Alkane Elimination Research Articles

Why are smaller fragments preferentially lost from radical cations at low energies and larger ones at high energies?: An experimental and theoretical study

A long-standing mystery in gas phase ion chemistry is: why are smaller fragments preferentially lost at low energies and larger fragments at higher energies in competing α-cleavages? This is addressed here by studies of dissociations of ethanol, 2-propanol, 2-butanol, 2-methylpropane and 2-butanone radical cations. The onsets and energy dependencies of the reactions were obtained by photoionization mass spectrometry. Stationary point geometries, critical energies and vibrational frequencies were generated by B3LYP/6-31G(d) and B3LYP/6-311G(d,p) theory (DFT). Dependencies of reaction rates on internal energy were calculated by Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Results obtained establish the generality of a previous finding that loss of H is slower than competing losses of polyatomic fragments. This is attributable to substantial lowering of the frequencies of the vibrations that are converted to rotations and translations in the transition states for the latter reactions, but not in transition states for H losses. It is concluded that lower dissociation thresholds produce preferred losses of smaller alkyl fragments at low energies and that looser transition states favor losses of larger fragments at higher energies. DFT results and abundances of parallel alkane eliminations (two step processes also initiated by simple CC bond cleavage) indicate that cleavage of the CC bond to the larger alkyl fragment is most frequent even below the threshold for complete dissociation (energies at which ion-induced dipole alkyl complexes are formed), i.e., at all energies of bond cleavage, even if not reflected in simple cleavage abundances near threshold. Obtaining realistic rates by RRKM theory required increasing the lowest transition state vibrational frequencies (for modes that become rotations and translations) to well above those obtained by DFT for alkyl losses, further evidence that the rates of those reactions are controlled by minimum entropy transition states occurring earlier than the single imaginary frequency transition states found by DFT. In addition, comparison of DFT critical energies, photoionization critical energies and critical energies that give the best RRKM rates support relaxation of the ground states to lower energy species with long CC bonds following ionization. Preferred losses of larger alkanes at low energies, dominant losses of larger alkyl groups at high energies and formation of long CC bonds to the larger alkyl in the ground state ion all appear to have a common origin.

Methane elimination from ionized 1-butene

The elimination of methane from the 1-butene cation is characterized by B3LYP/6-31G(d) and ab initio theories. This reaction effectively occurs in two steps (1) CC bond cleavage with migration of methyl to the bond between the second carbon and the hydrogen on that carbon, and (2) transfer of that H from allyl to methyl. This pathway differs from previously characterized alkane eliminations by radical cations in that both CH distances in C⋯H⋯C are short around the point where H is half transferred, and the associated angle is substantially bent. Also, the charge is delocalized over both fragments during the course of the reaction, giving the migrating methyl considerable cationic character. The net unpaired spin is almost completely on the allyl carbons, so the methyl has almost negligible radical character. The observed electron distribution is attributed to hybridization due to the similarity in energies of CH 2CCH 2 ++CH 3 and CH 2CCH 2+CH 3 +, the species the H is transferred between. Overlap population analysis reveals substantial bonding not only between the transferring H and the carbons it bridges, but also between the methyl carbon and the middle allyl carbon. In previously studied alkane eliminations from radical cations in the gas phase, H-transfer occurs with the C, H and C involved being close to “in line,” in contrast to the present H-abstraction essentially by attack by methyl on a CH bond.

Chemical, microstructural and thermal analyses of a naphthalene-derived mesophase pitch

A detailed characterisation of a synthetic naphthalene-derived mesophase pitch, in its as-received state and during pyrolysis, has been performed. The study has been conducted by means of various techniques and with a particular attention to Raman microspectroscopy. The Raman spectra show features in common with the naphthalene precursor, i.e., a broad and complex band at 1150–1500 cm −1 and a multicomponent G band at 1600 cm −1. These features correspond to the vibration modes of the molecules of the pitch and more especially to the non-aromatic C–C bonds involved in alkyl groups, aryl–aryl bonds or naphthenic rings. The pyrolysis of the pitch into coke takes place within a narrow temperature range (480–500 °C) through the elimination of hydrogen and light alkanes resulting from the breaking of homolytic C–H bonds and naphthenic cycles, respectively. This process initiates a swelling of the pitch. The analysis of the Raman features shows that the structure of the pitch is only slightly affected within this temperature range. Conversely, significant structural changes of the material (as shown by the vanishing of the multicomponent bands at 1600 and 1150–1500 cm −1) are evidenced beyond 750 °C, simultaneously with a hydrogen release and an increase of the true density. This phenomenon corresponds to the extension of the graphene layers of the coke and the formation of a distorted carbon network.

