Stereospecific fragmentation processes in cycloalkane/cycloalkene-fused isomers of saturated pyrrolo[2,1-b][1,3]oxazin-6-one derivatives

Electron ionization-induced fragmentations were studied in a set of saturated pyrrolo[2,1-b][1,3]oxazin-6-one derivatives fused to either cycloalkane or cycloalkene rings, including seven pairs of cis/trans and endo/exo annelation isomers. Fragmentation patterns were confirmed by accurate mass measurements and metastable ion spectra. A number of striking differences were observed between the mass spectra of cyclohexene-fused isomers due to highly stereospecific retro-Diels-Alder (RDA) fragmentation of their M^+· ions. The observed 90% stereospecificity is abnormally high in the light of the recent classification (A. Mandelbaum, 1994) of stereospecific RDA fragmentations according to the degree of substitution of the cyclohexene ring being cleaved. In the absence of RDA processes, the differences between the mass spectra of cyclohexane-fused isomers originated from heterocyclic fragmentations. The assumed mechanistic interpretation of the observed differences, e. g., in the formation of [M − C₃H₅O]⁺ ions, was consistent with the condensed-state conformations of these isomers determined previously by NMR and X-ray diffraction studies. Because of rapid RDA decompositions of their rather unstable M^+· ions, the spectra of the diendo/diexo norbornene-fused isomers were virtually identical.     

Ammonia chemical ionization mass spectrometry of alcohols: Structural,stereochemical, molecular-size and temperature effects

Ammonia chemical ionization (CI) mass spectra of various open-chain, cyclic and unsaturated C₅- to C₁₀-alcohols were obtained at source temperatures ranging from 60° to 250°C. The reactivity of the ammonia adduct ion MNH and its fragmentation channels are characteristic for substrate structure. Although strongly temperature-dependent, the spectra give nevertheless information on the OH-group environment as well as on the C-skeleton at any source temperature. Primary, secondary and tertiary alcohols as well as allylic and simple olefinic alcohols can be distinguished by their spectra, which show ammonium adduct ions [MNH₄]⁺, adduct dehydrogenation ions [MNH₄-H₂], ammonium substitution ions [MNH₄-H₂O]⁺ and [M-OH]⁺-ions as the main characteristic peaks. Moreover, konfigurational assignments of stereoisomeric alcohols are possible for larger substrate-size and source-temperature ranges than with isobutane CI mass spectrometry. Homologous M NH-ions show molecular-size control of fragmentation and linear MNH-ions are less stable than branched isomers due to incomplete energy randomization.     

Recherches dans la série des cyclitols XXXVII Etude par spectrométrie de masse

Low resolution mass spectra of cyclohexane-triols, -tetrols, -pentols and -hexols and of some of their deuterium labelled derivatives have been measured. The results indicate that for some geometrical isomers quantitative differences between ion intensities are significant, allowing to deduce the stereochemistry of the molecule from its mass spectrum. In all these compounds the electron impact induced elimination of water occurs mainly by interaction between OH groups. There is a correlation between the relative abundance of (M – H₂O)⁺ and (M ?2 H₂O)⁺ ions and the number of axial hydroxyls in the more stable conformation of the molecule. Difficulties encountered in deducing fragmentation patterns, and determination of number and position of deuterium atoms in the molecule of ms-inositol are discussed.     

