Heavy atom bonds

Three basis sets (minimal s-p, extended s-p and minimal s-p with d functions on the second row atoms) are used to calculate geometries and binding energies of 24 molecules containing second row atoms, d functions are found to be essential in the description of both properties for hypervalent molecules and to be important in the calculations of two-heavy-atom bond lengths even for molecules of normal valence. 

Scheme 28 [2+2] Cycloadditions of unsaturated heavy-atom bonds...

Table 5-2 Mean Absolute Errors in Heavy-Atom Bond Distances for Two- ...

Figure 5-1 6-311+G vs. Experimental Heavy-Atom Bond Distances in Two-Heavy-Atom, Main-Group Hydrides... 

This simple picture is supported by the results of MP2 calculations, which show bond lengthening (over Hartree-Fock models). The resulting bond distances are generally (but not always) longer than experimental values. This is clearly seen in Figure 5-2, which relates MP2/6-311+G to experimental heavy-atom bond distances. As with Hartree-Fock models, nearly identical results are provided with the smaller 6-3IG basis set. 

Calculated heavy-atom bond distances in molecules with three or more first and/or second-row atoms are tabulated in Appendix A5 molecular mechanics models (Table A5-21), Hartree-Fock models (Table A5-22), local density models (Table A5-23), BP, BLYP, EDFl and B3LYP density functional models (Tables A5-24 to A5-27), MP2 models (Table A5-28), and MNDO, AMI and PM3 semi-empirical models (Table A5-29). Results for STO-3G, 3-21G, 6-31G and 6-311+G basis sets are provided for Hartree-Fock models, but as in previous comparisons, only 6-3IG and 6-311+G basis sets are employed for local density, density functional and MP2 models. 

Table 5-7 Heavy-Atom Bond Distances and Skeletal Bond Angles in Hypervalent Molecules... 

Comparative data for heavy-atom bond lengths and skeletal bond angles for molecules incorporating one or more third or fourth-row, main-group elements are provided in Appendix A5 Table A5-39 for Hartree-Fock models with STO-3G, 3-2IG and 6-3IG basis sets. Table A5-40 for the local density model, BP, BLYP, EDFl andB3LYP density functional models and the MP2 model, all with the 6-3IG basis set, and in Table A5-41 for MNDO, AMI and PM3 semi-empirical models. 6-31G, local density, density functional and MP2 calculations have been restricted to molecules with third-row elements only. Also, molecular mechanics models have been excluded from the comparison. A summary of errors in bond distances is provided in Table 5-8. 

Table 5-16 Heavy-Atom Bond Distances in Anions... 

Table A5-39 Heavy Atom Bond Distances and Skeletal Bond Angles in Molecnles Incorporating Third and Fonrth Row, Main-Gronp Elements. Hartree-Fock Models (2)... 

Several variants of this useful and novel MCR thiazole synthesis have since been reported, including complexity-oriented syntheses of thiazolo-/> -lactams [18], in which two ring systems and five heavy atom bonds are newly formed (2 C-N, 2 C-S, 1 C-C), solid phase synthesis [19], and a Passerini variant of this reaction leading to 2-hydroxymethyl thiazoles [20], Scheme 3.13 illustrates some of the possibilities [21]. 

Proton transfer between electronegative atoms can be treated as a simple reaction dimension. This is known to be only an approximation but appears to be quite permissible if the reaction includes geometry change or heavy atom bond forming. 

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