Elimination of water from the molecular ion of cycloheptanol

Under electron impact cycloheptanol decomposes by four fragmentation paths: (1) α-cleavage with subsequent losses of C¹-C⁵ fragments, (2) elimination of water, (3) loss of the hydrogen atom from C-1 and (4) loss of the hydroxyl group. The mechanism of water elimination was investigated by means of deuterium labelling. 1,4-Elimination of water predominates in cycloheptanol, with the stereospecific cis-1,3-elimination also being operative. The loss of water is preceded by extensive exchange of the hydroxyl hydrogen with those of the ring. This is attributed to a very facile transannular interaction of the hydroxyl group with the C-3 to C-6 positions that are made accessible due to conformational properties of the 7-membered ring. A kinetic model is proposed, describing migrations of the ring hydrogen atoms.     

Reaction from isomeric parent ions in the dissociation of dimethylpyrroles

The major dissociation pathways of the [M-H]⁺ (loss of NH₃ or CH₄) and the [M+H]⁺ (loss of NH₃ or CH₃) ions from dimethylpyrroles have been determined to occur from isomeric parent ions. For the [M-H]⁺ ion (formed by loss of a methyl hydrogen), loss of NH₃ leads to the formation of the phenylium ion and is preceded by consecutive carbon ring expansions followed by a ring contraction to form protonated aniline. Loss of CH₄ occurs after the first carbon ring expansion, which forms protonated picoline. The relative partitioning between the two dissociation paths depends upon the internal energy content of the parent ion; the highest point on the potential energy surface is the second ring expansion step. The [M+H]⁺ ion reacts through a similar pathway via dihydro analogs of picoline and aniline. The proposed reaction pathways are supported by results of semiempirical molecular orbital calculations.     

Loss of hydrogen cyanide from monocyanopyridines upon electron impact: Dewar pyridine structures and ring opening—Ring closure reactions revealed by 13C and 15N Labelling

Extensive ¹³C and ¹⁵N labelling has shown that the molecular ions of 2-, 3- and 4-cyanopyridine with lifetimes up to 10^?6 s eliminate hydrogen cyanide originating predominantly from the ring (?65%). Moreover, this hydrogen cyanide loss occurs after an equilibrated positional interchange of the ring carbon atoms at positions interchange of the ring carbon atoms at positions 2, 4 and 6 via Dewar pyridine structures. In molecular ions with lifetimes of 10^?6–10^?5 s skeletal rearrangements have taken place in such a way that both nitrogen atoms have become equivalent prior to the loss of hydrogen cyanide. Arguments are put forward that this equivalence of nitrogen atoms is caused by the intermediacy of ions with a 1,4-dicyanobuta-1,3-diene structure. About 60% of these intermediate ions eliminate hydrogen cyanide in a fast process. The remaining 40% of these ions undergo ring closure again to a pyridine ring in which the carbon atoms of positions 2, 4 and 6 are positionally interchanged rapidly via Dewar pyridine structures followed by ring opening again and eventual loss of hydrogen cyanide. This interpretation of the ¹³C and ¹⁵N labelling results is further corroborated by a study of the loss of hydrogen cyanide from molecular ions of 1,4-dicyanobuta-1,3-diene labelled with ¹³C in both cyano groups.     

Translational energy release and stereochemistry of steroids. VIII-The mechanism of the elimination of water from metastable ions in epimeric 3-hydroxy steroids of the 5α-series

The mechanism of water elimination from metastable molecular, [M ? CH₃˙]⁺ and [M ? ring D]⁺˙ ions of epimeric 3-hydroxy steroids of the 5α-series has been elucidated. Deuterium labelling, the measurement of the translational energy released during the loss of water, and collision-induced decomposition mass-analysed kinetic energy spectrometry were the techniques used. It was found that the mechanisms of water loss from metastable M⁺˙ and [M ? ring D]⁺˙ ions is different from that from [M ? CH₃˙]⁺ ions.     

