Nitrogen Pairs Research Articles

Quantitative structure–activity relationships for the pre-steady-state inhibition of cholesterol esterase by 4-nitrophenyl-n-substituted carbamates

4-Nitrophenyl-N-substituted carbamates (1–6) are the pseudo-substrate inhibitors of porcine pancreatic cholesterol esterase. Thus, the first step of the inhibition (Ki step) is the formation of the enzyme–inhibitor tetrahedral adduct and the second step of the inhibition (kc) is the formation of the carbamyl enzyme. The formation of the enzyme–inhibitor tetrahedral adduct is further divided into two steps, the formation of the enzyme–inhibitor complex with the dissociation constant, KS, at the first step and the formation of the enzyme–inhibitor tetrahedral adduct from the complex at the second step. The two-step mechanism for the formation of the enzyme–inhibitor tetrahedral adduct is confirmed by the pre-steady-state kinetics. The results of quantitative structure–activity relationships for the pre-steady-state inhibitions of cholesterol esterase by carbamates 1–6 indicate that values of −logKS and logk2/k−2 are correlated with the Taft substituent constant, σ*, and the ρ* values from these correlations are −0.33 and 0.1, respectively. The negative ρ* value for the −logKS−σ*-correlation indicates that the first step of the two-step formation of the enzyme–inhibitor tetrahedral adduct (KS step) is the formation of the positive enzyme–inhibitor complex. The positive ρ* value for the logk2/k−2−σ*-correlation indicates that the enzyme–inhibitor tetrahedral adduct is more negative than the enzyme–inhibitor complex. Finally, the two-step mechanism for the formation of the enzyme–inhibitor tetrahedral adduct is proposed according to these results. Thus, the partially positive charge is developed at nitrogen of carbamates 1–6 in the enzyme–inhibitor complex probably due to the hydrogen bonding between the lone pair of nitrogen of carbamates 1–6 and the amide hydrogen of the oxyanion hole of the enzyme. The second step of the two-step formation of the enzyme–inhibitor tetrahedral adduct is the nucleophilic attack of the serine of the enzyme to the carbonyl group of carbamates 1–6 in the enzyme–inhibitor complex and develops the negative-charged oxygen in the adduct.

Small-angle X-ray scattering in type Ia diamonds

The results of a small-angle X-ray scattering (SAXS) investigation of natural and synthetic diamonds, with different types of nitrogen-related defects, are presented. They reveal a presence of impurity-rich spherical clusters with a diameter of approximately 90 Å in an IaB diamond. SAXS scattering in an IaA diamond has a much lower intensity, and originates from larger clusters with a considerable variation in size. It is likely that the dominant impurity in these clusters is nitrogen. Present data suggest the possibility that some of the nitrogen-related point defects (A- and B-centers) are not randomly distributed in the diamond lattice, but instead are concentrated in clusters. It is suggested that, although optical manifestations of the A-nitrogen are similar in different diamonds, IaA diamonds can be divided into two sub-groups. The IaA 1 group includes annealed, synthetic, and some natural diamonds. They contain randomly distributed nitrogen pairs, which do not influence mechanical properties and do not produce X-ray scattering. In the IaA 2 group, the diamond aggregation process went further and the formation of nitrogen-containing clusters began. The process of nitrogen aggregation is described in terms of the decay of a supersaturated solid solution of nitrogen in diamond. The driving force for the creation of impurity clusters is the lowering of the free energy of nitrogen dissolution in the diamond lattice, and the decrease of strain.

Optical transitions in the isoelectronically doped semiconductor GaP:N: An evolution from isolated centers, pairs, and clusters to an impurity band

In heavily nitrogen doped GaP, we show how isoelectronic doping results in an impurity band, and how this is manifested as a large band-gap reduction and an enhanced band-edge absorption. Heavily doped GaP:N or GaP{sub 1-x}N{sub x} exhibits properties characteristic of both direct and indirect gap semiconductors. Exciton bound states associated with perturbed nitrogen pair centers and larger GaN clusters are observed. This paper indicates that to properly describe the properties of an impurity band, a hierarchy of impurity complexes needs to be considered. Our data also suggest that the excitonic effect plays a role in the impurity band formation and band-gap reduction. (c) 2000 The American Physical Society.

