化学专业英语《Chemistry English》课程教学资源（PPT课件）Lecture 03

C2 in All Its GuisesRoaldHoffmannI love the chemistry that of its own subverts(推翻) theclassicalcompartmentalization(划分）of themolecularscience into organic, inorganic, biological, physical andanalytical subdisciplines.Here's an example.C2 is a simple diatiomic molecule. Just two carbonatoms. It's not very stable, quite unlike the familiar O2, N2orF2.Butwhenever anarcisstruckbetween twocarbonatoms,one gets a little C2(and a little soccer-ball shapedC6o, buckminsterfullerene. But that's another, marvelous,story). There is also a good bit of C2 in comets. And C, isresponsibleforthebluelight we seeinflames

1 C2 in All Its Guises I love the chemistry that of its own subverts(推翻) the classical compartmentalization( 划 分 ) of the molecular science into organic, inorganic, biological, physical and analytical subdisciplines. Here’s an example. C2 is a simple diatiomic molecule. Just two carbon atoms. It’s not very stable, quite unlike the familiar O2 , N2 or F2 . But whenever an arc is struck between two carbon atoms, one gets a little C2 (and a little soccer-ball shaped C60, buckminsterfullerene. But that’s another, marvelous, story). There is also a good bit of C2 in comets. And C2 is responsible for the blue light we see in flames

like aThe C, molecule looksfreedumbbell,andtheonlygeometrical variable it has’ is thedistance between the two carbons.That distance is 1.2425 angstroms inthe ground state, the stable form ofthe molecule.Any molecule, C, as well, alsoexists transiently in excited states.These result from absorption of lightby the molecule, or from the input ofenergy in other way.2

2 The C2 molecule looks like a dumbbell, and the only free geometrical variable it has is the distance between the two carbons. That distance is 1.2425 angstroms in the ground state, the stable form of the molecule. Any molecule, C2 as well, also exists transiently in excited states. These result from absorption of light by the molecule, or from the input of energy in other way

The C, molecule looks like a dumbbell,and the only free geometrical variable ithas is the distance between the twoThat distanceis1.2425carbons.angstroms in the ground state, the stableform of the molecule.Any molecule, C2 as well, also existstransiently in excited states. These resultfrom absorption of light by the molecule,or from the input of energy in other way.C分子看起来象哑铃，它具有的唯一的自由几何变量是两个碳原子间的距离。在基态，即分子的稳定状态该距离为1.2425埃。任何分子，包括C2，也短暂地处于激发态。激发态是由于分子吸收了光或通过其它途径获得了能量而形成的。3

3 C2 分子看起来象哑铃，它具有的唯一的自由几何变 量是两个碳原子间的距离。在基态，即分子的稳定状态， 该距离为1.2425埃。任何分子，包括C2, 也短暂地处于 激发态。激发态是由于分子吸收了光或通过其它途径获 得了能量而形成的。 The C2 molecule looks like a dumbbell, and the only free geometrical variable it has is the distance between the two carbons. That distance is 1.2425 angstroms in the ground state, the stable form of the molecule. Any molecule, C2 as well, also exists transiently in excited states. These result from absorption of light by the molecule, or from the input of energy in other way

C-CDistancesStateofCz(inangstroms)1.2425WLWIWIWWAE(ground state)1.3119,1.3184m1.3693S1.2661,1.25521.23,1.23801.5351Pha1.25291.3931.3579Figure Z:The archetypicalorganic moleculestethane,ethylene,acetylene.C-Cdistances in angstroms10-cen1.307C-Cdistance (angstroms)neters are indicatedcntheboeemIsRsACmputedpotential energycurvesforCCHTheoreticalOrganicPhysicalOganometallic271SlversurfanFigure6:TheGdoCClzstructureimcarbidsurfaceInorganicSolid state1995July-August309AmericanScientist,Volume83

