Raman Characterization of Phenyl-Derivatives: … L, et al. Raman Characterization of Phenyl-Derivatives: From Primary Amine to Introduction Diazonium Salts. J Org Inorg Chem. 2017,

2017Vol. 3 No. 1: 1

Research Article

DOI: 10.21767/2472-1123.100021

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Journal of Organic & Inorganic ChemistryISSN 2472-1123

Betelu S1, Tijunelyte I2, Boubekeur-Lecaque L3, Ignatiadis I, Schnepf AC1, Guenin E2, Bouchemal N2, Felidj N3, Rinnert E4 andLamy de la Chapelle M2

1 BRGM, Water Environment and Eco-Technology Division, F-45060 Orléans Cedex 02, France

2 University Paris 13, Sorbonne Paris Cité, CSPBAT laboratory, UMR 7244 CNRS, UFR SMBH, 74, Rue Marcel Cachin, 93017 Bobigny, France

3 University Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, 15 rue J-A de Baïf, 75205 Paris Cedex 13, France

4 IFREMER,BrittanyCenter,Measurements,DetectionandSensorsLaboratory, CS10070, 29280 Plouzané, France

Corresponding author: Betelu S

[email protected]

Water Environment and Eco-Technology Division, F-45060 Orléans Cedex 02, France.

Tel: 0033238643268Fax: 0033238644797

Citation: Betelu S, Tijunelyte I, Boubekeur-LecaqueL,etal.RamanCharacterizationofPhenyl-Derivatives:FromPrimaryAminetoDiazonium Salts. J Org Inorg Chem.2017,3:1.

IntroductionAryl-diazonium derivatives (ADD) are a class of highly usefulreaction intermediates, or reagents. Their salts are generallyobtained from the diazotization of aromatic amines in thepresence of tetrafluoroboric acid, hexafluorophosphate orhexafluoroantimonate[1].Aryl-diazoniumsaltsarewidelyusedin organic chemistry as reactants for different syntheses. Forinstance, the process of nitrogen elimination from diazoniumcationsisafundamentalstageoftheMeerwein[2]andSandmeyer[3,4]reactions.Forbothofthesereactions,improvingtheyield

depends on the application of diazonium salts that effectivelylosedinitrogen(N2).

The rise in popularity of aryl-diazonium salts has further resultedfromtheirefficiencyinsurfacefunctionalization,wherecovalently attached coatings on (semi)-conducting materialsbearawide rangeof functional groups [5-14]. Inaddition, thegrafting canbe accomplishedby either chemical (spontaneousgrafting),electrochemical,orphysicalmethods[5-12,15].Surface

Raman Characterization of Phenyl-Derivatives: From Primary

Amine to Diazonium Salts

AbstractTheobjectiveofthepresentworkistouseRamanspectroscopyforcharacterizing,thefateofphenyl-derivatives,fromphenyl-aminestoaryl-diazoniumderivatives(ADD). Four ADD were investigated: (i) benzene diazoniumtetrafluoroborate(DS), (ii) 4-decyl benzene diazoniumtetrafluoroborate (DS-C10H21), (iii)4-carboxybenzenediazoniumtetrafluoroborate(DS-COOH)and(iv)4-(aminoethyl)benzene diazoniumtetrafluoroborate (DS-(CH2)2NH2). Raman investigation oftheaboveADDconfirmedtheexistenceofanN≡Nbondstretchingintherangeof 2285-2305 cm-1. Moreover, the strong band related to CH in plane-bending and C-N-stretching modes in the 1073-1080 cm-1 range, is a signature of phenyl derivativesstemmingfromADD.Furthermore,weanalyzedanddiscusstheH-N-(ring) symmetric stretchingmodes and the ring-N, aswell as the benzene-ringvibrationalmodes,theC-Hrelatedvibrationsandthefunctionsinpara-positioncarriedbythearomaticring.Theeffectofstructuralchanges,theconformationalrearrangementsfromaminestoADDandtheinfluenceofthesubstituentlocatedin the para-position on Ramanmodes,were examined aswell. Finally, RamanexperimentssupportedbyDensityFunctionalTheory(DFT)modelingallowedusto determine the crystalline structure of DS-COOH.

