Molecular structure, anharmonic vibrational analysis and electronic spectra of o-, m-, p-iodonitrobenzene using DFT calculations

Journal of Molecular Structure 1059 (2014) 239–254

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Molecular structure, anharmonic vibrational analysis and electronic spectra of o-, m-, p-iodonitrobenzene using DFT calculations Mohammad Jane Alam, Shabbir Ahmad ⇑ Department of Physics, Aligarh Muslim University, Aligarh 202002, India

h i g h l i g h t s

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

 PES scan was carried out for o-, m-, p-

iodonitrobenzene.  Anharmonic frequencies are found in

excellent agreement with IR experiments.  Vibrational and electronic spectra were analyzed.

a r t i c l e

i n f o

Article history: Received 29 August 2013 Received in revised form 3 December 2013 Accepted 3 December 2013 Available online 11 December 2013 Keywords: Iodonitrobenzenes Anharmonic DFT FTIR Raman UV/Vis

a b s t r a c t In the present work, molecular geometry and anharmonic vibrational spectra of o-, m-, p-iodonitrobenzene have been studied. The anharmonic frequencies were calculated using second order perturbative (PT2) approach with basis set 3-21G⁄ on iodine and 6-311G(d,p) on other atoms at DFT(B3LYP) level of theory and were compared to experimental values. The assignments of vibrational modes of isomeric iodonitrobenzenes were done by using potential energy distribution (PED) and vibrational assignments of benzene, nitrobenzene and iodobenzene. The combination and overtone bands are also assigned. The electronic spectra were recorded as well as simulated using polarizable continuum model (PCM) at TD-B3LYP/6-311G(d,p)/3-21G⁄ level of theory. The vibrational and electronic spectra are interpreted. Moreover, atomic charges, MEP mapping, HOMO–LUMO, NBO analysis and various thermodynamics and molecular properties are reported. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Nitro aromatic compounds are widely used as intermediates in the synthesis of pharmaceuticals, dyes, polymers, pesticides, biologically active substances, explosives etc. The molecules, o-, m-, and p-iodonitrobenzen (OINB, MINB and PINB), have been chosen for the present studies because of its wide applications in biochemistry, organic chemistry and electrochemistry [1–6]. These

⇑ Corresponding author. Tel.: +91 9412501430. E-mail address: [email protected] (S. Ahmad). 0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.12.002

molecules play an important role in formation of radicals upon their electrochemical reduction [4,5]. MINB (1-iodo-3-nitrobenzene) is used in synthesis of N-phenyl-N0 -(2-chlroethyl) ureas which are antineoplastic agents [2]. OINB molecule shows high reactivity due to electron withdrawing and electron-donating substituents connected in the neighbouring positions of benzene ring, therefore, it plays a role in many chemical reactions for example in palladium-catalyzed N-arylation reaction of nucleobases for synthesis of DNA adducts with carcinogenic compounds [6]. The nitro and halogen substituents in the benzene ring affect its overall stability and various other properties due to their electron withdrawing nature. These are also molecules of scientific

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interest for understanding the deformation in benzene ring due to influence of nitro group and iodine atom. Spectroscopic techniques with aid of quantum chemical calculations are widely used to understand structure, vibrations and various other properties of molecules. Several authors [7–23] have studied structures and spectra of substituted nitrobenzenes. The molecular structures of several aromatic nitro compounds [12], o-, m-, p-iodonitrobenzenes [13,14], o-fluoronitrobenzene [15] have been measured using diffraction experiments. The structures of o-, m-, p-iodonitrobenzenes [14], o-fluoronitrobenzene [15], o-, m-, p-halonitrobenzene radical anions [16] were theoretically calculated. The electronic structures of chloro, bromo and iodonitrobenzene were studied by UV photoelectron spectroscopy [17,18]. Various descriptors such as LUMO energy, electron affinity, electrophilicity index and group electronegativity of 12 para substituted nitrobenzenes have been computed [22]. The vibrational spectra of 1,2-dichloro-4-nitrobenzene [19], p-fluoronitrobenzene [20] and isomeric chloronitrobenzenes [21] have been carried out by using FTIR, FT-Raman and quantum chemical calculations. The spectral studies of o-, m-, p-iodonitrobenzene are inadequate in the literature. The vibrational spectra of these molecules have been analyzed earlier using infrared spectra [10], Raman spectra [7,23] and normal coordinate analysis by Wilson’s GF matrix method [7]. To the best of our knowledge, harmonic and anharmonic quantum chemical calculations, combination and overtone bands, TD-DFT calculations, MEP mapping, HOMO–LUMO and NBO analysis of OINB, MINB and PINB have not been reported so far. Therefore, there is need to supplement the existing experimental and theoretical data to help in making correct assignments. In the present work, all these computations are carried out and theoretical as well as experimental studies of the FTIR, Raman and UV–Vis spectra of OINB, MINB and PINB have been reported. The theoretical vibrational spectra are simulated using the anharmonic frequencies obtained by PT2 method within DFT framework [24]. The fundamental, combination and overtone bands of the spectra have been assigned using anharmonic force field. Using PCM model at TD-B3LYP/6311G(d,p)/3-21G⁄ level of theory in ethanol solution, the electronic absorption spectra of the molecules are investigated. The effect of iodine substituent on the vibrational modes of nitrobenzene and benzene has been studied. Vibrational analyses of polyatomic molecules in the harmonic approximation have been performed by many workers and various scaling procedures [25–28] were used for compensating discrepancies. Harmonic calculations are of computationally low cost even for relatively large organic molecules but the accuracy is inadequate in particularly for stretching vibrations appearing at higher wavenumbers. The scaling methods are useful in many cases but have no first principles basis. Moreover, it does not represent nature of anharmonic part of the potential that is itself of great interest. In order to get reliable results without any scaling, a great effort has been made to carry out anharmonic computations [29–34]. Recently, vibrational spectra of polyatomic molecules [35–40] are well interpreted using anharmonic calculations. The calculations of vibrational spectra at the anharmonic level are of moderate computational cost and provide results in close agreement to the experimental data. These calculated frequencies can be used for assignment of modes without any manual scaling. The aim of this work is to use anharmonic force field method for analysis of the vibrational spectra of substituted benzenes and to test the capability of the method for medium sized molecules containing heavier atom like iodine. Present study encourages the use of anharmonic method within the DFT framework for calculating vibrational spectra of medium sized polyatomic molecules containing heavy elements.

2. Experimental details The samples of OINB, MINB and PINB, obtained from Koch-Light Laboratories and Colnbrook Bucks (England), were used for the spectral measurements. FTIR spectra of the samples were recorded on Tensor 37 Spectrometer (Bruker) in the region of 370–4000 cm1 with a spectral resolution of 1 cm1 using KBr pellet technique. A minimum of 32 scans were accumulated to increase the signal-tonoise ratio. The ultraviolet–visible (UV–Vis) spectra of compounds in ethanol solution were recorded in the region 175–800 nm on Lamda 950 UV–Vis spectrophotometer (Perkin–Elmer). 3. Computational details The quantum chemical calculations on compounds containing iodine atoms are not much accurate because of unavailability of basis sets for iodine atom in most of quantum mechanical programs, although various accurate theoretical methods (B3LYP, MPn and QCISD, etc.) have been developed. Gaussian type basis sets for the heavy elements beyond the fourth row in the periodic table, are relatively limited. In the present investigation, all the quantum chemical calculations were carried out at DFT/B3LYP [41,42] level of theory with 3-21G⁄ basis set on iodine and 6311G(d,p) on other atoms using Gaussian 03W software [43]. The basis set 3-21G⁄ on iodine atom is used because this is relatively more accurate and least expensive. The stable molecular structures of OINB, MINB and PINB in the ground state were obtained using Berny’s optimization algorithm under tight convergence criterion. To determine the stable conformations of these molecules, the torsional angle T (C–C–N–O) was varied from 0° to 180° with step size of 10° and the molecular geometry profiles were obtained. The optimized structural parameters have been computed for stable molecular structures and were used for the calculations of the harmonic and anharmonic vibrational frequencies at same level of theory [43]. The anharmonic frequencies were calculated using the second order perturbative method elaborated by Barone [24,43]. The combination and overtone frequencies were also obtained. All the real values of calculated frequencies confirm that the optimized molecular geometry corresponds to true energy minima on the potential energy surface. Vibrational spectra of the molecules have been assigned using PED values that were computed with the help of VEDA4 program [44]. Electronic spectra of the optimized molecules were calculated at PCM-TD-B3LYP/6311G(d,p)/3-21G⁄ level of theory in ethanol solution. Electronic properties like HOMO and LUMO energy Eigen values along with HOMO–LUMO gaps were also deduced for the stable structure at B3LYP/6-311G(d,p)/3-21G⁄ level of theory in gaseous state and solution. HOMO-LUMO energy gaps were also computed using PCM model by TD-DFT calculations. For the plots of simulated IR and Raman spectra, pure Lorentzian band shape was used with full width half maxima (FWHM) of 5 cm1 [45]. Raman intensities were computed by RAINT program [46]. The calculated harmonic IR and Raman intensities were used for the simulation of vibrational spectra. The gas phase thermodynamic properties such as entropy, heat capacity and enthalpy change have been calculated in the temperatures range 77–700 K at B3LYP/6-311G(d,p)/321G⁄ level of theory. The NBO analysis was performed by using the NBO 3.1 program [47]. 4. Results and discussions 4.1. Molecular geometry and potential energy surface scan The OINB, MINB and PINB molecules contain an iodine atom and a nitro group attached to a benzene ring. The optimized

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Fig. 1. The molecular structure with numbering scheme of OINB, MINB and PINB.

