Mass spectrometric decomposition processes of 2-methyl-1-hexene

Unimolecular decompositions of 2-methyl-1-hexene and several labelled analogues were studied following 70 eV electron impact (normal and metastable spectra) and field ionization (field ionization kinetic measurements). Molecules labelled with ¹³C in the 1-position and the methyl position were found to behave essentially identically. This is attributed to rapid transfer of a hydrogen atom mainly from C-5 to C-1 (γ-hydrogen shift). Loss of ethene, propene or propenyl do not involve loss of the methyl carbon or C-1. All three reactions are better than 90% specific in this respect under all conditions studied. At shorter times, C₃H₆ loss is the dominant reaction, while at longer times C₂H₆ loss accounts for >90% of the ion current. It is proposed that at least two distinct pathways for C₂H₄ loss operate in linear 1-alkenes, one of which (loss of carbons 1 and 2) is blocked by a 2-methyl substituent. The [C₆H₁₁]^+˙ and [C₅H₁₀]^+˙ ions formed from ¹³C labelled 2-methyl-1-hexenes fragment in an essentially statistical fashion.     

Unimolecular reactions of the isolated immonium ions CH3CH = NH+C4H9, CH3CH2Ch = NH+C4H9 and (CH3)2C = NH+C4H9

The reactions of ten metastable immonium ions of general structure R¹R²C?NH⁺C₄H₉ (R¹ = H, R² = CH₃, C₂H₅; R¹ = R² = CH₃) are reported and discussed. Elimination of C₄H₈ is usually the dominant fragmentation pathway. This process gives rise to a Gaussian metastable peak; it is interpreted in terms of a mechanism involving ion-neutral complexes containing incipient butyl) cations. Metastable immonium ions ontaining an isobutyl group are unique in undergoing a minor amount of imine (R¹R²C?NH) loss. This decomposition route, which also produces a Gaussian metastable peak, decreases in importance as the basicity of the imine increases. The correlation between imine loss and the presence of an isobutyl group is rationalized by the rearrangement of the appropriate ion-neutral complexes in which there are isobutyl cations to the isomeric complexes containing the thermodynamically more stable tert-butyl cations. A sizeable amount of a third reaction, expulsion of C₃H₆, is observed for metastable n-C₄H₉ ⁺NH?CR¹R² ions; in contrast to C₄H₈ and R¹R²C?NH loss, C₃H₆ elimination occurs with a large kinetic energy release (40–48 kJ mol^?1) and is evidenced by a dish-topped metastable peak. This process is explained using a two-step mechanism involving a 1,5-hydride shift, followed by cleavage of the resultant secondary open-chain cations, CH₃CH⁺ CH₂CH₂NHCHR¹R².     

Unimolecular reactions of [C5H10O]+ ˙ radical cations derived from4-penten-1-ol

The reactions of metastable [C₅H₁₀O]^{+ ˙} radical cations produced by ionization of 4-penten-1-ol are reported and discussed. These [C₅H₁₀O]^{+ ˙} species undergo mainly ethyl radical loss, with smaller contributions of methyl radical and water expulsion. ²H-Labelling studies reveal different specificities of hydrogen selection in these three fragmentations. The behaviour of these [C₅H₁₀O]^{+ ˙} ions is compared to those reported previously for isomeric radical cations containing linear alkenyl chains and a terminal hydroxyl group.     

Decompositions of metastable [C6H12O]+ ions with the oxygen on the third carbon: Ring-size preferences in [CnH2nO]+˙ ions

The isomerizations preceding the metastable decompositions in the mass spectrometer of a number of [C₆H₁₂O]⁺˙ ions with the oxygen on the third carbon are characterized utilizing deuterium labeling. Hydrogens are transferred in these ions by three-, five- and six-membered ring rearrangements, with propensities determined by features of the individual reactions. Three-membered ring hydrogen transfers between α and β-carbons are preferred to all five-membered ring hydrogen transfers. However, six-membered ring hydrogen transfers take place to the apparent exclusion of three-membered ring hydrogen transfers to enol carbons when the products are of comparable stability. The low-energy [C₆H₁₂O]⁺˙ isomerizations characterized are predictable from the behavior of their lower homologs. It is concluded that the determinants of these reactions are the same as those of other highly reactive organic intermediates.     