First-principles calculation of the interaction between nitrogen atoms and vacancies in silicon

Stability of several states of nitrogen in silicon were investigated by the first-principles calculation. A single nitrogen at a split interstitial site and bond center site is more stable than that at a tetrahedral and hexagonal site. A nitrogen pair is more stable than the single nitrogen at the split interstitial site by 4.3 eV, which indicates that the nitrogen pair exists even near the melting temperature of silicon. Binding energy between the nitrogen pair and two vacancies indicates that a complex consisting of the nitrogen pair and two vacancies can exist at the temperature of the void aggregation as far as considering thermal equilibrium. Formation of the complex can affect the concentration or the diffusibility of the vacancy.

Preparation and Spectroscopic Characterization of 2 : 1 Molecular Complexes of Tetracyanoethylene andmeso-Tetraphenylporphyrins

Different para-substituted meso-tetraphenylporphyrins (H2T(4-X)PP, X = H, Cl, CH3, CH(CH3)2, OCH3) react with various molar ratios of tetracyanoethylene (TCNE) in dichloromethane or benzene and produce only the 1 : 2 molecular complex, (TCNE)2H2T(4-X)PP. The remarkable agreement between the corresponding 1H, 13C solution (CDCl3) NMR resonances of the porphyrin moiety in these complexes with those of tetraphenylporphyrin dication, H4TPP2+, and also the strong resemblance between their UV-vis spectra and the spectra of the related diprotonated porphyrins indicates similar out-of-plane distorted porphyrin core structure in all of them. Based on these close spectral correspondences among the various molecular complexes and the related diprotonated porphyrins, and also consideration of the known structure of H4TPP2+Cl2- species, it is proposed that the two TCNEs in (TCNE)2H2T(4-X)PP complexes are bonded from above and below the mean porphyrin plane to the lone electron pairs of the pyrrolenine nitrogens of the...

Isoelectronic impurity states in GaAs:N

Using the one-band one-site Koster-Slater model, we explain the different behavior of isoelectronic impurities in GaAs:N and GaP:N in terms of their band-structure difference. We show that the two lowest nitrogen bound states, ${\mathrm{NN}}_{1}$ and ${\mathrm{NN}}_{2},$ are associated with the [220] and [110] nitrogen pairs, respectively, that the optical transition of the former is dipole allowed whilst the latter is forbidden in both systems, and that the order of the [220] and [110] pair levels are reversed in the two systems.

Mechanisms of nitrogen aggregation in nickel- and cobalt-containing synthetic diamonds

Abstract We present a study of the point defects observed in as-grown and annealed synthetic diamonds using electron paramagnetic resonance (EPR) and infrared spectroscopy. The diamonds were grown by the temperature gradient HPHT method in a split sphere apparatus using Fe–Ni–C or Fe–Co–C solvent catalysts at 1700 K and 5.5 GPa. We report for the first time the observation of the nitrogen-vacancy (W15) and W33 EPR centres in as-grown and annealed nickel- and cobalt-containing diamonds. The generation of interstitials and vacancies on transformation of substitutional nickel Ni − s into the NE4 defect with the structure of a double semivacancy, and on the reverse transformation, respectively, and the existence of different charge states of nickel and nitrogen defects, are reasons for aggregation of nitrogen at low annealing temperature. Most of the Ni − s is transformed into Ni–N complexes in the temperature range 1600–1900 K. Nitrogen aggregation observed at higher annealing temperatures is due to a third mechanism by enhancement of the mobility of N+ by the Coulomb field of negatively charged nickel-containing centres. Charge transfer induced by X-ray irradiation indicated the existence of nearest-neighbour N–N+ and separated nitrogen pairs N- - -N+ in diamonds. The decreasing content of neutral and positive charge states of nitrogen on X-ray irradiation is due to charge transfer processes between Ni − s , P1, A-centres and separated P1 pairs.

Experimental and theoretical studies of cobalt defects in diamond

Abstract Diamonds grown with a cobalt/iron solvent catalyst and annealed at 1800°C were cut and polished along the main symmetry directions. The photoluminescence spectrum showed at least four zero phonon lines. One line at 2.367 eV splits and shifts under uniaxial stress indicating that it originates at a transition between states of A and E symmetry at a trigonal defect. The same samples showed at least one new electron paramagnetic resonance (EPR) defect, called O4. The EPR spectrum showed a characteristic eight line hyperfine spectrum from a cobalt nucleus with I=7/2. Preliminary measurements suggest the centre has monoclinic symmetry. Ab initio theoretical modelling, using the local density approach (LDA), suggests that the optical centre originates at a cobalt–nitrogen complex with the impurity atoms on neighbouring sites. The structure of the EPR centre suggested from experiment consists of the cobalt atom at the centre of a divacancy, with a neighbouring nitrogen atom. This is supported by the modelling. We have applied three completely independent techniques to cobalt complexes in diamond, and conclude that at least two distinct simple cobalt–nitrogen pairs are present.

