On the mechanism of the ethyl elimination from the molecular ion of 6-methoxy-1-hexene

It is shown by ¹³C and D labelling that the ethyl radical elimination from the molecular ion of 6-methoxy-1-hexene is a very complex process involving at least two different channels. The major channel (80%) is induced by an initial 1,5-hydrogen shift in the molecular ion from C(5) to C(l) leading via a series of steps to methoxy-cyclohexnne, which then undergoes a ring contraction to 2-methyl-1-methoxycyclopentane, being the key intermediate for the ethyl loss. The same key intermediate is formed in the other, minor channel (20%) by ring closure directly following an initial 1,6-hydrogen shift in the molecular ion of 6-methoxy-1-hexene from C(6) to C(l). Collision-induced dissociation experiments on the [M ? ethyl]⁺ ion from 6-methoxy-1-hexene have further established that it has the unique structure of oxygen methyl cationized 2-methyIpropen-2-al. This ion is also generated by ethyl loss from the molecular ion of 2-methyl-1-methoxycyclopentane itself, as shown by collision-induced dissociation experiments, thus confirming the key role of the intermediate mentioned.     

Mass spectrometry of aralkyl compounds with a functional group—VIII: Comparison of the breakdown of 3-phenylnitropropane with that of 3-phenylpropyl nitrite upon electron-impact

Mass spectra of 3-phenylnitropropane and of its analogues, specifically deuterated in the aliphatic chain and in the phenyl ring, show that the molecular ion loses a molecule of water in two different modes, viz. either with both α-hydrogen atoms or with a γ- and an ortho-hydrogen atom. Moreover, a molecule of nitric oxide is eliminated from the molecular ion and the resulting [M - NO]⁺-ion appears to decompose further in many complicated ways. This loss of nitric oxide does not arise from an isomerization of the nitro group to a nitrite group, because the [M - NO]⁺-ion of 3-phenylpropyl nitrite breaks down in an altogether different manner than that of 3-phenylnitropropane. This is demonstrated by the spectra of specifically deuterated analogues of 3-phenylpropyl nitrite.     

Formation and structure of [C8H8O]+˙ ions,generated from gas phase ions of phenyl-cyclopropylcarbinol and 1-phenyl-1-(hydroxymethyl)cyclopropane

The structure and formation of [C₈H₈O]^+. ions generated from phenylcyclopropylcarbinol and 1-phenyl-1-hydroxymethylcyclopropane upon electron impact, have been studied using kinetic energy release measurements, by determination of ionization and appearance energies and by collisional activation. It is shown that the non-decomposing [C₈H₈O]^+˙ ions have exclusively the structure of the enol ion of phenylacetaldehyde, although it is less stable than the enol ion of acetophenone by about 45 kJ mol^?1. This has been interpreted as an indication that the [C₈H₈O]^+˙ ions from phenylcyclopropylcarbinol are formed by an attack of either the phenyl ring or the hydroxyl group upon the C-1? C-2 (or C-1? C-3) bond of the cyclopropane ring under a simultaneous expulsion of ethene and migration of the attacking group to the C-1 position. The [C₈H₈O]^+˙ ion from 1-phenyl-1-(hydroxymethyl)cyclopropane is formed by opening of the cyclopropane ring via a benzylic cleavage. A kinetically controlled hydrogen shift in the resulting ring opened ion prior to or during ethene loss then leads to the formation of [C₈H₈O]^+˙ ions which have the structure of the enol ion of phenylacetaldehyde.     

Elucidation of elctron-impact-induced fragmentations of hydrocinnamaldehyde by 13C labelling; evidence for an ‘ortho attack’ in the molecular ion

The mass spectra of hydrocinnamaldehydes, labelled with ¹³C in Position 1 and 3, have been studied in order to determine the origins of the carbon atoms in the moieties C₂H₂O, C₃H₄O, CH₃ and CO, expelled from the molecular ion.     

A gas phase study of the ions and [C3H3]− generated from the reaction of with propyne

It is reported that ions gererated in the gas phase by dissociative electron attachment to nitrous oxide react with propyne-d₃ (trideuteromethyl acetylene) to yield the ions ?D?C?C H, ?D₂? C?C^?, ^?CD₂C?CH and CD₃C?C^?. From their differing reactivity with methyl formate it is suggested that these four ions are distinct stable species.     

Coherent low-frequency motions of hydrogen bonded acetic acid dimers in the liquid phase

Ultrafast vibrational dynamics of cyclic hydrogen bonded dimers and the underlying microscopic interactions are studied in temporally and spectrally resolved pump-probe experiments with 100 fs time resolution. Femtosecond excitation of the O-H and/or O-D stretching mode gives rise to pronounced changes of the O-H/O-D stretching absorption displaying both rate-like kinetic and oscillatory components. A lifetime of 200 fs is measured for the v=1 state of the O-H stretching oscillator. The strong oscillatory absorption changes are due to impulsively driven coherent wave packet motions along several low-frequency modes of the dimer between 50 and 170 cm(-1). Such wave packets generated via coherent excitation of the high-frequency O-H/O-D stretching oscillators represent a clear manifestation of the anharmonic coupling of low- and high-frequency modes. The underdamped low-frequency motions dephase on a time scale of 1-2 ps. Calculations of the vibrational potential energy surface based on density functional theory give the frequencies, anharmonic couplings, and microscopic elongations of the low-frequency modes, among them intermolecular hydrogen bond vibrations. Oscillations due to the excitonic coupling between the two O-H or O-D stretching oscillators are absent as is independently confirmed by experiments on mixed dimers with uncoupled O-H and O-D stretching oscillators.     

