Investigation of radical cation in electrophilic fluorination by ESI-MS

Reaction solutions of selectfluor (1) with triphenylethylene (4a) and tetraphenylethylene (4b) were monitored by ESI-MS and ESI-MS/MS. Detection and characterization of the key radical cationic intermediates 5a.+ and 5b.+) fully supports the SET mechanism in electrophilic fluorination as depicted above. [reaction: see text]     

Radical identification by liquid chromatography/thermospray mass spectrometry

Radical adducts of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) with hydroxyl, methanol-derived, and ethanol-derived radicals were detected by a combination of liquid chromatography with either electron paramagnetic resonance or thermospray mass spectrometry (LC/EPR or LC/TSP-MS) in the Fenton system (with methanol or ethanol). One radical adduct was observed in the reaction of DMPO with the hydroxyl radical or the methanol-derived radical, while two adducts were detected in the reaction of DMPO with ethanol-derived radicals. The LC/TSP-MS spectra showed quasi-molecular ions [M + H]+ at m/z 146 and m/z 160 for the methanol-derived and ethanol-derived radical adducts, respectively, and an apparent molecular ion M+ at m/z 130 for the hydroxyl radical adduct. Use of methyl-D3 alcohol (CD3OH) and ethyl-D5 alcohol (CD3CD2OH) indicated that carbon-centered radicals are formed. Experiments with partially deuterated ethanol (CD3CH2OH and CH3CD2OH) indicated that the two adducts observed in the reaction of DMPO with ethanol-derived radicals correspond to the two diastereomeric adducts of DMPO with the alpha-hydroxyethyl free radical.     

Electrospray ionization mass spectrometry monitoring of indigo carmine degradation by advanced oxidative processes

The degradation of the dye indigo carmine in aqueous solution induced by two oxidative processes (H(2)O(2)/iodide and O(3)) was investigated. The reactions were monitored by electrospray ionization mass spectrometry in the negative ion mode, ESI(-)-MS, and the intermediates and oxidation products characterized by ESI(-)-MS/MS. Both oxidative systems showed to be highly efficient in removing the color of the dye aqueous solutions. In the ESI(-)-MS of the indigo carmine solution treated with H(2)O(2) and H(2)O(2)/iodide, the presence of the ions of m/z 210 (indigo carmine in its anionic form, 1), 216, 226, 235, and 244 was noticeable. The anion of m/z 235 was proposed to be the unprecedented hydroperoxide intermediate 2 formed in solution via an electrophilic attack by hydroxyl and hydroperoxyl radicals of the exocyclic C=C bond of 1. This intermediate was suggested to be rapidly converted into the anionic forms of 2,3-dioxo-1H-indole-5-sulfonic acid (3, m/z 226), 2-amino-alpha-oxo-5-sulfo-benzeneacetic acid (4, m/z 244), and 2-amino-5-sulfo-benzoic acid (5, m/z 216). In the ESI(-)-MS of the indigo carmine solution treated with O(3), two main anions were detected: m/z 216 (5) and 244 (4). Both products were proposed to be produced via an unstable ozonide intermediate. Other anions in this ESI(-) mass spectrum were attributed to be [4 - H + Na](-) of m/z 266, [4 - H](2-) of m/z 121.5, and [5 - H](2-) of m/z 107.5. ESI-MS/MS data were consistent with the proposed structures for the anionic products 2-5.     

The Fluorination of C−H Bonds: Developments and Perspectives

This Review summarizes advances in fluorination by C(sp²)?H and C(sp³)?H activation. Transition‐metal‐catalyzed approaches championed by palladium have allowed the installation of a fluorine substituent at C(sp²) and C(sp³) sites, exploiting the reactivity of high‐oxidation‐state transition‐metal fluoride complexes combined with the use of directing groups (some transient) to control site and stereoselectivity. The large majority of known methods employ electrophilic fluorination reagents, but methods combining a nucleophilic fluoride source with an oxidant have appeared. External ligands have proven to be effective for C(sp³)?H fluorination directed by weakly coordinating auxiliaries, thereby enabling control over reactivity. Methods relying on the formation of radical intermediates are complementary to transition‐metal‐catalyzed processes as they allow for undirected C(sp³)?H fluorination. To date, radical C?H fluorinations mainly employ electrophilic N?F fluorination reagents but a unique Mn^III‐catalyzed oxidative C?H fluorination using fluoride has been developed. Overall, the field of late‐stage nucleophilic C?H fluorination has progressed much more slowly, a state of play explaining why C?H ¹⁸F‐fluorination is still in its infancy.     

