IR, Raman and SERS spectral analysis and DFT calculations on the Herbicide O,S-Dimethyl phosphoramidothioate, metamidophos

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 114 (2013) 120–128

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IR, Raman and SERS spectral analysis and DFT calculations on the Herbicide O,S-Dimethyl phosphoramidothioate, metamidophos Guillermo Diaz Fleming a,⇑, Joao Villagrán a, Rainer Koch b a b

Molecular and Atomic Spectroscopy Laboratory, Department of Chemistry, Faculty of Sciences, University of Playa Ancha, Casilla 34-V, Valparaiso, Chile Institute for Pure and Applied Chemistry and Center of Interface Science, University of Oldenburg, P.O. Box 2503, D-26111 Oldenburg, Germany

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Raman and SERS study of pesticide

Infrared, Raman and SERS spectra of O,S-Dimethyl phosphoramidothioate, metamidophos, MAP, have been recorded. A DFT calculation was used for the optimization of the ground state geometry, simulation of the vibrational spectra and interaction MAP-metal surface. The comparison of SERS spectra obtained by using colloidal silver nanoparticles, with the corresponding Raman spectrum reveals enhancement and shifts in bands as well as information about the orientation of MAP on the nm-sized metal structures and the importance of the S atom on the SERS effect. DFT modelling of the SERS effect and Molecular Electrostatic Potentials (MEP) confirms the experimental information.

metamidophos, MAP, is reported.  Results on its structure, mode assignments and MAP-Ag surface are presented.  DFT, experimental and NCA with multiple scaled force constants are performed.  SERS modelling and electrostatic potentials evidence the interaction MAP-Ag surface.

a r t i c l e

i n f o

Article history: Received 7 January 2013 Received in revised form 4 May 2013 Accepted 6 May 2013 Available online 28 May 2013 Keywords: Methamidophos Infrared Raman SERS DFT SQM force field

a b s t r a c t Infrared, Raman and SERS spectra of O,S-Dimethyl phosphoramidothioate, metamidophos, MAP, have been recorded. Density Functional Theory, DFT, with the B3LYP functional was used for the optimization of the ground state geometry and simulation of the infrared and Raman spectra of this molecule. Calculated geometrical parameters fit very well with the experimental ones. Combining the recorded data, the DFT results and a Normal Coordinate Analysis based on a scaled quantum mechanical (SQM) force field approach, a complete vibrational assignment was made for the first time. The comparison of SERS spectra obtained by using colloidal silver nanoparticles, with the corresponding Raman spectrum reveals enhancement and shifts in bands as well as information about the orientation of MAP on the nm-sized metal structures and the importance of the S atom on the SERS effect. DFT modelling of the SERS effect and Molecular Electrostatic Potentials (MEP) confirms the experimental information. Ó 2013 Published by Elsevier B.V.

Introduction

⇑ Corresponding author. Tel.: +56 32 2500528. E-mail address: [email protected] (G.D. Fleming). 1386-1425/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.saa.2013.05.012

Compounds of phosphorus are one of the most important and widely used classes of the modern pesticides. Many derivatives

G.D. Fleming et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 114 (2013) 120–128

