Zum Mechanismus massenspektrometrischer Fragmentierungsreaktionen: XIII—Einfluß der Dissoziationsenergie auf intramolekulare aromatische Substitutionen in den Molekül-Ionen von N,N-Dimethyl-N′-phenylformamidinen

The effect of the dissociation energy of the C? X bond (X = H, F, Cl, Br, I) on the formation of benzimidazolium ions (b) by elimination of X from the molecular ions of ortho-substituted N,N-dimethyl-N′-phenylformamidines (I to V) has been investigated. No simple relation is observed between the intensities of ions b and the dissociation energy. Furthermore, the appearance potentials of ions b are not greatly affected by the dissociation energy, although differences of about 2.5 eV are expected for a simple cleavage reaction. The behaviour of the molecular ions of I to V is in accord with a two step addition-dissociation mechanism [M]⁺· → a → b, and the highest activation energy is required in the first addition step. Similar mechanisms are known for aromatic substitution reactions in the condensed phase, but have not been observed for mass spectrometric fragmentations. The detection of additional kinetic energy T in the reaction products by an analysis of the metastable transitions [M]⁺· → b corroborates the proposed mechanism.     

Mass spectra of aliphatic dicarboxylic acids and their dimethyl esters: Cyclic structures for the [M − H2O]+˙ ions from the diacids and [M − MeOH]+˙ ions from the dimethyl esters

Methyl 2-oxocycIoalkane carboxylate structures are proposed lor the [M ? MeOH]^+˙ ions from dimethyl adipate, pimelate, suberate and azelate. This proposal is based on a comparison of the metastable ion mass spectra and the kinetic energy releases for the major fragmentation reaction of these species with the same data for the molecular ions of authentic cyclic β-keto esters. The mass spectra of α,α,α′,α′-d₄-pimelic acid and its dimethyl ester indicate that the α-hydrogens are involved only to a minor extent in the formation of [M ? ROH]^+˙ and [M ? 2ROH]^+˙ ions, while these α-hydrogens are involved almost exclusively in the loss of ROH from the [M ? RO˙]⁺ ions (R = H or CH₃). The molecules XCO(CH₂)₇COOMe (X = OH, Cl) form abundant ions in their mass spectra with the same structure as the [M ? 2MeOH]^+˙ ions from dimethyl azelate.     

Ion-molecule reactions of oxygenated chemical ionization reagents with vincamine

The ion-molecule reactions of ions from acetone, dimethyl ether, 2-methoxyethanol, and vinyl methyl ether with vincamine were investigated. Reactions with dimethyl ether result in [M+13]⁺ and [M+45]⁺ products, reactions with 2-methoxyethanol produce [M+13]⁺ and [M+89]⁺ ions, and reactions with acetone or vinyl methyl ether ions generate predominantly [M+43]⁺ ions. Collision-activated dissociation and deuterium labeling experiments allowed speculation about the product structures and mechanisms of dissociation. The methylene substitution process was shown to occur at the hydroxyl oxygen and the phenyl ring of vincamine for dimethyl ether reactions, but the methylene substitution process was not favored at the hydroxyl oxygen for the 2-methoxyethanol reactions, instead favored at the 12 phenyl position. The reaction site is likely different for the 2-methoxyethanol ion due to its capability for secondary hydrogen-bonding interactions. For the [M+45]⁺ and [M+89]⁺ ions, evidence suggests that charge-remote fragmentation processes occur from these products. In general, the use of dimethyl ether ions or 2-methoxyethanol ions for ionmolecule reactions prove highly diagnostic for the characterization of vincamine; both molecular weight and structural information are obtained. Limits of detection for vincamine with dimethyl ether chemical ionization via this technique on a benchtop ion trap gas chromatography-tandem mass spectrometer are in the upper parts per trillion range.     

