Metastable Molecular Ions Research Articles

Unimolecular Gas-Phase Reactions of Diethyl Phthalate, Isophthalate, and Terephthalate upon Electron Ionization

Unimolecular gas-phase reactions of diethyl phthalate (1), isophthalate (2), and terephthalate (3), upon electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and deuterium labelling. The metastable molecular ions (1)+ decompose to give exclusively the ions m/z 176 ([M – CH3CH2OH]+) and not the ions by the loss of CH3CH2O as proposed earlier in the literature. The metastable molecular ions (2)+ and (3)+ fragment differently from (1)+ and lead not only to the formation of the major fragment ions m/z 194 ([M − CH2CH2]+) via a McLafferty rearrangement but also to minor fragment ions m/z 193 ([M – CH2CH3]+).Yet, molecular ions decomposing in the ion source all show as primary fragmentation channel the loss of CH3CH2O to give the ions at m/z 177, which further dissociate to give the ions at m/z 149 through the loss of C2H4 or CO, indicating the resulting ions are +COC6H4COOH and +C6H4COOCH2CH3. The +COC6H4COOH ions decompose into the m/z 121, 93, and 65 ions by the consecutive losses of three carbon monoxide molecules, respectively. Prior to the second CO loss, a migration of the OH group to the benzene ring occurs. During the metastable fragmentation of the +C6H4COOCH2CH3 ions no ethoxy migration occurs, in contrast to the methoxy migration taking place in the metastable decomposition of the lower homologue +C6H4COOCH3 ions.

Multiple Hydrogen Shifts Leading to Ammonia Loss from the Molecular Ions of Cyanocyclohexanes

The loss of ammonia from the metastable molecular ions of cyclic cyano compounds has been examined with the use of deuterium labeling and tandem mass spectrometry. Loss of ammonia is significant for ionized cyanocyclohexane, 1-methyl-, 4-methyl-, 4-cyano-and 4-phenyl-cyanocyclohexanes, 4-cyanopiperidine, cyanocycloheptane and 2-cyanonorbornane. By contrast, loss of ammonia is of minor importance (or absent) for the molecular ions of cyanocyclopentane, 2-methyl-cyanocyclohexane, 1-phenyl-cyanocyclohexane, 1-cyanocyclohexene, 4-cyanotetrahydrothiopyran, 2-cyano-5-norbornene and isocyanocyclohexane. Deuterium labeling of cyanocyclohexane reveals the occurrence of an H-shift from the 4-position to the cyano function, followed by a 1,2-H shift from the 1-position to the C-atom of the newly-formed–CNH group. Subsequently, a series of H-shifts leads to a distonic ion that is formulated as an N-protonated methylamine attached to a cyclohexadienyl radical. Loss of ammonia ensues and leads to ionized toluene as indicated by collision-induced dissociation experiments. For 4-phenyl-cyanocyclohexane, the metastable ions of the cis- and trans-isomers display, essentially, the same unimolecular chemistry. Briefly, the labeling of 4-phenyl-cyanocyclohexane indicates the following: (i) the H atom at the 4-position of the cyclohexane ring is incorporated, to a minor extent, in the ammonia molecule, (ii) loss of NHD2 predominates in the reactions of the molecular ions of 2,2,6,6-d4-4-phenyl-cyanocyclohexane and (iii) the ionized 3,3,5-d3-labeled species expels mainly NH2D. In addition, the metastable molecular ions of the 4-[d5-phenyl]-cyanocyclohexane expel NH3 and NH2D in a ratio of 35:65. A mechanistic scheme is proposed that is consistent with the labeling results for 4-phenyl-cyanocyclohexane as well as the indicated formation of ionized 4-methylbiphenyl as the product ion of ammonia loss.

