Gas-Phase Reactivity of Quinoline-Based Singlet Oxenium Cations.

Isomeric quinolyloxenium cations were generated in the gas phase in an ion trap mass spectrometer to explore their reactions. The structures of some products were identified via collision-activated dissociation experiments involving model compounds to demonstrate that they have the expected heavy atom connectivity. The lack of radical reactions suggests that the cations have closed-shell singlet electronic ground states. Calculations (CASPT2/CASSCF(16,14)/cc-pVTZ//CASSCF(16,14)/cc-pVTZ) predict that their closed-shell singlet (1A') ground states are lower in energy by ca. 25 kcal mol-1 than their lowest-lying excited states. All cations are reactive toward dimethyl disulfide, dimethyl sulfide, and allyl iodide and most toward water and moderately reactive toward cyclohexane, reflecting their strongly electrophilic nature. They form adducts with nucleophiles in exothermic reactions (ca. 50 kcal mol-1 for dimethyl sulfide) that can fragment or be stabilized via IR emission. Most water adducts spontaneously isomerize to lower-energy tautomers. The nucleophiles preferentially add to those carbon atoms in the benzene ring that have the greatest positive charge (but not the carbonyl carbon). The cations react with cyclohexane via hydride abstraction by the oxygen atom. This is the only reaction that initially involves the oxygen atom and hence reflects the formally positively charged, monovalent oxygen atom in these cations.

Gas-Phase Reactivity of Phenylcarbyne Anions.

Gas-phase reactivities of the phenylcarbyne anion and its four derivatives were studied using a linear quadrupole ion trap mass spectrometer. The phenylcarbyne anions were calculated to have a triplet ground state (singlet-triplet splittings of 4-9 kcal mol-1), with the exception of the 4-cyanophenylcarbyne anion that has a singlet ground state (singlet-triplet splitting of -1.9 kcal mol-1). Only the phenylcarbyne anions with a triplet ground state react with acetone and dimethyl disulfide via radical mechanisms. On the other hand, only the phenylcarbyne anion with a singlet ground state abstracts H2O and H2C═C═O from acetic acid via electrophilic addition of the reagents to the anion. Finally, two hydroxy-substituted phenylcarbyne anions (with triplet ground states) partially tautomerize with the assistance of reagent molecules to the more stable distonic phenylcarbene anions. This occurs via abstraction of a proton from the reagent by the phenylcarbyne anion to generate a neutral (triplet) phenylcarbene and a reagent anion, which is followed by proton abstraction from the hydroxyl group of the neutral phenylcarbene by the reagent anion to generate the distonic phenylcarbene anion in an excited triplet state. Experiments performed on deuterated hydroxy-substituted phenylcarbyne anions verified the mechanism. The reactivities of the distonic phenylcarbene anions were found to be quite different from those of the phenylcarbyne anions. For example, they were found to abstract CH2 from acetonitrile, which is initiated by C-H insertion─typical singlet carbene reactivity.

Spin-Spin Coupling Controls the Gas-Phase Reactivity of Aromatic σ-Type Triradicals.

Examination of the reactions of σ-type quinolinium-based triradicals with cyclohexane in the gas phase demonstrated that the radical site that is the least strongly coupled to the other two radical sites reacts first, independent of the intrinsic reactivity of this radical site, in contrast to related biradicals that first react at the most electron-deficient radical site. Abstraction of one or two H atoms and formation of an ion that formally corresponds to a combination of the ion and cyclohexane accompanied by elimination of a H atom ("addition-H") were observed. In all cases except one, the most reactive radical site of the triradicals is intrinsically less reactive than the other two radical sites. The product complex of the first H atom abstraction either dissociates to give the H-atom-abstraction product and the cyclohexyl radical or the more reactive radical site in the produced biradical abstracts a H atom from the cyclohexyl radical. The monoradical product sometimes adds to cyclohexene followed by elimination of a H atom, generating the "addition-H" products. Similar reaction efficiencies were measured for three of the triradicals as for relevant monoradicals. Surprisingly, the remaining three triradicals (all containing a meta-pyridyne moiety) reacted substantially faster than the relevant monoradicals. This is likely due to the exothermic generation of a meta-pyridyne analog that has enough energy to attain the dehydrocarbon atom separation common for H-atom-abstraction transition states of protonated meta-pyridynes.

