N1 Center Research Articles

HTHP treatment of diamonds containing N1 centers. Neutral state of the N1 center as a limiting step in the aggregation of substitutional nitrogen into A form

The step-by-step annealing of natural plastically deformed type IaAB diamond revealed new information on the N1 center, which is two nitrogen atoms separated by one carbon in an ionized state (S = 1/2). A slight increase in the intensity of the N1 electron paramagnetic resonance (EPR) spectrum after annealing at 1000 °C (10h) was observed, and the complete disappearance was detected at a temperature of 1300 °C (4 h). This behavior of the N1 paramagnetic center was associated with the annealing of electron acceptors, and we analyzed the conditions for the formation of complex vacancy defects. The temperature of 1000 °C corresponds to the formation of three vacancy chains from single vacancies and 1300 °C corresponds to their disappearance. We experimented with electron irradiation of the crystals (energy of 3 MeV and a fluence of 1018 e/cm2) followed by annealing at 1000 °C (for 10 h) and the N1 EPR spectrum appeared back with an intensity five times higher than in the initial crystal. The next heat treatment at 1400 °C (2 h) resulted in the expected vanish of the N1 EPR spectrum. In this case, electron irradiation followed by annealing at 1000 °C led to the restoration of a very weak EPR spectrum. The presence of the “EPR-silent” N1 centers in a neutral state in plastically deformed diamond prompted us to investigate their existence in crystals that were not subjected to plastic deformation. We generated acceptor centers in different diamonds by the above method and found N1 centers in the type IaAB natural and type IaA synthetic diamonds. The results obtained can suggest that the rate-limiting step in the aggregation of two impurity nitrogen atoms into close pair is the neutral N1 defect. In the ionized state of N1, the Coulomb repulsion between the unpaired extra electrons of nitrogen is eliminated, reducing the energy barrier in the final stage of transformation into the A center.

Molecular Modulation by σ Conjugated Spacer Enables Efficient Ultraviolet/Deep-Blue Emissive Organic Light-Emitting Diodes

Phenanthroimidazole (PI) has great proficiency to design efficient near UV deep blue emitting materials due to its bipolarity behavior. The tuning of N1 and C1 centers of the PI further reveals the improvement of the optical and electronical properties. Thorough investigation is also executed to explore the influence of the alkyl chain on the optical and electroluminescence (EL) properties of these emissive materials. The incorporation of the alkyl chain in the materials can tune the optical properties, leading to better solid state emission and enhancement of the photoluminescence quantum yield (PLQY) of thin film (∼74%) as compared to solution (∼65%). The EL spectra were also exhibited between 395 and 420 nm (in the near-UV spectral region). Furthermore, for boosting of the device efficiency and color purity, the OLED device is fabricated by doping CBP as host matrix. Selectively, CBP is used as a host because of the similar energy level (HOMO and LUMO) of the synthesized emitters which assist efficient charge trapping. Predominantly, the emitters in OLED device blended with 0.5 wt % concentration achieved brilliant device efficiency and luminance properties. Among all the synthesized fluorophores, PIPP (0.5 wt %) doped OLED device showed 4.4% maximum external quantum efficiency (EQEmax), 1202 cd/m2 highest luminance, 2.2 lm/W power efficiency (PE), 2.7 cd/A current efficiency (CE) with Commission International de L’Eclairage (CIE) coordinates x = 0.17, y = 0.10 (nearer the NTSC standard value).

New data on the N1 nitrogen paramagnetic center in brownish type IaAB diamonds from MIR pipe

In this work, two brownish crystals from the Mir pipe attributed to type IaAB have been examined by a complex of spectroscopic methods: electron paramagnetic resonance, infrared, and photoluminescence spectroscopies. A combination of features such as brownish color, optical system 490.7 nm, and paramagnetic centers W7 and 490.7 points out to plastic deformation of the crystals. The W7 is known to be formed as a result of destruction of A-aggregates during plastic deformation while part of the N3V centrers can be formed due to the disruption of the B-aggregates. The narrow-line EPR spectra from the nitrogen-related N3V centers and the P1 centers indicate that the crystals were annealed after plastic deformation. Another feature of the crystals studied is the observation of the well-known paramagnetic N1 center with only two magnetically inequivalent positions (i.e. with two magnetically inequivalent directions of the C1-N1 fragments) instead of the previously reported four. Possible transformation pathways of the W7 center (N1-C1-C2-N2+) into the N1 center (N1-C-N2+) during the post-deformation annealing are considered.

