Coordination of water to organolead(IV) nitrates: The examples of [PbPh2(NO3)(2)(H2O)(2)] and [PbMe3(NO3)(H2O)]

The compound [PbPh2(NO3)(2)(H2O)(2)] was synthesized and characterized by spectroscopic methods (IR; H-1, C-13 and Pb-207 NMR) and mass spectrometry. An X-ray diffraction study showed that the crystal is a supramolecular tridimensional network of hydrogen-bonded PbPh2(NO3)(2)(H2O)(2) units in which the Pb atom is octacoordinated and adopts a distorted hexagonal bipyramidal geometry, with four O (bidentate nitrate) and two O (water) atoms in equatorial positions and two C-phenyl atoms in axial positions. The crystal of [PbMe3(NO3)(H2O)], obtained as a byproduct in the synthesis of PbMe2(NO3)(2), contains chains of hydrogen-bonded PbMe3(NO3)(H2O) units in which the Pb atom is pentacoordinated with a slightly distorted trigonal bipyramidal environment. In this arrangement the three C-methyl atoms are equatorial and the O atoms from the monodentate nitrate and the water molecule are axial. (C) 2011 Elsevier Ltd. All rights reserved.

Anaerobic oxidation of ethanol over spinel mixed oxides: the first step in the steam-iron process for H2 production

The future hydrogen demand is expected to increase, both in existing industries (including upgrading of fossil fuels or ammonia production) and in new technologies, like fuel cells. Nowadays, hydrogen is obtained predominantly by steam reforming of methane, but it is well known that hydrocarbon based routes result in environmental problems and besides the market is dependent on the availability of this finite resource which is suffering of rapid depletion. Therefore, alternative processes using renewable sources like wind, solar energy and biomass, are now being considered for the production of hydrogen. One of those alternative methods is the so-called “steam-iron process” which consists in the reduction of a metal-oxide by hydrogen-containing feedstock, like ethanol for instance, and then the reduced material is reoxidized with water to produce “clean” hydrogen (water splitting). This kind of thermochemical cycles have been studied before but currently some important facts like the development of more active catalysts, the flexibility of the feedstock (including renewable bio-alcohols) and the fact that the purification of hydrogen could be avoided, have significantly increased the interest for this research topic. With the aim of increasing the understanding of the reactions that govern the steam-iron route to produce hydrogen, it is necessary to go into the molecular level. Spectroscopic methods are an important tool to extract information that could help in the development of more efficient materials and processes. In this research, ethanol was chosen as a reducing fuel and the main goal was to study its interaction with different catalysts having similar structure (spinels), to make a correlation with the composition and the mechanism of the anaerobic oxidation of the ethanol which is the first step of the steam-iron cycle. To accomplish this, diffuse reflectance spectroscopy (DRIFTS) was used to study the surface composition of the catalysts during the adsorption of ethanol and its transformation during the temperature program. Furthermore, mass spectrometry was used to monitor the desorbed products. The set of studied materials include Cu, Co and Ni ferrites which were also characterized by means of X-ray diffraction, surface area measurements, Raman spectroscopy, and temperature programmed reduction.