Protonated Ground-State Singlet meta-Pyridynes React from an Excited Triplet State.

The gaseous 2,6-didehydropyridinium cation and its derivatives transfer a proton to reagents for which the reaction for their singlet ground states is too endothermic to be observed. These reactions occur from the lowest-energy excited triplet states, which has not been observed (or reported) for other meta-benzyne analogues. Quantum chemical calculations indicate that the (excited) triplet states are stronger Brønsted acids than their (ground) singlet states, likely due to unfavorable three-center, four-electron interactions in the singlet-state conjugate bases. The cations have substantially smaller (calculated) singlet-triplet (S-T) splittings (ranging from ca. -11 to -17 kcal mol-1) than other related meta-benzyne analogues (e.g., -23.4 kcal mol-1 for the 3,5-isomer). This is rationalized by the destabilization of the singlet states (relative to the triplet states) by reduced (spatial) overlap of the nonbonding molecular orbitals due to the presence of the nitrogen atom between the radical sites (making the ring more rigid). Both the singlet and triplet states are believed to be generated upon formation of these biradicals via energetic collisions due to their small S-T splittings. It appears that once the triplet states are formed, the rate of proton transfer is faster than the rate of intersystem crossing unless the biradicals contain heavy atoms.

Measurement of the Proton Affinities of a Series of Mono- and Biradicals of Pyridine.

The proton affinity (PA) of a neutral molecule is defined as the negative of the enthalpy change for the gas-phase reaction between a proton and the neutral molecule to produce the (charged) conjugate acid of the molecule. PA is a fundamental property that is related to the structure of a molecule and affects its reactivity. Very few PA values are available for basic organic monoradicals and none for biradicals. Here, the PA values for several σ-type carbon-centered pyridine-based monoradicals and biradicals have been experimentally determined by monitoring proton transfer from the protonated mono- and biradicals to reference bases with known proton affinities as a function of time in Fourier-transform ion cyclotron resonance (FT-ICR) and linear quadrupole ion trap (LQIT) mass spectrometers. A procedure was developed for both instruments that permits differentiation between exo- and endothermic proton transfer reactions. The PA values of all the (bi)radicals studied were found to be lower than that of pyridine. This is rationalized based on the electron-withdrawing nature of the radical site(s). Thus, the PA values decrease in the order: pyridine > monoradicals > biradicals. The PA values of the monoradicals were also found to increase (making the protonated radicals less acidic) as the distance between the basic nitrogen atom and the radical site increases. Similar behavior was found for the biradicals, with one exception: 3,5-didehydropyridine has a larger PA (215.3 ± 3.3 kcal mol-1) than 3,4-didehydropyridine (PA = 213.4 ± 3.3 kcal mol-1) even though the latter biradical has one radical site farther away from the basic nitrogen atom. Quantum chemical calculations of the PAs of the (bi)radicals are in reasonably good agreement with the experimentally determined values. At the DFT (B3LYP), CCSD(T), and CASPT2 levels of theory, the mean unsigned errors are 2.3, 1.7, and 2.1 kcal mol-1.

Studies of the Fragmentation Mechanisms of Deprotonated Lignin Model Compounds in Tandem Mass Spectrometry.

Unlabeled and deuterium-labeled dimeric lignin model compounds with ß-O-4 linkages were evaporated and ionized using negative ion mode electrospray ionization, transferred into a linear quadrupole ion trap, isolated, and subjected to collision-activated dissociation (CAD; MS2 experiments). The elemental compositions of the fragment ions were determined by using a high-resolution Orbitrap mass analyzer, and their structures were examined using further CAD experiments (MSn experiments wherein n = 2-5). Data analysis was facilitated by determining the fragmentation pathways for several deprotonated model compounds. The structures of the key fragment ions of several pathways were determined by comparison of the CAD mass spectra measured for undeuterated and deuterated analogues and for deprotonated authentic compounds. Some of the proposed reaction mechanisms were tested by examining additional deprotonated synthetic model compounds. Quantum chemical calculations were used to delineate the most likely reaction pathways and reaction mechanisms. This work provides basic information needed for the design of tandem mass spectrometry-based CAD sequencing strategies for mixtures of lignin degradation products.