Hydrogen rearrangements of ionized propanoic acid and isomeric ions

Ionized propanoic acid and its enol isomer lose H^˙ in both normal and metastable decompositions. These C₃H₆O₂ ions lose their distinctness prior to undergoing metastable decomposition in the mass spectrometer. Deuterium labeling indicated that this results from the transfer of a β-hydrogen to oxygen in the propanoic acid ion followed by rapid exchange of hydrogen and deuteriums between the α- and β-carbons and slower exchange between the carbons and the oxygens. 3- and 5-Membered ring hydrogen rearrangements are preferred over 4-membered ring transfers. It is concluded that 1,2-hydrogen shifts following removal of hydrogen from alkyl chains may sometimes cause site-specific hydrogen rearrangements to appear non-site specific.     

Fragmentation of organosulfur compounds upon electron impact. Part III. Metastable decomposition of the molecular ions of methyl thioglycolate and ethyl thioglycolate

The metastable decompositions of the molecular ions of methyl thioglycolate (1) and ethyl thioglycolate (2) were investigated by means of mass analyzed ion kinetic energy (MIKE) spectra and deuterium labeling. The loss of methanol is the only metastable decomposition of 1^+·. This fragmentation occurs via two distinct pathways. The molecular ions of 2 decompose in a variety of ways, i.e., the losses of water, ethene, ethanol or ?₂H₃O₂. All of these decompositions, except the loss of ethene, occur through two distinct mechanisms. During the loss of ?₂H₃O₂, the ethyl group or ethene migrates from the oxygen to the sulfur atom. The loss of H?S, which corresponds to the loss of H?O with a concomitant double hydrogen transfer observed in the case of methyl glycolate (3), does not participate in the metastable decomposition of 1^+· and 2^+·. This is due to the energetic favorableness of the loss of methanol.     

The mass spectra of organic compounds. 7th Communication. 4,4-Dimethyl-1-pentene

Loss of CH, CH₄, C₂H₄, C₃H, C₃H₆ and C₃H₇ from the molecular ions of a number of ¹³C-labeled analogs of 4,4-dimethyl-1-pentene was studied both in normal (source) 70-eV electron impact (EI) spectra dn in metastable spectra. For loss of CH in the source, 96% of the methyl comes frm positions of 5, 5′ and 5″, while the remainder comes from position 1. In the metastable spectra, loss of C-1 (16%) and C-3 (9%) is increasing in importance. The loss of ethylene is a particular case: either C-1 or C-3 are lost with any other C-atom from positions 2,5,5′, and 5″ (8 × 10%) in the metastable spectra, the probability for simultaneous loss of C-1 and C-3 being 6%. If C-1 seems to these two positions become completely equivalent in the metastable time range. The T-values (kinetic energy release) for the different positions show small, but statisticaly different values and a small isotope effect. Loss of C₃H₅ (allylic cleavage) is 100% C-1, C-2 and C-3, i.e., no evidence for skeletal rearrangement is seen. This is also true for loss of C₃C₆ (McLafferty rearrangement) within the source, but in metastable decay the other positions gain in importance. The neutral fragment C₃H appears to be the the result of consecutive loss of CH and C₃H₄, rather than a one-step loss of propyl radical or the inverse reactions sequence. No metastable reaction can be seen for this reaction. Decomposition of labeled C₆H and C₅H secondary ions occurs in an essentially random fashion.     