Spectroscopic study of Bioceramics for Endodontic and Orthopaedics

This thesis was aimed at investigating the physical-chemical properties and the behaviour in physiological environment of two classes of bioceramics: calcium silicate-based dental cements and alumina-based femoral heads for hip joint prostheses. The material characterization was performed using spectroscopic techniques such as that allow to obtain information on the molecular structure of the species and phases present in the analyzed samples. Raman, infrared and fluorescence spectroscopy was principally used. Calcium silicate cements, such as MTA (Mineral Trioxide Aggregate), are hydraulic materials that can set in presence of water: this characteristic makes them suitable for oral surgery and in particular as root-end filling materials. With the aim to improve the properties of commercial MTA cements, several MTA-based experimental formulations have been tested with regard to bioactivity (i.e. apatite forming ability) upon ageing in simulated body fluids. The formation of a bone-like apatite layer may support the integration in bone tissue and represents an essential requirement for osteoconduction and osteoinduction. The spectroscopic studies demonstrated that the experimental materials under study had a good bioactivity and were able to remineralize demineralized dentin. . Bioceramics thanks to their excellent mechanical properties and chemical resistance, are widely used as alternative to polymer (UHMWPE) and metal alloys (Cr-Co) for hip-joint prostesis. In order to investigate the in vivo wear mechanisms of three different generations of commercial bioceramics femoral heads (Biolox®, Biolox® forte, and Biolox® delta), fluorescence and Raman spectroscopy were used to investigate the surface properties and residual stresses of retrieved implants. Spectroscopic results suggested different wear mechanisms in the three sets of retrievals. Since Biolox® delta is a relatively recent material, the Raman results on its retrievals has been reported for the first time allowing to validate the in vitro ageing protocols proposed in the literature to simulate the effects of the in vivo wear.

Studies of pure rotational spectra of isolated molecules and molecular adducts using pulsed jet Fourier transform microwave (PJ-FTM) spectroscopy

This thesis focuses on studying molecular structure and internal dynamics by using pulsed jet Fourier transform microwave (PJ-FTMW) spectroscopy combined with theoretical calculations. Several kinds of interesting chemical problems are investigated by analyzing the MW spectra of the corresponding molecular systems. First, the general aspects of rotational spectroscopy are summarized, and then the basic theory on molecular rotation and experimental method are described briefly. ab initio and density function theory (DFT) calculations that used in this thesis to assist the assignment of rotational spectrum are also included. From chapter 3 to chapter 8, several molecular systems concerning different kind of general chemical problems are presented. In chapter 3, the conformation and internal motions of dimethyl sulfate are reported. The internal rotations of the two methyl groups split each rotational transition into several components line, allowing for the determination of accurate values of the V3 barrier height to internal rotation and of the orientation of the methyl groups with respect to the principal axis system. In chapter 4 and 5, the results concerning two kinds of carboxylic acid bi-molecules, formed via two strong hydrogen bonds, are presented. This kind of adduct is interesting also because a double proton transfer can easily take place, connecting either two equivalent or two non-equivalent molecular conformations. Chapter 6 concerns a medium strong hydrogen bonded molecular complex of alcohol with ether. The dimer of ethanol-dimethylether was chosen as the model system for this purpose. Chapter 7 focuses on weak halogen…H hydrogen bond interaction. The nature of O-H…F and C-H…Cl interaction has been discussed through analyzing the rotational spectra of CH3CHClF/H2O. In chapter 8, two molecular complexes concerning the halogen bond interaction are presented.

Time-resolved surface enhanced resonance Raman spectro-electrochemistry of heme proteins

