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1、分子轨道理论 价键理论,化学键理论,价电子离域,价电子定域在两原子之间或成为孤对电子,每个原子提供一个价电子和一个轨道重叠成键,Molecular orbital theory and diatomic molecules 分子轨道理论和双原子分子Molecular orbital theory 分子轨道理论 a.Every electron in a molecule is in an average potential field of the nuclei and the other electrons.每个电子围绕原子核和其他电子组成的平均势场中运动,b.分子轨道近似为原子轨道线性组合
2、,组成分子轨道的原子轨道满足：*have comparable energy,能量相近*have compatible symmetry,对称性匹配*be able to have maximum overlap.轨道最大重叠,n个原子轨道线性组合形成n个分子轨道,n个原子轨道线性组合形成n个分子轨道,The overlap integral S may be positive(bonding),negative(antibonding)or zero(non-bonding interaction).,2.The characteristic distribution and classif
3、ication of molecular orbital 分子轨道分类 a.-orbital and-bond-轨道与-键,以“头碰头”的方式重叠；原子轨道重叠部分沿着键轴呈圆柱对称；成键时，原子轨道发生最大重叠。,s,s*,p,p*,2.The characteristic distribution and classification of molecular orbital 分子轨道分类 a.-orbital and-bond-轨道与-键,+,+,-,+,+,-,+,-,+,+,-,s,pz,ps,*ps,2.The characteristic distribution and cla
4、ssification of molecular orbital b.-orbital and-bond-轨道与-键,原子轨道平行或“肩并肩”重叠；重叠部分对通过一个键轴的平面具有镜面反对称性；轨道重叠程度比 键小，键能也小。,2.The characteristic distribution and classification of molecular orbital b.-orbital and-bond-轨道与-键,3.The structure of homonuclear diatomic molecules 同核双原子分子结构a.The ground state electron
5、ic configurations 基态电子组态,The aufbau(building-up)principle in molecules:Pauli exclusion principle Pauli不相容原理 The minimum energy priciple 能量最低原理 Hunds rule.Hund规则,Diatomic molecules:The bonding in H2(H2分子结构),H2=1g2,Diatomic molecules:The bonding in He2,He also has only 1s AO,so the MO diagram for the
6、molecule He2 can be formed in an identical way,except that there are two electrons in the 1s AO on He.,The bond order in He2 is(2-2)/2=0,so He2 may not exist as a covalently bounded molecule.However the cation He2+,in which one of the electrons in the*MO is removed,would have a bond order of(2-1)/2=
7、,so such a cation might be predicted to exist.The electron configuration for this cation is:He2+=1g21u*1,Diatomic molecules:Homonuclear Molecules of the Second Period,The bond order in Li2 is(4-2)/2=1,so the molecule could exists.In fact,a bond energy of 105 kJ/mol has been measured for this molecul
8、e.,The bond order in Be2 is(4-4)/2=0,so the molecule can not exist as a covalently bounded molecule.,Diatomic molecules:Homonuclear Molecules of the Second Period,This produces an MO over the molecule with a node between the F atoms.This is thus an antibonding MO of*u symmetry.,Diatomic molecules:Th
9、e bonding in F2,2pzA,+,2pzB,The combinations of symmetry:,3*u,This produces an MO over the molecule with a node on the bond between the F atoms.This is thus a bonding MO of u symmetry.,Diatomic molecules:The bonding in F2,2pyA,+,2pyB,The first set of combinations of symmetry:,1u,F,Energy,F,F2,2s,2s,
10、1g,1*u,2p,2p,2g,2*u,1u,1*g,(px,py),pz,O2:KK(2s)2(*2s)2(2p)2(2p)4(*2p)2,F2:KK(2s)2(*2s)2(2p)2(2p)4(*2p)4,For oxygen and fluorine,2p and 2s are well separated.,N2:KK(1g)2(1u)2(1u)4(2g)2,B,Energy,B,B2,2s,2s,1g,1*u,2p,2p,2g,2*u,1u,1*g,(px,py),pz,HOMO,LUMO,The effect of interactions between 2s and 2p.At
11、the start of the second row Li-N,we have mixing of 2s and 2p.The result is that 1u*is pushed down in energy whereas 2g is raised.,B2:KK(1g)2(1u)2(1u)2,Electronic configurations,2s-2pz mixing,2s-2pz mixing,3.The structure of hetronuclear diatomic molecules 异核双原子分子,组成分子轨道的原子轨道满足：*能量相近*对称性匹配*轨道最大重叠,Exa
12、mple:HF,The F(2s)is much lower in energy than the H(1s)so they do not mix.The F(2s)orbital makes a non-bonding MO.We certainly dont have to worry about the F(1s)because is MUCH lower in energy.The H(1s)and F(2p)s are close in energy and do interact.The 2px and 2py dont have the appropriate symmetry
13、though and therefore form nonbonding MOSOnly the 2pz and 1s mix.,K(2)2(3)2(1)4,电子组态,异核双原子分子,HX,MO diagram for HF,Simplified MO diagram of heteronuclear diatomic molecules,A B,A=B,异核双原子分子,YX,等电子规则:The MOs bond formation and electronic configurations are similar among the isoelectronic diatomic molecules.CO is isoelectronic with N2.KK(3)2(4)2(1)4(5)2,Molecule electrons electronic configuration,Simplified MO diagram of heteronuclear diatomic molecules,Energy,2s,2s,1g,1u,2p,2p,2g,2u,1u,1g,pz,HOMO,LUMO,CO is isoelectronic with N2.KK(3)2(4)2(1)4(5)2,
