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Issue 23, 2011
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A computational study on the structures of methylamine–carbon dioxide–water clusters: evidence for the barrier free formation of the methylcarbamic acid zwitterion (CH3NH2+COO) in interstellar water ices

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Abstract

We investigated theoretically the interaction between methylamine (CH3NH2) and carbon dioxide (CO2) in the presence of water (H2O) molecules thus simulating the geometries of various methylamine–carbon dioxide complexes (CH3NH2/CO2) relevant to the chemical processing of icy grains in the interstellar medium (ISM). Two approaches were followed. In the amorphous water phase approach, structures of methylamine–carbon dioxide–water [CH3NH2/CO2/(H2O)n] clusters (n = 0–20) were studied using density functional theory (DFT). In the crystalline water approach, we simulated methylamine and carbon dioxide interactions on a fragment of the crystalline water ice surface in the presence of additional water molecules in the CH3NH2/CO2 environment using DFT and effective fragment potentials (EFP). Both the geometry optimization and vibrational frequency analysis results obtained from these two approaches suggested that the surrounding water molecules which form hydrogen bonds with the CH3NH2/CO2 complex draw the carbon dioxide closer to the methylamine. This enables, when two or more water molecules are present, an electron transfer from methylamine to carbon dioxide to form the methylcarbamic acid zwitterion, CH3NH2+CO2, in which the carbon dioxide is bent. Our calculations show that the zwitterion is formed without involving any electronic excitation on the ground state surface; this structure is only stable in the presence of water, i.e. in a methyl amine–carbon dioxide–water ice. Notably, in the vibrational frequency calculations on the methylcarbamic acid zwitterion and two water molecules we find the carbon dioxide asymmetric stretch is drastically red shifted by 435 cm−1 to 1989 cm−1 and the carbon dioxide symmetric stretch becomes strongly infrared active. We discuss how the methylcarbamic acid zwitterion CH3NH2+CO2 might be experimentally and astronomically identified by its asymmetric CO2 stretching mode using infrared spectroscopy.

Graphical abstract: A computational study on the structures of methylamine–carbon dioxide–water clusters: evidence for the barrier free formation of the methylcarbamic acid zwitterion (CH3NH2+COO−) in interstellar water ices

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Publication details

The article was received on 28 Sep 2010, accepted on 18 Jan 2011 and first published on 10 Feb 2011


Article type: Paper
DOI: 10.1039/C0CP01962C
Citation: Phys. Chem. Chem. Phys., 2011,13, 11083-11098
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    A computational study on the structures of methylamine–carbon dioxide–water clusters: evidence for the barrier free formation of the methylcarbamic acid zwitterion (CH3NH2+COO) in interstellar water ices

    H. Kayi, R. I. Kaiser and J. D. Head, Phys. Chem. Chem. Phys., 2011, 13, 11083
    DOI: 10.1039/C0CP01962C

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