Quasistructural molecules

The concept of quasistructural molecules is introduced. For quasistructural molecules (a) the notion of a static equilibrium structure, corresponding to a minimum on the potential energy surface of the molecule, loses its strict meaning, (b) internal nuclear motions (rotations and vibrations) become dominant, resulting in an effective molecular structure often even qualitatively different from the equilibrium one, (c) separation of the internal nuclear motions breaks down, rotational and vibrational degrees of freedom cannot be separated from each other when interpreting even the lowest rovibrational eigenstates of the molecule, often resulting in effective rotational constants drastically different from the equilibrium ones even for the ground vibrational eigenstate, (d) classification of the rovibrational states requires the use of permutation‐inversion symmetry and molecular‐symmetry groups, and (e) some of the rovibrational eigenenergies assigned to a vibrational parent state exhibit unconventional (in the most striking cases “negative”) rotational contributions. Molecules showing quasistructural behavior include neutral species, such as dimethyl acetylene, charged species, such as H5+ and CH5+, van der Waals complexes, such as CH4·H2O, and molecular complexes held together by halogen bonds, like CF3Cl·CH3F. This article is categorized under: Structure and Mechanism > Molecular Structures Theoretical and Physical Chemistry > Spectroscopy Software > Quantum Chemistry Pictorial representation of the quantum graph applied to the quantum‐dynamical description of the low‐energy vibrations of CH5+. The 120 internal rotation and 60 flip edges connecting the 120 equivalent vertices (versions) are indicated by blue and red lines, respectively