Simple Bond Cleavage Research Articles

Theoretical exploration of the reactivity of cellulose models under non-thermal plasma conditions-mechanistic and NBO studies.

Mechanistic details of cellulose depolymerization by non‐thermal (atmospheric) plasma (NTAP) remains under‐explored given the complexity of the medium. In this study, we have investigated the reaction mechanism of glycosidic‐bond degradation triggered by reaction with hydroxyl radicals, considered as the principal reactive species in NTAP medium. In the first step of reaction sequence, H‐abstraction reactions by HO‧. radical on different C—H sites of the pyranose ring were found to be non‐selective and markedly exergonic giving rise to a set of cellobiosyl carboradicals likely to undergo further reactions. We then showed that cellobiosyl carboradicals are protected against direct hydrolysis, no activation of the (1–4)‐ β‐glycosidic bond being characterized. Interestingly, a simple homolytic bond cleavage allowed to obtain desired monomer. Among the 18 possible fragmentations, involving C—C and C—O bond breaking from cellobiosyl carboradicals, 14 transition states were successfully identified, and only three reaction pathways proved kinetically and thermodynamically feasible. Natural bond orbital (NBO) analysis was performed to shed light on electronic structures of different compounds.

What a little branching can do – Dissociative photoionization of two butanol isomers

The fragmentation processes of two internal energy-selected C4H10O+ isomers, 1-butanol and isobutanol cations, were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. The first dissociation channel leads to the formation of C4H8+ ions (m/z 56) by water loss in both isomers. Using statistical energy distribution and rate models including the isomerization of the parent ion, the 0 K appearance energies (E0) were determined to be 10.347 ± 0.015 eV and 10.57 ± 0.05 eV for 1-butanol and isobutanol, respectively. The second dissociation channel, the formation of CH3OH2+, quickly overtakes the water-loss channel in isobutanol with an E0 of 10.61 ± 0.02 eV. It appears only as a minor channel in 1-butanol with an E0 of 10.74 ± 0.09 eV. The methanol-loss channel, forming propylene ion, opens up at E0 = 10.94 ± 0.04 eV and 10.72 ± 0.02 eV in 1-butanol and isobutanol, respectively. The next two fragmentation pathways correspond to complementary pair formation of C3H7(+/o) and CH2OH(o/+). The former is assigned as the isopropyl (ion) in both butanol isomers. The channel corresponds to simple bond cleavage in isobutanol with the E0 difference corresponding to the ionization energy difference of the fragments at E0 = 10.97 ± 0.05 eV (C3H7+) and at E0 = 11.11 ± 0.20 eV (CH2OH+). However, there is an internal hydrogen shift necessary in 1-butanol and, therefore, the complementary ions appear at the same E0 = 11.10 ± 0.03 eV, which corresponds to a shared rate limiting transition state. The sequential dissociation product from m/z 56, C3H5+, appears above 11.6 eV as a minor channel in both isomers. Finally, we determined −279.1 ± 1.6 kJ mol−1 as the 298 K heat of formation of gaseous isobutanol. We also propose revised ionization energies for the two butanols isomers based on composite method calculations.

Gas-Phase Complexes of Americium and Lanthanides with a Bis-triazinyl Pyridine: Reactivity and Bonding of Archetypes for F-Element Separations.

Bis-triazinyl pyridines (BTPs) exhibit solution selectivity for trivalent americium over lanthanides (Ln), the origins of which remain uncertain. Here, electrospray ionization was used to generate gas-phase complexes [ML3]3+, where M = La, Lu, or Am and L is EtBTP 2,6-bis(5,6-diethyl-1,2,4-triazin-3-yl)-pyridine. Collision-induced dissociation (CID) of [ML3]3+ in the presence of H2O yielded a protonated ligand [L(H)]+ and hydroxide [ML2(OH)]2+ or hydrate [ML(L-H)(H2O)]2+, where (L-H)- is a deprotonated ligand. Although solution affinities indicate stronger binding of BTPs toward Am3+ versus Ln3+, the observed CID process is contrastingly more facile for M = Am versus Ln. To understand the disparity, density functional theory was employed to compute potential energy surfaces for two possible CID processes, for M = La and Am. In accordance with the CID results, both the rate determining transition state barrier and the net energy are lower for [AmL3]3+ versus [LaL3]3+ and for both product isomers, [ML2(OH)]2+ and [ML(L-H)(H2O)]2+. More facile removal of a ligand from [AmL3]3+ by CID does not necessarily contradict stronger Am3+-L binding, as inferred from solution behavior. In particular, the formation of new bonds in the products can distort kinetics and thermodynamics expected for simple bond cleavage reactions. In addition to correctly predicting the seemingly anomalous CID behavior, the computational results indicate greater participation of Am 5f versus La 4f orbitals in metal-ligand bonding.