Scatter profiling: A potentially useful technique for isomeric ion distinction and stability determination

A method to determine the extent of angular scattering of fragment-ion products of keV-collision-activated decomposition (CAD) reactions and, in particular, the collisional scatter incurred by the parent ions prior to their dissociation, is outlined. Since the half-widths of the collisional scatter profiles correlate approximately with the mean reaction endothermicities for some ‘test’ reactions, the method may, in principle, be used to estimate the stabilities of isomeric ion structures relative to a common fragmentation threshold level. For single-proton-loss CAD reactions of some [H₃, C, X]⁺ ions (X = F, Cl, OH) with either [H₃CX]⁺ or ylidion, [H₂CXH]⁺ structure, collisional scatter is found in each case to be greater for the isomeric ion with the more stable structure. The estimated magnitudes of the mean energy depositions occurring in the keV-collision-activation processes are generally much larger than the calculated minimum energy requirements, suggesting that survivable [M ? H]⁺ products can be formed with up to several eV of internal energy.     

Association reactions of trimethylsilyl ions

Adduct ions, [M + (CH₃)₃Si]⁺, were produced by bimolecular association reactions of trimethylsilyl ions, (CH₃)₃Si⁺, with acetone, cydohexaoone, anisole, dimethyl ether, 2,5-dimethylfuran, 2-methylfuran and furan in ion cyclotron resonance experiments at 300 K and at pressures of ～10^?7 Torr (1 Torr = 133.3 Pa). The rate constants, k_a, for the association reactions varied from 100% to 2% of the collision rate constants, k_c. The rate constants were independent of pressure, except for furan. Measurements were also made of bond dissociation energies for these adduct ions, D[(CH₃)₃Si⁺–X], from equilibrium measurements. The association efficiency, k_a/k_c, increased with increasing bond dissociation energy and with increasing numbers of degrees of freedom, in qualitative agreement with theoretical predictions. Observations pertinent to the dependence of k_a on reactant temperature and relative kinetic energy are discussed. The possibility of determining ion-neutral complex binding energies from radiative association rate constants is considered.     

Electron impact ionization of unstable enols: H2CCHOH,H2CC(OH)?CH3 and H2CC(OH)?C2H5

Ethenol, 2-hydroxypropene and 2-hydroxybutene-1 were prepared by low-pressure pyrolysis of cyclobutanol, 1-methylcyclobutanol and 1-ethylcyclobutanol, respectively. Mass spectra, ionization energies, appearance energies of metastable ions and kinetic energy releases were determined on a reverse Nier-Johnson double-focusing mass spectrometer. Mercury and CH₃ radicals from the pyrolysis of dimethylmercury were employed for calibration of the energy scale. The ionization energy of 2-hydroxybutene-1 is 8.55 ± 0.1 eV and the appearance energies of [C₂H₅CO]⁺ and [CH₃CO]⁺ from that molecule are 10.25 ± 0.1 and 10.40 ± 0.1 eV, respectively. Changes observed in metastable peak shapes for certain fragmentation reactions upon pyrolysis are discussed.     

The [M?OH]+ ions of m- and p-ethylnitrobenzene: A common structure?

The structures of the [M? OH]⁺ ions of m- and pethylnitrobenzene have been compared by measurements of metastable ion spectra, collisional activation spectra, kinetic energy releases and critical energies for the formation of these ions and their subsequent decomposition. Normalized rates of fragmentation of metastable molecular ions and metastable [M? OH]⁺ ions have been compared for ion lifetimes up to 30 μs. The energy measurements fail to distinguish between the structures of the [M? OH]⁺ ions, but the normalized fragmentation rates and the collisional activation spectra show their structures to be different.     

