Detection of relative dimer and rotamer concentrations of diacetamide in different solvents by FT-IR spectroscopy and DFT calculations

VibrationalSpectroscopy57 (2011) 294–299

ContentslistsavailableatSciVerseScienceDirect

Vibrational

Spectroscopy

j ou rna l h o me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / v i b s p e c

Detection

of

relative

dimer

and

rotamer

concentrations

of

diacetamide

in

different

solvents

by

FT-IR

spectroscopy

and

DFT

calculations

Sedat

Karabulut

a

_,

_Hilmi

_Namli

a,∗

_,

_Massimo

_Mella

b

a_{FacultyofArtsandSciences,DepartmentofChemistry,BalikesirUniversity,CagisTR-10145,Balikesir,Turkey} b_{SchoolofChemistry,CardiffUniversity,CardiffCF103AT,UnitedKingdom}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received6April2011

Receivedinrevisedform24August2011 Accepted26August2011

Available online 16 September 2011 Keywords: Diacetamide Dimerization Rotamerization FT-IR DFT

Relativeequilibriumconcentrations

a

b

s

t

r

a

c

t

Therelativerotamer,dimerandtautomerconcentrationsofdiacetamidehavebeenstudiedbymeansof infraredspectroscopy,withtherecordedspectrabeinganalyzedemployingresultsfromdensity func-tionaltheorycalculations.Itisobservedthatthecis–transmonomericformofdiacetamide(1)isfound tobethemoststableisomerinallstudiedsolvents,withtrans–transdiacetamide(2)beingfoundtobe 20%oftotaldiacetamideinmethanol.Whilethedimerformofdiacetamide(3)ispresentonlyin carbon-tetrachloride(about34%ofthetotal),itstautomericforms(4,5)arenotfavorableinanyofthestudied solvents.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Mostoftheorganicmoleculeshavedifferentstructural(dimeric,

tautomericor rotameric)propertiesin solventmedia.It is well

knownthatthesestructuraldifferencesinducedifferentchemical

propertiesandreactivity[1,2].Thus,abundanceandrelative

pro-portionsofthespecies(dimers,tautomers,rotamers)thatarein

equilibriuminsolutionisveryimportant[1,3].

Sincetautomericandrotamericinterconvertionsarequitefast

processes,theyaredifficulttostudy.Nevertheless,thevarietyand

importanceofapplicationsencompassingthesephenomena

con-tinuouslyencourageresearcherstoundertaketheirinvestigations

[2].TheNMRis oneofthecommonmethodusedtodetermine

equilibriumconstants[4,5]butinabilityofNMRtimescaleforthe

detectionof fastinterconversionsis acommonproblemand in

contrasttoNMRspectroscopy,thetimescaleofvibrational

spec-troscopicmeasurementsissuitableforsimultaneousdetectionof

coexistingspecies.Inthisrespect,infraredspectroscopyisoneof

themostversatilespectroscopicmethodsinthechemicalsciences

allowing studying processes such as theketo-enol equilibrium

[2,3,6].

Amongthemoleculesthatcanshowtautomerism,imidesare

strongcandidatesthatmaypresentstructuralfeaturesinsolution

∗ Correspondingauthor.

E-mailaddress:hnamli@balikesir.edu.tr(H.Namli).

differingfromsolidstateonesthankstothesimilaritywithamides

andthepresenceoftwocarbonylsattachedtothe–NH–group.

For thisreason, theabsorptionbandsof acyclicimiderotamers

inthecrystallinestateandinsolutionhavebeenstudied

exten-sivelybyFT-IRspectra[7–11].Besides,imidesareveryimportant

moleculesin molecularrecognition.Imide hydrogen-bondrules

canbealsoappliedtochemicalhomologuesandanaloguessuch

asuracilsandbarbituratestoinvestigatehost–guestinteractions

where predictableaggregationpatternsareveryoftenobserved

[12].

