Chemical ionization of phenyl n-propyl ether and methyl substituted analogs: Propene loss initiated by competing proton transfer to the oxygen atom and the aromatic ring

The mechanism of propene loss from protonated phenyl n-propyl ether and a series of mono-, di-, and trimethylphenyl n-propyl ethers has been examined by chemical ionization (CI) mass spectrometry in combination with tandem mass spectrometry experiments. The role of initial proton transfer to the oxygen atom and the aromatic ring, respectively, has been probed with the use of deuterated CI reagents, D₂O, CD₃OD, and CD₃CN (given in order of increasing proton affinity), in combination with deuterium labeling of the β position of the n-propyl group or the phenyl ring. The metastable [M + D]⁺ ions of phenyl n-propyl ether—formed with D₂O as the CI reagent—eliminate C₃H₅D and C₃H₆ in a ratio of 10:90, which indicates that the added deuteron is incorporated to a minor extent in the expelled neutral species. In the experiments with CD₃OD as the CI reagent, the ratio between the losses of C₃H₅D and C₃H₆ from the metastable [M + D]⁺ ions of phenyl n-propyl ether is 18:82, whereas the ratio becomes 27:73 with CD₃CN as the reagent. A similar trend in the tendency to expel a propene molecule that contains the added deuteron is observed for the metastable [M + D]⁺ ions of phenyl n-propyl ether labeled at the β position of the alkyl group. Incorporation of a hydrogen atom that originates from the aromatic ring in the expelled propene molecule is of negligible importance as revealed by the minor loss of C₃H₅D from the metastable [M + H]⁺ ions of C₆D₅OCH₂CH₂CH₃ irrespective of whether H₂O, CH₃OH, or CH₃CN is the CI reagent. The combined results for the [M + D]⁺ ions of phenyl n-propyl ether and deuterium-labeled analogs are suggested to be in line with a model that assumes that propene loss occurs not only from species formed by deuteron transfer to the oxygen atom, but also from ions generated by deuteron transfer to the ring. This is substantiated by the results for the methyl-substituted ethers, which reveal that the position as well as the number of methyl groups bonded to the ring exert a marked effect on the relative importances of the losses of C₃H₅D and C₃H₆ from the metastable [M + D]⁺ ions of the unlabeled methyl-substituted species.     

A chemical ionization study of deuteron transfer initiated propene loss from propoxypyridines

The mechanism of propene loss from the metastable [M + D](+) ions of isomeric 2-, 3-, and 4-n-propoxypyridines and the related isopropoxypyridines has been examined by chemical ionization (CI) and tandem mass spectrometry in combination with deuterium labeling. The [M + D](+) ions were generated with CD(3)OD, CD(3)CN, (CD(3))(2)CO, or pyrrole-D(5) (listed in order of increasing proton affinity) as the CI reagent. The results reveal that the deuteron added in the CI process is not interchanged with the hydrogen atoms of the propyl group prior to propene loss from the metastable [M + D](+) ions of the propoxypyridines. The site selective labeling of the alpha-, beta-, or gamma-position of the propyl group indicates that the [M + D](+) ions of 2-n-propoxypyridine expel propene with formation of an ion-neutral complex composed of a propyl carbenium ion and 2-pyridone. By contrast, the [M + D](+) ions of 3-n-propoxypyridine expel propene by: (1) Formation of ion-neutral complexes, and (2) a conventional 1,5-hydride shift from the beta-position of the n-propyl group to the ring and/or a 1,2-elimination type process. For the 4-isomer, the results suggest the occurrence of propene loss by a 1,2-elimination in addition to the intermediate formation of ion-neutral complexes. Loss of propene with one deuterium atom is the only reaction of the [M + D](+) ions of the isopropoxypyridines labeled at the alpha-position of the isopropyl group. The results for the isopropoxypyridines labeled with three deuterium atoms at the beta-position are consistent with: (1) The loss of propene by ion-neutral complex formation and the occurrence of a substantial isotope effect in the subsequent proton/deuteron transfer within the complex, and/or (2) the loss of propene by a 1,2-elimination type reaction.     

