NH Bond Research Articles

(E)-2-{[(2-Amino-phen-yl)imino]-meth-yl}-5-(benz-yl-oxy)phenol and (Z)-3-benz-yl-oxy-6-{[(5-chloro-2-hy-droxy-phen-yl)amino]-methyl-idene}cyclo-hexa-2,4-dien-1-one.

The title Schiff base compounds, C20H18N2O2 (I) and C20H16ClNO3 (II), were synthesized from 4-benz-yloxy-2-hy-droxy-benzaldehyde by reaction with 1,2-di-amino-benzene for (I), and condensation with 2-amino-4-chloro-phenol for (II). Compound (I) adopts the enol-imine tautomeric form with an E configuration about the C=N imine bond. In contrast, the o-hy-droxy Schiff base (II), is in the keto-imine tautomeric form with a Z configuration about the CH-NH bond. Neither mol-ecule is planar. In (I), the central benzene ring makes dihedral angles of 46.80 (10) and 78.19 (10)° with the outer phenyl-amine and phenyl rings, respectively, while for (II), the corresponding angles are 5.11 (9) and 58.42 (11)°, respectively. The mol-ecular structures of both compounds are affected by the formation of intra-molecular contacts, an O-H⋯N hydrogen bond for (I) and an N-H⋯O hydrogen bond for (II); each contact generates an S(6) ring motif. In the crystal of (I), strong N-H⋯O hydrogen bonds form zigzag chains of mol-ecules along the b-axis direction. Mol-ecules are further linked by C-H⋯π inter-actions and offset π-π contacts and these combine to form a three-dimensional network. The density functional theory (DFT) optimized structure of compound (II), at the B3LYP/6-311+G(d) level, confirmed that the keto tautomeric form of the compound, as found in the structure determination, is the lowest energy form. The anti-oxidant capacities of both compounds were determined by the cupric reducing anti-oxidant capacity (CUPRAC) process.

Intramolecular hydrogen bonding promoted excited state double proton transfer mechanism based on a typical molecule: Porphycene

The double excited state intramolecular proton transfer (ESIPT) mechanisms of porphycene, were theoretically studied. The primary bond lengths, IR vibrational spectra and hydrogen-bond energy indicate that the intramolecular hydrogen bonds were strengthened in the first excited state, which facilitate the ESIPT processes. To elucidate the proposed mechanism, the potential-energy surfaces of the ground state and first excited state were constructed as functions of NH bond lengths and its relative torsional angle rotation. The intramolecular proton transfer of prophycene is more likely to occur through lengthwise pathway and proceed in the concerted coordinated transfer manner.

Sustainable Disposal of Cr(VI): Adsorption–Reduction Strategy for Treating Textile Wastewaters with Amino-Functionalized Boehmite Hazardous Solid Wastes

The present work reports a sustainable adsorption–reduction strategy for disposal of Cr(VI)-bearing wastes. By cross-linking between boehmite and chitosan to prepare amino-functionalized boehmite adsorbents, Cr(VI) could be adsorbed from the Cr(VI)-containing solutions by the adsorbents with the maximum adsorption capacity of 120.2 mg/g, which was improved about 3 times compared to that of boehmite. Adsorption mechanism showed that the Cr(VI) was adsorbed by complexing with −NH2 bonds and exchanging with −OH groups of adsorbents. About 31% Cr(VI) was simultaneously reduced to Cr(III) by the reactive −OH and −NH2 electron acceptor of adsorbents. After adsorption, the Cr(VI/III) and boehmite of adsorbents were dissolved in 25% H2SO4 solution, the remaining Cr(VI) was reduced to Cr(III) with 100% reduction yield, and the CrxAl2–x(OH)2y(SO4)8–y·nH2O was prepared as product, while the insoluble chitosan was recycled in a closed loop. In application, column experiments results showed that the γ-AlOOH hazardous wastes could effectively dispose the Cr(VI)-containing textile wastewaters, and aluminum and chromium in the wastes could be comprehensively used to prepare chromium–aluminum tanning agent. The developed “waste-treating-waste” method leads to minimum pollutant emission and avoids the complex post-treatment problems.

