Search for ionic orthocarbonates: Ab initio study of Na4CO4

Abstract

Ab initio calculations on a variety of structures have been performed for the Na4CO4 stoichiometry to test stability of the orthocarbonate structure. We found that the orthocarbonate structure is a true minimum on the potential energy surface, but it is not the most stable one. The most stable structure can be characterized as a complex of sodium carbonate and sodium oxide. However, the orthocarbonate structure was found to be just 13.8 kcal/mol higher in energy at our highest level of theory (CCSD(T)/6-311+G(2df)), and it is separated from the global minimum structure by the 9.2 kcal/mol barrier (B3LYP/6-311+G*) and 13.9 kcal/mol (CCSDCT)/6-311+G(2df)//B3LYP/6-311+G*). We hope that by selecting appropriate countercations the orthocarbonate structure can be further stabilized and may even become the most stable one. We also believe that in the solid state under high-pressure conditions there is a chance to convert the carbonate structure into the orthocarbonate structure because of its smaller volume.