Keywords: Phenyl-amines; Diazonium salts; Synthesis and characterization;Raman;DFTcalculations

Abbreviations: ADD:Aryl-diazoniumderivatives;DS:Benzenediazoniumtetrafluo-roborate; DS-COOH: 4-Carboxybenzene diazoniumtetrafluoroborate; DS-C10H21:4-decyl benzenediazoniumtetrafluoroborate; DS-(CH2)2NH2: 4-(aminoethyl) ben-zenediazoniumtetrafluoroborate; DFT: Density functional theory; NBO: Naturalbond orbital.

Received: February 17, 2017; Accepted: February 28, 2017; Published: March 03, 2017

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functionalization using diazonium salts has thus become oneof the most powerful methods for coatings [8,13,14] throughlinking(bio)molecules[16,17],polymers[18-20]ornanoparticles[21]. The resultinghigh-performancematerials show improvedchemicalandphysicalpropertiesandfindwidespreadapplication[22].

Whatever the diazonium salt application, identification ofthe molecular bonds is of great importance for investigating,improving and/or controlling reaction pathways. Raman hasbeen selected as one of the most accurate spectroscopic techniquesfordeterminingspecificvibrationalmodesandthusforidentifyingthemolecularbondsandstructures.

Inthiswork,fourADDweresynthetizedandinvestigated:

-benzenediazoniumtetrafluoroborate(DS),

-4-carboxybenzenediazoniumtetrafluoroborate(DS-COOH),

-4-decylbenzenediazoniumtetrafluoroborate(DS-C10H21),and

- 4-(aminoethyl) benzene diazoniumtetrafluoroborate (DS-(CH2)2NH2).

Thechoiceofaryl-diazoniumsaltswasfurthermotivatedbythedevelopmentofrobust“long-life”(bio)sensors,astheycanbeused for surface functionalization in order to provide accuratepre-concentrationof(bio)organiccompoundsforsensitiveandreproducible sensing. The physical and chemical propertiesof the salts related to their interaction ability are summarizedhereafter.DSisthesimplestsalt;itwaschosenasareferenceandbecauseoftheavailabilityofthearomaticcycleforπ-πstacking.DS-C10H21hasalongnon-polaralkylchain,whoselipophilicandhydrophobic character serves to pre-concentrate non-polar molecules. DS-COOH and DS-(CH2)2NH2 can both be used for couplingbiomolecules,orforpre-concentratingpolarmolecules.

We used Raman spectroscopy for characterizing the fate of thesefourphenyl-derivatives,fromphenyl-aminederivativestoADD.OfparticularinterestareN≡Ngroupvibrations,H-N-(ring)symmetric stretching modes, ring-N as well as benzene-ringvibrational modes, C-H related vibrations, and para functionscarried by the aromatic ring, all of which are analyzed anddiscussed.Forsomebands,weexaminedtheeffectofstructuralchanges as well as of conformational rearrangements, fromaminetodiazoniumsalt.Theinfluenceofthesubstituentlocatedintheparapositionisdiscussed,especiallyintermsofthenatureandforceofthesubstituent[themesomericeffect(-M)providedbyCOOHaswellastheinductiveeffect(-I)providedbyNH2 and CH3 carried by CH2CH2NH2 and C10H21substituents].TheselectionofthesefourADDjustifieswhythemesomericeffectinducedbyelectron-donatinggroups(+M)hasnotbeeninvestigatedinthepresentwork.

In addition, Density Functional Theory (DFT) calculations andNatural Bond Orbital (NBO) charges were performed for DS-COOH in order to conduct the band assignment and to strengthen Raman-spectra interpretation.

Materials and Methods ReagentsThereagentsusedweresodiumnitrite(NaNO2),tetrafluoroboricacid(HBF4),diethylether(>98%,ACSreagent),aniline,4-docyl-aniline,4-aminobenzoicacid,4-(2-aminoethyl)-aniline,purchasedfrom Sigma Aldrich Chimie S.a.r.l (St. Quentin Fallavier, 38297France).

