A New Energy Ordering of Gas Phase Glycine and the Dipole Moment via Plane-wave Density Functional Theory Calculations

TL;DR

This study uses plane-wave density functional theory to accurately determine the energy ordering and dipole moment of glycine conformers, resolving previous discrepancies between theoretical predictions and experimental measurements.

Contribution

It provides the first plane-wave DFT calculations of glycine's conformers, revealing a stable conformer with a dipole moment consistent with microwave spectroscopy.

Findings

Most stable glycine conformer has a dipole moment of 5.76 Debye.

Energy ordering depends on the plane-wave energy cutoff.

Results reconcile theoretical predictions with experimental data.

Abstract

The abundance of glycine (Gly), the simplest amino acid, in meteorites leads us to the next question about its extraterrestrial origin. However, astronomers have not yet found glycine signature in interstellar medium. Laboratory microwave spectroscopy experiments report the most stable Gly conformer has a dipole moment of 4.5 - 5.45 Debye. Theoretical calculations, so far performed only with Gaussian basis functions, has predicted a dipole moment of about 1 Debye. This discrepancy has baffled astronomers. We study the energetics of glycine and its isomers and conformers via plane-wave density functional theory calculations. The geometric structures of the isomers and their conformers are identified, along with their relative stability and their dipole moment. In the case of glycine, we obtain the most stable conformer with a dipole moment of 5.76 Debye, close to the microwave spectroscopy experiments. If the plane wave energy cutoff is reduced to a lower value (~400 eV) on purpose, the energy ordering reverses to the case with Gaussian basis calculations.