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Vibrational Spectra of Small Methylamine Clusters Accessed by An Ab-Initio Anharmonic Approach

Abstract

Methylamine (MMA) is one of the simplest amines and vibrational spectra of its dimer have been recent measured experimentally. The NH stretch modes are well resolved, but complex vibrational features of CH3 group could not be fully accounted for even with the assistance of Ab Initio Molecular Dynamics (AIMD) with various density functional methods. In this work, we carried out anharmonic vibrational calculations on MMA clusters up to tetramer using MP2/aug-cc-pVDZ to examine the vibrational coupling among CH/NH and compute their vibrational spectra 2800~3500 cm-1. We found that the main origin in the complexity between 2800~3000 cm-1 is caused by the Fermi Resonance (FR) between the stretching and bending overtones of CH3. This spectral feature becomes simplier in trimer and tetramer. Furthermore, Fermi resonance in NH2 group is found to be also very strong. In MMA dimer, no noticeable FR features can be found, but in trimer and tetramer, the enhancement of hydrogen bond strength due to cooperative effect will cause N-H stretching mode to red-shift to revert the energy order of fundamental of 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 in 3200 and 3300 cm-1 should been seen.

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Publication details

The article was received on 24 Jan 2018, accepted on 13 Feb 2018 and first published on 13 Feb 2018


Article type: Paper
DOI: 10.1039/C8CP00533H
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    Vibrational Spectra of Small Methylamine Clusters Accessed by An Ab-Initio Anharmonic Approach

    Q. Huang, Y. C. Li, K. Ho and J. Kuo, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C8CP00533H

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