Destructive interference antibonding

The combination of hydrogen Is atomic orbitals to form MOs. The phases of the orbitals are shown by signs inside the boundary surfaces. When the orbitals are added, the matching phases produce constructive interference, which give enhanced electron probability between the nuclei. This results in a bonding molecular orbital. When one orbital is subtracted from the other, destructive interference occurs between the opposite phases, leading to a node between the nuclei. This is an antibonding MO. 

Overlap must be defined somewhat precisely to understand the second statement. Two orbitals overlap significantly if they both have appreciable amplitudes over the same region of space. The net overlap may be positive or zero, depending on the relative phases of the orbitals involved. Bonding orbitals arise from positive overlap (constructive interference), whereas antibonding orbitals result from zero overlap (destructive interference). 

Increased amplitude or electron density between atoms Antibonding orbital (destructive interference) ... 

In the bonding molecular orbital the electron density is greatest between the nuclei of the bonding atoms. In the antibonding molecular orbital, on the other hand, the electron density decreases to zero between the nuclei. We can understand this distinction if we recall that electrons in orbitals have wave characteristics. A property unique to waves allows waves of the same type to interact in such a way that the resultant wave has either an enhanced amplitude or a diminished amphtude. In the former case, we call the interaction constructive interference in the latter case, it is destructive interference (Figure 10.21). 

The plots clearly show the constructive interference that makes a bonding molecular orbital. Nodal planes created by destructive interference are clearly seen in the antibonding molecular orbitals. 

When the parallelp orbitals are combined with the positive and negative phases matched, constructive interference occurs, giving a bonding tt orbital. When the orbitals have opposite phases (the signs of one orbital are reversed), destructive interference occurs, resulting in an antibonding tt orbital. 

The mismatched phases produce destructive interference leading to a node of electron probability between the two nuclei. With electron distribution pushed to the outside, the energy of an antibonding orbital is higher than the energy of the atomic orbitals from which it is composed. 31. a. H2 has two valence electrons to put in the MO diagram whereas He2 has 4 valence electrons. 

At the equilibrium intemuclear separation of 106 pm, the overlap integral is equal to 0.59. Hence, = 0.56 and N = 1.10. A larger normalization constant is needed for the antibonding orbital because there is destructive interference of the two atomic orbitals between the two nuclei. 

Figure 10.23 shows the molecular orbital energy level diagram—that is, the relative energy levels of the orbitals produced in the formation of the H2 molecule— and the constructive and destructive interferences between the two Is orbitals. Notice that in the antibonding molecular orbital there is a nodal plane between the nuclei that signifies zero electron density. The nuclei are repelled by each other s positive... 

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