Reactivity and derivatization of five-coordinate, chelated aluminum.

The convenient five-coordinate starting materials, Salen((t)Bu)AlCl (Salen((t)Bu) = N,N'-alkylene (or arylene) bis (3,5-di-tert-butyl-2-hydroxybenzylideneamine) (1-4) can be used in a wide range of reactions to form five-coordinate aluminum compounds. Herein, these reagents were used to produce new five-coordinate azides, LAlN(3) (L = Salen((t)Bu) (5), Salpen((t)Bu) (6), and Salomphen((t)Bu) (7)) through trimethylsilylhalide elimination. The decomposition of the azides produce first hydroxide (LAlOH (L = Salen((t)Bu) (8)) and, subsequently in the presence of chlorotrimethylsilane, the siloxide compounds, LAlOSiMe(3) (L = Salen((t)Bu) (9), Salpen((t)Bu) (10), and Salomphen((t)Bu) (11)). Alkane elimination reactions may also be used to access this type of compound as evidenced by the formation of Salomphen((t)Bu)AlOSiPh(3) (12). Additionally, the first structurally characterized five-coordinate monomeric amide, Salcen((t)Bu)AlN(SiMe(3))(2) (13), can prepared by a salt elimination utilizing Salcen((t)Bu)AlCl (4). The compounds were characterized by spectroscopic methods ((1)H and (27)Al NMR, MS, and IR) and, in the case of 2 (Salpen((t)Bu)AlCl), 3 (Salomphen((t)Bu)AlCl) 9, 11, 12, and 13, by X-ray analysis. Several of the compounds were explored as potential catalysts for the living polymerization of propylene oxide.

Heats of formation for the propen-1-ol and propen-2-ol cations by photoionization mass spectrometry

Threshold photoionization mass spectrometry has been used to measure the appearance energies for C3H6O+· formed via alkene elimination from several ionized aldehydes and methyl ketones. The derived 298 K heats of formation for the propen-1-ol and propen-2-ol cations are 681.1 ± 1.8 kJ mol−1 and 676.6 ± 0.7 kJ mol−1, respectively. Although these values are ∼16 kJ mol−1 higher than those derived from previous electron ionization data, they are both in excellent agreement with high level G2 ab initio calculations.

Reaction of trimethylaluminium with main group hydroxides: a non-hydrolysis route to methylalumoxane†

Reaction of AlMe3 with [(tBu)2Ga(μ-OH)]3 or Ph3EOH (E = Sn, Pb) yields catalytically active MAO, [MeAlO]n, along with (tBu)2GaMe and Ph3EMe, respectively, in contrast, the reaction with Ph3EOH (E = C, Si, Ge) yields [Me2Al(μ-OEPh3)]2; the formation of MAO is proposed to occur via hydroxide exchange and the formation of unstable [Me2Al(μ-OH)]n; the propensity towards alkane elimination versus hydroxide exchange is controlled by the relative Bronsted acidity of the main group hydroxide.

ChemInform Abstract: Alkane Eliminations from Ions in the Gas Phase

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Effect of variations of the size and structure of the principal alkyl group on alkene elimination from the immonium ions CH 3CH 2CHN +(CH 3)R, CH 3CH 2CH 2CHN +(CH 3)R and CH 3CH 2CH 2CH 2CHN +(CH 3)R