Exclusive 2∶1 molecular complexation of 2,3-dichloro-5,6-dicyanobenzoquinone and para-substituted meso-tetraphenylporphyrins: spectral analogues for diprotonated meso-tetraphenylporphyrin

Various molar ratios of 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) and para-substituted meso-tetraphenylporphyrins (H2T(4-X)PP, X = H, Cl, CH3, CH(CH3)2, OCH3) in dichloromethane at room temperature in several days always afforded (DDQ)2H2T(4-X)PP molecular complexes as the sole products. The very close correspondence between the optical spectra of these 2∶1 molecular complexes and their related diprotonated porphyrins, and particularly the remarkable agreement of their corresponding 1H, 13C NMR resonances with those of tetraphenylporphyrin dication, H4TPP2+, strongly suggest similar distorted porphyrin core structures in all these compounds. 13C NMR and IR spectra of the complexed DDQs appear to be consistent with the interaction of one of their empty CN π* orbitals with a lone pair of a pyrrolenine nitrogen of the porphyrin. Also the loss of pyrrolic NH stretchings of the porphyrins in the IR spectrum of the molecular complexes indicates the occurrence of intramolecular H-bondings. It is proposed that coordination of DDQs in (DDQ)2H2T(4-X)PP complexes occurs from above and below the “plane” of the porphyrins, and they occupy the same positions as HCls in the structure of [H4TPP]2+, 2Cl− species.

Transition metals in diamond: experimental and theoretical identification of Co–N complexes

Diamonds grown using a cobalt/iron catalyst and annealed at 1800°C were studied using two experimental techniques. A zero-phonon line, observed in photoluminescence at 2.367eV, showed the splitting under uniaxial stress characteristic of an optical transition at a defect of trigonal symmetry. The same samples were measured by electron paramagnetic resonance (EPR) spectroscopy where they showed the defect labelled O4. This centre contains cobalt and possibly nitrogen, but has monoclinic symmetry. Ab initio modelling using the local density approach (LDA) with the code AIMPRO suggests that the optical transition is at a defect consisting of neighbouring substitutional cobalt and nitrogen atoms. A plausible model for the EPR data is a cobalt atom at the centre of a divacancy with a neighbouring nitrogen atom. We conclude that the defects giving rise to the 2.367eV photoluminescence and EPR O4 centres are both simple cobalt–nitrogen pairs.

The peroxy radical ammonia complex. A matrix isolation study

The infrared spectrum of the peroxy radical ammonia complex in argon matrices has been measured between 200 and 4000 cm −1 for several ammonia and peroxy radical isotopomers. The shifts of the intramolecular fundamentals indicate that the peroxy radical forms a strong hydrogen bond to the lone pair of the ammonia nitrogen. The H- to D-shift of the OH stretch of the complexed peroxy radical indicates a significant weakening of the OH bond.

Dipole moments and molecular conformations ofO-vinyl- andO-ethylacetoximes in solutions

Dipole moments of the molecules ofO-vinyl- andO-ethylacetoximes in octane and THF were measured at different temperatures. In octane, the equilibrium mixture of stableO-vinylacetoxime conformers consists of forms with different polarity. Polarity of these conformers, which are due to rotation about the C−O and N−O bonds, is mainly determined by the relative position of the lone electron pairs of nitrogen and oxygen atoms. The fraction of the polar conformer increases as temperature increases, as well as on going from octane to THF. Saturation of the vinyl group in theO-vinylacetoxime molecule nearly halves the molecular dipole moment in the nonpolar medium, which radically differentiates this compound from vinyl ethers.

A Vibrational Structural Analysis of Benzotriazole Adsorption and Phase Film Formation on Copper Using Surface-Enhanced Raman Spectroscopy

