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The previously proposed mechanism of the double hydrogen transfer in the molecular ion of 1-methoxy-2-propanol, which yields protonated dimethyl ether and a C 2 H 3 O radical [1], has been re-examined. To obtain a more refined picture of the structures of the intermediates and transition states as well as the energetics of this reaction, calculations using the CBS-QB3 model chemistry have been performed. In addition, collision-induced dissociative ionization (CIDI) experiments have been carried out which show that the eliminated C 2 H 3 O radical is solely the acetyl radical. Theory predicts that hydrogen-bridged radical cations (HBRCs) play a pivotal role in the reaction and that the lowest energy route involves loss of a CH 3 CO radical, by proton transport catalysis. Our mechanistic study also accounts for the minor H/D exchange observed in the DO-analogue of 1-methoxy-2-propanol and it proposes a revision of the energetics of the mechanisms of the earlier study [1].

The gas-phase negative-ion chemistry of 2-methylpropenal has been studied by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR). Deuterium labelling has shown that the gas-phase acidities of the methyl protons and the formyl proton are almost equal. The gas-phase reactions of the 2-formylallyl anion with several fluorinated substrates have been shown to proceed mainly via a cycloaddition mechanism.

The nucleophilic addition-elimination reaction on the carbonyl group of the substrates (CH3)3CCOCl, (CH3)3CCOSCH3 and (CH3)3CCON-CH = N-N = CH [(CH3)3CCOTAZ] has been studied using Fourier-Transform Ion-Cyclotron-Resonance (FT-ICR) Mass Spectrometry. The rates of reactions of (CH3)3CCOCl and (CH3)3CCOSCH3 appear to depend on the basicity of the nucleophiles with the more basic ones reacting most rapidly, while the reactions of (CH3)3CCOTAZ with various nucleophiles occur either at the collision rate for exothermic reactions or not at all for endothermic reactions. These results are for (CH3)3CCOCl and (CH3)3CCOSCH3 in agreement with a double-well potential model having the tetrahedral structure as central energy barrier. The results for the substrate (CH3)3CCOTAZ, however, can be explained very well with a single-well potential model, where the tetrahedral structure corresponds to an energy minimum.

In this article a retrospective overview will be presented on the development of ionic reaction mechanistic and catalytic studies over the last fifty years in mass spectrometry. The topic will be covered by a selection of sub-sections, including the McLafferty rearrangement, ion/molecule complexes during unimolecular dissociations of ions, distonic ions, H/D exchange in negative ion/molecule reactions, nucleophilic aromatic substitution, stereochemistry, small hydride solvated molecules and the radical anion H2O −, catalysis, reactive intermediates and reaction mechanistic studies in solution by use of electrospray ionization.

In this article a retrospective overview will be presented on the development of ionic reaction mechanistic and catalytic studies over the last fifty years in mass spectrometry. The topic will be covered by a selection of sub-sections, including the McLafferty rearrangement, ion/molecule complexes during unimolecular dissociations of ions, distonic ions, H/D exchange in negative ion/molecule reactions, nucleophilic aromatic substitution, stereochemistry, small hydride solvated molecules and the radical anion H2O −, catalysis, reactive intermediates and reaction mechanistic studies in solution by use of electrospray ionization.

It is shown on the basis of deuterium labeling that in the metastable molecular ions of N -(5-phenylvaleryl)-1-azacyclopentane-2-thione, decomposing by unimolecular fragmentation, both benzylic hydrogens migrate to the thiolactam ring to generate the protonated γ-thiobutyric lactam species with m / z 102. A mechanism is given for the formation of these ions that is proposed to be mediated by intramolecular acid–base reactions and an ion/molecule complex.

The metastable ion dissociations of the (M − COOCH 3 ) + ions ( m / z 135) generated upon electron ionization from dimethyl phthalate ( 1 ), isophthalate ( 2 ) and terephthalate ( 3 ), have been studied by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling. These ions all show as primary fragmentation channels the losses of methyl and carbon monoxide to give the ions m / z 120 and 107, respectively. The latter ions decompose further by the losses of another molecule of carbon monoxide or a molecule of formaldehyde to generate the ions at m / z 79 and 77, respectively. An additional dissociation channel is observed for the (M − COOCH 3 ) + ions from 2 + and 3 + , the loss of methanol to give the ions m / z 103. The dissociation mechanism of the (M − COOCH 3 ) + ions from 1 + is shown to be identical to that of the (M − CH 3 ) + ions from ionized 2-methoxyacetophenone. The latter ions have the 2-methoxybenzoyl cation structure demonstrating a methoxy migration in the (M − COOCH 3 ) + ions from 1 + precedes their metastable ion dissociations. Part of the (M − COOCH 3 ) + ions from 2 + and 3 + has also rearranged to the 2-methoxybenzoyl cation structure prior to dissociation, but the significantly enhanced loss of methyl and the loss of methanol from these ions occur from the unrearranged structures as indicated by comparison with the dissociation behavior of the (M − CH 3 ) + ions from ionized 3- and 4-methoxyacetophenones.

