Direct mass spectrometry of polymers. VII. Primary thermal fragmentation processes in polycarbonates

The primary fragmentation mechanisms in the thermal decomposition of several polycarbonates were studied by direct pyrolysis into the mass spectrometer. Our results indicate that ester exchange reactions predominate in the primary thermal fragmentation process of polycarbonates, causing the formation of cyclic oligomers.     

Direct mass spectrometry of polymers. VIII. Primary thermal fragmentation processes in polyurethanes

The primary fragmentation mechanisms in the thermal decomposition of several polyurethanes were studied by direct pyrolysis into the mass spectrometer. Ester exchange reactions predominate in the primary thermal fragmentation process, causing the formation of cyclic oligomers, which are subsequently cleaved to open-chain oligomers containing hydroxyl end groups.     

Direct mass spectrometry of polymers. XIV. Thermal fragmentation processes in poly-schiff bases

The thermal fragmentation processes in poly-Schiff bases have been investigated by direct pyrolysis–mass spectrometry. The mass spectral data show that the thermal fragmentation occurring in the polymers under investigation is characterized by hydrogen transfer reactions. In the case of a totally aromatic poly-Schiff base (polymer I ), the thermal fragmentation process involves hydrogen transfer irom the methyne group with formation of fragments bearing nitrile and/or phenyl end groups. In the case of aromatic-aliphatic poly-Schiff bases (polymers II–IV ), the hydrogen transfer process occurs from the aliphatic methylene groups. The latter process involves a lower energy and therefore occurs at lower temperatures with respect to the totally aromatic polymer I , with formation of thermal fragments bearing olefin and/or imine end groups. Beside these fragments, several thermal fragmentation compounds are also evolved by multiple hydrogen transfer reactions.     

Direct mass spectrometry of polymers. X. Primary thermal fragmentation processes in totally aromatic polyesters

The thermal decomposition of a series of isomeric poly-(oxphthaloyloxyphenylenes) (I–IV) and poly(m-hydroxybenzoic acid) (V) was studied by Direct Pyrolysis–Mass Spectrometry. The results indicate that intramolecular exchange reactions predominate in the primary thermal fragmentation processes, causing the formation of cyclic oligomers which are subsequently cleaved to open-chain fragments. The size and relative abundance of the cycles produced appear to be strongly influenced by steric factors, i.e., by the structure (para or meta) of the repeating unit in each polymer. Remarkably, in the case of poly(m-hydroxybenzoic acid) the formation of cyclic oligomers containing up to seven repeating units is observed.     

Hydrogen migration in the electron impact induced retro diels-alder fragmentation of N-aryl-4H-5,7a-epoxyisoindolines

The electron impact induced fragmentations of the C-5 unsubstituted and 5-methyl N-aryl-4H-5,7a-epoxyisoindolines (where aryl is pheny, p-tolyl, p-methoxyphenyl, o-methoxyphenyl and p-chlorophenyl) were investigated. The fragmentation patterns deduced were supported by exact mass measurements of prominent ions and by deuterium labelling. The retro Diels-Alder fragmentation turned out to be a predominant process in all the compounds investigated. In the mass spectra of the 5-methyl N-aryl-4H-5,7a-epoxyisoindolines hydrogen migration preceding fragmentation occurred. From the mass spectrum of the specifically deuterated compound it was concluded that the transferred hydrogen originates exclusively from the 5-methyl group.     

Metastable ion fragmentation processes in n-alkanes

The n-alkanes, propane to heptane, have been studied in a modified MS-9 mass spectrometer under conditions that enabled the daughter ions, resulting from metastable fragmentation processes occurring in the first field free region, to be observed with high sensitivity. The precursor ions in these fragmentations have been identified unambiguously and a general fragmentation scheme for the n-alkanes is proposed. The mass spectrometer incorporated a variable width monitor slit and narrow settings of this slit improved the energy resolution so that fine structure could be observed in some of the ‘metastable peaks’. The implications of this fine structure are discussed.     

Direct mass spectrometry of polymers. XII. Thermal fragmentation processes in poly-α-aminoacids

The thermal fragmentation processes in poly-α-aminoacids have been investigated by direct pyrolysis–Mass Spectrometry. The mass spectral data show that the pyrolytic breakdown of polyglycine, polysarcosine, and polyproline leads to the formation of cyclic oligomers. Polyalanine, polyphenylalanine, and polytyrosine decompose yielding compounds with olefin and nitrile end-groups. Finally, in the case of poly-α-methylglutamate, the primary thermal process is the loss of methanol with consequent formation, along the polymer chain, of pyroglutamic units, which yield cyclic dimer as main pyrolysis product.     

Direct mass spectrometry of polymers. XI. Primary thermal fragmentation processes in aromatic–aliphatic polyesters

The thermal decomposition of two series of isomeric aromatic–aliphatic polyesters was studied by direct pyrolysis-mass spectrometry. The results indicate that intramolecular exchange reactions predominate in the primary thermal fragmentation processes to cause the formation of cyclic oligomers. Several secondary thermal processes may occur after the primary step: hydrolytic cleavage of the ester bond, decarboxylation, and β-hydrogen transfer.     

