On the mode of hexacoordinated NO-binding to myo- and hemoglobin: Variable-temperature EPR studies at multiple microwave frequencies

Electron paramagnetic resonance (EPR) spectroscopy is performed on NO^?-ligated hemo- and myoglobin at temperatures between 5 and 300 K. Apart from the standard X-band (9.5 GHz), data are taken at K-(24 GHz), Q-(34 GHz), W-(94 GHz) and Y-band (285 GHz) frequencies, respectively. The spectra are delineated into contributions of two EPR signatures differing ing-tensor symmetry, an axial and a rhombic one. The state contributions are found to differ with temperature and the states show temperature-dependentg-factor variations. In MbNO the axial species (denoted state II) is the only species observed at 300 K with X-band. Analysis of its development with decreasing temperature shows a partial transformation into a rhombic state (I). For this, at an intermediate temperature range (240–100 K), irregular line shapes are apparent, indicating disordered geometries of the Fe-NO-heme group. Regular rhombic lineshapes develop increasingly at temperatures below about 70 K. At 5–10 K, the rhombic state I dominates the EPR spectra but state II is still present. A similar development of spectra is observed in HbNO, for which, however, a rhombic contribution is already present at 300 K. Subunit-associated variations in state contributions and EPR appearance are not resolved at any frequency. Also, lineshape irregularities with decreasing temperature are not manifest in HbNO. These findings are used to present a structure-based picture of states I and II and their transformations and to discuss the origin of the line shape irregularities in MbNO.     

Oxygen adsorption on the tungsten (110) plane. Electron impact and thermal desorption at high temperatures

When a layer of oxygen on the (110) plane of tungsten at coverages O/W≦0.5 is heated from 100 K, O⁺ evolution under electron impact becomes almost negligible at 600 K. On further heating, however, a slow, temperature-dependent evolution of O⁺ current is observed atT≳1500 K. For O/W>0.3 there is also desorption under massive bombardment. Once an equilibrium value of O⁺ current has been established, there is rapid adjustment to the appropriate equilibrium value when the temperature changes in the range 1500–1700 K. On cooling toT<1000 K, O⁺ decreases rapidly; on reheating toT>1500 K, O⁺ increases slowly again. Above 1700 K there is thermal desorption which is also reflected in the O⁺ signal. These facts indicate that there is a slow activated evolution of an electron sensitive state above 1500 K, from a reconstructed state formed by heating the low temperature layer toT≧1000 K. The latter state seems to be reformed on cooling below 1500 K.     

Ice XII in its second regime of metastability

We present neutron powder diffraction results which give unambiguous evidence for the formation of the recently identified new crystalline ice phase [2], labeled ice XII, in completely different conditions. Ice XII is produced here by compressing hexagonal ice I(h) at T = 77, 100, 140, and 160 K up to 1.8 GPa. It can be maintained at ambient pressure in the temperature range 1.5相似文献   

Probing the phase transition of methane with Mössbauer effect in 57Fe impurities

Mossbauer studies were performed in ⁵⁷Fe impurities (0.3% at.) in solid methane, in order to investigate the properties of the order-disorder phase transition taking place at T_c=20.4K. From the temperature dependence of the relative values of the recoil-free-fraction - f - it was found that near T_c a change occurs in the slope of f(T²) without discontinuity in the value of f. A consistent model is proposed by which immediately below T_c both phases, I and II coexist with a gradual evolution of the ordered state (II) on the account of the high temperature plastic state (I). The Debye temperature of phase II (θ_II = 101±7K) increases with respect to that of phase I (θ_I = 72±3K), indicating a hardening in the phonon spectrum and precluding the onset of rotational modes of the disordered molecules.     

High hydrostatic pressure effect on magnetic state of anion-deficient La0.70Sr0.30MnOx perovskite manganites

The magnetic state of La_0.70Sr_0.30MnO_x (x=2.85, 2.80) anion-deficient manganites is studied experimentally under hydrostatic pressure up to 5.2 GPa. These materials possessing the properties of a spin-glass state. The La_0.70Sr_0.30MnO_2.85 exhibits a coexistence of the rhombohedral (R3¯c) and tetragonal (I4/mcm) crystal structures and below T_f 50 K the spin-glass state is formed. The La_0.70Sr_0.30MnO_2.80 exhibits the tetragonal (I4/mcm) crystal structure. For this compound a phase-separated magnetic state below T_f is formed, involving coexistence of C-type antiferromagnetic (AFM) clusters within spin-glass matrix. In both compounds the crystal structure and magnetic states remain stable upon compression. Under the hydrostatic pressure up to 1 GPa, the La_0.70Sr_0.30MnO_2.85 is a spin glass with a smeared phase transition to the paramagnetic state. The freezing temperature T_f of the magnetic moments of the ferromagnetic clusters increases at a rate of 4.30 K/GPa and the magnetic ordering temperature T_MO increases at a rate of 12.90 K/GPa. The enhancement of the ferromagnetic properties of La_0.70Sr_0.30MnO_2.85 anion-deficient manganite under hydrostatic pressure is explained by the redistribution of oxygen vacancies and a decrease in the unit cell parameters.     

