NN Bond Research Articles

Identification of the Strongest Bonds in Chemistry

Increasing the effective electronegativity of two atoms forming a triple bond can increase the strength of the latter. The strongest bonds found in chemistry involve protonated species of hydrogen cyanide, carbon monoxide, and dinitrogen. CCSD(T)/CBS (complete basis set) and G4 calculations reveal that bond dissociation energies are misleading strength descriptors. The strength of the bond is assessed via the local stretching force constants, which suggest relative bond strength orders (RBSO) between 2.9 and 3.4 for heavy atom bonding (relative to the CO bond strength in methanol (RBSO = 1) and formaldehyde (RBSO = 2)) in [HCNH](+)((1)Σ(+)), [HCO](+)((1)Σ(+)), [HNN](+)((1)Σ(+)), and [HNNH](2+)((1)Σg(+)). The increase in strength is caused by protonation, which increases the electronegativity of the heavy atom and thereby decreases the energy of the bonding AB orbitals (A, B: C, N, O). A similar effect can be achieved by ionization of a nonbonding or antibonding electron in CO or NO. The strongest bond with a RBSO value of 3.38 is found for [HNNH](2+) using scaled CCSD(T)/CBS frequencies determined for CCSD(T)/CBS geometries. Less strong is the NN bond in [FNNH](2+) and [FNNF](2+).

The local structure of the azobenzene/aniline reaction intermediate on TiO2(110)

Scanned-energy mode photoelectron diffraction (PhD) and near-edge X-ray absorption fine structure (NEXAFS) have been used to study the surface species, previously proposed to be phenyl imide, C6H5N–, on rutile TiO2(110) following exposure to either azobenzene or aniline. All measurements are consistent with the two reactants forming a common surface species in the same local adsorption site. N K-edge NEXAFS confirms the scission of the NN bond in azobenzene, while C K-edge NEXAFS shows the phenyl ring to be intact with the molecular plane tilted relative to the surface normal and not aligned in either principle azimuth of the surface. N 1s PhD data indicate that the N atom bonds atop a surface five-fold-coordinated Ti atom, most probably at a TiN bondlength of 1.77±0.05Å, and not bridging two such atoms, as had been suggested. This atop geometry is favoured by recent density functional theory (DFT) calculations, but more quantitative aspects of the DFT result are not in agreement with the conclusions of our experimental study.

Sonophotocatalytic mineralization of antipyrine in aqueous solution

The mineralization of aqueous solutions of antipyrine, an emerging contaminant, using an innovative homogeneous sono-photocatalytic oxidation process (H2O2/UV/Fe/Ultrasound) was evaluated in an artificial UV lamp.At the selected operation conditions [H2O2]=1500ppm, pH=2.7, amplitude=100%, pulse length (cycles)=0.3 during 15min and then 1, 92% of TOC is removed after 50min treating an aqueous solution containing 50ppm of antipyrine. An important synergistic effect between sonolysis and photoFenton (UV/H2O2/Fe) of 45.4% was quantified using the first order rate constants for TOC.Comparison experiments of scavenger-loaded conditions demonstrate that the antipyirine photodegradation proceeds mainly through a radical mechanism probably beginning by the cleavage of the NN bond of penta heterocycle leading to the formation of aromatic acids (anthranilic and 1,4-benzenedicarboxylic acids) then followed by the opening of phenyl ring to form small molecular organic acids (mainly 2-butenedioic, 4-oxo-pentanoic and butanedioic acids), which may be decomposed further into CO2.

Synthesis, receptor binding and activity of iso and azakainoids

Two syntheses for the production of an unsubstituted azakainoid are described. The 1,3-dipolar cycloaddition of diazomethane with trans-dibenzyl glutaconate yields a 1-pyrazoline, which may be reduced directly to the pyrazolidine. An unexpected trans-cis isomerization is observed during Hg/Al reduction of the 1-pyrazoline NN bond. Alternatively, when TMS diazomethane is used as the dipole, the resulting 2-pyrazoline obtained after desilylation may be reduced with NaCNBH3 to provide the trans azakainate analog exclusively. The synthesis of an unsubstituted isokainoid via Michael addition is also described. Glutamate receptor binding assays revealed that the azakaniod has a moderate affinity for unspecified glutamate receptors. Membrane depolarization of Aplysia neurons upon application of the azakainoid demonstrates that it is an ionotropic glutamate receptor agonist.

