Raman and Surface-Enhanced Raman Scattering of some furan-2 carboxaldehide derivatives

STUDIA UNIVERSITATIS BABEŞ-BOLYAI, PHYSICA, SPECIAL ISSUE, 2001

ADSORPTION ON SILVER SURFACE OF 5-(3-NITRO-4-METHYL-PHENYL)-FURAN-2 CARBALDEHIDE
T. ILIESCU^a*, F. D. IRIMIE^b, M. BOLBOACA^c ,CS. PAIZS^b, W. KIEFER<sup>c

a</sup> Babes-Bolyai University, Physics Faculty, 3400 Cluj-Napoca, Romania.

^b Babes-Bolyai University, Chemistry Faculty, 3400 Cluj-Napoca Romania.

^c Institut für Physikaliche Chemie, Universität Würzburg, D-97074,Würzburg, Germany

ABSTRACT. FT Raman and SER spectra of 5-(3-nitro-4-methyl-phenyl)-furan-2 carbaldehide at different pH values were recorded. A theoretical vibrational analysis was made and compared with FT Raman spectra. At low pH values both samples present a good SER spectra. The shift to red of SERS bands by 10-15 cm^-1 is an indication of chemisorption of these species, which are bonded to the silver surface via nonbonding electrons of ring oxygen.

Keywords: Raman spectroscopy; SERS; 5 substituted-furan-2 carbaldehide

Introduction

Furan-2 carbaldehide derivatives are very important intermediates in organic synthesis. The bacteriostatic effects of these compounds were checked up with good results.

Surface Enhanced Raman Scattering (SERS) was observed for many molecular species. Because of suppression the of fluorescence, this nonradiativ transfer being determined by the roughness of the silver surface, this technique is very useful for molecules that present this phenomenon, especially for molecules of biological interest [1,2].

In this paper we present the investigation of 5-(3-nitro-4 methyl-phenyl)-furan-2 carbaldehide (5-(3-N-4MP)-F-2C) from analytical (FT-Raman spectroscopy) and theoretical (DFT calculation) point of view. We recorded SER spectra of this specie adsorbed on silver particles at different pH values in order to elucidate the form in which the adsorbed molecules may exist in adsorbed state. To our knowledge these data are not yet present in literature.

Experimental

The compounds were obtained from the corresponding diassonium salt and furfural in aqueous medium using CuCl₂ catalyst. [3] The products were purified with column chromatography on silica gel.

The FT Raman spectra were recorded using a Brucker IFS 120HR spectrometer with an integrated FRA 106 Raman module and a resolution of 2 cm^-1. Radiation of 1064 nm from Nd-YAG laser was employed for excitation. A Ge detector, cooled with liquid nitrogen, was used. SER spectra were recorded with a Spex 1404 double spectrometer using 514.5 nm and 300W output of Spectra physics argon ion laser. The detection of Raman signal was carried out with a Photometrics model 9000 CCD camera. Spectral resolution was 2 cm^-1. A citrate reduced Ag colloid was employed as SERS substrate. Ag citrate was prepared according to the literature [4]. Small amounts of 10 L of sample 10^-1mol L^-1 ethyl alcohol solution were added to 3-ml colloid. 0.1 ml 10^-1mol L^-1 NaCl solution was also added for producing a stabilization of the colloidal dispersion and a considerable enhancement of the SER spectra. The final concentration of the sample were approximately 3.2 10^-4 mol.l^-1. NaOH and H₂SO₄ were used to obtain different pH values.

The density functional theory (DFT) calculations were performed using GAUSSIAN98 [5]. Using a fully optimized geometry as reference geometry performed all calculation of harmonic wavenumber. The DFT geometry optimization was carried out with the combination of Becke's 1988 exchange functional [6] and the Perdew-Wang 91 gradient-corrected correlation functional [7] (BPW91). The 6-31+G* basis sets for all atoms has been employed in the geometry optimization and the vibrations calculations.

