Formaldehyde orbital numbering

For formaldehyde, molecular orbital number 8 is the HOMO, and molecular orbital number 9 is the LUMO. In this case, the energy also changes sign at the point separating the occupied from the unoccupied orbitals. 

In an electronic transition, an electron from one molecular orbital moves to another orbital, with a concomitant increase or decrease in the energy of the molecule. The lowest-energy electronic transition of formaldehyde promotes a nonbonding (ri) electron to the antibonding pi orbital (it ).1 1 There are two possible transitions, depending on the spin quantum numbers in the excited state (Figure 18-11). The state in which the spins are opposed is called a singlet state. If the spins are parallel, we have a triplet state. 

An electronic state is characterized by the resultant electron spin S and by the symmetry species of the complete electronic wavefunction. With few exceptions, stable molecules in their electronic ground states have completely filled orbitals and thus the resultant electron spin is zero but the excitation of one electron to a vacant orbital may occur either with retention or reversal of its spin so that the excited states of stable molecules may have S = 0 or 1. All free radicals have at least one unpaired electron (S = ), and a higher resultant spin (S = , ,...) is at least theoretically possible for some structures with an odd number of electrons. The quantity 2 + 1, known as the multiplicity of the state, is always an integer and is indicated by a superscript number preceding the species symbol for the electronic wavefunction. A state with zero resultant spin (S = 0 2 + 1 = 1) is described as singlet and states of multiplicity 2,3,4,... are referred to as doublets, triplets, quartets, etc. Thus the ordinary, ground state of, for example, formaldehyde in... 

FIGURE 17.1 Similarities between the orbital hybridization models of bonding in (a) ethylene and ( >) formaldehyde. Both molecules have the same number of electrons, and carbon Is sp -hybrldlzed in both. In formaldehyde, one of the carbons Is replaced by an sp -hybrldlzed oxygen (shown in red). Oxygen has two unshared electron pairs each pair occupies an sp -hybrldlzed orbital. Like the carbon-carbon double bond of ethylene, the carbon-oxygen double bond of formaldehyde Is composed of a two-electron cr component and a two-electron tr component. 

In our rules for orbital mixing, rule 5 states that similar molecules have similar MO diagrams. This is essentially true, but there are important differences that we must consider. Formaldehyde and ethylene are isoelectronic they have the same number of valence electrons and the same types of valence orbitals. Thus, we can expect similar MOs for formaldehyde and ethylene, but with some changes (more properly termed "perturbations") introduced by the oxygen. Experience has shown that the primary consequence of introducing heteroatoms into a hydrocarbon system is to alter orbital energies, as stated in rule 12. 

The number of electron groups around a central atom dictates the number of atomic orbitals that undergo hybridization. The carbon atom in formaldehyde contains three electron groups (remember, a double bond is counted as one group), which means that three atomic orbitals are hybridized to form three sp orbitals. We will follow the procedure shown in Figure 11-14. 

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