LCAO Procedure LCAO 2nd Period LCAO constructs approximate MO s from Hartree AO s Li2 1s2 2s1 1s and 2s di er by 52 eV unlikely that 1s and 2 s interact here Homonuclear second period diatomic molecules Mixing of AO s mixing of di erent orbitals occurs if energy states are close 1Rydberg 1Ry 13 6 eV Mixing occurs when orbitals of similar symmetry overlap Overlap of 2s and 2 p orbitals creating and bonds radial nodes visible Fig 6 13 p 255 formation of bonding and antibonding g2pz u2pz MO s from 2pz AO s NODE Fig 6 14 p 256 2px and 2py AO s forming g2p u2p MO s contain nodal plane in internuclear axis momentum Fig 6 15 p 257 MO s and their consNtuent AO s g u omiPed 1sA 1sB 1 s 1sA 1sB 1 s 2sA 2sB 2 s 2sA 2sB 2 s 2px A 2px B 2p 2px A 2px B 2p 2py A 2py B 2p 2py A 2py B 2p 2pz A 2pz B 2p 2pz A 2pz B 2p For Z 7 atoms and ions the degenerate 2p MO s are lower in energy than correlaNon diagram changes between N and O O 2p MO s are lower than 2s N 2p MO s are higher than 2s Fig 6 16 p 258 Drawing the CorrelaNon Diagram 2nd period Li Be B C N O F Ne 2s and 2p mixing con rmed by photoelectron spectroscopy F F bond order 1 What correlaNon diagrams can tell us Bond order CorrelaNon with Lewis diagram ParamagneNc or diamagneNc O2 6 valence electrons electrons O2 pick the correlaNon diagram Z 7 get the electron con guraNon plug them in the AO s Fill in the MO s Bond order 8 bonding 2 4 anNbonding 2 2 ParamagenNc or diamagneNc Has two unpaired electrons Hunds rule therefore paramagneHc Lewis predicts opposite liquid oxygen attracted by magnetic poles paramagentic Fig 6 18a p 260 N2 5 valence electrons liquid nirogen repelled by magnetic poles diamagentic Fig 6 18b p 260 Table 6 3 p 261 Fig 6 19 p 261 Heteronuclear Diatomic Molecules we drop the u g notaNon as symmetry changes no inversion symmetry in fact we can forget u and g in the exams now the energy levels of atom A and B di er energy of 1s 2s 2p are di erent in A and B Heteronuclear Diatomic Molecules 2s CA2sA CB2sB A B molecule 2s C A2sA C B2sB homonuclear CA CB C A C B heteronuclear CA CB C A C B If B is more electronegaNve than A CB CA for bonding and CB CA for anNbonding resembles more a 2sA AO mixing of di erent orbitals occurs if energy states are close 1Ry 13 6 eV and symmetry is similar 2 3 2 4 NO N 2s 2p O 2s 2p focus on 2 p electrons paramagneNc free radical bond order 2 5 Nitric oxide also known as nitrogen monoxide CorrelaHon Diagram for a Heteronuclear diatomic molecule AB B Fig 6 20 p 263 Fig 6 21 p 264 wierd Fig 6 21a p 264 Fig 6 21b p 264 Fig 6 21c p 264 Fig 6 21d p 264 Fig 6 22 p 265 HCl HF En 13 6 Ze 2 n2 eV Ze e ecNve atomic charge for the nucleus For approximate values one may use Z directly The 1s orbital energy level is 13 6 eV for hydrogen atoms measured as the ionizaNon energy of H Thus for the quantum number n 1 the energy level for 1s of He is approximately 54 eV 24 6eV Similarly the 1s energy level for F is 1101 eV 696 7 eV Photoelectron spectroscopy H2 H2 e IE 15 5 eV h photon 1 2 mv2 Eivib VibraNonal excitaNon approaches dissociaNon limit A photoemission from bonding orbital bond order decreases vibraNonal mode is souer B anNbonding orbital bond order increases vibraNonal mode higher sN er bond C non bonding orbital no change in vibraNonal frequency p 266 N1s N2 p 267 O1s O2 p 267 O1s N1s NO core electrons fairly insensiNve to bonding except chemical shiu CORE ELECTRONS p 267