Ion-Neutral complexes from protonated propyl phenyl ethers. Gas-phase solvolysis versus elimination-readdition

Ion-neutral complexes, well attested as intermediates in the expulsion of alkenes from M⁺? and MH⁺ ions from primary alkyl phenyl ethers, are shown to intervene in the decomposition of the MH⁺ ion of a secondary alkyl phenyl ether, (CD₃)₂CHOPh. Chemical ionization (CI) (methane reagent gas)-mass-analysed ion kinetic energy spectroscopy (MIKES) shows ions of both m/z 96 and 97, indicating that the proton deposited by the CI reagent exchanges with the methyl deuterium atoms. The ratio of daughter ion intensities, as well as the proportions of ions of m/z 95, 96 and 97 from the MH⁺ of CD₃CH₂CD₂OPh, agree with predictions based on the gas-phase solvolysis mechanism, in which [i-Pr⁺ PhOH] complexes form from the protonated parent via simple bond heterolysis. An alternative mechanism, elimination-readdition, would proceed via [propene PhOHD⁺] complexes. This latter mechanism predicts a ratio of daughter ion intensities that is very different from gas-phase solvolysis and which disagrees with experiment. The elimination-readdition pathway is effectively ruled out, while the gas-phase solvolysis mechanism is reinforced.     

Benzoaza-15-crown-5 ethers: synthesis,structure, and complex formation with metal and ethylammonium ions

Benzoaza-15-crown-5 ethers containing one or two nitrogen atoms in different positions of the macrocycle and bearing different substituents at these atoms were synthesized. The structures of azacrown ethers and their metal complexes were studied by X-ray diffraction. The stability constants of the complexes of azacrown ethers with Na⁺, Ca²⁺, Ba²⁺, Ag⁺, Pb²⁺, and EtNH₃ ⁺ ions were determined by ¹H NMR titration in MeCN-d₃. In free benzoazacrown ethers containing secondary nitrogen atoms bound to the benzene ring, as well as in N-acetyl derivatives, the N atoms are sp²-hybridized and have a planar geometry. The nitrogen lone pairs on the p orbitals are efficiently conjugated to the benzene ring or the carbonyl fragment of the acetyl group, which is unfavorable for the complex formation. In addition, the formation of complexes with benzoazacrown ethers containing secondary nitrogen atoms is hindered because the hydrogen atoms of the NH groups are directed to the center of the macrocyclic cavity. In benzoazacrown ethers bearing N-alkyl substituents or secondary nitrogen atoms distant from the benzene ring, the N atoms show a substantial contribution of the sp³-hybridized state and have a pronounced pyramidal configuration, which promotes the complex formation. The lead and calcium cations form the most stable complexes due to the high affinity of Pb²⁺ ions for O,N-containing ligands, a high charge density on these ions, and the better correspondence of the cavity size of the 15-membered macrocycles to the diameter of the Ca²⁺ ion. An increase in the stability of the complexes is observed mainly in going from monoazacrown ethers to diazacrown ethers containing identical substituents at the N atoms and in the following series of substituents: C(O)Me < H < Me < CH₂CO₂Et. In the case of the CH₂CO₂Et substituents, the carbonyl oxygen atom is also involved in the coordination to the cation. The characteristic features of the complexing ability of N-alkylbenzomonoaza-15-crown-5 ethers bearing the nitrogen atom conjugated to the benzene ring show that macro-cyclic ligands having this structure are promising as selective and efficient complexing agents for metal cations.     

On the loss of a benzyl radical from the molecular ion of α,ω -dibenzyloxyalkanes : Evidence for a benzyl cation transfer in an SNi-type reaction,revealed by a simultaneous D- and 18O-labelling

The molecular ions of the title compounds appear to lose a benzyl radical, which must be due to the presence of two benzyloxy groups, as benzylalkyl ethers do not exhibit such an expulsion upon electron impact. The results of the partition of the labels deuterium and ¹⁸O in the ions m/e 107 (protonated benzaldehyde) and [M-benzyl-benzaldehyde]⁺ put forward evidence that this process is initiated by a successive migration of a benzylic H atom to the opposite ether function and transfer of the benzyl cation from this protonated O atom to the uncharged O atom in an S_Ni-type reaction (cf Scheme 5).     

