The structure of acetonitrile−water mixtures has been investigated by X-ray diffraction with an imaging
plate detector and IR spectroscopy over a wide range of acetonitrile mole fractions (0.0 ≤ X
AN ≤ 1.0). Reichardt
E
T
N and Sone-Fukuda D
II,I values were also measured for the mixtures. It has been found from the X-ray
data that in pure acetonitrile an acetonitrile molecule interacts with two nearest neighbors by antiparallel
dipole−dipole interaction together with a small shift of the two molecular centers and that two acetonitrile
molecules in the second-neighbor shell interact with a central molecule through parallel dipole−dipole
interaction. Thus, acetonitrile molecules are alternately aligned to form a zigzag cluster. On addition of
water into pure acetonitrile, water molecules interact with acetonitrile molecules through a dipole−dipole
interaction in an antiparallel orientation. The IR spectra of O−D and C⋮N stretching vibrations, observed
for mixtures of acetonitrile AN and water containing 20% D2O, suggested that hydrogen bonds are also
formed between acetonitrile and water molecules in the mixtures at X
AN ≤ 0.8. The average numbers of the
first- and second-neighbor acetonitrile molecules gradually increase with increasing water content with an
almost constant first-neighbor distance and slightly decreased second-neighbor ones. Thus, acetonitrile
molecules are assembled to form three-dimensionally expanded clusters; the acetonitrile clusters are surrounded
by water molecules through both hydrogen bonding and dipole−dipole interaction. The X-ray radial distribution
functions and IR spectra suggest that the hydrogen bond network of water is enhanced in the mixtures at X
AN
< 0.6. The concentration dependence of E
T
N and D
II,I values determined reflects well the above-mentioned
behavior of water molecules in the mixtures. These findings suggest that both water and acetonitrile clusters
coexist in the mixtures in the range of 0.2 ≤ X
AN < 0.6, i.e., “microheterogeneity” occurs in the acetonitrile−water mixtures.
The structure of water in the liquid and supercritical states has been investigated with a newly developed rapid liquid and amorphous x-ray diffractometer using an imaging plate area detector. This new method has enabled us to reduce the measuring time to only one hour for each sample, which is less by a factor of about 100 than the time usually needed with a conventional θ–θ type diffractometer, and thus to measure x-ray scatterings of water at high temperatures and pressures, including supercritical state. In this study the temperature range of 300–649 K with pressures of 0.1–98.1 MPa was covered (Tc=647.3 K, Pc=22.12 MPa, ρc=0.322 g/cm3 for water). Densities of sample water were kept constant at 1.0, 0.95, 0.9, 0.8, and 0.7 g/cm3 by controlling temperature and pressure. The radial distribution functions (RDFs) have shown that the peaks for the second and further neighbors interactions disappear over 416 K and 0.95 g/cm3, showing the breakdown of local tetrahedral icelike structure in water. The analysis of the first peak of the RDFs has revealed that with increasing temperature the coordination number of the first neighbor interaction around 2.9 Å decreases from 3.1 at 300 K and 1.0 g/cm3 to 1.6 at 649 K and 0.7 g/cm3, whereas the interaction around 3.4 Å increases from 1.3 to 2.3 at the corresponding temperatures, resulting in a constant coordination number of around four in the first shell under the nearly constant densities. These findings are discussed with the recent results of computer simulation, neutron scattering, and Raman spectroscopic studies on water at high temperatures and pressures.
Extended x-ray absorption fine structure (EXAFS) measurements were performed for concentrated aqueous rare earth perchlorate solutions (R=28; R is the moles of water per mole of salt) in the liquid state at room temperature and in the glassy state at liquid nitrogen temperature. The quantitative analysis of the EXAFS data has revealed that the hydration number changes from about nine for light rare earth ions to about eight for heavy rare earth ions through the intermediate ions of Sm3+ ∼Eu3+ in both liquid and glassy states. The average Ln3+ –OH2 distances were determined and they are in agreement with previously reported values from x-ray and neutron diffraction. The Debye–Waller factor of the average Ln3+ –OH2 bonds for the light rare earth ions was larger than that for the heavy ions, suggesting that the hydration shell of the light rare earth ions is statically disordered, consisting of different Ln3+ –OH2 bonds.
Thermal properties, structure, and dynamics of
supercooled water in porous silica of two different pore
sizes
(30 and 100 Å) have been investigated over a temperature range from
298 down to 193 K by differential
scanning calorimetry (DSC), neutron diffraction, neutron quasi-elastic
scattering, and proton NMR relaxation
methods. Cooling curves by DSC showed that water in the 30 Å
pores freezes at around 237 K, whereas
water in the 100 Å pores does at 252 K. Neutron diffraction data
for water in the 30 Å pores revealed that
with lowering temperatures below 237 K hydrogen bond networks are
gradually strengthened, the structure
correlation being extended to 10 Å at 193 K. It has also been
found that crystalline ice is not formed in the
30 Å pores in the temperature range investigated, whereas cubic ice
(I
c) crystallizes in the 100 Å pores at
238
K. The self-diffusion coefficients of water protons in both pores
determined from the quasi-elastic neutron
scattering measurements showed that the translational motion of water
molecules is slower by a factor of two
in the 30 Å pores than in the 100 Å pores, the motion of water
molecules in the 100 Å pores being comparable
with that of bulk water. The self-diffusion coefficients of water
in both pores at different temperatures showed
that the translational motion of water molecules is gradually
restricted with decreasing temperature. The
spin-lattice relaxation time (T
1) and the
spin-spin relaxation time (T
2) data obtained by
the proton NMR
relaxation experiments suggested that the motions of water molecules in
the 100 Å pores are faster by a
factor of 2−3 than those of water molecules in the 30 Å pores.
The peak area, the half-width at half maximum,
the relaxation rates (T
1
-1 and
T
2
-1) of water molecules at the
various temperatures all showed an inflection
point at 238 and 253 K for the 30 and 100 Å pores, respectively,
suggesting the freezing of water in the
pores.
Aluminum coordination in the framework of USY and ZSM-5 zeolites containing charge-compensating cations (NH4+, H+, or Cu+) was investigated by Al K-edge EXAFS and XANES. This work was performed using a newly developed in-situ cell designed especially for acquiring soft X-ray absorption data. Both tetrahedrally and octahedrally coordinated Al were observed for hydrated H-USY and H-ZSM-5, in good agreement with 27Al NMR analyses. Upon dehydration, water desorbed from the zeolite, and octahedrally coordinated Al was converted progressively to tetrahedrally coordinated Al. These observations confirmed the hypothesis that the interaction of water with Brønsted acid protons can lead to octahedral coordination of Al without loss of Al from the zeolite lattice. When H+ is replaced with NH4+ or Cu+, charge compensating species that absorb less water, less octahedrally coordinated Al was observed. Analysis of Al K-edge EXAFS data indicates that the Al-O bond distance for tetrahedrally coordinated Al in dehydrated USY and ZSM-5 is 1.67 angstroms. Simulation of k3chi(k) for Cu+ exchanged ZSM-5 leads to an estimated distance between Cu+ and framework Al atoms of 2.79 angstroms.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.