Fragmentation of acylium ions from methyl levulinate. Isomerization processes involving carbon and oxygen atoms of both carbonyl groups

The fragmentations of the acylium ions O?C⁺? CH₂? CH₂? CO₂CH₃ and O?C⁺? CH₂? CH₂? COCH₃ generated from methyl levulinate are governed extensively by the interaction of the two carbonyl groups. Both species eliminate a molecule of CO unimolecularly and under CID conditions. The results derived from measurements of ¹³C and ¹⁸O labelled precursors, together with kinetic energy release values, have been used to study the mechanisms. In the first of these acylium ions, both carbonyl groups are equivalent; this phenomenon can be the result of a 1,4 methoxy shift. In the second acylium ion, only the oxygen atoms change their positions; this isomerization occurs via the [M? H]⁺ of γ-valerolactone. Some other fragmentation processes also discussed in relation to ²H labelling are the formation of the [M ? COOCH₃] ⁺ ion and the loss of HCOOCH₃ in the collision-induced dissociation mass spectra of the first acylium ion, and the formation of the [CH₃CO]⁺ ion and the loss of H₂O for the second one.     

Mass spectometry of aralkyl compounds with a functional group—VI1: Mass spectra of 1-phenylethyanol-1, 2-phenylethanol-1 and 1-phenylpropanol-2

Mass spectra of 1-phenylethanol-1 and its analogues, specifically deuterated in the aliphatic chain, suggest that the [M? CH₃]⁺ ion is represented partly by an α-hydroxybenzyl fragment. Moreover, the molecular ion loses successively—after scrambling of all hydrogen atoms, except those of CH₃? a hydrogen atom and C₆H₆, generation the CH₃CO⁺ ion. Diffuse peaks, found in the spectra of of 2-phenylethanol-1 and its analogues, specifically deuterated in the aliphatic chain and in the phenyl ring, show that the molecular ion loses C₂H₄O, possibly via a four-center mechanism, after an exchange of aromatic and hydroxylic hydrogens. Mass spectra of 1-phenylpropanol-2 and its analogues, specifically, deuterated in the aliphatic chain, demonstrate that in the molecular ion exclusively the hydroxyl hydrogen atom is transferred to one of the ortho-positions of the phenyl ring via a McLafferty rearrangement, generating the [M ? C₂H₄O]⁺ ion. Furtherore, an eight-membered ring structure is proposed for the [M ? CH₃]⁺ ion to explain the loss of H₂O and C₂H₂O from this ion after an extensive scrambling of hydrogen atoms.     

Rearrangements and fragmentations of phenyl styryl sulfides: 2—labelling studies

Metastable molecular ions of phenyl styryl sulfides may decompose by loss of CH₃^˙, SH^˙, CHS^˙, C₆H₅^˙, C₆H₆ or C₇H₇^˙. Labelling with carbon-13 and deuterium gave information about the mechanisms of these reactions. It appears that extensive rearrangements occur prior to most of these fragmentations. In the case of phenyl β-styryl sulfide both phenyl groups and both vinyl carbon atoms are found in the C₇H₇ fragment in comparable amounts. For phenyl α-styryl sulfide this fragmentation leads more specifically to the loss of the S-phenyl group and the β-vinyl carbon atom. It was concluded that rearrangements occur, partly via symmetric diphenyl ethene sulfide structures, to benzyl phenyl thione ions, from which the fragmentation occurs. For the loss of CHS^˙ an earlier proposed mechanism was confirmed. From both compounds the S-phenyl ring can be lost as C₆H₅^˙ or C₆H₆ as well as the C-phenyl ring. Fragmentation occurs from one of the initial structures as well as from benzyl phenyl thione. Loss of CH₃^˙ is thought to occur after ring closure with formation of dihydrobenzo[b]thiophenes followed by ring opening by rupture of a C? S bond. While phenyl β-styryl sulfide shows a strong tendency towards isomerization to a symmetric structure like 1,2-diphenylethene sulfide, phenyl α-styryl sulfide easily rearranges in an electrocyclic reaction with formation of benzyl phenyl thione.     

