CO clathrate hydrate: Near to mid-IR spectroscopic signatures

Carbon monoxide is the second most abundant molecule after H₂ in the molecular universe, and as such an abundant constituent of interstellar and Solar System ices. To trace the possibility of this molecule to be found in a clathrate hydrate inclusion compound, its pure phase FTIR spectrum is investigated. We confirm the formation of a type I clathrate structure whereas simple guest size estimates would favour a type II clathrate hydrate, revealing interactions of this molecule with its water network during clathrate formation. The observed cage vibrational downshift with respect to pure CO ice is within 5 cm⁻¹. The temperature dependent wavenumber separation between the two enclathrated CO vibrational transitions in the two distinct type I clathrate cages is less than a wavenumber below 140 K, implying that the spectral simplification for detailed spectroscopic analysis of the individual profiles is a difficult task. The dynamics of the CO molecules in its cage change considerably from 5 K to 140 K. At temperatures above 30 K, the molecule is extremely mobile in the cages, as revealed by the infrared profile, significantly different from CO entrapped in water ice and different from observed profiles in astrophysical objects.