Distribution by Scientific Domains
Distribution within Chemistry

Terms modified by NH

  • nh bond
  • nh group
  • nh groups
  • nh patient
  • nh proton
  • nh resident

  • Selected Abstracts

    Low temperature powder diffraction and DFT solid state computational study of hydrogen bonding in NH4VO3

    . Smr
    Abstract The crystal structure of NH4VO3 was refined by the geometry optimization done by total energy minimization in solid state using DFT/plane waves approach. The lattice parameters were derived by the Le Bail technique from the low temperature X-ray (40-293 K) and synchrotron (100-293 K) powder diffraction data. The structure is formed by the infinite chains of irregular VO4 tetrahedra running approximately parallel to the c -axis, which are interlinked by the ammonium ions placed between them. The ammonium ions link to the [VO4], chains through one linear, one bifurcated and two trifurcated N-H,O hydrogen bonds. Considering their stability there are six distinct N-H,O hydrogen bonds: two strong with the N-H,O bond angles close to the straight, two medium with the bond angles of 123° and 148° and two very bent (105° and 107°) and hence weak hydrogen bonds. There is a reasonable agreement between the energies of the stretching ,(NH) modes estimated using the optimised N,O contact distances and those obtained experimentally. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Long maternal separation has protective effects in rats exposed to activity-based anorexia

    O. Carrera
    Abstract This study examined the effect of three neonatal treatments of maternal separation during infancy in young adult rats exposed to standard activity-based anorexia (ABA) consisting of food restriction plus free access to an activity wheel. During the first 20 postnatal days of life rat pups were exposed to periods of either brief maternal separation (BMS, 15,min), long maternal separation (LMS, 180,min), or were non-handled (NH). Thereafter, male and female rats were exposed to ABA. Neonatal treatment produced no significant differences in the survival time of male rats, whereas survival was greater in female rats exposed to LMS than in NH rats under ABA procedure. In conclusion, prolonged maternal separation appears to promote resistance in female animals subjected to harsh ABA life-threatening conditions. © 2009 Wiley Periodicals, Inc. Dev Psychobiol 51: 616,624, 2009 [source]

    Early handling reduces vulnerability of rats to activity-based anorexia

    O. Carrera
    Abstract Resistance to restricted feeding with and without wheel access was tested in rats handled (H) for 20 days since birth. Weight loss produced by 1.5-hr restricted food access was less in H than in non-handled (NH) males when tested aged 41 days. At this age combining food restriction with access to a running wheel (a procedure commonly known as activity-based anorexia, ABA) produced very rapid weight loss and no effect of handling was detected. When 75-day females were tested in the same way, under the ABA procedure H rats took longer than NH controls to reach the removal criterion. Simply restricting food access in these females produced variable weight loss, without detection of any handling effect. No differences in food intake or running were detected between H and NH rats in either males or females. In conclusion, handling seems to have a direct effect on rats' later response to either food deprivation alone or to an ABA procedure. © 2006 Wiley Periodicals, Inc. Dev Psychobiol 48: 520,527, 2006. [source]

    Forest age, wood and nutrient dynamics in headwater streams of the Hubbard Brook Experimental Forest, NH

    Dana R. Warren
    Abstract Instream processing may substantially alter nutrient export from forested watersheds. This study tested how instream uptake of N and P were affected by successional differences in the accumulation of large wood and debris dams in a 66-year chronosequence formed by five watersheds within the Hubbard Brook Experimental Forest (HBEF), NH. Nutrient enrichment releases in summer 1998 were used to measure the uptake velocities of phosphate, nitrate and ammonium for five streams within HBEF, and results indicated that uptake of PO43, was closely associated with forest age. In 2004, we quantified volume and abundance of large wood in each stream to test whether large wood abundance could be linked to nitrate uptake as well as phosphate. The volume of instream wood increased with forest age, at an apparent rate of 0·03 m3 (100 m),1 per year for these early to mid-successional forests (r2 = 0.95); however, debris dam frequency did not. Instead, debris dam frequency, when controlled for stream size, followed a U-shaped distribution, with high dam frequency in very young forests, low frequency in forests around 20,30 years of age and increasing dam frequency again as forests matured. Phosphate uptake velocity increased strongly with both forest age and large wood volume (r2 = 0·99; p < 0·001 in both cases); however, nitrate and ammonium uptake were not related to either factor. We attribute the positive relationship between phosphate uptake velocity and forest age/large wood volume to increased abiotic adsorption of phosphate by the inorganic sediments retained by wood. Nitrogen uptake in these streams is primarily biologically driven and did not vary predictably with these structural features of channels. We expect wood abundance to increase in HBEF streams as the forest matures, with a subsequent increase in stream phosphate uptake capacity. Copyright © 2007 John Wiley & Sons, Ltd. [source]

    Analysis of Simulated Martian Regolith Using an Array of Ion Selective Electrodes