of the phosphorous, thiophosphorous, phosphoric, thio and dithio phosphoric, phosphonic and thiophosphonic acids possess pesticidal properties and have been successfully used in agriculture [1]. Organophosphorous compounds (OPC’s), extensively used as insecticides, are cholinesterase-inhibitors [2] due to the covalent interaction of the S-P part of the molecule with the enzyme active center. Most are highly toxic to humans and other mammals by all routes of exposure. When inhaled, the first effects are usually respiratory and may include bloody or runny nose, coughing, chest discomfort, difficult or short breath and wheezing due to constriction or excess fluid in the bronchial tubes. Due to their high chemical stability, OPC’s resist natural decomposition and biodegradation [3]. Moreover, owing to their high polarity and dissolution capacity, they can easily infiltrate the soil and migrate to other locations [4,5]. Metamidophos (O,S-Dimethyl phosphoramidothioate, MAP) is an OPC with a broad spectrum of activity as an insecticide–acaricide and is widely used on vegetables, corn, and some other crops [6]. This compound is derivative of thiophosphoric acid [7] and is a kind of high-effective organophosphate pesticide (OPP) that is persistent in the environment and has heavy toxicity; its LD50 (rat, oral) is 15–18 mg/kg [1]. The organophosphate neuropathy due to MAP is a biochemical and neurophysiological marker. Concerning its structure, Solovyov et al. [8] reported the crystal and molecular structure, while cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to study MAP and humic acid (HM) specifically adsorbed onto Pt and PtO films in pH 7.0 by Silva et al. [9] OPC’s in food are clearly dangerous to human health because of the increasing environmental damage as a consequence of its intensive and extensive use in agriculture. There exists a considerable interest in highly sensitive, selective, rapid, reliable, fielddeployable and cost-effective analytical methods or devices for OPC’s monitoring. In this sense, vibrational spectroscopy which has been scarcely used in the past for the qualitative and quantitative analysis of active principles in commercial pesticide formulations, is nowadays used commonly to study these toxic substances. Despite its importance vibrational studies of MAP have not been reported in the literature to the best of our knowledge. Like infrared spectroscopy, Raman spectroscopy also provides information on the distinct normal modes with a high degree of specificity in analysis [10]. The rather weak Raman effect can be greatly strengthened if the molecules are attached to nm-sized metal structures in the well-known Surface Enhanced Raman Scattering, SERS [11–17]. This method gives almost the same information on the molecules and their interactions as normal Raman spectroscopy, but ensures a great sensitivity. Raman as well as SERS techniques are usually complemented with theoretical methods for the computation of molecular structures, such as Density Functional Theory, DFT, which provides a promising cost-effective approach for calculating vibrational spectra of large molecules [18,19]. Therefore, the present work was carried out to perform the first complete vibrational study of this pesticide and to identify the various normal modes with high wavenumber accuracy. DFT calculations and experimental data were combined in a Normal Coordinate Analysis based on the scaled quantum mechanical (SQM) force field technique developed by Pulay et al. [20] which represent the limit in accuracy that can be achieved within the harmonic approximation, This allows better infrared and Raman bands assignments, throughout the Potential Energy Distribution, PED, of the distinct local symmetry coordinates constructed for the molecule under study. The SERS modelling will be accomplished by using a theoretical approach successfully employed on several other occasions [21–27]. Furthermore, the adsorption behaviour of MAP on colloidal silver surface is deduced from the

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SERS selection rules and analysis of the calculated molecular electrostatic potential (MEP).

Experimental and computational methods Samples and instrumentation MAP (C2H8NO2PS) of analytical grade was purchased from Arysta LifeScience Co. in aqueous solution like and was used to record the Raman and IR spectra. Stock solutions of MAP in water were prepared to a final concentration of 103 M. Aqueous stock solutions of the compounds were prepared in nanopure water. The pH of the solution was adjusted to 7.0 by dropwise addition of 2 M NaOH. A solution with an MAP concentration of 5 mM was also prepared in order to test the spectral stability with respect to the SERS spectrum. The weak Raman bands were less resolved but the strong bands showed the same position and relative intensity as in the less concentrated solutions. Colloidal silver nanoparticles were prepared by using hydroxylamine hydrochloride (NH2OHHCl) as reducing agent [28]. These nanoparticles have the advantage of a more uniformly distribution of size and shape together with the absence of interferences from residual oxidation product. The hydroxylamine hydrochloride reduced colloid was prepared by adding 10 mL of silver nitrate (102 M) to a 90 mL hydroxylamine hydrochloride (1.67  103 M) and sodium hydroxide (NaOH) (3.33  103 M) solution. This mixture was formed at room temperature under rapid stirring conditions. The final concentration in the mixture of silver nitrate and hydroxylamine/sodium hydroxide was 103 M and 1.5  103 M/ 3  103 M, respectively. SERS samples Diluted MAP solution was mixed with citrate reduced Ag colloidal solution to obtain a final solution MAP/metal colloid at 1:10 proportion; a drop of each system was deposited onto a quartz slide. The spectra were obtained with 1 and 50 objectives for SERS and micro-SERS measurements, respectively. Room temperature analyte-metal system was used for the SERS measurements. Raman spectra were registered in solid and aqueous solution 101 M. Instrumentation The IR spectrum was recorded in solid state using an ATR Buck spectrophotometer model M 500 equipped with a high energy optical design and a sensitive DLATGS detector. The Raman spectrum of the pure crystals of MAP was recorded with the Advantage 200A spectrophotometer on an aluminum foil using Right Angle Input Optics accessory at room temperature. When acquiring data, the system emits up to 3 mW at 633 nm of radiation through its optics. The spectral scanning conditions avoid sample degradation. Spectral resolution was 4 cm1. Integration and acquisition time were 1 and 15 s, respectively. SERS spectra of MAP were measured with a Renishaw micro-Raman system (RM2000) equipped with the 633 nm excitation laser lines, a Leica microscope and an electrically cooled CCD camera. The signal was calibrated by using the 520 cm1 line of a Si wafer and a 50 objective. The laser power on the MAP-Ag system was less than 2 mW. The resolution was set to 4 cm1. Spectra were recorded in the 200–3500 cm1 region. The sample was photo stable when irradiated with laser line at 633 nm.