Table 1 Optimized structural parameters of iodonitrobenzenes at B3LYP/6-311G(d,p)/3-21G⁄ level of theory. Parameters

OINB Calculated

Bond length (Å) C4–C5 C4–C12 C5–C6 C6–C8 C8–C10 C10–C12 hC–Cic C4–H14 C6–H7 C8–H9 C10–H11 C12–H13 hC–Hic N3–C5 O1–N3 O2–N3 hO–Nic C–I

a b c

1.399 1.398 1.394 1.386 1.393 1.390 1.393 1.081 1.083 1.084 1.082 1.083 1.482 1.220 1.224 1.222 2.115

Bond angle (°) O1–N3–O2 O1–N3–C5 O2–N3–C5 C5–C4–C12 C5–C4–H14 C12–C4–H14 N3–C5–C4 N3–C5–C6 C4–C5–C6 C5–C6–H7 C5–C6–C8 H7–C6–C8 C6–C8–H9 C6–C8–C10 H9–C8–C10 C8–C10–H11 C8–C10–C12 H11–C10–C12 C4–C12–C10 C4–C12–H13 C10–H12–C13 C–C–I C–C–I

123.1 115.5 121.4 118.3 120.1 121.6 120.0 119.3 120.7 120.3 120.3 119.3 121.2 119.0 119.8 125.8 116.5

Selected dihedral angle (°) O1–N3–C5–C4 O1–N3–C5–C6 02–N3–C5–C4 02–N3–C5–C6

31.7 147.8 149.5 91.0

125.2 117.9 116.9 117.6

MINB Experimentala

1.398

1.117 1.468

1.238 2.101

118.8 118.8 118.6

121.1 116.1 122.8 116.8 121.6 122.4 118.8 118.9 120.6 120.5

Calculated 1.390 1.391 1.389 1.391 1.394 1.396 1.392 1.080 1.081 1.083 1.083 1.082 1.484 1.222 1.223 1.223 2.115 125.1 117.5 117.4 118.3 119.4 122.3 118.4 119.0 122.6 119.6 118.1 122.3 119.8 120.6 119.6 119.9 120.0 120.1 120.4

PINB Experimentala

1.392

1.114 1.487

1.226 2.102

118.0 118.0 116.4 121.8 118.0 118.0 124.0 117.2 121.4 121.1 119.5 119.4 120.3 121.9

124.1

120.2 119.5

118.4 118.4

60.0

0.0 180.0 180.0 0.0

0.0

Electron diffraction data in gaseous phase (dynamical model), Ref. [14]. Deformation parameters on the benzene ring caused by nitro group and iodine atom, Ref. [14]. Mean value.

Calculated 1.390 1.390 1.390 1.390 1.396 1.396 1.392 1.081 1.081 1.082 1.082 1.082 1.478 1.223 1.223 1.223 2.112 125.0 117.5 117.5 118.9 119.6 121.5 119.0 119.0 121.9 119.6 118.9 121.5 119.8 119.8 120.4

Experimentalb

1.396

1.107 1.458

1.228 2.102

117.9 117.9 118.9 120.6

122.2 119.2

118.8

120.6

122.2

119.8 119.8 120.4 119.7 119.7

118.9

0.0 180.0 180.0 0.0

0.0

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molecular structures with labelling of atoms are shown in Fig. 1. The molecular structural parameters are listed in Table 1. The calculated structural parameters are in good agreement with data obtained from electron diffraction experiment [14]. In order to describe stable structure on potential energy surface, the energy profile has been achieved as a function of C4–C5–N3–O1 dihedral angle (Fig. 2). In the case of OINB, two maxima are found at 0° and 90° with relative energy values 4,597,341.675 and 4,597,335.400 kcal/mol respectively. The values of dihedral angle, O–N–C–C, (Table 1) show that aromatic rings are co-planar with nitro group for MINB and PINB. In OINB, the nitro group is twisted by 31.7° with the plane of aromatic ring for obtaining stable conformation. The angle of twist is confirmed by PES scan. The optimized molecular energies are 4,597,341.562, -4,597,347.807 and 4,597,348.243 kcal/mol for OINB, MINB and PINB respectively. The structure of PINB is more stable than OINB and MINB due to its predominant resonance structure and the larger separation between electron withdrawing nitro group and electronegative iodine atom. In the case of OINB, nitro group gets twist and the activation effect of nitro group is diminished by the steric effect and may result in a net deactivation [48,49]. The computed potential barrier heights (Vmax  Vmin) due to torsional motion of the nitro group of OINB are 3.14 and 0.94 kcal/ mol at 90° and 0° torsional angle respectively. The substitution of the iodine atom to the different positions in aromatic ring does not cause any noticeable change in C–C, C–H, C– N, N–O and C–I bond lengths in the investigated molecules (Table 1). It is also found that the N3–C5–C4, N3–C5–C6 and C–

C–I bond angles are approximately same in MINB and PINB but they differ in OINB. The differences in the bond angles for OINB can be attributed to steric effect of nitro group and iodine atom. 4.2. Vibrational analysis The FTIR and simulated Raman spectra of the molecules are shown in Figs. 3 and 4. The broad bands, appeared around 3400 cm1 in the FTIR spectra of MINB and PINB, are attributed to presence of water in KBr or semisolid state of the compounds at room temperature [7]. The experimental and theoretical vibrational frequencies along with their assignments are depicted in Tables 2–4. The far-IR and Raman wavenumbers are taken from the earlier reported spectra [7,23]. Under symmetry consideration, OINB, MINB and PINB belong to C1, CS and C2V point group respectively. The 36 normal modes of MINB are classified into the irreducible representations Cvib ¼ 25a0 þ 11a00 under CS point group, where a0 and a00 species represent in plane and out of plane mode of vibrations respectively. For PINB molecule, all the 36 fundamental modes of vibrations are distributed among four symmetry species of C2V point group as Cvib = 13a1 + 4a2 + 7b1 + 12b2. The vibrations of a1 and b2 symmetry species are the in-plane modes and those of a2 and b1 species are out of plane modes. Under C2V symmetry, a1, a2 and b1 vibrations are Raman and IR active while a2 vibrations are allowed in Raman but forbidden in infrared spectra. All the fundamental vibrations of OINB, MINB and PINB have been assigned using PED values, their animated modes and the vibrational assignments of benzene, nitrobenzene and

Fig. 2. PES scan for the selected T(C–C–N–O) dihedral angle of OINB, MINB and PINB.

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identified in mode 21 with major contribution (about 50%) in all the molecules. The symmetric NO2 in-plane bending vibration (Mode 21) in OINB has appeared at higher wavenumber with weak intensity in FTIR spectrum as compared with those in MINB and PINB. The wagging vibration of NO2 group normally occurs in the region 730–590 cm1. In the present work, mode 24 in OINB and mode 23 in MINB and PINB are assigned to wagging vibrations which have appeared with strong intensity in the FTIR spectra whereas with very weak intensity in the Raman spectra of the molecules. The wagging vibration is observed at lower wavenumber in OINB than those identified in MINB and PINB. The assignments of NO2 group vibrations are coherent with earlier reports [7,8,10,11,20,21,23,51]. All NO2 group vibrational frequencies have close similarity in magnitude in OINB, MINB and PINB.

