Distinct Depth-Discrete Profiles of Microbial Communities and Geochemical Insights in the Subsurface Critical Zone.

Microbial assembly and metabolic potential in the subsurface critical zone (SCZ) are substantially impacted by subsurface geochemistry and hydrogeology, selecting for microbes distinct from those in surficial soils. In this study, we integrated metagenomics and geochemistry to elucidate how microbial composition and metabolic potential are shaped and impacted by vertical variations in geochemistry and hydrogeology in terrestrial subsurface sediment. A sediment core from an uncontaminated, pristine well at Oak Ridge Field Research Center in Oak Ridge, Tennessee, including the shallow subsurface, vadose zone, capillary fringe, and saturated zone, was used in this study. Our results showed that subsurface microbes were highly localized and that communities were rarely interconnected. Microbial community composition as well as metabolic potential in carbon and nitrogen cycling varied even over short vertical distances. Further analyses indicated a strong depth-related covariation of community composition with a subset of 12 environmental variables. An analysis of dissolved organic carbon (DOC) quality via ultrahigh resolution mass spectrometry suggested that the SCZ was generally a low-carbon environment, with the relative portion of labile DOC decreasing and that of recalcitrant DOC increasing along the depth, selecting microbes from copiotrophs to oligotrophs and also impacting the microbial metabolic potential in the carbon cycle. Our study demonstrates that sediment geochemistry and hydrogeology are vital in the selection of distinct microbial populations and metabolism in the SCZ. IMPORTANCE In this study, we explored the links between geochemical parameters, microbial community structure and metabolic potential across the depth of sediment, including the shallow subsurface, vadose zone, capillary fringe, and saturated zone. Our results revealed that microbes in the terrestrial subsurface can be highly localized, with communities rarely being interconnected along the depth. Overall, our research demonstrates that sediment geochemistry and hydrogeology are vital in the selection of distinct microbial populations and metabolic potential in different depths of subsurface terrestrial sediment. Such studies correlating microbial community analyses and geochemistry analyses, including high resolution mass spectrometry analyses of natural organic carbon, will further the fundamental understanding of microbial ecology and biogeochemistry in subsurface terrestrial ecosystems and will benefit the future development of predictive models on nutrient turnover in these environments.

Natural organic matter composition and nanomaterial surface coating determine the nature of platinum nanomaterial-natural organic matter corona.

Natural organic matter corona (NOM corona) is an interfacial area between nanomaterials (NMs) and the surrounding environment, which gives rise to NMs' unique surface identity. While the importance of the formation of natural organic matter (NOM) corona on engineered nanomaterials (NMs) to NM behavior, fate, and toxicity has been well-established, the understanding of how NOM molecular properties affect NOM corona composition remains elusive due to the complexity and heterogeneity of NOM. This is further complicated by the variation of NOMs from different origins. Here we use eight NOM isolates of different molecular composition and ultrahigh resolution Fourier-transform ion cyclotron resonance-mass spectrometry (ESI-FT-ICR-MS) to determine the molecular composition of platinum NM-NOM corona as a function of NOM composition and NM surface coating. We observed that the composition of PtNM-NOM corona varied with the composition of the original NOM. The percentage of NOM formulas that formed PVP-PtNM-NOM corona was higher than those formed citrate-PtNM-NOM corona, due to increased sorption of NOM formulas, in particular condensed hydrocarbons, to the PVP coating. The relative abundance of heteroatom formulas (CHON, CHOS, and CHOP) was higher in the PVP-PtNM-NOM corona than in citrate-PtNM-corona which was in turn higher than those in the original NOM isolate, indicating preferential partitioning of heteroatom-rich molecules to NM surfaces. The relative abundance of CHO, CHON, CHOS, CHOP and condensed hydrocarbons in PtNM-NOM corona increased with their increase in NOM isolates. Furthermore, PtNM-NOM corona is rich with compounds with high molecular weight. This study demonstrates that the composition and properties of PtNM-NOM corona depend on NOM molecular properties and PtNM surface coating. The results here provide evidence of molecular interactions between NOM and NMs, which are critical to understanding NM colloidal properties (e.g., surface charge and stability), interaction forces (e.g., van der Waals and hydrophobic), environmental behaviors (e.g., aggregation, dissolution, sulfidation, etc.), and biological effects (e.g., uptake, bioaccumulation, and toxicity).

