Walsh diagram, orbital calculation

Figure 1. Walsh type orbital correlation diagram (EH MO calculations [6]) for transforming linear H3P-Pt-PH3 (D3h) to a bent (C2V) geometry. Only the valence MOs and the relevant metal contributions to the MO wave functions are shown. 

The stability of the trihalogen intermediate is greatest when the least electronegative atom is in the central position. This complex is expected to be bent by examination of the appropriate Walsh molecular orbital diagrams and it is possible that the reaction is constrained to take place only for certain preferred interaction geometries. Trajectory calculations [561] for Cl + Br2, I2 and Br + I2 could not reproduce the experimental results with any surface having a potential well that preferred collinear approach of the reagents. 

Computer-Aided Composition of Atomic Orbitals. Molecular orbital calculations with extended Hiickel method, orbital plots, and Walsh diagrams. Also available via anonymous ftp from cacao.issecc.fi.cnr.it (149.139.10.2). PCs. 

The use of Walsh diagrams, based on one-electron molecular orbitals, shows that on n —> 7t excitation the azobenzene molecule is stretched, which is the beginning of inversion. All calculations and suggestions for an inversion mechanism agree that the potential energy curve for inversion has a relatively steep slope at the E- and the Z- geometries. This is corroborated by the experimental evidence of a continuous n tc absorption band in both isomers. In fact, a structured n band in an azo compound that can isomerize has never been observed. 

Lewis acids are electron acceptors and Lewis bases electron donors. This means that the former, as we can see using BH3 as an example, have at least one low-lying unoccupied orbital. In the case of BH3, this is the LUMO, a pure boron p-orbital. The AMI-calculated energy for this MO is +1.6 eV, a low value for the LUMO of a neutral compound. This means that BH3 can accept an extra electron to form the BH3 radical anion. When the extra electron is added, the BH3 moiety becomes pyramidal, as shown in Sect. 2.5 (Walsh diagrams). We can now use ammonia as an example of a Lewis base. The lone pair HOMO has a calculated energy of -10.4 eV (i.e. a Koopmans theorem ionization potential of 10.4 eV). We can remove an electron from this MO to form the ammonia radical cation, which has a planar trigonal structure. 

Fig. 7 Calculated Walsh diagram for [Co(NO)(NH3)5] . The orbital primarily responsible for the bending distortion is shown in red. It corresponds to a 20-electron count for the linear geometry (shown in green) and 18-electron count for the bent geometry (shown in red)...

---