Gas‐Phase Synthesis of the Benzyl Radical (C6H5CH2)

Dicarbon (C₂), the simplest bare carbon molecule, is ubiquitous in the interstellar medium and in combustion flames. A gas‐phase synthesis is presented of the benzyl radical (C₆H₅CH₂) by the crossed molecular beam reaction of dicarbon, C₂(X¹Σ_g⁺, a³Π_u), with 2‐methyl‐1,3‐butadiene (isoprene; C₅H₈; X¹A′) accessing the triplet and singlet C₇H₈ potential energy surfaces (PESs) under single collision conditions. The experimental data combined with ab initio and statistical calculations reveal the underlying reaction mechanism and chemical dynamics. On the singlet and triplet surfaces, the reactions involve indirect scattering dynamics and are initiated by the barrierless addition of dicarbon to the carbon–carbon double bond of the 2‐methyl‐1,3‐butadiene molecule. These initial addition complexes rearrange via multiple isomerization steps, leading eventually to the formation of C₇H₇ radical species through atomic hydrogen elimination. The benzyl radical (C₆H₅CH₂), the thermodynamically most stable C₇H₇ isomer, is determined as the major product.     

Formation of phenanthrene via H-assisted isomerization of 2-ethynylbiphenyl produced in the reaction of phenyl with phenylacetylene

Model chemistry G3(MP2,CC)//B3LYP/6-311G(d,p) calculations of the potential energy surface for the reaction of phenyl radical (C₆H₅) with phenylacetylene (C₈H₆) have been carried out and combined with Rice-Ramsperger-Kassel-Marcus/Master Equation calculations of temperature- and pressure-dependent rate constants. The results showed that the reaction can serve as a viable source for the formation of phenanthrene via an indirect route involving a primary reaction of phenyl addition to the ortho carbon in the ring of phenylacetylene and H elimination producing 2-ethynylbiphenyl followed by secondary H-assisted isomerization of 2-ethynylbiphenyl to phenanthrene. In the secondary reaction, the H atom adds to the α carbon of the ethynyl side chain, then a six-member ring closure takes place followed by aromatization via an H loss. The channel of H addition to the side chain of 2-ethynylbiphenyl appears to be much faster than H addition to the ortho carbon in the ethynyl-substituted ring leading back to the initial C₆H₅ + C₈H₆ reactants. Rate constants for the primary C₆H₅ + C₈H₆2-ethynylbiphenyl ( p1 ) + H and secondary p1  + Hphenanthrene ( p2 ) + H reactions have been computed in the temperature range of 500-2500 K at pressures of 30 Torr, 1, 10, and 100 atm and fitted to modified Arrhenius expressions. The suggested kinetic scheme and rate constants are proposed as a prototype for the modeling of the growth of polycyclic aromatic hydrocarbons via the phenyl addition-dehydrocyclization (PAC) mechanism involving an addition of a PAH radical to an ethynyl-substituted PAH molecule.     

Metastable ion study of organosilicon compounds. Part III—trimethoxyphenylsilane

The unimolecular decomposition of trimethoxyphenylsilane (1) was investigated by mass-analysed ion kinetic energy (MIKE) spectrometry, deuterium-labelling studies and from high resolution data. The characteristic fragmentations of metastable molecular ion of 1 were losses of C₆H₆ and C₇H₇^· with rearrangements. Almost complete H/D scrambling occurred in the molecular ion prior to these decompositions. The other important fragmentation routes corresponded to expulsions of CH₃O^· and C₆H₅^·. These fragmentations were followed by consecutive elimination of an H₂CO molecule, as commonly observed in the mass spectra of alkoxysilanes. In these fragmentation processes, H/D scrambling increased as the internal energy of the molecular ion was lowered. The fragmentations of 1 were compared with those of its carbon analogue, α,α,α-trimethoxytoluene.     

