Isobutane Chemical Ionization Research Articles

Mapping Glyceride Species in Biodiesel by High-Temperature Gas Chromatography Combined with Chemical Ionization Mass Spectrometry.

Accurate and comprehensive identification of residual glycerides in biodiesel is an important part of fuel characterization due to the impact of glycerides on the fuel physicochemical properties. However, analysis of bound glycerol in biodiesel samples faces challenges due to lack of readily available standards of structurally complex glyceride species in nontraditional biodiesel feedstocks and a risk of misannotation in the presence of impurities in gas chromatographic separations. Here, we evaluate methane and isobutane chemical ionization-single quadrupole mass spectrometry combined with high-temperature gas chromatography separations for mapping monoacylglycerols, diacylglycerols, and triacylglycerols in biodiesel. Unlike electron impact ionization, which produces mostly in-source fragments, isobutane chemical ionization spectra of tetramethylsilyl-derivatized monoacylglycerols and diacylglycerols are dominated by molecular ions and M-SiO(CH3)3+ ions, which provide important diagnostic information. We demonstrate the utility of isobutane chemical ionization in identifying structurally complex glycerolipid standards as well as species in biodiesel samples from different plant and animal feedstocks.

Identification of volatile and semivolatile compounds in chemical ionization GC-MS using a Mass-To-Structure (MTS) Search Engine with integral isotope pattern ranking

The mass-to-structure or MTS Search Engine is an Access 2010 database containing theoretical molecular mass information for 19,438 compounds assembled from common sources such as the Merck Index, pesticide and pharmaceutical compilations, and chemical catalogues. This database, which contains no experimental mass spectral data, was developed as an aid to identification of compounds in atmospheric pressure ionization (API)-LC-MS. This paper describes a powerful upgrade to this database, a fully integrated utility for filtering or ranking candidates based on isotope ratios and patterns. The new MTS Search Engine is applied here to the identification of volatile and semivolatile compounds including pesticides, nitrosoamines and other pollutants. Methane and isobutane chemical ionization (CI) GC-MS spectra were obtained from unit mass resolution mass spectrometers to determine MH(+) masses and isotope ratios. Isotopes were measured accurately with errors of <4% and <6%, respectively, for A + 1 and A + 2 peaks. Deconvolution of interfering isotope clusters (e.g., M(+) and [M - H](+)) was required for accurate determination of the A + 1 isotope in halogenated compounds. Integrating the isotope data greatly improved the speed and accuracy of the database identifications. The database accurately identified unknowns from isobutane CI spectra in 100% of cases where as many as 40 candidates satisfied the mass tolerance. The paper describes the development and basic operation of the new MTS Search Engine and details performance testing with over 50 model compounds.

Regio-and Stereochemical Effects in the Chemical Ionization Mass Spectra of Isomeric Bicyclo|3, 2, O| Heptan-2-One Trimethylsiloxy Derivatives1

Isomers of bicyclo |3, 2, O| heptan-2-one trimethylsiloxy derivatives, produced by (π2+π2) photocycloaddition reactions between cyclopent-2-enones and trimethylsiloxy alkenes, have been investigated by methane and isobutane chemical ionization mass spectrometry. Head-to-head and head-to-tail regioisomers and the syn-and anti-epimers in each regioisomeric set can be identified by means of these ionization methods. Prominent fragmentation patterns are discussed.

Tandem mass spectrometry in food safety assessment: The determination of phthalates in olive oil

A gas chromatography–tandem mass spectrometry (GC–MS/MS) method for the detection of six phthalates in olive oil was developed. A gel permeation chromatography (GPC) clean-up step with cyclohexane:dichoromethane 7:3 as mobile phase was employed to remove the high-molecular mass species present in oil. Two ionization methodologies, i.e. electron (EI) and isobutane-chemical ionization (CI), were compared, in MS/MS mode, to achieve better analytical performances. An overall evaluation of all analytical parameters shows that the EI-MS/MS approach provides satisfactory results and is to be preferred to CI-MS/MS, at least in the case of the examined analytes. The observed accuracies, ranging from 71.7% to 112.2%, and the RSD values less than 9.7%, confirm the effectiveness of the proposed method in the assay of phthalate content in such a complex matrix as olive oil. The proposed protocol for the identification and assay of phthalates in olive oil might be of interest for the implementation of the QS (quality assurance scheme) for residue monitoring in food safety assessment.

