Isomerisation of hydrocarbon ions:I—Isomeric octanes: Acollisional actiation study

The molecular ions of isomeric octanes retain their structural identity, while their alkyl fragments [C_nH_2n+1]⁺ (n = 3 to 7) isomerise to common structures prior to decomposition. Structures for [C₆H₁₃]⁺ and [C₇H₁₅]⁺ ions are proposed.     

Isomerization of hydrocarbon ions. VI—parameters determining the isomerization of aliphatic hydrocarbon ions

The radical or non-radical character of aliphatic hydrocarbon ions determines the extent to which these ions equilibrate to a mixture of interconverting structures prior to decomposition. It is suggested that the radical (odd electron) ions have both lower thresholds for decomposition and higher barriers for isomerization that non-radical (even electron) ions, thus explaining their reduced tendency for isomerization. Moreover, the molecular size seems to be a major influencing factor for the isomerization of unsaturated hydrocarbon molecular ions. With decreasing molecular size isomerization prior to decomposition becomes more pronounced. Collisional activation spectra of fragment ions (formed by loss of H₂O from the corresponding alcohols) and of [C₅H₁₀]⁺ and [C₄H₈]⁺ molecular ions are reported in support of these conclusions.     

Stereochemical applications of mass spectrometry. II—Chemical ionization mass spectra of isomeric dicarboxylic acids and derivatives

The H₂ and CH₄, chemical ionization mass spectra of the cis dicarboxylic acids, maleic and citraconic acid, show much more extensive loss of H₂O from [MH]⁺ than the trans isomers, fumaric acid and mesaconic acid. Similarly, esters of maleic acid show a much more facile loss of ROH (R=alkyl or phenyl) from [MH]⁺ than do esters of fumaric acid. Similar differences are observed in the chemical ionization mass spectra of the isomeric phthalic and isophthalic acids and derivatives, where the ortho isomers show more extensive fragmentation of [MH]⁺ than the meta isomers. The facile fragmentation of [MH]⁺ for the cis and ortho isomers is attributed to ROH elimination involving interaction between the two carboxylate functions and forming the stable cyclic anhydride structure for the fragment ion. By contrast ROH elimination from [MH]⁺ for the trans and metu isomers requires a symmetry-forbidden [1,3]-H migration in the carboxyl protonated species and cannot lead to the cyclic anhydride structure. The chemical ionization mass spectra of cis and trans cyclohexane-1,2-dicarboxylic acids are essentially identical and show extensive fragmentation of the [IMH]⁺ ion. Experiments using deuterium labelling show extensive carboxyl group interactions for both isomers. The chemical ionization mass spectra of maleanilic and phthalanilic acids and of the related anhydrides and imides also are reported, as are the electron impact mass spectra of diphenyl maleate, diphenyl fumarate, diphenyl phthalate, maleanilic acid and phthalanilic acid.     

Metastable ion study of organosilicon compounds part II—hexamethyldisiloxane

Unimolecular reactions of the metastable silicenium ion (CH₃)₃SiOSi(CH₃)₂ ⁺ generated by dissociative ionization of bexamethyldisiloxane were investigated by mass-analysed ion kinetic energy (PUKE) Spectrometry. The characteristic fragmentations observed were losses of CH₄ and (CH₃)₂Si?O molecules. Complete scrambling of all methyl groups prior to these reactions was found by investigating the MIKE spectra of deuterium labelled analogues (CD₃)₃SiOSi(CH₃)₂⁺ and (CH₃)₃SiOSi(CD₃)₃⁺. The loss of methane was accompanied by a large kinetic energy release (T_0.5 = 482 meV). The MIKE spectra of silicenium ions were compared with those of their carbon analogues. The most predominant reaction of metastable (CH₃)₃COC(CH₃)₃⁺ ion was the loss of CH₂?C(C H₃)₂ leading to protonated acetone. Significant differences between the ion fragmention characteristics of silicon and carbon compounds were found.     

Boron–carbon coupling constants: II—the tetraphenylborate anion

Boron-carbon coupling constants were obtained from the ¹³C FT-NMR, ¹H noise decoupled spectra of sodium tetraphenylboron and tetra-n-butylammonium tetraphenylboron. The appearance of the spectra, as well as values obtained for J(BC), are discussed and contrasted with previously published data.     