Recovery of low temperature electron irradiation-induced damage in n-type gaas

Undoped n-type GaAa was irradiated near 5 and 77 °K with electrons having incident energies between 0.46 and 1.30 MeV. The recovery of the electrical resistivity and the Hall coefficient upon annealing from 4 to 520 °K was monitored. Changes which occurred upon annealing below 200 °K could be reversed by ionizing radiation. A small amount of irreversible ionization-induced recovery was observed after irradiation near 5 °K. Major irreversible recovery stages were centered near 235 (stage I), 280 (stage II) and 520 °K (stage III). Recovery in stage I and II obeyed first order kinetics. The activation energies of stages I and II were determined as 0.72 and 0.83 eV, respectively. The carrier concentration changes per unit irradiation dose corresponding to the three recovery stages differed in their energy dependence indicating that the defects which are removed in stage III have the lowest threshold energy. The carrier concentration changes per unit irradiation dose corresponding to stages I and III were higher for irradiation near 5 °K than for irradiation near 77 °K.     

The interplay between the lattice and magnetism in La(Fe_11.4Al_1.6)C_0.02 studied by powder neutron diffraction

The crystallographic structure and magnetic properties of La(Fe 11.4 Al 1.6 )C 0.02 are studied by magnetic measure- ment and powder neutron diffraction with temperature and applied magnetic field. Rietveld refinement shows that La(Fe 11.4 Al 1.6 )C 0.02 crystallizes into the cubic NaZn 13 -type with two different Fe sites: Fe I (8b) and Fe II (96i), and that Al atoms preferentially occupy the Fe II site. A ferromagnetic state can be induced at a medial temperature of 39 K–139 K by an external magnetic field of 0.7 T, and a large lattice is correspondingly found at 100 K and 0.7 T. In all other conditions, La(Fe 11.4 Al 1.6 )C 0.02 has no net magnetization in the paramagnetic (T > T N = 182 K) or antifer- romagnetic states, and thus keeps its small lattice. Analysis of the Fe–Fe bond length indicates that the ferromagnetic state prefers longer Fe–Fe distances.     

Electron Spin Resonance study of gamma irradiated MA-AMPS copolymer

Electron Spin Resonance (ESR) spectra of gamma irradiated methylacrylamide-2-acrylamido-2-methyl-propanesulphonic acid (MA-AMPS) are recorded to identify the radical species formed during the irradiation of the copolymer. The ESR spectrum observed for irradiated MA-AMPS copolymer at liquid nitrogen temperature (LNT) (77 r K) is an asymmetric triplet; while the spectrum observed at room temperature (RT) is a quintet. The intensity distribution of both the spectra deviated appreciably from the expected theoretical values. Computer simulations are employed to analyze the ESR spectra observed at different temperatures. The triplet spectrum observed at LNT is simulated to be a superposition of component spectra arising from macroradicals of the type～CH 2 - \dot {\rm C} H-CH 2 ～ (I), radicals of the type \dot {\rm C} H 2 SO 3 H (II), and peroxy radicals (III). In contrast, the RT spectrum is simulated to be a superposition of component spectra arising due to radicals I, II, III together with the component quartet assigned to methyl radicals ( \dot {\rm C} H 3 ). The formation of such free radicals in irradiated copolymer is discussed.     

Phase diagram of Bi up to 140 kbars

The phase diagram of Bi has been studied by resistometric techniques in the temperature range of 30 to 300°K up to pressures of 140 kbars. Using the original Bridgman phase notation, the phase transitions I–II, II–III, I–III, III–IV and V–VI were observed. Two new phases, designated VIII and IX were observed in this region. The triple points occurring between I–II–III near 29.5 kbars and 160°K, between IV–V–VIII near 55 kbars and 240°K, between V–VI–VIII near 72 kbars and 255°K and between VI–VIII–IX near 135 kbars and 250°K. Earlier measurements were adjusted to the 1970 Drickamer pressure scale and compared to the present results. A phase diagram is proposed for pressures to 140 kbars. Calculations of the volume changes and latent heats of transformation are made near the triple points I–II–III, IV–V–VIII and V–VI–VIII using the measured volume changes of Bridgman for the I–II, IV–V and V–VI transitions. The latent heat associated with the III–IV transition was calculated using the volume data of Bridgman to be less than ? 2 cal/mol.     