The membrane protein Cytochrome c Oxidase (CcO) is one of the most important functional bio-molecules. It appears in almost every eukaryotic cell and many bacteria. Although the different species differ in the number of subunits, the functional differences are merely marginal. CcO is the terminal link in the electron transfer pathway of the mitochondrial respiratory chain. Electrons transferred to the catalytic center of the enzyme conduce to the reduction of molecular oxygen to water. Oxygen reduction is coupled to the pumping of protons into the inter-membrane space and hence generates a difference in electrochemical potential of protons across the inner mitochondrial membrane. This potential difference drives the synthesis of adenosine triphosphate (ATP), which is the universal energy carrier within all biological cells. rnrnThe goal of the present work is to contribute to a better understanding of the functional mechanism of CcO by using time-resolved surface enhanced resonance Raman spectroscopy (TR-SERRS). Despite intensive research effort within the last decades, the functional mechanism of CcO is still subject to controversial discussions. It was the primary goal of this dissertation to initiate electron transfer to the redox centers CuA, heme a, heme a3 and CuB electrochemically and to observe the corresponding redox transitions in-situ with a focus on the two heme structures by using SERRS. A measuring cell was developed, which allowed combination of electrochemical excitation with Raman spectroscopy for the purpose of performing the accordant measurements. Cytochrome c was used as a benchmark system to test the new measuring cell and to prove the feasibility of appropriate Raman measurements. In contrast to CcO the heme protein cc contains only a single heme structure. Nevertheless, characteristic Raman bands of the hemes can be observed for both proteins.rnrnIn order to investigate CcO it was immobilized on top of a silver substrate and embedded into an artificial membrane. The catalytic activity of CcO and therefore the complete functional capability of the enzyme within the biomimetic membrane architecture was verified using cyclic voltammetry. Raman spectroscopy was performed using a special nano-structured silver surface, which was developed within the scope of the present work. This new substrate combined two fundamental properties. It facilitated the formation of a protein tethered bilayer lipid membrane (ptBLM) and it allowed obtaining Raman spectra with sufficient high signal-to-noise ratios.rnSpectro-electrochemical investigations showed that at open circuit potential the enzyme exists in a mixed-valence state, with heme a and and heme a3 in the reduced and oxidized state, respectively. This was considered as an intermediate state between the non-activated and the fully activated state of CcO. Time-resolved SERRS measurements revealed that a hampered electron transfer to the redox center heme a3 characterizes this intermediate state.rn

Non-Covalent Interaction: Revealed by Rotational Spectroscopy

The pulsed jet Fourier transform microwave spectroscopy have been applied to several molecular complexes involving H2O, freons, methane, carboxylic acids, and rare gas. The obtained results showcase the suitability of this technique for studying the intermolecular interactions. The rotational spectra of three water adducts of halogenated organic molecules, i.e. chlorotrifluoroethylene, isoflurane and alfa,alfa,alfa,-trifluoroanisole, have been investigated. It has been found that, the halogenation of the partner molecules definitely changes the way in which water will link to the partner molecule. Quadrupole hyperfine structures and/or the tunneling splittings have been observed in the rotational spectra of difluoromethane-dichloromethane, chlorotrifluorometane-fluoromethane, difluoromethane-formaldehyde and trifluoromethane-benzene. These features have been useful to describe their intermolecular interactions (weak hydrogen bonds or halogen bonds), and to size the potential energy surfaces of their internal motions. The rotational spectrum of pyridine-methane pointed out that methane prefers to locate above the ring and link to pyridine through a C-H•••π weak hydrogen bond, rather than the C-H•••n interaction. This behavior, typical of complexes of pyridine with rare gases, suggests classifying CH4, in relation to its ability to form molecular complexes with aromatic molecules, as a pseudo rare gas. The conformational equilibria of three bi-molecules of carboxylic acids, acrylic acid-trifluoroacetic acid, difluoroacetic acid-formic acid and acrylic acid-fluoroacetic acid have been studied. The increase of the hydrogen bond length upon H→D isotopic substitution (Ubbelohde effect) has been deduced from the elongation of the carboxylic carbons C•••C distance. The van der Waals complex tetrahydrofuran-krypton shows that the systematic doubling of the rotational lines has been attributed to the residual pseudo-rotation of tetrahydrofuran in the complex, based on the values of the Coriolis coupling constants, and on the type (mu_b) of the interstate transitions.

Crystal chemistry and hydrogen bonding of rustumite Ca10(Si2O7)2(SiO4)(OH)2Cl2 with variable OH, Cl, F

