Metastable ion and induced stable ion fragmentation of isomeric 5-methyl-3-tolyl-1,2,4-oxadiazoles

The electron ionization fragmentation patterns of 5-methyl-3-(o-, m- and p-tolyl)-1,2,4-oxadiazoles (1a—c) have been examined by metastable ion and high resolution mass spectrometry. The o-tolyl isomer loses CO and C₂H₂O from the metastable molecular ion whereas the m- and p-tolyl isomers lose only CH₃CN thus indicating a strong ortho effect in directing the fragmentation in 1a. Slight differences between o-, m- and p-tolyl isomers in the collisional activation fragmentation of stable [C₇H₆N]⁺ ions suggest that structural differences exist even after a series of extensive rearrangements of the molecular ions. Metastable ion kinetic energy (MIKE) and collisional activation (CA) spectra were very helpful in providing valuable information about many fragments.     

Intramolecular aromatic substitution reactions in substituted N,N-dimethylthiobenzamide ions

The molecular ions of N,N-dimethylthiobenzamide and its ortho substituted derivatives (substituents CH₃, Cl, Br, I) lose a hydrogen atom and/or the ortho substituent. The mechanism of this process has been studied by measurements of the ionization energies, appearance energies of the product ions m/z 164 and the kinetic energy release during this process. The structure of the product ions m/z 164 and relevant reference ions have been investigated by mass analysed ion kinetic energy spectra, B/E linked scan spectra and collision induced decompositions. The results show clearly the formation of two different kinds of product ions m/z 164 depending on the substituent lost. Type a ions are formed by loss of a H atom or the CH₃ substituent and correspond to protonated 3,4-benzo-N-methylpyroline-2-thione. The formation of these ions occurs by a hydrogen rearrangement followed by an intramolecular substitution via a 5-membered cyclic intermediate and is associated with a large release of kinetic energy. In contrast, the loss of the halogeno substituents to give type b ions probably occurs via a direct displacement reaction by the sulfur atom of the thioamide group giving rise to Gaussian shaped peaks mass analysed ion kinetic energy spectra.     

Formation and stability of gaseous tolyl ions

Collisional activation mass spectra confirm that tolyl ions can be produced from a variety of CH₃C₆H₄Y compounds. High purity o-, m- and p-tolyl ions are prepared by chemical ionization of the corresponding fluorides (Y=F) as proposed by Harrison. In electron ionization of CH₃C₆H₄Y formation of the more stable tropylium and benzyl ionic isomers usually accompanies that of the o-, m- and p-tolyl ions. Isomerization of low energy [CH₃C₆H₄Y]⁺? to [Y–methylenecyclohexadiene]⁺? is proposed to account for most [benzyl]⁺ formation, while the tropylium ion appears to arise from the isomerization of tolyl ions formed with higher internal energies, [o-, m-, p-tolyl]⁺→ [benzyl]⁺→ [tropylium]⁺, consistent with Dewar's predictions from MINDO/3 calculations.     

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.     

Fragmentations of protonated benzaldehydes via intermediate ion/molecule complexes

Ions a_p and a_m corresponding to protonated p- and m-methoxymethylbenzaldehydes have been generated in a mass spectrometer by electron impact fragmentation of the correspondingly substituted 1-phenylethanols (1 and 2). Metastable ions a_p and a_m (2nd FFR of a VG-ZAB-2F mass spectrometer) react by elimination of CH₃OH, loss of HCOOCH₃, formation of ions CH₂=OCH₃ and to a small extent by loss of CH₂O and CH₃OCH₃, respectively. The mechanisms of these reactions have been studied by extensive D-labelling, and it is shown that these fragmentations are initiated by a transfer of the proton located originally at the carbonyl group onto the aromatic ring. The formation of ions \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_2 = \mathop {{\rm OCH}_3 }\limits^ + $\end{document} and the loss of CH₃OH occurs via π-and σ-complexes. The elimination of HCOOCH₃ from ions a_p and a_m corresponds to a functional group interaction between distal side chains and occurs via intermediate ion/molecule complexes formed by a protolytic cleavage of the formyl group. The loss of CH₂O and CH₃OCH₃ proceeds also by intermediate ion/molecule complexes which are generated by a protolytic cleavage of the methoxymethyl side chain in ions a_p and a_m.     

