Carbonyl groups descriptions

Although the valence bond description of pyran-2-one could include a zwitterionic contribution involving the carbonyl group, thus making the ring aromatic, there is little evidence to support this. For example, the IR carbonyl frequency (1740 cm ) is typical of an unsaturated lactone, and the chemical shifts of the ring protons in the H NMR spectrum indicate that there is no ring current. 

Figure 16-1 Atomic-orbital description of the carbonyl group. The a bonds to carbon are coplanar, at angles near to 120° the two pairs of unshared electrons on oxygen are shown as occupying orbitals n.

Further analysis of the microwave spectra provides a precise description of the bond lengths and angles (Table 7). The C2C3 bond is unusually long and relatively weak as shown by the reactivity of substituted cyclopropanones in cycloaddition reactions (see Section 4.4). The carbon-oxygen bond is somewhat shorter than the average carbonyl group and this feature is reflected in the infrared properties of cyclopropanones outlined below. 

Attempts to achieve perfection may give poor results. Normal AOs are optimized for neutral molecules, so they are too small to give good descriptions of anions, particularly in the gas phase. A better model is obtained by a slight contraction of the Gaussian exponents. However, if these exponents had been completely optimized, the anions would become so stable that they would lose all reactivity for example, a hydride ion H will arrive at a carbonyl group, bounce on it and go away without reacting ... 

By analyzing an input molecule, substructures are found which point to chapters in the reaction library. There, descriptions of changes in these substructures are contained which will occur in the course of a certain reaction. Implementing these changes leads to synthetic precursors. Thus, the recognition of the substructure of a /3-hydroxy-carbonyl group would point to the chapter aldol condensation in the reaction library. There the structural changes of a retro-aldol condensation are contained and result in the two carbonyl compounds as precursors (see Fig. 1). As the reac-... 

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