Thermochemical determination of the structure of negative ions with the data from resonance electron capture spectrometry. 2. 5-Substituted 2-furancarboxylic acids and their esters

The energetics of isomerization and fragmentation of carboxylic acids and their esters from the furan series were investigated in conditions of resonance electron capture with the formation of negative ions. The enthalpies of formation of fragmentary negative ions were determined and the structure of some charged and neutral products of dissociation of these compounds was established. Schemes of the hydrogen and backbone rearrangements were proposed.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2049–2054, September, 1991.     

Specific features of the resonant electron attachment to the chlorodibenzo-p-dioxin molecules

The processes of resonant dissociative electron attachment to the molecules of dibenzo-p-dioxin and its chlorinated derivatives containing one to four chlorine atoms (totally eight compounds) were investigated. It was established that 2,3,7-trichlorodibenzo-p-dioxin; 1,2,3,4-tetrachlorodibenzo-p-dioxin; 1,3,7,8-tetrachlorodibenzo-p-dioxin, and 2,3,7,8-tetrachlorodibenzo-p-dioxin molecules are chatacterized by positive electron affinities. At electron energies below 2 eV, the electron attachment is caused by the shape resonances. Based on the energy correlation between the negative ion resonance peaks at 3—4 eV and the UV band maxima, it was suggested that electron attachment in this energy region occurs by the mechanism of inter-shell resonance with the molecular singlet-excited states as parents. The possibility for the rearrangement processes resulting in oxy-anionic structures to occur is substantiated.     

On the structure of negative ions formed by dissociative electron attachment by monochlorophenol molecules

The energetics of negative ion formation by resonant dissociative electron attachment by o-, m-, and p-chlorophenol molecules was studied. The structures of some fragment ions and their neutral partners were established. Hidden rearrangement processes leading to the formation of oxy anions by the detachment of chlorine atoms from molecular ions were found. The O—H bond dissociation energies for o-, m-, and p-chlorophenol molecules were 3.74±0.11, 3.72±0.17, and 3.94±0.11 eV, respectively.     

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.     

Formation of negative ions <Emphasis Type="Italic">via</Emphasis> resonant low-energy electron capture by cysteine and cystine methyl esters

The processes of resonance low-energy free electron attachment to methyl esters of some sulfur-containing amino acids were studied. The long-lived molecular negative ions of cystine dimethyl ester formed in the valence state via the Feshbach nuclear excited resonance mechanism were detected by mass spectrometry. The reactions of disulfide bond dissociation were identified in an electron energy range of 0—1 eV. They can be considered as model reactions regarding processes of peptide decomposition due to the resonance interaction with low-energy electrons. Predissociation of short-lived molecular ions of cysteine methyl ester formed by capture of electrons with energies of ~1.6 eV is accompanied by the intra-ionic transfer of negative charge from the carbonyl group to the sulfur atom leading to the elimination from the latter of hydrogen atom.     

Slow decay of negative molecular fluorofullerene ions in the electron autodetachment process

The mean lifetimes of negative molecular ions of C₆₀ fullerene and its fluoroderivatives C₆₀F₁₈ and C₆₀F₃₆ with respect to the autodetachment of electrons have been measured as functions of the ionizing electron energy by the method of mass spectrometry of electron resonance capture. The lifetimes of negative ions in the compounds under investigation are within the one-second range, and an increase in the number of addends leads to an increase in the lifetimes of negative ions by 1.5–2.5 orders of magnitude.     

On the energy dependence of the yield of doubly charged negative ions during the capture of free electrons by C60(CF3)12 trifluoromethylfullerene molecules

Peaks of C₆₀(CF₃) _n ^2? doubly charged negative ions (n = 6–12) have been observed in the mass spectra of the resonance electron capture by trifluoromethylfullerene C₆₀(CF₃)₁₂ molecules. It has been established that these ions are formed owing to the attachment of two free isoenergetic electrons. The autodetachment of an extra electron has been detected for the doubly charged molecular ions (n = 12). It has been established from the observation of the delayed fragmentation of the most abundant ions with n = 8 and 10 that the doubly charged negative ions, like their singly charged analogs, are metastable with respect to the separation of the CF₃ fragment(s). The yield of doubly charged negative ions has been obtained as a function of the electron energy. By comparing them with the analogous dependences for the singly charged ions, the specific features have been revealed which were associated with the presence of the repulsive Coulomb barrier and the regular effect of the doubled energy of two additional electrons on the energy dependence of the dissociative decay of the doubly charged negative ions. The absolute cross section for the formation of the C₆₀(CF₃) ₁₀ ^2? ions has been measured. At the energy of their yield maximum near the 5 eV, it is ～1 × 10^?19 cm².     

Determination of electron affinity of carbonyl radicals by means of negative ion mass spectrometry

Appearance energies of [M-H](-) ions from carbonyl compounds R-CO-R' (R,R' = H, CH(3), NH(2), OH) have been measured by means of negative ion mass spectrometry in resonant electron capture mode. Values of electron affinity of the corresponding radicals, CH(2)&dbond;C(X)O, NH&dbond;C(X)O and O&dbond;C(X)O, have been determined. Copyright 1999 John Wiley & Sons, Ltd.