Dissociation behavior of ionized valeramide: Part III. An unprecedented temperature effect on the C 3/C 2 branching ratio and its implications for metastable ion dissociations

The competing formations of C 2- and C 3-fragments in the unimolecular decay of mass-selected valeramide cation-radicals are analyzed in detail. It turns out that the different branching ratios observed in the various experiments are due not only to the internal energy of the ions but also to that of the neutral precursors. The latter is manifested by a pronounced effect of the temperature of the ion source on the C 3/C 2 branching ratios in the metastable ion spectra of ionized valeramide. Exploratory studies indicate that the effect is not limited to valeramide in that similar temperature phenomena are observed for the metastable molecular ions of hexanoic acid and its amide.

Isomeric dependence of the formation of ion/neutral complexes in dissociation reactions of ionized propoxypyridines

The mechanistic aspects of the dissociation reactions of the molecular ions of isomeric propoxypyridines have been examined with the use of deuterium labelling and tandem mass spectrometry. The ionized 2- and 3-propoxypyridines dissociate predominantly by the loss of propene, whereas the molecular ions of 4-propoxy- and 4-isopropoxypyridine react by the competing losses of a propene molecule and an allyl radical. The loss of propene from the metastable molecular ions of the 2-isomer involves a 1,5-H shift from the 2-position of the propyl group to the ring as evidenced by the labelling experiments. For the metastable molecular ions of 3-propoxypyridine, the results are in agreement with propene loss by a pathway involving formation of a [C 5H 4NO ·/(CH 3) 2CH +] complex which reacts further by proton transfer prior to dissociation. In contrast to these findings, interchange between the hydrogen atoms at the 2-position of the propyl group, and the 3- or 5-position of the pyridine ring, occurs in ionized 4-propoxypyridine. This interchange can precede the formation of a [C 5H 4NO ·/(CH 3) 2CH +] complex and the occurrence of proton transfer within this species. The [C 5H 5NO +·/C 3H 6] complex formed in the proton transfer step may either expel propene or react by hydrogen atom transfer prior to the loss of an allyl radical. For the metastable ions of 4-isopropoxypyridine, interchange between a hydrogen atom from one of the methyl groups of the alkyl chain and the aromatic ring appears to be of minor importance.

The rearrangement and simple cleavage of metastable octanamine radical cations

The isomerization and fragmentation of the straight-chain octanamine radical cations was studied with the aid of deuterium labeling and composite quantum chemical calculations with the G3 and G2 (MP2, SVP) methods. Metastable octan-1-amine molecular ions isomerize to octan-2-amine ions prior to cleavage, and metastable octan-2-amine molecular ions rearrange to the octan-3-amine isomer, but octan-1-amine ions do not isomerize to octan-3-amine, nor are the reverse reactions observed. The competition between cleavage and isomerization is determined by the respective energy barriers and by the internal energy distribution of the metastable molecular ions, which in turn depends on the critical energy for simple cleavage of the molecular ions.

Mass spectrometric fragmentation reactions of aliphatic carboxamides. A reinvestigation using metastable ions reveals new rearrangements

The mass spectrometric fragmentations of aliphatic carboxamides have been reinvestigated by using caproamide 2 and its N-methyl- and N,N-dimethyl derivatives 3 and 4 as model compounds and by studying the unimolecular reactions of metastable molecular ions. Whereas the EI (70 eV) mass spectra of caproic acid 1 and the amides 2-4 suggest a very similar behavior of the unstable molecular ions of these compounds, the mass analyzed ion kinetic energy(MIRE) spectra of metastable molecular ions indicate distinct differences. Of the three fragmentation pathways observed in the MIKE spectra, by far the most important for the molecular ions of the acid 1 is the loss of an ethyl radical which results in the formation of protonated methacrylic acid by a 1,2-shift of the protonated carboxyl group in intermediate distonic ions. The metastable molecular ions of the amides 2-4 also fragment to a significant extent by loss of a propyl radical and by the McLafferty rearrangement, the latter in particular for the tertiary molecular ions of 4. An even more significant difference is the formation of protonated vinylacetamides instead of protonated methacrylamides from the molecular ions of the amides 2-4. An analysis of the MIKE spectra of specifically deuterated derivatives of 2-4 shows that first, in contrast to the reactions of metastable molecular ions of the acid 1, the McLafferty rearrangement of the amide ions 2-4 occurs without significant H/D exchange reactions involving D atoms at the gamma-position. This implies that the McLafferty rearrangement of the amide ions is energetically more favored. Second, it is shown that If-shifts from the alkyl chain to the amide group are followed by 1,5- and 1,6-shifts (long-distance shifts) of the protonated amide group in the intermediate distonic ions. Thus, the metastable molecular ions of aliphatic carboxamides are mainly distinguished from those of the corresponding carboxylic acids and methyl esters by a preference for a fragmentation by the McLafferty rearrangement and for long-distance shifts of the protonated functional group in intermediate distonic ions.