Low-energy electron-induced decomposition of 5-trifluoromethanesulfonyl-uracil: A potential radiosensitizer.

5-trifluoromethanesulfonyl-uracil (OTfU), a recently proposed radiosensitizer, is decomposed in the gas-phase by attachment of low-energy electrons. OTfU is a derivative of uracil with a triflate (OTf) group at the C5-position, which substantially increases its ability to undergo effective electron-induced dissociation. We report a rich assortment of fragments formed upon dissociative electron attachment (DEA), mostly by simple bond cleavages (e.g., dehydrogenation or formation of OTf-). The most favorable DEA channel corresponds to the formation of the triflate anion alongside with the reactive uracil-5-yl radical through the cleavage of the O-C5 bond, particularly at about 0 eV. Unlike for halouracils, the parent anion was not detected in our experiments. The experimental findings are accounted by a comprehensive theoretical study carried out at the M06-2X/aug-cc-pVTZ level. The latter comprises the thermodynamic thresholds for the formation of the observed anions calculated under the experimental conditions (383.15 K and 3 × 10-11 atm). The energy-resolved ion yield of the dehydrogenated parent anion, (OTfU-H)-, is discussed in terms of vibrational Feshbach resonances arising from the coupling between the dipole bound state and vibrational levels of the transient negative ion. We also report the mass spectrum of the cations obtained through ionization of OTfU by electrons with a kinetic energy of 70 eV. The current study endorses OTfU as a potential radiosensitizer agent with possible applications in radio-chemotherapy.

VUV Dissociative Photoionization of Quinoline in the 7–26 eV Photon Energy Range

Abstract The dissociative photoionization of quinoline was studied by photoionization mass spectrometry and ion yield measurements over a synchrotron photon excitation energy range 7–26 eV. The ionic and neutral products were identified with the aid of thermochemical calculations that, in some cases, led to deeper understanding of photodissociation pathways and the determination of upper limits of heats of formation of ionic and neutral dissociation products. A detailed comparison between the 20 eV photon excitation and 70 eV electron impact mass spectra, coupled with estimation of thermochemical appearance energies, leads to assignment of the dissociative ionization cation and neutral products for each detected ion. Reaction schemes for formation of these products are proposed in a number of cases. Ion intensities in the photon and electron impact mass spectra were used to consider extending a rule of charge retention in simple bond cleavage to more complex cases of dissociative ionization.

Interpretation and Simulation of Negative Ion Mass Spectra of Some Phosphorus Organoelement Compounds

Negative ion mass spectra for a series of organophosphorus compounds were obtained and negative ion fragmentation processes were treated theoretically. Using O-isopropyl and O-pinacolyl methylphosphonofluoridates as examples, electron affinities of molecules and their fragments were estimated using the UB3LYP/6-311+G(d,p) quantum-chemical approach and energetically more favorable and characteristic routes of dissociative electron attachment, including simple bond cleavage and rearrangements, were determined. Based on the obtained experimental and theoretical data, hypothetic fragmentation patterns were proposed and a special algorithm was compiled to predict negative ion mass spectra for some groups of organophosphorus compounds, such as О-alkyl methylphosphonofluoridates, О,О-alkyl phosphonodichloridates, and О,О′-dialkyl phosphonochloridates. The simulated mass spectra showed a good agreement with the experimental ones, confirming reasonable reliability of the proposed algorithm.

Desorption Energy of Oxygen Molecule from Anionic Gold Oxide Clusters, AunO2–, Using Thermal Desorption Spectrometry

We determined the desorption energies of O2 molecules from AunO2– (n = 2, 4, 6, 8, 10, 12, and 14) by gas phase thermal desorption spectrometry. Cluster anions, AunO2–, prepared in the gas phase were heated inside a temperature-controlled copper tube. Formation of Aun– from the desorption of O2 was observed as a function of temperature. Temperature dependence of the intensities of AunO2– and Aun– was analyzed using the Arrhenius equation, estimating the desorption energy of O2 from AunO2–. Considering the geometrical structure of AunO2–, the desorption energy must be the same as or slightly higher than the binding energy of O2 to Aun–, because the desorption of O2 proceeds by a simple bond cleavage of Aun––O2, without significant rearrangement of atoms in the cluster. The collision-induced dissociation experiments show that the desorption energy was nearly equal to or slightly lower than the threshold energy for n = 2, 4, and 6.