THz spectra and corresponding vibrational modes of DNA base pair cocrystals and polynucleotides

The generalized energy-based fragmentation (GEBF) approach has been applied to study the THz spectra and vibrational modes of base pair cocrystals under periodic boundary conditions (denoted as PBC-GEBF). Results of vibrational mode reveal that hydrogen bonds play a pivotal role in the pairing process of base crystals, where most NH and CH bonds stretch to some extent. We also found that hydrogen bonds of a self-made A:T cocrystal completely break in a transition from liquid to the solid state, while self-made C:G cocrystal is different and easier to form a cocrystal, as confirmed by X-ray diffraction (XRD) and terahertz (THz) spectra. Furthermore, we have studied DNA polynucleotides (in both A and B forms) found that the vibrational modes changed a lot during the process of their forming double strand. Despite the key role played by hydrogen bonds, the key contribution originates from collective motions of the main skeleton. A comparative study of the spectra of some stranded fragments suggests that different sequences or forms have similar spectra in THz band. They distinguish from each other mainly in the low-frequency regions, especially below 1 THz. This study would make great contributions to the molecular dynamics model based DNA long-chain structure simulation in the future study.

Using Brønsted-Evans-Polanyi relations to predict electrode potential-dependent activation energies

Various elementary (de-)hydrogenation reactions at transition metal surfaces in heterogeneous catalysis have shown a linear Brønsted-Evans-Polanyi relation (BEP) between the activation energy and the reaction energy across metal surfaces. Using Density Functional Theory (DFT), we investigate if these BEP relations can be extended to elementary electrochemical reduction reactions involving transfer of a proton-electron pair. We examine the effect of the applied electrode potential on the BEP relations. We focus in particular on elementary electrochemical CH, OH and NH bond formation reactions on 7 close-packed transition metal surfaces, including Ag (111), Au (111), Cu (111), Ni (111), Pt (111), Pd (111) and Rh (111). The potential-dependent activation energies used to construct the BEP relations are calculated using a Marcus theory based approach. The role of interfacial water in the kinetics and the reaction mechanism of CH, OH and NH bond formation is explored. Specifically, two mechanisms are considered, a Tafel-like scheme involving direct surface hydrogenation and a Heyrovsky-like mechanism in which the proton is shuttled via an explicit water molecule. The potential-dependent elementary kinetics across three reduction reaction series, C* → CH4, O* → H2O and N* → NH3, are also discussed.The Heyrovsky-like scheme is the preferred mechanism for CH, OH and NH bond formation on all but two metals. BEP relations hold at the same applied electrode potential for CH, OH and NH formation for both examined mechanisms. However, BEP parameters differ among CH, OH, and NH reactions, indicating that elementary reaction energies alone are not sufficient for comparing the activity of catalysts when electrocatalytic reactions involve such different elementary steps.

Estimating the energy of intramolecular bifurcated (three-centered) hydrogen bond by X-ray, IR and 1H NMR spectroscopy, and QTAIM calculations

The structure of a series of the 2-substituted pyrroles containing the intramolecular NH⋯OC hydrogen bond and the 2,5-disubstituted pyrroles having the three-centered intramolecular CO⋯H(N)⋯OC hydrogen bond was studied by X-ray, IR, NMR spectroscopy, and quantum chemical calculations, including calculations in terms of Quantum Theory of Atoms in Molecules (QTAIM). The X-ray diffraction data show that the intramolecular rH … O and RN … O distances increase upon conversion from a two-centered hydrogen bond to a three-centered hydrogen bond, indicating a weakening of the last interaction. The topological parameters of the hydrogen bond obtained from the QTAIM calculations (electron density ρBCP and potential energy density V at the critical point of the hydrogen bond) made it possible to quantify the hydrogen bond energy in the studied compounds. The energy of intramolecular NH⋯OC hydrogen bond in the 2-substituted pyrroles is in the range from 5.5 to 6.0 kcal mol−1 (moderate hydrogen bond), whereas the energy of the components of the three-centered intramolecular CO⋯H(N)⋯OC hydrogen bond in the 2,5-disubstituted pyrroles is reduced to 2.5–3.5 kcal mol−1 (weak hydrogen bond). For weak components of the three-centered hydrogen bond, the principle of the additivity of effects on the spectral parameters influenced by the hydrogen bond (change in the vibration frequency of the NH bond, ΔνN‒H, and the chemical shift of bridge proton, ΔδNH) was proposed. For this reason, the ½ΔνN-H and ½ΔδNH values also were used for estimating the energy of the two-centered components of the three-centered CO⋯H(N)⋯OC hydrogen bond. These estimates are in good agreement with those obtained from the ρBCP and V parameters. Consequently, the ½ΔνN-H and ½ΔδNH parameters can be used as an energy descriptor for two-centered components in the case of a symmetric intramolecular three-centered hydrogen bond.

