Effect of alkali metal cationization on the collision-induced decomposition of alkyl per-O-acetyl-2-deoxy-2-halo-α-O-mannopyranosides

The effect of alkali metal cationization on the collision-induced decomposition of alkyl per-O-acetyl-2-deoxy-2-bromo-and-iodo-α-O-mannopyranosides was studied. The bromo sugars gave fairly abundant MH⁺, whereas for the iodo sugars the MH⁺ ions were insignificant. However, both the bromo and the iodo derivatives gave abundant M + alkali metal ion complexes. In contrast to the behaviour of the MH⁺ ion, the [M + Li]⁺, [M + Na]⁺ and [M + K]⁺ ions of these compounds do not decompose by loss of the C(1) substituent. Elimination of AcOH is the preferred fragmentation pathway of [M + Cat]⁺. Elimination of HX occurs only after loss of AcOH and CH₂CO from MH⁺, whereas [M + Cat]⁺ directly loses HX. The elimination of HX is more pronounced from [M + Na]⁺ and [M + K]⁺ than from [M + Li]⁺. Loss of AcOLi is an additional fragmentation route observed in the case of the decomposition of [M + Li]⁺ ion.     

Positive chemical ionization triple‐quadrupole mass spectrometry and ab initio computational studies of the multi‐pathway fragmentation of phthalates

We report the first positive chemical ionization (PCI) fragmentation mechanisms of phthalates using triple‐quadrupole mass spectrometry and ab initio computational studies using density functional theories (DFT). Methane PCI spectra showed abundant [M + H]⁺, together with [M + C₂H₅]⁺ and [M + C₃H₅]⁺. Fragmentation of [M + H]⁺, [M + C₂H₅]⁺ and [M + C₃H₅]⁺ involved characteristic ions at m/z 149, 177 and 189, assigned as protonated phthalic anhydride and an adduct of phthalic anhydride with C₂H₅⁺ and C₃H₅⁺, respectively. Fragmentation of these ions provided more structural information from the PCI spectra. A multi‐pathway fragmentation was proposed for these ions leading to the protonated phthalic anhydride. DFT methods were used to calculate relative free energies and to determine structures of intermediate ions for these pathways. The first step of the fragmentation of [M + C₂H₅]⁺ and [M + C₃H₅]⁺ is the elimination of [R? H] from an ester group. The second ester group undergoes either a McLafferty rearrangement route or a neutral loss elimination of ROH. DFT calculations (B3LYP, B3PW91 and BPW91) using 6‐311G(d,p) basis sets showed that McLafferty rearrangement of dibutyl, di(‐n‐octyl) and di(2‐ethyl‐n‐hexyl) phthalates is an energetically more favorable pathway than loss of an alcohol moiety. Prominent ions in these pathways were confirmed with deuterium labeled phthalates. Copyright © 2010 John Wiley & Sons, Ltd.     

Structure and fragmentation of [C6H13O]+ ions formed by chemical ionization

The structure and fragmentation of eight [C₆H₁₃O] ⁺ ions formed by protonation of C₆H₁₂O carbonyl compounds in the gas phase have been investigated using isotopic labeling and metastable ion studies to investigate the fragmentation reactions and collisional dissociation studies to probe ion structures. Protonated 3-methyl-2-pentanone and protonated 2-methyl-3-pentanone readily-interconvert by pinacolic-retro-pinacolic rearrangements; the remaining six ions represent stable ion structures, although in many cases fragmentation is preceded by pinacolic-type rearrangements. Unimolecular (metastable ion) fragmentation of the [C₆H₁₃O] ⁺ species occurs by elimination of H₂O, C₃H₆, C₄H₈ and C₂H₄O. The last three elimination reactions appear to occur through the intermediacy of a proton-bound complex of a carbonyl compound and an olefin, with the proton residing with the species of higher proton affinity on decomposition of the complex.     

