Pyramidal Inversion

Abstract Pyramidal inversion is discussed from the point of view of recent theoretical and experimental investigations in an attempt to provide a unified description of this process. Quantum mechanical studies of pyramidal molecules indicate that the origin of the inversion barrier may be dependent on the degree of angular constraint. Effects due to the electronegativity of substituents on the inversion center, to the presence of adjacent lone pairs, and to inclusion of d‐type functions in the basis set are discussed. The utility and limitations of molecular orbital calculations, vibrational spectroscopy, microwave spectroscopy, direct kinetic measurements, and dynamic nuclear magnetic resonance (DNMR) spectroscopy as means for determining barriers to pyramidal inversion are discussed in context with a review of the highlights of experimental observations on the subject. Ambiguities that arise in the interpretation of barriers determined by DNMR are explored in detail. Factors that affect the magnitude of inversion barriers are discussed separately in four broad categories: steric effects; effects of conjugation (including (p–d) π conjugation) and hyperconjugation; effects of angular constraint; and effects of heteroatomic substitution. In the last category, critical reference is made to the question of electronegativity vs. lone pair‐lone pair repulsions, the problem of rotation vs. inversion, and the role of d orbitals.