Isomerization of hydrocarbon ions. VIII—The electron impact induced decomposition of n-dodecane

The literature on the mass spectrometry of ²H and ¹³C labelled higher alkanes is reviewed and the decomposition behaviour of both the molecular and the fragment ions of n-dodecane, n-dodecane-1, 12-[¹³C₂] and n-dodecane-1,1,1,12,12,12-[²H₆] studied with special emphasis on metastable decompositions. It is shown that the elimination of alkane molecules and alkyl radicals from the n-dodecane molecular ion occurs primarily by simple splitting of the C? C bond. In addition, both small alkane molecule and alkyl radicals are eliminated with low probability from centreal parts of the molecular ion. The alkane elimination is less specific than the alkyl elimination. The methyl elimination shows an exceptionally high non-specificity, but is of negligible abundance in the 70 e V electron impact spectrum. The metastable ion spectra suggest, but do not prove unambiguously, that those small alkyl ions (with up to four carbon atoms) originating directly from the molecular ion, may be formed both by direct cleavage of the terminal groups and from central parts of the molecular ion. However, the majority of the small alkyl fragment ions in the 70 eV spectrum are formed by secondary decomposition explaining their apparent non-specific formation. The strikingly different fragmentation behaviour of even electron, [C_nH_2n+1]⁺, and odd electron fragment ions, results from differences in the product stabilities. Using collisional activation and metastable ion spectra it is shown that the odd electron fragments have the structure of the linear alkene (most probably the 1-alkene) molecular ion. In contrast to the molecular ions, alkyl fragment ions decompose with complicated skeletal rearrangements, which lead to substantial, but not complete, carbon randomization. The terminal hydrogen atoms, however, show little scrambling.     

Negative ions in organic mass spectrometry. II—Negative ion mass spectra of simple thio and dithio esters.

The 70 eV negative ion mass spectra of some simple aromatic and aliphatic thio and dithio esters are discussed. A characteristic fragmentation process, a rearrangement analogous to the nitro-nitrite conversion of aromatic nitro compounds, is observed for aromatic thio esters.     

NMR study of 2-alkoxy-1,3,2-dioxaphospholanes. II—molecular conformation of alkoxy substituents

The conformation of some 2-σ-1,3,2-dioxaphospholanes (σ=OAlkyl, CI) is studied. The determination of the ²J(POC) and ³J(POCC) coupling constants, which are influenced by the bulk of the alkoxy group, is a means of obtaining information about the rotation around the P? OR bond, which is dependent on steric interactions between the phosphorus lone pair, the alkoxy group and the substituents on the ring. When σ is a tert-butoxy group, a direct ‘through-space’ interaction is found between the phosphorus atom and one of the primary carbons of the OR group. If there is no substituent on the ring, the ³¹P chemical shifts are little affected by changes in OR; a diamagnetic effect is observed, however, in the case of the tert-butoxy group which is enhanced for the 4,4,5,5-tetramethyl derivatives.     

Determination of the fragmentation mechanisms of organophosphorus ions by H2O and D2O atmospheric-pressure ionization tandem mass spectrometry II—dialkyl alkylphosphonate ions

Diethyl methylphosphonate (DEMP), diisopropyl methylpbosphonate (DIMP), diethyl isopropylphosphonate (DEIP) and diethyl ethylphosphonate (DEEP) were characterized by H₂O and D₂O atmospheric pressure ionization tandem mass Spectrometry (API-MS/MS). Collision-induced dissociation (CID)/fragmentation pathways included alkyl ions by direct cleavage, alkyl radical and water loss processes and McLafferty and McLafferty-type rearrangements by six- and five-membered ring transition states, respectively. D₂O API proved particularly useful in that certain decomposition pathways (i.e. water and methanol neutral losses) had a statistical distribution as to the loss of an acid deuteron and proton(s). This phenomenon was manifested by two pairs of ions in the D₂O API daughter-ion mass spectrum for each phosphonate compound (e.g. both m/z 79/80 and 65/66 for DEMP and DIMP). The observed ion intensity ratios for these pairs of ions served as guides in the determination of their predicted ion relative abundance ratios and CID decomposition pathways. Water neutral losses as opposed to ether and alcohol neutral losses were favored for most of the protonated organophosphonate molecular ion decomposition schemes.     

Mass spectroscopy of natural products. II—allogibberic acid—A model for the skeletal fragmentation of the molecular ions of GA3 type gibberellins

The positive ion mass spectrum of allogibberic acid was examined and fragmentation routes involving the carbon skeleton are proposed on the basis of mass measurements and metastable ion observations. The results are compared with the mass spectra of 3-hydroxy-epiallogibberic acid, as well as gibberellin A₃ (GA₃), 3-epi-GA₃, 3-dehydro-GA₃ and iso-GA₃, the latter compounds all being capable of undergoing aromatization of ring A giving a key ion corresponding to the molecular ions of the allogibberic acid and 3-hydroxy-epiallogibberic acid models, respectively. All the gibberellin derivatives investigated show the same fragmentation of the skeleton. Thus, the behaviour under electron impact of the GA₃ type gibberellins follows a general pattern.     

Mass spectral studies of cycloheptatrienes. II—The monochlorocycloheptatrienes

The mass spectra of isomeric chlorocycloheptatrienes have been studied at high and low electron beam energies. Ion kinetic energy spectrometry was utilized to examine the major decomposition pathways. The extreme similarity found in the spectral characteristics of these compounds provides information about the structure of the decomposing molecular ion.     

