Slide Number 1Bond Dissociation EnergiesMore Bond Dissociation EnergiesVideo : OxygenSlide Number 5Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12MO’s for H2 moleculeSlide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Orbitals of same symmetry, either on same atom or on adjacent atoms can mix. Thus sigma MO’s from s + s overlap or from pz + pz overlap can affect each other if sufficiently close in energy. How do we know if they are?Slide Number 22Slide Number 23Slide Number 24Slide Number 25Slide Number 26Slide Number 27Slide Number 28Slide Number 29Slide Number 30Slide Number 31Slide Number 32Slide Number 33Slide Number 34Slide Number 35Slide Number 36Slide Number 37Superoxide Dismutase (SOD)Slide Number 39Slide Number 40Slide Number 41Slide Number 42Slide Number 43Slide Number 44Light Emission from CometsSlide Number 46Slide Number 47Slide Number 48Slide Number 49Slide Number 50Slide Number 51Slide Number 52Slide Number 53Slide Number 54Slide Number 55Slide Number 56sigma bonds in BenzenePi-bonds C6H6 Slide Number 59Pi-bonds C6H6 Slide Number 61Slide Number 62Slide Number 63Slide Number 64The following slides were mainly a gift from Professor Martyn Poliakoff Of the Department of Chemistry in Nottingham, England. Tom Poliakoff also used these slides and prepared them, to my knowledge. You might also check The MIT open courseware lecture to refresh your memory of molecular orbitals. Chemistry 362; spring 2017 Marcetta Y. Darensbourg, Professor Xuemei Yang, Graduate Assistant Pokhraj Ghosh, Graduate Assistant http://www.chemtube3d.com/orbitalsCO.htm https://www.youtube.com/watch?v=llaa-iEYDLI https://www.youtube.com/watch?v=GD5CrjyAKx4 https://www.youtube.com/watch?v=estiedAlXII HF B2H6 MIT Open Courseware lecture CO Molecular Orbital Approach to Bonding DiatomicsBond Dissociation Energies Can you account for these trends? Yes, you can.More Bond Dissociation EnergiesVideo : Oxygen http://www.periodicvideos.com/videos/008.htmMOLECULAR ORBITAL APPROACHBasis of VB approach: overlap orbitals in each bond separately. Each bond is LOCALISED between two atoms.In molecular orbital (MO) approach - overlap orbitals for the whole molecule - bonding is therefore DELOCALISED. We will look first at DIATOMIC MOLECULES and only later move on to POLYATOMIC MOLECULES.MOLECULAR ORBITAL THEORY FOR DIATOMIC MOLECULESIn principle, set up Schrödinger wave equation for molecule and solve it.Valence Bond Approach: Localized Bonds, just like Lewis Structures and VSEPR Molecular Orbital Approach: De-Localized Orbitals and Electrons in them  Energy Levels, MagnetismSolution will involve molecular orbitals - similar to atomic orbitals - but centred around all of the nuclei in molecule. Each defined by sets of quantum numbers, with electron probability density determined by ψ2, where ψ = molecular wave function.Approximate method:At any moment, electron near one nucleus - approximate behaviour like electron in atomic orbital for that atom. Over time - electron associated with other nuclei in molecule. Therefore construct molecular orbitals (m.o.'s) by forming:Linear Combination of Atomic OrbitalsSimplest example - H2: two H atoms HA and HBOnly two a.o.'s (1sA, 1sB) to form linear combinations.General rule: n a.o.'s n m.o.'sSo we can only construct 2 m.o.'s for H2 - and these are:ψb = 1sA + 1sB and ψa = 1sA - 1sBi.e. the sum (ψb) and the difference (ψa) of the constituent a.o.'s.Consider the electron distribution in each of these:It is this, LCAO, method which we will use to construct m.o's.Consider in each case the INTERNUCLEAR REGIONProbability of finding electron there is: ψb > 1sA, 1sB > ψaElectron in this region attracted to BOTH nuclei, therefore most favourable position. Hence, electron in ψb will be at lower energy than in non-interacting a.o.'s, and electron in ψa will be at higher energy still.Thus an electron in ψb will hold the nuclei together, one in ψa will push them apart.ψb is a BONDING m.o., ψa is an ANTI-BONDING m.o.+ ++++–Two non-interactingH atomsψb = 1sA + 1sBψa = 1sA - 1sBAtom AAtom BNODESigns refer to sign of ψThus we can draw ENERGY LEVEL DIAGRAM for m.o.'s of H2 :1sA1sBψbψaHA H2 HBBy aufbau & Pauli principles - the 2 electrons go into ψb - with paired spins.+++s/sσ2s..s + s overlap everywhere positive→ BONDING M.O.++––σ*2s. .s – s overlap everywhere negative→ ANTI-BONDING M.O.MO’s for H2 moleculeM.O.'s for homonuclear diatomics (A2) for elements of first row of the Periodic TableFor Li2, Be2, B2 etc., more complex than for H2, HHe - more available a.o.'s - 1s, 2s, 2p. Are there restrictions on overlap?(1) VALENCE electrons only - core electrons too close to nucleus, too tightly bound(2) Most efficient overlap between orbitals of same energy, i.e. for homonuclear diatomics this means 2s/2s, 2p/2p (for heteronuclear diatomics - see later)(3) SYMMETRY RESTRICTIONSThese are best shown pictoriallyLet us see how this works for 2s and 2p orbitals.BOND ORDERBy Lewis/V.B. theory - one pair of electrons = one bond.To be consistent, in M.O. theory, define BOND ORDER as follows:Bond order = [(No. of electrons in bonding m.o.'s) – (No. of electrons in antibonding m.o.'s)]/2Thus, for H2, bond order = (2 - 0)/2 = 1(i.e. a single bond - as expected) Magnetic Properties of MoleculesAll electrons paired - repelled by magnetic field - DIAMAGNETICOne or more unpaired electrons - attracted into magnetic field - PARAMAGNETICH2 is diamagnetic.A AA22s2sσ2sσ*2sRemember: 1s orbitals effectively non-bonding,M.O.Energy Level Diagram for A2 (A = Li, Be)Li2Only two valence electrons, i.e. σs2σ*s0.Bond order = 1. DiamagneticLi2 exists in gas phase over metallic lithium."Be2"σs2σ*s2Bond order = 0 - no net bonding energy, so molecule does not exist.Beryllium in gas phase is monatomic.Use Aufbau, Pauli, Hund - just as in filling atomic orbitals+++––––pz/pzσ2pz. .pz + pz overlap everywhere positive→ BONDING M.O.+ +++–––σ*2pz..–pz – pz overlap everywhere negative→ ANTI-BONDING M.O.p orbitals: pz + pz overlap => sigma orbitals, bonding and anti-bonding in same Area as sigma derived from s-orbital overlap.px, py orbitals are perpendicular to axis, but can still interact+++–––px/pxorpy/pyπ2px or π2py. .px + px