Results and Discussion

By rotation of the (CHO) group in 5-(3-N-4MP)-F-2C, two rotational isomers, syn-form and anti-form, were obtained. Theoretical calculations at the BPW91/6-31+G* level of theory were performed for both isomers. The optimized geometry of these rotamers with the labeling of their atoms are shown in Fig.1.

a

b
Fig.1. Structural formula for the two isomers of 5-(3-nitro-4-methyl-phenyl)-furan2 carbaldehide. (a) syn-form isomer, (b) anti-form isomer

The optimized structure of both conformers is not planar. Thus the C3NO3 and C4NO4 angles have 117^ while the dihedral angle C4C3NO3 and C4C3NO4 have -86^and 93^ respectively. Further, the analytical harmonic vibrational modes have been calculated in order to ensure that the optimized structure correspond to minima on the potential energy surface. The total energy for the syn-form isomer, including zero-point correction, are found to be –817.825627 and –817.826568 hartree at the BPW91/6-31+G* level of theory. Therefore, at this level of calculation, the anti-form isomer was found to be more stable than syn-form by 2.995 KJ.mol^-1.

The observed bands in IR, FT-Raman and SER spectra at pH 1 of 5-(3-N-4MP)-F-2C together with theoretical calculation of both isomers are summarized in Table 1.

Table 1

Experimental (infrared, FT-Raman) and calculated wavenumbers (cm^-1) (anti/syn forms) of 5-(3-nitro-4-methyl-phenyl)-furan-2 carbaldehide.

IR	Raman	Calc.^1 anti/syn	SERS pH 1	Assignment
	179m	170/180		(C4,5,7) + (C9,10,11)
	267w	246/285		(C2C12) + (C10C11)
505m	386m 505m	378/378 500/496		ring 1^b out of plane def.
551sh 560vw 619w 672m	558sh 567w 620w 675m	560/559 598/608 635/629 670/681		ring 1 + ring 2^c out of plane def
757m	756sh	741/757	724w	(C3N)
768sh	771m	779/772		(CH) (ring 2)
801m	800m	784/784	796w	(O3NO4) + (C1,2,3) + (C4,5,6)
810sh	811m	817/822		(CH) (ring 1)
934m	932m	937/937	903w	(CN) + (O1C7,8) + (C4,5,6)
967m	967m	964/961		(CH) (COH)
1039m 1048sh	1040m	1031/1039	1032m	(CH) (ring 2) + (CH) ( CH3)
1096w	1106w	1081/1081		(C1,2,3) + (C4,5,6) +(C8,7O1)
1211m 1217sh	1212m 1218sh	1205/1201 1220/1244	1211vw	(CH) (ring 1 + ring 2) + (C2,12) + (C5,7) + (C10,11)
1250m	1251w	1264/1270	1224w-m	(CH) (ring 1)
1278sh	1278sh	1278/1292		(C5,7) + (CH) (ring 1)
1294m	1295m	1313/1313	1294w-m	(O3NO4)
1355m	1355s	1334/1331	1331s	ring 1 stretching + (C8,9)
1391m	1392m	1392/1391	1375s	ring 1 + ring 2 stretching
1405sh	1402sh	1413/1415		(CH) (ring 2 + CH3)
1469m	1470s	1469/1471	1453w-m	(CH) (CH3)
1524s	1524vs	1514/1514	1519s	ring 1 + ring 2 stretching
1533s	1533s	1533/1531		(O3NO4) + (C9,10,11) + (C1,6,5)
1579m 1620s	1579m 1621vs	1571/1568 1619/1619	1573s 1603s 1634sh	ring 1 + ring 2 stretching
1664sh 1674s	1665m 1674vs	1687/1690		(CO) (COH) anti-isomer (CO) (COH) syn-isomer
2820m 2916sh	2820m	2862/2844		(CH) (COH)
2928m	2928m	2999/2999		(CH) (CH3)
3066m 3078sh	3058sh 3069m	3066/3067 3076/3077		(CH) (ring 1)
3114w	3112w	3204/3199		(CH) (ring 2)

^Abbreviation: w-weak, m-medium, s-strong, v-very, sh-shoulder, -stretching, -bending, -rocking, -twist, -wagging,.

