Electrostatic potential minima

Wiener, J. J. M., J. S. Murray, M. E. Grice, and P. Politzer. 1997. Relationships Between Bond Dissociation Energies, Electronic Density Minima and Electrostatic Potential Minima. Mol. Phys. In press. 

We have shown, for several families of oxygen- and nitrogen-containing molecules taken separately, that good relationships exist between the electrostatic potential minima (Vmin)... 

Other graphical uses of the electrostatic potential have been developed by Davis et al. (413), who were able to graphically align cyclic AMP and cyclic GMP, based on the superimposition of their respective electrostatic potential minima, and by Weinstein et al. (414), who oriented 5-hydroxytryptamine and 6-hydroxy-tryptamine based on the alignment of an electrostatically derived "orientation vector."... 

We have recently explored the possibility of using the NH3 molecule as a basis for measuring the electron-attracting tendencies of substituents We computed V, for the amine nitrogens in a series of NH2-X molecules at the STO-5G level. In order to focus on electronic factors, the HNX angles were constrained to their STO-3G values in NH3. Table 2 lists the resulting electrostatic potential minima of 31 NH2-X molecules, along with the inductive and resonance substituent constants, o, and for the substituents X. 

Table 2 Calculated STO-5G Electrostatic Potential Minima of NH2-X Molecules and Inductive and Resonance Substituent Constants ...

Table 3 Hydrogen Bond Acceptor Parameters P and STO-5G Calculated Electrostatic Potential Minima, V ,i (0), for Some Molecules Containing Double-Bonded Oxygens...

IlyperChetn displays the electrostatic potential as a contour plot when you select th e appropriate option in th e Con tour Plot dialog box. Choose the values for the starting contour and the contour increment so that you can observe the minimum (typically about 0.5 for polar organ ic molecules) and so that the zero potential line appears. 

Equation (7) guarantees that the minimum in the electrostatic potential is attained at the critical point r defining a sphere 5(0, r ), which contains an electron density amount equal to the atomic number Z. From eq (7) we may conclude that the charge normalization condition for neutral atoms may only be satisfied for r —> oo, whereas for cations it is never satisfied. 

A speculative proposal was made thirty years ago by Schmid and Krenmayr77, namely that a nitrosyl ion solvated, but not covalently bonded, by a water molecule may be involved in these systems. This hypothesis was investigated theoretically in 1984 by Nguyen and Hegarty78 who carried out ab initio SCF calculations of structure and properties employing the minimal STO-3G basis set, a split-valence basis set plus polarization functions. Optimized geometries of six planar and two nonplanar forms were studied for the nitrosoacidium ion. The lowest minimum of molecular electrostatic potential... 

In order to stably levitate an object, the net force on it must be zero, and the forces on the body, if it is perturbed, must act to return it to its original position. The object must be at a local potential minimum that is, the second derivatives with respect to all spatial coordinates of the potential must be positive. This may seem, at first sight, to be trivial to arrange. However, any system whose potential is a solution to Laplace s equation is automatically unstable A statement in words of Laplace s equation is that the sum of the second partial derivatives of the potential is zero, and so not all can be simultaneously positive. This has long been known for electrostatic potentials, having been stated by Earnshaw(n) Millikan s scheme for suspending charged particles is thus only neutrally stable, since the fields within a Millikan capacitor provide no lateral constraint. 

The average local electrostatic potential V(r)/p(r), introduced by Pohtzer [57], led Sen and coworkers [58] to conjecture that the global maximum in V(r)/p(r) defines the location of the core-valence separation in ground-state atoms. Using this criterion, one finds N values [Eq. (3.1)] of 2.065 and 2.112 e for carbon and neon, respectively, and 10.073 e for argon, which are reasonable estimates in light of what we know about the electronic shell structure. Politzer [57] also made the significant observation that V(r)/p(r) has a maximum any time the radial distribution function D(r) = Avr pir) is found to have a minimum. 

MFP Minimum energy path Molecular electrostatic potential... 

The question of methanol protonation was revisited by Shah et al. (237, 238), who used first-principles calculations to study the adsorption of methanol in chabazite and sodalite. The computational demands of this technique are such that only the most symmetrical zeolite lattices are accessible at present, but this limitation is sure to change in the future. Pseudopotentials were used to model the core electrons, verified by reproduction of the lattice parameter of a-quartz and the gas-phase geometry of methanol. In chabazite, methanol was found to be adsorbed in the 8-ring channel of the structure. The optimized structure corresponds to the ion-paired complex, previously designated as a saddle point on the basis of cluster calculations. No stable minimum was found corresponding to the neutral complex. Shah et al. (237) concluded that any barrier to protonation is more than compensated for by the electrostatic potential within the 8-ring. 

Hydrogen bond basicity is of much relevance to the problem of drug design. Hydrogen bond basicity was shown to correlate with the location of the electrostatic potential local minimum along the axis of the nitrogen lone pair in a series of heterocycles (94JCS(P2)199). The experimental and calculated basicities for oxazole, 2,4,5-trimethyloxazole, and pyridine are shown in Table 2. 

Correlations have been shown to exist between hydrogen bond accepting parameters (P) and calculated minimum electrostatic potentials Vmin near electron-rich sites, e.g. nitrogen and oxygen, for several molecules by Murray et al. [37], In order to examine if MEF values can also be correlated with the (P) parameters, we have computed MEF maps for two types of molecules which... 

This detailed picture of the movement of the atom during manipulation was achieved with the aid of simulations [6]. The atom moves in a local potential minimum on the surface. This potential is the sum of the surface potential and the tip potential. The surface potential can be expressed by the migration barrier while the tip potential describes the direct interaction via chemical or electrostatic forces. The local potential minimum is not identical with the adsorption site, in the limit of close tip-atom separation this minimum always resides below the tip resulting in the sliding mode. The atom is slowly pushed/pulled by the tip out of the adsorption site until it jumps into the next local potential minimum. The jump to the next potential minimum proceeds on a timescale of picoseconds [7,8] whereas typical tip speeds are of the order of 0.5-2.5nm/s. 

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