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CorrelationbetweentheVibrationalFrequenciesoftheCarboxylateGroupandtheTypesofItsCoordinationtoaMetalIon:AnabInitioMolecularOrbitalStudyMasayukiNara,†HajimeTorii,andMitsuoTasumi*DepartmentofChemistry,SchoolofScience,TheUniVersityofTokyo,Bunkyo-ku,Tokyo113,JapanReceiVed:May30,1996;InFinalForm:September10,1996XThestructuresandvibrationalfrequenciesoftheacetateioninteractingwithametalion(Na+,Mg2+,andCa2+)intheunidentate,bidentate,bridging,andpseudobridgingformsarestudiedbyabinitiomolecularorbitalcalculations.Effectsofawatermoleculecoordinatingtoeithertheacetateionorthemetalionarealsoexamined.Thecalculationsarecarriedoutbyusingtheself-consistentreactionfieldmethodattheHartree-Focklevelwiththe6-31+G**basisset.Forthespeciesinteractingwithadivalentmetalcation,thelengthsofthetwoCObondsoftheacetateionarenearlyequalinthebidentateformbutaresignificantlydifferentintheunidentateform.ThefrequencyoftheCOO-antisymmetricstretchoftheunidentatespeciesishigherthanthatoftheionicspecies,whichisinturnhigherthanthatofthebidentatespecies.ThereverseisthecasefortheCOO-symmetricstretch.Asaresult,thefrequencyseparations(¢îa-s)betweentheCOO-antisymmetricandsymmetricstretchesfortheunidentate,bidentate,andionicspeciesareinthefollowingorder:¢îa-s(unidentate)¢îa-s(ionic)¢îa-s(bidentate).ItisdemonstratedthatsuchacorrelationbetweenthevibrationalfrequenciesoftheCOO-groupandthetypesofitscoordinationtoadivalentmetalcationisrelatedtochangesintheCObondlengthsandtheOCOangle.Theresultsofthepresentstudyclarifythephysicalbasisoftheempiricalstructure-frequencycorrelation,whichhasbeenusedintheanalysisoftheinfraredspectraofCa2+-bindingproteins.1.IntroductionTherearethreerepresentativetypesofcoordinationofthecarboxylate(COO-)grouptometalion(s):1,2unidentate,bi-dentate,andbridging.AmetalioninteractsequallywiththetwooxygenatomsoftheCOO-groupinthebidentateform,whereasitinteractswithonlyoneofthoseoxygenatomsintheunidentateform.Inthebridgingform,ametalioninteractswithoneofthoseoxygenatoms,andanothermetalioninteractswiththeotheroxygenatom.Whenoneofthesemetalionsisreplacedbyahydrogenatomofawatermolecule,thesystemisinthepseudobridgingform.CorrelationbetweentheCOO-stretchingfrequenciesandthetypesofCOO-coordinationhasbeeninvestigatedinseveralstudies.1-8DeaconandPhillips1haveexaminedthestructuresandvibrationalfrequenciesobservedforanumberofacetatesaltsinthesolidstateandhavefoundanempiricalruleforthecorrelationbetween¢îa-s(frequencyseparationbetweentheCOO-antisymmetricandsymmetricstretches)andthetypesofcoordinationoftheCOO-grouptodivalentmetalcations,whichisexpressedas¢îa-s(unidentate)¢îa-s(ionic)¢îa-s(bridging)¢îa-s(bidentate).However,thetheoreticalbasisforthiscorrelationhasnotbeenfullyclarified.InteractionsbetweentheCOO-groupanddivalentmetalcationsplayimportantrolesinsomebiologicallyactivemol-ecules,suchascalmodulinandparvalbuminwhichareknownasCa2+-bindingproteins.We(M.N.andM.T.)haverecentlyshown9,10thattheexistenceoftheCOO-antisymmetricstretchingbandat1553cm-1andtheCOO-symmetricstretchingbandat1424cm-1isafeaturecharacteristicofCa2+-boundcalmodulinandparvalbumin.Inordertoderiveinformationonthemetal-ligandinteractionsinsuchmetal-ion-bindingsitesfromvibrationalspectra,itisimportanttounderstandthefactorsgivingrisetothecorrelationbetweentheCOO-stretchingfrequenciesandthetypesofcoordinationoftheCOO-grouptoCa2+.Inthepresentstudy,thestructuresandvibrationalfrequenciesoftheacetateioninteractingwithametalion(Na+,Mg2+,andCa2+)intheunidentate,bidentate,bridging,andpseudobridgingformsarestudiedbyabinitiomolecularorbital(MO)calcula-tions.Sincetheinfraredspectraofthesesystemsaremeasurableonlyinthecondensedphases,abinitioMOcalculationsforisolatedspeciesinvacuummaynotbeappropriatetoobtainvibrationalfrequenciesthatcanbecomparedwithexperimentalresults.Sincetheacetateionhasalargedipolemoment(5D),interactionwithsurroundingmoleculesisexpectedtobesignificant.Wongetal.11-14haveshownthattheself-consistentreactionfield(SCRF)methodisusefulforstudyingmolecularpropertiesofdipolarspeciesinthecondensedphases.ThismethodisbasedonOnsager’sreactionfieldtheory15ofelectrostaticsolute-solventinteractions.Weusetheresults(structuresandvibrationalfrequencies)calculatedbythismethodfordiscussingthecorrelationbetweentheCOO-stretchingfrequenciesandthetypesofcoordinationoftheCOO-grouptoametalion.2.ComputationalProcedureAbinitioMOcalculationshavebeenperformedbyusingtheGaussian92program16onaHewlett-Packardworkstation(Apollo9000seriesmodel735)attheResearchCenterforSpectrochemistryoftheUniversityofTokyo.TheSCRFmethodhasbeenemployedtotakeintoaccounttheeffectsofsurroundingdielectricmedia.Insomecalculations,astronglyinteractingwatermoleculehasbeenexplicitlyincluded.AllthecalculationshavebeencarriedoutattheHartree-Fock(HF)level.FortheatomsintheacetateionandthewatermoleculeaswellasforNa+andMg2+,the6-31+G**basissethasbeenused.ForCa2+,wehaveusedtheSVDPbasissetdevelopedbyScha¨feretal.17†Presentaddress:WaterResearchInstitute,Sengen2-1-6,Tsukuba305,Japan.XAbstractpublishedinAdVanceACSAbstracts,November15,1996.19812J.