Teaching the Right Reasons: Lessons from the Mistaken Origin of the Rotational Barrier in Ethane

Abstract

when will chemistry textbooks begin to serve as aids, rather than barriers, to this enriched quantum-mechanical perspective... F. Weinhold (2001) When I was first properly introduced to chemistry in the 9th grade, electrons (™chemical glue∫) were simply described as single dots around atoms. Magically, sharing the dots meant a ™chemical bond∫ and one had only to count up to eight to figure out how many electrons were shared and how many bonds you would have. Easy. We counted ions, balanced redox reactions, and had fun with oxidation numbers for inorganic and simple organic species. Then things became more complicated. We were introduced to Bohr×s model, in which you could only have two electrons in the innermost shell and eight in the next; beyond that, who knows what. That all had to do with some complicated quantum theory that we could not possibly understand in detail, so we were asked just to accept it. We did, until two years later when suddenly we were told that Bohr×s model is really too much of an approximation and that chemical bonding is far more complicated. It should be expressed using the intricate laws of quantum mechanics, namely, in terms of molecular orbitals. All bond types (multiple, metallic, back-bonding, etc.) suddenly became unique and things were really complicated mathematically. I remember well the funny-looking dumbbell-shaped, colorful styrofoam models of the various d orbitals, and thought that nature could not possibly be that complicated; in any case, two years from then they would tell us otherwise anyway. So I decided not to accept anything that had to do with this awkward quantum theory, only to be quizzed in the next class session. Needless to say, I received the worst grade in my chemical life and was embarrassed in front of the entire class. Shortly thereafter I decided to become a chemist.The reason that I am telling this story is because there are, as always, two sides to it. Teachers, on the one hand, can confuse and frustrate their students by trying to depict chemistry in the most up-to-date, albeit complicated fashion. On the other hand, motivated students will despise simple models if more appropriate ones continually replace them. Frustration may be the final result. The simplification approach may be acceptable at the high school level because easily understandable explanations, although possibly inadequate, do get the message across. At higher scholastic levels, however, this is no longer acceptable because we owe it to the students to teach them our current knowledge and interpretation of science. The convenience of visually pleasing explanations for intricate chemical phenomena must not outweigh proper physical descriptions. These are available through modern quantum mechanics within the Born ± Oppenheimer approximation that allows approximate solutions of the electronic Schrˆdinger equation in the form of potential energy hypersurfaces. Hence, quantum mechanics provides us with an accurate theoretical description. The subsequent interpretation is a model that may ...

Thermal rearrangement of substituted difluoro(methylene)cyclopropane

“…The nature of the stability difference can be discussed by hyperconjugation effects [11]. It was quantitatively examined by the second-order perturbation theory analysis of the Fock matrix in the NBO basis (Table 2).…”

Structural systematics of aryl-1,3-dithiane derivatives: crystal and energy-minimised structures, and Hirshfeld surface analysis

“…1 and 2, where rotation about the C2-C7 bond results in the same final conformation, the barrier to rotation was approximately 3.5 kcal.mol -1 . These values are in the same order of magnitude as rotation about a C-C single bond of common aliphatic hydrocarbons, even with carbon atoms bonded to bulky substituents [62][63][64]. In order to evaluate the reasons for the energy barrier additional calculations were performed.…”

Comments on “Is It Time To Retire the Hybrid Atomic Orbital?”

“…These are powerful instructional "degrees of freedom" for teachers of lower-level chemistry, helping students to gain their first intuitive glimpses of what lies on the forward path to professionalism (rather than mnemonic rationalizations that must be "unlearned" at the next level 16 ). Why would we remove such qualities from our textbooks and curriculum?…”