Diacetamideisthesimplestopenchainmoleculesthatcontain

a–CONHOC–functionalgroup.Whilethetwocarbonylsandimide

nitrogenareplanar,therearethreepossibleconfigurations

accord-ingtotherelativeorientationofcarbonylsandhydrogenonthe

nitrogen.Itwasconcludedthatbothcis–trans(1)(namedfromthe

positionofthecarbonylgroupsrelativetothegroupattachedto

thenitrogen)andtrans–trans(2)rotamersofdiacetamide(Fig.1)

arepresentinthesolidstate,withthefirstonebeingthemost

sta-bleconformation[13].Thelessstablecis–cis(6)conformationof

diacetamide(Fig.1)hasneverbeenobservedinsolidstateorin

solution[14].

TheconformationsofdiacetamideinCCl4werealsostudiedand

theexistence ofthedimer (3) and monomercis–trans (1) (Fig. 1)

geometry wasreported[15,16].Existence of dimericstructures

innon-polarsolventswasexplainedinvokingareductionofthe

dipolemomentupondimerization[16].Theeffectonthegeometry

andbondingofdiacetamidewithmetals(Mn(II),Fe(II),Co(II),Ni

0924-2031/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2011.08.008

Fig.1. Allpossiblerotamer,tautomeranddimerstructuresofdiacetamide.

(II),andZn(II))werealsostudiedbyFT-IR[17].Infraredspectra

oftrans-cisdiacetamideanditsC-andN-deuteratedcompounds

werecomparedandthevibrationalassignmentsperformed[18].

Diacetamidewasalso foundtobea versatile cocrystallizing

agent,formingat leasttendifferentcocrystal pairsand

crystal-lizesintwodifferentpolymericforms,thestableformcontaining

moleculeshaving thecis–transconformation. In thecrystalline

state,themoleculesareheldtogetherbycentrosymmetricNH···O

hydrogenbonds.Thecis–trans(1)conformerisalsothemoststable

forminsolution[12].Besides,thelackofspectralfeatures

indicat-ingtautomerisminthepublishedstudyagreeswellwiththeoretical

relativeenergiesshowingthattheiminol(4)tautomer(Fig.1)of

the1,3-dicarbonylimidestructureisindeednotfavorable[15].

Despiteallthequalitative characterizationcarriedoutinthe

past,thereissubstantiallackofquantitativeinformationonthe

solutionequilibriaaffordedbydiacetamide,agapthatwestriveto

fillinthisstudy.Themainideabehindthisworkisthatdifferent

compounds(tautomers,dimer–monomer,androtamers)shouldbe

expectedtohavedifferentFT-IRspectra.Thus,anexperimental

FT-IRspectrumofacompoundindynamicalisomericequilibrium

inanappropriatesolventshouldbethesumofthespectraofall

componentsintheequilibriumthankstothedifferenceintime

scalebetweenisomericconversionandinternalvibrations.

Obvi-ously,absorptionbandintensitiesoughttobeproportionaltotheir

relativeconcentrations.Formostoftheequilibriumsystems,itis

clearlyimpossibletoisolateeachcompoundinordertoinvestigate

itsindividualspectralproperties.Itisinsteadpossibletocalculate

theirtheoreticalFT-IRspectrumwithelectronicstructuremethods

suchasDensityFunctionalTheorymadeavailablein,e.g.,

Gaus-sian03[19].Wethusdecidedtoexploitsuchpossibilityinorderto

obtainsemi-quantitativeinformation.

In this study,we have thus employed a matching approach

betweenthediacetamideFT-IRspectraobtainedwiththeoretical

meansandexperimentally. Forthis purpose, calculationsonall

possiblespecies(1,2,3,4and5)insolution(Fig.1)havebeen

carriedoutwithasuitablemethodandbasisset.Theassignment

oftheabsorptionbands,supplementedwithcalculatedelectronic

296 S.Karabulutetal./VibrationalSpectroscopy57 (2011) 294–299

orabsenceofanyspeciesinequilibriumsystem.Theoretical

inten-sitiesofspecieslikelytobepresentinsolutionwerescaledwith

appropriateconstants(thesumoftheconstantsbeingequalto1)

andaddedtogethertogeneratethe“bestmatchingsynthetic

spec-trum”totheexperimentalFT-IRspectraasawaytoextractrelative

concentrationsandequilibriumconstants.

2. Materialsandmethods

AllsolventsanddiacetamidewerepurchasedfromAldrichor

Flukaasanalyticalpurityandnofurtherpurificationhasbeendone.

