Electron density distributions molecular structure aspect

The " N nucleus, because of its widespread occurrence in all types of systems (especially biologically active systems), is of particular interest in studying electron density distribution, molecular reorientations and intermolecular time-dependent interactions. It seems that such studies will acquire more and more importance in the future and will occur more frequently, especially with the availability of double resonance spectrometers and new data processing techniques such as the maximum entropy method. The examples discussed do not, of course, exhaust the potential of NQR as a tool for structure and chemical bonding. These are only simple illustrations of the applied aspects of NQR spectroscopy. 

Molecular orbitals are characterized by energies and amplitudes expressing the distribution of electron density over the nuclear framework (1-3). In the linear combination of atomic orbital (LCAO) approximation, the latter are expressed in terms of AO coefficients which in turn can be processed using the Mulliken approach into atomic and overlap populations. These in turn are related to relative charge distribution and atom-atom bonding interactions. Although in principle all occupied MOs are required to describe an observable molecular property, in fact certain aspects of structure and reactivity correlate rather well with the nature of selected filled and unfilled MOs. In particular, the properties of the highest occupied MO (HOMO) and lowest unoccupied MO (LUMO) permit the rationalization of trends in structural and reaction properties (28). A qualitative predictor of stability or, alternatively, a predictor of electron... 

Because all of these electronic aspects of aromaticity are ultimately derived from the electron distribution, we might ask whether representations of electron density reveal any special features in aromatic compounds. The electron density of the IT electrons can be mapped through the MESP (molecular electrostatic potential, see Section 1.4.5). The MESP perpendicular to the ring is completely symmetrical for benzene, as would be expected for a delocalized structure and is maximal at about... 

The spatial distribution of the electron cloud in a molecular system is investigated quantitatively through the single-particle electron density,which has served as a basic variable in the so-called density functional theory (DFT), an approach that bypasses the many-electron wavefunction, which is the usual vehicle in conventional quantum chemistry or electronic structure theory (for a review of modem developments in quantum chemistry, see reference 7). The theoretical framework of DFT is well known for the associated conceptual simplicity as well as for the computational economy it offers. Another equally important aspect of DFT is its ability to rationalize the existing concepts in chemistry as well as to give birth to newer concepts, which has led to the important field of conceptual DFT. ... 

Several issues remain to be addressed. The effect of the mutual penetration of the electron distributions should be analyzed, while the use of theoretical densities on isolated molecules does not take into account the induced polarization of the molecular charge distribution in a crystal. In the calculations by Coombes et al. (1996), the effect of electron correlation on the isolated molecule density is approximately accounted for by a scaling of the electrostatic contributions by a factor of 0.9. Some of these effects are in opposite directions and may roughly cancel. As pointed out by Price and coworkers, lattice energy calculations based on the average static structure ignore the dynamical aspects of the molecular crystal. However, the necessity to include electrostatic interactions in lattice energy calculations of molecular crystals is evident and has been established unequivocally. 

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