Chemical formula representation

Two-Dimensional Representation of Chemical Structures. The lUPAC standardization of organic nomenclature allows automatic translation of a chemical s name into its chemical stmcture, or, conversely, the naming of a compound based on its stmcture. The chemical formula for a compound can be translated into its stmcture once a set of semantic rules for representation are estabUshed (26). The semantic rules and their appHcation have been described (27,28). The inverse problem, generating correct names from chemical stmctures, has been addressed (28) and explored for the specific case of naming condensed benzenoid hydrocarbons (29,30). 

An important approach to the graphic representation of molecules is the use of a connection table. A connection table is a data base that stores the available bond types and hybridizations for individual atoms. Using the chemical formula and the connection table, molecular stmctures may be generated through interactive graphics in a menu-driven environment (31—33) or by using a linear input of code words (34,35). The connection table approach may be carried to the next step, computer-aided molecular design (CAMD) (36). 

The student conceptions that were displayed could be categorised into three main types, namely (1) confusion between macroscopic and submicroscopic representations, (2) extrapolation of bulk macroscopic properties of matter to the submicroscopic level and (3) corrfusion over the multi-faceted significance of chemical symbols, chemical formulas as well as chemical and ionic equations. Student conceptions held by at least 10% of the students who were involved in the alternative instractional programme were identified. Several examples of student conceptions involving the use of the triplet relationship are discussed in the next section. 

A model is one of the main outcomes of ary scientific enquiry and hence is a major contributor to philosophy of science. A model may be defined as a simplified representation of a phenomenon (an object, system, event, process) or idea produced for the specific purpose of providing an explanation of that entity, the most important outcomes of which are the production of successful predictions of how it will behave under a range of circumstances (Gilbert, Boulter, Elmer, 2000). Entities can be modelled at the three levels at the macroscopic, by representing some of the aspects of the entity that can be seen at the sub-microscopic, by representing the ideas produced to explain the constitution and behaviour of the particles that constitute the entity and at the symbolic, by representing the symbols created to simplify the reference to such particles (as, for instance, chemical formulae and chemical equations). 

In the usual chemical formulas written for chain polymers the sue-cessive units are projected as a co-linear sequence on the surface of the sheet of paper. This form of representation fails to convey what is perhaps the most significant structural characteristic of a long polymer chain, namely, its capacity to assume an enormous array of configurations. This configurational versatility is a consequence of the considerable degree of rotational freedom about single bonds of the chain. In the simple polymethylene chain, for example, the conventional formula... 

Fig. 2.1. Formula representation (left), calculated geometries (middle) and calculated distributions of formal charge (right) in the pnra-tolyl ion, [C7H7]. Reproduced from Ref. [26] with permission. American Chemical Society, 1977.

Graphic representations (chemical formulae) of macromolecules are used extensively in the scientific literature on polymers including lUPAC documents on macromolecular nomenclature. This document establishes rules for the unambiguous representation of macromolecules by chemical formulae. The rules apply principally to synthetic macromolecules. Insofar as is possible, these rules are consistent with the formulae given in lUPAC documents [2-4] and they also cover the presentation of formulae for irregular macromolecules [5], copolymer molecules [1, 6] and star macromolecules. 

In comparison with chemical formulae of low-molecular-weight compounds, the graphical representations of which have been addressed in a recent lUPAC document [7], chemical formulae of polymers must additionally reflect the multiplicity of constitutional units in a macromolecule and the various possibilities for connecting the constitutional units in a macromolecule. 

Graphic representations (chemical formulae) of macromolecules (lUPAC Recommendations 1994), Pure Appl. Chem. 66, 2469-2482 (1994). Reprinted as Chapter 18, this edition. 

Tins symbols employed in chemical formula to-day are, with a few alterations and additions, those used by Berzelius. The formula of simple compounds were represented by writing the symbols of the elements contained in the compound side by side, and this simple representation served for some time. The formulae used, however, did not denote the proportion of the atoms of one kind to that of another kind, and numerals were therefore introduced to denote the number of each kind of atoms in the molecule. This arose naturally when it was found that more than one compound might contain the same elements, and that the different properties of the compounds were due to the proportion of the elements present in the molecule as, for example, the two compounds of carbon and oxygen, carbon monoxide and carbon dioxide. 

Figure 3.11—Separation on a cyclodextrin-boimd stationary phase. Chromatogram of a racemic mixture chemical formula of /f-cyclodextrin (diameter, 1.5 nm cavity, 0.8 nm height, 0.8 nm) partial representation of cyclodextrin bonded to a silica gel bead through an alkyl chain linker arm side view of a cyclodextrin molecule with a hydrophobic cavity.

We distinguish between the chemical reaction (the actual process) and the chemical equation, which is a symbolic representation of the reaction in terms of chemical formulas. 

The intrinsic language of chemistry is formed by the vocabulary of chemical formulae and structural representations connected by the syntax of their interconversions. It describes a tangible reality it is a reification of the word and the text its signs are engraved into matter [1.18,1.19]. 

Some other examples of chemical reactions and their representation by chemical formulae are ... 

Nowadays, more than 4000 types of descriptors are known.17 There exist different ways to classify them. With respect to the type of molecular representation used for their calculations—chemical formula, molecular graph, or spatial positions of atoms—one speaks about ID, 2D, and 3D descriptors, respectively. Descriptors can be global (describing the molecule as a whole) and local (only selected parts are considered). One could distinguish information-based descriptors, which tend to code the information stored in molecular structures, and knowledge-based (or semiempir-ical) descriptors issued from the consideration of the mechanism of action. Most of those descriptors can be obtained with the DRAGON, CODESSA PRO, and ISIDA programs. 

Figure 9.39 Schematic representations of metal 4,4 -bipyridyl porous networks. Lines represent the bipyridyl ligand except for vertical lines in (c), which represent Ag - Ag bonds (2.977 A long), and horizontal lines in (d) which represent Cu---Cu. The chemical formula, framework dimensionality, structure type, degree of interpenetration and pore aperture are listed under each representation. (Reproduced with permission from Reference 38).

In these non-alternating compounds the M.O. representation, which foregoes the setting up of special chemical formulae and gives only bond orders and charge distribution for the various bonds, is greatly preferable to the very artificial V.B. interpretation in this case. 

Chemical formula the representation of a molecule in which the symbols for the elements are used to indicate the types of atoms present and subscripts are used to show the relative numbers of atoms. (2.7)... 

Many different approaches to QSAR have been developed since Hansch s seminal work. As briefly discussed above, the major differences between these methods can be analyzed from two viewpoints (1) the types of structural parameters that are used to characterize molecular identities, starting from different representation of molecules, from simple chemical formulas to three-dimensional conformations and (2) the mathematical procedure that is employed to obtain the quantitative relationship between these structural parameters and biological activity. 

The simplest molecular representation is the chemical formula, which is the list of the different atom types, each accompanied by a subscript representing the number of... 

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