Tests of Nonbonded Interactions

In the following sections, further experimental tests of nonbonded interactions will be discussed. Specifically, we shall examine the following topics ... 

In Part II we examined in detail various experimental tests of nonbonded interactions. Here, we again focus on specific physical and reactivity probes in order to test the importance of n—a interactions in organic problems. Specifically, we shall examine the following two areas ... 

We have seen that for our calculations essentially two types of force fields have to be considered VFF- and UBFF-expressions. The main difference with repect to spectroscopic force fields consists in the superposition of nonbonded interactions. The force fields used so far for our purposes are almost exclusively simple valence force fields without cross terms, and a veriety of UB-force fields. Only recently could experiences be gathered with a valence force field that includes a number of important cross terms (79). Vibrational spectroscopic force fields of both types have been derived and tested with an overwhelming amount of experimental data. The comprehensive investigations of alkanes by Schachtschneider and Snyder (26) may be mentioned out of numerous examples. The insights gained from this voluminous spectroscopic work are important also when searching for suitable potentials for our force-field calculations. 

The simple cycloalkanes (CH2)n with n = 5 to 12 are the compounds most frequently studied by force field calculations (8, 9, 11, 12,17, 21). This preference results from their simple structure, from the abundant available experimental material (structural (46), thermo-chemical (47) and vibrational spectroscopic (27, 48, 49) data), and from the fact that, apart from bond length deformations, all other strain factors (angle deformations, unfavourable torsion angles, strongly repulsive nonbonded interactions) are important for the calculation of their properties. The cycloalkanes are thus good candidates for testing force fields. For a more detailed discussion we choose cyclodecane, a so-called medium-ring compound. 

Comment Computational test is needed to determine the sign of sigma nonbonded interaction. 

The Diels Alder reaction has been an important testing ground for qualitative theories for many years256,257K In this section we consider the importance of nonbonded attractive interactions on Diels Alder reactivity. Our model reaction system is butadiene and the three isomers of dicyanoethylene ... 

Figure 10.1. Additivity test for Coulomb nonbonded interaction energies, Eq. (10.8). The radii of the degles represent an uncertainty of 0.03 kcal/mol. The points correspond to the numbering used in Table 10.1. (From Ref. 207.)...

The success of the ligand-ligand repulsion model prompted its adoption as an element of a molecular mechanics program. In the resulting approach the valence angles around the metal ion are modeled solely by nonbonded interactions, using the usual van der Waals potential (for example, Eq. 2.9 kg = 0 in Eq. 2.7 Urey-Bradley approach)136. 6 Again, the fact that the electronic effects responsible for the directionality of bonds are not explicitly modeled here may seem questionable but extensive tests have shown the model to be reliable 371. An explanation for this apparent contra-... 

The initial series of simulations was performed for 0 < p < 1. Because the purpose of these simulations was to test the common assumption that the nonbonded interactions did not contribute to the deviatoric stress, the sum in Eq. (6) was separated as in Eq. (9) to show the bonded and nonbonded interactions explicitly. The first sum is over all atom pairs that interact through u/, the second is over all pairs that interact through unh. 

There is a strong correlation between the parameters of different potential energy functions so that they should not be developed or refined in isolation. For example, the barrier to rotation about a bond can be modified by changing the explicit torsion angle term or by changing the nonbonded interactions. Thus, the effect of any change on a force field parameter needs to be tested extensively, i.e.,... 

The extent to which this kind of calculation is able to predict the effect of the solvent on conformational properties of carbohydrates has been thoroughly tested on 2-substituted oxane derivatives, D-glucopyranose, and methyl a- and y -D-glucopyranoside. In the model applied, the cavity term in Eq. 7 is based on an expression taken from the Scaled Particle Theory, and the electrostatic term is calculated according to the reaction held theory. The dispersion term takes into account both attractive and repulsive nonbonding interactions by using a combination of Lx)ndon dispersion energy and Bom-type repulsion. ... 

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