For example, Di Stefano and co-workers reported the operation of catenane molecular switch 8 using carboxylic acid 9H. The dominant co-conformation of 8 under neutral conditions is that in which the phenanthroline units are far apart. Protonation of 8 gives rise to 8H + with concomitant relative rotation of the macrocycles to give a new dominant co-conformation (Figure ). Carboxylic acid 9H, once deprotonated to give 9 -, undergoes spontaneous decarboxylation to give anion 10 -which can then be reprotonated to give 10H. Combining catenane 8 and acid 9H gives rise to salt 8H + •9 -. Carboxylate anion 9 -can then undergo decarboxylation to give a new salt 8H + •10 -. It is at this point that two different extreme pathways can be envisaged; 10 - can be protonated by either another molecule of 9H (Figure ) or by the same molecule of 8H + that initially accepted the proton (Figure ). Proton transfer is often mechanistically unimportant, but if one pathway is followed exclusively it has consequences for how this system should be interpreted.