Untargeted metabolomic profiling of Sphagnum fallax reveals novel antimicrobial metabolites.

Sphagnum mosses dominate peatlands by employing harsh ecosystem tactics to prevent vascular plant growth and microbial degradation of these large carbon stores. Knowledge about Sphagnum-produced metabolites, their structure and their function, is important to better understand the mechanisms, underlying this carbon sequestration phenomenon in the face of climate variability. It is currently unclear which compounds are responsible for inhibition of organic matter decomposition and the mechanisms by which this inhibition occurs. Metabolite profiling of Sphagnum fallax was performed using two types of mass spectrometry (MS) systems and 1H nuclear magnetic resonance spectroscopy (1H NMR). Lipidome profiling was performed using LC-MS/MS. A total of 655 metabolites, including one hundred fifty-two lipids, were detected by NMR and LC-MS/MS-329 of which were novel metabolites (31 unknown lipids). Sphagum fallax metabolite profile was composed mainly of acid-like and flavonoid glycoside compounds, that could be acting as potent antimicrobial compounds, allowing Sphagnum to control its environment. Sphagnum fallax metabolite composition comparison against previously known antimicrobial plant metabolites confirmed this trend, with seventeen antimicrobial compounds discovered to be present in Sphagnum fallax, the majority of which were acids and glycosides. Biological activity of these compounds needs to be further tested to confirm antimicrobial qualities. Three fungal metabolites were identified providing insights into fungal colonization that may benefit Sphagnum. Characterizing the metabolite profile of Sphagnum fallax provided a baseline to understand the mechanisms in which Sphagnum fallax acts on its environment, its relation to carbon sequestration in peatlands, and provide key biomarkers to predict peatland C store changes (sequestration, emissions) as climate shifts.

Interdisciplinary Round-Robin Test on Molecular Spectroscopy of the U(VI) Acetate System.

A comprehensive molecular analysis of a simple aqueous complexing system-U(VI) acetate-selected to be independently investigated by various spectroscopic (vibrational, luminescence, X-ray absorption, and nuclear magnetic resonance spectroscopy) and quantum chemical methods was achieved by an international round-robin test (RRT). Twenty laboratories from six different countries with a focus on actinide or geochemical research participated and contributed to this scientific endeavor. The outcomes of this RRT were considered on two levels of complexity: first, within each technical discipline, conformities as well as discrepancies of the results and their sources were evaluated. The raw data from the different experimental approaches were found to be generally consistent. In particular, for complex setups such as accelerator-based X-ray absorption spectroscopy, the agreement between the raw data was high. By contrast, luminescence spectroscopic data turned out to be strongly related to the chosen acquisition parameters. Second, the potentials and limitations of coupling various spectroscopic and theoretical approaches for the comprehensive study of actinide molecular complexes were assessed. Previous spectroscopic data from the literature were revised and the benchmark data on the U(VI) acetate system provided an unambiguous molecular interpretation based on the correlation of spectroscopic and theoretical results. The multimethodologic approach and the conclusions drawn address not only important aspects of actinide spectroscopy but particularly general aspects of modern molecular analytical chemistry.

Online supercritical fluid extraction mass spectrometry (SFE-LC-FTMS) for sensitive characterization of soil organic matter.