The adsorption of benzotriazole (BTAH) on copper in aqueous electrolytes is examined by means of a vibrational analysis utilizing surface-enhanced Raman spectroscopy (SERS) as a function of solvent deuteration, pH, and electrode potential in order to examine adsorbate speciation, surface bonding, and the potential-dependent onset of CuIBTA phase film formation. In acidic media at pH > 1, the monoprotonated acid BTAH constitutes the predominant adsorbate, being replaced by the diprotonated BTAH2+ species by pH 0, and the basic BTA- form in alkaline electrolytes. The analysis of adsorbate speciation is facilitated by the presence of several coupled ring modes yielding vibrational bands in the 1050−1350 cm-1 region that are sensitive to deuteration of the triazole ring nitrogen(s). Insight into the adsorbate molecular geometry and mode of surface attachment is obtained by examining the adsorption-induced alterations in frequency, bandwidth, and relative intensity of the various benzotriazole ring modes. These changes are especially prevalent for triazole ring vibrations and some deuteration-sensitive modes, the latter apparently associated with Fermi resonance effects. The former torsional/bending ring vibrations also exhibit potential-dependent frequencies (i.e., an electrochemical Stark effect), reflecting copper−triazole electronic interactions. Taken together, the SER spectral features for BTAH and BTA- suggest surface attachment via a pair of triazole nitrogens with a tilted (or vertical) orientation; the single nitrogen binding site anticipated for BTAH2+ is consistent with the milder vibrational perturbations observed for this adsorbate. The transformation from adsorbed benzotriazole to a CuIBTA film at higher potentials is readily diagnosed from the SER spectra, including the observed lack of a D/H vibrational effect on the latter species. The structure of benzotriazole films in air is also examined; emersion of adsorbed BTAH from solution spontaneously yields a CuIBTA layer via a coupled O2-induced electrochemical pathway. The thermal stability of copper-benzotriazole films in ambient-pressure O2 and N2 from 25 to 300 °C was also probed with SERS.

Silver(I) Cation Complexation with 3α,3'α-Bis(pyridine-n-carboxy) Lithocholic Acid 1,2-Ethanediol Diesters (n = 2–4): 1H, 13C and 15N NMR Spectral Studies and Molecular Orbital Calculations

Three isomeric molecular clefts: 3α,3'α-bis(pyridine-n-carboxy) lithocholic acid 1,2-ethanediol diesters (n = 2–4) 1–3 have been synthesized and their structures ascertained by 1H, 13C NMR and MALDI TOF MS. Their complex formation with Ag+-cation (added as AgO3 SCF3) have been investigated by means of NMR and molecular orbital calculations. The coordination behaviour of the silver(I) cation is dependent on the isomerism of the pyridine-n-carboxy moiety. In 1 (pyridine-2-carboxylato = picolinato) both NMR and theoretical calculations strongly suggest that the coordination occurs with the lone electron pairs of the pyridine nitrogen and carbonyl oxygen in both of the arms of the molecular cleft separately. In 2 and 3 (pyridine-3-carboxylato = nicotinato and pyridine-4-carboxylato = isonicotinato) where the distance between the pyridine nitrogen and carbonyl oxygen is too large to allow the same type of coordination as in 1, η3-complexation with the pyridine ring and carbonyl oxygen of the different arms of the molecular cleft simultaneously is suggested by molecular orbital calculations and supported also by NMR. No Ag+-cation coordination was observed with the 1,2-ethanediol oxygens in 1–3.

Degradation of commercial high-brightness GaP:N green light emitting diodes

Deep level transient spectra, electroluminescence spectra, and light output versus current were measured on diodes degraded by a current stress. It has been shown that the drop in electroluminescence efficiency can be related to a new deep level formed by recombination enhanced reaction from the electron trap attributed to the nitrogen pair in the phosphorus site with silicon in the nearest gallium site.

Effects of Metal Particle Size in Gas-Phase Hydrogenation of Acetonitrile over Silica-Supported Platinum Catalysts

The gas-phase hydrogenation of acetonitrile was studied with silica-supported platinum catalysts of which the degrees of metal dispersion were widely changed by reduction conditions. The activities were found to decrease gradually during the course of reaction for all the catalysts examined. The initial rate of reaction increased with an increase in the degree of platinum dispersion, D. Triethylamine was the only main product irrespective of D and period of reaction time. The initial turnover frequency, TOF0, was shown to be smaller for larger D values. This dependence of TOF0 on D was explained by the electronic state of the surface of the platinum particles and the state of acetonitrile molecules adsorbed on them on the basis of X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy measurements. The surface layer of smaller particles is more favorable for the adsorption of acetonitrile. The acetonitrile is adsorbed by platinum with the electron lone pair of nitrogen in the antibon...