Effects of the Distance between Radical Sites on the Reactivities of Aromatic Biradicals.

Coupling of the radical sites in isomeric benzynes is known to hinder their radical reactivity. In order to determine how far apart the radical sites must be for them not to interact, the gas-phase reactivity of several isomeric protonated (iso)quinoline- and acridine-based biradicals was examined. All the (iso)quinolinium-based biradicals were found to react slower than the related monoradicals with similar vertical electron affinities (i.e., similar polar effects). In sharp contrast, the acridinium-based biradicals, most with the radical sites farther apart than in the (iso)quinolinium-based systems, showed greater reactivities than the relevant monoradicals with similar vertical electron affinities. The greater distances between the two radical sites in these biradicals lead to very little or no spin-spin coupling, and no suppression of radical reactivity was observed. Therefore, the radical sites can still interact if they are located on adjacent benzene rings and only after being separated further than that does no coupling occur. The most reactive radical site of each biradical was experimentally determined to be the one predicted to be more reactive based on the monoradical reactivity data. Therefore, the calculated vertical electron affinities of relevant monoradicals can be used to predict which radical site is most reactive in the biradicals.

Quinoline Triradicals: A Reactivity Study.

The gas-phase reactivities of several protonated quinoline-based σ-type (carbon-centered) mono-, bi-, and triradicals toward dimethyl disulfide (DMDS) were studied by using a linear quadrupole ion trap mass spectrometer. The mono- and biradicals produce abundant thiomethyl abstraction products and small amounts of DMDS radical cation, as expected. Surprisingly, all triradicals produce very abundant DMDS radical cations. A single-step mechanism involving electron transfer from DMDS to the triradicals is highly unlikely because the (experimental) adiabatic ionization energy of DMDS is almost 3 eV greater than the (calculated) adiabatic electron affinities of the triradicals. The unexpected reactivity can be explained based on an unprecedented two-step mechanism wherein the protonated triradical first transfers a proton to DMDS, which is then followed by hydrogen atom abstraction from the protonated sulfur atom in DMDS by the radical site in the benzene ring of the deprotonated triradical to generate the conventional DMDS radical cation and a neutral biradical. Quantum chemical calculations as well as examination of deuterated and methylated triradicals provide support for this mechanism. The proton affinities of the neutral triradicals (and DMDS) influence the first step of the reaction while the vertical electron affinities and spin-spin coupling of the neutral triradicals influence the second step. The calculated total reaction exothermicities for the triradicals studied range from 27.6 up to 29.9 kcal mol-1.

Relative Reactivities of Three Isomeric Aromatic Biradicals with a 1,4-Biradical Topology Are Controlled by Polar Effects.

Unexpectedly, the 5-dehydroquinoline radical cation was formed in the gas phase from the 5-iodo-8-nitroquinolinium cation upon ion-trap collision-activated dissociation. This reaction involves the cleavage of a nitro group to generate an intermediate monoradical, namely, the 8-dehydro-5-iodoquinolinium cation, followed by rearrangement through abstraction of a hydrogen atom from the protonated nitrogen atom by the radical site. Dissociation of the rearranged radical cation through elimination of an iodine atom generates the 5-dehydroquinoline radical cation. The mechanism was probed by studying isomeric biradicals and performing quantum chemical calculations. The 5-dehydroquinoline radical cation showed greater gas-phase reactivity toward dimethyl disulfide, cyclohexane, and allyl iodide than the isomeric 5,8-didehydroquinolinium cation, which is more reactive than the isomeric 5,8-didehydroisoquinolinium cation studied previously. All three isomers have a 1,4-biradical topology. The order of reactivity is rationalized by the vertical electron affinities of the radical sites of these biradicals instead of their widely differing singlet-triplet splittings.

Spin-Spin Coupling Between Two meta-Benzyne Moieties In a Quinolinium Tetraradical Cation Increases Their Reactivities.

