Raman Characterization of Phenyl-Derivatives: From Primary Amine to Diazonium Salts

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

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

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

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

4IFREMER, Brittany Center, Measurements, Detection and Sensors Laboratory, CS10070, 29280 Plouzané, France

*Corresponding Author:
Betelu S
Water Environment and Eco-Technology Division
F-45060 Orléans Cedex 02, France
Tel: 0033238643268
Fax: 0033238644797
Email: [email protected]

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

Citation: Betelu S, Tijunelyte I, Boubekeur- Lecaque L, et al. Raman Characterization of Phenyl-Derivatives: From Primary Amine to Diazonium Salts. J Org Inorg Chem. 2017, 3:1.

 
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Abstract

The objective of the present work is to use Raman spectroscopy for characterizing, the fate of phenyl-derivatives, from phenyl-amines to aryl-diazonium derivatives (ADD). Four ADD were investigated: (i) benzene diazoniumtetrafluoroborate (DS), (ii) 4-decyl benzene diazoniumtetrafluoroborate (DS-C10H21), (iii) 4-carboxybenzene diazoniumtetrafluoroborate (DS-COOH) and (iv) 4-(aminoethyl) benzene diazoniumtetrafluoroborate (DS-(CH2)2NH2). Raman investigation of the above ADD confirmed the existence of an N≡N bond stretching in the range of 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 derivatives stemming from ADD. Furthermore, we analyzed and discuss the H-N- (ring) symmetric stretching modes and the ring-N, as well as the benzene-ring vibrational modes, the C-H related vibrations and the functions in para-position carried by the aromatic ring. The effect of structural changes, the conformational rearrangements from amines to ADD and the influence of the substituent located in the para-position on Raman modes, were examined as well. Finally, Raman experiments supported by Density Functional Theory (DFT) modeling allowed us to determine the crystalline structure of DS-COOH.

Keywords

Phenyl-amines; Diazonium salts; Synthesis and characterization; Raman; DFT calculations

Abbreviations

ADD: Aryl-diazonium derivatives; DS: Benzenediazoniumtetrafluoroborate; DS-COOH: 4-Carboxybenzene diazoniumtetrafluoroborate; DS-C10H21: 4-decyl benzenediazoniumtetrafluoroborate; DS-(CH2)2NH2: 4-(aminoethyl) benzenediazoniumtetrafluoroborate; DFT: Density functional theory; NBO: Natural bond orbital.

Introduction

Aryl-diazonium derivatives (ADD) are a class of highly useful reaction intermediates, or reagents. Their salts are generally obtained from the diazotization of aromatic amines in the presence of tetrafluoroboric acid, hexafluorophosphate or hexafluoroantimonate [1]. Aryl-diazonium salts are widely used in organic chemistry as reactants for different syntheses. For instance, the process of nitrogen elimination from diazonium cations is a fundamental stage of the Meerwein [2] and Sandmeyer [3,4] reactions. For both of these reactions, improving the yield depends on the application of diazonium salts that effectively lose dinitrogen (N2).

The rise in popularity of aryl-diazonium salts has further resulted from their efficiency in surface functionalization, where covalently attached coatings on (semi)-conducting materials bear a wide range of functional groups [5-14]. In addition, the grafting can be accomplished by either chemical (spontaneous grafting), electrochemical, or physical methods [5-12,15]. Surface functionalization using diazonium salts has thus become one of the most powerful methods for coatings [8,13,14] through linking (bio)molecules [16,17], polymers [18-20] or nanoparticles [21]. The resulting high-performance materials show improved chemical and physical properties and find widespread application [22].

Whatever the diazonium salt application, identification of the molecular bonds is of great importance for investigating, improving and/or controlling reaction pathways. Raman has been selected as one of the most accurate spectroscopic techniques for determining specific vibrational modes and thus for identifying the molecular bonds and structures.

In this work, four ADD were synthetized and investigated:

- benzene diazoniumtetrafluoroborate (DS),

- 4-carboxybenzene diazoniumtetrafluoroborate (DS-COOH),

- 4-decyl benzene diazoniumtetrafluoroborate (DS-C10H21), and - 4-(aminoethyl) benzene diazoniumtetrafluoroborate (DS- (CH2)2NH2).