Relaxation and Dissociation Following Photoexcitation of the (μ-N2)[Mo(N[t-Bu]Ar)3]2 Dinitrogen Cleavage Intermediate

Frequency resolved pump-probe spectroscopy was performed on isolated (μ-N(2))[Mo(N[t-Bu]Ar)(3)](2) (Ar = 3,5-C(6)H(3)Me(2)), an intermediate formed in the reaction of Mo(N[t-Bu]Ar)(3) to bind and cleave dinitrogen. Evidence is presented for 300 fs internal conversion followed by subpicosecond vibrational cooling on the ground electronic state in competition with bond dissociation. Fast cooling following photoexcitation leads to a relatively low overall dissociation yield of 5%, in quantitative agreement with previous work [Curley, J. J.; Cooke, T. R.; Reece, S. Y.; Mueller, P.; Cummins, C. C. J. Am. Chem. Soc. 2008, 130, 9394]. Coupling of vibrational modes to the excitation and internal conversion results in a nonthermal distribution of energy following conversion, and this provides sufficient bias to allow the nitrogen cleavage reaction to compete with breaking of the Mo-NN bond despite a higher energetic barrier on the ground state.

The Decomposition of Hydrazine in the Gas Phase and over an Iridium Catalyst

Hydrazine is an important rocket fuel, used as both a monopropellant and a bipropellant. This paper presents theoretical results to complement the extensive experimental studies of the gas phase and Ir catalyzed decompositions involved in the monopropellant applications of hydrazine. Gas phase electronic structure theory calculations that include electron correlation predict that numerous molecular and free radical reactions occur within the same energy range as the basic free radical pathways: NN bond breaking around 65 kcal/mol and NH bond breaking around 81 kcal/mol. The data suggest that a revision to existing kinetics modeling is desirable, based on the energetics and the new elementary steps reported herein. A supported Ir 6 octahedron model for the Shell 405 Iridium catalyst used in thrusters was developed. Self-Consistent Field and electron correlation calculations (with core potentials and associated basis sets) find a rich chemistry for hydrazine on this catalyst model. The model catalyst provides dramatically lower NN and NH bond cleavage energies and an even smaller barrier to breaking the NH bond by NH 2 abstractions. Thus, the low temperature decomposition over the catalyst is interpreted in terms of consecutive NH 2 abstractions to produce ammonia and nitrogen. The higher temperature channel, which has hydrogen and nitrogen products, may be due to a mixture of two mechanisms. These two mechanisms are successive NH cleavages with surface H + H recombinations, and the same type of assisted H 2 eliminations found to occur in the gas phase part of this study.

Ab initio MCSCF study on several azide molecules: energy component analysis of the pseudo-Jahn–Teller effect

An exploration of the most stable geometrical structures for the azide molecules R–N3 (R = H, BeH, BH2, CH3, C6H5) has been carried out by using the ab initio multi-configuration self-consistent field (MCSCF) method together with the aug-cc-pVTZ basis sets. The unknown hydridoberyllium azide (R = BeH) is predicted to suffer no pseudo-Jahn–Teller (JT) distortion and, hence, to assume a linear structure of C∞v symmetry. In contrast, each of the others is found to adopt a trans bent Cs structure with regard to an interior NN bond. To elucidate the nature of the pseudo-JT distortions, energy component analyses have been carried out at all of the stationary points for the relevant molecules. The results show that pseudo-JT stabilizations are classified into two groups, one in which the stability of the bent structure about the azido group arises from lowering of the interelectronic and internuclear repulsion energies and the other in which the stability results from lowering of the electron-nuclear attraction energy. These energy behaviors are accounted for in terms of an elongation or a contraction of the heavy-atom skeleton and a charge migration from some part of the molecule to the other ones. Moreover, it is shown that the theoretical structural characteristics for the known molecules are in good agreement with available experimental facts.