Gas-phase fragmentation reactions of protonated glycerol and its oligomers: Metastable and collision-induced dissociation reactions,associated deuterium isotope effects and the structure of [C3H5O]+, [C2H5O]+, [C2H4O]+. and [C2H3O]+ ions

The chemistry of glycerol subjected to a high-energy particle beam was explored by studying the mass spectral fragmentation characteristics of gas-phase protonated glycerol and its oligomers by using tandem mass spectrometry. Both unimolecular metastable and collision-induced dissociation reactions were studied. Collision activation of protonated glycerol results in elimiation of H₂O and CH₃OH molecules. The resulting ions undergo further fragmentations. The origin of several fragment ions was established by obtaining their product and precursor ion spectra. Corresponding data for the deuterated analogs support those results. The structures of the fragment ions of compositions [C₃H₅O]⁺, [C₂H₅O]⁺, [C₂H₄O]^+. and [C₂H₃O]⁺ derived from protonated glycerol were also identified. Proton-bound glycerol oligomers fragment principally via loss of neutral glycerol molecules. Dissociation of mixed clusters of glycerol and deuterated glycerol displays normal secondary isotope effects.     

Mechanism of CO loss for protonated 1-indanone and isomeric [C9H9O]+ ions

In order to establish the mechanism of CO loss occurring during metastable decomposition of protonated 1-indanone, fragmentations of monocyclic [C₉H₉O]⁺ isomers have been studied. These ions of known structure were prepared by CI protonation and fragmentation of the corresponding acids chlorides. It is demonstrated that the wide component of the [MH? CO]⁺ metastable peak induced by protonated 1-indanone fragmentation is the result of fragmentation of the [C₆H₅CH₂CH₂CO]⁺ isomer ion.     

Why Are B ions stable species in peptide spectra?

Protonated amino acids and derivatives RCH(NH₂)C(+O)X · H⁺ (X = OH, NH₂, OCH₃) do not form stable acylium ions on loss of HX, but rather the acylium ion eliminates CO to form the immonium ion RCH = NH ₂ ⁺ . By contrast, protonated dipeptide derivatives H₂NCH(R)C(+O)NHCH(R′)C(+O)X · H⁺ [X = OH, OCH₃, NH₂, NHCH(R″)COOH] form stable B₂ ions by elimination of HX. These B₂ ions fragment on the metastable ion time scale by elimination of CO with substantial kinetic energy release (T _1/2 = 0.3–0.5 eV). Similarly, protonated N-acetyl amino acid derivatives CH₃C(+O)NHCH(R′)C(+O)X · H⁺ [X = OH, OCH₃, NH₂, NHCH(R″)COOH] form stable B ions by loss of HX. These B ions also fragment unimolecularly by loss of CO with T _1/2 values of ～ 0.5 eV. These large kinetic energy releases indicate that a stable configuration of the B ions fragments by way of activation to a reacting configuration that is higher in energy than the products, and some of the fragmentation exothermicity of the final step is partitioned into kinetic energy of the separating fragments. We conclude that the stable configuration is a protonated oxazolone, which is formed by interaction of the developing charge (as HX is lost) with the N-terminus carbonyl group and that the reacting configuration is the acyclic acylium ion. This conclusion is supported by the similar fragmentation behavior of protonated 2-phenyl-5-oxazolone and the B ion derived by loss of H-Gly-OH from protonated C₆H₅C(+O)-Gly-Gly-OH. In addition, ab initio calculations on the simplest B ion, nominally HC(+O)NHCH₂CO⁺, show that the lowest energy structure is the protonated oxazolone. The acyclic acylium isomer is 1.49 eV higher in energy than the protonated oxazolone and 0.88 eV higher in energy than the fragmentation products, HC(+O)N⁺H = CH₂ + CO, which is consistent with the kinetic energy releases measured.     

A study of metal chelation of dinucleotide analogs in the gas phase by fast-atom bombardment mass spectrometry

