Computational studies of ion pairing. 10. Ion pairing between tetrabutylammonium ion and inorganic ions: a general motif confirmed.

Until recently, little has been known about the nature of the factors governing the stereochemistry of the interactions between organic ions, even though such information should be directly relative to issues of molecular recognition and supermolecular self-assembly. The present study of the preferred structures of the ion pairs between tetrabutylammonium and 22 common inorganic ions, a continuation of previous studies in this series, brings to 93 the number of such species that have been examined in this way. In every one of these ion pairs, the minimum energy orientation of the cation relative to the anion is the same, reinforcing the conclusion that this structural motif is completely general. This is the first such pattern to be identified for the mode of association between organic cations and polar species in solution.

Electrochemical and quantum chemical investigation of tetranitrocalix[4]arenes: molecules with multiple redox centers.

The calix[4]arene skeleton is electrochemically inactive, but it is a useful stable frame for building "smart" molecules and supramolecular assemblies. Suitable substitution on the upper (and/or lower) rim leads to unusual and surprising properties in this system. Polynitrocalix[4]arenes with reducible nitro groups located at the upper rim represent molecules with multiple redox centers where the potential for interactions between them is the focus of interest. The title compounds are promising precursors, e.g., for design and synthesis of sensors. In this work, the stepwise reduction of two tetranitrocalixarenes was investigated electrochemically, and the results were correlated with quantum chemical calculations. The order of individual electron transfers was described as a consequence of molecular geometry. Two independent pairs of equivalent nitro groups were identified whose reduction potential depends upon their respective locations in the molecule. All nitro groups are electronically isolated and thus are reduced independently yielding poly radical ions. The increasing charge has negligible impact on the geometry of the calixarene, which maintains its pinched shape even when carrying an overall molecular charge of -4.

Computational studies of ion pairing. 8. Ion pairing of tetraalkylammonium ions to nitrosobenzene and benzaldehyde redox species. A general binding motif for the interaction of tetraalkylammonium ions with benzenoid species.

Very little data is available on the detailed structures of ion pairs in solution, since few general experimental methods are available for obtaining such information. For this reason, computational methods have emerged as the method of choice for determining the structures of organic ion pairs in solution. The present study examines the ion pairs between a series of tetraalkylammonium ions and several redox forms of nitrosobenzene and a series of substituted benzaldehydes. The structures, though previously unexpected, are chemically reasonable and fit into a previous pattern of ion pairing described in previous publications in this series. To date in these studies, a total of 73 ion pairs and related species have in fact been identified having exactly the same unusual orientation of the tetraalkylammonium component with respect to the donor species. The results are pertinent to topics of general current interest, including self-assembly, molecular recognition, and supramolecular assembly.

Computational studies of ion pairing. 7. Ion-pairing and association effects between tetraalkylammonium ions and nitrobenzene redox species. "Ion pairing" to neutral substances.

The structures of few ion pairs are known with any degree of certainty. In this work, the structures and energies of attraction in acetonitrile of nitrobenzene and its anion radical and dianion with a series of tetraalkylammonium ions of increasing size were computed by density functional theory. The resulting associated molecular pairs all exhibit the same unusual but chemically reasonable structure resulting from attraction between hydrogen atoms of the ammonium cation and the oxygen atoms of the nitro group. The cations form weak complexes with neutral nitrobenzene, stronger ion pairs with the monoanion, and strongly bound ion pairs with the dianion. The results delineate subtle issues governing the formation of such complexes that are directly relevant to issues of molecular recognition, self-assembly, and supramolecular chemistry.

Experimental and computed absolute redox potentials of polycyclic aromatic hydrocarbons are highly linearly correlated over a wide range of structures and potentials.