Molecular and crystal structure of 1,1′-diamino-2,2′-bipyridinium and 1,1′-diamino-4,4′-bipyridinium dimesitylenesulphonates: A combined experimental and theoretical study

The purpose of the present work is to study molecular and crystal structures of 1,1′-diamino-2,2′-bipyridinium (1a) and 1,1′-diamino-4,4′-bipyridinium (1b) dimesitylenesulphonates by experimental (X-ray, NMR) and theoretical (DFT, RI-B3LYP and RI-MP2) methods. The NNH2 bond lengths determined by X-ray diffraction are close to that of single NN bond and considerably longer than that of double NN bond. The bipyridine fragment in dications is slightly asymmetric (σ–π distortion). The dihedral angle between the two pyridine planes in dication (1a) is essentially larger than that in dication (1b). The NH2 groups have a pyramidal structure. The crystal structure of 1,1′-diamino-2,2′-bipyridinium dimesitylensulphonate is a stack of layers of cations and anions. The basic supramolecular synthon is NH⋯O. The directed along a+b axis zig-zag ribbons of cations in a crystal of 1,1′-diamino-4,4′-bipyridinium dimesitylenesulphonate are formed due to such a synthon and pseudo-para arrangement of amino groups. DFT and RI-MP2 calculations give geometry of the dications similar to the experimental one. The conformational analyses of dications (1a) and (1b) have been carried out using RI-B3LYP method. Non-planar conformation of dication (1a) in DMSO solution is confirmed by 1H NMR data. Calculation of HOMAd index for the dications from the experimental bond lengths gives values which indicate their aromatic character.

Transducer influence of polycyclic aromatic hydrocarbons (PAHs) on photoisomerisation of 1-alkyl-2-(arylazo)imidazoles

The UV light irradiation to a solution of 1-alkyl-2-(arylazo)imidazole (L) in toluene shows transformation of trans to cis configuration about NN bond. The rate of photo-isomerisation and quantum yields are influenced by internal parameters like – nature of substituents, coordination to metal ions, steric and electronic effect, protonation etc. and the external factors like solvent (polarity, viscosity, dipole moment etc.), presence of innocent (to be chemically noninteracting) and noninnocent (chemically interacting) ions or molecules. In this work the influence of polycyclic aromatic hydrocarbons (PAH) such as – napththalene, anthracene, phenanthrene, pyrene – an innocent system, on the photochromic efficiency of L has been examined. The PAHs are π-donor and may form a π–π continuum with L which can significantly affect the motional (vibration, rotation) properties of the analyte and hence the photochromic activity. The rate and quantum yield of trans-to-cis isomerisation of L decrease with increasing [PAH] and also with increasing number of the fused phenyl ring(s) in PAH. Thus the photoisomerisation follows the rate sequence: no PAH>napththalene>anthracene–phenanthrene>pyrene. The reverse change, cis-to-trans is very slow upon light irradiation while appreciably fast in thermal process at dark. The activation energy (Ea) also decreases with [PAH] and number of fused phenyl rings.

Electrochemical behaviour of bacterial nitric oxide reductase—Evidence of low redox potential non-heme FeB gives new perspectives on the catalytic mechanism

Nitric oxide reductase (NOR) is a membrane bound enzyme involved in the metabolic denitrification pathway, reducing nitric oxide (NO) to nitrous oxide (N2O), subsequently promoting the formation of the NN bond. Three types of bacterial NOR are known, namely cNOR, qNOR and qCuNOR, that differ on the physiological electron donor. cNOR has been purified as a two subunit complex, the NorC, anchored to the cytoplasmic membrane, with a low-spin heme c, and the NorB subunit showing high structural homology with the HCuO subunit I, comprising a bis-histidine low-spin heme b and a binuclear iron centre. The binuclear iron centre is the catalytic site and it is formed by a heme b3 coupled to a non-heme iron (FeB) through a μ-oxo bridge. The catalytic mechanism is still under discussion and three hypotheses have been proposed: the trans-mechanism, the cis-FeB and the cis-heme b3 mechanisms. In the present work, the Pseudomonas nautica cNOR electrochemical behaviour was studied by cyclic voltammetry (CV), using a pyrolytic graphite electrode modified with the immobilised protein. The protein redox centres were observed and the formal redox potentials were determined. The binuclear iron centre presents the lowest redox potential value, and discrimination between the heme b3 and FeB redox processes was attained. Also, the number of electrons involved and correspondent surface electronic transfer rate constants were estimated. The pH dependence of the observed redox processes was determined and some new insights on the NOR catalytic mechanism are discussed.