Synthesis and purification of diazonium saltsDiazoniumsaltswereobtainedbyoxidationofthecorrespondingaryl-amine at 0°C and using sodium nitrite [7]. For the fourdifferent diazonium salts, 4mmol of the corresponding amineweremixedwith2mLoftetrafluoroboricaciddissolvedin7mLofmilli-Qwater.Themixtureswerethencooledinicefor15min(1hourforthatcontaining4-docyl-aniline).Afterwards,asolutionof300mg(600mgforthemixturecontaining4-docyl-aniline)ofsodiumnitritedissolved in less than1mLofmilli-Qwaterwasaddeddrop-by-drop.During the reaction the temperaturewasmaintainedat0°C.Aftertwohoursofreaction(96hoursforthemixture containing 4-docyl-aniline), the mixture was filteredthrough 0.2 μm cellulose ester filters (Whatman, France) andthoroughlywashedwithcolddiethylether.

Diazonium salt purification consisted in dissolving the crude solid in a small amount of deionized water, followed byrecrystallizationindiethylether.Therecrystallizationprocedurelasted for48hours at 6°C.Recrystallizeddiazonium saltswerethenfiltered(0.2µmofcelluloseesterfilters,Whatman,France),dried and stored at -20°C.

Raman measurementsRaman measurements were recorded on pure chemicals (aspowderunlessotherwise stated)witha LabramHR800Ramanmicrospectrometer (HORIBA Jobin Yvon SAS, 59650 Villeneuved'Ascq,France).Forbothaminesanddiazoniumsalts,arobustlaserdiodeat691nm(Ondax)thatissuitableformanyRamanapplications [23] was used to avoid molecular resonance. Fordiazoniumsaltsandthe4-aminobenzoicacid,thescatteredlightwascollectedbya100xobjectivewithanumericalaperture(N.A.)of 0.9. For liquid compounds (aniline, 4-(2-aminoethyl) anilineand 4-decylaniline), a water immersion objective (100x,N.A.=1.0)wasused.Spectralresolutionwaslessthan2cm-1 and spectral calibration was performed daily via a silicon sample.All presented spectrawere baseline corrected and normalizedusing the spectrometer software (LabSpec, HORIBA JobinYvon).Eachspectrumwasnormalizedtoitsmaximumintensity.For calculations, spectra were normalized regarding the C=Cstretchingmode[24]observedbetween1570and1611cm-1.

Nuclear Magnetic Resonance (NMR) measurementsSynthesizeddiazoniumsaltsandtheirprimaryamineswerealsocharacterized by NMR measurements. Spectra were recordedusing aBrukerAvance III 400MHz instrument (BrukerUK Ltd,Coventry CV4 9GH, United Kingdom). The procedure, NMR

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spectra and 1Hchemical shiftsarepresented in theSupportingInformation.Thestructureofthesaltsaswellastheindexationof protons is presented in Scheme 1. RMN spectra of both amines and diazonium salts are presented on Figure S1 in SupportingInformation.

Computational detailsAll calculations were carried out using the Gaussian 09 suiteof programs [25] using the B3LYP exchange and correlationfunctional [26,27] along with the 6-311++G(d,p) basis set forall atoms. The structures were optimized without symmetryconstraint (see Supporting Information). The vibrationalwavenumbers and normalmodeswere calculatedwithin theirharmonicapproximation.Ascalingfactorof0.976waschosenonthebasisofpreviouslypublishedwork[28].

Results and Discussion Figures 1-4 compare Raman spectra obtained for A) primaryaminesandB)correspondingsynthesizeddiazoniumsalts.

ThewavenumbersofimportantpeaksarereportedinTable 1 and theirassignmentswereprovidedfromtheliterature[24,29,30].Thespectrumobtainedforaniline is inagoodagreementwiththeoneprovidedbyBadawi[24];mostoftheirbandassignments

Structures of amine and diazonium salt withtheindexationofprotons.

Scheme 1

Raman spectraof aniline (black spectrum) and thecorresponding diazonium salt benzene diazonium tetrafluoroborate,DS(redspectrum).Eachspectrumwasnormalizedtoitsmaximumintensity.

Figure 1

Ramanspectraof4-aminobenzoicacid(blackspectrum)and the corresponding diazonium salt 4-carboxy benzene diazoniumtetrafluoroborate,DS-COOH(redspectrum).Eachspectrumwasnormalizedtoitsmaximumintensity.

Figure 2

Raman spectra of 4-(2-aminoethyl) aniline (blackspectrum) and the corresponding diazonium saltDS-(CH2)2NH2 (red spectrum). Each spectrum wasnormalized to its maximum intensity.

Figure 3

Raman spectra of 4-decylaniline (black spectrum) andthe corresponding diazonium salt 4-decyl benzene diazoniumtetrafluoroborate,DS-C10H21(redspectrum).Eachspectrumwasnormalizedtoitsmaximumintensity.