Rearrangements and fragmentations of [C2H5S]+ ions

From deuterium labelling experiments it was concluded that metastable molecular ions of ethyl methyl sulfide lose a methyl radical with the formation of both [CH₃S?CH₂]⁺ amd [CH₃CH?SH]⁺˙ The fragmentation reactions of metastable ions generated with these structure are losses of C₂H₂, H₂S and CH₄. These reactoins and the preceding isomerizations have also been studied by means of deuterium labelling. From the results it is concluded that the three fragmentation reactions most probably occur from ions with a C? C? S skeleton. Appearance energy measurements for ions generated with the two structures above and all give rise to the same ΔH_f value for these three isomeric forms. Ab initio molecular orbitals calculations confirm that these three ions fortuitously have very similar heats of formation. A potential energy diagram rationalizing the isomerizations and the principal fragmentation reaction is presented.     

Stereochemical applications of mass spectrometry 4. Energy-resolved study of the fragmentation of esters of maleic and fumaric acids

The fragmentation of the dimethyl and diethyl esters of maleic and fumaric acids have been studied as a function of the internal energy of the molecular ions using charge exchange techniques and metastable ion studies in combination with isotopic labelling. The dimethyl ester molecular ions show distinctive behaviours at both low and high internal energies, indicating that interconversion of the molecular ions does not occur. The fumarate molecular ion fragments by elimination of CH₂O and (CO₂ + CH₃) in the metastable ion time-frame, while the maleate ester fragments primarily by loss of CH₃O. At higher internal energies both molecular ions fragment primarily by loss of CH₃O but the fragment ion from the maleate ester shows a greater stability, presumably because it assumes the cyclic cationated maleic anhydride structure. The diethyl maleate and diethyl fumarate molecular ions show identical metastable ion characteristics; in addition the [COS]⁺· charge exchange mass spectra are very similar. These results indicate that low-energy molecular ions interconvert. At higher internal energies interconversion does not occur, and, although both moiecular ions fragment by loss of C₂H₅O, the resultsint fragment ions show different stabilities and fragmentation reactions.     

Ab initio study of 4-monosubstituted pyrimidines in ground and excited n → π* states

Ab initio SCF and SCF -CI calculations have been performed to investigate substituent effects on ground- and excited-state properties of 4-R-pyrimidines, and to compare these with substituent effects in 2- and 4-R-pyridines, with R including the π donating and σ withdrawing groups CH₃, NH₂, OH, F, and C₂H₃ and the σ and π electron-withdrawing groups CHO and CN. Substitution leads to significant changes in the internal angles of the pyrimidine ring, which are independent of the nature of the substituent. The geometry of the pyrimidine ring is more sensitive to substitution in the 4 position than the pyridine ring geometry is to substitution in either the 2 or the 4 position. The isodesmic reaction energies for substituent transfer from the 4 position of pyrimidine to the 2 or 4 position of pyridine indicate that all R groups except CN have a relative stabilizing effect in pyrimidine. The presence of a π donating group leads to an increase in the n→π* transition energy of 4-R-pyrimidines, while the π withdrawing group CN leads to a decrease in the transition energy relative to pyrimidine. Orbital energy differences and virtual excitation energies tend to correlate with n→π* transition energies of 4-R-pyrimidines with saturated R groups, but such correlations are masked by π conjugation, n orbital interaction, and configurational mixing when the unsaturated groups C₂H₃, CHO, and CN are present. The electronic effects of a π donating group are stronger when the group is bonded to pyrimidine than to pyridine, but those of a π withdrawing group are weaker when the group is bonded to pyrimidine.     

Loss of CO from the molecular ions of o-, m- and p-anisoyl fluorides,CH3OC6H4COF,with fluorine atom migration

Loss of CO from the molecular ions ([CH₃OC₆H₄COF]⁺˙) of o-, m- and p-anisoyl fluorides has been investigated by mass-analysed ion kinetic energy (MIKE) spectrometry. This reaction involves fluorine atom migration from the carbonyl group to the benzene ring. In the cases of o- and p-anisoyl fluorides, the fluorine atom migrates via a three-membered transition state to form the molecular ions ([CH₃OC₆H₄F]⁺˙) of o- and p-fluoroanisoles, respectively. On the other hand, in the case of m-anisoyl fluoride, the fluorine atom migrates from the carbonyl group to the benzene ring via a three- or four-membered transition state.     

Carbon-13 NMR study of substituted benzyl N,N-dimethylcarbamates

The ¹³C NMR chemical shifts of m- and p-substituted benzyl N,N-dimethylcarbamates were measured in CDCl₃. The meta and para ¹³C substituent chemical shifts were analysed by means of dual substituent parameter (DSP) equations. Good correlations were obtained, especially for the para-carbon substituent chemical shifts. The computed transmission coefficients, ρ_I and ρ_R, are consistent with the general features of the fitting parameters. It has been shown that no significant electron demand is imposed by the ? CH₂OCON(CH₃)₂ substituent.     