We report a novel technical approach for subcritical fluid extraction (SFE) for organic matter characterization in complex matrices such as soil. The custom platform combines on-line SFE with micro-solid phase extraction, nano liquid chromatography (LC), electrospray ionization and Fourier transform mass spectrometry (SFE-LC-FTMS). We demonstrated the utility of SFE-LC-FTMS, including results from both Orbitrap and FTICR MS, for analysis of complex mixtures of organic compounds in a solid matrix by characterizing soil organic matter in peat, a high-carbon soil. For example, in a single experiment, >6000 molecular formulas can be assigned based upon FTICR MS data from 1-50 µL of soil samples (roughly 1-50 mg of soil, dependent on soil density), nearly twice that typically obtained from direct infusion liquid solvent extraction (LSE) from an order of magnitude larger volume of the same soil. The detected species consisted predominately of lipid-like, lignin-like and protein-like compounds, based on their O/C and H/C ratios, with predominantly CHO and CHONP molecular compositions. These results clearly demonstrate that SFE has the potential to effectively extract a variety of molecular species and could become an important member of a suite of extraction methods for studying SOM and other natural organic matter. This is especially true when comprehensive coverage, minimal sample volumes, and high sensitivity are required, or when the presence of organic solvent residue in residual soil is problematic. The SFE based extraction protocol could potentially enable spatially resolved characterization of organic matter in soil with a resolution of ∼1 mm3 to facilitate studies probing the spatial heterogeneity of soil.

Microbial Interactions With Dissolved Organic Matter Drive Carbon Dynamics and Community Succession.

Knowledge of dynamic interactions between natural organic matter (NOM) and microbial communities is critical not only to delineate the routes of NOM degradation/transformation and carbon (C) fluxes, but also to understand microbial community evolution and succession in ecosystems. Yet, these processes in subsurface environments are usually studied independently, and a comprehensive view has been elusive thus far. In this study, we fed sediment-derived dissolved organic matter (DOM) to groundwater microbes and continually analyzed microbial transformation of DOM over a 50-day incubation. To document fine-scale changes in DOM chemistry, we applied high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and soft X-ray absorption spectroscopy (sXAS). We also monitored the trajectory of microbial biomass, community structure and activity over this time period. Together, these analyses provided an unprecedented comprehensive view of interactions between sediment-derived DOM and indigenous subsurface groundwater microbes. Microbial decomposition of labile C in DOM was immediately evident from biomass increase and total organic carbon (TOC) decrease. The change of microbial composition was closely related to DOM turnover: microbial community in early stages of incubation was influenced by relatively labile tannin- and protein-like compounds; while in later stages the community composition evolved to be most correlated with less labile lipid- and lignin-like compounds. These changes in microbial community structure and function, coupled with the contribution of microbial products to DOM pool affected the further transformation of DOM, culminating in stark changes to DOM composition over time. Our study demonstrates a distinct response of microbial communities to biotransformation of DOM, which improves our understanding of coupled interactions between sediment-derived DOM, microbial processes, and community structure in subsurface groundwater.

Formularity: Software for Automated Formula Assignment of Natural and Other Organic Matter from Ultrahigh-Resolution Mass Spectra.

Ultrahigh resolution mass spectrometry, such as Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS), can resolve thousands of molecular ions in complex organic matrices. A Compound Identification Algorithm (CIA) was previously developed for automated elemental formula assignment for natural organic matter (NOM). In this work, we describe software Formularity with a user-friendly interface for CIA function and newly developed search function Isotopic Pattern Algorithm (IPA). While CIA assigns elemental formulas for compounds containing C, H, O, N, S, and P, IPA is capable of assigning formulas for compounds containing other elements. We used halogenated organic compounds (HOC), a chemical class that is ubiquitous in nature as well as anthropogenic systems, as an example to demonstrate the capability of Formularity with IPA. A HOC standard mix was used to evaluate the identification confidence of IPA. Tap water and HOC spike in Suwannee River NOM were used to assess HOC identification in complex environmental samples. Strategies for reconciliation of CIA and IPA assignments were discussed. Software and sample databases with documentation are freely available.

Sequential extraction protocol for organic matter from soils and sediments using high resolution mass spectrometry.