3-Aroyl-5-hydroxyflavones: synthesis and mechanistic studies by mass spectrometry

The synthesis and mass spectra of three 3-aroyl-5-hydroxyflavones are reported. The interpretation of the mechanistic pathways for the fragmentation of the metastable molecular ions of these compounds was achieved through the analysis of their mass-analysed ion kinetic energy (MIKE) spectra and of the B 2 /E spectra of a few fragment ions. Labelling of the hydroxyl proton with deuterium and the analysis of the MIKE spectra of a model compound with chlorine atoms in the 2',6'- and 2,6-positions led to a mechanism for the losses of OH . and HCO . which involves hydrogen migration from the 2- or 6-position to the 4-carbonyl oxygen atom. A mechanism for the loss of a neutral molecule of anisole from the molecular ion of the 3-aroyl-5-hydroxyflavone with a methoxyl group in the 4'- and 4-positions is also suggested. For the flavones with hydrogen or chlorine substituents at these positions, loss of a phenyl (or chlorophenyl) radical occurs instead.

Measurement of Decay Lifetime of Metastable Dye Molecular Ions by Using Ion Trap

We have demonstrated the existence of metastable excited rhodamine molecular ions when created in laser desorption ionization. Rhodamine ions were stored in a rf ion trap, and photodissociation was used to probe the metastable population as a function of trapping time. From the dependence of the population decay on buffer gas pressure and by extrapolating it to zero pressure we have obtained the lifetime of the metastable state of the ions in isolated condition.

Mass spectrometry of tert-butylnaphthalenes–a comparison with the unimolecular fragmentation of tert-butylbenzene

The unimolecular fragmentation of ionized 1- and 2-tert-butylnaphthalenes 1 and 2 have been investigated using mass analyzed ion kinetic energy (MIKE) spectrometry and specifically deuterated and 13 C labeled derivatives. Generally, the mass spectrometric fragmentations of 1 and 2 follow closely the reactions known for ionized tert-butylbenzene 3. Metastable molecular ions of 1, 2 and 3 lose exclusively a methyl radical yielding 2-naphthyl-2-propyl cations 1a and 2a or 2-phenyl-2propyl cations 3a. Further fragmentations of 1a and 2a include, as the main processes, the elimination of C 2H4 and of CH3 • . The loss of C2H4 is associated with a large kinetic energy release giving rise to a broad flat-topped peak in the MIKE spectra (as in the case of 3a) while loss of CH3 gives rise to a Gaussian shaped signal. The latter reaction is not observed for metastable 2-phenyl-2-propyl cations 3a. The fragmentations of labeled derivatives of 1 and 2 show that H/D exchange in 1a and 2a, between the methyl groups of the side chain and the naphthalene ring, preceding the losses of C 2H4 and of CH3 • , is much more extensive than in 3a. This H/D exchange decreases for metastable 1a and 2a decomposing in the second field-free region, and the interchange of the C atoms within the C 3H6 is less than for 3a. These observations are explained by H migrations between the side chain and the naphthalene rings of 1a and 2a, before the rearrangements within the side chain by the series of reversible 1,2-H shifts, of a 1,2-aryl shift, and of a 1,2-methyl shift known for the reactions of 3a. The differences observed for the reactions of 1a and 2a and of 3a are explained by the more basic aryl group of 1a and 2a. These fragmentation mechanisms are corroborated by an AM1 calculation of the minimum energy reaction pathways (MERP).

