Importance of weak interactions and conformational equilibrium in N-butyl-N-methylpiperidinium bis(trifluromethanesulfonyl) imide room temperature ionic liquids: Vibrational and theoretical studies

(1)

Importance

of

weak

interactions

and

conformational

equilibrium

in

N-butyl-N-methylpiperidinium

bis(tri

ﬂuromethanesulfonyl)

imide

room

temperature

ionic

liquids:

Vibrational

and

theoretical

studies

Madhulata

Shukla

a

_,

_Hemanth

_Noothalapati

b

_,

_Shinsuke

_Shigeto

b

_,

_Satyen

_Saha

a,

_*

a

DepartmentofChemistry,FacultyofScience,BanarasHinduUniversity,Varanasi221005,India

b

DepartmentofAppliedChemistryandInstituteofMolecularScience,NationalChiaoTungUniversity,Hsinchu30010,Taiwan

ARTICLE INFO Articlehistory: Received10May2014

Receivedinrevisedform19September2014 Accepted25October2014

Availableonline30October2014 Keywords: Ionicliquid Piperidiniumcation Bis(triﬂuoromethanesulfonyl)imideanion Ramanspectroscopy Vibrationalmodes H-bondinginteraction Interactionenergy Densityfunctionaltheory

ABSTRACT

Piperidiniumcation-basedroomtemperatureionicliquids(RTILs)constituteanimportantclassofILs because of their unique electrochemicalproperties as wellas non-aromatic nature of thecation. However,detailedstructuralstudiesareyettobedone.Inthispaper,wediscussthemolecularstructure and vibrational spectra of N-butyl-N-methylpiperidinium bis(triﬂuromethanesulfonyl) imide, (PIP14NTf2; where, PIP14 is N-butyl-N-methylpiperidinium and NTf2 is bis(triﬂuromethanesulfonyl)

imide),obtainedwithacombinedapproachofinfrared(IR)andRamanspectroscopiesintheliquidstate and density functional theory (DFT) and Hartree–Fock (H–F) based theoretical calculations. DFT calculations,whicharefoundtoproducethemoststablegeometrycomparedtoothertwomethods(MP2 and H–F), reproduce theexperimentalIR and Ramanspectra reasonablywell. Ourﬁndings reveal structuralpropertiesthatprofoundlyinﬂuenceintermolecularinteractionsandmeltingpoint.There existsalargevariationinthemeltingpointoftheILsstudied.Whilethebromidesaltofthepiperidinium derivative(PIP14Br)issolidwithveryhighmeltingpoint(241C),thecorrespondingNTf2saltislow

viscous liquid at roomtemperature (mp: 25_C). _bmimBr _(bmim₌_{1-butyl-1-methylimidazolium)}

exhibitsasubstantiallylowermeltingpointof79CthanPIP14Br,suggestingthatmorenumberofstrong

classicalhydrogenbondinginteractionsinthelatterisprimarilyresponsibleforthemuchhighermelting point.Inaddition,involvementofthealkylgroupinPIP14inH-bondinginteractionprovidesadditional

rigidityinn-butylchainwhichisotherwiseabsentinbmimBr.InteractionenergyforPIP14Brisfoundto

behigherthanPIP14NTf2,showingapositivecorrelationbetweeninteractionenergyandmeltingpoint.A

blueshiftinC—HstretchingwavenumberasevidentfromIRandRamanspectraofPIP14BrILisaclear

indication of the stronger hydrogen bonding as compared to PIP14NTf2 IL. Furthermore, we

experimentallyobservetheexistenceofcisoid–transoidconformationalequilibriumofNTf2anionin

theRamanspectrumofPIP14NTf2fortheﬁrsttimeanddeterminedthattransoidNTf2aniontobemore

stablethanthecorresponding cisoidconformerby1.04kcal/molusingDFT.Examinationofvarious conformationalpossibilitiesofthecationshowsthatthebutylgrouppreferentiallyexistsingauche conformation.

ã2014ElsevierB.V.Allrightsreserved.

1.Introduction

In recent years, ionic liquids (ILs) have become a rapidly

expandingtopicofchemicalresearchonaccountoftheirunique propertiesthatincludeanegligiblevapor pressure,non

ﬂamma-bility and good ability to dissolve organic and inorganic

compounds,and evenpolymeric materials[1,2]. These unusual

propertiesmeanthatILsaresuperiormediaforabroadrangeof

potential uses, such as environmentally friendly solvent for

chemical synthesis or biocatalysis. They are also used in

electrochemical devices and processes, such as rechargeable

lithiumbatteriesandelectrochemicalcapacitors,etc.Rechargeable

lithium batteries are a ubiquitous energy device that is being

worldwideinmanytypesofportableelectronicequipment,suchas cellularphones,laptopcomputers,anddigitalcameras.Cyclicalkyl

quaternary ammonium (QA) based cations,

N-alkyl-N-methyl-piperidinium (PIP1n; where, n indicatesthe number of carbon

atomsinthelinearalkylchain)areaclassofcationswhoseroom

temperature ILs (RTILs) are very promising in the ﬁeld of

electrochemical applications due to their high thermal and

* Correspondingauthor.Tel.:+919935913366;fax:+915422368127. E-mailaddress:ssahabhu@yahoo.com(S.Saha).

http://dx.doi.org/10.1016/j.vibspec.2014.10.006

0924-2031/ã2014ElsevierB.V.Allrightsreserved.

ContentslistsavailableatScienceDirect

Vibrational

Spectroscopy

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electrochemical stabilities [3–18]. As an example, PIP1nNTf2;

where, NTf2 stands for bis(triﬂuoromethanesulfonyl) imide, is

recentlyproposedfor high-voltage supercapacitors and lithium

batteries[10–13].Electrochemicalstabilityisoneofthereasonsfor

recentgrowing interest in QAbased ILsin addition tothelow

cathodicpotentialsoftheQAcationscomparedto

1,3-dialkylimi-dazoliumcations.Becauseofthewideelectrochemicalwindows,

these ILs are used as solvent-free supporting electrolytes in

electrochemicaldevicesandalsoincreasetheirsafety.

