Raman and SERS study on atrazine, prometryn and simetryn triazine herbicides

Journal of Molecular Structure 1040 (2013) 139–148

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Raman and SERS study on atrazine, prometryn and simetryn triazine herbicides Sergio Bonora a, Enrico Benassi b, Assimo Maris c, Vitaliano Tugnoli a, Stefano Ottani d, Michele Di Foggia a,⇑ a

Dept. Biochemistry, University of Bologna, via Belmeloro 8/2, 40126 Bologna, Italy S3 Center, CNR Institute of Nanoscience, via G. Campi 213/a, 41125 Modena, Italy c Dept. Chemistry, University of Bologna, via Selmi 2, 40126 Bologna, Italy d Istituto per la Sintesi Organica e la Fotoreattività, ISOF-CNR, via P. Gobetti 101, 40129 Bologna, Italy b

h i g h l i g h t s " Combined and very accurate spectroscopic and theoretical structural model of triazines. " Discussion of the triazines spectra in different solvents and interaction with silver colloids (SERS conditions). " Triazines interact mainly through N aromatic atoms.

a r t i c l e

i n f o

Article history: Received 27 September 2012 Received in revised form 9 February 2013 Accepted 22 February 2013 Available online 4 March 2013 Keywords: Atrazine Triazines Raman spectroscopy SERS DFT calculation

a b s t r a c t In the present study, we considered the Raman spectra of atrazine, prometryn and simetryn, in the solid form and in polar and apolar solvents, extending the investigation in the very diluted aqueous solutions (ppm) range by using the SERS technique. We performed theoretical calculations at the B3LYP/aug-ccpVQZ level on the three triazines, alone and in solution with polar and apolar solvents. An excellent agreement between theoretical and experimental frequencies was reached, with differences lying within few wavenumbers. The small differences observed can be ascribed to the solid crystalline phase and can be caused by local deviations in the uniformity of the crystalline field or to a coupling with lattice vibrations. Also the theoretical and experimental peak intensities well agreed and in most cases lied within ±10%, the differences being ascribed to the local non-homogeneity of dielectric properties in the crystal. Moreover, this behavior confirmed the rigidity of the molecules and that their structure was not involved during the solution process. The theoretical SERS spectra at B3LYP/6-311+G(d,p) level of triazines bound to an Ag2 metal cluster offered an acceptable qualitative agreement with the experimental ones, suggesting that the stronger interaction site of triazines with Ag2 was on the less sterical hindered aromatic nitrogen atom, namely forming the N6


Ag2 molecular complex with atrazine, and the N2


Ag2 or N4


Ag2 molecular complexes with simetryn and prometryn. The agreement between calculated and experimental SERS spectra was not as good as that observed for the Raman spectra of pure compounds, but the trend of the theoretical spectra offered a useful guideline for the comprehension of the interaction sites and of the structural modification after adsorption on silver particles. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The s-triazines group of herbicides is well known and has been widely used for more than 60 years in the world. These chemicals inhibit photosynthesis in plants by blocking the electron transfer at the reducing site of chloroplast photosystem II electron transport [1] and act as selective persistent herbicides in forestry and in agricultural soils [2,3]. The use of some of these herbicides is now forbidden in some countries as a consequence of the ⇑ Corresponding author. Address: Dept. of Biochemistry ‘‘G. Moruzzi’’, Section of Chemistry and Propaedeutics Biochemistry, University of Bologna, via Belmeloro 8/ 2, 40126 Bologna, Italy. Tel.: +39 051 2094280 81; fax: +39 051 243119. E-mail address: [email protected] (M. Di Foggia). 0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.02.025

resistance of many s-triazines against chemical and biological degradation, leading to their accumulation in the environment [4]. As a result of their massive use, s-triazines and their metabolites have been detected in alarmingly high concentrations in soils, groundwater, rivers and lakes [5,6]. The activity of s-triazines in soils was recognized to be controlled by soil sorption processes, and, for this reason, the sorption mechanism of s-triazines is a topic of great interest [7,8]. Atrazine (Atr) (Fig. 1A), [6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine], a white crystalline powder, is probably the most common and well-known member of the s-triazine herbicides family. It is one of the most widely applied herbicides in the world and has been used since 1958 in the agricultural production. Unfortunately,

