A theoretical investigation of the low energy conformers of the isomers glycine and methylcarbamic acid and their role in the interstellar medium‡

We have theoretically investigated the low energy conformers of neutral glycine (NH2CH2COOH) and its isomer methylcarbamic acid (CH3NHCOOH) in the gas phase. A total of 16 different levels of the theory, including CCSD(T), MP2 and B3LYP methods with various Pople and Dunning type basis sets with and without polarization and diffuse functions were used. We found eight low energy glycine conformers, where the heavy atoms in three have a planar backbone, and four low energy methylcarbamic acid conformers all with non-planar backbones. Interestingly at all levels of theory, we found that the most stable methylcarbamic acid conformer is significantly lower in energy than the lowest energy glycine conformer. The MP2 level and single point CCSD(T) calculations show the lowest energy methylcarbamic acid conformer to be between 31 to 37 kJ mol−1 lower in energy than the lowest energy glycine conformer. These calculations suggest that methylcarbamic acid might serve as a precursor to glycine formation in the Interstellar Medium (ISM). We also report the theoretical harmonic vibrational frequencies, infrared intensities, moment of inertia, rotational constants and dipole moments for all of the conformers. In order to understand how glycine or methylcarbamic acid might be formed in the ISM, larger calculations which model glycine or its isomer interacting with several surrounding molecules, such as water, are needed. We demonstrate that B3LYP method should provide a reliable and computationally practical approach to modeling these larger systems.