Sigma-bond metathesis reactions of Sc(OCD3)2+ with water,ethanol, and 1-propanol: measurements of equilibrium constants,relative bond strengths,and absolute bond strengths

Fourier transform ion cyclotron resonance (FTICR) mass spectrometry has been used to examine the reactions of Sc(OCD₃)₂⁺ with water, ethanol, and 1-propanol. Sigma-bond metathesis resulting in the elimination of CD₃OH is the initial reaction observed, with further solvation of the metal center and subsequent elimination of hydrogen occurring as additional reaction channels. These processes are facile at room temperature and involve little or no activation energy. Measured equilibrium constants for the reaction Sc(OCD₃)₂⁺ +ROH ⇌ CD₃OScOR⁺ +CD₃OH with R =H, ethyl, and n-propyl are 0.013 ±0.004, 0.5 ±0.15, and 0.7 ±0.2, respectively. For the reaction ROScOCD₃⁺ +ROH ⇌ Sc(OR)₂⁺ +CD₃OH with R =H and ethyl the measured equilibrium constants are 0.013 ±0.004 and 0.3 ±0.1, respectively. ΔS is estimated for these processes using theoretical calculations and statistical thermodynamics, and in conjunction with the measured equilibrium constants we have evaluated ΔH for these reactions and the relative and absolute bond strengths of the Sc⁺–OR bonds, R =H, methyl, ethyl, and n-propyl. The relative bond strengths, D₂₉₈^o(CD₃OSc⁺–OR)–D₂₉₈^o(CD₃OSc⁺–OCD₃), for R =H, methyl, ethyl, and n-propyl are +11.9, 0, −0.1, and −1.4 kcal mol⁻¹, respectively. The absolute bond strengths for HOSc⁺–OCD₃, CD₃OSc⁺–OCD₃, CD₃OSc⁺–OC₂H₅, CD₃OSc⁺–OCH₂CH₂CH₃, and H₅C₂OSc⁺–OC₂H₅ are 115.0, 115.0, 114.9, 113.6, and 114.7 kcal mol⁻¹, respectively. Theoretical calculations with an LAV3P1 ECP basis set at the level of localized second-order Møller–Plesset perturbation theory were performed to evaluate ΔS and ΔG for the specific equilibria Sc(OH)₂⁺ +CD₃OH ⇌ CD₃OScOH +H₂O, CD₃OScOH +CD₃OH ⇌ Sc(OCD₃)₂⁺ +H₂O, and Sc(OCD₃)₂⁺ +C₂H₅OH ⇌ CD₃OScOC₂H₅⁺ +CD₃OH. The theoretically determined ΔG values agree reasonably well with the experimentally determined ΔG values. In accordance with earlier theoretical predictions, these metathesis reactions are consistent with an allowed four-center mechanism similar to that of a 2_σ +2_σ cycloaddition.     

On the structure of CH5+; A study of hydron transfer reactions from CH4H+ and CD4H+ tandem-ICR

Relative rates of proton and deuteron transfer from CH₄D⁺ and CD₄H⁺ to a number of molecules were examined in a tandem-ion cyclotron resonance instrument. The results were in conflict with the recent work of Sefcik et al. and support a randomized model.     

Mass spectrometry of aralkyl compounds with a functional group—VII. On the structure of the ions C8H9+ from C6H5C2H4X- and C9H11+ from C6H5C3H6X-compounds

The C₈H₉⁺-ion, formed from the molecular ions of 2-phenyl-1-bromoethane, 1-phenyl-1-bromoethane and of 1-phenyl-1-nitroethane by loss of the bromine atom and of the nitro group, splits off a molecule of acetylene after an almost complete randomization of hydrogens, as proved by deuteration. An eight-membered ring structure for the C₈H₉⁺-ion is proposed to explain these results. By loss of the nitro group from the molecular ions of 1-phenyl-1-nitropropane and of 1-phenyl-2-nitropropane the well-known phenylated cyclopropane ion3 (C₉H₁₁)⁺ is generated. Mass spectra of analogues, specifically deuterated in the side-chain, show that in this ion a randomization of hydrogen atoms in the cyclopropane ring as well as a hydride transfer from the cyclopropane ring to the phenyl cation occur.     