Isomerization of the molecular ion of cyclohexyl cyanide to an ionized bicycloheptane imine species competing with direct ring-opening

The mechanisms of the dissociation reactions of the molecular ion of cyclohexyl cyanide have been studied with nitrogen-15, carbon-13 and deuterium labelling in combination with tandem mass spectrometric experiments. The most important unimolecular reactions of ionized cyclohexyl cyanide involve the losses of CH • 3, NH 3, HCN, C 3H 3N and C 4H • 7. These reactions are all significant for the molecular ions dissociating in the ion source, whereas the metastable molecular ions react only by the competing losses of CH • 3, NH 3 and HCN. The results of the labelling experiments reveal that the carbon atoms at the 2- and 6-positions of the ring become structurally indistinguishable from the carbon atoms at the 3- and 5-positions prior to the losses of CH • 3 and HCN. This finding is suggested to be a result of a rearrangement of the molecular ion of cyclohexyl cyanide with formation of a symmetrical and ionized [2,2,1]-bicycloheptane-7-imine as the key intermediate. The elimination of C 3H 3N is indicated to involve a direct ring-opening of the molecular ion of cyclohexyl cyanide followed by dissociation to yield a C 4H +· 8 ion. The loss of a C 4H •; radical may involve the same initial step followed by an H-shift in the ring-opened species prior to dissociation.

Unimolecular HF Loss from the Molecular Ions of Fluorophenols and Fluoroanilines. A 'Ring-walk' Mechanism of a Fluorine Atom

The loss of HF is one of the main fragmentation processes in the metastable molecular ions of fluorophenols and fluoroanilines. By investigation of D-labelled compounds, it could be established that the eliminated HF molecule contains the hydroxylic or amino hydrogen atom. The other ring hydrogen atoms are not involved in the fragmentation. In order to rationalize this process, a ring-walk' mechanism of the fluorine atom is proposed.

Unimolecular HF Loss from the Molecular Ions of Fluorophenols and Fluoroanilines. A 'Ring‐walk' Mechanism of a Fluorine Atom

The loss of HF is one of the main fragmentation processes in the metastable molecular ions of fluorophenols and fluoroanilines. By investigation of D-labelled compounds, it could be established that the eliminated HF molecule contains the hydroxylic or amino hydrogen atom. The other ring hydrogen atoms are not involved in the fragmentation. In order to rationalize this process, a "ring-walk' mechanism of the fluorine atom is proposed.

Competing Reactions Leading to Propene Loss from the Molecular Ions of Aryln-Propyl Ethers

The mechanisms of the elimination of a propene molecule from the molecular ions of a series of 3- and 4-substituted aryl n-propyl ethers (YC 6 H 4 OC 3 H 7 , Y = H, CH 3 , CF 3 , NO 2 , CH 3 S and CH 3 O) were examined with the use of deuterium labelling and tandem mass spectrometry. Propene loss dominates in the ion-source reactions and is the exclusive process observed for the metastable molecular ions of most of the aryl n-propyl ethers. This process is concluded to proceed by two distinct pathways on the basis of the relative importances of the losses of C 3 H 4 D 2 and C 3 H 5 D from the ionized ethers labelled with two deuterium atoms at the β-position of the n-propyl group in combination with the results of an analysis of the product ion structures. One pathway involves intermediate formation of an ion-neutral complex composed of a YC 6 H 4 O . radical and a + CH(CH 3 ) 2 corbenium ion. This complex reacts further by proton transfer prior to dissociation with the formation of product ions, which have the same structure as the corresponding ionized and substituted phenols, YC 6 H 4 OH +. . The second pathway involves a reversible 1,5-H shift from the β-position of the propyl group to the 2- or 6-position of the aromatic ring with the formation of a distonic ion, which expels propene to afford the molecular ion of a substituted cyclohexa-2,4-dienone species. The first pathway prevails for most of the ionized ethers with the exception of the molecular ions of the 3-methoxy- and 3-methylthio-substituted ethers, which expel propene largely by the second pathway. In addition, the 1,5-H shift-initiated propene loss is particularly pronounced for the metastable molecular ions of these latter two ethers, suggesting that this reaction is associated with a lower critical energy than the reaction involving formation of an ion-neutral complex.