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The UV absorption data was collected using a Jasco Model V-530 double bean spectro-photo-meter. The scanning region is from 190 to 1100 nm, and the spectral bandwidth is 0.3 nm.

the Gaussian 03 program and adjusted with MOLVIB during the scaling procedure, were subsequently converted to relative Raman intensities (Ii) using the following relationship derived from the basis theory of Raman scattering [44–47].

Colloid analysis Experiments were conducted to determine the sensitivity of the colloids to provide a SERS enhancement on analytes known to produce a strong SERS signal. The capacity for SERS enhancement by the prepared colloids was carried out by performing UV absorption tests. Computational details All calculations were performed with the program package Gaussian 03 [29]. Density Functional Theory (DFT) with the B3LYP functional [30,31] were used for the optimizations of the ground state geometries, computation of molecular electrostatic potential (MEP) and simulation of the vibrational spectra. The default convergence criteria and integration grid of the program were used. The basis set was a combination of Pople’s (6-311+G(d,p) [32–35] for the MAP atoms and SDD [36,37] for silver atoms together with the SDD pseudo-potentials. Vibrational spectra were obtained without anharmonic corrections and Raman activities were determined using numerical differentiation. The eighteen atom neutral silver cluster was constructed as a closed-shell singlet based on the experimental bulk geometry [38] data with 12 atoms on the surface plane and 6 atoms on a parallel lower layer, representing the Ag [1,1,0] surface. The resulting z-matrix was constrained to have four independent variables; the distance between two adjacent Ag atoms parallel to the surface, the shortest distance between Ag atoms between surface parallel layers, the angle between three adjacent atoms in the surface plane and the angle between two atoms on the surface plane and one on the next plane. The attached MAP had no geometry constraints. The B3LYP hybrid functional frequencies are usually higher than the experimental ones. The systematical overestimations of the calculated harmonic wavenumbers arise from well-known factors such as neglecting anharmonicity characters of the normal modes, basis set super position error, basis set truncation effect as well as the deficiencies arising from the calculation method used itself. Therefore multiple scaling of the force field by SQM procedure [20,39] has been performed to obtain a better agreement between the theory and the experiment. This procedure not only modifies the frequencies but also the normal modes of vibrations (the vibrational eigenvectors) and the corresponding total energy distributions (TEDs). A Normal Coordinate Analysis, NCA, has been performed in order to obtain the detailed interpretation of the fundamental modes using Sundius’ MOLVIB program version 7.0 [40,41] in which the internal force constants fij extracted from Gaussian calculation are transformed in fij0 through the expression:

Ii ¼

f ð v 0  v i Þ 4 Si

v i ½1  expð hckTv Þ i

where m0 is the exciting frequency in cm1 units (m0 = 15797.79 cm1 corresponding to the wavelength of 633 nm of the HeANe laser used in this study), mi is the vibrational wave number of the ith normal mode; h, C and k are fundamental constants, T is temperature in Kelvin, and f is an appropriate normalization factor for all peak intensities, obtained through the relationship:

f ¼ ð24 p4 =45Þ  ðh=8p2 cv i Þ The simulated IR and Raman Spectra have been plotted using pure Lorentzian band shapes with the full width at half height (FWHH) of 10 cm1.