The ionized methylene transfer from the distonic radical cation +CH2-O-CH2 to heterocyclic compounds. A pentaquadrupole mass spectrometric study

Ion-molecule reactions of the mass-selected distonic radical cation ⁺CH₂-O-CH ₂ ^· (1) with several heterocyclic compounds have been investigated by multiple stage mass spectro- metric experiments performed in a pentaquadrupole mass spectrometer. Reactions with pyridine, 2-, 3-, and 4-ethyl, 2-methoxy, and 2-n-propyl pyridine occur mainly by transfer of CH ₂ ^+· to the nitrogen, which yields distonic N-methylene-pyridinium radical cations. The MS³ spectra of these products display very characteristic collision-induced dissociation chemistry, which is greatly affected by the position of the substituent in the pyridine ring. Ortho isomers undergo a δ-cleavage cyclization process induced by the free-radical character of the N-methylene group that yields bicyclic pyridinium cations. On the other hand, extensive CH ₂ ^+· transfer followed by rapid hydrogen atom loss, that is, a net CH⁺ transfer, occurs not to the heteroatoms, but to the aromatic ring of furan, thiophene, pyrrole, and N-methyl pyrrole. The reaction proceeds through five- to six-membered ring expansion, which yields the pyrilium, thiapyrilium, N-protonated, and N-methylated pyridine cations, respectively, as indicated by MS³ scans. Ion 1 fails to transfer CH ₂ ^+· to tetrahydrofuran, whereas a new α-distonic sulfur ion is formed in reactions with tetrahydrothiophene. Unstable N-methylene distonic ions, likely formed by transfer of CH ₂ ^+· to the nitrogen of piperidine and pyrrolidine, undergo rapid fragmentation by loss of the α-NH hydrogen to yield closed-shell immonium cations. The most thermodynamically favorable products are formed in these reactions, as estimated by ab initio calculations at the MP2/6-31G(d,p)//6-31G(d,p) + ZPE level of theory.     

Free radicals formed by ion beam neutralization of [DCO]+, [DOCO] +, [CD3OD2]+ and [(CD3)2COD] + on metal targets

A series of radicals formed when a fast beam of deuterated ions, [DCO] ⁺, [DOCO] ⁺, [CD₃OD₂]⁺ and [(CD) ₂COD] ⁺, is neutralized by electron transfer from K. Na and Zn atoms has been studied by a combination of charge stripping and beam scattering techniques For all systems studied, some fraction of The radicals from charge exchange are formed in excited dissociative states. Radical decomposition products have been identified and fragmentation energies have been measured. A metastable slate of CD3GD2 is observed with a lifetime greater than 0.5 μs. Stable radicals of DCO and (CD₃)₂ COD are formed by ion–Zn charge exchange. Indirect evidence indicates that [DCO] ⁺–K charge exchang; leads to an excited radiative state of the radical.     

Determination of orders of relative alkali metal ion affinities of crown ethers and acyclic analogs by the kinetic method

Ladders of relative alkali ion affinities of crown ethers and acyclic analogs were constructed by using the kinetic method. The adducts consisting of two different ethers bound by an alkali metal ion, (M₁ + Cat + M₂)⁺, were formed by using fast atom bombardment ionization to desorb the crown ethers and alkali metal ions, then collisionally activated to induce dissociation to (M₁ + Cat)⁺ and (M₂ + Cat)⁺ ions. Based on the relative abundances of the cationized ethers formed, orders of relative alkali ion affinities were assigned. The crown ethers showed higher affinities for specific sizes of metal ions, and this was attributed in part to the optimal spatial fit concept. Size selectivities were more pronounced for the smaller alkali metal ions such as Li⁺, Na⁺, and K⁺ than the larger ions such as Cs⁺ and Rb⁺. In general, the cyclic ethers exhibited greater alkali metal ion affinities than the corresponding acyclic analogs, although these effects were less dramatic as the size of the alkali metal ion increased.     

High resolution mass spectrometry—V: Cyclic esters of aliphatic α-hydroxy acids

The main fragmentation sequences of glycollide and its homologues are initiated by fission of a CO? O bond, leading to the formation of fragment ions of low, m/e, such as [R¹CO]⁺ and [CR¹R²CCO]⁺. When a hydrogen atom is present on a ring carbon atom, 1,3 hydrogen migration occurs to produce [CHR²OH]⁺. In case where a ring carbon atom carries an alkylchain ? C₂H₅, a McLafferty rearrangement occurs with the adjacent carbonyl group. When both ring carbon atoms are dimethyl substituted, a 1,4 hydrogen migration must be invoked to account for the observed fragmentation sequence.     

