Analyzing the frequency shift of physiadsorbed CO2 in metal organic framework materials

TL;DR

This study combines computational and experimental methods to analyze how CO2 vibrational frequencies shift when physiadsorbed in Mg-MOF74 and Zn-MOF74, revealing system-specific differences and underlying causes.

Contribution

It provides a detailed analysis of the factors affecting CO2 vibrational frequency shifts in MOFs, combining first-principles simulations with experimental data.

Findings

Frequency shifts differ significantly between Mg-MOF74 and Zn-MOF74.

Three main factors influence the frequency shift: molecule length change, asymmetric distortion, and metal center effects.

The study offers a fundamental understanding of physiadsorption-induced vibrational changes.

Abstract

Combining first-principles density functional theory simulations with IR and Raman experiments, we determine the frequency shift of vibrational modes of CO2 when physiadsorbed in the iso-structural metal organic framework materials Mg-MOF74 and Zn-MOF74. Surprisingly, we find that the resulting change in shift is rather different for these two systems and we elucidate possible reasons. We explicitly consider three factors responsible for the frequency shift through physiabsorption, namely (i) the change in the molecule length, (ii) the asymmetric distortion of the CO$_2$ molecule, and (iii) the direct influence of the metal center. The influence of each factor is evaluated separately through different geometry considerations, providing a fundamental understanding of the frequency shifts observed experimentally.