Effect of configuration and conformation on the spin multiplicity in xylylene type biradicals

The singlet-triplet splitting energy gap DeltaE(S.T) = E-S - E-T is calculated for the ortho-, meta-, and para-xylylenes and their heteroatomic analogous by means of AM1-CI approach. It is shown that when the radical centers R-.(R-.=H2C.-,H2N.+- or HN.-) are twisted sufficiently Tar out of conjugation with the benzene ring, DeltaE(S.T) tends to zero or is negative, i.e, ortho-, meta-, and para-phenylenes turn into weak ferromagnetic or antiferromagnetic coupling unit, while they are strong ferromagnetic (meta-isomers) or antiferromagnetic (ortho-, para-isomers) coupling units under planar conformation. It is suggested that serious twisted conformation is not recommended candidate for the design of novel high-spin molecules with stable high-spin ground states by ortho- or para-phenylene coupling unit.

X-RAY PHOTOELECTRON-SPECTRA OF SOME BIOINORGANIC COMPLEXES OF THE RARE-EARTHS WITH N-ACETYLALANINE

X-Ray photoelectron spectra of some bioinorganic complexes of La, Ce, PT, Nd, Sm and Eu with N-acetylalanine have been measured and the 3d5/2 and 3d3/2 main peaks and their satellites have also been assigned. ne spin-orbit splitting between the 3d5/2 and 3d3/2 core-level of the rare earth ion in these complexes becomes slightly larger than that of the free rare earth atom due to the effect of the crystal field. The satellite for the 3d main peaks of La in the solid state complex are in higher binding energy region and may be attributable to the L --> 4f charge-transfer shake-up process. The satellites for the 3d main peaks of Ce, Pr, Nd, Sm and Eu are in the lower binding energy region and may be attributable to the 4f --> L charge-transfer shake-down process.

PHOTOELECTRON-SPECTRA OF VARIOUS CORE LEVELS IN SOME RARE-EARTH N-ACETYLVALINE BIOINORGANIC COMPLEXES

X-ray photoelectron spectra of some bioinorganic complexes of La, Pr, Nd, Sm, and Gd with N-acetylvaline have-been measured. The complex formation does not give any detectable influence on the binding energy of the N 1s peak in the amino group, but has some appreciable effect on the binding energy of the C 1s peak and the O 1s peak in the carboxyl and carbonyl group of the biological ligand. The spin-orbit splitting between the 3d5/2 and 3d3/2 core level of the rare earth ion in these bioinorganic complexes also becomes slightly larger than that of the free rare earth atom due to the effect of the crystal field from the biological ligands.

MOSSBAUER-EFFECT STUDIES OF THE DIMER OF A BIMETALLIC EU-AL COMPLEX

Mossbauer spectra of the dimer of a bimetallic Eu-Al complex, [(CF3COO)3EuHAl(C2H5)2 . 2 THF]2 are measured at different temperatures (81 to 166 K) and some Mossbauer parameters, such as isomer shift, electric quadrupole splitting and asymmetric parameter, are derived from the experimental spectra. The Debye temperature of the compound determined by a Debye model is 128 K. The results indicate that europium in the organo-europium compound is trivalent and that a bridging hydrogen atom between two europium atoms exists in the dimer. The low Debye temperature implies that the weaker binding force between the europium atom and the lattice may be related to the structure and the chemical bonding in the organometallic compound of europium.

THE STUDY OF MOSSBAUER-EFFECT IN EUBA2CU3O7-Y

EuBa2(Cu1-xFex)3O7-y has been investigated by the Fe-57 and Eu-151 Mossbauer effect. The Fe-57 Mossbauer spectra of the EuBa2(Cu1xFex)3O7-y without or with DC electric current (the current strength I = 0.5A) around the superconducting transition temperature have been measured. The results indicate that the isomer shift (IS) and the quadrupole splitting (QS) of the Fe replacing the Cu(2) vary neither with increasing the Fe content nor with the small DC eletric current passing the superconductor and that the IS and the QS of the Fe replacing the Cu(1) vary with the Fe content. Especially, the IS and the QS of the Fe (D3) replacing the Cu(1) are changed when the small electric current passes the superconductor at 80K.

MOSSBAUER-EFFECT STUDIES OF THE PHASE-TRANSITION PROCESS IN EUROPIUM PENTAPHOSPHATE

Mossbauer spectra of europium pentaphosphate are measured at various temperatures (126 to 200-degrees-C). Some Mossbauer parameters, such as isomer shift, electric quadrupole splitting, and asymmetry parameter of the EFG at Eu-151 nuclei are derived from the experimental spectra. The lattice parameters of the crystal are determined at several temperatures. The experimental results indicate that the crystal structure of europium pentaphosphate changes from monoclinic to orthorhombic. All of the temperature dependences of the Mossbauer parameters provide evidence of a phase transition of the crystal. The phase transition temperature can be determined from the curve of the asymmetry parameter of EFG versus temperature to 165-degrees-C.

OBSERVATION OF MOSSBAUER-EFFECT IN THE HIGH-TEMPERATURE SUPERCONDUCTOR EUBA2CU3O7-X WITH DC ELECTRIC-CURRENT

The difference between the Mossbauer parameters for EuBa2Cu3O7-x with dc electric current and those without dc electric current at 83 K has been observed. The change in isomer shift, electric quadrupole splitting and the asymmetry parameter of the electric field gradient at the Eu-151 nucleus may be caused by the movement of a mass of conduction electrons along a certain direction in the EuBa2Cu3O7-x crystal with a layered structure.