Structure of NO dimer monolayer on Rh(1 1 1)

The first-principle technique has been employed to determine the structure of the NO dimer, the NO dimer molecular chain, the NO monolayer, and the (NO)2/Rh(111) system. Their potential structures have also been proposed. CASTEP calculation shows that the NO dimer and the monolayer are both self-assembly systems. The model of the NO dimer monolayer on Rh(111) is not unique because five models possibly exist. At this time, NO is a dimer, not two monomers. The best ones are the NN bond in the NO dimer parallel to the surface in fcc-hollow and hcp-hollow sites.

Theoretical study of the COLin complexes: Interaction between carbon monoxide and lithium clusters of different sizes

A theoretical study was carried out about the complexes composed of carbon monoxide and small lithium clusters, namely COLi2, COLi4, COLi6, COLi8, COLi5+, and COLi7+. The complexation of carbon monoxide with Li2 is found to be endothermic and not spontaneous. As for Li4 and Li5+, they can only reduce CO to single bond length. By comparison with the N2Lin complexes where a minimum of eight lithium atoms are needed to cleave the triple bonded nitrogen completely, the Lin cluster behaves more efficiently in activating the CO molecule because the Li6 cluster is sufficient to cleave the CO triple bond in a highly exothermic way. Besides, the degree of reduction of CO gradually increases with Lin becoming larger, and the Li7+ and Li8 clusters can cleave the CO triple bond to greater extent than Li6 does. Due to the ability of lithium clusters to cleave the NN and CO triple bonds in effective ways, they offer a promising approach to activate other strong chemical bonds.

Thermal behavior of 1,7-diamino-1,7-dinitrimino-2,4,6-trinitro-2,4,6-triazaheptane

This paper describes the synthesis and thermal behavior of 1,7-diamino-1,7-dinitrimino-2,4,6-trinitro-2,4,6-triazaheptane (APX). The non-isothermal kinetics of APX was calculated by using the differential isoconversional method based on thermo-gravimetric/differential thermal analyzer results. It was found that with the extent of conversion increase, the activation energy increase from 136.6kJ/mol to the highest value of 294.0kJ/mol at the conversion of 40%, and then decrease to 248.2kJ/mol; In addition, the decomposition mechanism of APX was systematically studied by in situ pyrolytic Fourier transform infrared spectroscopy, electron impact ionization analysis and density functional theory. The final decomposition products of APX were conformed to include N2O, NO2, CONH, H2CO, etc. The lowest bond dissociation energy of APX is NN bonds and these may be decomposed initially. Finally, the most probable decomposition pathway based on the N6N7 bond homolysis was proposed and the theoretical results are in good agreement with experimental data.

Photoexcited structural dynamics of an azobenzene analog 4-nitro-4′-dimethylamino-azobenzene from femtosecond stimulated Raman

Azobenzenes are used in many applications because of their robust and reversible light induced trans⇋cis isomerization about the N=N bond, but the mechanism of this ultrafast reaction has not been conclusively defined. Addressing this problem we have used Femtosecond Stimulated Raman Spectroscopy (FSRS) to determine the structural transients in the trans→cis photoisomerization of the azobenzene derivative, 4-nitro-4'-dimethylamino-azobenzene (NDAB). Key marker modes, such as the 1570/1590 cm(-1) NO(2) stretch and the 1630 cm(-1) C-N(Me)(2) stretch, enable the separation and analysis of distinct trans and cis photoproduct dynamics revealing the 400 fs Frank-Condon relaxation, the 800 fs timescale of the cis product formation and the 2 ps emergence and 8 ps relaxation of the unsuccessful ground state trans species. Based on these observations, we propose a reaction mechanism, including initial dilation of the CNN bend later joined by quick movement along the CCNN, CNNC and NNCC torsional coordinates that constitutes a mixed inversion-rotation mechanism.