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it has a long persistence (due to the presence of the C–Cl linkage) and its half-life varies upon the environmental conditions, reaching values up to 3–5 years [9]. The solubility of atrazine in water is quite low (35 mg/L at 25 °C) [10] and the molecule is fairly resistant to hydrolysis at neutral pH. In strong acids or alkaline solutions and at high temperatures (>70 °C), Atr hydrolyzes to hydroxyatrazine losing its herbicidal properties [10]. As a consequence of the widespread use (about 40 million kg worldwide) and moderate to high persistence in the environment, it has been frequently detected in ground and surface waters [11–14]. Concentrations levels over 30 lg/l were detected in surface waters [15–17]. Prometryn (Prom) [N2,N4-diisopropyl-6-methylsulfanyl-1,3,5triazine-2,4-diamine] (Fig. 1B) and simetryn (Sim) [N2,N4diethyl-6-methylsulfanyl-1,3,5-triazine-2,4-diamine] (Fig. 1C) are also widely used herbicides, similar to Atr in the biological properties [18,19]. Prom and Sim were introduced in the late ’60 as a response to the necessity of short half-life herbicides; nevertheless, in some countries, as a consequence of the massive use, they are probably the most important sources of environmental water pollution by pesticides [20]. Both compounds are white crystalline solids, with a low to moderate water solubility (33 and 450 mg/L, respectively) and with half-lives of the order of 1–2 months [21]. The pKa value of Atr, Prom and Sim in aqueous solution is 64.1 [22], ensuring thus that at pH P 6, almost all molecules are present in the neutral form. Raman spectroscopy has been widely used to study structure and interactions of organic compounds; however, the traditional Raman spectroscopy is a technique with a too low sensitivity to operate at the bioactive concentrations (ppm). In the late ’70, it was discovered that an enhancement in the Raman scattering signal of many substances took place when adsorbed over a rough silver substrate [23], originating thus the surface-enhanced Raman spectroscopy (SERS). The main advantage of SERS lies in the extremely large enhancement of the Raman signal (usually 6–7 orders of magnitude) when a substance is adsorbed on the rough surface of metal structures (Ag, Au, and Cu, etc.) or on metallic colloids in solution. Two models has been developed to explain the experimental results: the electromagnetic model [24], that involves the very large laser-induced electromagnetic fields near the metal surface, and the chemical model [25,26], that involves the charge transfer when molecules interact with the roughened metal surface, forming an adsorbate complex. Under certain conditions, the enhancement mechanism is so efficient to allow the ‘single molecule’ detection [27,28], rendering SERS a very promising technique for biomedical and environmental researches [29,30]. Recently, the group of Costa published two papers on the SERS study of triazines: the first on ametryn [31] (similar to Prm and Sim), the second on two chlorinated triazines (Atrazine and Simazine) [32]. However both studies were limited on a narrow spectral range (600–1800 cm 1), and using a B3LYP hybrid functional DFT basis set in the theoretical calculus. Moreover, to simulate the interaction with a silver surface, they used only one Ag atom and did not consider the effect of water as a solvent that was demonstrated to be of paramount importance in the correct interpretation of SERS theoretical spectra [33]. In the present study, we compared the complete theoretical Raman spectra (100–3500 cm 1), obtained by a very extended quantum-mechanics ab initio basis set with the experimental ones of Atr, Prom and Sim, both in the solid form as well as in polar and apolar solvents. Investigations were extended to very diluted aqueous solutions, at ppm range, by using the SERS technique, mimicking thus the bioactive concentrations. In the SERS theoretical calculus, we used the simplest silver cluster (Ag2) as a model of the silver surface structure: the Ag2 cluster posses a directionality,

making it more similar to a surface than a single atom that has a spherical symmetry, moreover the dimer can show vibrations. In conclusion, to our opinion, the choice of the Ag2 cluster was the best compromise between a sufficiently corrected model to simulate a surface of Ag atoms and the computing potential at our disposal. 2. Materials and methods 2.1. Synthesis and spectroscopic measurements The polycrystalline sample of atrazine (Fig. 1A) was a Sigma–Aldrich product; whereas prometryn and simetryn (Fig. 1B and C) were supplied by Riedel-de Haen, Seelze (D). The purity of all samples, checked by HPLC techniques, was found >98% and then used without further purification. Double distilled water and other ‘analytical grade’ or ‘spectral grade’ Merck, Darmastadt (D), reagents were also used. As apolar solvent, we used carbon tetrachloride (CCl4), in which large concentrations of Prom and Sim can be easily reached, while, as polar solvent, we used DMSO-d6, in which all compounds are highly soluble (Raman spectra were recorded on DMSO solutions at the concentration of 1 M). The use of the deuterated analogous allowed to let the mC–H spectral region free from solvent vibrations overlapping. Raman spectra of solids and solutions were recorded with a Bruker IFS 66 spectrometer equipped with a FRA-106 Raman module and a Ge diode detector. The excitation source was a Nd:YAG laser (1064 nm) in the backscattering (180°) configuration. The focused laser beam diameter was 100 lm, the spectral resolution 4 cm 1. The laser power on the sample was 50 mW. SERS spectra were recorded using a Jasco NRS-2000C instrument in the back-scattering conditions with 4 cm 1 spectral resolution. The excitation source was an Ar+ laser (Innova Coherent 70), using the 514.5 nm line (power of about 40 mW). The detector was a liquid nitrogen 160 K frozen CCD from Princeton Instruments Inc. Final spectra were 10 measure averages. For SERS measurements, a hydroxylamine hydrochloride-reduced silver colloid 10 3 M was prepared by the method described by Leopold and Lendl [34] and the pH of the colloidal solutions was adjusted to 6.0 (with small amounts of diluted HCl). This pH value offered an acceptable compromise between the good stability of the colloid (4 < pH < 6), and the pH with a biological significant (pH P 6), whereas the concentration of added s-triazines to the colloidal solutions was 10 4 M. Both Raman and SERS spectra were recorded at room temperature (20 ± 1) °C and without adding any aggregating substance (like NaCl or KNO3) to the Ag colloidal solution. 2.2. Computational details A full geometry optimization of the electronic ground state of Atr, Prom and Sim was obtained at Density Functional Theory (DFT) level using the Becke three-parameter Lee–Yang–Parr (B3LYP), exchange–correlation functional [35] with 6-311G(d,p), 6-311+G(d,p), and aug-cc-pVQZ (augmented correlation-consistent Quadrupole Zeta) basis sets. This was the first time that such a high level of theory has been applied for the prediction of the properties of these three molecules. The optimized geometries, reported in Fig. 1, were then submitted to vibration calculation in order to calculate the Raman scattering wavenumbers and intensities. No anharmonic or intensity correction was used. Subsequently, starting from the previously optimized structures, the molecular complexes of the considered triazines with Ag2 were built aligning the silver cluster to the lone pairs of the aromatic nitrogen atoms (N2, N4, and N6). Ag2 is the smallest silver