The reactions of three homologous series of metastable immonium ions of general structure, CH 3(CH 2) m CHN +(CH 3)R, [ m = 1–3; R = n-C n H 2 n+1 , n = 1–10, 12, and 14, iso-C 5H 11, CH 2CH(CH 3)C 2H 5, neo-C 5H 11, and CH 2CH 2C(CH 3) 3] are reported and discussed. The first member of each homologous series is anomalous in undergoing several reactions, including elimination of molecular hydrogen and a methyl radical. In contrast, higher members of all three series expel either or both alkenes, C n H 2 n or C n−1 H 2 n−2 derived from the principal alkyl group, R. The influence of the size and structure of R on these alkene eliminations is analysed. Loss of C n H 2 n produces an approximately Gaussian metastable peak; the associated kinetic energy release is relatively small and varies only slightly as the homologous series are ascended. In contrast, C n−1 H 2 n−2 elimination releases a much larger amount of kinetic energy, particularly for n = 3, when the associated metastable peak is dished; but the kinetic energy release declines dramatically on ascending the homologous series. Loss of C n−1 H 2 n−2 is favoured by branching at the γ-carbon atom, provided there is a γ-hydrogen atom, but it does not occur when R = CH 2CH 2C(CH 3) 3 because γ-hydrogen transfer is impossible. Keywords: Metastable ions; Degree of freedom effects; Immonium ions; Alkene elimination; kinetic energy release

Formation and Reactions of Tetracarbonyl Intermediates of the Fischer Carbene Complex (CO)5WC(OMe)Ph. A Laser Flash Photolysis Study Using Time-Resolved Infrared and UV/Vis Spectroscopy

The results of nanosecond laser flash photolysis (XeCl excimer irradiation at 308 nm) of the Fischer carbene complex [(CO)5WC(OMe)Ph] (CZ) in di-n-butyl ether and n-hexane are reported. Spectrokinetical detection of the intermediates was based on transient absorptions simulaneously recorded in the UV/vis and the IR spectral regions. The primary photoproducts are the unstable isomer CE produced with a quantum yield of about 30% and a tetracarbonyl complex I−S, probably of structure [(CO)4(S)WC(OMe)Ph], where S is a solvent molecule. The quantum yield of CO photoelimination was estimated as 1.7%. The CE isomer was found to return to the stable CZ isomer within a few microseconds, confirming previous results of McGarvey and co-workers. The tetracarbonyl species reacts with cosolutes present in the solution such as N2, H2O, acetonitrile, and methyl trans-crotonate to form secondary complexes of cis-tetracarbonyl complex structure [(CO)4(L)WC(OMe)Ph] (I−L). The tetracarbonyl complexes react readily with CO. For I−N2 in n-hexane the rate constant is 3.5 × 108 M-1 s-1. The rate constants of the corresponding reactions with acetonitrile (MeCN) and the olefin are larger by a factor of 2. In the presence of CO the complexes I−MeCN and I−olefin revert to CZ in a dissociative reaction mechanism. The rate constants for the elimination of MeCN or olefin are 5 and 105 s-1, respectively. The I−olefin complex irreversibly decomposes with a rate constant of 8 × 103 s-1.

Contributions of C3H6O+. ions with the oxygen on the middle carbon to gas phase ion chemistry

The numerous ways in which studies of C3H6O+. ions with the oxygen on the second carbon have added to our knowledge of gas phase ion chemistry are reviewed. The enol form of this ion (1) first attracted interest during early investigations of the mechanism of the McLafferty Rearrangement and later in characterizing the double McLafferty Rearrangement. Next, it was found that 1 isomerizes to the higher energy acetone ion (2). This discovery sparked studies of the relative stabilities of ionized and neutral enol and ketone species. It also led to the discovery that 1-->2 surmounts a substantial barrier, and that 2 dissociates faster than the excess energy deposited in it by the isomerization can become randomly distributed; i.e., dissociation following 1-->2 is nonergodic. This fact is manifested by a more abundant loss of the methyl formed by isomerization and a greater associated translational energy release. Propene oxide and methyl vinyl ether ions also appear to ionize to 2 and to dissociate nonergodically. Methane is eliminated from 2 through a methyl-acetyl ion complex. Characterization of this reaction by photoionization mass spectrometry, ab initio theory, and RRKM calculations helped to establish that complex-mediated alkane eliminations are generally confined to a region just above threshold. At higher energies, because attractions between ions and nonpolar neutrals are weak, simple dissociation is too fast for complex-mediated H-transfer to compete with it. Studies of the collision-induced dissociations of 2 demonstrate that the first electronically excited A state of 2 is very long-lived and efficiently releases its energy into translational energy when the ion collides with a neutral at low impact energies. Finally, ion-molecule reactions of 2 and acetone-containing ion clusters in the gas phase are described.

