Published values for the potential parameters σ and ε/k of the Lennard–Jones (12–6) and Stockmayer (12–6–3) potentials as based on viscosity measurements are reviewed, with particular reference to the problem of indeterminacy inherent to such calculations. A number of correlation techniques, calibrated on viscosity-based potential parameters, are critically reviewed; where possible, priority rules for the use of these correlations have been developed. In addition, several other criteria (i.e. not based on viscosity data) for the acceptance or rejection of σ and ε/k parameter values are also discussed. Upon application of the various criteria and priority rules it has been possible to give recommended σ and ε/k parameter values for 75 molecules.

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Following the general theory for medium NMR shifts in gases as outlined in Part I of this series, two improvements are proposed. For the calculation of the van der Waals interaction a repulsive term has been added to the customary attractive term. Wherever intermolecular distances are required, a properly averaged distance between the site of the measured proton in the solute molecule and the center of the solvent molecule is used rather than the distance between the centers of the molecules. The implications of these corrections for nonpolar molecules are discussed, and new values for the bond parameters B are proposed.

INDO–MO calculations have been performed for all the proton–proton coupling constants in ethylene, propylene, and cis- and trans-2-butene, both at their equilibrium geometry and at geometries deviating from equilibrium, thus resulting in [Formula: see text] parameters for all coupling constants as a function of all valence angles[Formula: see text]. It is shown that such valence angle effects can be appreciable, particularly for two- and three-bond couplings. The valence angle effects are demonstrated to be independent and additive. The calculated data, in combination with the exact geometrical data and experimental values for the coupling constants allow the separation of (inductive) substitution and rehybridization effects. It is also shown that for three-bond couplings the effects of in-path C—C—H bond angle changes are minor in comparison to the exo-path C=C—C valence angle effects. The new data provide a consistently good model to explain certain empirical trends in two- and three-bond coupling constants as a function of the size of the substituents, assuming a simple steric hindrance induced rehybridization mechanism. On the other hand it is also found that such a mechanism cannot account for similar trends in the H—C=C—CH3 allylic and the CH3—C=C—CH3 homoallylic coupling constants.

Presently accepted methods for calculating the expected response for combinations of agents such as herbicides or other growth regulators from the responses of individual agents can lead to unacceptable results. This may be true even if responses within the linear region of the individual response curves are used in such calculations. A solution to this problem is to assign parameters to response curves of individual agents and to define the expected response function for combinations of agents by the weighted algebraic means of these parameters.