Calc¹: BPW91/6-31+G^, ring 1^b-phenyl ring, ring 2^c-furan ring.

The assignment of the normal modes of vibration for the molecules was accomplished mainly by comparison with related molecules [8-11] and using the wavenumber (unscaled values) and intensities as obtained by BPW91 method. The theoretical results obtained with the 6-31+G* basis set reproduce closest the experimental values. A strict comparison between the experimental and calculated wavenumbers and intensities is not possible in this case because the experimental data were obtained for the solid state, whereas the theoretical results are for gas phase. It is well known that the calculated wavenumbers are obtained by using the harmonic approximation, whereas the experimental wavenumber are anharmonic by nature. Nevertheless, the quality of quantum chemical results at the presented theoretical level is sufficient to be useful for the assignment of the experimental data.

From solid state sample only strong fluorescence could be obtained for visible excitation, therefore IR excitation was necessary. F T Raman (Ordinary Raman Spectrum-ORS) of solid 5-(3-N-4MP)-F-2C is presented in Fig.2a. SER spectra in silver sol at different pH values (1and 13) are indicated in Fig.2b and 2c (to simplify the figure only spectra at two pH values was chosen).

Low concentration (3.2 10^-4 molL^-1) of utilized 5-(3-N-4MP)-F-2C to obtain SERS is a proof that we have an enhancement of Raman signal (ORS from solutions of such concentration cannot be obtained)

In the spectrum of 5-(3-N-4MP)-F-2C bands that appear in the 1600-1500 cm^-1 region are broad and split in two components (1524-1533, 1664-1674 cm^-1). This splitting can be explained by the presence of two rotational isomers. Same splitting is present in the Raman spectrum of furfural [8]. In the ORS of 5-(3-N-4MP)-F-2C the bands at 1664 and 1674 cm^-1 were attributed to the (C=O) stretching vibration of anti-form and syn-form isomers respectvely determined by the substituent group. presented in Fig.1.

The significant differences between FT Raman and SER spectra concerning the relative intensities and the bandwidth indicate a strong interaction between metal and adsorbate, this causing a quite different derivative of molecule's polarizability tensor.

There are two possibilities of molecule adsorption on metal surface, namely physisorption and chemisorption. The spectrum of physisorbed molecules is practically the same as that of free molecules, small change being observed only for the bandwidths. This situation corresponds to a relative large distance between metal surface and adsorbed molecules [12].

Fig. 2. FT Raman (a) and SER spectra at pH 1 (b) and 13 (c) of 5-(3-nitro-4-methyl-phenyl)-furan-2 carbaldehide
When the molecules are chemisorbed, there is an overlapping of the molecular and metal orbital, the molecular structure being modified and in consequence, the position of the bands and their intensities are dramatically changed [13].

Comparing the SER spectrum of 5-(3-N-4MP)-F-2C to the corresponding ORS (Fig.2) a shift to the lower wavenumber could be observed. Therefore, we conclude that there is a chemisorption of 5-(3-N-4MP)-F-2C molecules on the silver surface.

The enhancement of Raman signal in SERS can be explained through the two main mechanisms [13,14], the electromagnetic one, which implies the increase of the electric field near the silver surface through the resonance plasmon excitation. This mechanism explains the enhancement of the physisorbed molecules.

The chemical mechanism supposes the binding of the molecules on the silver surface, when the charge transfer between molecule and metal can occur. In this case a new specie metal-molecule SERS complex is formed. This can explain the band shift and the change in the relative intensities of the SER spectra compared to the corresponding ORS.

In our case is very difficult to separate these two mechanisms, both contributing to the enhancement of the Raman signal. Additionally, is possible to have some resonance Raman contribution to the total enhancement because the excitation wavelength 514.5 nm falls in the wing of the absorption band of both samples.

