•Reduction potentials of organic substances are strongly linearly correlated with their electron affinities computed by density functional theory; oxidation potentials are similarly correlated with computed ionization potentials.•A single equation permits correlation and prediction of reduction and oxidation potentials of unsaturated hydrocarbons on a single plot by using their computed absolute potentials.•The effects of changing the supporting electrolyte upon the measured reduction potentials of polycyclic aromatic hydrocarbons can be explained by changes in the association constant for ion pairing of the electrolyte cation to the hydrocarbon dianion.Two features of particular interest in organic electrochemistry are the reduction and/or oxidation potentials of organic substances and the structures of electrochemically generated intermediates. The development of accurate quantum chemical methods for determining the in recent years, together with continued improvements in computing technology with which to implement such methods, has resulted in opportunities to solve problems in chemistry that cannot be solved by experiment. Two major areas which have benefited most by the availability of computational technology are improved methods for (a) prediction of redox potentials of organic species and (b) determination of the structure of electrochemically generated ion pairs. The ability to predict the reduction or oxidation potential of an unknown substance can be of value in several ways: in planning for carrying out a reaction with a mediator of electron transfer, one can compute the redox potentials of likely compounds to choose the best candidate before setting out on what might be a lengthy synthetic program and when trying to analyze a complex reaction mechanism, one can compute the redox potentials of likely short-lived intermediates. The structures of electrogenerated intermediates can offer new insights into the binding between charged species.Download high-res image (74KB)Download full-size image

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.

•Tetraalkylammonium ions are pseudo-planar and effectively not bulky.•Positive charge is localized on the four methylene groups bound to nitrogen.•However, only two of these are located where they can aid ion pairing.•Heptyl4N+ has sixty hydrogen atoms but only four are actually used in ion pairing.The second reduction potentials of unsaturated hydrocarbons move to increasingly negative potentials with increasing size of the tetraalkylammonium ion used as supporting electrolyte. This has been interpreted as due to increasing steric inhibition of ion pair formation between the cation and electrochemically generated dianion with increasing cation size. We report computations that show that this interpretation is incorrect. Rather, the positive charge on the hydrogen atoms responsible for ion pairing decreases as the size of the electrolyte cation increases. Another unsuspected phenomenon is brought to light by the computations: the positive charge in a tetraalkylammonium ion is not distributed more or less uniformly, but rather it is concentrated on the methylene groups attached to the central nitrogen. It is these groups that are responsible for ion pair formation.

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.

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.

Anodic oxidation of cyclooctatetraene (COT) was carried out in acetonitrile containing excess allyltrimethylsilane. Under these conditions COT is oxidized preferentially to afford the COT cation radical, which then undergoes nucleophilic attack by the silane followed by further anodic oxidation. The process results in a substance in which the original eight carbons of COT are highly differentiated by the addition of three allyl groups and the formation of two new rings.

Anodic oxidation of 1,2-,5,6-bis[trimethylene]cyclooctaetraene in methanol affords as the major product a substance formed by a complex sequence of carbon–carbon bond cleavages and concomitant aromatization.

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 Et4N+ through Pr4N+ to Bu4N+. 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 Me4N+ exhibits anomalous behavior: even though Me4N+ is much smaller than Et4N+, ΔE° is greater with Me4N+ than with Et4N+ 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 Me4N+ ion is present in solution as a tetrasolvate [Me4N+/(DMF)4]. The PAH dianion (Ar−2) reacts with Me4N+/(DMF)4 to displace a molecule of DMF and produce the species Me4N+/(DMF)3/Ar−2. The computed pairing association constant Kion-pairing for the anthracene species is 35 M−1, compared with a value of 50000 M−1 for association of the bare Me4N+ ion with the dianion; the corresponding values for perylene are computed to be 4400 and 3.5 M−1, respectively.

The free energies of each of 80 tetraalkylammonium ion/solvent complexes [R4N+/(solv)n], with R ranging from methyl through butyl and n ranging from 1 through 4, were computed by density functional theory (DFT) in five common electrochemical solvents: dimethylformamide (DMF), dimethylsulfoxide (DMSO), acetonitrile (AN), dichloromethane (DCM), and methanol (MeOH). The energies of the complexes were computed both with and without their solvation energies. Additional computations of the energies of the individual components, both solvated and unsolvated, were also carried out. The resulting data permit construction of a thermodynamic cycle for each R4N+/solvent pair that in turn allows the determination of the extent of general and specific solvation energies for that pair. An additional series of computations for pentane as solvent were carried out. Since this solvent should not coordinate with tetraalkylammonium ions, these computations provide a test of the validity of the computational method. This work represents a useful new general protocol for assessing the relative importance of general and specific solvation in chemical systems.

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 (R2 = 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.

Density functional quantum chemical computations reveal the fact that a number of tetraalkylammonium ions hold an inner shell of tightly bound solvent molecules in N,N-dimethylformamide and dimethylsulfoxide.

The conformational properties of α-dimethylsilyl esters, synthesized in excellent yields by reaction of ester enolates with chlorodimethylsilane (DMSCl), have been explored by both molecular mechanics calculations and a variety of 1H- and 29Si-NMR spectroscopic experiments.Treatment of esters successively with LiN(i-Pr)2 and Me2SiHCl affords α-dimethylsilyl esters in excellent yields. The conformational properties of esters explored by both molecular mechanics calculations and a variety of 1H- and 29Si-NMR spectroscopic experiments demonstrate that these esters exist as a mixture of conformations with low interconversion barriers.

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 Et4N+ through Pr4N+ to Bu4N+. 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 Me4N+ exhibits anomalous behavior: even though Me4N+ is much smaller than Et4N+, ΔE° is greater with Me4N+ than with Et4N+ 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 Me4N+ ion is present in solution as a tetrasolvate [Me4N+/(DMF)4]. The PAH dianion (Ar−2) reacts with Me4N+/(DMF)4 to displace a molecule of DMF and produce the species Me4N+/(DMF)3/Ar−2. The computed pairing association constant Kion-pairing for the anthracene species is 35 M−1, compared with a value of 50000 M−1 for association of the bare Me4N+ ion with the dianion; the corresponding values for perylene are computed to be 4400 and 3.5 M−1, respectively.