5.1 Basic observations about MOs. When two AOs of the same symmetry interact, a bonding MO is formed which is lower inenergy than the lowest energy AO and an antibonding MO is formed which is higher inenergy than the highest energy AO.out-of-phasegreatestcontributionoverlapfromAantibondingloweringinenergybondingin-phasesmalleroverlaploweringevensmallerloweringBNote:thesizeofanAOalsogreatestcontributionmattersinbonding!increasingenergyseparationbetweenAOsfromB
5.1 Basic observations about MOs • When two AOs of the same symmetry interact, a bonding MO is formed which is lower in energy than the lowest energy AO and an antibonding MO is formed which is higher in energy than the highest energy AO. Note: the size of an AO also matters in bonding!
一5.1 Basic observations about MOs.Whenseveral AOs interacttoformMOs,thenumber of theMOsis the sameasthenumberof the AOshigherthanhighestAOdetailed calcneededtofindenergieslowerthanAOsMOsAOsMOsAOsMOslowestAO. In this more complex case it remains true that a particular MO will have the greatestcontribution from the AOs which are closest to it in energy
5.1 Basic observations about MOs • When several AOs interact to form MOs, the number of the MOs is the same as the number of the AOs. • In this more complex case it remains true that a particular MO will have the greatest contribution from the AOs which are closest to it in energy
Representing MOs: To draw MOs, we need to show the result of the in-phase or out-of-phase overlap, as wellastherelativecontributionsmadebythedifferentAOsOut-of-phase overlapgreatestcontributionfromatomAequal contributionIn-phase overlapgreatestcontributionfromatomB(white~positive,black~negative)
Representing MOs • To draw MOs, we need to show the result of the in-phase or out-of-phase overlap, as well as the relative contributions made by the different AOs. (white ~ positive, black ~ negative) In-phase overlap greatest contribution from atom A. greatest contribution from atom B. Out-of-phase overlap equal contribution
5.2 MO diagram for watero-x2OJEC2vC2: Example: H,O (point group C2v)1x2;y2;22111AlZ:TheO ls AOis too contracted and too low in11-1A2-1Rzxyenergy, transforming as A,B-11-1R,1xXzB21-1-11Rxyyz·0: 2s(spherical) as Aj:0202(Sa, St)T=A, @B2pz(z-like) as Ar.By inspection! (ForAOs without equivalentAOs)2px(x-like) as Bi2py(y-like) as B,. 2H: (sa ss) A, B, (Already considered in chapter 2)as A,0, = (sa+ sb)(x-like) as B)0,=(Sa-St)
5.2 MO diagram for water • Example: H2O (point group C2v) • The O 1s AO is too contracted and too low in energy, transforming as A1 . • O: 2s 2pz 2px 2py • 2H: (sa , sb ) 1 = (sa+ sb ) as A1 2= (sa -sb ) (x-like) as B1 (Already considered in chapter 2) (spherical) as A1 ; (z-like) as A1 . (x-like) as B1 (y-like) as B2 By inspection! (For AOs without equivalent AOs) A1 B1 (sa , sb ) 2 0 2 0 = A1 B1 b a
5.2 MO diagram for water: A rough sense of the relative energies of the AOs involved is needed to draw up the MOdiagram0O2sAO<2pAOD322名SS&SiHD0CHeIBePAr· H 1s AOs ~the oxygen 2p AOs23151840-203.5-40Ae/Koioue-60-80-100-120S-140
5.2 MO diagram for water • A rough sense of the relative energies of the AOs involved is needed to draw up the MO diagram. • O 2s AO < 2p AO, • H 1s AOs ~ the oxygen 2p AOs