Understanding Fermi resonances in the complex vibrational spectra of the methyl groups in methylamines.

Vibrational spectra of the methyl groups in mono-methylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA) monomers and their clusters were measured in three experimental set-ups to capture their complex spectral features as a result of bend/umbrella-stretch Fermi resonance (FR). Multiple bands were observed between 2800 and 3000 cm-1 corresponding to the methyl groups for MMA and DMA. On the other hand, the corresponding spectrum of TMA is relatively simple, exhibiting only four prominent bands in the same frequency window, even though TMA has a larger number of methyl groups. The discrete variable representation (DVR) based ab initio anharmonic algorithm with potential energy surface (PES) at CCSD/aug-cc-pVDZ quality is able to capture all the experimentally observed spectral features across all three amines, and the constructed vibrational Hamiltonian was used to analyze the couplings that give rise to the observed FR patterns. It was observed that the vibrational coupling among CH stretch modes on different methyl groups is weak (less than 2 cm-1) and stronger vibrational coupling is found to localize within a methyl group. In MMA and DMA, the complex feature between 2850 and 2950 cm-1 is a consequence of closely packed overtone states that gain intensities by mixing with the stretching modes. The simplification of the spectral pattern of TMA can be understood by the red-shift of the symmetric CH3 stretching modes by about 80 cm-1 relative to MMA, which causes the symmetric CH3 stretch to shift outside the FR window.

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

Barrierless intermolecular proton transfer from a CH bond has recently been reported in the vertical ionization of the trimethyl amine (TMA) dimer. This result indicates the remarkable enhancement of the proton-donating ability of the CH bond in its cationic state. In the present study, we have carried out an infrared spectroscopy of the neutral and cationic TMA in the CH stretch region and their theoretical calculations to investigate the mechanism of enhancement of the proton-donating ability in the cationic state. In the spectrum of the cation, the CH stretch band shows a long tail of up to 2600 cm-1. This tail component is attributed to the CH bond hyperconjugated with the nonbonding orbital at the nitrogen atom through geometry deformation (excitation of molecular vibrations) with the excess energy upon photoionization. This hyperconjugation causes the delocalization of the σ electron of the CH bond to the singly occupied nonbonding orbital so that the proton-donating ability of the CH is enhanced. It is shown that the excitation of the CN stretching vibration is especially effective in promoting the hyperconjugation.

Hyperconjugation in diethyl ether cation versus diethyl sulfide cation.

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.

The large variation in acidity of diethyl ether cation induced by internal rotation about a single covalent bond.

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.

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

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.