The choice of aryl-diazonium salts was further motivated by the development of robust “long-life” (bio) sensors, as they can be used for surface functionalization in order to provide accurate pre-concentration of (bio) organic compounds for sensitive and reproducible sensing. The physical and chemical properties of the salts related to their interaction ability are summarized hereafter. DS is the simplest salt; it was chosen as a reference and because of the availability of the aromatic cycle for π-π stacking. DS-C10H21 has a long non-polar alkyl chain, whose lipophilic and hydrophobic character serves to pre-concentrate non-polar molecules. DS-COOH and DS-(CH2)2NH2 can both be used for coupling biomolecules, or for pre-concentrating polar molecules.

We used Raman spectroscopy for characterizing the fate of these four phenyl-derivatives, from phenyl-amine derivatives to ADD. Of particular interest are N≡N group vibrations, H-N-(ring) symmetric stretching modes, ring-N as well as benzene-ring vibrational modes, C-H related vibrations, and para functions carried by the aromatic ring, all of which are analyzed and discussed. For some bands, we examined the effect of structural changes as well as of conformational rearrangements, from amine to diazonium salt. The influence of the substituent located in the para position is discussed, especially in terms of the nature and force of the substituent [the mesomeric effect (-M) provided by COOH as well as the inductive effect (-I) provided by NH2 and CH3 carried by CH2CH2NH2 and C10H21 substituents]. The selection of these four ADD justifies why the mesomeric effect induced by electron-donating groups (+M) has not been investigated in the present work.

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

Materials and Methods

Reagents

The reagents used were sodium nitrite (NaNO2), tetrafluoroboric acid (HBF4), diethyl ether (>98%, ACS reagent), aniline, 4-docylaniline, 4-aminobenzoic acid, 4-(2-aminoethyl)-aniline, purchased from Sigma Aldrich Chimie S.a.r.l (St. Quentin Fallavier, 38297 France).

Synthesis and purification of diazonium salts

Diazonium salts were obtained by oxidation of the corresponding aryl-amine at 0°C and using sodium nitrite [7]. For the four different diazonium salts, 4 mmol of the corresponding amine were mixed with 2 mL of tetrafluoroboric acid dissolved in 7 mL of milli-Q water. The mixtures were then cooled in ice for 15 min (1 hour for that containing 4-docyl-aniline). Afterwards, a solution of 300 mg (600 mg for the mixture containing 4-docyl-aniline) of sodium nitrite dissolved in less than 1 mL of milli-Q water was added drop-by-drop. During the reaction the temperature was maintained at 0°C. After two hours of reaction (96 hours for the mixture containing 4-docyl-aniline), the mixture was filtered through 0.2 μm cellulose ester filters (Whatman, France) and thoroughly washed with cold diethyl ether.

Diazonium salt purification consisted in dissolving the crude solid in a small amount of deionized water, followed by recrystallization in diethyl ether. The recrystallization procedure lasted for 48 hours at 6°C. Recrystallized diazonium salts were then filtered (0.2 μm of cellulose ester filters, Whatman, France), dried and stored at -20°C.

Raman measurements

Raman measurements were recorded on pure chemicals (as powder unless otherwise stated) with a Labram HR800 Raman microspectrometer (HORIBA Jobin Yvon SAS, 59650 Villeneuve d'Ascq, France). For both amines and diazonium salts, a robust laser diode at 691 nm (Ondax) that is suitable for many Raman applications [23] was used to avoid molecular resonance. For diazonium salts and the 4-aminobenzoic acid, the scattered light was collected by a 100x objective with a numerical aperture (N.A.) of 0.9. For liquid compounds (aniline, 4-(2-aminoethyl) aniline and 4-decylaniline), a water immersion objective (100x, N.A.=1.0) was used. Spectral resolution was less than 2 cm-1 and spectral calibration was performed daily via a silicon sample. All presented spectra were baseline corrected and normalized using the spectrometer software (LabSpec, HORIBA Jobin Yvon). Each spectrum was normalized to its maximum intensity. For calculations, spectra were normalized regarding the C=C stretching mode [24] observed between 1570 and 1611 cm-1.

