Are longer carbon carbon bonds always weaker?

A fascinating Nature paper (vol. 477 pp. 308 – 311 ’11, 15 September) stands what we thought we knew about carbon carbon bonding on its head.  How many transition state diagrams where bonds are stretched to their breakpoint have you seen?  The statement is made [ Nature vol. 456 pp. 45 – 47 ’08 ] that breaking bonds in a transition state extend by only .2 Angstroms — impressive since most bonds are over 1 Angstrom long. So the presumption is that the longer the bond is, the weaker it becomes.  Not so for the compounds described in this paper.

It involves super adamantanes (which the authors call diamondoids).  Consider the diamond structure — all the carbons are tetrahedral.  Adamantanes are the same way.  Extend the adamantane structure by forming further tetrahedral carbon carbon bonds and you get diamondoids.  Structures 6, 7 and 8 show this clearly.

Now take a pair of diamondoids and link them together by a carbon carbon bond (losing 2 hydrogen atoms in the process).  The resultant C – C bonds range in length from 1.647 to 1.704 Angstroms.  So, like cyclobutadiene, they have an evanescent existence, correct?   Wrong !  They don’t break up until heated to over 200 C.  The problem isn’t the instability of the resultant radical or ionic monomers.

The explanation given is that van der Waals effects between the apposed hydrogens on the two diamondoids overcome the energetic penalty of the long bond.  Just when you thought you knew everything.

Organic chemistry — the gift that keeps on giving.

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  • Curious Wavefunction  On September 23, 2011 at 10:40 am

    I am actually not too surprised by this result and in fact it seems to precisely embody the interplay of competing effects that lies at the heart of any chemical phenomenon. The whole point of the game in chemistry (and computational chemistry in particular) is to figure out which effect wins.

  • luysii  On September 23, 2011 at 11:44 am

    This sort of thing also tells us that protein protein interactions in the cell are likely to be pretty strong, even when no charges or hydrogen bonds are involved. Think of the amount of surface involved. The interface between the subunits in the nicotinic acetyl choline receptor is 2700 Angstroms^2 (of the snail, but so what) [ Nature vol. 411 pp. 261 – 268, 269 – 276 ’01 ]. I’m sure the strengths of protein/protein interactions have been measured many times.

    Here’s a bit more [ Proc. Natl. Acad. Sci. vol. 101 pp. 16437 – 16441 ’04 ] The hot spot theory of protein protein interactions says that the binding energy between two proteins is governed in large part by just a few critical residues at the binding interface. In a typical interface of 1000 – 2000 square Angstroms, only 5% of the residues from each protein contribute more than 2 kiloCalories/mole to the binding interaction.

  • Wavefunction  On September 26, 2011 at 3:48 pm

    Protein-protein interactions ARE quite strong and nanomolar binding affinities are quite common. The hot spot theory seems to be the current guiding principle. In some of my work with protein-protein interfaces this is precisely what I looked at using Rosetta; mutating (computationally) every single residue at the interface to alanine to see which ones are important. Sometimes it works. Also, 2 kcal/mol can contribute a lot as you know.

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