Stereochemical analysis of deuterated alkyl chains by MS/MS

Vicinally deuterated sec-alkyl phenyl ethers, CH(3)(CH(2))(m)()CH(OPh)CHD(CH(2))(n)()CH(3), display significant differences in mass spectra between threo and erythro stereoisomers. MS/MS experiments, in which parent ions of a single mass are selected and their fragmentation patterns subsequently measured, show that alkene expulsion represents virtually the only decomposition pathway. Two types of MS/MS experiment are reported: mass-analyzed ion kinetic energy (MIKE) spectroscopy of metastable ions and collisionally activated decomposition (CAD) of stable ions. The expulsion of a deuterated alkene from a monodeuterated precursor yields ionized phenol, PhOH(*)()(+) (m/z 94). The expulsion of an undeuterated alkene yields PhOD(*)()(+) (m/z 95). Without exception, the ratios (PhOD(*)()(+)/PhOH(*)()(+)) from precursors in the threo series have values greater than their diastereomers in the erythro series. The ratio of ratios, r = PhOD(*)()(+)/PhOH(*)()(+) for the threo divided by PhOD(*)()(+)/PhOH(*)()(+) for the erythro, has a value of 1.2 for the 2-phenoxy-3-deuteriobutanes and larger values for all of the higher homologues up through the monodeuterated phenoxyoctanes (m + n = 4). The highest degree of stereoselectivity, r = 5.8, is measured for 3-phenoxy-4-deuteriohexane. Experiments with multiply deuterated analogues show that alkene elimination is highly regioselective, unlike the corresponding decompositions of ionized sec-alcohols or their acetates. The fact that a large fraction of ionized sec-alkyl phenyl ethers undergo stereospecific syn-elimination means that mass spectrometry has a useful capacity to distinguish one isotopically labeled diastereomer from another.

Fragmentation reactions of alkylphenylammonium ions

The fragmentation reactions of a variety of alkylphenylammonium ions, C6H5NH3 −nRn+ (n ≥ 1, R = CH3, C2H5, i-C3H7, n-C4H9) were studied by energy-resolved mass spectrometry. Ionization was by fast atom bombardment (FAB) or electrospray ionization. Energy-resolved fragmentation data were obtained by low-energy collision-induced dissociation (CID) in the quadrupole cell of a hybrid sector/quadrupole instrument following FAB ionization and by cone-voltage CID in the interface region of the electrospray/quadrupole instrument. A comparison of the two methods of obtaining energy-resolved data showed that very similar results are obtained by the two methods. The fragmentation reactions of the alkylphenylammonium ions are rationalized in terms of competitive formation of an [R+—NC6H5H3−nRn−1] complex or a [C6H5H3−nRn−1N+·—·R] complex. The former complex fragments by internal proton transfer to yield C6H5H3 −nRn −1NH+ and [R −H] whereas the latter complex fragments to form C6H5H3 −nRn −1N+ and an alkyl radical. Alkane elimination, which is very prominent for tetraalkylammonium ions, most likely involves sequential elimination of an alkyl radical and either an H atom or an alkyl radical for the phenyl-substituted ammonium ions. Copyright © 1999 John Wiley & Sons, Ltd.

Formation of Separated versus Contact Ion Triplets in Magnesium Thiolates. Synthesis and Characterization of [Mg(15-crown-5)(SCPh(3))(2)] and [Mg(15-crown-5)(THF)(2)][S-2,4,6-tBu(3)C(6)H(2))](2).

The synthesis and structural characterization of two magnesium thiolate complexes are described. Both compounds were prepared by analogous reactions routes, namely, alkane elimination and transamination, but using differently sized ligands. Utilization of the smaller HSCPh(3), resulted in the formation of [Mg(15-crown-5)(SCPh(3))(2)], 1, displaying a seven-coordinate magnesium center with two magnesium-sulfur bonds. Employment of the sterically more encumbered HS-2,4,6-tBu(3)C(6)H(2) yielded a separated cation and anions as displayed in [Mg(15-crown-5)(THF)(2)][S-2,4,6-tBu(3)C(6)H(2)](2), 2. Compound 2 exhibits a seven-coordinate cation, completing its coordination environment with five metal-crown-ether interactions in addition to two tetrahydrofuran (THF) contacts. The compounds were characterized by IR and NMR spectroscopy in addition to X-ray crystallography. Crystal data with Mo Kalpha (lambda = 0.710 73 Å) are as follows: 1, C(48)H(50)MgO(5)S(2), a = 8.4785(4) Å, b = 9.3595(5) Å, c = 13.8306(6) Å, alpha = 87.332(1) degrees, beta = 85.929(1) degrees, gamma = 68.733(1) degrees, Z = 1, triclinic; 2, C(54)H(94)MgO(7)S(2), a = 13.6755(2) Å, b = 12.8045(6) Å, c = 32.7127(12) Å, beta = 97.87(1) degrees, Z = 4, monoclinic.

