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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.

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 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.

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.

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)].

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.

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.

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.

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.

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.

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 mechanisms of the elimination of a propene molecule from the molecular ions of a series of 3- and 4-substituted aryl n-propyl ethers (YC 6 H 4 OC 3 H 7 , Y = H, CH 3 , CF 3 , NO 2 , CH 3 S and CH 3 O) were examined with the use of deuterium labelling and tandem mass spectrometry. Propene loss dominates in the ion-source reactions and is the exclusive process observed for the metastable molecular ions of most of the aryl n-propyl ethers. This process is concluded to proceed by two distinct pathways on the basis of the relative importances of the losses of C 3 H 4 D 2 and C 3 H 5 D from the ionized ethers labelled with two deuterium atoms at the β-position of the n-propyl group in combination with the results of an analysis of the product ion structures. One pathway involves intermediate formation of an ion-neutral complex composed of a YC 6 H 4 O . radical and a + CH(CH 3 ) 2 corbenium ion. This complex reacts further by proton transfer prior to dissociation with the formation of product ions, which have the same structure as the corresponding ionized and substituted phenols, YC 6 H 4 OH +. . The second pathway involves a reversible 1,5-H shift from the β-position of the propyl group to the 2- or 6-position of the aromatic ring with the formation of a distonic ion, which expels propene to afford the molecular ion of a substituted cyclohexa-2,4-dienone species. The first pathway prevails for most of the ionized ethers with the exception of the molecular ions of the 3-methoxy- and 3-methylthio-substituted ethers, which expel propene largely by the second pathway. In addition, the 1,5-H shift-initiated propene loss is particularly pronounced for the metastable molecular ions of these latter two ethers, suggesting that this reaction is associated with a lower critical energy than the reaction involving formation of an ion-neutral complex.

The reactions of the atomic oxygen radical anion O −• with CH 3 S CH 2 CN in the gas phase have been examined with Fourier transform ion cyclotron resonance in combination with tandem mass spectrometric experiments performed with a double-focusing quadrupole hybrid instrument. Deuterium labeling has revealed that the O −• ion reacts with CH 3 S CH 2 CN by proton abstraction from the methylene group as well as by competing 1,1- and 1,3-H 2 +• abstractions to afford isomeric radical anions. High kinetic energy (8 keV) collision-induced charge reversal experiments indicate that the 1,1-H 2 +• -abstraction leads to a CH 3 S C CN carbene ion, whereas the 1,3-H 2 +• abstraction yields a novel sulfur-containing distonic radical anion, which is formulated as CH 2 S CH CN.

Singly and doubly charged xenon ions have been generated by electron impact of xenon gas in the cylindrical cell of a Bruker CMS 47X Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Subsequently, xenon ions with a particular mass and charge state have been selected by ejection of all other ions from the FT-ICR cell and allowed to react with neutral xenon atoms. The rate coefficient for the charge transfer reaction (1) 136Xe+ + Xe → 136Xe + Xe+ (1) has been determined to be 2.5 × 10−10 cm3 s−1, which agrees very well with values reported in the literature. In the interactions between Xe2+ ions and neutral xenon atoms not only a single electron transfer [reaction (2a)], but also a double electron transfer [reaction (2b)] is observed. 136Xe2+ + Xe → 136Xe+ + Xe+ (2a) 136Xe2+ + Xe → 136Xe + Xe2+ (2b) The overall rate coefficient for the charge transfer reaction [(2a) + (2b)] has been measured to be 2.7 × 10−10 cm3 s−1. From this and the branching ratio of 0.8 between reactions (2a) and (2b) the rate coefficients for single and double electron transfer from Xe to Xe2+ have been calculated to be 1.2 × 10−10 cm3 s−1 and 1.5 × 10−10 cm3 s−1, respectively.

Unimolecular metastable ion decompositions of acetyltrimethyl silane, CH 3 COSi(CH 3 ) 3 (MW: 116, (1)) generated by electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometryand D-labeling. In the metastable time window, the molecular ions 1 + decompose exclusively into the ions at m/z 101, [M-CH 3 ] + . A considerable extent of H/D exchange occurs prior to this [M-CH 3 ] + formation. However, the H/D exchange is hardly observed for the [M-CH 3 ] + formation in the ion source. The generated m/z 101 ions from 1 + consist of two isomeric ions. A trimethylsilyl ion, (CH 3 ) 3 Si + , at m/z 73 is generated by at least three different routes.

