Resonant Electron Capture Research Articles

Resonance electron capture by perylene molecules. Relation with negative differential conductance

Five resonant states of long-lived negative molecular ions were defined during the gas-phase resonant electron capture by perylene. Three states are the ground state (0.05 eV) and two shape resonances (0.4 eV and 0.7 eV). Core-excited Feshbach resonance (1.4 eV) transits into a quartet state, which delays the autodetachment of additional electron and causes the negative differential conductance in tunneling transition due to the spin prohibition. Inter-shell resonance (2.5 eV) is mixed with the ground state with the same symmetry and is likely to be stabilized through the light emission with simultaneous transition to a quartet state.

Resonant electron capture by polycyclic aromatic hydrocarbon molecules: Effects of aza-substitution.

Resonant electron capture by aza and diaza derivatives of phenanthrene (7,8-benzoquinoline and 1,10-phenanthroline) and anthracene (acridine and phenazine) at incident free electron energies (Ee) in the range of 0-15eV was studied. All compounds except 7,8-benzoquinoline form long-lived molecular ions (M-) at thermal electron energies (Ee ∼ 0eV). Acridine and phenazine also form such ions at epithermal electron energies up to Ee = 1.5-2.5eV. The lifetimes (τa) of M- with respect to electron autodetachment are proportional to the extent of aza-substitution and increase on going from molecules with bent geometry of the fused rings (azaphenanthrenes) to linear isomers (azaanthracenes). These regularities are due to an increase in the adiabatic electron affinities (EAa) of the molecules. The EAa values of the molecules under study were comprehensively assessed based on a comparative analysis of the measured τa values using the Rice-Ramsperger-Kassel-Marcus theory, the electronic structure analysis using the molecular orbital approach, as well as the density functional calculations of the total energy differences between the molecules and anions. The only fragmentation channel of M- ions from the compounds studied is abstraction of hydrogen atoms. When studying [M-H]- ions, electron autodetachment processes were observed, the τa values were measured, and the appearance energies were determined. A comparative analysis of the gas-phase acidity of the molecules and the EAa values of the [M-H]· radicals revealed their proportionality to the EAa values of the parent molecules.

Resonant Capture of Electrons by Molecules of Uracile, Cytosene, and Their 5-Methyl- and 5-Hydroxymethyl Derivatives

Resonant Capture of Electrons by Molecules of Uracile, Cytosene, and Their 5-Methyl- and 5-Hydroxymethyl Derivatives

Resonant Capture of Electrons by Molecules of Uracile, Cytosene, and Their 5-Methyl- and 5-Hydroxymethyl Derivatives

A study is performed of electron-induced reactions with uracil, thymine, 5-hydroxymethyluracil, cytosine, 5-methylcytosine, and 5-hydroxymethylcytosine. The processes responsible for the formation of negative ions in the studied objects are identified. Features of the formation of the mass spectra of hydroxymethyl derivatives are associated with the abstraction and destruction of substituents. The cross sections of [M–H]− ions are determined in the 3 eV range of energies: (1.1–2.6) × 10−18 cm2 for uracils and (3.6–5.0) × 10−19 cm2 for cytosines.

Resonance Electron Capture by 5-Methyluridine and 3'-Deoxythymidine Molecules

Resonance Electron Capture by 5-Methyluridine and 3'-Deoxythymidine Molecules

Resonance Electron Capture by 5-Methyluridine and 3'-Deoxythymidine Molecules

Negative ion mass spectrometry is used to study processes of resonant electron attachment by 5‑methyluridine and 3'-deoxythymidine nucleoside molecules in the electron 0–14 eV range of energies. It is established that they are similar to those in nucleosides studied earlier (uridine, deoxyuridine, thymidine). The main channels of the fragmentation of molecular ions are revealed, and the absolute cross sections for the formation of fragment ions are determined. It is found that the intensity of the breaking the glycosidic bond in 3'-deoxythymidine in the region of low energies is two and a half orders of magnitude below the one in stavudine, testifying to the prospect of replacing the antiretroviral drug stavudine with 3'-deoxythymidine if radiation therapy is required for oncological diseases contracted as complications of HIV.

