Andrej Krzan and Janez Mavri have been wondering about the occasional thin line between stable molecules and unstable molecules (DOI). Take for instance stable oxalic acid (HOC(O)(O)COH) and the non-existent carbonic acid dimer HOC(O)O(O)COH or the biodegradable biopolymer poly(glycolic acid) and the non-existent polycarbonic acid. When too many electron-withdrawing groups make a demand on carbon so it seems the molecule starts falling apart. But how to quantify stability?
An Atoms in Molecules sweep among a range of molecules related to oxalic acid did not yield a trend in the critical bond point (BCP) position. On the other hand a strong correlation was found between atomic charge and atomic volume. In one extreme the central carbon atom in the very stable isobutylene has no net charge and a large atomic volume of 70 cubic Bohr units. In the other extreme the central carbon atom in the hypothetical carbonic acid has a charge of +2.7 and an atomic volume approaching 10. Two molecules dodge the trend. Carbon dioxide and formic acid have higher volumes than predicted from atom charge and this makes Krzan and Mavri believe that volume (anything higher than 20) is the new stability decider.