A vast number of organic compounds are present in soil organic matter (SOM) and play an important role in the terrestrial carbon cycle, facilitate interactions between organisms, and represent a sink for atmospheric CO2. The diversity of different SOM compounds and their molecular characteristics is a function of the organic source material and biogeochemical history. By understanding how SOM composition changes with sources and the processes by which it is biogeochemically altered in different terrestrial ecosystems, it may be possible to predict nutrient and carbon cycling, response to system perturbations, and impact of climate change will have on SOM composition. In this study, a sequential chemical extraction procedure was developed to reveal the diversity of organic matter (OM) in different ecosystems and was compared to the previously published protocol using parallel solvent extraction (PSE). We compared six extraction methods using three sample types, peat soil, spruce forest soil and river sediment, so as to select the best method for extracting a representative fraction of organic matter from soils and sediments from a wide range of ecosystems. We estimated the extraction yield of dissolved organic carbon (DOC) by total organic carbon analysis, and measured the composition of extracted OM using high resolution mass spectrometry. This study showed that OM composition depends primarily on soil and sediment characteristics. Two sequential extraction protocols, progressing from polar to non-polar solvents, were found to provide the highest number and diversity of organic compounds extracted from the soil and sediments. Water (H2O) is the first solvent used for both protocols followed by either co-extraction with methanol-chloroform (MeOH-CHCl3) mixture, or acetonitrile (ACN) and CHCl3 sequentially. The sequential extraction protocol developed in this study offers improved sensitivity, and requires less sample compared to the PSE workflow where a new sample is used for each solvent type. Furthermore, a comparison of SOM composition from the different sample types revealed that our sequential protocol allows for ecosystem comparisons based on the diversity of compounds present, which in turn could provide new insights about source and processing of organic compounds in different soil and sediment types.

Unambiguous identification and discovery of bacterial siderophores by direct injection 21 Tesla Fourier transform ion cyclotron resonance mass spectrometry.

Under iron-limiting conditions, bacteria produce low molecular mass Fe(iii) binding molecules known as siderophores to sequester the Fe(iii), along with other elements, increasing their bioavailability. Siderophores are thought to influence iron cycling and biogeochemistry in both marine and terrestrial ecosystems and hence the need for rapid, confident characterization of these compounds has increased. In this study, the type of siderophores produced by two marine bacterial species, Synechococcus sp. PCC 7002 and Vibrio cyclitrophicus 1F53, were characterized by use of a newly developed 21 T Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FTICR MS) with direct injection electrospray ionization. This technique allowed for the rapid detection of synechobactins from Synechococcus sp. PCC 7002 as well as amphibactins from Vibrio cyclitrophicus 1F53 based on high mass accuracy and resolution allowing for observation of specific Fe isotopes and isotopic fine structure enabling highly confident identification of these siderophores. When combined with molecular network analysis two new amphibactins were discovered and verified by tandem MS. These results show that high-field FTICR MS is a powerful technique that will greatly improve the ability to rapidly identify and discover metal binding species in the environment.

Aligning the Measurement of Microbial Diversity with Macroecological Theory.

The number of microbial operational taxonomic units (OTUs) within a community is akin to species richness within plant/animal ("macrobial") systems. A large literature documents OTU richness patterns, drawing comparisons to macrobial theory. There is, however, an unrecognized fundamental disconnect between OTU richness and macrobial theory: OTU richness is commonly estimated on a per-individual basis, while macrobial richness is estimated per-area. Furthermore, the range or extent of sampled environmental conditions can strongly influence a study's outcomes and conclusions, but this is not commonly addressed when studying OTU richness. Here we (i) propose a new sampling approach that estimates OTU richness per-mass of soil, which results in strong support for species energy theory, (ii) use data reduction to show how support for niche conservatism emerges when sampling across a restricted range of environmental conditions, and (iii) show how additional insights into drivers of OTU richness can be generated by combining different sampling methods while simultaneously considering patterns that emerge by restricting the range of environmental conditions. We propose that a more rigorous connection between microbial ecology and macrobial theory can be facilitated by exploring how changes in OTU richness units and environmental extent influence outcomes of data analysis. While fundamental differences between microbial and macrobial systems persist (e.g., species concepts), we suggest that closer attention to units and scale provide tangible and immediate improvements to our understanding of the processes governing OTU richness and how those processes relate to drivers of macrobial species richness.