The decompositions of metastable [C4H8O]+˙ ions and the [C4H8O]+˙ potential surface

Collisionally activated decomposition (CA) spectra of [C₄H₈O]⁺˙ ions and the products of their metastable decompositions are used to refine a previously presented picture of the reactions of [C₄H₈O]⁺˙ ions. Metastable [C₄H₈O]⁺˙ isomers predominantly rearrange to the 2-butanone ion and decompose by loss of methyl and ethyl, although up to 38% of the methyl losses take place by other pathways to form \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm{CH}}_{\rm{2}} = {\rm{CHCH = }}\mathop {\rm{O}}\limits^{\rm{ + }} {\rm{H}}{\rm{.}} $\end{document} . The CA spectra of many of the [C₄H₈O]⁺˙ ions with the oxygen on the first carbon are very similar, consistent with those ions isomerizing largely to common structures before or after collision. However, several of these ions have unique CA spectra, so they must remain structurally distinct from the majority of the [C₄H₈O]⁺˙ ions below energies required for decomposition. The CA spectra of ions with the oxygen on the second carbon are distinct from those of ions with the oxygen on the first carbon, so there is limited interconversion of the non-decomposing forms of the two types of ions. A potential energy diagram for the reactions of metastable [C₄H₈O]⁺˙ ions is constructed from appearance energy measurements. As would be expected, the relative importances of most of the [C₄H₈O]⁺˙ isomerizations seem to be inversely related to the activation energies for those processes. Some parallels between the isomerizations of [C₄H₈O]⁺˙ ions and those of related ions are pointed out.     

Pyrolysis of 2-methylbut-1-ene-3-yne between 375 and 450°c

The pyrolysis of 2-methylbut-1-ene-3-yne (C₅H₆) has been studied from 375 to 450°C in a quartz reaction vessel in the absence and presence of O₂ or NO. From 375 to 425°C, the rates of disappearance of reactant and of formation of dimers are second order in C₅H₆. The major product is polymer, with the dimers accounting for about 3% of the C₅H₆ consumed. In addition, toluene and p-xylene are produced, their production coming, at least in part, from decomposition of the C₅H₆ dimers (C₁₀H₁₂). Also, trace amounts of CH₄, C₂H₄, C₂H₆, are formed. The rate coefficients for C₅H₆ removal and C₁₀H₁₂ formation in the absence of O₂ or NO are where the uncertainties are one standard deviation. The reaction mechanism for dimer formation is analogous to that in vinyl acetylene (C₄H₄) pyrolysis (5), except that in the C₄H₄ system cyclooctatetraene is seen as an unstable product that isomerizes to styrene, whereas in the C₅H₆ system, the dimethylcyclooctatetraene apparently is too unstable to be detected. The dimers detected were 2,6-dimethylstyrene (P4), p-isopropenyltoluene (P5), and two other unidentified dimers (P3) with nearly identical gas chromatographic retention times. From the effect of the radical scavengers and by comparison of the C₄H₄ and C₅H₆ systems, the following mechanistic characteristics were determined: (1) The direct formation of styrene in the C₄H₄ system comes from a head-to-head modified Diels-Alder six-member cycloaddition that proceeds through a diradical intermediate. (2) There is no positive evidence for a direct head-to-tail modified Diels-Alder six-member cycloaddition. However, if it does occur, it does not involve diradicals but must be concerted. (3) Cyclooctatetraene is formed in concerted, non-free-radical mechanisms that may proceed both by head-to-head and head-to-tail eight-member cycloadditions. For the C₅H₆ system, the head-to-head adduct isomerizes to P3, whereas the head-to-tail adduct isomerizes to P3, P4, and/or P5. and/or P5. Kinetic data suggest that P3 is not produced from the cyclooctatetraene intermediate, in which case, head-to-head addition would not occur. It appears that the head-to-head additions are free radical in nature and proceed mainly through a six-membered ring intermediate, while head-to-tail additions are a concerted molecular process and proceed mainly through an eight-membered ring intermediate.     

