Molecular Orbitals for Homonuclear Diatomics CHEM 2060 Lecture 26: MO theory contd L26-1 Molecular orbitals can be formed pictorially by looking at the way in which atomic orbitals overlap. Let s look at how s orbitals and p orbitals can overlap π* orbitals do not change sign upon inversion (g) p and π orbitals change sign upon inversion (u)
CHEM 2060 Lecture 26: MO theory contd L26-2 Note: On an exam, I would prefer that you illustrate the MOs as the linear combination of their respective atomic orbitals. + = σ + = σ - = σ - = σ + = π + =?? - = π HOMEWORK: Can you draw a σ bond between an s and a d orbital?
CHEM 2060 Lecture 26: MO theory contd L26-3 MOs for 2s orbitals The MO energy level diagrams for two interacting 2s atomic orbitals in a homonuclear diatomic look the same as those for two 1s orbitals. What happens when we change the internuclear distance? (i.e., vary the amount of overlap between the atomic orbitals) σ less antibonding lower energy σ 2s 2s 2s 2s σ less bonding higher energy σ
CHEM 2060 Lecture 26: MO theory contd L26-4 MOs for 2p orbitals Consider the isolated set of 2p orbitals in a homonuclear diatomic. σ π π p x p y p z p z p y px π π σ
CHEM 2060 Lecture 26: MO theory contd L26-5 O 2 and F 2 Treating the 2s and 2p orbitals as isolated works well (to a first approx n) for O 2 and F 2. The energy difference between 2s and 2p increases across a period. Note the 1s orbitals are much lower in energy (denoted by the wavy line). They are core orbitals. Chemistry happens at valence orbitals, so we can ignore the 1s orbitals except that we must remember that they are FILLED!
CHEM 2060 Lecture 26: MO theory contd L26-6 Rules for filling molecular orbitals: 1. Fill e- s from the bottom up; lower energy first! 2. Each MO can accommodate a maximum of two (spin-paired) electrons (Pauli exclusion principle) 3. MO s of equal energy (degenerate) are each filled with a single electron before electron pairing begins (Hund s Rule). O 2 Atomic oxygen has 6 valence electrons, so diatomic oxygen has 12 valence electrons. 2 valence electrons are in 2σ g 2 valence electrons are in 2σ u The other 8 are filled as shown. PARAMAGNETIC!
CHEM 2060 Lecture 26: MO theory contd L26-7 Paramagnetism of O 2 The MO diagram predicts that the ground state of dioxygen is paramagnetic! [Def.] Species with unpaired electrons are attracted to a magnetic field. This is called a paramagnetic response. Therefore species with unpaired electrons are paramagnetic. Species with no unpaired electrons are repelled by a magnetic field. This is a diamagnetic response. Because paramagnetism is a response to an applied H, it can be verified experimentally. Liquid O 2 is in fact attracted to an applied magnetic field!
Liquid O 2 poured through the an electromagnet CHEM 2060 Lecture 26: MO theory contd L26-8 Liquid oxygen is pale blue (hard to see in this photo) and is attracted to the poles of a magnet. O 2 has a double-bond and a triplet ground state (2 unpaired electrons). M.O. theory predicts this nicely. You can t predict this using VSEPR theory! So-called singlet oxygen refers to the 1 O 2 (a 1 Δ g ) state. This is an excited state (higher energy electron configuration) in which two electrons are paired in one of the π* orbitals. It is diamagnetic (repelled by an applied H).
CHEM 2060 Lecture 26: MO theory contd L26-9 Singlet oxygen is generated in the atmosphere by photo-excitation. A small energy difference (about 1 ev) allows for this. Singlet oxygen has a long lifetime (relaxation to the triplet ground state is spinforbidden).
CHEM 2060 Lecture 26: MO theory contd L26-10 Singlet oxygen is very reactive (same quantum state as most other species). This has massive ramifications for life on earth. Singlet oxygen causes oxidative damage to cellular structures (oxidation of cell membranes, proteins). When the accumulation of oxidative damage exceeds a threshold level, cells die. Some therapeutic strategies take advantage of Reactive Oxygen Species (singlet oxygen) to target cancer cells.