Nuclear Magnetic Resonance (NMR) measurements

Synthesized diazonium salts and their primary amines were also characterized by NMR measurements. Spectra were recorded using a Bruker Avance III 400 MHz instrument (Bruker UK Ltd, Coventry CV4 9GH, United Kingdom). The procedure, NMR spectra and 1H chemical shifts are presented in the Supporting Information. The structure of the salts as well as the indexation of protons is presented in Scheme 1. RMN spectra of both amines and diazonium salts are presented on Figure S1 in Supporting Information.

organic-inorganic-indexation

Scheme 1: Structures of amine and diazonium salt with the indexation of protons.

Computational details

All calculations were carried out using the Gaussian 09 suite of programs [25] using the B3LYP exchange and correlation functional [26,27] along with the 6-311++G(d,p) basis set for all atoms. The structures were optimized without symmetry constraint (see Supporting Information). The vibrational wavenumbers and normal modes were calculated within their harmonic approximation. A scaling factor of 0.976 was chosen on the basis of previously published work [28].

Results and Discussion

Figures 1-4 compare Raman spectra obtained for A) primary amines and B) corresponding synthesized diazonium salts.

The wavenumbers of important peaks are reported in Table 1 and their assignments were provided from the literature [24,29,30]. The spectrum obtained for aniline is in a good agreement with the one provided by Badawi [24]; most of their band assignments were used as reference for this work.

Raman wavenumber/cm-1 Assignment
Amines DS  
Aniline COOH (CH2)2NH2 C10H21 DS DS-COOH DS-(CH2)2NH2 DS-C10H21  
386               Aromatic ring-N in plane bending [24]
619 638 643 644 617 637 640 639 Ring deformation [24]
755       743       p CH wagging [24,29]
814 828 822 822   796 820 819 o; m CH wagging [24,029]
827 845 845 844   814 849 837 Ring breathing [24]
995
1027
      997
1021
      Ring deformation + ring breathing for benzene and mono-substituted derivatives [24,29]
CC stretching vibration (coupled with CH in plane bending vibration) [30,31]
    1067 1062     1082 1079 C-C stretching (aliphatic chain) [29]
        1073 1075 1076 1080 CH in plane bending for p-substituted and mono-substituted benzenes [12,29] coupled with C-N stretching
1154       1142       m; p CH in plane bending [24,29]
1175 1179 1177 1179 1174 1170vw 1198vw 1190vw m; o CH in plane bending [24,29]
1279 1286 1279 1265         Ring-N str + ring breathing + o-CH in plane bending [24]
    1469 1459     1454 1456 CH2 scissoring [32]
1602 1600 1611 1611 1570 1591 1588 1585 C=C stretching [24]
  1748       1708
1732
    C=O stretching
        2296 2305 2285 2294 NºN stretching [24,29]
    2813-2986 2813-2986     2813-2986 2813-2986 Symmetric and antisymmetric CH stretch of
n-alkanes [29]
3053-3071 3049-3063 3014-3051 3016-3057 3087 3096 3076 3066
3077
Aromatic CH stretch of benzene derivative [24,29]

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

In addition, interactions can occur between two salts in a crystalline state, especially for DS-COOH because carboxyphenyl-derivatives are both H-bonding donor and acceptor. Thus, they preferentially self-organize as dimers in the crystalline state [33-35]. To investigate the dimer formation and obtain a clear assignment of the DS-COOH Raman bands, DFT calculations and Natural Bond Orbital (NBO) charges were carried out, examining two models. The first one considered monomer units (named CBN2) whereas the second one considered DS-COOH dimers (named CBN2 dimer). Figure 5A shows a comparison between the Raman spectrum for DS-COOH and the calculated vibrational features of the monomer (CBN2) and the dimeric arrangement via H-bonds (CBN2 dimer). The Raman band assignments of important peaks are shown on Table 2. The crystalline state of DS-COOH regarding the CBN2 and CBN2 dimer models is discussed over the presentation of the different investigated modes of phenyl-derivatives.