Gas phase reactions of protonated 1,3-diphenylpropyne and some isomeric [C15H13]+ ions

Metastable (3-phenyl-2-propynyl)benzenium ions, generated by electron impact induced fragmentation from the appropriately substituted 1,4-dihydrobenzoic acid, react by loss of ˙CH₃ and C₆H₆. The study of deuterated derivatives reveals that hydrogen/deuterium exchanges involving all hydrogen and deuterium atoms precede the fragmentations. The results suggest a skeletal rearrangement by electrophilic ring-closure reactions giving rise to protonated phenylindene and protonated 9,10-methano-9,10-dihydroanthracene prior to the elimination of C₆H₆ and ˙CH₃, respectively. A study of isomeric [C₁₅H₁₃]⁺ ions by collision-induced decomposition and by deuterium labelling shows that these ions interconvert by hydrogen migrations and skeletal rearrangements.     

Zur lokalisierung funktioneller gruppen mit hilfe der massenspektrometrie—VIII : 6-hydroxy-androstan-3,17-dione und 6,17β-dihydroxy-androstan-3-one

5β-androstan-3-ones carrying a 6α-OH group show in their mass spectra a key-ion indicating the loss of water and C-1 to C-4 as C₄H₅O? particle. 6β-OH isomers lose instead C-1 to C-4 in form of C₄H₇O?.In 6α-hydroxy-androstan-3-ones differentiation between the connection of the A/B-ring system is possible, because in 5α-isomers the loss of C-3 to C-7 occurs as a C₅H₆O₂ particle, while the 5β-isomers lose the same C atoms as a C₅H₇O? unit.Compounds with a 6β-OH group in an A/B trans connected ring system show a tendency for thermal water elimination. After rearrangement of the double bond in 4,5 position the typical fragments for 3-keto-Δ⁴-steroids are obtained.Occasionally a strong influence of a 6-OH group on fragmentation reactions in the D-ring system is observed: The presence of a 6α-OH group in an androstan-3,17-dione enhances the loss of C-16 and C-17 in the form of acetaldehydenol. Also the connection of the A/B-ring system may have a considerable influence on this type of reaction: In 6,17β-dihydroxy-androstan-3-ones only by trans connection of the A/B-ring system, C-16 and C-17 are lost with high probability after water elimination.     

Protonation Sites in Gaseous Pyrrole and Imidazole: A Neutralization–Reionization and ab initio Study

Mild gas-phase acids C₄H₉⁺ and NH₄⁺ protonate pyrrole at C-2 and C-3 but not at the nitrogen atom, as determined by deuterium labeling and neutralization–reionization mass spectrometry. Proton affinities in pyrrole are calculated by MP2/6–311G(2d, p) as 866, 845 and 786 kJ mol^-1 for protonation at C-2, C-3 and N, respectively. Vertical neutralization of protonated pyrrole generates bound radicals that in part dissociate by loss of hydrogen atoms. Unimolecular loss of hydrogen atom from C-2-and C-3-protonated pyrrole cations is preceded by proton migration in the ring. Protonation of gaseous imidazole is predicted to occur exclusively at the N-3 imine nitrogen to yield a stable aromatic cation. Proton affinities in imidazole are calculated as 941, 804, 791, 791 and 724 for the N-3, C-4, C-2, C-5 and N-1 positions, respectively. Radicals derived from protonated imidazole are only weakly bound. Vertical neutralization of N-3-protonated imidazole is accompanied by large Franck–Condon effects which deposit on average 183 kJ mol^-1 vibrational energy in the radicals formed. The radicals dissociate unimolecularly by loss of hydrogen atom, which involves both direct N-H bond cleavage and isomerization to the more stable C-2 H-isomer. Potential energy barriers to isomerizations and dissociations in protonated pyrrole and imidazole isomers and their radicals were investigated by ab initio calculations.     

Translational energy release and stereochemistry of steroids. IX-The mechanism of the elimination of water from metastable ions in epimeric 3-hydroxy steroids of the 5β-series

The mechanism of water elimination from metastable molecular and [M ? CH₃˙]⁺ ions, as well as from ions deprived of ring D, in epimeric 3-hydroxy steroids of the 5β-series has been elucidated by deuterium labelling, by the measurements of the translational energy released during loss of water, and by collision-induced decomposition mass-analysed ion kinetic energy spectrometry. It was found that the dehydration of the metastable molecular ion in 3α-hydroxy steroids of the 5β-series occurs mostly regiospecifically as an elimination of the 3α-hydroxyl together with the 9α-hydrogen atom. The ring A in the molecular ion has to flip to the boat conformation to make this reaction possible. In the metastable molecular ion of 3β-hydroxy steroids of the 5β-series a different dehydration mechanism operates, with very little participation of the 9α-hydrogen atom. The mechanisms of water loss from metastable [M ? CH₃˙]⁺ ions and from ions deprived of ring D differ from that of the molecular ion.     

