Experimental spectroscopic (FTIR, FT-Raman, FT ...

Journal of Molecular Structure 1003 (2011) 92–102

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Experimental spectroscopic (FTIR, FT-Raman, FT-NMR, UV–Visible) and DFT studies of 2-amino-5-chlorobenzoxazole V. Arjunan a,⇑, P.S. Balamourougane b, C.V. Mythili c, S. Mohan d a

Department of Chemistry, Kanchi Mamunivar Centre for Post-Graduate Studies, Puducherry 605 008, India Centre for Research and Development, PRIST University, Thanjavur 613 403, India c Department of Chemistry, Rani Anna Government College for Women, Thirunelveli 627 008, India d Department of Mathematical and Physical Sciences, Hawasa University, Hawasa, Ethiopia b

a r t i c l e

i n f o

Article history: Received 2 June 2011 Received in revised form 22 July 2011 Accepted 22 July 2011 Available online 29 July 2011 Keywords: FTIR FT-Raman 2-Amino-5-chlorobenzoxazole DFT NMR UV–Visible

a b s t r a c t The solid phase FTIR and FT-Raman spectra of 2-amino-5-chlorobenzoxazole have been recorded in the regions 4000–400 and 3500–100 cm1, respectively. The spectra were interpreted in terms of fundamentals modes, combination and overtone bands. The normal coordinate analysis was carried out to confirm the precision of the assignments. The structure of the compound was optimised and the structural characteristics were determined by density functional theory (DFT) using B3LYP method with 6-31G(d,p), 6-311++G(d,p) and cc-pVDZ basis sets. The molecular properties were also determined by HF/6-311++G(d,p) level. The vibrational frequencies were calculated in all these methods and were compared with the experimental frequencies which yield good agreement between observed and calculated frequencies. The infrared and Raman spectra were also predicted from the calculated intensities. 1H and 13 C NMR spectra were recorded and 1H and 13C nuclear magnetic resonance chemical shifts of the molecule were calculated using the gauge independent atomic orbital (GIAO) method. UV–Visible spectrum of the compound was recorded and the electronic properties HOMO and LUMO energies were measured by time-dependent TD-DFT approach. The geometric parameters, energies, harmonic vibrational frequencies, IR intensities, Raman activities chemical shifts and absorption wavelengths were compared with the available experimental data of the molecule. The influences of chlorine atom and the amino group on the skeletal modes and on the proton chemical shifts have been investigated. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction The benzoxazole nucleus and its derivatives are known to play extremely crucial roles in the structures and functions of a number of biologically important molecules, generally by virtue of their being coordinated to metal ions. Various benzoxazole derivatives were extensively studied for their antibacterial and antifungal activity [1–5], anticancer activity [6,7], also as new non-nucleoside topoisomerase I poison [8] and HIV-1 reverse transcriptase inhibitors [9,10]. Benzoxazoles are also interesting fluorescent probes which show high Stokes shift and present thermal and photophysical stability due to an excited state intramolecular proton transfer mechanism [11,12]. Since they interfere with biosynthesis of coloured carotenoids by inhibiting the enzyme phytoene desaturase, they are studied as potential bleaching herbicides [13]. Benzoxazoles can be considered as structural bioisosters of naturally occurring nucleotides such as adenine and guanine, which allow them to ⇑ Corresponding author. Tel.: +91 413 2211111, mobile: +91 9442992223; fax: +91 413 2251613. E-mail address: [email protected] (V. Arjunan). 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.07.043

interact easily with the biopolymers of a living system. They have shown low toxicity in warm-blooded animals [14]. Reda et al. synthesised some novel benzoxazole derivatives as anticancer, anti-HIV-1 and antimicrobial agents [15,16]. These different applications have attracted many experimentalists and theoreticians to investigate the spectroscopic, structural and biological properties of benzoxazole derivatives [17–21]. Vibrational spectroscopy (IR, Raman) and NMR techniques are widely used to study the structural and dynamical aspects of molecular systems. Among various types of density functional available, those which use Becke’s three-parameter hybrid functional (B3LYP) with large basis set such as 6-31G(d,p), 6-311++G(d,p) and cc-pVDZ have been found to be the most promising in providing excellent results of vibrational and electronic structure of the molecule. In this work, to shed more light on the accurate prediction of the geometry, the thermodynamical properties and to determine the energy of the compound under study, interest resides in correlating the theoretically predicted optimised geometrical parameters, harmonic vibrations, NMR parameters and thermodynamic properties of 2A6FBT with the accurate experimental FTIR, FT-Raman and FT-NMR results.

V. Arjunan et al. / Journal of Molecular Structure 1003 (2011) 92–102

2. Experimental The compound 2-amino-5-chlorobenzoxazole (2A5CBO) was obtained from Aldrich Chemicals, USA and used as such to record FTIR and FT-Raman spectra. The FTIR spectrum of the crystalline compound has been recorded by KBr pellet method in the region 4000–400 cm1 using Bruker IFS 66V spectrometer with a Globar source, Ge/KBr beam splitter, and a TGS detector. The frequencies for all sharp bands are accurate to 2 cm1. The FT-Raman spectrum was also recorded in the range 3500–100 cm1 by the same instrument with FRA 106 Raman module equipped with Nd:YAG laser source with 200 mW power operating at 1064 nm. A liquid nitrogen cooled-Ge detector was used. The spectral resolution is 2 cm1. 1H and 13C nuclear magnetic resonance (NMR) (400 MHz; CDCl3) spectra were recorded on a Bruker HC400 instrument. Chemical shifts for protons are reported in parts per million scales (d scale) downfield from tetramethylsilane. The electronic absorption spectrum of the compound was also recorded with the Shimadzu UV/Visible spectrophotometer. The band width on half height is 3.0 nm.

3. Computational details To provide complete information regarding to the structural characteristics and the fundamental vibrational modes of 2A5CBO the LCAO–MO–SCF restricted Hartree–Fock and DFTB3LYP correlation functional calculations have been carried out. The calculations of geometrical parameters in the ground state were performed using the Gaussian 03 [22] program, invoking gradient geometry optimisation [23] on Intel core i3/2.93 GHZ processor. The geometry optimisation was carried out at Hartree–Fock and DFT methods adopting 6-31G(d,p), triple-f 6-311++G(d,p) and Dunning’s cc-pVDZ basis sets to characterise all stationary points as minima. The optimised structural parameters of 2A5CBO were used for harmonic vibrational frequency calculations resulting in IR and Raman frequencies together with intensities and Raman depolarisation ratios. In DFT methods, Becke’s three parameter exact exchange-functional (B3) [24–26] combined with gradient-corrected correlational functional of Lee, Yang and Parr (LYP) [27] by implementing the split-valence polarised 631G(d,p), triple-f 6-311++G(d,p) and Dunning’s cc-pVDZ basis sets [28,29] have been utilised for the computation of molecular structural properties, vibrational frequencies, thermodynamic properties and energy of the optimised structure. The force field obtained from B3LYP/6-311++G(d,p) method has also been utilised to perform normal coordinate analysis using Wilson’s FG matrix [30–32] method with the perturbation program written by Fuhrer et al. [33]. The potential energy distribution corresponding to each of the observed frequencies shows the reliability and accuracy of the spectral analysis. The isotropic chemical shifts are frequently used as an aid in identification of organic compounds and accurate predictions of molecular geometries are essential for reliable studies of magnetic properties. The B3LYP method allows calculating the shielding constants with accuracy and the GIAO method is one of the most common approaches for calculating nuclear magnetic shielding tensors. The 1H and 13C NMR isotropic shielding were calculated with the GIAO method [34,35] using the optimised parameters obtained from B3LYP/6-311++G(d,p) method. The effect of solvent on the theoretical NMR parameters was included using the default model IEF-PCM provided by Gaussian 03. The isotropic shielding values were used to calculate the isotropic chemical shifts d with respect to tetramethylsilane (TMS). diso(X) = rTMS(X) – riso(X), where diso – isotropic chemical shift and riso – isotropic shielding. UV–Visible spectra, electronic transitions, vertical excitation