Specific and random processes in the fragmentation of cyclohexanol

The mass spectra of cyclohexanol and eleven deuterated analogues have been studied. Detailed mechanisms, consistent with peak intensity and defocused metastable abundance data, have been proposed for the loss of water from the molecular ion and for the subsequent loss of a methyl radical. The importance of specific eliminations and the absence of any processes involving complete hydrogen atom scrambling in these fragmentations has been demonstrated. Scrambling has also been shown to be absent in the formation of the base peak m/e 57, the [C₃H₅O]⁺ ion.     

Hydrogen transfer in the retro Diels-Alder fragmentation of oxygen-containing heterocyclic Compounds. I—chromanones and 4-hydroxychromans

The mass spectra of 2,2-dimethylchromanones and 2,2-dimethyl-4-hydroxychromans show peaks corresponding to the ions formed by the retro Diels-Alder reaction with or without hydrogen transfer to the ion. The hydroxychromans also show peaks corresponding to an ion formed by the loss of hydrogen from the ion formed by the retro Diels-Alder reaction. Deuterium labelling showed that the hydrogen transferred in the chromanones arises from the gem-dimethyl groups on C-2, and the hydrogen atom lost in the case of the hydroxychromans is from C-4, the carbinyl carbon.     

Synthesis of heteroorganic derivatives of barene

Conclusions It was shown by the example of phenylbarenyllithium and heteroorganic halides of silicon, tin, mercury, and phosphorus that lithium derivatives of barene can be successfully used to obtain heteroorganic compounds of a new class, where the heteroatom is directly bonded to the barene ring.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8 pp. 1539–1540, August, 1964     

Ortho effects in mass spectrometric fragmentation processes

The mass spectra of the phenylhydrazones and 2,4-dinitrophenylhydrazones of ortho substituted benzaldehydes and acetophenones (X = I, Br, Cl, OCH₃, OH) show characteristic [M ? X]⁺ ions which allow the ortho derivatives to be distinguished from their meta and para isomers.     

Direct mass spectrometry of polymers. XIII.. Thermal fragmentation processes in copoly-α-amino acids

The thermal fragmentation processes in L-Proline—Sarcosine and in L-Proline—L-Alanine copolymers have been investigated by direct pyrolysis—mass spectrometry. The mass spectral data show that proline—sarcosine copolymers decompose in three steps, yielding cyclic oligomers. The first step is characterized by the presence of the Sar—Sar, Sar—Pro, and Pro—Pro cyclic dimers. In the second step the formation of several series of cyclic oligomers up to nonamers—decamers can be observed, while the third step of thermal decomposition process leads to the almost exclusive formation of Pro—Pro cyclic dimer. The proline—alanine copolymers, instead, show a single-step thermal degradation process leading to the formation both of cyclic oligomers and compounds with olefin and nitrile end-groups. Our results show that the thermal decompostion of alanine units is affected by the neighboring units along the polymer chain. Sarcosine units act as thermally labile probes, causing the selective thermal cleavage of the Pro—Sar copolymer.     

Various energy minima and corresponding fragmentation processes: Alkylsilanes

Fragmentation of alkylsilanes, in particular trimethylethylsilane, were studied by mass-analysed ion kinetic energy (MIKE) and collision-induced decomposition MIKE techniques. Ab initio and semi-empirical molecular orbital calculations were applied to explain the main fragmentation processes. These calculations indicate that more than one minimum can be located on the potential energy surface of a given ground-state molecule ion. These differ from each other mainly in the length of the silicon–carbon bonds. The structures can be adequately described as complexes of a trivalent silyl ion and an alkyl radical. Each of these complexes fragments by the loss of the weakly bound alkyl radical. The calculated energetics of these reactions were found to be in good agreement with the observed energy dependence of the mass spectra.     

Organomelemental fragmentations : I. Direct and retro fragmentations in organometallic benzyl compounds. Aromatisational metallation

Organometallic benzyl compounds are known to react with eletrophilic agents via two paths, viz., by direct substitution of the metal by the agent, and by attack of the ortho- and para-positions of the ring. The latter reaction leads to expulsion of the metal and formation of unstable triene intermediates (1-methylene-2,4-cyclohexadienes and 1-methylene-2,5-cyclohexadienes). By attributing these processes to electrophilic fragmentations (referred to as F_E) a new process may be predicted to occur, electrpohilic addition of metal salts to the exo-methylene site of trienes of this type. This process can be regarded as the reverse of the fragmentation reaction. The F_E representation proposed here forms the basis for a new organometallic synthesis, aromatisational metallation. It is shown that interaction of 1-methylene-6-ethyl-2,4-cyclohexadiene and 1-methylene-4-ethyl-2,5-cyclohexadiene with mercury dichloride in ether results in the formation of o- and p-ethylbenzylmercury chlorides, respectively.     

2-substituted-2,3-dihydro-4H-pyrans: Competition between ‘retro Diels-Alder’ fragmentation and substituent loss

The mass spectra of two series of 2-substituted-2,3-dihydro-4H-pyrans (six ethers and six carbonyl derivatives) showed a marked dependence upon the nature of the substituent. α-Cleavage (substituent loss) was the only primary decomposition observed in the carbonyl series, while ‘retro Diels-Alder’ reactions competed with α-cleavage in the ether series. An unambiguous interpretation of this difference in behaviour, based on the consequence of charge localization and thermochemical data, is proposed.