Nitrogen Pairs Research Articles

Unusual Excited State Characteristics of 6-aminobenzothiazole with β-cyclodextrin

The interaction between 6-aminobenzothiazole (6ABT) and β-cyclodextrin (β-CDx) has been investigated in solution and solid state. The stoichiometry and binding constants of the complex between 6ABT and β-CDx in solution have been determined by steady state and time-resolved fluorescence spectroscopy. Fluorescence intensity of neutral form decreases during complexation which is contrary to the usual observation. This is explained by hydrogen bonding interaction between lone pair of nitrogen with OH groups of β-CDx. The FT-IR spectral study and SEM images of solid complex confirmed the formation of inclusion complex. Excited state acidity constants for 6ABT have been determined in aqueous and β-CDx medium and discussed.

Effect of annealing at high pressures and temperatures on the defect-admixture structure of natural diamonds

The changes in the set and concentration of optically active defects in brown natural diamonds subjected to high pressure and high temperature annealing are studied. The darker brown color of the studied samples corresponds to stronger plastic deformation. The activation energy of the dissociation of the nitrogen pairs (defects A) decreases from 6.0 ± 0.4 eV in colorless samples with weak plastic deformation to 3.7 eV in dark brown samples with strong deformation. Possible defect transformation mechanisms are discussed.

Molecular Complexation between Iodine(III) Chloride and Meso‐tetraarylporphrins: Synthesis, Spectroscopic Characterization and Photoluminescence Study

AbstractIodine(III) chloride and free base para‐substituted meso‐tetraarylporphyrins (H2t(4‐Xp)p, X = OCH3, CH3,CH(CH)3, H, Cl) react at mild conditions for formation of [(ICl3)H2t(4‐Xp)p] molecular complexes. The very close correspondence between the UV‐VIS and (1H, 13C) NMR spectra of these complexes and molecular complexes of porphyrins with various acceptors indicated that two lone pairs of the pyrrolenine nitrogens in the tilted porphyrin macrocycle bonded to an iodine(III) center of ICl3.

Photochemical NO-removal and NO x-release in the presence of Fe–EDTA complexes. DFT calculations of electronic structure and spectroscopy of the [Fe(edta)(NO)] 2− complex

The calculations for iron complexes with EDTA and NO, [Fe II(edta)(NO)] 2− and [Fe III(edta)(NO)] −, were performed with the DFT/B3LYP method. The lowest energy structure for the Fe(II) complex is six-coordinated with one carboxylate group unbound. The Fe–NO bond is here short and strengthen additionally by the charge shift to form Fe IIINO. The low stability of the [Fe III(edta)(NO)] − complex can be interpreted in terms of the weak Fe–NO bond calculated for this species. Photoreduction of the [Fe III(edta)(H 2O)] − or [Fe III(edta)(OH)] 2− complex by EDTA in the NO atmosphere was studied and is suggested as convenient method of the [Fe(edta)(NO)] 2− synthesis. The results of the [Fe(edta)(NO)] 2− irradiation connotes also a possibility of alternative release NO or N 2O in acidic or alkaline solution, respectively. The calculated electronic spectrum of [Fe(edta)(NO)] 2− showed that low and medium energy bands are mainly of π NO ∗ / d → d / π NO ∗ , while transitions from free electron pairs of nitrogen and oxygen atoms to metal d and nitrosyl π NO ∗ orbitals are responsible for the high energy band. The former favors homolytic dissociation of the neutral NO molecule, whereas the latter heterolytic Fe–NO bond cleavage. In both cases excitation leads to weakening of the Fe–NO bond, enabling the pH effect on dissociation of neutral NO molecule or NO anion. This outcome allows to control the NO x delivery by two stimuli: light and solution pH.