Fig. 3. Experimental and simulated FTIR spectra of OINB, MINB and PINB.

iodobenzene (given in Supplementary materials Table 5S). In the FTIR spectra of OINB, MINB and PINB, the combination and overtone bands are also assigned with the help of calculated anharmonic frequencies. The calculated and experimental wavenumbers of non-fundamental bands are given in Table 5. The most of the observed non-fundamentals are combination bands. The assignments of the fundamental normal modes of the molecules are discussed in the following sections. 4.2.1. Nitro group vibrations The characteristic infrared frequencies of organic nitro compounds show strong absorptions due to the asymmetric and symmetric stretching vibrations of the nitro group in the region 1600–1510 cm1 and 1385–1325 cm1 respectively [8,10,11,50]. The strong infrared bands observed in the respective ranges are assigned to NO2 asymmetric (mode 7) and symmetric stretching (mode 10) vibrations for OINB, MINB and PINB and they have more than 70% contributions in these modes. The average error between calculated anharmonic and experimental wavenumbers is about 2% for NO2 asymmetric stretching vibrations. This could be related to either limitations of the calculations or higher order effects neglected in PT2 computations of anharmonicity. For symmetric stretching vibrations, the differences are relatively low which give an average error of 0.3%. The Raman band corresponding to NO2 asymmetric stretching vibration is stronger in OINB while NO2 symmetric stretching vibrations have appeared with strong intensity in all the isomers. The wavenumbers corresponding to NO2 stretching vibrations for OINB, MINB and PINB have appeared consistently with similar magnitude of frequencies. The NO2 deformation vibrations appear in the low wavenumber region. It is

4.2.2. C–H vibrations of the aromatic ring The IR spectra of aromatic compounds contain multiple weak peaks due to the C–H stretching vibrations in the region 3100– 3000 cm1 [50]. In Raman spectra, these vibrations may appear as strong bands. The bands observed in FTIR at 3084 cm1 in OINB, at 3095 and 3035 cm1 in MINB and at 3093 and 3051 cm1 in PINB are assigned to C–H stretching vibrations. They are also found with strong to medium intensity in Raman spectra. These modes are pure C–H stretching vibrations as evident from corresponding PED values given in Tables 2–4. The calculated anharmonic frequencies corresponding to C–H stretching vibrations for MINB and PINB are found at higher wavenumber than those for the OINB. The average deviation of calculated frequencies from experimental values for C–H stretching vibrations is about 1% in the case of anharmonic calculations. On the other hand, it is 4% for the harmonic frequencies. The improvement in results of anharmonic DFT calculations over harmonic one is largest for these modes. The deviations for C–H stretching vibrations in isomeric iodonitrobenzenes are larger in comparison to those of benzene, nitrobenzene and iodobenzene (Table 5S in Supplementary materials). The analysis of data also shows that some vibrational modes in OINB, MINB and PINB molecules involve stronger anharmonic effects as compared to benzene, nitrobenzene and iodobenzene. The aromatic C–H in plane bending vibrations occur in the region 1290–990 cm1 [7,8,50]. These bands are observed with weak to medium intensities in FTIR and Raman spectra and are assigned (Tables 2–4). The C–H out of plane bending vibrations appear in the region 950–600 cm1 [21]. These modes are also identified in iodonitrobenzenes and they have higher PED contributions than in plane bending vibrations (Tables 2–4). 4.2.3. C–X vibrations C–NO2 stretching vibrations normally show their presence in the region 1180–865 cm1 in aromatic nitro compounds [7,8,10,50]. In IR spectrum of nitrobenzene, C–NO2 stretching vibration, mixed with ring vibrations, is reported at 1108 and 392 cm1 [51]. According to PED values, the maximum contributions of C–NO2 stretching vibrations (about 45%) are obtained in the mode 30 for OINB and MINB. In the case of PINB, the contribution of this vibration is 58% in mode 29. The vibrational band due to C–NO2 stretching vibration is observed at lower wavenumbers in OINB and MINB than that for PINB. The pure torsional vibrations of C–NO2 are found at very low wavenumbers in mode 36 for OINB, MINB and PINB. For the nitrobenzene, it was observed at 51 cm1 [51]. A comparison of C–NO2 vibrational frequencies revealed close similarity in magnitude in all these molecules. It should be emphasized that the calculated frequencies for these vibrations are in good agreement with the corresponding experimental values. Due to the large mass of iodine atom, the C–I stretching vibration appears in low wavenumber region, 610–200 cm1 and it mixes with ring vibrations [19,50,52,54]. The C–I stretching vibration is reported at 269 cm1

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Fig. 4. Simulated Raman spectra of OINB, MINB and PINB.

in the Raman spectrum of 2-iodopyridine [52]. This vibration has been calculated at 109 cm1 [53] in p-iodobenzene sulfonyl chloride, at 238 cm1 in 5-iodouracil [54] and at 262 cm1 in 2-iodopyridine [52] using DFT/B3LYP theory. In the present investigation, C–I stretching vibrations have high PED contribution in mode 32 for OINB, MINB and in mode 33 for PINB. These vibrations are observed with weak intensities in FTIR and Raman spectra of all the molecules except Raman spectrum of MINB. In the present work, the C–I stretching vibrations are found at very low wavenumbers as a contradiction to spectral region of 500 ± 100 cm1 reported for other compounds containing iodine [55]. The remarkable reduction in the frequencies may be due to interaction of C–I stretching vibration with ring vibrations [54]. The C–I in plane bending vibrations have been calculated at 151, 126, 152 cm1 in OINB, MINB and PINB respectively. The computed frequencies at 193, 184 and 71 cm1 with very weak IR intensities in OINB, MINB and PINB, respectively, are assigned to C–I out of plane bending vibrations and are in good agreement with the experimental values.

4.2.4. Ring vibrations The C–C stretching vibrations of the phenyl ring occur in the region 1650–1100 cm1 [56,22]. In the present FTIR and Raman spectra, these vibrational bands are observed with variable intensity in the region 1600–1000 cm1 in OINB, MINB and PINB. As compared to range cited in literature, noticeable decrements in frequencies are found. These vibrations are affected by the nature of attached substituent (heavy iodine atom) with the phenyl ring [53]. The

calculated anharmonic frequencies are found to be in good agreement with experimental values. Other observed skeletal vibrations are mixed to a considerable extent in all investigated molecules (Tables 2–4). The ring breathing and Kekule modes appear at 993 and 1309 cm1 in benzene molecule respectively [57,58]. From the PED values and animated modes, the IR bands due to ring breathing and Kekule modes are assigned, respectively, at 1015 and 1261 cm1 in iodobenzene; at 1039 and 1290 cm1 in OINB; at 1052 and 1303 cm1 in MINB; and at 1053 and 1307 cm1 in PINB. Kekule mode is assigned at 1325 cm1 in nitrobenzene [51] and around 1300 cm1 in monohalogenated nitrobenzenes [7]. The ring twist mode is found at 1350 cm1 in benzene molecule [58]. The infrared bands observed at 1321, 1316, 1265 and 1272 cm1 in iodobenzene, nitrobenzene, MINB and PINB, respectively, are assigned to the ring twist modes. In the case of OINB, it is observed at 1428 cm1 with very low PED value. The other important mode of vibration ‘‘Star of David (trigonal ring breathing)’’ is assigned with the help of PED and visual inspection that can be seen in Tables 2–4 and Table 5S. This mode is reported at 1000 (1001) cm1 in the IR (Raman) spectra of 3-fluoroaniline [36].

4.3. Electronic spectra The TD-DFT method has become one of the most popular and widely used approaches for the calculation of properties such as excitation energies, oscillator strengths and excited state geometries of medium to large molecular systems [59]. Electronic spectra

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Table 2 The observed FTIR, Raman and calculated wavenumbers (cm1) along with their relative intensities and assignments for OINB. Mode

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

B3LYP/6-311G(d,p)/3-21G⁄

Experimental a

FTIR

Raman

3084vw

3091m 3069m

1582s

1580s 1560m 1525m 1460vw

1525vvs 1462w 1428w 1334vs 1290m 1243m 1165w 1136w 1101m 1039w 1019m 946w 930w 852m 872w 779s 727vs 692s 639w 556w 466w 424w 379vw 250w 238w 202w 139w 70w

1341vs 1250vw 1158w 1135w 1104w 1035s 1016w

854w 778vw 691vw 635w 465w 420w 244w 244m 200w 132w 92vs

x

har

3220 3207 3195 3179 1639 1617 1598 1489 1462 1380 1331 1286 1189 1164 1127 1065 1043 1030 993 910 871 811 762 724 714 653 571 487 430 406 302 249 198 153 113 51 50.25 39.06

x

anh

3059 3069 3066 3053 1604 1563 1562 1459 1433 1350 1304 1261 1178 1145 1109 1048 1023 943 940 854 862 779 724 692 703 644 558 471 415 403 296 244 193 151 109 57 15.05 10.97

Assignments and PED (%) values IR

RA

I

I

1.9 2.8 7.2 2.8 176.4 34.4 66.4 25.4 5.2 225.3 10.3 7.2 0.9 7.4 10.4 0.8 31.8 0.2 2.2 0.2 35.1 11.7 45.6 12.5 13.7 5.2 0.5 5.6 0.4 1.9 1.2 0.3 0.2 1.8 1.0 0.2