Asthepropertiesofanymaterialdependonthestructureof moleculesindifferentphases,itisveryimportanttounderstand thestructuralfeaturesofILsindepth.Theubiquitouspropertiesof ILsaregovernedbythetypeandstrengthofinteractionbetween theirconstituents[19–21].Recently,thepossibilitiesofexistence oforderedlocalstructure,i.e.,microheterogeneityinionicliquids havegot considerableattentions[22–25].Ingeneral, liquidsare muchlessunderstoodthangasesandcrystals.Structureinthegas phasecanbeaccuratelydeterminedbyelectrondiffractionor high-resolution rotationally-resolved spectroscopy, and solid/crystal structurecanbedeterminedbyX-rayand/orneutrondiffraction.In contrast,thediffractiontechniqueshavelimited applicabilityto elucidateliquid structure.Understandingthemicroscopic inter-actionsatthemolecularlevelisindeedachallengeinparticularfor ILs.Whilevastamountofstudieshavebeenmadeonimidazolium basedILs,veryfewliteraturesareavailablerelatedtostructural andvibrationalstudiesontheseusefulPIP1n-basedILs.Herewe

reportadetailedstructuralstudyofPIP14-basedILsusingdensity

functionaltheory(DFT)calculationsandvibrationalspectroscopy (bothIRandRaman).Theoreticalcalculations,inparticularDFT,are ofverymuchimportanceinpredictingthestructureofdifferent RTILs [26,27]. DFTcalculation also helps us to understandthe interactionpresentamongcationandanioninthemoleculeaswell asthetypeofbondingpresentinthemolecule.TheDFTcalculated

vibrational spectra gives us a strong base to analyze the

experimental spectra (both IRand Raman) including theeffect

ofinteractioncausingshiftinginRaman/IRbands[28].

Amongvariousphysicalproperties,meltingpointisthemost importantforILs.Thispropertyisdeterminedbythestrengthofits crystallatticepacking,whichisinturncontrolledbythreemain factors:(i)molecularsymmetry,(ii)intermolecularinteractions,

and (iii) conformational degrees of freedom of the molecule.

AccordingtoFuminoandLudwig[29],althoughILsconsistssolely

of cations and anions and Coulomb forces are the dominating

interaction,alocalanddirectionalinteractionsuchashydrogen bondinghassigniﬁcantinﬂuenceonthestructureandproperties ofionicliquids.IthasalsobeenexplainedbyKempteretal.[30]

that in ILs, hydrogen bonding is the major interaction which

controlthephysicalpropertiesofILs.StrengthofH-bondingcanbe

well studied by the vibrational spectroscopy [26,29]. As the

interactionstrengthincreasesbetweencation andanion, vibra-tionalfrequenciesshifttohigherwavenumberduetoincreasing forceconstantsindicating strongerinteraction betweentheion

species [29]. A shift in the wavenumber of intermolecular

vibrational bands can be correlated to the change in force

constantsorreducedmassesofanion.Largerandheavieranions

interact weakly with the cation, whereas, smaller and lighter

anionsprovide strong interaction potential due to theirhigher surfacechargedensity.Thus,bothparameters;thereducedmass aswellastheforceconstant,leadtoshiftintothesamedirection, namelytolowerorhigherfrequencies,respectively.Thisstatement hasbeenwelljustiﬁedbyourpreviouspublication[26].

Inthispaperwechoosetwosetsofionicsalts:(1)PIP14Brand

1-butyl-3-methylimidazoliumbromide(bmimBr)(anionsame)and

(2)PIP14BrandPIP14NTf2(cationsame)withanaimtopinpointthe

majorinteractionwhichdeterminesthephysicalstateofa salt. ExcepttheveryrecentpublicationbySiqueiraetal.[31]where

theyhavestudiedN,N-butyl-methylpiperidinium,incombination

with1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium

and 1-butyl-2,3-dimethylimidazolium cations based ILs and

evaluated the effects of chain length, ability of developing

hydrogen bond, electronic delocalization of their positively

charged region, no other vibrational studies can be found on

piperidinium ionic liquid despite their signiﬁcant importance. Theyhavemadethestructuralstudiesofallthesecationsthrough

XRDandDFTstudiesaswellasvibrationalstudiesusingRaman

spectroscopy.Alsotheyhaveinvestigatedthatverysimilarlocal

environment is experienced byall theseorganiccations in the

interlayerspace andin ionic liquidswithNTf2 anion [31]. This

studyrevealsthatitisindeedtheH-bondinginteractionalongwith conformational varietyof constituentsthat largely controls the physicalstateofthesalt.WeestablishthatPIP14Brisacolorless

solidandmorethanoneinteractionispresentbetweencationand anion.ThisfactcontrastswithbmimBr,whereonlyoneinteraction ispresentbetweencationandanionandisfoundtoexistaslow meltingsolid(mp:79_C)._Further,_Raman_and_DFT_studies_show thatNTf2anionfoundtoexistincisoid–transoidconformational

equilibrium in PIP14NTf2 IL analogous to n-butyl chain in

imidazoliumcation.Wealsopresentstudiesofnon-ionicspecies ofcorrespondingionicconstituentstounderstandthestabilization duetoformationofionicliquids.Toourbestofknowledge,thisis the ﬁrst reporton the vibrational–theoretical studies of inter-actionsforpiperidiniumcationicionicliquids.

2.Methodology

2.1.Reagentsandinstrumentation

N-Methylpiperidine(Sigma–Aldrich,redistilled,>99%),

bromo-butane (Merck, Germany), bis(triﬂuoromethanesulfonyl) imide

(Sigma–Aldrich,redistilled,>99%)wereusedasreceived forthe synthesis. Acetonitrile(HPLCgrade),ethylacetate (EtOAc,HPLC

grade)wereprocuredfromMerck,Germanyandwereusedafter

puriﬁcationfollowingstandardprocedures.IRspectraofPIP14Br

andPIP14NTf2weremeasuredwithVarianFTIR3100intheregion

400–3500cm1 using neat samples. 300MHz NMR (JEOL) was

usedtomeasurethe1_H_NMR._Raman_spectra_were_measured_with_a

home-made Ramanmicrospectrometer setup with CW 633nm

(He–Ne) excitation. Details of the experimental setup were

described elsewhere [32]. Raman spectra were obtained with

lowlaserpower(1.2mW)with10sexposure.TheRamanspectra presentedhereareafterbackgroundcorrection.

