[C6H6, C3H7 +]and [C6H7 +, C3H6] ion-neutral complexes intermediate in the reactions of protonated propylbenzenes

From the mass-analysed ion kinetic energy spectra of labelled ions, kinetic energy releases and thermodynamic data, it is proved that protonated n-propylbenzene (1) isomerizes into protonated isopropyl benzene (2). It is also shown that the dissociation of the less energetic metastable ions of (2), leading to [iso-C₃H₇]⁺ and [C₆H₇]⁺ product ions, is preceded by H exchange. This H exchange involves two interconverting ion-neutral complexes [C₆H₆, iso-C₃H₇⁺] (2π) and [C₆H₇⁺, C₃H₆] (2α).     

The mass spectra of alkyl phenyl tellurides

The mass spectra of several alkyl phenyl tellurides, C₆H₅TeR (R = CH₃, CD₃, C₂H₅, n-C₃H₇, i-C₃H₇ and n-C₄H₉) have been studied with special emphasis on the fragmentation patterns involving cleavage of the alkyl and aryl tellurium–carbon bonds. Each compound exhibited intense parent ions. The rearrangement ions [C₆H₆Te]⁺? and [C₆H₆]⁺? were found in the spectra of phenyl ethyl and higher tellurides. Two other rearrangement ions [HTe]⁺ and [C₇H₇]⁺ were observed in the spectrum of each compound. Examination of the mass spectrum of phenyl methyl-d₃ telluride demonstrated that the [HTe]⁺ ions derive hydrogen from the phenyl group.     

Energy dependence of the fragmentation of some isomeric [C6H12]+˙ ions

The charge exchange mass spectra of 14 C₆H₁₂ isomers have been determined using [CS₂]^+˙, [COS]^+˙, [Xe]^+˙, [CO]^+˙, [N₂]^+˙ and [Ar]^+˙ as the major reactant ions covering the recombination energy range from ∼10.2 eV to ∼15.8 eV. From the charge exchange data breakdown graphs have been constructed expressing the energy dependence of the fragmentation of the isomeric [C₆H₁₂]^+˙ molecular ions. The electron impact mass spectra are discussed in relation to these breakdown graphs and approximate internal energy distribution functions derived from photoelectron 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.     

Massenspektrometrische Untersuchungen an Organophosphorverbindungen. II—Elektronenstoßinduzierte Fragmentierungen von Tetraorganodiphosphandisulfiden

The behaviour under electron impact (70 eV) which includes some rearrangement processes of some tetraorganodiphosphanedisulfides R₂P(S)-P(S)R₂ (R ? CH₃, C₂H₅, n-C₃H₇, n-C₄H₉, C₃H₅, C₆H₅) and CH₃RP(S)–P(S)CH₃R (R ? C₂H₅, n-C₃H₇, n-C₄H₉, C₆H₅, C₆H₅, C₆H₅,CH₂) is reported and discussed. Fragmentation patterns which are consistent with direct analysis of daughter ions and defocusing metastable spectra are given. The atomic composition of many of the fragment ions was determined by precise mass measurements. In contrast to compounds R₃P(S) loss of sulphur is not a common process here. The first step in the fragmentation of these compounds is cleavage of one P–C bond and loss of a substituent R?. The second step is elimination of RPS leading to [R₂PS]⁺ from which the base peaks in nearly all the spectra arise. The phenyl substituted compounds give spectra with very abundant [(C₆H₅)₃P]^+. and [(C₆H₅)₂CH₃P]^+. ions respectively, resulting from [M]^+. by migration of C₆H₅. Rearrangement of [M]^+. to a 4-membered P-S ring system prior to fragmentation is suggested.     

The structures of [C5H5O]+ ions formed from isomers of nitrophenol in the gaseous phase

Translational energy release measurments on metastable ions are used in the comparison of the structures of isomeric ions. Metastable ions, m₂⁺, formed from m₁⁺ ions as the result of a high energy process in the ion source are compared with isomeric metastable ions formed as daughters from fragmentation of metastable m₁⁺ ions in a field. In the case of o-, m- and p-nitrophenol the structure of the [C₅H₅O]⁺ ions formed from [C₆H₅O]⁺ ions by these two independent methods is different as verified by comparison of the behaviour of [C₅H₅O]⁺ ions formed from several other compounds.     

