Robert A. Kromhout’s research while affiliated with Florida State University and other places

We derive the van der Waals induced moment using a reaction field approach based on charge density susceptibilities of several bodies interacting through Coulomb operators. We extend previous results to include nonadditive contributions from up to four bodies. We then expand the Coulomb operators in a moment T-tensor series. The lowest hyperpolarizability terms yielding nonzero values are products of the dipole-dipole quadrupole hyperpolarizability, B, for the body whose dipole moment is being evaluated and the dipole-dipole polarizabilities, alpha, of the other interacting bodies. We then introduce the image approximation for the case where one body is a solid and apply our results to the cases of argon and xenon on magnesium and on palladium. By closure, we are able to separate the frequency dependencies of the various factors so that this contribution can be reduced to a numerical factor that is then calculated by numerical integration. Our present results show that the work function reductions of magnesium by argon and xenon can be accounted for by our calculations. Our calculated reductions for palladium suggest that in the high work-function metals another mechanism enters in an important way, as has been suggested by others. The relationship between our present results and earlier estimates by ourselves and others is discussed.

By fitting the density of zeros of the grand partition function, in the neighborhood of transition and critical points, to functional forms consistent with the qualitative behavior expected from the Yang–Lee Theory of phase transitions, we show that the Yang–Lee theory implies (or is at least consistent with) scaling. We show that in scaling experimental data the reference density and chemical potentials should not be, in general, the critical values, but rather values associated with coexistence or with maxima in the isothermal compressibility. As an example we display data for water.

Our statistical-mechanical formulation for calculating heat-capacity and enthalpy functions in conformational changes of biomolecules is extended to include free-energy and entropy functions and generalized to treat multidomain systems. Each conformant may consist of one or several independent sections or domains. The treatment is based on a two-state model in which all residues within a domain are fully cooperative. The thermodynamic functions can be evaluated from the ratio of the effective partition functions of the residues in the different conformants, the difference in their zeros of energy, and the temperature derivatives of these partition functions. The former two parameters were evaluated from published optical density data of the conformants within the transition region; the last item was obtained from published heat-capacity data of the pure conformants outside the transition region. Three types of thermodynamic functions were considered:  (1) total functions, which include contributions from both folded (conformant A) and unfolded (conformant B) states; (2) excess functions, which are the functions in excess of pure A; (3) conformational functions, which do not include contributions from either pure A or pure B. The theory was applied to solutions of ribonuclease, exemplifying single-domain systems, and to solutions of poly-γ-benzyl-L-glutamate, exemplifying multidomain systems. All thermodynamic functions, including the heat capacities, in the transition region were calculated. The results are compared with published experimental values and are in reasonably good agreement with the measured results. The excess and conformational thermodynamic functions are compared and discussed.

Statistical mechanical calculations are presented of the conformational enthalpy and heat capacity of a solution of poly-γ-benzyl-l-glutamate using the partition function of Zimm and Bragg for helix−coil transitions. We determine the temperature dependence of the statistical parameter s from optical data and treat the parameter σ as a fitting parameter. We note, however, that consistency with the Zimm−Bragg theory requires that σ not be less than the square of the reciprocal of the number of residues per molecule. We utilize that data given by Ackermann and Rüterjans [Ackermann, Th.; Rüterjans, H. Ber. Bunsen-Ges. Phys. Chem. 1964, 68, 850] and find the same value for σ as they, σ = 2.5 × 10-5 for their solution of 0.257 mol of residue per liter in a solvent of dichloracetic acid and 1,2-dichlorethane. We consider our calculation to be in satisfactory agreement with that of the experiment but point out some problems associated with the determination of the temperature dependence of s.

The Blume-Capel (1966) model in a transverse field is studied in the mean field approximation. It is found that a tricritical point exists in the transverse field-temperature plane for a range of values of anisotropy to exchange interaction ratio. This is in contrast with the original Blume-Capel model which shows the tricritical phenomena only at a single value of this ratio. The most interesting feature, however, is that the ordering field, which is the externally applied field along the longitudinal direction, is experimentally accessible and can be set at one's will. This allows an experimentalist to investigate all aspects of a system describable by this model. Equations of the lines of critical points including the wing boundaries are derived. Phase diagrams for the whole range of values of anisotropy are described, and the experimental situation is discussed.

A statistical mechanical treatment is presented for determining the heat capacity and enthalpy changes with temperature in conformational biomolecular (e.g. folding−unfolding of proteins) transitions. The theory is formulated in terms of subunit partition functions, which are the average partition functions of the residues modulated by their interactions with each other and the solvent. The theory is specialized to a system of two types of conformants:  type A (for example, protein in the folded state) consisting of subunits of type “a”, and type B (for example, protein in the unfolded state) consisting of subunits of type “b”, assuming complete cooperativity. It is shown that for such a model, the heat capacity and enthalpy functions obtained from the isothermal−isobaric partition function of the biomolecular system can be calculated from the ratio of the partition functions of the “a” and “b” subunits, their temperature derivatives, and the difference between the lowest enthalpy levels of the subunits. The temperature variation of the partition functions of the subunits can be evaluated from thermal data of the pure conformants A and B outside the transition range. The other parameters may be inferred from the population ratio of the conformants inside the transition region. The theory is applied to lysozyme of pH 2.0, pH 2.25, and pH 3.5, using published optical density data (Khechineshvili, N. N.; Privalov, P. L.; Tictopulo, E. I. FEBS Lett. 1973, 30, 57) of the conformants within the transition range, and heat capacity data of the pure conformants outside the transition range. Kidokoro and Wada (Kidokoro, S.-I.; Wada, A. Biopolymers 1987, 26, 213) published heat capacity data for lysozyme at pH 2.0 with which we compared our calculated new capacity. The calculated heat capacity curve agrees well with the published one.

As has been recognized for some time, the dispersion interaction of molecules, particularly large molecules, cannot be adequately represented by a simple London-type expression based on the total molecule polarizabilities. We investigate two models consisting of several anisotropic polarizabilities associated with molecular subgroups. In one model intramolecular interactions among the groups are included; in the other only intermolecular interactions are included, but different ''effective'' polarizabilities are used. These models are generalizations of models considered previously by others. We calculate the interaction energy as a function of molecular separation for several different relative orientations of the molecular axes of model PAA (p-azoxyanisole) molecules. We find that (1) at very short distances intramolecular interaction is negligible; (2) at intermediate distances the distance dependence is approximately inverse fifth power; (3) the two models yield almost identical results over all but the very shortest distances; (4) at molecular separations greater than a few times the largest molecular dimension, the distance dependence is essentially the simple inverse sixth power London interaction based on total molecular polarizability. We compare these qualitative results with those found previously by others.

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.

Our formulation of ordered systems in terms of the single-particle distribution and the two-particle (total) correlation function, g(x, x’), is applied to hard ellipsoids of revolution with an aspect ratio of 5. It is found that such a system exhibits an isotropic-nematic phase transition at &rgr;v0=0.3806 (ordered) and 0.3791 (disordered), where &rgr; is the particle density and v0 the molecular volume, with an order parameter of 0.20.