2.2.SynthesisofN-methyl-N-butylpiperidiniumbromide(PIP14Br)

A general synthesis procedure for synthesizing PIP14Br was

reported in literature [6,14]. A modiﬁed form of the reported procedurewasfollowed:insteadofusinghightemperature,lower temperaturewasusedwithalongertimeofstirring.Thisexcludes majorpossibilitiesofinclusionofimpuritiesinresultantILs.The schemeforthesynthesisofPIP14BrisshowninFig. 1.20mLofethyl

acetatewastakenin100mLroundbottom(RB)ﬂask.N-Methyl

piperidine(8.0g,82mmol)wasaddedtoitwithstirringandthen

bromobutane(12.0g,90mmol)wasaddedslowlywithcontinuous

stirringat20Cinnitrogenatmosphere.Themixturewasstirred for16hat30Cfollowedbystirringat50Cfor3h.Thereaction

mixturewas thenwashedwith150mLdrydistilled EtOAc. The

solvent was then evaporated on a rotavapour under reduced

pressuretogetwhitesolidpowder,whichwasfurtherkeptunder highvacuumat50Cfor3h(yield:90%).Meltingpoint(mp)ofthe saltwasfoundtobe241CandthereforeisnotanILaccordingto

standard deﬁnition.The productwas conﬁrmedby 1_H _NMR ₍

d

, ppm,1.01(t,3H),1.47(q,2H),1.72(8H),3.63(s,3H),3.66(4H),3.81

(3)

(2H));andbyIR(569,673,904,940,1030,1227,1369,1464,2874, 2959cm1).

2.3.SynthesisofN-methyl-N-butylpiperidiniumbis (triﬂuoromethanesulfonyl)imide(PIP14NTf2)

PIP14BrwastakenasaprecursorforthesynthesisofPIP14NTf2.

TheschemeforpreparationofPIP14NTf2isshowninFig.2.PIP14Br

(4.5g,20mmol)wastakeninaRBﬂaskcontaining10mLoftriple distilled(TD)water.LiNTf2(6.0g,21mmol)dissolvedin10mLTD

waterwasaddedtoitat35_C_and_stirring_was_continued_for₄_h.

150mLdichloromethane(DCM)wasusedtoextracttheproduct,

whichwasrepeatedlywashedwithcoldTDwatertoremoveany

ionicimpurity.DCMwasthenevaporatedonavacuumrotavapour

andkeptunderhighvacuumfor2hat50C.Lightyellowliquid wasobtained(yield:88%).Relativelyhighertemperature(80_C) providesmoreyield[6,14]butwasfoundtoproducedarkeryellow coloredliquid.Thelightyellowcoloredliquidwasfurtherdissolved in10mLofpureacetonitrile(ACN)totreatwithactivatedcharcoal for decolorization.The mixture was stirredfor 4hfollowedby

ﬁltrationthrougha sinteredcolumnpackedwithfreshcharcoal

andactivatedalumina.Theresultantsolutionwasevaporatedon rotavapouratreducedpressuretogetacompletelycolorlessliquid product.Theproductwascon_ﬁrmedasdesiredionicliquidby1_H

NMR(

_d

,ppm,1.01(t,3H),1.44(q,2H),1.74(8H),3.42(s,3H),3.56 (4H),3.85(2H));andbyIR(570,619,1054,1139,1197,1348,1474, 2881,2966cm1).

2.4.Computationaldetails

Quantum chemical calculations were used for geometry

optimization and vibrational frequencies calculation on cation,

anion and ionpairs using Gaussian 03 package[33].Geometry

optimizationwasperformedusingdensityfunctionaltheory(DFT)

[34], second Møller–Plesset (M–P2) and Hartree–Fock (H–F)

methodstoexaminethemoststablestructure.DFTcomputations

wereperformedatBecke_’sthree-parameterhybridmodelusing

Lee–Yang–Parrcorrelationfunctional(B–3L–Y–P)leveloftheory

[35,36].6–31++G(d,p)basissetwasusedtoobtaintheoptimized

geometryand calculated IRand Raman frequencies.Two other

popular methods, MP2 and H–F, were examined for better

predictionsofexperimental IRand Ramanfrequencies [37].No

imaginaryvibrationalfrequencieswereobtainedforthe calcula-tionreportedhere,indicatingthatthegeometriesoftheionswere

at the minimum of the potential surface. Correlation between

experimental and calculated vibrationalfrequencies was

estab-lished based on the literature values of similar derivatives by

investigating the calculated frequencies through GaussView

5.0 software[38] andmatchingboth thecalculatedfrequencies

andRamanactivitieswithexperiment.SincetheRamanactivities obtainedfromGaussiancalculationscannotbecompareddirectly totheexperimentalRamanintensity[39–41],recentlypublished

methodology [42] was employedto convertthe DFTcalculated

Raman activitiesto DFTcalculated Ramanintensity. Calculated

Ramanspectraarethuscompareddirectlywiththeexperimental

Ramanspectraandpresentedinvariousﬁgures.

3.Resultsanddiscussion

3.1.Geometryoptimizationofcationandanion

Primarily the structure of cation and anion is optimized

independently at the B–3L–Y–P/6–31++G(d,p) level of theory.

The piperidine ring is found to be most stable in chair

conformationwhencomparedwithotherpossibleconformations

such as boat or twist boat forms. This led us to use chair

conformationastheinitialinputfortheoptimizationofPIPcation aswellasPIPbasedILs.AccordingtoSiqueiraetal.[31]alsothe

most stable conformation of piperidine ring is the chair

conformation like cyclohexane. The n-butyl chain attached to

piperidiniumcationiskeptingaucheform,whichisfoundtobe morestablethancisandtransforms[17].Anenergyleveldiagram

for optimized piperidine (unsubstituted neutral molecule),

N-methyl piperidine (substituted neutral molecule),

N-methyl-N-butylpiperidiniumcation(PIP14+),PIP14Br(salt),andPIP14NTf2 is

depictedinFig.3.Asexpected,neutralorcationic specieshave higherenergiesthantheionpairsandperhapsillustratethereason fortheformationofionicsaltfromconstituents.

