The fragmentation of 5- and 3-substituted thiophene-2-carboxamides under electron impact

The 70 eV electron impact mass spectra of twelve 5- and 3-substituted thiophene-2-carboxamides are discussed with the aid of exact mass measurements and labelling experiments. All mass spectra exhibit pronounced molecular ions. Some isomeric 5- and 3-substituted title compounds can be differentiated by mass spectrometry. The fragmentation is influenced by a strong ‘ortho-effect’ which activates the NH₃ elimination. In the other cases the most important fragmentation is NH₂˙ loss, followed by CO elimination.     

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.     

Ultrafast dynamics in ammonia clusters: analysis of protonated and unprotonated cluster ion signals

The dynamics of ammonia clusters excited to the à state with 160 fs laser pulses of 6.2 eV was studied by pump-probe experiments with a low probe photon energy of 3.1 eV. Protonated as well as unprotonated cluster ion signals have been observed. The time evolution of both species is characteristic of the intermediate rearrangement and fragmentation processes. The observations strongly support a previously developed kinetic model for this dynamics with the signal at long delay times>6 ps reflecting the species involved in the absorption dissociation ionization (ADI) mechanism. Strong evidence is found for the formation of an internally ‘quasi protonated’ excited state and of ammoniated NH₄ radicals.     

Unimolecular and collision-induced fragmentation of protonated nitroarenes

The unimolecular fragmentation reactions of 28 protonated nitroarenes, occurring on the metastable ion time-scale, are reported. In addition, the collision-induced fragmentation of the same species have been studied at 10 eV and at 50 eV collision energy. When an OH, COOH or NH₂ substituent is ortho to the nitro function, the dominant fragmentation involves loss of H₂O, for both unimolecular and collision-induced reactions. When there is an electron-releasing substituent ortho or para to the litro group, loss of OH is the dominant fragmentation reaction both on the metastable ion time-scale and for ions activated by collision. When the electron-releasing substituent is meta to the nitro group, loss of NO₂ is the dominant low-energy unimolecular fragmentation reaction while loss of HNO₂ is the most important fragmentation for ions activated by 50 eV collisions. Elimination of NO from [MH]⁺ occurs to a significant extent in the unimolecular fragmentation of protonated nitrobenzene and those protenated nitrobenzenes containing electron- attracting substituents. In the collision-induced dissociation of these species loss of HNO₂ occurs at the expense of loss of NO. The results are consistent with protonation predominantly at the nitro group. The results are discussed in terms of the use of neutral loss scans in tandem mass spectrometry to monitor complex mixtures for nitroarenes.     

Ultrafast fragmentation and ionisation dynamics of ammonia clusters

The formation of protonated and unprotonated ammonia cluster ions is studied by femtosecond two colour two photon pump-probe techniques applied to (NH₃) _n and (ND₃) _n clusters withn up to 8. The fourth harmonic (～ 200 nm, 6.2 eV, 160 fs) of a Ti: Sapphire laser pulse is used to excite the clusters in a state corresponding to theà state of NH₃ while the third harmonic (267 nm, 4.65 eV) is used for the subsequent ionisation step. Employing a combination of the optical Bloch equations for the excitation process and rate equations for the cluster dynamics we calibrate the zero time delay and carefully analyse the time dependence of the pump-probe signal. Several distinct intermediate steps in the time evolution can be distinguished, having characteristic time constants ranging from 40 fs to over 100 ps. They are discussed in a consistent scheme for the excitation, ionisation and protonation dynamics, accounting also for characteristic differences observed between deuterated and undeuterated species. A particularly remarkable time dependence of the homogeneous (NH₃) ₂ ⁺ cluster ion signal is interpreted as a fingerprint of internally protonated neutral precursors of the type NH₃NH₂NH₄.     

Diagnosing the Protonation Site of <Emphasis Type="BoldItalic">b</Emphasis><Subscript><Emphasis Type="BoldItalic">2</Emphasis></Subscript> Peptide Fragment Ions using IRMPD in the X–H (X = O,N, and C) Stretching Region

Charge-directed fragmentation has been shown to be the prevalent dissociation step for protonated peptides under the low-energy activation (eV) regime. Thus, the determination of the ion structure and, in particular, the characterization of the protonation site(s) of peptides and their fragments is a key approach to substantiate and refine peptide fragmentation mechanisms. Here we report on the characterization of the protonation site of oxazolone b ₂ ions formed in collision-induced dissociation (CID) of the doubly protonated tryptic model-peptide YIGSR. In support of earlier work, here we provide complementary IR spectra in the 2800–3800 cm^–1 range acquired on a table-top laser system. Combining this tunable laser with a high power CO₂ laser to improve spectroscopic sensitivity, well resolved bands are observed, with an excellent correspondence to the IR absorption bands of the ring-protonated oxazolone isomer as predicted by quantum chemical calculations. In particular, it is shown that a band at 3445 cm^–1, corresponding to the asymmetric N–H stretch of the (nonprotonated) N-terminal NH₂ group, is a distinct vibrational signature of the ring-protonated oxazolone structure.     