93

energies and oscillator strengths were computed with the timedependent DFT method. The electronic properties such as HOMO and LUMO energies were determined by time-dependent DFT (TD-DFT) approach, with solvent effect [36–39]. A better agreement between the computed and experimental frequencies can be obtained by using different scale factors for different types of fundamental vibrations. To determine the scale factors, the procedure used previously [57–65] have been followed that minimises the residual separating experimental and theoretically predicted vibrational frequencies. The scale factors used in this study minimised the deviations between the computed and experimental frequencies. A uniform scaling factor of 0.98 is thus recommended for all frequencies 2700 cm1 were scaled by two different scale factors [63,64]. A scaling factor of 0.92 and 0.96 for N–H stretching and C– H stretching, respectively have been utilised to obtain the scaled frequencies in B3LYP methods. Similarly, a scale factor of 0.87 and 0.92 for N–H and C–H stretching modes in HF method. In B3LYP methods 0.98 is used for modes with wavenumbers >1500 cm1 and 1.0 for all other vibrational modes observed in the lower wave number region and compared with the experimentally observed frequencies. In HF method 0.96 and 0.98 were used in the lower frequency region. The correction factors used in B3LYP methods brings the theoretical frequencies are much closer to the experimental values and more reliable than HF method.

4. Results and discussion 4.1. Structural properties The structure and numbering of the atoms of 2A5CBO is shown in Fig. 1. The geometry of the molecule under investigation is considered by possessing Cs point group symmetry. The 42 fundamental modes of vibrations of each compound are distributed into their reducible representations under Cs symmetry as Cvib = 29A0 + 13A00 . All vibrations are active in both IR and Raman. All the frequencies are assigned in terms of fundamental, overtone and combinational bands. The optimised structural parameters bond length and bond angle for the thermodynamically preferred geometry of 2A5CBO determined at HF/6-311G(d,p) and B3LYP with 6-311++G(d,p), cc-pVDZ and 6-31G(d,p) basis sets are presented in Table 1 in accordance with the atom numbering scheme of the molecule shown in Fig. 1. Since the geometry of the molecule obtained by

Fig. 1. The stable geometry and atom numbering of 2-amino-5-chlorobenzoxazole.

94


Table 1 Structural parameters of 2-amino-5-chlorobenzoxazole calculated by B3LYP/6-311++G(d,p), HF/6-311++G(d,p), B3LYP/6-31G(d,p), B3LYP/cc-Pvdz methods. Structural parameters

B3LYP/6-311++G(d,p)

HF/6-311++G(d,p)

B3LYP/6-31G(d,p)

B3LYP/cc-Pvdz

Internuclear distance (Å) O1–C2 O1–C9 C2–N3 C2–N14 N3–C8 C8–C9 C4–C8 C4–C5 C5–C6 C5–Cl10 C6–C7 C7–C9 C–H (ring)b N–H (amino)b

1.37 1.39 1.3 1.35 1.39 1.4 1.39 1.39 1.4 1.76 1.39 1.38 1.08 1.0

1.33 1.37 1.28 1.34 1.39 1.38 1.38 1.39 1.39 1.75 1.39 1.37 1.08 0.99

1.37 1.39 1.3 1.34 1.4 1.4 1.39 1.4 1.4 1.76 1.4 1.38 1.08 1.0

1.37 1.39 1.3 1.35 1.4 1.41 1.4 1.4 1.4 1.77 1.4 1.38 1.09 1.01

1.38 1.38 1.29

Bond angle (°) C2–O1–C9 O1–C2–N3 O1–C2–N14 N3–C2–N14 C2–N3–C8 C5–C4–C8 C5–C4–H11 C8–C4–H11 C4–C5–C6 C6–C5–Cl10 C4–C5–Cl10 C5–C6–C7 C7–C6–H13 C5–C6–H13 C6–C7–C9 C6–C7–H13 C9–C7–H13 C4–C8–C9 C4–C8–N3 C9–C8–N3 C7–C9–C8 C7–C9–O1 C8–C9–O1 C2–N14–H15 C2–N14–H16 H15–N14–H16

103.4 116.5 115.6 128.0 103.7 116.6 121.7 121.7 123.1 118.4 118.5 120.2 120.2 119.6 116.2 121.5 122.3 119.7 131.1 109.2 124.1 128.7 107.2 120.7 118.9 120.4

104.2 116.7 115.7 127.7 103.6 116.6 121.6 121.8 123 118.5 118.5 120.2 120.2 119.7 116.2 121.5 122.3 119.9 131.4 108.7 124.2 129 106.8 120.4 119.0 120.5

103.2 116.8 115.4 127.8 103.3 116.7 121.7 121.7 123.1 118.4 118.5 120.2 120.2 119.6 116.2 121.5 122.3 119.6 131 109.5 124.2 128.6 107.2 120.7 118.8 120.6

103.2 116.9 115.5 127.7 103.3 116.8 121.7 121.6 123.1 118.4 118.5 120.2 120.3 119.5 116.3 121.5 122.2 119.6 131 109.4 124.1 128.7 107.3 120.5 118.8 120.7

104.0 115.1

1.4 1.38 1.38 1.37 1.39 1.73 1.38 1.36

104.3 117.6

120.5 123.3 116.1 122.8

114.9

120.0 130.7 109.3 124.2 128.5 107.3

Values taken from Ref. [41]. Mean value.

