An ion-neutral complex in chemistry is an aggregate of an ion with one or more neutral molecules in which at least one of the partners has a rotational degree of freedom about an axis perpendicular to the intermolecular direction [1] In chemistry, the dissociation of a molecule into two or more fragments can take place in the gas phase, provided there is sufficient internal energy for the requisite barriers to be overcome.
In 1958, Allan Maccoll suggested that the decomposition of alkyl halides (RX) might take place via the intermediacy of ion pairs, [R+ X−], in which the charged fragments were no longer covalently bonded but were held together by electrostatic attraction.
[2] Maccoll and coworkers subsequently examined chlorine isotope effects in the thermal decomposition of chloroethane and concluded that the data did not support that interpretation;[3] however, he had provided the germ of an idea that came to fruition two decades later in the study of decompositions of electrically charged molecules.
More recently several research groups have provided evidence that revives Maccoll's original hypothesis, but with the variation that the fragments that sojourn in the presence of one another are both electrically uncharged.
[7] In other words, dissociations of a neutral molecule RX can take place in the gas phase via the intermediacy of radical pairs [R· X·], where X· can be as small as a hydrogen atom.