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The experimental electron density of ammonium dihydrogen phosphate (ADP) crystal in the paraelectric phase (155 K) as well as antiferroelectric phase (100 K) is obtained from its high resolution X-ray diffraction data using the maximum entropy method. Marked redistribution of electron density has been observed in ADP crystals as the crystal temperature is lowered below the phase transition temperature Tc = 148 K. The nature of very strong O–H–O hydrogen bonds between phosphate anions changes from an ideal covalent interaction to a polar covalent interaction as the temperature is altered from 155 to 100 K. The influence of intermolecular interaction like the dipolar interaction on the electron density particularly in the intermolecular region is clearly visible in the electron density maps. One of the most striking features of the electron density of ADP is the presence of non nuclear maxima (NNM) within the “ab” planes. It is argued that the appearance of these NNMs is a normal consequence of the chemical bonding between homonuclear groups in ADP. The manuscript describes the experimental electron density of ammonium dihydrogen phosphate (ADP) obtained from its high resolution X-ray diffraction data recorded at two crystal temperatures namely 100 and 155 K.

The proportion of chemical elements passing through vegetation prior to being exported in a stream was quantified for a forested tropical watershed(Mule Hole, South India) using an extensive hydrological and geochemical monitoring at several scales. First, a solute annual mass balance was established at the scale of the soil-plant profile for assessing the contribution of canopy interaction and litter decay to the solute fluxes of soil inputs (overland flow) and soil outputs (pore water flow as seepages). Second, based on the respective contributions of overland flow and seepages to the stream flow as estimated by a hydrological lumped model, we assigned the proportion of chemical elements in the stream that transited through the vegetation at both flood event (End Member Mixing Analysis) and seasonal scales. At the scale of the 1D soil-plant profile, leaching from the canopy constituted the main source of K above the ground surface. Litter decay was the main source of Si, whereas alkalinity, Ca and Mg originated in the same proportions from both sources. The contribution of vegetation was negligible for Na. Within the soil, all elements but Na were removed from the pore water in proportions varying from 20% for Cl to 95% for K: The soil output fluxes corresponded to a residual fraction of the infiltration fluxes. The behavior of K, Cl, Ca and Mg in the soil-plant profile can be explained by internal cycling, as their soil output fluxes were similar to the atmospheric inputs. Na was released from soils as a result of Na-plagioclase weathering and accompanied by additional release of Si. Concentration of soil pore water by evapotranspiration might limit the chemical weathering in the soil. Overall, the solute K, Ca, Mg, alkalinity and Si fluxes associated with the vegetation turnover within the small experimental watershed represented 10-15 times the solute fluxes exported by the stream, of which 83-97% transited through the vegetation. One important finding is that alkalinity and Si fluxes at the outlet were not linked to the ``current weathering'' of silicates in this watershed. These results highlight the dual effect of the vegetation cover on the solute fluxes exported from the watershed: On one hand the runoff was limited by evapotranspiration and represented only 10% of the annual rainfall, while on the other hand, 80-90% of the overall solute flux exported by the stream transited through the vegetation. The approach combining geochemical monitoring and accurate knowledge of the watershed hydrological budget provided detailed understanding of several effects of vegetation on stream fluxes: (1) evapotranspiration (limiting), (2) vertical transfer through vegetation from vadose zone to ground surface (enhancing) and (3) redistribution by throughfalls and litter decay. It provides a good basis for calibrating geochemical models and more precisely assessing the role of vegetation on soil processes. (C) 2014 Elsevier Ltd. All rights reserved.

Crystallization of the completely deuterated glycine silver nitrate (DGSN) from D2O gave a new polymorph with the crystal structure significantly differing from that of the nondeuterated form (GSN): space group P21/c with polymeric two-dimensional layers parallel to the ab plane. In DGSN, the silver ions are mono-nuclear, whereas in GSN Ag–Ag dimers are present. In contrast to GSN, DGSN does not undergo any phase transitions on cooling at least till 100 K.

A new complex of 2-aminopyridine and malonic acid in a 1:1 stoichiometric ratio is obtained. The complex crystallizes in a centrosymmetric P21/c space group with a = 3.8522(2)A, b = 17.5226(8)A, c = 13.7317(7)A and β = 94.418(3)°. This complex is held by network of hydrogen bonds between the protonated 2-aminopyridinium cation and the malonate anion. There is no phase transition observed in this complex, which is verified using DSC and single crystal X-ray measurement studies. Unlike 2-aminopyridinium maleate, this complex crystallizes in the centrosymmetric space group and hence, this complex will not exhibit any non-linear optical property.

The OH of COOH can acts as both donor and acceptor of hydrogen bond. OH of COOH as an acceptor was primarily observed in Oxalic acid Amide complexes. In order to further understand the packing in these complexes, oxalic acid was complexed with 2-tricholoroacetamide. This crystallization resulted in the formation of ammonium tetraoxalate dehydrate. A result similar to what was observed in complexation of oxalic acid with amide containing amino acids (asparagine and glutamine). Interestingly in all these cases, the amide bond is broken, to form the ammonium ion when trying to complex with oxalic acid.