Correlation between the negative differential conductance of single molecules and gas-phase long-lived negative ions formed during resonant electron capture by the same molecules

A correlation is shown between the negative differential conductance of single molecules recorded using scanning tunneling microscopes and gas-phase long-lived negative ions formed upon resonant electron capture by the same molecules with a multiplicity of four (4TNI#) and a spin prohibition on the rapid autodetachment of an additional electron. It is assumed that negative differential conductance in the studied compounds is associated with the formation of 4TNI# ions in scanning tunneling microscopes, which suppress the tunneling current due to a similar spin prohibition. Remarks are made about a possible mechanism for the formation of 4TNI# ions in scanning tunneling microscopes.

Xanthine and Methyl Xanthine Molecules in Reactions with Low-Energy Electrons

Intense signals of [M–H]– ions have been detected in the low-energy region of the resonance electron capture mass spectra of xanthine and its methyl derivatives. It was assumed that xanthine molecules can act as radioprotectors in living cells. It was concluded that the discovered release of the carbonyl and methyl groups is associated with xanthine metabolism reactions in the tissues of living organisms.

ДИССОЦИАТИВНЫЙ ЗАХВАТ ЭЛЕКТРОНОВ МОЛЕКУЛАМИ ДИГИДРОКУМАРИНА И 5,6-МЕТИЛЕНДИОКСИ-1-ИНДАНОНА

The molecules of dihydrocoumarin and 5,6-methylenedioxy-1-indanone were studied by the negative ion resonant electron capture mass spectrometry for slow (0-15 eV) electrons. Such studies of the structure of vacant molecular orbitals (VMO) and dissociative electron attachment (DEA) for isolated molecules of prospective organic semiconducting materials, which are of great interest in terms of use as elementary components of organic nanoelectronics devices, are the most promising, since a correspondence between the energies of vacant MO isolated molecules and the maxima of the density of states in the conduction band of thin films was revealed earlier. Also, there is an idea developing in modern science, that the results of studies of vacant orbitals of individual molecules in the gas phase, in particular, using the DEA method, are necessary to identify the features of the energy structure of the conduction band of molecular organic condensates, starting from energies below the vacuum level, by the amount of binding energy of molecules in the film. In particular, it was shown that the substances studied in this work cannot be used as monomeric compounds in the design of electrically conductive polymers in the future, since they do not form negative molecular ions in the gas phase at energies close to thermal.

Resonant electron capture by 5-Br-2'-deoxyuridine.

The results of the study of resonant electron capture by molecules of 5-Br-2'-deoxyuridine (BrdUrd) over the range of electron energies from near zero to 14eV are described. In the thermal energy range, long-lived molecular negative ions, unstable with respect to autoneutralization and dehalogenation, have been registered. Examination of the kinetics of these decay processes led us to the conclusion that the most probable structure for molecular negative ions is that with an extended C-Br bond, which was predicted earlier using quantum-chemical calculations. Estimates have shown that the BrdUrd molecule owns a significant electronic affinity of 0.93-1.38eV. The most intense fragmentation channel leads to the abundant formation of Br- ions. The dissociative electron attachment cross section for Br- ions formation was estimated to amount to no less than 1.65 × 10-15cm2, indirectly implying a fairly intense formation of complementary highly reactive deoxyuridine-5-yl particles. These particles are known to be responsible for the radiosensitizing properties of BrdUrd.

