conditions of the source have on the chemical inventory also remains unclear. Here we overview the chemical pathways for COM formation in hot cores. We also present new modeling results that begin to narrow down the possible routes for production of COMs based on the observed relative abundances of methyl formate (HCOOCH3) and its C2H4O2 structural isomers.

Differentiation of Protonated Aromatic Regioisomers Related to Lignin by Reactions with Trimethylborate in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

Several lignin model compounds were examined to test whether gas-phase ion-molecule reactions of trimethylborate (TMB) in a FTICR can be used to differentiate the ortho-, meta-, and para-isomers of protonated aromatic compounds, such as those formed during degradation of lignin. All three regioisomers could be differentiated for methoxyphenols and hydroxyphenols. However, only the differentiation of the ortho-isomer from the meta- and para-isomers was possible for hydroxyacetophenones and hydroxybenzoic acids. Consideration of the previously reported proton affinities at all basic sites in the isomeric hydroxyphenols, and the calculated proton affinities at all basic sites in the three methoxyphenol isomers, revealed that the proton affinities of the analytes relative to that of TMB play an important role in determining whether and how they react with TMB. The loss of two methanol molecules (instead of one) from the adducts formed with TMB either during ion-molecule reactions, or during sustained-off resonance irradiated collision-activated dissociation of the ion-molecule reaction products, revealed the presence of two functionalities in almost all the isomers. This finding supports earlier results suggesting that TMB can be used to count the functionalities in unknown oxygen-containing analytes.

Mild route to generate gaseous metal anions

Gaseous metal anions such as Na(-), K(-), Cs(-), and Ag(-) can be generated at ambient temperatures by the collision-induced dissociation of the anions of several dicarboxylic acid salts, including oxalate, maleate, fumarate, succinate, and glutamate salts. The formation of gaseous metal anions in this way is unprecedented because the metal is initially present in its cationic form. The mild process described here could facilitate novel applications of metal anions as selective reagents for gas-phase ion-molecule and ion-ion reactions. Ab initio calculations were used to describe the dissociation process for anions of the oxalate salts. The formation of alkalides occurs via production of a metal-carbon dioxide anion intermediate with a bidentate three-center two-electron bond to the metal. The metal atom acquires a partial negative charge in the intermediate structure.

Gas-phase ion-molecule reactions using regioselectively generated radical cations to model oxidative damage and probe radical sites in peptides

Collision induced dissociation (CID) of sodiated peptide derivatives containing a nitrate ester functionality was used to regiospecifically generate three isomeric radicals of the model peptide Bz-Ala-Gly-OMe corresponding to radicals formed at: C(α) of the alanine residue [4+Na](+); C(α) of the glycine residue [5+Na](+); and the side chain of alanine [6+Na](+). The ion-molecule reactions of these peptide radicals were examined to model oxidative damage to peptides and to probe whether the radical sites maintain their integrity or whether they isomerise via intramolecular hydrogen atom transfer (HAT). Only [6+Na](+) is reactive towards O(2), forming the peroxyl radical [7+Na](+), which loses O(2), HO˙ and HO(2)˙ under CID. The radical ion [7 + Na](+) abstracts a hydrogen atom from 4-fluorothiophenol to form the hydroperoxide [8+Na](+), which upon CID fragments via the combined loss of HO˙ and CH(2)O. In contrast, all three of the isomeric sodiated radicals react with NO˙ and NO(2)˙ to form adducts. CID of the NO adducts only regenerates the radicals via NO˙ loss, thus providing no structural information. In contrast, CID of the NO(2) adducts gives rise to a range of product ions and the spectra are different for each of the three adducts, suggesting that the isomeric radicals [4+Na](+), [5+Na](+) and [6+Na](+) are produced as discrete species. Finally, CID of the NO(2) adducts was used to probe the rearrangement of the radicals [4+Na](+), [5+Na](+) and [6+Na](+) prior to their reaction with NO(2)˙: [6 + Na](+) rearranges to a mixture of [4+Na](+) and [5+Na](+) while [5+Na](+) rearranges to [4+Na](+).