Cascade Dissociations of Peptide Cation-Radicals. Part 2. Infrared Multiphoton Dissociation and Mechanistic Studies of z-Ions from Pentapeptides

Dissociations of z(4) ions from pentapeptides AAXAR where X=H, Y, F, W, and V produce dominant z(2) ions that account for >50 % of the fragment ion intensity. The dissociation has been studied in detail by experiment and theory and found to involve several isomerization and bond-breaking steps. Isomerizations in z(4) ions proceed by amide trans→cis rotations followed by radical-induced transfer of a β-hydrogen atom from the side chain, forming stable C(β) radical intermediates. These undergo rate-determining cleavage of the C(α)-CO bond at the X residue followed by loss of the neutral AX fragment, forming x(2) intermediates. The latter were detected by energy-resolved resonant excitation collision-induced dissociation (CID) and infrared multiphoton dissociation (IRMPD) experiments. The x(2) intermediates undergo facile loss of HNCO to form z(2) fragment ions, as also confirmed by energy-resolved CID and IRMPD MS(4) experiments. The loss of HNCO from the x(2) ion from AAHWR is kinetically hampered by the Trp residue that traps the OCNH radical group in a cyclic intermediate.     

A cooperative allylic fluorination: combination of nucleophilic and electrophilic fluorine sources

A one step, regioselective allylic fluorination of alkenes is reported in which electrophilic and nucleophilic sources of fluorine act synergistically to afford rearranged allylic fluorides over alternative vicinal dihalides. The reaction occurs under exceptionally mild conditions and without need for prefunctionalization or transition metal catalysts. The fluorination of cyclic alkenes and monoterpenes is highlighted, and preliminary mechanistic experiments reveal that dual (radical and ionic) pathways operate simultaneously.     

Alkaline liquid chromatography/electrospray ionization skimmer collision-induced dissociation mass spectrometry for phosphopeptide screening.

A rapid on-line method for the identification of phosphorylated peptides in enzymatic protein digests by specific marker ion signals is described. In our study we investigated the use of alkaline conditions together with a previously described method for selective and sensitive detection of phosphopeptide ions combining high-performance capillary liquid chromatography (LC) and electrospray ionization mass spectrometry (ESI-MS). Phosphorylation-specific marker ions (m/z 79, PO(3)(-), and m/z 97, H(2)PO(4)(-)) were generated by skimmer collision-induced dissociation (sCID) in the negative-ion mode. The method was evaluated and validated for mono-phosphorylated synthetic peptides using different alkaline pH values and CID offsets. Alkaline conditions (pH 10.5) enhance the generation of phosphopeptide-specific fragment ions from serine- and tyrosine-phosphorylated peptides, and enable the use of m/z 79 (PO(3)(-)) and m/z 97 (H(2)PO(4)(-)) as phosphorylation-specific marker traces. Note that HPLC separation in trifluoroacetic acid containing solvents impairs the use of m/z 97 (C(2)F(3)O(-) fragment ion at m/z 97) as a phosphorylation-specific marker. The optimized method was applied for the detection of phosphorylated peptides in a tryptic beta-casein digest. The expected mono- and tetra-phosphorylated peptides were detected and rapidly identified by (mu)LC/ESI-sCID-MS and (mu)LC/ESI-MS analysis.     

Isomeric recognition by ion/molecule reactions: The ionized phenol-cyclohexadienone case

The isomerization process between ionized phenol and ionized cyclohexadienone is studied by performing ion/molecule reactions with several alkyl nitrites in a hexapole collision cell inserted in a six-sector mass spectrometer. The distinction between both isomeric species is readily achieved on the basis of the completely different reactivity patterns observed for them in subsequent reactions. When reacting with alkyl nitrite, ionized phenol undergoes two competitive reactions corresponding to the formal radical substitution of the hydroxylic hydrogen atom by respectively (i) the nitrosyl radical (m/z 123) and (ii) an alkoxyl radical (m/z 138 if alkyl=ethyl). Both reactions were theoretically demonstrated by density functional theory calculations [B3LYP/6-311++G(d,p)+ZPE] to involve hydrogen-bridged radical cations as key intermediates. The ion/molecule reaction products detected starting from ionized cyclohexadienone as the mass-selected ions arise from *OAlkyl, *OH, and NO2* radical additions. The occurrence of a spontaneous ring-opening of cyclohexadienone radical ion into a distonic species is suggested to account for the observed ion/molecule reaction products. We also demonstrated that ionized cyclohexadienone is partly isomerized during a proton-transfer catalysis process into ionized phenol inside the Hcell with ethyl nitrite as the base. The molecular ions of phenol generated in such conditions consecutively undergo reactions producing m/z 123 and 138 radical cations. The proposed mechanism is supported by results of quantum chemical calculations.     

