columbia大学有机化学课件01_12_14 html

1、1.12Electron Waves and Chemical BondsThe Lewis model of chemical bonding predates the idea that electrons have wave properties.There are two other widely used theories of bonding that are based on the wave nature of an electron.Valence Bond TheoryMolecular Orbital TheoryModels for Chemical BondingEx

2、amine how the electrostatic forces change as two hydrogen atoms are brought together.These electrostatic forces are: attractions between the electrons and the nucleirepulsions between the two nuclei repulsions between the two electrons+e+eFormation of H2 from Two Hydrogen Atomsweak net attraction at

3、long distancesPotentialenergyH+HH HInternuclear distanceFigure 1.14attractive forces increase faster than repulsive forcesas atoms approach each otherPotentialenergyH+HH HH HH HInternuclear distanceFigure 1.14ion energy)ar distancePotentialenergyH+HH-436 kJ/molH2Internuclear distancemaximum net attr

4、act(minimum potential at 74 pm internucle74 pmHH HHH Figure 1.141s1sHH2 H atoms:each electron feelsattractive force of one protonHHH2 molecule:each electron feelsattractive force of both protonsFigure 1.14reasee forces than 74 pmPotentialenergyH+HH-436 kJ/molH2Internuclear distancerepulsive forces i

5、ncfaster than attractiv at distances closer74 pmHH HHH Figure 1.14Valence Bond Theoryconstructive interference between two electron waves is basis of shared-electronbondMolecular Orbital Theoryderive wave functions of moleculesby combining wave functions of atomsModels for Chemical Bonding1.13Bondin

6、g in H2:The Valence Bond ModelElectron pair can be shared when half-filledorbital of one atom overlapshalf-filled orbital of another.in phase withValence Bond Model1s1sHHin-phase overlap of two half-filledhydrogen 1s orbitalsHHo bond of H2Valence Bond Modelo Bond:orbitals overlap alonginternuclear a

7、xisCross section of orbital perpendicular to internuclear axis is a circle.HHValence Bond ModelFigure 1.17(a)The 1s orbitals of two separated hydrogenatoms are far apart.Essentially no interaction.Eachelectron is associated with a single proton.Valence Bond Model of H2Figure 1.17(b)As the hydrogen a

8、toms approach each other,their 1s orbitals begin to overlap and each electron begins tofeel the attractive force of both protons.Valence Bond Model of H2Figure 1.17(c)The hydrogen atoms are close enough sothat appreciable overlap of the the two 1s orbitals occurs. The concentration of electron densi

9、ty in the region betweenthe two protons is more readily apparent.Valence Bond Model of H2Figure 1.17(d)A molecule of H2.The two hydrogen 1sorbitals have been replaced by a new orbital thatencompasses both hydrogens and contains both electrons.Valence Bond Model of H21.14Bonding in H2:The Molecular O

10、rbital ModelElectrons in a molecule occupy molecular orbitals (MOs) just as electrons in an atom occupy atomic orbitals (AOs).Two electrons per MO, just as two electrons per AO.Express MOs as combinations of AOs.Main IdeasLinear combination of atomic orbitals method expresses wave functions of molecular orbitals as sums and differences of wave functionsof atomic orbitals.Two AOs yield two MOsBonding combinationAntibonding combinationyMO=y(H)1s -y(H)1syMO=y(H)1s + y(H)1sMO Picture of Bonding in H21s1sAOAOFig. 1.19:Energy-Level Diagram for H2 MOsMOs*antibondingsbondingMOF