    ELECTROANALYSIS, Issue 15-16 2005
    Abstract A prototype miniature array of polymer membrane and solid state ion selective electrodes was developed for the purpose of performing an in-situ analysis of the soluble ionic species in Martian regolith (soil). The array contains a total of 27 electrodes for K+, Na+, Ca2+, Mg2+, NH, Ba2+, NO, Cl,, and Li+, each in triplicate. Barium electrodes were used to indirectly monitor sulfate through precipitation by the addition of barium chloride while the lithium electrodes served as a reference for the array by having a constant lithium concentration as a background for all solutions. The array was tested with several types of simulants, soils, and sawdust from a Mars meteorite, all with varying salt content, meant to approximate the various hypotheses regarding the ionic composition of the Martian soil. The activities of anions and cations determined with the array were compared to ion chromatography data. [source]

    Reductive transformation of hexahydro-1,3,5-trinitro-1,3,5-triazine, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, and methylenedinitramine with elemental iron

    Seok-Young Oh
    Abstract Reductive (pre)treatment with elemental iron is a potentiallyuseful method for degrading nitramine explosives in water and soil. In the present study, we examined the kinetics, products, and mechanisms of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) degradation with elemental iron. Both RDX and HMX were transformed with iron to formaldehyde, NH, N2O, and soluble products. The yields of formaldehyde were relatively constant (71% ± 5%), whereas the yields of NH and N2O varied, depending on the nitramine and the mechanism. The reactions most likely were controlled by a surface process rather than by external mass transfer. Methylenedinitramine (MDNA) was an intermediate of both RDX and HMX and was transformed quantitatively to formaldehyde with iron. However, product distributions and kinetic modeling results suggest that MDNA represented a minor reaction path and accounted for only 30% of the RDX reacted and 14% of the formaldehyde produced. Additional experiments showed that RDX reduction with elemental iron could be mediated by graphite and Fe2+ sorbed to magnetite, as demonstrated previously for nitroaromatics and nitrate esters. Methylenedinitramine was degraded primarily through reduction in the presence of elemental iron, because its hydrolysis was slow compared to its reactions with elemental iron and surface-bound Fe2+. Our results show that in a cast iron-water system, RDX may be transformed via multiple mechanisms involving different reaction paths and reaction sites. [source]

    Variations of nitrate and sulfate in the atmosphere on days of high and low particulate matters

    Chung-Yih Kuo
    Abstract Particulate matters with aerodynamicdiameter less than 10 ,m (PM10) were collected with high-volume samplers for four periods of consecutive samplings. Each period included a high PM10 (defined as PM10 ,110 ,g/m3) episode. The concentrations of all ionic species of high PM10 day (HPD) samples were higher than those of low PM10 (defined as PM10 < 110,g/m3) d (LPD) samples. Using the ionic concentrations of HPD samples at each station divided by those of LPD samples at respective stations, the results showed thatNO increased most from LPD to HPD among the eight ionic species examined. A high mean value (3.15) of NO/elemental carbon (EC) ratio of HPD divided by that of LPD indicates that concentration variations due to chemical formation and nonchemical factors apparently were higher than those of the concentration variations due to nonchemical factors alone. The NO/SO ratio of HPD divided by that of LPD ranged from 1.62 to 3.92 for the four periods. The results indicate that more nitrate than sulfate had formed during high PM10 episodes. Multiple linear regression analysis showed that the percentage of NH associated with nitrate and sulfate decreased and the percentage that could be explained by Ca2+ and Na+ increased on HPD. The reactions of HNO3 and H2SO4 with sea salt particles and with aqueous carbonates on soil particles during HPD were considered the main causes leading to these variations. [source]

    Nitrate- and Nitrite-Assisted Conversion of an Acetonitrile Ligand Into an Amidato Bridge at an {Mo2(Cp)2(,-SMe)3} Core: Electrochemistry of the Amidato Complex [Mo2(Cp)2(,-SMe)3{,-,1,,1 -OC(Me)NH}]+

    Marc Le Hénanf
    Abstract Treatment of [Mo2(Cp)2(,-SMe)3(MeCN)2]+ (1+) with NO3, or NO2, results in the conversion of one terminally bound acetonitrile ligand into an amidato bridge. The reaction produces [Mo2(Cp)2(,-SMe)3{,-,1,,1 -OC(Me)NH}]0/+ (20/+) and involves the formation of an intermediate, which was detected by cyclic voltammetry but which could not be isolated, and which likely arises from the substitution of the NOx anion for one MeCN ligand. The electrochemical behaviour of 2+ was studied by cyclic voltammetry in THF and MeCN. The reduction of 2+ in the presence of acid (HBF4/H2O or HBF4/Et2O) in these solvents leads to the release of the amidate bridge. Controlled-potential electrolysis of 2+ in MeCN in the presence of acid produces 1+ quantitatively; the charge consumed (>1 F,mol,1 of 1+) indicates that electrons are also used to reduce protons. This was confirmed by the formation of 2+ (in variable amounts depending on the conditions) on treating 2 with acid. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

    Towards Cationic Gallium Derivatives: Metallacycles from the Reactions of Organogallium Compounds with Tetraorganodichalcogenoimidodiphosphinates and a New N -(Diphenylthiophosphinyl)thioureato Ligand

