Thermochemical determination of structure of negative ions on the basis of data from resonance electron capture mass spectrometry. 1. Heterocyclic analogs of cyclopentadiene

Appearance energies have been measured for negative ions formed in the gas phase upon dissociative capture of electrons by molecules of cyclopentadiene, pyrrole, selenophene, thiophene, furan, and furan derivatives. From an analysis of the enthalpies of formation of the fragmentation products, it has been established that electron capture by molecules of C₄H₄X (X=O, S, Se, NH, CH₂) leads to the formation of ions with cyclic and acyclic structures.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 590–592, March, 1991.     

Thermochemical determination of the structure of negative ions on the basis of data from resonance electron capture mass spectrometry. Phenol, its chlorinated derivatives and a thioanalogue

Appearance energies for [M - H](-) ions from phenol (I), 4-chlorophenol (II), pentachlorophenol (III) and pentachlorothiophenol (IV) were measured. The following thermochemical data were deduced from experiment: DeltaH(acid) values of 343.3, 335.7, 317.1 and 317.1 kcal mol(-1) for RH molecules (I, II, III, and IV, respectively) and electron affinities (EAs) of R(.) free radicals 2.55, 2.90, 3.79, and 3.65 eV, respectively. Our data for phenol (I) and 4-chlorophenol (II) demonstrate a higher stabilization of ArO(-) anions than was previously accepted. Using the enthalpic shift procedure for molecules and a series of isodesmic reactions for free radicals, earlier elaborated by the authors, a new Delta$bf H_bf fbf 0$ values for the following gas-phase species were obtained (kcal mol(-1)): C(6)Cl(5)Br (5.0), C(6)Cl(5)SH (8.5), p-ClC(6)H(4)C(.) (2. 0), C(6)Cl(5)C(.) (-15), C(6)Cl(5)S(.) (44). Copyright 2000 John Wiley & Sons, Ltd.     

Resonant electron capture by some amino acids and their methyl esters

Resonant electron capture mass spectra of aliphatic and aromatic amino acids and their methyl esters show intense [M-H](-) negative ions in the low-energy range. Ion formation results from a predissociation mechanism mediated by the low-energy pi*oo resonant state. Methylation in general has little influence on the electronic structure according to quantum chemical calculations, but the corresponding ions from the methyl esters, [M-Me](-), could be ascertained to arise only at higher resonance energies. Aromatic amino acids are characterized by an additional low-energy fragmentation channel associated with the generation of negative ions with loss of the side chain. The complementary negative ions of the side chains are more efficiently produced at higher energies. The results have significant implications in biological systems as they suggest that amino acids can serve as radiation protectors since they have been found to efficiently thermalize electrons.     

Chemoenzymatic synthesis of optically active 4-methyl-tetrahydro-5-oxo-2-furancarboxylic acids and esters

Enantiomerically pure 4-methyl-tetrahydro-5-oxo-2-furancarboxylic acids and esters are prepared by enzymatic resolution of the chiral racemic esters. Their stereochemistry as well as their absolute configurations have been established by chemical correlation. The influence of the alkoxycarbonyl group at C-2 and that of the methyl group at C-4 on the sign of the Cotton effect in their CD spectra have been investigated. Formation of enantiomerically pure hydroxydiesters, precursors of the above-mentioned γ-lactones, by baker’s yeast reduction of the corresponding ketodiesters was unsatisfactory.     

Feasibility of electron impact and electron capture negative ionisation mass spectrometry for the trace determination of tri- to deca-brominated diphenyl ethers in human samples

The applicability of two different MS ionisation modes (EI and ECNI) for the determination of PBDEs at low-trace levels in small-size (up to 1 mL) human samples was compared. The instrumental precision, expressed as R.S.D., obtained for both ionisation modes was similar and lower than 6% (repeatability) and 12% (intermediate precision) for all congeners investigated, except PBDE 209. The LODs obtained when using the ECNI-MS operation mode (6-507 fg) were lower than those found in EI-MS experiments (9 and 10,909 fg), mainly for those congeners with a high bromination degree, i.e., hepta- to deca-BDEs. The selectivity of the ECNI-MS method proposed in the present work was improved by using two ions of the [M−H_xBr_y]⁻ cluster as both qualifier and quantifier ions. For the final validation of the methods, serum and breast milk samples from two different inter-laboratory exercises were analysed. A good agreement was found between the results obtained using the proposed methods and the results provided by the different inter-laboratory organisations, but only ECNI-MS provided the low-LODs necessary for the quantification of high brominated congeners (mainly, PBDEs 196, 197 and 209) at low concentration levels in small-size human samples. Finally, the ECNI-MS method was applied to real-life samples obtained from the Spanish population and the preliminary results obtained were in the same range as those found in other European and Asian regions and lower than the concentrations reported in USA populations.     