Figure 4

wereusedasreferenceforthiswork.

In addition, interactions can occur between two salts in acrystalline state, especially for DS-COOH because carboxyphenyl-

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Raman wavenumber/cm-1 AssignmentAmines DS

Aniline COOH (CH2)2NH2 C10H21 DS DS-COOH DS-(CH2)2NH2 DS-C10H21

386 Aromaticring-Ninplanebending[24]619 638 643 644 617 637 640 639 Ringdeformation[24]755 743 pCHwagging[24,29]814 828 822 822 796 820 819 o;mCHwagging[24,29]827 845 845 844 814 849 837 Ringbreathing[24]

9951027

9971021

Ringdeformation+ringbreathingforbenzeneandmono-substitutedderivatives[24,29]

CCstretchingvibration(coupledwithCHinplanebendingvibration)[30,31]

1067 1062 1082 1079 C-Cstretching(aliphaticchain)[29]

1073 1075 1076 1080CHinplanebendingforp-substitutedandmono-substitutedbenzenes[12,29]coupledwithC-N

stretching1154 1142 m;pCHinplanebending[24,29]1175 1179 1177 1179 1174 1170vw 1198vw 1190vw m;oCHinplanebending[24,29]

1279 1286 1279 1265 Ring-Nstr+ringbreathing+o-CHinplanebending[24]

1469 1459 1454 1456 CH2 scissoring[32]1602 1600 1611 1611 1570 1591 1588 1585 C=Cstretching[24]

1748 17081732 C=O stretching

2296 2305 2285 2294 N≡Nstretching[24,29]

2813-2986 2813-2986 2813-2986 2813-2986 SymmetricandantisymmetricCHstretchof

n-alkanes[29]3053-3071

3049-3063 3014-3051 3016-

3057 3087 3096 3076 30663077 AromaticCHstretchofbenzenederivative[24,29]

Table 1 Universal band assignments for Raman spectra obtained experimentally for primary amines and for the corresponding diazonium salts.

derivativesarebothH-bondingdonorandacceptor.Thus,theypreferentially self-organize as dimers in the crystalline state[33-35]. To investigate thedimer formationandobtain a clearassignmentoftheDS-COOHRamanbands,DFTcalculationsandNaturalBondOrbital(NBO)chargeswerecarriedout,examiningtwomodels. The first one consideredmonomer units (namedCBN2) whereas the second one considered DS-COOH dimers(named CBN2 dimer). Figure 5A shows a comparison betweentheRamanspectrumforDS-COOHandthecalculatedvibrationalfeaturesof themonomer (CBN2) and thedimeric arrangementvia H-bonds (CBN2 dimer). The Raman band assignments ofimportantpeaksareshownonTable 2. The crystalline state of DS-COOH regarding the CBN2 and CBN2 dimer models is discussed over the presentation of the different investigated modes ofphenyl-derivatives.

N≡N group vibrationsTheparticularityofdiazoniumsaltsistheirN2

+functionobtainedby oxidation of the corresponding amine. The strong peaksobserved at 2296, 2305, 2285 and 2294 cm-1 for DS, DS-COOH, DS-(CH2)2NH2 and DS-C10H21, respectively, (Figures 1-4; Table 1)on the salt spectra compared to aniline ones, are due to the N≡Nbond[12,29].Comparedtotheexperimentaldataacquiredforeachcompound,theDFTcalculationsonDS-COOHrevealedstrongsimilaritiesinthisspectralrange(Figure 5, Table 2). The calculated position for theN≡N stretchingmodewas found at2293 cm-1 for CBN2 and at 2304 cm-1 for CBN2 dimer. The similarity

between the latter theoretical position and the experimentalone(theDS-COOHN≡Nstretchingmodeisrecordedat2305cm-

1)strengthensthefactthatthebestmodelforreproducingthevibrational features of the normal Raman spectra is a dimericarrangementofthediazoniumcationviaH-bonds.