380.6 284.1 425.7 203.2 571.1 238.5 138.3 7.1 6.5 848.3 107.0 25.3 98.9 177.3 229.3 455.0 190.2 2.2 2.2 15.8 167.3 5.4 33.2 19.2 21.5 94.8 56.5 32.3 9.8 284.9 128.4 218.3 263.3 41.7 649.2 1093.8

mCH (87) mCH (83) mCH (77) mCH (86) mCC (71) + bCNO (7) mCC (50) + bHCC (10) + bCCC (9) masNO2 (73) bHCC (51) + bCCC (25) bHCC (53)b+mCC (19) msNO2 (75) + mNC (8) mCC (63)c+bHCC (8) bHCC (56) + mCC (21) bHCC (80) + mCC (9) mCC (37) + bHCC (28) bCCC (20) + bHCC (18) + mCC (10) + mNC (8) mCC (53)d+bCCC (15) + bHCC (12) bCCC (50)e+mCC (22) sHCCC (57) + sCCCC (18) sHCCC (78) sHCCC (76) bNO2 (53) + mNC (11) + bCCC (8) cNCCC (42) + sHCCC (20) + sCCCC (12) sHCCC (53) + cNCCC (21) + sCCCC (6) xNO2 (73) + sHCCC (10) bCCC (40) + bNO2 (18) + mNC (8) bCCC (72) bCNO (45) + sHCCC (9) sC–C–C–C (69) + sHCCC (9) sCCCC (49) + bCNO (12) + sHCCC (12) + cOCON (6) mNC (42) + bCCC (11) bCNO (67) mCI (49) + bCCI (18) c[NCCC + ICCC] (78) b[CCI + CCN] (68) sNCCC (57) + cCCCC (19) + sONCC (7) sNO2 (82)

Abbreviation used: m-stretching, ma, ms-asymmetric and symmetric stretching, b-bending, s-torsional motion, c-out of plane motion, x- wagging motion; vw-very weak, w-weak, m-medium, s-strong, vs-very strong, vvs-very very strong, RMS-Root Mean Square, MAD-Mean Absolute Deviation, xhar: harmonic wavenumber, xanh: anharmonic wavenumber, IIR-IR intensity (km/mole), IRA-Raman intensity(arb. unit). a Refs. [7,23]. b Ring twist. c Kekule. d Ring breathing. e Trigonal ring breathing.

of OINB, MINB and PINB are recorded in ethanol solution and analyzed using TD-DFT(B3LYP)/6-311G(d,p)/3-21G⁄ level of theory in combination with PCM model. The vertical excitation energies, oscillator strengths (f) and absorption wavelengths along with their assignments are given in Table 6. The experimental and simulated electronic spectra are shown in Fig. 5. The TD-DFT calculations in ethanol solution show two intense bands at 313 (f = 0.0321) and 234 nm (f = 0.0648) in OINB; three bands at 368 (f = 0.0173), 248 (f = 0.1325) and 239 nm (f = 0.2057) in MINB and three intense bands at 340 (f = 0.3033), 239.8 (f = 0.0230) and 239.7 nm (f = 0.1164) in PINB. These wavelengths are found in good agreement with the experimental values. In the experimental electronic spectrum of OINB, the prominent broad bands observed at 220 and 300 nm are assigned to HOMO-1 ? LUMO + 2 (50%) and HOMO-2 ? LUMO (55%) transitions respectively. In the UV–Vis spectrum of MINB, HOMO ? LUMO + 2 (81%), HOMO-4 ? LUMO (87%) and HOMO ? LUMO (94%) transitions are observed, respectively, at 224, 253 and 300 nm. Three absorption bands have appeared at 200, 215 and 285 nm in the spectrum of PINB which are assigned to HOMO-4 ? LUMO (90%), HOMO ? LUMO + 2 (81%) and HOMO ? LUMO (91%) transitions respectively.

4.4. Frontier molecular orbitals The frontier orbitals, HOMO and LUMO indicate the ability to donate and accept the electron respectively. These orbitals are important to determine the way; the molecules interact with other molecular species. The calculated frontier orbitals (HOMO and LUMO) are shown in Fig. 6. The HOMO–LUMO energy gap determines the chemical stability of the molecule. The HOMO and LUMO energy eigen values are useful for computing various global chemical reactivity indices (chemical hardness–softness, electronic chemical potential and electrophilicity) and Fukui functions [60] which predict relative stability and reactivity of the molecules [61,62]. In iodonitrobezes, the most of the part of HOMOs are located over iodine atoms and benzene rings whereas the LUMOs are delocalized over the whole C–C bonds and NO2 groups (Fig. 6). The HOMO–LUMO excitations in these molecules imply an electron density transfer from iodine atoms to NO2 groups and redistribution of the charge density on aromatic part. The HOMO and LUMO energy Eigen values along with their gaps are computed at PCM-B3LYP/6-311G(d,p)/3-21G⁄ level of theory for OINB, MINB and PINB and are presented in Table 7. It is clear from

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Table 3 The observed FTIR, Raman and calculated wavenumbers (cm-1) along with their relative intensities and assignments for MINB. Mode

Species

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

a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a00 a0 a00 a00 a0 a00 a00 a0 a00 a0 a0 a00 a00 a0 a0 a0 a00 a00 a0 a00

B3LYP/6-311G(d,p)/3-21G⁄

Experimental

3095m

Raman

a

3092s 3072m

3035w 1599m 1527vvs 1456m 1414w 1348vvs 1303m 1265m 1178w 1101m 1072m 1052m 963w 1009w 928w 885s 863s 776sh 729vs 711s 659s 639m 529w 485w 422m 381w 252w 238w 212w 140m 70w

1563s 1524w 1454vw 1412vw 1345vvs

1108m 1067m 1044w 1010vvs 890vw 854w 725vw 706w 654s 638m

425vw 390vw 250w

140s 128m 68m

x

har

3235 3229 3203 3185 1651 1614 1600 1495 1448 1380 1337 1303 1196 1123 1116 1082 1021 1011 973 946 882 823 740 732 694 659 538 505 438 400 302 257 188 143 127 45 52.70 39.63

x

anh

3091 3098 3077 3054 1613 1575 1561 1462 1421 1350 1307 1278 1178 1100 1090 1062 961 1002 921 888 871 798 737 722 660 651 526 489 422 394 301 253 184 143 126 62 14.40 10.11

Assignments and PED (%) values IR

RA

I

I

7.5 3.6 3.1 4.1 39.6 7.0 228.0 24.5 7.1 291.3 10.1 5.2 0.0 56.7 12.0 11.5 0.1 6.3 4.7 5.8 29.0 22.0 29.8 41.0 14.6 4.0 1.3 4.3 0.9 0.9 0.7 0.4 1.5 0.1 1.1 0.1

89.2 249.1 323.1 252.1 12.8 374.1 148.6 6.0 11.9 1281.7 28.9 9.8 42.6 303.2 86.0 113.0 0.2 584.1 13.1 3.8 64.6 15.8 35.7 17.2 1.3 124.3 49.7 8.2 0.5 70.3 84.7 442.4 241.5 235.0 211.7 273.8

mCH (100) mCH (97) mCH (87) mCH (88) mCC(67) + b CNO (7) mCC (55) + bCCC (13) + bHCC (7) masNO2 (83) bHCC (60) mCC (49) + bHCC (24) msNO2 (79) + mNC (7) mCC (69)c+bHCC (10) bHCC (75)b+mCC (7) bHCC (72) + mCC (6) bHCC (27) + m[CC + NC] (25) + bCCC (8) bHCC (26) + mCC (15) + bCCC (14) bCCC (16) + bHCC (14) + mCC (12)d sHCCC (71) + sCCCC (15) bCCC (45)e+mCC (32) sHCCC (73) sHCCC (65) + sCCCC (13) bNO2 (48) + mNC (14) + bCCC (6) sHCCC (74) xNO2 (80) + sHCCC (10) bCCC (33) + bNO2 (26) + mIC (12) + mCC (9) + mNC (7) sCCCC (55) + sHCCC (15) bCCC (53) + mIC (13) bCNO (77) [sCCCC + cNCCC] (81) + sHCCC (6) sCCCC (70) + sHCCC (19) mNC (45) + bCCC (20) bCNO (90) mIC (58) + bCCC (21) [sCCCC + cICCC] (80) cICCC (80) + sCCCC (10) b[CCI + CCN] (78) sNO2 (91)

Abbreviation used: m-stretching, ma, ms-asymmetric and symmetric stretching, b-bending, s-torsional motion, c-out of plane motion, x- wagging motion; vw-very weak, w-weak, m-medium, s-strong, vs-very strong, vvs-very very strong, RMS-Root Mean Square, MAD-Mean Absolute Deviation, xhar: harmonic wavenumber, xanh: anharmonic wavenumber, IIR-IR intensity (km/mole), IRA-Raman intensity(arb. unit). a Refs. [7,23]. b Ring twist. c Kekule. d Ring breathing. e Trigonal ring breathing.