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Fig. 1. Atomic numbering (above) and sketch of the molecular conformation (below) of of Atr (A), Prom (B), and Sim (C).

cluster and has been chosen as a computational target because it was the best compromise between computational cost of the calculation and an acceptable model of the silver surface [36]. Full geometry optimizations and harmonic frequency calculations of the adducts were performed at the B3LYP level with a mixed basis set: 6-311+G(d,p) on C, N, S, Cl, H, lanl2dz (Los Alamos National Laboratory 2 double f) effective core potential on the Ag core (the inner 28 electrons) and lanl2dz basis set on the Ag valence (the outer 19 electrons) [37]. To simulate the effect of the water as solvent, the same kind of calculations was carried out using a conductor-like polarizable continuum model (CPCM) [38]. Gaussian 09 computational package [39] was used in all these calculations. There was a remarkable difference in the accuracy of the results obtained by using the 6-311G(d,p) or 6-311+G(d,p) Pople’s bases (which gave similar predictions), and those obtained by using the aug-cc-pVQZ basis, especially concerning with the intensities. Only this last correlation-consistent basis set provided an excellent prediction of both experimental wavenumbers and experimental relative intensities. In contrast, the two Pople’s basis sets described quite well the experimental wavenumbers (although shifted up to 50–70 cm 1), but were poor in the calculation of bands intensities. Nevertheless, the simpler results obtained with the two Pople’s basis sets were sufficiently accurate to identify with certainty the interaction site between the triazine molecules and the Ag2 cluster. 3. Results and discussion The vibrational spectra (mostly IR but Raman also) of most common pure triazine compounds have been recorded since the ‘60 and the vibrations attribution have been made tentatively by examining the frequency shift caused by the substitution of atoms or groups [40–46]. The availability of high-power computing

machines and of very extended basis of calculus made us able to obtain theoretical spectra that reproduced very closely the experimental ones on a wide spectral range. In this paper we attributed with a high level of confidence each experimental frequency with its generating vibrational modes, thus improving the data on attributions of the triazines Raman spectra previously reported in the literature.

3.1. Pure compounds Figs. 2–4 show respectively the Raman spectra of Atr, Prom and Sim, in the 3500–2700 cm 1 and 1700–100 cm 1 spectral ranges for pure crystalline solids (a) and in solutions of CCl4 (b) or deuterated dimethylsulfoxide (DMSO-d6) (c). As a consequence of the too low solubility of Atr in CCl4, only the spectra of the solid in the DMSO-d6 solution were reported (Fig. 2). Tables 1a–b report the experimental and calculated values of wavenumber and intensity for the main bands of the spectra, while their attribution is discussed in detail in the following sections. It is noteworthy to underline that a very good superimposition between theoretical and experimental wavenumbers and intensities (most within 5 cm 1 and ±10%, respectively) was achieved. Figs. S1–S3 in the supplementary materials, show graphically this excellent agreement between theoretical and experimental data for the three triazines Raman spectra.

3.1.1. 3500–2500 cm 1 spectral region The medium intensity band at a wavenumber greater than 3200 cm 1 (3261 cm 1 in Atr, 3252 cm 1 in Prom and 3265 cm 1 in Sim), was attributed to the symmetrical N–H stretching vibration, confirming the previous attribution in literature [40]; This band was found to be of medium to weak intensity in the Raman spectra, as a consequence of the great variation in dipolar moment

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and in the 1750–200 cm

1

Fig. 3. Raman spectra in the 3500–2500 cm 1 and in the 1750–200 cm regions in which the spectra are free from the solvent bands.

1

Fig. 2. Raman spectra in the 3500–2500 cm spectrum is free from the solvent bands.