For 5-(3-N-4MP)-F-2C (Fig.2a) the split of the bands (1533.1-1524.5; 1664.2-1674 cm ^–1) suggests that both isomers are Raman active. In SER spectrum of this sample, at pH 1 (Fig.2b) the broadening of the bands and dramatically changes of the relative intensities are accompanied by the shift to the lower wavenumber by 10-15 cm ^–1 , especially for the bands in the 1450-1650 cm ^–1.region. These spectral changes can be explained by the adsorption on silver surface and by the presence of two Raman active isomers, the bands specific to each isomer being not resolved after adsorption.

The absence of the bands due to the C=O stretching mode corresponding to the both isomers in SER spectra can be a consequence of the hydration of the C=O bond according to the reaction
H₂O

(CH₃-NO₂-C₆H₄-C₄H₂O)-CHO (CH₃-NO₂-C₆H₄-C₄H₂O)-CH(OH)₂

The shoulder at 1634.1 cm^-1, present in SER spectrum has no correspondence in the bulk spectrum and is probably given by surface complex formed by adsorption.

In alkaline medium SER spectrum of 5-(3-N-4MP)-F-2C (Fig.2c) presents a very broad bands.

Spectral modification at alkaline pH can be explained by transformation of the aldehides according to reaction (Cannizzaro reaction)

2

SER spectra being probably a superposition of both species, namely, R-CH₂OH and R-COOH.

The orientation of the adsorbed 5-(3-N-4MP)-F-2C molecules can be determined with the help of surface selection rules for Raman scattering. These rules for roughened surface have been described by Creighton [15] and by Moskovitz and DiLella [16]. If the molecular z-axis is normal to the surface, then vibration of the adsorbed molecules, which have a polarizability tensor component along this axis, will be preferentially enhanced. Gao and Weaver [17] observed significant (~25 cm^-1) red shift relative to the bulk spectra in the ring band of the flat adsorbed substituted benzene derivatives, due to backdonation of electron density from the metal to the ^*antibonding orbital. A similar phenomenon was observed in the case of pyridine derivative [18]. These conclusions could be extended to the furan ring.

The 5-(3-N-4MP)-F-2C molecule has low symmetry and all vibrations will have a polarizability tensor component along the z-axis. .

In SER spectrum of 5-(3-N-4MP)-F-2C, at low values of the pH, the stretching modes of the furan and phenyl rings (see Table 1) are more enhanced than other modes and red shifted by 10-13 cm^-1 comparing to the bulk spectrum. The out of plane modes of the adsorbed molecules are weak enhanced. If the molecular plane would be vertical to the metal surface then a blue shift of the ring breathing modes is expected [19]. In our case the ring plane of the 5-(3-N-4MP)-F-2C molecule is probably not perfectly vertical but rather inclined to the silver surface, the molecules being bonded on the metal surface through the nonbonding electrons of the ring oxygen.

Because of the broadening of the bands at alkaline pH for both samples is very difficult to tell which of the species R, R'-CH₂OH or R,R'-COOH are adsorbed and which are the orientation of these species to the silver surface.

Conclusions

A vibrational analysis was performed on 5-(3-nitro-4-methyl-phenyl)-furan-2 carbaldehide. Both isomers, syn and anti-form of 5-(3-nitro-4-methyl-phenyl)-furan-2 carbaldehide are Raman active. The sample presents good SER spectra at low pH values but at large pH the enhancement is very weak and bands are more broadened. The red shift by 10-15 cm^-1 of the SERS bands, comparing to the ordinary Raman spectrum, indicates the chemisorption to the silver surface. The absence of band due to the C=O stretching mode corresponding to the both isomers in SER spectrum was explained by hydration of C=O bond. The molecular species are tilted oriented to the metal surface and bonded to it via nonbonding electrons of the ring.
Acknowledgements

The authors wish to thank Professor Dr. I. A. Silberg from Chemistry Institute Cluj-Napoca for fruitful discussion.

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