A more rigorous theoretical treatment of methods previously used to correlate computed energy values with experimental redox potentials, combined with the availability of well-developed computational solvation methods, results in a shift away from computing ionization potentials/electron affinities in favor of computing absolute reduction potentials. Seventy-nine literature redox potentials measured under comparable conditions from 51 alternant and nonalternant polycyclic aromatic hydrocarbons are linearly correlated with their absolute reduction potentials computed by density functional theory (B3LYP/6-31+G(d)) with SMD/IEF-PCM solvation. The resulting correlation is very strong (R(2) = 0.9981, MAD = 0.056 eV). When extrapolated to the x-intercept, the correlation results in an estimate of 5.17 ± 0.01 eV for the absolute potential of the ferrocene-ferrocenium redox couple in acetonitrile at 25 °C, indicating that this simple method can be used reliably for both calculating absolute redox potentials and for predicting relative redox potentials. When oxidation and reduction data are evaluated separately, the overall MAD value is improved by 50% to 0.028 eV, which improves relative potential predictions, but the computed values do not extrapolate to a reasonable estimate of the absolute potential of the ferrocene-ferrocenium ion reference.

The effect of tetramethylammonium ion on the voltammetric behavior of polycyclic aromatic hydrocarbons: computations explain a long-standing anomaly.

Cyclic voltammograms of several polycyclic aromatic hydrocarbons (PAH's) in highly purified N,N-dimethylformamide are known to exhibit two reversible reduction waves. To a good approximation, the potential of the first wave is independent of the nature of the supporting electrolyte, but the potential of the second wave is highly dependent upon the nature of the electrolyte. The spacing ΔE° between the first and second waves increases as the size of the cation of the electrolyte is increased from Et(4)N(+) through Pr(4)N(+) to Bu(4)N(+). This is typically interpreted as due to decreasing strength of ion-pairing between the cation and the dianion of the PAH with increasing size of the electrolyte cation. However, it has been known for many years that Me(4)N(+) exhibits anomalous behavior: even though Me(4)N(+) is much smaller than Et(4)N(+), ΔE° is greater with Me(4)N(+) than with Et(4)N(+) for anthracene and in fact greater than any of the larger electrolytes with perylene. It is now shown that this behavior arises out of the fact that Me(4)N(+) ion is present in solution as a tetrasolvate [Me(4)N(+)/(DMF)(4)]. The PAH dianion (Ar(-2)) reacts with Me(4)N(+)/(DMF)(4) to displace a molecule of DMF and produce the species Me(4)N(+)/(DMF)(3)/Ar(-2). The computed pairing association constant K(ion-pairing) for the anthracene species is 35 M(-1), compared with a value of 50,000 M(-1) for association of the bare Me(4)N(+) ion with the dianion; the corresponding values for perylene are computed to be 4400 and 3.5 M(-1), respectively.

Electrocatalytic oxidative cleavage of electron-deficient substituted stilbenes in acetonitrile-water employing a new high oxidation potential electrocatalyst. An electrochemical equivalent of ozonolysis.

A series of symmetrical and unsymmetrical stilbenes bearing two or more strong electron-withdrawing groups were oxidatively cleaved to the corresponding aldehydes in high yield by electrocatalytic anodic oxidation in aqueous acetonitrile employing a new high oxidation potential triphenylamine electrocatalyst. The oxidations apparently involve the corresponding 1,2-diols, which are also converted to aldehydes in high yield under the same conditions.

Experimental/computational study of the electrochemical oxidation of cyclooctatetraene in protic media. Solvent effects.

[reaction: see text] Electrochemical oxidation of cyclooctatetraene (COT) in methanol affords a ring-contracted acetal in high yield. Ring contraction is only a minor process in acetic acid. Ab initio computational methods were applied to understand and explain the effect of solvent. The computations show considerable differences between the thermodynamic and kinetic preferences for ring contraction vs bicyclo[4.2.0] product formation.

Construction of electrocatalytic electrodes bearing the triphenylamine nucleus covalently bound to carbon. A halogen dance in protonated aminotriphenylamines.

