Nonplanar property study of antifungal agent tolnaftate-spectroscopic approach

Spectrochimica Acta Part A 79 (2011) 993–1003

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Nonplanar property study of antifungal agent tolnaftate-spectroscopic approach D. Arul Dhas a , I. Hubert Joe b,∗ , S.D.D. Roy a , S. Balachandran c a

Department of Physics, Nesamony Memorial Christian College, Marthandam 629 165, Tamil Nadu, India Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College, Thiruvananthapuram 695015, Kerala, India c Department of Chemistry, M.G College, Thiruvananthapuram 695004, Kerala, India b

a r t i c l e

i n f o

Article history: Received 21 December 2010 Received in revised form 7 April 2011 Accepted 8 April 2011 Keywords: NBO Raman DFT ICT Bioactivity

a b s t r a c t Vibrational analysis of the thionocarbamate fungicide tolnaftate which is antidermatophytic, antitrichophytic and antimycotic agent, primarily inhibits the ergosterol biosynthesis in the fungus, was carried out using NIR FT-Raman and FTIR spectroscopic techniques. The equilibrium geometry, various bonding features, harmonic vibrational wavenumbers and torsional potential energy surface (PES) scan studies have been computed using density functional theory method. The detailed interpretation of the vibrational spectra has been carried out with the aid of VEDA.4 program. Vibrational spectra, natural bonding orbital (NBO) analysis and optimized molecular structure show the clear evidence for electronic interaction of thionocarbamate group with aromatic ring. Predicted electronic absorption spectrum from TD-DFT calculation has been compared with the UV–vis spectrum. The Mulliken population analysis on atomic charges and the HOMO–LUMO energy were also calculated. Vibrational analysis reveals that the simultaneous IR and Raman activation of the C–C stretching mode in the phenyl and naphthalene ring provide evidence for the charge transfer interaction between the donor and acceptor groups and is responsible for its bioactivity as a fungicide. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Vibrational spectroscopic investigation with the help of quantum chemical computational method has been used as an effective tool for the structural analysis of pesticide molecules [1–4]. The toxicologic and pharmacologic study of a new antitrichophyton agent Tolnaftate (TNF) [(2-naphthyl-N-methyl-N-(3-tolyl) thionocarbamate] has been reported [5–8]. TNF has been widely used as topical antifungal drug in the treatment of cutaneous diseases [9–11]. The present investigation aims to understand the molecular structure regarding intramolecular charge transfer (ICT) and electron delocalization like ␲-conjugation and hyperconjugation of the compound using FT-Raman and IR along with the density functional theory (DFT) calculation. The time-dependent density functional theory (TD-DFT) calculation and natural bond orbital (NBO) analysis have been used to study the electronic effects. 2. Experimental The compound Tolnaftate was purchased from Sigma–Aldrich (U.S.A.). The NIR-FT Raman spectrum in the region 3500–50 cm−1

∗ Corresponding author. Tel.: +91 4712351053; fax: +91 4712530023. E-mail addresses: [email protected], [email protected] (I. Hubert Joe). 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.04.011

was taken on a Bruker RFS 66v NIR FT-Raman Spectrometer. The FTIR spectrum in the region 4000–400 cm−1 was recorded using a Perkin–Elmer Spectrum One FT-IR Spectrometer with samples in the KBr. The UV–visible absorption spectrum was recorded using Jastrow V-550 UV–Visible Spectrophotometer. 3. Computational details The DFT computation has been performed using Gaussian 03W program package [12] using Becke’s three-parameter hybrid method with the Lee, Yang and Parr’s correlation functional methods with the standard 6-31G (d) basis set. The simulated IR and Raman spectra were plotted using pure Lorentizian band shapes with a bandwidth of full width half-maximum (FWHM) of 10 cm−1 . An empirical scaling factor of 0.9614 [13] was used to offset the systematic error caused by neglecting an harmonicity and electron correlation. The distributions of assignment of the calculated wavenumbers were aided by VEDA program [14]. The Raman activities calculated by the Gaussian’03 program have been converted to relative Raman intensities using the basic theory of Raman scattering [15,16]. The natural bonding orbital (NBO) calculation was performed using NBO 3.1 program [17] and was implemented in the Gaussian’03 package at the DFT/B3LYP level. The hyperconjugative interaction energy was deduced from the second order perturbation approach [18–20].

994

D. Arul Dhas et al. / Spectrochimica Acta Part A 79 (2011) 993–1003

Fig. 1. The optimized structure of TNF calculated at B3LYP/6-31G (d).

4. Results and discussion 4.1. Geometry optimization The TNF structure was optimized using DFT method with 6–31 G (d) basis set. The optimized structural parameters are given in Table 1. The optimized molecular structure of the compound with atom numbering scheme adopted in the computation is shown in Fig. 1. The molecule consists of two separate ring systems viz, the naphthalene ring and tolyl ring connected by thionocarbamate group and two methyl groups me1 (attached to thionocarbamate group) and me2 (attached to tolyl group). The calculated structural parameters are in good agreement with the experimental values reported in closely related compounds such as methyl 2-(7-benzyloxy-1-naphthyl)-2-oxacetate [21] and o-methyl nphenylthiocarbamate [22]. In o-methyl n-phenylthiocarbamate C–O bond length is found out to be 1.443 A˚ and that for TNF (i.e. ˚ which indicates an extension of conjugation of C12 –O18 ) is 1.397 A, the aromatic ring of naphthalene to oxygen. The C–N bond lengths ˚ C22 –N21 (1.472 A) ˚ and C26 –N21 (1.441 A) ˚ are C19 –N21 (1.357 A), shorter than the normal C–N single-bond length 1.480 A˚ [23]. The shortening of these C–N bonds reveal the effect of resonance in thionocarbamate part of the molecule. The van-der Walls repulsion of H32 and H39 with H23 causes a steric hindrance in achieving coplanarity for the tolyl ring with the carbamate group and thus a twisting of tolyl group about N21 –C26 bond was noticed. Hence the tolyl group assumes the dihedral angles C22 N21 C26 C31 (−95.4◦ ) and C22 N21 C26 C27 (80.6◦ ). In the compound the hetero atoms O18 and N21 are slightly distorted from the plane and the two rings are flipped in such a way that the whole structure appears in the form of a boat with the S atom inside the boat. The methyl group attached to N21 and the sulphur atom attached to C19 is above and below the plane. Computational geometry optimization reveals that the nonplanar conformation is preferred by a minimum energy −3314526 kJ mol−1 . 4.2. PES scan studies The TNF molecule can adopt different conformations, mainly by spatially orienting the tolyl and methyl groups attached to N21 and flipping the naphthyl group with respect to the ether oxygen (O18 ). The thionyl carbon is Sp2 hybridized and thus N–(C S)–O assume a planar configuration with 120◦ bond angle (Table 1). Rotation at C19 –N21 bond of dihedral angle C26 –N21 –C19 –S20 leads to two conformers, the CH3 group attached to N21 is cis and trans to the thionocarbonyl group and are depicted in Fig. 2(A) and (C). The rotation at C18 –O19 of dihedral angle C12 –O18 –C19 –S20 results in cis and trans naphthyl conformers with respect to thionocarbonyl group and are shown in Fig. 2(B) and (D). The four conformers can be assigned as cis–cis (A), cis–trans (B), trans–cis (C) and trans–trans (D). The rotational conformers in this kind of compounds containing two aromatic rings, attached to two different hetero atoms,

Fig. 2. Possible conformers of TNF.