The unimolecular metastable decompositions of ethoxytrimethylsilane ((CH3)3SiOCH2CH3; 1, MW: 118) and its fluorine analogue, 2,2,2-trifluoroethoxytrimethylsilane ((CH3)3SiOCH2CF3, 2; MW: 172) induced by electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling. Both molecular ions are formed in low abundance. The fragmentation of 1+ · is different from that of 2+ · , except for the loss of CH3 from the corresponding molecular ions. The MIKE spectra of the [M - CH3]+ ions from 1+ · and 2+ · gave five and three fragment ions, respectively. Two of the five reactions for 1 leading to the formation of the ion at m/z 101, (CH3)2Si+OCH = CH2, and m/z 61, CH3Si+HOH, concern new mechanistic pathways, which were not described in previous reports. Skeletal rearrangement by among others F and CF3 migrations, occurs during the fragmentation of the [M - CH3]+ ions from 2+ · . The m/z 61 ions from 2+ · are both CH2OCF+ (or CH2FCO+ etc.) and CH3Si+HOH.

The gas-phase ion/molecule reactions of organic molecules containing several functional groups, including amino, hydroxy and carboxy groups, have been investigated under nitromethane chemical ionization conditions. Three main reaction channels are observed in the ion source: (a) proton transfer, (b) electron transfer and (c) hydride abstraction. The product ion [M+NO]+ is also formed, but in a very low abundance. Initial electrophilic attack of the nitrosonium ion on the aromatic ring is postulated to explain the elimination of HNO from the [M+NO]+ adduct ions, observed for all substrates studied. Elimination of water is a characteristic fragmentation pathway for all substrates possessing a benzylic hydroxy group. Fragment ions resulting from cleavage of the molecular ions of the amines, formed by charge transfer, react with the neutral molecules forming two types of adduct ions: [M+immonium]+ and [M+C7H7]+, which have been characterized through the study of their unimolecular decompositions. The latter provide strong evidence for the existence of two types of structures: a covalent and an ion/molecule complex, that is a non-covalent structure.

The unimolecular metastable decompositions of methyl and ethyl trifluoroacetoacetates, CF 3 COCH 2 COOCH 3 (MW: 170 ( 1 )) and CF 3 COCH 2 COOCH 2 CH 3 (MW: 184 ( 2 )) induced by electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling. In the metastable time window, the molecular ions 1 + decompose to give exclusively the ions at m / z 101 [ M −CF 3 ] + . However, the metastably decomposing ions 2 + lead not only to the formation of the major fragment ion m / z 115 [ M −CF 3 ] + , but also to three minor fragment ions m / z 164 [ M −HF] + , m / z 156 [ M −C 2 H 4 ] + and m / z 87. A large part of the metastably decomposing ions 1 + and 2 + has the enol form. The loss of CO 2 from the ions m / z 101 and m / z 115 occurs through migration of the methyl and ethyl groups, respectively. The source-generated m / z 69 ions from 1 + and 2 + are most abundant and consist of both CF 3 + and OCCHCO + . The latter ion, a protonated carbon suboxide, is generated by at least three and four different fragmentation routes from 1 + and 2 + , respectively. The m / z 43 ion, C 2 H 3 O + , from 2 + is formed by at least two different routes.

The unimolecular metastable decompositions of trimethylsilylacetic acid, (CH3)3SiCH2COOH (1), and its methyl ester, (CH3)3SiCH2COOCH3 (2), were investigated by mass-analyzed ion kinetic energy (MIKE) spectrometry in conjunction with thermochemical data. The abundance of the molecular ions of both compounds, generated by electron ionization, is extremely low. However, the abundance of the ions generated by the loss of .CH3 and observed at m/z 117 and 131 is moderate. These fragment ions further decompose to form the most abundant m/z 75 and 89 ions, respectively, by the loss of CH2CO through a (CH3)2Si group migration. The loss of CH2CO is also observed to occur from 2+. and its fragment ion at m/z 115 generated by the loss of .OCH3. The former reaction is proposed to occur via an ion–radical complex. Copyright © 2002 John Wiley & Sons, Ltd.