According to the literature [29,36], the position of the N≡Nvibrationmodedependsuponthenatureoftheringsubstituentlocatedinthepara-positionofadiazoniumsalt.OurresultsshowthatsubstituentshavearatherloweffectontheN≡Nfragmentfollowing the observedwavenumber shifts (Table 1; Figure S2 in Supporting Information). ForDS-COOH, however, theCOOHelectron-withdrawingtendencywasconfirmedbycalculationofthe CN≡N NBO charges. Indeed, the CN≡N(DS-COOH)NBO charge is equal to 0.012 compared to the CN≡N(DS)NBO charge that is 0.003. As a result,ashiftof+9cm-1of theN≡Nvibrationmodewas foundfor DS-COOH relative to DS. The electron-withdrawing groups(i.e.,COOH)induceanincreaseofthecontributionofastructurelike N+ N N+ N-+and andhenceinduceashiftoftheN≡Nstretchingvibrationtohigherwavenumbers[29,36].Thisagreeswith theN≡Nbond-lengthdecrease [36]and isalsoconsistentwith thepolarizability increaseof theN≡Nbond.ExperimentaldatashowinfactthattherelativeintensityoftheN≡Nbondisincreased by 23% for DS-COOH compared with that obtainedfor DS, DS-C10H21 and DS-CH2CH2NH2 (Figure S2 and Table S1 in SupportingInformation)

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On the contrary, electron-donating groups located in para-positionsinduceanincreaseinthecontributionofastructurelike

N+ N N+ N-+and and,hence,adecreaseoftheN≡Nstretchingvibration wavenumber. Such shifts are effectively observed inourexperimental results since shiftsof -11 cm-1 and -2 cm-1 of theN≡N vibrationmodewere evaluated for DS-(CH2)2NH2 and DS-C10H21, respectively, compared to that observed for DS. Ofthe two electron-donating groups CH3 and NH2, NH2 has the higher inductive effect [NH2 substituent constants (σp)=-0.66;CH3 substituent constants (σp)=-0.17] [37,38]. Moreover, theinductiveeffectdropsoff rapidlywith thenumberofσbonds.Thisiswhy:

(i) The negative shift in wavenumber observed for N≡Nvibration forDS-C10H21 is not significantly different fromthe one observed for DS, andwhy

(ii)The negative shift in wavenumber observed for N≡NvibrationofDS-(CH2)2NH2 is higher than that observed for DS-C10H21.

H-N-(ring) symmetric stretching modeInadditiontotheoccurrenceofanN≡Nstretchingvibration,theN-H symmetric stretching disappears in the highwavenumberrange 3250-3400 cm-1 for primary amines (Figure 6). This indicates thesuccessfulsynthesisofdiazoniumsaltswithahighproductionrate,whichwasconfirmedbyNMRstudywiththedisappearanceoftheηbandontheNMRspectra(SupportingInformation).Eachspectrumwasnormalizedtoitsmaximumintensity.

Ring-N vibrational modesAccording toBadawi [24] andWojciechowski [30], thepeakat386 cm-1 fortheanilinemolecule(Figure 1), corresponds to planar

ComparisonoftheexperimentalRamanspectrumrecordedforDS-COOHandthecalculatedvibrationalfeaturesofthesalt,byconsideringmonomer(CBN2)anddimericarrangementsviaH-bonds(CBN2 dimer).

Figure 5

DS-COOH CBN2 CBN2 dimerExperimental/cm-1 Calculated/cm-1 Assignment Calculated/cm-1 Assignment

2305 2293 v(N≡N) 2304 v(N≡N)1778 v(C=O)

1708 1684 v sym(C=O)1732 × v asym(C=O)1591 1588 v(C=C) 1590 v(C=C)

1293wwv 1344 v(CO-H) 1271 v sym(CO-H)1165 v(C-H)inplaneandv(CO-H)

1127ww 1122 v(C-H)inplane

1075 1093 v(C-H)inplane,v(C-N)andv(C-O) 1090 v(C-H)inplane,v sym(C-N)

814 797 Ring breathing 807 Ring breathing

Table 2ComparisonofexperimentalandcalculatedwavenumbersconsideringCBN2 and CBN2-dimer for modeling. Assignments of the main peaks. × Calculated CBN2 dimerfeaturesacenterofsymmetryimposingthemutualexclusionruleforvibrationalmodes.Inthisframework,thevsym(C=O)isRamanactivewhilevasym(C=O)isRamaninactive.

