Molecular Structure and Interaction

In Chapter 3, we introduced Lewis dot structures as a tool with which to predict the bonding patterns in a molecule, hi Chapter 4, we will study the shape, or geometry, of molecules. Geometry has an important influence on the physical and chemical properties of molecules, such as melting point, boiling point, and reactivity. We will see that we can predict the shapes of molecules with considerable accuracy using a simple method based on Lewis structures. 

in Chapter 3 we introduced the concept of molecular-orbital theory to explain bonding in diatomic molecules. In Chapter 4, we will extend this useful quantum-mechanical concept to polyatomic molecules. In addition, in the final section of Chapter 4, we will examine how molecular shape and bonding affect the interactions of molecules with one another. 

1 The Basic Three-Dimensional Structure of a Molecule Can Be Predicted Using the VSEPR Model 

Within the VSEPR model, the basic molecnlar geometry about a given central atom is nniqnely determined from two nnmbers  

Ns = ( of lone pairs) -i- ( of atoms bonded to central atom) 

---

We carry out computer simulations in the hope of understanding bulk, macroscopic properties in temis of the microscopic details of molecular structure and interactions. This serves as a complement to conventional experiments, enabling us to leam something new something that cannot be found out in other ways. 

Figure B3.3.1. Simulations as a bridge between the microscopic and the macroscopic. We mput details of molecular structure and interactions we obtain predictions of phase behaviour, structural and time-dependent properties.

Tager JM. Organelles in Eukaryotic Cells. Molecular Structure and Interactions, Plenum Press, New York, 1990. 

Computational Center for Molecular Structure and Interactions, Department of Chemistry... 

A great deal of work has been done on nuclear spin-spin coupling in pyridine derivatives. The variety and scope of this work is illustrated in Table 9, which shows a sample of the results obtained over the last few years, and indicates the type of information available, and the way in which it can be correlated with other facets of molecular structure and interactions. 

Computational Center for Molecular Structure and Interactions, Department of Chemistry, P. O. Box 17910, Jackson State University, Jackson, MS 39217, USA... 

The thermodynamic and physical properties of pure steam are well estabUshed over the range of pressures and temperatures used. The chemical properties of steam and of substances in steam, their molecular structures, and interactions with the solid surfaces of containments need to be more fully explored. 

Yano, J., and K. Sato, FT-IR Studies on Polymorphism of Fats Molecular Structures and Interactions, Food Res. Inti. 32 249-259 (1999). 

The success of the application of quantum mechanical methods to the description of physical and chemical properties of molecules is evident in every facet of modern chemical research and is well documented. It may be of special interest to point out some of the very recent uses of quantum chemical approaches applied as a tool in the study of molecular structure and interactions. The examples below were not selected for their relative importance in the field of quantum chemistry nor for the use of the most advanced methodology. Each represents one of many illustrations of a specific application. 

In recent years, spectroscopic techniques have become invaluable tools for probing the molecular structures and interactions in CEPs. Three of the most useful are the UV-visible, circular dichroism, and Raman spectroscopy techniques. In each of these cases, absorption of the incident radiation by water (and many other electrolytes) is minimal, meaning that it is possible to obtain the spectra by reflection or transmission of light from the sample while it is immersed in an electrolyte as part of an electrochemical cell. Another valuable technique has been electron spin resonance (ESR) spectroscopy, which involves the absorption of microwaves. 

Feeney, K.A., WeUner, N., Gilbert, S.M. et al. (2(X)3) Molecular structures and interactions of repetitive peptides based on wheat glutenin subunits depend on chain length. Biopolymers, 72, 123-131. 

Raman spectroscopy is, in principle, amenable to dynamic studies, which affords additional information about electrochemical interfaces. The conceptually most straightforward extensions into this field are time-resolved investigations. Here, changes in the molecular structure and interaction at the surface, initiated by some external activation at a given time, are monitored. 

In practice, dynamic oscillatory measurements are sensitive probes of molecular structure and interactions, for example, in emulsions and dispersions. Oscillatory measurements probe emulsion structure without destroying it. This is accomplished by applying very small sinusoidal displacements or strains to the emulsion at controlled amplitude and frequency. In general, there are two mechanisms for a material to respond to a deformation ... 

---