    Virginia Montiel-Palma
    Abstract The organometallic complexes of general formulae [Me2Ga{,2 - E,E, -[R2P(E)NP(E,)R,2]}] [R = R, = Ph, E = E, = O (1); R = R, = Ph, E = E, = S (2); R = R, = Ph, E = E, = Se (3); R = R, = Ph, E = O, E, = S (4); R = Me, R, = Ph, E = S, E, = O (5)] and [Me2Ga{,2 - S,S, -[Ph2P(S)NC(S)(C9H10N)]}] (6) were obtained by facile methane elimination reactions from GaMe3 and the acidic ligands L1H [(XPPh2)2NH (X = O, S, Se), (OPPh2)(SPPh2)NH, and (OPMe2)(SPPh2)NH] and L2H [Ph2P(S)NHC(S)(C9H10N)] in toluene. Replacement of one phosphorus atom by a carbon atom in the ligand skeleton of L1H gave the new ligand L2H, which, upon reaction with GaMe3, gave compound 6, which shows no significant structural differences with respect to 1,5. Therefore, L2H does not induce partial planarity in the six-membered ring, indicating the necessity for replacing both phosphorus atoms of the ligand by carbon atoms, as in the ,-diketonate-type derivatives, in order to impose ring planarity. Thus, despite originating from a variety of ligands with differing donor atoms and substituents at the phosphorus atoms, all complexes show little structural differences. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

    Syntheses and Crystal Structures of Tetrakis(arylamidine)nickel(II) Chloride and Bis[2,4-dipyridyl-1,3,5-triazapentadienato]nickel(II)

    Jianping Guo
    Abstract The reaction of LiN(SiMe3)2 with arylnitrile, followed by the addition of anhydrous NiCl2 gives ionic complexes of the general formula [Ni{H2NC(Ar)=NH}4]Cl2 (Ar = Ph 1, p -tolyl 2). When the above reaction is carried out with cyanopyridine instead of arylnitrile under the same reaction conditions, neutral complexes of the general formula [{HN=C(Py)N=C(Py)NH}2Ni] (Py = 4-pyridyl, 3; 3-pyridyl, 4] are obtained. Compound 1 undergoes a metathesis reaction with sodium benzoate to give the neutral complex [(PhCO2)2Ni {H2NC(Ph)=NH}4] (5). Magnetic susceptibility measurements show that 1,4 are diamagnetic and that 5 is paramagnetic with two unpaired electrons. These results suggest that 1,4 are d8 square-planar complexes and 5 is an octahedral complex. The solid state structures of compounds 1,5 were determined by X-ray crystallography. Structural analyses reveal that 1 and 2 form a one-dimensional network through charge-assisted hydrogen bonds; whereas 5 forms a one-dimensional network through hydrogen bonds only. In complexes 3 and 4, the 2,4-dipyridyl-1,3,5-triazapentadienyl ligand behaves as a bidentate ligand forming a six-membered ring with the metal ion. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

    Synthesis of Pyrazoles by Treatment of 3-Benzylchromones, 3-Benzylflavones and Their 4-Thio Analogues with Hydrazine,

    Albert Lévai
    Abstract The synthesis of pyrazoles 13,24 has been accomplished by treatment of 3-benzylchromones 1,5, 3-benzylflavones 6,12 and their 4-thio analogues 25,29 with hydrazine hydrate in hot pyridine. A plausible reaction mechanism for the formation of pyrazoles 13,24 is discussed. A 1H NMR study in [D6]DMSO allowed the presence of both pyrazole annular tautomers to be observed, due to the presence of intramolecular hydrogen bonds in each tautomer (OH--N and NH--O). GIAO/B3LYP/6-311++G** calculations were carried out on some model pyrazoles to provide a theoretical basis for the NMR experimental observations.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source]

    Modeling of tRNA-assisted mechanism of Arg activation based on a structure of Arg-tRNA synthetase, tRNA, and an ATP analog (ANP)

    FEBS JOURNAL, Issue 17 2009
    Michiko Konno
    The ATP,pyrophosphate exchange reaction catalyzed by Arg-tRNA, Gln-tRNA and Glu-tRNA synthetases requires the assistance of the cognate tRNA. tRNA also assists Arg-tRNA synthetase in catalyzing the pyrophosphorolysis of synthetic Arg-AMP at low pH. The mechanism by which the 3,-end A76, and in particular its hydroxyl group, of the cognate tRNA is involved with the exchange reaction catalyzed by those enzymes has yet to be established. We determined a crystal structure of a complex of Arg-tRNA synthetase from Pyrococcus horikoshii, tRNAArgCCU and an ATP analog with Rfactor = 0.213 (Rfree = 0.253) at 2.0 Å resolution. On the basis of newly obtained structural information about the position of ATP bound on the enzyme, we constructed a structural model for a mechanism in which the formation of a hydrogen bond between the 2,-OH group of A76 of tRNA and the carboxyl group of Arg induces both formation of Arg-AMP (Arg + ATP , Arg-AMP + pyrophosphate) and pyrophosphorolysis of Arg-AMP (Arg-AMP + pyrophosphate , Arg + ATP) at low pH. Furthermore, we obtained a structural model of the molecular mechanism for the Arg-tRNA synthetase-catalyzed deacylation of Arg-tRNA (Arg-tRNA + AMP , Arg-AMP + tRNA at high pH), in which the deacylation of aminoacyl-tRNA bound on Arg-tRNA synthetase and Glu-tRNA synthetase is catalyzed by a quite similar mechanism, whereby the proton-donating group (,NH,C+(NH2)2 or ,COOH) of Arg and Glu assists the aminoacyl transfer from the 2,-OH group of tRNA to the phosphate group of AMP at high pH. [source]