Gas-phase interactions between lead(II) ions and cytosine: tandem mass spectrometry and infrared multiple-photon dissociation spectroscopy study.

Gas-phase interactions between Pb(2+) ions and cytosine (C) were studied by combining tandem mass spectrometry, infrared multiple photon dissociation spectroscopy, and density functional theory (DFT) calculations. Both singly and doubly charged complexes were generated by electrospray. The [Pb(C)-H](+) complex was extensively studied, and this study shows that two structures, involving the interaction of the metal with the deprotonated canonical keto-amino tautomer of cytosine, are generated in the gas phase; the prominent structure is the bidentate form involving both the N1 and O2 electronegative centers. The DFT study also points out a significant charge transfer from the nucleobase to the low-lying p orbitals of the metal and a strong polarization of the base upon complexation. The various potential energy surfaces explored to account for the fragmentation observed are consistent with the high abundance of the [PbNH2](+) fragment ion.

ChemInform Abstract: Enantiomerically Pure Dihydropyrimidinones as Reagents and Auxiliaries for Asymmetric Synthesis.

We report herein full experimental details of the synthesis, structure, and reactivity of (R)- and (S)-2-tert-butyl-1-carbomethoxy-2,3-dihydropyrimidin-4(1H)-one (1). The synthesis employs asparagine as the starting material and provides 1 in 55% yield without the need for chromatographic purification. The structure of 1, as determined by X-ray crystallographic analysis, demonstrates significant pyramidalization at the C4 (carbonyl) and N1 centers, with little evidence of conjugation of N1 with the α,β-unsaturated (vinylogous urea) system

EPR, ESE and pulsed ENDOR study of the nitrogen donor pairs on quasi-cubic lattice sites in 6H SiC

Abstract D-band EPR, X-band FS ESE and pulsed ENDOR studies of the additional nitrogen (N) related centers observed in highly compensated n-type 6H-SiC wafers are presented. The D-band EPR and X-band FS ESE spectrum consists of the 14N hyperfine (hf) triplet lines of the N donors incorporated at the two quasi-cubic (Nc1, Nc2) sites with the intensity ratio of INc1/INc2=0.7 and three additional triplet lines. X-band pulsed ENDOR spectra have shown intense 14N ENDOR signals of Nc1, Nc2 centers and new 14N lines due to the N related centers N1, N2, N3 with the isotropic hf splitting: 21.04, 26.43, 29.77 MHz, respectively. It was found that the g-tensors of the N2 and N3 triplet lines coincide with those of N on quasi-cubic Nc2 site. The g-tensor of the third N1 triplet lines corresponds to the average value of the Nc1 and Nc2 spectrum: g ( N 1 ) ≈ 1 2 [ g ( N c 1 ) + g ( N c 2 ) ] . It was suggested that N1 center with Aiso=21.04 MHz is due to the spin coupling between N on two quasi-cubic Nc1 and Nc2 sites while two others are tentatively attributed to the N pairs formed between N atoms on one quasi-cubic Nc2 site.

Structural defects in natural plastically deformed diamonds: Evidence from EPR spectroscopy

Structural defects formed as a result of plastic deformation in natural diamond crystals have been studied by EPR spectroscopy. The spectra of brown, pink-brown, black-brown, pink-purple, and gray plastically deformed diamonds of type Ia from deposits in Yakutia and the Urals were recorded. The results of EPR spectroscopy allowed us to identify various deformation centers in the structure of natural diamonds and to show that nitrogen centers were transformed under epigenetic mechanical loading. Abundant A centers, consisting of two isomorphic nitrogen atoms located in neighboring structural sites, were destroyed as a result of this process to form a series of N1, N4, W7, M2, and M3 nitrogen centers. Such centers are characterized by an anisotropic spatial distribution and a positive charge, related to the mechanism of their formation. In addition, N2 centers (probably, deformation-produced dislocations decorated by nitrogen) were formed in all plastically deformed diamonds and W10 and W35 centers (the models have not been finally ascertained) were formed in some of them. It has been established that diamonds with various types of deformation-induced color contain characteristic associations of these deformation centers. The diversity of associations of deformation centers indicates appreciable variations in conditions of disintegration of deep-seated rocks, transfer of diamonds to the Earth’s surface, and formation of kimberlitic deposits. Depending on the conditions of mechanical loading, the diamond crystals were plastically deformed by either dislocation gliding or mechanical twinning. Characteristic features of plastic deformation by dislocation gliding are the substantial prevalence of the N2 centers over other deformation centers and the occurrence of the high-spin W10 and W35 centers. The attributes of less frequent plastic deformation by mechanical twinning are unusual localization of the M2 centers and, in some cases, the N1 centers in microtwinned lamellae. Numerous data on models of deformation centers in natural diamonds, including the M2 and M3 centers, which were observed in the studied collection for the first time, are discussed.

