Electrostatic , generally field

In Chapter 4, we will discuss the relative importance of inductive effects and field effects on reactivity. Generally, field effects appear to be the dominant mechanism for the transmission of electrostatic effects of polar bonds to other parts of a molecule. 

More recently, with the significant increases in computer power even on desktop PCs, methods for directly matching 3-D features of molecules have become more prevalent. Features here generally refer to various types of molecular fields, some such as electron density ( steric ) and electrostatic-potential fields are derived from fundamental physics (30,31) while others such as lipophilic potential fields (32) are constructed in an ad hoc manner. Molecular fields are typically represented as continuous functions. Discrete fields have also been used (33) albeit somewhat less frequently except in the case of the many CoMFA-based studies (34). 

An alternative approach (30) treats the steric and electrostatic potential fields directly. The steric field is generally given by an expression similar to that in Eq. 2.70,... 

Any computational treatment of TM systems must account for the LFSE. QM methods achieve this implicitly but d-electron effects must be explicitly added to MM (4). Some effects can be modeled within conventional MM. For example, low-spin d8 complexes are planar by virtue of the LFSE (21,22), but a planar structure can also be enforced using a normal out-of-plane term (22). However, the simplest general model for describing d-orbital energies is ligand field theory (LFT) (23) which was itself derived from the earlier electrostatic crystal field theory (CFT) (24) approach. 

Generally, upon adsorption, the intensity of UV-VIS bands is significantly altered caused by an increase or a decrease in the extinction coefficient e. This effect depends mainly on the adsorption geometry, i.e., whether the electronic vector of the adsorbed species is parallel or perpendicular to the electrostatic surface field. In addition, e may follow a direct or inverse variation with coverage 0, some-... 

In 2001, van Duin (Van Duin et al, 2001) proposed a technique of reaction force field, this method was applied to carbohydrates, the geometry data of compounds from simulation are consistent with the literature well, and the bond parameters agree to the results of quantum chemistry, but the calculation time is much less than the time required for quantum chemical calculations. Subsequently, researchers come to realize the advantages of the reaction force field method compared with quantum chemistry and electrostatic force field methods. Reaction force field parameters that are suitable for other materials have gradually been developed, such as silicon oxide, platinum, and titanium. The reaction force field parameters are theoretically universal and general. When Kim et al (2013) studied the interaction between the titanium oxide and water, sodium ions, chloride ions, methanol, and formic acid and other substances, the interaction parameter of Cl/O/H and Na/O/H is from the hteratures of different systems. 

The potential importance of the double layer to adsorption in charged systems can be seen from the fact that the relationship in Eq. (10.1) predicts that potential interactions between surface charges and ions in solution will be significantly affected by the conditions in the solution phase. Double-layer theory is most important in the general field of colloidal stability, in which the concentration and valence of ions present can dramatically affect the stability and utility of an electrostatically stabilized colloidal system. 

Kirkwood generalized the Onsager reaction field method to arbitrary charge distributions and, for a spherical cavity, obtained the Gibbs free energy of solvation in tenns of a miiltipole expansion of the electrostatic field generated by the charge distribution [12, 1 3]... 

MMl, MM2, MM3, and MM4 are general-purpose organic force fields. There have been many variants of the original methods, particularly MM2. MMl is seldom used since the newer versions show measurable improvements. The MM3 method is probably one of the most accurate ways of modeling hydrocarbons. At the time of this book s publication, the MM4 method was still too new to allow any broad generalization about the results. However, the initial published results are encouraging. These are some of the most widely used force fields due to the accuracy of representation of organic molecules. MMX and MM+ are variations on MM2. These force fields use five to six valence terms, one of which is an electrostatic term and one to nine cross terms. 

The Merck molecular force field (MMFF) is one of the more recently published force fields in the literature. It is a general-purpose method, particularly popular for organic molecules. MMFF94 was originally intended for molecular dynamics simulations, but has also seen much use for geometry optimization. It uses five valence terms, one of which is an electrostatic term, and one cross tenn. 

MOMEC is a force field for describing transition metal coordination compounds. It was originally parameterized to use four valence terms, but not an electrostatic term. The metal-ligand interactions consist of a bond-stretch term only. The coordination sphere is maintained by nonbond interactions between ligands. MOMEC generally works reasonably well for octahedrally coordinated compounds. 

AMBER, BIO-h and OPLS scale 1 van der Waals and 1 electrostatic interactions. Although the value of the 1 nonbonded scale factors is an option in HyperChem, you should generally use recommended values. This is because during parameterization, the force field developers used particular values for the 1 nonbonded scale factors, and their parameters may not be correct for other scale factors. 

Dynamic-field mass spectrometer. A mass spectrometer in which the separation of an ion beam depends essentially on the use of a field or fields that vary with time. These fields are generally electrostatic or magnetic. 

The source requited for aes is an electron gun similar to that described above for electron microscopy. The most common electron source is thermionic in nature with a W filament which is heated to cause electrons to overcome its work function. The electron flux in these sources is generally proportional to the square of the temperature. Thermionic electron guns are routinely used, because they ate robust and tehable. An alternative choice of electron gun is the field emission source which uses a large electric field to overcome the work function barrier. Field emission sources ate typically of higher brightness than the thermionic sources, because the electron emission is concentrated to the small area of the field emission tip. Focusing in both of these sources is done by electrostatic lenses. Today s thermionic sources typically produce spot sizes on the order of 0.2—0.5 p.m with beam currents of 10 A at 10 keV. If field emission sources ate used, spot sizes down to ca 10—50 nm can be achieved. 

Forces of Adsorption. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. In physical adsorption the forces are relatively weak, involving mainly van der Waals (induced dipole—induced dipole) interactions, supplemented in many cases by electrostatic contributions from field gradient—dipole or —quadmpole interactions. By contrast, in chemisorption there is significant electron transfer, equivalent to the formation of a chemical bond between the sorbate and the soHd surface. Such interactions are both stronger and more specific than the forces of physical adsorption and are obviously limited to monolayer coverage. The differences in the general features of physical and chemisorption systems (Table 1) can be understood on the basis of this difference in the nature of the surface forces. 

Noncontacting Electrostatic Measurements These measurements are made by instruments that respond to the electric fields at their sensing electrodes. Gonsiderable care must be taken in the interpretation of the measurements. The three general types of devices are described as follows. 

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