Nuclear electronic structure relationship

In this chapter we will familiarize ourselves with basic concepts in molecular symmetry [17]. The presence or absence of symmetry has consequences on the appearance of spectra, the relative reactivity of groups, and many other aspects of chemistry, including the way we will make use of orbitals and their interactions. We will see that the orbitals that make up the primary description of the electronic structure of molecules or groups within a molecule have a definite relationship to the three-dimensional structure of the molecule as defined by the positions of the nuclei. The orientations of the nuclear framework will determine the orientations of the orbitals. The relationships between structural units (groups) of a molecule to each other can often be classified in terms of the symmetry that the molecule as a whole possesses. We will begin by introducing the basic termi-... 

As we have discussed many times elsewhere, the magnetic and electric nuclear hyperfine constants provide information about the electronic structure of the molecule. The latter was outlined and summarised briefly in (8.391), where the unpaired electron is placed in a 7r-type molecular orbital, which may be regarded as a linear combination of the N and O atomic 2p orbitals. Dousmanis [ 140] was among to first to show the relationships between the dipolar hyperfine constants and the electronic wave function. The orbital hyperfine constant, a, which in NO is found to have the value 84.20378 MHz, is given by the expression... 

Modern highly accurate electronic structure calculations are undertaken with a view to elucidating molecular structure. For this to be possible the relationship of nuclear motion to electronic structure must be specified. An outline of how this specification might be made and some of the problems... 

It seems that there are still difficulties in the way of specifying precisely how nuclear motions are related to electronic structure and, at present, it is not clear that the relationship can be determined with any confidence. 

Wo have already made the general statement that when any two metals are used to form an alloy the actual structure of the alloy depends on whether, for the medals concerned, the tendency to produce a solid solution is greater than the opposing tendency to form an intermetallic compound. Since the formation of solid solutions is mainly governed by the relative sizes of the two atoms, while compound formation involves relationships between the atoms and their extra-nuclear electrons, it is probably more convenient to discuss first the sizes of metal atoms, usually expressed in terms of atom diameters. We have recently shown that in all types of pure metal crystals there are certain directions of atom close packing, and if, as is convenient, we continue to regard atoms as spheres, it is readily seen that the shortest interatomic distances are of the same magnitude as the respective atomic diameters. We shall therefore proceed to discuss the closest distance of approach of atoms in the typical metallic lattices. 

Experimental techniques for the measurement of Si nmr spectra (mainly for solutions) are briefly discussed, followed from considerations of the nuclear-spin relaxation mechanisms with emphasis on the Si nucleus. Chemical shifts 5 Si and indirect nuclear spin-spin coupling constants j( Si,x), the most prominent nmr parameters, are discussed in more detail The increasing reliability of quantum chemical methods for the calculation of these parameters is pointed out. The relationship between the electronic structure and the nmr parameters b Si and j( Si,x) is indicated... 

Nowadays, computational techniques have become useful interpretative and predictive tools to investigate environmental effects on properties and processes in supramo-lecular systems of increasing complexity. The purpose of this chapter is to show the capabilities of such techniques, focussing particularly on the simulation of spectroscopic properties, since they allow a direct comparison between calculated and experimental data. Moreover, the computation of the spectroscopic response permits an analysis of the relationship between the nuclear and electronic structure of the molecular probes and the interactions with the environment These ideas are illustrated with case studies involving different spectroscopic techniques and various molecular and environmental systems. 

Quantitative structure-physical property relationships (QSPR). There are two types of physical properties we must consider ground state properties and properties which depend on the difference in energy between the ground state and an excited state. Examples of the former are bond lengths, bond angles and dipole moments. The latter include infrared, ultraviolet, nuclear magnetic resonance and other types of spectra, ionization potentials and electron affinities. 

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