Apparent Molar Volumes of Multicharged Cations in Dimethyl Sulfoxide Solutions at 25°C

Densities for DMSO solutions of iron(III), aluminium(III), beryllium(II) and magnesium(II) perchlorates and silver nitrate are reported. Densities for DMSO solutions of tetraethylammonium perchlorate and nitrate and tetrabutylammonium perchlorate and tetraphenylborate are also presented. The partial molar volumes of the DMSO-solvated cations are derived and discussed in terms of variation with the charge number.     

Comparison of solubility of gases and vapours in wet and dry alcohols,especially octan‐1‐ol

Equations for the solubility of gases and vapours into dry alcohols from methanol to decan‐1‐ol and into water‐saturated alcohols from butan‐1‐ol to decan‐1‐ol have been compared through the use of the Abraham solvation equation. It is shown that there are noticeable differences in solvation into the dry and wet alcohols, and that these differences become larger as the alcohols become smaller and take up more water. The two main factors that lead to the differences in solvation are the solute hydrogen‐bond basicity, B, and solute size, L. Increase in solute hydrogen‐bond basicity favours the wet alcohols and increase in solute size favours the dry alcohols. Solute hydrogen‐bond acidity plays no part, because the hydrogen‐bond basicity of water, wet alcohols and dry alcohols is almost the same. Copyright © 2008 John Wiley & Sons, Ltd.     

Chelation of Na+ cations in nonaqueous solvents. Structural requirements and methodological considerations

Intrinsic formation constants for complexes of Na⁺ ions with a series of polyamines have been determined from²³Na NMR measurements in binary mixtures of tetrahydrofuran (THF) and amines. The results show in a totally unambiguous manner a regular decrease in the magnitude of the chelate effect with the number of atoms intervening between the nitrogen ligators. The key assumption, tetracoordination of the cation, is critically examined, and experimental evidence is adduced in its support.     

Conformational analysis of methylphenidate: comparison of molecular orbital and molecular mechanics methods

Summary Methylphenidate (MP) binds to the cocaine binding site on the dopamine transporter and inhibits reuptake of dopamine, but does not appear to have the same abuse potential as cocaine. This study, part of a comprehensive effort to identify a drug treatment for cocaine abuse, investigates the effect of choice of calculation technique and of solvent model on the conformational potential energy surface (PES) of MP and a rigid methylphenidate (RMP) analogue which exhibits the same dopamine transporter binding affinity as MP. Conformational analysis was carried out by the AM1 and AM1/SM5.4 semiempirical molecular orbital methods, a molecular mechanics method (Tripos force field with the dielectric set equal to that of vacuum or water) and the HF/6-31G* molecular orbital method in vacuum phase. Although all three methods differ somewhat in the local details of the PES, the general trends are the same for neutral and protonated MP. In vacuum phase, protonation has a distinctive effect in decreasing the regions of space available to the local conformational minima. Solvent has little effect on the PES of the neutral molecule and tends to stabilize the protonated species. The random search (RS) conformational analysis technique using the Tripos force field was found to be capable of locating the minima found by the molecular orbital methods using systematic grid search. This suggests that the RS/Tripos force field/vacuum phase protocol is a reasonable choice for locating the local minima of MP. However, the Tripos force field gave significantly larger phenyl ring rotational barriers than the molecular orbital methods for MP and RMP. For both the neutral and protonated cases, all three methods found the phenyl ring rotational barriers for the RMP conformers/invertamers (denoted as cte, tte, and cta) to be: cte, tte> MP > cta. Solvation has negligible effect on the phenyl ring rotational barrier of RMP. The B3LYP/6-31G* density functional method was used to calculate the phenyl ring rotational barrier for neutral MP and gave results very similar to those of the HF/6-31G* method.     

The parameterization and validation of generalized born models using the pairwise descreening approximation

Generalized Born Surface Area (GBSA) models for water using the Pairwise Descreening Approximation (PDA) have been parameterized by two different methods. The first method, similar to that used in previously reported parameterizations, optimizes all parameters against the experimental free energies of hydration of organic molecules. The second method optimizes the PDA parameters to compensate only for systematic errors of the PDA. The best models are compared to Poisson-Boltzmann calculations and applied to the computation of potentials of mean force (PMFs) for the association of various molecules. PMFs present a more rigorous test of the ability of a solvation model to correctly reproduce the screening of intermolecular interactions by the solvent, than its accuracy at predicting free energies of hydration of small molecules. Models derived with the first method are sometimes shown to fail to compute accurate potentials of mean force because of large errors in the computation of Born radii, while no such difficulties are observed with the second method. Furthermore, accurate computation of the Born radii appears to be more important than good agreement with experimental free energies of solvation. We discuss the source of errors in the potentials of mean force and suggest means to reduce them. Our findings suggest that Generalized Born models that use the Pairwise Descreening Approximation and that are derived solely by unconstrained optimization of parameters against free energies of hydration should be applied to the modeling of intermolecular interactions with caution.     