Tautomerization in gas‐phase ion chemistry of isomeric C‐8 deoxyguanosine adducts from phenol‐induced DNA damage

Collision-induced dissociation (CID) of 8-(4''-hydroxyphenyl)-2'-deoxyguanosine and 8-(2''-hydroxyphenyl)-2'-deoxyguanosine was investigated using sequential tandem mass spectrometry. These adducts represent biomarkers of DNA damage linked to phenolic radicals and were investigated to gain insight into the effects of chemical structure of a C-8 modification on fragmentation pathways of modified 2'-deoxyguanosine (dG). CID in MS(2) of the deprotonated molecules of both the isomers generated the same product ion having the same m/z values. CID in MS(3) of the product ion at m/z 242 and CID in MS(4) experiments carried out on the selected product ions at m/z 225 and m/z 218 afford distinct fragmentation patterns. The conformational properties of isomeric product ions from CID showed that the ortho-isomers possess the unique ability to tautomerize through an intramolecular proton transfer between the phenolic OH group and the imine nitrogen (N7). Tautomerization of ortho-isomers to their keto-tautomers led to differences in their system of conjugated double bonds compared with either their enol-tautomer or the para-isomer. The charge redistribution through the N-7 site on the imidazole ring is a critical step in guanosine adduct fragmentation which is disrupted by the formation of the keto-tautomer. For this reason, different reaction pathways are observed for 8-(4''-hydroxyphenyl)-2'-deoxyguanosine and 8-(2''-hydroxyphenyl)-2'-deoxyguanosine. We present herein the dissociation and the gas-phase ion-molecule reactions for highly conjugated ions involved in the CID ion chemistry of the investigated adducts. These will be useful for those using tandem mass spectrometry for structural elucidation of C-8 modified dG adducts. This study demonstrates that the modification at the C-8 site of dG has the potential to significantly alter the reactivity of adducts. We also show the ability of tandem mass spectrometry to completely differentiate between the isomeric dG adducts investigated.

A Preliminary Approach to Mimic the Formation of POPs in the Atmosphere：Chlorination of Benzene in the Gas Phase

The process of persistent organic pollutants( POPs) formation was studied by using the collision chamber of tandem mass spectrometer instead of atmospheric environment. It is found that chlorobenzenes can be formed through gas phase ion-molecule reaction in the collision chamber. And the formation of chlorobenzenes in the ion source of the mass spectrometer is also observed. When benzene content is 4 × 10 -3 Pa and acetyl chloride content is 4 × 10 -4 Pa,the chlorobenzene content reaches 5 × 10 -8 Pa,while the background of chlorobenzene is only 5 × 10 -9 Pa. So it is possible that the formation of chlorobenzene by gas phase ion-molecule reactions,because chlorobenzene is formed easier in atmosphere than that in the collision chamber.

Electrospray MS‐based characterization of β‐carbolines – mutagenic constituents of thermally processed meat

The beta-carbolines 1-methyl-9H-pyrido [3,4-b]indole and 9H-pyrido[3,4b]indole have been implicated as having causative roles in a number of human diseases, such as Parkinson's disease and cancer. As they can be formed during the heating of protein-rich food, a number of analytical methodologies have been proposed for their detection and quantification in foodstuff. For this purpose, LC-MS and LC-MS/MS have emerged as the most specific analytical methods, and the quantification is based on the occurrence of unusual ions, such as [M+H-(H(*))]+ and [M+H-2H]+. In this study, we have investigated the formation of these ions by accurate-mass electrospray MS/MS and demonstrated that these ions are formed from gas-phase ion-molecule reactions between water vapor present in the collision cell and the protonated molecule of 1-methyl-9H-pyrido [3,4-b]indole and 9H-pyrido[3,4b]indole. Although this reaction has been previously described for heterocyclic amine ions, it has been overlooked in the most of recent LC-MS and LC-MS/MS studies, and no complete data of the fragmentation are reported. Our results demonstrate that additional attention should be given with respect to eliminating water vapor residues in the mass spectrometer when analysis of beta-carbolines is performed, as this residue may affect the reliability in the results of quantification.