Table 7, PINB molecule is most stable in gaseous state because of its largest HOMO–LUMO gap among the investigated molecules as well as it is least reactive than those having smaller gaps. The calculated excitation energies at TD-DFT level for transitions between HOMOs and LUMOs are reasonable approximations to the HOMO–LUMO gaps. The HOMO–LUMO gaps calculated at TD-DFT level have more physical meaning than the derived directly from the energy differences between HOMO and LUMO energy eigen values obtained in DFT calculations. The least value of HOMO– LUMO gap in OINB predicts its high reactivity in chemical reactions. 4.5. Thermodynamic properties Thermodynamic data are important for understanding the chemical processes. The density functional theory is a well-established and efficient tool to predict various statistical thermodynamic properties of molecules. Nowadays, due to theoretical predictions of thermodynamic properties, their experimental measurements are possible with high accuracy. For ideal gases, the thermodynamic functions such as molar entropy, heat capacity

and enthalpy change can be predicted easily from the molecular partition function. These calculations are used to convert molecular energy levels into macroscopic properties. Molecular energy arises from molecular translation, rotation, vibration and electronic excitation. This information constitutes the spectroscopy of the molecule of interest [63]. In the present investigations, the parameters such as SCF energies, zero-point vibrational energies, specific heat capacities, rotational constants, rotational temperatures, dipole moments, entropies and enthalpies of OINB, MINB and PINB are computed at B3LYP/6-311G(d,p)/3-21G⁄ level of theory (available in Supplementary materials). Based on harmonic vibrational analysis, the statistical thermodynamic functions: molar entropy (S0m ), molar enthalpy change (DH0m ) and molar heat capacity (C 0p;m ) for the molecules are calculated in the temperature range, 77–700 K (available in Supplementary materials). It can be seen that these thermodynamic functions are increasing with temperature because the molecular vibrational intensities increase with temperature [64]. The statistical thermodynamic functions and ZPE with anharmonic corrections at 298.15 K are also presented. The correlation equations for entropies, enthalpy changes and heat capacities are fitted

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M.J. Alam, S. Ahmad / Journal of Molecular Structure 1059 (2014) 239–254 Table 4 The observed FTIR, Raman and calculated wavenumbers (cm1) along with their relative intensities and assignments for PINB. Mode

Species

Experimental FTIR

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

a1 b2 b2 a1 b2 a1 b2 a1 b2 a1 b2 b2 a1 b2 a1 a1 a1 a2 b1 b1 a1 a2 b1 a1 b1 b2 b2 b1 a1 a2 b2 b1 a1 b2 b1 a2

B3LYP/6-311G(d,p)/3-21G* Raman

a

3078m 3093m 3051w 1595m 1570m 1513vvs 1470w 1394vw 1343vvs 1307m 1272w 1179m 1104m 1073w 1053w 1008m

1179w 1104s 1067m 1052vs 1002w

915vw 837s 851sh

835vw 845w

734s 700vw 671w 621w 530vw 468w 455w 266m 248s 222w 125w 70w

1590w 1562s 1510w 1463w 1404w 1340vvs

732vw 698w 618w

460w 400vw 250w 216w 130w 79w

x

har

3225 3224 3201 3201 1649 1616 1593 1506 1423 1377 1339 1316 1206 1124 1120 1079 1027 1019 1016 872 869 860 751 719 710 638 533 477 465 427 296 257 215 151 74 57 55.23 39.84

x

anh

Assignments and PED (%) values IR

3086 3094 3064 3084 1610 1577 1554 1470 1392 1347 1313 1285 1186 1103 1092 1055 1009 955 925 843 859 828 725 710 677 631 535 466 457 411 292 250 214 152 71 56 12.91 9.27

I

IRA

3.0 1.5 1.5 0.2 101.7 69.6 135.2 34.1 8.1 414.4 9.3 2.4 6.6 9.0 38.5 22.1 28.7 0.00 0.3 39.1 78.7 0.0 14.8 0.7 17.5 0.5 1.5 8.5 6.3 0.0 0.8 2.9 0.0 1.7 2.0 0.0

451.4 75.9 116.4 139.5 14.9 1702.3 66.3 14.2 7.2 4712.0 3.1 0.4 40.5 0.4 1574.2 634.4 43.0 0.10 12.7 22. 232.1 25.4 26.9 57.7 13.3 145.2 78.7 0.2 3.2 0.7 18.9 87.3 151.3 127.4 2.4 51.6

mCH (91) mCH (94) mCH (94) mCH (91) mCC (68) + bCNO (9) mCC (69) + bHCC (17) + bCCC (12) masNO2 (76) bHCC (67) + bCCC (11) mCC (32) + bHCC (32) + bCCC (16) msNO2 (79) + mNC (7) mCC (40)c+bHCC (31) bHCC (57)b+mCC (29) bHCC (68) + mCC (17) bHCC (65) + mCC (23) mCC (37) + mNC (17) + bHCC (10) bCCC (34) + bHCC (18) + mCC (13)d bCCC (61)e+mCC (16) sHCCC (74) + sCCCC (25) sHCCC (68) + sCCCC (25) sHCCC (85) bNO2 (54) + mCC (12) + mNC (6) sHCCC (100) [sCCCC + xNO2] (67) + sHCCC (16) bCCC (42) + m[NC + IC] (15) sCCCC (64) + sHCCC (14) + cOCON (7) bCCC (81) + b CCI (7) bCNO (77) [sCCCC + cNCCC] (92) mNC (58) + bCCC (14) sCCCC (73) + sHCCC (25) bCNO (87) cNCCC (84) mIC (61) + bCCC (22) bCCI (82) c[NCCC + ICCC] (90) sNO2 (98)

Abbreviation used: m-stretching, ma, ms-asymmetric and symmetric stretching, b-bending, s-torsional motion, c-out of plane motion, x-wagging motion; vw-very weak, wweak, m-medium, s-strong, vs-very strong, vvs-very very strong, RMS-Root Mean Square, MAD-Mean Absolute Deviation, xhar: harmonic wavenumber, xanh: anharmonic wavenumber, IIR-IR intensity (km/mole), IRA-Raman intensity(arb. unit). a Refs. [7,23]. b Ring twist. c Kekule. d Ring breathing. e Trigonal ring breathing.

by quadratic polynomial in temperature. The corresponding fitting equations with fitting factors (r2) are given as follows: For OINB, the corresponding fitting equations are:

S0p;m

4 2

¼ 57:0595 þ 0:1405T  0:4799  10 T ðr ¼ 0:9996Þ

DH ¼ 0:2965 þ 0:0102T þ 0:3579  104 T 2 ðr 2 ¼ 0:9998Þ: For MINB, the corresponding fitting equations are 4 2

DH ¼ 0:2933 þ 0:0101T þ 0:3578  104 T 2 ðr 2 ¼ 0:9998Þ:

2

C p ¼ 5:4169 þ 0:1031T  0:4318  104 T 2 ðr 2 ¼ 0:9989Þ

S0p;m

C p ¼ 5:4278 þ 0:1027T  0:4271  104 T 2 ðr2 ¼ 0:9989Þ

All the computed gaseous phase thermodynamic data will be helpful for calculating other thermodynamic energies and for predicting directions of chemical reactions using second law of thermodynamics [65]. 4.6. Atomic charges

2

¼ 57:5931 þ 0:1408T  0:4815  10 T ðr ¼ 0:9996Þ

C p ¼ 5:4801 þ 0:1026T  0:4256  104 T 2 ðr 2 ¼ 0:9989Þ

DH ¼ 0:2764 þ 0:0101T þ 0:3582  104 T 2 ðr 2 ¼ 0:9998Þ: For PINB, the corresponding fitting equations are

S0p;m ¼ 57:606 þ 0:1405T  0:4792  104 T 2 ðr 2 ¼ 0:9996Þ

In the present work, natural atomic charges of isomeric iodonitrobenzenes are calculated at B3LYP/6-311G(d,p)/3-21G⁄ level of theory. The values of computed natural atomic charges are available in Supplementary materials. The plots of atomic charges on different atoms for the molecules are shown in Fig. 7. Atomic charges have a significant importance in the application of quantum chemical calculations for molecular system because they are related to dipole moment, electronic structure, molecular polarizability and other various properties of the molecules.