The reactivity of a carbon-centered σ,σ,σ,σ-type singlet-ground-state tetraradical containing two meta-benzyne moieties was examined in the gas phase. Surprisingly, the tetraradical showed higher reactivity than its individual meta-benzyne counterparts. The reactivity of meta-benzynes is controlled by their (calculated) distortion energy ΔE2.3 , singlet-triplet spitting ΔES-T , and electron affinity (EA2.3 ) of the meta-benzyne moiety at the transition state geometry for hydrogen-atom abstraction reactions. The addition of a second meta-benzyne moiety to a meta-benzyne does not significantly change EA2.3 . However, ΔE2.3 is substantially decreased for both meta-benzyne moieties in the tetraradical, and this explains their higher reactivities. The decrease in ΔE2.3 for each meta-benzyne moiety in the tetraradical is rationalized by stabilizing spin-spin coupling between one radical site in each meta-benzyne moiety. Therefore, spin-spin coupling between the meta-benzyne moieties in this tetraradical increases its reactivity, whereas spin-spin coupling within each meta-benzyne moiety decreases its reactivity.

Polar Effects Control the Gas-Phase Reactivity of para-Benzyne Analogs.

We report herein a gas-phase reactivity study on a para-benzyne cation and its three cyano-substituted, isomeric derivatives performed using a dual-linear quadrupole ion trap mass spectrometer. All four biradicals were found to undergo primary and secondary radical reactions analogous to those observed for the related monoradicals, indicating the presence of two reactive radical sites. The reactivity of all biradicals is substantially lower than that of the related monoradicals, as expected based on the singlet ground states of the biradicals. The cyano-substituted biradicals show substantially greater reactivity than the analogous unsubstituted biradical. The greater reactivity is rationalized by the substantially greater (calculated) electron affinity of the radical sites of the cyano-substituted biradicals, which results in stabilization of their transition states through polar effects. This finding is in contrast to the long-standing thinking that the magnitude of the singlet-triplet splitting controls the reactivity of para-benzynes.

Effects of hydrogen bonding on the gas-phase reactivity of didehydroisoquinolinium cation isomers.

Two previously unreported isomeric biradicals with a 1,4-radical topology, the 1,5-didehydroisoquinolinium cation and the 4,8-didehydroisoquinolinium cation, and an additional, previously reported isomer, the 4,5-didehydroisoquinolinium cation, were studied to examine the importance of the exact location of the radical sites on their reactivities in the gas phase. The experimental results suggest that hydrogen bonding in the transition state enhances the reactivity of the 1,5-didehydroisoquinolinium cation towards tetrahydrofuran but not towards allyl iodide, dimethyl disulfide or tert-butyl isocyanide. The observation of no such enhancement of reactivity towards tetrahydrofuran for the 4,8-didehydroisoquinolinium and 4,5-didehydroisoquinolinium cations supports this hypothesis as these two biradicals are not able to engage in hydrogen bonding in their transition states for hydrogen atom abstraction from tetrahydrofuran. Quantum chemical transition state calculations indicate that abstraction of a hydrogen atom from tetrahydrofuran by the 1,5-didehydroisoquinolinium cation occurs at the C-1 radical site and that the transition state is stabilized by hydrogen bonding.

Dehydration Pathways for Glucose and Cellobiose During Fast Pyrolysis.

A full understanding of all possible elementary reactions applicable to cellulose fast pyrolysis is key to developing a comprehensive kinetic model for fast pyrolysis of cellulose. Since water is an observed product of fast pyrolysis of cellulose, the energetics of the dehydration reactions of cellulose were explored computationally by using density functional theory. Glucose and cellobiose were selected as the cellulose model compounds. The four water loss mechanisms studied are Maccoll elimination, Pinacol ring contraction, cyclic Grob fragmentation, and alcohol condensation, some of which have not been considered previously in the literature. Levoglucosan formation via alcohol condensation has the lowest calculated free-energy barrier (50.4 kcal mol-1) for glucose dehydration. All other water loss reactions have calculated free-energy barriers greater than 60 kcal mol-1. Cellobiose dehydration shows similar trends to those of glucose, suggesting that these reactions are applicable to glucooligosaccharides with higher degrees of polymerization. Secondary reactions of dehydrated glucose and dehydrated cellobiose via retro-Diels-Alder and aldol rearrangement mechanisms are also explored computationally.

Substituent Effects on the Reactivity of the 2,4,6-Tridehydropyridinium Cation, an Aromatic σ,σ,σ-Triradical.