1

spectral ranges of: pure solid Atr (a) and of Atr solution in DMSO-d6 (b), in the region in which the

spectral ranges of: pure solid Prom (a) and of Prom solutions in DMSO-d6 (b), and in CCl4 (c), in the

of the molecules, that made this vibration very intense in the IR spectra [40,42,43]. Because of the absence in all the studied compounds of aromatic C–H bonds, all other bands in this spectral region arose from the aliphatic mC–H stretching modes only. On the basis of our theoretical model, we attributed, in Atr, the band at 3112 cm 1 to the mC31–H + mC32–H coupled with the mC52–H + mC53–H synchro-

nous vibrations; the 2974 and 2939 cm 1 bands to the ms and masC51–H vibrational modes, respectively, and the 2871 cm 1 peak to the msC–H vibrations in the CH2 coupled with those in the CH3 of the ethyl moiety. The weak peak at a lower wavenumber (2719 cm 1), arose from the msC–H of the CH3 groups of the isopropyl moiety, whereas the weak bands at 2686 and 2491 cm 1 were generated by the simultaneous msC–H vibration on the CH3 of both

S. Bonora et al. / Journal of Molecular Structure 1040 (2013) 139–148

Fig. 4. Raman spectra in the 3350–2500 cm 1 and in the 1750–200 cm regions in which the spectra are free from the solvent bands.

1

143

spectral ranges of: pure solid Sim (a) and of Sim solutions in DMSO-d6 (b), and in CCl4 (c), in the

Table 1a Wavenumbers and intensities, both theoretical and experimental, of the Raman bands of Atrazine. Theoretical values were cut off after the first decimal. Atrazine Experimental wavenumber (cm 1)

Theoretical wavenumber (cm 1)

Experimental intensity (I/ I2939  100)

Theoretical intensity (I/ I2939  100)

195 253 324 370 416 546 648 684 839 924 964 993 1084 1127 1168 1251 1310 1343 1378 1388 1448 1550 1599 1610 1741 2491 2686 2719 2871 2939 2974 3112 3261 3519

192.3 257.6 319.9 363.4 411.0 560.3 646.1 677.8 849.9 917.9 976.8 1005.8 1101.4 1131.8 1169.2 1256.6 1311.7 1341.5 1376.0 1388.7 1442.7 1589.0 1604.6 1624.9 1755.3 2515.7 2662.1 2733.5 2876.1 2945.2 2976.3 3114.5 3258.8 3517.9

19 15 56 19 22 15 22 26 22 22 81 19 11 11 7 33 13 12 7 4 41 15 26 15 2 3 2 7 52 100 96 7 22 8

20.0 16.1 55.9 20.4 21.5 13.3 20.2 29.0 22.1 20.4 79.4 21.2 10.9 13.3 8.6 31.3 7.1 12.6 4.5 2.6 40.3 12.5 23.5 12.6 1.9 6.2 1.2 7.0 54.9 100.0 92.1 6.6 23.0 7.7

ethyl and isopropyl groups coupled to a stretching mode involving the C51–C53 and C31–C32 bonds. In Prom the 3182 cm 1 peak was attributed to the ms mode of the CH3 groups of the sulfanyl moiety; the 3087 and 3019 cm 1 bands were attributed to the mC52(53)–H and to the mC51–H modes, whereas the 2982 and 2972 cm 1 bands arose from the same vibrations, but relative to the C31, C32 and C33 moiety. The 2935 and 2913 cm 1 bands were attributed to the mixed mode mC31–H + mC32–H + mC33–H and to the mC51–H + mC52–H + mC53– H, respectively, whereas the 2874 cm 1 band was attributed to a mode involving the simultaneous stretching of the three C–H bond in the CH3 groups. The weaker components received the following attributions: the 2760 cm 1 band to the stretching of all C–H bond of a moiety superimposed to a dH–C–C bending on the same moiety; the 2717 cm 1 band to the same mode, but relative to the other moiety and the 2461 cm 1 band to the symmetric stretching mC31–H composed with a stretching C31–C32. A similar behavior was found in the Sim Raman spectrum: the 3100 cm 1 band was attributed to the mC52–H + mC51–H modes superimposed with the mC32–H + mC31–H; the 2976 and the 2938 cm 1 bands to the msC52(C32)–H and to the msC51(C31)–H modes, respectively. The 2877 cm 1 band originated from a mode involving the simultaneous stretching of the three C–H bond in the CH3 groups; whereas the 2746 cm 1 and the 2457 cm peaks arose from the same mC–H superimposed to a dH–C–C bending or a mC–C stretching modes involving the same CH3 group, respectively. The broadness of the coupling in most observed modes confirmed the flatness of the molecules and, in Prom and Sim, the difficulty to rotate around the C1–S bond. In the polar and apolar solutions, the mC–H stretching modes wavenumbers were unchanged (Figs. 2–4), confirming that these modes were not involved in the interaction with the solvents and were independent from the polarity of the solvent itself. On the contrary, the intensity of the peaks exhibited an intensity decrease in the higher frequency component and this effect could be ascribed to the dielectric properties of the solvents. In the supplementary material three tables (Tables S1-S3) with a list of the

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Table 1b Wavenumbers and intensities, both theoretical and experimental, of the Raman bands of Prometryn and Simeryn. Theoretical values were cut off after the first decimal. Prometryn Experimental wavenumbers (cm 1)

Simetryn Theoretical wavenumbers (cm 1)

Experimental intensity (I/ I2935  100)

Theoretical intensity (I/ I2935  100)