Fragmentation dynamics of ammonia cluster ions after single photon ionisation

A reflecting time of flight mass spectrometer (RETOF) is used to study unimolecular and collision induced fragmentation of ammonia cluster ions. Synchrotron radiation from the BESSY electron storage ring is used in a range of photon energies from 9.08 up to 17.7 eV for single photon ionisation of neutral clusters in a supersonic beam. The threshold photoelectron photoion coincidence technique (TPEPICO) is used to define the energy initially deposited into the cluster ions. Metastable unimolecular decay (µs range) is studied using the RETOF's capacity for energy analysis. Under collision free conditions the by far most prominent metastable process is the evaporation of one neutral NH₃ monomer from protonated clusters (NH₃) _{n ? 2}NH ₄ ⁺ . Abundance of homogeneous vs. protonated cluster ions and of metastable fragments are reported as a function of photon energy and cluster size up ton=10.     

Fragmentation of doubly charged ammonia cations NH 3 ++ studied by the photoion-photoion coincidence (PIPICO) method

Doubly charged NH ₃ ⁺⁺ cations were produced by double photoionization of neutral ammonia molecules by using the synchrotron radiation from ACO as a photon source of variable energy in the 35–49 eV energy range. The fragmentation of NH ₃ ⁺⁺ was studied by the photoion-photoion coincidence (PIPICO) method. NH ₃ ⁺⁺ cations were produced in the \(\tilde X^1 \) A ₁ and \(\tilde B^1 \) electronic states of which the onset energies were measured at, respectively, 35.4±0.5 eV and 44.5±0.5 eV. It was shown that the NH ₃ ⁺⁺ ions, initially produced in their \(\tilde X^1 \) A ₁ state, rapidly dissociate (in less than 50 ns), into NH ₂ ⁺ + H⁺. Furthermore, the comparison with results obtained by other methods indicates that NH ₃ ⁺⁺ ions can either be long-lived (τ>10 µs) or slowly dissociating (1 µs<τ<10 µs) or rapidly dissociating (τ<50 ns), depending on their geometry and/or internal energy in their \(\tilde X^1 \) E A ₁ electronic state.     

Structure of water (H2O) 6 − and ammonium (NH3) 13 − clusters with an excess electron: Quantum chemical studies

The B3LYP method within DFT and the ab initio MP2 method with an extended 6-311++G(3df,3pd) basis set are employed to calculate the adiabatic bound state of an excess electron in (H₂O) ₆ ^? water and (NH₃) ₁₃ ^? . ammonium clusters. Adiabatic electron affinity of (H₂O)₆ and (NH₃)₁₃ clusters is 0.03–0.18 eV and 0.18 eV respectively. The calculated vertical binding energies of the excess electron in anionic clusters ((H₂O) ₆ ^? 0.37÷0.66 eV and (NH₃) ₁₃ ^? 0.26 eV) agree well with the experimental values of 0.50 eV and 0.22 eV obtained from photoelectron spectra. A cavity model of solvated electrons in water and ammonium is considered.     

Scattering of neutral NH3 clusters off LiF(100): angular distributions of NH3 and small clusters

The scattering behavior of neutral ammonia clusters off a LiF(100) surface is studied. Ammonia clusters are produced by a coexpansion of NH₃ and Kr with an average kinetic energy of 48 meV per monomer molecule. Using single photon VUV laser ionization at λ = 118 nm (hv = 10.49 eV) the mass distribution of scattered particles is obtained in a reflecting time-of-flight mass spectrometer. Compared with the incoming cluster beam the average cluster size of the scattered particles is drastically decreased. The angular distribution of NH ₃ ⁺ and NH ₄ ⁺ after scattering reveals a strong inelastic interaction between the clusters and the LiF(100) surface which is described in the context of a thermokinetic model and a phonon excitation along the (001) azimuth of the LiF(100) surface.     

Theoretical studies of excited states of H3O and of core-excited NH3: Comparisons with equivalent core analysis of NH3 core-excitation spectra and implications for the radiation chemistry of aqueous systems

The H₃O radical has been studied within the ab initio LCAO SCF MO model. A flexible basis set including diffuse basis functions at both O and H has been used in order to represent the excited states adequately. Calculated excitation energies are 1.87, 2.87–3.16, and 3.36–3.47 eV; the calculated ionization energy is 4.75 eV. These represent well the experimental values (good to ±0.3 eV) of 1.6, 2.9, and 3.5 eV for excitation, and 5.0 eV for ionization, deduced by equivalent core analysis of high energy electron impact energy loss studies of NH₃. Similar explicit calculations on the N-1s core-excited states of NH₃ have also been made to examine directly the equivalent core concept. “Excited states” (relative to the lowest bound core-excited state) at 1.72, 2.85–3.09, and 3.32 eV, and the “ionization energy” of 4.68 eV, agree well with experiment and support the equivalent core concept. The possible significance of these H₃O results in the radiation chemistry of aqueous media is discussed in view of the fact that the maximum in the absorption spectrum of the hydrated electron lies near that of H₃O.     