Intramolecular aromatic substitution and amino‐Claisen rearrangement in substituted N‐(2‐propynyl)anilines on electron impact

AbstractN‐(2‐Propynyl)anilines undergo amino‐Claisen rearrangement to a minor extent in the ion source, losing a molecule of HCN under electron impact conditions. However, metastable molecular ions with energies closer to threshold undergo Claisen rearrangement giving rise to more abundant [M − HCN]+· ions in the first field‐free region. Loss of a hydrogen from the molecular ion gives rise to the base peak in the mass spectrum of N‐(2‐propynyl)aniline. The hydrogen that is expelled for the formation of the [M − H]+ ion is observed to be from the amino nitrogen, propargylic carbon and the ortho‐carbon of the ring. The last process leads to a cyclic fragment involving intramolecular aromatic substitution. Double oxygen migration from the nitro group to the triple bond, due to the ortho effect, yields an abundant ion at m/z 105 in N‐(2‐propynyl)‐o‐nitroaniline. The proposed fragmentation pathways and ion structures are substantiated by high‐resolution data, B/E and B2/E linked‐scan spectra, collisionally activated dissociation–B/E linked‐scan spectra and deuterium isotopic labelling.

Formation and electrocyclization of gaseous α-phenyl- ortho-quinodimethane radical cations

α-Aryl- ortho-quinodimethane radical cations are generated by the loss of water via a two-step process (“ortho-effect”) from metastable molecular ions of 2-methylbenzhydrol derivatives. A competing fragmentation is the loss of the methyl substituent from metastable molecular ions which, however, is also observed for the corresponding meta and para isomers and which does not interfere with the formation of α-phenyl- ortho-quinodimethane radical cations. The reaction of these latter ions was studied with the aid of specifically labelled derivatives. The results can be interpreted by an electrocyclization of the α-phenyl- ortho-quinodimethane radical ions to form dihydroanthracene radical cations which decompose by loss of a hydrogen atom or a substituent from one of the phenyl rings in the case of substituted α-phenyl- ortho-quinodimethane radical cations. However, in competition with this cyclization process via a six-membered transition state the loss of a methyl radical from the α-phenyl- ortho-quinodimethane radical cations is observed which is the consequence of a cyclization via a five-membered transition state. This latter process is best depicted as an intramolecular aromatic substitution by a radical.

Fragmentation of the metastable molecular ion of methyl lactate: The formation of oxygen-protonated methanol [CH 3OH 2+ involving double hydrogen atom transfers

The spontaneous unimolecular dissociation reaction of methyl lactate (1) ionized by electron impact was investigated by a combination of mass-analyzed ion kinetic energy spectrometry and deuterium labeling. The metastable ions 1(+·) decompose in a variety of ways: four fragment peaks are observed at m/z 89, 76, 61, and 45, which correspond to the losses of ĊH3, CO, CH3ĊO, and ĊOOCH3, respectively. Double hydrogen atom transfer occurs in the third reaction.The source-generated m/z 61 ions decompose into oxygen-protonated methanols at m/z 33 ([CH3OH 2 (+) ]) by the loss of CO with double hydrogen atom migration. Both hydroxyl and methyne hydrogen atoms in 1 (+·) are present in the resultant protonated methanols.

