Spectrochimica Acta Part A 94 (2012) 30– 35 Contents lists available at SciVerse ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy j our na l ho me p age: www.elsev ier .co Theore y o 1-(4-br eth J. Clemy Department of Tamil a r t i c l Article history: Received 21 Ja Received in re Accepted 22 M Keywords: Urea FTIR Raman DFT MEP PED cidal ration scaled resp liken senc by th 1. Introdu In recent years, the family of phenyl urea herbicides (PUHs) attains particular attention because of its high biotoxicity and possible carcinogenic properties [1]. PUHs are extensively used for general weed control on non-crop area and as selective pre- emergence The study o compounds addition to areas or as targeting ph QA (primar ondary qui compound is chemical 3-methyl u to inhibit ph ing the Hill molecule ac of chlorbrom bial activity soil [9]. Surp hence it has mal quantit ∗ Correspon E-mail add e ove using experimental and theoretical spectroscopic data in order to unravel the physiochemical properties responsible for the stability of the molecule. Nowadays, density functional theory (DFT) method becomes the most popular and versatile quantum mechanical modeling 1386-1425/$ – http://dx.doi.o on crop such as citrus, asparagus, and bush fruits [2]. f haemotoxic effects on rats of chronic exposure to these were carried out using three substituted PUHs [3]. In these, the PUHs are also used as total herbicides in urban algicides in paints and coatings [4]. These herbicides otosystem II (PSII) block the electron transfer beyond y quinone electron acceptor) by binding to the QB (sec- none electron acceptor) site [5–7]. Among PUHs, the taken for present investigation is chlorbromuron which ly known as 1-(4-bromo-3-chlorophenyl)-3-methoxy- rea. Chlorbromuron is a urea-based herbicide that acts otosynthesis and the oxidation of water to oxygen dur- reaction. Also, the formation of hydrogen bonds in the ts as an active site in the protein [8]. The concentration uron depends on several factors such as rains, micro- and temperature since the herbicide is bound to the risingly, chlorbromuron remains in the soil surface and no adverse effects on the crops [10]. Exceeding a nor- y and frequent use will generate accumulation of soil ding author. Tel.: +91 9443746555; fax: +91 4652229800. ress:
[email protected] (C. James). method to investigate the electronic structure. Normal coordinate analysis (NCA) has been done to provide the complete and reli- able information about the fundamental modes of the molecular system. Also, the natural bond orbital analysis (NBO) has been per- formed to get more precise information regarding the hybridization and hydrogen bond interaction energies. In addition to these, the HOMO–LUMO energy gap has been calculated to give the informa- tion about the charge transfer within the molecule. 2. Experimental details The compound chlorbromuron was purchased from Sigma–Aldrich (USA) and it was used without further purifi- cation. The FTIR spectrum in the region 4000–400 cm−1 was recorded using a Perkin Elmer RXI spectrometer with the spectral resolution of 1 cm−1. The sample was prepared as a KBr disc. The FT-Raman spectrum in the region 3500–10 cm−1 was recorded on a BRUKER IFS 66 V model interferometer equipped with an FRA-106 FT Raman accessory. The 1064 nm line of Nd: YAG laser was used as the exciting source with an output power of about 150 mW. Raman spectra were collected for samples with 1000 scan accumulated for over 30 min duration with the resolution of 1 cm−1. see front matter © 2012 Elsevier B.V. All rights reserved. rg/10.1016/j.saa.2012.03.069 tical and experimental vibrational stud omo-3-chlorophenyl)-3-methoxy-3-m Monicka, C. James ∗ Physics and Research