Tandem mass spectrometric approach for determining the structure of molecular species of ceramide in the marine sponge Haliclona cribricutis

Ceramides are important intracellular second messengers that play a role in the regulation of cell growth, differentiation and programmed cell death. Analysis of these second messengers requires sensitive and specific analytical method to detect individual ceramide species and to differentiate between them. Eight molecular species of ceramide were identified from the marine sponge Haliclona cribricutis using electrospray ionization tandem mass spectrometry (ESI-MS/MS). From this marine sponge N-hencicosanoyl (N21:0) to N-hexasanoyl (N26:0) Octadecasphing-4 (E)-enine have been reported for the first time. The ESI-MS spectra gave several strong protonated molecular ion [M+H](+) with the corresponding bis (2-ethyl hexyl) phthalate adduct [M+H+DHEP](+). The collision induced dissociation (CID) on ceramides at m/z 622.7337, 636.7645, 650.7789, 664.7925 and 678.8130 conducted at low-collision energy produced well characteristic product ions at m/z 252.31, 264.32, 278.33, 282.33 and 296 .35 for d18:1 sphingosine regardless of the length of the fatty chain. The MS/MS of the Phthalate adduct [M+H+DHEP](+) at m/z 1013.1820, 1027.1971, 1041.2176, 1055.2394 and 1069.2573 also yielded characterizing product ions for sphingosine and confirmed the molecular ion at m/z 391 for bis (2-ethyl hexyl) phthalate. The major ions in the [M+H](+) and [M+H+DHEP](+) were due to neutral loss of [M+H-H(2)O](+) and [M+H(H(2)O)(2)](+).     

Tamoxifen metabolism in rat liver microsomes: identification of a dimeric metabolite derived from free radical intermediates by liquid chromatography/mass spectrometry

Tamoxifen has been shown to be a potent liver carcinogen in rats, and generates covalent DNA adducts. On-line high performance liquid chromatography/electrospray ionisation mass spectrometry (HPLC/ESI-MS) has been used to further study the metabolites of tamoxifen formed by rat liver microsomes in the presence of NADPH with a view to identifying potential reactive metabolites which may be responsible for the formation of DNA adducts, and liver carcinogenesis. A metabolite has been detected with a protonated molecule at m/z 773. The mass of this compound is consistent with a dimer of hydroxylated tamoxifen (m/z 388). Analysis of 4-hydroxytamoxifen incubated with a rat liver microsomal preparation showed the formation of a similar metabolite with an apparent MH+ ion at m/z 773, believed to be a dimer of 4-hydroxytamoxifen formed by a free radical reaction. The retention time for this metabolite from 4-hydroxytamoxifen is identical to that of the tamoxifen metabolite, suggesting that these two compounds are the same. The levels of the dimer were higher when 4-hydroxytamoxifen was used as substrate and, in addition, two isomers were detected. It is proposed that tamoxifen was first converted to arene oxides which react with DNA or to 4-hydroxytamoxifen, either directly or via 3,4-epoxytamoxifen, which then undergoes activation via a free radical reaction to give reactive intermediates which can then react with DNA and protein, or with themselves, to give the dimers (m/z 773).     

Electrophilic fluorination with N,N′-difluoro-2,2′-bipyridinium salt and elemental fluorine

N,N′-Difluoro-2,2′-bipyridinium bis(tetrafluoroborate) (MEC-31) was shown to be a highly reactive electrophilic fluorinating agent with the highest effective fluorine content in its class. We have developed the perfect recycled fluorination system with MEC-31 for the lower-cost industrial fluorination and for an environment. MEC-31 can be completely recycled including the counter-anion. We found the fluorination of 2-naphthol in liquid CO₂ with MEC-31 in the presence of catalytic amount of NaOTf proceeded quantitatively without the generation of by-product.

In the fluorination of 1,3-dicarbonyl compounds with elemental fluorine, we found the introduction method of fluorine gas would be very important in order to make a reaction efficient. As fluorination goes on, the quantity of 1,3-dicarbonyl compounds of the starting material is reduced gradually, and therefore the quantity of fluorine must be reduced by the method to control the flow rate or the concentration of fluorine gas diluted with nitrogen, together the fluorination to proceed efficiently.     