Three samples of the skarn mineral rustumite Ca10(Si2O7)2(SiO4)(OH)2Cl2, space group C2/c, a ≈7.6, b ≈ 18.5, c ≈ 15.5 Å, β ≈ 104°, with variable OH, Cl, F content were investigated by electron microprobe, single-crystal X-ray structure refinements, and Raman spectroscopy. “Rust1LCl” is a low chlorine rustumite Ca10(Si2O7)2(SiO4)(OH1.88F0.12)(Cl1.28,OH0.72) from skarns associated with the Rize batholith near Ikizedere, Turkey. “Rust2F” is a F-bearing rustumite Ca10(Si2O7)2(SiO4)(OH1.13F0.87) (Cl1 96OH0.04) from xenoliths in ignimbrites of the Upper Chegem Caldera, Northern Caucasus, Russia. “Rust3LClF” represents a low-Cl, F-bearing rustumite Ca10(Si2O7)2(SiO4)0.87(H4O4)0.13(OH1.01F0.99) (Cl1.00 OH1.00) from altered merwinite skarns of the Birkhin massif, Baikal Lake area, Eastern Siberia, Russia. Rustumite from Birkhin massif is characterized by a significant hydrogarnet-like or fluorine substitution at the apices of the orthosilicate group, leading to specific atomic displacements. The crystal structures including hydrogen positions have been refined from single-crystal X-ray data to R1 = 0.0205 (Rust1_LCl), R1 = 0.0295 (Rust2_F), and R1 = 0.0243 (Rust3_LCl_F), respectively. Depletion in Cl and replacement by OH is associated with smaller unit-cell dimensions. The substitution of OH by F leads to shorter hydrogen bonds O-H⋯F instead of O-H⋯OH. Raman spectra for all samples have been measured and confirm slight strengthening of the hydrogen bonds with uptake of F.This study discusses the complex crystal chemistry of the skarn mineral rustumite and may provide a wider understanding of the chemical reactions related to contact metamorphism of limestones.

Oxygen isotopes and hydrocarbon composition of gas hydrate and hydrate water from ODP Leg 165 sites

Ocean Drilling Program (ODP) Leg 164 recovered a number of large solid gas hydrate from Sites 994, 996, and 997 on the Blake Ridge. Sites 994 and 997 samples, either nodular or thick massive pieces, were subjected to laboratory analysis and measurements to determine the structure, molecular and isotopic composition, thermal conductivity, and equilibrium dissociation conditions. X-ray computed tomography (CT) imagery, X-ray diffraction, nuclear magnetic resonance (NMR), and Raman spectroscopy have revealed that the gas hydrates recovered from the Blake Ridge are nearly 100% methane gas hydrate of Structure I, cubic with a lattice constant of a = 11.95 ± 0.05 angström, and a molar ratio of water to gas (hydration number) of 6.2. The d18O of water is 2.67 per mil to 3.51 per mil SMOW, which is 3.5-4.0 heavier than the ambient interstitial waters. The d13C and dD of methane are -66 per mil to -70 per mil and -201 per mil to -206 per mil, respectively, suggesting that the methane was generated through bacterial CO2 reduction. Thermal conductivity values of the Blake Ridge hydrates range from 0.3 to 0.5 W/(m K). Equilibrium dissociation experiments indicate that the three-phase equilibrium for the specimen is 3.27 MPa at 274.7 K. This is almost identical to that of synthetic pure methane hydrate in freshwater.

Direct measurement of hydrogen bonding in DNA nucleotide bases by atomic force microscopy.

We have used self-assembled purines and pyrimidines on planar gold surfaces and on gold-coated atomic force microscope (AFM) tips to directly probe intermolecular hydrogen bonds. Electron spectroscopy for chemical analysis (ESCA) and thermal programmed desorption (TPD) measurements of the molecular layers suggested monolayer coverage and a desorption energy of about 25 kcal/mol. Experiments were performed under water, with all four DNA bases immobilized on AFM tips and flat surfaces. Directional hydrogen-bonding interaction between the tip molecules and the surface molecules could be measured only when opposite base-pair coatings were used. The directional interactions were inhibited by excess nucleotide base in solution. Nondirectional van der Waals forces were present in all other cases. Forces as low as two interacting base pairs have been measured. With coated AFM tips, surface chemistry-sensitive recognition atomic force microscopy can be performed.