Deciphering ocean carbon in a changing world.

Dissolved organic matter (DOM) in the oceans is one of the largest pools of reduced carbon on Earth, comparable in size to the atmospheric CO2 reservoir. A vast number of compounds are present in DOM, and they play important roles in all major element cycles, contribute to the storage of atmospheric CO2 in the ocean, support marine ecosystems, and facilitate interactions between organisms. At the heart of the DOM cycle lie molecular-level relationships between the individual compounds in DOM and the members of the ocean microbiome that produce and consume them. In the past, these connections have eluded clear definition because of the sheer numerical complexity of both DOM molecules and microorganisms. Emerging tools in analytical chemistry, microbiology, and informatics are breaking down the barriers to a fuller appreciation of these connections. Here we highlight questions being addressed using recent methodological and technological developments in those fields and consider how these advances are transforming our understanding of some of the most important reactions of the marine carbon cycle.

Advanced solvent based methods for molecular characterization of soil organic matter by high-resolution mass spectrometry.

Soil organic matter (SOM), a complex, heterogeneous mixture of above and belowground plant litter and animal and microbial residues at various degrees of decomposition, is a key reservoir for carbon (C) and nutrient biogeochemical cycling in soil based ecosystems. A limited understanding of the molecular composition of SOM limits the ability to routinely decipher chemical processes within soil and accurately predict how terrestrial carbon fluxes will respond to changing climatic conditions and land use. To elucidate the molecular-level structure of SOM, we selectively extracted a broad range of intact SOM compounds by a combination of different organic solvents from soils with a wide range of C content. Our use of electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) and a suite of solvents with varying polarity significantly expands the inventory of the types of organic molecules present in soils. Specifically, we found that hexane is selective for lipid-like compounds with very low O/C ratios (<0.1); water (H2O) was selective for carbohydrates with high O/C ratios; acetonitrile (ACN) preferentially extracts lignin, condensed structures, and tannin polyphenolic compounds with O/C > 0.5; methanol (MeOH) has higher selectivity toward compounds characterized with low O/C < 0.5; and hexane, MeOH, ACN, and H2O solvents increase the number and types of organic molecules extracted from soil for a broader range of chemically diverse soil types. Our study of SOM molecules by ESI FTICR MS revealed new insight into the molecular-level complexity of organics contained in soils. We present the first comparative study of the molecular composition of SOM from different ecosystems using ultra high-resolution mass spectrometry.

Influence of alkaline co-contaminants on technetium mobility in vadose zone sediments.

Pertechnetate was slowly reduced in a natural, untreated arid sediment under anaerobic conditions (0.02 nmolg(-1)h(-1)), which could occur in low permeability zones in the field, most of which was quickly oxidized. A small portion of the surface Tc may be incorporated into slowly dissolving surface phases, so was not readily oxidized/remobilized into pore water. In contrast, pertechnetate reduction in an anaerobic sediment containing adsorbed ferrous iron as the reductant was rapid (15-600 nmolg(-1)h(-1)), and nearly all (96-98%) was rapidly oxidized/remobilized (2.6-6.8 nmolg(-1)h(-1)) within hours. Tc reduction in an anaerobic sediment containing 0.5-10mM sulfide showed a relatively slow reduction rate (0.01-0.03 nmolg(-1)h(-1)) that was similar to observations in the natural sediment. Pertechnetate infiltration into sediment with a highly alkaline water resulted in rapid reduction (0.07-0.2 nmolg(-1)h(-1)) from ferrous iron released during biotite or magnetite dissolution. Oxidation of NaOH-treated sediments resulted in slow Tc oxidation (∼0.05 nmolg(-1)h(-1)) of a small fraction of the surface Tc (13-23%). The Tc remaining on the surface was Tc(IV) (by XANES), and autoradiography and elemental maps of Tc (by electron microprobe) showed Tc was present associated with specific minerals, rather than being evenly distributed on the surface. Dissolution of quartz, montmorillonite, muscovite, and kaolinite also occurred in the alkaline water, resulting in significant aqueous silica and aluminum. Over time, aluminosilicates, cancrinite, zeolite and sodalite were precipitating. These precipitates may be coating surface Tc(IV) phases, limiting reoxidation.

