Novel bis-arene (Sandwich) Complexes with NO\u3csup\u3e+\u3c/sup\u3e Acceptor. Isolation, X-ray Crystallography and Electronic Structure

The unusual charge-transfer complexes of various arene donors (ArH) with the nitrosonium cation (NO+) resulting from bimolecular [1 ∶ 1] associations can be extended at suitably high ArH concentrations to termolecular processes leading to the analogous [2 ∶ 1] complexes. Spectral analyses of the intense color changes accompanying the arene interaction with NO+ provide optimum conditions for the isolation of pure crystalline ternary complexes. Single crystal X-ray crystallographic determinations establish the unique sandwich structure consisting of the NO moiety interposed (parallel) between a pair of cofacial arene donors—reminiscent of the well-known transition metal sandwich complexes with aromatic ligands. The electronic structure associated with the arene binding to NO in the ternary complex is analyzed by the application of the semi-empirical LCAO molecular-orbital methodology and the Mulliken (charge-transfer) formulation of the electronic (UV–VIS–NIR) transitions. The resultant evaluation of the electronic coupling (matrix) elements HAB indicates strong donor/acceptor interactions of the frontier orbitals of the arene donor (HOMO) and nitrosonium acceptor (LUMO) that are only slightly less than those extant in the corresponding binary [1 ∶ 1] complexes

Molecular Recognition of NO/NO\u3csup\u3e+\u3c/sup\u3e via Multicenter (Charge-Transfer) Binding to Bridged Diarene Donors. Effect of Structure on the Optical Transitions and Complexation Thermodynamics

Bridged diarenes form very strong [1:1] complexes with nitrosonium/nitric oxide in which the NO moiety is optimally sandwiched in the cleft between a pair of cofacial aromatic rings which act as a molecular “Venus flytrap”. The spectral features of these associates are generally similar to those for [1:1] and [2:1] nitrosonium complexes with mononuclear alkyl-substituted benzenes, and they are appropriately described within the LCAO molecular-orbital methodology and the Mulliken (charge-transfer) formulation of donor/acceptor electronic transitions. The thermodynamics study indicates that the efficient binding is determined by (i) the close matching of the donor/acceptor redox potentials and (ii) the ability of bridged diarenes for multicentered interactions with a single NO moiety. The best fit of the electronic and structural parameters is provided by a calixarene host that allows the interacting centers to be arranged in a manner similar to those extant in [2:1] nitrosonium complexes with analogous (nonbridged) aromatic donors; this results in its very strong noncovalent binding with nitrosonium/nitric oxide with the formation constant of KB ≈ 108 M-1 and free-energy change of −ΔG° = 45 kJ mol-1. Such strong, selective, and reversible bindings of nitrosonium/nitric oxide by (cofacial) aromatic centers thus provide the basis for the development of efficient NO sensors/absorbents and also suggest their potential relevance to biochemical systems

X-ray Structure Analysis and the Intervalent Electron Transfer in Organic Mixed-Valence Crystals with Bridged Aromatic Cation Radicals

X-ray crystallography identifies the aromatic donor group D = 2,5-dimethoxy-4-methylphenyl to be a suitable redox center for the construction of organic mixed-valence crystals owing to its large structural change attendant upon 1e oxidation to the cation−radical (D•+). The combination of cyclic voltammetry, dynamic ESR line broadening, and electronic (NIR) spectroscopy allows the intervalence electron transfer between the redox centers in the mixed-valence system D-br-D•+ [where br can be an aliphatic trimethylene or an aromatic (poly)phenylene bridge] to be probed quantitatively. Independent measures of the electronic coupling matrix element (H) for D/D•+ electron exchange via Mulliken−Hush theory accord with the X-ray crystallographic databoth sufficient to consistently identify the various D-br-D•+ according to the Robin−Day classification. Thus, the directly coupled biaryl D−D•+ is a completely delocalized cation in class III with the charge distributed equally over both redox centers. The trimethylene- and biphenylene-bridged cations D(CH2)3D•+ and D(ph)2D•+ with highly localized charge distributions are prototypical class II systems involving moderately coupled redox centers with H ≈ 400 cm-1. The borderline region between class II/III is occupied by the phenylene-bridged cation D(ph)D•+; and the X-ray, CV, and NIR analyses yield ambivalent H values (which we believe to be) largely a result of an unusually asymmetric (20/80) charge distribution that is polarized between the D/D•+ redox centers

Intervalence (Charge-Resonance) Transitions in Organic Mixed-Valence Systems. Through-Space versus Through-Bond Electron Transfer between Bridged Aromatic (Redox) Centers