1.8 1.7 1.6

B3LYP/6-311++G(d,p) HF/311++G(d,p) B3LYP/6-31G(d,p) B3LYP/cc-pVDZ

1.5 1.4 1.3 1.2 1.1 1.0 0.9 O1-C2 O1-C9 C2-N3 C2-N14 N3-C8 C8-C9 C4-C8 C4-H11 C4-C5 C5-CL10 C6-C7 C6-H12 C7-C9 C7-H13 N14-H15 N14-H16 --

B3LYP/6-311++G(d,p) method is energetically most stable, hence the theoretical values of this method are taken for correlation and is more reliable. It is observed that the calculated C–C bond distances and the C–H bond lengths are found to be nearly the same at all levels of calculations. The bond distances calculated by HF method are little smaller in magnitude than that obtained by DFT methods. The bond angles are all well agreed in all the methods. The influence chlorine on the C–C bond distance of ring carbon atoms seems to be negligibly small. The C–H bond distances are in the range 1.07–1.09 Å. The shorter the bond length of C2–N3 indicates that the benzene ring exerts larger attraction on valence electron cloud of nitrogen atom resulting easy delocalisation of lone pair of electrons into the ring and thereby increase in force constant. The bond lengths determined by HF and DFT methods were correlated in Fig. 2. The calculated bond angles in all the methods are very close to each other. The bond angle at the point of amino group substitution, O1–C2–N3 is 116.5°. With the electron donating substituents on the benzene ring, the symmetry of the ring is distorted, yielding ring angles smaller than 120° at the point if substitution and slightly larger than 120° at the ortho and meta positions [40]. In 2A5CBO molecule the bond angle C4–C5–C6 is 123.1° while at

Bond length(A0)

a b

Experimentala

2-Amino-5-chlorobenzoxazole

Fig. 2. The correlation of bond lengths of 2-amino-5-chlorobenzoxazole determined by HF and DFT methods.

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V. Arjunan et al. / Journal of Molecular Structure 1003 (2011) 92–102 Table 3 The charges of the atoms determined from natural bond orbital analysis (NBO) by B3LYP/ 6-311++G(d,p) method.

Bond angle (Degree)

130 125 120 115 110 B3LYP/6-311++G(d,p) HF/311++G(d,p) B3LYP/6-31G(d,p) B3LYP/cc-pVDZ

C2-O1-C9 O1-C2-N3 O1-C2-N14 N3-C2-N14 C2-N3-C8 C5-C4C8 C5-C4-H11 C8-C4-H11 C4-C5-C6 C6-C5-CL10 C4-C5-CL10 C5-C6-C7 C7-C6-H13 C5-C6-H13 C6-C7-C9 C6-C7-H13 C9-C7-H13 C4-C8-C9 C4-C8-N3 C9-C8-N3 C7-C9-C8 C7-C9-O1 C8-C9-O1 C2-N14-H15 C2-N14-H16 H15-N14-H16 --

105

Fig. 3. The correlation of bond angles of 2-amino-5-chlorobenzoxazole determined by HF and DFT methods.

ortho, C5–C4–C8 is 116.6° and meta position angle is 116.2°. This is clearly due to the high electronegativity (–I effect) of chlorine. More distortion in bond parameters is observed in the hetero ring than in the benzene ring. The variation in bond angle depends on the electro negativity of the central atom. In 2A5CBO the bond angles C2–O1–C9 and C2–N3–C8 are very small and close to each other due to the high electro negativity of nitrogen and oxygen. The computed structural parameters were correlated with the structurally related benzoxazole derivative [41]. The correlation of bond angles of 2A5CBO determined by HF and DFT methods is depicted in Fig. 3. The thermodynamic parameters of the compound, total thermal energy, vibrational energy contribution to the total energy, the rotational constants and the dipole moment values obtained from HF method is DFT methods are presented in Table 2. The energy of the compound 2A5CBO determined by B3LYP/6-311++G(d,p) method is –914.827 Hartrees is the least and the corresponding geometry is most stable than the geometry of 2A6FBT obtained by other methods. The large dipole moment (4.509 D) of 2A5CBO is due to the presence of charge separation by electronegative atoms. The charges of the atoms determined by natural bond orbital (NBO) analysis by B3LYP/6-311++G(d,p) method is presented in the Table 3. The more positive charge on C2 carbon atom is due

Atom

Charge

O1 C2 N3 C4 C5 C6 C7 C8 C9 Cl10 H11 H12 H13 N14 H15 H16

–0.4897 0.7077 –0.5510 –0.2308 –0.0410 –0.2352 –0.2216 0.1141 0.2566 –0.0009 0.2281 0.2297 0.2207 –0.7852 0.4007 0.4077

to the highly electronegative nitrogen and oxygen attachment with that carbon atom. This is caused by the –I effect of nitrogen and oxygen atoms. When compared the charges of the aromatic ring carbon atoms, less positive charge is observed in the C8 and C9 carbon atoms which is attached to the highly electronegative nitrogen (N3) and oxygen (O1), respectively. The high negative charge at N14 (–0.7852) and a positive charge at the carbon atom C2 reveal the high electron attracting nature of the nitrogen atom and possible delocalisation of electrons towards it. Electrostatic potential maps, also known as electrostatic potential energy maps, or molecular electrical potential surfaces, illustrate the charge distributions of molecules three dimensionally. The purpose of finding the electrostatic potential is to find the reactive site of a molecule. These maps allow us to visualise variably charged regions of a molecule. Knowledge of the charge distributions can be used to determine how molecules interact with one another. Molecular electrostatic potential (MESP) mapping is very useful in the investigation of the molecular structure with its physiochemical property relationships [42–45]. Total SCF electron density surface mapped with molecular electrostatic potential (MESP) of 2A5CBO are shown in Fig. 4. The molecular electrostatic potential surface MESP which is a 3D plot of electrostatic potential mapped onto the iso-electron density surface simultaneously displays molecular shape, size and electrostatic potential values. The colour scheme for the MSEP surface is red-electron rich or partially

Table 2 The calculated thermodynamic parameters of 2-amino-5-chlorobenzoxazole employing B3LYP/6-311++G(d,p), HF/6-311++G(d,p), B3LYP/6-31G(d,p), B3LYP/cc-Pvdz methods. Thermodynamic parameters (298 K)

SCF energy (Hartree) Total energy (thermal), Etotal (kcal mol1) Heat capacity at const. volume, Cv (cal mol1 K1) Entropy, S (cal mol1 K1) Vibrational energy, Evib (kcal mol1) Zero-point vibrational energy, E0 (kcal mol1) Rotational constants (GHz) A B C

2-Amino-5-chlorobenzoxazole B3LYP/6-311++G(d,p)

HF/6-311++G(d,p)

B3LYP/6-31G(d,p)

B3LYP/cc-Pvdz

–914.827 75.22 32.22 89.98 73.44 70.08

–911.377 80.1 29.7 87.8 78.32 75.32

–914.675 75.55 32.24 90.05 73.77 70.39

–914.738 75.33 32.16 89.97 73.55 70.19

3.28 0.52 0.45

3.35 0.53 0.46

3.23 0.52 0.45

3.25 0.52 0.45

–4.25 1.52 0 4.51

–4.15 1.77 0 4.51

–4.54 1.48 0 4.77

–4.69 1.34 0 4.87

Dipole moment (Debye)

lx ly lz ltotal

96


Figs. 6 and 7. The theoretical spectra were obtained from the B3LYP/6-311++G(d,p) method using Lorentzian band shape with band width on half-height 10 cm1. This reveals good correspondence between theory and experiment in main spectral features. The observed FTIR and FT-Raman spectra along with the theoretical infrared and Raman spectra of 2A5CBO B3LYP/6-311++G(d,p), HF/6-311++G(d,p), B3LYP/6-31G(d,p) and B3LYP/cc-pVDZ methods along with their relative intensities, probable assignments and potential energy distribution (PED) are summarised in Tables 4 and 5.

