Prof. William Karney

Professor Karney will give the 1^st Liebig-College seminar and lectures on 

(I) Annulenes With a Twist: Topology, Aromaticity, and Reactivity

(II) Beneath the Surface: Tunneling in Organic Reactions

Abstract:

(I)

The fully conjugated hydrocarbon rings we know as annulenes have long been studied for what they teach us about π-delocalization and aromaticity. Medium-to-large size annulenes also undergo a variety of dynamic processes, including conformation change, π-bond shifting, configuration change (cis/trans isomerization), and electrocyclic ring closure. When annulenes adopt Möbius topology, Hückel’s rule is turned upside down. At the crossroads of aromaticity, topology, and reaction mechanisms, we can find explanations for reactions that for many years were poorly understood, such as thermal cis/trans isomerization of C=C bonds at surprisingly low temperatures. In addition, despite the unappealing sound of an “anti-aromatic transition state”, some molecules have no choice but to react via such a path.

(II)

From the standpoint of transition state theory, we think of chemical reactions in terms of reactants passing over a potential energy barrier to reach products. Due to quantum mechanics, however, there is some probability that reactants will pass through the barrier rather than over it. We will discuss factors that affect the tunneling probability, such as atomic mass and barrier width, as well as different types of experimental and computational evidence for tunneling. While there are many examples of hydrogen tunneling in chemistry and biochemistry, due to the small mass of hydrogen, tunneling by heavy atoms such as carbon is less common. We will see highlights and explore the limits of heavy-atom tunneling in organic reactions.