Ion–neutral complex formation and hydrogen‐shifts in the alkene loss from ionized alkyl phenyl thioethers in the gas phase

AbstractThe mechanistic aspects of alkene loss from ionized ethyl‐and n‐propyl phenyl thioethers have been studied by use of deuterium labelling and tandem mass spectrometry. Loss of ethene from the molecular ion of ethyl phenyl thioether proceeds predominantly by a specific H‐shift from the β‐position of the ethyl group to the remaining part of the ion. In contrast, deuterium labelling reveals that elimination of propene from the molecular ions of the n‐propyl phenyl and n‐propyl 2,6‐dimethylphenyl thioethers involves three competing reactions: (i) a 1,2‐hydride shift assisted cleavage of the bond between the propyl group and the sulfur atom yielding an ion–neutral complex of a thiophenyoxy radical and a secondary propyl carbenium ion, which reacts further by proton transfer prior to dissociation; (ii) a partially reversible 1,5‐H shift from the β‐position to the 2‐ or 6‐position of the ring; and (iii) a 1,3‐H shift from the β‐position to the sulfur atom. The preference for transfer of a hydrogen atom from the β‐position is particularly pronounce for the reactions of the metastable molecular ions of the n‐propyl phenyl and n‐propyl 2,6‐dimethylphenyl thioethers. This indicates that the critical energies of the processes involving H‐shifts are lower than the critical energy for propene loss with intermediate formation of an ion–neutral complex. In addition to ethene elimination, ionized ethyl phenyl thioether expels a HS+· radical. This reaction is preceded by an interchange between the hydrogen atoms at the α‐methylene group and the hydrogen atoms at the 2‐ and 6‐positions of the ring as observed also for ionized methyl phenyl thioether.

Rearrangement by intermediate ion/neutral complexes during the McLafferty fragmentation of unsaturated ketones

The fragmentation of 1-phenylhex-5-ene-1-one 1 was studied by mass analyzed ion kinetic energy spectrometry of its deuterium and ''carbon labelled derivatives as a further example of interactions within ion/neutral complexes, The analysis of the MIKE spectra demonstrate two important reaction channels of metastable molecular ions, The first one is a McLafferty rearrangement by elimination of a 1,3-butadiene molecule, The second energetically even more favoured fragmentation corresponds to the loss of an ethyl radical. The MIKE spectra of labelled molecular ions, especially that of the (13)carbon labelled compound, indicate a skeletal rearrangement specifically of the butenyl moiety of the alkyl side chain by interchanging the terminal carbon atoms C-6 and C-5 with the inner C atoms C-3 and C-4 prior to the loss of the ethyl radical, This specific skeletal rearrangement is explained by an internal return within an intermediate ion/neutral complex f, consisting of the acetophenone enol radical cation and the neutral butadiene, by addition of the enol radical cation to the remote double bond of the diene, The second reaction channel is the irreversible cyclization of an intermediate distonic ion c leading to the formation of an ionized ethyl-alpha-dihydronaphthol d. This ion is stabilized by loss of an ethyl radical resulting in protonated alpha-naphthol e.

Metastable-ion mass spectra of insect pheromones

The laws of the fragmentation of the metastable molecular and key fragmentary ions of mono- and dienic alcohols, acetates, aldehydes, hydrocarbons, and epoxy compounds forming components of the pheromones of the turnip, tomato, and bollworm moths and other pests of agricultural crops are discussed, and the use of the laws found in the identification of compounds of this class has been demonstrated.

Influence of laser focusing in matrix‐assisted laser desorption/ionization

AbstractThe influence of laser focusing on the target in matrix‐assisted laser desorption/ionization (MALDI) has been studied. The emission of stable and metastable molecular ions of bovine insulin depends on both the laser energy and the distance between the target and the laser focal plane. The results clearly show that the laser fluence is the important parameter in MALDI. The fluence threshold of metastable‐ion emission is higher than that of stable‐ion emission. Beyond this threshold, the emission of stable ions reaches a plateau. This is due to the spatial inhomogeneity of the laser energy distribution. A phenomenological model including two fluence thresholds for the stable‐ and metastable‐ion emission has been developed and compared to the experimental data.