The mechanism of elimination of an H2O molecule in desmethylencecalin under electron impact

The mass spectrum of desmethylencecalin has been measured and a fragmentation mechanism for the loss of a water molecule from the [M? CH₃]⁺ ion is proposed on the basis of isotope labelling and metastable peak observations. The eliminated water has been shown to involve the carbonyl oxygen and hydrogen atoms of the hydroxyl and acetyl groups.     

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.     

Gas‐phase dissociation study of erythrinian alkaloids by electrospray ionization mass spectrometry and computational methods

Alkaloids from plants of the genus Erythrina display important biological activities, including anxiolytic action. Characterization of these alkaloids by mass spectrometry (MS) has contributed to the construction of a spectral library, has improved understanding of their structures and has supported the proposal of fragmentation mechanisms in light of density functional calculations. In this study, we have used low‐resolution and high‐resolution MSⁿ analyses to investigate the fragmentation patterns of erythrinian alkaloids; we have employed the B3LYP/6‐31+G(d,p) model to obtain their reactive sites. To suggest the fragmentation mechanism of these alkaloids, we have studied their protonation sites by density functional calculation, and we have obtained their molecular electrostatic potential map and their gas‐phase basicity values. These analyses have indicated the most basic sites on the basis of the proton affinities of the nitrogen and oxygen atoms. The protonated molecules were generated by two major fragmentations, namely, neutral loss of CH₃OH followed by elimination of H₂O. High‐resolution analysis confirmed elimination of NH₃ by comparison with the losses of H₂ and •CH₃. NH₃ was eliminated from compounds that did not bear a substituent on ring C. The benzylic carbocation initiated the dissociation mechanism, and the first reaction involved charge transfer from a lone pair of electrons in the oxygen atoms. The second reaction consisted of ring contraction with loss of a CO molecule. The presence of hydroxy and epoxy groups could change the intensity or the occurrence of the fragmentation pathways. Given that erythrinian alkaloids are applied in therapeutics and are promising leads for the development of new drugs, the present results could aid identification of several analogues of these alkaloids in biological samples and advance pharmacokinetic studies of new plant derivatives based on MSⁿ and MS/MS analyses. Copyright © 2017 John Wiley & Sons, Ltd.     

Mechanism of the “abnormal” pyrolytic 1,3-elimination of 2-adamantyl methane sulphonate

Pyrolysis of syn-2-adamantyl-4-d₁ methane-sulphonate 8 in a silanised pyrex reactor leads to protoadamantene with about 0.25 atoms ²H at each of C-2, -7 and -9 and little labelling elsewhere according to ²H and ¹³C NMR. This indicates complete epimerisation of 8 before elimination. In the concurrently formed dehydroadamantane, unequal partitioning of label between positions C-6/8, -9 and -10 suggests only limited epimerisation before elimination. Pyrolysis of the 4,4-dideuteriated sulphonate 11, gave analogous results. The syntheses of 8 and 11 are described and the relevance of the analytical results to possible mechanisms is considered.     

An electron impact study of HCN elimination from indole by use of 13C labelling

The study of the loss of HCN from the molecular ions of [2-¹³C]indole and [3-¹³C]indole shows that, to a good approximation, only the two carbon atoms of the pentagonal ring are involved in this fragmentation process, contrary to the behaviour of the H atoms; the C-2 atom is eliminated predominantly, chiefly in the ion source (85–90%) and a little less in the metastable energy range (75–80%). The losses of ¹³CCH₃^˙ and C₂H₃^˙ from the [M? H¹²CN]^+˙ ions of the two compounds suggest the occurrence of different structures, providing evidence for several mechanisms of HCN elimination.     