264 308

264.9 299.4

46 23

42.0 20.3

424

421.7

28

30.1

705

708.8

38

32.7

836 906 951 971

820.7 893.2 951.1 957.4

25 16 16 84

22.0 13.4 14.6 81.2

1132 1182 1277 1314

1113.8 1174.0 1266.3 1301.4

10 12 21 18

10.3 10.3 20.9 22.7

1350

1341.3

16

14.7

1420 1453 1462 1522 1547 1581 1606 1621 2461 2717 2760 2874 2913 2935 2972 2982 3019 3087 3182 3252

1409.0 1452.2 1456.3 1510.2 1524.2 1580.7 1606.0 1619.0 2450.2 2715.3 2758.1 2869.9 2900.1 2930.9 2966.0 2979.3 3014.7 3095.3 3183.1 3248.6

14 25 24 4 6 5 4 4 2 5 6 48 66 100 70 63 18 8 8 15

13.0 21.4 22.8 4.4 5.6 4.5 2.1 4.5 0.7 3.9 5.7 48.3 63.7 100.0 68.3 64.4 22.1 5.9 12.5 14.3

main bands of the three triazines alone and in CCl4 and DMSO-d6, showed the above mentioned effects. 3.1.2. 1650–150 cm 1 spectral region 3.1.2.1. Atrazine. The 1610 cm 1 peak arose mainly from a deformation in plane of the ring +dC3–N3–H, the 1599 cm 1 peak from a deformation in plane of the ring +dC5–N5–H; whereas the 1550 cm 1 from the simultaneous mC5–N6 and mC3–N2, in phase and in the plane, mixed with dC3–N3–H + dC5–N5–H. Both the 1448 cm 1 and the 1388 cm 1 component could be mainly ascribed to the deformation modes dCH3 in both acylic chains; in particular, the latter was ascribed mainly to a symmetrical deformation of the CH3 groups on the isopropyl moiety. The 1378 cm 1 and the 1343 cm 1 bands were ascribed to the dC–C– H modes on the aliphatic moieties mixed with the in plane deformation of the triazine ring and with the same deformation plus the mC–Cl stretching, respectively. The 1310 cm 1 band was attributed to the rocking vibration of the terminal aliphatic chains (mostly of the ethyl chain) and the 1168 cm 1 arose from the dC–C–H of the terminal aliphatic chains mixed with the out of plane deformation of the ring plus the mC–Cl mode. The 1127 cm 1 peak was attributed mostly to the dC–C–H on the terminal groups of the aliphatic chains.

Experimental wavenumbers (cm 1)

Theoretical wavenumbers (cm 1)

Experimental Intensity (I/ I2935  100)

Theoretical Intensity (I/ I2935  100)

233 300 356 401 493 566 648 695 727 870

232.2 306.5 353.7 399.4 499.9 563.6 647.1 684.9 736.6 862.6

2 44 22 3 4 2 33 17 22 17

1.4 42.4 22.1 3.6 2.8 1.2 30.8 15.2 20.1 16.1

949 993 1080

948.0 988.4 1085.5

39 39 17

37.4 37.9 16.3

1274 1312 1322 1342 1389 1432 1459 1486

1269.7 1307.4 1320.9 1329.3 1387.4 1436.9 1457.1 1476.5

17 11 11 17 56 22 33 11

16.9 10.4 10.5 15.9 54.2 21.6 31.1 11.5

1553 1586 1607 1621 2457

1551.4 1585.2 1610.0 1619.7 2454.7

6 6 6 6 2

4.8 7.2 5.0 6.2 1.8

2746 2877

2744.0 2870.2

6 56

5.1 53.9

2938 2976

2930.3 2968.3

100 39

100.0 37.8

3100

3103.3

5

5.4

3265

3249.0

11

10.4

Other bands (1084 and 993 cm 1) had a noticeable component from the deformation in plane of the ring, but, in any case, mixed with dN–C–H or dC–C–H modes. It is worth mentioning that many peaks (1343, 1310, 1251, and 1168 cm 1) showed a small component arising from the mC1–Cl stretching modes or from a deformation between the ring and the Cl atom, while the 964 cm 1 appeared as the ‘ring breathing’ mode, confirming, in this case, the previous existent attribution in literature [44,45]. The presence of the mC–Cl vibration played an enhanced role in the 924 cm 1 peak, in which it was mixed with an asymmetric in plane deformation of the ring, localized mainly on the N atoms and in the 416 cm 1 vibration, in which the mC– Cl was mixed with a rocking (q) mode of the isopropyl groups. At wavenumbers lower than 700 cm 1, the deformation out of plane of the ring played the main role, and the 684 cm 1 peak was mainly generated by this mode, confirming the older attribution [41,42]. The 648 cm 1 peak arose from the synchronous asymmetric deformation dN6–C1–N2 and dC5–N4–C3 and the 546 cm 1 band appeared to be generated by the twisting of the ring in plane mixed with the simultaneous bending modes on the ethyl and isopropyl groups. The lower wavenumbers modes appeared of a great complexity, involving the simultaneous movement of about all the atoms in the chains; it is interesting to note that the 253 cm 1 peak arose from

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S. Bonora et al. / Journal of Molecular Structure 1040 (2013) 139–148 Table 2 Calculated structures (B3LYP/6-311+G(d,p), and lanl2dz ECP basis set on silver atoms), relative energy and zero-point energy corrected values (DE/DE0 kJ mol molecular complexes of atrazine, simetryn and prometryn. The same results for the water solvated forms are listen in the second row. Atrazine

