Investigating the α-Effect in Gas-Phase S_N2 Reactions of Microsolvated Anions

Abstract

The α-effect-enhanced reactivity of nucleophiles with a lone-pair adjacent to the attacking center-was recently demonstrated for gas-phase S(N)2 reactions of HOO(-), 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(-)(H2O) to that of microsolvated normal alkoxy nucleophiles, RO(-)(H2O), in reaction with CH3Cl using a flowing afterglow-selected ion flow tube instrument. The results reveal enhanced reactivity of HOO(-)(H2O) 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 βnuc 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(-) 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 CH3Cl can lead to formation of either the bare Cl(-) anion or the Cl(-)(H2O) cluster. The product distributions show preferential formation of the Cl(-) anion even though the formation of Cl(-)(H2O) would be favored thermodynamically. Although the structure of the HOO(-)(H2O) cluster resembles HO(-)(HOOH), we demonstrate that HOO(-) is the active nucleophile when the cluster reacts.

Probing the reactivity of microhydrated α‐nucleophile in the anionic gas‐phase SN2 reaction

“…The latter then breaks down to give the products complex (Pro‐COM) CH 3 Nu(H 2 O) n ⋯Cl − . Subsequently, the Pro‐COM dissociates into CH 3 Nu and Cl − (H 2 O) n through a low‐barrier water‐migration TS, [CH 3 Nu⋯(H 2 O) n ⋯Cl] − ≠ , that is the more exothermic reaction channel . The energetics of reactants complex, inversion TS, products complex, and final products relative to separated reactants are listed in Table S1 (see Supporting Information).…”

Effect of higher order solvation and temperature on SN2 and E2 reactivity

“…However, the reaction rate constant measured for F À (CH 3 OH) 2 + C 2 H 5 Br was larger than the reaction rate constant measured for F À (CH 3 OH) + C 2 H 5 Br at both 200 and 250 K. Interestingly, the decrease in efficiency with increasing solvation was also somewhat muted for the reactions with C 2 H 5 Br, though the reasons behind these observations are unknown. Table 1 Reaction rate constants, reaction efficiencies, product ion branching ratios (BR), and exothermicities for S N 2 and E2 pathways for each reaction at 300 K. Reaction k (10 À10 cm 3 molec À1 s À1 ) at 300 K Reaction efficiency (k/k col ) Ionic product BR at 300 K DH rxn S N 2 (kJ mol À1 ) DH rxn E2 (kJ mol À1 ) F À + CH 3 …”

“…The addition of a small number of solvent molecules to gas-phase ions provides insight into this transition. Recent investigations of microsolvated ions have addressed the impact of solvation on mechanisms [2], reactivity [3], reaction dynamics [4][5][6][7][8], vibrational energy transfer [9,10], and delocalization of charge [11].…”

Nucleophilic Substitution (SN2): Dependence on Nucleophile, Leaving Group, Central Atom, Substituents, and Solvent

“…A few years later, however, the same laboratory arrived at the opposite conclusion and obtained experimental validation of the α ‐effect in the gas‐phase from a thorough thermochemical analysis . The influence of solvation was then reinvestigated Bierbaum and co‐workers, through the investigation of the gas‐phase reactivity of monosolvated nucleophiles. It was found that coordination of a single water molecule to the nucleophile resulted in a smaller PA and higher barrier compared to unsolvated nucleophiles.…”