DS-COOH CBN2 CBN2 dimer
Experimental/cm-1 Calculated/cm-1 Assignment Calculated/cm-1 Assignment
2305 2293 v(NºN) 2304 v(NºN)
  1778 v(C=O)    
1708     1684 vsym(C=O)
1732     × vsym(C=O)
1591 1588 v(C=C) 1590 v(C=C)
1293 wwv 1344 v(CO-H) 1271 vsym(CO-H)
  1165 v(C-H) in plane and v(CO-H)    
1127 ww     1122 v(C-H) in plane
1075 1093 v(C-H) in plane, v(C-N) and v(C-O) 1090 v(C-H) in plane, vsym(C-N)
814 797 Ring breathing 807 Ring breathing

Table 2: Comparison of experimental and calculated wavenumbers considering CBN2 and CBN2-dimer for modeling. Assignments of the main peaks. × Calculated CBN2 dimer features a center of symmetry imposing the mutual exclusion rule for vibrational modes. In this framework, thevsym(C=O) is Raman active while vsym(C=O) is Raman inactive.

N≡N group vibrations

The particularity of diazonium salts is their N2+ function obtained by oxidation of the corresponding amine. The strong peaks observed 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≡N bond [12,29]. Compared to the experimental data acquired for each compound, the DFT calculations on DS-COOH revealed strong similarities in this spectral range (Figure 5, Table 2). The calculated position for the N≡N stretching mode was found at 2293 cm-1 for CBN2 and at 2304 cm-1 for CBN2 dimer. The similarity between the latter theoretical position and the experimental one (the DS-COOH N≡N stretching mode is recorded at 2305 cm-1) strengthens the fact that the best model for reproducing the vibrational features of the normal Raman spectra is a dimeric arrangement of the diazonium cation via H-bonds.

According to the literature [29,36], the position of the N≡N vibration mode depends upon the nature of the ring substituent located in the para-position of a diazonium salt. Our results show that substituents have a rather low effect on the N≡N fragment following the observed wavenumber shifts (Table 1; Figure S2 in Supporting Information). For DS-COOH, however, the COOH electron-withdrawing tendency was confirmed by calculation of the 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, a shift of +9 cm-1 of the N≡N vibration mode was found for DS-COOH relative to DS. The electron-withdrawing groups (i.e., COOH) induce an increase of the contribution of a structure like and hence induce a shift of the N≡N stretching vibration to higher wavenumbers [29,36]. This agrees with the N≡N bond-length decrease [36] and is also consistent with the polarizability increase of the N≡N bond. Experimental data show in fact that the relative intensity of the N≡N bond is increased by 23% for DS-COOH compared with that obtained for DS, DS-C10H21 and DS-CH2CH2NH2 (Figure S2 and Table S1 in Supporting Information) On the contrary, electron-donating groups located in parapositions induce an increase in the contribution of a structure like and, hence, a decrease of the N≡N stretching vibration wavenumber. Such shifts are effectively observed in our experimental results since shifts of -11 cm-1 and -2 cm-1 of the N≡N vibration mode were evaluated for DS-(CH2)2NH2 and DS-C10H21, respectively, compared to that observed for DS. Of the 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, the inductive effect drops off rapidly with the number of σ bonds. This is why:

i) The negative shift in wavenumber observed for N≡N vibration for DS-C10H21 is not significantly different from the one observed for DS, and why

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

H-N-(ring) symmetric stretching mode

In addition to the occurrence of an N≡N stretching vibration, the N-H symmetric stretching disappears in the high wavenumber range 3250-3400 cm-1 for primary amines (Figure 6). This indicates the successful synthesis of diazonium salts with a high production rate, which was confirmed by NMR study with the disappearance of the η band on the NMR spectra (Supporting Information). Each spectrum was normalized to its maximum intensity.

