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CHEM 230 (1)

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CHEM 230
Jennifer Warriner

:r;e. ,:---,: :,::''-a:::s,,.erel:cnl; fonilsr-roratomricorlEen ere knorr,n.lnLcludins O--" (:).-,:: il,'-T":e rtemluclear O-O distance can be conrenientlr correlatedrrith the b'-':i-,ri:::3;:;Iei br themolecular orbital model. asshou-n inthe follorvins table. Bond order Internucleadistanc(pm) - O- dier:itgenvtl: 2.5 ttz.3 O- rdiorveen,t6 2.0 120.07 O^- tsuperoxide)7 1.5 128-132 O,r-(peroxide)7 1.0 150-115 The effectof mixing is not sufficientin O, to push the o,(2n) orbital tohigher energy thanthe ftorbitals.The order of molecular orbitals shown is consistentwith the photoelectron spectrumdiscussed Section 5-2-4. F rlo Itu'nn*'nn*' Qill n2'nu' The MO picture of F, showsa diamagnetic molecule having a singlefluorine-fluorine bond,in agreementwith experimental dataon this veryreactivemolecule. Thenet bondorder inN, O, andF, is thesame whether or notmixing is taken into account,but the order of thefilled orbitalssdifferent.The switching of the orderof the o r(2iland n,(2p) orbitalscanoccur because these orbitalsaresoclosein energy;minor changesin eitherorbitalcanswitch their order.Theenergy differencebetweenthe 2s and 2p orbitalsof theatomsincreaseswith increasingnuclear charge from 5.7eY inboronto 2l .1eY in fluorine(detailsareinSection 5-4-l). Because thedifferencebecomes greater, thes-p interactiondecreases and the "normal" order of molecular orbitalsreturns.The higher o, orbitalis seenagainin CO, describedlaterin Section5-4-2. Ne, All the molecular orbitalsare filled, thereareequal numbersof bonding andantibond- ing electrons, and the bond order is thereforezero. The Ne, molecule is a transient speciesif it existsat all. One triumph of molecular orbitaltheorywasits prediction of two unpairedelec- tronsfor Or. It hadlong been known thatordinary oxygenis paramagnetic, but the ear- lier required of aspecial "three-electron phe- bondingtheories use bond"8to explainthis nomenon. On the other hand, the molecular orbital descriptionshowsthe unpaired electronsdirectly.In the othercasesdescribed previously, the experimental facts(para- magneticB, diamagneticCr) requirea shift of orbitalenergies,raising onaboveft,,but theydo not requireanydifferentkind of orbitalsor bonding. 5-2.4PHOTOELECTRO SNPECTROSCOPY It would be useful to establish the MO order experimentally to check the theoretical results.Photoelectronspectroscopye is oneof themost directmethodsfor determining 5G.Herzber, MoleculaSpectrandMolecular StructuI: TheSpectraofiatomic MoleculeYan NostrandReinholdNew York, 1950,.366. 65.L.Miller andC. H. Townes,ys.Rev.,1953,90537. 7N.-G.Vannerberg,Prog. Inorg. Chem.,,4, 125Oxygen--oxygendistancesin Or- andOr2- are influencedby the cation. 8L.PaulingTheNature of the Chemicalnd, 3rd eCornellUniversityress,IthacN.Y.,1960, pp.34V354. eD.N. Hendrickson,in. S.Drago, Physical Methoin Chemistry,W.B. Saunders,Philadelphia, 1977,pp. 566-584 12O Chapter 5 MolecularOrbitals Nitrogen or(Zn) nu(2n) o rrl ou* (2s) FIGURE 5-7 PhotoelectronSpec- trum and MoleculOrbitalnergy Levelsof Nr.Photoelectrspec- trum reproducewithpermission from J. L. Gardner andJ. A. R. Sam- son,,I. Chem.Phys.,L975,62, 1447- r4s2.) orbital energies,but there are differences of opinion about the interpretation of the results.In this technique,UV light or X-rays dislodgeelectronsfrom molecules: Ort hv(photons) + Or+ * e- The kinetic energy of the expelled electrons can be measured.The difference between the energy of the incident photons and this kinetic energy is the ionization energy (binding energy) of the electron: Ionization energy : ftv(photons) - kinetic energyof expelledelectron Ultravioletlight removes outer electrons, usually from gases; X-rays are more energeticandremoveinner electrons aswell, from any physical state.Figures 5-7 and 5-8 show photoelectron spectra for N, and O, and the relative energiesof the highest occupiedorbitalsof the ions.The lower energy peaks (atthetop in thefigure) arefor the higher
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