via a thionocarbonyl group and two different groups attached to the heteroatoms are influenced by different structural factors, such as steric, dipolar, mesomeric and hyperconjugative effects including hydrogen bonding interactions. The optimized TNF structure appears in the trans–cis conformer (C) where the tolyl and naphthyl rings are slightly out of plane (Table 1) to form of a boat with the hetero S20 atom inside the boat. The potential energy scan has been performed on the optimized geometry by rotating different spatially important groups with respect to single bond present in TNF between 0 and 360◦ with the increment of 10◦ . The possible maxima and minima relative energies and the corresponding dihedral angles are listed in Table S1 (Supporting Information). The rotation with respect to other dihedral angles (C12 O18 C19 S20 , C13 C12 O18 C19 and O18 C19 N21 C22 ) shows a restriction due to much interaction between the various groups. The PES scan plot and the corresponding conformers for the rotation of S20 C19 N21 C22 with respect to the bond C19 –N21 are given in Fig. S1 (Supporting Information). The staggered condition (160◦ ) corresponds to the trans–cis conformer (C) and the eclipsed condition (0◦ ) to cis–cis conformer (A). Comparing to the eclipsed structure the trans–cis staggered conformer, which corresponds to the optimized structure, is stabilized by 296,189 kJ mol−1 (Table S1; Supporting Information). At the maximal position the methyl group attached to N21 and the ether oxygen O18 groups are cis to each other and H24 . . .O18 and H25 . . .O18 distances are 2.492 and 2.615 A˚ (Table S2a; Supporting Information), respectively. The electrostatic interaction and the double bond character of C19 –N21 bond due to the mesomeric shift of lone pair of electrons from N21 along with the shift of double bonded electrons of C S group to S attribute to the energy reduction of conformer (C) in comparison to that of (A). The ether oxygen O18 can also interact via electron donating mesomeric effect and as a result the thionocarbamte group will have an extended conjugation with both the aromatic rings via the tolyl and naphthyl. The steric strain however does not allow a planar structure which reduces the resonance stabilization via the extended conjugation and can be observed by the various dihedral angles given in the optimized structure (Table 1). The energy curve for the dihedral angle C12 O18 C19 N21 (O18 –C19 bond) of TNF is shown in Fig. S2 (Supporting Information). It is observed that the conformer at 0◦ corresponds to trans–cis conformer (C) and the rotation did not cause any hindrance till it reaches 280◦ . The steric interactions due to van der Waals’ repulsion is comparatively weak since the distances between the interacting

D. Arul Dhas et al. / Spectrochimica Acta Part A 79 (2011) 993–1003

995

Table 1 Selected optimized geometric parameters of fungicide TNF on B3LYP/6-31G (d) basis set. Bond angles (◦ )

Bond lengths (Å)

C11 –C12 C12 –C13 C13 –C14 C11 –H15 C19 –N21 C22 –N21 C22 –H23 C22 –H24 C22 –H25 C26 –N21 C12 –O18

a b

B3LYP/6-31G(d)

Experi mental

1.371 1.413 1.374 1.086 1.357 1.472 1.090 1.093 1.095 1.441 1.397

1.375a 1.418a 1.363a 0.930a 1.331b 1.337b 0.970b 0.970b 0.970b 1.420b 1.361a

H15 –C11 –C6 H16 –C13 –C12 O18 –C12 –C13 C22 –N21 –C19 H23 –C22 –N21 H24 –C22 –N21 H25 –C22 –N21 C26 –N21 –C19 C27 –C26 –N21 C26 –N21 –C22 N21 –C19 –O18 N21 –C19 –S20 S20 –C19 –O18 C31 –C26 –N21 C27 –C26 –N21

Dihedral angles (◦ ) B3LYP/6-31G(d)

Experi mental

120.3 119.4 119.5 121.3 107.8 110.9 111.2 121.0 119.6 117.6 108.5 126.5 124.9 119.8 119.6

120.0a 120.0a 1125.2a 116.9b 109.5b 109.5b 109.5b 126.1b 117.0b 1115.6b 1112.5b 1123.4b 1124.0b 1117.0b 1123.9b

H16 –C13 –C12 –C11 H12 –C13 –C14 –C17 C22 –N21 –C19 –O18 C22 –N21 –C26 –C27 C22 –N21 –C26 –C31 H23 –C22 –N21 –C19 H24 –C22 –N21 –C19 H25 –C22 –N21 –C19 H39 –C27 –C26 –N21 H27 –C26 –N21 –C19 C19 –O18 –C12 –C11 C26 –N21 –C19 –S20 C26 –N21 –C19 –O18 C22 –N21 –C19 –S20 C12 –O18 –C19 –S20 C12 –O18 –C19 –N21 O18 –C19 –N21 –C22

B3LYP/6-31G(d)

Experimental

−179.8 179.8 0.9 80.6 −95.4 175.4 55.7 −64.9 4.2 −101.9 −111.1 3.2 −176.4 −179.5 4.9 −175.5 0.9

−175.9a 142.7a −30.5b 176.6b −19.7b 70.0b 46.5b 180.0b 0.0b −151.8b −153.5b −177.6b 3.0b −177.0b 5.7b −174.9b −177.0b

Taken from [21]. Taken from [22].

groups are larger as shown in Table S2b (Supporting Information). The extended conjugation is the reason which causes a larger barrier for rotation 103,804 kJ mol−1 (Table S1; Supporting Information). The conformers B and D are not observed during the rotation since the groups interact very strongly and re-orient themselves to attain stable trans–cis conformation. When the tolyl group is rotated by keeping the remaining part fixed with respect to N21 –C26 bond, the possible conformers and the PES curve are given in Fig. S3 (Supporting Information). Throughout the rotation of tolyl group, the structure remains in conformer (C). Two energy barriers of 54 kJ mol−1 and 60 kJ mol−1 (Table S1; Supporting Information) are observed at 20◦ and 200◦ exactly at 180◦ difference, which is due to van der Waals’ repulsion between ˚ and H32 . . .H23 (1.689 A) ˚ (Table S2c; SupportH39 . . .H23 (1.689 A) ing Information). At 80◦ and 130◦ and at 270◦ and 320◦ there are two additional lowering of energy of only less than 5 kJ mol−1 (Table S1; Supporting Information) which can be ascribed to the possible van der Waals’ attraction of S20 with H39 and S20 with H32 . A half-chair conformation is observed (Fig. S4; Supporting Information), when the naphthyl group is rotated with respect to C12 –O18 bond (dihedral angle C11 C12 O18 C19 ) keeping the tolyl and thionocarbamate groups stationary. A barrier of 2917 kJ mol−1 (Table S1; Supporting Information) is observed at eclipsed condition and is due to the steric and electrostatic interactions between H15 and H24 and S20 and H16 atoms. The extended conjugation may also contribute to the increased energy barrier (Table 2d; Supporting Information). There are two minimum energy areas which can ascribe as before as due to the electrostatic attraction of S20 with H, at 80◦ S20 –H16 interaction and at 220◦ it is S20 –H15 interaction. The conformer at 220◦ has the calculated energy nearest (larger by 677 kJ mol−1 ) to the optimized structure even though there are number of other conformers whose values are close to this value (Table S1; Supporting Information). The possible conformations during the rotation of the compound by keeping me1 (methyl attached to nitrogen) group fixed and rotating the remaining part with respect to N21 –C22 bond is shown in Fig. S5(a) (Supporting Information). In fact, it displays maxima for those conformations where the steric effect arising due to weak van der Waals repulsive interaction arises between the positively charged methyl hydrogen and phenyl ring hydrogen atoms (H23 . . .H39 = 2.827 A˚ at 50◦ and H24 . . .H39 = 2.879 A˚ at 170◦ ) (Table S2e; Supporting Information). The maximal interaction is for the rotation of 120◦ and

Fig. 3. Combined IR spectrum of TNF – (a) computed and (b) experimental.

is the bond angle of H23 C22 H24 , for the remaining 240◦ there is no repulsive interaction. The similar effect can be observed in me2 (methyl attached to benzene ring) with respect to C30 –C33 bond (dihedral angle C29 C30 C33 H36 ), where the maxima’s are ˚ and 330◦ at 90◦ (H34 . . .H32 = 2.487 A˚ and H35 . . .H37 = 2.511 A) ˚ (Table S2f; Support(H36 . . .H32 = 2.496 A˚ and H34 . . .H37 = 2.505 A) ing Information). The barrier for me1 is 16,489 and 15,049 kJ mol−1 and me2 is having the barrier 21,558 and 30,556 kJ mol−1 (Table S1; Supporting Information). The energy barrier value shows that the rotation about me1 group is easy while comparing to me2 group, since there are more number of repulsive interactions in me2 .