2,4,6-Tridehydropyridinium cation (7) undergoes three consecutive atom or atom group abstractions from reagent molecules in the gas phase. By placing a π-electron-donating hydroxyl group between two radical sites, their reactivity can be quenched by enhancing their through-space coupling via a favorable resonance structure. Indeed, 3-hydroxy-2,4,6-tridehydropyridinium cation (8) abstracts only one atom or group of atoms from reagents. On the other hand, an electron-withdrawing cyano group between two of the radical sites (9) destabilizes the analogous resonance structure and diminishes through-space coupling between the radical sites, resulting in abstraction of three atoms, just like 7. However, the cyano-substituent also increases acidity to the point that 9 reacts pre-dominantly via proton transfer instead of undergoing radical reactions. Therefore, acidic triradicals may undergo nonradical, barrierless proton transfer reactions faster than radical reactions, which are usually accompanied by barriers. Examination of the analogous cyano-substituted mono-and biradicals revealed behavior similar to that of the corresponding unsubstituted species, with the exception of substantially greater reactivities due to their greater (calculated) vertical electron affinities. Finally, the 3-cyano-2,6-didehydropyridinium cation with a singlet ground state (S-T splitting: -11.9 kcal mol-1) was found to react exclusively from the lowest-energy triplet state by fast proton transfer reactions.

(-)ESI/CAD MSn Procedure for Sequencing Lignin Oligomers Based on a Study of Synthetic Model Compounds with ß-O-4 and 5-5 Linkages.

Seven synthesized G-lignin oligomer model compounds (ranging in size from dimers to an octamer) with 5-5 and/or ß-O-4 linkages, and three synthesized S-lignin model compounds (a dimer, trimer, and tetramer) with ß-O-4 linkages, were evaporated and deprotonated using negative-ion mode ESI in a linear quadrupole ion trap/Fourier transform ion cyclotron resonance mass spectrometer. The collision-activated dissociation (CAD) fragmentation patterns (obtained in MS2 and MS3 experiments, respectively) for the negative ions were studied to develop a procedure for sequencing unknown lignin oligomers. On the basis of the observed fragmentation patterns, the measured elemental compositions of the most abundant fragment ions, and quantum chemical calculations, the most important reaction pathways and likely mechanisms were delineated. Many of these reactions occur via charge-remote fragmentation mechanisms. Deprotonated compounds with only ß-O-4 linkages, or both 5-5 and ß-O-4 linkages, showed major 1,2-eliminations of neutral compounds containing one, two, or three aromatic rings. The most likely mechanisms for these reactions are charge-remote Maccoll and retro-ene eliminations resulting in the cleavage of a ß-O-4 linkage. Facile losses of H2O and CH2O were also observed for all deprotonated model compounds, which involve a previously published charge-driven mechanism. Characteristic "ion groups" and "key ions" were identified that, when combined with their CAD products (MS3 experiments), can be used to sequence unknown oligomers.

Reactivity Controlling Factors for an Aromatic Carbon-Centered σ,σ,σ-Triradical: The 4,5,8-Tridehydroisoquinolinium Ion.

The chemical properties of the 4,5,8-tridehydroisoquinolinium ion (doublet ground state) and related mono- and biradicals were examined in the gas phase in a dual-cell Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. The triradical abstracted three hydrogen atoms in a consecutive manner from tetrahydrofuran (THF) and cyclohexane molecules; this demonstrates the presence of three reactive radical sites in this molecule. The high (calculated) electron affinity (EA=6.06 eV) at the radical sites makes the triradical more reactive than two related monoradicals, the 5- and 8-dehydroisoquinolinium ions (EA=4.87 and 5.06 eV, respectively), the reactivity of which is controlled predominantly by polar effects. Calculated triradical stabilization energies predict that the most reactive radical site in the triradical is not position C4, as expected based on the high EA of this radical site, but instead position C5. The latter radical site actually destabilizes the 4,8-biradical moiety, which is singlet coupled. Indeed, experimental reactivity studies show that the radical site at C5 reacts first. This explains why the triradical is not more reactive than the 4-dehydroisoquinolinium ion because the C5 site is the intrinsically least reactive of the three radical sites due to its low EA. Although both EA and spin-spin coupling play major roles in controlling the overall reactivity of the triradical, spin-spin coupling determines the relative reactivity of the three radical sites.