The nitrogen isotopic composition of dissolved nitrate in the Yangtze River (Changjiang) estuary, China

The nitrogen isotopic composition of dissolved nitrate (delta N-15-NO3-) in surface water of the Yangtze River estuary was determined in four seasons of 2006. delta N-15-NO3- ranged from 0.4 parts per thousand to 6.5 parts per thousand and varied with seasons and geographic regions, reflecting the dynamics of nitrogen cycling in the estuarine ecosystem. delta N-15-NO3- was markedly lower in February than in other seasons and exhibited conservative mixing, which was probably attributed to the NO3- being sourced from the atmospheric deposition and agricultural fertilizer. In the upper estuary, the influence of riverine inputs was important during all surveys. in the turbidity maximum zone, nitrification was found with nitrate depleted in N-15 in May, whereas denitrification resulting in heavy delta N-15-NO3- played an important role in August. More enriched delta N-15-NO3- values coinciding with losses of nitrate concentrations based on the conservative mixing model were found in the adjacent marine area in May, and may reflect obvious phytoplankton assimilation of dissolved nitrate. In this manner, delta N-15-NO3- may be a sensitive indicator of nitrogen sources and biogeochemical processing existing in this estuary in conjunction with the variations of dissolved nitrate and other environmental factors. (C) 2009 Elsevier Ltd. All rights reserved.

A nitrogen budget of the Changjiang river catchment

Based on 1997-1998 field investigations in the Changjiang river mouth, rain sampling from the river's upper reaches to the mouth, historical data, and relevant literature, the various sources of Total Nitrogen (TN) and Dissolved Inorganic Nitrogen (DIN) in the Changjiang river catchment and N transport in the Changjiang river mouth were estimated. The export fluxes of various form of were mainly controlled by the river runoff, and the export fluxes of NO3-N, DIN and TN in 1998 (an especially heavy flood year) were 1438 103 tonnes (t) yr(-1) or 795.1 kg km(-2) yr(-1) 1746 10(3) t yr(-1) or 965.4 kg km(-2) yr(-1) and 2849 10(3) t yr(-1) or 1575.3 kg km(-2) yr(-1), respectively. The TN and DIN in the Changjiang river came mainly from precipitation, agricultural nonpoint sources, N lost from fertilizer and soil, and point sources of industrial waste and residential sewage discharge, which were about 56.2% and 62.3%, 15.4% and 18.5%, 17.1% and 14.4%, respectively, of the N outflow at the Changjiang river mouth; maximum transport being in the middle reaches.

The effect of land management on carbon and nitrogen status in plants and soils of alpine meadows on the Tibetan plateau

Large-scale grassland rehabilitation has been carried out on the severely degraded lands of the Tibetan plateau. The grasslands created provide a useful model for evaluating the recovery of ecosystem properties. The purposes of this research were: (1) to examine the relative influence of various rehabilitation practices on carbon and nitrogen in plants and soils in early secondary succession; and (2) to evaluate the degree to which severely degraded grassland altered plant and soil properties relative to the non-disturbed native community. The results showed: (1) The aboveground tissue C and N content in the control were 105-97 g m(-2) and 3.356gm(-2), respectively. The aboveground tissue C content in the mixed seed treatment, the single seed treatment, the natural recovery treatment and the severely degraded treatment was 137 per cent, 98 per cent, 49 per cent and 38 per cent, respectively, of that in the control. The corresponding aboveground tissue N content was 109 per cent, 84 per cent, 60 per cent and 47 per cent, respectively, of that in the control. (2) Root C and N content in 0-20 cm depths of the control had an 2 2 average 1606 gm(-2) and 30-36 gm(-2) respectively. Root C and N content in the rehabilitation treatments were in the range of 26-36 per cent and 35-53 per cent, while those in the severely degraded treatment were only 17 per cent and 26 per cent of that in the control. (3) In the control the average soil C and N content at 0-20 cm was 11307 gm(-2) and 846 gm(-2), respectively. Soil C content in the uppermost 20 cm in the seeded treatments, the natural recovery treatment and the severely degraded treatment was 67 per cent, 73 per cent and 57 per cent, respectively, while soil N content in the uppermost 20cm was 72 per cent, 82 per cent and 79 per cent, respectively, of that in the control. The severely degraded land was a major C source. Restoring the severely degraded lands to perennial vegetation was an alternative approach to sequestering C in former degraded systems. N was a limiting factor in seeding grassland. It is necessary for sustainable utilization of seeding grassland to supply extra N fertilizer to the soil or to add legume species into the seed mix. Copyright (c) 2005 John Wiley & Sons, Ltd.

Formation of the V center on H-type zeolites with thermal treatment

The effect of thermal treatment on H-MCM-22 and H-ZSM-5 zeolites was investigated using the electron spin resonance technique. A six-line signal (denoted as A, g = 2.048, A = 22. 15 G) was detected on H-MCM-22 after He purging at high temperatures, whose intensities increased with the treating temperature. The same signal was also found on H-ZSM-5 zeolites with different crystal sizes. The paramagnetic center was identified as a V center, namely, a hole of an electron trapped on an oxygen atom bonding to a nearby aluminum atom. These signals appeared only on a dealuminated sample or a sample concomitantly with dealumination. The formation of the hole might involve an electron transferring from the lattice oxygen to a nonframework aluminum species, and the hyperfine splitting is caused by the interaction between the electron hole locating on the p orbit of oxygen and the framework aluminum bonding with the oxygen. The signal disappeared after the sample was exposed to air or oxygen at room temperature. However, the process was reversible. A new set of signals (denoted as B, g(1) = 2.008, g(2) = 2.003, g(3) = 1.9985) was observed after oxygen adsorption on the H-MCM-22 pretreated with He at 973 K or He purging at 973 K on the H-MCM-22 pretreated with oxygen at 813 K, which was attributed to the O- species.