Huckel counting rule

A-4. For each of the following, determine how many tt electrons are counted toward satisfying Huckel s rule. Assuming the molecule can adopt a planar conformation, is it aromatic ... 

The AdNDP analysis revealed four lone pairs on Hg atoms with ON = 1.94 e, as well as delocalized p-AO based 4c-2e a-radial bond with ON = 2.00 e, p-AO based 4c-2e a-tangential bond with ON = 2.00 e, and p-AO based 4c-2e n-bond with ON = 2.00 e. Thus Hg4 should be considered as both p-radial-AO and p-tangential based a-aromatic species. Therefore the Hg4 cluster satisfies the 4 - - 4 counting rule for a-aromaticity in cyclic systems with even numbers of atoms (see Table 2). In addition, Hg " should be considered as n-aromatic on the basis of the 4 -I- 2 Huckel rule applied to its rr-subsystem (see Table 2). Thus, Hg4 is a doubly o- and Jt-aromatic system. 

Therefore the X3 (X = Sc, Y, La) clusters satisfy the 4n -f 2 counting rule for a-aromaticity and the 4n - - 2 Huckel rule for Tt-aromaticity in cyclic systems with odd numbers of atoms (see Table 1). Thus, all three anions are d-orbital based doubly (a- and rr-) aromatic systems. The obtained via the AdNDP method 3c-2e d-AO based Or-bond and 3c-2e d-AO based rtf-bond are similar to the 3c-2e la i and la model molecular orbitals presented in Figs. 5 and 11, respectively. 

Figure 7 shows where various planar carbon rings lie on the curve of Fig. 6. Notice that the results encompass Huckel s 4n + 2 rule ) quite nicely. The electron counts predicted by this rule to give stable structures lie in regions where the m ring is stabilized with respect to the molecule which contains no such loops. Thus we find stability for C He but not stability for C4HI2 but not C4H4 itself, etc. The moments method has...

Hiickel s MO work [4] in this area of course stands preeminent. But it has been much elaborated first, as regards the possibility of a Mobius cycle (with one P>0) whence [39,41] the electron-count conditions for stability and reactivity are interchanged, as indicated in Table 1 following second, as regards the interpretation of Huckel rule as a basis for the Woodward-Hoffmann rules (at least for electrocyclic reactions) and third, as regards the rules applicability [39] to cycles even when embedded in more extensive n-networks. 

As an aromatic molecule, a ir-electron count of the porphyrin macrocycle reveals 22 ir-electrons, which conforms nicely with Huckel s [4 =2] rule for aromaticity (n = 5). However, only 18 -ir-electrons are considered to lie in... 

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