Atomic-molecular interaction

Figure 6-8. Correlation of electronic terms for the atomic-molecular interaction N2-O electronic terms...

The diffusion activation energy is defined by the values of electron densities of migrating particle and particles surrounding it at the distance of atom-molecular interaction radius (R), that is, such interaction is carried out via the power field of particles evaluated by the values of their P-parameters [16], In such a model the diffusion process has basically the similar nature in any aggregate state and its energy and directedness are defined by three main factors ... 

Thus we assume that the radius of sphere of atom-molecular interaction during the particle diffusion in the sublimation process is defined as follows ... 

On the top part of liquid surface layer the volume of the sphere of atom-molecular interaction and the number of particles in it is practically two times... 

Koide A, Meath W J and Allnatt A R 1981 Second-order charge overlap effects and damping functions for isotropic atomic and molecular interactions Chem. Phys. 58 105... 

The long-range interaction V(R) between two atomic/molecular species can be decomposed into... 

Molecular mechanical force fields use the equations of classical mechanics to describe the potential energy surfaces and physical properties of molecules. A molecule is described as a collection of atoms that interact with each other by simple analytical functions. This description is called a force field. One component of a force field is the energy arising from compression and stretching a bond. 

Arestraint (not to be confused with a Model Builder constraint) is a user-specified one-atom tether, two-atom stretch, three-atom bend, or four-atom torsional interaction to add to the list of molecular mechanics interactions computed for a molecule. These added interactions are treated no differently from any other stretch, bend, or torsion, except that they employ a quadratic functional form. They replace no interaction, only add to the computed interactions. 

Molecular Interaction. The examples of gas lasers described above involve the formation of chemical compounds in their excited states, produced by reaction between positive and negative ions. However, molecules can also interact in a formally nonbonding sense to give complexes of very short lifetimes, as when atoms or molecules collide with each other. If these sticky collisions take place with one of the molecules in an electronically excited state and the other in its ground state, then an excited-state complex (an exciplex) is formed, in which energy can be transferred from the excited-state molecule to the ground-state molecule. The process is illustrated in Figure 18.12. 

The use of molecular and atomic beams is especially useful in studying chemiluminescence because the results of single molecular interactions can be observed without the complications that arise from preceding or subsequent energy-transfer coUisions. Such techniques permit determination of active vibrational states in reactants, the population distributions of electronic, vibrational, and rotational excited products, energy thresholds, reaction probabihties, and scattering angles of the products (181). 

When a gas comes in contact with a solid surface, under suitable conditions of temperature and pressure, the concentration of the gas (the adsorbate) is always found to be greater near the surface (the adsorbent) than in the bulk of the gas phase. This process is known as adsorption. In all solids, the surface atoms are influenced by unbalanced attractive forces normal to the surface plane adsorption of gas molecules at the interface partially restores the balance of forces. Adsorption is spontaneous and is accompanied by a decrease in the free energy of the system. In the gas phase the adsorbate has three degrees of freedom in the adsorbed phase it has only two. This decrease in entropy means that the adsorption process is always exothermic. Adsorption may be either physical or chemical in nature. In the former, the process is dominated by molecular interaction forces, e.g., van der Waals and dispersion forces. The formation of the physically adsorbed layer is analogous to the condensation of a vapor into a liquid in fret, the heat of adsorption for this process is similar to that of liquefoction. 

Selected art in the text is supported by dynamic media. Students can view motion, three-dimensional effects, and atomic and molecular interactions to learn to visualize as chemists do—at a molecular level. 

In their initial stndies, Pallant and Tinker (2004) found that after learning with the molecular dynamic models, 8th and 11th grade students were able to relate the difference in the state of matter to the motion and the arrangement of particles. They also used atomic or molecular interactions to describe or explain what they observed at the macroscopic level. Additionally, students interview responses included fewer misconceptions, and they were able to transfer their understanding of phases of matter to new contexts. Therefore, Pallant and Tinker (2004) concluded that MW and its guided exploration activities could help students develop robust mental models of the states of matter and reason about atomic and molecular interactions at the submicro level. 

Atomic orbitals interact with each other to give bond orbitals (Sect. 1), which mutually interact to give molecular orbitals (Sect. 2). Here we will examine interactions of molecular orbitals, especially those of frontier orbitals important for chemical reactions. 

Since the idea that all matters are composed of atoms and molecules is widely accepted, it has been a long intention to understand friction in terms of atomic or molecular interactions. One of the models proposed by Tomlinson in 1929 [12], known as the independent oscillator model, is shown in Fig. 13, in which a spring-oscillator system translates over a corrugating potential. Each oscillator, standing for a surface atom, is connected to the solid substrate via a spring of stiffness k, and the amplitude of the potential corrugation is. ... 

Both of the above approaches rely in most cases on classical ideas that picture the atoms and molecules in the system interacting via ordinary electrical and steric forces. These interactions between the species are expressed in terms of force fields, i.e., sets of mathematical equations that describe the attractions and repulsions between the atomic charges, the forces needed to stretch or compress the chemical bonds, repulsions between the atoms due to then-excluded volumes, etc. A variety of different force fields have been developed by different workers to represent the forces present in chemical systems, and although these differ in their details, they generally tend to include the same aspects of the molecular interactions. Some are directed more specifically at the forces important for, say, protein structure, while others focus more on features important in liquids. With time more and more sophisticated force fields are continually being introduced to include additional aspects of the interatomic interactions, e.g., polarizations of the atomic charge clouds and more subtle effects associated with quantum chemical effects. Naturally, inclusion of these additional features requires greater computational effort, so that a compromise between sophistication and practicality is required. 

It is due to the already significant surface atom-molecule interaction present in the case of ammonia and water before reaction, when the molecules adsorb strongly through their respective molecular lone-pair orbitals. 

If the atomic or molecular interaction potentials are known, the surface energy for very thin films can be calculated by assuming additivity and integrating over the finite thickness of the film below the surface plus that of the semi-infinite substrate underneath. 

---