质谱法研究互变异构体英文TAUTOMERIC EQUILIBRIA STUDIES BY MASS SPECTROMETRY.docx
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质谱法研究互变异构体英文TAUTOMERIC EQUILIBRIA STUDIES BY MASS SPECTROMETRY.docx
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质谱法研究互变异构体英文TAUTOMERICEQUILIBRIASTUDIESBYMASSSPECTROMETRY
TAUTOMERICEQUILIBRIASTUDIESBYMASSSPECTROMETRY
PatriciaE.Allegretti1,M.delasMercedesSchiavoni1,EduardoA.Castro2*andJorgeJ.P.Furlong1
1LaboratoriodeEstudiodeCompuestosOrgánicos(LADECOR),DivisiónQuímicaOrgánica,DepartamentodeQuímica,FacultaddeCienciasExactas,UniversidadNacionaldeLaPlata,(1900)LaPlata,BuenosAires,Argentina.
2INIFTA(UNLP-CONICET-CIC),DepartamentodeQuímica,FacultaddeCienciasExactas,UniversidadNacionaldeLaPlata,Diagonal113y64,Suc.4,C.C.16,(1900)LaPlata,BuenosAires,Argentina.
KEYWORDS:
MassSpectrometry–Tautomerism-CarbonylicCompoundsThiocarbonylicCompounds-AbundanceRatios-Keto-EnolEquilibrium-TheoreticalCalculations.
*Corresopndingauthor(eacast@,castro@quimica.unlp.edu.ar)
ABSTRACT
Tautomerisminorganicchemistryhasbeenextensivelystudiedincondensedphasebyspectrometricmethods,mainlybyIRandNMRtechniques.Massspectrometrystudiesstart40yearsagobutjustrecentlyithasbeenrecognizedtheimportanceofthemassspectraldataforthestudyoftautomerisminthegasphase.
Massspectrometrycanprovidevaluableinformationinregardtotautomericequilibriawhenstudyingmassspectraamongthemembersofdifferentfamiliesoforganiccompounds.
Therelevanceofthemassspectraldataresidesonseveralfactsbuttherearetwothatareofkeyimportance:
1-Massspectralfragmentationassignmentsshouldbetautomerspecificsincethecorrespondingabundancesratiosaresupposedtobecorrelatedtotheketo/enolcontents.
2-Ionizationintheionsourceitissupposedtohavenoeffectonthepositionoftheequilibriumsothattheresultsreflectthetautomerscontentinthegasphaseprevioustoionization.
Someofthecarbonyliccompoundsdonotexhibitnoticeabletautomerismsothefragmentabundancesassignedtotheenolformisverylowornotmeasurable.Sinceenolizationismorenoticeableinthecaseofthio-derivatives(whichcorrelatesadequatelywiththeoxygenatedanalogues),thestudyoftheirmassspectraisaninterestingchoicetoreachsomedegreeofgeneralization.
Inaddition,experimentalfindingsaresupportedbysemiempiricaltheoreticalcalculations,whichprobedtobeadequatenotonlyforsupportingtendencycorrelationsamongthemembersofacompoundfamilybutalsotocalculateheatsoftautomerizationingasphase.
Reportsusingmassspectrometryfortautomerismarebecominglesscommon.OneofthereasonsisthatnowitwouldappearthattheinterpretationofMSresultsisnotasstraightforwardasitwasoncebelieved,eventhoughinarecentreviewitwaswrittenthat:
“Massspectrometryisthemostinformativeandpracticalmethodforstudyingandidentifyingtautomersinthegasphase”[1].
Infact,massspectrometryseemstobeveryinformativeforstudyingandidentifyingtautomers,becauseinthiscaseexternalfactorslikesolvents,intermolecularinteractions,etc.,canbeexcludedbytransferringthetautomericsystemintogasphase,wheretheprocessbecomestrulyunimolecular[1].
ThisreviewcoversthestudyofTautomerismbyMassSpectrometryinthelastfourdecades.
TAUTOMERISMBEFORE/AFTERIONIZATION
Acriticalaspectinthestudyofchemicalreactionsistounderstandwheretheequilibriumtakesplaceintheinstrumentalsetup.
Ithasbeenshownthattheexchangeofenolizablehydrogenatomsfordeuteriumatomstakesplaceintheinletsystemofamassspectrometer[2][3],whichindicatesthattheketo-enolequilibriummightbeestablishedintheinletsystem.
Equation1
MacLeodstudiedthepossibleoperationoftautomerismbeforeandafterelectron-impactinducedfragmentationofmolecularions[4].
AstudyofthedoubleMcLaffertytransferprocessinthemassspectraoftwocyclicketonesandasubstitutedmalonicester,atbothhighandlowelectronenergies,hasshownthatlittletautomerismoftheintermediatesingleMcLaffertyenolionoccursinthesecases(Scheme1).Inaddition,tautomerismofthemolecularioninanumberofcarbonylcompoundsappearsnottobeaprerequisitefor-cleavageintheirmassspectra.