Centre, Scott Christian College (Autonomous), Nagercoil 629003, e i n f o nuary 2012 vised form 8 March 2012 arch 2012 a b s t r a c t FT-Raman and IR spectra of the herbi The detailed interpretation of the vib dinate analysis (NCA) following the intramolecular interactions which are natural bond orbital analysis. The Mul energy were also calculated. The pre clearly exposed from the IR spectrum ction residu m/locate /saa f the phenyl urea herbicide ylurea Nadu, India compound chlorbromuron have been recorded and analyzed. al spectra has been carried out with the aid of normal coor- quantum mechanical force field methodology. The various onsible for the stabilization of the molecule were revealed by population analysis on atomic charges and the HOMO–LUMO e of strong N H· · ·O intermolecular hydrogen bonding was e red shifting of NH stretching wavenumber. © 2012 Elsevier B.V. All rights reserved. r the days. Complete structural analysis has been made J.C. Monicka, C. James / Spectrochimica Acta Part A 94 (2012) 30– 35 31 Fig 3. Comput Electron of the mole 311G(d,p) b functionals tion functio [11]. Based internal coo distribution Detailed vib given on the simulated I Lorenztian b cies were s (SQM) proc tions [15] h level using package in calization. T from the se E(2) = −n� ε� where 〈�| F NBO orbital is the popul chemical d gap and mo explain the 4. Results 4.1. Geome The opt charges plo is represen 09 was com ening of N1 and 0.029 A˚ ble bond ch that extend are in the in intramolecu angles C6 as a consequence of steric interaction [19] arises due to repulsion of amide hydrogen with aromatic hydrogen H7, with the non- bonded distance of H7· · ·H14 (2.303 A˚). The calculated bond angles C5 C4 Br9 (122.3◦) and C4 C5 Cl10 (121.3◦) are greater than the ngle port ···O d by ith H antl atom n co the lar h O an ana bet tion erac y the 2). T oxyg H11) ·O an utio rine a the of 6 tron E (2 ges t �* N e � 14 bo f bon A˚). T �*(C onan .85 kJ ectro hest pied itals tical ener MO– . 1. Theoretical geometrical structure of chlorbromuron. ational details ic structure investigation and geometry optimization cule have been done by the DFT method employing 6- asis set and Becke’s three parameter hybrid exchange with Perdew and Wang’s gradient-corrected correla- nals (B3PW91) using the Gaussian‘09 software package on Pulay’s recommendations [12,13] a complete set of rdinates has been defined to calculate potential energy (PED) for each normal mode with the aid of NCA. rational assignments of the normal modes have been basis of PED using the MOLVIB (7.0) program [14]. The R and Raman spectra have been obtained using a pure and profile (FWHM = 10 cm−1). The calculated frequen- caled down based on the scaled quantum mechanical edure to offset the systematic errors. The NBO calcula- ave been performed at the DFT/B3PW91/6-311G (d,p) NBO 3.1 as implemented in the Gaussian’09 software order to represent the measure of intramolecular delo- he hyperconjugative interaction energy can be deduced cond order perturbation approach. 〈�| F |�〉2 ∗ − ε� = n� F2 ij �E (1) |�〉2 or F2 ij is the Fock matrix element between i and j s, ε� and ε�* are the energies of � and �* NBO’s and n� ation of the donor � orbital. Finally, significant quantum escriptors (net atomic charges, HOMO–LUMO energy lecular electrostatic potential) have been described to biological activity of the compound. ideal a in this lar C H expose occur w signific the O betwee around molecu 4.2. NB NBO actions interac the int lated b (Table of the �*(C6 C H· · contrib of chlo ring vi energy its elec higher of char bonds that th N12 C ening o (1.402 orbital the res of 154 4.3. El Hig unoccu lar orb and op orbital The HO and discussion try imized