Drying process of microcrystalline cellulose studied by attenuated total reflection IR spectroscopy with two-dimensional correlation spectroscopy and principal component analysis

Molecular interactions between microcrystalline cellulose (MCC) and water were investigated by attenuated total reflection infrared (ATR/IR) spectroscopy. Moisture-content-dependent IR spectra during a drying process of wet MCC were measured. In order to distinguish overlapping O–H stretching bands arising from both cellulose and water, principal component analysis (PCA) and, generalized two-dimensional correlation spectroscopy (2DCOS) and second derivative analysis were applied to the obtained spectra. Four typical drying stages were clearly separated by PCA, and spectral variations in each stage were analyzed by 2DCOS. In the drying time range of 0–41 min, a decrease in the broad band around 3390 cm−1 was observed, indicating that bulk water was evaporated. In the drying time range of 49–195 min, decreases in the bands at 3412, 3344 and 3286 cm−1 assigned to the O6H6cdots, three dots, centeredO3′ interchain hydrogen bonds (H-bonds), the O3H3cdots, three dots, centeredO5 intrachain H-bonds and the H-bonds in Iβ phase in MCC, respectively, were observed. The result of the second derivative analysis suggests that water molecules mainly interact with the O6H6cdots, three dots, centeredO3′ interchain H-bonds. Thus, the H-bonding network in MCC is stabilized by H-bonds between OH groups constructing O6H6cdots, three dots, centeredO3′ interchain H-bonds and water, and the removal of the water molecules induces changes in the H-bonding network in MCC.

Zinc deposits and related mineralization of the Burketown mineral field, including the world-class Century Deposit, Northern Australia: Fluid inclusion and stable isotope evidence for basin fluid sources

The stratiform Century Zn-Pb deposit and the discordant Zn-Pb lode deposits of the Burketown mineral field, northern Australia, host ore and gangue minerals with primary fluid inclusions that have not been affected by the Isan orogeny, thus providing a unique opportunity to investigate the nature of the ore-forming brines. All of the deposits are hosted in shales and siltstones belonging to the Isa superbasin and comprise sphalerite, pyrite, carbonate, quartz, galena, minor chalcopyrite, and minor illite. According to Pb model ages, the main ore stage of mineralization at Century formed at I575 Ma, some 20 m.y. after deposition of the host shale sequence. Microthermometry on undeformed, primary fluid inclusions hosted in porous sphalerite shows that the Zn at Century was transported to the deposit by a homogeneous, Ca2+- and Na+-bearing brine with a salinity of 21.6 wt percent NaCl equiv. delta D-fluid of the fluid inclusion water ranges from -89 to -83 per mil, consistent with a basinal brine that evolved from meteoric water. Fluid inclusion homogenization temperatures range between 74 degrees and 125 degrees C, which are lower than the 120 degrees to 160 degrees C range calculated from vitrinite reflectance and illite crystallinity data from the deposit. This discrepancy indicates that mineralization likely formed at 50 to 85 Mpa, corresponding to a depth of 1,900 to 3,100 m. Transgressive galena-sphalerite veins that cut stratiform mineralization at Century and breccia-filled quartz-dolomite-sphalerite-galena veins in the discordant Zn-Pb lodes have Pb model ages between 1575 and 1485 Ma. Raman spectroscopy and microthermometry reveal that the primary fluid inclusions in these veins contain Ca2+, Na+. but they have lower salinities between 23 and 10 wt percent NaCl equiv and higher delta D-fluid values ranging from -89 to -61 per mil than fluid inclusions in porous sphalerite from Century. Fluid inclusion water from sphalerite in one of the lode deposits has delta O-18(fluid) values of 1.6 and 2.4 per mil, indistinguishable from delta O-18(fluid) values between -0.3 to +7.4 per mil calculated from the isotopic composition of coexisting quartz, dolomite, and illite. The trend toward lower salinities and higher delta D-fluid values relative to the earlier mineralizing fluids is attributed to mixing between the fluid that formed Century and a seawater-derived fluid from a different source. Based on seismic data from the Lawn Hill platform and paragenetic and geochemical results from the Leichhardt River fault trough to the south, diagenetic aquifers in the Underlying Calvert superbasin appear to have been the most likely sources for the fluids that formed Century and the discordant lode deposits. Paragenetically late sphalerite and calcite cut sphalerite, quartz, and dolomite in the lode deposits and contain Na+-dominated fluid inclusions with much lower salinities than their older counterparts. The isotopic composition of calcite also indicates delta O-18(fluid) from 3.3 to 10.7 per mil, which is larger than the range obtained from synmineralization minerals, supporting the idea that a unique fluid source was involved. The absolute timing of this event is unclear, but a plethora of Pb model, K-Ar, and Ar-40/Ar-39 ages between 1440 and 1300 Ma indicate that a significant volume of fluid was mobilized at this time. The deposition of the Roper superbasin from ca. 1492 +/- 4 Ma suggests that these late veins formed from fluids that may have been derived from aquifers in overlying sediments of the Roper superbasin. Clear, buck, and drusy quartz in veins unrelated to any form of Pb-Zn mineralization record the last major fluid event in the Burketown mineral field and form distinct outcrops and ridges in the district. Fluid inclusions in these veins indicate formation from a low-salinity, 300 degrees +/- 80 degrees C fluid. Temperatures approaching 300 degrees C recorded in organic matter adjacent to faults and at sequence boundaries correspond to K-Ar ages spanning 1300 to 1100 Ma, which coincides with regional hydrothermal activity in the northern Lawn Hill platform and the emplacement of the Lakeview Dolerite at the time of assemblage of the Rodinia supercontinent.

