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1、J.Phys.Chem.A2003,107,11517-1152411517ProtonDonorIsMoreImportantThanProtonAcceptorinHydrogenBondFormation:AUniversalEquationforCalculationofHydrogenBondStrengthLiBian*CharterSchoolofWilmington,Wilmington,Delaware19805ReceiVed:May25,2003;InFinalForm:Sep
2、tember28,2003Analysisshowsthatprotondonorismoreimportantthanprotonacceptorindetermininghydrogenbondformationandstrength,andaclearexplanationisgivenforthis.Withinonesystem,hydrogenbondstrength,¢H,canbecalculatedby¢H)kx-R,wherekandRareconstantsandxisther
3、atioofprotonaffinitiesoftheacceptor[PA(A)]tothedonor[PA(D)].Iftheprotondonorisfixedandtheprotonacceptorisvariedwhenonesystemisfitintothelinearfunction,thekandRsoobtainedarefunctionsofdonor'sprotonaffinity,PA(D).Therefore,itispossibletoobtainauniversale
4、quationtocalculatehydrogenbondstrengthinavarietyofsystems.Oneexamplehastheformof¢H)aPA(D)+dx+cPA(A)+e,wherea,c,d,andeareconstants.Thisequationhasbeentestedwithmorethan300hydrogenbondpairswithpositivecharge.Theaverageabsoluteerrorforthetestis1.9kcal/mol
5、.Otherformsoftheuniversalequationhavealsobeendiscussed.Introductionaffinity(PA)isdefinedasanegativeenthalpyofthefollowingreaction:2Hydrogenbondingisoneoftheimportantinteractionsbetweenmoleculesinbiologyandchemistry.1ThehydrogenB+H+)BH+bondcanberoughlyc
6、ategorizedasweakbonding,usuallyhavingabondstrengthbelow10kcal/molandformingbetweenPA(B))-¢Hrxn(1)twoneutralpartners,orstrongbonding,usuallyhavingabondAcidityofanacid,¢Hacid,isdefinedastheenthalpyofthestrengthfrom10to50kcal/molandformingbetweenaneutralf
7、ollowingreaction:3moleculeandachargedpartner,eitherpositiveornegative.Differentapproacheshavebeenandarebeingusedtobetter-+AH)A+Hunderstandandpredicthydrogenbondingstrength.Oneoftheapproachesisbasedonfittingexperimentalvalues,which¢Hacid)¢Hrxn(2)usually
8、providesusefulequationsforpredictingtheactualbondstrength.Thepresentstudyfallsintothiscategory,onthebasisBecausetheacidityofanacidequalstheprotonaffinityoftheoftheunderstandingofthedifferentsignificancesofahydrogenacid'sconjugatebase,A-
