An ion-neutral complex in

chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

is an aggregate of an

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

with one or more neutral

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

s in which at least one of the partners has a rotational degree of freedom about an axis perpendicular to the intermolecular direction ''Advances in Gas Phase Ion Chemistry'', Volume 4 by L.M. Babcock, N.G. Adams 2001 In chemistry, the dissociation of a molecule into two or more fragments can take place in the

gas phase In the physical sciences, a phase is a region of material that is chemically uniform, physically distinct, and (often) mechanically separable. In a system consisting of ice and water in a glass jar, the ice cubes are one phase, the water is a ...

, provided there is sufficient

internal energy The internal energy of a thermodynamic system is the energy of the system as a state function, measured as the quantity of energy necessary to bring the system from its standard internal state to its present internal state of interest, accoun ...

for the requisite barriers to be overcome. For many years, it was assumed that the fragments of a gas phase dissociation simply fly apart. In 1958, Allan Maccoll suggested that the decomposition of

alkyl halides The haloalkanes (also known as halogenoalkanes or alkyl halides) are alkanes containing one or more halogen substituents of hydrogen atom. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalka ...

(RX) might take place via the intermediacy of

ion pair In chemistry, ion association is a chemical reaction whereby ions of opposite electric charge come together in solution to form a distinct chemical entity. Ion associates are classified, according to the number of ions that associate with each ...

s, ⁺ X⁻ in which the charged fragments were no longer

covalently bonded A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...

but were held together by

electrostatic attraction Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that calculates the amount of force between two electrically charged particles at rest. This electric force is conventionally called the ''electrostatic f ...

. Maccoll and coworkers subsequently examined

chlorine Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...

isotope effects in the

thermal decomposition Thermal decomposition, or thermolysis, is a chemical decomposition of a substance caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic ...

chloroethane Chloroethane, commonly known as ethyl chloride, is a chemical compound with chemical formula CH3CH2Cl, once widely used in producing tetraethyllead, a gasoline additive. It is a colorless, flammable gas or refrigerated liquid with a faintly sweet ...

and concluded that the data did not support that interpretation; 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. In the late 1970s three research groups—in England, the United States, and France—independently provided evidence for the occurrence of ion-neutral complexes (sometimes called ion-dipole complexes or ion-molecule complexes) in the unimolecular dissociations of positive ions under the conditions associated with

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...

(i.e. as isolated species in a vacuum). The general idea is that a charged species, RY⁺, can give rise to dissociation fragments via a transient complex, ⁺ Y in which the electrically charged partner, R⁺, can undergo molecular rearrangements at the same time as it rotates relative to the neutral partner, Y. Similarly, the neutral partner, Y, can also rotate relative to the charged partner, as well as having the ability to exchange

Hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

s and internal energy with it. 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. In other words, dissociations of a neutral molecule RX can take place in the gas phase via the intermediacy of

radical Radical (from Latin: ', root) may refer to: Politics and ideology Politics *Classical radicalism, the Radical Movement that began in late 18th century Britain and spread to continental Europe and Latin America in the 19th century *Radical politics ...

pairs · X· where X· can be as small as a hydrogen atom. In the gas phase such intermediates are often called roaming radicals. Bowman, J.M.; Shepler, B.C. Roaming Radicals ''Annual Review of Physical Chemistry'' 2011, 62, 531-553 {{doi, 10.1146/annurev-physchem-032210-103518

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