Peptide-based low molecular weight gelators for the preparation of self-assembled materials

Low molecular weight gelators (LMWGs) based on pseudo-peptides are here studied for the preparation of supramolecular materials. These compounds can self-assemble through non-covalent interactions such as hydrogen bonds and π-π stacking, forming fibres and gels. A wide variety of materials can be prepared starting from these building blocks, which can be tuned and functionalised depending on the application. In this work, derivatives of the three aromatic amino acids L-Phenylalanine, L-Tyrosine and L-DOPA (3,4-dihydroxiphenylalanine) were synthesised and tested as gelators for water or organic solvents. First, the optimal gelating conditions were studied for each compound, varying concentration, solvent and trigger. Then the materials were characterised in terms of mechanical properties and morphology. Water remediation from dye pollution was the first focus of this work. Organogels were studied as absorbent of dyes from contaminated water. Hydrogels functionalised with TiO2 nanoparticles and graphene platelets were proposed as efficient materials for the photo-degradation of dyes. An efficient method for the incorporation of graphene inside hydrogels using the gelator itself as dispersant was proposed. In these materials a high storage modulus coexists with good self-healing and biocompatibility. The incorporation of a mineral phase inside the gel matrix was then investigated, leading to the preparation of composite organic/inorganic materials. In a first study, the growth of calcium carbonate crystals was achieved inside the hydrogel, which preserved its structure after crystal formation. Then the self-assembled fibres made of LMWGs were used for the first time instead of the polymeric ones as reinforcement inside calcium phosphate cements (CPCs) for bone regeneration. Gel-to-crystal transitions occurring with time in a metastable gel were also examined. The formation of organic crystals in gels can be achieved in multicomponent systems, in which a second gelator constitutes the independent gel network. Finally, some compounds unable to gelate were tested as underwater adhesives.

