Vibrational spectra of small methylamine clusters accessed by an ab initio anharmonic approach

Abstract

Methylamine (MMA) is one of the simplest amines, and the vibrational spectra of its dimer have recently been obtained experimentally. The vibrational spectra of NH stretch modes were well resolved, but the complex features of the CH group could not be fully accounted for even with the assistance of ab initio molecular dynamics (AIMD) with various density functional methods. In this study, we carried out anharmonic vibrational calculations on MMA clusters up to tetramers using MP2/aug-cc-pVDZ to examine vibrational coupling among CH/NH and compute the vibrational spectra of these clusters between 2800 and 3500 cm. We found that the main origin of the complexity between 2800 and 3000 cm was caused by Fermi resonance (FR) between the stretching and bending overtones of the CH group. This spectral feature becomes simpler in trimers and tetramers. Furthermore, Fermi resonance in the NH group is found to be very strong. In the MMA dimer, no noticeable FR features can be found; however, in its trimers and tetramers, the enhancement of hydrogen bond strength due to the cooperative effect will cause the N-H stretching mode to red-shift to revert the energy order of the fundamental of the N-H stretch and overtone of N-H bending between n = 3 and n = 4. Therefore, significant re-distribution of the intensities of the bands at 3200 and 3300 cm should be seen.

Strong Fermi Resonance Associated with Proton Motions Revealed by Vibrational Spectra of Asymmetric Proton‐Bound Dimers

“…In the last decade, there have been many theoretical analyses of the FR of O‐H and C‐H vibrations [6–11] based on anharmonic terms of the potential energy surface (PES) and the coupling strength of the FR is found to be typically in the order of 30 to 40 cm −1 . Recently, there are also a few works using ab initio methods to access the anharmonic vibrational coupling behind vibrational spectra of clusters of ammonia, amine, and anilines, and the similar coupling strength is reported [12–14] …”

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

“…VCI developed by some of us will be applied to the same QP for comparison. The details of the implementation of our VCI + QP algorithms and their applications to F-R in N H, C H, and O H have been reported previously [7,[14][15][16][17]19] and are not repeated here. All of the electronic structure calculations used here were calculated with Gaussian 09 or Gaussian 16 packages.…”

“…Kuo and coworkers succeeded in analyzing F-R of the X H stretching vibration by using ab initio anharmonic algorithms using reduced-dimensional potential energy surfaces (PESs) up to CCSD(T)/aug-cc-pVTZ level of theory. [7,[13][14][15][16][17][18][19] It is well known that many of the IR spectra in the C H stretching region of aromatic molecules show very complicated features due to F-R. For example, benzene exhibits three absorption bands in the 3,020-3,130 cm À1 region due to F-R between an IR active C H stretching vibration (ν 20 ) and the combination bands of C H bending vibrations. [20,21] For the mono-substituted benzene, toluene exhibits more than nine bands [22] in the aromatic C H stretching vibration region, and phenol exhibits more than 15 bands in the C H stretching region.…”

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

“…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.…”