Self-Assembled Binuclear Cu(II)-Histidine Complex for Absolute Configuration and Enantiomeric Excess Determination of Naproxen by Tandem Mass Spectrometry.

Naproxen is one of the most consumed nonsteroidal anti-inflammatory drugs and marketed as S-naproxen since R-naproxen is hepatotoxic. In this study, chiral recognition of naproxen has been investigated by tandem mass spectrometry (MS/MS). Among all diastereomeric complexes formed between naproxen and the examined chiral selectors, including cyclodextrins (α/β/γ-CD), modified phenylalanines ( N-acetyl-phenylalanine, N-t-butoxycarbonyl-phenylalanine, N-9-fluorenylmethyloxycarbonyl-phenylalanine), amino acids (Trp, Phe, Tyr, His), glucose, tartaric acid, and vancomycin, a novel binuclear metal bound diastereomeric complexes [(M(II))2( S/ R-naproxen)(l-His)2-3H]+ (M = Cu, Ni, or Co with Cu being the best) could allow effective identification of the absolute configuration of naproxen and determination of its enantiomeric excess ( ee) through MS/MS analysis. The key candidate structure of [(Cu(II))2( S/ R-naproxen)(l-His)2-3H]+ has been revealed by means of collision-induced dissociation, ion mobility mass spectrometry and density functional theory calculations, indicating an interesting and unusual self-assembled compact geometry with the two Cu(II) ions bridged closely together (Cu-Cu distance is 3.04 Å) by the carboxylate groups of the two histidines. It was shown that the difference in dissociation efficiency between the two diastereomers was attributed to the interaction between the NH2 bond of the amino group of one histidine and the naphthyl ring of naproxen. The present report is the first to observe and characterize the complex of (Cu(II))2(His)2 with aromatic acid, which could contribute to the chiral recognition of other chiral aromatic acids, design of catalysts based on binuclear copper bound complex, as well as the better understanding of metal ion complexation by His or His-containing ligands.

NH3 molecule adsorption on spinel-type ZnFe2O4 surface: A DFT and experimental comparison study

Ammonia (NH3) is a caustic environment pollutant which contributes to haze formation and water pollution. Zinc ferrite (ZnFe2O4) exhibits good catalytic activity in NH3 removal. The density functional theory (DFT) was applied to explore the interaction mechanism of NH3 molecule adsorption on spinel-type ZnFe2O4 (1 1 0) surface with GGA-PW91 method in atomic and electronic level. The results indicated that NH3 molecule preferred to adsorb on surface Zn atom with the formation of H3NZn coordinate bond over ZnFe2O4 (1 1 0) surface. The H3NZn state was exothermic process with adsorption energy of −203.125 kJ/mol. About 0.157e were transferred from NH3 molecule to the surface which resulted in strong interaction. Higher activation degree occurred in H3NZn configuration with two NH bonds elongated and NH3 structure became more flat on the surface. The PDOS change of NH3 molecule was consistent with the result of adsorption energy. It was concluded that s orbital of NH3 (N) and s, p orbitals of Zn atom overlapped at −0.619 Ha. The p orbital of NH3 (N) has interaction with d orbital of Zn atom suggesting the hybridization between them. Based on NH3 removal experimental and XPS spectra results, NH3ZnFe2O4 interaction was mainly depended on the coordination between Zn atom and NH3 molecule. The DFT calculations have deepened our understanding on NH3ZnFe2O4 interaction system.