An investigation of the decomposition of the intermediate ions produced by electron-impact. II—[C7H7]+ ions from several halogenotoluenes

The structure and decomposition of the [C₇H₇]⁺ ions produced by electron-impact from o-, m- and p-chlorotoluene, o-, m- and p-bromotoluence, and p-iodotoluence, have been investigated. By determining the relative abundance of normal and metastable ions, these [C₇H₇]⁺ ions at electron energy of 20 eV are shown to be so-called ‘tropylium ions’. The amount of the internal energy of the [C₇H₇]⁺ ion estimated by the relative ion abundance ratios, ? [C₅H₅]⁺/[C₇H₇]⁺ and m*/[C₇H₇]⁺ for the decomposition \documentclass{article}\pagestyle{empty}\begin{document}$ [{\rm C}_{\rm 7} {\rm H}_{\rm 7}]^ + \mathop \to \limits^{m^* } [{\rm C}_{\rm 5} {\rm H}_{\rm 5}]^ + + {\rm C}_{\rm 2} {\rm H}_{\rm 2} $\end{document}, is in the order iodotoluene > bromotoluene > chlorotoluene. The heats of formation of the activated complexes for the reaction \documentclass{article}\pagestyle{empty}\begin{document}$ [{\rm C}_{\rm 7} {\rm H}_{\rm 7}]^ + \mathop \to \limits^{m^* } [{\rm C}_{\rm 5} {\rm H}_{\rm 5}]^ + + {\rm C}_{\rm 2} {\rm H}_{\rm 2} $\end{document} were estimated. The values suggest that the decomposing [C₇H₇]⁺ ions from various halogenotoluenes are identical in structure.     

Heuristic programming as an ion generator in mass spectrometry. II—Generation of primary ions from simple cyclic structures

The Ion Generator computer program has now the ability to fragment cyclic structures Examples of mechanisms which are proposed by the program for the fragmentation of simple ring compounds are presented.     

Stereoisomeric effects on mass spectra—II: The isomeric methyldecalins

Mass spectra of the four stereoisomeric 1-methyldecalins and the four 2-methyldecalins have been measured. For each set of stereoisomers, the order of relative intensity of the parent-less-CH₃ ion, but not of the parent ion, parallels the order of relative stability of the molecules. The correlation suggests that loss of the methyl group from the members of each set of stereoisomers leads to a common product, with the differing conformational energies converting into differences in vibrational energy, which in turn lead to differing extents of further decomposition.     

Fragmentation D'azoles Sous L'impact Electronique. V—Isomerisation Du Benzimidazole Avant Degradation

The mass spectrum of benzimidazole is investigated using deuterium and carbon-13 labelling. The [M – HCN]⁺· ion is the result of two competitive reaction. Isomerisation into a cyanoaniline structure which eliminates HCN from the amino group and also from the nitrile group (by an ortho effect) is confirmed by ions abundance ratios. Hydrogen scrambling does not occur to a significant extent in the ions under consideration.     

Electron impact mass spectrometry of nuphar alkaloids. II—Thiohemiaminals

The appearance of m/e 228 and 176 ions, useful in ascertaining the presence of the hemiaminal hydroxyl, are discussed for six thiaspirane-type hemiaminals isolated from Nuphar luteum and three epimeric pairs of α-thiohemiaminals derived from (–)-deoxynupharidine. Other groups of ions useful in ascertaining stereochemical differences of the sulfur moiety in α-thiohemiaminals are discussed for the same compounds. One such group of ions is comprised of 248, 246 and 231; a second is 192 and 191.     

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.     

Etude conformationnelle de la dimethyl-3,4 cyclopentanone trans: II—Spectres de RMN

The NMR spectra of trans-3,4-dimethylcyclopentanone and trans-3,4-dimethyl-2,2,5,5-tetradeuteriocyclopentanone are analysed. The coupling constants, calculated for the deuterated compound, are in good agreement with a diequatorial conformation of the methyl groups.     

Nuclear magnetic resonance spectroscopy of Schiff bases: II—imines extracted from aqueous solution

Nuclear magnetic resonance spectroscopy is used to study netural imines formed from various aldehydes and primary amines in aqueous solution. The imines are then extracted into CDCl₃ and their formation constants in the aqueous phase are calculated as a function of pD. The results are in agreement with previous studies. Decoupling experiments are used to prove that additional splitting of the proton spectrum is due to long range coupling rather than syn-anti isomerism. The possibility of utilizing the formation of Schiff bases in aqueous solution for synthetic purposes is discussed and the ¹³C magnetic resonance spectrum of the imine, isolated from the reaction of 2-thiophenecarbaldehyde and n-butylamine, is reported.