Mass spectra of organometallic compounds: XIV—Some monometallic olefing manganese tricarbonyl derivatives

The metastable ion supported fragmentation process in the mass spectra of the cyclohexadienyl derivative C₆H₇Mn(CO)₃, the cycloheptadienyl derivative C₇H₉Mn(CO)₃, the 1,2,3,4,5-and 1,2,3,5,6-pentahaptocyclootadienyl derivatives C₈H₁₁Mn(CO)₃, the cyclooctatrienyl derivative C₈H₉Mn(CO)₃ and the substituted cyclopentadienyl derivative (CH₃)₂NCH₂C₅H₄Mn(CO)₃, are described. Losses of carbonyl groups, generally stepwise, from the molecular ions to give the corresponding [M – 3CO]⁺· ions are first observed. Further fragmentation of the carbonyl-free [M – 3CO]⁺· ions can involve a variety of processes such as the following: (a) elimination of a neutral manganese atom to give a hydrocarbon fragment; (b) elimination of a neutral hydrocarbon fragment to give an [MnH]⁺· ion; (c) dehydrogenation; (d) elimination of a 2-carbon C₂H₂ or C₂H₄ fragment; (e) elimination of a C₃H₄ or C₃H₆ fragment as a neutral species when it is bridging two carbon atoms bonded to manganese, as in C₈H₉Mn(CO)₃ and 1,2,3,4,5,h⁵-C₈H₁₁Mn(CO)₃, respectively. Fragmentation of the [M – 3CO]⁺· ion in (CH₃)₂NCH₂C₅H₄Mn(CO)₃ presents the following additional features: (a) elimination of C₆H₆ with a nitrogen shift from carbon to manganese; (b) elimination of a neutral dimethylamino fragment to give [C₆H₆Mn]⁺·, which then loses neutral C₆H₆, C₆H₅ or Mn fragments and thus is formulated tentatively as [(fulvene)Mn]⁺· or [C₆H₅MnH]⁺· rather than [(benzene)Mn]⁺·.     

The effect of ion preparation on kinetic energy release: Substituent and leaving group effects on the reactivity of the [C6H5CO]+ ion

Ion kinetic energy spectrometry has been utilized to investigate the loss of CO from [p-Z-C₆H₄-CO]⁺ and [C₆H₅CO]⁺ ions generated from substituted acetophenones and benzo-phenones. The variation of the leaving group in forming the [C₆H₅CO]⁺ ion has no detectable effect on the kinetic energy release observed in the subsequent decomposition reaction. A substituent effect consistent with the assumption that substituted benzoyl ions have no memory of their origin is also observed.     

Electron impact studies. CXII—The properties of positive ions produced by charge stripping from negative ions. [C6H5O]+, [C6H5S]+ and [C7H7S]+

The dissociative spectrum of the [C₆H₅S]⁺ ion derived by charge inversion from [C₆H₅S]^?, shows a variety of fragmentations including the competitive losses of H?, C₃H₄ and the formation of [CHS]⁺. The spectrum of a deuteriated derivative shows that these three processes are preceded or accompanied by H/D scrambling. The corresponding [C₆H₅O]⁺ species also undergoes hydrogen scrambling prior to fragmentation. In marked contrast, the ion [p-MeC₆H₄S]⁺ does not undergo hydrogen randomization between the methyl and aryl groups, and positional integrity is retained during fragmentation. These results are compared with the properties of the same ions produced by conventional ionization.     

A comparison of the unimolecular decomposition pathways of some C7 and C8 ions in the gas phase

[CnH_2n?3]⁺ and [C_nH_2n?4]⁺·(n = 7, 8) ions have been generated in the mass spectrometer from C_nH_2n?3 Br (n = 7, 8) precursors and from two steroids. The relative abundances of competing ‘metastable transitionss’ indicate (partial) isomerization to a common structure (or mixture of structures) prior to decomposition in most examples of all four types of ions. In contrast, [C₈H₁₀O]⁺· and [C₈H₁₂O]⁺· ions, generated from different sources as molecular ions and by fragmentation of steroids, do not decompose through common-intermediates.     

