Rings orbital calculations

A number of derivatives of antimonin are also known (141,142). The potential aromaticity of this ring system has aroused considerable interest and has been investigated with the aid of spectroscopy as well as ab initio molecular orbital calculations (143). There seems to be no doubt that antimonin does possess considerable aromatic character. 

The [S3N3]" cation is of interest as an example of an antiaromatic eight TT-electron system (Section 4.4). Ab initio molecular orbital calculations indicate that a triplet cation, with a planar ring, is more stable than the singlet cation. The [S3N3]" cation has been obtained as the norbomene adduct 5.16, but salts of the free cation have not been isolated. ... 

Most electrophilic substitutions in benzimidazole (31 R = H) occur primarily in the 5-position. In multiple bromination the order followed, 5 > 7 > 6,4 > 2, parallels molecular orbital calculations. In benzimidazole itself the 4(7)- and 5(6)-positions are tautomerically equivalent. Fusion of a benzene ring deactivates C-2 to electrophilic attack to such an extent that it is around 5000 times less reactive than the 2-position of imidazole. Strong electron donors at C-5 direct halogenation to the 4-position, whereas electron-withdrawing groups favor C-4 or C-6 substitution (84MI21). 

Simple resonance theory predicts that pentalene (48), azulene (49), and heptalene (50) should be aromatic, although no nonionic canonical form can have a double bond at the ring junction. Molecular orbital calculations show that azulene should be stable but not the other two, and this is borne out by experiment. Heptalene has been prepared but reacts readily with oxygen, acids, and bromine, is easily hydrogenated, and polymerizes on standing. Analysis of its NMR spectrum shows that it is... 

Ab initio molecular orbital calculations (using the Gaussian 80 computer program) on the barrier to pseudorotation (for the furanose ring) of two model compounds, 2-deoxy-/ -D- /) cero-tetrofuranosylamine (781) and 2-deoxy-2-fluoro-)3-D-erythrofuranosylamine (782) were reported. Al-... 

Semi-empirical molecular orbital calculations have been carried out on the model phosphorane HaPiCHa. Besides the expected transfer of charge, the inclusion of the phosphorus 2>d orbitals showed a significant hyperconjugative interaction between the CHg orbitals and a 2>d orbital of appropriate symmetry on phosphorus. Calculations on cyclopropylidene-phosphorane revealed a similar interaction between the Walsh orbitals of the ring and an in-plane phosphorus 2>d orbital. 

The considerable number of molecular orbital calculations which have recently been made for sandwich compounds are however considered in some detail in Section 6. This has been done in order to make clear the relationship between the ligand field and molecular orbital approaches, and also to indicate the need for the use of a more sophisticated molecular orbital scheme than that adopted in this Introduction, i.e. one in which the a-framework of the rings is specifically included in the basis set as well as the rr-type orbitals. 

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