Non-ambient solid-state characterization of crystalline molecular organic semiconductors

The research project is focused on the investigation of the polymorphism of crystalline molecular material for organic semiconductor applications under non-ambient conditions, and the solid-state characterization and crystal structure determination of the different polymorphic forms. In particular, this research project has tackled the investigation and characterization of the polymorphism of perylene diimides (PDIs) derivatives at high temperatures and pressures, in particular N,N’-dialkyl-3,4,9,10-perylendiimide (PDI-Cn, with n = 5, 6, 7, 8). These molecules are characterized by excellent chemical, thermal, and photostability, high electron affinity, strong absorption in the visible region, low LUMO energies, good air stability, and good charge transport properties, which can be tuned via functionalization; these features make them promising n-type organic semiconductor materials for several applications such as OFETs, OPV cells, laser dye, sensors, bioimaging, etc. The thermal characterization of PDI-Cn was carried out by a combination of differential scanning calorimetry, variable temperature X-ray diffraction, hot-stage microscopy, and in the case of PDI-C5 also variable temperature Raman spectroscopy. Whereas crystal structure determination was carried out by both Single Crystal and Powder X-ray diffraction. Moreover, high-pressure polymorphism via pressure-dependent UV-Vis absorption spectroscopy and high-pressure Single Crystal X-ray diffraction was carried out in this project. A data-driven approach based on a combination of self-organizing maps (SOM) and principal component analysis (PCA) is also reported was used to classify different π-stacking arrangements of PDI derivatives into families of similar crystal packing. Besides the main project, in the framework of structure-property analysis under non-ambient conditions, the structural investigation of the water loss in Pt- and Pd- based vapochromic potassium/lithium salts upon temperature, and the investigation of structure-mechanical property relationships in polymorphs of a thienopyrrolyldione endcapped oligothiophene (C4-NT3N) are reported.

Diffusion anomaly and dynamic transitions in the Bell-Lavis water model

In this paper we investigate the dynamic properties of the minimal Bell-Lavis (BL) water model and their relation to the thermodynamic anomalies. The BL model is defined on a triangular lattice in which water molecules are represented by particles with three symmetric bonding arms interacting through van der Waals and hydrogen bonds. We have studied the model diffusivity in different regions of the phase diagram through Monte Carlo simulations. Our results show that the model displays a region of anomalous diffusion which lies inside the region of anomalous density, englobed by the line of temperatures of maximum density. Further, we have found that the diffusivity undergoes a dynamic transition which may be classified as fragile-to-strong transition at the critical line only at low pressures. At higher densities, no dynamic transition is seen on crossing the critical line. Thus evidence from this study is that relation of dynamic transitions to criticality may be discarded. (C) 2010 American Institute of Physics. [doi:10.1063/1.3479001]

The isotropic nuclear magnetic shielding constants of acetone in supercritical water: A sequential Monte Carlo/quantum mechanics study including solute polarization

The nuclear isotropic shielding constants sigma((17)O) and sigma((13)C) of the carbonyl bond of acetone in water at supercritical (P=340.2 atm and T=673 K) and normal water conditions have been studied theoretically using Monte Carlo simulation and quantum mechanics calculations based on the B3LYP/6-311++G(2d,2p) method. Statistically uncorrelated configurations have been obtained from Monte Carlo simulations with unpolarized and in-solution polarized solute. The results show that solvent effects on the shielding constants have a significant contribution of the electrostatic interactions and that quantitative estimates for solvent shifts of shielding constants can be obtained modeling the water molecules by point charges (electrostatic embedding). In supercritical water, there is a decrease in the magnitude of sigma((13)C) but a sizable increase in the magnitude of sigma((17)O) when compared with the results obtained in normal water. It is found that the influence of the solute polarization is mild in the supercritical regime but it is particularly important for sigma((17)O) in normal water and its shielding effect reflects the increase in the average number of hydrogen bonds between acetone and water. Changing the solvent environment from normal to supercritical water condition, the B3LYP/6-311++G(2d,2p) calculations on the statistically uncorrelated configurations sampled from the Monte Carlo simulation give a (13)C chemical shift of 11.7 +/- 0.6 ppm for polarized acetone in good agreement with the experimentally inferred result of 9-11 ppm. (C) 2008 American Institute of Physics.