Reactions in Nitroimidazole and Methylnitroimidazole Triggered by Low-Energy (0-8 eV) Electrons.

Low-energy electrons (0-8 eV) effectively decompose 4-nitroimidazole (4NI) and the two methylated isomers 1-methyl-5-nitroimidazole and 1-methyl-4-nitroimidazole via dissociative electron attachment (DEA). The involved unimolecular decompositions range from simple bond cleavages (loss of H(•), formation of NO2(-)) to complex reactions possibly leading to a complete degradation of the target molecule (formation of CN(-), etc.). At energies below 2 eV, the entire rich chemistry induced by DEA is completely quenched by methylation, as demonstrated in a previous communication (Tanzer, K.; Feketeová, L.; Puschnigg, B.; Scheier, P.; Illenberger. E.; Denifl, S. Angew. Chem., Int. Ed. 2014, 53, 12240). The observation that in 4NI neutral radicals and radical anions are formed via DEA at high efficiency already at threshold (0 eV) may have significant implications for the development of nitroimidazole-based radiosensitizers in tumor radiation therapy.

Electron-impact-induced tryptophan molecule fragmentation

The fragmentation of a gas-phase tryptophan molecule by a low-energy (<70 eV) electron impact was studied both experimentally and theoretically. Various positively charged fragments were observed and analyzed. A special attention was paid to the energy characteristics of the ionic fragment yield. The geometrical parameters of the initial molecule rearrangement were also analyzed. The fragmentation observed was due to either a simple bond cleavage or more complex reactions involving molecular rearrangements.

Reactions in nitroimidazole triggered by low-energy (0-2 eV) electrons: methylation at N1-H completely blocks reactivity.

Low-energy electrons (LEEs) at energies of less than 2 eV effectively decompose 4-nitroimidazole (4NI) by dissociative electron attachment (DEA). The reactions include simple bond cleavages but also complex reactions involving multiple bond cleavages and formation of new molecules. Both simple and complex reactions are associated with pronounced sharp features in the anionic yields, which are interpreted as vibrational Feshbach resonances acting as effective doorways for DEA. The remarkably rich chemistry of 4NI is completely blocked in 1-methyl-4-nitroimidazole (Me4NI), that is, upon methylation of 4NI at the N1 site. These remarkable results have also implications for the development of nitroimidazole based radiosensitizers in tumor radiation therapy.

Reactions in 1,1,1-trifluoroacetone triggered by low energy electrons (0-10 eV): from simple bond cleavages to complex unimolecular reactions.

The impact of low energy electrons (0-10 eV) to 1,1,1-trifluoroacetone yields a variety of fragment anions which are formed via dissociative electron attachment (DEA) through three pronounced resonances located at 0.8 eV, near 4 eV, and in the energy range 8-9 eV. The fragment ions arise from different reactions ranging from the direct cleavage of one single or double bond (formation of F(-), CF3(-), O(-), (M-H)(-), and M-F)(-)) to remarkably complex unimolecular reactions associated with substantial geometric and electronic rearrangement in the transitory intermediate (formation of OH(-), FHF(-), (M-HF)(-), CCH(-), and HCCO(-). The ion CCH(-), for example, is formed by an excision of unit from the target molecule through the concerted cleavage of four bonds and recombination to H2O within the neutral component of the reaction.

Photofragmentation of Isoleucine by Vacuum Ultraviolet Photoionization

Vacuum ultraviolet photon-induced ionization and dissociation of isoleucine are investigated with synchrotron radiation photoionization mass spectroscopy and theoretical calculations. The main fragment ions at m/z=86, 75, 74, 69, 57, 46, 45, 44, 41, 30, 28, and 18 from isoleucine are observed in the mass spectrum at the photon energy of 13 eV. From the photoionization efficiency curves, appearance energies for the principal fragment ions C5H12N+ (m/z=86), C2H5NO2+ (m/z=75), C5H9+ (m/z=69), C4H9+ (m/z=57), and CH4N+ (m/z=30) are determined to be 8.84±0.07, 9.25±0.06, 10.20±0.12, 9.25±0.10, and 11.05±0.07 eV, respectively, and possible formation pathways are established in detail by the calculations at the B3LYP/6-31++G(d, p) levels. These proposed channels include simple bond cleavage reactions as well as reactions involving intermediates and transition structures. The experimental and computational appearance energies or barriers are in good agreement.