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Table 5 Assignments of combination and overtone bands in the FTIR spectra of iodonitrobenzenes. OINB

MINB

FTIR (cm

1

x

)

2367

anh

1

(cm

2382 2396 2368 2319 1993

2345 2332 1975

)

Assignment

779 + 1582 1039 + 1334 1101 + 1243 872 + 1462 639 + 1334

PINB 1

FTIR (cm

x

)

1623

692 + 946

1495 1445 1382

1498 1438 1382

639 + 852 424 + 1019 2692

1196

1202

556 + 639

1057 805 760

1059 817 804

639 + 424 424 + 379 2379

(cm

1

)

Assignment

FTIR (cm1)

xanh (cm1)

Assignment 1570 + 1272 1394 + 1053 1343 + 1104 1272 + 1104

3016 2862 2457

3026 2888 2481

1599 + 1414 1599 + 1265 1527 + 928

2848 2442

2365

2377

1265 + 1101

2370

2856 2446 2450 2386

2344

2340

1414 + 928

2342

2375

1272 + 1073

1988

1456 + 529 1348 + 639 2963 1265 + 659 1178 + 711 1178 + 639

1924

1925 1919

1470 + 455 1394 + 530

1880 1816

1993 2002 1904 1938 1901 1830

1801

1772

1773

1348 + 422

1780

1753

1759

863 + 885

1755

1726 1707

1742 1710

8632 1178 + 529

1722 1705

1690 1661

1696 1661 1681 1673 1612 1572 1482 1423 1402 1293 1218 1159 1045 842

963 + 729 1009 + 659 776 + 885 1265 + 381 963 + 659 928 + 639 1052 + 422 1009 + 422 863 + 529 863 + 422 776 + 422 729 + 422 639 + 381 2422

1688 1657

1805 1812 1779 1772 1764 1754 1731 1718 1713 1702 1653

1343 + 455 1272 + 530 1053 + 734 1307 + 468 1053 + 700 915 + 837 1053 + 671 2851 1179 + 530 837 + 851 1179 + 468

1625

1641

1008 + 621

1443 1412

1451 1396

915 + 530 734 + 671

1148

1157

530 + 621

1925

1641

anh

1641 1626 1568 1478 1429 1395 1288 1198 1152 1025 839

xanh – Calculated anharmonic frequencies at B3LYP/6-311G(d,p)/3-21G⁄ level of theory.

Table 6 Assignment of the electronic bands of iodonitrobenzenes in ethanol solution. OINB kcalc (nm)

MINB kobs (nm)

PINB

kcalc (nm)

kobs (nm)

f

E (eV)

Assignment

kcalc (nm)

kobs (nm)

f

E (eV)

Assignment

PCM-TDDFT(B3LYP)/6-311G(d,p)/3-21G⁄ 375 0.0127 3.31 H?L (76%)+H-1?L (18%) 354 0.0387 3.50 H-1?L (67%) + H?L (20%)

368 324

300

0.0173 0.0000

3.37 3.83

340 325

285

0.3033 0.0000

3.64 3.81

H?L (91%) H-3? L (52%)+ H-1?L (43%)

313

314

0.0000

3.95

H-1?L (55%)+ H-3?L (41%)

295

0.0168

4.20

0.0000 0.0011

4.44 4.75

H-2?L (89%) + H?L + 2 (5%) H-5?L (92%) H?L + 1 (94%)

0.0230 0.1164

5.17 5.17

H?L + 2 (81%) H-4?L (90%) H-1?L + 1 (85%)+H?L + 2 (6%) H-1?L + 2 (100%)

f

E (eV)

Assignment

0.0321

3.97

H-2?L (55%) + H-4?L (26%)

318

0.0000

3.90

273

0.0011

4.54

H-3?L (34%) + H-5?L (28%) + H-6?L (27%)

279

0.0001

4.45

H?L (94%) H-1?L (78%) + H-3?L (19%) H-3?L (74%) + H-1?L (20%) H-5?L (91%)

271 266

0.0012 0.0617

4.57 4.66

278 267

0.1379 0.0010

4.45 4.64

H-2?L (85%) H?L + 1 (94%)

280 261

255 246

0.0036 0.0405

4.85 5.05

0.0619

5.19

H-4?L (87%) H-1?L + 1 (88%) H?L + 2 (81%)

239.8 239.7

239

H?L + 1 (83%) H-4?L (34%) + H-2?L (25%) + H-3?L (13%) H-1?L + 1 (87%) H-3?L (43%) + H-4?L (22%) + H-5?L (14%) H?L + 2 (48%) + H-1?L + 2 (38%)

238

0.0003

5.21

0.0648

5.29

H-1?L + 2 (99%)

224

0.0004

5.53

234

300

220

H-1?L + 2 (50%) + H?L + 2 (20%) + H?L + 3 (13%)

248 240

253

0.1325 0.0173

4.99 5.16

239

224

0.2057

5.19

0.0001

5.38

230

Abbreviation used: H-HOMO, L-LUMO, f-oscillator strength, k-excitation wavelength, E-excitation energy.

215 200

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M.J. Alam, S. Ahmad / Journal of Molecular Structure 1059 (2014) 239–254

Table 7 SCF, HOMO, LUMO Energies and HOMO–LUMO energy gaps (kcal/mol) for OINB, MINB and PINB in ethanol solution. Parameters

SCF HOMO LUMO HOMO–LUMO gap

a b

PCM-B3LYP/6-311G(d,p)/3-21G⁄ OINB

MINB

PINB

4597349.110 4597341.562b 157.649 159.682b 63.253 60.605b 94.396 99.077b 76.330a

4597354.848 4597347.807b 157.599 162.217b 66.378 65.154b 91.221 97.063b 77.711a

4597355.520 4597348.243b 159.136 164.401b 66.008 64.872b 93.129 99.529b 83.942a

HOMO ? LUMO excitation energy at PCM-TD-B3LYP/6-311G(d,p)/3-21G⁄. Values in gas at B3LYP/6-311G(d,p)/3-21G⁄.

Among all carbon atoms in the ring, the C5 atom has less positive charge in all the iodonitrobenzenes. This may be due to attached electron withdrawing group, NO2, on C5 atom. The charges on N3 atoms in the iodonitrobenzenes are found larger than other atoms due to electron withdrawing inductive effect of NO2. 4.7. Molecular electrostatic potential map

Fig. 5. Experimental and simulated UV–Vis spectra in ethanol solution for the OINB, MINB and PINB.

The molecular electrostatic potential (MEP) is a tool for understanding and predicting the reactive behavior of molecules [66]. It is a real physical property that can be observed experimentally by diffraction methods as well as theoretically by ab-initio and DFT calculations. There are many applications of the electrostatic potential in the fields such as molecular recognition, hydrogen bonding and understanding of variety of physiochemical properties

Fig. 6. The HOMO and LUMO orbitals of OINB, MINB and PINB.

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M.J. Alam, S. Ahmad / Journal of Molecular Structure 1059 (2014) 239–254

In the MEP, the maximum negative regions are preferred site for electrophilic attack indicated by deep red color. The deep blue color surfaces represent maximum positive regions that are preferred site for nucleophilic attack. The almost neutral part, having electrostatic potentials close to zero, is indicated by green color. Electrostatic potential contour maps are also presented in Fig. 9. In the iodonitrobenzenes, negative regions are localized on surface around the oxygen while positive regions are confined over hydrogen atoms. The surfaces over the aromatic ring and iodine atom are showing neutral part (green color). 4.8. NBO analysis

Fig. 7. The correlation of atomic charges of OINB, MINB and PINB.

related to molecular interactions. The MEP maps for iodonitrobenzenes are shown in Fig. 8. The values of electrostatic potentials on the surface increase in the order red < yellow < green < blue. In the MEP plots, the color-coded maximum potential values (a.u.) are shown over the isodensity surfaces. The electrostatic potential values on the surface are specified in the range 0.0442 a.u. (deepest red) to 0.0442 a.u. (deepest blue) in the present molecules (Fig. 8). In any particular region around a molecule, the sign of the electrostatic potential depends upon whether the effects of the nuclei or electrons are dominant. It is a key to assessing its reactivity there.

Natural bond orbital (NBO) analysis is an important technique for studying intra and intermolecular bonding, hybridization and charge transfer in the molecular systems. The NBOs calculations also perform energetic analysis of NBO interactions based on the one electron effective energy operator b F i.e. Fock matrix. All the possible interactions between filled NBOs (donor) and empty NBOs (acceptor) with their interaction energy are estimated by second order perturbation theory. The two electron stabilization energy E(2) associated with delocalization, i ? j, for each donor NBO (i) and acceptor NBO (j) is computed by second order perturbation theory using equation: 2

Eð2Þ ¼ DEij ¼

qi Fði; jÞ ðj  i Þ

Fig. 8. Molecular electrostatic potential map (MEP) for OINB, MINB and PINB.

Fig. 9. Electrostatic potential contour map for (a) OINB, (b) MINB and (c) PINB.