molecular structure along with the atomic tted on each atom of the molecule chlorbromuron ted in Fig. 1. The optimized geometry from Gaussian pared (Table 1) with the XRD data [8]. The short- 2 C14, C14 N16, N16 C17 bonds about 0.1088, 0.078 from its normal value (1.48 A˚) show the typical dou- aracter as a consequence of resonance interaction [16] ed over the whole urea fragment. The N C bondlengths creasing order as N12 C14 < C14 N16 < N16 C17 due to lar electronic effects [17,18]. The increase in the bond C1 N12 and C1 N12 C14 by 3.3◦ and 7.7◦ from 120◦, stability ind to low stabi the scale of muron imp with the tar for the mol HOMO ene LUMO ene HOMO–LU Both the rings indica type orbital of the mole ure shows of 120◦ which indicates the steric and charge repulsion ion of the molecule. The existence of the intramolecu- and N H···O hydrogen bonding interactions have been the intramolecular contacts to H11···O15 and H13···O21 ···O distances of 2.19 and 2.03 A˚ respectively, which are y shorter than the Vander Waals separation between and H atom (2.72 A˚) [20]. There are slight deviations mputed and experimental values of the dihedral angles central urea fragment owing to the neglect of inter- ydrogen bonding. alysis lysis provides a clear description of the stabilizing inter- ween filled and unoccupied orbitals and destabilizing s between filled orbitals [21,22]. The energy values for tion between the filled i and vacant orbital j, calcu- second order perturbation theory have been tabulated he hyperconjugative interactions of p-type lone pair en atoms LP (2) O15 and LP (2) O21 with the remote and �*(N12 H13) antibonds shows the occurrence of d N H· · ·O intramolecular interactions whose energy ns are 4.72 and 8.11 kJ/mol respectively. The lone pair atom, LP (3) Cl10 donate its �-electrons to the phenyl �-antibonding orbital of C4 C5, being stabilized by 2.63 kJ/mol. This �*(C4 C5) NBO will further transfer s to the adjacent �* orbitals of C1 C6, C2 C3 with the ) of 585.38 and 579.81 kJ/mol, respectively. Magnitude ransferred from the lone pair LP (2) O15 into the anti- 12 C14 (0.07511 e) and �* C14 N16 (0.09088 e) shows electrons tend to be mostly localized in the vicinity of nd. This is well reflected in the geometry as the short- dlength N12 C14 (1.371 A˚) when compared to C14 N16 he electron donation from LP (1) N12 to the antibonding 1 C6) is an important (n → �*) interaction related with ce in the molecule, results to a maximum stabilization /mol. nic properties of chlorbromuron occupied molecular orbitals (HOMOs) and the lowest molecular orbitals (LUMOs) are the Frontier molecu- (FMOs) which play an important role in the electric properties, as well as in chemical reactions [23]. The gies for both HOMO and LUMO have been calculated. LUMO energy gap is an important reactivity index and ex for the molecule in which smaller energy gap leads lity and high reactivity. Since the energy gap �E reflects activity, the higher energy gap (5.461 eV) of chlorbro- lies high stability and less interaction of the herbicide get site. The 3D shapes of the HOMO–LUMO distribution ecule is depicted in Fig. S1 (supplementary material). rgy = −6.3092 eV rgy = −0.8479 eV MO energy gap, �E = 5.461 eV HOMO and LUMO are mainly localized on the phenyl ting that the HOMO–LUMO is mostly the �-antibonding s. When HOMO and LUMO are located on the same side cule, herbicidal activity strongly decreases [24]. The fig- that HOMO lies on the phenyl ring and the secondary 32 J.C. Monicka, C. James / Spectrochimica Acta Part A 94 (2012) 30– 35 Table 1 Selected geometrical parameters of chlorbromuron. Bond lengths Calc (Å) Expt (Å) Bond angles Calc (◦) Expt (◦) Dihedral angles Calc (◦) Expt (◦) C1 N12 