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bending ofanaromaticring-N.However,thisvibrationalmodeislessobviousforpara-substitutedamines.Apossibleexplanationis that aromatic-ring-vibration modes depend on the mass ofthe substituent [29].Thus, in the rangeof200-580cm-1, many vibrationalmodescanbeobserveddependingonthenatureofthe para-substituted group (Figures 2-4). The same conclusion is valid for the range of 1250-1290 cm-1, another rangewherea band corresponding to C-N vibration can be located. Ramanbands for the amines observed between 1265 and 1279 cm-1 (Table 1) and assigned to ring-N stretching coupled to ring breathing and to ortho-CH in plane bending [24], is no longerseen in the diazonium salt spectra.

This effect may be due to significant structural changes andconformational rearrangements, as the theoretical andexperimental data provided by literature indicate that the neutralanilineinitsgroundelectronicstate(1A1)isnon-planar.ThedihedralanglebetweentheNH2 plane and the C6H5N plane wasdeterminedtobe37±2°,assumingthattheC-Nbondmakesan angle of 1.5-2.3° [30]. In comparison, the correspondingdiazonium salt (aryl-diazonium) is a quasi-planar molecule[39], assuming that theC-Nbondmakesanangleof0.5° [40].The main factor causing this quasi-planarity is undoubtedly the strong through-bond interaction between the ring, which isknowntobeastrongelectrondonor,andtheN2group,whichisthestrongestknownelectronacceptor[39].Inagreementwiththeliterature[30,39,41],thisbondlengthenssothatco-planarityismaintained.Anadditionalexplanationcouldberelatedtotheelectron-charge density redistribution in the C-N bond. This isconsistentwiththeobservationthatthediazoniumgroupisoneofthemostpowerfulσ-electronandπ-electronacceptors[41].

Benzene-ring and C-H related vibrationsBy comparing Raman spectra from amines and diazonium salts, several common bands related to benzene-ring vibrations canbe observed. First of all, a strong band related to the benzene-ring stretching mode (Table 1) occurs on all Raman spectra in the range of 1585-1611 cm-1.AccordingtoWojciechowski[30],thisbandisessentiallyrelatedtoortho-metaC-Cstretchingmodes.Fromphenyl-aminederivatives todiazoniumsalts, thisband isthemostsignificantsignatureofthebenzenering.Comparedtotheprimaryamines,thisbandisshiftedtoalowerwavenumberforallsalts.Theseshiftsarecloseto25cm-1 for salts except DS-COOH,maybeduetothehigherweightoftheN2 group compared to NH2thatinducesadecreaseofthevibrationfrequency.InthecaseofDS-COOH,theshiftoflessthan10cm-1 may be due to the dimericformofthesaltthatreducestheweighteffectoftheN2.

Two strong and distinct peaks again assigned to the ring-deformation and ring-breathing modes-coupled with CH inplane-bending vibration of benzene and of mono-substitutedderivatives,arefoundintheRamanspectrumofanilinearound1000 cm-1 (Figure 1; Table 1, trigonal ring breathing vibration)and 1027 cm-1 (Figure 1; Table 1)[30,31].ForDS,thesebandsareslightlyshiftedto997 and 1021 cm-1.Incaseofpara-substitutedaminesanddiazoniumsalts(di-substitutedbenzenederivatives)thesevibrationalmodesarenotallowed[29].

Bandsassignedtoringdeformation(C=C-Cinplanedeformation)in the region of 617-644 cm-1 (Table 1) are present for both amines anddiazoniumsalts.InagreementwithSocrates[29],thebandsshift to a higher wavenumber for para-substituted aromaticcompoundscomparedtomono-substitutedcompounds.

According to Badawi [24], the vibrational mode at 827 cm-1 (Table 1) for aniline can be assigned to a ring-breathing mode, coupledwithring-Nstretchingandringdeformation.Thisbandis shifted to higher wavenumbers (844-845 cm-1) for para-substitutedamines.However,shiftsof-31cm-1 and -7 cm-1towardlower wavenumbers are observed for DS-COOH and DS-C10H21 respectively,comparedtotheirprimaryamines,whereasintheRamanspectrumofDS-(CH)2NH2ashiftof+4cm

-1wasfound.Incommonforallsubstituteddiazoniumsalts,theintensityofthisvibrationalmodestronglydecreasesand isno longerobservedin the Raman spectrum of DS. Another vibrational mode at814 cm-1assignedo,m-CHwagging(Table 1 and Figures 2-4)wasfound to behave in similar manner as described above for the ring-breathingmode. The shift of these bandsmay be due tostructural changes and conformational rearrangements caused by the presence of the N2

+ [39,41].