Some synthetically obtained linear and cyclic phosphopeptides of low molecular weight have been studied by fast-atom bombardment and tandem mass spectrometry to verify the position of the phosphate group in these compounds. Based upon the occurrence/non-occurrence of loss of phosphoric acid from low abundance fragment ions induced by low- and high-energy collisions with target gases, it is shown that the position of the phosphate group in the phosphopeptides studied can be determined unequivocally.

The NH ion reacts with fluoro- and chlorobenzene in the cell of a Fourier-transform ion cyclotron resonance instrument only by proton abstraction, whereas the reaction with bromobenzene also yields Br− ions. The deprotonation of these halogen-substituted benzenes occurs the 2-, 3- or 4-positions as revealed by deuterium labelling in combination with high kinetic energy (8keV) collision-induced dissociation (CID) experiments performed with a double-focusing quadrupole hybrid mass spectrometer. The 2-fluoro- and 3- fluorophenyl anions eliminate HF following collision with an oxygen molecule. By contrast, the collisions between 4-fluorophenyl anions and O2 do not yield detectable amounts of negatively charged fragment ions owing to the exclusive occurrence of electrom detachment. Electron detachment is also the only process observed in the 8 keV CID experiments with chlorophenyl anions and 3- and 4-bromophenyl anions, whereas the ion formed by proton abstraction from the 2-position in bromobenzene dissociates to Br− and 1,2-dehydrobenzene. The 8 keV collision-induced charge-reversal spectra of the anions formed by proton abstraction from the 2-, 3- or 4-positions in a given halogen-substituted benzene are virtually identical, indicating that this method does not provide insight into the position of the charge with respect to the halogen atom in the substituted phenyl anions.

The electron impact-induced fragmentation patterns of 2,1-benzisothiazoline 2,2-dioxide nitro derivatives were studied. The rationalizations proposed for the fragmentations are supported by accurate mass measurements, daughter ion (mass analysed ion kinetic energy and B/E linked-scan), parent ion, and constant neutral loss spectra in metastable and collision-induced dissociation modes and deuterium labelling.

The reaction of metal carbonyl anions (M(CO) n − ; M = Cr, Mn and Fe; n = 1-3) with n-heptane, water and methanol were studied with use of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with an external ion source. The M(CO) n − ions were formed in the FT-ICR cell by collision-induced dissociation of the most abundant primary ion generated by electron impact of the appropriate metal carbonyl compound present in the external ion source. The M(CO) n − ions were allowed subsequently to undergo non-reactive collisions with argon in order to remove possible excess internal/translational energy prior to the ion/molecule reaction. Only the Cr(CO) 3 − , Mn(CO) 3 − and Fe(CO) 2 − ions react with n-heptane

Decomposition of protonated β-haloethanols was found to proceed predominantly via the loss of water. The mechanism was studied in the decomposition of metastable and collisionally activated isotopically labelled protonated β-haloethanols. The study involved the characterization of the resulting fragment ions by collisionally activated decomposition and by low-pressure ion/molecule probe reactions. The results indicate that loss of water from protonated β-haloethanols proceeds via two competing mechanisms. The competition between these mechanisms appears to be governed by the nature of the halogen and by the internal energy of the decomposing protonated β-haloethanol. In the first mechanism, loss of water is considered to be initiated by a concerted 1,2-elimination of H3O+ resulting in an ion–molecule complex between H3O+ and haloethene. Rapid and reversible proton transfers between the two species in the complex is followed by a relatively slow dissociation ofthe intermediate complex resulting in the formation of an water molecule and an α-haloethyl cation. In competition with this mechanism, loss of water can be initiated by carbon–oxygen bond cleavage, which is assisted by halogen–carbon bond formation resulting in the formation of a water molecule and cyclic ethylenehalonium ion.