AsurveyoftheliteraturerelatedtotheconformationofNTf2

revealsthat bothcisoidandtransoidconformationsarepossible

[43–51].WhilecisoidconformationhasbeenproposedbyArnaud etal.[48]basedonabinitiocalculationonLiNTf2,Johanssonetal.

[49]andLopesandPadua[50]havereportedtransoidasamore

stableconformer.Ontheotherhand,incrystallinestate,transoid

N

C

4

H

9

Br

+

N

C

4

H

9

LiNTf

2

PIP

₁₄

Br

PIP

14

NTf

2

Water/

RT

4hStirring

Whitesolid Colorlessliquid

N

S

O

O O

F

₃

C

CF

₃

Fig.2.SchemeforthesynthesisofN-methyl-N-butylpiperidiniumbis(triﬂuoromethanesulfonyl)imide(PIP14NTf2).

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NTf2wasobservedbyForsythetal.[51].Itisplausiblethatwhile

NTf2ispresentpredominantlyintransoidform,cisoidformcan

alsobepresentasminorspecies.Recently,Fariaetal.[52]inan impressivearticlehavepointedoutthroughRamanspectroscopic

studiesthat, during crystallization of butyltrimethylammonium

NTf2 IL, slowcooling process led to formation of cisoid NTf2,

whereas,fastcoolingprocessledtoformation oftransoid NTf2.

Similarinterestingobservationsarealsoreportedfor

butylimida-zolium NTf2 (hbimNTf2) derivative by Moschovi et al. [53].

Accordingtothem,incrystallinephase(i.e.,atlowtemperature),

NTf2 adopts trans conformation, while upon melting, both

conformationsremainsinequilibrium(enthalpyoftheequilibrium 8.5kJ/mol). Interestingly, at elevated temperatures, NTf2

pre-dominantly remains in cis conformation. This variety of

con-formations of NTf2 anion makes ILs comprising NTf2 an

interestingtopicofstudy.Hence,itisimportanttoknowwhich conformationofNTf2doesexistpredominantlyinthePIP14NTf2.

DFTcalculateddipolemomentvaluesforthecisoidandtransoid

conformers are found to be quite different, 4.12 and 0.05 D,

respectively.Thesedipolemomentvaluescloselymatchwiththose reportedbyFujiietal.[43].Thesigniﬁcantlylargervalueforcisoid plausiblyimpliesthatwhenthecounterion(cation)issmall,the cisoid geometry is preferred due to dipole–dipole interactions.

Since, in the present study, the cation is reasonably large

(comparedtoLi+,etc.),itisexpectedthattransoidconformation

is predominant. Our DFT calculationfurther indicated that the

transoidform(1146638.63kcal/mol)oftheanionismorestable thanthecorrespondingcisoid(1146637.59kcal/mol)by1.04kcal/ mol(Fig.4).Thesepiecesofevidence ledus toperformallthe

calculationsonNTf2anionbasedILswiththetransoid

conforma-tion.

3.2.Geometryoptimizationofcation–anionionpair

Basedontheinformationobtainedfromtheindividualcation andanionspecies,theinputstructureofPIP14BrandPIP14NTf2ion

pairsaredetermined.Whilethechairformofpiperidiniumistaken forcation,thetransoidformofNTf2anionisconsideredforthe

constructionofPIP14NTf2ionpairforcalculations.Fig.5depictsthe

optimizedmoleculargeometryofPIP14BrobtainedatDFT/B–3L–Y–

P/6–31++G(d,p) level calculation in gas phase. The important calculatedparametersaretabulatedinTable1.TheN—CandC—C

bond lengths are found to be in the 1.51–1.54Å range, which

indicates single bond lengths. The C7—N1—C8 and N1—C8—

C9bondanglesarefoundtobe110and116,respectively,which deviatefromthetetragonalgeometry(109)insigniﬁcantly. The

N—C8—C9—C10 torsional angle is observed to be 163, which

deviateby just 2 fromthat reportedbyReichert et al.for the crystalstructuralofPIP14I[17].Thisexplainsthatthebutylgroupis

ingaucheconformationsimilartothatinPIP14I.TheC8—C9—C10—

C11torsionalangleiscalculatedtobe178_,_indicating_the_trans formofthebutylchain.Withrespecttointeractionspresentinthe salt,Fig.5showsthatthesinglebromideionisinvolvedinthree hydrogenbonds(2.45,2.54,and2.57Å)withthehydrogensofthe cation.Thisindicatesthepresenceofastronginteractionbetween

monoatomicbromideanionandthecation.Thisperhapsleadsto

tightﬁttingoftheionsinthecrystallatticeandhence,toahigh meltingpointofPIP14Br(i.e.,241C).

Fig.3. DFT-calculatedenergiesforoptimizedmoleculesoftheneutralandchargedconstituentsandthesalts:(a)piperidine,(b)N-methylpiperidine,(c) N-methyl-N-butylpiperidiniumcation(PIP14+),(d)cis-PIP14NTf2,(e)trans-PIP14NTf2,and(f)PIP14Br.Calculateddipolemomentsarefoundtobe0.892,0.466,3.288,16.787,16.816,and

13.994Debyefor(a)–(f),respectively.

Fig.4.CisoidandtransoidconformationsofNTf2anion.TheterminalCF3rotatesalongtheS—Nbondtogiverotationalisomers.Thetwistangle(C4—S2—S3—C5)forcisoidis

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Molecular geometry optimization of PIP14NTf2 ion pair was

carriedout usingthree differentmethods:DFT/B3–L–Y–P, MP2,

andH–F,withanaimtoﬁndoutthebestmethodforthiscategory ofILs.Theimportantbondlengths,bondangles,torsionalangles, andhydrogenbondingparametersaretabulatedinTable1.While

bondlengthandbondangleparametersforpiperidiniumcation

arefoundtobesimilartothoseforPIP14Br,theN1—C8—C9—C10

torsionalanglesarefoundtobe178,177,and171fromDFT,MP2, andH_–Fcalculationsrespectively,whicharedifferentfrom163

observed for PIP14Br. This indicates that the butyl group in

PIP14NTf2ionpairexistsintransconﬁguration.TheC8—C9—C10—

C11 torsional angle is observed to be 179 _from _DFT _and

MP2 calculation, indicating its trans conformation around the

C9—C10bond.Anexcellentcorrelationisalsoobservedbetween experimentalandcalculatedgeometricparametersofNTf2anion.