Structure of [C3H5O]+ ions produced from unimolecular decomposition of several [C4H8O]+˙ metastable ions

It is demonstrated by means of collisionally activated decomposition (CAD) that [C₃H₅O]⁺ originating from metastable [C₄H₈O]^+˙ ions are either acylium [C₂H₅CO]⁺ (a) or hydroxycarbenium [CH₂CHCHOH]⁺ (b). Butanone gives exclusively a but 2-methyl-2-propen-1-ol, 2-buten-1-ol, 3-buten-1-ol, butanal and 2-methylpropanal lead to ion b. Both structures a and b are produced from 3-buten-2-ol. These results are discussed in conjunction with experimental and calculated (MINDO/3) thermodynamic data.     

Reaction of ammonia with accelerated benzoyl ions under multiple-collison conditions in a triple quadrupole instrument

The reaction of benzoyl ion with ammonia in multiple-collision conditions in the second quadrupole assembly of a triple quadrupole mass spectrometer at (laboratory) ion kinetic energies from 0 to 20 eV produced the even-electron ions [C₆H₅]⁺, [C₆H₅NH₃·(NH₃)_m]⁺ (m = 0, 1) and [C₆H₅CONH₃·(NH₃)_n]⁺ (n = 0, 1, 2, 3) and the odd-electron ions [C₆H₄NH₃·(NH₃)_p]⁺· (p = 0, 1). Thermochemical information could not be obtained under multiple-collision conditions: both exothermic and endothermic reactions were observed, with no translational-energy onset measurable for the endothermic processes, nor decrease in the yield of the exothermic processes at high energies. The behaviour of cluster-ion intensities as pressure varied was qualitatively as expected. There are pressure and energy regions where spectra change little; if this feature were to be general, it would point to some utility for these conditions in qualitative analysis.     

Alkane loss from collisionally activated alkylmethyleneimmonium ions

The collision-induced dissociation (CID) spectra of five alkylmethyleneimmonium ions (H₂C-N⁺R¹R², (a) R¹ = R² = C₂H₅, (b) R¹ = n-C₃H₇, R² = H, (c) R¹ = n-C₃H₇, R² = CH₃, (d) R¹ = n-C₃H₇, R² = C₂H₅, (e) R¹ = R² = n-C₃H₇) are reported and discussed in terms of the mechanism of alkane loss. The most abundant alkane losses result from 2-azaallylic bond cleavages within R¹ and R² leading to daughter ions of m/z 84. Ion d (R¹ = n-C₃H₇, R² = C₂H₅) was chosen for a deuterium-labelling study because it exhibited methane loss nearly free from interferences with other fragmentations. The methane lost consists to a great extent (95%) of the methyl moiety of R². Whereas the methyl moiety obviously stays intact during the fragmentation process, the hydrogen additionally needed originates from all positions of R¹ and the double-bonded methylene in an approximately random distribution, suggesting extensive hydrogen migrations preceding the transfer step.     

Isomerization and fragmentation of methylfuran ions and pyran ions in the gas phase

The mutual interconversion of the molecular ions [C₅H₆O]⁺ of 2-methylfuran (1), 3-methylfuran (2) and 4H-pyran (3) before fragmentation to [C₅H₅O]⁺ ions has been studied by collisional activation spectrometry, by deuterium labelling, by the kinetic energy release during the fragmentation, by appearance energles and by a MNDO calculation of the minimum energy reaction path. The electron impact and collisional activation mass spectra show clearly that the molecular ions of 1–3 do not equilibrate prior to fragmentation, but that mostly pyrylium ions [C₅H₅O]⁺ arise by the loss of a H atom. This implies an irreversible isomerization of methylfuran ions 1 and 2 into pyran ions before fragmentation, in contrast to the isomerization of the related systems toluene ions/cycloheptatriene ions. Complete H/D scrambling is observed in deuterated methylfuran ions prior to the H/D loss that is associated with an iostope effect k_H/k_D = 1.67–2.16 for metastable ions. In contrast, no H/D scrambling has been observed in deuterated 4H-pyran ions. However, the loss of a H atom from all metastable [C₅H₅O]⁺ ions gives rise to a flat-topped peak in the mass-analysed ion kinetic energy spectrum and a kinetic energy release (T₅₀) of 26 ± 1.5 kJ mol^?1. The MNDO calculation of the minimum energy reaction path reveals that methylfuran ions 1 and 2 favour a rearrangement into pyran ions before fragmentation into furfuryl ions, but that the energy barrier of the first rearrangement step is at least of the same height as the barrier for the dissociation of pyran ions into pyrylium ions. This agrees with the experimental results.     