1.396 1.400 C1 N12 C14 127.7 125.4 C2 C1 N12 C14 175.46 −157.7 C4 Br9 1.892 1.885 C6 C1 N12 123.3 123.5 C2 C1 N12 H13 2.03 22.7 C5 C6 1.390 1.370 C3 C4 Br9 118.8 119.5 C1 N12 C14 O15 −2.41 6.6 C5 Cl10 1.735 1.742 C5 C4 Br9 122.3 122.3 C1 N12 C14 N16 174.78 177.9 N12 H13 1.009 0.860 C4 C5 Cl10 121.3 121.0 H13 N12 C14 O15 171.15 −173.8 N12 C14 1.371 1.356 C6 C5 Cl10 117.6 116.9 H13 N12 C14 N16 −11.63 1.4 C14 O15 1.214 1.227 C1 N12 H13 117.2 117.4 N12 C14 N16 C17 150.65 154.8 C14 N16 1.402 1.387 H13 N12 C14 114.6 117.1 N12 C14 N16 O21 15.17 20.1 N16 C17 1.451 1.452 N12 C14 O15 125.9 124.9 – – – N16 O21 1.407 1.417 N12 C14 N16 113.3 115.4 – – – O21 C22 1.426 1.429 O15 C14 N16 120.6 119.4 – – – amide moiety whereas LUMO is mainly located at the phenyl ring. Since the HOMO and LUMO lies on the same side of the molecule the herbicidal activity of the title compound decreases [24]. Mulliken’s population analysis [25] provides a partitioning of either a total charge density or an orbital density. The bond length between the atoms either increases or decreases as a consequence of the distribution of positive or negative charges over the atoms. The oxygen large negat All the hyd attachment ticular, the shows the p the molecu charge mai oxygen ato possesses m This is due electron tow The mo charged po by the mole MEP plots were obtai ent values are represe most negat most positi of zero pot red < orange potential su distribution showing re culated 3D region (pro (A) Experimental FT-Raman spectrum of chlorbromuron, (B) simulated n spectrum of chlorbromuron, (C) experimental FT-IR spectrum of chlor- n, and (D) simulated FT-IR spectrum of chlorbromuron. ary amide carbonyl group and probably this proton acceptor otential binding site. brational analysis recorded FTIR and Raman spectra for the title compound parison with the simulated spectra are as shown in Fig. 2. romuron has 69 normal vibration modes and the detailed ments of the experimental bands corresponding to the 69 Table 2 Second order p Donor (i) gy (j) (a.u.) E(2)a (kJ/mol) E(j) − E(i)b (a.u.) F(i,j)c (a.u.) LP (2) O15 6089 4.72 0.72 0.027 LP (2) O15 LP (2) O15 LP (2) O21 LP (3) Cl10 �* C4–C5 �* C4–C5 LP (1) N12 LP (1) N12 LP (1) N16 a E(2) mean b E(j) − E(i) m c F(i,j) is the (O15, O21) and the nitrogen (N12, N16) atoms have a ive charge (Fig. 1) and behaved as electron acceptors. rogen atoms possess a net positive charge due to its with the more electronegative carbon atoms; in par- hydrogen atom H11 has large net positive charge which ossibility of forming intramolecular hydrogen bond in le. The carbon atom C14 possesses maximum atomic nly due to the attachment of more negatively charged m and two nitrogen atoms. The carbonyl oxygen O15 ore negative charge than the other oxygen atom O21. to the delocalization of the nitrogen (N12) lone pair of ards the carbonyl end. st probable regions for the electrophilic attack of int-like reagents on organic molecules are represented cular electrostatic potential (MEP) maps [26]. The 3D (Fig. S2, supplementary material) of chlorbromuron ned based on the DFT optimized result. The differ- of the electrostatic potential at the molecular surface nted by different colours: red represents regions of ive electrostatic potential, blue represents regions of ve electrostatic potential, and green represents regions ential. Electrostatic potential increases in the order: < yellow < green < blue. The shape of the electrostatic rface is influenced by the structure and charge density s in the molecule with sites close to the oxygen atom, gions of most negative electrostatic potential. The cal- MEP contour map shows that the negative potential ton acceptor site) is generated by the oxygen atom of