Polymers and oligomers of carbon dioxide: ab initio and semiempirical calculations

Abstract

Listen

Translate article

Ab initio calculations have been performed on the D 2 h cyclic dimer (CO)O(CO)O (1,3-dioxacyclobutane-2,4-dione, 1,3-dioxetanedione), the D 3 h cyclic trimer (CO)O(CO)O(CO)O (1,3,5-trioxacyclohexane-2,4,6-trione, 1,3,5-trioxanetrione), the acyclic oligomers HO(CO 2) n H, n = 2–6 (dicarbonic to hexacarbonic acid) and the “infinite” polymer(CO 2) n -(poly(carbon dioxide)) of CO 2. The cyclic molecules were studied at the AM1, HF/3-21G, HF/6-31G ∗ and MP2(FC)/6-31G ∗ levels, and the acyclic molecules at the AM1 and HF/3-21G levels. At the levels used, all these species are minima on the potential energy surface. The cyclic dimer and trimer are thermodynamically unstable with respect to CO 2 by 216 and 283 kJ mol −1, respectively, at the MP2(FC)/6-31G ∗ level, i.e. 108 and 94 kJ mol −1 per CO 2 unit, while the limiting ( n → ∞) value for the acyclic polymer is 57 kJ mol −1 per CO 2 unit at the AM1 and 82 kJ mol −1 at the 3-21G level. The barrier for the concerted decomposition of the cyclic dimer and trimer to CO 2 is 41 and 80 kJ mol −1, respectively, at the MP2(FC)/6-31G ∗ level, which corresponds to half-lives at room temperature of 1.1 × 10 −6 and 7.4 s and at 77 K of 4.6 × 10 15 and 1.3 × 10 41s, respectively.