Thevibrationalabsorptionspectraofdiacetamideinallsolutions

weremeasuredusingaPerkinElmer1600BX2FT-IR

spectropho-tometer.Theresolutionandtheintervalvalueswerechosenas4

and2inallFTIRmeasurements.Thespectraofpuresolventswere

recordedasabackgroundandstoredonthecomputerforeach

mea-surement.Solutionspectraweremeasuredusinga0.015mmpath

lengthinCaF2cellwithanaverage32scans.InallFT-IR

measure-ments,theconcentrationsofthediacetamidewere0.0100mol/L,

except in carbon tetrachloride. To get more information about

themonomer–dimerequilibrium, threedifferentconcentrations

(0.0100,0.0050,0.0025mol/L)ofdiacetamidehavebeenstudied.

TheGibbsfreeenergyofallpossiblemolecules(1,2,3,4and5)

ineachsolventhasbeenseparatelycalculatedandtheseenergies

wereusedtoassigntheexistenceofthesespeciesinequilibrium.

Theseexistingassignmentswerealsosupportedbymatchingthe

experimentalandcalculatedFT-IRspectra.Whilethefrequenciesof

theFT-IRabsorptionbandsgivescluefortheexistence,the

compar-isonoftheintensitiesalsoprovidevaluableinformationaboutthe

relativeconcentrationsofthesespeciesintheequilibriumsystem.

Calculationsforallpossiblestructures;tautomers(4,5),dimer

(3)androtamer(1,2)(Fig.1)werecarriedoutwiththeGaussian

03[19]setofprograms.Alltheinputfilesgeneratedwith

Chem-Draw2008andpotentialenergysurfacecalculationsweredone

withsemi-empiricalAM1methodtopreparethebestinput

geom-etry.The geometryoptimizations and frequencycalculations of

tautomers,dimersandrotamersofdiacetamidewerecarriedout

attheDFTB3LYP/6-311G++(2d,2p)level[20–24]indifferent

sol-vents,employingtheconductor-likepolarizablecontinuummodel

(CPCM)[25]. Thus, allstationarypoints ofthe differentspecies

involvedinthisstudywereoptimizedbothinthegasphaseand

usingtheCPCMmodelwiththeappropriatedielectricconstants.

3. Resultsanddiscussion

Toobtaintherelativestabilitiesbetweenrotamers,dimersand

tautomersofdiacetamideatheoreticalstudyhasbeenperformed

forallpossibleketoandenolicformsofdiacetamide(Fig.1)in7

differentsolvents.Energydifferencesbetweenallpossible

struc-turesandthemoststableconformer(1)wereobtainedsubtracting

theenergyofthelatterfromtheoneoftheothermolecules(2,

3,4and5).Thus,themoststableconformer(1)isusedas

refer-enceandgivenzeroenergyvaluewithallsolvents.Theresultsare

summarizedinTable1.Specieswithaenergydifferencelessor

equal5kcal/molareconsideredtodynamicallyconvertbetween

eachotheratambienttemperature[2].Specieswithanenergy

dif-ferencehigherthan5kcal/molhavenotbeentakenintoaccountin

ourdiscussion.AsitcanbeseenfromTable1,theenergydifference

ofmolecules4and5fromthemoststablemolecule1ishigherthan

5kcal/mol.Thus,theeffectsdueto4and5couldbeneglectedinall

studiedsolvents[15,26].

Forthesakeofcomparingtheoreticalandexperimental

frequen-ciesandintensitiesinthespectrummatchingmethodemployed

inthis work,therecorded experimentaldatawereexportedas

data tables. Scalingfactor for the theoretical vibrational bands

Table1

Theenergydifferencebetweenthepossiblestructuresinsolvents.(Forthedimer structure,theobtainedenergyfromtheoreticalcalculationdividedbytwo.)

