3-21G level calculations

Hartree-Fock calculations with the 3-21G and 6-31G basis sets have been performed to study the structure and energetics of Na+, K+ and Al+ -azole complexes. Structures have been fully optimized at the 3-21G level. Calculated X+ (X = H, Li, Na, K, Al) binding energies of 1,2,3-triazole show that cation association energies follow the sequence Li+ > Al+ > Na+ > K+, and all of them are much smaller than the corresponding protonation energies (92JPC3022). 

Figure 1 Contour map of the negative of the molecular electrostatic potential for acetamide at the HF/3-21G( ) level calculated from the full molecular wavefunction. Shading indicates approximate value of the potential in the region. Thus, the MEP near the oxygen is negative, and the MEP near the amide hydrogens (not shown) is positive. The basis set has polarization functions only on second-row atoms.

Table 1 Net Atomic Charges for Heavy Atoms and Amide Hydrogens in Acetamide at the HF/3-21G( ) Level Calculated with the CHELP Model...

Calculations at the 6-31G and 6-31G level provide, in many cases, quantitative results considerably superior to those at the lower STO-3G and 3-21G levels. Even these basis sets, however, have deficiencies that can only be remedied by going to triple zeta (6-31IG basis sets in HyperChem) or quadruple zeta, adding more than one set of polarization functions, adding f-type functions to heavy atoms and d-type functions to hydrogen, improving the basis function descriptions of inner shell electrons, etc. As technology improves, it will be possible to use more and more accurate basis sets. 

Theoretical calculations have also permitted one to understand the simultaneous increase of reactivity and selectivity in Lewis acid catalyzed Diels-Alder reactions101-130. This has been traditionally interpreted by frontier orbital considerations through the destabilization of the dienophile s LUMO and the increase in the asymmetry of molecular orbital coefficients produced by the catalyst. Birney and Houk101 have correctly reproduced, at the RHF/3-21G level, the lowering of the energy barrier and the increase in the endo selectivity for the reaction between acrolein and butadiene catalyzed by BH3. They have shown that the catalytic effect leads to a more asynchronous mechanism, in which the transition state structure presents a large zwitterionic character. Similar results have been recently obtained, at several ab initio levels, for the reaction between sulfur dioxide and isoprene1. ... 

Addition of carbon nucleophiles to furfural tosylhydrazone provides 5-substituted 2E,4E-pentadienyls in good yields <00TL2667>. The ab initio calculations at the RHF/3-21G level have been utilized to study the origins of diastereoselectivity of the vinylogous Mannich reaction of 2-methylfuran with pyrrolinium ion . A simple procedure for isomerization of 2-furylcarbinols to cyclopentenones under neutral condition was reported and a new mechanism was proposed <00H(52)185>. 

These calculations were performed at the HF/3-21G level of theory. 

The above data indicate that the association energy for dimerization of stacked dimeric species G is —26.3 (—29.2) kcalmoU (energies at the 6-31G and 3-21G level of theory). The energies for the polymerization of the species C and F are —40.4 (—47.0) kcalmoU and —40.6 (—46.1) kcalmoU, respectively. This calculated energy of aggregation suggests the formation of a polymeric ladder as more favorable than the stacked dimer. In the experiment, it is seen that the stacked dimer crystallizes only on rapid cooling... 

The first ab initio MO calculation of the reaction between MeLi and H2C=0 was carried out mostly at the HF/3-21G level of theory . The comparison of the reaction profiles for the monomer and dimer species revealed that the first complex formation was less exothermic and the barrier for the addition step was lower for the dimer reaction, but the overall barrier height was similar for the two systems. The overall reaction was more exothermic for the dimer reaction. 

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