Benzene as a selective chemical ionization reagent gas

Dilute mixtures of C₆H₆ or C₆D₆ in He provide abundant [C₆H₆]^+· or [C₆D₆]^+· ions and small amounts of [C₆H₇]⁺ or [C₆D₇]⁺ ions as chemical ionization (CI) reagent ions. The C₆H₆ or C₆D₆ CI spectra of alkylbenzenes and alkylanilines contain predominantly M^+· ions from reactions of [C₆H₆]^+· or [C₆D₆]^+· and small amounts of MH⁺ or MD⁺ ions from reactions of [C₆H₇]⁺ or [C₆D₇]⁺. Benzene CI spectra of aliphatic amines contain M^+·, fragment ions and sample-size-dependent MH⁺ ions from sample ion-sample molecules reactions. The C₆D₆ CI spectra of substituted pyridines contain M^+· and MD⁺ ions in different ratios depending on the substituent (which alters the ionization energy of the substituted pyridine), as well as sample-size-dependent MH⁺ ions from sample ion-sample molecule reactions. Two mechanisms are observed for the formation of MD⁺ ions: proton transfer from [C₆D₆]^+· or charge transfer from [C₆D₆]^+· to give M^+·, followed by deuteron transfer from C₆D₆ to M^+·. The mechanisms of reactions were established by ion cyclotron resonance (ICR) experiments. Proton transfer from [C₆H₆]^+· or [C₆D₆]^+· is rapid only for compounds for which proton transfer is exothermic and charge transfer is endothermic. For compounds for which both charge transfer and proton transfer are exothermic, charge transfer is the almost exclusive reaction.     

Fragmentation of the metastable molecular ion of methyl lactate: The formation of oxygen-protonated methanol [CH3OH2]+ involving double hydrogen atom transfers

The spontaneous unimolecular dissociation reaction of methyl lactate (1) ionized by electron impact was investigated by a combination of mass-analyzed ion kinetic energy spectrometry and deuterium labeling. The metastable ions 1^+· decompose in a variety of ways: four fragment peaks are observed at m/z 89, 76, 61, and 45, which correspond to the losses of ?H₃, CO, CH₃?O, and ?OOCH₃, respectively. Double hydrogen atom transfer occurs in the third reaction. The source-generated m/z 61 ions decompose into oxygen-protonated methanols at m/z 33 ([CH₃OH ₂ ⁺ ]) by the loss of CO with double hydrogen atom migration. Both hydroxyl and methyne hydrogen atoms in 1 ^+· are present in the resultant protonated methanols.     

Mass spectrometry of pyridine compounds–IV: The formation and decomposition of the [M – methyl]+ ion [C8H10N]+ from 4-t-butylpyridine in comparison with that of the [M – methyl]+ ion [C9H11]+ from t-butylbenzene,as studied by D- and 13C-labelling

A strong secondary isotope effect is observed in the preferred loss of methyl vs. trideutero-methyl from the molecular ions of appropriately labelled 4-t-butylpyridine and t-butylbenzene decomposing in the first and second field free regions of a double focusing mass spectrometer. This has been rationalised by invoking the theory of radiationless transitions², which can account for the higher population of activated states responsible for loss of methyl vs. that for trideuteromethyl. ¹³C-Labelling at the central carbon atom of the t-butyl group indicates that the [M – methyl]⁺ ions, decomposing further by elimination of ethylene, cannot be represented exclusively by a pyridylated (or phenylated) cyclopropane ion if present at all. It is concluded that ions with structures generated by 1,2-hydrogen-, 1,2-pyridyl- (or 1,2-phenyl-) and 1,2-methyl shifts must also play a role. D-labelling further shows an extensive randomisation of side-chain hydrogen atoms in the [M-methyl]⁺ ions of 4-t-butylbenzene; in this case, however, the expelled ethylene also contains ring hydrogen atoms (≤2). Presumably this is caused by exchange between the side-chain and ortho-hydrogen atoms in the initially generated phenyldimethylcarbinyl carbenium ion.     

3,6-Bis(dimethylamino)-10-propylacridinium iodide

In the title compound, C₂₀H₂₆N₃⁺·I⁻, the acridinium moiety shows mirror symmetry about the central C—N vector. The fused tricyclic system is only approximately planar and the geometry is affected by the presence of both di­methyl­amino groups and the propyl substitution at the central N atom. The propyl chain adopts an extended trans conformation and the plane through the chain C atoms is perpendicular to the mean plane through the rings. The I⁻ ion is involved in short-range hydrogen-bonding interactions with two centrosymmetrically related cations via three activated acridinium C atoms. Stacks of acridinium cations propagate through the crystal along the c direction. The ring overlap is partial, but the di­methyl­amino groups also participate in the stacking.     