Fig. 4. The total electron density surface mapped with electrostatic potential of 2amino-5-chlorobenzoxazole.

Fig. 5. The contour map of electrostatic potential of the total density of 2-amino-5chlorobenzoxazole.

negative charge; blue-electron deficient or partially positive charge; light blue-slightly electron deficient region; yellowslightly electron rich region, respectively. Areas of low potential, red, are characterised by an abundance of electrons. Areas of high potential, blue, are characterised by a relative absence of electrons. Nitrogen has a higher electronegativity value would consequently have a higher electron density around them. Thus the spherical region that corresponds to nitrogen atom would have a red portion on it. The MESP of 2A5CBO clearly indicates the electron rich centres of nitrogen, oxygen and the areas covering the C4, C5, C6, C7 and Cl atoms. The contour map of electrostatic potential of 2A5CBO has been constructed by the DFT method and is shown in Fig. 5 also confirms the different negative and positive potential sites of the molecule in accordance with the total electron density surface. 4.2. Vibrational analysis The observed FTIR and FT-Raman spectra of 2A5CBO along with the simulated infrared and Raman spectra are shown in

4.2.1. Skeletal stretching vibrations The carbon–carbon vibrations are more interesting if the double bond is in conjugation with the ring. The actual positions of the C– C stretching modes are determined not so much by the nature of substituents but by the form of the substitution around the ring [46]. The C–C stretching in 2A5CBO are assigned to the bands appeared in the infrared spectrum at 1618, 1556, 1471, 1449, 1380 and 1174 cm1 while in Raman spectrum at 1623, 1562, 1465, 1450 and 1390 cm1. The strong bands occurring at 1686 and 1301 cm1 in IR are assigned to C@N and C–N stretching vibrations. The strong bands observed in the infrared spectrum at 1427 and 955 cm1, respectively is assigned to O–C–O asymmetric and symmetric stretching modes. The C–C–C trigonal bending and ring breathing modes of benzene ring are attributed to the strong bands 923 and 848 cm1. All other modes are presented in Tables 4 and 5. The normal coordinate analysis predicts that the C–C–C inplane bending vibrations significantly mixed with the C–H in-plane bending modes. 4.2.2. C–H Vibrations The aromatic C–H stretching vibrations are normally found between 3100 and 3000 cm1. In this region the bands are not affected appreciably by the nature of substituents. The aromatic C–H stretching frequencies arise from the modes observed at 3062 (a1g), 3047 (e2g), 3060 (b1u) and 3080 (e1u) cm1 of benzene and its derivatives [47]. In 2A5CBO, these modes are observed at 3111 and 3093 cm1. The aromatic C–H in-plane bending modes of benzene and its derivatives are observed in the region 1300– 1000 cm1. Studies on the spectra of benzene shows that there are two degenerate e2g (1178 cm1) and e1u (1037 cm1) and two non-degenerate b2u (1152 cm1) and a2g (1340 cm1) vibrations involving the C–H in-plane bending [47]. The C–H out of plane bending modes [48,49,40] usually medium intensity arises in the region 950–600 cm1. In the case of 2A5CBO the bands observed at 1113 and 1048 cm1 in IR and at 1114 and 1054 cm1 in Raman spectrum are assigned to the C–H in-plane bending vibrations. The C–H out of plane bending results from b2g (985 cm1), e2u (970 cm1), e1g (850 cm1) and a2u (671 cm1) modes of benzene [47]. The aromatic C–H out of plane bending vibrations of 2A5CBO are assigned to the medium to weak bands observed at 871 and 780 cm1 in the Raman spectrum. The aromatic C–H inplane and out of plane bending vibrations have substantial overlapping with the ring C–C–C in-plane and out of plane bending modes, respectively. 4.2.3. C–Cl vibrations The C–Cl absorption is observed in the broad region between 850 and 550 cm1. Thus, the strong band in IR at 700 cm1 having a Raman counterpart at 698 cm1 is assigned to the C–Cl stretching mode of 2A5CBO. This assignment is well agreed with the theoretically calculated C–Cl bond distance of 2A5CBO by HF and DFT methods. Due to the longer bond length of C–Cl reduction in force constant occurs and the C–Cl stretching falls in the lower frequency region. The C–Cl in-plane bending mode is observed at 233 cm1. These assignments are in good agreement with the


97

Fig. 6. (a) Observed FTIR and (b) Theoretical infrared spectra of 2-amino-5-chloro-benzoxazole.

Fig. 7. (a) Observed FT-Raman and (b) Theoretical Raman spectra of 2-amino-5-chloro-benzoxazole.

literature of N-(chloro substituted phenyl)-2,2-dichloroacetamides [50], N-substituted aniline [51] and dichloroanilines [52]. 4.2.4. Amino group vibrations It is stated that in amines, the N–H stretching vibrations occur in the region 3500–3300 cm1. The asymmetric –NH2 stretching vibration appears from 3500 to 3420 cm1 and the symmetric –NH2 stretching is observed in the range 3420–3340 cm1. With the above reference, the vibrational frequencies observed at 3469 and 3339 cm1 in the infrared spectrum are assigned to the –NH2

asymmetric and symmetric stretching modes, respectively. For primary amino group the in-plane –NH2 deformation vibration occur in the short range 1650–1580 cm1 region of the spectrum. Therefore the medium band observed in IR at 1600 cm1 is assigned to the deformation mode of the amino group. The amino in-plane bending rocking mode normally appears in the range 1150–900 cm1 while the wagging bands between 850 and 500 cm1. Therefore, the bands at 599 and 445 cm1 are attributed to the wagging and twisting modes of amino group, respectively. The rocking mode is calculated at 1088 cm1 by B3LYP/6-311++G(d,p) method. The –NH2

98 Table 4 The observed FTIR, FT-Raman and calculated frequencies using B3LYP/6-311++G(d,p) and HF/6-311++G(d,p) force field along with their relative intensities, probable assignments and potential energy distribution (PED) of 2-amino-5chlorobenzoxazole. Species

A0 A0 A0 A0 A0

a

HF/6-311++G(d,p) calculated wavenumber

B3LYP/6-311++G(d,p) calculated wavenumber

FTIR

Unscaled (cm1)

Scaled (cm1)

IR intensity

Unscaled (cm1)

Scaled (cm1)

IR intensity

Raman activity

3989 3850

3470 3350

102.09 171.18

3761 3632

3460 3341

77.99 178.81

3377 3372 3356

3107 3102 3088

3215 3212 3198

3086 3084 3070

0.00 1.81 1.34

1844 1800 1769 1723 1621 1571 1536 1467 1376 1303 1271 1206 1189 1149 1076 1054 1000 982 933 910 838 817 772 766 679 613 556 486 464 415 401 354 305 225 205 107 85

1678 1638 1610 1568 1475 1445 1413 1350 1266 1199 1169 1110 1094 1057 990 970 920 903 858 837 771 752 710 705 625 564 523 457 436 390 377 333 287 212 193 101 85