Identification of fragile microscopic structures during mineral transformations in wet supercritical CO2.

This study examines the nature of highly fragile reaction products that form in low water content supercritical carbon dioxide (scCO2) using a combination of focus ion beam/scanning electron microscopy, confocal Raman spectroscopy, helium ion microscopy (HeIM), and transmission electron microscopy (TEM). HeIM images show these precipitates are fragile rosettes. Using the TEM revealed details on the interfacial structure between the newly formed surface precipitates and the underlying initial solid phases. Detailed microscopy analysis revealed that growth of the precipitates either followed a tip growth mechanism, with precipitates forming directly on the forsterite surface if the initial solid was nonporous (natural forsterite) or growth from the surface of the precipitates, where fluid was conducted through the porous (nanoforsterite) agglomerates to the growth center. Identification of the mechanism of formation of hydrated/hydroxylated magnesium carbonate compound phases is a key factor in unraveling the impact of water recycling on mineral reactivity in low water content scCO2 solutions, which has received a great deal of attention as a result of the potential for CO2 to act as an atmospheric greenhouse gas. Techniques used here to examine these fragile structures are also used to examine a wide range of fragile material surfaces.

Spectral signatures for the classification of microbial species using Raman spectra.

In general, classification-based methods based on confocal Raman microscopy are focused on targeted studies under which the spectral libraries are collected under controlled instrument parameters, which facilitate analyses via standard multivariate data analysis methods and cross-validation. We develop and compare approaches to transform spectra collected at different spectral ranges and varying levels of resolution into a single lower-dimension spectral signature library. This will result in a more robust analysis method able to accommodate spectra accumulated at different times and conditions. We demonstrate these approaches on a relevant test case; the identification of microbial species from a natural environment. The training data were based on samples prepared for three unique species collected at two time points and the test data consisted of blinded unknowns prepared and analyzed at a later date with different instrument parameters. The results indicate that using reduced dimension representations of the spectral signatures improves classification accuracy over basic alignment protocols. In particular, utilizing the microbial species partial least squares discriminant analysis classifier on the blinded samples based on alignment achieved ~78 % accuracy, while both binning and peak selection approaches yielded 100 % accuracy.

Neutron powder diffraction and molecular simulation study of the structural evolution of ammonia borane from 15 to 340 K.

The structural behavior of (11)B-, (2)H-enriched ammonia borane, ND(3)(11)BD(3), over the temperature range from 15 to 340 K was investigated using a combination of neutron powder diffraction and ab initio molecular dynamics simulations. In the low temperature orthorhombic phase, the progressive displacement of the borane group under the amine group was observed leading to the alignment of the B-N bond near parallel to the c-axis. The orthorhombic to tetragonal structural phase transition at 225 K is marked by dramatic change in the dynamics of both the amine and borane group. The resulting hydrogen disorder is problematic to extract from the metrics provided by Rietveld refinement but is readily apparent in molecular dynamics simulation and in difference Fourier transform maps. At the phase transition, Rietveld refinement does indicate a disruption of one of two dihydrogen bonds that link adjacent ammonia borane molecules. Metrics determined by Rietveld refinement are in excellent agreement with those determined from molecular simulation. This study highlights the valuable insights added by coupled experimental and computational studies.

Experimental and computational studies on collective hydrogen dynamics in ammonia borane: incoherent inelastic neutron scattering.