The unimolecular metastable decompositions of bis(2,2,2-trifluoroethyl) ether, CF3CH2OCH2CF3 (1), and ethyl 2,2,2-trifluoroethyl ether, CH3CH2OCH2CF3 (2), induced by electron ionization, have been investigated by mass-analyzed ion kinetic energy (MIKE) spectrometry and energy-dependent collision-induced dissociation (CID) spectra in conjunction with thermochemical data. In the metastable time window, the molecular ions of 1 decompose into the ions at m/z 113 by the loss of CF3 and m/z 83 by the consecutive losses of CF3 and CH2O, while those of 2+ • decompose into the ions at m/z 113 and m/z 59 by the elimination of CH3 and CF3 , respectively. The decomposition of the latter fragment ion is similar to that of the m/z 59 ion from the corresponding fluorine-free analogue, diethyl ether, CH3CH2OCH2CH3 (3), that is, fragment ions m/z 41 and m/z 31 are observed, which correspond to the competitive losses of H2O and C2H4, respectively. The metastable ions m/z 113 from 1+ • and 2+ • decompose in a variety of ways: in addition to the fragment ion m/z 65, which is generated by the elimination of CHFO, four other fragment ions with m/z 83, 63, 61, and 31 are observed , which correspond to the competitive losses of CH2O, (CH2O+HF), CH2F2, and C2HF3 (or C2H4F2O), respectively.

The structural characterization of the ions generated by the electron ionization-induced loss of ammonia from the molecular ions of α,ω-diamines, using ion/molecule reactions in combination with collision-induced dissociation (CID) studies, is described. The results of the experiments of ion/molecule reactions using dimethyl disulfide exclude the distonic radical cation structure for those long-lived ions proposed earlier by other authors for ions generated within a few microseconds following ionization. The unimolecular and CID characteristics of the ions [M---NH3]√+ of 1,4-diaminobutane and 1,5-diaminopentane and of their fragment ion m/z=56, are discussed in terms of the structures CH3CH2CH=CHNH2√+ and CH3CH2CH2CH=CHNH2√+ for the ions [M---NH3]√+, respectively. The structure of the closed shell ions resulting from loss of ammonia from the protonated α,ω-diamines was also probed through the CID spectra of model ions prepared by chemical ionization with methane in the chemical ionization source of the mass spectrometer.

The m/z 75 fragment ions are formed by the loss of a neutral species with 43 Da from the molecular ions of ethyl lactate [CH 3 CH(OH)COOCH 2 CH 3 ]. In contrast to the previously reported results, it is shown that the population of these ions consists of at least three different ionic species, which are CH + (OH)OCH 2 CH 3 (protonated ethyl formate), CH 3 CH(OH)O + =CH 2 , and CH + (OH)COOH (protonated glyoxylic acid). The protonated ethyl formate species decomposes into the m/z 47 ion (protonated formic acid) by the loss of ethylene, the second ion decomposes into the m/z 45 ion (protonated acetaldehyde) by the loss of formaldehyde and the protonated glyoxylic acid eliminates two molecules of carbon monoxide to generate the m/z 19 ion (protonated water).

The unimolecular metastable decomposition of 1-methoxy-2-propanol, CH3OCH2CH(OH)CH3 (mol. wt. 90) induced by electron ionization, has been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling in conjunction with thermochemical data. In the metastable time window, the molecular ions decompose almost exclusively into ions at m/z 47 [i.e. protonated dimethyl ether, CH3O+(H)CH3] by the loss of a C2H3O radical species following a double hydrogen atom transfer (DHT). Until now, only one DHT mechanism has been proposed, involving and accounting for the loss of an acetyl radical, C2H3O. In the present study it is shown that more DHT mechanisms are operative, leading to the losses of isomeric C2H3O radicals. The results obtained are best explained by the formation of the key intermediate ion–molecule complexes [CH3OCH3+•, CH3CHO] and [CH3OCH3, CH2=C(H)OH+•] following unimolecular metastable dissociation of the molecular ion. Subsequent hydrogen atom abstraction channels by CH3OCH3+• in the former complex and proton abstraction channels by CH3OCH3 in the latter complex lead eventually to the formation of protonated dimethyl ether with m/z 47.

This paper gives a brief description of the recently formalized International Mass Spectrometry Society (IMSS). It is presented here in order to increase awareness of the opportunities for collaboration in mass spectrometry in an international context. It also describes the recent 15th International Mass Spectrometry Conference, held August/September 2000, in Barcelona. Each of the authors is associated with the IMSS. The 15th Conference, which covers all of mass spectrometry on a triennial basis, was chaired by Professor Emilio Gelpi of the Instituto de Investigaciones Biomedicas, Barcelona. The outgoing and founding President of the IMSS is Professor Graham Cooks, Purdue University, and the incoming President is Professor Nico Nibbering, University of Amsterdam. Similar material has been provided to the Editors of other journals that cover mass spectrometry. Copyright © 2001 John Wiley & Sons, Ltd.

The molecular structure of a series of hyperbranched polyesteramides was studied using size exclusion chromatography (SEC), mass spectrometry (MS) and small-angle neutron scattering (SANS). Products with increasing molecular mass were obtained through the polycondensation of in situ produced AB2-like monomeric units. Electrospray ionization MS indicates that narrow SEC fractions consist of different isomers, the number of which increases with molecular mass. Molecular mass moments and polydispersity numbers increase with polycondensation degree. The SANS measurements and SEC−DV data yield values for the polymers fractal dimension, which can be rationalized in the framework of percolation theory that originally was designed for randomly branched polymers. Randomly branched polymer behavior in this particular case possibly results from a side reaction involving reactivity among B-end groups.