Morpholinization of aminoacyloxy acids and hydroxyacylamino acids and their esters under mass spectrometry conditions

Summary 1. Free hydroxyacylamino acids and their methyl esters, and also free aminoacyloxy acids, morpholinize under mass spectrometry conditions.2. In the mass spectrometry of methyl esters of aminoacyloxy acids and tert-butyl esters of aminoacyloxy acids and hydroxyacylamino acids no morpholinization takes place. These compounds behave similarly to the esters of acylated peptides and depsipetides.Khimiya Prirodnykh Soedinenii, Vol. 3, No. 5, pp. 325–327, 1967     

Electronic structure of SiH2, PH2, and their positive and negative ions

Ab initio LCAO-MO-SCF calculations on SiH₂ and PH₂ and their positive and negative ions are reported, using a variety of Gaussian basis sets. The charge distribution, bonding, and molecular properties of these ions are compared with the available experimental data.     

Stable aziridinium salts of oxalic and 2,5-dihydro-4,5,5-trimethyl-2-oxo-5-furancarboxylic acids

Conclusions Stable salts of aziridine, 2-methylaziridine, and 2,2-dimethylaziridine with the oxalic and 2,5-dihydro-4,5,5~trimethyl-2-oxo-5-furancarboxylic acids were obtained.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 192–194, January, 1983.     

Dissociative double electron capture,dynamics of fragmentation of the water and hydrogen sulfide negative ions

The technique of ion kinetic energy spectrometry has been used to observe the unimolecular decompositions of H₂O^?? and H₂S^?? generated by charge exchange of the corresponding high velocity positive ions. The method involves dissociative double electron capture by a high velocity ion and allows the study of unstable negative ions that may be directly observable by conventional electron capture techniques. Information on the energetics of the reaction is obtained from the kinetic energy of the product ion. The reactions under consideration are shown in (1) and (2) where X = O or S.

The kinetic energy releases accompanying the reactions given in (1) and (2) have been measured and compared to those for the collision-induced reactions which produce the corresponding positive ions. The results have been used to deduce that the sequence of steps in the formation of the fragment negative ions is that given in (1) and (2). The cross section of OH^? formation is observed to be somewhat greater than for O^? production. This result is in contrast with dissociative electron capture cross sections from the neutral species and is interpreted on the basis of the energetic requirements for the reactions under consideration. H₂O^? reacts from different electronic states in yielding OH^? on the one hand and O^? on the other. The energy partitioning associated with reaction (2) suggests that the neutral productions 2H' rather than H₂. The kinetic energy losses accompanying excitation and kinetic energy releases upon fragmentation were similar for the corresponding reactions of the sulfur and oxygen-containing ions indicating related mechanisms in the two sets of reactions.     

Mass spectrometry of N-(Pyrimidin-2-yl)amino acids and their methyl esters

N(Pyrimidin-2-yl)-glycine, -alanine and -phenylalanine (1-3) and their methyl esters (4-6) were investigated using electron impact (EI) mass Spectrometry. The results showed that EI-induced decomposition occurs on the carboxylic group or involves the loss of R₂OH. In contrast to earlier investigations on N-(pyrimidin-4-yl)amino acids, elimination of water (in 1-3) or methanol (in 4-6) was found to be of EI-induced nature. The loss of 'COOH from M⁺ of ester 4 suggests the occurrence of a skeletal rearrangement leading to the isomeric N-methylamino acid.     

Synthesis of 5-arylpyrimidine-2-carboxylic acids and the liquid-crystal characteristics of their aryl esters

5-Arylpyrimidine-2-carboxylic acids were synthesized by the hydrolysis of 5-aryl-2-cyanopyrimidines and the oxidation of 5-aryl-2-styrylpyrimidines under the conditions of phase-transfer catalysis. The aryl esters of the acids were obtained, and their liquid-crystal characteristics were studied. The p-substituted aryl esters of 5-phenylpyrimidine-2-carboxylic acid do not exhibit mesomorphism, but the introduction of a butyloxy group at the p position of the phenyl residue leads to the appearance of nematic characteristics. Aryl 5-phenylpyrimidinylcarbonyloxy-benzoates are nematic liquid crystals with a thermally stable meso phase and an existence range of 50–80 °C.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 380–388 March, 1986.The authors express their gratitude to M. F. Grebenkin and V. T. Lazareva for useful discussion of the results.     

Reaction of 2-aminopyrimidines with αhalocarboxylic acids and their esters

N-Pyrimidylcarboxylic acids and their esters, which can be cyclized to tetrahydromidazo-[1,2-a]pyrimidine-2,5-diones, are formed from 2-aminopyrimidine and its C-substituted derivatives by reaction with -halo acids and their esters under mild conditions. Alkylation proceeds at the N₁ and N₃ atoms of the pyrimidine ring during the action of unsymmetrical 2-aminopyrimidines with -halo acids.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1140–1142, August, 1974.     

