Topic 2 Molecular Orbital Theory
Professor David L. Van VrankenChemistry 201: Organic Reaction Mechanisms ITopic 2: Molecular Orbital TheoryReading: Ch. 1 of your sophomore organic chemistry textbookI. Fleming Molecular Orbitals and Organic Chemical Reactions, Ch. 2 & 3Bradley, J. D.; Gerrans, G. C.“Frontier molecular orbitals. A link between kinetics and bonding theory.”J. Chem. Educ. 1973, 50, 463.
The Need For Orbitalsn Heisenberg said we can't specify the location of electronsn We need orbitals to describe where the pairs of electrons want to ben Orbital phases help us see how one electron avoids the other, even though they are in the same orbitaln There are three basic types of orbitals1. Atomic Orbitals2. Hybrid Atomic Orbitals3. Molecular OrbitalsLet's review them
1. Atomic Orbitals - Reviewn There are four types of atomic orbitals:s, p, d, fn We can rationalize everything in this class using combinations of s and p orbitalsn p orbitals are way higher in energy than s orbitalsn electronegativity decreases orbital energyThere’s a spherical node inside the 2s orbital, but let’s ignore it.
2. Hybrid Atomic Orbitals - Reviewn Mixing Rule:When you mix two orbitals, you get two orbitalsThe reason you get two orbitals is because there are always two arbitrary phasing combinationsn Three ways to mix one 2s and three 2p orbitals of 2nd row atoms to give non-bonding orbitals, nignore 1sNote: electrons inhigher energy orbitalsare more reactiven Atom geometry correlates with hybridization (VSEPRtheory). If you could force ammonia to be planar, the lone pairwould end up in a super high energy p orbital. sp 3Nplanardistortion. pNsupernucleophilic
2. Hybrid Atomic Orbitals – Differences in Reactivity Based on p Charactern Assess p character inmolecular orbitalscorresponding to everybond and every lone pairbecause it predicts thereactivity of the electrons.n More p character more basic and more nucleophilicn The magnitude of the effect isless pronounced for oxygen, whichis less reactive overall thannitrogen.
2. Quantitative Differences in Reactivity based on p Charactern More p character in C-H sigma bonds correlates with lowerBond Dissociation Energies. (Compare only C-H bonds)n BIG Caution: assign hybridizationAFTER considering resonance. If youdon’t consider resonance then you’re notreally thinking about molecular orbitals.BDE is defined as theenergy required for homolysisin a hypothetical reaction:
3. Molecular Orbitals – Six Types of “Frontier” Molecular Orbitalsn Arrows start from filled orbitals and end on un-filledorbitals. There are six canonical classes of frontiermolecular orbitals that are used for arrow pushing.n Commit these canonical orbitals, andtheir relative energies to memory.n Since there are only three types of filled FMOs and three types of unfilled FMOs, that meansthat there are only 3x3 9 types of non-concerted elementary chemical reactions. We’ll spend therest of this quarter talking about these nine types of interactions between filled and un-filled orbitals.n Note that unfilledmolecular orbitals arealways higher in energythan filled molecularorbitals.(Aufbau principle).
3. Molecular Orbitals - M.O. Interaction Diagramsn Perturbation theory saysthat you get more orbitalInteraction Energy, (I.E.) bymixing MOs that are closerin energyOrbitalInteraction Energyorbital overlapE filled-E emptyn M.O. Interaction diagrams are used to graphically depict the energetic consequences that result fromperturbation of molecular orbitals through pair-wise mixing.n If you could predict which filled orbitals are higher in energy and which unfilled orbitals are lower inenergy, then you could predict which reactions would be fast and which reactions would be slow. You’llspend the rest of the quarter practicing those predictions.
3. Molecular Orbitals – FRONTIER Molecular Orbitalsn When two reactants interact, every filled orbital in one reactant interacts with every filled orbital inthe other reactant. We can quantify that with perturbation theory resulting in a mathematicalequation with lots of terms.n When two reactants interact,most of the orbital interactionsare not energetically favorable.The summed energy fromorbital interactions usuallycomes from a singleinteraction: between thehighest occupied molecularorbital (HOMO) in one reactant,and the lowest unoccupiedmolecular orbital (LUMO) in theother reactant. The HOMO andLUMO are the frontierorbitals.H 3NH 3CBretc.A complex summation oforbital interactions.Setc.all MOsEMOorbital overlapE filled-E emptyLUMOHOMOreduces to.etc.etc.orbital overlapEHOMO -ELUMO
3. Molecular Orbitals - The Importance of Orbital Overlapn Correct symmetry is required for effective overlap. Graphically, like phases lead to constructiveinteractions, but unlike phases lead to destructive interactions.GOODssBADp or spnp or spnppenergy loweringp pno net changen Bredt’s Rule: Bridgehead olefins are unstablen p orbitals overlap more effectively when they are closer together.Longer bonds are less stable and more nucleophilicp overlap C CpE CE C1.46 ÅC5H11C5H11closer p orbitals better overlap more stable pi bond less reactive pi bondC C1.20 Å86%mCPBAROCapon, R. J.; Barrow, R. A. J. Org. Chem. 1998, 63, 75-83REMOb3lyp/6-311 G**H 2CCH2HC CH-0.285 ev-0.306 ev