Fast-atom bombardment (FAB) mass spectrometry was used to investigate the interaction of proton and alkali metal ions with dinucleotide analogs such as T-n-T (T = thymine moiety, n = polyether chain, e.g., triethylene, tetraethylene, pentaethylene, and hexaethylene ether 1–4), A-n-T (A = adenine unit 5–8), and T-n-OMe (9–12) in 3-nitrobenzyl alcohol matrix. The [M + H]⁺ ion is the most abundant ion for the A-n-T series, whereas in 1–4 and 9–12 the (TC₂H₄)⁺ ion is the most abundant. Formation of [M + H -C₂H₄O]⁺ ions, a characteristic fragmentation of crown ethers under electron ionization, is observed for compounds 1–12 and is more pronounced in 6 and 7. An abundant [M ? H]^? ion is observed for all the compounds studied under negative ion FAB due to the presence of the (-CO-NH-CO-) group of thymine, an indication of existence of intramolecular H bonding. The FAB mass spectra of 1–12 with alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) showed formation of abundant metal-coordinated ions ([M + Met]⁺ and [TC₂H₄ + Met]⁺). Compounds 3, 4, 6, 7, and 10–12 showed ions due to the substitution of the thymine moiety by a hydroxyl group ([M + Met ? 108]⁺, Met = metal ion). For compound 3 alone, substitution of two thymine groups ([M + Met - 216]⁺) was observed. Metastable ion studies were used to elucidate the structures of these potentially significant ions, and the ion formule were confirmed with high resolution measurements. Selectivity toward metal complexation with ligand size was seen in the T-n-T and A-n-T series and was even more pronounced in A-n-T series. These dinucleotide analogs fall in the following order of chelation of alkali metal ions, acyclic glymes < dinucleotide analogs (acyclic glymes substituted with nitrogen bases) < crown ethers, which places them in perspective as receptor models.     

Mass spectrometric fragmentation of isomeric 2-alkyl-substituted 1,3-indandiones and 3-alkylidenephthalides: a seven-step consecutive isomerization of regular and distonic molecular radical cations

The electron impact-induced fragmentation of 2,2-dimethyl- and 2-ethyl-1,3-indandione, 1 and 2, and their isomers, 3-isopropylidene- and 3-propylidenephthalide, 3 and 4, respectively, was studied in detail by mass-analysed ion kinetic energy (MIKE) and collision-induced dissociation (CID-MIKE) spectrometry, including ²H and ¹³C. labelled analogues of 1 and 2. In all regimes of internal energy, the molecular ions 1^+. ? 4^+. interconvert by up to seven consecutive, reversible isomerization steps prior to the main fragmentation processes, viz. loss of CH₃^. and C₂H₄. 1,3-Indandione and 3-methylenephthalide ions with identical alkylidene moieties (i.e. 1^+.?3^+. and 2^+.?4^+.) equilibrate rapidly and completely prior to fragmentation, whereas these pairs of isomers interconvert only slowly via a five-step rearrangement of the indandione ions 1^+.?2^+.. Distinct from the behaviour of simpler ionized carbonyl species, a 1,2-C shift of a (formally) neutral carbonyl group is found to occur along with that of a protonated one. Also distinct from simpler cases, methyl loss does not take place from the ionized enol intermediates formed within the interconversion 1^+.?2^+. of the diketone ions but rather from the n-propylidenephthalide ions 4^+.. This follows from CID-MIKE spectrometry of the [M ? CH₃]⁺ ions of 1–4 and two reference C₁₀H₇O₂⁺ (m/z 159) ions of authentic structures (protonated 2-methylene-1,3-indandione and protonated 1,4-naphthoquinone). The characteristic CID fragmentation of the C₁₀H₇O₂⁺ ions is rationalized. Finally, the multistep isomerization of ionized 1,3-indandiones apparently also extends to higher homologues [e.g. 5^+. from 2-ethyl-2-methyl-1,3-indandione (5) and 6^+. from 2,2-diethyl-1,3-indandione (6)]: the ionized phthaloyl group of 1,3-indandione radical cations 1^+., 2^+., 5^+. and 6^+., originally attached with its two acyl functionalities to the same carbon of the aliphatic chain, performs, in fact, a ‘multi-step migration’.     