On the mechanism of the electrochemical conversion of ammonia to dinitrogen on Pt(1 0 0) in alkaline environment

The electrochemical oxidation of ammonia to dinitrogen is a model reaction for the electrocatalysis of the nitrogen cycle, as it can contribute to the understanding of the making/breaking of NN, NO, or NH bonds. Moreover, it can be used as the anode reaction in ammonia electrolyzers for H2 production or in ammonia fuel cells. We study here the reaction on the N2-forming Pt(1 0 0) electrode using a combination of electrochemical methods, product characterization and computational methods, and suggest a mechanism that is compatible with the experimental and theoretical findings. We propose that N2 is formed via an ∗NH + ∗NH coupling step, in accordance with the Gerischer-Mauerer mechanism. Other NN bond-forming steps are considered less likely based on either their unfavourable energetics or the low coverage of the necessary monomers. The NN coupling is inhibited by strongly adsorbed ∗N and ∗NO species, which are formed by further oxidation of ∗NH.

Corrosion inhibition of copper particles on ITO with 1,2,4-triazole-3-carboxylic acid

Copper particles electrodeposited on ITO/glass (Cu/ITO) have been prepared and used to explore the inhibitory capability of 1,2,4-triazole-3-carboxylic acid (TCA) against the copper oxidation. The protective effect of TCA for the Cu/ITO is demonstrated in CH3OH solution with NaOH and in H2O solution with NaCl, using cyclic voltammetry. The surface species responsible for the corrosion inhibition are analyzed using X-ray photoelectron and infrared spectroscopies. The spectroscopic results strongly suggest that the doubly dehydrogenated TCA (TCA-2H), from the NH and OH bond scission, plays an important role for the copper protection. The density-functional-theory calculations of the TCA-2H on Cu(100) and Cu(111) model surfaces show that it is adsorbed with a perpendicular orientation, via the 1N, 2N and one of the carboxylate oxygens. Moreover, the calculated vibrational frequencies of the TCA-2H on Cu(100) agree with the observed ones from the species exerting the inhibitory effect.

Borenium and boronium ions of 5,6-dihydro-dibenzo[c,e][1,2]azaborinine and the reaction with non-nucleophilic base: trapping of a dimer and a trimer of BN-phenanthryne by 4,4′-di-tert-butyl-2,2′-bipyridine

Abstract 6-Chloro-5,6-dihydro-dibenzo[c,e][1,2]azaborinine (1) reacts with pyridine derivatives to borenium or boronium ions. The boronium ion obtained from reaction with an excess of pyridine could be characterized structurally and transforms into the borenium ion in solution. The reaction of 1 with 2,2′-bipyridine (a) and derivatives (b: 6,6′-dimethyl; c: 4,4′-di-tert-butyl) results in spirocyclic boronium ions 8a–c of which 8b could be characterized by X-ray crystallography. Despite the chelate effect, the spirocyclic boronium ions readily undergo hydrolysis or alcoholysis. Treatment of the spirocyclic boronium ion 8c with potassium hexamethyl disilazide (KHMDS) results in neutral products that are monomers, dimers, or trimers of dibenzo[c,e][1,2]azaborinine (“BN-phenanthryne”) trapped with 4,4′-di-tert-butyl-2,2′-bipyridine by formation of B–C, B–N, or dative B–N bonds, indicative of deprotonation of NH and CH bonds of the boronium ion by KHMDS.