4.3. Vibrational spectral analysis The vibrational spectral analysis has been performed on the basis of the characteristic group vibrations of naphthalene ring, phenyl ring, thionocarbamate group and methyl group. The experimental and scaled wavenumbers along with their respective dominant normal modes and the corresponding (Potential Energy Distribution) PED’s are presented in Table 2. The observed and simulated FT-IR and Raman spectra are given in Figs. 3 and 4, respectively.

996

D. Arul Dhas et al. / Spectrochimica Acta Part A 79 (2011) 993–1003

Table 2 FT IR, Raman and computed vibrational bands of TNF and their assignments based on VEDA 4 program. IR intensity (arbitrary units)

Raman intensity (arbitrary units)

Assignments with PED (%)

3104 3089 3086

5.76 11.67 9.04

2.47 3.99 6.22

3085

32.93

4.01

C13 H16 Nap(94) 2C27 H39 Ph (68) + 2C28 H38 Ph (28) C11 H15 Nap (69) + C2 H8 Nap (29) + C3 H9 Nap (11) C3 H9 Nap (32) + C11 H15 Nap (30) + C2 H8 Nap (29) 7a C31 H32 Ph (80) 20b C28 H38 Ph (41) + 20b C27 H39 Ph (22) + 20b C31 H32 Ph (19) + 20b C29 H37 Ph (18) C3 H9 Nap (30) + C2 H8 Nap (29) + C1 H7 Nap (21) + C4 H10 Nap (18) C14 H17 Nap (89) C1 H7 Nap (49) + C4 H10 Nap (32) + C2 H8 Nap (13) 13 C29 H37 ph (74) + 13 C28 H38 ph (23) C4 H10 Nap (42) + C3 H9 Nap (21) + C1 H7 Nap (20) + C2 H8 Nap (16) C22 H25 me1 asy (85) C22 H23 me1 asy (50) + C22 H24 me1 asy (49) C33 H36 me2 asy (51) + C33 H34 me2 asy (49) C33 H35 me2 asy (43) + C33 H34 me2 asy (29) + C33 H36 me2 asy (28) C22 H23 me1 sy (43) + C22 H24 me1 sy (43) + C22 H25 me1 sy (15) C33 H35 me2 sy (57) + C33 H34 me2 sy (22) + C33 H36 me2 sy (21) C11 C12 Nap (19) + C5 C4 Nap (15) + C14 C13 Nap (14) + C1 C6 Nap (14) 8b C29 C28 Ph (28) + 8b C30 C31 Ph (15) + 8b C31 C26 Ph (11) C2 C1 Nap (20) + C11 C12 Nap (19) + C4 C3 Nap (17) + C14 C13 Nap (13) 8a C26 C27 Ph (34) + 8a C29 C28 Ph (11) C3 C2 Nap (20) + C14 C13 Nap (12) + ıC5 C4 C3 Nap (16) C4 C5 Nap (10) ıH24 C22 H23 me1 asy (19) + 1ıH37 C33 C30 Ph (15) ıH24 C22 H23 me1 asy (30) + ıH23 C22 H25 me1 asy (12) + ıH25 C22 H24 me1 asy (11) ıH34 C33 H35 me2 asy (28) + ıH35 C33 H36 me2 asy (24) + H35 C33 C30 C31 (10) ıH36 C33 H34 me2 asy (62) + H34 C33 C30 C31 (10) ıH17 C14 C13 Nap (13) + ıH8 C2 C3 Nap (12) + ıH7 C1 C2 Nap (10) + ıH16 C13 C14 Nap (10) ıH23 C22 H25 me1 asy (40) + ıH25 C22 H24 me1 asy (38) + H25 C22 N21 C19 (14) C19 N21 (23) + ıH24 C22 H23 me1 sy (30) + ıH25 C22 H24 me1 sy (16) + ıH23 C22 H25 me1 sy (14) C4 C5 Nap (10) + ıH9 C3 C4 Nap (22) + ıH10 C4 C3 Nap (11) 19a C31 C30 Ph (19) + 19a C26 C31 Ph (15) ıH35 C33 H36 me2 sy (38) + ıH34 C33 H35 me2 sy (37) + ıH36 C33 H34 me2 sy (19) C19 N21 (29) C19 N21 (11) + C14 C13 Nap (17) C4 C3 Nap (26) + C2 C1 Nap (23) + C1 C6 Nap (10) ıH16 C13 C14 Nap (13) + ıH7 C1 C2 Nap (11) 14C28 C29 ph (16) + 14C27 C28 ph (16) + 14C30 C31 ph (11) C26 N21 (23) 3C31 C26 ph (16) + 3C30 C31 ph (14) ıH10 C4 C3 Nap (15) + ıH7 C1 C2 Nap (14) + ıC5 C4 C3 Nap (12) C5 C4 Nap (20) O18 C19 (21) + O18 C12 (18) + 9bıH38 C28 C29 Ph (37) + 9bıH37 C29 C28 Ph (17) + 9bıH39 C27 C28 Ph (17) + ıH15 C11 C12 Nap (16)

Calculated wavenumbers (cm−1 )

Experimental wavenumbers (cm−1 ) IR

Raman

3081 3079

3058mbr

3060vs

2.46 28.42

2.17 1.98

3074

3036w

3031vw sh

34.78

2.77

7.62 0.51

2.57 2.01

8.10 2.82

2.11 0.22

19.72 14.07 16.64 19.43

2.09 1.74 1.79 2.24

38.27

3.95

3067 3060 3058 3056

3010w

3055 3006 3005 2982

2982vw

3010vw

2950w

2947 2936

2926vmbr

2915mbr

26.48

5.01

1624

1621wsh

1629w

21.53

4.06

1600

1600m

1607w

19.80

3.78

32.73

0.30

14.45 3.63

3.96 3.66

23.54 95.73 39.19

0.07 1.30 2.59

6.31

0.93

1460 1458

6.75 25.19

2.88 12.27

1456

5.61

2.39

171.64

1.45

1.49

4.59

15.32 0.42

1.55 4.14

1354vw

260.92 171.74 9.72

5.76 19.76 19.96

1335vw 1301w

13.61 1.99

0.04 0.22

1595 1580 1569

1586w

1505 1482 1477

1505w sh

1465

1463vs

1443

1445vs

1583s 1536vw

1467s

1441m

1431 1417 1388

1371 1365 1357

1384vvs

1370vvs

1342 1304

1371s sh

1295 1275 1244

1299m 1278vw 1240m

1277w 1243w

99.79 0.96 5.99

7.56 0.06 0.15

1202 1150

1214vs 1159vs

1218m 1153m

136.91 322.45

0.50 3.04

D. Arul Dhas et al. / Spectrochimica Acta Part A 79 (2011) 993–1003

997

Table 2 (Continued) Calculated wavenumbers (cm−1 )

Experimental wavenumbers (cm−1 ) IR

1142 1135 1133 1110 1100

1097 1081 1040 1011 986 980

IR intensity (arbitrary units)

