Benzene as a selective chemical ionization reagent gas
摘要:
AbstractDilute mixtures of C6H6 or C6D6 in He provide abundant [C6H6]+· or [C6D6]+· ions and small amounts of [C6H7]+ or [C6D7]+ ions as chemical ionization (CI) reagent ions. The C6H6 or C6D6 CI spectra of alkylbenzenes and alkylanilines contain predominantly M+· ions from reactions of [C6H6]+· or [C6D6]+· and small amounts of MH+ or MD+ ions from reactions of [C6H7]+ or [C6D7]+. Benzene CI spectra of aliphatic amines contain M+·, fragment ions and sample‐size‐dependent MH+ ions from sample ion‐sample molecules reactions. The C6D6 CI spectra of substituted pyridines contain M+· and MD+ ions in different ratios depending on the substituent (which alters the ionization energy of the substituted pyridine), as well as sample‐size‐dependent MH+ ions from sample ion‐sample molecule reactions. Two mechanisms are observed for the formation of MD+ ions: proton transfer from [C6D6]+· or charge transfer from [C6D6]+· to give M+·, followed by deuteron transfer from C6D6 to M+·. The mechanisms of reactions were established by ion cyclotron resonance (ICR) experiments. Proton transfer from [C6H6]+· or [C6D6]+· is rapid only for compounds for which proton transfer is exothermic and charge transfer is endothermic. For compounds for which both charge transfer and proton transfer are exothermic, charge transfer is the almost exclusive reaction.
The rate of back electron transfer following photoexcitation of ground-state complexes between ClO and aromatic molecules in nitrile solvents is examined. Both solvent effects on a single molecular complex and a series of complexes within a single solvent are analyzed in terms of commonly used theoretical models, For a single molecular complex (ClO-benzene), the rate of back electron transfer decreases with decreasing solvent dielectric constant and is temperature independent. This indicates that the reaction rate decreases with increasing exothermicity, behavior consistent with that expected for reactions in the Marcus inverted region. Investigation of the dependence of the reaction rate on exothermicity using a series of substituted benzenes as acceptor molecules in a single solvent revealed increasing reaction rates with increasing driving force, opposite to that observed upon varying solvent. Studies of deuteration effects on the reaction rate constant suggest that the origin of these disparate predictions arises from the assumptions made in carrying out the data analysis on the series of donor-acceptor complexes studied. In particular, both the inner-sphere and outer-sphere contributions to the reorganization energy are not constant for the set of molecules studied. This study demonstrates the difficulty in extracting accurate information on the reaction exothermicity, reorganization energy, and electronic coupling from measured rate constants.