    Escherichia coli cyclophilin B binds a highly distorted form of trans -prolyl peptide isomer

    FEBS JOURNAL, Issue 18 2004
    Michiko Konno
    Cyclophilins facilitate the peptidyl-prolyl isomerization of a trans -isomer to a cis -isomer in the refolding process of unfolded proteins to recover the natural folding state with cis -proline conformation. To date, only short peptides with a cis -form proline have been observed in complexes of human and Escherichia coli proteins of cyclophilin A, which is present in cytoplasm. The crystal structures analyzed in this study show two complexes in which peptides having a trans -form proline, i.e. succinyl-Ala- trans -Pro-Ala- p -nitroanilide and acetyl-Ala-Ala- trans -Pro-Ala-amidomethylcoumarin, are bound on a K163T mutant of Escherichia coli cyclophilin B, the preprotein of which has a signal sequence. Comparison with cis -form peptides bound to cyclophilin A reveals that in any case the proline ring is inserted into the hydrophobic pocket and a hydrogen bond between CO of Pro and N,2 of Arg is formed to fix the peptide. On the other hand, in the cis -isomer, the formation of two hydrogen bonds of NH and CO of Ala preceding Pro with the protein fixes the peptide, whereas in the trans -isomer formation of a hydrogen bond between CO preceding Ala-Pro and His47 N,2 via a mediating water molecule allows the large distortion in the orientation of Ala of Ala-Pro. Although loss of double bond character of the amide bond of Ala-Pro is essential to the isomerization pathway occurring by rotating around its bond, these peptides have forms impossible to undergo proton transfer from the guanidyl group of Arg to the prolyl N atom, which induces loss of double bond character. [source]

    Patterns of rhizosphere carbon flux in sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis) saplings

    Richard P. Phillips
    Abstract Despite its importance in the terrestrial C cycle rhizosphere carbon flux (RCF) has rarely been measured for intact root,soil systems. We measured RCF for 8-year-old saplings of sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis) collected from the Hubbard Brook Experimental Forest (HBEF), NH and transplanted into pots with native soil horizons intact. Five saplings of each species were pulse labeled with 13CO2 at ambient CO2 concentrations for 4,6 h, and the 13C label was chased through rhizosphere and bulk soil pools in organic and mineral horizons for 7 days. We hypothesized yellow birch roots would supply more labile C to the rhizosphere than sugar maple roots based on the presumed greater C requirements of ectomycorrhizal roots. We observed appearance of the label in rhizosphere soil of both species within the first 24 h, and a striking difference between species in the timing of 13C release to soil. In sugar maple, peak concentration of the label appeared 1 day after labeling and declined over time whereas in birch the label increased in concentration over the 7-day chase period. The sum of root and rhizomicrobial respiration in the pots was 19% and 26% of total soil respiration in sugar maple and yellow birch, respectively. Our estimate of the total amount of RCF released by roots was 6.9,7.1% of assimilated C in sugar maple and 11.2,13.0% of assimilated C in yellow birch. These fluxes extrapolate to 55,57 and 90,104 g C m,2 yr,1 from sugar maple and yellow birch roots, respectively. These results suggest RCF from both arbuscular mycorrhizal and ectomycorrhizal roots represents a substantial flux of C to soil in northern hardwood forests with important implications for soil microbial activity, nutrient availability and C storage. [source]

    Preparation of Functional Hybrid Glass Material from Platinum (II) Complexes for Broadband Nonlinear Absorption of Light

    Roman Zieba
    Abstract The synthesis of trans -di(arylalkynyl)diphosphine platinum(II) complexes bearing trialkoxysilane groups is described, as well as the preparation of siloxane-based hybrid materials from organometallic chromophores through a modified sol,gel process. Glass materials prepared from trans -[P(n,Bu)3]2Pt[(C,C,p,C6H4,C,C,p,C6H4,CH2O(CO)NH(CH2)3Si(OC2H5)3]2 generally show spectral transmittance, absorption and luminescence similar to that of solutions reported in the literature. Measurements of optical power limiting for the hybrid glass are carried out, and show broadband nonlinear absorption throughout the whole visible wavelength range with clamping values in the range 0.2,7,µJ at 120,mM chromophore concentration. The sol,gel process using urethane-propyltriethoxysilane-functionalized chromophores as precursors appears to be a valid method for formation of robust silicate materials with grafted diarylethynyl Pt(II) complexes for OPL devices. [source]

    Interannual to decadal changes in area burned in Canada from 1781 to 1982 and the relationship to Northern Hemisphere land temperatures