Raman intensity (arbitrary units)

Assignments with PED (%)

6.18

0.31

61.18 16.06 29.76 3.77

1.73 0.61 0.22 0.98

114.37 19.14 2.32 1.01 0.72 5.78

0.83 0.70 1.82 5.60 0.65 10.85

37.80 0.19

0.65 0.18

929vw

2.15 1.61

0.63 0.31

895vw

3.31

1.91

ıH8 C2 C3 Nap (23) + ıH7 C1 C2 Nap (19) + ıH9 C3 C4 Nap (14) + ıH10 C4 C3 Nap (10) ıH9 C3 C4 Nap (11) + O18 C19 (16) ıH16 C13 C14 Nap (25) + ıH17 C14 C13 Nap (23) ıH16 C13 C14 Nap (10) ıH25 C22 H24 me1 asy (16) + ıH23 C22 H25 me1 asy (13) + H25 C22 N21 C19 (30) + H24 C22 N21 C19 (24) + H23 C22 N21 C19 (14) H23 C22 N21 C19 (18) + O18 C19 (12) 18aC29 C28 ph (19) + 18aC27 C28 ph (19) C22 N21 (46) + C20 S19 (10) ıCH2 rock me2 (16) 14C30 C31 Ph (10) + H35 C33 C30 C31 Nap (29) 6bıC29 C28 C27 Ph (32) + 6bıC26 C27 C28 Ph (14) + 6bıC31 C26 C27 Ph (14) C12 C13 Nap (21) + C11 C12 Nap (12) H38 C28 C27 C26 Ph (51) + H37 C29 C28 C27 Ph (21) + H39 C27 C28 C29 Ph (13) H7 C1 C2 C3 Nap (10) + H10 C4 C3 C2 Nap (10) H17 C14 C13 C12 Nap (16) + H16 C13 C14 C5 Nap (14) H39 C27 C28 C29 Ph (26) + H37 C29 C28 C27 Ph (24) + H32 C31 C30 C33 Ph (21) ıC4 C3 C2 Nap (17) H15 C11 C12 C13 Nap (55) + H8 C2 C3 C4 Nap (11) + H15 C11 C12 C13 Nap (14) H32 C31 C30 C33 Ph (54) + H39 C27 C28 C29 Ph (14) H10 C4 C3 C2 Nap (15) + H9 C3 C2 C1 Nap (14) 4C26 C27 C28 Ph (15) + C30 C33 (12) H17 C14 C13 C12 Nap (25) + H16 C13 C14 C5 Nap (35) + H7 C1 C2 C3 Nap (11) H39 C27 C28 C29 Ph (33) + H37 C29 C28 C27 Ph (16) C3 C2 C1 Nap (23) + C11 C12 O18 (12) C4 C3 C2 C1 Nap (15) H9 C3 C2 C1 Nap (25) + H8 C2 C3 C4 Nap (24) + H10 C4 C3 C2 Nap (21) ıC12 O18 C19 (10) S20 N21 O18 C19 (59) S20 N21 O18 C19 (59) + ıC4 C3 C2 Nap (12) C14 C13 C12 C11 Nap (12) ıC13 C12 O18 (24) 6bıC30 C31 C26 Ph (32) + C30 C33 (16) C3 C2 C1 C6 Nap (13) + C13 C11 O18 C12 (18) 16bC33 C30 C29 Ph (13) + ıN21 C26 C31 (34) C1 C6 C11 Nap (18)+ C3 C2 C1 C6 Nap (16) + C2 C1 C6 C11 Nap (11) + C13 C11 O18 C12 (17) ıC22 N21 C19 (36)+ 16bıC33 C30 C29 Ph (24) + C1 C6 C11 Nap (11) C1 C6 C11 Nap (25) + C11 C12 O18 (14) ıC19 N21 C26 (10) + ıS20 C19 O18 (19) + C31 C26 C27 C28 Ph (24) + C30 C31 C26 C27 Ph (10) C30 C31 C26 C27 Ph (18) + C31 C26 C27 C28 Ph (21) + C33 C31 C29 C30 Ph (30) ıO18 C19 N21 (35) + O18 C12 (16) C3 C2 C1 C6 Nap (12) but Nap (24) + ıC12 O18 C19 (29) + ıS20 C19 O18 (12) H25 C22 N21 C19 (30) + ıN21 C26 C27 (14) H24 C22 N21 C19 (15) ıC19 N21 C26 (21) + C6 C11 C12 O18 (14) + C2 C1 C6 C11 Nap (11) + N21 C31 C27 C26 (33) H34 C33 C30 C31 Ph (33) + H36 C33 C30 C31 Ph (20) + H35 C33 C30 C31 Ph (29) C11 C12 O18 C19 (47)

Raman 1140vw

1134vw 1126w 1115vw

1083wsh

1123w br

1000vw

1084w 1021m 1002vs

963m

964vw

960 938

938vw

925 914

911vw

886 874 866

888m

880vw

7.42 8.30

0.21 3.32

860 842 799 793

859w

862vw

805vs sh 809s

806vw

1.48 13.73 2.90 17.43

0.75 1.22 1.43 0.11

16.41 2.02 0.11 26.44

1.31 17.92 2.27 3.23

5.79 8.48 0.37 3.70 0.35 0.45 12.52 1.09 3.25 0.73

8.01 0.86 0.29 2.30 5.59 4.19 1.25 0.40 0.93 7.19 1.61 1.23 3.43 3.96

774 751 742 739 670 613 609 591 511 504 469 439 423 390

779w 753s

769vs

730vw

730vw 686m

592vw 520s 446vw

369w

321 300 273 224

329m

231m

0.60 0.10 2.23 3.46

205

209m

1.57

2.79

184 182 143

195sh 158w

0.71 1.19 2.27

4.15 10.30 9.69

115vvs 95s sh

0.50 0.13 1.16

2.40 1.29 48.34

42

0.43

4.67

17

0.13

100.00

131 100 84

 – stretching, ␦ – in-plane-bending, ␥ – out-of-plane-bending, ␶ – torsion, but – butterfly, asy – asymmetric, sy – symmetric, vvs – very very strong, vs – very strong, s – strong, m – medium, sh – shoulder, br – broad w – weak, vw – very weak, Ph – phenyl ring, Nap – naphthalene ring.

998

D. Arul Dhas et al. / Spectrochimica Acta Part A 79 (2011) 993–1003

ber range 3000–3100 cm−1 [29]. The observed band at 3031 cm−1 in Raman and 3036 cm−1 in IR are assigned to C–H stretching vibration.

Fig. 4. Combined Raman spectrum of TNF – (a) computed and (b) experimental.