Competitive Ion-Molecule Reactions within V+(CH3COOCH3)nClusters

The importance of transition metal ions in catalyzing the chemical reactions of various organic molecules has prompted extensive experimental and theoretical studies aimed at under- standing the catalytic activity of such ions. In this context, the examination of gas phase ion-molecule reactions within hetero- clusters, which avoids the complications arising from solvent environments and crystalline forces, has great potential for the detailed elucidation of the catalytic activities of transition metal ions and of the changes in the reaction pathways as a function of cluster size. 1

Identification and Counting of Carbonyl and Hydroxyl Functionalities in Protonated Bifunctional Analytes by Using Solution Derivatization Prior to Mass Spectrometric Analysis Via Ion-Molecule Reactions

A mass spectrometric method has been developed for the identification of carbonyl and hydroxyl functional groups, as well as for counting the functional groups, in previously unknown protonated bifunctional oxygen-containing analytes. This method utilizes solution reduction before mass spectrometric analysis to convert the carbonyl groups to hydroxyl groups. Gas-phase ion-molecule reactions of the protonated reduced analytes with neutral trimethylborate (TMB) in a FT-ICR mass spectrometer give diagnostic product ions. The reaction sequence likely involves three consecutive steps, proton abstraction from the protonated analyte by TMB, addition of the neutral analyte to the boron reagent, and elimination of a neutral methanol molecule. The number of methanol molecules eliminated upon reactions with TMB reveals the number of hydroxyl groups in the analyte. Comparison of the reactions of the original and reduced analytes reveals the presence and number of carbonyl and hydroxyl groups in the analyte.

An experimental guided-ion-beam and ab initio study of the ion-molecule gas-phase reactions between Li+ ions and iso-C3H7Cl in their ground electronic state

Reactive collisions between Li(+) ions and i-C(3)H(7)Cl molecules have been studied in the 0.20-12.00 eV center-of-mass energy range using an octopole radio frequency guided-ion beam apparatus recently developed in our laboratory. At low collision energies, dehydrohalogenation reactions giving rise to Li(C(3)H(6))(+) and Li(HCl)(+) are the main reaction channels, while at higher ones C(3)H(7)(+) and C(2)H(3)(+) become dominant, all their reactive cross sections having been measured as a function of the collision energy. To obtain information about the potential energy surfaces (PESs) on which the reactive processes take place, ab initio calculations at the MP2 level have been performed. For dehydrohalogenations, the reactive ground singlet PES shows ion-molecule adduct formation in both the reactant and product sides of the surface. Following the minimum energy path connecting both minima, an unstable intermediate and the corresponding barriers, both lying below the reactant's energy, have been characterized. The entrance channel ion-molecule adduct is also involved in the formation of C(3)H(7)(+), which then generates C(2)H(3)(+) via an CH(4) unimolecular elimination. A qualitative interpretation of the experimental results based on ab initio calculations is also included.

Ion-molecule reaction mechanism of SF + + PSCl 3

The ion-molecule reaction mechanism of SF + with PSCl 3 is explored theoretically at the CCSD(T)/6-311+G(2d) level using the B3LYP/6-311G(d), and B3LYP/6-311++G(d) geometries. Various possible reaction pathways are probed. It is shown that eight dissociation products P 1 (S 2F + + PCl 3), P 2 (S 2Cl + + PFCl 2), P 3 (SPCl 2 + + FSCl), P 4 (PCl 2 + + FSSCl), P 5 (S 2 + + PFCl 3), P 6 (FSPCl + + SCl 2), P 7 (FSPClS + + Cl 2) and P 8 (PSCl 3 + + SF) are both thermodynamically and kinetically accessible. Among the eight dissociation products, P 8 is the most favorable product with the largest yields, whereas P 1 is much less competitive followed by the least feasible products P 2 and P 3 . The remaining products, P 4 , P 5 , P 6 , and P 7 may have negligible yields. Significant discrepancies on the product distributions are found between our calculated results and previous experimental observations. Therefore, future experimental reinvestigation on the SF + + PSCl 3 reaction is desirable to clarify the mechanism of the title reaction. The present work may be helpful for understanding the analogous ion-molecule reactions in gas phase.