Exploring the nature of the H-bonds between the human class II MHC protein, HLA-DR1 (DRB*0101) and the influenza virus hemagglutinin peptide, HA306-318, using the quantum theory of atoms in molecules

The nature of the H-bonds between the human protein HLA-DR1 (DRB*0101) and the hemagglutinin peptide HA306-318 has been studied using the Quantum Theory of Atoms in Molecules for the first time. We have found four H-bond groups: one conventional CO··HN bond group and three nonconventional CO··HC, π··HC involving aromatic rings and HN··HCaliphatic groups. The calculated electron density at the determined H-bond critical points suggests the follow protein pocket binding trend: P1 (2,311) >> P9 (1.109) > P4 (0.950) > P6 (0.553) > P7 (0.213) which agrees and reveal the nature of experimental findings, showing that P1 produces by a long way the strongest binding of the HLA-DR1 human protein molecule with the peptide backbone as consequence of the vast number of H-bonds in the P1 area and at the same time the largest specific binding of the peptide Tyr308 residue with aromatic residues located at the binding groove floor. The present results suggest the topological analysis of the electronic density as a valuable tool that allows a non-arbitrary partition of the pockets binding energy via the calculated electron density at the determined critical points.

Protein Rotational Dynamics in Aligned Lipid Membranes Probed by Anisotropic T1ρ NMR Relaxation

A membrane-bound form of Pf1 coat protein reconstituted in magnetically aligned DMPC/DHPC bicelles was used as a molecular probe to quantify for the viscosity of the lipid membrane interior by measuring the uniaxial rotational diffusion coefficient of the protein. Orientationally dependent 15N NMR relaxation times in the rotating frame, or T1ρ, were determined by fitting individually the decay of the resolved NMR peaks corresponding to the transmembrane helix of Pf1 coat protein as a function of the spin-lock time incorporated into the 2D SAMPI4 pulse sequence. The T1ρ relaxation mechanism was modeled by uniaxial rotational diffusion on a cone, which yields a linear correlation with respect to the bond factor sin4θB, where θB is the angle that the NH bond forms with respect to the axis of rotation. Importantly, the bond factors can be independently measured from the dipolar couplings in the separated local-field SAMPI4 spectra. From this dependence, the value of the diffusion coefficient D|| = 8.0 × 105 s−1 was inferred from linear regression of the experimental T1ρ data even without any spectroscopic assignment. Alternatively, a close value of D|| = 7.7 × 105 s−1 was obtained by fitting the T1ρ relaxation data for the assigned NMR peaks of the transmembrane domain of Pf1 to a wavelike pattern as a function of residue number. The method illustrates the use of single-helix transmembrane peptides as molecular probes to assess the dynamic parameters of biological membranes by NMR relaxation in oriented lipid bilayers.

Theoretical Models for Quantitative Description of the Acid-Base Equilibria of the 5,6-Substituted Uracils.

The acidities of 18 5,6-substituted uracils have been numerically estimated as pKa values in terms of three theoretical models. The first scheme includes the calculation of the gas-phase acidity of uracil with the G3MP2B3 method and taking into account the solvent effect using the polarizable continuum approximation PCM(SMD)-TPSS/aug-cc-pVTZ. The second model is one step and implies calculation of the free Gibbs energies of the hydrate complex of uracil (and its anion) with 5 water molecules by the TPSS/aug-cc-pVTZ method. This model accounts for the solvent effect corresponding to both specific and nonspecific solvation. The third scheme required high time and computational resources and includes the strong features of the two previous schemes. Here, the theoretical estimation of pKa is performed by the CBS-QB3 composite method. As in the second approach, both specific (as pentahydrate) and nonspecific solvent effects are determined. We have analyzed the advantages and model restrictions of the considered schemes for the pKa calculations. All models have systematic errors, which have been corrected with the linear empirical regression relations. In the presented model, the absolute mean deviations of the pKa values of uracils dissociating via the N1-H bonds diminish to 0.25, 0.28, and 0.23 pKa units (respectively, for I, II, and III models), which corresponds to ∼0.3 kcal/mol on the energy scale. The applicability of our computational schemes to uracils dissociating via N3-H, O-H (orotic acids) and C-H bonds is discussed.