Dissociative electron attachment to the silane derivatives trichlorovinylsilane (SiCl3C2H3), tetravinylsilane (Si(C2H3)4) and trimethylvinylsilane (Si(CH3)3C2H3)

The formation of negative ions following low energy (0–12eV) electron attachment to the title compounds is studied in a crossed electron-molecular beam experiment applying mass spectrometric detection of the anions. All silane derivatives presently studied are weak electron scavengers which also holds for trichlorovinylsilane. The behaviour in the silanes is hence in strong contrast to the hydrocarbons where chlorination usually results in a strong increase of the cross section for dissociative electron attachment (DEA) yielding intense Cl− signals already at very low electron energies (near 0eV). The different behaviour can be explained by the appreciably stronger Si–Cl binding energy compared to that of C–Cl making the DEA reaction leading to Cl− endothermic for trichlorovinylsilane while the corresponding reaction is exothermic for most chlorinated hydrocarbons. Trichlorovinylsilane and tetravinylsilane undergo various further DEA reactions via different resonances at higher energy (in the range between 4 and 10eV) leading to a variety of ionic fragments. These reactions range from simple bond cleavages to considerably more complex unimolecular decompositions involving multiple bond cleavages, hydrogen transfer in the temporary negative ion and formation of new bonds. Trimethylvinylsilane yields only one single fragment due to the loss of a neutral hydrogen atom ((M−H)−).

Fragmentation of valine and proline in resonant free electron capture reactions

The processes of resonant electron capture by molecules of valine and proline were studied. The main dissociative decay channels of molecular ions and the structures of fragment ions were established. The reactions of simple bond cleavage and H-shift rearrangement processes were revealed. The dissociative electron attachment cross-sections for [M - H]− ions were measured in the region of 1.25 eV. The metastable decay of [M - H]− ions were found in the energy region of 4.5–8 eV. The structures of decomposing parent ions were determined using the statistical model of metastable fragmentation.

Using density functional theory to rationalise the mass spectral fragmentation of maraviroc and its metabolites

Tandem mass spectrometry (MS/MS) is widely used for the identification of metabolites at all stages of the pharmaceutical discovery and development process. The assignment of ions in the product ion spectra can be time-consuming and hence delay feedback of results that may influence the direction of a project. A deeper understanding of the processes involved in generation of the product ions formed via collision-induced dissociation may allow development of chemically intelligent software to aid spectral interpretation. Current commercially available spectral interpretation software takes a mainly arithmetical approach resulting in extensive lists of numerically plausible ions, many of which may not be chemically feasible. In this study, high-resolution MS/MS spectra were obtained for maraviroc and two of its synthetic metabolites, and structures for the product ions proposed. Density functional theory (DFT) based on in silico modelling was undertaken to investigate whether the fragmentation observed was potentially a result of bond lengthening (and hence weakening) as a consequence of protonation of the molecule at the most thermodynamically stable site(s). It was determined that for all three compounds, where the product ions resulted from simple bond cleavages (not rearrangements), the bonds that cleaved had been calculated to elongate after protonation. It was also noted that the protonated molecule may represent a mixture of singly charged protonated species and that the most basic sites in the molecule may not necessarily be the most thermodynamically stable for protonation.

Guided Ion Beam and Theoretical Studies of the Reactions of Pd+with CS2: Thermochemistry of PdS+and PdCS+

The gas-phase reactivity of the atomic palladium cation with CS(2) is investigated using guided-ion beam mass spectrometry. Endothermic reactions forming PdS(+) and PdCS(+) are observed. Analysis of the kinetic energy dependence of the cross sections for formation of the two products yields the 0 K bond energies of D(0)(Pd(+)-S) = 2.36 +/- 0.11 eV and D(0)(Pd(+)-CS) = 2.67 +/- 0.21 eV. Quantum chemical calculations are used to investigate the electronic structure of the two product ions. In addition, detailed calculations of the potential energy surfaces reveal that the mechanism involves oxidative addition of a CS bond to the metal cation followed by simple bond cleavages. Formation of the ground-state PdS(+) ((4)Sigma(-)) product is spin-forbidden from ground-state Pd(+) ((2)D) whereas PdCS(+) ((2)Sigma(+)) formation is spin-allowed. Crossing points between the doublet and quartet surfaces are located in the region of the SPd(+)(CS) intermediate, which suggests that coupling between the surfaces is efficient, consistent with experimental indications.

