Monopole charges

We assume that the double bonds in 1,3-butadiene would be the same as in ethylene if they did not interact with one another. Introduction of the known geometry of 1,3-butadiene in the s-trans conformation and the monopole charge of 0.49 e on each carbon yields an interaction energy <5 — 0.48 ev between the two double bonds. Simpson found the empirical value <5 = 1.91 ev from his assumption that only a London interaction was present. Hence it appears that only a small part of the interaction between double bonds in 1,3-butadiene is a London type of second-order electrical effect and the larger part is a conjugation or resonance associated with the structure with a double bond in the central position. 

The atom-centered monopole charges have been determined by fitting the quantum mechanically derived electrostatic potential in the region... 

Kollman suggested that the electron density could be partitioned into a set of monopole charges based on the electrostatic potential. At the same time, Momany showed that a set of monopole charges could be fit to the electrostatic potential. Atom-centered point charges that best fit the overall MEP were determined by iterative minimization of the following function ... 

Table III summarizes some calculations carried out to explore what effects contribute to the high reactivity of SOD. For the native-llke model with a monopole charge of A, inclusion of the (non-centrosymmetric) quadrupole increases the reaction rate by 40%. The quadrupole evidently helps to steer O2 into the active site. Parallel simulations were also carried out in which monopole charges of 0 and +4 were used. Although increasing the monopole charge from -4 to 0 to +4 increased the rates by factors of 2.5 and 5, respectively, the steering effect is present in each case. This suggests that the enhancement in rate due to steering by local electrostatic interactions will persist in the presence of added salt, which will suppress the effects of the monopole field more strongly than those of the shorter-ranged quadrupole field.

Table 10 shows MPA results for the crystal structures of pyrimidine and s-tetrazine. " ° In the MPA calculations, all parameters not fixed by the observed space group symmetry were varied simultaneously to find the minimum of the lattice energy. The monopole charges were multiplied by a scale factor of 0.83 so that they approximately reproduce experimental dipole moments. In the MPA calculations foreshortened C —H bond lengths were used, and monopole values were adjusted to retain the nonforeshortened bond dipole value. 

An infinite number of centers and the monopoles (charges) only... 

Using the monopole charges is a very crude way to determine electrostatic potentials, but it is capable of giving qualitatively correct values, as we show for a few sites on formamide using a 431G wave function. The exact potential is listed at possible H-bond sites (1), (2), (3) and the Mulliken population derived potential value is in parentheses next to it. 

The electrostatic interaction only between monomers with nonzero monopole charges, with or without condensed ions, is considered till now in this work (the third-term in the polymer free energy Fs). Further, Fs considers only the monopole contribution of each ion-pair or ion-triplet. For example, a monomer-Na pair and a monomer-Ba +-Cl triplet would contribute identically to Fs, although they have different electrostatic effects. Similarly, a monomer-Ba pair would be simply treated as a +1 charge, although the pair has additional dipole effects. These additional dipole or higher order multipole effects are critical when the average... 

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