The adsorption of molecular oxygen species on Ag(110)

The adsorption of oxygen on the Ag(110) surface was examined at temperatures down to 123 K. In addition to the dissociatively adsorbed state which desorbed at 590 K, a second oxygen state desorbed at 190 K following adsorption at 150 K and below. This high temperature state appeared to form prior to the development of the low temperature state. The ratio of coverages of the two states was a strong function of both exposure and adsorption temperature. Isotopic exchange experiments indicated that the low temperature state was molecularly adsorbed. The desorption of the molecularly adsorbed oxygen exhibited complex kinetics due to interaction with adsorbed oxygen atoms.     

Substitution of manganese by trivalent and tetravalent elements in the CMR perovskites Pr1-x(Ca,Sr)xMnO3

The substitution of trivalent (M=In, Ga) and tetravalent elements (M=Sn, Ti) for Mn(III) and Mn(IV) respectively has been studied in the colossal magnetoresistant (CMR) perovskites Pr_0.7Ca_0.2Sr_0.1MnO₃ (type I) and Pr_0.5Sr_0.5MnO₃ (type II). For the former compound, whatever the element, the temperature transition (T_max or T_C) separating the ferromagnetic metallic (FM) state and the paramagnetic semiconductinc (PSC) state decreases dramatically when the substituted element content is increased, the saturated magnetic moment at low temperature being slightly decreased. For these type I perovskites the maximum magnetoresistance is achieved for Pr_0.7Ca_0.2Sr_0.1Mn_0.99Ga_0.01O₃, reaching a resistance ratio of 600 at 127rK against 275 at 151rK for the pristine sample. These results show that by decreasing or increasing the hole concentration via M(IV) or M(III) substitutions on the manganese site the Curie temperature T_C is always decreased. In contrast to the type I perovskites, two different effects are evidenced for the Pr_0.5Sr_0.5Mn_1-xM_xO₃ substituted type II phases depending on the M valence. On one hand for M=Ga, In when x increases the antiferromagnetic semiconducting (AFSC) state and the PSC state are favoured at the expense of ferromagnetism. On the other hand for M=Sn, Ti the low temperature AFSC state tends to disappear and for x>0.04 only a FM to PSC transition still exits similarly to that observed in type I perovskites. Correspondingly the magnetization versus temperature curves evolve from the bell shape curve typical of the charge ordering state to that of a ferromagnetic compound. The effect of valence and d⁰, d¹⁰ electronic configurations in both type I an II CMR perovskites is discussed.     

Dynamical properties of Cr-Fe amorphous powders obtained from mechanical milling

Mössbauer measurements have been performed in situ in the temperature range of 25 to 550°C for three Cr-Fe samples: Cr₇₂Fe₂₈ crystalline alloy (I), as-milled amorphous Cr₆₅Fe₃₅ powders made by mechanical grinding (II) and the same powder annealed at 420°C for 2 h (III). The temperature dependence of isomer shifts for samples I and III follows the variation predicted by the second-order Doppler effect, while for the sample II a deviation has been observed. The Debye temperatures have been determined to be 420, 400 and 380 K for sample I, III and II, respectively. A linear dependence onT ^3/2 of quadrupole splitting of the annealed powder is observed.     

High oxygen pressures and the stabilization of a new mixed valence Cu(III)/Cu(IV)

Abstract

The Cu(III) oxides were investigated first for the unusual copper oxidation state and then for the high-Tc superconductivity. In the last case this property is observed when a mixed copper valence is stabilized : Cu(I)/Cu(II) or Cu(II)/Cu(III). A new mixed valence Cu(III)/Cu(IV) has been stabilized in the three dimensional lattice of the perovskite-type oxide Lal_-xSr_xCuO₃ (0 ≥ ′ ≥ 0,25). Chemical analysis and XPS study confirm the mixed valence. This compound shows metallic properties with an broadening of the conduction band compared to LaCu0₃. Such a behaviour can be explained either by the increase of the covalence of the average Cu-0 bond or by the decrease of the structural distortion induced by the substitution of Sr(II) for La(III). No superconducting property has been detected down to 4K.     

Investigation of In-Gap States in SmB6

We have investigated the low temperature properties of the narrow-gap semiconductor SmB₆ by means of electrical resistivity and specific heat measurements. Results imply that the residual resistivity below about 3 K is non-activated and the corresponding state, which is formed within the in-gap states, has a metallic-like nature. Heat capacity measurements confirmed the metallic-like properties of the in-gap states and revealed, moreover, an enhancement of the specific heat below 2 K which is more expressive for the sample with a lower amount of impurities. The observed behaviour can be attributed to the formation of a coherent state within the in-gap states of this compound.     