ε-Keggin-based coordination networks: Synthesis, structure and application toward green synthesis of polyoxometalate@graphene hybrids

Four coordination networks based on the {ε-PMo(V)(8)Mo(VI)(4)O(40)(OH)(4)Zn(4)} Keggin unit (εZn) have been synthesized under hydrothermal conditions. (TBA)(3){PMo(V)(8)Mo(VI)(4)O(36)(OH)(4)Zn(4)}[C(6)H(4)(COO)(2)](2) (ε(isop)(2)) is a 2D material with monomeric εZn units connected via 1,3 benzenedicarboxylate (isop) linkers and tetrabutylammonium (TBA) counter-cations lying between the planes. In (TPA)(3){PMo(V)(8)Mo(VI)(4)O(37)(OH)(3)Zn(4)}[C(6)H(3)(COO)(3)] (TPA[ε(trim)](∞)), 1D inorganic chains formed by the connection of εZn POMs, via Zn-O bonds, are linked via 1,3,5 benzenetricarboxylate (trim) ligands into a 2D compound with tetrapropylammonium (TPA) cations as counter-cations. (TBA){PMo(V)(8)Mo(VI)(4)O(40)Zn(4)}(C(7)H(4)N(2))(2)(C(7)H(5)N(2))(2)·12H(2)O (ε(bim)(4)) is a molecular material with monomeric εZn POMs bound to terminal benzimidazole (bim) ligands. Finally, (TBA)(C(10)H(10)N(4))(2)(HPO(3)){PMo(V)(8)Mo(VI)(4)O(40)Zn(4)}(2)(C(10)H(9)N(4))(3)(C(10)H(8)N(4)) (ε(2)(pazo)(4)) is a 1D compound with dimeric (εZn)(2) POMs connected by HPO(3)(2-) ions and terminal para-azobipyridine (pazo) ligands. In this compound an unusual bond cleavage of the central N[double bond, length as m-dash]N bond of the pazo ligand is observed. We report also a green chemistry-type one-step synthesis method carried out in water at room temperature using ε(2)(pazo)(4) and ε(isop)(2) as reducing agent of graphite oxide (GO) to obtain graphene (G). The POM@G hybrids were characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, powder X-ray diffraction, energy dispersive X-ray analysis, infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and cyclic voltammetry.

Synthesis and properties of five unexpected copper complexes with ring-cleavage of 3,6-di-2-pyridyl-1,2,4,5–tetrazine by one pot in situ hydrothermal reaction

Solvothermal reactions of CuSO4·6H2O and Cu(Ac)2·H2O with 3,6-di-2-pyridyl-1,2,4,5-tetrazine (DPTZ) in the presence of different acetonitrile/water ratios (from 1:3 to 3:1) and temperatures (from 40 to 120 °C) successfully produce five coordination polymers: [Cu4(SO4)2(H2O)5(L2)2] (1), [Cu2SO4(H2O)3(L2)] (2), [Cu2(μ-CN)2(L1)] (3), [Cu2(bdc0.5)2(L2)(H2O)] (4, H2bdc = p-phthalic acid) and [Cu(L3)2] (5) (the known mononuclear compound). Unexpectedly three ligands: 2,5-bis(2-pyridyl)-1,3,4-oxodiazole (L1), N-bis-(R-hydroxyl-2-pyridyl)ketazine (H2L2) and pyridine-2-carboxylate (HL3), were obtained through NN bond cleavage of DPTZ. Complexes 1, 2 and 3 own one-dimensional (1D) structures, complex 4 shows a two-dimensional (2D) framework. The new compounds were characterized by elemental analysis, IR spectroscopy, and X-ray single-crystal diffraction. We focus our study on the ring-opening reaction mechanism, the occurrence process of CuII → CuI, the C–CN cleavage reaction of acetonitrile under lower temperatures (40–60 °C) and the influence of temperature and the ratio of solvent. In addition, magnetic susceptibility measurements indicate that 1, 2 and 4 show strong antiferromagnetic interactions between the adjacent CuII ions.

