Carbonyl bond electrostatic component

Qualitatively, the CNDO/2 results in Fig. 3 show that it is easier to stretch and break the bond to the 3 substituent when a nucleophile is in the vicinity of or attached to the 3-lactam carbonyl carbon. One reason for this is that the 3-lactam amide and the enamine systems can transmit electron density changes from the carbonyl carbon to the 3 group. Another reason is that it is easier to break OAc away from an anionic parent (the TS models) than from the neutral cephem because the latter would leave behind a cation. This electrostatic component of the total energy change is seen as the gap between the top curve in Fig. 3 at 3.5 A and the corresponding curve segment at infinity. 

Figure 17.2 compares the electrostatic potential surfaces of ethylene and formaldehyde and vividly demonstrates how oxygen affects the electron distribution in formaldehyde. The electron density in both the o and tt components of the carbon-oxygen double bond is displaced toward oxygen. The carbonyl group is polarized so that carbon is partially positive and oxygen is partially negative. 

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