2 research outputs found
Investigating the α‑Effect in Gas-Phase S<sub>N</sub>2 Reactions of Microsolvated Anions
The α-effectenhanced
reactivity of nucleophiles with
a lone-pair adjacent to the attacking centerî—¸was recently demonstrated
for gas-phase S<sub>N</sub>2 reactions of HOO<sup>–</sup>,
supporting an intrinsic component of the α-effect. In the present
work we explore the gas-phase reactivity of microsolvated nucleophiles
in order to investigate in detail how the α-effect is influenced
by solvent. We compare the gas-phase reactivity of the microsolvated
α-nucleophile HOO<sup>–</sup>(H<sub>2</sub>O) to that
of microsolvated normal alkoxy nucleophiles, RO<sup>–</sup>(H<sub>2</sub>O), in reaction with CH<sub>3</sub>Cl using a flowing
afterglow-selected ion flow tube instrument. The results reveal enhanced
reactivity of HOO<sup>–</sup>(H<sub>2</sub>O) and clearly demonstrate
the presence of an α-effect for the microsolvated α-nucleophile.
The association of the nucleophile with a single water molecule results
in a larger Brønsted β<sub>nuc</sub> value than is the
case for the unsolvated nucleophiles. Accordingly, the reactions of
the microsolvated nucleophiles proceed through later transition states
in which bond formation has progressed further. Calculations show
a significant difference in solvent interaction for HOO<sup>–</sup> relative to the normal nucleophiles at the transition states, indicating
that differential solvation may well contribute to the α-effect.
The reactions of the microsolvated anions with CH<sub>3</sub>Cl can
lead to formation of either the bare Cl<sup>–</sup> anion or
the Cl<sup>–</sup>(H<sub>2</sub>O) cluster. The product distributions
show preferential formation of the Cl<sup>–</sup> anion even
though the formation of Cl<sup>–</sup>(H<sub>2</sub>O) would
be favored thermodynamically. Although the structure of the HOO<sup>–</sup>(H<sub>2</sub>O) cluster resembles HO<sup>–</sup>(HOOH), we demonstrate that HOO<sup>–</sup> is the active
nucleophile when the cluster reacts
Substituent Effects on the Nonradical Reactivity of 4-Dehydropyridinium Cation
Recent studies have shown that the reactivity of the
4-dehydropyridinium
cation significantly differs from the reactivities of its isomers
toward tetrahydrofuran. While only hydrogen atom abstraction was observed
for the 2- and 3-dehydropyridinium cations, nonradical reactions were
observed for the 4-isomer. In order to learn more about these reactions,
the gas-phase reactivities of the 4-dehydropyridinium cation and several
of its derivatives toward tetrahydrofuran were investigated in a Fourier
transform ion electron resonance mass spectrometer. Both radical and nonradical
reactions were observed for most of these positively charged radicals.
The major parameter determining whether nonradical reactions occur
was found to be the electron affinity of the radicalsî—¸only
those with relatively high electron affinities underwent nonradical
reactions. The reactivities of the monoradicals are also affected
by hydrogen bonding and steric effects