Can 1,3-dimethylcyclobutadiene and carbon dioxide co-exist inside a supramolecular cavity?

Abstract

The calculated free energy barrier at 175 K between 1,3-dimethylcyclobutadiene and carbon dioxide inside a calixarene host (ωB97XD/6-311G(d,p)+polarizable continuum solvent model) has the low value of ∼8–10.5 kcal mol⁻¹. This value casts doubt on the recently claimed isolation and X-ray structure determination at 175 K of 1,3-dimethylcyclobutadiene and carbon dioxide as separate species inside such a cavity.