The uni- and bimolecular chemistry of C 2 H + 7 ions have been studied in the gas phase using the methods of sector and Fourier transform ion cyclotron resonance mass spectrometry. Unimolecular decomposition of the C 2 H + 7 ions predominantly shows the elimination of a hydrogen molecule which proceeds without a significant kinetic energy release. However, the elimination of a hydrogen molecule is found to suffer from a very large isotope effect, which has been rationalized by the difference in Gibbs free energy change for H 2 , HD and D 2 loss from the various isotopomers of protonated ethane. In general, long-lived C 2 H + 7 ions can be generated either by proton transfer to ethane, methyl cation transfer to methane or by association of C 2 H + 5 and H 2 . Conversely, C 2 H + 7 ions can react as a proton or a methyl cation donor, or eliminate an H 2 molecule. In contrast to CH + 5 , C 2 H + 7 displays an ambident chemical behavior, which shows a balanced competition between a proton and a methyl cation donor. Both the uni- and bimolecular reactivity of C 2 H + 7 reveal that the proton accepted in an exothermic protonation of ethane randomizes with the original hydrogen atoms of ethane. This intramolecular randomization is found to be a very fast process which precedes decomposition of the metastable C 2 H + 7 ions as well as the bimolecular processes of the long-lived C 2 H + 7 ions.

The gas-phase ion/molecule chemistry of the z−CHNC and z−CHCN radical anions formed in the reactions of the Oz− ion with CH3NC and CH3CN respectively, has been studied by Fourier transform ion cyclotron resonance mass spectrometry. The gas-phase acidity, ΔH0acid, of the zCH2NC radical is determined to be 1582 ± 10 kJ mol−1, while the zCH2CN radical is considerably more acidic in the gas phase (ΔH0acid = 1563 ± 3 kJ mol−1). The z−CHNC ion does not react with the parent compound by proton abstraction in accordance with the gas-phase acidity of CH3NC, which is determined to be 1589 ± 8 kJ mol−1 on the basis of a proton-transfer equilibrium between −CH2NC and CH3OH. The z−CHNC and z−CHCN radicals anions react with HCO2CH2CH3, CF3CO2CH2CH3, CF3COSCH2CH3 and CH3SSCH3 to form product ions of the same type, but in significantly different abundance ratios. In particular, the reaction of the z−CHNC ion with HCO2CH2CH3 proceeds by loss of an ethoxy radical and an ethanol molecule from the collision complex, whereas the reaction of the z−CHCN ion with this substrate results almost exclusively in elimination of an ethoxy radical.

The intramolecular secondary isotope effects on the α-cleavage of deuterium-labelled N-methyldipentylamine radical cations have been studied as a function of ion lifetime by field ionization kinetics. The isotope effects observed are all normal and increase in magnitude with increasing ion lifetime, with the exception of the δ-labelled compound which shows an inverse effect (predominant loss of the labelled radical) at times shorter than 10−9 s, and a normal effect at longer times. The isotope effects reflect differences in zero-point energies of the transition states as well as the influence of slight reductions of isotope-dependent frequencies on the state sums–a statistical weight effect. The latter is particularly important at high ion energies and is the primary reason for the occurrence of the inverse isotope effect. The time dependence of the normal and inverse isotope effect is reproduced by QET/RRKM calculations.

The regioselectivity of methyl cation transfer from (CH3)2F+, (CH3)2Cl+ and (CH3)3O+ to 1-phenyl-2-aminopropane was studied by Fourier transform ion cyclotron resonance in combination with collision-induced dissociation and neutralization-reionization mass spectrometry of the stable [M + CH3]+ ions formed in a chemical ionization source. The (CH3)2F+ ion transfers a methyl cation to the NH2 group and the phenyl ring with almost equal probability. Predominant CH3+ transfer to the NH2 group is observed for the (CH3)2Cl+ ion whereas the (CH3)3O+ ion reacts almost exclusively at the amino group. The preference for methylation at NH2 is discussed in terms of a lower methyl cation affinity of the phenyl ring than of the amino group and the existence of an energy barrier for methylation of the phenyl moiety.

