In the IR spectrum of the diethyl ether cation, an extraordinarily intense band, with an extremely broad bandwidth, was observed at 2700 cm(-1), much lower frequency than normal CH stretch frequencies. This band is assigned to the stretch band of the CH bond, which is hyperconjugated with the singly occupied molecular orbital of the oxygen atom. The hyperconjugation causes the delocalization of the σ electron of the CH bond so that it enhances the acidity of the CH bond as well as the CH stretch band intensity. Theoretical simulation shows that the strength of hyperconjugation varies greatly with internal rotation of the ethyl group, and this is reflected in the large width of the observed CH stretch band. These results indicate that the DEE cation drastically changes its property from aprotic to highly acidic by the rotational isomerization of the ethyl group.
Vibrational spectra of the methyl groups in mono-methylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA) monomers and their clusters were measured to capture their spectral features as a result of bend/umbrella-stretch Fermi resonance (FR).
Infrared spectroscopy of the hydrated clusters of cationic pentane, which are generated through the vacuum ultraviolet photoionization in the gas phase, is carried out to probe the acidic properties of their CH bonds. The monohydrated pentane cation forms the proton-shared structure, in which the proton of CH in cationic pentane is shared between the pentyl radical and water molecule. In the di- and trihydrated clusters, the proton of CH is completely transferred to the water moiety so that the clusters are composed of the pentyl radical and protonated water cluster. These results indicate that two water molecules are enough to cause the proton transfer from CH of cationic pentane, and thus its acidity is highly enhanced with the ionization.
Ionization of a molecule can greatly alter its electronic structure as well as its geometric structure. In this collaborative experimental and theoretical study, we examined variance in hyperconjugation upon ionization of diethyl ether (DEE) and diethyl sulfide (DES). We obtained the experimental gas phase vibrational spectra of DEE, DES, DEE(+), DES(+), DEE(+)-Ar, and DES(+)-Ar in the wavenumber region of 2500 to 3600 cm(-1). For DEE(+) and DEE(+)-Ar, we observed a greatly red shifted CH stretching peak at 2700 cm(-1), while the lowest CH stretching peaks for DEE, DES, DES(+) and DES(+)-Ar were observed around 2850 cm(-1). For DEE(+), we calculated a drastic red shifted CH stretching peak at 2760 cm(-1), but for DEE, DES, and DES(+) the lowest CH stretching peaks were calculated to be at 2860, 2945, and 2908 cm(-1), respectively. In addition, for DEE, the minima (maxima) geometry in the neutral state becomes a maxima (minima) geometry in the cationic state, while similar minima geometries are seen in neutral and cationic states of DES. These experimental and theoretical findings were rationalized through the natural bond orbital analysis by quantifying the hyperconjugation between the σCH orbital and the ionized singly occupied p orbital of the oxygen (sulfur) in DEE(+) (DES(+)). This study showed how orientation with the ionized orbital can greatly affect the neighboring CH bond strength and its polarity, as well as the geometry of the system. Furthermore, this change in the CH bond strength between DEE(+) and DES(+) is quantified from the energies for intramolecular proton transfer in the two cations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.