    GLOBAL ECOLOGY, Issue 5 2007
    Martin P. Girardin
    ABSTRACT Aim, Temporal variability of annual area burned in Canada (AAB-Can) from ad 1781 to 1982 is inferred from tree-ring width data. Next, correlation analysis is applied between the AAB-Can estimates and Northern Hemisphere (NH) warm season land temperatures to link recent interannual to decadal changes in area burned with large-scale climate variations. The rationale in this use of tree rings is that annual radial increments produced by trees can approximate area burned through sensing climate variations that promote fire activity. Location, The statistical reconstruction of area burned is at the scale of Canada. Methods, The data base of total area burned per year in Canada is used as the predictand. A set of 53 multicentury tree-ring width chronologies distributed across Canada is used as predictors. A linear model relating the predictand to the tree-ring predictors is fitted over the period 1920,82. The regression coefficients estimated for the calibration period are applied to the tree-ring predictors for as far back as 1781 to produce a series of AAB-Can estimates. Results, The AAB-Can estimates account for 44.1% of the variance in the observed data recorded from 1920 to 1982 (92.2% after decadal smoothing) and were verified using a split sample calibration-verification scheme. The statistical reconstruction indicates that the positive trend in AAB-Can from c. 1970,82 was preceded by three decades during which area burned was at its lowest during the past 180 years. Correlation analysis with NH warm season land temperatures from the late 18th century to the present revealed a positive statistical association with these estimates. Main conclusions, As with previous studies, it is demonstrated that the upward trend in AAB-Can is unlikely to be an artefact from changing fire reporting practices and may have been driven by large-scale climate variations. [source]

    Community and Individual Race/Ethnicity and Home Health Care Use among Elderly Persons in the United States

    James B. Kirby
    Objective. To investigate whether the interaction between individual race/ethnicity and community racial/ethnic composition is associated with health-related home care use among elderly persons in the United States. Data Sources. A nationally representative sample of community-dwelling elders aged 65+ from the 2000 to 2006 Medical Expenditure Panel Survey (N=23,792) linked to block group-level racial/ethnic information from the 2000 Decennial Census. Design. We estimated the likelihood of informal and formal home health care use for four racial/ethnic elderly groups (non-Hispanic [NH] whites, NH-blacks, NH-Asians, and Hispanics) living in communities with different racial/ethnic compositions. Principal Findings. NH-Asian and Hispanic elders living in block groups with ,25 percent of residents being NH-Asian or Hispanic, respectively, were more likely to use informal home health care than their counterparts in other block groups. No such effect was apparent for formal home health care. Conclusions. NH-Asian and Hispanic elders are more likely to use informal home care if they live in communities with a higher proportion of residents who share their race/ethnicity. A better understanding of how informal care is provided in different communities may inform policy makers concerned with promoting informal home care, supporting informal caregivers, or providing formal home care as a substitute or supplement to informal care. [source]

    2-(2,4-Dinitrobenzyl)pyridine (DNBP): A Potential Light-Activated Proton Shuttle

    Catrin Goeschen
    Abstract The well-known photochromic tautomerism of 2-(2,4-dinitrobenzyl)pyridine (1; CH; Scheme,1) was re-investigated by flash photolysis in aqueous solution in view of its potential application as a light-activated proton pump. Irradiation of 1 yields the enamine tautomer NH (,max=520,nm) that rapidly equilibrates with its conjugate base CNO, (,max=420,nm). The pH,rate profile for the first-order decay of NH and CNO, provides a direct determination of the acidity constant of NH, pK=5.94±0.12 (I=0.1M) and serves to clarify the mechanisms of proton transfer prevailing in aqueous solutions. The acidity constant of protonated 1 (CHNH+), pK=4.18±0.02, was determined by spectrophotometric titration. [source]

    A Theoretical Study of Correlation between Hydrogen-Bond Stability and J -Coupling through a Hydrogen Bond

    Shun-ichi Kawahara
    trans -Hydrogen-bond hyperfine splitting via magnetic interaction, which is observed as J -coupling in NMR experiments, was theoretically studied. trans -Hydrogen-bond hyperfine splitting should be closely related to the orbital interaction between the lone-pair orbital of the H-bond acceptor and the antibond orbital of the H-bond donor. A linear relationship was observed between magnetic interaction hyperfine splitting through a H-bond and the H-bond strength. The relationship was dependent on the type of the nucleus forming the H-bond; linear correlation was observed in NH,,,O/N type or OH,,,N type H-bonded complexes, but not in OH,,,O type H-bonded complexes. [source]

    Hetero-,-systems from 2 + 2 cycloreversion, part 2.1Ab initio thermochemical study of heterocyclobutanes 2 + 2 cycloreversion to form heteroethenes H2C=X (X=NH, O, SiH2, PH, S),

