作者	N.S.Isaacs
出版社
出版时间	1997-01-01

特色：

片断：Anunderstandingofchemicalreactivitybeginswithanunderstandingofchemicalbonding,theforceswhichrendercertainaggregatesofatoms(i.e.thefamiliarmolecules)morestablethanothers.1-3Itisonthisbasisthatchemicalreactions-changesinbonding-maybeapproachedandarationalandconsistenttheoryoforganicchemistrydevised.Twomilestonesintheunderstandingofbondingmaybequoted.Thefirst,therecognitionoftheelectron-paircovalentbondbyLewis4andbyLangmuir5in1919,stillprovidesamodelforthedescriptionofmolecularstructureadequateformostpurposesandwillbeextensivelyemployedinthefollowingtext.Accordingtothisconcept,valenceelectronsaresharedsoastocreatefilledshellconfigurationsandareregardedasessentiallylocalizedintheintemuclearspace.Forthefirstrowelementsofwhichorganiccompoundsarealmostentirelycomposed,thisistheoctet(2s2,2p6);forhydrogen,lsl.ThesecondleapinunderstandingwasmadebytheintroductionofquantummcchanicstochemistryfollowingthemolecularorbitaldescriptionofbondinginthehydrogenmoteculebyHeitlerandLondon,6in1929.Thisapproachsupersededtheconceptoflocalizedelectronsandpavedthewaytoquantitativeunderstandingofbonding,thesatisfactorycalculationsofbondingenergies,optimumbondlengthsandgeometries.Itwillbenecessarytoturntothesemethods,despitethenecessityofsomewhatlengthycomputation,whentheneedarisestoconsiderspecificmolecularorbitalproperties(forexample,inthetheoryofpericyclicreactions,Chapter14).Nonetheless,quantumconceptspermeateanydescriptionsofchemicalbonding,thoughtworatherdistinctmodelsmaybeusedwhichwillnowbebrieflydescribed.I.I.IThevalencehond(VB)model'Weknowthestructureofamoleculeinthatitcontainsdefinedatomslocatedpreciselyinspace.Onebeginswiththisdeterminatepartofmolecularstructure(whichcanbeobtainedaccurately,byX-raycrystaldiffractionforinstance)bysettingallnucleiintheircorrectspatialpositions.Theindeterminatepart,thedispositionofthebondingelectrons,12SS*1534isthenaccomplishedbyaddingtheseinpairssuchthatnoatomexceedsitsclosedshellnumber.Thereareinevitablymanywaysinwhichthiscanbeachieved,eachlocalizedstructure(knownasa'contributing'structureor,intheolderliterature,a'canonical'structure)beingregardedascontributing,insomemeasuredeterminedbyitsenergy,tothetruestructure.Themoleculeisconceivedasa'resonancehybrid'ofmanycontributingstructureswhosecontributioncanbeexpressedasafractionenteringintothewhole.Therelationshipbetweencontributingstructures,whichdifferonlyinthedistributionofvalenceelectrons,isexpressedbythedoublearrow,.Althoughforthepurposesofexactcalculationsofmolecularenergies,forinstance,manycontributingstructuresareneededevenforamoleculesuchasH2,itisfrequentlyfoundthatasingleVBstructuresufficestodescribethestructureadequatelyforqualitativepurposes.Forexample,methanemayberepresentedas1andcontributionsfromsuchstructuresas2ignoredfortheinterpretationofreactionmechanisms.