Isobutane chemical ionization mass spectra of unsaturated alcohols

Analysis of the isobutane chemical ionization mass spectra of hexenols, cyclohexenols and various syn/anti pairs of bicyclic and tricyclic homoallylic alcohols shows that: (i) the spectra of the allylic alcohols are dominated by [M + H – H₂O]⁺ and [M + C₄H₉–H₂O]⁺ ions and contain traces of [M + H]⁺ ions; (ii) [M + H]⁺ ions are prominent in the spectra of acyclic and certain cyclic homoallylic alcohols; and (iii) [M + H]⁺ ions dominate the spectra of other acyclic unsaturated alcohols. The [M + H]⁺ ions may result from either: (a) protonation of the hydroxyl group, followed by a very rapid intramolecular proton transfer from the protonated hydroxyl group to the carbon–carbon double bond or internal solvation of the protonated hydroxyl group by the carbon–carbon double bond; and/or (b) direct protonation of the carbon–carbon double bond with significant internal solvation of the resulting carbocation by the hydroxyl group, which may lead to carbon–oxygen bond formation to give a protonated cyclic ether. The consequences of placing various geometric constraints on the possible intramolecular interactions between the hydroxyl group and the carbon–carbon double bond in unsaturated alcohols are explored.     

Benzene as a selective chemical ionization reagent gas

Dilute mixtures of C₆H₆ or C₆D₆ in He provide abundant [C₆H₆]^+· or [C₆D₆]^+· ions and small amounts of [C₆H₇]⁺ or [C₆D₇]⁺ ions as chemical ionization (CI) reagent ions. The C₆H₆ or C₆D₆ CI spectra of alkylbenzenes and alkylanilines contain predominantly M^+· ions from reactions of [C₆H₆]^+· or [C₆D₆]^+· and small amounts of MH⁺ or MD⁺ ions from reactions of [C₆H₇]⁺ or [C₆D₇]⁺. Benzene CI spectra of aliphatic amines contain M^+·, fragment ions and sample-size-dependent MH⁺ ions from sample ion-sample molecules reactions. The C₆D₆ CI spectra of substituted pyridines contain M^+· and MD⁺ ions in different ratios depending on the substituent (which alters the ionization energy of the substituted pyridine), as well as sample-size-dependent MH⁺ ions from sample ion-sample molecule reactions. Two mechanisms are observed for the formation of MD⁺ ions: proton transfer from [C₆D₆]^+· or charge transfer from [C₆D₆]^+· to give M^+·, followed by deuteron transfer from C₆D₆ to M^+·. The mechanisms of reactions were established by ion cyclotron resonance (ICR) experiments. Proton transfer from [C₆H₆]^+· or [C₆D₆]^+· is rapid only for compounds for which proton transfer is exothermic and charge transfer is endothermic. For compounds for which both charge transfer and proton transfer are exothermic, charge transfer is the almost exclusive reaction.     

Substituent effects in the mass spectra of diethyl phenyl phosphates

The mass spectra of diethyl phenyl phosphates show substituent effects with electron-donating groups favouring the molecular ion M⁺˙, and the [M? C₂H₄]⁺˙, [M – 2C₂H₄]⁺˙ and [XPhOH]⁺˙ ions. The [PO₃C₂H₆]⁺ (m/z 109) and [PO₃H₂]⁺ (m/z 81) ions are favoured by electron-withdrawing groups. Results suggest that the formation of the [XPhC₂H₃]⁺˙ ion involves rearrangement of C₂H₃ to the position ortho to the phosphate group. Ortho effects are also observed.     

Theoretical study on structures and vibrational spectra of M+(H2O)Ar (M = Cu,Ag, Au)

A theoretical study on the structures and vibrational spectra of M⁺(H₂O)Ar_0‐1 (M = Cu, Ag, Au) complexes was performed using ab initio method. Geometrical structures, binding energies (BEs), OH stretching vibrational frequencies, and infrared (IR) absorption intensities are investigated in detail for various isomers with Ar atom bound to different binding sites of M⁺(H₂O). CCSD(T) calculations predict that BEs are 14.5, 7.5, and 14.4 kcal/mol for Ar atom bound to the noble metal ion in M⁺(H₂O)Ar (M = Cu, Ag, Au) complexes, respectively, and the corresponding values have been computed to be 1.5, 1.3, and 2.1 kcal/mol when Ar atom attaches to a H atom of water molecule. The former structure is predicted to be more stable than the latter structure. Moreover, when compared with the M⁺(H₂O) species, tagging Ar atom to metal cation yields a minor perturbation on the IR spectra, whereas binding Ar atom to an OH site leads to a large redshift in OH stretching vibrations. The relationships between isomers and vibrational spectra are discussed. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Electron-impact studies—LV: Skeletal rearrangement and hydrogen scrambling processes in the positive and negative-ion mass spectra of phenyl derivatives of elements of group IV and V

