A Shift in Bonding

This past week, I’ve shifted my attention to thinking about chemical bonding, motivated by the benzene paper mentioned in my last blog post. It’s a nice break from my sabbatical research projects, especially since I get to think about teaching! I’ve been contemplating how to introduce some ideas from the benzene paper into my quantum chemistry course.

Looking at my most recent syllabi, I’ve typically spent 4-5 class periods (out of 42) on chemical bonding concepts not emphasized in the standard textbook I use for quantum chemistry (McQuarrie). I introduce the Heitler-London (HL) wavefunction for H₂ and discuss how it differs from the molecular orbital (MO) description. We delve into hybridization and the surprising case of bond angles in simple hydrides; sometimes we venture into so-called hypervalent bonding. There’s some Huckel theory and symmetry concepts related to the nodal theorem.

I’d like to approximately double this amount. What would I add? Largely concepts learned from Valence Bond (VB) theory beyond simple Lewis structures. I’ve already started with the HL wavefunction and some simple surprises. I’d like to show more connections between MO and VB approaches, emphasize the importance of Pauli repulsion, and further cement the notion of how different models are used to describe the ‘unseen’ behavior of electrons in chemical bonds. I’d also like to tackle the different types of bonds that are encountered. There are the strangely weak O–O and F–F bonds, the Rundle-Pimentel three-center-four-electron bonds, the one-electron metal bonds, and what goes on in high-strain structures.

To cover these, I’m thinking of centering these discussions around Charge-Shift Bonds (CSB). You’ve never heard of them? CSB proponents argue they are a unique class, and complement the two better-known classes: ionic and covalent bonds. I’ve known about CSB for a while, but with a literature search I came across and excellent recent review article (Angew. Chem. Int. Ed. 2020, 59, 984-1001) written by some of the stalwarts in the field. Is this new class of bonds warranted? The authors present arguments to answer two questions: “Do the bonds belonging to the new class have clearly different features than bonds that belong to formerly defined classes? Is the definition of the new class useful, and does it simulate chemists to make new predictions?” They conclude yes after providing some evidence, and I’m inclined to agree with them. What’s nice about their approach is that it brings in both MO and VB approaches, and takes Pauli repulsion very seriously. There’s also a nice connection to the virial ratio, which the students encounter when solving for the kinetic and potential energy of the hydrogen atom.

All this sounds nice, but if I’m going to add material, what should I cut? My course is already jam-packed with lots of good stuff. (The students might debate ‘good’; they certainly would agree with ‘jam-packed’.) Looking at my syllabus, I could cut the general derivation of the wave equation, reduce time spent on term symbols, cut out some of the math in the spherical harmonics derivation (Legendre polynomials) and in the Hartree-Fock wavefunction for H₂. That may or may not be 4-5 classes worth. I might be able to shave off other bits here and there, but then this brings up the question of whether to keep the textbook. The more I deviate from it, the more students don’t like it. That’s why I don’t use a textbook for second-semester P-Chem (statistical thermodynamics and kinetics). I’m seriously thinking of doing the same for quantum and writing up worksheets for every class. It will be a lot of work, but it might also be helpful should we have another coronavirus-like situation. (Another factor: the textbook has also increased significantly in price.)

I’m not slated to teach quantum next semester since I’m doing a special topics class on the chemical origins of life! Thus, I have time to think, plan, and work on an overhaul – I’m likely to teach quantum in Fall 2021. In the meantime, I’ve been enjoying thinking about teaching innovations. Last week I wrote out four examples of journal and glossary entries after pondering this as a key semester-long assignment for my special topics class. I’m also reminded, amidst a sabbatical mostly doing research, why I chose to be a professor at a liberal arts college rather than a research university – the love of teaching still excites me after so many years and I’ve been missing it this year!