Concerning the C-H ring related vibrations, bands assigned top-CHwagging-coupledwithring-Nwaggingando-CHwaggingat755 and 743 cm-1 (Table 1)aswellasm,p-CHbending-coupledwithringdeformationat 1154 and 1142 cm-1 (Table 1) are only observedforanilineandDS,duetomono-substitution.

The very strong band observed around 1075 cm-1 and assigned toCHinplanebending-coupledwithring-Nstretchingmode for mono-andpara-substitutedbenzenesindiazonium-saltspectra,

Comparison of aromatic C-H stretching and N-Hstretching modes between the four synthesizeddiazonium salts (B red lines) and the correspondingprimary amines (A black lines). Roman numbers I toIVcorrespondtoDS,DS-COOH,DS-(CH2)2NH2 and DS-C10H21,respectively.

Figure 6

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isofparticularinterest(redspectraonFigures 1-4; Table 1).Theappearance of this band is an astonishing phenomenon, explained by electron-charge density redistributionwithin the structurescaused by the presence of N2

+. It is also the indisputable signature ofphenyl-derivativesstemmingfromdiazoniumsalts.

According to the literature, aromatic C-H symmetric andantisymmetricstretchingmodesoccurabove3000cm-1 andshowamultiplicity of weak tomoderate bands [42]. In comparisonwith amines, CH stretching (Table 1) shifts toward the higherwavenumber fordiazoniumsalts (Figure 6).Thisshift indicatesastrengtheningoftheC-Hbond,possiblyduetoanewchargedistributionontheringafterthesubstitutionofNH2 by N2

+[30].Thisresult is ingoodagreementwiththeonesintheliteratureforanilineanditscorrespondingdiazoniumsalt[30,39,41].

Functions in para- position carried by the aromatic ringConcerningthedifferentfunctionsinpara-positioncarriedbythearomaticring(COOH,(CH2)2NH2 and C10H21),somecharacteristicbands have been identified. For (CH2)2NH2, C10H21 and their corresponding diazonium salts, symmetric and antisymmetricCH3stretchesofn-alkanesaswellassymmetricCH2 stretches of n-alkanes are clearly identified in the rangeof 2813-2986 cm-1 (Figures 3 and 4; Table 1).Inaddition,C-Cstretchingmodeoccursin the range of 1062-1082 cm-1 (Table 1) and CH2 scissoring in the range of 1454-1469 cm-1 (Table 1).Somesignificantchangesin peak position or intensity occur, meaning that the simplemodificationofthemoleculefromanilineformtodiazoniumsaltcouldhave a strong influenceon the vibrationalmodesof thealkylchaininpara-position.

Concerning aminobenzoic acid derivatives (Figure 2; Table 1), theweakbandsinthe1700-1750cm−1regionarecharacteristicfeaturesofthecarboxylicgroupduetoC=Ostretchingvibration.Theywererespectivelyassignedtosymmetricandantisymmetricstretchingvibrationsat1708and1732cm-1.Theantisymmetricstretchisusuallyseenatahigherwavenumberthanthesymmetricone[43].WithregardstoDFTcalculations(Figure 5; Table 2), the CBN2-model simulated spectrum predicts a single C=O stretching mode, v(C=O)CBN2, at 1778 cm-1. This DFT numerical modeling is ingoodagreementwiththeliterature[29]:whencarbonyldoesnot interact with a hydrogen bond (the monomer case), thestretching bands appear in the range 1760-1735 cm-1.However,thedimershouldbecharacterizedbytwoC=Ostretchingmodes,thesymmetricC=Oone(vsym(C=O)CBN2 dimer)at1683cm

-1 and the antisymmetric C=O one (vasym(C=O)CBN2 dimer) at 1729 cm

-1. With regard to Raman calculations of the C=O stretching mode inthe CBN2 model(vsym(C=O)CBN2 at 1778 cm-1),thetheoreticalshifttoward the lowerwavelengthof thesymmetricC=Ostretchingmode in the dimer [vsym(C=O)CBN2 dimer at 1683 cm-1] is due to achange in polarizability of the CO group induced by hydrogen bonding. The simulated Raman spectrum for CBN2 dimer only showsthislattersymmetricmode[vsym(C=O)CBN2 dimer]at1683cm

-

1.ThisconcurswiththefactthattheCBN2 dimer features a center of symmetry imposing a mutual exclusion rule for vibrationalmodes.Inthisframework,thevsym(C=O)CBN2 dimerisRamanactive,

while vasym(C=O)CBN2 dimerisRamaninactive.ThismutualexclusionruleexplainswhythesimulatedRamanspectrumforanisolatedCBN2 dimeronlyshowssymmetricC=Ostretching.