The reactions of metastable [C5 H10 O]+˙ ions with the oxygen on the second carbon

Nearly all [C₅H₁₀O]⁺˙ isomers with the oxygen on the second carbon are shown to interconvert with each other and lose methyl and ethylene at the threshold for dissociation. The methyls contain the carbons from the 1- and 5-positions with about equal frequency, and C(3) or perhaps C(4) about half as often as either terminal carbon. CH₃ CH₂ CH₂ CO⁺ is formed by loss of the C(1) methyl and by loss of the C(5) methyl. Hydorgen transfer between C(5) and the oxygen and between the oxygen and C(4) are facile, and 1,2-hydrogen transfers between C(3) and C(4) occur with high frequency. Extensive skeletal rearrangements also take placae by 1,2-shifts between C(2), C(3) and C(4). We attribute the occurrence of teh three-center shifts between C(2), C(3) and C(4) to the presence of considerable charge density on C(2) and C(3) in many of the [C₅H₁₀O]⁺˙ isomers. The isomerizations of [C₅H₁₀O]⁺˙ can be considered a mixture of free radical and carbocation reactions. Strong similarities exist between the isomerizations of metastable [C₅H₁₀O]⁺˙ ions with the oxygen on the second carbon and those of isomers of ionized butanoic acid, methyl butanoate and n-butanal.     

Unimolecular reactions of ionized methyl allyl ether

The fragmentation of CH₂?CHCH₂OCH₃^+· cation-radicals has been investigated by means of ²H- and ¹³C-labelling experiments and by analysis of collision-induced dissociation spectra. Metastable C₄H₈O^+· species decompose via one of three main channels which involve loss of (a) a hydrogen atom, (b) a methyl radical or (c) a formaldehyde molecule. Extensive, but not complete, exchange of the hydrogen and deuterium atoms in specifically labelled C₄H₈-_nD_nO^+· analogues precedes each of the three fragmentation pathways. The role of distonic ions in the rearrangement steps which bring about hydrogen exchange is discussed. The influence of isotope effects on the relative rates of the major reactions and the associated kinetic energy releases is examined. Only loss of a hydrogen atom is subject to a substantial isotope effect. Elimination of a methyl radical releases a large amount of kinetic energy, as is shown by the broad and dish-topped appearance of the corresponding metastable peak (T_1/2 ≈ 42 kJ mol^?1). The carbon atom of the original methoxy group is specifically expelled in this process. Both the large T_1/2 value and the unusual site selectivity are atypical of methyl and other alkyl radical losses from ionized alkenyl methyl ethers. The carbon atom of the methoxy group also participates specifically in formaldehyde elimination, but the two hydrogen atoms are not always selected from the three contained in the initial methoxy group. The implications of these labelling results for the synchronicity of concert of formaldehyde loss, which can be formu lated as a pericyclic process, is analysed.     

Metastable ion studies. III—composite metastable peaks in fragmentations of some small hydrocarbon cations

Composite metastable peaks are generated in the unimolecular fragmentations (i) [C₃H₅]⁺ → [C₃H₃]⁺ + H₂ (flat-top upon flat-top) and (ii) [C₄H₉]⁺ → [C₃H₅]⁺ + CH₄ (flat-top and gaussian). The measurement of appearance potentials and kinetic energy releases lead us to conclude, in agreement with earlier proposals, that in (i) the components can arise from the generation of the isomeric cyclopropenium and propargyl daughter cations. In (ii) the components are proposed to arise from the fragmentation of tert- and sec-butyl cations yielding allyl as the common daughter ion. The composite peak observed in the fragmentation (iii) [C₃H₄]⁺· → [C₃H₃]⁺ + H· is shown to be present only if the decomposing molecular ion is large enough to also produce [C₆H₈]²⁺ ions. The second component in (iii) then arises from the reaction [C₆H₈]²⁺ → [C₆H₆]²⁺ + H₂.     

