Bimodal Distribution & Complexity
From WikidChem
The previous slides were talking about the structural analysis of molecules. Here we can take a closer look at the N-C(aromatic) bond length and attempt to explain why it's distribution is what it is. First of all, when statistically analyzing the frequency of various bond lengths (shown here on the graph ranging from 1.31. to 1.46, we see two peaks. Why? Well, we see from the slide that the structure of this molecule does have resonance. One of the structures is planar, due to a double bond, while the other is pyramidal, because the lone pair on N makes the N-C bond twist to avoid poor pi overlap. Thus, we get two peaks, one centering around the shorter bond length as in the double bond resonance structure, and one around the longer bond length as in the single bond twisted resonance structure.
In the database referenced on the previous wiki, there are fairly accurate bond lengths for all different kinds of bonds. But how accurate does a chemical theorist really have to be when trying to come up with a model? The purpose of a model is 1.) to explain current structural behavior, and 2.) to predict unknown structural behavior. If there are so many (682) different kinds of bonds, differing due to attached functional groups, configuration, hybridization, etc., does a theorist really need to take into account each specific type of bond?
It is possible to predict a bond distances within 1% on the basis of precedent, but to do so one must use a very close analogy. To supply sufficiently close precedents Cambridge researchers defined 682 different classes of bonds in organic molecules. In making the model to explain chemical behavior, the key to understanding chemical behavior of a compound is its structure and bond energies. By using energy, we can predict rates and equilibrium, and thus can connect structure with energy.
Everything looks alright to me. These were pretty straight-forward slides. I think they might be off by one, though. Maybe the PowerPoint was changed after these assignments were made. -RLM