Fig. 2. FT-Rama bromuro second is the p 4.4. Vi The in com Chlorb assign erturbation theory analysis of Fock matrix in NBO basis. ED (i) (e) Energy (i) (a.u.) Acceptor (j) ED (j) (e) Ener 1.83967 −0.26264 �* C6 H11 0.01549 0.4 1.83967 −0.26264 �* N12 C14 0.07511 0.43519 1.83967 −0.26264 �* C14 N16 0.09088 0.39133 1.93758 −0.35579 �* N12 H13 0.02238 0.39978 1.91631 −0.32132 �* C4 C5 0.47208 −0.00919 0.47208 −0.00919 �*C1 C6 0.37910 0.01791 0.47208 −0.00919 �* C2 C3 0.33269 0.01650 1.68532 −0.27935 �*C1 C6 0.37910 0.01791 1.68532 −0.27935 �*C14 O15 0.20810 0.27452 1.80139 −0.32693 �*C14 O15 0.20810 0.27452 s energy of hyperconjugative interactions. eans the energy difference between donor and acceptor i and j NBO orbitals. Fock matrix element between i and j NBO orbitals. 99.83 0.70 0.118 108.74 0.65 0.118 8.11 0.76 0.035 62.63 0.31 0.068 585.38 0.03 0.084 579.81 0.03 0.084 154.84 0.30 0.095 52.76 0.55 0.077 47.90 0.60 0.074 J.C. Monicka, C. James / Spectrochimica Acta Part A 94 (2012) 30– 35 33 Table 3 Calculated vibrational wavenumbers, observed IR and Raman frequencies and their assignments with PED % for chlorbromuron. �IR (cm−1) �Raman (cm−1) B3PW91/6-311G (d,p) level Assignment with PED % (≥10%) �Scal (cm−1) IIRa IRb 3275 vs 3275 m 3275 91.77 10.50 �sNH (99) 3101 w 3109 vw 3085 19.59 4.89 �sCH (99) – 3077 s 3051 1.18 11.90 �sCH (99) – – 3018 9.13 11.11 �sCH (99) 3008 w 3010 w 3014 1.23 9.77 �a′ CH3 I (93) 2972 m 2977 w 2969 15.08 10.90 �a′ CH3 II (53) + �a′′ CH3 II (45) 2934 m 2935 s 2939 41.31 17.40 �a′′ CH3 II (50) + �a′ CH3 II (45) – – 2936 22.03 16.80 �a′′ CH3 I (78) + �sCH3 I (18) 2893 w 2893 w 2875 35.64 37.40 �sCH3 II (71) + �sCH3 I (17) 2812 w 2814 w 2873 57.44 33.70 �sCH3 I (56) + �sCH3 II (22) + �a′′ CH3 I (14) 1663 vs 1663 s 1663 239.31 48.20 �sC O (63) + � ′ NCN (12) – 1590 s 1611 79.45 100.00 �sCC (63) + � ′ CH (16) + Phasyd (10) 1576 s – 1584 188.76 13.00 �sCC (59) + � ′ NH(14) 1521 s 1520 w 1519 945.11 29.00 �asNC (30) + � ′ NH (28) + �sCC (21) 1474 w – 1468 35.61 4.16 �sCC (40) + � ′ CH (39) – – 1456 12.33 15.60 � ′ CH3 I (51) + � ′′ CH3 II (31) 1448 vw 1453 w 1454 3.14 14.50 � ′′ CH3 II (38) + � ′ CH3 I (26) + � ′ CH3 II (14) – – 1439 31.70 9.50 � ′′ CH3 I (80) – 1422 vw 1430 4.36 8.42 � ′ CH3 II (72) + � ′′ CH3 II (16) 1384 m 1394 w 1389 125.20 1.25 �sCC (48) + � ′ CH (26) – – 1371 13.19 3.80 ˇCH3 II (90) – – 1342 6.25 5.16 ˇCH3 I (83) 1319 m – 1316 33.52 32.50 �sCC (83) 1295 m 1295 s 1292 97.77 11.90 �asCN (37) + �sNC (11) + �sCC (11) + � ′ CH (10) 1248 m – 1237 47.10 6.59 � ′ CH (33) + �sCC (19) + � ′ NH (13) + �asCN (13) 1226 s 1226 vs 1221 21.70 39.30 � ′ CH(37) + �sCC (27) + �asNC (13) 1180 s 1182 vw 1147 15.06 3.33 ıCH3 II (21) + � ′ CH (17) – – 1146 8.01 3.36 ıCH3 II (33) + � ′ CH (16) + �CH3 II (10) 1149 w 1153 vw 1138 57.63 2.44 � ′ CH (28)+ �sCC (12)+ ıCH3 I (11) + �CH3 I (11) 1112 vs 1117 w 1108 36.32 16.20 �sCN (22)+ �CH3 II (13) + �sNO (12) + �sCC (11) – – 1105 13.27 18.60 �sCC (33) + � ′ CH (28) + �CH3 II (15) – – 1101 5.45 13.60 �CH3 II (41) + ıCH3II (17) 1032 s – 1069 92.56 6.12 ıCH3I (61) + �CH3 I (13) 1019 m 1014 s 1006 52.00 5.81 �sOC (56) + �sNO (13) 984 s 981 vw 992 19.46 26.40 Phtrid (59) + �sCC (23) 950 w – 951 78.25 6.84 �sOC (25) + �CH3 I (16) + �sCN (13) 921 vw 922 s 905 24.78 6.94 �sCCl (18) + �sCC (15) + �sNC (11) + Phtrid (11) + � ′ CNC (10) 875 vs 876 vw 876 1.61 0.56 ωCH (89) 841 vw 842 w 843 18.76 2.24 ωCH (75) + Phpuck (15) 817 vs – 826 0.59 7.84 �sNO (24) + ıCO (20) 755 w 757 vw 754 35.32 1.40 ωCH (81) 713 vs – 709 40.80 2.03 ωCO (77) 694 s – 693 29.08 1.56 Phasydo (28) + �sCCl (17) + Phasyd (12) 663 m 663 vs 656 8.60 30.60 Phasyd (25) + Phasydo (23) + �sCN (10) – – 646 3.14 8.08 Phpuck (63) + ωCCl (12) + ωCN (11) 