Slide Number 1Slide Number 2Slide Number 3Slide Number 4Slide Number 5Slide Number 6Slide Number 7Slide Number 8MO’s for H2 moleculeSlide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Slide Number 21Slide Number 22Slide Number 23Slide Number 24Slide Number 25Superoxide Dismutase (SOD)Slide Number 27Slide Number 28Slide Number 29Slide Number 30Slide Number 31Slide Number 32Light Emission from CometsSlide Number 34Slide Number 35Slide Number 36Slide Number 37Slide Number 38Slide Number 39Slide Number 40Slide Number 41Slide Number 42Slide Number 43Slide Number 44sigma bonds in BenzenePi-bonds C6H6 Slide Number 47Pi-bonds C6H6 Slide Number 49Slide Number 50The 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 2016 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 BondingMOLECULAR 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 VSEPRSolution 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.+ ++++–Two non-interactingH atomsψb = 1sA + 1sBψa = 1sA - 1sBAtom AAtom BNODESigns refer to sign of ψConsider 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.Thus 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.MO’s for H2 moleculeBOND 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.HETERONUCLEAR DIATOMIC MOLECULESSimplest would be HHe. Differs from H2 in two ways:(1) A.O. energies for H, He different. He - greater nuclear charge, electrons more tightly bound.(2) Now three electrons to feed into m.o.'s.Energy level diagram is now:1sH1sHeψbψaaverage energyof a.o.'sH HHe HeFor heteronuclear diatomics, m.o.'s formed symmetrically above and below AVERAGE energy of constituent a.o.'sFor HHe, bond order = (2 - 1)/2 = 1/2 i.e. v. wk. "1/2" bond - not formed under normal conditions - v. unstable.Unpaired electron, PARAMAGNETIC.Note for "He2" - extra electron in antibonding m.o. - therefore bond order = 0. Molecule does not exist - no force to hold atoms together.He is monatomic gas.M.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.+++s/sσ2s..s + s overlap everywhere positive→ BONDING M.O.++––σ*2s. .s – s overlap everywhere negative→ ANTI-BONDING M.O.For p orbitals - three per atom. Define z-axis as molecular axis. Hence pz orbitals can overlap in same way as s orbitals.+++––––pz/pzσ2pz. .pz + pz overlap everywhere positive→ BONDING M.O.+ +++–––σ*2pz..–pz – pz overlap everywhere negative→ ANTI-BONDING M.O.px, py orbitals are perpendicular to axis, but can still interact+++–––px/pxorpy/pyπ2px or π2py. .px + px overlap everywhere positive→ BONDING M.O.++++––––π*2px or π*2py. .px – px overlap everywhere negative→ ANTI-BONDING M.O.Also exactly analogous pair from py.Need to consider all possibilities (could be needed for heteronuclear diatomics)++++––––π*2px or π*2py. .+++–––px/pxorpy/pyπ2px or π2py. .s/pz gives bonding