Functional group-selective ion-molecule reactions of ethylene glycol and its monomethyl and dimethyl ethers

The selective methylation and methylene substitution reactions of dimethyl ether ions with ethylene glycol, ethylene glycol monomethyl ether, and ethylene glycol dimethyl ether were investigated in a quadrupole ion trap mass spectrometer. Whereas the reactions of ethylene glycol and ethylene glycol monomethyl ether with the methoxymethylene cation 45⁺ gave only [M + 13]⁺ product ions, the reaction of ethylene glycol dimethyl ether with the same reagent ion yielded exclusively [M + 15]⁺ ions. The relative rates of formation of these products and those from competing reactions were examined and rationalized on the basis of structural and electronic considerations. The heats of formation for various relevant species were estimated by computational methods and showed that the reactions leading to the [M + 13]⁺ ions were more energetically favorable than those leading to the [M + 15]⁺ products for cases in which both reactions are possible. Finally, the collision-induced dissociation behavior of the [M + H]⁺, [M + 13]⁺, and [M + 15]⁺ ions indicated that the and [M + H]⁺ rons dissociated by analogous pathways and were thus structurally similar, whereas the [M + 13]⁺ ions possessed distinctly different structural characteristics.     

Breaking and Making of Carbon–Carbon Bonds by Lanthanides and Third‐Row Transition Metals

Carbon‐atom extrusion from the ipso‐position of a halobenzene ring (C₆H₅X; X=F, Cl, Br, I) and its coupling with a methylene ligand to produce acetylene is not confined to [LaCH₂]⁺; also, the third‐row transition‐metal complexes [MCH₂]⁺, M=Hf, Ta, W, Re, and Os, bring about this unusual transformation. However, substrates with substituents X=CN, NO₂, OCH₃, and CF₃ are either not reactive at all or give rise to different products when reacted with [LaCH₂]⁺. In the thermal gas‐phase processes of atomic Ln⁺ with C₇H₇Cl substrates, only those lanthanides with a promotion energy small enough to attain a 4fⁿ5d¹6s¹ configuration are reactive and form both [LnCl]⁺ and [LnC₅H₅Cl]⁺. Branching ratios and the reaction efficiencies of the various processes seem to correlate with molecular properties, like the bond‐dissociation energies of the C?X or M⁺?X bonds or the promotion energies of lanthanides.     

Gas phase substitution under ammonia chemical ionization

Evidence for the isomerization of the [M + NH₄]⁺ adducts of nitrobenzene and triphenylnitrocyclohexenes to the isonitroso derivatives in the first field free region has been obtained. The isomerized adducts undergo successive loss of ?O and ?H or NH₃ to give ions corresponding to the substitution products. Mass analysed ion kinetic energy and collisionally activated dissociation/mass analysed ion kinetic energy data reveal that the adduct decomposes by similar pathways in the second field free region. In the ion source an addition elimination reaction and nucleophilic substitution contribute to the formation of the substituted product ion.     

Potential energy profiles for the unimolecular reactions of [C3H7S]+ Ions

Appearance energies were measured for several types of [C₃H₇S]⁺ ions. From these appearance energy values the heats of formation of the ions were calculated. For the isomeric ions that could not be generated in the mass spectrometer, the heats of formation were estimated by means of the isodesmic substitution method. Transition state energies for the decomposition to C₂H₄ and [CH₃S]⁺ along two pathways were determined from the appearance energies of these ions. Using the energy values, potential energy diagrams were constructed for the rearrangements and decompositions of the [C₃H₇S]⁺ ions. A supplementary ¹³C Clabelling experiment is described for the determination of the rearrangement pathway of [CH₃CH₂CH?SH]⁺ ions prior to decomposition.     

Substitution reactions under NH3 chemical ionization conditions in cis-/trans-1,2-dihydroxybenzosuberans

The nucleophilic substitution reaction under NH₃ chemical ionization (CI) conditions in cis- and trans-1,2-dihydroxybenzosuberans (1–4) has been studied with the help of ND₃ CI and metastable data. The results indicate that in the parent diols 1 (cis) and 2 (trans), the substitution ion [M_sH]⁺, is produced mainly by the loss of H₂O from the [MNH₄]⁺ ion (S_Ni reaction) while in their 7-methoxy derivatives 3 and 4, the ion-molecule reaction between [M? OH]⁺ and NH₃ seems to be the major pathway for the formation of [M_sH]⁺. The substitution ion from 1 and 2 and the [MH]⁺ ion from trans-1-amino-2-hydroxybenzosuberan give similar collision-induced dissociation mass-analysed ion kinetic energy spectra. Interestingly, their diacetates do not undergo the substitution reaction.     