Table 8 NBO analysis of OINB, MINB and PINB at B3LYP/6-311G(d,p)/3-21G⁄ level of theory. Occupancy OINB

MINB

PINB

EDA% OINB

MINB

PINB

OINB

MINB

PINB

OINB

MINB

PINB

r(O1–N3)

1.99483

1.99498

1.99503

51.63

51.55

51.52

48.37

48.45

48.48

1.98566

1.98630

59.23

59.94

40.77

40.06

r(O2–N3)

1.99382

1.99482

51.61

51.54

48.39

48.46

0.7180 sp2.87(74.05)O + 0.6961 sp2.11 (67.78)N 0.7742 sp1.00(99.85)O + 0.6329 sp1.00(99.74)N 0.7179 sp2.87(74.06)O + 0.6961 sp2.11(67.80)N

0.7177 sp2.88(74.11)O + 0.6963 sp2.12 (67.86)N

p(O1–N3)

0.7185 sp2.90(74.23)O + 0.6955 sp2.10 (67.68)N 0.7696 sp99.99(99.57)O + 0.6385 sp99.99(99.65)N 0.7184 sp2.94(74.53)O + 0.6956 sp2.12(67.90)N

0.7886 sp1.82(64.52)N + 0.6149 sp3.26(76.41)C 0.6991 sp1.59(61.42)C + 0.7150 sp1.56(60.88)C

0.7905 sp1.81(64.43)N + 0.6124 sp3.21(76.17)C 0.7037 sp1.88(65.26)C + 0.7105 sp1.64(62.05)C

0.7096 sp1.58(61.14)C + 0.7046 sp1.77(63.84)C 0.7227 sp1.00(99.96)C + 0.7046 sp1.00(99.95)C 0.7620 sp3.40(77.23)C + 0.6475 sp7.66(88.19)I

0.7125 sp1.72(63.13)C + 0.7016 sp1.56(60.98)C 0.6750 sp1.00(99.95)C + 0.7378 sp1.00(99.98)C

p(O2–N3)

1.99503

EDB%

1.98601

51.52

NBO (% p character)

60.02

48.48 39.98

r(N3–C5)

1.98852

1.98834

1.98904

62.19

62.49

62.43

37.81

37.51

37.57

r(C4–C5)

1.97603

1.96764

1.97586

48.87

49.52

49.16

51.13

50.48

50.84

p(C4–C5)

1.64363

r(C4–

1.97563

1.97587

C12) p(C4–C12)

1.66179

1.66553

r(C4–I)

1.97329

r(C12–I)

1.96680

50.36

50.77

52.23

45.56

1.97464

49.64

49.23

47.77

54.44

56.61

1.97366

1.97409

1.96533

1.97612

1.97586

p(C5–C6)

1.65159

1.64754

r(C6–H7)

1.97346

1.97519

r(C6–C8)

1.97556

1.97482

p(C6–C8)

54.90 49.92

41.93

1.97517

r(C4–

50.08

58.07

r(C10–I)

H14) r(C5–C6)

45.10

43.39 56.47

62.77

62.17

51.27

50.96

50.84

54.44

54.28

1.97409

61.91

62.12

1.96680

50.55

50.48

1.64608

0.7524 sp3.61(78.23)C + 0.6587sp7.36(87.78)I 43.53

37.23

37.83

48.73

49.04

49.16

45.56

45.72

62.17

38.09

37.88

37.83

50.08

49.45

49.52

49.92

48.52

1.97851

1.97731

1.97744

60.77

60.82

61.32

39.23

39.18

38.68

r(C8–

1.97970

1.97148

1.98077

49.98

49.60

50.17

50.02

50.40

49.83

C10) p(C8–C10)

1.62914

1.63908

51.10

51.03

48.90

48.97

r(C10–

1.97778

1.97778

60.67

61.20

39.33

38.80

1.97277

1.98099

49.58

50.11

50.42

49.89

1.98077 1.65650

1.97689

1.98139 1.89240

1.97744

1.98135 1.89660

1.98142 1.89746

49.83 52.13

61.23

61.32

0.7160 sp1.68(62.68)C + 0.6981sp1.87(65.12)C 0.7379 sp1.00(99.97)C + 0.6750 sp99.99(99.92)C 0.7868 sp2.51(71.50)C + 0.6172 sp0.00(0.05)H 0.7110 sp1.73(63.31)C + 0.7032 sp1.80(64.23)C

0.7923 sp2.50(71.35)C + 0.6101 sp0.00(0.01)H 0.7139 sp1.62(61.76)C + 0.7003 sp1.88(65.21)C 0.7368 sp1.00(99.98)C + 0.6762 sp1.00(99.94)C 0.7882 sp2.43(70.83)C + 0.6154 sp0.00(0.05)H 0.7105 sp1.77(63.85)C + 0.7037 sp1.78(64.05)C

0.7795 sp2.52(71.57)C + 0.6264 sp0.00(0.05)H 0.7070 sp1.79(64.10)C + 0.7073 sp1.78(63.95)C 0.7148 sp1.00(99.95)C + 0.6993 sp1.00(99.95)C 0.7789 sp2.58(72.05)C + 0.6271 sp0.00(0.05)H 0.7041 sp1.77(63.91)C + 0.7101 sp1.76(63.73)C

0.7799 sp2.57(71.95)C + 0.6260 sp0.00(0.05)H 0.7042 sp1.77(63.92)C + 0.7100 sp1.79(64.07)C 0.7144 sp1.00(99.95)C + 0.6998 sp1.00(99.95)C 0.7823 sp2.60(72.18)C + 0.6229 sp0.00(0.04)H 0.7079 sp1.75(63.61)C + 0.7063 sp1.54(60.59)C

51.48

r(C8–H9)

H11) r(C10– C12) p(C10– C12) r(C12– H13) Lone Pairs LP1O1 LP2O1

50.17 47.87

38.77

0.7177 sp2.88(74.11)O + 0.6963 sp2.12(67.86)N 0.7747 sp1.00(99.85)O + 0.6323 sp1.00(99.74)N 0.7901 sp1.80(64.29)N + 0.6129 sp3.15(75.83)C 0.7011 sp1.86(65.01)C + 0.7130 sp1.64(62.06)C 0.6716 sp1.00(99.94)C + 0.7409 sp1.00(99.99)C 0.7077 sp1.76(63.71)C + 0.7065 sp1.80(64.28)C

38.68

0.7825 sp2.62(72.31)C + 0.6226 sp0.00(0.05)H sp0.35(25.82) sp1.00(99.92)

sp0.35(25.74) sp99.99(99.92)

0.7515 sp3.65(78.41)C + 0.6598 sp7.24(87.62)I 0.7885 sp2.47(71.17)C + 0.6151 sp0.00(0.05)H 0.7130 sp1.64(62.06)C + 0.7011 sp1.86(65.01)C

0.7885 sp2.47(71.17)C + 0.6151 sp0.00(0.05)H 0.7077 sp1.76(63.71)C + 0.7065 sp1.80(64.28)C 0.6965 sp1.00(99.94)C + 0.7175 sp1.00(99.95)C 0.7831 sp2.58(72.02)C + 0.6220 sp0.00(0.04)H 0.7083 sp1.75(63.57)C + 0.7059 sp1.55(60.68)C

M.J. Alam, S. Ahmad / Journal of Molecular Structure 1059 (2014) 239–254

Bond (A–B)

0.7059 sp1.55(60.68)C + 0.7083 sp1.75(63.57)C 0.7220 sp1.00(99.97)C + 0.6919 sp1.00(99.95)C 0.7831 sp2.58(72.02)C + 0.6220 sp0.00(0.04)H sp0.35(25.69) sp99.99(99.91) 251

(continued on next page)

sp0.35(25.73) sp99.99(99.92) sp1.00(99.87) sp0.13(11.42) sp1.00(99.99) sp1.00(99.98)

sp0.13 (11.58) sp1.00(99.99) sp1.00(99.98)

€ and j ¼ hjjFjji € where qi is the donor orbital occupancy; i ¼ hijFii € are the off diagonal eleare diagonal elements and Fði; jÞ ¼ hijFjji ments of NBO Fock matrix. This equation describes the donoracceptor interactions in the NBO analysis [67,68]. The natural bond orbital (NBO) analysis has been carried out at the DFT/B3LYP level of theory using NBO 3.1 program implemented in the Gaussian 03 package [43,47]. NBO analysis transforms the delocalized molecular orbitals into localized molecular orbitals that are closely related to chemical bonding concepts. The hybridization of filled orbital is shown in Table 8. The second order perturbation energy values, E(2), corresponding to the important interactions between the electron donors and acceptors, are presented in Table 9. The NBO for a localized bond rpq between atoms p and q is formed from orthonormal hybrids (NHOs), hp and hq, using the relation, rpq = cphp + cqhq where cp and cq are polarization coefficients. The larger value of polarization coefficients represents larger electron density value (%) of NBO which shows higher electronegativity (Table 8). It should be noted that a decrease in occupancy of third lone pair of oxygen and iodine atoms has been found in iodonitrobenzenes. The interaction energies E(2) representing delocalization of electron density between Lewis (bond or lone pair) and non Lewis (anti bond) NBO orbital’s are given in Table 9. The larger the E(2) value, the more intensive is the interaction between electron donors and electron acceptors. The important donor–acceptor interactions between NBO’s with large value of E(2) are n(LP2O1) ? r⁄(O2–N3), n(LP2O1) ? r⁄(N3–C5), n(LP2O2) ? r⁄(O1–N3) and n(LP2O2) ? r⁄(N3–C5) in OINB, MINB and PINB; p(O1–N3) ? n(LP3O2) and n(LP3O2) ? p⁄(O1–N3) in OINB and PINB; n(LP3O1) ? p⁄(O2–N3) and n(LP3I) ? p⁄(C10–C12) in PINB; n(LP3I) ? r⁄(C4–C12) in OINB and n(LP3I) ? p⁄(C4–C12) in MINB.

sp0.34(25.56) sp1.00(99.93) sp99.99(99.55) sp0.14(12.52) sp65.89(98.50) sp99.99(99.96)

OINB

4.9. Other molecular properties

OINB

Abbreviation used: ED – Electron Density.