Results and discussion Optimized structure Fig. 1 shows the DFT-optimized molecular structure of MAP. In Table 1, calculated angles and bond distances are compared with X-ray data obtained by Solovyov et al. [8]. The O,S-Dimethyl phosphoramidothioate crystallizes in the monoclinic space group P21/n with cell dimensions a = 5.374(3), b = 9.220(4), c = 13.847(5) Å and b = 101.08(5)° at 100 °C. The structures were solved by direct methods and refined by least-squares to R = 0.0493(1) and 0.0482(2). The coordination around P of this molecule is distorted tetrahedrally. This molecule has nearly planar HCSP = O and HNPOC moieties. The HCSP = O moiety has trans-orientation in the HCSP, CSP = O and NPOC groups. The angle between these planes is 85.3°. Experimental geometrical characteristics of MAP are in very good agreement with those ones obtained through our DFT calculations. Calculated parameter show a very good correlation for linear regression in the case of bond distances (R2 = 0.996). Correlation for angles deviates (R2 = 0.89) mainly for small and medium values.

fij0 ¼ ðsi sj Þ1=2 fij ; where the symbols si and sj denote the scale factors for the diagonal force constants fii and fjj. The multiple scale factors are obtained in a least square iterative refinement process of the force constants with participation of the experimental frequencies and applied subsequently to a suitable set of internal coordinates, regarding more the local symmetry coordinates recommended by Pulay et al. [20] and complemented with those reported by Shimaouchi [42] and Matsuura et al. [43] which are based on the principles of locality, local pseudosymmetry and elimination of redundancy. DFT calculations yield Raman scattering amplitudes, which cannot be taken directly to be the Raman intensities. In order to simulate the Raman spectrum, the activities (Ai) calculated with

Fig. 1. Optimized structure of MAP with atom labels and numbers.

G.D. Fleming et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 114 (2013) 120–128 Table 1 Selected B3LYP/6-311+G(d,p)-optimized bond lengths (Å), bond angles (°) of MAP and comparison with X-ray data (atom labelling is shown in Fig. 1).a Ref. [1]

DFT

Distances (Å) PAS 2.085 P@O2 1.467 PAO3 1.566 O3AC4 1.439 SAC 1.792 PAN 1.609 NAH15 1.013 NAH14 1.013

2.055 1.468 1.600 1.435 1.818 1.666 0.800 0.900

Angles (°) SAPAO2 SAPAO3 O3APAO2 PAO3AC4 PASAC9 O3APAN O2APAN SAPAN PANAH14 PANAH15

Ref. [1]

DFT

105.7 106.7 116.2 120.7 100.9 113.3 113.3 114.7 113.0 116.0

115.5 102.1 115.9 118.1 99.60 112.8 112.5 106.9 112.8 115. 0

a Calculated CAH bonds and HACAH bond angles show averaged values of 1.1 A and 110°, respectively, in good agreement with the experimental ones.

Vibrational analysis IR and Raman spectra Metamidophos consists of 15 atoms with a total of 39 normal modes of vibration. These modes have been assigned based on the Normal Coordinate Analysis following the scaled quantum mechanical force field methodology regarding the detailed vibrations of the individual atoms. For this purpose, 51 internal valence coordinates, including 13 redundancies, are defined in Table 2 and are used to derive local symmetry coordinates according to Pulay’s recommendation (Table 3). Unscaled DFT and calculated multiple scaled force constants (mdyn/Å) as well as the corresponding scale factor obtained during the Normal Coordinate Analysis are reported in Table 4.

