Tiling, propensity for planarity of the group-XIV pentatomic dihydrides XYZH₂(X, Y, Z = C, Si, Ge), and beyond

A general (3+4)-atom partitioning scheme, quasi-molecule theory or simply tiling, is briefly reviewed and used anew for rationalizing the propensity to planarity of the families of Group-XIV pentatomic dihydrides. <i>Ab initio</i> molecular orbital theory is used to calculate the structures of such dihydrides in their electronic ground states, as well as the tiles [tri- and tetratomic quasi-molecules (quasi-radicals)] embedded on them. Using cc-pVXZ and aug-cc-pVXZ basis sets up to quadruple-<i>ζ</i> in conjunction with coupled-cluster theory, and its explicitly correlated variant, including single, double, and perturbative triple excitations, a brief study of the involved potential energy surfaces is presented, including equilibrium geometries and harmonic vibrational frequencies of many reported stationary points. Unveilled are the structural shapes of the title species, aiming in particular to explain why they all tend to assume planar forms. Although a cyclic structure is frequently the global minimum, in striking similitude with the also unusual purely carbonated structure recently conjectured to be present in the atmosphere of Titan, other variants turn out to be more stable in some cases. The relative stabilities of the isomers of the title species are also determined. Although not at the focal point, optimizations of other molecules and radicals were also done aiming at put in perspective recent work while providing further benchmark tests on linearity, planarity or otherwise.