Molecular structure formal charge

From the early advances in the quantum-chemical description of molecular electron densities [1-9] to modem approaches to the fundamental connections between experimental electron density analysis, such as crystallography [10-13] and density functional theories of electron densities [14-43], patterns of electron densities based on the theory of catastrophes and related methods [44-52], and to advances in combining theoretical and experimental conditions on electron densities [53-68], local approximations have played an important role. Considering either the formal charges in atomic regions or the representation of local electron densities in the structure refinement process, some degree of approximate transferability of at least some of the local structural features has been assumed. 

In this chapter, procedures for drawing molecular structures have been illustrated, and a brief overview of structural inorganic chemistry has been presented. The structures shown include a variety of types, but many others could have been included. The objective is to provide an introduction and review to the topics of VSEPR, hybrid orbitals, formal charge, and resonance. The principles discussed and types of structures shown will be seen later to apply to the structures of many other species. 

It can be concluded from the discussions in Sections IIIA-D that B=N double bonds in aminoboranes and B N triple bonds in iminoboranes represent a realistic picture. It is here recommended, therefore, to indicate these bonds in structural formulas as usual, but to omit erroneous formal charges, e.g., amine-borane X3B—NR3 aminoborane X2B=NR2i iminoborane XB=NR. [Note that R3N-BX3 is recommended as the correct molecular formula for amine-boranes (46), but one is not bound to rules in constructing structural formulas, e.g., X3B—NR3.]... 

Chemists commonly include formal charges as part of structural formulas. While such a practice might be viewed simply as chemical bookkeeping , it serves as well to anticipate molecular properties and chemical reactivity. However useful charges may be, the fact is that they may neither be determined from any experiment nor calculated in a unique manner. (Discussion of why this is so is provided in Chapter 16.) The closest one can get is a measure of overall molecular polarity as contained in the dipole moment. This is a measurable and calculable quantity. ... 

We have already mentioned the necessity of having the hydrogens and formal charges explicitly accounted for in a molecular model prior to calculations. A program will handle a structure with, e.g., -COO-, differently from a structure with -COOH. If you attempt to do an MM calculation on an incomplete model, some programs will allow the calculation to proceed, but the results will be substandard. 

Know the meaning of molecular formula, structural formula, structural (or constitutional) isomers, continuous and branched chain, formal charge, resonance, contributing structures, sigma (a) bond, sp3-hybrid orbitals, tetrahedral carbon. 

Given a simple molecular formula, draw the electron-dot formula and determine whether each atom in the structure carries a formal charge. 

The most sophisticated methods developed to date to treat solvent effects in electronic interactions and EET are those reported by Mennucci and co-workers [47,66,67], Their procedure is based on the integral equation formalism version of the polarizable continuum model (IEFPCM) [48,68,69], The solvent is described as a polarizable continuum influenced by the reaction field exerted by the charge distribution of the donor and acceptor molecules. In the case of EET, it is the particular transitions densities that are important. The molecules are enclosed in a boundary surface that takes a realistic shape as determined by the molecular structure. 

Which of these two structures is correct Both of them satisfy the octet rule and neither has formal charges, so both are predicted to be of comparable stability. On the basis of what we have discussed so far, we cannot predict which is more stable. In fact, both of these compounds are quite stable and can be put in a bottle. But they are different compounds. Ethyl alcohol is the alcohol found in beverages. It is a liquid at room temperature. In contrast, dimethyl ether is a gas at room temperature and is quite poisonous. As was mentioned in Section 1.7, compounds such as these, with the same molecular formula but different arrangements of bonded atoms (different structures or different connectivities), are called constitutional isomers (or structural isomers). Constitutional isomerism is very common in organic compounds. This is another reason why it is necessary to show the structure of the compound under discussion rather than just the molecular formula. 

To express the collective solvent reaction coordinate as in equation (6), it is necessary to define the specific diabatic potential surface for the reactant and product state. This, however, is not a simple task, and there is no unique way of defining such diabatic states. What is needed is a method that allows the preservation of the formal charges of the fragments of reactant and product resonance states. At the same time, solvent effects can be incorporated into electronic structure calculations in molecular dynamics and Monte Carlo simulations. Recently, we developed a block-localized wave function (BLW) method for studying resonance stabilization, hyperconjugation effects, and interaction energy decomposition of organic molecules.20-23 The BLW method can be formulated to specify the effective VB states.14... 

Draw Lewis structures for the nine isomers having molecular formula CsHgO, with all atoms having a zero formal charge. 

Formal charges can be used to help in the assessment of resonance structures and molecular topology. The use of formal charges is presented here as a simplified method of describing structures, just as the Bohr atom is a simple method of describing electronic configurations in atoms. Both of these methods are incomplete and newer approaches are more accurate, but they can be useful as long as their limitations are kept in mind. 

First, in the relativistic formulation the number of particles is not conserved. Electron-positron creation processes, which conserve the total charge of the system but not the number of particles, are permitted in the relativistic formalism. The use of second quantized methodology is therefore mandatory in a fully relativistic formulation of the molecular structure problem. 

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