Lewis-Langmuir charge

Figure 13 Methyl group charges for seven groups (Li to F) using seven different methods of computing them. Note that Lewis-Langmuir charges (LLa) give values that are roughly the average of the six other methods...

Why do we want to model molecules and chemical reactions Chemists are interested in the distribution of electrons around the nuclei, and how these electrons rearrange in a chemical reaction this is what chemistry is all about. Thomson tried to develop an electronic theory of valence in 1897. He was quickly followed by Lewis, Langmuir and Kossel, but their models all suffered from the same defect in that they tried to treat the electrons as classical point electric charges at rest. 

Molecular Properties. Corresponding to its nuclear charge number, the nitrogen atom possesses seven shell electrons. One electron pair in in the ground state 1 s(K shell), and five electrons are distributed over the four orbitals with the principal quantum number 2 (L shell). Of these, one electron pair occupies the 2 s level and three unpaired electrons, respectively, a half of the remaining three levels, 2 px, 2 pv, 2 pz. The unpaired electrons can enter into electron-pair bonds with the 1 s electron of three hydrogen atoms. Thus, the three half occupied orbitals of the L shell become about fully accupied (formation of an octet of the neon type in accordance with the octet theory of Lewis-Langmuir). 

A R = Allred and Rochow BPI = bond polarity index CE = configuration energy DHF = Dirac-Hartree-Fock DHHF = Dirac-hyper-Hartree-Fock El = energy index FC = formal charge LLa = Lewis-Langmuir atomic charges ON = oxidation number OS = oxidation state. 

Figure 14 Lewis-Langmuir atomic charges. Top examples using the simplest formulation. Middle plot of the electronegativity ratio that interpolates between fonnal charge and oxidation number. The dotted line is the formal charge limit and (0,0) is the oxidation number limit. Bottom improved LLa formula that corrects for charge on nearest neighbor atom...

Figure 15 Correlation diagram between Lewis-Langmuir and natural population analysis (NPA) charges for carbon atoms in 15 organic molecules a, acetone (Me) b, acetaldehyde (Me) c, acetonitrile (Me) d, ethane e, vinylamine (CH2) f, nitromethane (Me) g, vinylamine (CH) h, formaldehyde i, acetonitrile (CN) j, acetaldehyde (CO) k, acetone (CO) 1, difluoromethane m, formamide n, fluoroform o, carbon tetrafluoride...

For F3SN, the standard Lewis stractures are of types (21)-(23). These structures satisfy the Lewis-Langmuir octet rule, but each carries a formal charge of +2 on the sulphur atom. 

Association of molecules changes their As as has been proven for metal complexes, charge transfer complexes, Lewis acids, etc. In any case, complex formation can be followed by the variation of the CD. Molecular association is the first step to an aggregation and, furthermore, is the presupposition for supramolecular structures. Compounds with inter-molecular exciton interaction, e.g., carotenoids or cyanine dyes, have been successfully analyzed. The spontaneous association to chiral associates from achiral compounds, e.g., cyanines, have been proven and analyzed with the help of ECD spectroscopy. In this context, the successful CD analyses with Langmuir-Blodgett films as well as the analyses of membranes have to be mentioned where suprastruc-tural chirality can also emerge from achiral monomers. 

He was the first to observe the very stable adsorbed monatomic films on tungsten and platinum filaments, and was able, after experiments with oil films on water, to formulate a general theory of adsorbed films. He also studied the catalytic properties of such films. Langmuir s work on space charge effects and related phenomena led to many important technical developments which have had a profound effect on later technology. In chemistry, his interest in reaction mechanism caused him to study structure and valence, and he contributed to the development of the Lewis theory of shared electrons. 

This absence of effect, Lewis considered logically and scientifically objectionable for that state of motion which produces no physical effect whatsoever may better be called a state of rest. [Lewis, 1916]. And he was not alone in his scepticism. J. J. Thomson was trying to work a static atom theory as late as 1923 as were Langmuir, Davey, Born, Lande and Parson. All involved were trying to modify Coulomb s force law to get a theory in which the electrons in an atom remained still and were distributed at the corners of a cube. But in his Nobel lecture in 1922, Bohr delivered a sharp attack on static atom theories, pointing out that Earnshaw s theorem showed that a static distribution of charges must be unstable if Coulomb s law applies and that the argument could be extended to cover the proposed modified potentials. 

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