A re-examination of the highly floppy melamine molecule and predictability of quasi-molecule theory

With atoms assumed as points, the query is whether melamine is 2 D or 3 D . The answer from this work is 3 D at its equilibrium geometry but it may assume both forms when the zero-point harmonic vibrational energy is considered. Reasons for melamine’s high floppiness are discussed and the role of the “tiles” embedded on it examined using popular quantum chemistry methods in the spirit of quasi-molecule theory and a fragmentation approach. It is argued that melamine is yet another case of spontaneously broken symmetry. The validity of the reverse of the Lemma that grounds the theory for planarity is also discussed. • It is shown up to the highest affordable ab initio accuracy that gas phase ground-strate melamine is quasi-planar at its equilibrium geometry. • It can be planar and quasi-planar (non-planar) under physical conditions (beyond 0 K). The role of ZPE is addressed, and the molecule classified as spontaneously broken symmetry. • The above, supported by experimental data, has been deciphered with quasi-molecule theory. The role played by the embedded CNH 2 tiles (quasi-molecules) is pinpointed. • The validity of the reverse of the Lemma that grounds quasi-molecule theory for planarity is discussed for CH 4 and C 2 H 6 . • Quasi-molecule theory is claimed predictive and contrasted with atomic orbital hybridization.