Kinetic studies in mass spectrometry—VI: The [M ? CH3] reaction in substituted ethylbenzenes

The [M] → [M ? CH₃] reaction in a series of m- and p-X substituted ethylbenzenes has been studied by wide range electron energy kinetics and metastable ion characteristics techniques. By this approach, qualitative measures of activation evergy differences between [XC₆H₄CH₂]⁺ ions derived from m- and p-X isomer substrates have been secured, for both their formation and further decomposition. These evergy differences are consistent with (but do not prove) ion structures that have been suggested by previous work in this area, involving the use of isotope labeling, and ionization and appearance potential methods.     

Effect of the Electronic Structure of Substituents at the Silicon Atom on the Structure and Bond Energies of Methylhydrosilanes and Silenes

Ab initiocalculations with full geometry optimization were performed for methylhydrosilanes R₂HSiCH₃, dimethylsilanes C₂Si(CH₃)₂, and silenes R₂Si = CH₂ (R = H, CH₃, SiH₃, CH₃O, NH₂, Cl, F). The enthalpies of dehydrogenation methylhydrosilanes into silenes and of dehydrocondesation of methylhydrosilanes into dimethylsilanes were calculated. The enthalpies of dehydrogenation and dehydrocondensation increase with the electronegativity of substituent R. The Si-C and Si = C bond energies were calculated. As the electronegativity of the substituent increases, the Si-C bond shortens and strengthens, while the Si = C bond shortens and weakens.     

Carbon–13 NMR studies of tautomerism in some 2-substituted imidazoles and benzimidazoles

The ¹³C NMR spectra of several 2-substituted imidazoles and benzimidazoles have been measured. The substituent was CH₃, COOH and CONHR, where R = H, n-Bu, p-tolyl or m-chlorophenyl. Carbons 4 and 5 in the imidazoles and the carbon pairs 8/9, 4/7 and 5/6 become equivalent by proton transfer from N-1 to N-3, possibly through intermolecular association. The rate of this proton exchange increases with concentration and temperature. It decreases with extension of the 2-substituent (rate CH₃?CONH-phenyl > CONH-p-tolyl ? CONH-m-chlorophenyl ? CONH-n-butyl) due to steric hindrance at the site of the (benz)imidazole nitrogen.     

The loss of ortho halogeno substituents from substituted thiobenzamide ions

The loss of ortho substituents (CH₃, Cl, Br, I) from molecular ions of substituted thiobenzamides has been investigated by determination of the critical energy and kinetic energy released during this process to obtain some further insight into the corresponding reaction of N,N-dimethylthiobenzamide ions. In contrast to the latter compounds the ortho methyl substituent is not eliminated from the molecular ions of o-methylthiobenzamide, but the loss of ortho halogeno substituents occurs with identical reaction characteristics in both series of compounds. It is concluded that the loss of halogeno substituents from molecular ions in both series corresponds to a direct substitution reaction via a 4-membered transition state.     

Mass spectra of some (Z)-α-Phenyl-β-(2-thienyl)acrylonitriles

The mass spectra of some (Z)α-(4-R′-phenyl)-β-(2-thienyl-5-R)acrylonitriles (R = H, CH₃, Br; R′ = H, CH₃O, CH₃, Cl, NO₂) at 70 eV are reported. Mass spectra exhibit pronounced molecular ions. The compound's where R = H, and CH₃ are characterized by the occurrence of a strong [M - H]⁺ peak. Moreover, in all the compounds a m/z 177 peak occurs. In the compounds where R = H, [M - HS]* and [M - CHS]* ions are present except the nitroderivatives. Where R = CH₃, [M - HS]⁺ ion occurs.     

Fragmentation selectivity in electron impact ionization mass spectra of substituted dimethoxybenzenes

Several 1-X-sabstitirted-3-methoxy-4-trideuteromethoxybenzens were synthesized and their electron impact ionization mass spectra were measured with an ionizing energy of 20 eV. From the peak intensity ratio of [M ? CD₃ ] and [M ? CH₃] the fragmentation-directing ability of the substituent X was evaluated. The most powerful group was found to be NH₂, which expelled a methoxy methyl group only from its para position. The CH₃ group and four halogen atoms, F, Cl, Br and I, exerted a moderate effect Electron-withdrawing groups such as NO₂, CHO and CN had only a little influence on the fragmentation selectivity. These results were interpreted in terms of the effect of X on the distribution of both the unpaired electron and the positive charge in the molecular ion.     