Fore.g.,N—S,S¼O,S—C,andC—Fbondlengthscalculatedusing

DFT show very minor deviation (0.05, 0.04, 0.07, and 0.01Å,

respectively) fromthereported crystaldata [43].Though bond

lengthscalculatedbyH–Fmethodareinbetteragreementwith

crystalsdata,itsbondangleandtorsionalanglespredictionsare grosslydifferent(videTable1).Ontheotherhand,B–3L–Y–Plevel oftheoryfairlywellreproducebondanglesandtorsionalangles

(e.g.,seeS—N—Sbondangle,C—S—S—C,andS—N—S—Ctorsional

anglesinTable1).MP2calculationalsoproducessimilarresultsas

DFT, but it consumed nearly three times computational time

comparedtothatofDFTcalculation.Therefore,itappearsfromour

studies,that DFT ismostreasonable forgeometryoptimization

studiesof piperidinium basedsalts.Classicalhydrogen bonding interactionpresentbetweencationandanionareshowninFig.6as

broken lines.C7—H—N12,C7—H—O15,C9—H—O15,and C2—H—

N12hydrogenbondsarefoundtohavebondlengthsof2.64,2.34,

2.54, and 2.63Å, respectively. Among the two salts examined,

PIP14BrwasfoundtobemorestablethanPIP14NTf2by590kcal/mol

(Fig.3).BoththepossiblestructuresofPIP14NTf2havingthecisoid

ortransoidformofNTf2anionhavealsobeenstudied

theoreti-cally and transoid-PIP14NTf2 is found to have a slightly lower

energy(by0.38kcal/mol)comparedtocisoid-PIP14NTf2(seeFig.3).

3.3.Hydrogenbondinginteractionandinteractionenergies

Studies of hydrogen bonding interaction present in ILs are

important as the number of H-bond donors, the interaction

strength,andthetypeofnetworkformationcanbetailoredtogeta Fig. 5.DFT optimized structure of N-butyl-N-methylpiperidinium bromide,

(PIP14Br).ThedistanceswithinthesumofvanderWaalsradiiarealsoshown.

Table1

Selectedbondlengths(Å),bondangles()anddihedralangles()foroptimizedstructureofPIP14BrusingDFTmethodandPIP14NTf2usingDFT,MP2,andH–Fmethodsaswell

asitsreportedcrystaldata.

Parameter PIP14Br Parameter/PIP14NTf2 CrystaldataofNTf2anion[25] DFTresults MP2results H–Fresults

N1—C2 1.52Å Anion N1—C7 1.51Å N12—S14 1.57Å 1.62Å 1.62Å 1.57Å N1—C6 1.53Å S14—O17 1.42Å 1.46Å 1.46Å 1.42Å C2—C3 1.54Å S14—C20 1.83Å 1.89Å 1.88Å 1.83Å C2—H—Br 2.45Å C20—F24 1.32Å 1.33Å 1.34Å 1.32Å C7—H—Br 2.57Å S13—N12—S14 125 ₁₂₆ ₁₂₄ ₁₂₉ C9—H—Br 2.54Å S13—N12—S14—C20 92 ₈₉ ₁₀₁ ₁₀₃ C2—H—Br 153 _{S14—N12—S13—C19} ₉₂ ₈₉ ₉₃ ₈₉ C7—H—Br 155 _{C20—S14—S13—C19} ₁₇₂ ₁₆₅ ₁₇₁ ₁₆₁ C9—H—Br 156 Cation C7—N1—C8 110 C7—N1—C8 – 109 109 109 N1—C8—C9 116 _N1—C8—C9 _– ₁₁₆ ₁₁₅ ₁₁₆ N—C8—C9—C10 163 _{N—C8—C9—C10} _– ₁₇₈ ₁₇₇ ₁₇₁ C8—C9—C10—C11 176 _{C8—C9—C10—C11} _– ₁₇₉ ₁₇₉ ₁₇₇ Cation–anioninteraction C7—H—N12 – 2.64Å 2.57Å 2.70Å C7—H—O15 – 2.34Å 2.32Å 2.42Å C9—H—O15 – 2.54Å 2.29Å 2.69Å C2—H—N12 – 2.63Å 2.38Å 2.72Å

Fig.6.DFToptimizedstructureofN-butyl-N-methylpiperidinium bis(triﬂuoro-methanesulfonyl)imide,(PIP14NTf2).ThedistanceswithinthesumofvanderWaals

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varietyofILs[28].Inthisstudy,wehavechosenaparticularcation withtwodistinctly differentanionshaving quite differentsize, ﬂexibility,interacting sites,and H-bondingformation capability. Thesetwoanions,BrandNTf2,ontheotherhand,representtwo

extremecasesinthesensethatwhiletheformerisasingleatom basedanion,thelatterisamultidentate,conformationallyﬂexible,

considerably largeranion. The difference in hydrogen bonding

patternispresentedinFigs.5and6.TherearethreeH-bonding interactionsfoundinPIP14BrwithH—Brdistances2.57,2.45,and

2.54Å,whicharesmallerthanthesumofvanderWaalradiusofH andBr(3.05Å)[54].ThecorrespondinganglesC7—H—Br,C2—H—

Br, and C9_—H—Br were found to be 155_, ₁₅₃_, _and ₁₅₆

respectively,indicatingrelativelystrongerH-bondinginteractions.