Prometryn

Simetryn

13.0/13.6 11.1/11.5w

1.5/1.3 0.3/0.2

2.9/2.5 0w

9.9/11.0 9.6/9.7w

0 0w

0 0.5/0.2w

0 0w

4.6/4.7 3.3/3.4w

6.2/5.6 3.8/4.6w

1

) of the N:Ag2

Ag


N2

Ag


N4

Ag


N6

the simultaneous rocking of the C53–H and C52–H bonds in the isopropyl moiety; whereas the 195 cm 1 component was attributed to the simultaneous rocking C32–H on the ethyl moiety mixed with a out of plane deformation of the triazine ring, involving mainly the C5–N4–C3 bond. In DMSO-d6 solution the peaks involving the N–H vibration were noticeably affected, whereas less affected appeared those involving the ring motion and we attributed these differences to the high polarity of the solvent. As a matter of fact, the mN–H peak appeared strongly broadened in solution, as a consequence of the setting up of a H-bond network between solute and solvent molecules. Nevertheless, the substantial constancy of the peak wavenumber suggested that the strength of the N–H


O bond was of the same order of magnitude of the N–H


H hydrogen-bond in the solid state. The spectral region 1500–1625 cm 1 appeared to be modified, exhibiting a noticeable shift (from 1550 cm 1 to 1589 cm 1) of the peak in which the dC–N–H modes played an important role. On the contrary, small influence was observed in the lower wavenumbers (up to 950 cm 1), in which the deformation in plane of the ring, mC–H or mC–Cl vibrations were mostly involved (see Table S1). 3.1.2.2. Prometryn and simetryn. In Prom and Sim the weak peaks found in the region 1625–1520 cm 1 have the same attributions as for Atr: the 1621 cm 1 peak arose from the deformation in plane of the ring mixed with the deformation dC3–N3–H, the 1606 cm 1 (1607 cm 1 in Sim) from the ring deformation in plane +dC5–N5–

H, the 1581 and 1547 cm 1 (1586 and 1553 cm 1 in Sim) from the simultaneous mC5–N6 and mC3–N2, out of phase and in phase, mixed with dC3–N3–H + dC5–N5–H vibrations. The 1462 cm 1 (1486 cm 1 in Sim) component could be ascribed to the deformation modes dCH3 on the sulfanyl group, confirming the previous attribution [46]. To the same group could be attributed the 1350 cm 1 peak (1342 cm 1 in Sim) generated from the symmetrical bending of the sulfanyl CH3 group mixed with the deformation dC–C–H on the lateral chains. Other bands up to 1000 cm 1 had a noticeable component from the in plane deformation of the ring, mixed with dN–C–H or dC–C– H modes or with the stretching mC–S, as in the 1314 and 1277 cm 1 bands (1312 and 1274 cm 1 in Sim). The simultaneous stretching of the C–S bond occurred in many vibrational modes of the molecules, as in the 1182 and 1133 cm 1 bands of Prom, in which it is coupled to synchronous deformation modes on the isopropyl group (the same modes appeared as a single peak at 1080 cm 1 in Sim). The intense peaks at 971 cm 1 in Prom and at 993 cm 1 in Sim appeared as the ‘ring breathing’ modes, whereas other peaks up 800 cm 1 (951, 906 and 836 cm 1 in Prom; 949 and 870 cm 1 in Sim) arose from complex modes, in which the in plane deformation modes of the ring were mixed with different modes on the two lateral chains. At lower wavenumbers, the out of plane deformation of the ring appeared at 705 cm 1 (695 cm 1 in Sim), and it was mixed with the simultaneous stretching of the C1–S bond. As already said, a

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about 30 kJ mol 1 and than the one with the sulfur atom by 15–20 kJ mol 1; (5) when the intermolecular bond is forced to bend by steric hindrance effects, the corresponding dissociation energy lowers; this is the case of 1-thiomethyl-s-triazine: where the methyl group is in the cis arrangement instead of the trans arrangement with respect the C1–N2 bond. In this case the dissociation energy of the N2:


Ag2 bond is about 14 kJ mol 1 lower. (6) the hydration process plays an important role to definite the most probable binding site (Table 2).

detailed discussion of the lower wavenumber modes appeared to be of relative interest, because these modes involve the simultaneous movement of about all the atoms in the molecules. Some modification can be detected in the solutions and, as a general feature, they were more evident in DMSO-d6 solution (see Table S2, S3 in the supplementary material). Like in Atr, the spectral region 1500–1625 cm 1 of both Prom and Sim was noticeably affected in DMSO-d6 solution, exhibiting wavenumber and intensity changes in the peaks in which the dC3(5)–N3(5)–H modes played an important role. Unfortunately, the intensity of the peaks in this region in CCl4 solutions was too low to appreciate variation. Even the vibrations related to the sulfanyl group or ring motions appeared to be influenced by the interaction with solvent molecules, as confirmed, for example, by the downshift of the 1459 cm 1 in Sim and of the 1462 cm 1 in Prom bands to 1442 cm 1 and 1452 cm 1, respectively, and by the upshift of the 695 cm 1 in Sim and 705 cm 1 in Prom bands to 723 cm 1 in both compounds and solvents.