Fragmentation reactions of alkylphenylammonium ions

The fragmentation reactions of a variety of alkylphenylammonium ions, C(6)H(5)NH(3 -n)R(n)(+) (n >/= 1, R = CH(3), C(2)H(5), i-C(3)H(7), n-C(4)H(9)) were studied by energy-resolved mass spectrometry. Ionization was by fast atom bombardment (FAB) or electrospray ionization. Energy-resolved fragmentation data were obtained by low-energy collision-induced dissociation (CID) in the quadrupole cell of a hybrid sector/quadrupole instrument following FAB ionization and by cone-voltage CID in the interface region of the electrospray/quadrupole instrument. A comparison of the two methods of obtaining energy-resolved data showed that very similar results are obtained by the two methods. The fragmentation reactions of the alkylphenylammonium ions are rationalized in terms of competitive formation of an [R(+)-NC(6)H(5)H(3-n)R(n-1)] complex or a [C(6)H(5)H(3-n)R(n-1)N(+.)-(.)R] complex. The former complex fragments by internal proton transfer to yield C(6)H(5)H(3 -n)R(n -1)NH(+) and [R -H] whereas the latter complex fragments to form C(6)H(5)H(3 -n)R(n -1)N(+) and an alkyl radical. Alkane elimination, which is very prominent for tetraalkylammonium ions, most likely involves sequential elimination of an alkyl radical and either an H atom or an alkyl radical for the phenyl-substituted ammonium ions. Copyright 1999 John Wiley & Sons, Ltd.

Elimination of alkanes from off‐gases using biotrickling filters containing two liquid phases

AbstractBiological techniques are highly cost‐effective for the treatment of off‐gases containing low concentrations of pollutants (< 5 g/m3). They may also be attractive for the elimination of higher concentrations of explosive hydrocarbons (when compared to incineration). Conventional techniques such as biofilters have low elimination capacities for hydrophobic compounds caused by a poor mass transfer from the gas to the aqueous phase.To overcome solubility problems, a novel biotrickling filter was developed and is characterized by the use of an organic solvent. In such a biotrickling filter, a mixture of an organic solvent and water is continuously trickled over a packed bed, while the polluted gas passes counter‐current to the liquid. The microorganisms exist on the packing material and in the circulating liquid. The alkanes are absorbed in the oil phase of the liquid, transfer red to the microorganisms and biodegraded.Laboratory‐scale experiments at 201 scale with hexane as a model pollutant and silicon oil as a solvent revealed that a 90% elimination efficiency can be reached at a volumetric loading rate of 100g hexane per m3 filterbed per hour. The influent gas contained 10 g hexane per m3 and had a temperature of 29°C.The method described in this paper can be a cost‐effective way to treat gases containing hexane, other alkanes or other strongly hydrophobic compounds. Biological co‐oxidation of other biodegradable pollutants from the gas is to be expected.

Distinction among isomeric unsaturated fatty acids as lithiated adducts by electrospray ionization mass spectrometry using low energy collisionally activated dissociation on a triple stage quadrupole instrument

