A combined NMR crystallographic and PXRD investigation of the structure-directing role of water molecules in orotic acid and its lithium and magnesium salts

Despite the abundance of hydrates, their multifaceted nature and hydration/dehydration behaviour is still not fully understood. For the example of orotic acid monohydrate and its lithium and magnesium hydrate salts, we show how NMR crystallography, namely a combination of solid-state NMR with a focus here on 1H Magic Angle Spinning (MAS) NMR experiments and first-principles DFT GIPAW (gauge-including projector augmented wave) calculations, can play a valuable role in the characterization of hydrate systems. Starting from lithium orotate monohydrate, a rigid system with a limited number of tightly bound water molecules, the general feasibility of this approach was demonstrated. Moving onto more complex hydrate structures, mobility in the orotic acid monohydrate was observed, while for the most complex hydrate, magnesium orotate octahydrate, a loss of associated water molecules was observed after an overnight MAS NMR experiment. A combined study by experimental MAS NMR, powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA) revealed changes after vacuum drying as well as after storage of a vacuum dried sample under ambient conditions. Specifically, TGA showed the vacuum dried sample to correspond to a dihydrate, for which no structure has yet been determined by single-crystal diffraction. An NMR crystallography analysis showed that a combination of putative symmetric and asymmetric dihydrate structures explains the observed changes in the experimental MAS NMR spectra

Nighttime chemistry at a high altitude site above Hong Kong

Nighttime reactions of nitrogen oxides influence ozone, volatile organic compounds, and aerosol and are thus important to the understanding of regional air quality. Despite large emissions and rapid recent growth of nitrogen oxide concentrations, there are few studies of nighttime chemistry in China. Here we present measurements of nighttime nitrogen oxides, NO3 and N2O5, from a coastal mountaintop site in Hong Kong adjacent to the megacities of the Pearl River Delta region. This is the first study of nighttime chemistry from a site within the residual layer in China. Key findings include the following. First, highly concentrated urban NOx outflow from the Pearl River Delta region was sampled infrequently at night, with N2O5 mixing ratios up to 8 ppbv (1 min average) or 12 ppbv (1 s average) in nighttime aged air masses. Second, the average N2O5 uptake coefficient was determined from a best fit to the available steady state lifetime data as γ(N2O5) = 0.014 ± 0.007. Although this determination is uncertain due to the difficulty of separating N2O5 losses from those of NO3, this value is in the range of previous residual layer determinations of N2O5 uptake coefficients in polluted air in North America. Third, there was a significant contribution of biogenic hydrocarbons to NO3 loss inferred from canister samples taken during daytime. Finally, daytime N2O5 mixing ratios were in accord with their predicted photochemical steady state. Heterogeneous uptake of N2O5 in fog is determined to be an important production mechanism for soluble nitrate, even during daytime. Key Points Large (up to 12 ppbv N2O5) but infrequent nocturnal NOx outflow from the Pearl River Delta Average N2O5 uptake coefficients 0.014 ± 0.007, in line with residual layer measurements in the U.S. Daytime N2O5 follows predicted steady state but rapidly produces soluble nitrate in fog.Department of Civil and Environmental Engineerin

Heterolytic H_2 Activation Mediated by Low-Coordinate L_3Fe-(µ-N)-FeL_3 Complexes to Generate Fe(µ-NH)(µ-H)Fe Species

The diiron μ-nitride complexes, {L_3Fe^(II)(μ-N)Fe^(II)L_3}- and L_3Fe^(III)(μ-N)Fe^(II)L_3, heterolytically activate hydrogen (1 atm) at ambient temperature in solution (L_3 = [PhB(CH_2PPh_2)_3]-). These transformations lead to structurally unique {L_3Fe^(II)(μ-NH)(μ-H)Fe^(II)L^3}- and L_3Fe^(III)(μ-NH)(μ-H)Fe^(II)L_3 products. X-ray data establish a marked reduction in the Fe−Fe distance upon H_2 uptake, and spectroscopic data establish both Fe^(II)Fe^(II) species to be diamagnetic, whereas the Fe^(III)Fe^(II) species, L_3Fe^(III)(μ-N)Fe^(II)L_3 and L_3Fe^(III)(μ-NH)(μ-H)Fe^(II)L_3, populate doublet ground states with thermally accessible higher spin states

Molecular architecture of softwood revealed by solid-state NMR

Economically important softwood from conifers is mainly composed of the polysaccharides cellulose, galactoglucomannan and xylan, and the phenolic polymer, lignin. The interactions between these polymers lead to wood mechanical strength and must be overcome in biorefining. Here, we use 13C multidimensional solid-state NMR to analyse the polymer interactions in never-dried cell walls of the softwood, spruce. In contrast to some earlier softwood cell wall models, most of the xylan binds to cellulose in the two-fold screw conformation. Moreover, galactoglucomannan alters its conformation by intimately binding to the surface of cellulose microfibrils in a semi-crystalline fashion. Some galactoglucomannan and xylan bind to the same cellulose microfibrils, and lignin is associated with both of these cellulose-bound polysaccharides. We propose a model of softwood molecular architecture which explains the origin of the different cellulose environments observed in the NMR experiments. Our model will assist strategies for improving wood usage in a sustainable bioeconomy

Draft Genome Sequence for Desulfovibrio africanus Strain PCS.