Influence of Orientational Disorder on the Optical Absorption Properties of the Hybrid Metal-Halide Perovskite CH3NH3PbI3

An experimental and theoretical investigation is reported to analyze the relation between the structural and absorption properties of CH₃NH₃PbI₃ in the tetragonal phase. More than 3000 geometry optimizations were performed to reveal the structural disorder and identify structures with the lowest energies. The electronic structure calculations provide an averaged band gap of 1.674 eV, which is in excellent agreement with the experimental value of about 1.6 eV. The simulations of the absorption spectrum for three representative structures with lowest energy reproduced the absorption shoulders observed in the experimental spectra. These shoulders are assigned to excitations having similar orbital characters and involving transitions between hybridized 6s(Pb)/5p(I) orbitals and 6p(Pb) orbitals. The geometries of the three structures were analyzed and the effects of the inorganic frame and the CH₃NH₃⁺ cations on the absorption properties were estimated. It was found that both changes in the inorganic frame and the CH₃NH₃⁺ cations orientations impact the absorption spectra, by modifying the transitions energies and intensities. This highlights the role of CH₃NH₃⁺ cation in influencing the absorption properties of CH₃NH₃PbI₃ and demonstrates that CH₃NH₃⁺ cation is one of the key elements explaining the broad and nearly constant absorption spectrum in the visible range.     

A Lead Iodide Perovskite Based on a Large Organic Cation for Solar Cell Applications

Methylammonium (CH₃NH₃⁺) and formamidinium ((NH₂)₂CH⁺) based lead iodide perovskites are currently the two commonly used organic–inorganic lead iodide perovskites. There are still no alternative organic cations that can produce perovskites with band gaps spanning the visible spectrum (that is, <1.7 eV) for solar cell applications. Now, a new perovskite using large propane‐1,3‐diammonium cation (1,3‐Pr(NH₃)₂²⁺) with a chemical structure of (1,3‐Pr(NH₃)₂)_0.5PbI₃ is demonstrated. X‐ray diffraction (XRD) shows that the new perovskite exhibits a three‐dimensional tetragonal phase. The band gap of the new perovskite is about 1.6 eV, which is desirable for photovoltaic applications. A (1,3‐Pr(NH₃)₂)_0.5PbI₃ perovskite solar cell (PSC) yields a power conversion efficiency (PCE) of 5.1 %. More importantly, this perovskite is composed of a large hydrophobic cation that provides better moisture resistance compared to CH₃NH₃PbI₃ perovskite.     

High-pressure photoionization mass spectrometry. Effect of internal energy and density on the ion–molecule reactions occurring in methyl,dimethyl, and trimethylamine

The ion–molecule reactions of CH₃NH₂⁺, (CH₃)₂NH⁺, and (CH₃)₃N⁺ with the respective amines have been investigated at thermal kinetic energies in a high-pressure photoionization mass spectrometer at several wavelengths (energies) in the vacuum ultraviolet. The absolute rate coefficient for proton transfer from (CH₃)₃N⁺ to (CH₃)₃N decreases from 8.2 × 10^?10 cm³/molecule · sec at 147.0 nm (8.4 eV) to 4.9 × 10^?10 cm³/molecule. sec at 106.7-104.8 nm (11.7 eV). In dimethylamine, the rate coefficient decreases from 11.6 × 10^?10 cm³/molecular. sec at 8 4 eV to 10.2 × 10^?10 cm³/molecule osec at 11.7 eV, while no significant effect of energy was detected in methylamine. The reactions of several fragment ions are also reported. Experiments were also carried out at pressures up to 0.5 torr in order to investigate the further solvation of CH₃NH₂⁺, (CH₃)₂NH₂⁺, and (CH₃)₃NH⁺. It was found that the maximum proton solvation numbers in methyl-, dimethyl-, and trimethyl-amine are 4, 3, and 2, respectively, under these conditions.     

A kinetic study of proton transfer reactions of NH+4, CH3NH+3, and PH+4 with calcium atoms

Cross section measurements for the proton transfer reactions of NH⁺₄, CH₃NH⁺₃, and PH⁺₄ with Ca(g) have been obtained over a range of low ion kinetic energies. For all reactions studied the cross sections drop sharply with increase in ion kinetic energy, indicating exothermic behavior. The results show that Ca(g) is an unusually strong base with a proton affinity in excess of 9.2 eV. Cross sections for the PH⁺₄Ca reaction are an order to magnitude higher than those for the NH⁺₄Ca reaction for ion energies between one and three eV. This effect is not explained by simple theories of ion-induced dipole interactions. It is suggested that the enhanced rate of the PH⁺₄Ca reaction may be due to d-orbital participation.     