Determination of the Characteristics of the Molecular Structure of Trialkyl Phosphates by the Joint Application of Electronic Ionization Mass Spectra with Registration of Positive Ions and Negative Ions of Resonance Coupling

Trialkylphosphates are of considerable interest from the military-chemical point of view as precursors of highly toxic organophosphorus toxicants, possible products of their decontamination, complexing agents in processes of reprocessing of used nuclear fuel, etc. Meanwhile, the development of ways to identify these substances is still in its infancy. In addition, trialkylphosphates are an interesting subject for the search of a solution to the analytical problem of establishing the structure of O-alkyl fragments in molecules of M(O) (OR) n esters of multibasic acids, where M is the central atom (central core), n is the basicity of the acid, R are alkyl radicals with an unknown and in general case different number of carbon atoms in each of them. No general solution to this problem could be found in the book sources. The aim of this work is to develop an algorithm for in-depth identification of trialkyl phosphates, which includes establishing the belonging of the investigated substance to this class, as well as a general and reliable method for determining the number of carbon atoms in each of the O-alkyl radicals of the investigated substances. On the basis of the revealed regularities of the decay of positive molecular ions of trialkyl phosphates, as well as negative ions of resonant capture of electrons of the same compound obtained under conditions of electron ionization, an algorithm for in-depth identification of trialkyl phosphates is proposed, which includes establishing the belonging of the investigated substance to this class, as well as a general and reliable method determining the number of carbon atoms in each of the O-alkyl radicals of the substance molecule

Anisotropy and polarization of x-ray line emissions in the dielectronic recombination of hydrogenlike Fe25+ ions

We present a systematic study of the anisotropy and polarization of x-ray line emissions in $KLL$ dielectronic recombination of hydrogenlike ${\mathrm{Fe}}^{25+}$ ions. For the $KLL$ dielectronic recombination considered, it consists of a resonant electron capture plus a two-step radiative decay. While the anisotropy and polarization of the first-step decay photons are calculated by using the flexible atomic code, those of the second-step decay photons are obtained with two methods, i.e., the formalism of the deorientation factor and density matrix theory. Resonance strengths are presented as well to synthesize the dielectronic satellite line spectra. The synthetic spectra including the correction of anisotropy and polarization effects show better agreement with existing experiments than previous calculations. This indicates that these effects are important for the intensity distribution of the dielectronic satellite lines.

Resonance Capture of Electrons by Molecules near the Threshold of Ionization

Resonance Capture of Electrons by Molecules near the Threshold of Ionization

Three New Quinazoline-Containing Indole Alkaloids From the Marine-Derived Fungus Aspergillus sp. HNMF114.

By feeding tryptophan to the marine-derived fungus Aspergillus sp. HNMF114 from the bivalve mollusk Sanguinolaria chinensis, 3 new quinazoline-containing indole alkaloids, named aspertoryadins H–J (1–3), along with 16 known ones (4–19), were obtained. The structures of the new compounds were elucidated by the analysis of spectroscopic data combined with quantum chemical calculations of nuclear magnetic resonance (NMR) chemical shifts and electron capture detector (ECD) spectra. Structurally, compound 3 represents the first example of this type of compound, bearing an amide group at C-3. Compounds 10 and 16 showed potent α-glucosidase inhibitory activity with IC50 values of 7.18 and 5.29 μM, and compounds 13 and 14 showed a clear activation effect on the ryanodine receptor from Spodoptera frugiperda (sfRyR), which reduced the [Ca2+]ER by 37.1 and 36.2%, respectively.

Resonant electron capture negative ion mass spectrometry: the state of the art and the potential for solving analytical problems

Resonant electron capture negative ion mass spectrometry: the state of the art and the potential for solving analytical problems