Provitamin D and vitamin D isomerizations in the mass spectrometer. Translational energy release and collision-induced dissociation studies

Collision-induced dissociation mass-analysed ion kinetic energy spectrometry and translational energy release studies have established that provitamin D₃ (7-dehydrocholesterol), provitamin D₂ (ergosterol), vitamin D₃ and vitamin D₂ isomerizations in the mass spectrometer occur at the fragment ion level leading to [M? H₂O? CH₃]⁺ ions of identical structure. It was found that the reaction, [M]⁺˙→[M? H₂O]⁺˙, plays a central role in these isomerizations.     

Differentiation of the isomeric 1,2-cyclopentanediols by ion-molecule reactions in a triple-quadrupole mass spectrometer

Collisionally activated decompositions and ion-molecule reactions in a triple-quadrupole mass spectrometer are used to distinguish between cis- and trans-1,2-cyclopentanediol isomers. For ion kinetic energies varying from 5 eV to 15 eV (laboratory frame of reference), qualitative differences in the daughter ion spectra of [MH]⁺ are seen when N₂ is employed as an inert collision gas. The cis ?1,2-cyclopentanediol isomer favors H₂O elimination to give predominantly [MH- H₂O]⁺. In the trans isomer, where H₂O elimination is less likely to occur, the rearrangement ion [HOCH₂CHOH]⁺ exists in significantly greater abundance. Ion-molecule reactions with NH₃ under single-collision conditions and low ion kinetic energies can provide thermochemical as well as stereochemical information. For trans ?1,2-cyclopentanediol, the formation of [NH₄]⁺ by proton transfer is an exothermic reaction with the maximum product ion intensity at ion kinetic energies approaching 0 eV. The ammonium adduct ion [M + NH₄]⁺ is of greater intensity for the trans isomer. In the proton transfer reaction with the cis isomer, the formation of [NH₄]⁺ is an endothermic process with a definite translational energy onset. From this measured threshold ion kinetic energy, the proton affinity of cis ?1,2-cyclopentanedioi was estimated to be 886 ± 10 kJ mol^?1.     

A simple method to estimate relative stabilities of polyethers cationized by alkali metal ions

Dissociation of doubly cationized polyethers, namely [P + 2X]²⁺ into [P + X]⁺ and X⁺, where P = polyethylene glycol (PEG), polypropylene glycol (PPG) and polytetrahydrofuran (PTHF) and X = Na, K and Cs, was studied by means of energy‐dependent collision‐induced dissociation tandem mass spectrometry. It was observed that the collision voltage necessary to obtain 50% fragmentation (CV₅₀) determined for the doubly cationized polyethers of higher degree of polymerization varied linearly with the number of degrees of freedom (DOF) values. This observation allowed us to correlate these slopes with the corresponding relative gas‐phase dissociation energies for binding of alkali ions to polyethers. The relative dissociation energies determined from the corresponding slopes were found to decrease in the order Na⁺ > K⁺ > Cs⁺ for each polyether studied, and an order PPG ≈ PEG > PTHF can be established for each alkali metal ion. Copyright © 2011 John Wiley & Sons, Ltd.     

Electron‐induced dissociation of doubly protonated betaine clusters: controlling fragmentation chemistry through electron energy

The [M₂₁+2H]²⁺ cluster of the zwitterion betaine, M = (CH₃)₃NCH₂CO₂, formed via electrospray ionisation (ESI), has been allowed to interact with electrons with energies ranging from >0 to 50 eV in a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer. The types of gas‐phase electron‐induced dissociation (EID) reactions observed are dependent on the energy of the electrons. In the low‐energy region up to 10 eV, electrons are mainly captured, forming the charge‐reduced species, {[M₂₁+2H]^{+ .} }*, in an excited state, which stabilises via the ejection of an H atom and one or more neutral betaines. In the higher energy region, above 12 eV, a Coulomb explosion of the multiply charged clusters is observed in highly asymmetric fission with singly charged fragments carrying away more than 70% of the parent mass. Neutral betaine evaporation is also observed in this energy region. In addition, a series of singly charged fragments appears which arise from C? X bond cleavage reactions, including decarboxylation and CH₃ group transfer. These latter reactions may arise from access of electronic excited states of the precursor ions. Copyright © 2009 John Wiley & Sons, Ltd.     