Identification of N-Oxide and Sulfoxide Functionalities in Protonated Drug Metabolites by Using Ion-Molecule Reactions Followed by Collisionally Activated Dissociation in a Linear Quadrupole Ion Trap Mass Spectrometer.

The in vivo oxidation of sulfur and nitrogen atoms in many drugs into sulfoxide and N-oxide functionalities is a common biotransformation process. Unfortunately, the unambiguous identification of these metabolites can be challenging. In the present study, ion-molecule reactions of tris(dimethylamino)borane followed by collisionally activated dissociation (CAD) in an ion trap mass spectrometer are demonstrated to allow the identification of N-oxide and sulfoxide functionalities in protonated polyfunctional drug metabolites. Only ions with N-oxide or sulfoxide functionality formed diagnostic adducts that had lost dimethyl amine (DMA). This was demonstrated even for an analyte that contains a substantially more basic functionality than the functional group of interest. CAD of the diagnostic product ions (M) resulted mainly in type A (M - DMA) and B fragment ions (M - HO-B(N(CH3)2)2) for N-oxides, but sulfoxides also formed diagnostic C ions (M - O═BN(CH3)2), thus allowing differentiation of the functionalities. Some protonated analytes yielded abundant TDMAB adducts that had lost two DMA molecules instead of just one. This provides information on the environment of the N-oxide and sulfoxide functionalities. Quantum chemical calculations were performed to explore the mechanisms of the above-mentioned reactions. The method can be implemented on HPLC for real drug analysis.

Fast pyrolysis of 13C-labeled cellobioses: gaining insights into the mechanisms of fast pyrolysis of carbohydrates.

A fast-pyrolysis probe/tandem mass spectrometer combination was utilized to determine the initial fast-pyrolysis products for four different selectively (13)C-labeled cellobiose molecules. Several products are shown to result entirely from fragmentation of the reducing end of cellobiose, leaving the nonreducing end intact in these products. These findings are in disagreement with mechanisms proposed previously. Quantum chemical calculations were used to identify feasible low-energy pathways for several products. These results provide insights into the mechanisms of fast pyrolysis of cellulose.

Polar Effects Control the Gas-phase Reactivity of Charged para-Benzyne Analogs.

The gas-phase reactivity of charged para-benzynes is entirely unexplored as they and/or their precursors tend to undergo ring-opening upon their generation. We report here a gas-phase reactivity study of two such benzynes, the 2,5-didehydropyridinium and 5,8-didehydroisoquinolinium cations, generated in a modified dual-linear quadrupole ion trap (DLQIT) mass spectrometer. Both biradicals were found to form diagnostic products with organic molecules, indicating the presence of two radical sites. As opposed to earlier predictions that the singlet-triplet (S-T) splitting controls the radical reactivity of such species, the 2,5-didehydropyridinium cation reacts much faster in spite of its larger S-T splitting. Calculated vertical electron affinities of the radical sites of the para-benzynes, a parameter related to the polarity of the transition states of their reactions, appears to be the most important reactivity controlling factor.

Mass spectrometric studies of fast pyrolysis of cellulose.

A fast pyrolysis probe/linear quadrupole ion trap mass spectrometer combination was used to study the primary fast pyrolysis products (those that first leave the hot pyrolysis surface) of cellulose, cellobiose, cellotriose, cellotetraose, cellopentaose, and cellohexaose, as well as of cellobiosan, cellotriosan, and cellopentosan, at 600°C. Similar products with different branching ratios were found for the oligosaccharides and cellulose, as reported previously. However, identical products (with the exception of two) with similar branching ratios were measured for cellotriosan (and cellopentosan) and cellulose. This result demonstrates that cellotriosan is an excellent small-molecule surrogate for studies of the fast pyrolysis of cellulose and also that most fast pyrolysis products of cellulose do not originate from the reducing end. Based on several observations, the fast pyrolysis of cellulose is suggested to initiate predominantly via two competing processes: the formation of anhydro-oligosaccharides, such as cellobiosan, cellotriosan, and cellopentosan (major route), and the elimination of glycolaldehyde (or isomeric) units from the reducing end of oligosaccharides formed from cellulose during fast pyrolysis.