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1、分子轨道理论分子轨道理论 价键理论价键理论化学键理论化学键理论价电子离域价电子离域价电子定域在两原子价电子定域在两原子之间或成为孤对电子之间或成为孤对电子每个原子提供一个价电每个原子提供一个价电子和一个轨道重叠成键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1.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 ot
2、her 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 (no
3、n-bonding interaction). 2. The characteristic distribution and classification of molecular orbital 分子轨道分类 a. -orbital and -bond -轨道与-键 以以“头碰头头碰头”的方式重叠；的方式重叠；原子轨道重叠部分沿着键轴呈圆柱对称；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。成键时，原子轨道发生最大重叠。ss*pp*2. The characteristic distribution and classification of molecular orbi
4、tal 分子轨道分类 a. -orbital and -bond -轨道与-键 +-+-+-+-spzps*ps2. The characteristic distribution and classification of molecular orbital b. -orbital and -bond -轨道与-键 原子轨道平行或原子轨道平行或“肩并肩肩并肩”重叠；重叠；重叠部分对通过一个键轴的平面具有镜面反对称性；重叠部分对通过一个键轴的平面具有镜面反对称性；轨道重叠程度比轨道重叠程度比 键小，键能也小。键小，键能也小。2. The characteristic distribution
5、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 electronic configurations 基态电子组态The aufbau (building-up) principle in molecules:l Pauli exclusion principle Pauli不相容原理l The minimum energy priciple 能量最
6、低原理l Hunds rule. Hund规则HEnergyHH21s1sY+=1gY-=1*uDiatomic molecules: The bonding in HDiatomic molecules: The bonding in H2 2 (H (H2 2分子结构分子结构分子结构分子结构) )H2 = 1g2Bond order(Bond order(键级键级键级键级) of H) of H2 2+ +=1/2; bond order of H=1/2; bond order of H2 2=1.=1.For HFor H2 2, the stabilization energy is
7、 432 kJ/mol., the stabilization energy is 432 kJ/mol.n: Electrons in bonding orbitals 成键n*:Electrons in antibonding orbitals反键Diatomic molecules: The bonding in He2He also has only 1s AO, so the MO diagram for the molecule He2 can be formed in an identical way, except that there are two electrons in
8、 the 1s AO on He.HeEnergyHeHe21s1s1g1*u 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 = , so such a cation might be predicted to exist. The
9、 electron configuration for this cation is:He2+ = 1g21u*1Diatomic molecules: Homonuclear Molecules of the Second PeriodThe 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 molecule.LiEnergyLiLi21s1s1g1*u2s2s2g2*uThe bond
10、order in Be2 is (4-4)/2 = 0, so the molecule can not exist as a covalently bounded molecule.BeEnergyBeBe21s1s1g1*u2s2s2g2*uDiatomic molecules: Homonuclear Molecules of the Second PeriodThis produces an MO over the molecule with a node between the F atoms. This is thus an antibonding MO of *u symmetr
11、y.Diatomic molecules: The bonding in F22pzA+2pzBThe combinations of symmetry:This produces an MO around both F atoms and has the same phase everywhere and is symmetrical about the F-F axis. This is thus a bonding MO of g symmetry.2pzA-2pzB3g3*uThis produces an MO over the molecule with a node on the
12、 bond between the F atoms. This is thus a bonding MO of u symmetry.Diatomic molecules: The bonding in F22pyA+2pyBThe first set of combinations of symmetry:1uThis produces an MO around both F atoms that has two nodes: one on the bond axis and one perpendicular to the bond. This is thus an antibonding
13、 MO of *g symmetry.-2pyA2pyB1*gFEnergyFF22s2s1g1*u2p2p2g2*u1u1*g(px,py)pzO2: 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 (1 u)4 (2g)2 BEnergyBB22s2s1g1*u2p2p2g2*u1u1*g(px,py)pzHOMOLUMOThe effect
14、of interactions between 2s and 2p. At 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 (1 u)2Electronic configurations2s-2pz mixing MoleculeLi2Be2B2C2N2O2F2Ne2 Bond Order10123210Bond Length ()2.67 n/
15、a1.591.24 1.01 1.21 1.42 n/aBond Energy (kJ/mol)105n/a289609941494155n/aDiamagnetic (d)/ Paramagnetic (p)dn/apddpdn/a2s-2pz mixing3. 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 Example: HFThe F (2s) is much lower in energy than the H (1s) so they do
16、 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 though and therefore form nonbonding MOSOnly the 2pz and
17、1s mix.K(2)2 (3)2 (1)4LiH4K(2)2BeH5K(2)2 (3)1CH7K(2)2 (3)2 (1)1NH8K(2)2 (3)2 (1)2OH9K(2)2 (3)2 (1)3HF10K(2)2 (3)2 (1)4电子组态电子组态电子组态电子组态异核双原子分子异核双原子分子异核双原子分子异核双原子分子, HX, HXMO diagram for HFSimplified MO diagram of heteronuclear diatomic moleculesSimplified MO diagram of heteronuclear diatomic molecule
18、sA A B BA = BA = BBeO 12KK(3)2 (4)2 (1)4CN13KK(3)2 (4)2 (1)4 (5)1CO14KK(3)2 (4)2 (1)4 (5)2NO15KK(3)2 (4)2 (1)4(5)2 (2)1异核双原子分子异核双原子分子异核双原子分子异核双原子分子, YX, YX等电子规则等电子规则: The MOs bond formation and electronic configurations are similar among the isoelectronic diatomic molecules.CO is isoelectronic with
19、N2.KK(3)2 (4)2 (1)4 (5)2 LiH4K(2)2BeH5K(2)2 (3)1CH7K(2)2 (3)2 (1)1NH8K(2)2 (3)2 (1)2OH9K(2)2 (3)2 (1)3HF10K(2)2 (3)2 (1)4BeO , BN12KK(3)2 (4)2 (1)4CN ,BeF13KK(3)2 (4)2 (1)4 (5)1CO14KK(3)2 (4)2 (1)4 (5)2NO15KK(3)2 (4)2 (1)4 (5)2 (2)1Molecule electrons electronic configurationSimplified MO diagram of heteronuclear diatomic moleculesSimplified MO diagram of heteronuclear diatomic moleculesEnergy2s2s1g1u2p2p2g2u1u1gpzHOMOLUMOCO is isoelectronic with N2. KK(3)2 (4)2 (1)4 (5)2
