Provisional structure

The first four steps in our procedure lead to a provisional Lewis structure that contains the correct bonding framework and the correct number of valence electrons. Although the provisional stmcture is the correct structure in some cases, many other molecules require additional reasoning to reach the optimum Lewis structure. This is because the distribution of electrons in the provisional structure may not be the one that makes the molecule most stable. Step 3 of the procedure places electrons preferentially on outer atoms, ensuring that each outer atom has its full complement of electrons. However, this step does not always give the optimal configuration for the inner atoms. Step 5 of the procedure addresses this need. 

You should be able to follow the first four steps of the procedure to obtain the following provisional structure ... 

Because the inner atoms are electron-deficient in the provisional structure, we should pay particular attention to their electron count in the final stmcture. Each of the carbon atoms has an octet composed of four bonds, in accordance with line stmcture mles. 

To convert from the provisional structure of the nitrate anion to complete the nitrogen octet, any of the three oxygen atoms can supply a pair of electrons. ... 

Although the inner phosphorus atom has its octet of electrons, a formal charge calculation indicates that we can shift electrons to optimize the provisional structure ... 

A provisional structure for gramicidin was proposed by Gavrilov and Akimova30. 

If methanol is employed as reaction medium, quinolizines (e.g., 306) are not formed but relatively high yields of deep red 1 3-molar adducts from the quinoline and DMAD are obtainable.379 Two isomeric types of adducts are formed, which are called first and second red adducts because of their elution order on chromatography. These adducts proved extremely difficult to separate completely. A provisional structure (317), for which many geometrical isomers are possible, was put forward379 to account for the properties of the compounds, but an X-ray diffraction study has now shown that the... 

Rauwolfia serpentina Benth. is the source of several related alkaloids. Ajmaline, ajmalicine, ajmalinine, serpentine, serpentinine, isoajmaline, and neoajmaline have been identified. Ajmaline and serpentine have been given provisional structural formulas (81, 82). Other members of the Apocynaceae yield alkaloids of similar properties. Although Rauwolfia preparations have been used in Ayurvedic medicine for hypertension, there appear no indications that they have been used in clinical cardiac arrhythmia. Yet, incomplete pharmacologic studies indicate properties for these alkaloids which should prompt additional studies toward their possible use in auricular fibrillation. 

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