The effect of steric hindrance on the relative rates of anchimerically assisted alcohol eliminations from MH + ions of 2-substituted 1,4-dialkoxybutanes upon CI and CID : Experiment and theory

1,4-Dialkoxybutanes afford very abundant [MH–ROH] + ions upon isobutane chemical ionization (CI) and collision induced dissociation (CID), in contrast to other primary mono-ethers or 1, ω-diethers of long chain diols. This behavior suggests involvement of anchimeric assistance in the mechanism of alcohol elimination from the MH + ions of 1,4-dialkoxybutanes. This concerted mechanistic pathway finds support in the CI and CID behavior of the [MH–ROH] + ions, obtained from isomeric mixed 1,4-dialkoxybutanes (1-ethoxy-4-methoxy- and 1-methoxy-4-ethoxy) substituted at position 2 with alkyl groups or with deuterium atoms. Density functional (DFT) calculations at B3LYP/6-31G** level of theory also support the anchimerically assisted elimination mechanism. The isomeric mixed 1,4-dialkoxybutanes, substituted at position 2 with alkyl groups of variable bulkiness, exhibit preferential elimination of alcohol from position C-4 rather than from C-1, and this tendency increases with the size of the substituents. This steric effect is explained by a more hindered transition state involved in the anchimerically assisted elimination of alcohol from C-1 due to interaction of the substituent(s) at position 2 with the protonated alkoxy group at position 1. Calculations show the energies of transition states resulting in 4-elimination products are lower than those leading to the elimination of alcohol from position 1, and the difference significantly increases with the increase of the bulkiness of the 2-substituents.

Dicarboxylic degradation products of nonylphenol polyethoxylates: synthesis and identification by gas chromatography–mass spectrometry using electron and chemical ionization modes

The synthesis, mass spectra and detectability of four selected dicarboxylic degradation products (CAPECs) of nonylphenol polyethoxylates (NPEOs) are reported. The selected isomers have an α,α-dimethyl configuration (expressed as “dm” in their abbreviation), five to eight C atoms and a carboxyl group in the alkyl chain, and a carboxymethoxy acid group (dm-CA 5–8P1ECs). The synthesis was successfully accomplished via a reaction sequence that started from anisole. After trimethylsilylation with N, O-bis(trimethylsilyl)acetamide or methylation with (trimethylsilyl)diazomethane, the derivatives of the dm-CA 5–8P1ECs were subjected to a GC–electron ionization (EI)–MS and GC–isobutane chemical ionization (CI)–MS. In EI–MS, ion peaks at m/ z = 265 and 207, corresponding to the α,α-dimethyl structures via the benzyl cleavage of carboxyalkyl chain, were the most significant ions of the trimethylsilyl and methyl derivatives, respectively. In CI–MS, the main ion peaks of dm-CA 5-, dm-CA 6-, dm-CA 7-, and dm-CA 8P1EC after methylation were at m/ z = 129, 143, 157, and 171, respectively, corresponding to the loss of methyl phenoxyacetate from [ M + H] +; meanwhile significant peaks were detected at 321, 335, 349, and 363, corresponding to the loss of the trimethylsilanol after trimethylsilylation. The potential for the identification and quantification of individual branched carboxyalkyl isomeric mixtures of CA 5-, CA 6-, CA 7-, and CA 8P1EC metabolites based on corresponding dm-CA 5–8P1ECs revealed the advantage of the GC–CI–MS although the detection limits in CI were clearly higher than those in EI.

Intramolecular electrophilic aromatic substitution in gas-phase-protonated difunctional compounds containing one or two arylmethyl groups.