Photophysics of 9-aminoacridinium hydrochloride

The steady state and time domain spectroscopy of 9-aminoacridinium hydrochloride have been studied. 9-aminoacridine (I), 9-aminoacridinium (II) and the double protonated ion (III) follow Waterman’s principle with respect to their spectra and lifetimes. It is established that the excimer formation in (II) is from the ground state dimers which undergo either thermally-activated radiationless excimer conversion or a geometrical rearrangement in the excited state at room temperature. The quenching of monomer emission by dimers or foreign quenchers (KI) is found to be of transient nature as suggested by the continuum model. The authors felicitate Prof. D S Kothari on his eightieth birthday and dedicate this paper to him on this occasion.     

An NQR study of the polymorphism of triphenylchloromethane

This work is a study of the polymorphic transformations of triphenylchloromethane (TPCM) as well as the structure and dynamics of this molecular solid. Crystalline TPCM has been studied by a variety of techniques and many of its physical properties have been characterized. Previous crystallographic studies have reported the existence of two crystalline phases. Phase II, stable above 372 K, is trigonal with space group P3; and Z = 6. The compound associates pairwise (halogen-to-halogen and triphenylmethyl-to-triphenylmethyl) and linearly with all carbon-halogen bonds aligned with C3 and S6 axes of the lattice. Below 372 K, a triclinic modification (phase III) is found (P1;, Z = 10) where the molecules are also aligned pairwise with close Cl cdots, three dots, centered Cl interactions. However, the C-Cl bonds were no longer parallel. On the other hand, DTA studies have also reported a phase transition at 381 K (to phase I) that can been obtained from slow evaporation of pentane or ether solutions. Crystallographic information about these phases has not been obtained in the literature.Five NQR lines have been observed from liquid nitrogen temperature to 372 K in good agreement with phase III that has Z = 10. Above 372 K, three lines are observed which are related to phase II. Any effort to observe phase I through NQR failed; phase II remains until the sample is melted. Instead, if phase II is cooled down, a second-order phase transition to a new phase at 259 K is observed through NQR studies. The temperature dependence of experimental data in the phase transition region is well explained assuming the transitions occur when molecules in the crystal lattice change their direction uniformly with a change in temperature and the liberational modes, coupled in an anharmonic way with the uniform mode, change their frequencies in such a way that entropy compensates the unfavorable potential energy increase due to the uniform angular tilt.     

金属玻璃Ni30Zr70的动态粘度和稳态粘度

本文测量了金属玻璃Ni₃₀Zr₇₀的动态粘度和稳态粘度,结果表明,由于结构弛豫的影响,动态粘度随升温速率的减小而线性增加,在玻璃转变温度T_g以下,动态粘度与温度的关系可以用Arrhenius方程进行描述,稳态粘度与温度的依赖关系满足Fulcher-Vogel方程。从4种不同升温速率的高温(结晶开始温度T_x)动态粘度值,给出金属玻璃Ni₃₀Zr₇₀的平衡粘度。在630—670K     

CX2(X=F, Cl, Br)与CH3CHO中C－C键插入和环加成的理论模拟

采用密度泛函[DFT]和自然键轨道理论[NBO]及高级电子耦合簇[CCSD(T)]和电子密度拓扑分析[AIM]方法, 研究了单重态二卤卡宾CX2(X=F, Cl, Br)与乙醛CH3CHO 中C—C键的插入反应及其环加成的反应机理. 在B3LYP/6-31G(d)水平上优化了各驻点构型, 用频率分析和内禀反应坐标法(IRC)对过渡态进行了验证, 计算了各物种的CCSD(T)/6-31G(d, p)单点能量. 用经Wigner校正的Eyring过渡态理论分别计算了1大气压下主反应通道的热力学与动力学性质, 并对反应通道中构型进行了自然键轨道及电子密度拓扑分析. 结果表明, CF2与CH3CHO反应的主产物是P2F[CH3CF2CHO: 插入CH3CHO中C-C键, 反应I(2)], 而CCl2及CBr2与CH3CHO反应的主产物是P1Cl[Cl2COCHCH3: 成环反应II(1)]及P1Br[Br2COCHCH3: 成环反应III(1)], 1大气压下, 反应I(2)和II(1)及III(1)进行的适宜温度范围分别为400~1300K和400~1000K.     

On the magnetic ordering of CeAl2

The magnetic behaviour of CeAl₂ at low temperature is not apparent : contradictory experiments have assumed either an antiferromagnetic ordering or a moment reduction of the Ce³⁺ ion. Using the multidetector D1B at the I.L.L. high flux reactor, we have measured neutron diffraction diagrams, above and below the transition temperature of 3.8 K. At 1.9 K, we have found very weak magnetic reflections corresponding to an antiferromagnetic structure sinusoǐdally modulated, according to the propagation vector (0.612, 0.388, 0.5). The moment reduction in such a modulated structure at 1.9 K can hardly be explained by a thermal partial disordering of the moments. It is rather due to the existence of a singlet ground state resulting from the negative exchange between the 4f electron of Ce³⁺ and the conduction electrons.