NBu4][ReNCl4]: Facile synthesis, structure, electron paramagnetic resonance spectroscopy and reactions

The compound [NBu4][ReNCl4] has been found to be easily accessible by the reaction of [NBu4][ReO4] with sodium azide and HCl gas in ethanol and represents an excellent precursor for the synthesis of further rhenium nitrido complexes. It has been studied by X-ray crystallography. Rhenium(VI) compounds of the general formula [ReNX4(5)]–,2– (X = Br or NCS) were obtained when [NBu4][ReNCl4] reacts with HBr or KSCN. The products as well as mixed-ligand intermediates which are formed during the ligand exchange have been studied by EPR spectroscopy. The EPR parameters of the individual mixed-ligand compounds are clearly correlated to the composition of the co-ordination sphere and can be used to characterize the mixed-ligand species unambiguously. This is shown by a nearly linear dependence of g0, g||, a0Re and A||Re on the spin–orbit coupling constants of the equatorial donors. Reactions of [NBu4][ReNCl4] with most organic ligands yielded rhenium(V) compounds. A number of phosphine complexes and chelates with sulfur-containing ligands have been prepared and spectroscopically characterized. A crystallographic study on [ReN(Hdetcb)2] (Hdetcb– = N 2-diethylthiocarbamoylbenzamidinate) shows cis co-ordination for the chelating S,N-donor ligands. Weak intermolecular hydrogen bridges have been found between the co-ordinated nitrogen atoms and the lone pair of the nitrido nitrogen. A comparison between the crystal structural data of [NBu4]2[ReN(mnt)2] (mnt2– = 1,2-dicyanoethene-1,2-dithiolate), prepared from [NBu4][ReNCl4] and Na2mnt, and [NBu4][ReO(mnt)2] shows longer Re–S bonds in the nitrido anion but markedly higher O–Re–S bond angles indicating higher steric requirements of the oxo ligand compared with ‘N3–’.

Structure of [{Li(OEt2)}2Ni(CH2NMe2)4], a homoleptic nickelate complex

Abstract A crystal of the title compound [{Li(OEt2)}2 Ni(CH2NMe2)4] (1), grown from diethyl ether, was investigated by single-crystal X-ray analysis. 1 crystallizes in the monoclinic space group P21/n with a = 9.100(2) Å, b = 11.721(2) Å, c = 13.773(3) Å, β = 93.47(3)°, and Z = 2. The crystal structure was solved by the heavy atom method (wR2 = 0.1290 for 3532 reflections; R1 = 0.0453 for 2458 observed reflections). The molecular structure of the very air sensitive compound was found to be centrosymmetric with a square planar arrangement of the four methylene carbon atoms and nickel. The Ni–C bond lengths amount to 2.028(2) Å and 2.037(2) Å, respectively. The lone pair of nitrogen is strongly involved in the lithium coordination (Li–N bond lengths: 1.954(5) Å, 1.995(5) Å). This fact and the absence of β-hydrogen atoms seem to be responsible for an unusual thermal stability of complex 1 (T dec. = 363–388 K).

Influence of Heterocyclic Anils on Corrosion Inhibition and Hydrogen Permeation Through Mild Steel in Acid Chloride Environments

Abstract The heterocyclic anils 2-salicylideneaminobenzothiazole (SABT), 2-salicylideneamino-6-chlorobenzothiazole (SACLBT), and 2-salicylideneamino-6-methoxybenzothiazole (SAMEOBT) were investigated to evaluate their efficiency as corrosion inhibitors for mild steel (MS) in 1 M hydrochloric acid (HCl) at concentrations from 100 ppm to 500 ppm and temperatures from 35°C to 60°C. SACLBT performed best, giving an inhibition efficiency as high as 98% even at a very low concentration and higher temperature. Potentiodynamic polarization studies showed inhibition was under cathodic control. These compounds reduced hydrogen permeation through MS in 1 M HCl solution. Adsorption on the MS surface followed Temkin's adsorption isotherms. Surface analysis of the MS surface exposed with SACLBT proved adsorption of this compound on the surface occurred through interaction of lone pairs of nitrogen, sulfur, and chlorine atoms with the metal surface.

Electron paramagnetic resonance of nitrogen pairs and triads in 6H-SiC: Analysis and identification

A study of electron paramagnetic resonance (EPR) of nitrogen-doped 6H-SiC is reported here. By use of both first- and second-harmonic EPR detection, the predicted spectral fingerprints for nitrogen pairs and triads were found at low temperatures. The pair spectrum was present in the second harmonic, while the spectral feature for both nitrogen pairs and triads was evidenced in the first harmonic. At least one nitrogen donor of the identified pairs and triads is located at one of the two available quasicubic carbon sites, and the other(s) could be at any one of the two quasicubic sites and the hexagonal carbon site.