Comment on “A cooperation molecular recognition study: syntheses and analysis of supramolecular assemblies of tetrafluoroterephthalic acid with some aza compounds” by L. Wang, L. Zhao, Y. Hu, W. Wang, R. Chen and Y. Yang, CrystEngComm, 2013, 15, 2835

Abstract

This is a Commentary on a paper by Lei Wang et al. (CrystEngComm, 2013, 15, 2835). The protonation state of 2-aminopyrimidine in the crystal structure deposited as REXNIT has been re-examined using periodic density functional theory (DFT) calculations. Motivated by recent literature suggestions questioning the assignment of protonation at the exocyclic amino group, several alternative protonation scenarios were systematically explored within the experimental crystal framework, allowing full relaxation of atomic positions and unit cell parameters. The results demonstrate that protonation at the ring nitrogen N1 represents the only robust and energetically preferred configuration, corresponding to a well-defined global minimum. Protonation at N3 leads to an unstable local minimum that is highly sensitive to the initial proton placement, while protonation at oxygen atoms results in spontaneous proton transfer to N1 during geometry optimization, in some cases yielding locally trapped lattice configurations. These findings provide quantitative support for reconsidering the protonation assignment in REXNIT and illustrate the value of periodic theoretical methods for validating crystallographic models involving proton transfer.