Determination of orders of relative alkali metal ion affinities of crown ethers and acyclic analogs by the kinetic method

Ladders of relative alkali ion affinities of crown ethers and acyclic analogs were constructed by using the kinetic method. The adducts consisting of two different ethers bound by an alkali metal ion, (M₁ + Cat + M₂)⁺, were formed by using fast atom bombardment ionization to desorb the crown ethers and alkali metal ions, then collisionally activated to induce dissociation to (M₁ + Cat)⁺ and (M₂ + Cat)⁺ ions. Based on the relative abundances of the cationized ethers formed, orders of relative alkali ion affinities were assigned. The crown ethers showed higher affinities for specific sizes of metal ions, and this was attributed in part to the optimal spatial fit concept. Size selectivities were more pronounced for the smaller alkali metal ions such as Li⁺, Na⁺, and K⁺ than the larger ions such as Cs⁺ and Rb⁺. In general, the cyclic ethers exhibited greater alkali metal ion affinities than the corresponding acyclic analogs, although these effects were less dramatic as the size of the alkali metal ion increased.     

Intermetallic Competition in the Fragmentation of Trimetallic Au–Zn–Alkali Complexes

Cationization is a valuable tool to enable mass spectrometric studies on neutral transition‐metal complexes (e.g., homogenous catalysts). However, knowledge of potential impacts on the molecular structure and catalytic reactivity induced by the cationization is indispensable to extract information about the neutral complex. In this study, we cationize a bimetallic complex [AuZnCl₃] with alkali metal ions (M⁺) and investigate the charged adducts [AuZnCl₃M]⁺ by electrospray ionization mass spectrometry (ESI‐MS). Infrared multiple photon dissociation (IR‐MPD) in combination with density functional theory (DFT) calculations reveal a μ³ binding motif of all alkali ions to the three chlorido ligands. The cationization induces a reorientation of the organic backbone. Collision‐induced dissociation (CID) studies reveal switches of fragmentation channels by the alkali ion and by the CID amplitude. The Li⁺ and Na⁺ adducts prefer the sole loss of ZnCl₂, whereas the K⁺, Rb⁺, and Cs⁺ adducts preferably split off MCl₂ZnCl. Calculated energetics along the fragmentation coordinate profiles allow us to interpret the experimental findings to a level of subtle details. The Zn²⁺ cation wins the competition for the nitrogen coordination sites against K⁺, Rb⁺, and Cs⁺ , but it loses against Li⁺ and Na⁺ in a remarkable deviation from a naive hard and soft acids and bases (HSAB) concept. The computations indicate expulsion of MCl₂ZnCl rather than of MCl and ZnCl₂.     

Destabilized carbenium ions: α-carbomethoxy-α,α-dimethyl-methyl cations

Tertiary α-carbomethoxy-α,α-dimethyl-methyl cations a have been generated by electron impact induced fragmentation from the appropriately α-substituted methyl isobutyrates 1–4. The destabilized carbenium ions a can be distinguished from their more stable isomers protonated methyl methacrylate c and protonated methyl crotonate d by MIKE and CA spectra. The loss of I^— and Br˙ from the molecular ions of 1 and 2, respectively, predominantly gives rise to the destabilized ions a, whereas loss of Cl˙ from [3]^{+ ˙} results in a mixture of ions a and c. The loss of CH₃˙ from [4]⁺˙ favours skeletal rearrangement leading to ions d. The characteristic reactions of the destabilized ions a are the loss of CO and elimination of methanol. The loss of CO is associated by a very large KER and non-statistical kinetic energy release (T₅₀ = 920 meV). Specific deuterium labelling experiments indicate that the α-carbomethoxy-α,α-dimethyl-methyl cations a rearrange via a 1,4-H shift into the carbonyl protonated methyl methacrylate c and eventually into the alkyl-O protonated methyl methacrylate before the loss of methanol. The hydrogen rearrangements exhibit a deuterium isotope effect indicating substantial energy barriers between the [C₅H₉O₂]⁺ isomers. Thus the destabilized carbenium ion a exists as a kinetically stable species within a potential energy well.     