The noncovalent synthesis of "layer-block" metallodendrimers containing hydrophobic shells of covalent dendritic wedges at the periphery is described. Starting from first- and second-generation Fréchet wedges having phosphines at their focal point, convergent dendritic growth yields third- and fourth-generation metallodendrimers in which the coordination of nitriles, pyridines, and phosphines to SCS Pd(II) pincers is used as the assembly motif. In this convergent growth, the number of terminal hydrophobic phosphine wedges increases with generation. The solubility of the dendritic structures in apolar organic solvents such as chloroform and dichloromethane increases accordingly, in contrast to previously reported metallodendrimers. All dendritic structures were characterized by (1)H and (31)P NMR spectroscopy, elemental analysis, and MALDI-TOF mass spectrometry.

The role of various mass spectrometric methods, including electron ionization, collisional activation, metastable peak shapes, analysis of neutrals from ionic unimolecular dissociations, field ionization kinetics, drift cell, and Fourier transform ion cyclotron resonance spectrometry, in ionic reaction mechanistic studies is described. This is illustrated by selected examples of research performed in the author’s group over the last three decades. They comprise inter alia intramolecular acid–base, anchimeric assistance, nucleophilic attack, isomerization, cycloaddition, S N 2, and hydride ion transfer reactions.

The gas-phase reactions of dimethyl ether (DME) ions with a number of biologically active arylalkylamines of the general formula R(1)R(2)C(6)H(3)CHR(3)(CH(2))(n)NR(4)R(5), where R(1) = H or OH, R(2) = H, F, NO(2), OH or OCH(3), R(3) = H or OH, R(4) and R(5) = H or CH(3), have been studied by means of chemical ionization mass spectrometry. Under the experimental conditions used, the most abundant DME ion is the methoxymethyl cation (CH(3)OCH(2)(+), m/z 45). The unimolecular metastable decompositions of the [M + 45](+), [M + 13](+) and [M + 15](+) adducts formed have been interpreted in terms of the initial site of reaction with the amines and the presence of different functional groups in the molecule. This has permitted establishment of general fragmentation patterns for the adducts, and their correlation with structural features of the molecules. The main site of reaction of the ion CH(3)OCH(2)(+) with the amines seems to be the amino group, particularly if the amine is primary, although a competition with attack on the aromatic ring and especially on the benzylic hydroxy group is observed. In a few cases the reaction mechanisms have been elucidated through the use of deuterated amines obtained by H/D exchange with D(2)O.

There is a general fascination with regard to the origin of life on Earth. There is an intriguing possibility that prebiotic precursors of life occurred in the interstellar space and were then transported to the early Earth by comets, asteroids and meteorites. It is probable that some part of the prebiotic molecules may have been generated by gas-phase ion/molecule reactions. Here we show experimentally that gaseous ion/molecule reactions of the amino acids, Glu and Met, may promote the synthesis of protonated dipeptides such as (Glu-Glu)H(+) and (Glu-Met)H(+) and their chemical growth to larger protonated peptides.

It is shown by the use of Fourier transform ion cyclotron resonance mass spectrometry that indene radical cations excited by laser light of 514.5 nm are more reactive toward reference bases than ground state indene radical cations. Such excitation effects proton transfer from the indene radical cation to reference bases where a reaction in the ground state of the reaction partners is endothermic up to a maximum of 48 kJ/mol.

Hyperbranched polyesteramides (DA2), prepared from hexahydrophthalic anhydride (D) and diisopropanolamine (A) have been characterized, by use of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), field desorption (FD)-MS, and electrospray ionization (ESI)-MS. MALDI of polyesteramides produces protonated molecules. The spectra show a complex chemical composition distribution and end-group distribution which are mainly composed of two series of homologous oligomers DnA(n)+1 - mH2O and DnA(n) - mH2O, where m = 1-2. Signals from protonated molecules DnAn+1 and DnAn are almost absent in the MALDI spectrum, whereas these ions are responsible for the base peak of DnA(n)+1 - mH2O and DnA(n) - mH2O (m = 1-2) clusters in the ESI spectrum. The absence of -OH end-groups signals in the MALDI spectrum is due to a metastable decay of protonated DnA(n)+1 and DnAn ions in the ion source of the MALDI mass spectrometer prior to ion extraction. In-source decay results in the formation of protonated lower DnA(n)+1 - mH2O and DnA(n) - mH2O oligomers and their corresponding neutrals, leading to wrong conclusions concerning the relative end-group distribution as a function of the degree of polymerization and the chemical composition.

