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

Abstract

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).

Understanding Fermi resonances behind the complex vibrational spectra of the methyl groups in simple alcohol, thiol, and their ethers

“…In the latter group, the frequency of umbrella modes is close to the bending modes and CH stretching fundamental has red‐shifted, both factors result in better energy‐matching conditions that give rise to more complex vibrational features between 2800 and 3000 cm −1 . It is also worth mentioning that the FR patterns in CH 3 OH/CH 3 OCH 3 are similar to their counterparts in MMA, DMA, and TMA [ 20 ] with main difference in the position and intensity of the three CH stretching fundamentals.…”

“…Our spectrum is broadened by the rotational profile, which is similar to an early work on MMA monomer. [ 20 ] Bernstein and coworkers have identified four bands at 3010, 2956, 2925, and 2837 cm −1 and assigned them to be ν 2 , ν 9 , combination/overtone state, and ν 3 , respectively. Vibrational spectrum of CH 3 OCH 3 , measured in this study, is shown in Figure 2d in which four well‐resolved groups of peaks at 2816, 2880, 2922, and 2998 can be seen.…”

“…Recent studies on the FR pattern of methyl groups on monomers and clusters of monomethylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA) have been investigated by both theoretical and experimental teams to reveal the involvement of the umbrella modes in the FR pattern of the methyl group on these amines. [ 17–20 ] Since the frequency of the umbrella mode in CH 3 OH is similar to those of MMA, DMA, and TMA, it is legitimate to ask whether the vibrational coupling between overtone of the umbrella mode and stretch fundamentals could play a role in methanol. Furthermore, we found from the studies on MMA, DMA, and TMA that an increase in the number of methyl groups on the amines leads to the red shift of the CH fundamental.…”

“…Both PES and DMS on these grid points are evaluated with high‐level electronic structure methods. In an earlier work on the vibrational spectra of the methyl groups of amines, [ 20 ] it was shown that five GH grid points are adequate along all selected stretch/bending/umbrella modes.…”

Infrared–vacuum ultraviolet spectroscopy of the CH stretching vibrations of jet‐cooled aromatic azine molecules and the anharmonic analysis

“…[1] F-R is later observed in the C═O stretching vibration of ketone [2] and in the X H stretching vibration, where X refers mostly to O, N, and C atom. [3][4][5][6][7][8][9][10] Sibert and coworkers analyzed the F-R between C H stretching and overtone C H bending vibrations of alkyl group by model Hamiltonian based on scaled frequencies obtained from DFT calculations. [4] For the larger molecules, Roy et al applied the vibrational self-consistent field with second-order perturbation correction (VSCF-PT2) algorithm with HF/MP2 potentials to analyze the vibrational spectra of N-acetyl tryptophan amide and its derivative with an extended peptide backbone.…”