Hydrogen bonding in the protic ionic liquid triethylammonium nitrate explored by density functional tight binding simulations

TL;DR

This study evaluates the density functional tight binding (DFTB) method for modeling hydrogen bonds and IR spectra in the protic ionic liquid triethylammonium nitrate, demonstrating its reliability compared to high-level theories.

Contribution

It demonstrates the applicability of DFTB to hydrogen bond dynamics and IR spectroscopy in a protic ionic liquid, validating its use against more computationally intensive methods.

Findings

DFTB potential energy curves match high-level coupled cluster results.

Geometric correlations in hydrogen bonds are quantified using DFTB.

IR spectra reveal signatures of hydrogen bonding in specific vibrational ranges.

Abstract

The applicability of the density functional based tight binding (DFTB) method to the description of hydrogen bond dynamics and infrared spectroscopy is addressed for the exemplary protic ionic liquid triethylammonium nitrate. Potential energy curves for proton transfer in gas and liquid phase are shown to be comparable to high level coupled cluster theory in the thermally accessible range of bond lengths. Geometric correlations in the hydrogen bond dynamics are analyzed for a cluster of six ion pairs. Comparing DFTB and regular DFT data lends further support for the reliability of the DFTB method. Therefore, DFTB bulk simulations are performed to quantify the extent of geometric correlations in terms of Pauling's bond order model. Further, infrared (IR) absorption spectra are obtained and analyzed putting emphasis on the signatures of hydrogen bonding in the NH-stretching and far IR hydrogen bond range.