The role of geometry on regioselectivity and rate of fluorination of fluorene and diphenylmethane with Selectfluor F-TEDA-BF4

Diphenylmethane and fluorene were used as target molecules in an investigation of the effect of the geometry of aromatic molecules on the regioselectivity and rate of fluorination with 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (Selectfluor F-TEDA-BF4). In acetonitrile at 80 degrees C ring fluorination of diphenylmethane was accompanied by oxidation of the saturated carbon atom, while in trifluoroacetic acid only ring fluorination with an ortho-para regioselectivity of 1.8:1 was observed. Fluorene was converted in acetonitrile as well as in trifluoroacetic acid into 2- and 4-fluoro substituted products in the relative ratio of 2:1 and 1.2:1, respectively. The reactions in acetonitrile obey a simple rate equation: v = d[F-TEDA]/dt = k2 x [F-TEDA] x [Substrate] and the second order rate constants for the reactions in acetonitrile at 65 degrees C were determined; values of 0.6 x 10(-4) M-1 s-1 for diphenylmethane and 35.5 x 10(-4) M-1 s-1 for fluorene were obtained. The reaction rates for the various functionalisations of fluorene relative to those for diphenylmethane were found to be considerably influenced by the type of functionalisation. Relative rate factors (k(rel) = k2(fluorene)/k2(diphenylmethane)) with values between 59 for fluorination and 712 for chlorination were determined, while the corresponding data for the biphenyl/diphenylmethane pair were only slightly dependent on the type of functionalisation. A reaction pathway involving electron transfer, thus forming cation radical intermediates, was proposed as the main process in the case of fluorination of fluorene with F-TEDA-BF4.     

Analysis of benzo[a]pyrene diol epoxide-DNA adducts by capillary zone electrophoresis- electrospray ionization-mass spectrometry in conjunction with sample stacking

Benzo[a]pyrene diol epoxide (BPDE) was reacted in vitro with (2'-deoxy)nucleotides and with calf thymus DNA. The modified DNA was enzymatically hydrolyzed to the 5'-monophosphate nucleotides using deoxyribonuclease I (DNA-ase I), nuclease P1 and snake venom phosphodiesterase (SVP). Most of the unmodified nucleotides were removed using solid phase extraction (SPE) in a polystyrene divinylbenzene copolymer. Three adducts could be detected and identified using capillary zone electrophoresis(negative)-ion electrospray ionization-mass spectrometry (CZE-(-)-ESI-MS) in conjunction with sample stacking. This way, not only a BPDE-dGMP adduct [(M-H)(-) at m/z 648], a well-known nucleotide adduct, could be retrieved, but also a BPDE-dAMP [(M-H)(-) at m/z 632] and a BPDE-dCMP adduct [(M-H)(-) at m/z 608] could be detected for the first time. The presence of the prominent ion at m/z 195 (the deoxyribose-phosphate ion) in the three low-energy collision-activated decomposition (CAD) spectra indicated that the adducts were formed through base-alkylation. CZE-positive ion ESI-MS/MS experiments were performed to obtain further information on base-alkylation. The absence of the loss of NH(3) from the nucleobase in each CAD spectrum points to an alkylated exocyclic NH(2) position of the nucleobase. So, the three adducts could be identified as BPDE-N(2)-dGMP, BPDE-N(6)-dAMP and BPDE-N(4)-dCMP using CZE-ESI-MS and CZE-ESI-MS/MS.     

Mass spectrometric analysis of leukotriene A4 and other chemically reactive metabolites of arachidonic acid

The biosynthesis of prostaglandins and leukotrienes proceeds through the formation of chemically reactive intermediates leukotriene A4 (LTA4) and prostaglandin H2 (PGH2) which in aqueous solutions have chemical half-lives of 3 s and 3 min, respectively. Prostacyclin (PGI2) is another chemically reactive prostanoid that has a chemical half-life of 3-4 min. The recent development of reversed phase HPLC stationary phases that are stable to elevated pH (pH 10-12) without significant column damage has permitted direct analysis of these acid-sensitive eicosanoids. Using electrospray ionization, molecular anions [M - H]- of these compounds were observed at m/z 317 for LTA4 and m/z 351 for both PGH2 and PGI2. The mechanism of formation of ions derived from collisional activation of LTA4 was studied using stable isotope labeled and chemical analogs of LTA4 and found to involve formation of highly conjugated anions at m/z 261 and 163. The collisional activation of the molecular anion of PGH2 yielded a product ion spectrum identical to that observed for the isomeric prostaglandins PGE2 and PGD2. However, it was possible to baseline separate PGE2, PDG2, and PGH2 by reversed phase HPLC using basic HPLC mobile phases. The collisional activation of PGI2 led to a family of abundant ions including highly conjugated carbon-centered and oxygen-centered radical species (m/z 245 and 205) likely derived from the attack of the carboxylate anion on the cyclic enolether of PGI2 as well as the most abundant product ion (m/z 215) which formed following loss of neutral hexanal and water. The structures of these product ions were consistent with high resolution measurements measured in a quadrupole time-of-flight mass spectrometer.     

