Simulating Relaxation Channels of CO2 in Clathrate Nanocages

P R Dahoo,A Lakhlifi,C Meis,J D Gale,R Puig

doi:10.1088/1742-6596/738/1/012072

Abstract

The energy levels of CO2 in the small (s) and large (l) nano-cages of cubic sI clathrates are calculated in the Born-Oppenheimer approximation using pairwise atom-atom interaction potentials. In the s cage, the centre of mass of CO2 oscillates with small amplitudes, asymmetrically about the cage center with harmonic frequencies around 100 cm-1. In the l cage, oscillations are anharmonic with large amplitude motions in a plane parallel to the hexagonal faces of the cage and the corresponding frequencies are calculated to be 55 cm-1 and 30 cm-1. Librational harmonic frequencies are calculated at 101.7 cm-1 and 56.0 cm-1 in the 5 cage and at 27.9 cm-1 and 46.4 cm-1 in the l cage. Results show that the coupling between the CO2 molecule and the nano-cage is quite different for the low frequency translational, rotational or librational modes and the high frequency vibrational modes, which consequently leads to different relaxation channels.

Highlights

A clathrate is a crystalline solid with a cage-like structure of water molecules surrounding atoms or small molecules
Results show that the coupling between the CO2 molecule and the nano-cage is quite different for the low frequency translational, rotational or librational modes and the high frequency vibrational modes, which leads to different relaxation channels
The unit cell of sI consists of 46 water molecules with two types of cages; a small cage (s) that is a polyhedron composed of 12 pentagonal faces, and a large cage (l) that is a polyhedron consisting of 12 pentagonal faces and two opposite hexagonal faces [2]

Summary

Introduction

A clathrate is a crystalline solid with a cage-like structure of water molecules surrounding atoms or small molecules. The energy levels of CO2 in the small (s) and large (l) nano-cages of cubic sI clathrates are calculated in the Born-Oppenheimer approximation using pairwise atom-atom interaction potentials. Results show that the coupling between the CO2 molecule and the nano-cage is quite different for the low frequency translational, rotational or librational modes and the high frequency vibrational modes, which leads to different relaxation channels.

Results

Conclusion