Electron density for 2-aminopyridinium maleate molecule has been obtained from high resolution low temperature single crystal X-ray diffraction data using the maximum entropy method. Since this method is not commonly used in regular charge density studies we also present an extensive comparison with the theoretical electron density of the molecule obtained using density-functional theory. Study shows that maximum entropy method is not only capable of revealing the finer details of influence of chemical bonding on electron density but might also give a more realistic picture of the chemical bonds, since it is model independent and can in principle reproduce any radial deformation present in the molecular electron density.

Charge-density analysis of hydrogen-bonded complexes of glycine is undertaken in order to elucidate the chemical nature of very strong hydrogen bonds present in these complexes. We have chosen two glycine complexes triglycine sulphate (TGS) and Bis (glycinium) oxalate (DGO) which are known to exhibit very strong hydrogen bonding. Electron density maps demonstrate the covalent nature of very strong O–H...O bond found in DGO and TGS complexes. Strong hydrogen bonds in TGS have significant covalent character. This charge-density analysis confirmed the existence of the bifurcated acceptor hydrogen bond which is responsible for the coupling between the two components of structural disorder found at high temperatures (T > 49 °C) in TGS crystals, namely disorder of amino group of one of the glycine ions (GI) and the hydrogen atom disorder between the carboxylic group of other two glycines GII and GIII. Recent charge density studies have demonstrated that hydrogen bonds are more convincingly reproduced by the model independent maximum entropy method (MEM) than by multipole models, hence we have used MEM procedure in our charge-density analysis.

A new anomalous salt in 2:3 ratio of 2-aminopyridine and maleic acid was obtained. This salt, 2-aminopyridinium maleate maleic acid crystalllizes in monoclinic C2/c space group. 2-Aminopyridinium maleate maleic acid is isostructural to 2-amino 3-methyl pyridiniminium maleate maleic acid and is the third complex crystallized from 2-aminopyridine and maleic acid in aqueous solution in a different stochiometric ratio. This salt is stabilized by N H…O and O H…O bydrogen bonds apart from π–π interactions.

A new complex of 2-aminopyridine and maleate in 1:1 stoichiometric ratio was obtained. The complex crystallizes in non-centrosymmetric space group Pc. This complex is held by network of hydrogen bonding between the protonated 2-aminopyridinium cation and maleate anion and also by pi–pi stacking. There is no phase transition observed in this complex, which has been verified using DSC and single crystal X-ray measurement studies. A comparison between 2-aminopyridinium maleate and bis(2-aminopyridinium) maleate structure shows that the positive charge resides on the ring nitrogen in the present complex unlike the case in bis(2-aminopyridinium) maleate where the charge is on the amino nitrogen. This complex has higher meting point compared to that of bis(2-aminopyridinium) maleate which is due to the difference in hydrogen bonding pattern between the two complexes.

We revisit the problem of the spin-Peierls instability in a one dimensional spin-1/2 chain coupled to phonons. The phonons are treated within the mean field approximation. We use bosonization techniques to describe the gapped spin chain and then use the Thermodynamic Bethe Ansatz to obtain quantitative results for the thermodynamics of the spin-Peierls system in a whole range of temperature. This allows us to predict the behavior of the specific heat and the magnetic susceptibility in the entire dimerized phase. We study the effect of small magnetic fields on the transition. Moreover, we obtain the parameters of the Landau-Ginzburg theory describing this continuous phase transition near the critical point., 16 pages, 4 EPS figures v2: corrected discussion of logarithmic corrections, some typos corrected, references updated

We study thermal transport in one dimensional spin systems both in the presence and absence of impurities. In the absence of disorder, all these spin systems display a temperature dependent Drude peak in the thermal conductivity. In gapless systems, the low temperature Drude weight is proportional to temperature and to the central charge which characterizes the conformal field theory that describes the system at low energies. On the other hand, the low temperature Drude weight of spin gap systems shows an activated behavior modulated by a power law. For temperatures higher than the spin gap, one recovers the linear T behavior akin to gapless systems. For temperatures larger than the exchange coupling, the Drude weight decays as 1/T^2. We argue that this behavior is a generic feature of quasi one dimensional spin gap systems with a relativistic-like low energy dispersion. We also consider the effect of a magnetic field on the Drude weight with emphasis on the commensurate-incommensurate transition induced by it. We then study the effect of nonmagnetic impurities on the thermal conductivity of the dimerized XY chain and the spin-1/2 two leg ladder. Impurities destroy the Drude peak and the thermal conductivity exhibits a purely activated behavior at low temperature, with an activation gap renormalized by disorder. The relevance of these results for experiments is briefly discussed., 13 pages, 6 eps figures, RevTeX 4

We discuss the phase transitions of fermions in one dimension with a narrow Feshbach resonance described by the boson–fermion resonance model. By means of the bosonization technique, we derive a low-energy Hamiltonian of the system and show that a strongly correlated state exists, where the order parameters of the Bose condensation and superfluidity decay with the same critical exponent. We also show that density fluctuations near the Fermi wavevector are strongly suppressed as a consequence of a spin gap and a gap against the formation of phase slips. We find a Luther–Emery point where the phase slips and the spin excitations can be described in terms of pseudofermions, providing closed form expressions of the density–density correlations and the spectral functions. The relevance of our results for experiments with ultracold atomic gases subject to one-dimensional confinement is also discussed.