Formation of N2O greenhouse gas during SCR of NO with NH3 by supported vanadium oxide catalysts

Selective catalytic reduction of NO by NH3 over supported vanadium oxide catalysts has been studied for decades, but the studies mostly concentrated on the dominant N2 product with much less attention paid to the formation of the undesired N2O product. In the present study, fundamental aspects of the N2O formation reaction were elucidated by a series of temperature-programmed surface reaction studies with isotopic labelled reactants. The surface vanadium oxide species on the TiO2 support are active sites for the N2O formation reaction, while tungsten species mainly function as promoters. Oxygen from NO, gaseous O2 and catalyst oxygen all function as oxygen sources for formation of N2O (∼50%, ∼30% and ∼20%, respectively). The rate-determining-step for N2O formation involves breaking of the ammonia NH bond. These new molecular level insights have the potential to guide the rational design of improved SCR catalysts for diesel engines with reduced N2O produced.

Investigation of pyramidal inversion of nitrogen atom in carbamate and thiocarbamate ions formed at the reaction of СО2, СOS, CS2 with 2-aminoethanol

Dynamic NMR was applied to measuring the value ΔG≠ characterizing the height of the barrier to the pyramidal inversion of the nitrogen atom in carbamate and thiocarbamate anions formed at the reaction of 2-aminoethanol with CO2 and COS. The refinement was introduced in formerly suggested cyclic structures of anions containing an intramolecular hydrogen bond NH···O(S), which contradicted the found large values of the barrier of inversion (ΔG ≠ ∼ 70 kJ mol–1). The hydrogen bond in the cyclic anions of carbamates and thiocarbamates is two-electron and three-center. Analogous cyclic structure with a multicenter hydrogen bond does not form in the case of dithiocarbamate anion that is the product of 2-aminoethanol reaction with CS2.

SYNTHESIS AND INVESTIGATION OF THE PHYSICOCHEMICAL PROPERTIES OF THE MECHANOCOMPOSITES OF ARABINOGALACTAN WITH CYCLOPHOSPHAMIDE

Methods of the creating systems for directional transport in the organism with the help of various carriers have recently been developed to improve the effectiveness and selectivity of the action of known drug substances (DS). Especially, the relevant problem is the toxicity reducing and selectivity increasing of antitumor DS. In recent years, studies are conducted on the creation of DS conjugates with natural polymers, in particular, with non-starch polysaccharides, many of which exhibit antitumor and immunomodulatory properties. Due to the unique combination of physico-chemical and biological properties, larch arabinogalactan (AG) is one of the most promising for the development of complex drugs with increased bioavailability and reduced toxicity.The aim of this work is to obtain mechanocomposites of cyclophosphamide (CPA) with the initial (molecular weight 16645 Da) and fragmented (molecular weight 8380 Da) arabinogalactan and to study their physico-chemical properties.According to IR and NMR 13C, 15N and 31P spectroscopy data, the long-term mechanochemical treatment does not lead to the change of the chemical composition of CPA molecules, only the formation of hydrogen bonds NH•••OH is observed. According to the data of X-ray diffraction analysis, IR and multi-nuclear NMR spectroscopy, the disordering of the crystal CPA structure and its molecular dispersion in the polysaccharide matrix without chemical interaction of the components are occurred during the mechanochemical treatment of CPA-AG mixtures at mass ratios of 1 : 5 and 1 : 10 for 4–12 hours.

Model Catalytic Studies of Novel Liquid Organic Hydrogen Carriers: Indole, Indoline and Octahydroindole on Pt(111).

Indole derivatives were recently proposed as potential liquid organic hydrogen carriers (LOHC) for storage of renewable energies. In this work, we have investigated the adsorption, dehydrogenation and degradation mechanisms in the indole/indoline/octahydroindole system on Pt(111). We have combined infrared reflection absorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS) and DFT calculations. Indole multilayers show a crystallization transition at 200 K, in which the molecules adopt a strongly tilted orientation, before the multilayer desorbs at 220 K. For indoline, a less pronounced restructuring transition occurs at 150 K and multilayer desorption is observed at 200 K. Octahydroindole multilayers desorb already at 185 K, without any indication for restructuring. Adsorbed monolayers of all three compounds are stable up to room temperature and undergo deprotonation at the NH bond above 300 K. For indoline, the reaction is followed by partial dehydrogenation at the 5-membered ring, leading to the formation of a flat-lying di-σ-indolide in the temperature range from 330-390 K. Noteworthy, the same surface intermediate is formed from indole. In contrast, the reaction of octahydroindole with Pt(111) leads to the formation of a different intermediate, which originates from partial dehydrogenation of the 6-membered ring. Above 390 K, all three compounds again form the same strongly dehydrogenated and partially decomposed surface species.

