Però què és, una molècula?


A finals de gener el blog Chemistry World va publicar l’entrada What is a Molecule on es fa una reflexió sobre la definició de molècula, i fins i tot sobre la tradició en química del concepte de substància pura. De fet, tot prové de l’afirmació de Stuart Cantrill, de Nature, sobre els rotaxans i catenans com a molècules i no pas com a entitats supramoleculars.

Parteix d’al menys dues definicions:

  1. ‘The simplest unit of a chemical compound that can exist, consisting of two or more atoms held together by chemical bonds.’
  2. ‘The smallest particle of a substance that retains the chemical and physical properties of the substance and is composed of two or more atoms; a group of like or different atoms held together by chemical forces.’

I llavors es pregunta…

So how can we differentiate molecules from supermolecules or condensed phases? If molecules mustn’t easily fall apart into, and establish an equilibrium with, their separate but individually stable components, then what are we going to call weak acids or indeed water? If rotaxanes (where a cyclic molecule remains threaded on a linear one because of bulky capping groups on the thread) are molecules, what about pseudorotaxanes, where there are no end caps and the threading is merely statistical, or held in place by weak interactions? One can clearly engineer a progressive free-energy barrier to unthreading with increasingly bulky end groups; is that then a gradual transition from supermolecule to molecule? At what temperature? In which solvent?

I la darrera frase de l’article fa referència a la diferent tradició de precisió en física, en química i en biologia:

Scientific words and phenomena overlap well enough to permit discussion and theorising; we scarcely need be surprised or disconcerted when we find that the correspondence isn’t always perfect. Chemistry makes more use of fuzzy but valuable concepts (oxidation state, bond, electronegativity) than does physics (phases, forces); biology probably uses even more (species, allostery, ecosystem, gene). There’s no pressing need to police the boundaries of these words, other than to entertain ourselves. Mindful of which, I ask: how do you define a molecule?

I després de reflexionar i fer-se preguntes… segueix qüestionant…

How do you define a molecule?

I jo crec que la veritat és que tampoc no importa pas gaire.

M’agrada la imatge de l’article esmentat: semblant a la clàssica “Ceci n’est pas une pipe”