Hydrogen migrations in mass spectrometry. II—single and double hydrogen migrations in the electron impact fragmentation of n-propyl benzoate

The sources of the migrant hydrogen atom(s) in reactions (a) and (b) in the electron impact mass spectrum of n-propyl benzoate have been investigated: (a) [C₆H₅CO₂C₃H₇]⁺ →[C₆H₅CO₂H]⁺ + C₃H₆; (b) [C₆H₅CO₂C₃H₇]⁺ → [C₆H₅CO₂H₂]⁺ + C₃H₅^sdot;. Deuterium labelling of the propyl group showed that, for reaction (a) at 70 eV ionizing energy 3 ± 1% of the hydrogen originates from C-1 of the propyl group, 86 ± 4% from C-2 and 11 ± 3% from C-3. The specificity of the transfer from C-2 increases as the internal energy of the fragmenting ions decreases, indicating that the results cannot be rationalized in terms of H/D interchanges between positions in the propyl group, but rather that the reaction involves specific, competing, H transfer reactions from each propyl position, in contrast to the high site specificity characteristic of the McLafferty rearrangement. Reaction (b) involves, almost exclusively, transfer of one hydrogen from C-2 and one from C-3 with only very minor participation of C-1 hydrogens. The [C₆H₅COOH]⁺ ion produced in reaction (a) fragments further to [C₆H₅CO]⁺ + OH. and the labelling results indicate some interchange of the carboxylic hydrogen with (ortho) ring hydrogens for those ions fragmenting in the first drift region. The extent of interchange is less than that observed for fragmentation of the same ion produced by direct ionization of benzoic acid or by reaction (a) in ethyl benzoate.     

Further examples of skeletal rearrangements of the Wagner-Meerwein type in chemical ionization mass spectrometry: The case of [C6H9]+ ions

The [C₆H₉]⁺ ions produced either via unimolecular H₂O loss from 13 [C₆H₁₁O]⁺ precursors or direct protonation of 1,3- and 1,4-cyclohexadiene have identical collisional activation mass spectra. The kinetic energy release data for the process [C₆H₁₁O]⁺→[C₆H₉]⁺+H₂O are also very similar (on average T_0.5=24 meV) irrespective of the constitution of the precursor. From the proton affinities of 1,3-cyclohexadiene (PA=837.2 kJ mol^?1) using ion cyclotron resonance mass spectrometry the heat of formation of the [C₆H₉]⁺ ion is determined to 804.6 kJ mol^?1. This value taken together with the results of molecular orbital calculations (MNDO) and the structure indicative losses of CH₃^. and C₂H₄ upon collisional activation suggest that the [C₆H₉]⁺ ion has the structure of the 1-methylcyclopentenylium ion f and not that of the slightly less stable cyclohexenylium ion g. The generator of an easily interconverting system of isomeric [C₆H₉]⁺ ions is unlikely to be due to the high barrier separating the various isomers.     