Low-energy structures of clusters supported on metal fcc(110) surfaces

The low-energy structures (LESs) of adatom clusters on a series of metal face-centered cubic (fcc) (110) surfaces are systematically studied by the genetic algorithm, and a simplified model based on the atomic interactions is developed to explain the LESs. Two different kinds of LES group mainly caused by the different next nearest-neighbor (NNN) adatom-adatom interaction are distinguished, although the NNN atomic interaction is much weaker than the nearest-neighbor interaction. For a repulsive NNN atomic interaction, only the linear chain is included in the LES group. However, for an attractive one, type of structure in the LES group is various and replace gradually one by one with cluster size increasing. Based on our model, we also predict the shape feature of the large cluster which is found to be related closely to the ratio of NN and NNN bond energies, and discuss the surface reconstruction in the view of atomic interaction. The results are in accordance with the experimental observations.PACS: 68.43.Hn; 68.43.Fg.

Ultrafast cis-to-trans photoisomerization of a bridged azobenzene through nπ∗ excitation: Rotational pathway is not restricted

A bridged azobenzene was recently found by Siewertsen et al. to exhibit highly efficient photoisomerization through nπ∗ absorption. In the work of the present Letter, cis-to-trans photoisomerization of this molecule in the gas phase was examined in detail via density-functional-based dynamical simulations. Our results indicate that rotation around the central NN bond is the dominant mechanism when this reaction proceeds via nπ∗ excitation, and the bridging feature was found not to hinder the rotation. Nonadiabatic de-excitation occurred at an avoided crossing between the ground state and lowest singlet excited state near the midpoint of the rotational pathway.

A comparative study on the photocatalytic efficiency of ZnO thin films prepared by spray pyrolysis and sol–gel method

Nanocrystalline ZnO thin films are deposited on aluminum foil and glass substrates by two different chemical methods: (i) polymer modified spray pyrolysis and (ii) sol–gel dip-coating from zinc acetate complex solutions. The ZnO films are characterized by means of scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. The films obtained by procedure (i) possess a porous structure, while the films obtained by procedure (ii) have ganglia on the surface. The influence of the thermal treatment temperature (350 or 450°C) on the film microstructure and photocatalytic efficiency toward the organic dyes malachite green (MG) and reactive black (RB5) is also investigated. The films obtained by both methods have a better photocatalytic activity in the degradation of RB5 compared to MG due to the weaker NN bond in comparison with the CC bond between the central carbon atom and N,N-dimethylaminobenzyl in MG.

Bond elongation in the anion radical of coordinated tetrazine ligands: A crystallographic, spectroscopic and computational study of a reduced {Re(CO)3Cl} complex

The binuclear complex [(μ-Me2BPTZ)(Re(CO)3Cl)2] (1), where Me2BPTZ=3,6-(5-methyl-pyridyl)-1,2,4,5-tetrazine, can be reduced by addition of bis(η5-pentamethylcyclopentadienyl) iron(II) (decamethylferrocene, Fc∗), to obtain a stable radical anion form 1−. A single-crystal X-ray diffraction study of the radical anion (1−)(Fc∗+) was conducted and compared with a computational model of the same compound in the neutral and reduced states. As such, this work presents the first structural analysis of a reduced diimine ligand that is coordinated to {Re(CO)3Cl} moieties. Bond-length changes within the tetrazine ring system were consistent with previously reported examples of tetrazine radicals and with calculated structures that show clear elongation of the azo-type NN bond. Consistently atomic charge calculations indicate that the extra electron in the radical anion resides largely at the tetrazine core. A negligible change in the Re–Cl bond length is observed and computed.