Low-Noise Pulsed Pre-Polarization Magnet Systems for Ultra-Low Field NMR

A liquid cooled, pulsed electromagnet of solenoid configuration suitable for duty in an ultra-low field nuclear magnetic resonance system has been designed, fabricated and successfully operated. The magnet design minimizes Johnson noise, minimizes the hydrogen signal and incorporates minimal metal and no ferromagnetic materials. In addition, an acoustically quiet cooling system permitting 50% duty cycle operation was achieved by designing for single-phase, laminar flow, forced convection cooling. Winding, conductor splicing and epoxy impregnation techniques were successfully developed to produce a coil winding body with integral cooling passageways and adequate structural integrity. Issues of material compatibility, housing, coolant flow system and heat rejection system design will be discussed. Additionally, this pulsed electromagnet design has been extended to produce a boiling liquid cooled version in a paired solenoid configuration suitable for duty in an ultra-low field nuclear magnetic resonance system. This pair of liquid nitrogen cooled coils is currently being tested and commissioned. Issues of material compatibility, thermal insulation, thermal contraction, housing and coolant flow design are discussed.

Nitrogen Centers in Nanodiamonds: EPR Studies

Electron paramagnetic resonance (EPR) and electron spin echo (ESE) at X-band (9.4 GHz) and W-band (94 GHz) have been used to study defects in natural diamond nanocrystals, detonation nanodiamond (ND) with a size of  4.5 nm and detonation ND after high-pressure high-temperature (HTHP) sintering with a size of  8.5 nm. Based on identification of atomic nitrogen centers N0 and nitrogen pairs N2+ detected by means of the high frequency EPR and ESE in natural diamond nanocrystals, atomic nitrogen centers N0 have been discovered in nanodiamond core in detonation ND and detonation ND after sintering. In addition EPR signal of multi-vacancy centers with spin 3/2 seems to be observed in diamond core of detonation ND.

DFT Study on the Interactions Between Au<SUB><I>n</I></SUB> (<I>n</I> = 2 … 4) and Adenine

Binding of gold clusters with adenine was studied using density functional method (DFT). Geometries of neutral adenine with Au(n) (n = 2, 3 and 4) clusters were optimized using the B3LYP approach. The 6-31G+(d, p) basis set was used for adenine and the Stuttgart/Dresden effective core potential (ECP) basis set SDD was employed for the gold atom. Structural parameters and energy properties were discussed for the optimized complexes stabilized with the anchoring Au-N and the nonconventional Au...H-N hydrogen bond. The calculated interacting energy results show that the binding of Au(n) is the most strong at N3, whereas its binding at N6 displays relatively weaker stability. Furthermore, analysis of the charge distributions of the optimized complexes by using the natural bond orbital analysis indicates charge is transferred from the lone pair of nitrogen in adenine to the n* and sigma* orbitals of Au during the interaction between the Au(n) and adenine. The calculated second-order perturbation stabilization energies show that the interaction Au-N anchoring bond plays an important role compared with the nonconventional Au...H-N hydrogen bond.

Zooming into π-Stacked Manifolds of Nucleobases: Ionized States of Dimethylated Uracil Dimers

The electronic structure of 1,3-dimethyluracil and its dimer is characterized by ab initio calculations. The methylation eliminates the H-bonded isomers and allows one to focus on the pi-stacked manifold. In the neutral species, methylation increases the binding energy by 3-4 kcal/mol and reduces the lowest ionization energy (IE) by 0.6 eV. Other valence IEs are also red-shifted and the relative state ordering is the same as in uracil; however, the magnitude of the effect varies from 0.37 to 0.86 eV. The largest shifts are observed for the states with large contributions from lone pairs of nitrogens, which are primary substitution sites. The effect of stacking interactions on IEs is similar in methylated and non-methylated dimers: the lowest IE is red-shifted by 0.37 and 0.35 eV relative to the respective monomers. The splittings between other pairs of dimer states derived from the in-phase and out-of-phase combinations of the monomers' states are also similar to non-methylated uracil dimers, except for the states that include a large weight of nitrogen lone pairs. Because of the nonuniform effect on both monomers' levels and the shifts, the relative order of the ionized states in the dimer changes, relative to that of the non-methylated uracil dimer. The ionized stacked isomers show two different relaxation patterns--several isomers form structures with the delocalized hole stabilized by the orbital overlap, whereas others relax to the structures with the localized hole stabilized by electrostatic interactions. Electronic spectra of the ionized species at the neutral and cation geometries are presented and discussed.