1717 1657 1621 1587 1484 1457 1410 1364 1272 1266 1189 1133 1088 1066 953 933 926 879 864 808 755 735 718 712 610 570 514 445 423 379 372 328 281 209 188 98 87

1683 1624 1589 1555 1484 1457 1410 1364 1272 1266 1189 1133 1088 1066 953 914 907 861 847 808 755 735 718 712 610 570 514 445 423 379 372 328 281 209 188 98 87

658.77 40.41 204.72 76.32 150.56 23.90 76.62 19.13 21.58 50.00 41.12 1.44 16.56 27.18 29.33 1.57 57.46 18.35 2.32 2.85 37.67 3.34 17.43 19.25 9.07 14.28 0.30 6.88 0.04 4.40 3.64 2.99 3.52 1.46 4.05 4.66 3.59

FTR

3469 s 3339 m 3269 m 3111 s 3093 m 2974 s 1686 vs 1618 s 1600 m 1556 vs 1471 s 1449 s 1427 s 1380 vs 1301 s 1251 vs 1174 m 1113 m

1114 w

1048 m 955 vs

1054 vw 955 m

923 s

926 w 871 m

1663 m 1623 m 1562 m 1465 w 1450 w 1430 w 1390 w 1307 w 1249 vs

848 s 780 vs 736 w 700 s 599 m 567 s 527 m 460 w 445 vw 405 w

698 s 569 w 530 s

396 s 347 vs 292 w 233 w 203 vw 139 m

0.127 1.85 1.11 667.34 127.49 286.59 192.32 204.09 24.61 146.53 59.99 20.89 8.80 48.82 10.43 18.22 16.98 25.71 1.93 57.05 28.56 0.98 12.09 47.45 0.01 1.04 38.09 8.59 15.81 0.73 7.30 0.01 2.48 3.13 3.32 2.69 0.98 3.22 4.17 5.24

Depolarisation ratio

Assignment

%PED

0.24 1.0

0.75 0.14

maNH2 msNH2

94mNH 92mNH

0.48 0.56 0.27

0.25 0.18 0.61

0.25 0.25 0.07 0.12 0.07 0.10 0.09 0.06 0.02 0.51 0.01 0.01 0.03 0.06 0.01 0.001 0.04 0.001 0.09 0.002 0.001 0.002 0.05 0.04 0.001 0.01 0.01 0.001 0.0002 0.004 0.03 0.03 0.002 0.004 0.001 0.002 0.0002

0.32 0.55 0.17 0.20 0.39 0.48 0.38 0.73 0.08 0.18 0.65 0.75 0.24 0.06 0.26 0.75 0.40 0.75 0.06 0.75 0.75 0.75 0.13 0.12 0.75 0.68 0.41 0.75 0.75 0.75 0.23 0.34 0.75 0.75 0.75 0.75 0.74

2 1600 mCH mCH mCH 1556 + 1427 mC@N mCC dNH2 mCC mCC mCC mCO mCC mC–N mC–N(H2) mCC bC–H qNH2 bC–H mCO bC–H bCCC cC–H bCCC cC–H cC–H bCCC bCOC mC–Cl xNH2 bNCO cCCC cCCC sNH2 cCOC cCCC bC–N(H2) cNCO bC–Cl cC–NH2 cC–Cl cCCC

95mCH 90mCH 93mCH 92mCN 94mCC 93dNH2 91mCC 93mCC 90mCC 91mCO 92mCC 93mCN 90mCN 91mCC 79bCH 73qNH2 + 15bNCO 77bCH 89mCO 75bCH 74bCCC + 12bCH 66cCH + 21cCCC 69bCCC + 16bCH 65cCH + 18cCCC 64cCH + 20cCCC 67bCCC + 18bCH 69bCOC + 21bCN 92mCCl 60xNH2 + 26s NH2 65bNCO + 22bCN 66cCCC + 21cCH 64cCCC + 22cCH 62sNH2 + 28xNH2 65cCOC + 18cCN 64cCCC + 20cCH 67bCN + 18bNCO 69cNCO + 16cCN 67bCCl + 15bCCC 65cCN + 22cCCC 66cCCl + 18cCCC 62cCCC + 25cCH

m – stretching; b – in-plane bending; d – deformation; q – rocking; c – out of plane bending; x – wagging and s – twisting, wavenumbers, (cm1); IR intensities, (kM/mole); Raman scattering activities, (Å)4/(a.m.u).


A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A00 A0 A00 A00 A0 A0 A0 A00 A0 A00 A00 A00 A00 A00 A0 A00 A0 A00 A00 A00

Observed wavenumber (cm1)

Table 5 The observed FTIR, FT-Raman and calculated frequencies using B3LYP/6-31G(d,p) and B3LYP/cc-pVDZ force field along with their relative intensities, probable assignments and potential energy distribution (PED) of 2-amino-5chlorobenzoxazole. Species

Observed wavenumber (cm1) FTIR

A0 A0 A0 A0 A0

a

3093 m 2974 s 1686 vs 1618 s 1600 m 1556 vs 1471 s 1449 s 1427 s 1380 vs 1301 s 1251 vs 1174 m 1113 m

1663 m 1623 m 1562 m 1465 w 1450 w 1430 w 1390 w 1307 w 1249 vs 1114 w

1048 m 955 vs

1054 vw 955 m

923 s

926 w 871 m

848 s 780 vs 736 w 700 s 599 m 567 s 527 m 460 w 445 vw 405 w

698 s 569 w 530 s

396 s 347 vs 292 w 233 w 203 vw 139 m

Unscaled (cm

1

)

Scaled (cm

1

)

B3LYP/6-31G(d,p) Calculated wavenumber 1

)

1

IR intensity

Unscaled (cm

3759 3619

3458 3329

79.68 178.70

3793 3655

3471 3344

Scaled (cm

)

77.81 178.88

3228 3224 3208

3099 3095 3080

0.05 1.68 1.72

3237 3231 3217

3108 3102

0.03 2.08 1.46

1727 1669 1627 1559 1486 1462 1406 1383 1280 1254 1195 1124 1082 1063 956 942 925 893 864 816 767 737 716 711 614 570 511 445 421 380 372 327 281 209 190 99 75

1692 1636 1594 1559 1486 1462 1406 1383 1280 1254 1195 1124 1082 1063 956 942 925 893 864 816 767 737 716 711 614 570 511 445 421 380 372 327 281 209 190 99 75

623.81 44.26 262.59 11.48 164.50 11.62 64.62 14.86 72.17 0.79 31.63 1.61 20.56 20.46 24.93 0.97 52.08 14.65 2.20 27.12 1.88 3.80 17.94 15.25 5.23 12.30 0.27 3.99 0.00 3.12 3.68 3.59 3.54 1.28 3.27 3.93 26.60

1731 1671 1632 1591 1497 1469 1411 1382 1283 1276 1203 1140 1087 1073 959 928 927 878 866 807 739 728 717 712 606 571 511 441 416 377 372 327 281 209 190 99 73