Hydrogen migrations in mass spectrometry. I—The loss of olefin from phenyl-n-propyl ether following electron impact ionization and chemical ionization

Using specifically labelled compounds we have made a detailed study of the source of the hydrogen transferred in the elimination of C₃H₆ from the molecular ion of phenyln-propyl ether following electron impact ionization and from the protonated (and ethylated) molecule following chemical ionization. The migrating hydrogen originates from all three positions of the npropyl group but not in the ratio expected for randomization of the alkyl hydrogens prior to transfer. The source of the migrating hydrogen is similar for both electron impact ionization and chemical ionization, indicating that the factors governing the rearrangement are the same for both modes of ionization. From a comparison of the results for labelled 2,6-dimethyl phenyl n-propyl ethers with the results for the unsubstituted ether it is concluded that hydrogen transfer occurs only to the ether oxygen and not to the phenyl ring. A two-step mechanism involving a set of competing reversible hydrogen transfer reactions followed by C? O bond cleavage is proposed to explain the results.     

Liquid-liquid extraction of copper(I) by cyclic tetrathio ethers

The liquid-liquid extraction of copper(I) with 12-, 13-, 15- and 16-membered cyclic tetrathio ethers ([n]aneS₄, where n represents the total number of carbon and sulphur atoms in the cyclic ligand ring) was examined stoichiometrically using picrate ion (Pic^?) for the formation of the ion pair. Copper(I) was extracted with four ligands (L) into 1,2-dichloroethane as the ion-pair compound, [Cu(I)L]⁺Pic^?. The extraction constant, K_ex, with each ligand was determined. As the ring size of cyclic tetrathio ethers increases, the log K_ex values, including that previously reported for [14]aneS₄, increase from 7.7 to 9.4. The value of Δ log K_ex, which represents the increase in log K_ex due to the addition of one carbon atom to the macrocyclic ring, was large between [13]aneS₄ and [14]aneS₄ (Δ log K_ex=1.0) and small between [14]aneS₄ and [15]aneS₄ (Δ log K_ex=0.1).     

Effects of functional group interactions on the gas-phase methylation and dissociation of acids and esters

Functional group interactions have been observed to affect gas-phase ion-molecule chemistry in a quadrupole ion trap mass spectrometer. Gas-phase methylation and collisionactivated dissociation reactions of a series of related acids and esters allows an evaluation of the structural factors that influence reactivity and functional group interactions of these compounds. Examination of the [M+H]⁺ or [M+15]⁺ product ions by collision-activated dissociation has provided insight into the conformations from which diacids and diesters undergo electrophilic addition. Collision-activated dissociation has provided not only more detailed information on the structures of the ions, but also the data necessary for confident mechanistic interpretation. Labeling studies were done to probe fragmentation pathways. Upon activation of the [M+CD₃]⁺ products of dimethyl maleate and dimethyl succinate, formed from reaction of the neutrals with CD₃OCD ₂ ⁺ ions, a rapid interfunctional group methyl transfer causes scrambling of the methyls prior to elimination of dimethyl ether or methanol. The [M+15]⁺ ions of dimethyl maleate are believed to lose dimethyl ether through a rate-determining 1,6-methyl transfer, whereas the [M+15]⁺ ions of dimethyl succinate eliminate methanol through a rate-determining 1,5-proton transfer.     

Substituent effects in the binding of bis(4-fluorobenzyl)ammonium ions by dianilino[24]crown-8

A series of para-substituted dianilino[24]crown-8 (DA24C8) macrocycles were synthesized and their ability to form host-guest complexes with bis(4-fluorobenzyl)ammonium ions (DFA⁺) were investigated. Although these crown ethers contain weakly hydrogen bonding aniline motifs, they do bind DFA⁺ in CDCl₃/CD₃NO₂ solution, presumably in a pseudorotaxane-like manner. A plot of the values of the relative binding strengths (log[K_a(R)/K_a(H)]) versus the Hammett substituent constants σ⁺ of the groups at the para-position of the aniline units suggests that a linear free energy correlation exists for this self-assembly process. The strength of the binding between the crown ether and the thread-like ion can be fine-tuned over a narrow range by judicious choice of the substituting groups.