Features of tandem mass spectra of dilithiated adduct ions of unsaturated fatty acids obtained by electrospray ionization mass spectrometry with low-energy collisionally activated dissociation (CAD) on a triple stage quadrupole instrument are described. These spectra distinguish among isomeric unsaturated fatty acids and permit assignment of double-bond location. Informative fragment ions reflect cleavage of bonds remote from the charge site on the dilithiated carboxylate moiety. The spectra contain radical cations reflecting cleavage of bonds between the first and second and between the second and third carbon atoms in the fatty acid chain. These ions are followed by a closed-shell ion series with members separated by 14 m/z units that reflect cleavage of bonds between the third and fourth and then between subsequent adjacent pairs of carbon atoms. This ion series terminates at the member reflecting cleavage of the carbon–carbon single bond vinylic to the first carbon–carbon double bond. Ions reflecting cleavages of bonds distal to the double bond are rarely observed for monounsaturated fatty acids and are not abundant when they occur. For polyunsaturated fatty acids that contain double bonds separated by a single methylene group, ions reflecting cleavage of carbon–carbon single bonds between double bonds are abundant, but ions reflecting cleavages distal to the final double bond are not. Cleavages between double bonds observed in these spectra can be rationalized by a scheme involving a six-membered transition state and subsequent rearrangement of a bis-allylic hydrogen atom to yield a terminally unsaturated charge-carrying fragment and elimination of a neutral alkene. The location of the β-hydroxy-alkene moiety in ricinoleic acid can be demonstrated by similar methods. These observations offer the opportunity for laboratories that have tandem quadrupole instruments but do not have instruments with high energy CAD capabilities to assign double bond location in unsaturated free fatty acids by mass spectrometric methods without derivatization.

Syntheses and Structures of Five-Coordinate Zirconium Alkyl Complexes Supported by Diketiminate Ligands

An alkane elimination reaction generates the diketiminate compound (TTP)Zr(CH2Ph)3 (1) from Zr(CH2Ph)4 and TTPH (TTPH = 2-p-tolylamino-4-p-tolylimino-2-pentene). The molecular structure of 1 was so...

Half-Sandwich Alkyl and Hydrido Complexes of Yttrium: Convenient Synthesis and Polymerization Catalysis of Polar Monomers

Direct preparation of alkyl half-sandwich complexes of the heavier rare earth elements (such as 1) is possible by alkane elimination from the corresponding cyclopentadiene derivative and the trialkyl complex [Y(CH2SiMe3)3(thf)2]. Hydrogenation of 1 gives the highly fluxional dimeric hydride complex 2. Both complexes catalyze the polymerization of the polar monomers tert-butyl acrylate and acrylonitrile.

Half-Sandwich Alkyl and Hydrido Complexes of Yttrium: Convenient Synthesis and Polymerization Catalysis of Polar Monomers

Direct preparation of alkyl half-sandwich complexes of the heavier rare earth elements (such as 1) is possible by alkane elimination from the corresponding cyclopentadiene derivative and the trialkyl complex [Y(CH2SiMe3)3(thf)2]. Hydrogenation of 1 gives the highly fluxional dimeric hydride complex 2. Both complexes catalyze the polymerization of the polar monomers tert-butyl acrylate and acrylonitrile.

EMS Account: Alkane eliminations from ions in the gas phase

A review of reactions involving the elimination of an alkane from a variety of even and odd electron ions in the gas phase is presented. Particular attention is focused on the mechanisms of these reactions and the role of ion–neutral complexes. Alkane eliminations from open shell species derived by ionisation of ketones, ethers, alcohols, amines, alkanes and alkyl-substituted cycloalkanes are considered, together with ethane loss from the distonic ion CH2=O+CH2CH2•. These reactions are, without obvious exception, complex-mediated. Elimination of alkanes from an assortment of closed shell ions, including protonated acetaldehyde, dialkylmethylene immonium ions, onium ions, quaternary ammonium cations, alkoxide anions, enolate anions and deprotonated amines are also discussed. These reactions tend to be complex-mediated when heterolytic bond cleavages are possible. When such cleavages are not energetically feasible, alkane eliminations can take place by asynchronous, concerted mechanisms. Several factors affect the relative rates of alkane elimination compared to those of competing alkyl radical loss in the dissociations of these radical-cations; these factors include the internal energy of the ion, the size of the ionic and neutral partners, the initial orientation of these components and the binding energy of the complex. In addition, primary and secondary isotope effects influence both alkyl radical and alkane elimination. When alkanes are eliminated from radical-cations, reaction between the partners is often quickly overtaken in importance by direct separation of the components if the internal energy is raised either by increasing the energy of the ionisation process or by prior isomerisation over an energy barrier. This trend demonstrates that the attractions between the partners are easily overcome by quite small amounts of excess energy in the system. Typical binding energies between the ionic and non-polar neutral partners lie in the range 0–30 kJ mol−1 at distances at which covalent forces cease to be significant. Illustrative examples are given to show how studies of reactions involving ion–neutral complexes offer a means of obtaining information on the dynamics of short range interactions between ions and neutrals, including the motions of the partners relative to each other.