4.3.1. Methyl group vibration The position of the CH3 vibration is almost entirely dependent up on the nature of the element to which the methyl groups are attached. The asymmetric C–H stretching mode of me2 is expected around 2980 cm−1 and the symmetric stretching is expected at 2870 cm−1 [24,25]. The me2 asymmetric stretching is observed as a weak band in IR at 2950 cm−1 and the symmetric stretching mode is observed as a medium broad band at 2926 cm−1 in IR and at 2915 cm−1 in Raman. The blue shifting (56 cm−1 ) of me2 symmetric stretching is due to the electron donating inductive effect and hyper conjugative effect of methyl group attached to the aromatic ring [26,27]. These effects imply electron delocalization, which may be taken in to account by a molecular orbital approach, this can point to changing polarizability and dipole moment due to electron delocalization [28]. The asymmetric C–H stretching of me1 is expected in the range 2820–2760 cm−1 [29]. The asymmetric stretching mode is shifted towards higher wavenumber 2982 cm−1 (PED 85%) in the IR spectrum. This blue shifting (162 cm−1 ) of methyl stretching wavenumber is due to the hyperconjugation interaction between the  (C22 –H24 ) → * (N21 –C26 ) bond. The asymmetric deformation mode of the hydrogen atoms of me2 group results very intense band at 1463 cm−1 in IR and 1467 cm−1 in Raman which are extremely stable, since the methyl group is attached to another carbon atom. The symmetric bending vibrations of me2 group are expected to appear in the region 1390–1370 cm−1 [24,25]. The intense band at 1384 cm−1 in Raman is correlated to symmetric bending mode. The relatively large Raman intensity of the symmetric bending mode suggests a large positive charge localized on the hydrogen atom of me2 group, which further supports the presence of hyperconjugation. 4.3.2. Naphthalene ring vibrations Naphthalene ring vibrations are found to make a major contribution in IR and Raman [30]. Naphthalene ring stretching vibrations usually occur in the region 1640–1400 cm−1 . C C stretching is an important marker band and its wavenumber is a genuine measure of the degree of conjugation through the ␲-electron chain [31]. In this compound the C C stretching mode is identified at 1621 and 1214 cm−1 in IR and at 1629, 1536, 1354 and 1218 cm−1 in Raman, respectively. Simultaneous activation of C C stretching mode of the naphthalene ring (1621:1629, 1214:1218 cm−1 ) in the IR and Raman spectra provides evidence for the charge transfer interaction between the donor and acceptor groups through the ␲ system [32]. The characteristic C–H stretching vibrations of heteromatic structure are expected to appear in the wavenum-

4.3.3. Thionocarbamate group vibrations In thionocarbamate related compound [33], C S stretching mode is expected at 1050 cm−1 . The medium band observed at 1021 cm−1 in Raman is assigned to C S stretching mode. The C–O stretching vibration is expected at 1149 cm−1 [34]. In TNF, the C–O stretching vibration is observed as very strong band at 1159 cm−1 in IR and medium band at 1153 cm−1 in Raman. The vibrational band corresponds to C–N stretching in tertiary amine occur in the range 1250–1020 cm−1 [24]. In TNF compound C19 –N21 stretching vibration is observed as very strong band at 1445 cm−1 in IR and medium band at 1441 cm−1 in Raman. This blue shift of about 195 cm−1 is due to the charge transfer from electron donor me1 group to acceptor aromatic rings through an extended hyperconjugation involving the thionocarbonyl group. Also a very strong band of C19 –N21 stretching vibration is observed at 1370 cm−1 in IR and 1371 cm−1 in Raman. The stretching vibration N21 –C26 is observed as a medium band in IR at 1299 cm−1 , which is blue shifted (49 cm−1 ) due to hyperconjugation interaction between the  (C22 –H24 ) → * (N21 –C26 ) bond. These hyperconjugative charge transfer interactions contribute to the fungicidal activity of TNF. 4.3.4. Phenyl ring vibration Various normal modes of vibration of the substituted phenyl ring have been comprehensively studied according to Wilson’s numbering convention [29,35]. TNF consists of meta disubstituted phenyl ring whose normal modes of C–H in-plane bending are classified as 3, 9b, 18a and 18b. The observed weak bands at 1277 cm−1 (IR) and 1278 cm−1 (Raman) are correlated to the C–H in-plane bending mode3. The benzene mode 9b is expected in the region 1149–1166 cm−1 [29], and is observed as medium band at 1153 cm−1 in Raman and as very intense band at 1159 cm−1 in IR. The corresponding calculated value lies at 1150 cm−1 (very intense). The intense IR band is due to strong electron–donor methyl group attached to benzene ring [29]. The ring mode 18a appears at 1083 and 1084 cm−1 as a weak band in IR and Raman respectively. The in-plane bending mode 3, 9b and 18a are found to be simultaneously active in both IR and Raman because of symmetry lowering of the molecule; in addition such deformation changes the bond length between the carbon atoms, which build the charge transfer axis and the polarizability of such ␲ conjugated system. The selection rule allows five normal modes 8a, 8b, 19a, 19b and 14 for the tangential C–C stretching mode in asymmetrically disubstituted benzene derivatives. In meta disubstituted benzene the wavenumber of 8a is smaller than that of 8b [29]. DFT computation shows vibrational modes 8a and 8b at 1580 and 1600 cm−1 ; such splitting can be observed experimentally and is allowed for the selection rule for asymmetrically meta disubstituted ring. The phenyl ring mode 8a manifests as very strong band in Raman at 1583 cm−1 and weak band in IR at 1586 cm−1 and its companion 8b appears as weak and medium band at 1607 and 1600 cm−1 in Raman and IR, respectively. The calculated values also agree well with the experimental data. The mode 20b is observed as very intense band in Raman at 3060 cm−1 and medium broad band in IR at 3058 cm−1 . 5. Electronic absorption spectra Electronic transitions have been investigated by UV–visible spectroscopy. Absorption maximum (max ) was calculated by TD-

D. Arul Dhas et al. / Spectrochimica Acta Part A 79 (2011) 993–1003

999

222 nm. The very strong band at 222 nm is a characteristic peak of aromatic system due to ␲–␲* transition (E band) [36]. TD-DFT calculation shows that the corresponding band lies at 234 nm. The weak band at 258 nm is due to n–␲* transition (R-band), which is attributed to sulphur atom [37]. The corresponding calculated value lies at 245 nm.

5.1. HOMO–LUMO

Fig. 5. Combined UV–visible absorption spectrum of TNF – (a) computed and (b) experimental.

DFT method. The combined UV–visible absorption spectrum of the sample is shown in Fig. 5. The electronic transitions and the corresponding excitation energies are listed in Table 3. The UV–visible spectrum was measured in methanol solution and it is found that the absorption bands are observed at 258 and

Spatial distribution of molecular orbitals, especially those of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are excellent indicators of electron transport in molecular systems. In the molecule HOMO–LUMO energy gap is 222 kJ mol−1 . The lowering of the HOMO–LUMO band gap is essentially a consequence of the large stabilization of LUMO due to strong electron-accepting ability of the electron-acceptor group. The atomic orbital components of the frontier molecular orbitals are shown in Fig. 6. The absorption band in the long wavelength region (258 nm) is primarily due to HOMO → LUMO + 2 transition, in which the electronic charge density is withdrawn from the thionocarbamate group (i.e. 3p-orbit of S atom in C S group) to the tolyl ring, i.e. the absorption band near 258 nm of UV–visible spectra resulted from the electron transitions from the initial state is mainly contributed by HOMO and the final state is contributed by LUMO. The

Table 3 Calculated electronic absorption spectrum of TNF molecule. Excitation

Excited state 1 HOMO − 2 → LUMO HOMO − 1 → LUMO HOMO − 1 → LUMO + 1 HOMO − 1 → LUMO + 2 HOMO → LUMO HOMO → LUMO + 1 Excited state 2 HOMO − 2 → LUMO + 1 HOMO − 1 → LUMO HOMO − 1 → LUMO + 1 HOMO → LUMO HOMO → LUMO + 1 Excited state 3 HOMO − 4 → LUMO HOMO − 4 → LUMO + 1 HOMO − 4 → LUMO + 2 HOMO − 4 → LUMO + 4 Excited state 4 HOMO − 2 → LUMO + 1 HOMO − 1 → LUMO HOMO → LUMO HOMO → LUMO + 1 HOMO → LUMO + 2 Excited state 5 HOMO − 5 → LUMO + 4 HOMO − 3 → LUMO + 2 HOMO − 3 → LUMO + 3 HOMO − 2 → LUMO + 2 HOMO − 2 → LUMO + 4 HOMO − 1 → LUMO + 2 HOMO → LUMO + 1 HOMO → LUMO + 2 Excited state 6 HOMO − 6 → LUMO HOMO − 4 → LUMO HOMO − 2 → LUMO + 3 HOMO − 1 → LUMO + 1 HOMO − 1 → LUMO + 5 HOMO → LUMO + 1

CI expansion coefficient

0.40514 0.35848 0.27321 −0.10113 0.37352 −0.19625 −0.24525 0.50836 −0.16720 −0.35227 −0.24615 0.13291 0.18287 0.59163 0.21633 0.33508 0.23511 −0.11335 0.36662 −0.11859 −0.12652 −0.11597 −0.23275 −0.33378 0.10735 0.18375 0.21031 0.36655 0.31953 −0.11200 0.11891 0.43584 0.10474 −0.11449

LUMO is No.82 Orbital. HOMO is No.81 Orbital.