605 m – 601 26.52 1.83 OCN (14) + ωNH (11) 576 m 576 vw 587 32.44 5.42 ωNH (19) + ıCO (16) + ˇOC (11) + OCN (10) – – 556 0.11 2.53 ωCN (30) + ωCCl (24) + Phasyt (21) 507 w 507 w 500 5.33 19.10 � ′ CNCO (40) + � ′ NCN (14) 444 s 445 m 439 3.43 13.30 ˇOC (26) + �sCCl (15) + � ′ Br (13) – – 429 50.40 32.60 ˇOC (19) +�sCCl (17) + � ′ CNCO (12) + ωCCNO (10) – – 420 7.87 9.42 Phasyto (53) + ωCCl (12) + ωCH (10) – – 412 2.66 5.87 � ′ Cl (29) + � ′ CN (12) + � ′ Br (10) + ˇOC (10) – – 368 23.72 8.99 � ′ CNC (20) + �sBr (16) + � ′ CN (14) + ıCO (12) – 334 m 345 6.80 4.48 � ′′ CNOC (54) – – 302 1.01 7.80 ωCBr (38) + Phasyt (22) + Phpuck (11) – 221 w 229 10.39 4.63 � ′ NCN (20) + ωCCNO (12) – – 213 3.03 22.40 �sBr (19) + Phasyd (17) + ωCCNO (16) – – 192 1.73 12.60 Phasyto (17) + ωCCNO (13) + ωCCl (11) +ωCH (11) – – 180 0.94 10.20 CH3 (27) + ωCCNO (17) + NH3 (15) – – 166 0.33 18.00 NH3 (54) + CH3 (12) + � ′ CNCO (10) – 153 m 155 0.51 66.20 � Br (50) + � ′ Cl (24) – – 154 0.89 67.00 CH3 (40) + ωCCNO (31) – – 122 1.23 23.30 OCNC (33) + ωNH (20) + Phasyt (12) + NC (10) – – 93 1.66 56.90 CNOC (38) + OCNC (15) + � ′ CN (12) – – 84 2.93 64.70 OCNC (41) + Phasyt (19) + ωCCNO (10) – – 70 2.11 45.70 CNOC (48) + � ′ CNC (13) – – 31 0.19 388.00 ωNH (30) + OCN (28) + Phasyt (17) – – 25 0.66 519.00 NC (63) vs: very strong; s: strong; w: weak; vw: very weak Ph: phenyl ring; �s: symmetric stretching; �a′ : in plane stretching; �a′′ : out of plane stretching; ˇ: symmetric bending; � ′ : in plane bending; � ′′ : out of plane bending; �: in plane rocking; ı: out of plane rocking; trid: trigonal deformation; asyd: asymmetric deformation; asydo: out of plane asymmetric deformation; puck: puckering; ω: gauche; : torsion; asyt: asymmetric torsion; asyto: out of plane asymmetric torsion. a Calculated IR intensities. b Calculated Raman intensities 34 J.C. Monicka, C. James / Spectrochimica Acta Part A 94 (2012) 30– 35 expected normal modes have been made on the basis of PED. Non-redundant set of internal coordinates (Table S1, supplemen- tary material) for chlorbromuron has been defined. The computed wavenumbers have been selectively scaled and the scale factors were tabula The exp intensities been tabula bands have molecules [ 4.4.1. Phen Normal been assign The five C are 8a, 8b, 1 resolved in 1560–1610 Strong Ram the ring mo has been as (1611 and 8b show th with the ex 19a is 1370 at 1384 cm− been assign 19b appear band. A med trum has be of C H str 3000–3120 ing bands 2 20a is activ tively. Also, in the Ram fied as C H 1226 cm−1 to the mod the IR spec a very weak mal modes vibrations a very strong band appea mode 10b. 4.4.2. C X The vibr the ring an mixing of v ular symme assigned th quency ran band obser 921 cm−1 h C Br bendi 153 cm−1. 4.4.3. Secon The mo secondary usually app ing mode 3275 cm−1 (∼75 cm−1) for monosubstituted ureas [27]. This red shifting of N H stretch- ing mode provides the spectral evidence for the existence of strong N12 H13· · ·O15 intermolecular hydrogen bonding in the title molecule. The C O stretching band (amide-I band) is the charac- sec 1630 m−1 is sim he lo njuga rly o ∼0.0 ng st (1) N n be d. N ed a -pla ing v at 1 ed at amid tivel on (a on oc tchin 5 cm m in hic ed in man Meth thoxy prov exhi hen at fo on of quen 850 t 72 cm nd 2 etric appe e sh ttribu the w H13 e NB 3 an een a band ama CH3 met 280 2 cm m h clus chlo 1 m try i ted (Table S2, supplementary material). erimental and calculated frequencies, IR and Raman and the PED based on the 6-311G(d,p) basis set have ted (Table 3). Detailed assignments of the observed been made in comparison with the spectra of related 27–29] and are discussed below. yl ring vibrations modes of asymmetric trisubstituted phenyl ring have