Thymidine and stavudine molecules in reactions with low-energy electrons

The negative ion formation and fragmentation processes in thymidine and stavudine were studied by means of resonant electron capture mass spectrometry in the electron energy range 0–14 eV. Stavudine is a nucleoside analog of thymidine, that differs from the latter by the presence of double bond in the deoxyribose moiety. In the energy range below 5 eV transient negative ions have been found to arise via shape resonances resulting in electron attachment into the empty π*-orbitals. The main fragmentation channels for molecular ions are the H-atom loss and N-glycosidic bond cleavage between sugar and a nucleobase moieties. The dissociative electron attachment cross sections for the fragment negative ions were measured. The structures for some observed anions and neutral counterparts were suggested basing on the thermochemical approach for dissociative reactions energy balance under the support of quantum chemical calculations. The processes of ion formation in thymidine were found to be similar to those revealed previously for deoxyuridine nucleoside. In contrast, very strong dissociative channel was found for stavudine yielding [M–R]– ions (where R is a sugar moiety) in the energy range <3 eV. The efficient fragmentation found for stavudine in the low energy electron attachment reactions may have implications for the design of nucleoside radiosensitizers for cancer therapy.

Desorption processes in the measurement of weak currents

It was previously shown that the experimental measurement of the lifetime of gas-phase negative ions formed in autoionization states during resonant electron capture by molecules is inherently fraught with inaccuracy due to adsorption of ions on the walls of the ionization chamber, with some of them surviving there and staying stabilized as ‘eternal’ till they eventually desorb back into the vacuum and merge with the total ion flow; this leads to an upward distortion of lifetime measurements. This paper describes a technique for removing desorbed ions from the total flow using an ion filter that lets only gas-phase ions pass into the detection system but detains the desorbed ions due to the differences among their kinetic energies.

Comparison of sensitivity for methods of positive and negative ion mass spectrometry, exemplified by nitrotoluenes

For a model series of nitro aromatic compounds comprising mono-, di- and trinitrotoluenes, and dinitrobenzene, using the same commercial experimental set up consisting of gas chromato-graphy/mass spectrometry (GC/MS) instrument equipped with a quadrupole analyzer, the two types of mass spectra were recorded, of positive ions (in the conventional, standard mode of electron ionization at 70 eV) and of negative ions (NI, generated at low, pre-ionization electron energies by resonant electron capture, REC). The comparative analysis of characteristic fragmentation processes was carried out for molecular ions of both signs. The sensitivity of REC NI MS method was established to enhance upon increasing the number of electron-withdrawing nitro groups, reaching or even outperforming that of the classical method of electron ionization (EI). Thus, the detection limit for trinitrotoluene in a solution was estimated as 5 ppm, which is by an order of magnitude lower as compared to the conventional EI mode of analysis. Comparison of the ion composition of NI mass spectra and of the relative intensities of ion peaks with the corresponding data obtained previously for the studied group of compounds by means of different types of mass spectrometers, has revealed a satisfactory inter-instrumental reproducibility of the NI mass spectra. The results reported herein indicate that the NI generation/registration mode, which can technically be readily available even without modifications of design of commercial GC/MS instrument in their basic configuration, is suitable to be introduced in laboratory practice as the additional tool for increasing the selectivity and reliability in detection/identification of trace amounts of compounds possessing a high electron af nity and for solving extraordinary structural and analytical problems.

Mass Spectra Electron Ionization of Tetraalkyl Pyrophosphates in the Mode of Registration of Positive Ions and Negative Ions of Resonance Electron Capture. Group Identification and Determination of the Number of Carbon Atoms in Alkyl Radicals of These Compounds

he decomposition paths of tetraalkyl pyrophosphates under conditions of electron ionization in the mode of registration of positive ions correspond to the general pattern of fragmentation of phosphoric acids esters elucidated earlier by the authors. Electron ionization mass spectra in the mode of registration of negative ions by resonant electron capture are an effective means for extracting information on the number of carbon atoms in the alkyl radicals of tetraalkyl pyrophosphates and full esters of other phosphoric acids

Mixing of States with the Same Symmetry in Negative Ions

An analysis is performed of the mass spectrum of resonance electron capture by tetracyanoquinodimethane molecules, in which long-lived negative molecular ions are detected. At 3.6 eV, the ions can be attributed to an electron-excited Feshbach resonance with an admixture of a long-lived ground state whose contribution to the superposition of two states yields a measurable ion lifetime in the microsecond range.