Unimolecular dissociations of excited C3H6O+: A comparative photoelectron—photoion coincidence study of 1,2-epoxypropane and acetone

The unimolecular fragmentation of internal energy selected 1,2-epoxypropane cations has been studied by fixed-wavelength photoelectron—photoion coincidence spectroscopy. Branching ratios for the prominent fragment ions are reported up to an ionization energy of I = 14 eV. It is shown that 1,2-epoxypropane cations initially formed with none or only little vibrational excitation in the electronic ground state do not dissociate, though their excess energy with respect to the lowest energetic fragmentation pathway is 1.25 eV. As the internal energy is increased, slow fragmentation into several dissociation channels is observed. This is used to explain a comparably slow dissociation process observed in the case of acetone molecular ions initially excited to their electronic à state. CH₂C(OH)CH₃⁺ and/or CH₃CHCHOH⁺ are proposed as precursors for these low-rate unimolecular reactions.     

Can Li Atoms Anchored on Boron- and Nitrogen-Doped Graphene Catalyze Dinitrogen Molecules to Ammonia? A DFT Study

The most successful electrochemical conversion of ammonia from dinitrogen molecule reported to date is through a Li mediated mechanism. In the framework of the above fact and that Li anchored graphene is an experimentally feasible system, the present work is a computational experiment to identify the potential of Li anchored graphene as a catalyst for N₂ to NH₃ conversion as a function of (a) minimum number of Li atoms needed for anchoring on graphene sheets and (b) the role of chemical modification of graphene surfaces. The studies bring forth an understanding that Li anchored graphene sheets are potential catalysts for ammonia conversion with preferential adsorption of N₂ through end-on configuration on Li atoms anchored on doped and pristine graphene surfaces. This mode of adsorption being characteristic of Nitrogen Reduction Reaction (NRR) through enzymatic pathway, examination of the same followed by analysis of electronic properties demonstrates that tri-Li atoms (Tri Atom Catalysts, TACs) are more efficient as catalysts for NRR as compared to two Li atoms (Di Atom Catalysts, DACs). Either way, the rate determining step was found to be *NH₂→*NH₃ step (mixed pathway) with ΔG_max=1.02 eV and *NH₂−*NH₃→*NH₂ step (enzymatic pathway) with ΔG_max=1.11 eV for 1B doped TAC and DAC on graphene sheet, respectively. Consequently, this work identifies the viability of Li anchored graphene based 2-D sheets as hetero-atom catalyst for NRR.     

Mass spectra of derivatives of N-alkyl- and N-alkoxyalkylideneimine-2-carboxylic acids

The fragmentation of compounds of the RN(CH₂)_nCHCOX (n=1–5) and RNCH₂C(COX)₂ type, where X=OAlk and NH₂ and R=H, D, OAlk, and Cl, under electron impact was studied. When n=2–5, the chief fragmentation process is amine fragmentation, and the (M–COX)⁺ ion peak is the principal peak in the spectra at 30 and 12 eV. The fragmentation of three-membered heterocycles differs radically. The dominant fragmentation for 1-alkylaziridine-2-carboxylic and -2,2-dicarboxylic esters is splitting out of a radical from the ester group. This process is absent when R=H, D, C, and OAlk. Fragmentation with splitting out of the elements of alcohol for the esters and of ammonia for the amines is characteristic in the case of derivatives of 1-alkoxyaziridine-2-carboxylic and-2,2-dicarboxylic acids.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 624–633, May, 1977.     

A Photoionization Reflectron Time-of-Flight Mass Spectrometric Study on the Detection of Ethynamine (HCCNH2) and 2H-Azirine (c-H2CCHN)

Ices of acetylene (C₂H₂) and ammonia (NH₃) were irradiated with energetic electrons to simulate interstellar ices processed by galactic cosmic rays in order to investigate the formation of C₂H₃N isomers. Supported by quantum chemical calculations, experiments detected product molecules as they sublime from the ices using photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS). Isotopically-labeled ices confirmed the C₂H₃N assignments while photon energies of 8.81 eV, 9.80 eV, and 10.49 eV were utilized to discriminate isomers based on their known ionization energies. Results indicate the formation of ethynamine (HCCNH₂) and 2H-azirine (c-H₂CCHN) in the irradiated C₂H₂:NH₃ ices, and the energetics of their formation mechanisms are discussed. These findings suggest that these two isomers can form in interstellar ices and, upon sublimation during the hot core phase, could be detected using radio astronomy.