The α-effect in gas-phase SN2 reactions of microsolvated anions: methanol as a solvent.

The α-effect, an enhanced reactivity of nucleophiles with a lone-pair adjacent to the reaction center, has been studied in solution for several decades. The gas-phase α-effect has recently been documented in studies of SN2 reactions as well as in competing reactions for both bare and microhydrated anions. In the present work we extend our studies of the significance of microsolvation on the α-effect, employing methanol as the solvent, in the expectation that the greater stability of the methanol cluster relative to the water cluster will lower the reactivity and thereby allow studies over a wider efficiency range. We compare the gas-phase reactivity of the microsolvated α-nucleophile HOO(-)(CH3OH) to that of microsolvated normal alkoxy nucleophiles, RO(-)(CH3OH) in reactions with CH3Cl and CH3Br. The results reveal enhanced reactivity of HOO(-)(CH3OH) toward both methyl halides relative to the normal nucleophiles, and clearly demonstrate the presence of an α-effect for the microsolvated α-nucleophile. The highly exothermic reactions with methyl bromide result in a smaller Brønsted ßnuc value than observed for methyl chloride, and the α-effect in turn influences the reactions with methyl chloride more than with methyl bromide. Computational investigations reveal that reactions with methyl bromide proceed through earlier transition states with less advanced bond formation compared to the related reactions of methyl chloride. In addition, solvent interactions for HOO(-) are quite different from those with the normal nucleophiles at the transition state, indicating that differential solvation may well contribute to the α-effect. The greater thermodynamic and kinetic stability of the anion-methanol clusters relative to the anion-water clusters accounts well for the differences in the influence of solvation with the two protic polar solvents.

Investigating the α-effect in gas-phase S(N)2 reactions of microsolvated anions.

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 ßnuc 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.

Intramolecular interactions in 2-aminoethanol and 3-aminopropanol.

Gas-phase vibrational spectra of 2-aminoethanol and 3-aminopropanol were recorded up to the third OH-stretching overtone using Fourier transform infrared spectroscopy, cavity ringdown spectroscopy, and intracavity laser photoacoustic spectroscopy. The experimental investigation was supplemented by local mode calculations, and the intramolecular interactions were investigated using atoms in molecules (AIM) and noncovalent interactions (NCI) theories. All calculations were performed at the CCSD(T)-F12a/VDZ-F12 level of theory. For both compounds the most abundant conformer has a structure that allows for hydrogen bond interaction from the OH group to the N atom of the amino group (OH-N). The spectra show signals from both hydrogen bonded and free OH stretches, implying the presence of several conformers. We observe hydrogen-bond-like interactions in both compounds. The red shift of the bonded OH-stretching frequency and intensity enhancement of the fundamental transition suggest that the hydrogen bond interaction is more pronounced in 3-aminopropanol. AIM analysis supports the presence of a hydrogen bond in 3-aminopropanol but not in 2-aminoethanol, whereas NCI analysis shows hydrogen bonding in both compounds with the stronger interaction found in 3-aminopropanol.

Pseudo-bimolecular [2+2] cycloaddition studied by time-resolved photoelectron spectroscopy.

The first study of pseudo-bimolecular cycloaddition reaction dynamics in the gas phase is presented. We used femtosecond time-resolved photoelectron spectroscopy (TRPES) to study the [2+2] photocycloaddition in the model system pseudo-gem-divinyl[2.2]paracyclophane. From X-ray crystal diffraction measurements we found that the ground-state molecule can exist in two conformers; a reactive one in which the vinyl groups are immediately situated for [2+2] cycloaddition and a nonreactive conformer in which they point in opposite directions. From the measured S(1) lifetimes we assigned a clear relation between the conformation and the excited-state reactivity; the reactive conformer has a lifetime of 13 ps, populating the ground state through a conical intersection leading to [2+2] cycloaddition, whereas the nonreactive conformer has a lifetime of 400 ps. Ab initio calculations were performed to locate the relevant conical intersection (CI) and calculate an excited-state [2+2] cycloaddition reaction path. The interpretation of the results is supported by experimental results on the similar but nonreactive pseudo-para-divinyl[2.2]paracyclophane, which has a lifetime of more than 500 ps in the S(1) state.

Single Solvent Molecules Induce Dual Nucleophiles in Gas-Phase Ion-Molecule Nucleophilic Substitution Reactions.

Direct dynamics simulation of singly hydrated peroxide ion reacting with CH3Cl reveals a new product channel that forms CH3OH + Cl- + HOOH, besides the traditional channel that forms CH3OOH + Cl- + H2O. This finding shows that singly hydrated peroxide ion behaves as a dual nucleophile through proton transfer between HOO-(H2O) and HO-(HOOH). Trajectory analysis attributes the occurrence of the thermodynamically and kinetically unfavored HO--induced pathway to the entrance channel dynamics, where extensive proton transfer occurs within the deep well of the prereaction complex. This study represents the first example of a single solvent molecule altering the nucleophile in a gas-phase ion-molecule nucleophilic substitution reaction, in addition to reducing the reactivity and affecting the dynamics, signifying the importance of dynamical effects of solvent molecules.

The α-effect and competing mechanisms: the gas-phase reactions of microsolvated anions with methyl formate.

The enhanced reactivity of α-nucleophiles, which contain an electron lone pair adjacent to the reactive site, has been demonstrated in solution and in the gas phase and, recently, for the gas-phase S(N)2 reactions of the microsolvated HOO(-)(H2O) ion with methyl chloride. In the present work, we continue to explore the significance of microsolvation on the α-effect as we compare the gas-phase reactivity of the microsolvated α-nucleophile HOO(-)(H2O) with that of microsolvated normal alkoxy nucleophiles, RO(-)(H2O), in reactions with methyl formate, where three competing reactions are possible. The results reveal enhanced reactivity of HOO(-)(H2O) towards methyl formate, and clearly demonstrate the presence of an overall α-effect for the reactions of the microsolvated α-nucleophile. The association of the nucleophiles with a single water molecule significantly lowers the degree of proton abstraction and increases the S(N)2 and B(AC)2 reactivity compared with the unsolvated analogs. HOO(-)(H2O) reacts with methyl formate exclusively via the B(AC)2 channel. While microsolvation lowers the overall reaction efficiency, it enhances the B(AC)2 reaction efficiency for all anions compared with the unsolvated analogs. This may be explained by participation of the solvent water molecule in the B(AC)2 reaction in a way that continuously stabilizes the negative charge throughout the reaction.