The thermal hydrogen/deuterium exchange behaviors of GlyProH + , ProGlyH + , and ValGlyH + in the reactions with D 2 O have been studied in a Fourier transform ion cyclotron resonance mass spectrometer. The analyzed behaviors are consistent with our earlier conclusions that more basic N-terminal amino acids lower the overall efficiency of the hydrogen atom exchange in protonated alkyl dipeptides. It appears that the mobility of a proton over the basic sites in peptides can be restricted drastically by N-terminal proline, relative to peptides with other N-terminal alkyl amino acids.

The reactivity of the nitrosonium ion, NO+, with the amino acid Glu and the dipeptide γ‒Glu–Met in the gas phase has been investigated using the combination of chemical ionisation mass spectrometry and MS/MS. It is shown that NO+reacts efficiently with both Glu and Glu–Met leading to the formation of the nitroso‒group containing ions atm/z159 and 288.The formation ofm/z159, (GluNO‒18)+, is rationalized by a mechanism involving an electrophilic attack of NO+upon the carbonyl oxygen atom of one of the carboxylic groups of Glu and the N‒terminal carboxylic group of Glu–Met leading to the neutral losses of H2O and Met, respectively. The unimolecular decompositions of the metastable and collisionally activatedm/z159 ions lead primarily to the elimination of the neutral species HNO (major) and the (Glu‒18) residue (minor). The formation of them/z288 ions can be described by a mechanistic scheme which involves the ion‒molecule interaction ofm/z159 with Glu and Glu–Met and subsequent losses of H2O and Met, respectively. Unimolecular and collisionally activated dissociations ofm/z288 suggest the formation of the proton‒bridged ion‒neutral complex [(GluNO‒18)···H+···(Glu‒18)].

It is shown using (high-resolution) mass spectrometry that treatment of 4,5-dihydrobenz[ l ]acephenanthrylene ( 2 , C 20 H 14 ) and S 8 in the melt gives the insoluble 532 a.m.u. (C 40 H 20 S) and 750 a.m.u. (C 60 H 30 ) bisbenz[ l ]acephenanthrothiophenes ( 4a–c ) and, trinaphtho[2,1- b ,2,1- m ,2,1- x ]- ( 3a ) and trinaphtho[2,1- b ,2,1- m ,1,2- a ′]decacyclene ( 3b ), respectively (estimated ratio 4a–c / 3a–b 20:1). Whereas 3a fits the `C 60 Schlegel diagram' and might be converted into C 60 (720 a.m.u.) after full cyclodehydrogenation, 3b can only give a curved C 60 H 12 /C 60 H 10 (732:730 a.m.u.). MALDI time-of-flight mass spectrometry shows that cyclodehydrogenations of 3a–b down to 732:730 a.m.u. occur; no evidence for C 60 formation was found. This is attributed to the preferred formation of 3b , the lack of cyclodehydrogenation selectivity and the occurrence of C 2 extrusions.

C70O dissolved in toluene transforms to as yet undetermined compounds when exposed to light at room temperature. After 140 days, only 21% of the original C70O remained. C70 was not formed. C139, or C140O were not found. One possible explanation is that the compounds formed were very insoluble in toluene and that their solids either remained in colloidal solution or attached themselves to the glass walls of the container bottles.

A Fourier transform ion cyclotron resonance (FTICR) mass spectrometer has been used to study near thermal collision energy symmetric charge transfer reactions. The systems Xen+/Xe, Krn+/Kr and Arn+/Ar have been measured at ambient temperature, where n = 1, 2. In the interactions between doubly charged rare gas ions and their corresponding neutral gas atoms the double electron capture (DEC) has been observed to be more efficient than the single electron capture (SEC). From the measured overall rate coefficient of 3.4 × 10−10 cm3 s−1 and the branching ratio of 1.1 between the DEC reaction and the SEC reaction the rate coefficients for double and single electron transfer from Xe to Xe2+ have been calculated to be 1.8 × 10−10 cm3 s−1 and of 1.6 × 10−10 cm3 s−1. In the experiments with doubly charged krypton and doubly charged argon ions in their parent gases the symmetric double charge transfer is observed to be even more favored in comparison with the Xe2+/Xe system at near thermal collision energies.

Experimental and theoretical studies of gas-phase isomeric C4H5− anions have been performed. Deprotonation of 1,2-butadiene is expected to occur mainly at the 3-position, giving an anion which also is obtained after deprotonation of 2-butyne. The gas-phase acidities (ΔGacid°) of 1,2-butadiene and 2-butyne have been determined from the equilibria with methanol and pyridine and have led to ΔG acid°=1574±10 kJ mol −1 and ΔGacid° =1596±10 kJ mol−1, respectively. Deprotonated 1,2-butadiene and 2-butyne have been found to react similarly with N2O, neo-pentyl nitrite and methyl formate. These reactivity studies have indicated that the charge of the anion is delocalized over 2 carbon atoms. Ab initio calculations have been applied to isomeric C4H5 − anions and have confirmed this hypothesis. The calculations have shown the existence of six stable C4 H5− anions. The reaction between carbon-3 deprotonated 1,3-butadiene with methyl formate has shown that the 2-buta-1,3-dienyl anion has a delocalized charge at carbon-1 and carbon-3. Copyright © 1999 John Wiley & Sons, Ltd.