In conclusion, we can state that the preferential interaction sites of the silver cluster Ag2 were the nitrogen aromatic atoms. On these basis, we considered the molecular complexes of Ag2 with Atr, Prom and Sim, in their stable molecular conformation (Fig. 1), in the three aromatic N atoms. The obtained results, reported in Table 2, confirmed the data concerning the simplest molecules examined above: the most stable forms were those in which the Ag2 cluster was collinear to the nitrogen lone pair direction. In addition, we observed that the bonding of Ag2 to the N2 atom of Sim induced the rotation of the ethyl group. The agreement within the calculated Raman spectra of triazines in their corresponding N2


Ag2, N4


Ag2 and N6


Ag2 molecular complexes and the corresponding experimental spectra (Figs 5 and 6) was worse than for the pure compounds. This was a consequence of the increased complexity of the systems, that forced to use a more simplified basis set; nevertheless, the trend of the theoretical spectra offered a useful guideline for the comprehension of the interaction sites and of the structural modification in geometry after adsorption. Moreover, the use of the Ag2 cluster in the DFT calculus of the SERS interaction enabled a sensible improvement of the agreement between the theoretical and the experimental SERS spectrum, compared to the single Ag atom used by other authors [31,32,36]. Fig. 5 reports the simulated (B3LYP/6311+G(d,p), and lanl2dz ECP basis set on silver atoms) SERS spectra of the water solvated N:


Ag2 molecular complexes of Atr (N6:


Ag2 interaction), Prom (N2:


Ag2 interaction) and Sim (N2:


Ag2 interaction) in the 1600–100 cm 1 spectral region.

3.2. SERS spectra In each of the investigated molecules, there are six possible electron-donor sites available for the interaction with silver: the heteroatoms (sulfur or chlorine), the three aromatic nitrogen atoms and the two aliphatic nitrogen atoms. In order to probe the strength of the different kinds of interactions we submitted to the theoretical calculus the three amino-, chloro- and thiomethyl- monosubstituted s-triazine molecules, considering their plausible molecular complexes with Ag2. The results can be summarized in few points: (1) it has not been possible to obtain a local minimum with the silver atom bound to the chlorine atom; (2) if steric hindrance effects are not present, the Ag2 cluster is almost collinear to the direction of the heteroatom lone pair; (3) the planar amino group bends when the nitrogen atom binds to silver; (4) the intermolecular bond of the silver atom with the aromatic nitrogen atoms is stronger then that with the aminic one by

10

atrazine prometryn simetryn

Intensity / arb. units

8

6

4

2

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

1600

0

Wavenumbers / cm-1 Fig. 5. Simulated (B3LYP/6-311+G(d,p), and lanl2dz ECP basis set on silver atoms) SERS spectra of the water solvated N:


Ag2 molecular complexes of (a) Atr (N6:


Ag2 interaction), (b) Prom (N2:


Ag2 interaction) and (c) Sim (N2:


Ag2 interaction) in the 1600–100 cm 1 spectral region. An arbitrary 5 cm 1 FWHM for the predicted band was chosen for the spectra visualization.

S. Bonora et al. / Journal of Molecular Structure 1040 (2013) 139–148

Fig. 6. SERS spectra in the 3400–2700 cm

1

1700–200 cm

1

spectral range of 10

Fig. 6 exhibits the experimental SERS spectrum of 10 4 M water solutions of the some compounds in the 3200–2700 and in the 1700–200 cm 1 spectral regions. The spectra obtained with a 10 4 M concentration of the compounds were almost identical to that of 5  10 5 M, confirming that the saturation of the binding sites on the silver surfaces was reached. In the mC–H spectral region (3100–2700 cm 1), the experimental SERS spectra (Fig. 6) resemble the spectra in DMSO-d6, confirming that the acylic chains maintained the same freedom degrees and structure, apart from the silver presence. On the contrary, the mN–H in all spectra was shifted to a lower wavenumber (from 3250 cm 1 to 3210 cm 1), and this behavior can be ascribed to the setting up of strong H-bond N–H


O between the aminic groups and water molecules. Differences were observed in the 1500–1100 cm 1 region of the experimental SERS spectra of the considered compounds, well evident on the vibrational modes involving the ring. This behavior confirmed that triazines adsorb on the silver particles interacting through the aromatic N atoms, as it was suggested, in the Atr SERS spectrum, by the shift from 1599 cm 1 to 1580 cm 1 and by the setting up of a new intense SERS peak at 1340 cm 1. Noticeable changes leading to the same conclusions were observed in the 1550–1500 cm 1 spectral region of Prom and in Sim. Furthermore, the shift of the 1314 and 1277 cm 1 bands in pure Prom (1312 and 1274 cm 1 in Sim), arising from the in plane deformation of the ring, to 1307 and 1255 cm 1 in SERS spectra of Prom (1308 and 1258 cm 1 in Sim SERS spectra) supported this hypothesis. The same effect was responsible of the shift of the 940–965 cm 1 peaks in the pure triazines Raman spectra, attributed to the breathing mode of the ring, to higher SERS wavenumbers (970–990 cm 1) and with increased intensity. The energy calculations of the triazines