An Ion/Molecule Reaction for the Identification of Analytes with Two Basic Functional Groups

A mass spectrometric method is presented for the identification of analytes with two basic functionalities and PA between 222 and 245 kcal/mol, including diamines. This method utilizes gas-phase ion-molecule reactions of protonated analytes with neutral 1,1-diethoxyethene (DEE) in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR). A variety of protonated mono-, bi-, and trifunctional analytes containing different functional groups, namely, amido, amino, N-oxide, hydroxy, carboxylic acid, keto, thio, thioether, alkene, phosphite, and phosphonate, were tested in the FT-ICR. The results demonstrate that basic protonated bifunctional compounds (PA between 222 and 245 kcal/mol) react selectively with DEE by forming a specific addition/elimination product ion (adduct - EtOH) (this product was also observed for lysine with three functionalities). The diagnostic reaction sequence involves proton transfer from the protonated analyte to the basic vinyl group in DEE, followed by addition of one of the functional groups of the analyte to the electrophilic alpha-carbon in protonated DEE. The next step involves proton transfer from this functionality to the other analyte functionality, followed by proton transfer to DEE and elimination of ethanol. Since the mechanism involves proton transfer between two functional groups of the analyte, the reaction does not occur for analytes where the two functionalities cannot be in close proximity (i.e., meta-phenylenediamine), and where no proton is available (i.e., dimethylaminoketone).

Ion–molecule reactions facilitate the identification and differentiation of primary, secondary and tertiary amino functionalities in protonated monofunctional analytes in mass spectrometry

A mass spectrometric method is presented that facilitates the identification and differentiation of primary, secondary and tertiary amino functionalities in protonated monofunctional analytes. This method utilizes gas-phase ion–molecule reactions of protonated analytes with neutral hexamethylphosphoramide (HMPA) and diethylmethylphosphonate (DEMP) in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR). A variety of protonated analytes containing different functional groups, namely, amino, amido, N-oxide and various oxygen-containing functional groups, were examined to demonstrate that protonated primary and secondary amines can be identified and differentiated by reactions with HMPA and DEMP. However, differentiation of tertiary amines from some N-oxides requires additional experiments. First, protonated secondary and tertiary amines can be differentiated from protonated primary amines, amides and oxygen-containing functionalities, as well as from each other (but not from protonated N-oxides), by using HMPA. Protonated primary amines, amides, some N-oxides and oxygen-containing analytes, most with a proton affinity (PA) < 229 kcal/mol, only transfer a proton to HMPA (PA = 229 kcal/mol). In contrast, protonated secondary amines also form two stable hydrogen-bound adducts (MH + + HMPA, MH + + 2HMPA; M: amine), and tertiary amines and some N-oxides (with PA ≥ 222 kcal/mol) react with HMPA by forming just one stable hydrogen-bound adduct (MH + + HMPA). Further, ion–molecule reactions with the other reagent, DEMP, allow the differentiation of protonated primary and secondary amines from tertiary amines and N-oxides and from protonated oxygen-containing analytes and amides. Protonated primary amines and secondary amines (most with PA ≥ 220 kcal/mol) react with DEMP (PA = 219 kcal/mol) by forming two stable hydrogen-bound adducts (MH + + DEMP and MH + + 2DEMP), while protonated oxygen-containing analytes and amides (with PA ≤ 221 kcal/mol) solely transfer a proton to DEMP. Protonated tertiary amines and N-oxides react yet differently, and yield only one stable hydrogen-bound adduct (MH + + DEMP) with DEMP. Protonated N-oxides can be differentiated from protonated tertiary amines by using previously reported methods based on diagnostic ion–molecule reactions of protonated N-oxides with 2-methoxypropene, dimethyl disulfide and/or tri(dimethylamine)borane.