The collision-induced dissociation (CID) of deprotonated arylalkylamines of general formula R(1)C(6)H(4)CHR(2)CH(2)NR(3)(2) (where R(1) = H, OH, F or NO(2); R(2) = H or OH; R(3) = H or CH(3)) generated by negative chemical ionization with H(2)O and D(2)O as ionizing reagents, is discussed. The negative chemical ionization mass spectra show that, in the absence of a hydroxy group in the aromatic ring, deprotonation takes place at the benzylic position whereas the proton is lost from the OH group when present. The nitro compound forms only M(-.) ions. The CID spectra of the deprotonated molecules show that fragmentations are strongly dependent on the structural features of the molecules, namely the presence or absence of substituents in the aromatic ring or aliphatic chain. Copyright 1999 John Wiley & Sons, Ltd.

It is shown by use of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry that photoexcitation of protonated naphthalene by visible laser light of 488 nm can effect a proton transfer from this ion to acetonitrile. The reaction of the ground state reaction partners is endoergic by 31 kJ/mol.

Fourteen metabolites of methylprednisolone have been analysed by gradient-elution high-performance liquid chromatography coupled with tandem mass spectrometry (LC-MS-MS). The compounds were separated on a Cp Spherisorb 5 μm ODS column connected to a guard column packed with pellicular reversed phase. The mobile phase was an acetonitrile-1.0% aqueous acetic acid gradient at a flow rate of 1.5 mL min−1. The analysis gave a complete picture of parent drug, prodrugs and metabolites, and the α/β stereochemistry was resolved. The short (1-2h) elimination half-life of methylprednisolone is explained by extensive metabolism. The overall picture of the metabolic pathways of methylprednisolone is apparently simple—reduction of the C20 carbonyl group and further oxidation of the C20, C21 side chain (into C21COOH and C20COOH), in competition with or in addition to oxidation at the C6 position.

The molecular ion of 3-phenyl-1-bromopropane is known to eliminate ethylene containing the methylene groups of positions 1 and 2 following an exchange between the hydrogen atoms from position 1 and the ortho positions of the phenyl ring. During this reaction migration of the bromine atom to some position in the molecular ion must take place. The eventual position to which the bromine atom has migrated has been probed by studying the metastable behaviour and collision-induced dissociation reactions of the (M–C 2 H 4 ) •+ ion. From comparison with appropriate reference ions it is found that the (M–C 2 H 4 ) •+ ion has the structure of ionized benzyl bromide. The mechanistic implication of this finding is briefly discussed.

We have prepared the dimer radical cations (R2S)2+· of a series of dialkyl sulfides R2S (= Me2S, MeEtS, Et2S, Et-n-PrS, n-Pr2S, i-Pr2S, n-Bu2S) in an FT-ICR cell. These have been found to react with neutral dialkyl sulfides by ligand exchange. By studying the (R2S)2+· + 2Et2S ⇄ (Et2S)2+· + 2 R2S equilibria the associated Gibbs energy changes ΔG0 have been determined. From these, the sulfur–sulfur bond dissociation enthalpies (BDE) of the dimers could be estimated using the relationship BDE[(R2S)2+·] = −ΔG0 − TΔS0 + IE[Et2S] − IE[R2S] + BDE[Et2S)2+·] and taking literature values for BDE[(Et2S)2+·] and the ionization energies IE[R2S]. The entropies of reaction ΔS0 have been estimated using statistical thermodynamics and computationally optimized structures. The 298 K sulfur–sulfur BDE values thus obtained are 115, 112, 115, 107, 103, and 97 kJ mol−1 for (Me2S)2+·, (MeEtS)2+·, (Et2S)2+·, (Et-n-PrS)2+·, (n-Pr2S)2+· and (n-Bu2S)2+·, respectively. The trend in S–S bond strength as well as the various contributions to ΔS0 are discussed.

The high affinity of Ag+ ions for aromatic pi donors and cyano groups is exploited in a novel MALDI-TOF mass spectrometric method for the identification of hydrogen-bonded assemblies. The interaction with the Ag+ ions - which, for example, can be complexed by two phenyl groups in a sandwich-type manner (see drawing on the right) - provides positively charged assemblies in a nondestructive way.

