Talk:Mechanical bond

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The definition of mechanical bond was edited firstly to make it more concise (although I agree it could have more links) and secondly to clarify what category mechanically interlocked molecules belong to.

If the example of a [2]catenane is considered first. Normally assembled using non-covalent template strategies (such as Stoddart's, Sauvage's, Leigh's and others) once the ring forming reaction is complete there is absolutely no way to distort the molecule to separate the two rings from one another without cleaving a covalent bond. The species is thus a single molecule that is furthermore topologically non-trivial. In the case of a [2]rotaxane once assembled it is held together by a mechanical bond (i.e. covalent bond cleavage is required to separate the two species) but the molecule is topologically trivial. Simply by imagining the rotaxane was made of highly elastic rubber, no matter how big the stoppering groups are you will always be able to pull the ring off the thread in a [2]rotaxane. This scenario does not apply to a rotaxane where the ring is comprised of atoms held together by covalent bonds which can only be stretched so far. If the[2]rotaxane in question can be separated by these means it must be a psuedo-rotaxane and indeed all rotaxanes assembled by 'slippage' fall into this category. If the term mechanical bond is to mean anything it must be applied where cleavage of covalent bonds occurs prior to component dissociation. If the two fragments of a rotaxane can dissociate (or are indeed formed by) slippage the species must be considered a host-guest complex or stable supramolecular complex for which there are already definitions.

In short, mechanical bonding cannot be likened to non-covalent covalent bonding.

I basically agree with your revisions to make the definition more concise. However, I think the case of the rotaxane is more complicated. A molecule could be considered a rotaxane at a lower temperature and a pseudo rotaxane at a higher temperature. It is conceivable that multiple covalent bonds could be stretched or compressed to allow the macrocyle to slip off. The components could therefore separate without actually breaking of the covalent bonds. Unlike catenanes separation for rotaxanes could simply have a high energy transition state.

Classification of the mechanical bond as non-covalent is simply due to the fact that is not covalent. However, this is probably not an important point. --M_stone 15:12, 8 April 2006