Ag2 molecular complexes allowed predicting more exactly the N atom interested in the interaction, concluding that the most probable sites are the N6 in Atr and N2 or N4 in Prom and Sim. Indeed, the intense SERS

4

147

M solutions of Sim (a), Prom (b), and Atr (c).

experimental peak at 1450 cm 1 of all triazines was found, at about the same wavenumbers and intensity, only, in the theoretical SERS spectra relative to the previously noted binding sites (Figs. 5 and 6). Moreover, the weak experimental SERS peak of Atr at 1355 cm 1 was present at 1362 cm 1 in the theoretical (N6 binding site) and the peak at 1308 cm 1 in Prom (experimental) was found at 1302 cm 1 in the theoretical (N2 binding site). Similar, and within the error limits, was the behavior of the Sim corresponding peak (1258 cm 1 experimental; 1272 cm 1, theoretical, N2 binding site). The intense peak at about 970 cm 1 in the experimental SERS spectra was due to the symmetrical expansion mode of the ring and it was predicted, even of reduced intensity, in the 940–970 cm 1 range in the theoretical ones, but without noticeable differences between the different N atoms as binding sites. The intense theoretical peak at about 670 cm 1 characterized the binding site N2 of Prom e Sim (Fig. 5); whereas the N4 site was characterized by a strong peak at about 450 cm 1. The simultaneous presence of both peaks at about the same intensity in the experimental SERS spectra of Prom and Sim (Fig. 6b, e and c) led us to hypothesize the presence of an equilibrium between the two binding sites. Since the chosen basis set did not give a satisfactory approximation about the theoretical intensities, it was not possible to make an accurate prevision on the equilibrium ratio, even if the behavior of the higher frequency peak may suggest that the N2 binding site is the most probable. These results were in partial agreement with the study of Costa et al. on ametryn (whose structure is intermediate between Prm and Sim), where they speculated an interaction with the N4 atom [31], while they did not agree with the other paper of Costa et al. on atrazine [32]. In this last work, the authors hypothesized an interaction with N2, assuming that the interaction with N6 was prevented by the bulky isopropyl moiety. However, the Atr calculated structure did not present the tilted isopropyl group as in our case, and this can be attributed to the simpler basis set adopted by Costa et al. Moreover, in their paper, they could not observe the strong SERS

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calculated band at 1339 cm 1, marker of the N2 interaction [32]. Therefore we estimate that our theoretical approach can better describe the Ag–Atr interaction. The intense experimental SERS peak observed at 235–225 cm 1 (Fig. 6), characteristic of most SERS spectra on colloidal Ag obtained using hydroxylamine HCl as reducing agent, has been attributed to a mAg–Cl stretching vibration [34]. In the same region of the theoretical spectra, we observed an intense peak at about 180 cm 1 in the all the Ag2


triazine adducts (Fig. 5): this peak was generated by the stretching vibration m(Ag–Ag) coupled with a simultaneous skeletal vibration involving the whole triazine molecule. However, this band came from an oversimplified model of the Ag surface and it seems unlikely that could be related to the intense experimental 230 cm 1 band. 4. Conclusions The comparison of our theoretical attributions, resulting from the use of a very extended basis set, with the previous one, showed that the use of less refined basis sets or atom/group substitution method [31,32,40–44] led to an excessive simplification of the vibrational modes, overestimating the localized vibrations role. In fact, excluding the mN–H vibrations, no ‘true’ single bond localized vibrations exist, whereas they arose from a combination of different modes. The comparison between theoretical and experimental frequencies (reported in Tables 1a and 1b), showed an excellent agreement. The differences in most cases lay within few wavenumbers, supporting the correctness of the molecular structure geometries reported in Fig. 1. The small differences observed can be ascribed to the solid crystalline phase as stemming from local deviations in the uniformity of the crystalline field or from a coupling with the lattice vibration. Moreover, theoretical and experimental peak intensities were also in good agreement, generally within ±10%; discrepancies can be attributed to the local non-homogeneity of the dielectric properties at the crystal, an effect not comprised in the DFT level. The role of the adsorption of the molecules was highlighted since interaction of triazines on a metal surface could mimic the adsorption of these substances on a colloidal clay particle or on carrier protein molecules. The agreement within the calculated Raman spectra of Atr, Prom and Sim in their corresponding N2


Ag2, N4


Ag2 and N6


Ag2 molecular complexes and the corresponding experimental spectra was worse than for the pure compounds, consequence of the increased complexity of the systems that forced to use a more simplified basis set. Nevertheless, from the comparison between experimental and theoretical data (in particular the energy calculations), we can conclude that the interaction site of Atr was on the N6 atom, whereas in Prom and Sim two interaction sites were possible: the N2 atom (the most probable) and the N4. Moreover, the theoretical SERS spectra using as a model the simplest silver cluster, suggest that Ag2 may be considered a sufficiently good probe to identify the principal intermolecular interactions between metal particles and adsorbing molecules. Acknowledgment This study was supported by a Grant of MIUR ex 60% to Sergio Bonora and Vitaliano Tugnoli. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2013.02.025.

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