Initiation Mechanism of Alternating Copolymerization of Styrene with Some Electron-Accepting Monomers in the Presence of Zinc Chloride by Means of Spin Trapping Technique

Abstract

The initiation mechanism of spontaneous alternating copolymerizations of styrene (St) and some electron-accepting monomers such as methyl methacrylate (MMA), methyl acrylate (MA), methacrylonitrile (MAN), and acrylonitrile (AN) in the presence of ZnCl₂ was studied by the spin trapping technique, in which 2-methyl-2-nitrosopropane (BNO) was used as a spin trapping reagent. When this technique was applied to the alternating copolymerization systems of St-MMA-ZnCl₂, St-MA-ZnCl₂, and St-MAN-ZnCl₂, the 2-phenylvinyl radical (·CH[dbnd]CH[sbnd]C₆H₅) was trapped as nitroxide. The structure of this nitroxide, which showed a large coupling constant (19～20 G) by β-hydrogen, was confirmed by comparison with the result of authentic experiment Accordingly it was concluded that this nitroxide was formed through proton migration from the St cation radical to the acceptor monomer anion radical in the charge- or electron-transfer complex, followed by reaction with BNO.

In the St-AN-ZnCl₂ system, however, a nitroxide derived from a cyclic radical was observed together with the nitroxide from 2-phenylvinyl radical. This cyclic radical seemed to be produced via the Diels-Alder adduct between St and AN.     

The time dependence of the competing processes leading to the loss of C2H4 from tetralin

Field ionization kinetic measurements indicate that loss of C₂H₄ from the molecular ion of tetralin occurs via competing processes, viz. (a) at short times the retro Diels-Alder decomposition which is a high energy process, (b) loss from positions 1+2 (=3+4) and (c) loss after complete C and H scrambling which gains in importance for long ion lifetimes.     

Unimolecular and collision-induced dissociation reactions of peracetylated methyl glucopyranoside

The mechanism of the collision-induced fragmentation of peracetylated methyl-α-D-glucopyranoside was investigated using deuterium-labelled acetates and sequential mass spectrometry. Loss of the substituent at C(1), the anomeric carbon, yields an ion of m/z 331, [C₁₄H₁₉O₉]⁺. This ion further dissociates via two pathways, the first including m/z 271, [C₁₂H₁₅O₇]⁺, 169, [C₈H₉O₄]⁺ and 109, [C₆H₅O₂]⁺, and the second including m/z 211, [C₁₀H₁₁O₅]⁺, 169, [C₈H₉O₄]⁺ and 127 [C₆H₇O₃]⁺. The first path proceeds via loss of acetate at C(3), followed by a single-step concerted loss of acetates from C(2) and C(4), and ending with loss of acetate from C(6). The second path proceeds predominantly via loss of acetates from C(3) and C(4), elimination of ketene from the C(2)-acetate and finally loss of ketene from the acetate at C(6). This path is also characterized by an ill-defined series of parallel decomposition reactions involving acetates from other sites on the molecule. At low collision energy, and in the absence of collision gas (unimolecular reaction conditions), the former pathway predominates; m/z 331 dissociates via loss of acetate at C(3), followed by a single-step concerted loss of acetates from C(2) and C(4).     

Chemical synthesis with metal atoms: the reaction of chromium and nickel atoms with styrene

The preparation of C₈H₈Cr(CO)₃ (or C₈H₈Cr(PF₃)₃) and polystyrenechromium tricarbonyl (or polystyrenechromium tris(trifluorophosphine) from chromium atoms, styrene and carbon monoxide (or trifluorophosphine) is described; nickel atoms and styrene produce tristyrenenickel, which reacts with α,α′-bipyridyl to yield bipyridylstyrenenickel.     

Doubly charged fragment ions in the field ionization mass spectra of alkylbenzenes

It is shown that the radical [C₆H₅C_mH_2m]²⁺ fragment ions found in the field ionization mass spectra of alkylbenzenes are formed via a different adsorption state of the singly charged species than in the case of the formation of [M]²⁺ molecular ions. It is further demonstrated that the primary fragmentation of molecules by the cleavage of C? C bonds results not only from decompositions of molecular ions in the gas phase but also from surface reactions.