Solvents ␦Gfvaluesforpossiblespecies(kcal/mol)

1ref. 2 3 4 5 MeOH 0.0 1.6 6.2 14.6 18.4 DMSO 0.0 0.4 5.0 12.4 17.3 CH3CN 0.0 0.9 5.0 13.4 17.3 THF 0.0 3.0 5.7 12.8 18.2 DCM 0.0 1.5 5.5 12.6 18.9 CHCl3 0.0 2.2 4.5 13.6 18.3 CCl4 0.0 3.3 2.7 11.6 18.1 GasPhase 0.0 4.8 0.4 7.7 17.9

intheexperimentallyscannedfrequencyrangeweredetermined

by leastsquare fit ofselectedfrequencies against experimental

resultsinordertoallowadirectcomparisonalsofromthevisual

pointofview.ObtainedR2_{valueswereanindicationforfrequency}

regression.Withthetheoreticalfrequenciesappropriatelyscaled

tomatchtheexperimentalones,alinearregressionbetween

the-oreticalandexperimentalintensitiescanbecarriedouttoextract

relativeconcentration.Inotherwords,assuminggoodaccuracyfor

thetheoreticalabsorptioncoefficients,thelinearregression

pro-videsonewiththerelativefractionstobeassignedtoallspeciesin

ordertoreconstructtheexperimentallyrecordedspectra.

Fromthe experimentalFT-IRspectra(Fig. 2)and theoretical

energy calculations of diacetamide (Table 1), it is obviousthat

therearenoindicationsforthepresenceofenol(5)andiminol

(4)tautomersinanysolvent.Sincetautomerizationseemsnotto

occur,dimerizationandrotamerizationshouldbeheld

responsi-bleforthedifferencesbetweentheFT-IRspectraofdiacetamidein

differentsolvents,especiallyintheregion1660cm−1–1800cm−1

(Fig.2).Thus,weaimtoobtainsemi-quantitativeinformationon

therelevantequilibriawiththeaforementionedspectrum

match-ingapproach.In thisrespect,wechosetodiscussin detailsthe

approach for the cases represented by diacetamide in carbon

tetrachlorideandmethanol.Thesearegoodrepresentativemodel

systemsthankstothefactthatCCl4istheonlysolventwherethe

dimerstructure(3)hasbeendetectedwhilemethanolisthesolvent

thatcontainsthehighestamount(20%)oftrans–trans(2)

configu-ration.Themethodwasidenticallyappliedtothecaseoftheother

solventsandresultsaresummarizedinTable2.

ThecalculatedgasphaseIRfrequenciesofexistingmolecules

(1,2and3)havebeencomparedwiththeIRfrequenciesin

sol-vent.Itisobservedthatthestretchingbandsofhighlypolarized

bonds(C O,N–H)shiftconsiderablymorethanthoseofless

polar-izablebondscomparinggasphase andsolventmedia.Thiskind

ofshiftshasalsobeenobservedbytheincreaseinpolarityofthe

2034 1950 19001850 1800 17501700165016001550150014501400 1350 1305 CCl4 CHCl3 DCM THF CH₃CN MeOH DMSO Transmiance (au) Wavenumber (cm-1₎

Table2

Selectedexperimental(Vexp)andrelatedtheoretical(Vtheo)frequencies,correlationcoefficientbetweenVexpandVtheo(R2v),experimentalabsorbances(Aexp)andrelated theoreticalintensities(Atheo)(multipliedwithappropriaterelativeconcentrationvalues),correlationcoefficientbetweenAexpandAtheo(R2_A),relativeconcentrationof1 (%cis–trans),2(%trans–trans)and3(%dimer).

Solvents Vexp Vtheo Sourceofband R2_V Aexp Atheo R2_A %(1) %(2) %(3)

CCl4 1376 1412 3 0.99 0.020 79 0.98 32 0 34 1464 1506 1 0.012 65 1506 1556 3 0.014 87 1694 1702 3 0.063 598 1716 1730 1 0.034 246 1744 1770 3 0.024 143 CHCl3 1426 1466 1 0.99 0.003 99 0.99 100 0 0 1470 1508 1 0.011 193 1710 1714 1 0.031 688 1736 1756 1 0.008 198 CH3CN 1378 1406 1 0.99 0.028 84 0.99 99 1 0 1448 1466 1 0.017 56 1706 1698 1 0.099 936 1736 1742 1 0.031 199 1764 1774 1 0.016 84 DCM 1428 1464 1 0.99 0.006 88 0.99 96 4 0 1472 1512 1 0.021 245 1710 1706 1 0.071 849 1736 1752 1 0.019 196 1768 1786 2 0.004 45 THF 1708 1722 1 0.99 0.097 865 0.99 96 4 0 1736 1764 1 0.032 202 1770 1804 2 0.008 35 DMSO 1508 1512 1 0.99 0.010 248 0.95 90 10 0 1698 1698 1 0.052 856 1730 1746 1 0.018 197 1758 1776 2 0.009 92 MeOH 1522 1524 2 0.99 0.009 184 0.99 80 20 0 1696 1698 1 0.041 714 1738 1748 1 0.012 172 1760 1776 2 0.012 217