[reaction: see text]. The triarylamine nucleus has been attached to a carbon fiber electrode by diazotization of an aminotriphenylamine followed by electrochemical reduction. The resulting electrodes can electrocatalyze the oxidation of organic substrates. In acid, 4-amino-4',4' '-dibromotriphenylamine undergoes dismutation into a mixture of amines containing from 0 to 3 bromine atoms.

Anodic oxidation of methyl alpha-dimethylsilyldihydrocinnamate. A novel silicon gamma-aryl effect.

The anodic oxidation of methyl 3-phenyl-2-dimethylsilylpropionate occurs at a potential almost 1 V positive of that required to oxidize other alpha-silyl esters. Semiempirical and ab initio calculations on the model compound 1-phenyl-2-trimethylsilylethane indicate that electron removal from these two compounds is highly stereoelectronically dependent. Both molecules exist almost exclusively in a conformation in which the phenyl group and silicon atom are anti and the side chain is perpendicular to the aromatic ring. This conformation has a higher energy HOMO orbital and lower computed ionization potential than the only other significantly populated conformation of 1-phenyl-2-trimethylsilylethane. Finally, the ab initio calculations show that in the cation radical of this model compound the ipso carbon of the aromatic ring and the side chain carbon bound to silicon draw significantly closer together than in the neutral species; an electrostatic potential map of the cation radical shows that the ipso carbon bears the highest degree of positive charge of any of the benzenoid carbons. We interpret these data, taken together, as an indication that this cation radical is stabilized by overlap of the rear lobe of the carbon-silicon bond with the p-orbital of the ipso carbon.

Equilibrium Acidities and Homolytic Bond Dissociation Enthalpies of the Acidic C-H Bonds in P-(Para-substituted benzyl)triphenylphosphonium Cations and Related Cations.

Equilibrium acidities (pK(HA)) of six P-(para-substituted benzyl)triphenylphosphonium (p-GC(6)H(4)CH(2)PPh(3)(+)) cations, P-allyltriphenylphosphonium cation, P-cinnamyltriphenylphosphonium cation, and As-(p-cyanobenzyl)triphenylarsonium cation, together with the oxidation potentials [E(ox)(A(-))] of their conjugate anions (ylides) have been measured in dimethyl sulfoxide (DMSO) solution. The acidifying effects of the alpha-triphenylphosphonium groups on the acidic C-H bonds in toluene and propene were found to be ca 25 pK(HA) units (34 kcal/mol). Introduction of an electron-withdrawing group such as 4-NO(2), 4-CN, or 4-Br into the para position of the benzyl ring in p-GC(6)H(4)CH(2)PPh(3)(+) cations resulted in an additional acidity increase, but introduction of the 4-OEt electron-donating group decreases the acidity. The equilibrium acidities of p-GC(6)H(4)CH(2)PPh(3)(+) cations were nicely linearly correlated with the Hammett sigma(-) constants of the substituents (G) with a slope of 4.78 pK(HA) units (R(2) = 0.992) (Figure 1). Reversible oxidation potentials of the P-(para-substituted benzyl)triphenylphosphonium ylides were obtained by fast scan cyclic voltammetry. The homolytic bond dissociation enthalpies (BDEs) of the acidic C-H bonds in these cations, estimated by combining their equilibrium acidities with the oxidation potentials of their corresponding conjugate anions, showed that the alpha-Ph(3)P(+) groups have negligible stabilizing or destabilizing effects on the adjacent radicals. The equilibrium acidity of As-(p-cyanobenzyl)triphenylarsonium cation is 4 pK(HA) units weaker than that of P-(p-cyanobenzyl)triphenylphosphonium cation, but the BDE of the acidic C-H bond in As-(p-cyanobenzyl)triphenylarsonium cation is ca 2 kcal/mol higher than that in P-(p-cyanobenzyl)triphenylphosphonium cation.