Chemistry 125: Lecture 2 Force Laws, Lewis Structures, and Resonance Newton suggested searching for a special force law to describe very short range attraction. Neither direct (Hooke’s Law) nor inverse (Coulomb, Gravity) dependence on distance will do – a composite like the Morse potential is needed. G. N. Lewis devised a “cubic-octet” theory based on the newly discovered electron, and developed it into a shared pair model to explain bonding. Lewis-dot notation and formal charges can be useful, but in some “singleminimum” cases the Lewis formalism is inadequate and salvaging it required introducing the confusing concept of “resonance.” Synchronize when the speaker finishes saying “…last time we got to the point of asking…” Synchrony can be adjusted by using the pause(||) and run(>) controls. Except for cited third-party material Chem 125 Lectures by J. M. McBride are licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Are There Atoms & Molecules? What Force Holds Atoms Together? Isaac Newton (1643-1727) Force : Gravity Attraction at a Distance vs. Cartesian blocked repulsion Newton : Force r-2 (How about mass?) Isaac Newton (1643-1727) Query 31 in Opticks (1717) Have not the small Particles of Bodies certain Powers, Virtues, or Forces by which they act at a distance, not only upon the Rays of Light for reflecting, refracting and inflecting them, but also upon one another for producing a great part of the Phaenomena of Nature? For it's well know that Bodies act one upon another by the Attractions of Gravity, Magnetism and Electricity; and these Instances shew the Tenor and Course of Nature, and make it not improbable but that there may be more attractive Powers than these. For Nature is very consonant and conformable to her self. Query 31 How these Attractions may be perform'd, I do not here consider. What I call Attraction may be perform'd by impulse, or by some other means unknown to me. I use that Word here to signify only in general any Force by which Bodies tend towards one another, whatsoever be the Cause. For we must learn from the Phaenomena of Nature what Bodies attract one another, and what are the Laws and Properties of the attraction, before we enquire the Cause by which the Attraction is perform'd, The Attractions of Gravity, Magnetism and Electricity, react to very sensible distances, and so have been observed by vulgar Eyes, and there may be others which reach to so small distances as hitherto escape observation; and perhaps electrical Attraction may react to such small distances, even without being excited by Friction. Query 31 The Parts of all homogeneal hard Bodies which fully touch one another, stick together very strongly. And for explaining how this may be, some have invented hooked Atoms, which is begging the Question; and others tell us that Bodies are glued together by rest, that is, by an occult Quality, or rather by nothing; and others that they stick together by conspiring Motions, that is, by2 relative rest Maybe not F 1/r ? amongst themselves. I had rather infer from their Cohesion, that their Particles attract one another by some Force, which in immediate Contact is exceeding strong, at small distances performs the chymical Operations above mention'd, and reaches not far from the Particles with any sensible Effect. Query 31 …the Attraction [between glass plates separated by a thin film of Oil of Oranges] may be proportionally greater, and continue to increase until the thickness do not exceed that of a single Particle of the Oil. Query 31 There are therefore Agents in Nature able to make the Particles of Bodies stick together by very strong Attractions. And it is the business of experimental Philosophy to find them out. (This business will take us nearly five weeks) Binding Energies from Various Sources Coulombic q1q2/r : Proton-Electron at 1.54Å (216 kcal/mol) Magnetic 12/r3 : Electron Spins at 1.54Å (0.0014 kcal/mol) “Strong” Binding : Proton-Neutron in Deuterium Nucleus (5 x 107 kcal/mol) Gravitational m1m2/r : C atoms at 1.54Å (3 x 10-32 kcal/mol) Chemical Bond : C-C at 1.54Å (90 kcal/mol) (similar to 1 e Coulombic) -3 0 3 6 (What of Kinetic Energy?) Log (Potential Energy) kcal/mol Is there a Chemical Force Law? How far can you Stretch a Chain of Atoms before it Snaps? Force Laws & Molecular Structure Spring (ut tensio sic vis) F = -k x E = k/2 (x)2 nd Spring =F 2Slope (weaker, opposing) x minimum Balanced Single Minimum Potential Energy sum 0 0 Electrical Charges (gravity, etc.) F = k / (x)2 E = -k/(2|x|) sum Balancedx minimum ! rd 3 Stronger DoubleBody Minimum Demonstration with Magnets Thus with springs you might make a stable polyatomic molecule from point atoms. (but not with ions or magnets) However Hooke’s Law bonds would never break. Morse Potential Mathematically convenient approximation for realistic bond energies (proposed 1929) Fixed Neighbor Sum Second Fixed Neighbor Morse Potential Snaps at Inflection Point (Change from direct to inverse force) What ARE bonds? 