Robert A. Kromhout

Proton‐Transfer Studies by Nuclear Magnetic Resonance. I. Diffusion Control in the Reaction of Ammonium Ion in Aqueous Acid

Rate constants were determined by precise nuclear‐magnetic‐resonance techniques for the reactions: NH4++H2O−→ lim k4NH3+H3O+;NH3+H3O+−→ lim k‐4NH4++H2O; and N lim −H4++NH3−→ lim k6N lim −H3+NH4+ in aqueous acid at 25°C. For 0.25–3.00M NH4Cl, k—4 and k6 were 4.3×1010 sec—1M—1 and 1.17×109 sec—1M—1 respectively, independent of NH4Cl concentration. Both rate constants increase on addition of KCl. The rate constant, k4, was 24.6 sec—1 at zero ionic strength and decreased sharply with NH4Cl concentration. The values of KA needed in the kinetic analysis were measured also. The reaction of NH4+ with H2O was investigated also in 50–60 wt % H2SO4‐H2O mixtures, where the half‐life for isotopic exchange was between two and 20 minutes. The data allowed us to estimate the rate constant (1/τD+1/τR) for the rupture of the H3N···HOH hydrogen bond, by diffusion (D) or rotation (R). The value obtained was 51×1010 sec—1. Using Einsteins theory of Brownian motion, the value of 1/τD was estimated to be 34×1010 sec—1, leaving...

Polarizability and Second Dielectric Virial Coefficient of Interacting He Atoms

The polarizability α of the pair of interacting He atoms has been calculated by Hartree–Fock perturbation theory, both coupled and uncoupled. A basis of 16 Gaussians was used, eight centered on each atom. Good convergence was obtained in the region of overlap between one and six atomic units internuclear separation, R. Values of α for large separations were obtained by extrapolation using an R−6 dependence as predicted by previous long‐range calculations. The effect of the interatomic interaction on the second dielectric virial coefficient was then calculated using the radial distribution function based on empirical Lennard‐Jones parameters. The results show that the effect of overlap and exchange do not sufficiently counteract the positive deviation caused by the van der Waals interaction to produce the substantial negative value as measured by Orcutt and Cole.

van der Waals induced dipoles

Expressions are derived for calculating the induced dipole moment of an arbitrary molecule A interacting with an arbitrary system B through first‐ and second‐order Coulomb interaction. The theory is formulated in terms of linear and quadratic charge‐density susceptibilities and takes account of charge penetration but not exchange between the systems. The theory is specialized to the interaction of two nonoverlapping molecules and to a molecule interacting with a nonferroelectric solid, metallic, or crystalline. In the case of two interacting molecules, the induced moment is developed in inverse powers of R, the distance between the centers of the molecules, up to and including R−7; the coefficients of the series are given in terms of the total charges, permanent moments, polarizabilities, and hyperpolarizabilities. In the case of the solid, the results are given in terms of the molecule–solid distance z0, the dielectric function of the solid, and the permanent moments, polarizabilities, and hyperpolarizab...

Proton‐Transfer Studies by Nuclear Magnetic Resonance. II. Rate Constants and Mechanism for the Reaction CH3NH3++OH2+NH2CH3 in Aqueous Acid

Rate constants were measured by precise nuclear‐magnetic‐resonance techniques for the reactions CH3N lim −H3++NH2CH3−→ lim k6CH3N lim −H2+HN lim +H2CH3,CH3N lim −H lim +3+OH2+NH2CH3−→ lim k7CH3N lim −H2+HOH+HN lim +H2CH3, in aqueous acid at 25°. The ratio k6/k7 remained virtually constant between 1.7 and 8.1M concentration of CH3NH3Cl. The rate constants were inversely proportional to the viscosity of the solution, and were extrapolated on this basis to infinite dilution to yield the values k60=4.0×108 sec—1 M—1 and k70=5.3×108 sec—1 M—1 at 25°. Acid dissociation constants, densities, and viscosities for 1.7 to 9M solutions of CH3NH3Cl in water were measured also. Measurements of the water activity of these solutions showed that the mean ionic molal activity coefficients of CH3NH3Cl were virtually constant over the entire range. The magnitude of k7, as well as the constancy of k7/k6, indicated that the most probable rate‐determining step for the reaction with rate constant k7 is proton transfer from a wat...

