Huckel MO Method

A thiepin is formally isoelectronic with the 8ic-electron 1,3,5,7-cyclooctatetraene and 1,3,5-cycloheptatrienide ion and, if planar, may actually be antiaromatic. Recently, the question of the antiaromaticity of thiepin has been the subject of interest for both synthetic and theoretical chemists. The apparent instability of the thiepin ring system is in good agreement with theoretical calculations. Dewar and Trinajstic 68) have reported that the thiepin is considered to be weakly antiaromatic (RE = — 1.45 kcal mol-1) based on PPP SCF MO calculations. On the other hand, Hess Jr. and Schaad 69) have found it to be substantially antiaromatic (RE = —0.232 J) by using the Huckel MO method. This result was also supported by a graph-theoretical treatment by Aihara 70). 

If calculated corrections for the anisotropic contribution 8 (C,) are introduced into the experimentally measured values of 5" (Cj), then the corrected values of the chemical shifts reveal their common linear dependence on the values of jt-electron deficiency calculated by the Huckel MO method... 

Wade and collaborators more recently continued to use extended Huckel MO methods to investigate stability patterns in deltahedral borane chemistry. Calculations were carried out for published experimental geometries and also for hypothetical structures in which all boron atoms lie on a spherical surface and all deltahedral faces are equilateral triangles with B—B = 1.70 A and B—H = 1.19 A. These calculations were found to reproduce at least semiquantitatively the patterns of chemical stability for the observed B H 2 (6 < n < 12) ions and... 

---