Multiple ionization,charge separation and charge stripping reactions involving polycyclic aromatic compounds

The 70 eV electron ionization mass spectra of polycyclic aromatic compounds are characterized by the presence of relatively stable multiply charged molecular ions [M]ⁿ⁺ (n=2–4). When generated from the compounds benzene, napthalene, anthracene, phenanthrene, 2,3-benzanthracene, 1,2-benzanthracene, chrysene, 9,10-benzophenanthrene and pyrene, the relative abundances of the multiply charged ions increase dramatically with the number of rings. These compounds form multiply charged molecular ions (n=2, 3) which undergo unimolecular decompositions indicative of considerable ionic rearrangement. The main charge separation processes observed here [M]²⁺→m₁⁺+m₂⁺, [M]³⁺˙→m₃⁺+m→+m₄²⁺) involve, in almost every case, one or more of the products [CH₃]⁺, [C₂H₃]⁺˙ and [C₃H₃]⁺. This suggests the existence of preferred structures amongst the metastable parent ions. Information on the relative importance of the various fragmentation pathways is presented here along with translational energy release data. Some tentative structural information about the metastable ions has been inferred from the translational energy release on the assumption that the released energy is due primarily to coulombic repulsion within the transition state structure. For the triply charged ions these interpretations have necessitated the use of a coulombic repulsion model which takes account of an extra charge. Vertical ionization energies for the process [M]ⁿ⁺+G→[M](ⁿ⁺¹)+G+e^? (charge stripping) have also been determined where possible for n=1 and 2 and the results from these experiments allow the derivation of simple empirical equations which relate successive ionization energies for the formation of [M]²⁺ and [M]³⁺˙ to the appearance energy of [M]⁺˙.     

Two competing fragmentation processes in dimethoxybenzenes depending on their positional isomers: Elimination of CH3 and CHnO (n = 1–3) and formation of methoxycyclopentadienyl and protonated phenol ions

All the metastable transitions observed above m/z 39 in the first field-free region were compared for the three positional isomers of dimethoxybenzene. The observed isomer-dependent fragmentation processes, in particular the formation and decomposition of the m/z 95 (C₆H₇O)⁺ ion, are discussed in terms of two competing fragmentations [elimination of CH₃ and CH_nO (n = 1–3) and formation of methoxycyclopentadienyl and protonated phenol ions] and the relative energies of several isomers of the C₆H₇O⁺ ion calculated with molecular orbital theory.     

Substituent effects on ion abundances and energetics in substituted acetophenones

Based on the quasi-equilibrium theory of mass spectra it is shown that the intensity ratio [A]⁺/[M]⁺, where [A]⁺ is a fragment ion and [M]⁺ is the molecular ion, is given by [A]⁺/[M]⁺ = f′ (k₁/k_t) ((1/f) ? 1), where f is the fraction of molecular ions with insufficient energy to fragment, f′ is the fraction of [A]⁺ ions with insufficient energy to fragment, and k₁/k_t is the fraction of fragmenting molecular ions which form [A]⁺. For substituted acetophenones it is shown that f depends on the substituent present and that f′ k₁/k_t is also substituent dependent for formation of both [CH₃CO]⁺ and [YC₆H₄CO]⁺. It is also shown that no direct information concerning the effect of a substituent on the rate of a particular fragmentation reaction can be obtained from intensity studies. The ionization potentials of the parent molecules and the appearance potentials of the [YC₆H₄CO]⁺ fragment ions have been measured for fifteen substituted acetophenones and the correlations with substituent constants are discussed.     

Proximity effects in the mass spectra of crowded bis(dimethylamino)arenes: Part II—Rearrangements of the molecular radical cations of ‘proton sponges’ by reciprocal methyl/hydrogen transfer

The EI induced fragmentation pathways of 4,5-bis(dimethylamino)fluorene, 4-(d₆-dimethylamino)-5-(dimethylamino)fluorene, 4,5-bis(d₆-dimethylamino)fluorene, 4-(dimethylamino)-5-methylaminofluorene, 4,5-bis-(methylamino)fluorene, 4-amino-5-methylaminofluorene, 1,8-bis(dimethylamino)naphthalene, 1,8-bis(d₆-dimethyl-amino)-naphthalene and 1,8-bis(dimethylamino)-2,7-dimethoxynaphthalene were investigated. A mechanism is pro-posed for the surprising elimination of CH₃? NH₂ from the molecular ion, followed by loss of C₂H₅·, C₂H₄ and CH₃CN and for the accompanying cyclizations to stable heterocyclic ions: prior to fragmentation the molecular radical ion rearranges to new, distonic radical ions by reciprocal H and CH₃ transfers between the adjacent dimethylamino groups. Each of these new, isomeric molecular ions decomposes subsequently in a characteristic way.