Isolated Water Molecules Research Articles

Effect of molecular interactions on the O-H stretching force constants for associated water species

Semi-empirical molecular-orbital calculations are reported for the O-H stretching force constants of different hydrogen-bonded species of water involving dimeric, trimeric, tetrameric and pentameric associations. These calculations show that - in general - a water molecule bonded to another water molecule through oxygen lone-pair electrons behaves as a stronger acid than an isolated water molecule, and a water molecule bonded to two water molecules through its two oxygen lone pairs is still stronger. This is reflected in the force constants of hydrogen-bonded O-H bonds. These results explain why O-H stretching bands in IR and Raman spectra of liquid water appear at significantly lower frequencies than for solutions of water in organic bases, even though water is a weaker base.

Vibrational spectra of α-molybdic acid-MoO3.H2O

Their and Raman spectra of molybdenum trioxide—monohydrate are studied assuming an effective tetrahedral Mo-O coordination and isolated water molecules, although the crystallographic coordination is six-fold with two long Mo-O distances. Based onCt symmetry, the group theoretical analysis has been carried out and a vibrational assignment is proposed. The nature of hydrogen bonding and the librational modes of water molecules are discussed. The factor group splitting forv3, the asymmetric stretching mode of MoO4= ion, is large indicating strong interchain coupling.

The hydration of anions in nonaqueous media

A very simple isopiestic method based on that of S. Christian is used for measuring the salting-in of water into nonpolar, low-volatility solvents by tetraalkylammonium salts. The quantity of excess water which is dissolved in such solvents is directly proportional to the salt concentration and is sharply dependent on the nature of the anion but is nearly insensitive to that of the R4N+ cation. The hydration ratioH, which we define as the moles of excess solubilized water per mole of R4N+ X−, is directly relatable to the enthalpy of hydration of the anion X− in several solvents and in the gas phase. The quantityH is also correlated with many free-energy terms including those for the Hofmeister lyotropic series, for the ability of the anions to salt nonelectrolytes out of water, for the free-energy terms for separation of these ions by reverse osmosis membranes, and for their nucleophilicities. A surprising (but not unprecedented) feature of the hydration ratio is that it, rather than its logarithm, behaves as a free-energy term. It is proposed that all these properties have in common the free energy of hydration of the anions, and this notion is supported by a close correspondence between the anionic hydration ratio and their hydrogen-bonding energies with proton donors in aprotic solvents. The results support scattered observations by other workers that isolated water molecules do not have an unusual inherent affinity for anions. Accordingly, large anionic hydration energies in bulk aqueous media reflect extensive cooperative interactions in the solvent. Implications for nucleophilic activity in phase transfer catalysis and enzyme activity are mentioned.

An accurate potential for deformable water molecules

An atom-atom interaction potential for the water dimer is presented. The model has been fitted to spectroscopic data for the isolated water molecule, and to the second virial coefficient of steam. In addition it has the correct long range interaction and is in reasonable agreement with the static lattice energy of ice. An account is given of the ability of the potential to predict molecular beam scattering data. The model is compared with previously suggested pair potentials and is shown to be more satisfactory.

Hydrogen bonding and the Compton profile of water

The effect of the hydrogen bond interaction on the Compton profile for water is estimated by observing the consistent trend in the water polymer (monomer, dimer, trimer, tetramer and pentamer) profiles obtained from various quality wavefunctions. The profile of the isolated water molecule is found to be lowered by hydrogen bonding.

The crystal structure of rösslerite MgHAsO4 · 7 H2O

Abstract Rösslerite MgHAsO4 · 7H2O crystallizes in the monoclinic system with cell dimensions a = 6,687 ± 0,003, b = 25,73 ± 0.01, c = 11,531 ± 0,005 Å, β = 95° 7′ ± 5′. The space group is C2/c and there are eight molecules per unit cell. The structure has been solved from a Patterson synthesis and partially phased Fourier syntheses and least-squares refinement has been completed on three-dimensional data (2065 structure factors within the CuKα sphere). The final residual is 11.2%. The structure consists of AsO2(OH½)2 tetrahedra, Mg · 6H2O octahedra in two independent positions and isolated water molecules held together by hydrogen bonds. The position of the hydrogen bonds have been inferred from stereochemical and crystallographic evidence. The AsO2(OH½)2 tetrahedron is distorted, with a mean (uncorrected) arsenic–oxygen bond length of 1,691 ± 0,003 Å and the Mg · 6H2O octahedra are also distorted, with a mean (uncorrected) magnesium–water oxygen of 2,077 ± 0,003 Å. However in this case if the water oxygen is a receptor atom for a hydrogen bond the magnesium–water oxygen bond is lengthened. The thermal vibrations have been analyzed and the mean corrected bond lengths are arsenic–oxygen 1,695, magnesium–water oxygen 2,086 (assuming riding motion in each case).

Nachweis von strukturellen L-Defekten im innerkristallinen Wasser von hydratierten Vermikuliteinkristallen mittels dielektrischer Messungen / Detection of Structural L-Defects in Intracrystalline Water of Vermiculites by Dielectric Measurements

Homoionic single crystals of vermiculite in different hydration stages (isolated water molecules, discrete monomolecular or bimolecular water layers respectively) have been used for dielectric measurements within the frequency of 30 Hz and 10 MHz. The results are interpreted by the diffusion of isolated structural L-defects. The activation energy of the process has been determined to be 2.3 kcal/mole.

Nuclear Magnetic Resonance of 1H and 27Al and Al–Si Order in Low Cordierite, Mg2Al4Si5O18 · nH2O

Proton resonance data in the low-temperature form of cordierite indicate the presence of isolated water molecules in the channel formed of the AlSi5O18 rings, with H–H directions along the crystallographic c axis and with large vibration amplitudes. Two crystallographically nonequivalent aluminum sites have been observed. The quadrupole coupling constant e2qQ/h, the asymmetry parameter η, and the principal axes directions are: AlI: | e2qQ/h |=10.6 MHz; η=0.38; X ‖ c; Z and Y, 30° away from a and b.AlII: | e2qQ/h |=5.6 MHz; η=0.34; Y ‖ c; Z and X, 20° away from a and b.The AlI and AlII sites correspond to the distorted tetrahedral T1 site outside the six-membered ring and T5 site within the ring in the cordierite structure. The AlI NMR data also agree with Fe3+ paramagnetic resonance data, indicating that the Fe3+ ions preferentially occupy the larger T1 sites. The 27Al NMR results indicate a high degree of Al–Si order in low cordierite.

Anharmonic force constants of water

The observed vibrational energies and rotational constants have been used in a least-squares determination of the force constants in the general quartic force field of the isolated water molecule. The force constants have been examined in the formalism proposed by Machida and Overend and attempts have been made to simplify the potential function in order to develop a model with general applicability to bent triatomic molecules.