However,thesamedimerconsideredinthecrystalpackinghasalowersymmetrythatsuspendsthemutualexclusionrule,bothv sym(C=O)CBN2 dimer and vasym(C=O)CBN2 dimerbecomingRamanactive.ThisisconsistentwiththetwopeaksontheexperimentalRamanspectrumofDS-COOHforboththesymmetricandantisymmetricC=O stretchingmodes. The shift toward ahigherwavenumberobserved experimentally for symmetric C=O stretching from the theoretical vsym(C=O)CBN2 dimer value calculated at 1683 cm-1

to the experimentally measured vsym(C=O)DS-COOH value at 1708 cm-1 is probably caused by the presence of BF4

- in the crystalline structure of DS-COOH. This agrees with the effect providedby electronegative atoms, which increase the C=O stretchingvibrationwavenumber[44,45].InadditiontoandinagreementwiththeresultsshownonTable 2,thesimilaritiesbetweentheexperimental and the theoretical position of v(N≡N), v(C=C),ring breathing and v(C-H) in plane, vsym(C-N), vsym(CO-H) aswell as v(C-H) in plane confirm the fact that the best modelfor reproducing the vibrational features of the normal Ramanspectra is a dimeric arrangement of DS-COOH via H-bonds.

ConclusionsTheworkconsisted inaRamancharacterizationofsynthesizeddiazoniumsalts,comparingthemwithRamanspectraprovidedforamines.RamaninvestigationofsynthesizeddiazoniumsaltsconfirmedthestretchingoftheN≡Nbondintheexpectedspectralrange of 2285-2305 cm-1 (2296, 2285, 2305, and 2294 cm-1 for DS, DS-COOH, DS-(CH2)2NH2 and DS-C10H21, respectively). ThedisappearanceoftheH-N-(ring)symmetricstretchingobservedin the range 3299-3361 cm-1 for the primary amines, is another evidence of the effectiveness of salt synthesis. Moreover,our results indicate that the very strong band related to the combinationofbothCH in-planebending formono-andpara-substitutedbenzenesaswellasC-Nstretchingintherange1073-1080 cm-1,constitutesanactualspectralsignatureofthephenyl-derivativesstemmingfromdiazoniumsalts.

Furthermore, the fate of phenyl-derivatives has also beenexaminedbyusingRamanspectroscopy;thiscoveredN≡Ngroupvibrations, H-N-(ring) symmetric stretching modes, ring-N aswell as benzene-ring vibrationalmodes, C-H-related vibrationsandpara functions carriedby thearomatic ring.Weespeciallyconsidered both

1) Theeffectof structuralchangesand theconformationalrearrangements from amine to diazonium salt, and

2) Theinfluenceofthesubstituentlocatedinaparaposition,i.e.,thenatureandforceofthemesomericeffectofthesubstituent,ontheRamanmodes.

Comparison between Raman experiments and DFT modelingshowedthatDS-COOHself-organizedasdimersinthecrystallinestate.

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This paper reports on a preliminary study designed as a lead-in to further examinations for investigating, improving and/or controlling reaction pathways using synthesized diazoniumsalt(i.e.,toexamineandstrengthenthenatureoftheinterfacebetween Gold NanoStructures (GNS, electron-beam nano-lithographiedSERSsubstrates)andorganiccoatings)[46].

Author ContributionsAll authors have given approval to the final version of themanuscript. SB and IT contributed equally.

AcknowledgmentsThisworkwasfundedbyANRECOTECH(Productiondurableettechnologiesdel'environnement);REMANTASproject:EnhancedRamanscatteringforaquaticmedia:anewtechnologyforon-siteanalysis 2011-2014 (REMANTAS project; ANR-11-ECOT-0010).TheEnglishtextwasproofreadbyDr.HMKluijver.

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