The normalized unimolecular rate constant for loss of a methyl radical from pent-3-en-2-ol molecular ions with lifetimes ranging from 10−11 to 10−9 s was studied by field ionization kinetics (FIK). The normalized rate curve shows maxima at molecular ion lifetimes of 10−10.5 and 10−10.1 s. Based on results for deuterium and 13C-labelled pent-3-en-2-ol, the maximum at 10−10.5 s is ascribed to loss of the methyl group at the 1-position by a direct cleavage reaction. The maximum at 10−10.1 s is attributed to a 1,2-H shift-initiated rearrangement of the molecular ion, which leads to loss of the methyl group at the 5- and 1-positions. The time dependence of the processes in the form of the maxima on the normalized rate curve is discussed qualitatively in terms of a lower critical energy and a tighter transition state of the 1,2-H shift than those of the direct cleavage reaction. Collision-induced dissociation of the [C4H7O]+ product ions in combination with FIK provides evidence that at molecular ion lifetimes corresponding to the first maximum on the rate curve protonated crotonaldehyde is formed, whereas protonated methyl vinyl ketone and the butyryl cation are formed at times corresponding to the second maximum.

The formation of singly protonated water clusters in the positive-ion mode and singly deprotonated water clusters as well as singly negatively charged water clusters in the negative-ion mode with a thermospray system running on pure water and in the discharge-on mode is described. The influence of the potential at the repeller electrode opposite to the sampling cone recalls previous mechanistic discussions on the repeller influence. Application of these water cluster ions to tuning and calibration in the thermospray mode is proposed.

Tandem mass spectrometry has been applied to solve the question as to whether stable ethane dications can be generated by electron-impact ionization. A reaction sequence of charge exchange followed by collision-induced dissociation (electron-capture-induced-decomposition-type reactions) was performed on m/z 15 ions of ethane in order to probe the amount of hydrogens incorporated in the initial doubly charged cations.

The unimolecular gas-phase chemistry of CNCN +· and NCCN · has been investigated by electron-impact mass spectrometry, mass-analysed ion kinetic energy mass spectrometry and a theoretical density-functional method. The CN + formation appears to compete with the C +· 2 formation in the microsecond time scale, in spite of an energy gap of 3.5 eV (CNCN) and 3.0 eV (NCCN) between the two processes. This observation is explained by assuming that the fragmentation to CN + proceeds via an isolated excited electronic state of the molecular ion. The C +· 2 fragment ion must be formed from both isomeric C 2 N 2 +· parent ions by a rearrangement reaction. While NCCN +· is more stable than CNCN +· by 0.6 eV, the kinetic energies released in both rearrangements and theoretical calculations indicate that the corresponding intermediates are not only much closer in energy, but even reversed in energetic order with respect to their precursor ions.

Molecular ionic dimers of water, hydrochloric acid, ethane, cyanogen and acetonitrile are produced via a well-controlled successive two-step molecule exchange reaction from noble gas ionic dimers in a low pressure Fourier transform ion cyclotron resonance (FT-ICR) cell. To this end, the ionic dimers of argon, krypton and xenon have been generated under high pressure chemical ionization conditions in an external ion source located outside the influence of the ICR magnetic field and subsequently injected into the FT-ICR cell, where they have been allowed to react with gaseous water, hydrochloric acid, ethane, cyanogen and acetonitrile in the absence of stabilizing collisions at very low pressure. Endothermic electron transfer accompanying these molecule exchange reactions suggests that possibly higher bound electronic states of the studied noble gas ionic dimers may be involved. The generated ionic dimers of water, hydrochloric acid, ethane, cyanogen and acetonitrile in the reaction with their neutral monomers all exhibit chemical behaviour which is identical to that of their ionic monomers, suggesting that at least the reactive structure of the above-mentioned ionic dimers resembles that of the corresponding solvated ionized monomer.

The collision-induced dissociation reactions of the monocarboxylate anions, generated from the dimethyl and diethyl esters of glutaric acid and its 3,3-dimethyl analogue in a chemical ionization source, were studied as a function of the potential applied to the collision cell in combination with 2 H labelling experiments. It was shown that many of the product anions are formed in consecutive steps. The mechanisms associated with these steps appear to be initiated by a functional group interaction between the carboxylate anion and the ester group, 1,5-hydrogen migrations both to the carboxylate anion and the uncharged ester group and charge remote fragmentation. In one of the collision-induced dissociation channels a methyl anion is generated as a granddaughter product anion, which contains the hydrogen atoms exclusively originating from positions 2 and 4 as shown by the applied 2 H labelling