    Leonid E. Gusel'nikov
    Ab initio and DFT thermochemical study of diradical mechanism of 2 + 2 cycloreversion of parent heterocyclobutanes and 1,3-diheterocyclobutanes, cyclo -(CH2CH2CH2X), and cyclo -(CH2XCH2X), where X = NH, O, SiH2, PH, S, was undertaken by calculating closed-shell singlet molecules at three levels of theory: MP4/6-311G(d)//MP2/6-31G(d)+ZPE, MP4/6-311G(d,p)//MP2/6-31G (d,p)+ZPE, and B3LYP/6-311+G(d,p)+ZPE. The enthalpies of 2 + 2 cycloreversion decrease on going from group 14 to group 16 elements, being substantially higher for the second row elements. Normally endothermic 2 + 2 cycloreversion is predicted to be exothermic for 1,3-diazetidine and 1,3-dioxtane. Strain energies of the four-membered rings were calculated via the appropriate homodesmic reactions. The enthalpies of ring opening via the every possible one-bond homolysis that results in the formation of the corresponding 1,4-diradical were found by subtracting the strain energies from the central bond dissociation energies of the heterobutanes CH3CH2,CH2XH, CH3CH2,XCH3, and HXCH2,XCH3. The latter energies were determined via the enthalpies of the appropriate dehydrocondensation reactions, using C,H and X,H bond energies in CH3XH calculated at G2 level of theory. Except 1,3-disiletane, in which ring-opening enthalpy attains 69.7 kcal/mol, the enthalpies of the most economical ring openings do not exceed 60.7 kcal/mol. The 1,4-diradical decomposition enthalpies found as differences between 2 + 2 cycloreversion and ring-opening enthalpies were negative, the least exothermicity was calculated for , CH2SiH2CH2CH2. The only exception was 1,3-disiletane, which being diradical, CH2SiH2CH2SiH2, decomposed endothermically. Since decomposition of the diradical containing two silicon atoms required extra energy, raising the enthalpy of the overall reaction to 78.9 kcal/mol, 1,3-disiletane was predicted to be highly resisting to 2 + 2 cycloreversion. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:704,720, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20377 [source]

    Hydrochemical behaviour of dissolved nitrogen and carbon in a headwater stream of the Canadian Shield: relevance of antecedent soil moisture conditions

    Julie M. L. Turgeon
    Abstract This paper examines the impact of contrasting antecedent soil moisture conditions on the hydrochemical response, here the changes in dissolved nitrogen (NO3,, NH4+ and dissolved organic nitrogen (DON)) and dissolved organic carbon (DOC) concentrations, of a first-order stream during hydrological events. The study was performed in the Hermine, a 5 ha forested watershed of the Canadian Shield. It focused on a series of eight precipitation events (spring, summer and fall) sampled every 2 or 3 h and showing contrasted antecedent moisture conditions. The partition of the eight events between two groups (dry or wet) of antecedent moisture conditions was conducted using a principal component analysis (PCA). The partition was controlled (first axis explained 86% of the variability) by the antecedent streamflow, the streamflow to precipitation ratio Q/P and by the antecedent groundwater depth. The mean H+, NO3,, NH4+, total dissolved nitrogen and DOC concentrations and electrical conductivity values in the stream were significantly higher following dry antecedent conditions than after wetter conditions had prevailed in the Hermine, although the temporal variability was high (17 to 138%). At the event scale, a significantly higher proportion of the changes in DON, NO3,, and DOC concentrations in the stream was explained by temporal variations in discharge compared with the seasonal and annual scales. Two of the key hydrochemical features of the dry events were the synchronous changes in DOC and flow and the frequent negative relationships between discharge and NO3,. The DON concentrations were much less responsive than DOC to changes in discharge, whereas NH was not in phase with streamflow. During wet events, the synchronicity between streamflow and DON or NO3, was higher than during dry events and discharge and NO3, were generally positively linked. Based on these observations, the hydrological behaviour of the Hermine is conceptually compatible with a two-component model of shallow (DON and DOC rich; variable NO3,) and deep (DON and DOC poor; variable NO3,) subsurface flow. The high NO3, and DOC levels measured at the early stages of dry events reflected the contribution from NO3, -rich groundwaters. The contribution of rapid surface flow on water-repellent soil materials located close to the stream channel is hypothesized to explain the DOC levels. An understanding of the complex interactions between antecedent soil moisture conditions, the presence of soil nutrients available for leaching and the dynamics of soil water flow paths during storms is essential to explain the fluxes of dissolved nitrogen and carbon in streams of forested watersheds. Copyright © 2007 John Wiley & Sons, Ltd. [source]

    State-state transitions for CCl2(X1A1, A1B1, a3B1) radical and collisional quenching of CCl2(A1B1 and a3B1) by O2, N2, NO, N2O, NH3, and various aminated molecules

    Yide Gao
    CCl2 free radicals were produced by a pulsed dc discharge of CCl4 in Ar. Ground electronic state CCl2(X) radicals were electronically excited to the A1B1 (0,4,0) vibronic state with an Nd:YAG laser pumped dye laser at 541.52 nm. Experimental quenching data of excited CCl2(A1B1 and a3B1) by O2, N2, NO, N2O, NH3, NH(CH3)2, NH(C2H5)2, and N(C2H5)3 molecules were obtained by observing the time-resolved total fluorescence signal of the excited CCl2 radical in a cell, which showed a superposition of two exponential decay components under the presence of quencher. The quenching rate constants kA of CCl2(A) state and ka of CCl2(a) state were derived by analyzing the experimental data according to a proposed three-level model to deal with the CCl2(X1A1, A1B1, a3B1) system. The formation cross sections of complexes of electronically excited CCl2 radicals with O2, N2, NO, N2O, NH3, and aminated molecules were calculated by means of a collision-complex model. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 351,356, 2002 [source]