The mass spectra of many triphenyl/tetraphenyl derivatives of the Group IV and V elements exhibit the processes [M⁺˙ ? C₁₂H₁₀] and/or [M⁺˙ ? C₆H₅· ? C₁₂H₁₀]. These fragmentations are not preceded by hydrogen scrambling between all the phenyl rings. Hydrogen scrambling does occur in certain fragment ions prior to fragmentation in both the positive and negative-ion spectra. The process [M⁺˙ ? C₁₂H₁₀] occurs in the negative-ion mass spectrum of tetraphenylsilane.     

Collision spectroscopy in isomer characterization: The case of (E)- and (Z)-p-nitrophenyldiazo tert-butyl sulphides

The structural characterization of the E and Z isomers of p-nitrophenyl-diazo tert-butyl sulphide has been achieved by means of different ionization methods (electron impact, fast atom bombardment, positive-ion chemical ionization) and collision experiments performed under different kinetic energy regimes (high- and low-energy collisions, angle-resolved mass spectrometry and energy-resolved mass spectrometry). The two compounds give rise to identical fragmentation patterns. Collision experiments, both at low energy and in the keV range at a scattering angle of 0°, in M^+· species obtained by electron impact on the two isomers, do not show any significant differences; the same experiments performed with 8 keV ions at a scattering angle of 2° indicate a clear difference in the absolute abundances of the two main daughter ions. High-energy collisions of MH⁺ ions obtained by fast atom bombardment lead to different spectra at both 0° and 2° scattering angles, proving that the energy deposition in the preselected species is an important parameter. Low-energy collisions with argon of MH⁺ ions generated by positive-ion chemical ionization (NH₄⁺) give rise to almost identical energy-resolved mass spectra, whereas the same experiments using helium as target gas lead to a clear distinction between the two isomers.     

Fragmentation of pyrazolecarbaldehyde thio- and dithioacetals under electron impact and chemical ionization

The mass spectra of 1-substituted 3,5-dimethyl-1H-pyrazole-4-carbaldehyde bis(2-hydroxyethyl) dithioacetals and thioacetals were studied for the first time. The main fragmentation pathways of their molecular ions generated under electron impact and chemical ionization were similar. Primary decomposition of the molecular ions of bis(2-hydroxyethyl) dithioacetals involves elimination of 2-sulfanylethanol molecule with formation of the corresponding 1,3-oxathiolane radical cation. Fragmentation of the molecular ions [M]⁺ · and [M + H]⁺ derived from 2-(3,5-dimethyl-1H-pyrazol-4-yl)-1,4,6-oxadithiocanes includes cleavage of the eight-membered heteroring and elimination of C₄H₉OS ·. Substituents in the heteroring of pyrazolecarbaldehydes inhibit decomposition processes related to the aldehyde group.     

Neutralization-reionization mass spectrometry (NRMS). Structural information from vertical neutralization and reionization efficiencies

The neutralization-reionization mass spectra of alkane radical ions indicate significant differences between the structures and geometries of alkane molecules and their molecular ions, confirming recent ab initio predictions. Ionic isomers that are indistinguishable by collisionally-activated dissociation because of easy interconversion can be characterized by neutralization-reionization if the corresponding neutrals show different reactivities, as is demonstrated for the [C₂H₅]⁺/C₂H₅˙ system and for [C₂H₄O₂]⁺˙ isomers. For identification of mixtures of more than one neutral species, the relative efficiency for reionizing each neutral must be determined; e.g. the O₂ reionization efficiency of ˙CH₂OH radicals is ～4 times greater than that of CH₃O˙. This information and reference reionization spectra of CH₃O˙ and ˙CH₂OH show that metastable or collisionally activated methyl acetate cations lose CH₃O˙, not ˙CH₂OH as previously reported; the newly-formed CH₃O˙ undergoes partial (～20%) isomerization to ˙CH₂OH in the ～10^?6s before reionization. Similar results are obtained for [B(OCH₃)₃]⁺˙.     