A variety of dibenzyl esters and ethers undergo a rearrangement process upon isobutane chemical ionization and collision-induced dissociation of their MH(+) ions, whereby a new bond is formed between the two benzyl groups, giving rise to abundant [C(14)H(13)](+) (m/z 181) ions. This rearrangement has been explained as an intramolecular electrophilic substitution in the gas phase occurring in an ion-neutral complex formed by the cleavage of one of the benzyl-oxygen bonds. A similar highly efficient intramolecular electrophilic substitution takes place in di-alpha- and beta-naphthylmethyl adipates affording m/z 281 [C(22)H(17)](+) ions, but not in the sterically hindered di-9-anthracylmethyl adipate. An analogous efficient rearrangement occurs in benzyl alpha- and beta-naphthylmethylcyclohexane-1,4-dicarboxylates and in benzyl alpha- and beta-phenylethylcyclohexane-1,4-dimethanol ethers. The analogous rearrangement is much less efficient in benzylallyl, benzylpropargyl and benzyl-9-anthracylmethyl derivatives, even less in benzylisopropyl and benzylacetyl analogs, and it is absent in benzyltetrahydropyranyl derivatives. The distinctive behavior of the protonated difunctional benzyl derivatives is interpreted in terms of the energy requirements of the O-R bond heterolysis of the protonated functionalities, the ability of the neutral R' groups (non-dissociated from the oxygen atom) to play the role of the nucleophile in the intramolecular electrophilic substitution processes and the electrophilicity of the R(+) ions.

The role of hydride migration in the mechanism of alcohol elimination from protonated ethers upon chemical ionization: Experiment and theory

An enhanced elimination of methanol under isobutane-chemical ionization (CI) conditions, resulting in highly abundant [MHCH 3OH] + ions, has been observed in several primary and secondary methyl ethers having a tertiary β-position (methine), as compared with those with β-methylene. This elimination is stereospecific in stereoisomeric 2-methyl-1-methoxycyclohexanes and in other ethers affording significantly more abundant [MHCH 3OH] + ions in the cis-isomers than in their trans-counterparts. These findings suggest involvement of a 1,2-hydride migration from the β- to α-position in the course of the alcohol elimination from the MH + ions of the above cis-ethers, resulting in stabilized tertiary carbocation structures. The possible pathways of methanol elimination from protonated cis-2-methyl-1-methoxycyclohexane were explored by density functional calculations at the B3LYP/6-31+G(d,p) level of theory. The transition states for MeOH elimination involving 1,2-hydride migration were located and the activation energy of the process was evaluated. The activation barrier of the alcohol elimination assisted by 1,2-hydride migration is lower by ∼10 kcal/mol than the simple bond cleavage (9.6 kcal/mol vs. 20.5 kcal/mol). These computational results support the mechanistic pathway involving the 1,2-hydride transfer. A step-wise mechanistic pathway is proposed for the less efficient elimination of methanol from protonated trans-2-methyl-1-methoxycyclohexane.

Determination of pentachlorophenol by negative ion chemical ionization with membrane introduction mass spectrometry.

Pentachlorophenol (PCP) was used as a model compound to explore the potential of desorption chemical ionization (DCI) in the determination of polychlorinated pesticides using membrane introduction mass spectrometry (MIMS). A direct insertion membrane probe was modified so that a chemical ionization plasma could be established at the membrane surface. Using selected ion monitoring (SIM) in a tandem triple quadrupole mass spectrometer with isobutane chemical ionization (CI), the PCP detection limit under positive chemical ionization is 20 ppb whereas negative CI gives detection limits in the low ppb range. This performance is achieved without any pre-treatment or derivatization of the sample. Negative ion CI gives a signal that is linear over a concentration range of 2-1000 ppb. Comparison of data obtained with low ppb samples of 2,4,6-trichlorophenol, 2,3,4,6-tetrachlorophenol and pentachlorophenol suggests that the sensitivity of this analytical procedure increases with increase in the number of electronegative substituents in the molecule.