Excitation energies (kJ mol−1 )

Wavelength (nm)

Oscillator strength (f)

Assignment

Calc.

Expt.

359

333

–

0.0941

–

374

320

–

0.0492

–

435

275

–

0.0012

–

480

249

–

0.1171

488

245

258

0.3337

n→␲*

512

234

222

0.4398

␲ → ␲*

–

1000

D. Arul Dhas et al. / Spectrochimica Acta Part A 79 (2011) 993–1003

weak intramolecular C–H. . .S hydrogen bonds due to the interaction between the lone pair of sulphur n1 (S20 ) with the antibonding orbitals * (C13 –H16 )(6.3 kJ mol−1 ) and (* (C27 –H39 )(9.6 kJ mol−1 ) have been confirmed by the results of NBO analysis. This was substantiated by PES analysis during the rotation with respect to N21 –C26 bond. The most important interaction energy related to the resonance of the molecule is electron donation from n1 (N21 ) (1.625 e) to the antibonding acceptor ␲* (C26 –C27 ) of the tolyl ring (124.9 kJ mol−1 ) and ␲* (C19 –S20 ) of thionocarbamate group (245.8 kJ mol−1 ). These interactions lead to stability and in turn to the bioactivity of fungicide. 6.1. NHO directional analysis

Fig. 6. HOMO–LUMO plot of TNF.

charge transfer from thionocarbamate group (i.e. 3p-orbit of S atom in C S group) to naphthalene ring is also very pronounced in the HOMO − 1 → LUMO + 1 transition, which predominantly occurs in the band observed at 222 nm. The HOMO and LUMO plot shows that both the HOMO and LUMO orbitals predominantly localize on the naphthalene ring, thionocarbamate group and phenyl ring. The small HOMO–LUMO gap (222 kJ mol−1 ) reflects the chemically reactive [38] nature with a low electron density at the sulphur atom which accounts its fungicidal activity. 6. NBO analysis NBO results showing the formation of Lewis and non-Lewis orbital by the valence hybrids corresponding to the intramolecular bonds are given in Table 4. In addition the most important interactions between ‘filled’ (donors) Lewis-type NBOs and ‘empty’ (acceptors) non-Lewis NBOs are reported in Table 5. There is strong intramolecular hyperconjugative interaction of ␲ electrons in the aromatic ring, i.e. C3 –C4 and C12 –C13 bonds conjugate to the ␲* (C5 –C14 ) bond of the naphthalene ring. The electron density (ED) 0.480 e leads to a stabilization energy 79.5 kJ mol−1 . This enhancement of ␲* (C5 –C14 ) NBO further conjugates with ␲* (C3 –C4 ) and ␲* (C6 –C11 ) resulting to an enormous stabilization energy of 260.8 and 432.4 kJ mol−1 , respectively. ˚ is due to rehyThe weakening of C22 –H24 bond (1.093 A) bridization, which is revealed by low value of ED (0.012 e) in the * (C22 –H24 ) orbitals. The hyperconjugative interaction energy (12.6 kJ mol−1 ), due to interaction between  (C22 –H24 ) and * (N21 –C26 ) leads to blue shifting (162 cm−1 ) of methyl asymmetric stretching wavenumber, which is confirmed by vibrational spectral analysis. This is further supported by the blue shifting of about 49 cm−1 in the N21 –C26 stretching (Table 2). The hyperconjugative interaction between the oxygen lone pair and C–C antibonding orbital of the naphthalene ring is maximum, i.e. n1 (O18 ) → ␲* (C12 –C13 ) increases ED in C–C antibonding orbital ˚ (0.506 e) that weakens the respective bond (C12 –C13 = 1.413 A) leading to stabilization energy of 16.4 kJ mol−1 . The existence of

The bending angles of different bonds expressed as the angle of deviation from the direction of the line joining the two nuclei centers are given in Table 6. For maximum resonance interaction to occur O18 should be in the same plane with naphthalene ring, but due to the weak C22 H24 . . .O18 interaction (2.6 kJ mol−1 ) oxygen of  CO is bent away from the line of C12 –O18 centers by 17.8◦ as a result of lying in the strong charge transfer path towards naphthalene ring, whereas the carbon NHO is approximately aligned with the C–O axis. A little lower bending effect is also noticed at the C12 –C13 (14.8◦ ) and C5 –C14 (13.0◦ ) bonds of the naphthalene ring. The bending of the bonds within the naphthalene ring, phenyl ring and thionocarbamate group attributes to the increase of strains at the active center of the compound. Also in  (O18 –C19 ), the bent of  OC bond orbital from the line of O–C centers decreases (1.8◦ ) due to the presence of sulphur atom close to it. 7. Atomic net charges A comparative study of Mulliken’s net charges and the atomic natural charges for TNF have been carried out by B3LYP/6-31 G (d) basis set and the results are shown in Table S3 (Supporting Information). The Mulliken population analysis shows the presence of three large electronegative atoms such as O18 , S20 and N21 , which impose more positive charge on C19 (0.373 e). The C22 –N21 bond length connecting the donor-subunit with ␲ conjugating path is computed to ˚ while for the acceptor moiety C26 –N21 bond distance be 1.472 A, ˚ The shortening of C26 –N21 is mainly due to the is about 1.441 A. hyperconjugative interaction between phenyl ring and the strong electron donating group (N-methyl). Since the H24 has comparatively high positive charge, this leads to increase the distance of H24 ˚ The increase of electron density and O18 atoms by about 2.497 A. on O18 (1.949 e) seems to indicate a strong interaction of electron donating methyl group towards O18 . It is observed that the interaction between H24 and O18 atoms contribute extra stability to the inclined structure. 8. Bioactivity The thionocarbamate antimycotic agent, tolnaftate has potent antidermatophytic activity [39], which is primarily inhibiting the ergosterol biosynthesis in the fungus. The exact mode of action has not been reported, but similar to that of allylamines naftifine and terbinafine, TNF is known to inhibit selectively the fungal squalene epoxidase reaction, i.e., the conversion of squalene into 2,3-oxidosqualene catalyzed by squalene epoxidase, ultimately resulting in the lack of ergosterol, which is an indispensable lipid component of fungal cell membrane. The thionocarbamate moiety is the active center as in the carbamate pesticides which are cholinesterase inhibitor [40–44]. The electron density variation of C O group is well studied experimentally in carbamate and the reduction in electron density at the O atom of C O enhances its

D. Arul Dhas et al. / Spectrochimica Acta Part A 79 (2011) 993–1003

1001

Table 4 NBO results showing the formation of Lewis and non-Lewis orbitals by the valence hybrids corresponding to the intramolecular bonds of TNF. Bond (A–B)

ED/Energy (a.u.)

