Chapter 9. Covalent onding: Orbitals Models to explain the structures and/or energies of the covalent molecules Localized Electron (LE) onding Model Lewis Structure Valence Shell Electron Pair Repulsion (VSEPR) Model Valence ond Theory (ybridization) Delocalized Electron onding Model What do the atomic orbitals for forming molecules? Valence ond Theory (VT) : ybridization First attempt of quantum mechanical explanation of chemical bonding Ψ = φ Α (1)φ Β ()φ Α ()φ Β (1) Ψ = φ Α (1)φ Β () Each electron is free to migrate to the other atom. Probability to find e s between two nuclei is high. bonding Think forming of a bond as Overlap of atomic orbitals
Valence ond Theory (VT) : ybridization Cl C 4 tetrahedral, 4 equivalent bonds 4 C C Cl σ bond σ bond 90 o C 90 o 90 o and one at not defined position??? A σ bond centers along the internuclear axis. Valence ond Theory (VT) : ybridization ybridization : the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. sp 3 C 4 109.5 o four sp 3 orbitals
Valence ond Theory (VT) : ybridization C 4 sp 3 109.5 o sp 3 hybrid a.o.s: 4 C C(sp 3 ) tetrahedral σ(sp 3 C 1s ) N 3 N O O O N s p s sp 3 sp 3 hybridized p 3 lone pair in sp a.o. σ(sp 3 N 1s N ) sp 3 sp 3 hybridized lone pairs in sp 3 a.o.s σ(sp 3 O O 1s ) Valence ond Theory (VT) : ybridization sp F 3 trigonal planar, 3 equivalent bonds
Valence ond Theory (VT) : ybridization sp C 4 σbond πbond all six atoms lie in the same plane A π bond occupies the space above and below the internuclear axis. Valence ond Theory (VT) : ybridization sp linear
Valence ond Theory (VT) : ybridization sp linear C C linear Valence ond Theory (VT) : ybridization dsp 3 PCl 5 A A trigonal bipyramid PCl 5
Valence ond Theory (VT) : ybridization d sp 3 SF 6 A A octahedral Valence ond Theory (VT) : ybridization The Localized Electron Model : A Summary Draw the Lewis structure(s) Determine the arrangement of electron pairs (VSEPR model). Specify the necessary hybrid orbitals. Ex) What is the hybridization of each indicated atom in the following molecule? ow many σ and π bonds are in the molecule? sp sp O C C C C sp N sp 3 O sp C sp 3 1 σ bonds and 4 π bonds ow many lone pair electrons?
LE is great to predict bondings and structures of molecules UT There are some short points. No concept of resonance. No paramagnetic properties. No information of bond energy. O is paramagnetic! O O Lewis structure VSEPR Valence bond theory sp hybridized lone pairs in sp hybrid orbitals bonding pairs in σ and π bonds Paramagnetism unpaired electrons attracted to induced magnetic field much stronger than diamagnetism All show all electrons paired. 1protonand 1electron Schröedinger Eq => Atomic orbitals protons and electrons Schröedinger Eq => Molecular orbitals ut, no way to solve => LCAO (linear combination of atomic orbitals) φ Α Α Β φ φ Β φ Constructive overlap enhance e density between the nuclei attract the nuclei bonding orbital Overlap of wave functions: φ Α φ φ Β Α Β φ destructive overlap node(s) between the nuclei repel each other antibonding orbital Molecular orbitals : Analagous to atomic orbitals for atoms, MOs are the quantum mechanical solutions to the organization of valence electrons in molecules. constructive overlap destructive overlap
higher energy φ Α Α Β φ φ Β φ Constructive overlap enhance e density between the nuclei attract the nuclei bonding orbital φ Α φ Β Α Β φ lower destructive energy overlap node(s) between the nuclei repel each other antibonding orbital φ node atomic orbitals MO = LCAO (linear combination of atomic orbitals) σ* 1s antibonding m.o. σ 1s bonding m.o. e node b.o. = 1 b.o. = 0.5 b.o. = 0 (σ 1s ) (σ 1s ) (σ 1s *) 1 (σ 1s ) (σ 1s *)
node Li s s Li atomic orbitals In order to participate in MOs, atomic orbitals must overlap in space.(therefore, only valence orbitals of atoms contribute significantly to MOs.) e e e e b.o. = 1 (σ s ) (σ s ) (σ s *) b.o. = 0 node s s atomic orbitals p orbitals => Interactions between p orbitals => π π σ
expected : (σ s ) (σ s *) (σ p ) expected b.o = 1 The stronger overlap between AOs, the lower MO. node s s s atomic orbitals paramagnetic must have unpaired electron(s) Diamagnetic material expected : (σ s ) (σ s *) (σ p ) expected b.o = 1 Paramagnetic material Paramagnetism unpaired electrons attracted to induced magnetic field much stronger than diamagnetism Caused by σ s *σ p mixing (σ s ) (σ s *) (π p ) b.o = 1
ndrow homonuclear diatomic molecules LUMO (lowest unoccupied MO) OMO (highest occupied MO) ig triumph of MO Theory!! In general, b.o, b.l., b.e. O in magentic field Ex) electron configurations and bond orders of O, O, O? O O O O : (σ s ) (σ s *) (σ p ) (π p ) 4 (π p *) b.o. = O : (σ s ) (σ s *) (σ p ) (π p ) 4 (π p *) 1 b.o. =.5 O : (σ s ) (σ s *) (σ p ) (π p ) 4 (π p *) 3 b.o. = 1.5
etero Nuclear Diatomic Molecules different two atoms similar two atoms NO NO and CN b.o. =.5 b.o. = 3 Combining the Localized Electron and Molecular Orbital Theory Resonance in Lewis structure C 6 6 6 equivalent CC bonds σ bonds delocalized π bonding all equivalent σ bonds 6 equivalent CC bonds
Combining the Localized Electron and Molecular Orbital Theory Resonance in Lewis structure O 3 NO 3 delocalized π bonding σ (sp O sp N)