Structure-bonding theory, nuclear

Linus Pauling (1901-1994) was born in Portland Ore gon and was educated at Oregon State University and at the California Institute of Technology where he earned a Ph D in chemistry in 1925 In addition to re search in bonding theory Pauling studied the structure of proteins and was awarded the Nobel Prize in chemistry for that work in 1954 Paul ing won a second Nobel Prize (the Peace Prize) in 1962 for his efforts to limit the testing of nuclear weapons He was one of only four scientists to have won two Nobel Prizes The first double winner was a woman Can you name her" ... 

Whereas the quantum-mechanical molecular Hamiltonian is indeed spherically symmetrical, a simplified virial theorem should apply at the molecular level. However, when applied under the Born-Oppenheimer approximation, which assumes a rigid non-spherical nuclear framework, the virial theorem has no validity at all. No amount of correction factors can overcome this problem. All efforts to analyze the stability of classically structured molecules in terms of cleverly modified virial schemes are a waste of time. This stipulation embraces the bulk of modern bonding theories. 

In valence-bond theory, in terms of which the nuclear spin coupling was first interpreted by the American chemist Martin Karplus (ref. 153), the delocalization of o electrons is dealt with by including resonant structures such as... 

An atom of hydrogen consists of one proton (constituting the nucleus) and one electron. This simplicity of atomic structure means that H is of great importance in theoretical chemistry, and has been central in the development of atomic and bonding theories (see Chapter 1). The nuclear properties of the hydrogen atom are essential to the technique of H NMR spectroscopy (see Section 2.11). 

In the previous chapter, we saw that molecular geometry was a consequence of the tradeoff between electronic effects (the electron-electron repulsions that result from the Pauli principle) and steric effects (the nuclear-nuclear repulsions between the ligands on the central atom). In this chapter, we are concerned with the determination of molecular symmetry. While molecular geometry is concerned with the shapes of molecules, molecular symmetry has to do with the spatial relationships between atoms in molecules. As we shall see, it is the three-dimensional shape of a molecule that dictates its molecular symmetry and we can use a mathematical description of symmetry properties, known as group theory, to describe the structure, bonding, and spectroscopy of molecules. 

Molecular mechanics force fields rest on four fundamental principles. The first principle is derived from the Bom-Oppenheimer approximation. Electrons have much lower mass than nuclei and move at much greater velocity. The velocity is sufficiently different that the nuclei can be considered stationary on a relative scale. In effect, the electronic and nuclear motions are uncoupled, and they can be treated separately. Unlike quantum mechanics, which is involved in determining the probability of electron distribution, molecular mechanics focuses instead on the location of the nuclei. Based on both theory and experiment, a set of equations are used to account for the electronic-nuclear attraction, nuclear-nuclear repulsion, and covalent bonding. Electrons are not directly taken into account, but they are considered indirectly or implicitly through the use of potential energy equations. This approach creates a mathematical model of molecular structures which is intuitively clear and readily available for fast computations. The set of equations and constants is defined as the force... 

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