A fast atom bombardment and field ionization/field desorption study of some isomeric unsaturated dicarboxylic acids

Geometrically isomeric dicarboxylic acids, such as maleic and fumaric acid and their methyl homologues, and the isomeric phthalic acids, have been investigated using fast atom bombardment, field ionization and field desorption mass spectrometry. The most intense peak in the positive ion fast atom bombardment spectra corresponds with the [M + H]⁺ ion. This ion, when derived from the E -acids, tragments either by successive loss of water and carbon monoxide or by elimination of carbon dioxide. In the case of the Z -acids only elimination of water from the [M + H]⁺ ions is observed to occur to a significant extent. The same is true for the [M + H]⁺ ions of the isomeric phthalic acids, that is the [M + H] ions derived from iso- and terephthalic acid exhibit more fragmentation than those of phthalic acid. All these acids undergo much less fragmentation upon field ionization, where not only abundant [M + H]⁺ ions, but also abundant [M]^+· ions, are observed. Upon field desorption only the [M + H]⁺ and [M + Na]⁺ ions are observed under the measuring conditions. Negative ion fast atom bombardment spectra of the acids mentioned have also been recorded. In addition to the most abundant [M? H]^? ions relatively intense peaks are observed, which correspond with the [M]^?˙ ions. The fragmentations observed for these ions appear to be quite different from those reported in an earlier electron impact study and in a recent atmospheric pressure ionization investigation.     

Chemical ionization of fluorophenyl n-propyl ethers: Loss of propene from the metastable [M + D]+ ions

Chemical ionization (CI) mass spectrometry with the reagents D₂O, CD₃OD, and CD₃CN (given in order of increasing proton affinity) has been used to generate metastable [M + D]⁺ ions of a series of mono-, di-, and trifluorophenyl n-propyl ethers and analogs labeled with two deuterium atoms at the β position of the alkyl group. Loss of propene is the main reaction of the [M + D]⁺ ions, whereas dissociation with formation of propyl carbenium ions is of minor importance. The combined results reveal that the deuteron added in the CI process can be incorporated in the propene molecules as well as in the propyl carbenium ions. The extent to which the added deuteron is exchanged with the hydrogen atoms of the propyl group is markedly dependent on the position of the fluorine atom(s) on the ring and the exothermicity of the initial deuteron transfer. For 3-fluorophenyl n-propyl ether, exchange is not observed if D₂O is the CI reagent, and occurs only to a minor extent in the experiments with the CI reagents CD₃OD and CD₃CN. Similar results are obtained for the 3,5-difluoro- and 2,4,6-trifluorophenyl ethers, whereas significant exchange is observed prior to the dissociations of the [M + D]⁺ ions of the 4-fluoro- and 2,6-difluorophenyl n-propyl ethers, irrespective of the nature of the CI reagent. These results are discussed in terms of the occurrence of initial deuteron transfer either to the oxygen atom or the aromatic ring followed by formation of an ion/neutral complex of a fluorine-substituted molecule and a secondary propyl carbenium ion. Initial deuteron transfer to the oxygen atom is suggested to yield complexes that can react by exchange between the added deuteron and the hydrogen atoms of the original propyl group prior to dissociation. By contrast, initial deuteron transfer to the ring is suggested to lead to complexes that react further by loss of propene molecules containing only the hydrogen/deuterium atoms of the original propyl entity.     

Insertion reactions of metal carbonyl anions with methyl formate in the gas phase as revealed by 13C- and D-labeling

School of Chemistry, University of New South Wales, Kensington, Australia Institute of Mass Spectrometry, University of Amsterdam, Nieuwe Achtergracht The gas-phase reactions of coordinatively unsaturated metal carbonyl anions (M(CO) _n ^? , M=Cr, Mn, Fe, Co; n=0-3 and Co(CO)_nNO^?, n=0-2) with unlabeled and D- and ¹³C-labeled methyl formate have been studied with Fourier transform ion cyclotron resonance mass spectrometry. The reactions proceed in most instances by loss of one or more CO molecules from the collision complex. In the reactions of the dicarbonyl and tricarbonyl anions with H¹³COOCH₃, part of the eliminated carbon monoxide molecules contain the label revealing the occurrence of initial insertion of the metal center into the bonds adjacent to the carbonyl function of the substrate with formation of five- or six-coordinate intermediates, respectively. In addition, the MnCCO) ₃ ^? , Fe(CO) ₂ ^? , and CoCCO) ₂ ^? ions react by the loss of methanol and a [C,H₂,O] neutral species. The D- and ¹³C-labeling show that methanol is expelled in a reductive elimination from a five- or six-coordinate species, whereas the [C,H₂,O] loss is a more complex process possibly involving the competing losses of formaldehyde and CO + H₂. In the reaction of Fe(CO) ₃ ^? with H ₁₃ ¹³ COOCH₃, a facile consecutive exchange of all three CO ligands of the reactant ion for ¹³CO is observed. This novel reaction appears to involve initial insertion into the H¹³CO—OCH₃-bond followed by facile hydrogen shifts from the formyl ligand to a CO Hgand prior to the loss of unlabeled methyl formate.     

On the structure of protonated methane

Results of a Fourier transform-ion cyclotron resonance study are reported concerning the reactivity of protonated perdeuteromethane and deuteronated methane, generated under varying pressure conditions in an external chemical ionization ion source, toward ammonia. The competition between proton and deuteron transfer from both protonated perdeuteromethane and deuteronated methane to ammonia exhibits chemically distinguishable hydrogens. The chemical behavior of protonated methane appears to be compatible with the theoretically predicted stable structure with C_S symmetry, involving a three-center two-electron bond associating two hydrogens and the carbon atom. Interconversion of this structure due to exchange between one of these hydrogens and one of the three remaining hydrogens appears to be a fast process that is induced by interactions with the chemical ionization gas.