Ring-N vibrational modes

According to Badawi [24] and Wojciechowski [30], the peak at 386 cm-1 for the aniline molecule (Figure 1), corresponds to planar bending of an aromatic ring-N. However, this vibrational mode is less obvious for para-substituted amines. A possible explanation is that aromatic-ring-vibration modes depend on the mass of the substituent [29]. Thus, in the range of 200-580 cm-1, many vibrational modes can be observed depending on the nature of the para-substituted group (Figures 2-4). The same conclusion is valid for the range of 1250-1290 cm-1, another range where a band corresponding to C-N vibration can be located. Raman bands 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 longer seen in the diazonium salt spectra.

This effect may be due to significant structural changes and conformational rearrangements, as the theoretical and experimental data provided by literature indicate that the neutral aniline in its ground electronic state (1A1) is non-planar. The dihedral angle between the NH2 plane and the C6H5N plane was determined to be 37 ± 2°, assuming that the C-N bond makes an angle of 1.5-2.3° [30]. In comparison, the corresponding diazonium salt (aryl-diazonium) is a quasi-planar molecule [39], assuming that the C-N bond makes an angle of 0.5° [40]. The main factor causing this quasi-planarity is undoubtedly the strong through-bond interaction between the ring, which is known to be a strong electron donor, and the N2 group, which is the strongest known electron acceptor [39]. In agreement with the literature [30,39,41], this bond lengthens so that co-planarity is maintained. An additional explanation could be related to the electron-charge density redistribution in the C-N bond. This is consistent with the observation that the diazonium group is one of the most powerful σ-electron and π-electron acceptors [41].

Benzene-ring and C-H related vibrations

By comparing Raman spectra from amines and diazonium salts, several common bands related to benzene-ring vibrations can be observed. First of all, a strong band related to the benzenering stretching mode (Table 1) occurs on all Raman spectra in the range of 1585-1611 cm-1. According to Wojciechowski [30], this band is essentially related to ortho-meta C-C stretching modes. From phenyl-amine derivatives to diazonium salts, this band is the most significant signature of the benzene ring. Compared to the primary amines, this band is shifted to a lower wavenumber for all salts. These shifts are close to 25 cm-1 for salts except DSCOOH, maybe due to the higher weight of the N2 group compared to NH2 that induces a decrease of the vibration frequency. In the case of DS-COOH, the shift of less than 10 cm-1 may be due to the dimeric form of the salt that reduces the weight effect of the N2.

Two strong and distinct peaks again assigned to the ringdeformation and ring-breathing modes-coupled with CH in plane-bending vibration of benzene and of mono-substituted derivatives, are found in the Raman spectrum of aniline around 1000 cm-1 (Figure 1; Table 1, trigonal ring breathing vibration) and 1027 cm-1 (Figure 1; Table 1) [30,31]. For DS, these bands are slightly shifted to 997 and 1021 cm-1. In case of para-substituted amines and diazonium salts (di-substituted benzene derivatives) these vibrational modes are not allowed [29].

Bands assigned to ring deformation (C=C-C in plane deformation) in the region of 617-644 cm-1 (Table 1) are present for both amines and diazonium salts. In agreement with Socrates [29], the bands shift to a higher wavenumber for para-substituted aromatic compounds compared to mono-substituted compounds.

According to Badawi [24], the vibrational mode at 827 cm-1 (Table 1) for aniline can be assigned to a ring-breathing mode, coupled with ring-N stretching and ring deformation. This band is shifted to higher wavenumbers (844-845 cm-1) for parasubstituted amines. However, shifts of -31 cm-1 and -7 cm-1 toward lower wavenumbers are observed for DS-COOH and DS-C10H21 respectively, compared to their primary amines, whereas in the Raman spectrum of DS-(CH)2NH2 a shift of +4 cm-1 was found. In common for all substituted diazonium salts, the intensity of this vibrational mode strongly decreases and is no longer observed in the Raman spectrum of DS. Another vibrational mode at 814 cm-1 assigned o,m-CH wagging (Table 1 and Figures 2-4) was found to behave in similar manner as described above for the ring-breathing mode. The shift of these bands may be due to structural changes and conformational rearrangements caused by the presence of the N2+ [39,41].