激光溅射金属等离子体与醇类分子束反应的研究：产物离子对分子束状态的依赖性

The gas phase reactions of metal plasma with alcohol clusters were studied by time of flight mass spectrometry （TOFMS） using laser ablation-molecular beam （LAMB） method. The significant dependence of the product cluster ions on the molecular beam conditions was observed. When the plasma acted on the low density parts of the pulsed molecular beam, the metal-alcohol complexes M^＋An （M=Cu, Al, Mg, Ni and A=C2H5OH, CH3OH） were the dominant products, and the sizes of product ion clusters were smaller. While the plasma acted on the high density part of the beam, however, the main products turned to be protonated alcohol clusters H^＋An and, as the reactions of plasma with methanol were concerned, the protonated water-methanol complexes H3O^＋（CH3OH）n with a larger size （n≤12 for ethanol and n≤24 for methanol）. Similarly, as the pressure of the carrier helium gas was varied from 1 × 10^5 to 5 × 10^5 Pa, the main products were changed from M^＋An to H^＋An and the sizes of the clusters also increased. The changes in the product clusters were attributed to the different formation mechanism of the output ions, that is, the M^＋An ions came from the reaction of metal ion with alcohol clusters, while H^＋An mainly from collisional reaction of electron with alcohol clusters.     

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.     

Repeller-Induced dissociation of alkyl alcohols in thermospray mass spectrometry

Six alkyl alcohols were studied using thermospray mass Spectrometry. Whereas the dominant ion in the spectrum up to a repeller potential of 120 V was [M + NH₄]⁺, above that potential [M + H]⁺ and fragment ions appeared. The fragments observed were largely due to hydrogen release from alkyl ions ([C_nH_2n+1]⁺ – H2 → [C_nH_2n-1]⁺) and loss of water or some other stable molecule from the same species. The results are compared with those from ionization of the same alcohols under electron impact and photoionization conditions and with results obtained for methanol under thermospray conditions.     

Metastable ion decomposition and collisional activation mass spectra of 1,2,3-triarylpropen-1-ones

Substituents have been found to have a marked influence on the metastable ion decompositions and collisionally activated (CA) fragmentations of the M⁺˙ ion of a number of 1,2,3-triarylpropen-1-ones. An attempt has been made to confirm the structures of the rearrangement ions, [C₁₄H₁₀]⁺˙, [C₁₃H₁₁]⁺˙, [C₁₃H₉]⁺ and [C₁₂H₈]⁺˙ by comparison of their CA spectra with those of the corresponding ions produced from reference compounds. The results imply that [C₁₄H₁₀]⁺˙ and the M⁺˙ ions of phenanthrene and diphenylacetylene have a common structure, [C₁₃H₉]⁺ and the fluorenyl cation have a common structure and [C₁₂H₈]⁺˙ and biphenylene molecular ion have a common structure. The available data indicate that the ion at m/z 167 consists of a mixture of structures, likely possibilities being diphenylmethyl, phenyltropylium and dihydrofluorenyl cations.     

Structure and formation of gaseous [C4H5O]+ Ions. II–Ions of the type HCC?\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}(OH)(CH3)

Loss of an alkyl group X? from acetylenic alcohols HC?C? CX(OH)(CH₃) and gas phase protonation of HC?C? CO? CH₃ are both shown to yield stable HC?C? \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}(OH)(CH₃) ions. Ions of this structure are unique among all other [C₄H₅O]⁺ isomers by having m/z 43 [C₂H₃O]⁺ as base peak in both the metastable ion and collisional activation spectra. It is concluded that the composite metastable peak for formation of m/z 43 corresponds to two distinct reaction profiles which lead to the same product ion, CH₃\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}?O, and neutral, HC?CH. It is further shown that the [C₄H₅O]⁺ ions from related alcohols (like HC?C? CH(OH)(CH₃)) which have an α-H atom available for isomerization into energy rich allenyl type molecular ions, consist of a second stable structure, H₂C?\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}? C(OH)?CH₂.     