Desulfovibrio africanus strain PCS is an anaerobic sulfate-reducing bacterium (SRB) isolated from sediment from Paleta Creek, San Diego, CA. Strain PCS is capable of reducing metals such as Fe(III) and Cr(VI), has a cell cycle, and is predicted to produce methylmercury. We present the D. africanus PCS genome sequence

The scenes and spaces of anxiety: Embodied expressions of distress in public and private fora

Psychological treatments of mental health issues have acquired a justifiable notoriety for their tendency to engage in generalisation and reductionism. By contrast, the emergent geographies of exclusion make visible the fine-grain material and spatial contours of the lives of individuals who experience mental health difficulties and distress. However, this can come at the cost of a relative neglect of the psychological. In this paper we propose a set of concepts for facilitating the study of intersecting planes of experience, which demonstrates the interdependency of the spatial, the psychological and the technological. Drawing on empirical work with participants who live with persistent anxiety, we demonstrate how online support networks mediate – that is transduct, intersect and transform – how experiences of anxiety are lived out. Attention to endogenous ‘tactics’ or ‘modes of normativity’ provides an interesting agenda for the emergent engagement of social psychology with social/cultural geography

The use of a selective saturation pulse to suppress t1 noise in two-dimensional 1H fast magic angle spinning solid-state NMR spectroscopy

A selective saturation pulse at fast magic angle spinning (MAS) frequencies (60+ kHz) suppresses t1 noise in the indirect dimension of two-dimensional 1H MAS NMR spectra. The method is applied to a synthetic nucleoside with an intense methyl 1H signal due to triisopropylsilyl (TIPS) protecting groups. Enhanced performance in terms of suppressing the methyl signal while minimising the loss of signal intensity of nearby resonances of interest relies on reducing spin diffusion – this is quantified by comparing two-dimensional 1H NOESY-like spin diffusion spectra recorded at 30 to 70 kHz MAS. For a saturation pulse centred at the methyl resonance, the effect of changing the nutation frequency at different MAS frequencies as well as the effect of changing the pulse duration is investigated. By applying a pulse of duration 30 ms and nutation frequency 725 Hz at 70 kHz MAS, a good compromise of significant suppression of the methyl resonance combined with the signal intensity of resonances greater than 5 ppm away from the methyl resonance being largely unaffected is achieved. The effectiveness of using a selective saturation pulse is demonstrated for both homonuclear 1H – 1H double quantum (DQ)/ single quantum (SQ) MAS and 14N – 1H heteronuclear multiple quantum coherence (HMQC) two-dimensional solid-state NMR experiments

Observations of nitryl chloride and modeling its source and effect on ozone in the planetary boundary layer of southern China

Nitryl chloride (ClNO2) plays potentially important roles in atmospheric chemistry, but its abundance and effect are not fully understood due to the small number of ambient observations of ClNO2 to date. In late autumn 2013, ClNO2 was measured with a chemical ionization mass spectrometer (CIMS) at a mountain top (957 m above sea level) in Hong Kong. During 12 nights with continuous CIMS data, elevated mixing ratios of ClNO2 (>400 parts per trillion by volume) or its precursor N2O5 (>1000 pptv) were observed on six nights, with the highest ever reported ClNO2 (4.7 ppbv, 1 min average) and N2O5 (7.7 ppbv, 1 min average) in one case. Backward particle dispersion calculations driven by winds simulated with a mesoscale meteorological model show that the ClNO2/N2O5-laden air at the high-elevation site was due to transport of urban/industrial pollution north of the site. The highest ClNO2/N2O5 case was observed in a later period of the night and was characterized with extensively processed air and with the presence of nonoceanic chloride. A chemical box model with detailed chlorine chemistry was used to assess the possible impact of the ClNO2 in the well-processed regional plume on next day ozone, as the air mass continued to downwind locations. The results show that the ClNO2 could enhance ozone by 5-16% at the ozone peak or 11-41% daytime ozone production in the following day. This study highlights varying importance of the ClNO2 chemistry in polluted environments and the need to consider this process in photochemical models for prediction of ground-level ozone and haze. Key Points First observation of ClNO2 in the planetary boundary layer of China Combined high-resolution meteorological and measurement-constrained chemical models in data analysis ClNO2 enhances daytime ozone peak by 5-16% in well-processed PRD air.Department of Civil and Environmental Engineerin

Probing intermolecular interactions in a diethylcarbamazine citrate salt by fast MAS 1 H solid-state NMR spectroscopy and GIPAW calculations

Fast magic-angle spinning (MAS) NMR is used to probe intermolecular interactions in a diethylcarbamazine salt, that is widely used as a treatment against adult worms of Wuchereria bancrofti which cause a common disease in tropical countries named filariasis. Specifically, a dihydrogen citrate salt that has improved thermal stability and solubility as compared to the free form is studied. One-dimensional 1H, 13C and 15N and two-dimensional 1H-13C and 14N-1H heteronuclear correlation NMR experiments under moderate and fast MAS together with GIPAW (CASTEP) calculations enable the assignment of the 1H, 13C and 14N/15N resonances. A two-dimensional 1H-1H double-quantum (DQ) –single-quantum (SQ) MAS spectrum recorded with BaBa recoupling at 60 kHz MAS identifies specific proton-proton proximities associated with citrate-citrate and citrate-diethylcarbamazine intermolecular interactions