Electron capture by pentafluoronitrobenzene and pentafluorobenzonitrile

Electron attachment to pentafluorobenzonitrile (C(6)F(5)CN) and pentafluoronitrobenzene (C(6)F(5)NO(2)) is studied in the energy range 0-16 eV by means of a crossed electron-molecular beam experiment with mass spectrometric detection of the anions. We find that pentafluoronitrobenzene exclusively generates fragment anions via dissociative electron attachment (DEA), while pentafluorobenzonitrile forms a long lived parent anion within a narrow energy range close to 0 eV and additionally undergoes DEA at higher energies. This is in contrast to the behaviour of the non-fluorinated analogues as in nitrobenzene the non-decomposed anion is formed while in benzonitrile only DEA is observed. The associated reactions involve simple bond cleavages but also complex unimolecular decompositions associated with structural and electronic rearrangement also resulting in the deterioration of the cyclic structure.

A Novel Salt Bridge Mechanism Highlights the Need for Nonmobile Proton Conditions to Promote Disulfide Bond Cleavage in Protonated Peptides Under Low-Energy Collisional Activation

The gas-phase fragmentation mechanisms of small models for peptides containing intermolecular disulfide links have been studied using a combination of tandem mass spectrometry experiments, isotopic labeling, structural labeling, accurate mass measurements of product ions, and theoretical calculations (at the MP2/6-311 + G(2d,p)//B3LYP/3-21G(d) level of theory). Cystine and its C-terminal derivatives were observed to fragment via a range of pathways, including loss of neutral molecules, amide bond cleavage, and S-S and C-S bond cleavages. Various mechanisms were considered to rationalize S-S and C-S bond cleavage processes, including charge directed neighboring group processes and nonmobile proton salt bridge mechanism. Three low-energy fragmentation pathways were identified from theoretical calculations on cystine N-methyl amide: (1) S-S bond cleavage dominated by a neighboring group process involving the C-terminal amide N to form either a protonated cysteine derivative or protonated sulfenyl amide product ion (44.3 kcal mol(-1)); (2) C-S bond cleavage via a salt bridge mechanism, involving abstraction of the alpha-hydrogen by the N-terminal amino group to form a protonated thiocysteine derivative (35.0 kcal mol(-1)); and (3) C-S bond cleavage via a Grob-like fragmentation process in which the nucleophilic N-terminal amino group forms a protonated dithiazolidine (57.9 kcal mol(-1)). Interestingly, C-S bond cleavage by neighboring group processes have high activation barriers (63.1 kcal mol(-1)) and are thus not expected to be accessible during low-energy CID experiments. In comparison to the energetics of simple amide bond cleavage, these S-S and C-S bond cleavage reactions are higher in energy, which helps rationalize why bond cleavage processes involving the disulfide bond are rarely observed for low-energy CID of peptides with mobile proton(s) containing intermolecular disulfide bonds. On the other hand, the absence of a mobile proton appears to "switch on" disulfide bond cleavage reactions, which can be rationalized by the salt bridge mechanism. This potentially has important ramifications in explaining the prevalence of disulfide bond cleavage in singly protonated peptides under MALDI conditions.

Energy selective excision of CN−following electron attachment to hexafluoroacetone azine ((CF3)2CN–NC(CF3)2)

Low energy electron attachment (DEA) to hexafluoroacetone azine (HFAA) leads to a remarkable energy selective excision of CN(-) within a pronounced resonance located at 1.35 eV. The underlying dissociative electron attachment (DEA) reaction involves multiple bond cleavages and rearrangement within the neutral products. A series of further fragment ions (F(-), CF(3)(-), (CF(3))(2)C(-) and (CF(3))(2)CN(-)) are observed from resonant features above 2 eV and only (CF(3))(2)CN(-) is additionally formed within a narrow resonance below 1 eV. In contrast to CN(-) all the remaining fragment ions can be formed by simple bond cleavages with (CF(3))(2)CN(-) being the result of a symmetric decomposition of the target molecule by cleavage of the (N-N) bond with the excess charge localised on either of the identical fragments. Our ab initio calculations predict an adiabatic electron affinity of HFAA close to 2 eV with the geometry of the relaxed anion considerably distorted with respect to that of the neutral molecule.

Thermochemistry of Organic and Heteroorganic Species. Part 13. Structural Aspects and Thermochemical Approach to Isomerization and Fragmentation of Negative Ions. Simple Bond Cleavage and Hydrogen Rearrangement Processes

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