Tricaesium tris(oxalato‐κ2O1,O2)chromate(III) dihydrate

The title compound, Cs₃[Cr(C₂O₄)₃]·2H₂O, has been synthesized for the first time and the spatial arrangement of the cations and anions is compared with those of the other members of the alkali metal series. The structure is built up of alternating layers of either the d or l enantiomers of [Cr(oxalate)₃]³⁻. Of note is that the distribution of the [Cr(oxalate)₃]³⁻ enantiomers in the Li⁺, K⁺ and Rb⁺ tris(oxalato)chromates differs from those of the Na⁺ and Cs⁺ tris(oxalato)chromates, and also differs within the corresponding BEDT‐TTF [bis(ethylenedithio)tetrathiafulvalene] conducting salts. The use of tris(oxalato)chromate anions in the crystal engineering of BEDT‐TTF salts is discussed, wherein the salts can be paramagnetic superconductors, semiconductors or metallic proton conductors, depending on whether the counter‐cation is NH₄⁺, H₃O⁺, Li⁺, Na⁺, K⁺, Rb⁺ or Cs⁺. These materials can also be superconducting or semiconducting, depending on the spatial distribution of the d and l enantiomers of [Cr(oxalate)₃]³⁻.     

Unimolecular and collision-induced dissociation reactions of peracetylated methyl glucopyranoside

The mechanism of the collision-induced fragmentation of peracetylated methyl-α-D-glucopyranoside was investigated using deuterium-labelled acetates and sequential mass spectrometry. Loss of the substituent at C(1), the anomeric carbon, yields an ion of m/z 331, [C₁₄H₁₉O₉]⁺. This ion further dissociates via two pathways, the first including m/z 271, [C₁₂H₁₅O₇]⁺, 169, [C₈H₉O₄]⁺ and 109, [C₆H₅O₂]⁺, and the second including m/z 211, [C₁₀H₁₁O₅]⁺, 169, [C₈H₉O₄]⁺ and 127 [C₆H₇O₃]⁺. The first path proceeds via loss of acetate at C(3), followed by a single-step concerted loss of acetates from C(2) and C(4), and ending with loss of acetate from C(6). The second path proceeds predominantly via loss of acetates from C(3) and C(4), elimination of ketene from the C(2)-acetate and finally loss of ketene from the acetate at C(6). This path is also characterized by an ill-defined series of parallel decomposition reactions involving acetates from other sites on the molecule. At low collision energy, and in the absence of collision gas (unimolecular reaction conditions), the former pathway predominates; m/z 331 dissociates via loss of acetate at C(3), followed by a single-step concerted loss of acetates from C(2) and C(4).     

Complexation of decamethylcucurbit[5]uril with alkali metal ions

Four coordination compounds, namely [Na(H₂O)(H₂O)₂⊂C₄₀H₅₀N₂₀O₁₀](C₆H₆O₂)₂Cl·8H₂O (1), [K₂(H₂O)₂(H₂O)⊂C₄₀H₅₀N₂₀O₁₀](C₆H₆O₂)₂Cl₂·7H₂O (2), [Rb₂(H₂O)₂(H₂O)⊂C₄₀H₅₀N₂₀O₁₀](C₆H₆O₂)₂Cl₂·7H₂O (3) and [Cs(H₂O)₂(H₂O⊂C₄₀H₅₀N₂₀O₁₀)](C₆H₆O₂)₂Cl·6H₂O (4), were obtained by the reactions of the corresponding alkali metal salts with decamethylcucurbit[5]uril (Me₁₀Q[5]) in the presence of hydroquinone, and their structures were determined by single-crystal X-ray diffraction. The results revealed that in compounds 1 and 4 each Me₁₀Q[5] ligand coordinates one Na⁺ or Cs⁺ ion to form a molecular bowl structure, while in compounds 2 and 3 each Me₁₀Q[5] ligand coordinates two K⁺ or Rb⁺ ions to form a closed molecular capsule structure, and adjacent molecular capsules bridge each other through water molecules to form 1D coordination polymers. In addition, we found that the coordination distances for the metal ions and the height of the metal ions out-of-portal-plane for the four compounds are in the same order, 1 < 2 < 3 < 4, which is attributed to the fact that the radius of alkali cations is in the order Na⁺ < K⁺ < Rb⁺ < Cs⁺. Although each portal of Q[6] binds with two alkali cations (not including Cs⁺), the Q[6]-based alkali cations complexes display similar structural trends.     