Hydrogen migrations in mass spectrometry. II—single and double hydrogen migrations in the electron impact fragmentation of n-propyl benzoate

The sources of the migrant hydrogen atom(s) in reactions (a) and (b) in the electron impact mass spectrum of n-propyl benzoate have been investigated: (a) [C₆H₅CO₂C₃H₇]⁺ →[C₆H₅CO₂H]⁺ + C₃H₆; (b) [C₆H₅CO₂C₃H₇]⁺ → [C₆H₅CO₂H₂]⁺ + C₃H₅^sdot;. Deuterium labelling of the propyl group showed that, for reaction (a) at 70 eV ionizing energy 3 ± 1% of the hydrogen originates from C-1 of the propyl group, 86 ± 4% from C-2 and 11 ± 3% from C-3. The specificity of the transfer from C-2 increases as the internal energy of the fragmenting ions decreases, indicating that the results cannot be rationalized in terms of H/D interchanges between positions in the propyl group, but rather that the reaction involves specific, competing, H transfer reactions from each propyl position, in contrast to the high site specificity characteristic of the McLafferty rearrangement. Reaction (b) involves, almost exclusively, transfer of one hydrogen from C-2 and one from C-3 with only very minor participation of C-1 hydrogens. The [C₆H₅COOH]⁺ ion produced in reaction (a) fragments further to [C₆H₅CO]⁺ + OH. and the labelling results indicate some interchange of the carboxylic hydrogen with (ortho) ring hydrogens for those ions fragmenting in the first drift region. The extent of interchange is less than that observed for fragmentation of the same ion produced by direct ionization of benzoic acid or by reaction (a) in ethyl benzoate.     

The electron impact induced fragmentation of aromatic aldoximes—III. On the loss of HCNO,OH⋅ and substituents and the role of substituent effects in the decomposition of cyclohexadiene type intermediate ions

The mechanisms for loss of HCNO, OH^˙ and the substituent X^˙ from aromatic aldosimes were elucidated with the aid of deuterium labelling, metastable ion characteristics and substituent effects. It is proposed that the loss of HCNO occurs through a cyclohexadiene type intermediate ion generated via a 6-membered ring hydroxyl hydrogen transer to the ortho position of the phenyl ring. This is followd by a second step which involves the trnsfer of a hydrogen atom from the ortho position to C-1. It is inferred from the corelation with the mesomeric effect (σ_R⁺) of substituents that this step is rate determining. Loss of OH^˙ and X^˙ proceed via the same cyclohexadiene type intermediate ion but, depending upon the substituent, other pathways are also followed.     

Structure and fragmentation mechanism of [C3H7S]+ ions

From a comparison of the metastable ion bundance ratios for loss of C₂H₄ and H₂S from [C₃H₇S]⁺ ions in a series of alkyl thio ethers and thiols it was concluded that in most compunds these ion s isomerize to a common structure prior to decomposition in the first field free region. The mechanism for C₂H₄ loss from the [C₃H₇S]⁺ ion gen erated from CH₃SCH₂CH₃ was investigated in detail using ¹³C and ²H labelling. A rearrangement with formation of a cyclic intermediate prior to the decompistion reaction is proposed. The fragmentation is preceded by extensive hydrogen scrabling. The carbon atoms of the expelled C₂H₄ molecule are those of the CH₂?CH₃ moiety.     

Synthesis and structural studies of hydrazino-crown ethers and thermodynamic studies of their interaction with transition,and post-transition metal ions and methyl ammonium ion

Hydrazino-crown ethers have been synthesized in only 3 or 4 steps starting from 1,2-diacetylhydrazine. The X-ray crystal structure of protonated hydrazino-19-crown-7 (2) showed that one of the hydrazino nitrogen atoms was directed outside the ring cavity. A solvent methanol molecule is held in the cavity of the host ligand by three hydrogen bonds involving two hydrogen atoms bonded to nitrogens of the ligand and the alcohol hydrogen of the methanol. The logK values for the interaction of2 with CH₃NH ₊ ³ , Ag⁺, Pb²⁺, and Cd²⁺ were much less than those for the interaction of symmetrical triaza-l8-crown-6 (5) with the same cations. Hydrazino-crown2 reduced silver ions to silver metal when a solution of2 and silver ions in DMSO was allowed to stand for several days.