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Families of Molecular Descriptors
Published in Mihai V. Putz, New Frontiers in Nanochemistry, 2020
Lorentz Jäntschi, Sorana D. Bolboacă
One of the outcomes of the molecular modeling is the charge distribution over the atoms in the molecule, or partial charges. There are different approaches are available: Born (seeBorn and Goppert-Mayer, 1931);Callen (seeCallen, 1949);Szigeti (seeSzigeti, 1949);Mulliken (seeMulliken, 1955 and thereafter);Coulson (seeCoulson et al., 1962);Politzer (seePolitzer, 1968)Löwdin (seeLöwdin, 1970);Hirshfeld (seeHirshfeld, 1977);Cioslowski (seeCioslowski, 1989);Bader (seeBader, 1990);An optimization method based on electrostatic potentials (see for instance Wang and Ford, 1994).
Filtrative Particle Removal
Published in Maik W. Jornitz, Filtration and Purification in the Biopharmaceutical Industry, 2019
The bond strength depends upon the two particular atoms involved, as does the degree of their sharing the bonding electrons. The sharing need not be equal, the propensity of different atoms for attracting electrons not being the same. Atoms are neutral in charge. An electron, by convention, is negatively charged. Therefore, the atom acquiring the greater share of the two bonding electrons than the one it contributed takes on a negative charge. The partner atom with its smaller share assumes a partial positive charge. The partial charges attract their opposite partially charged equivalents present in other molecules. Being only partial in their extent of charge, their attractions form bonds that are relatively weak. Their force extends over lesser distances, and when in opposition to repulsive forces arising from full charges are easily surpassed. Thus, they are called “secondary” bonds. However, they are important factors in adsorptive bonding such as are operative between organisms and filters that culminate in organism removals. (The symbol for the partial charge is the lower case Greek letter delta, δ.)
Chemical Bond III: Complements
Published in Franco Battaglia, Thomas F. George, Understanding Molecules, 2018
Franco Battaglia, Thomas F. George
This sharing, symmetric if the two atoms belong to the same element, is otherwise asymmetric, and entails a nonzero dipole moment associated with the bond (and with the entire molecule unless, due to the molecule symmetry, the individual-bond dipole moments compensate each other, as happens, for instance, in the CO2 molecule). In such a case, we say that the covalent bond is polar—in contrast to an apolar covalent bond—and the larger are the ionization energy and electron affinity differences between the two bonded atoms, the larger is the bond polarity, a circumstance that may be interpreted as a partial charge transfer between those atoms. In the limiting case where there is total electron charge transfer, the bond is said to be ionic. This is indeed a limit occurrence, and there is no diatomic molecule with a pure ionic bond.
Photosensitivity, substituent and solvent-induced shifts in UV-visible absorption bands of naphthyl-ester liquid crystals: a comparative theoretical approach
Published in Liquid Crystals, 2014
P. Lakshmi Praveen, Durga P. Ojha
The reactivity of a molecule may be inferred from its electron density distribution. One way to quantify such a distribution is by means of a charge population analysis. The charge at each atomic centre is assigned by the sum of its nuclear charge (atomic number) and the number of electrons occupying the orbitals belonging to that atom. An appropriate modelling of mesophases relies on the possibility of assigning a partial charge to all atomic centres. The atom-positioned partial charges are helpful to parameterise the molecular interactions for computational studies. Quantum chemical computations offer the possibility to take a detailed look at the electronic structure of the molecules. This can be done by determining atom-based partial charges, which are not quantum mechanical observables.