differentsolvents.For exampletheC Ostretchingbandof1 is

calculatedas1762cm−1,1730cm−1and1698cm−1 ingasphase,

CCl4 andCH3CNrespectively.Intrans–transform(2)sameband

hasbeencalculatedas1834cm−1,1806cm−1and1774cm−1.This

shiftin frequency hasbeen obtainedas 1718cm−1, 1702cm−1

and1686cm−1 for dimericstructure.TheN–Hstretchingbands

of 1 and 2 are significantly different from gas phase to

solu-tionbut thisis nottruefordimeric structure(3). Thereisonly

2cm−1differencebetweenthecalculatedN–Hstretchingband

fre-quencyof3ingasphase(3338cm−1)andCCl4(3340cm−1).Even

inpolarsolventsthefrequencyofthestretchingofN–Hisabout

thesame(3346cm−1forCH3CN).OnthedeterminationoftheN–H

0 200 400 600 800 1000 1500 1550 1600 1650 1700 1750 1800 Intensity 1 2 0 200 400 600 800 1500 1600 1700 1800 1 2 Intensity 0 200 400 600 800 1500 1600 1700 1800 Intensity Wavenumber (cm-1) Wavenumber (cm-1) Wavenumber (cm-1₎ Wavenumber (cm-1₎ 0.00 0.01 0.02 0.03 0.04 0.05 1500 1600 1700 1800 Absorbance 1748 cm-1 1698 cm-1 1776 cm-1 1524 cm-1 1760 cm-1 1522 cm-1 1696 cm-1 1738 cm-1

a

b

d

c

Fig.3.(a)TheoreticalIRspectraofcis–trans(1)andtrans–trans(2)rotamersofdiacetamidebetween1500cm−1_and1800cm−1_{inmethanol.(b)IRspectraof1(0.80)and2}

298 S.Karabulutetal./VibrationalSpectroscopy57 (2011) 294–299

stretchingbandfrequency,theC O···H–Nhydrogenbondisvery

effectivesothesolventcannotinteractwiththeprotichydrogens

asmuchasmonomericstructures1and2.

3.1. Theequilibriuminmethanol

Table 1 shows the energy difference between 2 and 1

(1.6kcal/mol),and3and1(6.2kcal/mol);thus,weexcluded3from

ourmodelspectrum.ThetheoreticalIRfrequenciesfor1and2in

methanolareshowninFig.3a.Thecalculatedintensitiesfor1and

2wereoverlaidwithequalweightonthesamefrequencyaxisfora

directcomparisonwiththeexperimentaldata(experimentaldata

areshowninFig.3d).

Focusing on the 1800–1650cm−1 region, the overlaid

spec-trum (Fig. 3a) shows three absorption bands, the latter being

assignedasasymmetricandsymmetricC Ostretchingbandsof

1,at 1698cm−1 and1748cm−1,and symmetricC Ostretching

of2at1776cm−1.Theexperimentalspectrum(Fig.3d)alsohas

threeabsorptionbandsintherelatedregionsupportingour

sug-gestionforthepresenceoftheequilibriumbetween1and2(Fig.1)

in methanol. The absorptionband at 1760cm−1 in the

experi-mentrelatestothesymmetricC Ostretchingof2(theoretically

at1776cm−1).

Aleastsquarefitofselectedexperimentalandtheoretical

fre-quencies(Table2),showsastronglinearrelationshipwithanR2

valueof0.99(Fig.4a)withoutapplyinganyscalingfactor.