Effect of thermal annealing on the blueshift of energy gap and nitrogen rearrangement in GaAsSbN

We report on the blueshift of energy gap and the nitrogen rearrangement resulting from the thermal treatment of GaAsSbN samples. The composition ranges of N and Sb are from 2% to 3% and from 7% to 11%, respectively. By comparing the photoluminescence peak energy and absorption energy gap, we found that the thermal annealing leads to compositional homogenization. When the annealing temperature is higher than 750 °C, the local vibration mode (LVM) absorbance of isolated substitutional nitrogen NAs starts to increase with the temperature. This finding supports the argument that nitrogen pairs thermally dissociate to isolated nitrogen. In additions, the increase of LVM absorbance accompanies the blueshift of energy gap, suggesting that the blueshift is the consequence of the nitrogen rearrangement. The decomposition of nitrogen pairs has also been observed by photoreflectance spectroscopy (PR). Multiple peaks were resolved from the PR spectra of as-grown GaAsSbN. They are relevant to transitions from regions with different nitrogen pairings and isolated nitrogen. As the annealing temperature increases, the peaks gradually merge to that of isolated nitrogen, indicating the decomposition of nitrogen pairings.

Photoluminescence from single isoelectronic traps in nitrogen delta-doped GaAs grown on GaAs(1 1 1)A

We have studied photoluminescence (PL) observed from single isoelectronic traps formed by nitrogen pairs in nitrogen δ-doped GaAs layers grown on GaAs(1 1 1)A substrates. The PL was composed of a single peak with a narrow linewidth of ∼80 μeV. Polarized PL measurements confirmed that the emission from single isoelectronic traps in nitrogen δ-doped GaAs(1 1 1) is unpolarized irrespective of nitrogen pair arrangements. These results can be explained by in-plane isotropy of the samples, which is consistent with the symmetrical property of GaAs(1 1 1), and demonstrate that utilizing (1 1 1) substrate is an effective means for obtaining unpolarized single photons, which are desirable for the application to quantum cryptography.

Interaction between rhodanine and silver species on a nanocolloidal surface and in the solid state

AbstractThis paper discusses and compares molecular interactions of rhodanine (Rd), the heterocyclic compound containing N, S, and O atoms, with two forms of silver species, i.e. Ag(I) ions and silver nanocolloidal particles. Vibrational spectroscopic and powder crystallography studies on coordination of Ag(I) ions in the solid state, supported by density functional theory, clearly indicate complexation through the nitrogen (the deprotonated imino group) and exocyclic sulfur atoms. Molecules of the complex are arranged in a one‐dimensional infinite polymeric chain structure that intertwines to give a helix motif. On the other hand, adsorption geometry of Rd on the silver surface was determined by using surface‐enhancement Raman scattering (SERS) spectroscopy. Observed differences between the SERS and the normal Raman spectra of Rd suggest that Rd adsorbs on the silver species after the rearrangement of the keto‐thione form into the enol‐thiol tautomer in acidic and neutral solutions. This leads to adsorption on this silver species through the thiolate ion. On the other hand, the alkaline pH forces first N‐deprotonation of Rd, and, in consequence, the obtained anion of the keto‐thione form interacts with the silver surface as a result of the presence of lone pairs of nitrogen and sulfur atoms, similar to the case discussed for the solid state. Copyright © 2009 John Wiley & Sons, Ltd.

