Chemical Bonding and Physical Properties

All suboxides and subnitrides described in the preceding sections are metallic. In the case of the alkali metal suboxides this property has been demonstrated by measurements of the electrical conductivity [58], CS7O, for example, exhibits a free electron like behavior in the temperature dependence of its resistivity rather similar to the element Cs itself. The characteristic colors of the alkali metal suboxides have been mentioned before, and spectroscopic investigations to be discussed in the following provide a more quantitative access to the metallic properties and the underlying chemical bonding. 

The decrease of the work function of Rb and Cs upon oxidation is also seen in quite different experiments. The wetting properties of liquid He on the metal surfaces is critically dependent on the mutual interaction, which is determined by the work function [63, 64]. Discrete He droplets are stable to higher temperature on Cs than on Rb, the latter in this property coming dose to Cs upon oxidation. 

Model calculations for the Cs suboxides in comparison with elemental Cs have shown that the decrease in the work function that corresponds to an increase in the Fermi level with respect to the vacuum level can be explained semi-quantitatively with the assumption of a void metal [65], The Coulomb repulsion of the conduction electrons by the cluster centers results in an electronic confinement and a raising of the Fermi energy due to a quantum size effect. 

Addison, The Chemistry of the Liquid Alkali Metals, Wiley Sons, Chichester, 1984. 

in Clusters and Colloids, from Theory to Application, G. Schmid, ed., VCh Weinheim, New York, Basel, Cambridge, Tokyo, 1994, p. 373. 

---

At present no microscopic theory exists which can answer questions like this, and there are few phenomenological descriptions either. What is badly needed is a common framework which will at least facilitate the development of incisive and informative phenomenological descriptions. I believe that polarizabilities can provide such a framework, and that they can be used to explain trends in chemical bonding and physical properties even when an absolute and rigorous connection is not demonstrated. For this to become so, however, it is necessary to understand the quantum mechanical meaning of polarizabilities in a more profound way than has generally been the case in the past. The story of how little this rather simple subject has been explored theoretically illustrates why it is that our understanding of the properties of materials is still at so primitive a level. 

Theories of molecular stracture attempt to describe the nature of chemical bonding both qualitatively and quantitatively. To be useful to chemists, the bonding theories must provide insight into the properties and reactivity of molecules. The stractural theories and concepts that are most useful in organic chemistry are the subject of this chapter. Our goal is to be able to relate molecular stracture, as depicted by stractural formulas and other types of stractural information, such as bond lengths and electronic distributions, to the chemical reactivity and physical properties of molecules. 

The two groups of substances thus have completely different structures and totally different paths leading to their formation. A similarity in the chemical properties is due to similarity of the chemical bonds. Some physical properties are also similar, such as color which is due to Si-Si bond. Each group of compounds will be described separately. 

Epoxies are especially reliable when used with epoxy-based composites because they have similar chemical characteristics and physical properties. Room temperature curing adhesives are often used to bond large composite structures to eliminate expensive fixtur-ing tools and curing equipment required of higher-temperature cure adhesives. However, room temperature epoxies require long cure times, so they are not suitable for large, highspeed production runs. Some of the lower-temperature composite materials are sensitive to the heat required to cure many epoxies. Epoxies are too stiff and brittle to use with flexible composites. 

Recently, HPMOs have been studied by several research groups. The main point of HPMOs is the introduction of different bridge-bonded organic groups into the framework, which could impart a unique chemical function and physical property to the material. 

Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

Substitution of fluorine for hydrogen in an organic compound has a profound influence on the compound s chemical and physical properties. Several factors that are characteristic of fluorine and that underHe the observed effects are the large electronegativity of fluorine, its small size, the low degree of polarizabiHty of the carbon—fluorine bond and the weak intermolecular forces. These effects are illustrated by the comparisons of properties of fluorocarbons to chlorocarbons and hydrocarbons in Tables 1 and 2. 

Phosphorus compounds exhibit an enormous variety of chemical and physical properties as a result of the wide range ia the oxidation states and coordination numbers for the phosphoms atom. The most commonly encountered phosphoms compounds are the oxide, haUde, sulfide, hydride, nitrogen, metal, and organic derivatives, all of which are of iadustrial importance. The hahde, hydride, and metal derivatives, and to a lesser extent the oxides and sulfides, are reactive iatermediates for forming phosphoms bonds with other elements. Phosphoms-containing compounds represented about 6—7% of the compound hstiugs ia Chemical Abstracts as of 1993 (1). 

Reactions of the Disulfide Group. Besides the thiol end groups, the disulfide bonds also have a marked influence on both the chemical and physical properties of the polysulftde polymers. One of the key reactions of disulfides is nucleophilic attack on sulfur (eq. 4). The order of reactivity for various thiophiles has been reported as (C2H O) P > R, HS , C2H5 S- >C,H,S- >C,H,P,... 

Heterocycles of type 3-14 containing either additional nonsulfur heteroatom or nonsulfone/sulfoxide functional groups (other than double bonds) within the ring skeleton, have been excluded from being treated because of the overwhelming amount of material and since we wanted to emphasize the effects which these two functional groups exert on the chemical and physical properties of the systems. 

The elements in a compound are not just mixed together. Their atoms are actually joined, or bonded, to one another in a specific way due to a chemical change (see Section A). The result is a substance with chemical and physical properties different from those of the elements that form it. For example, when sulfur is ignited in air, it combines with oxygen from the air to form the compound sulfur dioxide. Solid yellow sulfur and odorless oxygen gas produce a colorless, pungent, and poisonous gas (Fig. C.l). 

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. 

All the elements in a main group have in common a characteristic valence electron configuration. The electron configuration controls the valence of the element (the number of bonds that it can form) and affects its chemical and physical properties. Five atomic properties are principally responsible for the characteristic properties of each element atomic radius, ionization energy, electron affinity, electronegativity, and polarizability. All five properties are related to trends in the effective nuclear charge experienced by the valence electrons and their distance from the nucleus. 

---