    Winter snow depth variability over northern Eurasia in relation to recent atmospheric circulation changes

    V. Popova
    Abstract Mean snow depth time-series for February (1936,2001) over northern Eurasia (incl. Norway, Finland and the former USSR), interpolated into 5 × 5° grid points, are studied using empirical orthogonal function (EOF) analysis. First, five statistically significant rotated PCs are correlated to Northern Hemisphere (NH) teleconnection patterns at the 700 hPa height: North Atlantic Oscillation (NAO), Polar-Eurasia (Pol), Pacific-North American (PNA), West Pacific (WP), and Scandinavia (Scand). The impact of the NH circulation modes on snow depth variations is evaluated using the multiple stepwise backward regression (MSBR). Analyses of the snow depth PCs indicate that within the northern Eurasia territory, there are several regions with snow accumulation, respondent to certain circulation modes. PC1 describes low-frequency snow depth variation to the north from 55 to 60°N between the White Sea and the Lena river basin, and is positively correlated with NAO and negatively,with Scand. MSBR shows that in 1951,1974 the leading role in snow depth variability belongs to Scand. After 1975, Scand has passed over the leading role to NAO. Scand and NAO are also responsible for the surface air temperature changes over the northern Eurasia. Snow depth PC1 and wintertime temperature are closely related to each other. PC2 describes quasi-decadal snow depth variability over eastern Europe and is negatively correlated with NAO. For the Baltic and White Sea coasts, Fennoscandia, and the center of the East European plain, decrease of snow accumulation, related to a positive NAO phase, seems to be caused by mild winters. For the southwestern and central regions of eastern Europe, negative snow depth anomalies could also be caused by decrease of precipitation associated with the eastward shift of cyclone tracks related to the positive NAO phase. Two regions, where snow depth variations are described by PC1 and PC2, respectively, reveal the border between the opposite recent tendencies of snow accumulation. Copyright © 2007 Royal Meteorological Society [source]

    Modelling current trends in Northern Hemisphere temperatures

    Terence C. Mills
    Abstract Fitting a trend is of interest in many disciplines, but it is of particular importance in climatology, where estimating the current and recent trend in temperature is thought to provide a major indication of the presence of global warming. A range of ad hoc methods of trend fitting have been proposed, with little consensus as to the most appropriate techniques to use. The aim of this paper is to consider a range of trend extraction techniques, none of which require ,padding' out the series beyond the end of the available observations, and to use these to estimate the trend of annual mean Northern Hemisphere (NH) temperatures. A comparison of the trends estimated by these methods thus provides a robust indication of the likely range of current trend temperature increases and hence inform, in a timely quantitative fashion, arguments based on global temperature data concerning the nature and extent of global warming and climate change. For the complete sample 1856,2003, the trend is characterised as having long waves about an underlying increasing level. Since around 1970, all techniques display a pronounced warming trend. However, they also provide a range of trend functions so that extrapolation far into the future would be a hazardous exercise. Copyright © 2006 Royal Meteorological Society. [source]

    In search of simple structures in climate: simplifying EOFs

    A. Hannachi
    Abstract Empirical orthogonal functions (EOFs) are widely used in climate research to identify dominant patterns of variability and to reduce the dimensionality of climate data. EOFs, however, can be difficult to interpret. Rotated empirical orthogonal functions (REOFs) have been proposed as more physical entities with simpler patterns than EOFs. This study presents a new approach for finding climate patterns with simple structures that overcomes the problems encountered with rotation. The method achieves simplicity of the patterns by using the main properties of EOFs and REOFs simultaneously. Orthogonal patterns that maximise variance subject to a constraint that induces a form of simplicity are found. The simplified empirical orthogonal function (SEOF) patterns, being more ,local', are constrained to have zero loadings outside the main centre of action. The method is applied to winter Northern Hemisphere (NH) monthly mean sea level pressure (SLP) reanalyses over the period 1948,2000. The ,simplified' leading patterns of variability are identified and compared to the leading patterns obtained from EOFs and REOFs. Copyright © 2005 Royal Meteorological Society. [source]

    January northern hemisphere circumpolar vortex variability and its relationship with hemispheric temperature and regional teleconnections