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frequencies together with the respective Potential Energy Distributions (PEDs) and mode assignments are collected in Table 5. As seen in Table 5, the agreement between experimental and calculated frequencies for fundamental is good. Theoretical assignment is supported by current data reported in literature. According to the geometry and functional groups presented in MAP, Figs. 2 and 3 show two sets of prominent bands at ca. 3500–2800 cm1 and 400–1600 cm1. This vibrational profile features the specific vibrations of this type of organophosphorus compound. Thus, the PAOAR group gives rise to bands currently within the region 1088–920 cm1 [48–53]. These bands are usually very strong in the IR and medium-weak in the Raman spectra. They involve out-of-phase PAOAC stretching with greater participation of the OAC stretching than the PAO stretching. PAOACH3 groups have a band in the 1088–1015 cm1 region [48] and another IR band, usually somewhat weaker at 845–725 cm1, thought to involve in-phase PAOAC stretching, or simply mainly due to PAO stretching [48,51]. Further, PAS stretching bands are reported in the region 613–440 cm1 [48]. The P@O stretching vibration gives rise to a strong band in the IR and a medium band in the Raman. Most P@O compounds have a band in the region 1320–1140 cm1. The P@O stretching frequency in X3PO compounds varies chiefly with the sum of the inductive effects of the three substituents X on the P@O groups has been established by Thomas [48]. Unlike the C@O group, it is little affected by mesomeric effects such as conjugation or by being part of a strained ring. At this respect, Thomas and Chittenden [54] derived a relationship for the P@O stretching wavenumber on the basis of the Phosphorus Inductive Constants (P) for the substituents X Groups.

Experimental and theoretical infrared and Raman spectra

v ðP@OÞ ¼ 930 þ 40R<½cm1 

The experimental and the simulated spectra at our level of calculation are given in Figs. 2 and 3 for visual comparison. Detailed spectral assignments with the corresponding calculated

The P values for the substituents groups presented in MAP: 2.9 (OCH3), 1.5 (NH2) and 2.4 (SACH3) allow to assign v(P@O) at about 1200 cm1 for this compound.

Table 2 Definition of internal valence coordinates of MAP. Indices

Symbol

Description

Composition

Stretching 1–6 7 8–9 10 11 12 13 14

ri Ri ri Ri Qi Ri Ri Ri

HAC (methyl) NAP HAN PAS P@O OAP CAO CAS

H10AC9, H11AC9, H12AC9, H5AC4, H6AC4, H7AC4 N13AP1 H14AN13, H15AN13 P1AS8 O2AP1 O3AP1 C4AO3 C9AS8

ai bi bi bi ci bi bi bi bi

HACAH (methyl) PAO3AC4 O3APAS O3APAS NAPAS PASAC9 SACAH OACAH HANAH PANAH

H11AC9AH10, H12AC9AH10, H12AC9AH11, H6AC4AH5, H7AC4AH5, H7AC4AH6 P1AO3AC4 O3AP1AS8 O3AP1AS8 N13AP1AS8 P1AS8AC9 S8AC9AH10, S8AC9AH11, S8AC9AH12 O3AC4AH5, O3AC4AH6, O3AC4AH7 H15AN13AH14 P1AN13AH14, P1AN13AH15

si si si si si si si si

PASACAH (s(CH3) I PASACAH (s(CH3) II OPSC O(S)POC O3PNH SPNH O2PNH NPS(O)C

P1AS8AC9AH10, P1AS8AC9AH11, P1AS8AC9AH12 P1AO3AC4AH5, P1AO3AC4AH6, P1AO3AC4AH7 O2AP1AS8AC9, O3AP1AS8AC9 O2AP1AO3AC4, C4AO3AP1AS8 O3AP1AN13AH14, O3AP1AN13AH15 S8AP1AN13AH14, S8AP1AN13AH15 O2AP1AN13AH14, O2AP1AN13AH15 N13AP1AO3AC4, N13AP1AS8AC9

Bending 15–20 21 22 22 23 24 25–27 28–30 31 32–33 Torsion 34–36 37–39 40–41 42–43 44–45 46–47 48–49 50–51

ai

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Table 3 Definition of local symmetry coordinates based on the natural internal coordinates defined in Table 2. Number

