Repulsion nonbonding

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. 

We are now prepared to examine how attractive or repulsive nonbonded interactions determine molecular geometry. We shall discuss representative examples where pi and/or sigma nonbonded interactions obtain. In each case, we provide computational data in support of general theoretical arguments as well as pertinent experimental results. It should be mentioned that only crucial indices of nonbonded interactions are provided and the survey of the experimental work is by necessity incomplete, i.e., it would take volumes to consider all available data. Nonetheless, at the end of this chapter, the reader should be able to apply the key ideas to problems of direct interest to him. 

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. ... 

A comparison of 1-alkyladamantanes versus 2,2-methylalkylpropanes is given in Table 15. The preference for adamantyl substitution is probably due to a lessening of the repulsive nonbonded interactions between R and the ring because of a widening of the C—C—R angle in adamantane ( 110 vs. 109 in f-butyl). 

As such, for total energy the semiempirical AMI (Austin Model 1) and PM3 (re-parameterized AMI with less repulsive nonbonding interactions) were considered among the ab initio //Fand DFTmethods. The results are presented in Tables ill and 3.28. 

Types of Strain in Conformers Torsional strain is caused by electron-electron repulsions (nonbonded interactions or van der Waals forces) in adjacent bonds that can be reduced or eliminated by rotation around a single bond. In Figure 1.31, the energy prohle of rotation around the O -bond of butane is shown, where different levels of torsional strain are coursed by the placements of the substituents relative to each other. Different conformations have different energy levels and therefore different levels of... 

---