EDA (%)

EDB (%)

NBO

␲C3–C4

1.741 −0.259 1.581 −0.246 0.480 −0.002 0.506 −0.012 0.365 0.023 0.020 0.713 0.031 0.467 1.977 −0.706 1.949 −0.575 1.825 −0.352

48.12

51.88

62.05

37.95

57.51

42.49

37.95

62.05

48.95

51.05

38.16

61.84

36.52

63.48

␲C12–C13 ␲* C5–C14 ␲* C12–C13 ␲

*

C26–C27

␴* C13–H16 ␴* C27–H39 n1 (S20 ) n1 (O18 ) n2 (O18 )

s (%)

–

–

0.694 (sp 99.99 )C +0.720 (sp 99.99 )C 0.788 (sp 94.26 )C +0.616 (sp 99.86 )C 0.758 (sp99.99 )C −0.652 (sp18.88 )C 0.616 (sp16.58 )C −0.788 (sp99.99 )C 0.670 (sp 99.99 )C −0.715 (sp 99.99 )C 0.622 (sp 1.99 )C −0.783 (s)H 0.604 (sp 2.16 )C −0.797 (s)H sp99.99

–

–

sp1.44

–

–

sp17.38

P(%)

0.18 0.50 5.69 0.08 0.05 5.03 5.69 0.08 0.55 0.88 22.79 100.00 31.62 100.00 0.39

99.77 99.45 94.26 99.86 99.91 94.89 94.26 99.86 99.42 99.06 77.12 – 68.33 – 99.56

40.90

59.05

5.44

94.49

Table 5 Second order perturbation theory analysis of Fock matrix in NBO basis. ED(i) (e)

Acceptor NBO (j)

ED(j) (e)

E(2)(kJ mol−1 )

E(j)–E(i) (kJ mol−1 )a

F(i,j) (kJ mol−1 )b

␲ (C1 –C2 )

1.733

␲ (C3 –C4 )

1.741

␲ (C5 –C14 )

1.556

␲ (C12 –C13 )

1.581

␲ (C5 –C14 )

0.480

␲ * (C12 –C13 ) ␲ * (C26 –C27 )

0.506 0.365

␴ (C13 –C14 ) ␴ (C30 –C31 ) ␴ (C22 –H24 ) n1 (S20 )

1.976 1.972 1.994 1.977

n1 (O18 )

1.949

n1 (N21 )

1.625

␲ (C3 –C4 ) ␲ * (C6 –C11 ) ␲ * (C1 –C2 ) ␲ * (C5 –C14 ) ␲ * (C6 –C11 ) ␲ * (C12 –C13 ) ␲ * (C5 –C14 ) ␲ * (C6 –C11 ) ␲ * (C3 –C4 ) ␲ * (C6 –C11 ) ␲ * (C6 –C11 ) ␲ * (C28 –C29 ) ␲ * (C30 –C31 ) ␴* (C12 –O18 ) ␴* (N21 –C26 ) ␴* (N21 –C26 ) ␴* (C13 –H16 ) ␴* (C27 –H39 ) ␴* (C19 –S20 ) ␴* (C6 –C11 ) ␲ * (C6 –C11 ) ␴* (C12 –C13 ) ␲ * (C12 –C13 ) ␲ * (C26 –C27 ) ␴* (C26 –C31 ) ␴* (C19 –S20 ) ␲ * (C19 –S20 )

0.263 0.346 0.257 0.480 0.346 0.506 0.480 0.346 0.263 0.346 0.346 0.317 0.353 0.050 0.037 0.037 0.020 0.031 0.414 0.022 0.346 0.040 0.506 0.018 0.020 0.028 0.414

82.6 70.3 67.8 79.5 32.8 153.5 50.8 75.9 260.8 432.4 252.9 464.6 499.2 8.6 16.9 12.6 6.3 9.6 23.1 13.3 10.9 10.8 16.4 124.9 3.4 9.8 245.8

761 709 788 683 709 630 630 683 105 53 79 53 53 2652 2862 2416 2625 2863 2363 2967 1549 2862 1470 761 2127 1549 551

176 160 163 173 110 223 129 165 181 163 147 173 192 108 155 123 92 116 165 142 100 123 126 218 66 95 260

Donor NBO (i)

*

*

ED – electron density. E(2) means energy of hyperconjugative interactions (stabilization energy). a Energy difference between donor and acceptor i and j NBO orbitals. b F (i, j) is the Fock matrix element between i and j NBO orbitals. Table 6 NHO directionality and “bond bending” (deviations from line of nuclear centers) of TNF. NBO

Bond A–B

Line of centers ◦

Around thionocarbamate group ␴C12–O18 C12–O18 ␴ O18–C19 O18–C19 Around phenyl ring ␴C26–C31 C26–C31 ␴C28–C29 C28–C29 Around naphthalene ring ␴C5–C14 C5–C14 ␴C12–C13 C12–C13

Hybrid 1 ◦

◦

Hybrid 2 ◦

◦

( )

ϕ( )

␪( )

␸( )

Deviation at A ( )

 (◦ )

ϕ (◦ )

Devia tion at B (◦ )

124.4 72.8

161.2 200.2

108.6 72.4

170.6 198.3

17.8 1.8

49.0 107.0

340.4 18.9

6.6 1.3

73.6 73.9

133.2 132.9

78.4 80.5

128.2 134.7

6.8 6.8

109.2 110.7

317.3 309.9

4.8 5.4

99.6 102.2

227.7 298.7

106.5 88.7

232.6 292.3

8.4 14.8

91.1 70.6

55.3 116.2

13.0 7.6

1002

D. Arul Dhas et al. / Spectrochimica Acta Part A 79 (2011) 993–1003

pesticide activity. Analogues to carbamates the variation of electron density is observed in C S group when different groups are attached to the thionocarbamate group. The calculated Charge density, Dipole Moment, Mulliken Charge of S, SCF Energy and Homo–Lumo Energy gap for various thionocarbamate derivatives (Fig. S6; Supporting Information) are listed in Table S4 (Supporting Information). From Table S4 (Supporting Information), it is clearly observed that the parent compound (TNF) has the lowest charge density at S atom. The maximum charge density is observed for dimethyl thionocarbamate, where both the methyl groups are electron donating and enhances the charge at sulphur through the hyperconjugated structure. In carbamates due to higher electronegativity of oxygen (3.5) the ␲-electrons of C O is shifted more towards oxygen but in thionocarbamates the nearly equal electronegativites of carbon and sulphur (2.5) give equal probability for ␲-electron shift. The phenyl and naphthyl group on the other hand is electron withdrawing and thus decreases the electron density at C19 and which in turn abstract the ␲-electrons of C S. The near coplanarity of aromatic rings with hetero atoms O18 and N21 and with C19 (Table 1) allows an extended conjugation throughout the compound. The shifted electrons from sulphur towards the C19 carbon moves towards naphthyl group via oxygen atom while towards tolyl group via nitrogen atom, the lone pair of electron present in these hetero atoms help in these electron flow. The lesser electron density at S20 enhances its biological activity as shown experimentally [45]. The substitution of electron donating methyl group in phenyl ring increases the electron density in S when they are at ortho and para positions. When it is at meta position (TNF) there is comparatively smaller electron density at S (Table S4; Supporting Information). Thionocarbamates with electron-withdrawing substituent groups are highly fungicidal active [45]. In tolnaftate there are two electron-withdrawing substituent groups such as benzene and naphthalene ring, hence tolnaftate is highly fungicidal active compound. Also the large energy barriers in potential energy scan, the lowering of HOMO–LUMO energy gap, influence of electronic effect resulting from the hyperconjugation and induction of methyl group in the aromatic ring are the main factors which decide the bioactive nature of the compound.