Description

Composition

Stretching 1–2 3–4 5–6 7 8 9 10 11 12 13 14

CH3ss CH3ips CH3ops NAP NH2ss NH2ips PAS O@P O3AP CAO CAS

p p (r1 + r2 + r3)/ 3, (r4 + r5 + r6)/ 3 p p (2r1  r2  r3)/ 6, (2r4  r5  r6)/ 6 p p (r2  r3)/ 2, (r5  r6) 2 R7 p (r8 + r9)/ 2 p (r8  r9)/ 2 R10 Q11 R12 R13 R14

Bending 15–16 17–18 19–20 21–22 23–24 25 26 27 28 29 30 31

CH3sd CH3ipb CH3opb CH3ipr CH3opr PAO3AC4 O3APAS NAPAS PASAC9 NH2sci NH2twist NH2wagg

p p (a15 + a16 + a17 – b25  b26  b27)/ 6, (a18 + a19 + a20  b28  b29  b30)/ 6 p p (2a15  a16  a17)/ 6, (2a18  a19  a20)/ 6 p p (a16  a17)/ 2, (a19  a20)/ 2 p p (2b25  b26  b27)/ 6, (2b28  b29  b30)/ 6 p p (b26  b27)/ 2, (b29  b30)/ 2 b21 b22 c23 b24 p (2a31  b32  b33)/ 6 p (b32  b33)/ 2 p (a31  b32  b33)/ 3

Torsion 32 33 34 35 36 37 38 39

s(CH3) I s(CH3) II s OPSC s COPO s O3PNH s SPNH s O2PNH s NCOP

p (s34 + s35 + s36)/ 3 p (s37 + s38 + s39)/ 3 p (s40 + s41)/ 2 p (s42 + s43)/ 2 p (s44 + s45)/ 2 p (s46 + s47)/ 2 p (s48 + s49)/ 2 p (s50 + s51)/ 2

ss: Symmetrical stretching; ips: in-plane stretching; ipb: in-plane bending; sd: symmetrical deformation; ipr: in-plane rocking; opr: out-of-plane rocking; twist: twisting; ops: out-of-plane stretching; opb: out-of-plane bending; sci: scissoring; wagg: wagging.

On the other hand, compounds containing the PANH2 group have bands involving PAN stretching bands that are seen in the general region 1102–789 cm1 [48,55]. The NH2 group itself gives rise to bands at 3425–3012 cm1 and 1575–1538 cm1 for stretching and scissoring, respectively. Likewise, the out-of-plane bending NH2 wagging and twisting bands appear between 850 and 750 cm1 [56]. In general, the assignment of the band due to the CAS stretching vibration in different compounds is different in the infrared since the band is of variable intensity and may be found over the wide range 1035–245 cm1. In this sense, for the SACH3 group, the SAC stretching band in the region 750– 690 cm1 (weak in IR and strong in Raman) can be considered as quite characteristic [48]. The CH3 stretching and bending modes appear to be quite pure group vibrations. Considering the assignment of CH3 group frequencies, one can expect that nine fundamentals can be associate to each CH3 group (I and II), namely CH3 symmetrical stretching, CH3 in-plane stretching, CH3 out-of-plane stretching, CH3 in-plane bending, CH3 twisting hydrogen bending, CH3 out-of-plane bending, CH3 in-plane rocking, CH3 out-of-plane rocking and a single torsional mode (CACH3). Assignments given in Table 5 are well supported by the literature [57–59]. Adsorption of MAP on the silver surface Fig. 4 shows the normal Raman and SERS spectra of MAP recorded on Ag colloid. A notably increased intensity of the band assigned to m(PAS) and a moderate enhancement in the region concerning the m(CH3) is observed.