Stereospecific elimination of dihydropyran from protonated tetrahydropyranyl (THP) difunctional derivatives upon chemical ionization and collision-induced dissociation: intramolecular interactions in polyfunctional ions

Tetrahydropyranyl (THP) ethers of cis-1-methoxymethyl-, 1-benzyloxymethyl-, and 1-hydroxymethyl-4-cyclohexane methanols and of cis-1,4-dihydroxycyclohexane, and THP ester of cis-1,3-cyclohexane dicarboxylic acid (cis-3–cis-7, respectively) exhibit low abundance MH+ ions and undergo efficient elimination of dihydropyran (DHP) upon isobutane chemical ionization (CI). In contrast to this behavior, the trans isomers give rise to highly abundant MH+ ions, whereas the elimination of dihydropyran affords low abundance [MH−DHP]+ ions. A similar stereospecific behavior has been also observed under collision-induced dissociation (CID) conditions. Tetrahydropyranylium ion (m/z 85), obtained by a simple C-O bond dissociation, is abundant in the CI and CID mass spectra of both stereoisomers in all the examined systems. The high stereoselectivity suggests intermediacy of internal proton bridging of the two adjacent basic sites in the formation of the [MH−DHP]+ ions from the MH+ ions of the cis isomers. Thermochemical analysis indicates stabilized proton bridged structures for the [MH−DHP]+ ions. A mechanistic pathway has been proposed for the DHP elimination, based on the stereospecificity of this process and on its thermochemistry.

Mass-spectrometric differentiation of di exo- and di endo-fused isomers of norbornane/ene-condensed 2-thiouracil and 1,3-thiazino[3,2- a]pyrimidine derivatives: stereoselectivity of retro-diels-alder fragmentations under EI and CI conditions

The stereoisomers of the title compounds produce nearly identical electron ionization (EI) mass spectra, which are dominated in the case of the norbornene-condensed derivatives by retro-Diels-Alder (RDA) fragmentation of the hydrocarbon ring. The RDA fragmentation mainly occurs with H transfer and gives rise to [M–C 5H 5] +. For the norbornane-condensed derivatives, the main fragmentation routes include the formation of [M–C 5H 7] + (protonated thiouracil) and [M–C 7H 9] + (only from thiazinopyrimidines). The latter species are formed via RDA decomposition of the pyrimidone subunit of the heterocyclic system, a process previously observed for cyclohexane-condensed analogs of these compounds. Only minor differences could be detected between the EI spectra of the di exo and di endo isomers. Under chemical ionization (CI) conditions, the norbornane-condensed compounds produced no significant fragment peaks with either isobutane or methane as reagent gas. In contrast, the isobutane and methane CI spectra of the norbornene-condensed compounds exhibited prominent peaks of [MH–C 5H 6] + and [(M+C xH y)–C 5H 6] + originating from moderately stereoselective RDA fragmentations. The relative abundances of the RDA ions obtained from the respective stereoisomers with the same reagent gas were consistently different over a range of experimental conditions. The non-occurrence of RDA fragmentation of the thiazinopyrimidine ring under CI conditions suggested that its energy of activation is higher than that for either of the norbornene-ring RDA fragmentations (with or without H transfer) observed under EI and CI conditions.

Proton transfer via strained transition states in the elimination of alcohols from MH+ ions of stereoisomeric diethers and hydroxy esters upon chemical ionization and collision‐induced dissociation

AbstractThe isobutane chemical ionization (CI) mass spectra of cis‐ and trans‐1,4‐di(alkoxymethyl)cyclohexanes, with a tertiary alkoxy group ROH and a primary group R′OH, are identical, and they exhibit exclusive elimination of the alcohol ROH originating from the tertiary alkoxyl. The high abundance of the [MH − ROH]+ ion and the absence of [MH − R′OH]+ and MH+ ions is unexpected in the case of trans‐diethers, and it suggests proton transfer from the primary alkoxyl OR′ to the tertiary OR group prior to the elimination of ROH, despite the large distance between them in the trans configuration. Various isomerization pathways were explored in order to account for the similar behavior of the cis‐ and trans‐isomers upon chemical ionization. The results show that the hydrogens at positions 1 and 4 are not involved in the elimination of ROH. Methyl substitution at positions 1 and 4 leads to the competitive elimination of ROH and R′OH, indicating suppression of the proton transfer. Methyl substitution at position 1 adjacent to the primary alkoxy group has a minor effect on the chemical ionization behavior of the diethers. On the other hand, methyl substitution at position 4, adjacent to the tertiary alkoxy group, suppresses the proton transfer, (i.e. both [MH − ROH]+ and [MH − R′OH]+ are abundant in the CI mass spectra of the trans‐isomers), indicating an effect of steric hindrance. The results suggest that direct proton transfer via a strained proton‐bound transition‐state occurs, and methyl substitution at positions 1 and 4 can affect the relative stability of the transition‐state structures involved. Similar behavior has been observed in the CI and collision‐induced dissociation spectra of 1,4‐hydroxy ester cyclohexanes. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Stereochemical effects enhanced by using selective “self-ionization” under electron ionization conditions in a quadrupole ion trap mass spectrometer