Althoughthecalculatedfrequenciesofthesetwomolecules(1

and2)appearinagoodharmonywiththeexperimentalfrequencies

(Fig.4a),theintensitiesdonotforrelativeconcentrationsof50%

(Fig.4b).

Sinceitiswellknownthattheabsorptionbandintensitiesof

differentmoleculesinthesameFT-IRmeasurementaredirectly

relatedtotherelativeconcentrations,abettermatchingbetween

spectracanbeobtainedmodifyingtherelativeweightofthetwo

species.Webeginnoticingthat,whiletheexperimental

absorp-tionbandsat1738cm−1 and1760cm−1(Fig.3d)havesimilarin

intensity,itisnotsofortherelatedtheoreticalabsorptionbands

at 1748cm−1 and 1776cm−1 (Fig. 3a) for a 1:1 ratio between

1 and 2. Since the latter has higher intensity, it is clear that

the relative concentration of 2 should be decreased. Reducing

the relative concentrations of 2 while increasing the one of 1

shouldthereforeincreasethecorrelationcoefficientbetweenthe

R² =0.99 1500 1550 1600 1650 1700 1750 1800 1800 1750 1700 1650 1600 1550 1500

Experimental

Frequency

Theorecal Frequency R2= 0.34 0 200 400 600 800 1000 1200 0.05 0.04 0.03 0.02 0.01 0 Theorecal Intensity

Experimental

Intensity

R² =0.99 0 200 400 600 800 0.05 0.04 0.03 0.02 0.01 0 Theorecal Intensity

Experimental Intensity

a

b

c

Fig.4. (a)Correlationsofselectedexperimentalandtheoreticalfrequenciesfor1

and2inMeOH.(b)Correlationsofselectedexperimentalandtheoretical intensi-tiesfor1and2inMeOH.(c)Correlationsofselectedexperimentalandtheoretical intensitiesfor1and2inMeOH(relativeconcentrationvaluesof1(0.8)and2(0.2)).

theoreticalandexperimentalintensities(R2_{).Attherightrelative}

concentrations,R2_{fortheintensitiesshouldbeatmaximum.We}

foundthatmultiplyingtheintensitiesof1and2respectivelyby

0.80and0.20(Fig.3b)R2 _{reachesitsmaximumof0.99(Fig.4c)}

0 200 400 600 800 1300 1400 1500 1600 1700 1800 Intensity

Wavenumber (cm

-1

)

Wavenumber (cm

-1

)

Wavenumber (cm

-1

)

3 1 0 200 400 600 800 1300 1400 1500 1600 1700 1800 Intensity 0.00 0.02 0.04 0.06 0.08 1300 1400 1500 1600 1700 1800 Absorbance 1412cm-1 1702 cm-1 1506 cm-1 1730 cm-1 cm 1694 -1 1716cm-1 cm 1744 -1 1464 cm-1 1376 cm-1

a

b

c

R² =

0.99

1300 1400 1500 1600 1700 1800 1800 1700 1600 1500 1400 1300 Theorecal Frequency ExperimentalFrequency

R

2

=

0.98

0 100 200 300 400 500 600 700 0.08 0.06 0.04 0.02 0 Theorecal Intensity ExperimentalIntensity

a

b

Fig.6.(a)Correlationsofselectedexperimentalandtheoreticalfrequenciesfor1

and3inCCl4.(b)Correlationsofselectedexperimentalandtheoreticalintensities for1and3inCCl4relativeconcentrationvaluesof1(0.32)and3(0.34).

withanearlyperfectmatchbetweensyntheticandexperimental

spectra.