ed by adopting Wilson’s atom numbering scheme [30]. C stretching modes corresponding to the phenyl ring 4, 19a and 19b, respectively. The ring mode 8 has been to 8a and 8b modes in which 8a appears in the range cm−1 and 8b occurs in the range 1571–1642 cm−1 [30]. an band observed at 1590 cm−1 has been assigned to de 8b whereas a strong IR band observed at 1576 cm−1 signed to the ring mode 8a. Computed wavenumbers 1584 cm−1) ascribed to the vibrational modes 8a and e splitting of the vibrational mode 8 which agrees well perimental wavenumbers. The wave number range for –1450 cm−1 and for 19b it is 1460–1530 cm−1. IR band 1 and a weak intensity Raman band at 1394 cm−1 has ed to the C C stretching (Mode 19a) vibration. Mode s in the IR spectrum at 1474 cm−1 as a weak intensity ium intense band observed at 1319 cm−1 in the IR spec- en assigned to the ring mode 14. The frequency interval etching modes of the phenyl ring is expected to be cm−1 [30]. The selection rule allow three C H stretch- , 20a and 20b in the IR and Raman spectra. The mode e both in IR and Raman at 3101 and 3109 cm−1 respec- the mode 2 is identified as a strong band at 3077 cm−1 an spectrum. Normal modes 3, 15 and 18b are classi- in-plane bending vibrations. Strong bands observed at in both the IR and Raman spectrum have been assigned e 3. Vibrational mode 15 appears as a weak band in trum at 1149 cm−1 and its counterpart is identified as band at 1153 cm−1 in the Raman spectrum. The nor- 10a, 10b and 11 which allow C H out-of-plane bending re expected to be in the region 1000–700 cm−1 [31]. A IR band observed at 875 cm−1 and a very weak Raman rs at 876 cm−1 have been assigned to the vibrational vibrations (X = Cl, Br) ational assignments correspond to the bonds between d the halogen atoms are important to discuss, since ibrations are possible due to the lowering of the molec- try and the presence of heavy atoms. Mooney [32,33] e vibrations of C X group (X = C1, Br and I) in the fre- ge of 1129–480 cm−1. In FT-Raman spectrum, a strong ved at 922 cm−1 and a weak IR band observed at ave been assigned to the C Cl stretching vibration. The ng mode appears as a medium intensity Raman band at dary amide vibrations st common and important type of amides are the amides which has only one N–H stretching band ear in the region 3500–3300 cm−1 [34,35]. N–H stretch- appears as a strong and medium intensity band at in the IR and Raman spectrum which is relatively lower than the N H stretching mode (3350 cm−1) reported teristic 1680– 1663 c which [37]. T the co is clea gates ( resulti of LP jugatio �-bon observ N H in stretch spectra observ plane ( respec vibrati vibrati ric stre at 129 mediu mode w identifi the Ra 4.4.4. Me which bonds oxygen than th vibrati the fre from 2 and 29 ops) a asymm mode The blu been a well as of N12 from th at 145 have b strong weak R the O The region at 281 spectru 5. Con The B3PW9 geome ondary amide band that usually occurs in the region cm−1 [36]. Strong IR and Raman bands observed at have been assigned to the C O stretching vibration ilar in frequency as observed for related compounds wering (∼17 cm−1) of C O stretching mode is due to tion of carbonyl group with the amide nitrogen. This bserved in the geometry as the bond C14 O15 elon- 08 A˚) from the values reported earlier [19]. Also, the abilization energy (52.76 kJ/mol) from the interaction 12 → �* (C14 O15) shows the lesser degree of con- tween nitrogen lone pair electron and the carbonyl H in-plane bending (amide-II band) mode is usually s a strong band in the region 1570–1515 cm−1. The ne bending mode is coupled with the N C asymmetric ibration and is found active in both the IR and Raman 521 and 1520 cm−1, respectively. Very