Natural bond order calculations

Table 3.1 Natural bond orders for selected bonds calculated at the MP2/ 6-311+G(3df,3pd) level.

Table 12.4 shows the isomer shifts and quadrupole splittings. Comparing the data for 12.V11 and 12.V111 with the " Sn Mossbauer isomer shifts in Table 6.3 we see that the experimental values clearly favor the interpretation of the compounds as derivatives of Sn(IV). Comparison with the data for 12.IX and 12.X confirms this interpretation. We also see that the isomer shifts correspond nicely to the atomic charges of the Sn atoms calculated by quantum mechanical calculations and subsequent natural bond order analyses. 

Nitrones are a rather polarized 1,3-dipoles so that the transition structure of their cydoaddition reactions to alkenes activated by an electron-withdrawing substituent would involve some asynchronous nature with respect to the newly forming bonds, especially so in the Lewis acid-catalyzed reactions. Therefore, the transition structures for the catalyzed nitrone cydoaddition reactions were estimated on the basis of ab-initio calculations using the 3-21G basis set. A model reaction indudes the interaction between CH2=NH(0) and acrolein in the presence or absence of BH3 as an acid catalyst (Scheme 7.30). Both the catalyzed and uncatalyzed reactions have only one transition state in each case, indicating that the reactions are both concerted. However, the synchronous nature between the newly forming 01-C5 and C3-C4 bonds in the transition structure TS-J of the catalyzed reaction is rather different from that in the uncatalyzed reaction TS-K. For example, the bond lengths and bond orders in the uncatalyzed reaction are 1.93 A and 0.37 for the 01-C5 bond and 2.47 A and 0.19 for the C3-C4 bond, while those in... 

Bond order and valence indices are easy to calculate from wave functions generated in computational chemistry. The examples presented in this chapter indicate that the little computational cost can bring valuable information about changes in bonding during chemical reactions. Bond order indices help us to characterize the reactant- or product-like nature of the TS of reactions, follow the progress of the conversion of bonds, and find out whether the reaction is synchronous or not. 

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