1.99528 1.97706 1.92651

1.98131 1.89621 1.43956 1.99518 1.97673 1.93736 LP3O1 LP1O2 LP2O2 LP3O2 LP1I LP2I LP3I

1.98138 1.89510 1.43921 1.99547 1.97269 1.92405

1.44227 1.98142 1.89746

OINB MINB OINB

Occupancy

Bond (A–B)

Table 8 (continued)

PINB

EDA%

MINB

PINB

EDB%

MINB

PINB

NBO (% p character)

MINB

PINB

M.J. Alam, S. Ahmad / Journal of Molecular Structure 1059 (2014) 239–254

sp1.00(99.87) sp0.35(25.69) sp99.99(99.91)

252

DFT calculations are efficient tool for designing non-linear optical (NLO) molecules and predicting some related properties such as molecular dipole moments, polarizabilities and hyperpolarizabilities [69–73]. The computation of polarizabilities and hyperpolarizabilities of the organic molecules are of great importance to study the phenomenon induced by intermolecular interactions and nonlinear optical effects. Theoretical molecular polarizability and first order static hyperpolarizability values are reported (available in Supplementary materials). Measurements of electric dipole moment give the understanding about the degree of polarity. This important property is used to study the intermolecular interactions involving the non-bonded type dipole–dipole interactions because higher the dipole moment, stronger will be the intermolecular interactions. The calculated value of the dipole moment of OINB is largest among the investigated molecules. The polarizability and hyperpolarizability describe the response of molecular system in the presence of weak homogeneous electric field. The high values of dipole moment and first order hyperpolarizability are indicating nonlinearity of isomeric iodonitrobenzenes. The lower HOMO–LUMO gap and higher dipole moment of OINB show its higher activity and lesser stability in comparison to MINB and PINB. The order of stability increases as OINB < MINB < PINB. 5. Conclusion The optimization of geometry at B3LYP/6-311G(d,p)/3-21G⁄ level of theory and PES scan reveal that MINB and PINB are planer while OINB is non-planar in geometry due to steric effect. The substitution of iodine at different sites of the nitrobenzene ring leads to significant changes in structures and modes of vibration. It is also found that the N3–C5–C4, N3–C5–C6 and C–C–I bond angles are approximately same in MINB and PINB but they differ in OINB. In

253

M.J. Alam, S. Ahmad / Journal of Molecular Structure 1059 (2014) 239–254 Table 9 Second order perturbation theory analysis of Fock matrix in NBO basis for OINB, MINB and PINB at B3LYP/6-311G(d,p)/3-21G⁄ level of theory. Donor–acceptor interaction

E(2)a (kcal/mol) OINB

MINB

p(O1–N3) ? n(LP3O2) r(O2–N3) ? n(LP3O2) p(O2–N3) ? n(LP3O2) n(LP1O1) ? r⁄(O2–N3) n(LP1O1) ? r⁄(N3–C5)

12.31 1.50

12.66

2.57 4.20 0.63 19.09 13.93 0.52

2.40 4.16

2.48 4.12 18.29 0.99 14.09

2.41 4.15 18.69

159.82 3.35 1.66 1.26

166.93

n(LP2O1) ? n(LP3O2) n(LP2O1) ? r⁄(O2–N3) n(LP2O1) ? r⁄(N3–C5) n(LP2O1) ? r⁄(C5–C6) n(LP2O1) ? r⁄(C4–C12) n(LP2O1) ? r⁄(O1–N3) n(LP1O2) ? r⁄(O1–N3) n(LP1O2) ? r⁄(N3–C5) n(LP2O2) ? r⁄(O1–N3) n(LP2O2) ? p⁄(O1–N3) n(LP2O2) ? r⁄(N3–C5) n(LP2O2) ? r⁄(C5–C4) n(LP2O2) ? r⁄(C8–C6) n(LP3O1) ? p⁄(O2–N3) n(LP3O2) ? p⁄(O1–N3) n(LP3O2) ? r⁄(O2–N3) n(LP1I) ? r⁄(C4–C5) n(LP1I) ? r⁄(C4–C12) n(LP1I) ? r⁄(C10–C8) n(LP2I) ? r⁄(C10–C8) n(LP2I) ? r⁄(C4–C5) n(LP2I) ? r⁄(C4–C12) n(LP2I) ? r⁄(C10–C12) n(LP1I) ? r⁄(C10–C12) n(LP3I) ? r⁄(C4–C12) n(LP3I) ? p⁄(C4–C12) n(LP3I) ? p⁄(C10–C12) a b c

E(j)–E(i)b (a.u.)

18.74 13.89 0.68 0.52

13.83 0.70 0.54

PINB

12.56 2.36 4.20 18.75 13.56 0.69 0.55 2.36 4.20 18.75 13.56 0.69 0.55 166.29

1.36

OINB

MINB

0.19 0.81

0.18

1.22 1.05 0.02 0.72 0.55 0.83

1.22 1.05

1.22 1.05 0.72 0.17 0.55

1.23 1.05 0.73

0.15 0.70 1.39 1.40

0.14

0.73 0.56 0.84 0.84

0.56 0.84 0.85

2.67 2.59 1.43

2.63 1.41

8.17

PINB

0.18 1.22 1.06 0.73 0.56 0.84 0.85 1.22 1.06 0.73 0.56 0.84 0.85 0.14

1.43

1.41 2.63 3.92 1.73 0.53

F(i, j)c (a.u.) OINB

MINB

0.080 0.058

0.079

0.050 0.061 0.005 0.106 0.078 0.019

0.049 0.061

0.050 0.060 0.104 0.013 0.079

0.049 0.060 0.105

0.141 0.050 0.043 0.038

0.140

0.106 0.079 0.022 0.019

0.078 0.022 0.020

0.82 0.82 1.43

0.82 1.43

0.26 8.02

0.105 0.078 0.022 0.020 0.048 0.061 0.105 0.078 0.022 0.020 0.140

0.040 0.041 0.050 0.034 0.019

0.042 0.041 0.041

0.041 0.040

0.045 0.27

8.44

0.079 0.048 0.061

0.040

1.43 0.82 0.80 0.81 0.83

PINB

0.045 0.27

0.046

Energy of hyper conjugative interaction (stabilization energy). Energy difference between donor and acceptor i and j NBO orbitals. Fock matrix element between i and j NBO orbitals.

addition to fundamentals and combinations, some important vibrations such as Kekule, ring breathing, Star of David and ring twisting vibrations in OINB, MINB and PINB are assigned. The calculated harmonic frequencies for higher modes have shown large deviations from the experiment than those obtained in anharmonic approximation. Good agreement between the experimental and simulated anharmonic vibrational spectra has been achieved without any scaling. The differences between anharmonic and experimental wavenumbers for some modes are found large which may be either due to limitations of the calculations or neglect of higher order potential energy terms in PT2 approach. The PT2 method with 6-311G(d,p) and 3-21G⁄ basis set have performed reasonably well in reproducing the experimental spectra. The studies demonstrate that the 3-21G⁄ basis set is suitable for iodine atom in iodo-substituted molecules to understand their molecular properties as well as spectra. Electrostatic potential contour maps show that negative regions are localized on surface around the oxygen while positive regions are confined over hydrogen atoms. The surfaces over the aromatic ring and iodine atom are neutral in the all iodonitrobenzenes. The important donor-acceptor interactions between NBO’s are n(LP3O2) ? p⁄(O1–N3) in OINB and PINB and n(LP3O1) ? p⁄(O2– N3) in MINB with larger two electron stabilization energy (E(2)). The value of HOMO–LUMO energy gap for the OINB is least that attributes to high reactivity the molecule.

Acknowledgments We gratefully acknowledge the UGC (DRS) and DST (FIST) for the award of Grants for FTIR and UV–Vis-NIR spectrophotometer.

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