First-principles study of adsorption mechanism of NH3 on different ZnO surfaces on organics photocatalytic degradation purpose

ZnO nano-materials present excellent performance in catalysis and their different surfaces have different catalytic properties. In this work, the adsorption activities of NH3 and H2O on different crystal faces of ZnO are investigated by DFT theory, and the results show that the surface of (100) has absorbability of NH3 as well as the repellency of H2O. Meanwhile, its relatively low adsorption energy is good for the desorption of organics after the photocatalytic degradation process, which makes the ZnO nano materials easily reused and recycled. According to the calculation results, the adsorption energy value of (001)-a O-terminal surface is higher than that of other surfaces. Based on the electron density and density of state analysis, the NH3 loses electrons, leading to the NH bond’s weakening when the NH3 is adsorbed on the highest-energy-adsorption site. On the contrary, the adsorption energy is negative on all adsorption sites of (001)-b Zn-terminal surface, which may explain why the O-terminal surface is the most likely to be formed in the experiment by hydrothermal method.

Redox-Dependent Binding to an Electroactive Urea: Comparison of Ferrocene and Phenylenediamine Redox Couples with Two Different Guests

In previous work in our lab, the redox-dependent binding behavior of a phenylenediamine urea, UHH, was investigated in the presence of different guests in CH2Cl2. The urea functionality contains two good H-donors in the two urea NH bonds (a DD motif) that are capable of H-bonding with two appropriately spaced H-acceptors (AA motif). Examples are the cyclic diamide PZD (see Figure) and 1,8-naphthyridine, naph. 1H NMR studies indicated a modest Kbinding of 100 M-1 with PZD and 30 M-1 with naph in CH2Cl2. The expectation was that oxidation of the phenylenediamine couple in UHH to the radical cation would increase the acidity of one of the NH’s leading to stronger H-bonding with the guests and a negative shift in the observed redox potential upon addition of these guests. A small negative shift was observed with PZD, but further research indicated that the actual overall reaction occurring upon oxidation of UHH was not the simple 1 electron oxidation to the radical cation expected, but rather 2 electron oxidation of one UHH accompanied by proton transfer to a reduced UHH to give UH+ and HUHH+ as shown in the Figure. Addition of PZD does not change this overall reaction. An alternative explanation of the potential shift is that PZD is actually H-bonding to the protonated HUHH+, which would also be a stronger H-donor. Interestingly, similar magnitude shifts are observed upon oxidation of the ferrocene-containing urea, FcUHH. Since Fc is only capable of being oxidized by 1 electron, oxidation of the Fc also makes a +1 charge, which appears to have a similar effect on the H-bonding ability of the urea as does protonation. In contrast to the PZD, addition of naph causes the current for the oxidation of UHH to increase with little change in peak potential. This can be explained by proton transfer from the UHH radical cation to the naphthyridine. The resulting neutral radical will be immediately oxidized by a second electron leading to the increase in current. The proton transfer will change the H-bonding motif from AA-DD to AD-DA, which is inherently weaker because of unfavorable secondary interactions. In addition, because of the two electron oxidation, both binding partners in the resulting complex, (naphH+)(UH+), will be positively charged. Therefore it is not surprising that the H-bonding does not strengthen upon oxidation. It will be of interest to compare this behavior to that of FcUHH and naph. Cyclic voltammetry experiments will be run with FcUHH and naphthyridine to see if eliminating the possibility of the second electron transfer prevents proton transfer and results in the expected stronger H-bonding to the oxidized form. Figure 1