It is worthmentioningthat when compared with imidazolium

analoguebmimBr,themeltingpointofPIP14Brisrecordedtobe

higherby162C(bmimBrmp=79C,PIP14Brmp=241C)[55].The

cations are different in the sense that imidazolium cation is

aromatic in nature, whereas, piperidinium cation is a

non-aromatic,quaternaryammoniumsalt.Interestingly,eventhough

there is an additional possibility of having

_p

–

p

interaction in imidazoliumILsincontrasttopiperidiniumcationbasedILs,the

meltingpointoftheformerisfoundtobemuchlowerthanthe

latter.Sincethesubstituentsaresame(inbothcases,butyland methylgroups),itcanfairlybeassumedthatthereisnodrastic

change in hydrophobic interaction [56]. A close look at other

interactions between cation and anion provides interesting

differences. While in bmimBr, only one strong H-bonding

interaction was observed between cation and anion involving

C2—H—Brwithadistanceof2.19Åandangleof154[26,56],there

are three H-bonding interactions present between cation and

anioninPIP14Br(seeFig.5).TheH-bondingintheformerinvolving

mostacidic C2—H is quite wellknownand extensivelystudied

[56,57].The singleH-bondinbmimBris, however,foundtobe

quite stronger than any of the three individual H-bonding

interactions present in PIP14Br. The H-bonding interactions in

PIP14Brarequiteuniquecomparedtocorrespondingimidazolium

derivatives. In case of PIP14Br, the Br anion holds all three

segments (i.e., methyl group, n-butyl chain and piperidinium

moiety)ofPIP14cationthroughH-bondinginteractions,givingit

morecompact,lessﬂexiblecation–anionpairthancorresponding bmimBr(seeFig.5).Thistemptedustoinferthatmorenumberof H-bondingthoughweakeraremoresigniﬁcantindeterminingthe physical state than a stronger classical H-bonding interaction. Interestingly, when the anion in PIP14Br(having mp 241C) is

changedbyNTf2anion,thesaltbecomesacolorless,lessviscous,

RTIL(mp25_C).

TheDFToptimizedstructureofPIP14NTf2(Fig.6)showsfour

H-bonding present between cation and anion; two are with the

methylgroup,onewiththepiperidiniumring,andonewiththe

n-butyl chain. Although the number of H-bonding is larger in

PIP14NTf2(through multi-interacting sites),themelting pointof

PIP14BrissigniﬁcantlyhigherthanPIP14NTf2.Hence,theﬂexibility

andvolumeofanioninthiscaseappeartobemoreimportantthan H-bondinginteractions[58,59].This_ﬁndinghasmadeusstudythe

interaction between the cation and anion in more detail. The

interactionenergy(

D

E)isdeﬁnedasthedifferencebetweenthe

energyofthecation–anionpair(EAX)andthesumoftheenergiesof

theisolatedcationic(EA+)andanionic(EX)species.Theenergiesof

thecation,anionandcation–anionpairofPIP14NTf2arecalculated

usingDFT,MP2,andH–FmethodsandarepresentedinTable2.Itis

evidentthatthemoststablegeometrycanbeobtainedfromDFT

calculation compared with MP2 (by 7kcal/mol) and H–F (by

14kcal/mol)method.Ithasbeenpointedoutinourearlierreport thatinteractionenergycanprovideusafairlywellindicationof meltingpointofthesalt[26].Thelinearrelationshipbetweenthe

melting points and the interaction energies

D

E has also been

demonstratedveryrecentlybyLietal.[60].Theinteractionenergy calculatedforPIP14Brwasfoundtobe90kcal/mol,whilethatfor

PIP14NTf2wasobservedtobeonly70kcal/mol,clearlyshowingthe

linear relationship between themelting points and interaction

energy.

3.4.Understandingtheinteractionsthroughvibrationalspectral featuresforPIP14NTf2

Vibrationalspectroscopyisapowerfultooltogetthestructural featuresandinteractionspresentinILsregardlessoftheirphysical

states. Fumino and Ludwig [29] have demonstrated that far

infrared(FIR)spectroscopycanbeasuitablemethodforstudying thecation–anioninteractioninionicliquids.Signiﬁcantdifference

inwavenumberas wellas intensityof thepeakonvariationof

systematic variation of anion with imidazolium cation helped

them to understand speciﬁc anion–cation interaction. Their

experimentalresultssupportedbyDFTcalculationalsoindicates

linear relationship between stretching frequency and binding

energy.SincewehaveshownabovethatDFTcanreproducewell

theexperimentallyobtainedstructureandinteractions,itisused tounderstandandexplaintheexperimentalIRandRamanspectra ofsynthesizedPIP14NTf2.Fig.7showstheIRspectrumofPIP14NTf2

Table2

EnergiesofPIP14+cation,NTf2anionanditsionpair,calculatedusingDFT,MP2,andH–Fmethods.

Methods Energyofcation/kcal/ molx103

Energyofanion/kcal/ molx103

Totalenergy(cation+anion)/kcal/ molx103

(I)

Energyofionpair/kcal/ molx103

(II)

Interactionenergykcal/ mol

(II–I)

DFT 281.683 1146.638 1428.320 1428.390 70

MP2 280.753 1144.398 1425.151 1425.214 63

H–F 279.729 1142.838 1422.567 1422.623 56

Fig. 7. IR spectrum of neat PIP14NTf2 (continuous line) and DFT calculated

vibrationalbands(verticallines).Ascalingfactorof0.966wasrequiredtoreproduce theexperimentalobservationsatthehigherwavenumberregion.

(7)

along withDFTcalculated IR-active normal modes.The

experi-mental IRspectrum showsmajorpeaksat 570,619,1054,1139,

1197,1348,1474,2881,and2966cm1.Toassignthesepeaks,both

DFT/B–3L–Y–P and H–F calculation methods were used for

PIP14NTf2ionpair.TheefﬁciencyofDFTwasclearlyfoundtobe

betterinreproducingtheexperimental resultsthanthatofH–F

method.It canbeseenfromFig.8that H–Fmethodrequiresa

signiﬁcantlylargercorrectionfactor ascompared toDFTin the

higher wavenumber region. Similar indications have also been

reportedforawiderangeofmolecules[37,46].OnthebasisofDFT

calculation, experimental peaks have tentatively beenassigned

andarepresentedinTable3.Whilethebandsat570and1054cm1

are assigned to the O¼S¼O scissoring, and S¼O symmetric

stretching (weakly coupled with S—N asymmetric stretching,

respectively), the1139and1197cm1peakscorrespondtoC—F

stretching and symmetric bendings, respectively. The bands at

2881and2966cm1correspondtothesymmetricandasymmetric

C—Hstretchingmodesofthecation. TheIRfrequenciesderived

fromtheDFT/B–3L–Y–Pcalculationsagreereasonablywellwith

theexperimentalfrequencies(seeFig.8).Inaddition,theoretically

determined relative intensities are also found to be in good

agreement with experimental IR intensities. The correlated

Fig.8.Correlation diagramofexperimentalIRfrequencieswith DFTandH–F calculatedfrequenciesforPIP14NTf2.

