Low energy (0–10 eV) electron driven reactions in the halogenated organic acids CCl3COOH, CClF2COOH, and CF3CHNH2COOH (trifluoroalanine)

Abstract

Negative ion formation following resonant electron attachment to the three title molecules is studied by means of a beam experiment with mass spectrometric detection of the anions. All three molecules exhibit a pronounced resonance in the energy range around 1 eV which decomposes by the loss of a neutral hydrogen atom thereby generating the closed shell anion (M-H)(-) (or RCOO(-)), a reaction which is also a common feature in the non-substituted organic acids. The two chlorine containing molecules CCl(3)COOH and CClF(2)COOH exhibit an additional strong and narrow resonance at very low energy (close to 0 eV) which decomposes by the cleavage of the C-Cl bond with the excess charge finally localised on either of the two fragments Cl(-) and (M-Cl)(-). This reaction is by two to three orders of magnitude more effective than hydrogen loss. Apart from these direct bond cleavages (C-Cl, O-H) resonant attachment of subexcitation electrons trigger additional remarkably complex unimolecular decompositions leading, e.g., to the formation of the bihalide ions ClHCl(-) and ClHF(-) from CCl(3)COOH and CClF(2)COOH, respectively, or the loss of a neutral CF(2) unit from trifluoroalanine thereby generating the fluoroglycine radical anion. These reactions require substantial rearrangement in the transitory negative ion, i.e., the cleavage of different bonds and formation of new bonds. F(-) from both chlorodifluoroacetic acid and trifluoroalanine is formed at comparatively low intensity (more than three orders of magnitude less than Cl(-) from the chlorine containing molecules) and predominantly within a broad resonant feature around 7-8 eV characterised as core excited resonance.

Electron-induced damage of biotin studied in the gas phase and in the condensed phase at a single-molecule level

“…The CN − ion formation requires a complex reaction, since five covalent bonds need to be broken prior to its formation (N-H, two C-N bonds and a C O double bond). Nevertheless CN − is often observed after electron attachment to various molecules such as amino acids [51,54], trifluoroalanine [46], N-acetyl-glycine [55], hexafluoroacetone azine [56], formamide [57] and various amide derivatives [58] even at very low energies (0-3 eV). The low-energy thresholds for CN − formation are usually explained by the high electron affinity of the CN radical (3.8 eV), the additional two bonds that are formed within CN − , and a number of new bonds that must be formed in byproducts in the course of the reaction.…”

“…This fragment ion is most likely generated by hydrogen loss from the carboxyl group of Bt and represents a stable closed shell anion. Similar to other organic acids such as formic acid [44] and halogenated organic acids [45,46], the resonance at 1.3 eV is assigned to a π * shape resonance, i.e. the incoming electron occupies a formerly empty π * orbital located on the COOH group.…”

Electron driven processes in chlorodifluoroacetic acid methyl ester

“…Although the thermodynamic threshold for F − formation, as mentioned above, is quite low and F is characterized by an appreciable electron affinity, the F − ion intensity is much lower than Cl − . As discussed in our earlier paper [10] this is due likely to significant difference in polarizability between Cl and F atoms (2.18 × 10 −24 cm 3 and 0.557 × 10 −24 cm 3 , respectively [12]) which is an essential quantity determining the value of electron attachment cross section. This value increases downwards the 17th group and changes in the same line as the electron capacity [14], which is a quantity describing the ability of a halogen in a neutral molecule to accommodate extra charge.…”

“…It is noteworthy that for previously studied chloro-containing organic acids (CClF 2 COOH and CCl 3 COOH [10]) the complementary anionic fragment (M-Cl) − has been generated from DEA reactions. While for the CCl 3 COOH molecule (M-Cl) − was the second most intense anionic product, for CClF 2 COOH it was in fact the most intense product.…”

“…In addition, due to the high electron affinity of the halogen atoms further and often more intense fragments are generated at this low energy domain. As an example, in the case of CF 3 COOH [8] the loss of HF is observed as the second most dominant decomposition channel at around 1 eV, while for the chloro-containing compounds namely CClF 2 COOH [9,10] and CCl 3 COOH [10] there are two complementary fragments Cl − and (M-Cl) − , which are produced within the highest intensity already at energy close to 0 eV.…”