1696 1638 1599 1559 1467 1440 1411 1382 1283 1276 1203 1140 1087 1073 959 928 927 878 866 807 739 728 717 712 606 571 511 441 416 377 372 327 281 209 190 99 73

646.13 43.30 218.53 33.74 146.59 14.81 72.06 17.95 65.45 9.35 34.55 1.46 21.34 21.87 24.30 0.77 55.74 18.41 2.12 33.34 3.86 4.48 16.11 17.94 3.84 11.75 0.21 4.12 0.42 4.13 3.92 3.73 4.21 1.13 3.30 3.28 21.12

Assignments

% PED

maNH2 msNH2

92mNH 90NH

IR intensity

2 1600 Nch mCH mCH 1556 + 1427 mC@N mCC dNH2 mCC mCC mCC mCO mCC mC–N mC–N(H2) mCC bC–H qNH2 bC–H mCO bC–H bCCC cC–H bCCC cC–H cC–H bCCC bCOC mC–Cl xNH2 bNCO cCCC cCCC sNH2 cCOC cCCC bC–N(H2) cNCO bC–Cl cC–NH2 cC–Cl cCCC

93mCH 90mCH 91mCH 90mCN 92mCC 91dNH2 90mCC 92mCC 89mCC 92mCO 91mCC 92mCN 88mCN 89mCC 80bCH 72qNH2 + 16bNCO 75bCH 87mCO 74bCH 72bCCC + 16bCH 656cCH + 20cCCC 67bCCC + 18bCH 65cCH + 21cCCC 62cCH + 22cCCC 65bCCC + 21bCH 71bCOC + 16bCN 90mCCl 58xNH2 + 26s NH2 65bNCO + 22bCN 64cCCC + 21cCH 62cCCC + 22cCH 62sNH2 + 28xNH2 63cCOC + 18cCN 64cCCC + 20cCH 65bCN + 18bNCO 65cNCO + 16cCN 62bCCl + 18bCCC 61cCN + 20cCCC 62cCCl + 19cCCC 60cCCC + 24cCH


A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A00 A0 A00 A00 A0 A0 A0 A00 A0 A00 A00 A00 A00 A00 A0 A00 A0 A00 A00 A00

FTR

3469 s 3339 m 3269 m 3111 s

B3LYP/cc-pVDZ Calculated wavenumber

m – stretching; b – in-plane bending; d – deformation; q – rocking; c – out of plane bending; x – wagging and s – twisting, wavenumbers, (cm1); IR intensities, (KM/mole).

99

100


Fig. 8. 1H NMR spectrum of 2-amino-5-chlorobenzoxazole.

Fig. 9. 1C NMR spectrum of 2-amino-5-chlorobenzoxazole.

deformation vibrations are not much affected by the hetero ring. These amino vibrations are also in good agreement with literature values of aniline [40], 4-aminoquinaldine [53] and 5-aminoquinoline [54]. Considerable overlapping between the –NH2 wagging and twisting out of plane bending modes occurs and is confirmed from the PED. The vibrational assignment of all the fundamental modes is also supported by Gauss view molecular visualisation program [55,56].

5. NMR spectral studies The ‘‘gauge-independent atomic orbital’’ (GIAO) method has proven to be quite accepted and accurate, in particular when applied in the context of highly correlated ab initio methods, such as perturbation theory and coupled cluster theory. Both approaches are computationally expensive and time consuming and often cannot be applied to larger molecular systems. Density functional theory (DFT) shielding calculations are rapid and applicable to large systems, but the paramagnetic contribution to the shielding tends to be overestimated. In this sense, theoretical calculations of the chemical shifts may be used as an aid for the

Table 6 The Experimental and calculated chlorobenzoxazole.

1

H and

assignment of the experimental data and for the study of benzoxazole and its derivatives. The observed 1H and 13C NMR spectra of the compound 2A5CBO are given in the Figs. 8 and 9, respectively. The 1H and 13C theoretical and experimental chemical shifts, isotropic shielding tensors and the spectral assignments are presented in Table 6. 1 H atom is mostly localised on periphery of the molecules and their chemical shifts would be more susceptible to intermolecular interactions in the aqueous solutions as compared to that for other heavier atoms. Aromatic carbons give signals in overlapped areas of the spectrum with chemical shift values from 100 to 200 ppm [66,67]. The cumulative –I effect of nitrogen and oxygen in the hetero ring of 2A5CBO reduces the electron density of the carbon atom C2, thus its NMR signal is observed in the very downfield at 162.63 ppm. The carbon atoms C8 and C9 are also in the downfield due to the deshielding effect of the electronegative atoms N3 and O1. The oxygen atom is more electronegative than nitrogen and hence the chemical shift of C9 (148.25 ppm) is relatively in the more downfield than C8 (144.27 ppm). The carbon atoms C4, C5 and C6 are significantly observed in the upfield with chemical shift values 116.91, 129.61 and 121.49 ppm, respectively reveals that the influence of the electronegative chlorine atom shift the chemical shift of C5 to down field to some extent and their signal are observed in the normal range. The influence of electronegative chlorine, nitrogen and oxygen atoms on C7 is negligibly small and thus is observed in the upfield at 109.71 ppm. 1 H chemical shifts of 2A5CBO were obtained by complete analysis of the NMR spectrum and interpreted critically to quantify different effects acting on the chemical shifts of protons. The hydrogen atoms H11, H12 and H13 present in the benzene ring shows NMR peaks in the normal range of aromatic hydrogen atoms and are assigned to the chemical shift values 7.32, 7.04 and 7.48 ppm, respectively [68]. The effect of electronegative atoms O1 and N3 is most pronounced on the chemical shifts of the benzenoid ring protons H11 and H13. The more effect on H13 is to be expected since it is ortho to the electron releasing O1 and hence the magnitude of the effect on the H13 proton is significantly higher. The mesomeric (+M) effect of chlorine is another reason for the difference in chemical shifts of benzenoid protons. The amino protons are put into the upfiled at 5.5 ppm shows that these protons are under high magnetic shielding. The calculated and experimental chemical shift values are given in Table 6 shows a good agreement with each other. The protons are located on the periphery of the molecule and thus are supposed to be more susceptible to molecular solute–solvent effects than the carbon atoms and usually the agreement between the experimental and calculated shifts for 1H deviate more than for 13C [69].