Stereoelectronic effects on the retro-Diels-Alder fragmentation of ionized bicyclo[4.3.0]nona-3,7-dienes

The electron impact and collision-induced dissociation mass spectra of cis- and trans-annulated bicyclo[4.3.0]nona-3,7-dienes differ in their relative abundances of [C₅H₆]⁺˙ fragments formed by the retro-Diels-Alder decomposition. The formation of [C₅H₆]⁺˙ is not preceded by hydrogen migration in the short-lived and long-lived molecular ions. The appearance energy of [C₅H₆]⁺˙ from both annulation isomers is identical within experimental error: AE_cis([C₅H₆]⁺˙)=10.56±0.10 eV and AE_trans([C₅H₆]⁺˙)=10.54±0.15 eV. The barrier to the retro-Diels-Alder fragmentation lies 68–76 kJ mol^?1 above the thermo-chemical threshold corresponding to [C₅H₆]⁺˙ + C₄H₆. Investigation of the two-dimensional reaction coordinate by the Topological Molecular Orbital treatment shows that the lowest energy path for the retro-Diels-Alder reaction involves a two-step dissociation of the C(5)? C(6) and C(1)? C(2) bonds in the molecular ion, the latter step overcoming a barrier, calculated as 80 kJ mol^?1 above the thermochemical threshold. The stereochemical difference between the geometric isomers is due to stereoelectronic assistance of the π orbitals of the cis-annulated isomer in the cleavage of the C(5)? C(6) bond. Other mechanisms of the retro-Diels–Alder reaction are discussed.     

Isomeric [C3H4]+· ions: Their identification and generation in dissociative ionizations

The collisional activation (CA) and charge stripping (CS) mass spectra of the three [C₃H₄]^+· isomers, allene, propyne and cyclopropene, are reported. The extent of isomerization among these ions prior to collisional excitation depends on their internal energy content, but is small. Each [C₃H₄]^+· ion structure also can uniquely be generated via appropriate dissociative ionizations. Analysis of mixtures of [C₃H₄]^+· (daughter) ion structures is, in general, not possible from CA and CS mass spectra alone but may be aided by appearance energy measurements.     

The effect of configuration of gas phase protonated ethenedicarboxylates on their low energy collision induced dissociation behaviour

Low energy collision induced dissociation (CID) spectra were measured by a triple stage quadrupole mass spectrometer for the [MH]⁺ ions of diethyl and dimethyl esters of maleic, fumaric, citraconic and mesaconic acids. A very high degree of stereospecificity was observed for the geometrically isomeric diethyl esters. The cis esters give rise to very abundant [MH? EtOH]⁺ and [MH? EtOH? C₂H₄]⁺ ions, while the trans isomers exhibit very abundant [MH? C₂H₄]⁺ and [MH? 2 C₂H₄]⁺ ions. The highly stereospecific processes indicate that the double bond configuration is retained in the protonated species under the conditions of the experiment.     

Stereochemical effects in the electron impact mass spectra of cis–trans isomeric 1,2,3,5-tetramethylcyclohexanes and some trimethylcyclohexanes

The mass spectra of 10 isomeric trimethylcyclohexanes and six cis–trans isomeric 1,2,3,5-tetramethylcyclohexanes are discussed. The thermochemically stable isomers show a higher abundance of the [M? CH₃]⁺ ions and a lower abundance of the [M? C₂H₅]⁺ ions than the strained isomers. The log of the intensity ratio for loss of methyl and ethyl from the molecular ions correlates well with the strain energies and with the mean number of gauche arrangements of the stereoisomers.