Peptide-phospholipid cross-linking reactions: Identification of leucine enkephalin-alka(e)nal-glycerophosphatidylcholine adducts by tandem mass spectrometry

The covalent interactions between peptides and lipid oxidation products, with formation of Schiff and Michael adducts, are known to occur during free radical oxidative damage. In this study, leucine-enkephalin-glycerophosphatidylcholine alka(e)nal adducts were analyzed by electrospray tandem mass spectrometry (MS/MS). Upon collision-induced dissociation of the Leucine enkephalin-2-(9-oxo-nonanoyl)-1-palmitoyl-3-glycerophosphatidylcholine, an alkanal Schiff adduct observed at m/z 1187.7, the main product ions were attributed to the phosphocholine polar head and loss of the peptide. Also, product ions resulting from characteristic losses of phosphatidylcholines and cleavages of the peptide chain (mainly b-type) were observed. Additional product ions formed by combined peptide and phosphatidylcholine fragmentations were identified. The fragmentation pattern of the leucine enkephalin-alkanal Schiff adduct and the leucine enkephalin-alkenal phosphatidylcholine Schiff and Michael adducts were similar, although the loss of the peptide for the Michael adduct should occur through a distinct mechanism. These fragmentation pathways differ greatly from those described for peptide-lipid Schiff and Michael adducts, in which only peptide chain cleavages are reported, probably due to charge retention in the glycerophosphatidylcholine polar head in peptide-glycerophosphatidylcholine adducts.     

Thermochemistry of N-N containing ions: a threshold photoelectron photoion coincidence spectroscopy study of ionized methyl- and tetramethylhydrazine

The unimolecular dissociation reactions of the methylhydrazine (MH) and tetramethylhydrazine (TMH) radical cations have been investigated using tandem mass spectrometry and threshold photoelectron photoion coincidence spectroscopy in the photon energy ranges 9.60-31.95 eV (for the MH ion) and 7.74-29.94 eV (for the TMH ion). Methylhydrazine ions (CH3NHNH2(+*)) have three low-energy dissociation channels: hydrogen atom loss to form CH2NHNH2(+) (m/z 45), loss of a methyl radical to form NHNH2(+) (m/z 31), and loss of methane to form the fragment ion m/z 30, N2H2(+*). Tetramethylhydrazine ions only exhibit two dissociation reactions near threshold: that of methyl radical loss to form (CH3)2NNCH3(+) (m/z 73) and of methane loss to form the fragment ion m/z 72 with the empirical formula C3H8N2(+*). The experimental breakdown curves were modeled with Rice-Ramsperger-Kassel-Marcus theory, and it was found that, particularly for methyl radical loss, variational transition state theory was needed to obtain satisfactory fits to the data. The 0 K enthalpies of formation (delta(f)H0) for all fragment ions (m/z 73, m/z 72, m/z 45, m/z 31, and m/z 30) have been determined from the 0 K activation energies (E0) obtained from the fitting procedure: delta(f)H0[(CH3)2NNCH3(+)] = 833 +/- 5 kJ mol(-1), delta(f)H0 [C3H8N2(+*)] = 1064 +/- 5 kJ mol(-1), delta(f)H0[CH2NHNH2(+)] = 862 +/- 5 kJ mol(-1), delta(f)H0[NHNH2(+)] = 959 +/- 5 kJ mol(-1), and delta(f)H0[N2H2(+*)] = 1155 +/- 5 kJ mol(-1). The breakdown curves have been measured from threshold up to h nu approximately 32 eV for both hydrazine ions. As the photon energy increases, other dissociation products are observed and their appearance energies are reported.     

