MOLECULAR STRUCTURE CHAPTER 11 Experiments show O 2 is paramagnetic

Shapes of AB n molecules

Derivatives from AB n geometry CH 4 H2OH2ONH 3

Fig MO energy level diagram for H 2 constructed from overlap of H1s orbitals D o = 7.18 x J = 36,130 cm -1 λ o = 277 nm

Fig MO energy level diagram for He 2 constructed from overlap of H1s orbitals Bond order: b = ½(n – n*)

Fig Overlap of Period 2 valence orbitals All orbitals of appropriate symmetry contribute to a MO To build σ orbitals, all AOs with cylindrical symmetry about internuclear (z) axis are used. Four AOs form four MOs: 1σ2σ*3σ4σ*

Fig Sigma bonds formed from overlap of 2p z

Fig Pi bonds formed from overlap of two 2p x and two 2p y to form four MOs

Fig Overlap of an s and a p z orbital When two orbitals are far apart their overlap is small When two atoms are closer their orbital overlap will be greater Extent of orbital overlap given by the overlap integral: (Not orthogonal because s and p z are of “appropriate symmetry”)

Fig Overlap integral S between two H1s orbitals in H 2 + as a function of their separation S = 0.59 Typically for n = 2, S = 0.2 – 0.3 For two H1s orbitals:

Fig Overlap of a 2s with a 2p x or 2p y 2s2p+ + − S

Fig MO energy level diagram for (some) homonuclear diatomics p x and p y orbital overlap is off-axis, so... Pi orbitals less strongly bonding than sigma orbitals This diagram applies to O 2 and F 2

Fig Variation in MO energies across Period 2

Fig MO energy level diagram for homonuclear diatomics up to and including N 2