Dimer radical cations of methyl chloride have been prepared in a Fourier transform ion cyclotron resonance mass spectrometer using a consecutive exchange reaction of the xenon dimer radical cation with methyl chloride. Both theory and experiment have indicated the existence of two different stable methyl chloride dimer radical cations, that is a two-center three-electron (2c/3e) bonded dimer radical cation, CH3Cl∴ClCH3•+, with C2 symmetry and a methyl chloride chloronium ylidion structure, CH3Cl·H·ClCH2•+, with Cs symmetry. The CH3Cl∴ClCH3•+ species has been observed to react both as an electron acceptor and as a proton donor. The CH3Cl·H·ClCH2•+ structure, however, reacts only as an electron acceptor. Calculations of the CH3Cl∴ClCH3•+ structure at the MP2/6-311G(d,p) level of theory indicate a bond dissociation energy of 105.4 kJ mol-1 relative to its ionized and neutral monomer products, while experiments show a bond dissociation energy of 109 ± 5 kJ mol-1. The bond dissociation energy of the CH3Cl·H·Cl...

Die hohe Affinitat von Ag+-Ionen fur aromatische π-Donoren und Cyangruppen wird bei einer neuen MALDI-TOF-MS-Methode zur Identifizierung von H-Brucken-Aggregaten genutzt (MALDI-TOF-MS=Matrix-unterstutzte Laser-Desorptions-Ionisierungs-Flugzeit-Massenspektrometrie): Durch das Ag+-Ion, das z. B. sandwichartig von zwei Phenylgruppen komplexiert werden kann (siehe rechts), lassen sich die Aggregate - ohne Zersetzung - mit einer positiven Ladung versehen.

A simple procedure has been developed to select ions in a Fourier transform ion cyclotron resonance (FT-ICR) ion trap. This procedure involves the use of correlated single- and swept-frequency excitation fields to eject unwanted ions from the FT-ICR ion trap. Correlation with the effective cyclotron frequency of the ions which are to be selected leads to optimization of the parameters controlling the single- and swept-frequency excitation fields, as a result of which accumulation of off-resonance excitation energy is prevented during the ion ejection event. Accordingly, it is demonstrated that the correlated harmonic excitation field (CHEF) procedure, which readily can be implemented in the data acquisition system of FT-ICR instruments, can conveniently be used for the selection of thermally labile ions. Hence, the CHEF procedure can be a generally applicable experimental tool for MS-MS studies, especially for studies requiring thermal ions such as ion/molecule reaction kinetic studies, and determinations of absolute threshold energies of ion decomposition reactions. The CHEF procedure can minimize the duration of the ion selection event, which especially is beneficial for ion selections which require high mass resolution. It is experimentally shown that ion selections which require a mass resolution beyond 60 000 do not suffer from ion loss.

The gas-phase reaction of protonated alkyldipeptides with acetonylacetone has been studied in a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. The reaction is identified as the gas-phase analogue of Paal-Knorr pyrrole synthesis. Under thermal conditions, the reaction complex loses a water molecule during a condensation reaction, which couples the dipeptide to the acetonylacetone molecule via an imine bond. Low energy collisional activation of the long-lived imine product ion induces additional loss of a water molecule to form the protonated 2,5-dimethylpyrrole derivative of the dipeptide. Detailed insight into the mechanism is obtained by a comparison of the reactivity of various alkyldipeptides with model compounds with amino functional groups. The reaction is catalysed by the peptide carbonyl groups, which assist in the protonation of the acetonylacetone carbonyl oxygen atoms, making the acetonylacetone carbonyl carbon atoms susceptible to nucleophilic attack by the peptide amino group. From both the previously studied bimolecular hydrogen-deuterium exchange behaviour and the presently studied reaction with acetonylacetone, it follows that the bimolecular reactivity of protonated alkyldipeptides is related to the extent of mobility of the proton within the reactive complex.

Triphenyltin compounds are widely introduced into the Dutch aquatic environment. To be able to detect them in environmental samples, the ionization methods of electron ionization, chemical ionization, fast atom bombardment, field desorption, thermospray and electrospray have been applied to triphenyltin acetate, chloride, fluoride and hydroxide to find out which of these methods is best suited to obtain molecular weight information on the intact molecules. For this purpose, field desorption is shown to be the most appropriate method giving, without fragmentation, molecular ion peaks, with the exception of triphenyltin hydroxide. The latter compound gives rise to the base peak at mz 716, due to the formation of bis(triphenyltin)oxide. Field desorption tandem mass spectrometry, applied to the molecular ions, has shown that the main decomposition pathway corresponds to the loss of a phenyl radical. Subsequently, sediment and surface water samples from the Dutch inland water, without and with the use of clean-up procedures, have been analyzed by the application of field desorption in combination with tandem mass spectrometry. Within the limits of detection, no signals for the presence of triphenyltin compounds in these environmental samples has been found. Upon spiking these samples with triphenyltin acetate, chloride, fluoride and hydroxide, it has appeared that the covalently bonded non-aromatic substituent of the molecules is exchanged for hydroxyl.

