Diffusion of Squalene in Nonaqueous Solvents.

Capillary flow techniques have been used to determine the translational diffusion constant, D, of squalene in seven alkanes and five cyclohexanes. The alkanes are n-hexane, n-octane, n-decane, n-dodecane, n-tetradecane, 2,2,4,4,6,8,8-heptamethylnonane (isocetane), and 2,6,10,14-tetramethylpentadecane (pristane). The cyclohexanes are cyclohexane, n-butylcyclohexane, n-hexylcyclohexane, n-octylcyclohexane, and n-dodecylcyclohexane. When combined with published data in CD2Cl2, ethyl acetate, n-hexadecane, squalane, n-octane-squalane mixtures, and supercritical CO2, the 35 diffusion constants and viscosities, η, vary by factors of ∼230 and ∼500, respectively. A fit to the modified Stokes-Einstein equation (MSE, D/T = A SE/η p ) gives an average absolute percentage difference (AAPD) of 7.72% between the experimental and calculated D values where p and A SE are constants, T is the absolute temperature, and the AAPD is the average value of (102) (|D calcd - D exptl|/D exptl). Two other MSE fits using subsets of the 35 diffusion constants may be useful for (a) estimating the viscosity of the hydrophobic core of lipid droplets, where squalene is a naturally occurring component, and (b) providing estimates of the D values needed to design extraction processes by which squalene is obtained from plant oils. The Wilke-Chang equation also was considered and found to give larger AAPDs than the corresponding MSE fits.

Diffusion of Polymethylene Chain Molecules in Nonpolar Solvents.

The translational diffusion constants, D, for solutes with polymethylene chains are compared with the predictions of a hydrodynamic bead model based on Kirkwood-Riseman theory. The solute-solvent combinations include (a) n-alkanes in n-alkanes; (b) n-alkanes in benzene, toluene, tetralin, decalin, and CCl4; (c) 1-alkenes in n-alkanes; (d) 1-phenylalkanes in n-alkanes; and (e) 1-phenylalkanes in isocetane (2,2,4,4,6,8,8-heptamethylnonane), pristane (2,6,10,14-tetramethylpentadecane), and squalane (2,6,10,15,19,23-hexamethyltetracosane). The bead model gives good overall agreement with an average difference of less than 3% between 207 experimental and calculated diffusion constants that include published data as well as new D values determined for 1-alkenes and 1-phenylalkanes using capillary flow techniques. The calculated values are obtained using chain element (bead) radii that decrease as the solvent viscosity increases. The bead model's results are comparable to those obtained using cylinder and lollipop diffusion for many of the same solute-solvent systems; the three models are compared and discussed. The results for the 1-alkenes and n-alkanes in the n-alkanes are the first from a Kirkwood-Riseman analysis in a homologous series of solvents.

Diffusion of Benzene and Alkylbenzenes in Nonpolar Solvents.

The translational diffusion constants, D, of benzene and a series of alkylbenzenes have been determined in n-pentadecane, 2,6,10,14-tetramethylpentadecane (pristane), 2,2,4,4,6,8,8-heptamethylnonane (isocetane), and 2,6,10,15,19,23-hexamethyltetracosane (squalane) using capillary flow techniques. The solutes' D values are compared with the predictions of a cylinder diffusion model as are those for (a) benzene and alkylbenzenes in n-nonane, n-decane, n-dodecane, and supercritical CO2 and (b) n-alkanes and 1-alkenes in n-hexane, n-heptane, n-octane, benzene, and toluene. The D values for benzene and the alkylbenzenes also are compared with the predictions of lollipop diffusion for which the phenyl ring is the candy and the alkyl chain is the handle. Both models give an average difference of less than 4% between experimental and calculated diffusion constants in solvents whose viscosities vary by a factor of more than 600 when benzene and toluene (as solutes) are omitted; the comparisons include 150 and 85 D values for the cylinder and lollipop models, respectively. The differences increase when benzene and toluene are included and are most likely because of their shapes and the shapes assumed by the models. The agreement with the models indicates that the chains of the alkylbenzenes and 1-alkenes, like those of the n-alkanes, are relatively extended. The D values for several of the solutes also are fitted to a modification of the Stokes-Einstein relation that varies their dependence on viscosity instead of chain dimensions.

d-electron count, ion-pairing and diagonal twist angles in metallo-bis(dithiolene) complexes.

Electronic structure calculations for late transition metals coordinated by two dithiolene ligands are found to be consistent with existing structures and also predict the geometries of Ni(I) species for which no solid state structures have been reported. Of particular interest are the compounds [M(mnt)2 ]n- (M = Ni, Pd, and Pt with n = 1, 2, 3; M = Cu with n = 2). Calculations have been performed with and without ion-paring with M(diglyme)+ (M = Li, Na, K) and R4 N+ (R = Me, Bu). The diagonal twist angle between two NiS2 planes is found to depend on (i) the metal's d-electron count, spanning from 0° (planar d7 and d8 ), to 42° (d9 ), to 90° (pseudo-tetrahedral d10 ), and (ii) the identity of the ion-paired cations. Calculated ion-pairing energies are functions of the cation size and charge-density, being larger for alkali-metal coordinated diglyme and smaller for tetra-alkyl ammonium cations. © 2016 Wiley Periodicals, Inc.