Intervalence absorption bands appearing in the diagnostic near-IR region are consistently observed in the electronic spectra of mixed-valence systems containing a pair of aromatic redox centers (Ar•+/Ar) that are connected by two basically different types of molecular bridges. The through-space pathway for intramolecular electron transfer is dictated by an o-xylylene bridge in the mixed-valence cation radical 3•+ with Ar = 2,5-dimethoxy-p-tolyl (T), in which conformational mobility allows the proximal syn disposition of planar T•+/T redox centers. Four independent experimental probes indicate the large through-space electronic interaction between such cofacial Ar•+/Ar redox centers from the measurements of (a) sizable potential splitting in the cyclic voltammogram, (b) quinonoidal distortion of T•+/T centers by X-ray crystallography, (c) “doubling” of the ESR hyperfine splittings, and (d) a pronounced intervalence charge-resonance band. The through (br)-bond pathway for intramolecular electron transfer is enforced in the mixed-valence cation radical 2a•+ by the p-phenylene bridge which provides the structurally inflexible and linear connection between Ar•+/Ar redox centers. The direct comparison of intramolecular rates of electron transfer (kET) between identical T•+/T centers in 3•+ and 2a•+indicates that through-space and through-bond mechanisms are equally effective, despite widely different separations between their redox centers. The same picture obtains for 3•+ and 2a•+from theoretical computations of the first-order rate constants for intramolecular electron transfer from Marcus−Hush theory using the electronic coupling elements evaluated from the diagnostic intervalence (charge-transfer) transitions. Such a strong coherence between theory and experiment also applies to the mixed-valence cation radical 7•+, in which the aromatic redox S center is sterically encumbered by annulation

“Separated” versus “Contact” Ion-Pair Structures in Solution from Their Crystalline States: Dynamic Effects on Dinitrobenzenide as a Mixed-Valence Anion

Qualitative structural concepts about dynamic ion pairs, historically deduced in solution as labile solvent-separated and contact species, are now quantified by the low-temperature isolation of crystalline (reactive) salts suitable for direct X-ray analysis. Thus, dinitrobenzenide anion (DNB-) can be prepared in the two basic ion-paired forms by potassium-mirror reduction of p-dinitrobenzene in the presence of macrocyclic polyether ligands:  LC (cryptand) and LE (crown-ethers). The crystalline “separated” ion-pair salt isolated as K(LC)+//DNB- is crystallographically differentiated from the “contact” ion-pair salt isolated as K(LE)+DNB- by their distinctive interionic separations. Spectral analysis reveals pronounced near-IR absorptions arising from intervalence transitions that characterize dinitrobenzenide to be a prototypical mixed-valence anion. Most importantly, the unique patterns of vibronic (fine-structure) progressions that also distinguish the “separated” from the “contact” ion pair in the crystalline solid state are the same as those dissolved into THF solvent and ensure that the same X-ray structures persist in solution. Moreover, these distinctive NIR patterns are assigned with the aid of Marcus−Hush (two-state) theory to the “separated” ion pair in which the unpaired electron is equally delocalized between both NO2-centers in the symmetric ground state of dinitrobenzenide, and by contrast, the asymmetric electron distribution inherent to “contact” ion pairs favors only that single NO2-center intimately paired to the counterion. The labilities of these dynamic ion pairs in solution are thoroughly elucidated by temperature-dependent ESR spectral changes that provide intimate details of facile isomerizations, ionic separations, and counterion-mediated exchanges

Facile synthetic procedure for and electrochemical properties of hexa(2-thienyl)benzenes directed toward electroactive materials

In the presence of RhCl3 center dot 3H(2)O and i-Pr2NEt, the cyclotrimerization of di(2-thienyl)acetylenes proceeded smoothly to afford hexa(2-thienyl)benzenes. CV analysis of the hexa(2-thienyl)benzenes showed that they may be useful as electroactive materials.</p

Halide Recognition through Diagnostic “Anion–π” Interactions: Molecular Complexes of C\u3csup\u3el\u3c/sup\u3e−, Br\u3csup\u3e−\u3c/sup\u3e, and I\u3csup\u3e−\u3c/sup\u3e with Olefinic and Aromatic π Receptors

Intense colorations and new charge-transfer absorption bands are observed upon addition of a halide (Cl−, Br−, I−) to neutral organic π acceptors with electron-deficient olefinic and aromatic centers. These phenomena results from noncovalent anion–π interactions (shown schematically), which were confirmed by X-ray crystallography