strong IR band 713 and 817 cm−1 have been assigned to the C O out-of e-VI) and C O in-plane bending vibrations (amide-IV) y. Another important vibration is the C N stretching mide-III) which is difficult to identify since mixing of curs in the region 900–1300 cm−1 [38]. C N asymmet- g mode appears as a strong band in the Raman spectra −1 and it appears in the same wavenumber region with tensity in the IR spectra. The C N symmetric stretching h is coupled with the ring C C stretching mode has been the IR spectrum at 1112 cm−1 with the counterpart in spectrum at 1117 cm−1. oxy group (O CH3) and N CH3 vibrations group vibrations are the important vibrational modes ide more stabilization to the molecule [36]. The CH bit trans conformation with respect to the lone pair ce O–CH3 stretching bands spread over a larger region r the C CH3 group [39]. The C H asymmetric stretching O–CH3 group occurs between 2970 and 2920 cm−1 and cy domain of C H symmetric stretching mode extends o 2815 cm−1 [40]. IR bands observed at 2934 (CH3 ops) −1 (CH3 ips) and Raman bands observed at 2935 (CH3 977 cm−1(CH3 ips) have been assigned to the O CH3 stretching mode. The O CH3 symmetric stretching ars as weak bands in both IR and Raman at 2893 cm−1. ift (∼43 cm−1) of O CH3 symmetric stretching mode has ted to the N12 H13· · ·O21 intramolecular interaction as eak C H· · ·Cl intermolecular interaction. The presence · · ·O21 intramolecular interaction has been identified O analysis as discussed earlier. Raman bands observed d 1422 cm−1 and the IR band observed at 1448 cm−1 ssigned to the asymmetric deformation modes. Also, a identified at 1180 cm−1 in the IR spectrum and a very n band observed at 1182 cm−1 have been assigned to rocking modes. hyl symmetric stretch of N CH3 usually absorbs in the 5–2780 cm−1 [40]. In this study, weak band observed −1 in the IR spectrum and at 2814 cm−1 in the Raman ave been assigned to the symmetric stretching mode. ion rbromuron molecule is theoretically optimized using ethod with 6-311G(d,p) basis set and the optimized s tabulated in comparison with the experimental XRD J.C. Monicka, C. James / Spectrochimica Acta Part A 94 (2012) 30– 35 35 data and well discussed. Shortening of C N bond shows the effect of resonance in that part of the molecule. Also, the intramolecular hyperconjugative interactions (C6 H11· · ·O15 and N12 H13· · ·O21) responsible for the stability of the molecule are well identified the- oretically using the NBO analysis. The higher energy gap (5.461 eV) of chlorbromuron implies high stability and less interaction of the herbicide with the target site. The experimental vibrational spectra of the compound were studied in comparison with the sim- ulated spectra. The peaks of the FTIR spectrum show the existence of N12 H13· · ·O15 intermolecular interaction which influences the bioactivity of the compound and act as an active site in protein. MEP plot predicts the secondary amide oxygen as the potential binding site. Acknowledgement Authors gratefully acknowledge the Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram for the support in recording the FTIR and Raman spectra. Appendix A. 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Theoretical and experimental vibrational study of the phenyl urea herbicide 1-(4-bromo-3-chlorophenyl)-3-methoxy-3-methylurea 1 Introduction 2 Experimental details 3 Computational details 4 Results and discussion 4.1 Geometry 4.2 NBO analysis 4.3 Electronic properties of chlorbromuron 4.4 Vibrational analysis 4.4.1 Phenyl ring vibrations 4.4.2 CX vibrations (X=Cl, Br) 4.4.3 Secondary amide vibrations 4.4.4 Methoxy group (OCH3) and NCH3 vibrations 5 Conclusion Acknowledgement Appendix A Supplementary data References