From a previous study on the reactivity of the cis and trans (4-hydroxy-4-methyl-cyclohexyl)-benzoates (Mw = 234) under ammonia chemical ionization conditions in high pressure source, it was demonstrated that competitive decompositions, have specific orientations. In particular, highly stereospecific benzoic acid loss from the trans isomer takes place, suggesting that protonation by NH 4 + regioselectively occurred at the benzoate site rather than at the hydroxyl group. From the cis epimer elimination of water was only observed. This behavior could be explained by (1) possible proton transfer to both the functional groups and (2) decomposition occurring via water loss due to a higher rate constant compared with that of the benzoic acid loss (PA C 6H 5 COOH > PA H 2O ), which is not observed. These epimers are now studied under low pressure chemical ionization (CI) and electron impact (EI) conditions by using an ion trap mass spectrometer. Epimer differentiation from the stereospecific benzoic acid loss can be achieved independently of the gas-phase reagent used for chemical ionization, even if quasimolecular ions are entirely decomposed (i.e. in low pressure isobutane CI and methane CI). More spectacular differentiation is displayed in the ammonia CI mass spectra of epimers. Among the various quasimolecular species, the product [MNH 4–H 2O] + ions, termed substituted ions, usually containing a covalent C–NH 3 + bond are herein in fact a noncovalent form similar to the ammonia solvating protonated(4-hydroxy-4-methyl-cyclohexyl)-benzoate structure, as shown by deuterium labeling. Other product ions such as C 7H 11NH 3 + and {NH 3, C 6H 5COOH 2 +} are detected in contrast to those observed under high pressure source conditions. Collision induced dissociation spectra of the adduct MNH 4 + ions in addition to those of protonated molecules are investigated to obtain information about the location of ammonium (or proton) attachment. It appears for the trans isomer, a regioselective approach to the benzoate group, yielding [MH–C 6H 5COOH] +, takes place rather than attack at the OH site resulting into [MH–H 2O] +. This difference is at the origin of the observed stereospecific decomposition, which occurs via anchimeric assistance. Alternatively, these epimers give similar EI mass spectra, masking all the stereochemical effects, which reappear when using a residence time of 80 ms prior to apply the analytical scan. Under such conditions, ion–molecule reactions are enhanced, yielding formation of the epimeric MH + ions. These “ self-ionization” processes are induced through exothermical proton transfers from many EI fragment ions. Competitive decompositions of the product even-electron MH + species provide a direct cis/ trans differentiation. The diagnostic cleavages involved are comparable to those observed under low pressure CI conditions except that the loss of water occurs from both the precursors. The latter loss indicates that protonation is in competition and takes place at either of the basic sites, in contrast with that observed in chemical ionization. Furthermore, the exothermicity of proton transfer is preserved as internal energy of MH + which allows consecutive decompositions. At higher m/z ratio ranges, several adduct species are stereoselectively observed in the self-ionization mass spectrum of the trans epimer. These specific processes provide enhancement of the stereochemical cis/ trans differentiation.