Ternary complexes between cationic GdIII chelates and anionic metabolites in aqueous solution: an NMR relaxometric study

The (1)H and (17)O NMR relaxometric properties of two cationic complexes formed by Gd(III) with a macrocyclic heptadentate triamide ligand, L(1), and its Nmethylated analogue, L(2), have been investigated in aqueous media as a function of pH, temperature and magnetic field strength. The complexes possess two water molecules in their inner coordination sphere for which the rate of exchange has been found to be sensibly faster for the Nmethylated derivative and explained in terms of electronic effects (decrease of the charge density at the metal center) and perturbation of the network of hydrogen-bonded water molecules in the outer hydration sphere. The proton relaxivity shows a marked dependence from pH and decreases of about six units in the pH range 6.5 to 9.0. This has been accounted for by the displacement of the two water molecules by dissolved carbonate which acts as a chelating anion. The formation of ternary complexes with lactate, malonate, citrate, acetate, fluoride and hydrogenphosphate has been monitored by (1)H NMR relaxometric titrations at 20 MHz and pH 6.3 and the value of the affinity constant, K, and of the relaxivity of the adducts could be obtained. Lactate, malonate and citrate interact strongly with the complexes (log K > or =3.7) and coordinate in a bidendate mode by displacing both water molecules. Larger affinity constants have been measured for GdL(2). Acetate, fluoride and hydrogenphosphate form monoaqua ternary complexes which were investigated in detail with regard to their relaxometric properties. The NMR dispersion (NMRD) profiles indicate a large contribution to the relaxivity of the adducts from water molecules belonging to the second hydration shell of the complexes and hydrogen-bonded to the anion. A VT (17)O NMR study has shown a marked increase of the rate of water exchange upon binding which is explained by coordination of the anion in an equatorial site, thus leaving the water molecule in an apical position, more accessible for interactions with the solvent molecules of the second hydration shell which facilitate the exchange process.     

Formation of solvated ions in the atmospheric interface of an electrospray ionization triple-quadrupole mass spectrometer

A simple method capable of generating and investigating various solvent clusters and solvated ions was developed. The technique opens a door to studying these complexes on commercially available instruments. Formation of the desired solvated ion in the gas phase was achieved by introducing the appropriate volatile solvent vapour into the curtain gas stream. Capabilities of the technique are illustrated by generating alkali, alkaline earth and transition metal cations solvated by various volatile compounds such as water, methanol and acetonitrile. Depending on the ligands and on the experimental conditions, clusters of 2-100 molecules may be observed. Isotope labelling suggests that these are formed by a re-solvation process in the curtain gas region.     

Effect of H/D-isotope substitution on the solubility and solvation thermodynamics of krypton in methanol solutions of carbamide

The solubility of gaseous krypton in CO(NH₂)₂—CH₃OH (CD₃OH) and CO(ND₂)₂—CH₃OD solutions with carbamide concentrations of up to 1.5 solvomolality units (0.026 mole fractions) was measured at 278, 288, 298, 308, and 318 K and at a partial gas pressure of 101325 Pa. The thermodynamic functions of dissolution (solvation) of krypton and the standard Setchenov coefficients were calculated. The solvation of Kr molecules increases upon deuterium substitution and with an increase in the temperature and carbamide concentration. In these solutions, specific contacts between the carbamide and methanol molecules play the predominant role.     

DMRG studies on interchain coupling model for quasi-one-dimensional organic magnet

We introduce a solid-on-solid growth process which evolves by random deposition of dimers, surface diffusion, and evaporation of monomers from the edges of plateaus. It is shown that the model exhibits a robust transition from a smooth to a rough phase. The roughening transition is driven by an absorbing phase transition at the bottom layer of the interface, which displays the same type of critical behavior as the pair contact process with diffusion 2A↦3A, 2A↦. Received 14 October 2002 Published online 14 February 2003 RID="a" ID="a"e-mail: Haye.Hinrichsen@physik.uni-wuppertal.de