3.2. TheequilibriuminCCl4

Theabsenceof thepeculiar symmetricC Ostretchingband

of2at1750cm−1–1800cm−1 (Fig.5)andlowenergydifference

between1and 3 (2.7kcal/molinTable 1)allowsus toassume

thattheequilibriuminCCl4includesonly1and3.Inthisrespect,

thelowpolarityandaproticnatureofCCl4makeadvantageousfor

diacetamidetoformintermolecularH-bondsandtodimerize.As

aresultofthis,thediacetamidedimer3attainsitshighest

con-centrationinCCl4withrespecttoothersolventsasshownbythe

experimentalspectra.Thesameprocedureappliedinthemethanol

casewasfollowedtoobtaintherelativeconcentrationsof

diac-etamidedimer(3)andmonomer(1)inCCl4,thelatterbeingfound

34%and32%in0.0100mol/Lsolutionsrespectively.However,the

concentrationofthesolutionaffectstherelativedimerratios,

dilu-tionofthesolutionto0.0050and 0.0025mol/Lcausesdecrease

inrelativeratiosofthedimericstructureto27%and20%.Notice

thatrelativeconcentrationsmusttakeintoaccountthatdimer

con-tainstwomonomermolecules.Thisresultsupportsthepresence

ofdimericstructureinCCl4 becauseinanon-polarsolvent

diac-etamidepreferstointeractwithitself[27]whichistheonlychance

toformhydrogenbonds.

Thelinearrelationshipbetweenselectedexperimentaland

the-oreticalfrequenciesfordiacetamideinits1and3formsareshown

inFig.6a.Afterscalingtheintensities,goodagreementbetween

spectraisfound(Fig.6b).

As seen from Table 2, the concentrations of monomer

trans–trans(2) were changed according tothe solvent

proper-ties. The high polarity of DMSO (7.2)and theprotic hydrogen

effectofMeOHshouldberesponsibleforthehighconcentrationof

monomertrans–trans(2)inrelatedsolvents.Althoughthepolarity

ofmethanol(5.1)isrelativelylowerthanDMSO,theprotic

hydro-geneffectprevailsandtheconcentrationof2isobtainedas20%in

MeOH.ThepolarityofCH3CN(5.8)isnotashighasDMSOandquite

similartotheMeOH.Sinceitdoesnothaveanyprotichydrogenthe

concentrationof2isobtainedas1%.

4. Conclusions

The presence and relative concentrations of possible

diac-etamidetautomers,dimer and rotamerswere investigatedin 7

organicsolventsusingbothIRexperimentsandtheoretical

meth-ods. Equilibria were fully characterized in a semi-quantitative

mannerusingaspectra-matchingmethod.Forthelattertask,

the-oreticalIRspectrawereobtainedattheDFTB3LYP/6-311++G(2d,

2p)level;thegoodpartofdoingthisisthatthelatterresults

pro-videdthemolarabsorptioncoefficientsforthedifferentspecies

investigated.Asitshouldperhapsbeexpected,itisfoundthat

diac-etamidepreferstoformhydrogenbondswithpolarproticsolvents

likemethanolandpreferscis–transortrans–transconformations.

Innon-polarsolvents,diacetamidetendstodimerizeby

inter-molecular hydrogen bonding and giving rise to a dimer (3) –

monomer(1)equilibrium.Weconsidertheimprovedstabilityof

thedimerwithrespecttothemonomerinCCl4asduetotwo

hydro-genbondsandadditionallytwon–␲*transitionfromthenitrogen

atomstothecarbonyls.Theplanargeometryof–CONCO–,shorter

C–NandlongerC Obond distancesareevidencesforthen–␲*

transition.Withrespecttothepossibletautomerization,the

transi-tionandthehydrogenbondsseemtobeanacceptableexplanation

forthepreferentialformationofdimer(3)insteadofatautomer(4,

5)sincetheyreducethepotentialenergyofthesystemdespitethe

factthatanintramolecularhydrogenbondandconjugatedouble

bondsmayrepresentanadvantageforatautomeric(4)structure

(Fig.1).

References

[1] Y.C.Martin,J.Comput.AidedMol.Des.23(2009)693–704. [2] E.D.Raczynska,W.Kosinska,Chem.Rev.105(2005)3561–3612.

[3]V.A.Yaylayan,A.A.Ismail,S.Mandeville,Carbohyd.Res.248(1993)355–360. [4]R.M.Claramunt,C.Lopez,M.D.SantaMaria,D.Sanz,J.Elguero,Prog.Nucl.Magn.

Reson.Spectrosc.49(2006)169–206.

[5] V.L.Junior,M.G.Constantino,G.V.J.daSilva,A.C.Neto,C.F.Tormena,J.Mol. Struct.828(2007)54–58.

