Highest occupied molecular orbital hybridization

The highest occupied molecular orbital (HOMO) is a doubly occupied sp2 hybrid, while the lowest unoccupied molecular orbital (LUMO) is a carbon p orbital. 

Figure 5.4. The highest occupied molecular orbital of a Si911,2 dimer cluster. The top two silicon atoms comprise the surface dimer, and the remaining seven Si atoms contain three subsurface layers which are hydrogen terminated to preserve the sp3 hybridization of the bulk diamond cubic lattice. The up atom is nucleophilic and the down atom is electrophilic.

Klemperer and co-workers. 31) In this model the hydrogen bond is viewed as an electron donor-acceptor complex in which a pair of electrons from the highest occupied molecular orbital of the Lewis base is donated to the lowest unoccupied molecular orbital of the Lewis acid. If the donor electron pair is assumed to have the appropriate hybridization, and the acceptor orbital to be axially symmetric, the above structures can be rationalized as giving maximal overlap between the HOMO and LUMO. 

The nature of the frontier orbitals is also of interest and a Mulliken population analysis of the constituent orbitals establishes that the highest occupied molecular orbital (HOMO) of each phosphine consists primarily of a lone pair sp hybrid on phosphorus. The orbital energy ordering PMe3 < PH3 < PF3 also parallels the percentage phosphorus s-character of the HOMO of PMe3 (11% s and 60% p), PH3 (14% s, 67% p) and PF3 (29% s, 32% p). In each case the back lobe of the sp hybrid interacts with the substituent attached to phosphorus in a (T-bonding fashion. 

Photoelectron spectroscopy reveals that the highest occupied molecular orbital (HOMO) in NHC is the lone pair in the sp hybrid orbital of the carbene carbon atom (see Figure 1.18). However, a molecular orbital with jt symmetry centred around the N-C-N heteroallyl system is of almost equal energy. This gives the carbene the ability to engage in n-face donor interactions with the transition metal fragment. For unsaturated NHC these n-face donor interactions are more favourable than for saturated ones [104]. 

The relativistic highest occupied molecular orbital (RHOMO) 9 el/2> consists of a 7sa-6da hybrid on U with small contributions from 5f7T)9> and 5faa> U DFAOs. Using the relations (3.5a-3.5d) of Malli and Pyper (14), all the valence RMOs of UO can also be written... 

Carbocations are electron-deficient species that are the most important intermediates in several kinds of reactions. A common model for carbocation stmcture is a planar species exhibiting sp hybridization, as shown in Figure 2.4 for methyl cation. The p-orbital that is not utilized in the hybrids is empty and is often shown bearing the positive charge since it represents the orbital available to accept electrons. There is a vacant p orbital perpendicular to the plane of the molecule this is the LUMO (lowest unoccupied molecular orbital). In all reactions of carbocations there is an interaction between this LUMO and the HOMO (highest occupied molecular orbital) of another molecule. A structure with an empty p orbital should be more stable than a structure in which an orbital with s character is empty. In general, a carbocation is a purely ionic species. 

The position of the do-do absorption and emission bands were found to be phosphine-dependent. The trend going from 11 to 14 is explained by the increase in electronic density on the P-atom by inductive effect destabilizing the highest occupied molecular orbital (HOMO) when Ph, (CHj), (CHj), and (CH ) are compared. The stronger red-shift in absorption and emission between 12-14 and 15 is explained by the change in C hybridation state going from sp to sp, stabilizing the lowest... 

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