Secondary Guest-Driven Nucleation of N 2 O Clathrate Hydrate in Amorphous Ice Mixture under Ultrahigh Vacuum

We report the first observation of nitrous oxide (N₂O) clathrate hydrate (CH) under ultrahigh vacuum (UHV) conditions using infrared spectroscopy. While nucleation of CHs from water and guest molecules either spontaneously or in the presence of promoters is well-established under high-pressure conditions, their occurrence under cryogenic UHV remains less understood, particularly in multiguest systems. In this study, binary and ternary ice mixtures of N₂O, ethylene oxide (ETO), dimethyl ether (DME), tetrahydrofuran (THF), and H₂O were investigated via thermal annealing at 130 K. The binary N₂O-H₂O system showed no signs of hydrate formation after 50 h of annealing, whereas the addition of ETO, DME, or THF led to immediate CH formation under identical conditions. This demonstrates a clear promoter-assisted heterogeneous nucleation mechanism. In situ reflection absorption infrared (RAIR) spectroscopy confirms that ETO facilitates the formation of structure I hydrates, incorporating both ETO and N₂O in the cages, while DME and THF promote larger-cage CHs. Density functional theory calculations support experimental RAIR results, which predict stronger host-guest interactions and blue shifts for small cages and weaker shifts for large cages. These findings offer new mechanistic insights into multiguest hydrate nucleation and establish N₂O CH formation under UHV as a relevant process in cryogenic and astrochemical environments.