[6]J.Emsley,N.J.Freeman,J.Mol.Struct.161(1987)193–204. [7] T.Uno,K.Machida,Bull.Chem.Soc.Jpn.34(1961)545–550. [8] T.Uno,K.Machida,Bull.Chem.Soc.Jpn.34(1961)551–556. [9]T.Uno,K.Machida,Bull.Chem.Soc.Jpn.34(1961)821–826.

[10]T.Uno,K.Machida,I.Hamanaka,Bull.Chem.Soc.Jpn.34(1961)1448–1453. [11] T.Uno,K.Machida,Bull.Chem.Soc.Jpn.35(1962)1226–1232.

[12] M.C.Etter,S.M.Reutzel,J.Am.Chem.Soc.113(1991)2586.

[13]K.L.Gallaher,S.H.Bauer,J.Chem.Soc.FaradayTrans.2(71)(1975)1423–1435. [14]G.Nandini,D.N.Sathyanarayana,Spectrochim.ActaA60(2004)1115–1126. [15]F.Ramondo,S.NunzianteCesaro,L.Bencivenni,J.Mol.Struct.291(1993)

219–244.

[16]C.M.Lee,W.D.Kumler,J.Am.Chem.Soc.84(4)(1962)571–578. [17]C.S.Kraihanzel,S.C.Grenda,Inorg.Chem.4(7)(1965)1037–1042. [18] Y.Kuroda,Y.Saito,K.Machida,T.Uno,Spectrochim.Acta27A(1971)1493. [19]M.J.Frisch,G.W.Trucks,H.B.Schlegel,G.E.Scuseria,M.A.Robb,J.R.

Cheese-man,J.A.MontgomeryJr.,T.Vreven,K.N.Kudin,J.C.Burant,J.M.Millam,S.S. Iyengar,J.Tomasi,V.Barone,B.Mennucci,M.Cossi,G.Scalmani,N.Rega,G.A. Petersson,H.Nakatsuji,M.Hada,M.Ehara,K.Toyota,R.Fukuda,J.Hasegawa, M.Ishida,T.Nakajima,Y.Honda,O.Kitao,H.Nakai,M.Klene,X.Li,J.E.Knox,H.P. Hratchian,J.B.Cross,V.Bakken,C.Adamo,J.Jaramillo,R.Gomperts,R.E. Strat-mann,O.Yazyev,A.J.Austin,R.Cammi,C.Pomelli,J.W.Ochterski,P.Y.Ayala,K. Morokuma,G.A.Voth,P.Salvador,J.J.Dannenberg,V.G.Zakrzewski,S.Dapprich, A.D.Daniels,M.C.Strain,O.Farkas,D.K.Malick,A.D.Rabuck,K.Raghavachari, J.B.Foresman,J.V.Ortiz,Q.Cui,A.G.Baboul,S.Clifford,J.Cioslowski,B.B. Ste-fanov,G.Liu,A.Liashenko,P.Piskorz,I.Komaromi,R.L.Martin,D.J.Fox,T.Keith, M.A.Al-Laham,C.Y.Peng,A.Nanayakkara,M.Challacombe,P.M.W.Gill,B. John-son,W.Chen,M.W.Wong,C.Gonzalez,J.A.Pople,Gaussian03,RevisionD.01, Gaussian,Inc.,Wallingford,CT,2004.

[20]W.J.Hehre,L.Radom,P.v.R.Schleyer,J.A.Pople,AbInitioMolecularTheory, Wiley,NewYork,1986.

[21] F.Jensen,IntroductiontoComputationalChemistry,JohnWiley&Sons,London, 1999.

[22]R.G.Parr,W.Yang,DensityFunctionalTheoryofAtomsandMolecules,Oxford UniversityPress,NewYork,1989.

[23]C.Lee,W.Yang,R.G.Parr,Phys.Rev.37(1988)785. [24]A.D.Becke,Phys.Rev.B38(1988)3098.

[25]V.Barone,M.Cossi,J.Phys.Chem.A102(1998)1995.

[26] A.Güven,N.Kanis¸kan,J.Mol.Struct.(Theochem.)488(1999)125–134. [27] M.T.Nguyen,N.Leroux,T.Z.Huyskens,J.Chem.Soc.FaradayTrans.93(1)(1997)