Intrinsic Reactivity of Canonical MOs Used for Arrow-Pushing: l.p., pi bonds, sigma bondsn Generally the reactivity ofnucleophilic groups used for arrowpushing follows the order: l.p. pi sigma.n You usually won’t have a problemidentifying the most reactive pair ofelectrons in a molecule.lone pairsN:pi bondssigma bonds10 2120intrinsic nucleophilicity of MOs (ignoring charge)n Electronegativity plays an importantrole in determining nucleophilicity.log15O:1011.610 11.5C C105F:010 1.610 2.9C N10 -3.7C O-510 2.7C H10 0C C10 -3.2C N -5.110C O10 -8.7C F-10Based on MO energies calculated with B3LYP/6-31 G(d,p) versus the LUMO for H3CCH O. Assumes equal orbital overlap.sCC
Intrinsic Reactivity of Canonical MOs Used for Arrow-Pushingn How accurate are the intrinsic reactivitivities of thecanonical MOs? Maybe 105 ?n Lot’s of the lower energy FMOs will have similarreactivity, but usually, it won’t be difficult to identifythe most reactive frontier orbital.Suggest a plausible arrow-pushing mechanism:N HN CH3H 3CISSwarmexcessand NH 2ISGabriel, et al.Chem. Ber. 1890, 247815Hintrinsic nucleophilicity of MOs (ignoring charge)n Remember that generally: l.p. pi sigmalogonefunctionalgroup10510 12.4C N:HH l.p.H10 2.9C NH p H0sCC-5-10-15H10 -12.1C NONsssHHOFlots of MOs w/ similar reactivity
The Importance of Hybridization and p Charactern More p character more nucleophiliclogn Less p character less nucleophiliclone pairsN:pi bondssigma bonds10 21n p orbitals overlap more effectively when theyare closer together.Longer bonds are less stable and morenucleophilicintrinsic nucleophilicity of MOs (ignoring charge)20151011.5C10C10 8C C5N:10 2.910 2.7C H1.4C H 100N:10 -2.2-5-1010 -9.3C HsCC
Effect of Bond Length on the Energy of Frontier Orbitalsn Longer bonds are more nucleophilic.n Longer bonds are easier to break.n That is why: AlH4- is more nucleophilic than BH4n That is why: SN2 reactions with R—I are faster than SN2 reactions with R-Cl
The Effect of Conjugation on the Energy of Filled and Unfilled Frontier Orbitalsn Pi conjugation raises the HOMO and lowers the LUMOn That is why:even thoughconjugatednon-conjugatedR faster thanR : Nufaster than: Numore stable than
Summary of FMO Trends
Topic 2: Molecular Orbital Theory Reading: Ch. 1 of your sophomore organic chemistry textbook I. Fleming Molecular Orbitals and Organic Chemical Reactions, Ch. 2 & 3 Bradley, J. D.; Gerrans, G. C. “Frontier molecular orbitals. A link between kinetics and bonding theory.” J .
2 A BRAND OF ITW ORBITAL CUTTING & WELDING ORBITALUM TOOLS GMBH osef-Schuettler-Str. 17 78224 Singen Germany Tel. 49 (0) 77 31 792 - 0 Fax 49 (0) 77 31 792 - 524 tools@orbitalum.com www.orbitalum.com Orbital welding systems and accessories for high-purity process piping ORBIMAT 180 SW Orbital welding power supply More quality, safety and flexibility in orbital welding by Industry 4.0 technology:
Using Symmetry: Molecular Orbitals One approach to understanding the electronic structure of molecules is called Molecular Orbital Theory. MO theory assumes that the valence electrons of the atoms within a mole
’s point group atom X’s p x and p y atomic orbitals symmetry label given to identify the p x and p y orbitals of atom X (central atom) symmetry label given to identify the p z orbital of atom X (central atom) atom X’s p z atomic orbitals atom X’s s–orbital s-orbital symmetry label C 2 1 –1
to the understanding and application of approximate molecular orbital calculations to chemical problems. The remainder of this chapter is a cursory and informal discussion of certain quantum-mechanical principles and an introduction to the orbital description of electronic structure. In Chap. 2, the methods of molecular orbital theory are
Topic 5: Not essential to progress to next grade, rather to be integrated with topic 2 and 3. Gr.7 Term 3 37 Topic 1 Dramatic Skills Development Topic 2 Drama Elements in Playmaking Topic 1: Reduced vocal and physical exercises. Topic 2: No reductions. Topic 5: Topic 5:Removed and integrated with topic 2 and 3.
Timeframe Unit Instructional Topics 4 Weeks Les vacances Topic 1: Transportation . 3 Weeks Les contes Topic 1: Grammar Topic 2: Fairy Tales Topic 3: Fables Topic 4: Legends 3 Weeks La nature Topic 1: Animals Topic 2: Climate and Geography Topic 3: Environment 4.5 Weeks L’histoire Topic 1: Pre-History - 1453 . Plan real or imaginary travel .
referred to as "FMO-MD," is an ab initio MD method (Komeiji et al., 2003) based on FMO, a highly parallelizable ab initio molecular orbital (MO) method (Kitaura et al., 1999). Like any ab initio MD method, FMO-MD can simulate molecular phenomena involving electronic structure changes such as polarization, electron transfer, and reaction.
Alfredo López Austin “Rayamiento (Tlahuahuanaliztli)” p. 15-22 : Juegos rituales aztecas Alfredo López Austin (versión, introducción y notas) México Universidad Nacional Autónoma de México . Instituto de Investigaciones Históricas : 1967 . 94 p. (Cuadernos Serie Documental 5) [Sin ISBN] Formato: PDF Publicado en línea: 21 de noviembre de 2018 . Disponible en: www.historicas.unam .