Scheme1X
Larsenandco-workers[5]aretobecreditedwithinitiatinganewtypeofapplicationofmassspectrometry,whichconsistsinthestudyoftheeffectoftheinletsystemtemperatureonmassspectraandtheestimationofthecorrespondingheatsoftautomerization.Theymetseveraldifficultiesintheirstudy,mainlyduetotheverysmalltemperaturerange(26ºC)availabletothem.Theketo-enoltautomerisminthreeβ-diketoneshasbeenstudiedbymeansoftheeffectofvariationoftheinlettemperatureonthemassspectra.However,accordingtotheauthors,thecomparisonofdifferentlysubstitutedβ-diketoneshasthedisadvantagethattherelativeabundancesinthemassspectrawoulddependnotonlyontheketo-enoltautomerism,butalsoonthedifferencesinbondstrength.Notwithstanding,thechangesinthemassspectraofacetylacetone,3-methylacetylacetoneand3-allylacetylacetonewiththetemperatureoftheinletsystemcouldbecorrelatedwiththeexpectedchangesintheketo-enolequilibriaforthethreecompounds.
So,itwasconcludedthatitseemspossibletoderiveconsistentconclusionsregardingketo-enolequilibriaofβ-diketonesinthegasphasebyastudyoftheeffectoftheinlettemperatureonmassspectra.Certainlytoidentifypeaksasbeingformedalmostexclusivelyfromeithertheketoortheenolformofthemoleculesisabsolutelynecessaryalthoughitisnotsimpletodeterminewhetheramasspeakis“pure”.Thequantitativeapproachdescribedinthetext,whichallowsthedeterminationof∆Hvaluesinreasonableagreementwithdatadeterminedindependently,supportsthebeliefthatsomepeaksarisealmostexclusivelyfromelectron-impactoneithertheketoortheenolformwithonlyminorcontributionsfromtheotherform.
Byintroductionofthewell-knownvan´tHoffequation:
lnK=-∆H/RT+C,thefollowingequationisderived(respondfactorsforbothtautomersshouldbesimilar):
Equation2lnIenol=ln[enol]+a=lnK+a=-∆H+C+a
Iketoln[keto]RT
where“I”ispeakabundanceinthemassspectrumand“a”isaconstantofproportionality.
Aplotofln(Ienol/Iketo)versus1/T,whereTistheabsolutetemperatureatwhichtheratioIenol/Iketohasbeendeterminedshouldthereforegiveastraightlinefromwhich∆Hcanbeestimatedprovidedthegivenassumptionsarevalid.
Themassspectraofotherβ-diketoneshavebeenstudied[6-14]andbycomparisonofdifferentlysubstitutedcompoundsithasbeenfoundthatthefragmentationisinfluencedbytheketo-enolcontentofthecompounds(thismeansprevioustoionization).
Thediketoandketo-enoltautomersofaliphatic1,3-diketonescouldbeeasilyseparatedbygaschromatography[13],whichisnottheusualcasefortautomersmixtures.Themassspectraofthetautomersarequitedifferent,andthemainfragmentationpathwayscanbeeasilylinkedtothestructuresofthe(non-interconverting)tautomericmolecularion.Furthermore,isomersdifferingbythepositionofthesubstituentcouldalsobeidentifiedbytheirmassspectra.
Orlovandcoworkers[14]statedthattheketo-enolratioofthestartingdiketonescannotbefoundfrompeakintensitiesofthefragmentions.Theyresorttotheheatsofformationofthechargedfragmentsandneutral1,3-diketonesforthecalculationsoftheheatsofgas-phasereactions.
STUDIESONDIFFERENTCOMPOUNDSFAMILIES
Similarobservationrespecttotautomerisminvolvingneutralspecieshasbeenreportedforothercompoundfamilies.
Larsonandcoworkerscarriedoutmassspectrometricstudiesforamidesandthioamides,andpointedoutthetautomericequilibriumdisplacementtowardstheimidolforminthecaseofthioamides[15].
Equation3
Theresultsofaninvestigationofgas-phasetautomerismofoxazolidinesandβ-diketones[16]usingmassspectrometrysuggestedthatketo-enoltautomerisminacetylacetoneinvolveswallcollisionsand/oroccursbyanintramolecular(fourcentered)mechanism.Sincethisprocessoccursinthesourceitwouldoverrideanypriortautomerizationthatmayoccurintheheatedinletsystem.Thegoodcorrelationbetween∆Hvaluesandsubstituenteffectsontheoxazolidinering-chaintautomerisminthegasphaseandthoseinnon-polarsolutioninnowayrequirethatthedetailedmechanisminvolvedbesimilar(Eq.4).Withdataontwocompletelydifferentlytautomericprocessessomecommentsweremadeonthemeritsofthemassspectrometricmethod,particularlyvisavisnuclearmagneticresonance.Intermsofprovidingthermodynamicquantities,massspectrometryseemstobeveryrestricted,equilibriumconstantscannotbeobtainedandenthalpydifferencesareonlyapproximate.Animportantadvantageofthisapproach,however,isthefactthatsomeinsightintothemechanism(molecularity)ofthetautomerizationprocesscanbeobtained.Inmostsystemsassignmentoftautomerspecificfragmentionsshouldbepossible,althoughlabeledanaloguesmayberequired.
Equation4
Thefragmentationpatternsofthethioacylderivativesof2-aminothiazoleand2-aminobenzothiazoleandtheirfixedstructureiminoandaminotautomericformsgaveevidenceofthepredominanceoftheiminotautomerinthemolecularionsofthetrifluorothioacetylcompounds[17](Fig.1and2).Ontheotherhand,themolecularionsofthethioacetylandthiobenzoylderivativesaremainlytheaminotautomers.ArearrangementwitheliminationofRCNandformationofthe2-thiazolothione(or2-benzothiazolotione)ionwascharacteristicforallthecompoundsinvestigated(Scheme2).Additionally,evidencefortheinterpretationofthemainfragmentationp
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