19th Century Experiments led to VALENCE numbers 1861 Different # for different atoms: H(1), C(4), O(2), N(3)or 5?) NH3 and NH4Cl Why do Elements Differ? Gertrude and Robert Robinson (1917) Such slippery concepts “explain” so much convinceScheme you of nothing. Why/Whenthat theyReaction “latent” valence loop ? Why/When “partial dissociation”? reaction What does the loop mean? Might Latent Valence Loop explain trivalence of pentavalent N? Might Partial Dissociation explain amine/HCl reactivity? product Electron Discovered 1897 The Cubic Octet of G.N. Lewis as as Harvard Harvard Undergraduate Instructor ~1894 ~1902 © E. S. Lewis, by permission (1875-1946) Octet to "Explain" Periodicity & Electron Transfer (1902 teaching notes) Octet Predicts Shared Pair Bonding shared edge shared face ? Cubic Octet to to Tetrahedral TetrahedralOctet Octet (G. N. Lewis 1916) :N N: Tetrahedral distribution of the bonds from C had already been known in organic chemistry for 40 years! Good Theory should be Realistic & asSimple as possible In regard to Facts it should allow: Prediction Suggestion Explanation Classification & Remembering Postdiction: Realm of Lore From Number of Valence Electrons we would like to predict: Constitution (valence numbers for different atoms) Structure (distances & angles) Charge Distribution Energy Content Reactivity Lewis Explains Constitution “the nature and sequence of bonds” (Electrons, Valence, and Unshared Pairs) 1 H 3 • • B• • Why Octet? Why Pair for H / He? • 4 3 • •• C• • • N • •• • HN H •• H •• •• • 2 •• O •• 1 • • •• F•• •• HCN • HC N H C N •• • •• •• •• • • • •• H C N• •• • • • • Bookkeeping of + “Formal” Charges NH3 H3N-BH3 BH3 (each atom is assigned half-interest in bonding pairs) •• H• • H • • N• • H H H + •• •• •• • • • • • • H N• B• H • • HH Tetravalent N is positive. H• • B• • •H • H Puzzle: Tetravalent B 2 BH 3 B2H6 + ~40 kcal/mol What is the “glue”? (Answer in Lecture 16) is negative. + Surface Potential* of H3N-BH 3 (from Quantum-Mechanics) HIGH (+ 25 kcal/mole) (-41 kcal/mole) LOW *) Energy of a proton on the “molecular surface” Lewis Explains “Pentavalent” N. Actually Tetravalent - thus Charged. H+ H N H Cl H R• ++2 • • • R • • S• • O •• O •• one Sulfide oxide •• •• •• •• •• Amine Oxide •• R• + • •• R • • N• O • •• R Start Lewis-Drill Problems: Draw Lewis Dot Structures for: HNC (in the order shown) also for HCNO (CNO in all six linear orders plus ring) Start Memorizing Functional Groups EQUILIBRIUM vs. RESONANCE : : + N closer to C all octets H C N O shift to restore octet shift to eliminate charge sepn. •• •• charge sepn than to O equilibrium Geometric Implication? all octets • •- + N ~midway still charge sepn H C N O •• between C and O poorer site for - •• Energy •• but maybe in truth… left N position midway Double Minimum EQUILIBRIUM vs. RESONANCE + H C N O •• •• •• resonance + C N O •• •• H - •• single compromise position for N Energy Single Minimum left N position midway Choice between Resonance and Equilibrium must be based on experimental facts (or a better theory) that can distinguish single from double minimum Equilibrium vs. Resonance A B Two Real Species Compared to what? A B One Real Species Two “Reasonable” Structural Formulas Failure of Simplistic Notation Unusually Stable Equilibrium vs. Resonance H H C O •• • • • •• • •• O H CC H •• •• O O •••• • • C O Two Species LORE • • • • •••• O O O H •• O • •• • H H C C One Species? Species! Two One Nuclear Geometry! •• •• •• •• O • • (Evidence: Infrared Spectroscopy) (Evidence: Electron Paramagnetic Resonance) H End of Lecture 2 Sept 5, 2008 Except for cited third-party material Chem 125 Lectures by J. M. McBride are licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Cited third-party material may be subject to additional intellectual property notices, information or restrictions. Users are responsible for checking the credits within the lectures and course material to identify thirdparty material, as such material may pose additional restrictions..