The effect of dispersion interaction on nuclear magnetic shielding

Abstract A theory is developed for calculating the effect of the London-van der Waals interaction on the nuclear magnetic shielding of a closed shell atom. The effect of a single perturber at fixed internuclear distance is first obtained by the use of triple perturbation theory. In the closure approximation, this pair shift is proportional to the van der Waals energy and the unperturbed shielding constant. The solvent shift due to the fluid medium is then obtained statistically. The results are compared with existing heuristic expressions based on field simulation, and the parameters used in the field techniques are analyzed and interpreted. Numerical values are given for the van der Waals shifts of the noble gas liquids. Difficulties associated with the extension of the theory to polyatomic molecules are discussed, and appropriately modified parameters are suggested. Numerical estimates so obtained for CH4 and CF4 are in good agreement with experimental data.

On the dipole moments of physisorbed rare gas atoms

Abstract Rare gas adatom dipoles are shown to be small and incapable of explaining observed work function changes. Three-body dispersion forces yield values consistent with the reduction in the adatom pair potentials; contributions from adatom dipoles are negligible.

Role of dispersion potential anisotropy in the theory of nematic liquid crystals. I. The mean‐field potential

We reexamine the effect of the anisotropy in the intermolecular attractive (dispersion) interaction on the attractive mean‐field potential used in the generalized van der Waals theory of the nematic–isotropic transition. Contrary to results in the literature, we show (1) that the anisotropic part of the dispersion potential reduces the anisotropy of the mean field contributed by the isotropic part of the dispersion potential; (2) that the form of anisotropy in the attractive intermolecular interaction usually chosen for simplicity in thermodynamic calculations corresponds to a preaveraged form of the dispersion interaction. The anisotropy of both the dispersion and of the mean included angle potential (which corresponds to the mean‐field potential in a perfectly ordered nematic) are shown graphically, and the expansion coefficients for the latter in terms of even‐ordered Legendre polynomials are tabulated.

Nematic‐isotropic phase transition: Hard spheroids with London interaction

Cluster expansions for the thermodynamic functions of ordered systems are specialized to nematic systems and used in an augmented generalized van der Waals calculation based on hard spheroids of axial ratio 5 with full London attractive interaction. The convergence of the cluster expansion is accelerated by use of the y expansion [B. Barboy and W. M. Gelbart, J. Chem. Phys. 71, 3053 (1979)]. The correlation functions are hard spheroid Percus–Yevick values from the literature. The orientational distribution functions are determined by minimizing the Helmholtz free energy. An isotropic‐nematic transition is found at one atmosphere pressure with characteristics in approximate agreement with the experimental values for paraazoxyanisole (PAA). The importance of the attractive interaction for reproducing experimental pressures and temperatures is discussed, and some speculations are made as to how agreement with experiment might be further im‐ proved.

Role of anisotropic dispersion potential in the theory of nematic liquid crystals. II. Thermodynamic calculations

We reexamine the effect of the anisotropy in the intermolecular attractive (dispersion) interaction on the thermodynamic quantities characterizing the nematic–isotropic phase transition, as estimated from generalized van der Waals theory. Contrary to statements in the literature, this anisotropy significantly affects the transition temperature and the slope of the coexistence curve. The results obtained here, as in generalized van der Waals calculations in the literature, in general do not well reproduce the experimental data. Sources of these discrepancies are discussed briefly.

Two‐Temperature van der Waals Potentials

The Linder formulation of the van der Waals potential, expressed in terms of susceptibilities, between two weakly interacting molecular systems at different temperatures is applied to a simple model. The model molecules are assumed to have the low‐frequency characteristics of rigid rotors and the high‐frequency characteristics of harmonic oscillators. It is shown that the induction potential is always attractive; the orientation potential becomes repulsive when the temperature difference is large. The dispersion potential becomes repulsive when the temperature difference is large provided the hotter system has the higher natural frequency.