6. Electronic properties The lowest singlet ? singlet spin-allowed excited states were taken into account for the TD-DFT calculation in order to investigate the properties of electronic absorption. The energies of four

13

C isotropic chemical shifts (diso, ppm) with respect to TMS and isotropic magnetic shielding tensors (riso) of 2-amino-5-

Assignment

riso (1H)

Cal. (diso)

Expt. (diso)

Assignment

riso (13C)

Cal. (diso)

Expt. (diso)

H11 H12 H13 H (amino)

24.34 24.76 24.47 26.95

7.61 7.19 7.48 5.0

7.32 7.04 7.17 5.5

C2 C4 C5 C6 C7 C8 C9

16.18 61.92 39.9 58.0 69.55 31.22 27.91

167.95 122.21 144.23 126.13 114.58 152.91 156.22

162.63 116.91 129.61 121.49 109.71 144.27 148.25

101


Table 7 Experimental and calculated absorption wavelength (k), excitation energies (E), oscillator strength (f) and frontier orbital energies of 2-amino-5-chlorobenzoxazole by TD-DFT method. k (Expt., nm)

k (Cal., nm)

E (eV)

f

Assignment

EHOMO

ELUMO

EHOMO1

ELUMO+1

280 250

265.03 240.41 237.79

4.6782 5.1572 5.214

0.1845 0.1951 0.0051

n ? p⁄ p ? p⁄ p ? p⁄

–6.1866 eV

–0.8626 eV

–6.9283 eV

–0.4090 eV

important molecular orbitals of 2A5CBO: the second highest and highest occupied MO’s (HOMO and HOMO – 1), the lowest and the second lowest unoccupied MO’s (LUMO and LUMO + 1) were calculated and are presented in Table 7. The experimental kmax values are obtained from the UV/Visible spectra recorded in CHCl3. The Fig. 10 depicts the observed and the theoretical UV–Visible spectra of 2A5CBO. The calculations were also performed with CHCl3 solvent effect. The calculated absorption wavelengths (kmax) and the experimental wavelengths are also given in Table 7. The energy gap between HOMO and LUMO is a critical parameter in determining molecular electrical transport properties [70]. In the electronic absorption spectrum of 2A5CBO, there are two absorption bands with a maximum 280 and 250 nm. The intensity of absorption and the calculated oscillator strength shows that these two absorptions are almost equally prominent. The strong absorption band at 280 nm is caused by the n ? p⁄ transitions and the other intense band at 250 nm is due to p ? p⁄ transition [71,72]. The p ? p⁄ transitions are expected to occur relatively at lower wavelength, due to the consequence of the extended aromaticity of the benzene ring. Then in 2A5CBO molecule the n ? p⁄ transition is more significant due to the presence of lone pair of electrons in the oxygen and nitrogen atoms. The 3D plots of important molecular orbitals are shown in Fig. 11. The energy gap of HOMO–LUMO explains the eventual charge transfer interaction within the molecule, and the frontier orbital energy gap in case of 2A5CBO is found to be 5.324 eV obtained at TD-DFT method using 6-311++G(d,p) basis set. Closer examination (Fig. 11) shows the electron density in the HOMO mostly centred on the heterocyclic moiety and the benzene ring while in LUMO the electron density predominantly located on the benzene ring and on the hetero atoms; indicating a charge

Fig. 11. The HOMO, LUMO, HOMO 1 and LUMO + 1 orbitals of 2-amino-5chlorobenzoxazole.

transfer of the type p ? p⁄ and n ? p⁄ excitation are more prominent. While agreement between the calculated and experimental kmax values of 2A5CBO is evident, the calculated bands are redshifted by 15 and 10 nm.

7. Conclusions

Fig. 10. (a) Observed and (b) Theoretical UV–Visible spectra of 2A5CBO.

The geometry of 2A5CBO was optimised in different levels with DFT-B3LYP method using 6-31G(d,p), triple-f 6-311++G(d,p) and

102


Dunning’s cc-pVDZ basis sets and HF/6-311++G(d,p) method. The complete molecular structural parameters and thermodynamic properties of the optimised geometry of the compound have been obtained from ab initio and DFT calculations. The computed geometries are benchmarks for predicting crystal structural data of the molecule. The vibrational frequencies of the fundamental modes of the compound have been precisely assigned and analysed and the theoretical results were compared with the experimental vibrations. The observed and B3LYP/6-311++G(d,p), 6-31G(d,p) and cc-pVDZ) infrared frequencies show similar profiles in both position and intensities making normal mode assignments with confidence. 1H and 13C NMR spectra were recorded and the 1H and 13C NMR isotropic chemical shifts were calculated. The assignments made were compared with the experimental values. The UV–Visible spectrum was also recoded and the energies of important MO’s and the kmax of the compound were also determined from TD-DFT method using 6-311++G(d,p) basis set. The infrared and UV/Visible spectra are analysed in detail, and interpreted with the help of density functional theory (DFT) calculations. The relative stabilities, HOMO–LUMO gaps and implications of the electronic transitions are examined and discussed. Thus the present investigation provides complete vibrational assignments, structural informations, chemical shifts and electronic properties of the compound.

[23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]

[34] [35] [36] [37] [38] [39] [40] [41] [42]

References [43] [1] W.S. Saari, J.S. Wai, T.E. Fisher, C.M. Thomas, J.M. Hoffman, C.S. Roomey, A.M. Smith, J.H. Jones, D.L. Bamberger, M.E. Goldman, J.A. O’brien, J.H. Nunberg, J.C. Quintero, Q.A. Schleif, E.A. Emini, P.S. Anderson, J. Med. Chem. 35 (1992) 3792. [2] C. Ramalinghanm, S. Balasubramanian, S. Kabilan, M. Vasudevan, J. Eur. Med. Chem. 39 (2004) 527. [3] G. Turan-Zitouni, S. Demirayak, A. Ozdemir, Z.A. Kaplacikli, M.T. Yildiz, Eur. Med. Chem. 39 (2003) 267. [4] O. Temiz, I. Oren, E.A. Sener, I. Yalcin, N. Ucartuerk, Farmaco 53 (1998) 337. [5] I. Oren, O. Temiz, I. Yalcin, E.A. Sener, N. Altanlar, Eur. J. Pharm. Sci. 7 (1998) 153. [6] S.M. Rida, F.A. Ashour, S.A.M. El-Hawash, M.M. El-Semary, M.H. Badr, M.A. Shalaby, Eur. J. Med. Chem. 20 (2005) 949. [7] D. Kumar, M.R. Jacob, M.B. Reynolds, S.M. Kerwin, Bioorg. Med. Chem. 10 (2002) 3997. [8] J.S. Kim, Q. Sun, B. Gatto, C. Yu, A. Liu, L.F. Liu, E. LaVoie, J. Bioorg. Med. Chem. 4 (1996) 621. [9] J.M. Hoffman, A.M. Smith, C.S. Rooney, T.E. Fisher, J.S. Wai, C.M. Thomas, D.L. Bamberger, J.L. Barne, T.M. William, J. Med. Chem. 36 (1993) 953. [10] L. Perrin, A. Rakik, S. Yearly, C. Baumberger, S. Kinloch-deLoies, M. Pechiere, B. Hirschel, AIDS 10 (1996) 1233. [11] V. Kozich, J. Drezer, A. Vodchits, W. Wernere, Chem. Phys. Lett. 415 (2005) 121. [12] M.G. Holler, L.F. Campo, A. Brandelli, V. Stefani, J. Photochem. Photobiol. A: Chem. 149 (2002) 217. [13] B. Laber, G. Usunow, E. Wiecko, W. Franke, H. Franke, A. Koehn, Pesticide Biochem. Physiol. 63 (1999) 173. [14] D.W. Dunwell, D. Evans, J. Med. Chem. 20 (1977) 797. [15] S.M. Rida, F.A. Ashour, S.A.M. El-Hawash, M.M. ElSemary, M.H. Badr, M.A. Shalaby, Eur. J. Med. Chem. 40 (2005) 949. [16] Ikay Ören, Özlem Temiz, Smail Yalçın, Esinener, Nurten Altanlar, Eur. J. Pharm. Sci. 7 (1999) 153. [17] A.A. El-Azhary, Spectrochim. Acta 55A (1999) 2437. [18] W.B. Colier, T.D. Klots, Spectrochim. Acta 51A (1995) 1255. [19] T.D. Klots, W.B. Colier, Spectrochim. Acta 51A (1995) 1291. [20] Y. Sheena Mary, HemaTresa Varghese, C. Yohannan Panicker, Tugba Ertan, Ilkay Yildiz, Ozlem Temiz-Arpaci, Spectrochim. Acta 71A (2008) 566. [21] K.R. Ambujakshan, V.S. Madhavan, Hema Tresa Varghese, C. Yohannan Panicker, Ozlem Temiz-Arpaci, Betul Tekiner-Gulbas, Ilkay Yildiz, Spectrochim. Acta 69A (2008) 782. [22] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.

