Henry Rzepa's Blog Chemistry with a twist

July 22, 2021

Another very large anomeric effect – with a twist.

Filed under: crystal_structure_mining — Henry Rzepa @ 8:30 am

In the earlier post on the topic of anomeric effects, I identified a number of outliers associated with large differences in the lengths of two carbon-oxygen bonds sharing a common carbon atom.

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July 21, 2021

Dimerisation of cyclopropenylidene: what are the correct “curly arrows” for this process?

Filed under: Curly arrows,reaction mechanism — Henry Rzepa @ 3:45 pm

In another post, a discussion arose about whether it might be possible to trap cyclopropenylidene to form a small molecule with a large dipole moment. Doing so assumes that cyclopropenylidene has a sufficiently long lifetime to so react, before it does so with itself to e.g. dimerise. That dimerisation has an energy profile shown below, with a free energy of activation of 14.4 kcal/mol, so a facile reaction that will indeed compete with reaction with Ph-I+-CC.

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July 11, 2021

Molecules with very large dipole moments: cyclopropenium acetylide

Filed under: Interesting chemistry — Henry Rzepa @ 9:54 am

Occasionally, someone comments about an old post here, asking a question. Such was the case here, when a question about the dipole moment of cyclopropenylidene arose. It turned out to be 3.5D, but this question sparked a thought about the related molecule below.

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July 1, 2021

Two record breakers for the anomeric effect; one real, the other not.

Filed under: crystal_structure_mining — Henry Rzepa @ 12:41 pm

The classic anomeric effect operates across a carbon atom attached to oxygens. One (of the two) lone pairs on the oxygen can donate into the ŌÉ* orbital of the C-O of the other oxygen (e.g. the red arrows) tending to weaken that bond whilst strengthening the donor oxygen C-O bond. Vice versa means e.g. the blue arrows weakening the other C-O bond. This effect tends to increase charge separation and the question then arises: how large can this effect get? To try to find out, we are going to do some crystal structure mining in this post!

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