Ligand and electronic-structure effects in metal-mediated gas-phase activation of methane: A cold approach to a hot problem

Gas-phase ion-molecule reactions, complemented by DFT and wave-function based electronic structure calculations, are presented, which permit rather detailed descriptions of two fundamental processes related to the activation of methane at room temperature. (i) Recent examples are discussed, which shed light on the role of oxygen-centered radicals in the first step of the oxidative dimerization of methane, i.e. the homolytic C-H bond cleavage of methane. (ii) Thermal ligand exchange processes of the type ML(+) + CH(4) --> M(CH(3))(+) + LH are analyzed in detail with an emphasis on L = H and F attached to various transition-metal cations M(+). It will be demonstrated inter alia that similar systems, e.g. MH(+) (M = Ni, Pd, Pt) in their room-temperature reactions with methane, actually exhibit fundamentally different mechanistic scenarios.

Influence of Fragmentor Voltage and Solvent on Negative Ionization Behaviors of Uvinul 3039 Using LC/APCI-MS

Engineering plastics undergo degradation when exposed to the ultraviolet radiation in sunlight. This degradation process results in appearance changes such as discoloration, chalking, and crazing as well as reduction of physical properties. UV absorbers slow down the degradation process by absorbing harmful ultraviolet radiation and dissipating it as thermal energy. Analysis of UV absorbers is very important to understand polymer degradation studies and qualityassurance testing. 2-Ethylhexyl-2-cyano-3,3-diphenylacrylate (Uvinul 3039) is one of popular cyanoacrylate UV absorbers since it is aprotic and stable UV absorber. Uvinul 3039 has a single absorption band with a maximum at 305 nm. Analysis of UV absorbers is difficult because of requirement of a sensitive method for identification, high molecular weight, and high polarity. Thus, gas chromatography (GC) is not suitable for analysis of UV absorbers. Although high performance liquid chromatography (HPLC) with UV detection has been successfully used, this method is only applicable to compounds having UV-absorbable groups. These problems can be solved by the use of liquid chromatography/mass spectrometry (LC/MS), 1,2 especially LC/MS with atmospheric pressure chemical ionization (LC/APCIMS) 3 . Block and coworkers 3 analyzed antioxidants and UV stabilizers for polymeric materials using LC/APCI-MS. In APCI, a corona discharge makes solvent reactant ions that are used to ionize the sample by chemical ionization. Solute and sample are vaporized by pneumatic nebulization. APCI relies upon gas-phase ion-molecule reactions to place a charge on neutral analytes, so it is especially important to understand these reactions. Kostiainen and Bruins reported that a judicious selection of solvent can be made to promote ionization of the compound of interest. 4-7 When mobile phase transports analyte molecules, APCI can deliver a proton to the analyte. The solvent-derived ions serve as reagents for proton transfer ionization. Mass spectrometry with APCI gives a great deal of structural information in a short analysis time. In APCI-MS, fragmentation pattern can be obtained by varying the fragmentor voltage (or accelerating voltage) and some fragment ions are produced to identify the structure of an analyte. 8 A key parameter in tuning the instrument’s sensitivity is the fragmentor voltage (similar to cone voltage in other instruments). At higher fragmentor voltage settings, collisioninduced dissociation (CID) can be initiated in the region between the end of the transfer capillary and the first skimmer cone, so that fragmentation increases. 9 In this study, we analyzed Uvinul 3039 using LC/APCI-MS with negative ion mode and investigated the ionization behaviors such as formation of solvent-adducted ion and fragmentation by varying the fragmentor voltage and kind of solvent. Methanol, ethanol, and acetone were employed as the solvent and eluent. These solvents have been used for extraction of organic additives including UV absorbers from plastic polymers such as polyethylene (PE), polypropylene (PP), poly(ethylene terephthalate) (PET), and poly(vinyl

