Electron density bonding

The spectrochemical data on 1,2- and 2,1-benzisoxazoles were reviewed in 1962 and reported on in greater detail in 1967 62HC(l7)l,p. 159, 62HC(17)l, p. 213, 67AHC(8)277). These reviews discuss tt-electron densities, bond order calculations and dipole moment studies for these ring systems. 

Electron densities, bond densities, and spin densities, as well as particular molecular orbitals may be displayed as graphical surfaces. In addition, the value of the electrostatic potential or the absolute value of a particular molecular orbital may be mapped onto an electron density surface. These maps provide information about the environment around the accessible surface of a molecule. Electrostatic potential maps show overall charge distribution, while orbital maps reveal likely sites for electrophilic and/or nucleophilic attack. Surface displays may be combined with any type of model display. 

In addition to the obvious structural information, vibrational spectra can also be obtained from both semi-empirical and ab initio calculations. Computer-generated IR and Raman spectra from ab initio calculations have already proved useful in the analysis of chloroaluminate ionic liquids [19]. Other useful information derived from quantum mechanical calculations include and chemical shifts, quadru-pole coupling constants, thermochemical properties, electron densities, bond energies, ionization potentials and electron affinities. As semiempirical and ab initio methods are improved over time, it is likely that investigators will come to consider theoretical calculations to be a routine procedure. 

The differences just outlined cannot be explained by means of a classical mechanical model. However, they can be explained by considering chemical bonding. In particular, hardness depends in covalent crystals on the fact that the valence (bonding) electrons are highly localized as shown by electron-diffraction studies which can provide maps of electron densities (bonds). 

The electron densities, bond orders, first six excitation energies, oscillator strengths, and weighting factors of pyrido[l,2-f)]pyridazinium cation were calculated by the PPP semiempirical version of the SCFMO-CI method, which indicated that protonation is expected to take place at the nonbridgehead nitrogen, and nucleophilic substitution is predicted to occur at position 3 (68TCA417i... 

A mathematical analysis of all four isomeric thiadiazoles by the simple molecular orbital method has provided molecular diagrams of the free base and conjugate acid of each thiadiazole, with electron densities, bond orders, and free valencies. On this basis, predictions have been made concerning the reactivities of the six non-equivalent carbon atoms, the basicities of the nitrogen atoms, and the delocalization energies in these molecules. The 5-position in free 1,2,4-thiadiazole should possess maximum reactivity in nucleophilic substitution reactions. The treatment also accounts for the order of the polarographic half-wave potentials and the position of the absorption maxima in the ultraviolet region of the spectra of 1,2,4- and 1,3,4-thiadiazoles.4... 

In ethane, each carbon is surrounded by four regions of electron density bond angles are 10-9.5°. In ethylene, each carbon is surrounded by three regions of electron density bond angles are 120°. 

Fig. 1. Molecular diagrams of the 1,2-dithiolium cation.122-123 Model A a, j8ag = 0.5)S b, )Sgg=1.0)S. Model B c,, a = O.o/J d, /Jaa = 1.0/3. (Relative electron densities, bond orders, and free valences.) e, the nonoverlap case f, the overlap case. (Relative electron densities and bond orders.)...

The idea of the central role that refrachve index plays in describing all ophcal properties of materials has been developed. As already stated, it is not so much the central role of the refractive index, but the central role of Maxwell s equation that produces the phenomenological quantity refrachve index. From the perspechve of the optical materials engineer or producer, the refractive index clearly holds the key to producing desired materials. Those chemical factors (electron density, bond type) that infiuence the magnitude of the refrachve index have already been addressed. How the physical manipulation of a material infiuences its refractive index is now examined. 

Normal or generalized coordinate Predictive correlation coefficient Quadrupole moment Electron density Bond order... 

Also much is to be expected in the generalized use of computers as representation tools in chemistry, in particular the true advent of a new chemical semiotics (Costa Pereira, 1990a), not based as it has been on the all-powerful, although rough, concept of electronic duplet, but on more sound notions as electron densities, bond order, bond polarizabilities, etc., easily calculated and represented by the computer in a hypermedia environment. 

The VSEPR theory also predicts that the NPF ion has tetrahedral electronic geometry. Each region of high electron density bonds the central atom to another atom (H in ) at the comer of the tetrahedral arrangement. We describe the molecular geometry of this ion as tetrahedral. 

The X-ray diffraction pattern is photographed and computer software is used to calculate the positions of the particles within the lattice. This can be used to generate an electron density map (Figure 4.10) of the molecule in a molecular lattice. Each contour line connects points of the same electron density. Bond lengths and bond angles of the molecule may be obtained from the electron density map. 

Theoretical calculations, by the HMO and PPP methods, have provided information concerning electron densities, bond orders, and polarization energies of 3-, 4-, and 5-phenylisothiazoles. ... 

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