The effects of electron and chemical ionization modes on the MS profiling of whole bacteria

Free fatty acid profiling of whole bacteria [ Francisella tularensis, Brucella melitensis, Yersinia pestis, Bacillus anthracis (vegetative and sporulated), and Bacillus cereus] was carried out with direct probe mass spectrometry under 70-eV electron ionization (EI) and isobutane chemical ionization in both the positive (CI +) and negative modes (CI −). Electron ionization produced spectra that contained molecular ions and fragment ions from various free fatty acids. Spectra acquired with isobutane chemical ionization in the positive mode yielded molecular ions of free fatty acids as well as ions from other bacterial compounds not observed under EI conditions. Spectra obtained with negative chemical ionization did not contain as much taxonomic information as EI or CI +; however, some taxonomically significant compounds such as dipicolinic acid and poly(3-hydroxybutyrate) did produce negative ions. All ionization modes yielded spectra that could separate the bacteria by Gram-type when observed with principle components analysis (PCA). Chemical ionization in the positive ion mode produced the greatest amount of differentiation between the four genera of bacteria when the spectra where examined by PCA.

Role of hydrogen migration in the mechanism of acetic acid elimination from MH(+) ions of acetates on chemical ionization and collision-induced dissociation

The elimination of acetic acid from the MH(+) ions of acetates of stereoisomeric 2-methyl-1-cyclohexanols and 1-hydroxy-trans-decalins exhibits a significant degree of stereospecificity under isobutane chemical ionization and collision-induced dissociation (CID) conditions, resulting in more abundant [MH - AcOH](+) ions in the cis-isomers 4c and 5tc than in their trans-counterparts 4t and 5tt. These findings suggest the involvement of a 1,2-hydride shift from the beta- to the alpha-position in the course of the acetic acid elimination from the MH(+) ions of the above cis-acetates, resulting in tertiary carbocation structures. The proposed mechanism of the elimination is supported by a considerable deuterium isotope effect detected in beta-deuterium-labeled cis-2-methyl-1-acetoxycyclohexane and by a CID study of the structures of the [MH - AcOH](+) ions obtained from cis- and trans-1,2-diacetoxycyclohexanes. Copyright 1999 John Wiley & Sons, Ltd.

Intramolecular proton transfers in stereoisomeric gas-phase ions and the kinetic nature of the protonation process upon chemical ionization

The isobutane chemical ionization (CI) mass spectra of cis- and trans-1-butyl-3- and -4-dimethylaminocyclohexanols and of their methyl ethers exhibit abundant [MH - H(2)O](+) and [MH - MeOH](+) ions respectively. On the other hand, only the MH(+) ions of the cis-isomers exhibit significant [MH - H(2)O](+) and [MH - MeOH](+) ions under collision-induced dissociation (CID) conditions. The non-occurrence of water and methanol elimination in the CID spectra of the trans-isomers indicates retention of the external proton at the dimethylamino group in the MH(+) ions that survive after leaving the ion source and the first quadrupole of the triple-stage quadrupole ion separating system, and the trans-orientation of the two basic sites does not allow proton transfer from the dimethylamino group to the hydroxyl or methoxyl. Such transfer is allowed in the cis-amino alcohols and amino ethers via internal hydrogen-bonded (proton-bridged) structures, resulting in the elimination of water and methanol from the surviving MH(+) ions of these particular stereoisomers upon CID. The abundant [MH - ROH](+) ions in the isobutane-CI mass spectra of the trans-isomers indicates protonation at both basic sites, affording two isomeric MH(+) ions in each case, one protonated at the dimethylamino group and the other at the less basic oxygen function. These results show that the isobutane-CI protonation of the amino ethers and amino alcohols is a kinetically controlled process, occurring competitively at both basic sites of the molecules, despite the large difference between their proton affinities ( approximately 25 and approximately 35 kcal mol(-1); 1 kcal = 4.184 kJ). Copyright 1999 John Wiley & Sons, Ltd.