Pentafluorobenzyl chloroformate derivatization for enhancement of detection of amino acids or alcohols by electron capture negative ion chemical ionization mass spectrometry

Pentafluorobenzyl chloroformate (PFB-chloroformate) has been utilized as a derivatization reagent to impart electron affinity and provide structurally relevant fragmentation in electron capture negative ion chemical ionization mass spectrometry (ECNICI-MS). Phenylalanine (Phe) and decanol were used as model analytes. The conditions used for their derivatization and the chromatographic and mass spectrometric properties of the derivatives are reported. Phenylalanine in aqueous solution was derivatized in one step by using PFB-chloroformate and a mixture of water, ethanol, and pyridine. The phenylalanine N-pentafluorobenzyl-oxycarbonyl ethyl ester (N-PFBC-Phe-OEt) exhibited good gas chromatographic properties and in ECNICI-MS, a dominant [M — 181]^? fragment carries most of the ion current. Selected ion monitoring experiments on N-PFBC-Phe-OEt resulted in the facile detection of 400 fmol of material. Decanol was derivatized by using anhydrous conditions, and the resultant pentafluorobenzyl carbonate also exhibited a predominant [M — 181]^? ion in ECNICI-MS. Initial results indicate that the ECNICI-MS molar response of the decyl pentafluorobenzyl carbonate derivative is six-fold that of the decyl pentafluorobenzoate.     

Reaction of ethyl ester and nitrile of 5-methyl-2-furancarboxylic acid with mixture of nitric and sulfuric acids

The reaction of 70% HNO with ethyl 5-methyl-2-furancarboxylate in concentrated H₂SO₄ gave the corresponding 4-nitro derivative and ethyl 5-formyl-2-furancarboxylate. Only 4-nitro derivative, which was converted into amidoxime, was obtained from 5-methyl-2-furonitrile.Latvian Institute of Organic Synthesis, Riga LV-1006; e-mail: elmira@osi.lv. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 168–170, February 2000.     

Dissociative electron-capture mass spectrometry of fatty acids and their pyrrolidies and methyl esters

The regions of the energies of resonance capture of electrons by the molecules of fatty acids and their pyrrolidides and methyl esters have been established, which has permitted an explanation of the difference between the dissociative electron-capture mass spectra of these compounds and their chemical ionization and fast-atom bombardment negative-ion mass spectra. It has been shown that the dissociative electron-capture mass spectra of a high-energy resonance state characterized by the most far-reaching dissociation permit the unambiguous determination of the positions of multiple bonds in the initial molecules.Pacific Ocean Institute of Bioorganic Chemistry, Far Eastern Scientific Center, Academy of Sciences of the USSR. Translated from Khimiya Priordnykh Soedinenii, No. 3, pp. 348–353, May–June, 1987.     

Detection limits of electron and electron capture negative ionization-mass spectrometry for aldehydes derivatized with o-(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine hydrochloride

In contrast to common expectations, the differences in limits of detection (LODs) between electron capture negative ionization (ECNI) and electron ionization (EI) mass spectrometry (MS) were found to be insignificant for a wide range of aldehydes derivatized with o-(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine hydrochloride. Comparison of the two ionization methods based on LOD confidence intervals revealed that a traditional presentation of the LOD or limit of quantitation (LOQ) as a single value may over/underestimate the significance of obtained results. LODs were between 20 and 150 pg injected for the majority of tested derivatized carbonyls using both ionization methods. ECNI-MS improved LODs by ～10- to 20-fold only for two derivatized aldehydes, 4-hydroxybenzaldehyde and 5-(hydroxymethyl)furfural. Selectivity of ECNI did not appear to be beneficial when analyzing a wood smoke particulate matter (WS-PM) extract, possibly because the majority of interferences were removed during sample preparation (i.e., liquid-liquid extraction). The impact of four different data acquisition modes of transmission quadrupole (TQ)-MS on LODs and their precisions was also investigated. As expected, LODs in the selected ion monitoring (SIM) were ～two to four times lower than those obtained using total ion current (TIC) mode. More importantly, TQ-MS in the selected ion-total ion (SITI) mode (i.e., acquiring SIM and TIC data in a single analysis) provided signal-to-noise ratios and precisions, which were comparable to SIM alone.     

Identificatini and determination of aromatic nitro compounds by electron spin resonance spectrometry

Aromatic nitro compounds are quantitatively converted to the corresponding anion-radical form by electron tranfer at the surface of thermally activated magnesium oxide. The radicals are stable in the absorbed state, and the reaction is useful for the identification and determination of the parent compound. The method is applied to detect and quantify various nitrobenzenes, nitrotoluenes and teryl by electron spin resonance spectrometry. For 2,4,6-trinitrotoluene (TNT), the detection limit was ca. 10 ng; the r.s.d. at the 2 μg level was ± 1.8%. The analysis of hand-swab extracts that contained TNT at the trace levels demonstrateds a potentially important application of the method.