11/29/2014. Problems with Valence Bond Theory. VB theory predicts many properties better than Lewis Theory

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1 Problems with Valence Bond Theory VB theory predicts many properties better than Lewis Theory bonding schemes, bond strengths, bond lengths, bond rigidity however, there are still many properties of molecules it doesn t predict perfectly magnetic behavior of O Molecular Orbital Theory in MO theory, we apply Schrödinger s wave equation to the molecule to calculate a set of molecular orbitals in practice, the equation solution is estimated we start with good guesses from our experience as to what the orbital should look like then test and tweak the estimate until the energy of the orbital is minimized in this treatment, the electrons belong to the whole molecule so the orbitals belong to the whole molecule unlike VB Theory where the atomic orbitals still exist in the molecule 157 LCAO the simplest guess starts with the atomic orbitals of the atoms adding together to make molecular orbitals this is called the Linear Combination of method weighted sum because the orbitals are wave functions, the waves can combine either constructively or destructively 158 1

2 Molecular when the wave functions combine constructively, the resulting molecular orbital has less energy than the original atomic orbitals it is called a Bonding Molecular Orbital, most of the electron density between the nuclei when the wave functions combine destructively, the resulting molecular orbital has more energy than the original atomic orbitals it is called a Antibonding Molecular Orbital *, * most of the electron density outside the nuclei nodes between nuclei 159 Interaction of 160 Molecular Orbital Theory Electrons in bonding MOs are stabilizing Lower energy than the atomic orbitals Electrons in anti bonding MOs are destabilizing Higher in energy than atomic orbitals Electron density located outside the internuclear axis Electrons in anti bonding orbitals cancel stability gained by electrons in bonding orbitals 161 2

3 MO and Properties Bond Order = difference between number of electrons in bonding and antibonding orbitals only need to consider valence electrons may be a fraction higher bond order = stronger and shorter bonds if bond order = 0, then bond is unstable compared to individual atoms no bond will form. A substance will be paramagnetic if its MO diagram has unpaired electrons # Bond Elec.- #Antibond Elec. if Bond all electrons Order paired it is diamagnetic Hydrogen Orbital Dihydrogen, H 2 Molecular Hydrogen Orbital Since more electrons are in bonding orbitals than are in antibonding orbitals, net bonding interaction 163 H 2 bonding MO HOMO = highest occupied MO * Antibonding MO LUMO = lowest occupied MO 164 3

4 Helium Orbital Dihelium, He 2 Molecular Helium Orbital BO = ½(2 2) = 0 Since there are as many electrons in antibonding orbitals as in bonding orbitals, there is no net bonding interaction 165 Lithium Dilithium, Li 2 Molecular Lithium BO = ½(4 2) = 1 Any fill energy level will generate filled bonding and antibonding MO s; therefore only need to consider valence shell Since more electrons are in bonding orbitals than are in antibonding orbitals, net bonding i t ti 166 Li 2 bonding MO HOMO Antibonding MO LUMO 167 4

5 O 2 dioxygen is paramagnetic paramagnetic material have unpaired electrons neither Lewis Theory nor Valence Bond Theory predict this result 168 Paramag O 2 Diamag N 2 Interaction of p 169 Interaction of p 170 5

6 171 O 2 dioxygen is paramagnetic paramagnetic material have unpaired electrons neither Lewis Theory nor Valence Bond Theory predict this result 172 Paramag O 2 Diamag N 2 O 2 as described by Lewis and VB theory 173 6

7 Oxygen Oxygen Since more electrons are in bonding orbitals than are in antibonding orbitals, net bonding interaction BO = ½( 8 be 4 abe) BO = 2 O 2 MO s Since there are unpaired electrons in the antibonding orbitals, O 2 is paramagnetic 174 O 2 Antibonding MO HOMO Antibonding MO LUMO Bonding MO HOMO Antibonding MO LUMO+1 Draw a molecular orbital diagram of N 2 and predict its bond order and magnetic properties 176 7

8 Nitrogen Nitrogen BO = ½( 8 be 2 abe) b) BO = 3 N 2 MO s Since there are no unpaired electrons, N 2 is diamagnetic 177 Figure MO occupancy and molecular properties for B 2 through Ne Heteronuclear Diatomic Molecules the more electronegative atom has lower energy orbitals when the combining atomic orbitals are identical and equal energy, the weight of each atomic orbital in the molecular orbital are equal when the combining atomic orbitals are different kinds and energies, the atomic orbital closest in energy to the molecular orbital contributes more to the molecular orbital lower energy atomic orbitals contribute more to the bonding MO higher energy atomic orbitals contribute more to the antibonding MO nonbonding MOs remain localized on the atom donating its atomic orbitals 179 8

9 NO Free Radical Bonding MO mainly O s atomic orbital 180 * s The MO diagram for NO * p Energy p possible Lewis structures p 0 0 N O AO of N * s AO of O N O s MO of NO The MO diagram for HF Energy x y AO of H MO of HF AO of F 9

10 Polyatomic Molecules when many atoms are combined together, the atomic orbitals of all the atoms are combined to make a set of molecular orbitals which are delocalized over the entire molecule gives results that better match real molecule properties than either Lewis or Valence Bond theories 183 Ozone, O 3 Delocalized bonding orbital of O