Infrared Spectroscopic Investigation of the Acidic CH Bonds in Cationic n-Alkanes: Pentane, Hexane, and Heptane

Abstract

Radical cations of n-alkanes (pentane, hexane, and heptane) in the gas phase are investigated by infrared predissociation spectroscopy with the argon or nitrogen tagging. All-trans and gauche-involving conformers are identified for these cations by comparisons of observed infrared spectra and vibrational simulations. Intense CH stretch bands are observed in the frequency region lower than the normal alkyl CH stretch frequency. These low frequencies and high intensities of the CH stretch bands are caused by the CH bond weakening and the enhanced positive charge of the hydrogen atoms through the delocalization of the σ electron in the CH bonds. These effects of the delocalization of the σ electron result in the enhanced acidity of the CH bonds. The conformation as well as alkyl chain length dependence of the acidity of the CH bonds is demonstrated by the CH stretch frequency shift trend.

Infrared Spectroscopic Study of the Acidic CH Bonds in Hydrated Clusters of Cationic Pentane

“…Therefore, we have concluded that the pentane cation has the acidic CH bonds. The acidity enhancements of CH in cationic pentane, hexane, and heptane have been suggested by the CH stretch frequency shifts of their monomer cations . The present results support that cationic alkanes as well as cationic pentane are highly acidic.…”

“…Although CH bonds sometimes act as very weak proton donors in the CH···O/N interaction, they are generally regarded as aprotic. , On the contrary, proton transfer from positively charged CH bonds is frequently seen in gas-phase ion–molecule reactions, radiation chemistry, and organic synthesis. − However, while the acidities of cationic OH and NH have undergone a great deal of experimental and theoretical investigations, that of cationic CH has attracted much less attention. Recently, barrierless proton transfer from CH has been found in the ionized trimethylamine and dimethyl ether dimers. , These prove that the acidity enhancement of cationic CH and the reaction mechanism has been interpreted by delocalization of the positive charge through the hyperconjugation between the CH and nonbonding orbitals, the electron of which is ejected in the ionization. − Infrared (IR) spectroscopic signatures of acidity enhancement have been reported also for the cationic alkanes, which have no nonbonding orbital . This result implies that alkyl groups as well as alkanes can be acidic under the influence of positive charge.…”

“…10−12 Infrared (IR) spectroscopic signatures of acidity enhancement have been reported also for the cationic alkanes, which have no nonbonding orbital. 13 This result implies that alkyl groups as well as alkanes can be acidic under the influence of positive charge. Because of the ubiquity of alkyl groups in chemistry, elucidation of the property of charged CH would be highly important in understandings of diverse chemical processes involving carbocation and CH activation, in which (partially) charged CH arises through charge fluctuation and environmental perturbation.…”

Infrared Spectroscopic Study on Trimethyl Amine Radical Cation: Correlation between Proton-Donating Ability and Structural Deformation

“…This delocalization causes an enhancement of the proton-donating ability of CH. 36 A similar type of charge delocalization has been reported for the adamantane cation, which has a cage structure. 26 The previous studies on ethers and alcohols have demonstrated that the hyperconjugation between CH and SOMO at an oxygen atom is a key to understand the enhancement of the proton-donating ability of CH in their cation.…”

“…High proton-donating abilities of CH in radical cations have also been shown spectroscopically. − Recently, spectroscopic studies have revealed that barrierless proton-transfer reactions from CH occur in the ionization of the dimers of trimethyl amine (TMA) and ethers, and the hydrated clusters of alkane. − These barrierless proton-transfer reactions indicate the remarkable enhancement of the proton-donating ability of cationic CH. The enhancement mechanisms of the proton-donating ability in the cationic state have been analyzed for ethers, alcohols, and alkanes. − In the former two, a nonbonding electron of an oxygen atom is ejected in their ionization and thus the nonbonding orbital becomes the singly occupied molecular orbital (SOMO). A bonding σ electron of CH delocalizes to SOMO through hyperconjugation.…”