Formation of Complex Organics from Acetylene Catalyzed by Ionized Benzene

We present direct evidence for low temperature associative charge transfer (ACT) reactions of acetylene onto the benzene cation that catalyze the conversion of acetylene molecules into polymerized cations and for high temperature addition/elimination reactions that lead to the generation of naphthalene-type ions. At low temperatures acetylene molecules bind noncovalently to the benzene cation, where partial charge transfer from the ion activates an acetylene molecule for addition polymerization with other associated acetylene molecules, thus amounting to catalytic cyclization/polymerization of the acetylene molecules. At high temperatures the barrier of the covalent addition of acetylene to the benzene cation to form a styrene-type ion is measured as 3.5 kcal/mol. The second acetylene addition followed by H elimination to form a naphthalene-type ion is calculated to be highly exothermic and without a barrier. These reactions can explain the formation of complex organics by gas phase ion-molecule reactions under a wide range of temperatures and pressures in astrochemical environments.

Coupling of ion‐molecule reactions with liquid chromatography on a quadrupole ion trap mass spectrometer

We report for the first time a coupling of gas-phase ion-molecule reactions with chromatographic separations on a quadrupole ion trap mass spectrometer. The interface was accomplished by using a pulsed valve for the introduction of a volatile neutral into the ion trap. The pulsed valve controller is synchronized with the mass spectrometer software. The setup requires some minor modifications to the vacuum system of the commercial quadrupole ion trap but most of the modifications are external to the mass spectrometer. Two applications of this interface are described: differentiation between two phosphoglucose positional isomers and detection of a phosphopeptide in a peptide mixture. Both applications are using the reactivity of trimethoxyborate towards a phosphate moiety in the negative ion mode. The detection of phosphopeptides hinges on our findings that non-phosphorylated peptide anions do not react with trimethoxyborate. This LC/MS detection can be easily visualized in terms of selected reaction monitoring.

Constituents with independence from growth temperature for bacteria using pyrolysis-gas chromatography/differential mobility spectrometry with analysis of variance and principal component analysis

Four bacteria, Escherichia coli, Pseudomonas aeruginosa, Staphylococcus warneri, and Micrococcus luteus, were grown at temperatures of 23, 30, and 37 degrees C and were characterized by pyrolysis-gas chromatography/differential mobility spectrometry (Py-GC/DMS) providing, with replicates, 120 data sets of retention time, compensation voltage, and ion intensity, each for negative and positive polarity. Principal component analysis (PCA) for 96 of these data sets exhibited clusters by temperature of culture growth and not by genus. Analysis of variance was used to isolate the constituents with dependences on growth temperature. When these were subtracted from the data sets, Fisher ratios with PCA resulted in four clusters according to genus at all temperatures for ions in each polarity. Comparable results were obtained from unsupervised PCA with 24 of the original data sets. The ions with taxonomic features were reconstructed into 3D plots of retention time, compensation voltage, and Fisher ratio and were matched, through GC-mass spectrometry (MS), with chemical standards attributed to the thermal decomposition of proteins and lipid A. Results for negative ions provided simpler data sets than from positive ions, as anticipated from selectivity of gas phase ion-molecule reactions in air at ambient pressure.

Gas-Phase Ion Chemistry of GeH4/B2H6 Mixtures

The gas phase ion-molecule reactions in positively and negatively ionized germane/diborane mixtures have been studied by ion trap mass spectrometry. Reaction sequences and rate constants for the most interesting processes have been determined. In positive ionization, formation of Ge-B bonds exclusively occurs through condensation reactions of B(n)H(m)(+) ions with germane, followed by H(2) or BH(3) loss. No reactions of ions from germane with B(2)H(6) were observed under the experimental conditions used here. In negative ionization, the Ge(n)H(m)(-) (n = 1, 2) ion families react with diborane to yield the Ge(n)B(p)H(q)(-) (p = 1, 2) ions, again via dehydrogenation and BH(3) loss, while diborane anions proved to be unreactive. In both positive and negative ionization, Ge-B ions reach appreciable abundances. The present results afford fundamental information about the intrinsic reactivity of gas-phase ions and provide valuable indications about the first nucleation steps ultimately leading to amorphous Ge and B-doped semiconductor materials by chemical vapor deposition methods.