Paramagnetic N1 center in plastically deformed and differently colored crystals of natural diamond

Paramagnetic N1 center in plastically deformed and differently colored crystals of natural diamond

Heterocyclization of Amidophenacylation Products of Uracil.

When uracil is reacted with benzoylamino(chloro)acetophenone, we obtain two amidophenacylation products depending on the condensation conditions. The first product contains an amidophenacyl moiety at the N1 center, and in the second product two such moieties are located at the N1 and N3 centers of the uracil. Treatment of these accessible uracil derivatives with phosphorus oxychloride, thionyl chloride, or phosphorus pentasulfide leads to cyclization of the amidophenacyl side group, which is used to synthesize a number of modified pyrimidine bases with 2,5-diphenyloxazole or 2,5-diphenylthiazole residues.

Heterocyclization of amidophenacylation products of uracil

When uracil is reacted with benzoylamino(chloro)acetophenone, we obtain two amidophenacylation products depending on the condensation conditions. The first product contains an amidophenacyl moiety at the N1 center, and in the second product two such moieties are located at the N1 and N3 centers of the uracil. Treatment of these accessible uracil derivatives with phosphorus oxychloride, thionyl chloride, or phosphorus pentasulfide leads to cyclization of the amidophenacyl side group, which is used to synthesize a number of modified pyrimidine bases with 2,5-diphenyloxazole or 2,5-diphenylthiazole residues.

Initial stage of sodium cluster formation in NaCl after intense electron pulse irradiation

Optical absorption spectra of electron-irradiated NaCl after consecutive heating steps are measured. Thermal annealing of the localized electron centers and growth of the band (peaking at 2.10 eV) due to sodium clusters are examined using computer analysis of the optical absorption spectrum. It has revealed that the sodium clusters may be nucleated from the N1 centers, and that the 2.10 eV band is initially composed of two peaks appearing around 1.70 and 2.30 eV, respectively. The oscillation mode of metal aggregates on the basis of dipole-dipole interaction suggests that these two peaks are attributed to the surface plasma oscillation of planar aggregates being formed at the initial stage.

ENDOR studies on the N1 di-nitrogen centre in diamond

New ENDOR measurements on the N1 centre confirm the N-C-N+ model for the defect. The N+ is in a substitutional site with approximately tetrahedral symmetry. The N-C fragment of the centre resembles the P1 centre, with slightly larger unpaired electron density on the nitrogen, and a smaller quadrupole interaction.

Enantiomerically pure dihydropyrimidinones as reagents and auxiliaries for asymmetric synthesis

We report herein full experimental details of the synthesis, structure, and reactivity of (R)- and (S)-2-tert-butyl-1-carbomethoxy-2,3-dihydropyrimidin-4(1H)-one (1). The synthesis employs asparagine as the starting material and provides 1 in 55% yield without the need for chromatographic purification. The structure of 1, as determined by X-ray crystallographic analysis, demonstrates significant pyramidalization at the C4 (carbonyl) and N1 centers, with little evidence of conjugation of N1 with the α,β-unsaturated (vinylogous urea) system

Models for the di-nitrogen centres found in brown diamond

No new experiments are reported in this paper: the paper comprises a re-interpretation of published EPR data of the various di-nitrogen centres in brown diamond, including the new ENDOR results of the authors' previous paper on the W7 centre (see ibid., vol.3, p.3591, 1991). The new evidence about W7 provided by these measurements, particularly the identification of the site symmetry as C1, together with other data about the centre, has enabled them to propose a definitive model for W7: a puckered ring structure (comprising four C atoms, one N atom and N+ ion). This model for the W7 centre is shown to lead, by comparison of the properties of the two centres, to a second neighbour, N-C-N+, model for the N1 centre. This model for N1 is shown, in the appendix, to allow new information to be drawn from 3C hyperfine structure in the EPR spectrum of P1, the single substitutional nitrogen. A re-examination of published experimental EPR data of Welbourn (1978) for the N4 centre, showing that the two nitrogen atoms are equivalent, is used to propose a collinear N-C-+C-N model for N4. The occurrence of only some of the possible orientations of W7 and N4 centres allowed by the symmetry operations of the N4 centre, are explained in terms of a model of the formation of these two centres by plastic deformation.