The relative intensities of the bands from SERS spectra are expected to differ significantly from those of normal Raman spectra owing to the specific selection rules [60,61]. Surface selection rule suggests that intensity of Raman bands depends on the orientation of the scattering molecules and an enhancement of vibrational modes with a change in the polarizability component normal to the surface is most efficient. It is further seen that vibrations involving atoms that are close to the metal surface will be enhanced. Molecular Electrostatic Potentials (MEP) maps – plots of the electrostatic potential mapped onto an isoelectron density surface – are frequently used for interpreting and predicting the reactivity of a wide variety of chemical systems in both electrophilic and nucleophilic reactions [62,63]. The different values of the electrostatic potential are color-coded, usually red represents a negative electrostatic potential region (the preferred site for electrophilic attack), while blue color indicates the opposite. In this work MEP confirms our interpretation of the SERS spectrum, because the silver nanoparticle as an approaching electrophile will be attracted to negative regions, where the electron distribution effect is dominant. The mapping of the electrostatic potential of MAP (see Supplementary data for details) shows a region of negative charge mainly around the S atom. Thus, when added to the silver colloidal solution, the adsorption of MAP is supposed to occur through this atom and those ones close to it. Taking into account the SERS surface selection rules and the electrostatic potential of MAP, we can suppose that MAP could be located with the PAS bond almost perpendicular to the silver surface. These empirical and MEP considerations about the

G.D. Fleming et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 114 (2013) 120–128 Table 4 Diagonal unscaled DFT and SQM multiple scaled force constants (mdyn/Å), Normal Coordinate Analysis-derived scale factors. No.

Symbol

Unscaled (DFT) force constants

Scaled SQM force constants

Multiple scale factors from NCA

Stretching 1 2 3 4 5 6 7 8 9 10 11 12 13 14

CH3ss I CH3ss II CH3ips I CH3ips II CH3ops I CH3ops II NAP NH2ss NH2ips PAS O@P O3AP CAO CAS

5.151 5.103 5.154 5.153 5.112 5.680 3.889 7.184 7.631 2.575 10.01 5.150 5.231 3.146

5.015 4.712 5.156 4.880 5.112 4.680 2.889 5.521 5.631 2.865 9.008 3.485 4.231 2.086

0.93748 0.88560 0.98191 0.93582 0.97694 0.90994 0.71161 0.78207 0.79344 0.76803 0.96601 0.82512 0.87560 0.71284

Bending 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

CH3sd I CH3sd II CH3ipb I CH3ipb II CH3opb I CH3opb II CH3ipr I CH3ipr II CH3opr I CH3opr II PAO3AC4 O3APAS NAPAS PASAC9 NH2sci NH2twist NH2wagg

0.725 0.748 0.262 0.287 0.474 0.336 0.472 0.821 0.883 0.856 0.748 0.663 0.663 0.410 0.528 0.631 0.687

0.468 0.548 0.692 0.477 0.724 0.416 0.522 0.417 0.583 0.472 1.054 0.663 0.763 0.470 0.558 0.431 0.678

0.78048 0.77682 1.01834 1.16184 0.67464 0.91910 0.88374 0.69429 0.74459 0.71689 1.34516 1.07357 1.27342 1.18186 1.03726 0.86320 0.99560

Torsion 32 33 34 35 36 37 38 39

s(CH3) I s(CH3) II s OPSC s COPO s O3PNH s SPNH s O2PNH s NCOP

0.420 0.427 0.691 0.584 0.507 0.574 1.561 0.564

0.413 0.420 0.491 0.261 0.111 0.143 0.251 0.290

0.91517 1.04200 0.87328 0.99777 0.82877 0.47493 0.24683 0.63692

Fig. 2. Comparison of experimental (A) and theoretical IR spectra (B). Fig. 3. Comparison of experimental (A) and theoretical Raman spectra (B).

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Table 5 Experimental IR and Raman data, computed wavenumbers (cm1) and their assignments (PED). Mode

Calc. (DFT)

Calc. (SQM)

Exp. IR

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

3619 3518 3129 3122 3119 3095 3033 3020 1506 1493 1489 1479 1476 1470 1365 1241 1197 1177 1034 1005 997 990 855 745 684 608 538 441 420 392 311 268 259 198 184 169 147 103 53

3225.0 3111.0 3100.0 3086.0 3015.0 2952.1 2938.0 2844.0 1560.7 1458.9 1435.7 1340.6 1299.0 1278.4 1219.1 1195.2 1077.9 1031.4 947.3 802.5 725.4 667.2 650.3 606.2 580.2 578.9 551.5 489.2 403.4 387.0 308.5 276.5 244.6 187.6 179.1 168.0 144.4 106.0 61.0

3225 3112 3100 3086

Exp. Raman

w ww w w 3015 w

2952 w 2938 vs 2844 s 1562 m 1461 m 1437 m

1454 m

1203 vs

1216 mw

1018 vs 936 s 767 s 697 w

1066 m 1036 m 946 mw 786 s 712 s

615 m 576 m

Fig. 4. SERS spectrum of MAP on silver (A) and Raman spectrum of MAP (B).