Structural distortions in nitrogen-doped GaP and GaAs

Abstract Using the tight-binding approximation we investigated the effect of structural relaxation on nitrogen-induced states in GaP and GaAs. We show that deformation energies of the impurity depend on mutual orientation of nitrogen atoms and possibilities for them to relax. The calculated deformation energies that allowed us to remove the strain by changing atomic positions can determine the final position of cluster states. We show that the lowest nitrogen bound state is associated with the [1 1 0] nitrogen pair, while the upper bound state corresponds to the [2 0 0] pair. The contribution of Γ , L and X conduction band minimums into nitrogen-induced states is evaluated.

Detection and identification of nitrogen defects in nanodiamond as studied by EPR

Electron paramagnetic resonance (EPR) and electron spin echo (ESE) at X-band and at high-frequency W-band (95 GHz) have been used to study defects in natural diamond nanocrystals, detonation nanodiamond (ND) with a size of ∼4.5 nm and detonation ND after high-temperature, high-pressure sintering with a size of ∼8.5 nm. Atomic nitrogen centers N0 and nitrogen pairs N2+ have been detected and identified and their structure has been unambiguously determined by means of the high frequency EPR and ESE in natural diamond nanocrystals. In detonation ND and detonation ND after sintering atomic nitrogen centers N0 have been discovered in nanodiamond core. In addition EPR signal of multi-vacancy centers with spin 3/2 seems to be observed in diamond core of detonation ND.

A quantum chemical study of red-shift and blue-shift hydrogen bonds in bimolecular and trimolecular methylhydrazine-hydrate complexes

This study presents a theoretical discussion of the geometry and molecular parameters of the methylhydrazine-hydrate complex. Using B3LYP/6-311++G(d,p) calculations, two geometries for the methylhydrazine-hydrate complex were analyzed by considering the interaction between water in: (i) a lone nitrogen pair assisted by a methyl ( I); and (ii) the nitrogen of the NH 2 group ( II). These geometries were examined by examining the formation of (N···H) hydrogen bonds, which were characterized using ChelpG charge transfer amounts, as well as by means of topological parameters derived from Quantum Theory of Atoms in Molecules (QTAIM) calculations. In a qualitative evaluation, both complexes ( a) and ( b) were compared with the corresponding trimolecular system ( c) formed by methylhydrazine and two water molecules. A conclusion was then obtained by means of vibrational analysis, in which, in addition to δ υ(H–O) red-shifts in water molecules, the stretch frequency of the H–C bond of methyl group shifted upwards, indicating the formation of a blue-shifting hydrogen bond in the methylhydrazine-bihydrate complex.

Electrostatically driven complexes of SiF4 with amines

AbstractSome SiF4·polyamine complexes that have recently been prepared are investigated computationally and compared to their counterparts involving NH3. The formation of these systems can be explained satisfactorily in terms of electrostatic interactions between positive σ‐holes on the silicon, on the extensions of the FSi bonds, and the lone pairs of the amino nitrogens. The displacement of an F− ion en route to complexes with three amino groups can be understood as a means of avoiding an excessive and destabilizing accumulation of negative charge in the central portion of the system. Very good overall agreement is found between crystallographic and the corresponding computed structural data. The σ‐hole interpretationa is fully compatible with initial sp3 → d2sp3 rehybridization of the silicon, but it is not necessary to invoke the latter. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Electron spin resonance detection and identification of nitrogen centers in nanodiamonds

Individual nitrogen centers N0 and nitrogen pairs N2+ have been detected and identified in natural diamond nanocrystals by means of the high-frequency electron spin resonance method. The N0 nitrogen centers have been observed in synthetic diamond nanocrystallites with a size of less than 10 nm produced by high-temperature high-pressure sintering of detonation nanodiamonds. Thus, the possibility of the stable state of impurity nitrogen atoms in diamond nanoparticles has been demonstrated.