Study of ion structures produced by the reaction of 2-propyl cations with water and methanol; covalently bound v. Hydrogen-bridged adducts

In contrast to an earlier report,¹ the collisonally induced dissociation of protonated 2-propanol and t-butyl alcohol yields spectra that are indistinguishable from those of the corresponding [C₃H₇/H₂O]⁺ and [C₄H₉/H₂O]⁺ ions generated by the (formal) gas phase addition reactions in a high pressure ion source of [s-C₃H₇]⁺ and [t-C₄H₉]⁺ ions with the n-donor H₂O. Similarly, [s-C₃H₇/CH₃OH]⁺ ions generated by both gas phase protonation of n- and s-propyl methyl ethers and addition reactions of [C₃H₇]⁺ to CH₃OH display mode-of-generation-independent collisionally induced dissociation characteristics. However, analysis of the unimolecular dissociation (loss of propene) of the [C₃H₇/CH₃OH]⁺ system, including a number of its deuterium, ¹³C- and ¹⁸O-labelled isotopomers, supports the idea that prior to unimolecular dissociation, covalently bound [C₃H₇- O(H)CH₃]⁺ ions intercovert with hydrogen-bridged adduct ions, analogous to the behaviour of the distonic ethene-, propene- and ketene-H₂O radical cations.     

Mass spectrometry of transition metal π-complexes. XV—The effect of substituents on the fragmentation of benchrotrenyls under electron impact

Mass spectra of substituted benchrotrenyls RC₆H₅Cr(CO)₃ where R?H, F, CI, I, CH₃, OCH₃, COOCH₃, C₂H₅, N(CH₃)₂, NH₂, C₆H₅, C(CH₃)₃, p-C₆H₄NH₂, CH₂C₆H₅, CH₂CH₂C₆H₅), 1,3,5-(CH₃)₃C₆H₃Cr(CO)₃ and 1,2,3,5-(CH₃)₄C₆H₂Cr(CO)₃ have been studied. It has been found that for monosubstituted benchrotrenyls there is a linear dependence of the parameter log [Cr]⁺/[RC₆H₅Cr]⁺) on the number of degrees of freedom of the [RC₆H₅Cr]⁺ ion. Decarbonylation of the molecular ions is not affected by the nature of the substituent R. The results are interpreted in terms of the quasi-equilibrium theory of mass spectra.     

A photo-ionization study of organosulfur ring compounds: Thiirane,thietane and tetrahydrothiophene

The gas phase heats of formation of several organosulfur cations were determined from thiirane, thietane and tetrahydrothiophene precursor molecules by photoionization mass spectrometry. Heats of formation at 0 K and 298 K are reported for the following ions: [H₂CS]^+˙, [H₃CS]⁺, [C₂H₃S]⁺, [C₂H₄S]^+˙, [C₃H₅S]⁺, [C₃H₆S]^+˙, [C₄H₇S]⁺ and [C₄H₈S]^+˙. The [C₄H₇S]⁺ (m/z 87), [C₂H₄S]^+˙ (m/z 60), [C₂H₃S]⁺ (m/z 59), [C₄H₇]⁺ (m/z 55), [C₄H₆]^+˙ (m/z 54) and [CH₂S]^+˙ (m/z 46) ions are produced from metastable tetrahydrothiophene ions at photon energies between 10.2 and 10.7 eV. Decay rates of internal energy selected parent ions to the m/z 60, 59, 55 and 54 fragments were measured by threshold photoelectron-photoion coincidence, the results of which are compared to statistical theory (RRKM/QET) calculations. The [C₂H₄S]^+˙ ion from tetrahydrothiophene is found to have the thioacetaldehyde structure. From the measured [C₂H₄S]^+˙ onset a ΔH = 50±8 kJ mol^?1 was calculated for the thioacetaldehyde molecule.     

Rearrangements and fragmentations of [C2H5S]+ ions

[C₂H₅S]⁺ ions (m/e 61) with different initial structures were generated in the mass spectrometer from twelve precursor ions. Abundance ratios of competing metastable ion decompositions were used to determine whether these ions decompose through the same or different reaction channels. It was concluded that all [C₂H₅S]⁺ ions isomerize to a common structure or mixture of structures prior to decomposition in the first field free region. From ¹³C labelling experiments it was concluded that [C₂H₅S]⁺ ions generated from the molecular ions of 2-propanethiol-2-[¹³C], partially rearrange to a symmetrical structure before decomposition to [CHS]⁺ and CH₄, whereas in [C₂H₅S]⁺ ions generated from the the molecular ions of 1,2-bis-(thiomethoxy-[¹³C]) ethane, the two carbon atoms become fully equivalent before CH₄ loss occurs.