Behavioral Nudges as an Incentive Problem

Behavioral nudges have come under recent scrutiny due to meta-analyses showing that the impact of nudges may be over-stated in academic studies relative to scaled nudges conducted by government nudge units. Unfortunately, the economics literature provides few, if any, theoretical mechanisms for understanding the limits of behavioral nudges. We develop a theoretical model, which examines behavioral nudges from the perspective of an incentive design problem, where discrete choice architecture strategies are used rather than monetary incentives. We propose that nudge incentive power should satisfy incentive compatibility conditions. This provides researchers with a clear theoretical mechanism for understanding when we should expect nudges to induce a response and when they might have muted effects across different contexts. We also highlight that when the social planner has a “common value” payoff function and consumers are heterogeneous, even a nudge that appears successful in inducing a response at the population level may not increase welfare. This is because a nudge is likely to induce a larger response from non-targeted subgroups rather than the targed subgroup. This creates an illusion of success where distortions are created with the non-targeted group while generating only weak or no response from the targeted group. Finally, we show that an optimal monetary based contract can induce targeted subgroups to respond while not creating distortions by the non-targeted subgroup

Examining a Transition from Supramolecular Halogen Bonding to Covalent Bonds: Topological Analysis of Electron Densities and Energies in the Complexes of Bromosubstituted Electrophiles.

The transition from weak (noncovalent) interactions to fully developed covalent bonds is examined using the quantum theory of atoms in molecules in a series of halogen-bonded (XB) complexes of bromosubstituted electrophiles, RBr, with 1,4-diazabicyclo[2.2.2]octane (DABCO) and Cl- and Br- anions. The gradual decrease in the XB lengths in these associations, d Br···Y (where Y = Cl-, Br-, or N), was accompanied by the exponential increase in the binding energies and charge transfer, as well as electron densities and magnitudes of the kinetic and potential energy densities at the bond critical points (BCPs) on the Br···Y bond path. These indices, as well as characteristics of the adjacent bonds in the XB donor, followed remarkably close trend lines when plotted against the normalized XB length R BrY = d Br···Y/(r Br + r Y) (where r Br and r Y are the van der Waals radii) regardless of the methods [MP2/6-311+G(d,p) or M062X/6-311+G(d,p)], media (gas phase or dichloromethane), and nucleophiles (Cl-, Br-, or DABCO). In the systems with an R BrY higher than about 0.78, the energy densities H(r) at BCPs at the Br···Y bond path were small and positive, and XBs did not substantially affect the characteristics of the adjacent R-Br covalent bond in the XB donor. Accordingly, the XB can be identified as noncovalent in this range. In the complexes with R BrY values between about 0.67 and 0.78, energy densities H(r) at Br···Y BCPs were negative, and their magnitudes increased with the decrease in the Br···Y separation. In this range, formation of XBs was accompanied by the increase in the R-Br bond length in the XB donor and the decrease in the magnitude of the (negative) H(r) values at the BCPs of the R-Br bonds. XBs can be classified as partially covalent in this R BrY range. At an R BrY less than about 0.67, electron densities were larger, and energy densities were more negative at BCPs of the Br···Y bond than those at BCPs of the R-Br bond in the XB donor. This indicates that Br···Y bonds were stronger than R-Br bonds, and these (Br···Y) XBs can be regarded as essentially covalent. The synchronous change of a variety of (R-Br and Br···Y) bonding characteristics with R BrY suggests that the normalized XB bond length can be used as a basic parameter in the identification of the type of intermolecular interaction. A continuity of these characteristics suggests an inherent relationship between limiting (covalent and noncovalent) types of XBs and thus an onset of molecular-orbital interactions in the weaker bonds

Public Leaderboard Feedback in Sampling Competition: An Experimental Investigation

We investigate the role of performance feedback, in the form of a public leaderboard, in a sequential-sampling contest with costly observations. The player whose sequential random sample contains the observation with the highest value wins the contest and obtains a prize with a fixed value. We find that there exist parameter configurations such that in the subgame perfect equilibrium of contests with a fixed ending date (i.e., finite horizon), providing public performance feedback results in fewer expected observations and a lower expected value of the winning observation. We conduct a controlled laboratory experiment to test the theoretical predictions, and find that the experimental results largely support the theory. In addition, we investigate how individual characteristics affect competitive sequential-sampling activity. We find that risk aversion is a significant predictor of behavior both with and without leaderboard feedback, and that the direction of this effect is consistent with the theoretical predictions