Table3

SelectedIRfrequenciesofPIP14NTf2calculatedusingDFTandH–Fmethods.

Observedn(cm1₎ _DFT_n_(cm1₎ _H–F_n_(cm1₎ _Band_assignment_in_PIP 14NTf2

568 549 629 ScissoringinO¼S¼OandCF3

570 589 694 opbendingofNandScissoringinO¼S¼O

739 701 810 N—Ssymstretching

790 755 847 C—FandN—Ssymstretching

905 905 987 N—C(CH3)stretchingandtwistinginH—C—H

938 945 1025 RockingofH—C—H,predominantlyofpiperidineringCH2

1054 1086 1257 S¼OsymstretchingweaklycoupledwithS—Nasymstretching

1139 1152 1346 C—Fstretching

1197 1196 1379 C—Fsymbending

1469 1480 1569 H—C—HwaggingandumbrellabendinginCH3group

2881 3015 3162 symC—HstretchinginBugroup

2966 3089 3241 asymC—Hstretchinginpiperidiniumring

Sym–symmetric;asym–asymmetric.

(8)

experimentalIRspectrafor PIP14Brand PIP14NTf2 areshown in

Fig.9.

Although a conformational studyof cation and anion is not

possiblethroughIR,Ramanspectraarefoundtobeveryhelpful. TheexperimentalandsimulatedRamanspectraofPIP14NTf2are

shown in Fig.10. The experimental Raman spectra have been

correlated with the DFT calculated Raman intensity. Raman

activitiesobtainedbyGaussianprogramwereconvertedtoRaman intensityusingaproceduredescribedinliteratures[39_–42].The ﬁngerprintregionconsistsofonestrong,sharplineat740cm1, whichischaracteristicforNTf2anionandcorrespondstoS—Nand

C—Fstretching coupled withSO2 wagging motion. In addition,

relativelylessintensepeaksareseenat323,343,403,714,1081, 1135,1237,and1441cm1intheﬁngerprintregion,whileabroad

conglomerate of some bands appears in the 2800–3100cm1

interval. Raman spectra reported by Siqueira et al. [31] very

recently for PIP14NTf2 is found to correspond well with our

experimentalandtheoreticalresults.Themajorityofthesepeaks

have been assigned based on reported values and interpreted

satisfactorilyonthebasisofourDFT(6–31++G(d,p))calculation (see Table 4) [46]. More speciﬁcally, the S¼O symmetric and

asymmetric stretching bands appear at 1081 and 1276cm1,

respectively. Sharp bands at 2874and 2944cm1 along witha

humpat2979cm1arealsoobservedintheC—Hstretchingregion. CorrelationofRamanfrequenciescalculatedusingtwodifferent methods(DFTandH–F)withexperimentalpeakpositionsisshown inFig.11.DFTcalculationisfoundtoreproducetheexperimental

frequenciesbettercomparedtoH–Fmethod.TheRamanspectra

Fig.10.RamanspectrumofneatPIP14NTf2correlatedwithDFTcalculatedvibrationalbands(verticallines).Ascalingfactorof0.966wasrequiredabove1500cm1to

reproducetheexperimentalobservationsintheC—Hstretchandﬁngerprintregions.

Table4

SelectedRamanshifts(wavenumber)ofPIP14NTf2(transoidform)calculatedatDFTandH–Fmethods.Someofthecalculatedbandsarecloselymatchingwiththatof

experimentalbandswhichmightbejustcoincidentalconsideringthatthecalculationsweredoneingasphasewhileRamanspectrawererecordedinitsnormalphaseof existence.

Observedn(cm1) Transoidform Assignmentofbands

DFTn(cm1) H–Fn(cm1)

301 294 348 WagginginSO2coupledwithC—Fsymbending

323 324 348 TwistinginSO2

343 342 377 RockingofH—C—Hinpiperidiniumring

349 366 398 RockingofSO2andS—Nstretching

396 388 447 RockingofSO2

569 560 581 ScissoringinSO2andF—C—F

714 701 810 S—NstretchingandC—Fsymmetricstretching

740 755 866 S—NstretchingandC—FsymmetricstretchingcoupledwithSO2wagging

837 837 895 WaggingofH—C—Hinpiperidiniumring

909 912 977 C—Cstretchinginbutylgroup

1028 1060 987 N—CH3stretching

1081 1081 1239 S¼Osymstretching

1135 1152 1341 C—Fstretching

1237 1196 1386 C—FsymbendinginNTf2(umberallabending)

1276 1285 1440 S¼Oasymmetricstretching

1441 1488 1606 ScissoringofH—C—H

2873 3015 3162 symC—Hstretchinginbutylgroup

2945 3089 3241 asymC—Hstretchinginpiperidiniumring

(9)

forPIP14BrandPIP14NTf2intheﬁngerprintandC—Hregionsare

showninFigs.12and13,respectively.Whileinsigniﬁcantshiftis

observed in the ﬁngerprint region, the C—H region shows

signiﬁcantshiftsbetweenPIP14BrandPIP14NTf2ILsasevidentin

Fig.13.Blueshiftof(9cm1)isobservedforPIP14Brcomparedto

that of PIP14NTf2. This vibrational spectroscopic result clearly

indicatesthatthestrengthofhydrogenbondisstrongerincaseof PIP14Br when compared to PIP14NTf2. This result is also in

agreement with ourprevious report for similarderivatives but

withimidazoliumcation[26].HobzaandHavlashavedescribed

thisblueshiftsolelyasduetotheincreaseinstrengthofhydrogen

bonding between cation and anion [61]. Further, the Raman

spectrum interestingly indicates a cis–trans equilibrium of

NTf2–anion(showninFig.10;thecorrespondingbandisindicated

by an arrow) in liquid PIP14NTf2. The existence of two closely

spacedbands(between380and450cm1)indicatesthe

coexis-tence of two conformational forms. Details of cisoid–transoid

conformationsarediscussedinSection3.5.