    Robert V. Rohli
    Abstract Variability in the hemispheric-scale atmospheric circulation can be directly linked to variations in surface environmental features, such as temperature, precipitation, salinity of water bodies, and pollutant transport. One indicator of the behavior of the hemispheric-scale circulation is the circumpolar vortex (CPV). This research utilizes a geographic information system approach to characterize variability in the Northern Hemispheric (NH) CPV. Specifically, the area, shape, and centroid of the January NHCPV are analyzed for 1959,2001 because it may provide insight about relationships between hemispheric-scale circulation and global temperature change. We also use a new means of characterizing the hemispheric-scale circulation using a ,circularity ratio' (Rc). Results suggest that the January NHCPV has exhibited no long-term trends in area or shape, and that the mean centroid is positioned at approximately 85.3°N, 178.0°W. Regional patterns emerge, which suggest that the area and circularity are associated with variability in surface temperature and moist static energy. Furthermore, the area of the January NHCPV is associated with variability in the Arctic Oscillation, while the shape is tied to variability in the Pacific-North American teleconnection pattern. These results will facilitate understanding of the relationship between hemispheric-scale circulation, regional circulation, and local temperatures. Copyright © 2005 Royal Meteorological Society. [source]

    A need for a simplified approach to venous thromboembolism prophylaxis in acute medical inpatients

    D. P. J. Howard
    Summary Venous thromboembolism (VTE) is a major cause of morbidity and mortality in the UK. Studies have shown that pulmonary embolism causes or contributes to approximately 1 in 10 hospital deaths of medical patients admitted to general hospitals in the UK (Lindblad B, Sternby NH, Bergqvist D. BMJ 1991; 302: 709,11), with pulmonary embolus being the most common preventable cause of hospital death. Thromboprophylaxis is safe, highly effective and cost effective, but despite various current clinical guidelines, physicians fail to prescribe prophylaxis for the majority of medical inpatients at risk of VTE. This article outlines the current evidence for VTE prophylaxis in medical patients and discusses the reasons behind the insufficient use of prophylaxis in the acute medical setting. [source]

    Dementia and depression among nursing home residents in Lebanon: a pilot study

    L. M. Chahine
    Abstract Background The proportion of elderly in the Lebanese population is 7.1% and this is expected to increase to 10.2% by the year 2025. The nursing home (NH) population in Lebanon has not been studied. The aim of this study was to investigate the prevalence of dementia and depression among a portion of nursing home residents (NHR) in Lebanon and describe the characteristics of NHR afflicted with dementia and depression. Method Of 200 NHR from three NH in Lebanon, 117 were selected by random sampling. Data on demographics and medical history were collected. An Arabic version of the Mini-Mental Status Examination and Geriatric Depression Scale (GDS) were administered. Results Our final sample consisted of 102 NHR. Sixty-one (59.8%) had dementia of some kind. Seventeen (27.9%) had mild dementia, 14 (22.9%) had moderate dementia, and 30 (49.2%) had severe dementia. Forty-five (57.7%) of the NHR tested had depression as measured by a GDS score of more than 10. Conclusions Dementia and depression were present in more than half of the NHR in our sample. Our results have important implications, being the first to be collected in the Lebanese community. Screening NHR for dementia and depression on admission and at regular time intervals is a must. More studies targeting all aspects of the elderly population in Lebanon are needed. Copyright © 2006 John Wiley & Sons, Ltd. [source]

    Hydrogen bond of radicals: Interaction of HNO with HCO, HNO, and HOO

    Yong Yang
    Abstract Ab initio quantum mechanics methods are employed to investigate hydrogen bonding interactions between HNO and HCO, HOO radicals, and closed-shell HNO. The systems were calculated at MP2/6-311++G (2d, 2p) level and G2MP2 level. The topological and NBO analysis were investigated the origin of hydrogen bonds red- or blue-shifts. In addition, the comparisons were performed between HNO-opened-shell radical (HCO, HOO) complexes and HNO-corresponding closed-shell molecule (H2CO, HOOH) complexes. It is found that the stabilities of complexes increase from HNO-HCO to HNO-HOO. There are blue-shifts of NH, CH stretching vibrational frequencies and a red-shift of OH stretching vibrational frequency in the complexes. Rehybridization and electron density redistribution contribute to the blue-shifts of CH and NH stretching vibrational frequencies. Compared with the closed-shell H2CO, HCO is weaker proton donor and weaker proton acceptor. For the HOO, it is stronger proton donor and weaker proton acceptor than the HOOH is. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source]

    Hydrogen bonding characterization of XH2NH2···HNO(X = B, Al, Ga) complexes: A theoretical investigation

    Ying Liu
    Abstract The complexes of XH2NH2···HNO(X = B, Al, Ga) are characterized as head to tail with hydrogen bonding interactions. The structural characteristics can be confirmed by atoms in molecules (AIM) analysis, which also provide comparisons of hydrogen bonds strengths. The calculated interaction energies at G2MP2 level show that stability of complexes decrease as BH2NH2···HNO > AlH2NH2···HNO > GaH2NH2···HNO. On the basis of the vibrational frequencies calculations, there are red-shifts for ,(X1H) and blue-shifts for ,(NH) in the complexes on dihydrogen bonding formations (X1H···HN). On hydrogen bonding formations (NH···O), there are red-shifts for ,(NH) compared to the monomers. Natural bond orbital (NBO) analysis is used to discuss the reasons for the ,(X1H) and ,(NH) stretching vibrational shifts by hyperconjugation, electron density redistribution, and rehybridization. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source]