[44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72]

Morokuma, A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, 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, C. Gonzalez, J.A. Pople, Gaussian Inc., Wallingford, CT, 2004. H.B. Schlegel, J. Comput. Chem. 3 (1982) 214. A.D. Becke, Phys. Rev. A 38 (1988) 3098. A.D. Becke, J. Chem. Phys. 98 (1993) 5648. B.G. Johnson, M.J. Frisch, Chem. Phys. Lett. 216 (1993) 133. C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. Z. Dega-Szafran, A. Katrusiak, M. Szafran, J. Mol. Struct. 741 (2005) 1. W.J. Hehre, L. Radom, P.V.R. Schleyer, A.J. Pople, Ab initio Molecular Orbital Theory, Wiley, New York, 1989. E.B. Wilson Jr., J. Chem. Phys. 7 (1939) 1047. E.B. Wilson Jr., J. Chem. Phys. 9 (1941) 76. E.B. Wilson Jr., J.C. Decius, P.C. Cross, Molecular Vibrations, McGraw-Hill, New York, 1955. H. Fuhrer, V.B. Kartha, K.L. Kidd, P.J. Kruger, H.H. Mantsch, Computer Program for Infrared and Spectrometry, Normal Coordinate Analysis, vol. 5, National Research Council, Ottawa, Canada, 1976. R. Ditchfield, J. Chem. Phys. 56 (1972) 5688. K. Wolinski, J.F. Hinton, P. Pulay, J. Am. Chem. Soc. 112 (1990) 8251. E. Runge, E.K.U. Gross, Phys. Rev. Lett. 52 (1984) 997. F. Furche, R.J. Ahlrichs, J. Chem. Phys. 117 (2002) 7433. R. Bauernschmitt, R. Ahlrichs, Chem. Phys. Lett. 256 (1996) 1996. C. Jamorski, M.E. Casida, D.R. Salahub, J. Chem. Phys. 104 (1996) 5134. Y. Wang, S. Saebar, C.U. Pittman Jr., J. Mol. Struct.: Theochem. 281 (1993) 9l. H. Unver, O. Temiz Arpaci, D. Mehmet Zengin, T. Nuri Durlu, J. Mol. Struct. 609 (2002) 205. I. Fleming, Frontier Orbitals and Organic Chemical Reactions, John Wiley and Sons, New York, 1976. pp. 5–27. J.S. Murray, K. Sen, Molecular Electrostatic Potentials, Concepts and Applications, Elsevier, Amsterdam, 1996. J.M. Seminario, Recent Developments and Applications of Modern Density Functional Theory, vol. 4, Elsevier, 1996. pp. 800–806. T. Yesilkaynak, G. Binzet, F. Mehmet Emen, U. Florke, N. Kulcu, H. Arslan, Eur. J. Chem. 1 (2010) 1. G. Varsanyi, Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives, vol. I, Adam Hilger, London, 1974. L.J. Bellamy, The Infrared Spectra of Complex Molecules, third ed., Wiley, New York, 1975. H. Bohlig, M. Ackermann, F. Billes, M. Kudra, Spectrochim. Acta 55A (1999) 2635. E. Koglin, E.G. Witte, R.J. Meier, Vib. Spectrosc. 33 (2003) 49. V. Arjunan, S. Mohan, S. Subramanian, B. Thimme Gowda, Spectrochim. Acta 60A (2004) 1141. M.F. Budyka, T.S. Zyubina, A.K. Zarkadis, J. Mol. Struct.: Theochem. 594 (2002) 113. A.K. Rai, S. Kumar, Anitha Rai, Vib. Spectrosc. 42 (2006) 397. A. Altun, K. Gölcük, M. Kumru, J. Mol. Struct.: Theochem. 637 (2003) 155. V. Arjunan, S. Mohan, P.S. Balamourougane, S. Mohan, Spectrochim. Acta 74A (2009) 1225. R.I. Dennington, T. Keith, J. Millam, GaussView, Version 5.0.8, Semichem. Inc., Shawnee Mission, KS, 2008. G.A. Zhurko, D.A. Zhurko, Chemcraft Program Academic version 1.6, 2009. V. Arjunan, S. Mohan, J. Mol. Struct. 892 (2008) 289. V. Arjunan, S. Mohan, Spectrochim. Acta 72A (2009) 436. H.F. Hameka, J.O. Jensen, J. Mol. Struct. (Theochem) 362 (1996) 325. J.O. Jensen, A. Banerjee, C.N. Merrow, D. Zeroka, J.M. Lochner, J. Mol. Struct. (Theochem) 531 (2000) 323. A.P. Scott, L. Radom, J. Phys. Chem. 100 (1996) 16502. M.P. Andersson, P. Uvdal, J. Chem. Phys. A 109 (2005) 2937. M. Alcolea Palafox, M. Gill, N.J. Nunez, V.K. Rastogi, Lalit Mittal, Rekha Sharma, Int. J. Quant. Chem. 103 (2005) 394. M. Alcolea Palafox, Int. J. Quant. Chem. 77 (2000) 661. V. Arjunan, P. Ravindran, T. Rani, S. Mohan, J. Mol. Struct. 988 (2011) 91. H.O. Kalinowski, S. Berger, S. Braun, Carbon-13 NMR Spectroscopy, John Wiley & Sons, Chichester, 1988. K. Pihlaja, E. Kleinpeter (Eds.), Carbon-13 Chemical Shifts in Structural and Stereochemical Analysis, VCH Publishers, Deerfield Beach, 1994. F.L. Tobiason, J.H. Goldstein, Spectrochim. Acta 23A (1967) 1385. B. Osmiałowski, E. Kolehmainen, R. Gawinecki, Magn. Res. Chem. 39 (2001) 334. K. Fukui, Science 218 (1982) 747. R.H. Holm, F.A. Cotton, J. Am. Chem. Soc. 80 (1958) 5658. F.A. Cotton, C.W. Wilkinson, Advanced Inorganic Chemistry, third ed., Interscience Publisher, New York, 1972.