Article révisé par les pairs
Résumé : In this note we stress the relationship between the classification theory of molecular non-rigidity and some important observable spectroscopic properties (statistical weights and transition probabilities). The principles of this classification are illustrated for the case of PF5. We then show the relation between this classification (isomerization modes) and the well-known non-rigid molecule symmetry groups (Longuet-Higgins groups). In the case of PF5 we apply these concepts to the determination of the non-rigid molecular energy levels and their statistical weights. © 1975 Taylor & Francis Group, LLC.