Proton transfer between functional groups in MH + ions of stereoisomeric diethers on chemical ionization and collision-induced dissociation conditions

Proton transfer between functional groups in the MH + ions of stereoisomeric mixed methyl ethyl diethers of trans-tricyclo[6.2.2.0 2,7]dodeca-9-en-3,6-diols 1t and 2t, obtained on isobutane chemical ionization (CI), was investigated by a comparison of their CI and collision-induced dissociation (CID) mass spectra. Preferential elimination of the alcohol originating at the exo-alkoxyl was observed under CI conditions, whereas loss of the protonated endo-alkoxyl was predominant in the CID measurements of the MH + ions. These results indicate preferential initial CI protonation at the exo-alkoxyl group, followed by elimination of the alcohol originating at that position, and predominant retention of the proton at the endo-alkoxyl in the surviving MH + ions which are submitted to the collision cell. These findings lead to the conclusion that the external proton does not migrate from its original protonation site to the endo-alkoxyl to an appreciable extent, if at all, during the course of CI and CID experiments. This method of exploring the question of occurrence of interfunctional proton transfer processes in gas-phase protonated difunctional compounds may be applied in other systems.

Identification and biosynthesis of (E,E)-10,12-tetradecadienyl acetate in Spodoptera littoralis female sex pheromone gland

A minor component of the sex pheromone gland of the Egyptian cotton leafworm. Spodoptera littoralis, has been identified as ( E, E)-10,12-tetradecadienyl acetate. Structural elucidation has been carried out by isobutane-chemical ionization mass spectrometry of the fatty acyl biosynthetic precursor-derived methyl ester. To assign the stereochemistry of the double bonds, the four isomers of both 10,12-tetradecadienyl acetates and methyl 10,12-tetradecadienoates have been synthesized and their gas chromatography retention times and mass spectra have been compared to those of the corresponding natural compounds. Mass-labeling experiments showed that the ( E, E)-10,12-tetradecadienoyl moiety is biosynthetically derived from (Z)-11-tetradecenoic acid in the insect pheromone gland.

Chemistry and Stereochemistry of Benzyl-Benzyl Interactions in MH+ Ions of Dibenzyl Esters upon Chemical Ionization and Collision-induced Dissociation Conditions

Isobutane chemical ionization mass spectra of dibenzyl esters of a wide variety of aliphatic, olefinic, alicyclic and aromatic dicarboxylic acids exhibit abundant m/z 181 C 14 H + 13 ions, indicating a highly general rearrangement process involving the formation of a new bond between the two benzyl groups. An extensive collision-induced dissociation and deuterium labeling study suggested that these ions are an almost equimolar mixture of isomeric α-o-tolylbenzyl, α-p-tolylhenzyJ and p-benzylbenzyl cation structures, and this composition is identical for all the diesters examined. This structural assignment of the C 14 H 13 + ions suggests a mechanistic pathway for their generation, based on the formation of the new bond between the benzyl methylene group of the protonated benzoxycarb-onyl and the phenyl ring of the other ester moiety via π- (and/or ion-neutral) and α-complexe. Stereoisomeric diesters show an unusual steric effect: trans-isomers give rise to much more abundant C 14 H + 13 ions than the cis counterparts. This behavior is explained by stabilized proton-bridged structures of the MH+ ions of the cis-isomers.

Stereospecificity and Concertedness of Retro-Diels-Alder Fragmentation in Some Diester Systems Upon Chemical Ionization

Retro-Diels–Alder (RDA) fragmentation of cis- and trans-2,3-diethoxycarbonyl-5,6,7,8-dibenzo-bicyclo[2.2.2]octanes under isobutane and methane chemical ionization conditions is highly stereospecific, giving rise to protonated diethyl maleate and fumarate, respectively. This behaviour indicates a single-step concerted mechanism, analogous to the ground-state RDA process that occurs in neutral molecules in the condensed phase. The analogous dissociation is partially stereospecific in stereoisomeric endo-,exo- and trans-2,3-diethoxycarbonylbicyclo[2.2.1]heptanes and non-stereospecific in endo-,exo- and trans-2,3-diethoxycarbonyl-5,6-benzobicyclo[2.2.2]octanes, indicating the involvement of a stepwise mechanism in the latter two systems. The different behaviour of the above systems is explained in terms of the energy of the RDA fragmentation. The differentiation and quantitative estimates of protonated diethyl maleate and fumarate were obtained from collision-induced dissociation measurements.