3.5.Existenceofcisoid–transoidequilibriumoftheanioninPIP14NTf2

The experimental Raman spectrum of PIP14NTf2 shown in

Fig. 10containstwocloselyspacedbandsduetovibrationalmotion ofNTf2intheregionof380–450cm1.TointerrogatethisRaman

signature, we here focus on the calculated and experimental

spectra below 700cm1.Ramanfrequenciescalculated for both cisoidandtransoidNTf2anioncontainingPIP14ILsusingDFT/B3–

L–Y–P/6–31++G(d,p)inthelow-frequencyregionaretabulatedin

Table5andthesamehasalsobeenplottedinFig.14a.Thebands areassignedbasedontheliterature[53]andonobservingrelative

displacementsoftheatomsusingGaussViewprogram.Theﬁgure

clearly depicts that some of the bands have been shifted

considerablyonchangeofconformationofNTf2,whilesomehave

differentintensities.Theout-of-planebendingofNinNTf2anion

isobservedat207cm1intransoidconformation,whileitisred shifted to 188cm1 in cisoid-PIP14NTf2. The band at 278cm1,

whichisassignedtotheSO2twistinginthetransoidform,isshifted

to 257cm1 in thecisoid form. TheSO2 scissoring observed at

324cm1inthetransoidformisshiftedto304cm1inthecisoid form.TheSO2 rockinginthetransoidconformationobservedat

388cm1movesto406cm1inthecisoidconformation.TheS–N–

S in-plane bending observed at 602cm1 in the transoid

conformationis shiftedto627cm1in thecisoidconformation. Otherbandsarefoundtoshowamaximumshiftofabout10cm1.

Fig.14bcomparestheobservedRamanspectrum(solidcurve) withthecisoid(blueverticalline)andtransoidPIP14NTf2(green

vertical line) DFT-calculated frequencies in the 380–420cm1

region. This region represents Raman bands of the cisoid and

transoidconformersofNTf2,asdescribedinseveralliteraturesfor

imidazoliumbasedILs[43,53].Furthertwocloselyspacedbands, at617and651cm1,thathavebeenobservedexperimentallyare identiﬁedasscissoring ofS–N–Sintransand cisconformations,

respectively, with the help of DFT calculation. As there is no

calculated band present between 603_–701cm1 for trans

Fig.11.CorrelationdiagramforRamanspectrumofPIP14NTf2:experimentalversus

calculated Raman transition frequencies. (DFT and H–F methods). Scaling factors=0.966(B–3L–Y–P)and0.915(H–F).

Fig.12.ExperimentalRamanspectraofPIP14BrandPIP14NTf2(ﬁngerprintregion).

Fig.13.ExperimentalRamanspectraofPIP14BrandPIP14NTf2(C—Hregion).

Table5

ImportantRamanbandsobservedincisoid–transoidconformationofPIP14NTf2

anion.

transoidform cisoidform Assignmentofbands DFTn(cm1) DFTn(cm1)

207 188 OutofplanebendingofN

278 257 TwistingofSO2

324 304 SO2scissoring

388 406 RockingofSO2

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conformerandoneextrabandisobservedexperimentallyinthis region,itcanberecognizedforitsoriginfromthecisconformation fromDFTcalculation.DFT-calculatedRamanbandsinthisregion correspondtothewaggingmodesofSO2at388cm1fortransoid

and 406cm1 for cisoid conformations. The Raman frequency

calculation using DFT produces no vibrations between 389–

455cm1fortransoid-PIP14NTf2 andbetween379–405cm1for

cisoid-PIP14NTf2,asclearlyseeninFig.14a.Hence,weconﬁrmthat

NTf2intheliquidstateofPIP14NTf2consistsoftwoconformersin

equilibrium.ThefrequenciesweobtainedfromDFTcalculationfor theSO2 wagging is consistent with the literatureavailable for

imidazoliumcation based NTf2 ILs [43,53]. These transoid and

cisoidmarkerbandsarereportedtoappearat397and405cm1for hmimNTf2,almostatthesamepositionasinemimNTf2[53].

4.Conclusions

In N-butyl-N-methylpiperidiniumbis(tri

ﬂuoromethanesul-fonyl) imide (PIP14NTf2) optimized structure, the piperidinium

ringwasfoundtoexistinachairform,whichisthemoststable

conformation,when compared withboatand twist-boatforms.

Also the butyl group was observed to be stable in gauche

conformationwithrespecttothepiperidiniumring.InbmimBr,

noH-bondingwasobservedbetweentheanionandalkylchain.In contrast,inPIP14Br,twoH-bondingwiththealkylchainalongwith

one H-bonding with a hydrogen on the piperidine ring was

observed. Hence, this higher number of H-bonding present in

PIP14Bris one of themain reasons toits higher melting point

comparedtoimidazoliumanalog,bmimBr.Theinteractionenergy

forPIP14Brwas estimatedtobehigher than that for PIP14NTf2,

showinga positive correlation betweeninteraction energy and

meltingpoint.DFTcalculationgeneratedthemoststablegeometry forPIP14NTf2whencomparedwiththreedifferent(DFT,MP2,and

H–F) methods. The transoid conformation of NTf2 anion was

foundtobemorestablethanthecisoidconformationby1.04kcal/

mol. Interestingly, the experimental Ramanspectrum of liquid

PIP14NTf2 (between 380 and 450cm1) clearly indicates the

existenceofcisoid–transoidconformationalequilibriumofNTf2.

DFTcalculationpredicts the Raman bandarising from theSO2

waggingtoappearat388cm1(transoid)and 406cm1(cisoid).

Blue shift in C—H stretching frequency has been observed for

PIP14Br which clearly indicates stronger hydrogen bonding in

PIP14Br compared to PIP14NTf2 IL. Further, the existence of

rotamersoftheanioninPIP14NTf2mightaccountforthedrastic

decreaseinmeltingpointwhencomparedwithPIP14Br.

Acknowledgements

Financial assistance from DRDO (ARMREB/HEM/2012/140),

NewDelhi,Indiaisgratefullyacknowledged.Authorsalsothank

BHUandHyderabadCentralUniversity,Hyderabad,forproviding

computationalfacility.MLSthanksUGCforprovidingfellowship. SSwouldliketothanktheEditorforconstructivesuggestionsand Prof.D.Michalskaforhelpanddiscussionregardingcalculationof Ramanintensities.

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