Structural correlations with kinetic energy release during loss of chlorine from nineteen selected chlorinated biphenyls

Unimolecular and collision induced decompositions of the major ions of selected polychlorinated biphenyls in the field free region between the magnetic and electric sectors of a reversed Nier-Johnson instrument were studied. Loss of a single chlorine atom is associated with a wide range of kinetic energy releases but still can be correlated by a single reaction mechanism. Loss of two chlorines is interpreted as a rapid sequential loss from isomerized molecular ions for all but one compound. The decompositions which metastable ions undergo are not always the same as those of high energy ions in the source. Correlations between substituent positions and kinetic energy release can be made for the [M]^+·→[M? Cl]⁺ and [M? Cl₂]^+· processes.     

Dissociation dynamics of halotoluene ions,production of tolyl,benzyl and tropylium ([C7H7]+)ions

The dissociation rates and energetics of the loss of halogen atoms from energy-selected halotoluene ions were investigated by photoelectron photoion coincidence (PEPICO) and collisional activation (CA) mass spectrometric experiments. Dissociation onsets, determined from the dissociation rates measured as a function of the internal energy of the parent ion, revealed the formation of three [C₇H₇]⁺ isomers, which were identified, on the basis of the CA data, as the tolyl, benzyl and tropylium ions. All of the ions investigated produced a mixture of isomeric ions. Only iodotoluene ions produced any tolyl product ions by a direct bond cleavage. The bromo- and chlorotoiuene ions produced mixtures of benzyl and tropyl ions. The observed two-component decay rates of the iodotoluene ions revealed the participation of a lower energy [C₇H₇I]^{+ ˙} isomer in the dissociation process. The identity of this isomer is not known but it probably does not have the cycloheptatriene ion structure because considerable kinetic energy was released in this dissociation.     

Energy dependence of the fragmentation of ester enolate ions

The energy dependence of the fragmentation of a selection of ester enolate ions has been studied by variable, low-energy collision-induced dissociation experiments in the quadrupole collision cell of a hybrid BEQQ mass spectrometer. The dominant fragmentation reactions observed are where ΔH₁ ? ΔH₂=PA([RCCO]^?) ? PA([?O]^?) (PA=proton affinity). The anion of lowest proton affinity is formed preferentially at low internal energies with the yield of the anion of higher proton affinity increasing with increasing internal energy. The [CH₃OCOCOCH₂]^? anion derived from methyl pyruvate forms [CH₃OCO]^? by reaction (2); this anion readily fragments to [CH₃G]^?+ CO consistent with a structure represented by a dipole-stabilized cluster of [CH₃O]^? and CO. Comparison of the 8-keV with the 50-eV collision-induced dissociation mass spectra indicated that the average internal energy of the fragmenting ions is considerably lower in the high-energy collisional experiments than it is in the low-energy collisional experiments.     

Translational energy release and stereochemistry of steroids. VIII-The mechanism of the elimination of water from metastable ions in epimeric 3-hydroxy steroids of the 5α-series

The mechanism of water elimination from metastable molecular, [M ? CH₃˙]⁺ and [M ? ring D]⁺˙ ions of epimeric 3-hydroxy steroids of the 5α-series has been elucidated. Deuterium labelling, the measurement of the translational energy released during the loss of water, and collision-induced decomposition mass-analysed kinetic energy spectrometry were the techniques used. It was found that the mechanisms of water loss from metastable M⁺˙ and [M ? ring D]⁺˙ ions is different from that from [M ? CH₃˙]⁺ ions.