Synthesis of new sandwich intermediate sitting-atop complexes between meso-tetraarylporphyrins and germanium(IV) chloride

GeCl 4 and meso-tetraarylporphyrins (H 2TAPP) react in chloroform solvent for formation sandwich intermediate sitting-atop (i-SAT) complexes, [GeCl 4(H 2TAP) 2]. The various spectral data ( 1H NMR, 13C NMR, UV–Vis, FT-IR and elemental analysis) were used for characterization of the i-SAT complexes. In the sandwich complexes, the pyrrole rings of two porphyrin macrocycles are tilted up and down and act as electron donors by lone pairs of pyrrolic nitrogens to germanium center of GeCl 4. The spectral results of 1H NMR and FT-IR showed that in the i-SAT complexes, the hydrogen atoms of two pyrrolic nitrogens remained still on the porphyrin macrocycles.

Puckering transitions of pseudoproline residues

The puckering transitions of pesudoprolines such as oxazolidine and thiazolidine residues (Oxa and Thz dipeptides) with trans and cis prolyl peptide bonds were explored by optimizations along the endocyclic torsion angle chi(1) using quantum-chemical methods in the gas phase and in water. The overall shapes of the potential energy surfaces for Oxa and Thz dipeptides in the gas phase and in water are similar to those for the Pro dipeptide, although there are some differences in relative stabilities of local minima and in barriers to puckering transition. On the whole, the barriers to puckering transition for Oxa and Thz dipeptides are computed to be 0.8-3.2 kcal/mol at the B3LYP/6-311++G(d,p) level in the gas phase and in water, which are lower by 0.5-1.9 kcal/mol than those for the Pro dipeptide. The n --> sigma* interactions for the delocalization of the lone pair of the prolyl amide nitrogen into the antibonding orbitals that are anti to the lone pair appear to play a role in stabilizing the nonplanar puckered transition states over the corresponding planar structures. The calculated barriers indicate that the down-to-up puckering transition can proceed in the orders Pro < Oxa < Thz in the gas phase and Pro approximately Oxa < Thz in water. (c) 2009 Wiley Periodicals, Inc. Biopolymers 91: 444-455, 2009.

Single NN pair luminescence and single photon generation in nitrogen δ‐doped GaP

AbstractLuminescence from individual nitrogen pairs (NN) were studied in nitrogen δ‐doped gallium phosphide. Density of NN pairs (NN3,4,5,6) were directly measured by counting the spot density in μ‐photoluminescence images in samples with different nitrogen densities. It was found that all the NN densities depend on the square of nitrogen sheet density suggesting that NN3∼6 definitely consists of two nitrogen atoms, although quantitative discrepancy was found for NN5 and NN6. In high resolution polarized spectrum of single NN4, two different polarization patterns due to different N‐N orientation were observed. Single photon generation from a single NN4 was demonstrated under pulsed optical excitation at 30 K. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Self-assembly of hydrogen-bonded supramolecular structures of two copper(II) 2-bromobenzoate complexes with 4-pyridylmethanol and nicotinamide

The structures of [Cu(2-Brbz)2(4PM)2(H2O)] (1) and [Cu(2-Brbz)2(NIA)2] · 2H2O 2 [where 2-Brbz is the 2-bromobenzoate anion, 4-PM is the 4-pyridylmethanol and NIA is nicotinamide] have been determined by X-ray and characterized by EPR spectroscopy. The Cu2+ cation in 1 is coordinated by a pair of oxygens from monodentate 2-bromobenzoate anions by a pair of pyridine nitrogens from monodentate 4-pyridylmethanol ligands and finally by a water forming a tetragonal-pyramidal coordination polyhedron. The Cu2+ cation in 2 is coordinated by two pairs of oxygens from the asymmetric bidentate 2-bromobenzoate anions and by a pair of pyridine nitrogen atoms from the monodentate nicotinamide in trans positions, forming an extremely elongated bipyramid. The molecules of both complexes are linked by O–H ··· O, C–H ··· O and for 2 by N–H ··· O hydrogen bonds, which create three-dimensional hydrogen-bonding networks. EPR spectra of 1 and 2 are in agreement with X-ray data. Nicotinamide as well as 4-pyridylmethanol are suitable ligands for construction of hydrogen bonding coordination polymers.