Chemical bonding polar

Polarization function-supplemented basis functions (e.g. 6-31G, 6-31G(d), DZp, cc-pVXZ, and cc-pCVXZ) add polarization functions to incorporate the anisotropic nature of molecular orbitals originating from chemical bonds. Polarization functions usually have higher angular momenta than the highest angular momenta of the atomic orbitals that mainly make up the molecular... 

Graphitized carbon black with a monomolecular layer of a polar polymer (e.g. polyglycol or copper phthalocyanin) or, for example, silica with chemically bonded polar groups. These adsorbents may give strong selective interactions with alcohols and amines. 

Polarization functions can be added to basis sets to allow for nonuniform displacement of a charge away from the atomic nuclei, thereby improving descriptions of chemical bonding. Polarization... 

Chemical bonding, polar The chemical bond that results between two atoms or molecules that are oppositely polarized. 

Much of chemistry is concerned with the short-range wave-mechanical force responsible for the chemical bond. Our emphasis here is on the less chemically specific attractions, often called van der Waals forces, that cause condensation of a vapor to a liquid. An important component of such forces is the dispersion force, another wave-mechanical force acting between both polar and nonpolar materials. Recent developments in this area include the ability to measure... 

In the case of chemisoriDtion this is the most exothennic process and the strong molecule substrate interaction results in an anchoring of the headgroup at a certain surface site via a chemical bond. This bond can be covalent, covalent with a polar part or purely ionic. As a result of the exothennic interaction between the headgroup and the substrate, the molecules try to occupy each available surface site. Molecules that are already at the surface are pushed together during this process. Therefore, even for chemisorbed species, a certain surface mobility has to be anticipated before the molecules finally anchor. Otherwise the evolution of ordered stmctures could not be explained. 

Basis sets can be further improved by adding new functions, provided that the new functions represent some element of the physics of the actual wave function. Chemical bonds are not centered exactly on nuclei, so polarized functions are added to the basis set leading to an improved basis denoted p, d, or f in such sets as 6-31G(d), etc. Electrons do not have a very high probability density far from the nuclei in a molecule, but the little probability that they do have is important in chemical bonding, hence dijfuse functions, denoted - - as in 6-311 - - G(d), are added in some very high-level basis sets. 

We can combine our knowledge of molecular geometry with a feel for the polarity of chemical bonds to predict whether a molecule has a dipole moment or not The molec ular dipole moment is the resultant of all of the individual bond dipole moments of a substance Some molecules such as carbon dioxide have polar bonds but lack a dipole moment because their geometry causes the individual C=0 bond dipoles to cancel... 

Cehte or firebrick packing for glc columns is often treated with TMCS, DMCS, or other volatile silylating agents (see Table 1) to reduce tailing by polar organic compounds. A chemically bonded methyl siUcone support is stable for temperature programming to 390°C and allows elution of hydrocarbons up to C q (20). 

Another fundamental property of chemical bonds is polarity. In general, it is to be expected that the distribution of the pair of electrons in a covalent bond will favor one of the two atoms. The tendency of an atom to attract electrons is called electronegativity. There are a number of different approaches to assigning electronegativity, and most are numerically scaled to a definition originally proposed by Pauling. Part A of Table 1.6... 

Parallel with these trends and related to them is the increase in chemical reactivity which is further enhanced by the increasing bond polarity and the increasing availability of low-lying vacant orbitals for energetically favourable reaction pathways. 

At the molecular level, electric dipole moments are important because they give information about the charge distribution in a molecule. Examination of the experimental data for a few simple compounds reveals that the electric dipole moment is also a property associated with chemical bonds and their polarity. The... 

Polar bond A chemical bond that has positive and negative ends characteristic of all bonds between unlike atoms, 182-183... 

The variation in the repolarization character causes systematic changes in the properties of the materials. Particularly, the transition from onedimensional structure compounds to three-dimensional structure compounds is accompanied by a decrease in the spontaneous polarization value and in the compound s Curie temperature, and a change in the character of the compound s chemical bonds [390]. 

We have to refine our atomic and molecular model of matter to see how bulk properties can be interpreted in terms of the properties of individual molecules, such as their size, shape, and polarity. We begin by exploring intermolecular forces, the forces between molecules, as distinct from the forces responsible for the formation of chemical bonds between atoms. Then we consider how intermolecular forces determine the physical properties of liquids and the structures and physical properties of solids. 

What Do We Need to Know Already This chapter huilds on the introduction to acids and bases in Section J. It also draws on and illustrates the principles of thermodynamics (Chapters 6 and 7) and chemical equilibrium (Chapter 9). To a smaller extent, it uses the concepts of hydrogen bonding (Section 5.5), bond polarity (Section 2.12), and bond strength (Sections 2.14 and 2.15). 

The interchange energy of electrons is in general the energy of the non-polar or shared-electron chemical bond. 

The application of the quantum mechanics to the interaction of more complicated atoms, and to the non-polar chemical bond in general, is now being made (45). A discussion of this work can not be given here it is, however, worthy of mention that qualitative conclusions have been drawn which are completely equivalent to G. N. Lewis s theory of the shared electron pair. The further results which have so far been obtained are promising and we may look forward with some confidence to the future explanation of chemical valence in general in terms of the Pauli exclusion principle and the Heisenberg-Dirac resonance phenomenon. 

Throughout our discussion the crystals will be referred to as composed of ions. This does not signify that the chemical bonds in the crystal are necessarily ionic in the sense of the quantum mechanics they should not, however, be of the extreme non-polar or shared electron pair type.13 Thus compounds of copper14 and many other eighteen-shell atoms cannot be... 

We have learned the interactions of the same orbitals and chemical bonds between the same atoms. The orbital phase plays a crucial role in the energies and the spacial extensions of the bond orbitals. Here we learn interactions of different orbitals and amplitude of orbitals, using an example of polar bonds between different atoms. 

The lone pairs on the nitrogen and oxygen atoms make a significant difference in the chemical reactions (Scheme 17). P-Arylenamines undergo [2-t-2] cycloaddition reactions [93] whereas P-arylenol ethers undergo [2h-2h-2] cycloaddition reactions [94]. The mode selectivity was attributed [95] to the HOMO amplitude or the n bond polarity. 

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. 

The validity (or lack thereof) of the classical Zind formahsm as applied to less polar intermetallics, involving metals along the Zind border, is nicely probed by electron-poof trelides. Seminal work by Corbett [44] and Belin [48] recognized the proclivity of trelides (Ga, In, H) to form cluster-based anion structures. The apparent electron deficiency in the chemical bonding of these cluster com-... 

Our work described in this section clearly illustrates the importance of the nature of the cations (size, charges, electronegativities), electronegativity differences, electronic factors, and matrix effects in the structural preferences of polar intermetallics. Interplay of these crucial factors lead to important structural adaptations and deformations. We anticipate exploratory synthesis studies along the ZintI border will further result in the discovery of novel crystal structures and unique chemical bonding descriptions. 

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