Assignment (PED) NH2 ips 100% NH2 ss 100% CH3 ips 99% (I) CH3 ops 99% (I) CH3 ips 98% (II) CH3 ips 99% (I) CH3 ss 100% (I) CH3 ss 99% (II) NH2 sci 90% CH3 ipb (I) 80% CH3 ipb(II) 70% CH3 sd(II) 70% + CH3 opb (II) 28% CH3 opb (I) 69% + CH3 sd(II) 22% CH3 opr (II) 71% + mPO2 25% mPO2 85% + CH3 opr (I) 15% CH3 opr (I) 69% + mPO2 25% CH3 ipr (I) 76% CH3 ipr(II) 81% mCO3 83% + mO3P 15% mSC 79% + NH2 17% twist mO3P 65% + mCO3 28% mPN 67% + NH2 wagg 28% NH2 wagg 73% + mPN 13% mPS 60% + mNP 19% mCS 79% + mPS 20% NH2 twist 58% + mPN 30% s(CH3) I 87% s(CH3) II 75% dPO3C4 34% + dO3PS 25% + dPSC 20% dO3PS 35% + dNPS 23% + dPSC 12% dNPS 49% + dPSC 36% dPSC + dO3PS dO2PS + dPO3C4 sOPSC 67% + sO3PNH 34% sCOPO 53% + sO2PNH 45% sO3PNH 36% + sOPSC 29% sSPNH 69% + sNPOC 18% sO2PNH 79% + sCOPO 15% sNPOC 89%

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Appendix A. Supplementary material B3LYP/6-311+G(d,p)-calculated 3D electrostatic potential contour map of MAP. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2013.05.012.

References

Fig. 5. Most stable B3LYP/6-311+G(d,p)-computed orientation of MAP on a silver surface.

orientation of MAP on the Ag surface are supported by the DFT geometry optimization of the system MAP-Ag18 cluster. The most stable orientation of MAP on the silver surface model is shown in Fig. 5. This figure also shows the proximity of the CH3 moiety attached to the S atom to the metal surface, which explains the moderated enhancement of the band at about 2900 cm1. Furthermore, Fig. 5 would account the absence of enhancement in the region of the P@O stretching bond (ca. 1200 cm1) because of the null contribution of this normal mode to the azz polarizability component (z being the axis perpendicular to the surface). Conclusion Combining quantum chemical results, Normal Coordinate Analysis with symmetry considerations and IR and Raman literature data, it is possible to perform a complete vibrational frequency assignment with a high degree of confidence for such a complex molecule like metamidophos. This detailed band assignment is performed for the first time for this molecule and will be useful for the in situ identification of this toxic substance. The DFT level of calculation used in the present work together with the SQM treatment has proven to be an appropriate to support the assignment of the distinct normal modes of MAP. Generally, the application of the individual SQM scaling factors gives a much improved agreement between calculated and measured frequencies, usually within 10 cm1. Although somewhat more demanding, it is clearly a superior approach of scaling calculated normal modes compared to the application of a single scale factor as it is common for most quantum chemical calculations. The sets of scaled force constants are in very good agreement with those reported in the literature for molecules containing the same internal coordinates, The approach of comparing the calculated normal modes to the experimental ones obtained with the scaled force constants is – consequently – a test to prove the quality of the scale factors. Acknowledgments This work has been supported by FONDECYT project 1110106. GDF acknowledges DGI Universidad de Playa Ancha, Project CNEI 02-1011. Generous allocation of computer time on the CSC Oldenburg is gratefully acknowledged. GDF and RK are also indebted to the international collaboration program CONICYT-DFG 070 2008.

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