Diffusion of Benzene and Alkylbenzenes in n-Alkanes.

The translational diffusion constants, D, of benzene and a series of alkylbenzenes have been determined in four n-alkanes at room temperature using capillary flow techniques. The alkylbenzenes are toluene, ethylbenzene, 1-phenylpropane, 1-phenylpentane, 1-phenyloctane, 1-phenylundecane, 1-phenyltetradecane, and 1-phenylheptadecane. The n-alkanes are n-nonane, n-decane, n-dodecane, and n-pentadecane. Ratios of the solutes' D values are independent of solvent and in general agreement with the predictions of diffusion models for cylinders and lollipops. For the latter, an alkylbenzene's phenyl ring is the lollipop's candy; the alkyl chain is its handle. A model that considers the solutes to be spheres with volumes determined by the van der Waals increments of their constituent atoms is not in agreement with experiment. The diffusion constants of 1-alkene and n-alkane solutes in n-alkane solvents also are compared with the cylinder model; reasonably good agreement is found. The n-alkanes are relatively extended, and this appears to be the case for the alkyl chains of the 1-alkenes and alkylbenzenes as well.

Diffusion of squalene in n-alkanes and squalane.

Squalene, an intermediate in the biosynthesis of cholesterol, has a 24-carbon backbone with six methyl groups and six isolated double bonds. Capillary flow techniques have been used to determine its translational diffusion constant, D, at room temperature in squalane, n-C16, and three n-C8-squalane mixtures. The D values have a weaker dependence on viscosity, η, than predicted by the Stokes-Einstein relation, D = kBT/(6πηr). A fit to the modified relation, D/T = ASE/η(p), gives p = 0.820 ± 0.028; p = 1 for the Stokes-Einstein limit. The translational motion of squalene appears to be much like that of n-alkane solutes with comparable chain lengths; their D values show similar deviations from the Stokes-Einstein model. The n-alkane with the same carbon chain length as squalene, n-C24, has a near-equal p value of 0.844 ± 0.018 in n-alkane solvents. The values of the hydrodynamic radius, r, for n-C24, squalene, and other n-alkane solutes decrease as the viscosity increases and have a common dependence on the van der Waals volumes of the solute and solvent. The possibility of studying squalene in lipid droplets and membranes is discussed.

Molecular motion of the bis(maleonitriledithiolato)nickel trianion in solution.

Electron spin resonance (ESR) has been used to study the reorientational motion of the bis(maleonitriledithiolato)nickel trianion, [Ni(mnt)(2)](3-), in diethylene glycol dimethyl ether (diglyme). [Ni(mnt)(2)](3-) has one unpaired electron and was prepared by reducing the dianion, [Ni(mnt)(2)](2-), with potassium metal. The trianion and dianion are members of the redox series [Ni(mnt)(2)](n-) with n = 0, 1, 2, and 3. The monoanion, [Ni(mnt)(2)](-), also has S = 1/2 and its rotational diffusion in diglyme was the subject of previous ESR studies. This made possible the comparison of the reorientational data for two different oxidation states of the same planar complex in the same solvent. Differences were found; isotropic rotational diffusion produced agreement between the trianion's experimental and calculated spectra, whereas the monoanion's simulations required axially symmetric reorientation with diffusion about the long in-plane axis three times faster than that about the two perpendicular axes. At a given temperature, the monoanion's reorientation rates about the long in-plane axis and two perpendicular axes were faster than the trianion's isotropic rate by factors of ∼27 and ∼9, respectively. These differences suggest that [Ni(mnt)(2)](-) and [Ni(mnt)(2)](3-) have different shapes and sizes in solution; the monoanion is approximately a prolate ellipsoid, whereas the trianion is larger and more spherical. [Ni(mnt)(2)](3-) appears to be ion-paired, whereas in accord with results from other techniques, [Ni(mnt)(2)](-) is not.

Diffusion of organic solutes in squalane.

The translational diffusion constants, D, of 26 hydrocarbons have been determined in squalane (2,6,10,15,19,23-hexamethyltetracosane) at room temperature using capillary flow techniques. These new data and previously published room-temperature D values for the same solutes in some (or all) of the n-alkanes n-C(6)-n-C(16) constitute a study of solute diffusion in media spanning a 100-fold change in viscosity; at 23 °C, η = 0.31 cP for n-C(6), 3.2 cP for n-C(16), and 30 cP for squalane. The D values in the n-alkanes and squalane show deviations from the Stokes-Einstein relation, D = k(B)T/(6πηr); the values of r, a solute's hydrodynamic radius, decrease as the viscosity increases. The deviations increase as the solute size decreases and are analyzed by fitting the diffusion constants to the modified Stokes-Einstein equation, D/T = A(SE)/η(p). Fits involving the n-alkane-only and combined n-alkane-squalane D values give comparable results with values of p < 1 that increase as the solute size increases; p = 1 for the Stokes-Einstein limit. The deviations from Stokes-Einstein behavior also are discussed in terms of the relative sizes of the solutes, the n-alkanes, and squalane.