4 Organic Oganometallic Inorganic Solid state surface Physical Theoretical

Theinthisyou'vestructuresseenMarginalium come from different parts of themolecular enterprise: physical, theoretical, organicorganometallic, inorganic, solid state, surfacechemistry. I think what nature is saying to us, asclearly as possible through this dazzling richness,is “You guys may chop up chemistry as you wish,but I'm telling you the world is one. There are C2molecular units in each of these structures, actingout a dance of varying distances."I think this is beautiful. And seeing the unitytakes nothingaway form the diversity5

5 The structures you’ve seen in this Marginalium come from different parts of the molecular enterprise: physical, theoretical, organic, organometallic, inorganic, solid state, surface chemistry. I think what nature is saying to us, as clearly as possible through this dazzling richness, is ‘‘You guys may chop up chemistry as you wish, but I’m telling you the world is one. There are C2 molecular units in each of these structures, acting out a dance of varying distances.” I think this is beautiful. And seeing the unity takes nothing away form the diversity

Inhis2004PriestleyMedal address.published in Chemical and EngineeringNews,[4] E. J. Corey writes,"On May4,1964,I suggested to mycolleague R.B. Woodward a simple ex-planation involving the symmetry of theperturbed (HOMO) molecular orbitalsfor the stereoselective cyclobutene/l,3-butadieneand1,3,5-hexatriene/cyclohexadiene conversions that provided thebasisforthefurther developmentoftheseideas into what became known as theWoodward-Hoffmann rules."6

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When Woodward and I spoke of thematter-again.Icannotrememberwhen it was--I asked him directly ifCorey had a role in this work. He saidInthelettertomeof November16.“no"this was our(R.B.W.and R.H.)1981, E. J. Corey says that he told me inwork.May1964that hehad suggested toR.B.Woodwardthe HOMO explanation forI believe that E.J. Corey's percepthe stereochemistry of electrocyclic re-tion of the consequences of what hetoldactions.andthatBobhadtakenovertheR.B. Woodward is just that-what he.idea from him.I spoketo Corey often inCorey,believes.Based on the argumentsthatperiod,butIhavenorecollection ofI made above,and on what I remember.such a conversation;I think CoreysIdon't thinkthat the conversation instatement would have been so strikingquestion influenced in any significant(especially the part about Woodwardway the beginning of the orbital symtakingtheideafromCorey)thatIwouldmetry control story. And given thathaverememberedit.Corey's claim was (and is being)madeafterWoodward'sdeath-whentheclaim could havebeenmadefor15yearsin Woodward's lifetime-that claim isalso deeply and fundamentally unfair.7

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80W.nJmsTwwiloomJaF.B.ThnlluJACS8S,3309((963)1969TulkanthWuduend-to7方DLCxpomun?Figure1.May5,1964,entryin RoaldHoffmann's notebook.Theyear in thedate waswritten in8sometimelater

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Shapes of molecules IELECTRONPAIRREPULSIONThe shape of a molecule can be worked out using the followingrules about electron pairs:(i) the pairs of electrons in a molecule (both shared and lonepairs) behave as though they repel each other and move as farapart as possible;(i) lone pairs repel more than bonding pairs9

9 Shapes of molecules I The shape of a molecule can be worked out using the following rules about electron pairs: ELECTRON PAIR REPULSION (i) the pairs of electrons in a molecule (both shared and lone pairs) behave as though they repel each other and move as far apart as possible; (ii) lone pairs repel more than bonding pairs

ShapesofmoleculesISo, the number of electron pairs decides the shape as the tableshows:ShapeNameof shapeNo. of charge clouds2000lineartrigonal3planar4tetrahedraltrigonal5bipyramidaloctahedral610

10 Shapes of molecules I So, the number of electron pairs decides the shape as the table shows: Shape No. of charge clouds Name of shape 2 3 4 5 6 linear trigonal planar tetrahedral trigonal bipyramidal octahedral