Concerning the C-H ring related vibrations, bands assigned to p-CH wagging-coupled with ring-N wagging and o-CH wagging at 755 and 743 cm-1 (Table 1) as well as m, p-CH bending-coupled with ring deformation at 1154 and 1142 cm-1 (Table 1) are only observed for aniline and DS, due to mono-substitution.

The very strong band observed around 1075 cm-1 and assigned to CH in plane bending-coupled with ring-N stretching mode for mono- and para-substituted benzenes in diazonium-salt spectra, is of particular interest (red spectra on Figures 1-4; Table 1). The appearance of this band is an astonishing phenomenon, explained by electron-charge density redistribution within the structures caused by the presence of N2+. It is also the indisputable signature of phenyl-derivatives stemming from diazonium salts.

organic-inorganic-Raman-spectra

Figure 1: Raman spectra of aniline (black spectrum) and the corresponding diazonium salt benzene diazonium tetra fluoroborate, DS (red spectrum). Each spectrum was normalized to its maximum intensity.

organic-inorganic-Raman-spectra-aminobenzoic

Figure 2: Raman spectra of 4-aminobenzoic acid (black spectrum) and the corresponding diazonium salt 4-carboxy benzene diazonium tetra fluoroborate, DS-COOH (red spectrum). Each spectrum was normalized to its maximum intensity.

organic-inorganic-maximum-intensity

Figure 3: Raman spectra of 4-(2-aminoethyl) aniline (black spectrum) and the corresponding diazonium salt DS-(CH2)2NH2 (red spectrum). Each spectrum was normalized to its maximum intensity.

organic-inorganic-corresponding-diazonium

Figure 4: Raman spectra of 4-decylaniline (black spectrum) and the corresponding diazonium salt 4-decyl benzene diazonium tetra fluoroborate, DS-C10H21 (red spectrum). Each spectrum was normalized to its maximum intensity.

organic-inorganic-calculated-vibrational

Figure 5: Comparison of the experimental Raman spectrum recorded for DS-COOH and the calculated vibrational features of the salt, by considering monomer (CBN2) and dimeric arrangements via H-bonds (CBN2 dimer).

According to the literature, aromatic C-H symmetric and antisymmetric stretching modes occur above 3000 cm-1 and show a multiplicity of weak to moderate bands [42]. In comparison with amines, CH stretching (Table 1) shifts toward the higher wavenumber for diazonium salts (Figure 6). This shift indicates a strengthening of the C-H bond, possibly due to a new charge distribution on the ring after the substitution of NH2 by N2+ [30]. This result is in good agreement with the ones in the literature for aniline and its corresponding diazonium salt [30,39,41].

organic-inorganic-stretching-modes

Figure 6: Comparison of aromatic C-H stretching and N-H stretching modes between the four synthesized diazonium salts (B red lines) and the corresponding primary amines (A black lines). Roman numbers I to IV correspond to DS, DS-COOH, DS-(CH2)2NH2 and DSC 10H21, respectively.

Functions in para- position carried by the aromatic ring

Concerning the different functions in para-position carried by the aromatic ring (COOH, (CH2)2NH2 and C10H21), some characteristic bands have been identified. For (CH2)2NH2, C10H21 and their corresponding diazonium salts, symmetric and antisymmetric CH3 stretches of n-alkanes as well as symmetric CH2 stretches of n-alkanes are clearly identified in the range of 2813-2986 cm-1 (Figures 3 and 4; Table 1). In addition, C-C stretching mode occurs in the range of 1062-1082 cm-1 (Table 1) and CH2 scissoring in the range of 1454-1469 cm-1 (Table 1). Some significant changes in peak position or intensity occur, meaning that the simple modification of the molecule from aniline form to diazonium salt could have a strong influence on the vibrational modes of the alkyl chain in para-position.