9. Conclusions Predicted electronic absorption spectra from TD-DFT calculation have been analysed comparing with the experimental UV–visible spectrum and they are mainly derived from the contribution of n-␲* and ␲–␲* band. The lowering of HOMO–LUMO energy gap clearly explains the charge transfer interactions taking place within the molecule and which leads to its enhanced bioactivity. The simultaneous IR and Raman activation of the phenyl ring C–C stretching mode, in-plane bending mode and symmetric stretching mode of methyl groups also provide evidences for the intramolecular charge transfer interaction between the donor and acceptor group through the ␲ system. NBO analysis reveals that the most important interaction energy related to the resonance of the molecule is electron donation from n1 (N21 ) to the antibonding acceptor ␲* (C26 –C27 ) of the tolyl ring (124.9 kJ mol−1 ), and which leads to the stability and in turn to the bioactive nature of TNF fungicide. The near co-planarity of aromatic rings with hetero atoms O18 and N21 and with C19 allows an extended conjugation throughout the compound, the PES scan result also support this observation. The two electron-withdrawing substituent groups such as benzene and naphthalene increase the fungicidal activity of tolnaftate.

Acknowledgements The author D. Arul Dhas thanks the University Grants Commission (UGC), India, for the award of a Teacher Fellowship under FIP scheme leading to the Ph.D. degree. Authors also thank Prof. M.H. Jamroz, Institute of Industrial Chemistry Research, Warsaw, Poland, for providing VEDA 4 Program. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.saa.2011.04.011. References [1] S. De-Qing, Z. Xiao-Fei, S. Yuan-Zhi, Spectrochim. Acta Mol. Biomol. Spectrosc. A 71 (2008) 1011–1020. [2] V.L. Furer, I.I. Vandyukova, A.E. Vandyukov, S. Fuchs, J.P. Majoral, A.M. Caminade, V.I. Kovalenko, J. Mol. Struct. 932 (2009) 97–104. [3] M. Karabacak, D. Karagöz, M. Kurt, J. Mol. Struct. 892 (2008) 25–31. [4] D. Arul Dhas, I. Hubert Joe, S.D.D. Roy, T.H. Freeda, Spectrochim. Acta Part A 77 (2010) 36–44. [5] Y. Hashimoto, T. Noguchi, H. Kitagawa, G. Ohta, Toxicol. Appl. Pharmacol. 8 (1966) 380–385. [6] T. Noguchi, Y. Hashimoto, T. Makita, T. Tanimura, Toxicol. Appl. Pharmacol. 8 (1966) 386–397. [7] A.E. Czeizel, Z. Kazy, E. Puho, Reprod. Toxicol. 18 (2004) 443–444. [8] R. Munguia, S. Daniel, J. Interferon J. Ped. Otorhinolaryngol 72 (2008) 453–459. [9] F. Chapeland, R. Fritz, C. Lanen, M. Gredt, P. Leroux, Pest. Biochem. Phys. 64 (1999) 85–100. [10] B. Tang, X. Wang, G. Wang, C. yu, Z. Chen, Talanta 69 (2006) 113–120. [11] T. Bo, W. Xu, W. Jing, Y. Chengguang, C. Zhenzhen, D. Yi, J. Phys. Chem. B110 (2006) 8877–8884. [12] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R. E/Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, B. Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian 03, Revision C. 02, Gaussian, Inc, Wallingford CT, 2004. [13] A.P. Scott, L. Radom, J. Phys. Chem. 100 (1996) 16502–16513. [14] M.H. Jamroz, Vibrational Energy Distribution Analysis: VEDA 4 Program, Drug Institute, Warsaw, Poland, 2004. [15] G. Keresztury, S. Holly, J. Varga, G.A. Besenyei, Y. Wang, J.R. Durig, Spectrochim. Acta A 49 (1993), 2007–2017, 2019–2026. [16] G. Keresztury, J.M. Chalmers, P.R. Griffith, Raman Spectroscopy, Theory in Handbook of Vibrational Spectroscopy, vol. 1, John Wiley & Sons Ltd, New York, 2002, 71–87. [17] E.D. Glendening, A.E. Reed, J.E. Carpenter, F. Weinhold, NBO Version 3.1,TCI, University of Wisconsin, Madison, 1998. [18] X.-H. Li, R.-Z. Zhang, X.-Z. Zhang, Struct. Chem. 20 (2009) 1049–1054. [19] J. Chocholousova, V. Vladimin Spirko, P. Hobza, Phys. Chem. Chem. Phys. 6 (2004) 37–41. [20] A.E. Reed, L.A. Curtiss, F. Weinhold, Chem. Rev. 88 (1988) 899–926. [21] F. Hoong-Kun, S. Chantrapromma, L. Shu-Xian, L. Hua-Min, Acta Crystallogr. E64 (2008) 1409. [22] S.Y. Ho, C.S. Lai, E.R.T. Tiekink, Acta Crystallogr. E59 (2003) 1155–1156. [23] H. Arslan, U. Florke, N. Kulcu, G. Binzet, Spectrochim. Acta A 68 (2007) 1347–1355. [24] B. Smith, Infrared Spectral Interpretation. A Systematic Approach, CRC Press, New York, DC, 1999. [25] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, 3rd ed., Academic Press, New York, 1990. [26] Y. Huang, D.F.R. Gilson, I.S. Butler, J. Chem. Phys. 97 (1993) 1998–2001. [27] M. Gussoni, C. Castiglioni, M.N. Ramos, M.C. Rui, G. Zerbi, J. Mol. Struct. 224 (1990) 445–470. [28] V. Hernandz, C. Castiglioni, G. Zerbi, J. Mol. Struct. 324 (1994) 189–198. [29] G. Varsanyi, Vibrational Spectra of Benzene Derivatives, Academic Press, New York, 1969. [30] A.J. Barnes, Spectrochim. Acta A 41 (1985) 629–635. [31] L. Rimai, M.E. Heyde, D. Gill, J. Am. Chem. Soc. 95 (1973) 4493–4501. [32] M. Snehalatha, C. Ravikumar, N. Sekhar, V.S. Jayakumar, I. Hubert Joe, J. Raman Spectrosc. 39 (2008) 928–936. [33] G. Fairthorne, D. Fornasiero, J. Ralston, Int. J. Miner. Process. 46 (1996) 137–153. [34] H. Arslan, Ulrich, N. Kulcu, Spectrochim. Acta A 67 (2007) 936–943. [35] G. Socrates, Infrared Characteristic Group frequencies, Wiley-Interscience Publication, New York, 1980.

D. Arul Dhas et al. / Spectrochimica Acta Part A 79 (2011) 993–1003 [36] [37] [38] [39]

L. Gomati Devi, G. Krishnamurthy, J. Hazard. Mater. 162 (2009) 899–905. H.M. Pinheiro, E. Touraud, O. Thomas, Dyes Pigments 61 (2004) 121–139. N.S. Mills, A. Levy, B.F. Plummer, J. Org. Chem. 69 (2004) 6623–6633. K. Iwata, T. Yamashita, H. Uehara, Antimicrob. Agents Chemother. 33 (1989) 2118–2125. [40] W. Iwatani, T. Arika, H. Yamaguchi, Antimicrob. Agents Chemother. 37 (1993) 785–788. [41] M.H. Weiden, J. Bull. Org. Mond. Sante, Bull. World Health Org. 44 (1971) 203–213.

1003

[42] N.S. Ryder, I.F. Marie, C.D. Pont, Antimicrob. Agents Chemother. 29 (1986) 858–860. [43] M.J. Weinstein, E.M. Oden, E. Moss, Antimicrob. Agents Chemother. 10 (1964) 595–601. [44] K.J. Barretti-Bee, A.C. Lane, R.W. Turner, J. Med. Vet. Mycol. 24 (1987) 155–160. [45] M. Albores-Velsco, J. Thorne, R.L. Wain, J. Agric. Food Chem. 43 (1995) 2260–2261.