Gas phase reactivity of cation radicals and protonated species from isomeric C4H8S thioethers

Four isomeric thioethers, 2,3-dimethylthiirane ( 1 ), 2-methylthietane ( 2 ), tetrahydrothiophene ( 3 ), and allyl methyl thioether ( 4 ), have been subjected to mass spectrometric analysis in the gas phase, under electron impact (El) and chemical ionization (CI) conditions. The metastable molecular ions M⁺′ generated from 1-4 under EI (70 eV) conditions give distinct patterns of unimolecular fragmentation, thus indicating that isomer interconversion reactions are slower than dissociation (a possible exception, to some extent, is the case of [M₂]⁺′ and [M₂]⁺′). The change of the relative intensities of some prominent peaks with increasing ion lifetime (decomposition within the ion source, the first, and the second field-free regions of the mass spectrometer) is pointed out. Metastable [MH]⁺ ions, generated from 1-4 in chemical ionization experiments with CH₄, all eliminate H₂ and H₂S, although in different relative proportions. In addition to these processes protonated 4 also undergoes loss of C₂H₄ and C₃H₆, likely from a C-protonated structure.     

Localization of Fatty Acyl and Double Bond Positions in Phosphatidylcholines Using a Dual Stage CID Fragmentation Coupled with Ion Mobility Mass Spectrometry

A high content molecular fragmentation for the analysis of phosphatidylcholines (PC) was achieved utilizing a two-stage [trap (first generation fragmentation) and transfer (second generation fragmentation)] collision-induced dissociation (CID) in combination with travelling-wave ion mobility spectrometry (TWIMS). The novel aspects of this work reside in the fact that a TWIMS arrangement was used to obtain a high level structural information including location of fatty acyl substituents and double bonds for PCs in plasma, and the presence of alkali metal adduct ions such as [M?+?Li]⁺ was not required to obtain double bond positions. Elemental compositions for fragment ions were confirmed by accurate mass measurements. A very specific first generation fragment ion m/z 577 (M-phosphoryl choline) from the PC [16:0/18:1 (9Z)] was produced, which by further CID generated acylium ions containing either the fatty acyl 16:0 (C₁₅H₃₁CO⁺, m/z 239) or 18:1 (9Z) (C₁₇H₃₃CO⁺, m/z 265) substituent. Subsequent water loss from these acylium ions was key in producing hydrocarbon fragment ions mainly from the α-proximal position of the carbonyl group such as the hydrocarbon ion m/z 67 (+H₂C-HC?=?CH-CH?=?CH₂). Formation of these ions was of important significance for determining double bonds in the fatty acyl chains. In addition to this, and with the aid of ¹³C labeled lyso-phosphatidylcholine (LPC) 18:1 (9Z) in the ω-position (methyl) TAP fragmentation produced the ion at m/z 57. And was proven to be derived from the α-proximal (carboxylate) or distant ω-position (methyl) in the LPC.     

Functional group interaction in the fragmentation of protonated 2,7-octanedione

The fragmentation of 2,7-octanedione, induced by chemical ionization with methane as a reagent gas (CI (CH₄)), is shown to be extensively governed by the interaction of the two carbonyl groups. Tandem mass spectrometry reveals that a sequential loss of H₂O and C₂H₄O from the [M + H]⁺ ion competes with sequential loss of H₂O and C₆H₁₀, and that both processes occur via the same [MH - H₂O]⁺ intermediate. This intermediate is likely to be formed via intramolecular gas-phase aldol condensation and subsequent dehydration. The resulting C(1) protonated 1-acetyl-2-methylcyclopentene structure readily accounts for the observed further decomposition to CH₃C?O⁺ and 1-methylcyclopentene (C₆H₁₀) or, alternatively, to [C₆H₉]⁺ (e. g. 1-methylcyclopentenylium) ions and acetaldehyde (C₂H₄O). Support for this mechanistic rationale is derived from deuterium isotope labelling and low-energy collision-induced dissociation (CID) of the [MH - H₂O]⁺ ion. The common intermediate shows a CID behaviour indistinguishable by these techniques from that of reference ions, which are produced by gas-phase protonation of the authentic cyclic aldol or by gas-phase addition of an acetyl cation to 1-methylcyclopentene in a CI (CH₃COOCH₃) experiment.     

