The Science education unit at the ACS publication C&EN publishes its list of molecules of the year (as selected by the editors and voted upon by the readers) in December. Here are some observations about three of this year’s batch.
The atropisomerism involves restricted (high energy) rotation about an axis shown as a dotted red line, accompanied at a separate stage with rotation about the C-C single bond shown in blue. The polysiphenols showed similar two-stage atropisomerism.
I show an intrinsic reaction coordinate calculation of this process below, being respectively the energy response and the dihedral angle responses about the 16-17 bond (blue) and the axis 2-18 (dashed red) and ending with an animation of the process.
Note how the C-C rotation is much lower in energy than the about the red axis. It is also interesting to observe that pyramidal inversion at the chiral oxygen centre via a trigonal planar unit only happens at an IRC value of ~+30, well after both transition states have passed!
What does a calculation reveal (ωB97XD/Def2-TZVPP; DOI for data 10.14469/hpc/13532)? The Wiberg bond index calculated for the central carbon atom is 3.8328, which corresponds to 7.67 electrons. Far from four! The Wiberg bond orders along the chain are 1.54, 1.14, 1.83, 1.83, 1.12, 1.58 (the species is calculated as not quite symmetrical as an isolated molecule) which is a close match to structure 1e (not shown in the article). Dare I suggest that the tag line for this entry in the C&EN article is a good example of copywriters hyperbole, something designed to catch the interest and attention of the reader? Well, it certainly succeeded, but I venture to suggest that although the molecule is indeed interesting and unique, it does NOT break the octet rule. A good discussion point perhaps for a chemistry tutorial?
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