Formation of charged N1 centers in KCl and KBr

This paper reports the results of studies on charged N1 centers, produced using a technique in which additively colored KCl and KBr crystals are exposed to near ultraviolet excitation at low temperatures. In undoped crystals, absorption and emission data are presented showing several new charged N1 transitions along with information relating to the nature of the defects. In addition, this work demonstrates that in sodium-doped crystals, N1 centers can be ionized through electron transfer to FA centers. This technique has already been used in stabilizing (F+2)A centers in laser-active crystals and may have future application for lasing ionized N1 centers.

A Dynamic Jahn-Teller Effect in the ESR Spectrum of the N1 Centre in Diamond

AbstractThe N1 centre in Type Ia diamonds is a di-nitrogen centre with S = ½. One nitrogen's interaction with the unpaired electron at room temperature gives a large splitting, All = 46.4 G, A ⊥ = 32.2 G with <111> symmetry while the hyperfine splitting due to the other one has <110> symmetry with parameters All = 2.95 ±.03 G, A⊥ = 2.81 ±.03 G. As the temperature is increased averaging effects start; first some hyperfine lines broaden at about 150°C and disappear at 200°C. At still higher temperatures new lines appear and at about 850°C the averaging process is completed. The interpretation of the dynamics leads to a model of a non-planar N-C-C-N configuration with the electron jumping between the two N-C bonds. The measured activation energy is 0.4 eV which is about midway between the measured values for a similar effect in the isolated nitrogen centre, P1, (E = 0.7eV) and the N-C-N, W7, centre (E = 0.24 eV).

High-resolution Stark spectroscopy of the 6070 AA colour centre in NaF using spectral hole-burning

The nature of the 6070 AA colour centre in NaF has been investigated using high-resolution spectral hole-burning. Well resolved Stark splittings were observed in seven experimental geometries and it was found that the centre lacks inversion and has a plane of symmetry perpendicular to (110). The transition dipole is perpendicular to this symmetry plane and the difference of ground and excited-state permanent dipole moments lies in this plane along (1.000(5), 1.000(5), 1.445(10)). The previous assignment of this line to an N1 centre with C2h symmetry is not consistent with these results. Possible alternative defect structures are considered.

E.S.R. and optical study of the interstitial h centres in heavily neutron irradiated Lif

The paramagnetic spectra of the hole-centres created in LiF by a heavy neutron irradiation at 78°K have been studied by E.S.R. and optical techniques. Samples which have been irradiated at a dose at least equal to 10 19 solneutrons cm 2 exhibit , after they have been warmed at 200°K, several spectra related to 〈111〉-oriented asymmetric centres. These centres are stable up to 500°K. One of them, designated the H N1 centre, has been studied in detail. The analysis of the paramagnetic spectrum shows that this defect is an interstitial fluorine atom which forms an asymmetric ( F - −1 3 - F - −2 3 ) molecule ion with a lattice fluorine ion. Optical experiments have shown that the excited Σ level is at 4.1 eV from the ground Σ level. Two excited π levels have been found at 2.5 and 2.1 eV. This asymmetric H centre is formed from a primary symmetric H centre, designated the H N2 centre. The H N2 centre is 〈110〉-oriented. The transformation of the symmetric H N2 centre into the asymmetric H N1 one is associated with important changes in the values of the isotropic hyperfine interaction and of the g ⊥ shift.

The Strak Effect of M and N Color Centers in Alkali Halides

AbstractThe Stark effect of the M center in LiF, KCl, and NaCl and of the N centers in KCl and NaCl was measured with an alternating electric field along the 〈100〉 direction. The data for the Stark effect of the M band were analyzed to find the difference in energy of two mixing nondegenerate energy levels of opposite parity and to find the matrix element between these two energy levels when the electric field was applied. Measurements of the Stark effect for the N centers in KCl and NaCl were resolved into two components representing contributions from the N1 and N2 centers.

Paramagnetic resonance of an N centre in calcium oxide

A paramagnetic point defect has been observed in high purity single crystals of calcium oxide, after neutron irradiation. The electron paramagnetic resonance spectrum is described by an orthorhombic g tensor with principal axes approximately along [211]-, [111]- and [110]-type directions and with S = ½. It is suggested that the defect may be an F-aggregate centre similar to Pick's N1 centre.