An energy-resolved study of the fragmentation of ionized 1-Penten-3-ol

A detailed energy-resolved study of the fragmentation of CH₂?CHCH(OH)CD₂CD₃ (1-d₅) has been carried out using metastable ion studies and charge exchange techniques, combined with collision-induced dissociation studies to establish the structures of fragment ions. At low internal energies (metastable ions) the molecular ion of 1-d₅ rearranges to the 3-pentanone structure and fragments by loss of C₂H₅ or C₂D₅ leading to the acyl structure, [CH₃CH₂C?O]⁺ or [CD₃CD₂C?O]⁺, for the fragment ion. However, with increasing internal energy of the molecular ion this rearrangement process decreases rapidly in importance and loss of C₂D₅ by direct cleavage, leading to [CH₂?CHCH?OH]⁺, becomes the dominant fragmentation reaction. As a result the [C₃H₅O]⁺ ion seen in the electron impact mass spectrum of 1-penten-3-ol has predominantly the protonated acrolein structure.     

Chemical ionization mass spectra of selected C3H6O compounds

The chemical ionization mass spectra of five isomers of C₃H₆O (acetone, propionaldehyde, oxetane, propylene oxide and allyl alcohol) have been determined using a variety of reagent gases (H₂, D₂, N₂/H₂, CO₂/H₂ and CO/H₂). The [C₃H₇O]⁺ ions produced by protonation of these isomers undergo very similar reactions to those reported for analogous [C₃H₇O]⁺ metastable ions; however, decomposing ions generated by chemical ionization appear to have somewhat higher internal energies. The results of ²H labelling studies (D₂ reagent gas or labelled analogues of C₃H₆O) indicate that protonation occurs mainly on oxygen and are consistent with previous investigations of metastable oxonium ions. The protonated acetone ion is particularly stable, in agreement with the higher activation energies for fragmentation of this isomer than for other [C₃H₇O]⁺ structures. As the calculated heat of protonation of C₃H₆O is reduced by changing the reagent gas, so the extent to which fragmentation occurs decreases. This is discussed in the context of competition between fragmentation and collisional stabilization of the excited [C₃H₇O]⁺* ion. It is concluded that on average a large fraction (approaching 1) of the exothermicity of the protonation reaction resides in the [C₃H₇O]⁺* ions produced initially.     

Gas‐phase fragmentation reactions of protonated benzofuran‐ and dihydrobenzofuran‐type neolignans investigated by accurate‐mass electrospray ionization tandem mass spectrometry

We have investigated gas‐phase fragmentation reactions of protonated benzofuran neolignans (BNs) and dihydrobenzofuran neolignans (DBNs) by accurate‐mass electrospray ionization tandem and multiple‐stage (MSⁿ) mass spectrometry combined with thermochemical data estimated by Computational Chemistry. Most of the protonated compounds fragment into product ions B ([M + H–MeOH]⁺), C ([ B –MeOH]⁺), D ([ C –CO]⁺), and E ([ D –CO]⁺) upon collision‐induced dissociation (CID). However, we identified a series of diagnostic ions and associated them with specific structural features. In the case of compounds displaying an acetoxy group at C‐4, product ion C produces diagnostic ions K ([ C –C₂H₂O]⁺), L ([ K –CO]⁺), and P ([ L –CO]⁺). Formation of product ions H ([ D –H₂O]⁺) and M ([ H –CO]⁺) is associated with the hydroxyl group at C‐3 and C‐3′, whereas product ions N ([ D –MeOH]⁺) and O ([ N –MeOH]⁺) indicate a methoxyl group at the same positions. Finally, product ions F ([ A –C₂H₂O]⁺), Q ([ A –C₃H₆O₂]⁺), I ([ A –C₆H₆O]⁺), and J ([ I –MeOH]⁺) for DBNs and product ion G ([ B –C₂H₂O]⁺) for BNs diagnose a saturated bond between C‐7′ and C‐8′. We used these structure‐fragmentation relationships in combination with deuterium exchange experiments, MSⁿ data, and Computational Chemistry to elucidate the gas‐phase fragmentation pathways of these compounds. These results could help to elucidate DBN and BN metabolites in in vivo and in vitro studies on the basis of electrospray ionization ESI‐CID‐MS/MS data only.