Stereoselective glycal fluorophosphorylation: synthesis of ADP-2-fluoroheptose, an inhibitor of the LPS biosynthesis

Heptosides are found in important bacterial glycolipids such as lipopolysaccharide (LPS), the biosynthesis of which is targeted for the development of novel antibacterial agents. This work describes the synthesis of a fluorinated analogue of ADP-L-glycero-beta-D-manno-heptopyranose, the donor substrate of the heptosyl transferase WaaC, which catalyzes the incorporation of this carbohydrate into LPS. Synthetically, the key step for the preparation of ADP-2F-heptose is the simultaneous and stereoselective installation of both the fluorine atom at C-2 and the phosphoryl group at C-1 through a selectfluor-mediated (selectfluor=1-chloromethyl-4-fluorodiazoniabicyclo[2.2.2]octane bis(triflate)) electrophilic addition/nucleophilic substitution involving a heptosylglycal. Therefore, we detail in this article 1) the stereoselective preparation of the key intermediates heptosylglycals, 2) the development of a new fluorophosphorylation procedure allowing an excellent beta-gluco stereoselectivity with "all-equatorial" glycals, 3) the synthesis of the target ADP-2F-heptose, and 4) some comments on the contacts observed between the fluorine atom of the final molecule and the protein in the crystallographic structure of heptosyltransferase WaaC.     

Selectfluor: Mechanistic Insight and Applications

The replacement of hydrogen atoms with fluorine substituents in organic substrates is of great interest in synthetic chemistry because of the strong electronegativity of fluorine and relatively small steric footprint of fluorine atoms. Many sources of nucleophilic fluorine are available for the derivatization of organic molecules under acidic, basic, and neutral conditions. However, electrophilic fluorination has historically required molecular fluorine, whose notorious toxicity and explosive tendencies limit its application in research. The necessity for an electrophilic fluorination reagent that is safe, stable, highly reactive, and amenable to industrial production as an alternative to very hazardous molecular fluorine was the inspiration for the discovery of selectfluor. This reagent is not only one of the most reactive electrophilic fluorinating reagents available, but it is also safe, nontoxic, and easy to handle. In this Review we document the many applications of selectfluor and discuss possible mechanistic pathways for its reaction.     

Selectfluor: mechanistic insight and applications

The replacement of hydrogen atoms with fluorine substituents in organic substrates is of great interest in synthetic chemistry because of the strong electronegativity of fluorine and relatively small steric footprint of fluorine atoms. Many sources of nucleophilic fluorine are available for the derivatization of organic molecules under acidic, basic, and neutral conditions. However, electrophilic fluorination has historically required molecular fluorine, whose notorious toxicity and explosive tendencies limit its application in research. The necessity for an electrophilic fluorination reagent that is safe, stable, highly reactive, and amenable to industrial production as an alternative to very hazardous molecular fluorine was the inspiration for the discovery of selectfluor. This reagent is not only one of the most reactive electrophilic fluorinating reagents available, but it is also safe, nontoxic, and easy to handle. In this Review we document the many applications of selectfluor and discuss possible mechanistic pathways for its reaction.     

Unusual atmospheric pressure chemical ionization conditions for detection of organic peroxides

Organic peroxides such as the cumene hydroperoxide I (M(r) = 152 u), the di-tert-butyl peroxide II (M(r) = 146 u) and the tert-butyl peroxybenzoate III (M(r) = 194 u) were analyzed by atmospheric pressure chemical ionization mass spectrometry using a water-methanol mixture as solvent with a low flow-rate of mobile phase and unusual conditions of the source temperature (< or =50 degrees C) and probe temperature (70-200 degrees C). The mass spectra of these compounds show the formation of (i) an [M + H](+) ion (m/z 153) for the hydroperoxide I, (ii) a stable adduct [M + CH(3)OH(2)](+) ion (m/z 179) for the dialkyl peroxide II and (iii) several protonated adduct species such as protonated molecules (m/z 195) and different protonated adduct ions (m/z 227, 389 and 421) for the peroxyester III. Tandem mass spectrometric experiments, exact mass measurements and theoretical calculations were performed for characterize these gas-phase ionic species. Using the double-well energy potential model illustrating a gas-phase bimolecular reaction, three important factors are taken into account to propose a qualitative interpretation of peroxide behavior toward the CH(3)OH(2) (+), i.e. thermochemical parameters (DeltaHdegrees(reaction)) and two kinetic factors such as the capture constant of the initial stable ion-dipole and the magnitude of the rate constant of proton transfer reaction into the loose proton bond cluster.