Electron spin resonance studies of the reorientational motion of Ni(mnt)2(-).

Electron spin resonance studies of the planar bis(maleonitriledithiolato)nickel complex ion, Ni(mnt)(2)(-), have been carried out from the motional narrowing region to the glassy limit in a series of ethers: 2-methyltetrahydrofuran (MTHF), diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), and tetraethylene glycol dimethyl ether (tetraglyme). Analyses of the spectra show that Ni(mnt)(2)(-) is reorienting a factor of 3 faster about its long in-plane axis in all of these solvents; i.e., axially symmetric rotational diffusion produces agreement between the experimental and calculated line widths with D(parallel)/D(perpendicular) = 3.0 +/- 0.2; D(parallel) and D(perpendicular) are the diffusion constants for reorientation about the long in-plane (parallel) and perpendicular axes, respectively. The temperature dependence of the reorientational correlation time tau(2)(0) = (6D(perpendicular))(-1) determined from the widths is in agreement with the modified Stokes-Einstein-Debye model; the results indicate that Ni(mnt)(2)(-) has relatively strong (but not associative) interactions with the ethers. The experimental values of tau(2)(0) and the solvents' viscosities, self-diffusion constants, and dielectric relaxation times are compared and found to have a common temperature dependence. The ESR data also are compared with values of tau(solv), the correlation time obtained when a fluorescent probe is excited and its emission is monitored as the nonequilibrium solvent distribution relaxes. tau(solv) and tau(2)(0) are found to have a common temperature dependence in MTHF, tetraglyme, and two other solvents (ethyl alcohol and 1-butanol) in which Ni(mnt)(2)(-) has been studied. The factors determining these transport properties are discussed.

Self-exchange reaction of [Ni(mnt)2](1-,2-) in nonaqueous solutions.

The rate constant, k, for the homogeneous electron transfer (self-exchange) reaction between the diamagnetic bis(maleonitriledithiolato)nickel dianion, [Ni(mnt) 2] (2-), and the paramagnetic monoanion, [Ni(mnt) 2] (1-), has been determined in acetone and nitromethane (CH 3NO 2) using (13)C NMR line widths at 22 degrees C (mnt = 1,2-S 2C 2(CN) 2). The values of k (2.91 x 10 (6) M (-1) s (-1) in acetone, 5.78 x 10 (6) M (-1) s (-1) in CH 3NO 2) are faster than those for the electron transfer reactions of other Ni(III,II) couples; the structures of [Ni(mnt) 2] (1-) and [Ni(mnt) 2] (2-) allow for a favorable overlap that lowers the free energy of activation. The values of k are consistent with the predictions of Marcus theory. In addition to k, the spin-lattice relaxation time, T 1e, of [Ni(mnt) 2] (1-) is obtained from the NMR line width analysis; the values are consistent with those predicted by spin relaxation theory.

Molecular motion of a nickel-bis(dithiolato) complex in solution.

The molecular motion of the planar bis(maleonitriledithiolato)nickel anion, Ni(mnt)(2)(-), has been studied as a function of temperature using electron spin resonance (ESR) in several polar solvents; they are ethyl alcohol, eugenol, dimethyl phthalate, tri-n-butyl phosphate, tris(2-ethyl-hexyl)phosphate, diglyme, and a dimethylformamide-chloroform mixed solvent. Calculated spectra in agreement with the experimental X-band spectra are obtained using axially symmetric reorientation when the long in-plane axis is the unique (parallel) axis of the rotational diffusion tensor with D parallel/D perpendicular = 3.0-4.0; D parallel and D perpendicular are the diffusion constants for reorientation about the parallel and perpendicular axes, respectively. The reorientational model required for the simulations is either in or close to the Brownian rotational diffusion limit. In the slow motional (low temperature) region, the spectra can be simulated using the glassy g values. As the temperature increases, however, agreement is obtained only if the intermediate g factor, g(y), for the non-axially symmetric Zeeman interaction increases while g(x), g(z), and the motional model remain unchanged; this scheme and others for which gx and g(z) are possibly temperature-dependent are discussed. The values of D perpendicular from the simulations are in general agreement with those from earlier analyses of the width of the central spectral feature. The simulations and width analyses indicate (as do electrochemical, conductivity, and vapor-phase osmometry data) that the paramagnetic species reorienting in solution has a shape similar to that of the Ni(mnt)(2)(-) ion.