Concerning aminobenzoic acid derivatives (Figure 2; Table 1), the weak bands in the 1700-1750 cm-1 region are characteristic features of the carboxylic group due to C=O stretching vibration. They were respectively assigned to symmetric and antisymmetric stretching vibrations at 1708 and 1732 cm-1. The antisymmetric stretch is usually seen at a higher wavenumber than the symmetric one [43]. With regards to DFT calculations (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 in good agreement with the literature [29]: when carbonyl does not interact with a hydrogen bond (the monomer case), the stretching bands appear in the range 1760-1735 cm-1. However, the dimer should be characterized by two C=O stretching modes, the symmetric C=O one (vsym(C=O)CBN2dimer) at 1683 cm-1 and the antisymmetric C=O one (vsym(C=O)CBN2dimer) at 1729 cm-1. With regard to Raman calculations of the C=O stretching mode in the CBN2 model (vsym(C=O)CBN2dimer at 1778 cm-1), the theoretical shift toward the lower wavelength of the symmetric C=O stretching mode in the dimer [vsym(C=O)CBN2dimer at 1683 cm-1] is due to a change in polarizability of the CO group induced by hydrogen bonding. The simulated Raman spectrum for CBN2 dimer only shows this latter symmetric mode [vsym(C=O)CBN2dimer] at 1683 cm- 1. This concurs with the fact that the CBN2 dimer features a center of symmetry imposing a mutual exclusion rule for vibrational modes. In this framework, the vsym(C=O)CBN2dimer is Raman active, whilevsym(C=O)CBN2dimer dimer is Raman inactive. This mutual exclusion rule explains why the simulated Raman spectrum for an isolated CBN2 dimer only shows symmetric C=O stretching.

However, the same dimer considered in the crystal packing has a lower symmetry that suspends the mutual exclusion rule, both vsym(C=O)CBN2dimer and vsym(C=O)CBN2dimer dimer becoming Raman active. This is consistent with the two peaks on the experimental Raman spectrum of DS-COOH for both the symmetric and antisymmetric C=O stretching modes. The shift toward a higher wavenumber observed experimentally for symmetric C=O stretching from the theoretical vsym(C=O)CBN2dimer 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 provided by electronegative atoms, which increase the C=O stretching vibration wavenumber [44,45]. In addition to and in agreement with the results shown on Table 2, the similarities between the experimental 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) as well as v(C-H) in plane confirm the fact that the best model for reproducing the vibrational features of the normal Raman spectra is a dimeric arrangement of DS-COOH via H-bonds.

Conclusions

The work consisted in a Raman characterization of synthesized diazonium salts, comparing them with Raman spectra provided for amines. Raman investigation of synthesized diazonium salts confirmed the stretching of the N≡N bond in the expected spectral range of 2285-2305 cm- 1 (2296, 2285, 2305, and 2294 cm- 1 for DS, DS-COOH, DS-(CH2)2NH2 and DS-C10H21, respectively). The disappearance of the H-N-(ring) symmetric stretching observed in 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 combination of both CH in-plane bending for mono- and parasubstituted benzenes as well as C-N stretching in the range 1073- 1080 cm- 1, constitutes an actual spectral signature of the phenylderivatives stemming from diazonium salts.

Furthermore, the fate of phenyl-derivatives has also been examined by using Raman spectroscopy; this covered N≡N group vibrations, H-N-(ring) symmetric stretching modes, ring-N as well as benzene-ring vibrational modes, C-H-related vibrations and para functions carried by the aromatic ring. We especially considered both

1) The effect of structural changes and the conformational rearrangements from amine to diazonium salt, and

2) The influence of the substituent located in a para position, i.e., the nature and force of the mesomeric effect of the substituent, on the Raman modes.

Comparison between Raman experiments and DFT modeling showed that DS-COOH self-organized as dimers in the crystalline state.

This paper reports on a preliminary study designed as a leadin to further examinations for investigating, improving and/ or controlling reaction pathways using synthesized diazonium salt (i.e., to examine and strengthen the nature of the interface between Gold NanoStructures (GNS, electron-beam nanolithographied SERS substrates) and organic coatings) [46].

Author Contributions

All authors have given approval to the final version of the manuscript. SB and IT contributed equally.

Acknowledgments

This work was funded by ANR ECOTECH (Production durable et technologies de l'environnement); REMANTAS project: Enhanced Raman scattering for aquatic media: a new technology for on-site analysis 2011-2014 (REMANTAS project; ANR-11-ECOT-0010). The English text was proofread by Dr. HM Kluijver.

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