Abstract
AbstractThe 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 and , 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
Highlights
The classical-mechanics-based model of associating a static structure to quantum systems called molecules is rooted deeply in the minds of practitioners of chemistry, physics and molecular biology, as this simple model successfully explains a large number of phenomena of interest to them
This static picture forms the basis of the simplest dynamical picture of molecular motions: rigid rotations and small-amplitude vibrations (SAVs) about a well-defined structure
As the resolution and the accuracy of the traditional experimental techniques probing molecular structures and spectra in the gas phase improved and more and more new, sophisticated high-resolution spectroscopic techniques have become available, more and more molecular systems have been discovered where this picture does not hold and where assuming a single structure, corresponding to a minimum on the clamped-nuclei potential energy surface (PES) computed by means of standard electronic-structure theory, does not provide a good representation of the spectral features observable under high resolution
Summary
Since the existing usage of the possible terms “quasimolecule” and “fluxional molecule” is not compatible with our task of naming highly unusual structural cases from the point of view of high-resolution spectroscopy, one must invent a new term for molecules which do not have a characteristic equilibrium structure, whose internal nuclear dynamics becomes very complex, and where (some of the) vibrational motions cannot be separated even in zeroth order from the rotations as they have similar energy and time scales.
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