It is shown that the peaks at m/z 31, 29 and 15 in the mass-analyzed ion kinetic energy spectrum of the m/z 59 ions generated from methyl propanoate upon electron impact ionization originate from decompositions of m/z 59, i.e. C 2 H 3 O 2 + ions having the O=C=O + CH 3 structure, and of m/z 59, i.e. C 3 H 7 O + ions having the CH 3 CH=O+CH 3 structure. The latter ions contribute

The gas-phase reactions of the O -• radical anion with the halomethanes CH3X, CH2X2, CHX3, CX4, CF3X, CF2X2, CFX3 (X = Br and Cl) and CXClBr2 (X = Cl and F) have been examined at a low pressure (10 -5 -10 -4 Pa) with use of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. The overall reactions proceed essentially with the collision rate and lead to a variety of product ions dependent on the number and nature of the halogen atoms present in the substrate. For CH3Cl and CH3Br, the dominant pathways are H. abstraction, H2 +• abstraction and nucleophilic substitution. With CH2Cl2, proton transfer is an additional major pathway and in the reaction with CH2Br2 attack on a halogen atom with formation of BrO - ions also occurs. In the reactions with CHCl3 and CHBr3, proton transfer competes with attack on a halogen atom, whereas initial attack on a halogen atom dominates in the reactions of O -• with the CCl4 and CBr4 methanes. Attack on the carbon atom is the main process if CF3Cl is the substrate and results in F - , Cl - as well as FCl -• ions. For CF3Br, however, attack on the bromine atom with formation of BrO - ions dominates over the reaction at the carbon atom. In the reactions with CF2Cl2, CFCl3, CF2Br2 and CFBr3 as well as the CCl2Br2 and CFClBr2 methanes, most product ions are formed by competing attack on a chlorine and bromine atom. For some of the halomethanes, the present findings are compared with reported results obtained with use of the high pressure (< 70 PA) flowing afterglow and selected ion flow tube methods. © 1997 by John Wiley & Sons, Ltd.

The mechanisms of the dissociation reactions of the molecular ion of cyclohexyl cyanide have been studied with nitrogen-15, carbon-13 and deuterium labelling in combination with tandem mass spectrometric experiments. The most important unimolecular reactions of ionized cyclohexyl cyanide involve the losses of CH • 3 , NH 3 , HCN, C 3 H 3 N and C 4 H • 7 . These reactions are all significant for the molecular ions dissociating in the ion source, whereas the metastable molecular ions react only by the competing losses of CH • 3 , NH 3 and HCN. The results of the labelling experiments reveal that the carbon atoms at the 2- and 6-positions of the ring become structurally indistinguishable from the carbon atoms at the 3- and 5-positions prior to the losses of CH • 3 and HCN. This finding is suggested to be a result of a rearrangement of the molecular ion of cyclohexyl cyanide with formation of a symmetrical and ionized [2,2,1]-bicycloheptane-7-imine as the key intermediate. The elimination of C 3 H 3 N is indicated to involve a direct ring-opening of the molecular ion of cyclohexyl cyanide followed by dissociation to yield a C 4 H +· 8 ion. The loss of a C 4 H • ; radical may involve the same initial step followed by an H-shift in the ring-opened species prior to dissociation.

The research concerned with the ion/molecule chemistry of radical anions is reviewed together with the thermochemical properties of these ions and the related neutral species. Attention has been given to the formation of radical anions in the gas phase by various pathways, such as (dissociative) electron attachment and bimolecular reactions. A brief discussion of the electron affinity of the neutral species related to a given radical anion has been included, because this parameter is important for the discussion of the finding that an HA˙ radical is often more acidic than the related H2A molecule. In addition, the thermochemical properties form the basis for a discussion of the various processes that may occur in reactions of radical anions with organic molecules. In particular, electron transfer, proton transfer, and hydrogen atom abstraction reactions have been described for selected radical anions. Other processes that have been reviewed include SN2 substitution and an attack on a carbonyl function in an organic molecule. The interplay between these latter two processes has been described for the reactions of some halogen-substituted carbene radical anions with the methyl ester of trifluoroacetic acid and dimethyl carbonate. The structural characterization of radical anions, distonic radical anions in particular, is summarized with an emphasis on methods such as isomer-specific ion/molecule reactions and collision-induced dissociation or charge-reversal processes. © 1998 John Wiley & Sons, Inc., Mass Spectrom Rev 16, 181–200, 1997

Rate constants and product distributions have been measured for the reactions of O(+)˙((4)S) with the methyl halides (CH(3)F, CH(3)Cl, CH(3)Br, CH(3)I) and three perfluorinated compounds (CF(4), SF(6), SF(5)CF(3)) at 300 K in a flowing afterglow-selected ion flow tube (FA-SIFT). The reactions occur with high efficiency, despite the necessity for spin conversion in some processes. The mechanisms include charge transfer, atom abstraction, and fragmentation reactions. Computational modeling was carried out to provide insight into these processes.