Incoherent inelastic neutron scattering is used to probe the effects of dihydrogen bonding on the vibrational dynamics in the molecular crystal of ammonia borane. The thermal neutron energy loss spectra of (11)B enriched ammonia borane isotopomers ((11)BH(3)NH(3), (11)BD(3)NH(3), and (11)BH(3)ND(3)) are presented and compared to the vibrational power spectrum calculated using ab initio molecular dynamics. A harmonic vibrational analysis on NH(3)BH(3) clusters was also explored to check for consistency with experiment and the power spectrum. The measured neutron spectra and computed ab initio power spectrum compare extremely well (50-500 cm(-1)). Some assignment of modes to simple harmonic motion, e.g., NH(3) and BH(3) torsion in the molecular crystal is possible, and it is confirmed that the lowest modes are dominated by collective motion. We show that the vibrational dynamics as modeled with ab initio molecular dynamics provides a more complete description of anharmonic and collective dynamics in the low frequency region of the inelastic incoherent neutron scattering spectra when compared to the conventional harmonic approach.

Materials for hydrogen storage: structure and dynamics of borane ammonia complex.

The activation energies for rotations in low-temperature orthorhombic ammonia borane were analyzed and characterized in terms of electronic structure theory. The perdeuterated (11)B-enriched ammonia borane, (11)BD(3)ND(3), sample was synthesized, and the structure was refined from neutron powder diffraction data at 175 K. This temperature has been chosen as median of the range of previously reported nuclear magnetic resonance spectroscopy measurements of these rotations. A representative molecular cluster model was assembled from the refined geometry, and the activation energies were calculated and characterized by analysis of the environmental factors that control the rotational dynamics. The barrier for independent NH(3) rotation, E(a) = 12.7 kJ mol(-1), largely depends on the molecular conformational torsion in the solid-state geometry. The barrier for independent BH(3) rotation, E(a) = 38.3 kJ mol(-1), results from the summation of the effect of molecular torsion and large repulsive intermolecular hydrogen-hydrogen interactions. However, a barrier of E(a) = 31.1 kJ mol(-1) was calculated for internally correlated rotation with preserved molecular conformation. Analysis of the barrier heights and the corresponding rotational pathways shows that rotation of the BH(3) group involves strongly correlated rotation of the NH(3) end of the molecule. This observation suggests that the barrier from previously reported measurement of BH(3) rotation corresponds to H(3)B-NH(3) correlated rotation.

Molecular structure and dynamics in the low temperature (orthorhombic) phase of NH3BH3.

Variable temperature 2H NMR experiments on the orthorhombic phase of selectively deuterated NH3BH3 spanning the static to fast exchange limits of the borane and amine motions are reported. New values of the electric field gradient (EFG) tensor parameters have been obtained from the static 2H spectra of V(zz) = 1.652 (+/-0.082) x 10(21) V/m(2) and eta = 0.00 +/- 0.05 for the borane hydrogens and V(zz) = 2.883 (+/-0.144) x 10(21) V/m(2) and eta = 0.00 +/- 0.05 for the amine hydrogens. The molecular symmetry inferred from the observation of equal EFG tensors for the three borane hydrogens and likewise for the three amine hydrogens is in sharp contrast with the C(s) symmetry derived from diffraction studies. The origin of the apparent discrepancy has been investigated using molecular dynamics methods in combination with electronic structure calculations of NMR parameters, bond lengths, and bond angles. The computation of parameters from a statistical ensemble rather than from a single set of atomic Cartesian coordinates gives values that are in close quantitative agreement with the 2H NMR electric field gradient tensor measurements and are more consistent with the molecular symmetry revealed by the NMR spectra.

Spectroscopic studies of the phase transition in ammonia borane: Raman spectroscopy of single crystal NH3BH3 as a function of temperature from 88 to 330 K.

Raman spectra of single crystal ammonia borane, NH3BH3, were recorded as a function of temperature from 88 to 300 K using Raman microscopy and a variable temperature stage. The orthorhombic to orientationally disordered tetragonal phase transition at 225 K was clearly evident from the decrease in the number of vibrational modes. However, some of the modes in the orthorhombic phase appeared to merge 10-12 K below the phase transition perhaps suggesting the presence of an intermediate phase. Factor group analysis of vibrational spectra for both orthorhombic and tetragonal phase is provided. In addition, electronic structure calculations are used to assist in the interpretation and assignment of the normal modes.