Chapter – V

Analysis of 5-chloro-1-methyl-4-nitro-1H-imidazole (CMNI)

126

CHAPTER-V

INVESTIGATION OF MOLECULAR STRUCTURE VIBRATIONAL,

ELECTRONIC, NMR AND NBO ANALYSIS OF 5-CHLORO-1-

METHYL-4-NITRO-1H-IMIDAZOLE (CMNI) USING

AB INITIO HF AND DFT CALCULATIONS

ABSTRACT

This study represents the vibrational, electronic, NMR, NLO and structural

aspects of 5-chloro-1-methyl-4-nitro-1H-imidazole (CMNI). A detailed

interpretation of the FT-IR, FT-Raman, UV and NMR spectra were reported.

Theoretical calculations were performed by ab initio HF and density functional

theory (DFT)/B3LYP method using 6-311+G(d,p) basis sets. The electronic

properties was also studied and the most prominent transition corresponds to π→π*.

The lower frontier orbital gap of CMNI explains the eventual charge transfer

interaction taking place within the molecule. The stability and charge delocalization

of the molecule was studied by natural bond orbital (NBO) analysis. CMNI

exhibited good nonlinear optical activity and was 11 times greater than that of urea.

In addition, a molecular electrostatic potential map (MEP) of the title compound was

studied for predicting the reactive sites.

127

CHAPTER-V

INVESTIGATION OF MOLECULAR STRUCTURE VIBRATIONAL,

ELECTRONIC, NMR AND NBO ANALYSIS OF 5-CHLORO-1-

METHYL-4-NITRO-1H-IMIDAZOLE (CMNI) USING

AB INITIO HF AND DFT CALCULATIONS

5.1. Introduction

The imidazole scaffold is forecasted to be a good pharmacophore and

represents an important synthetic precursor in new drug discovery [1–3]. These

scaffolds play very important role as mediators in synthetic reactions, primarily for

preparing functionalized materials [4–6]. Imidazole nucleus forms the main structure

of some well known components of the amino acid (histidine), vitamin B12, DNA,

purines, histamine and biotin [7,8]. Recently heterocyclic imidazole derivatives have

attracted considerable attention exhibiting various biological applications as

antitumor [9], anti-HIV [10], antimicrobial [11], anticonvulsant [12], antitubercular

[13], anti-inflammatory, FTase and MAP kinase p38 inhibitory activities [14, 15]. In

particular, nitroimidazoles exhibit interesting therapeutic applications as anaerobic

antibacterials [1], antiprotozoal agents [16], radiosensitizers [17,18] and hypoxia

detecting chemosensitizers. Recently, nitroimidazole derivatives have emerged as

multifunctional “drug-biomarker monitors” with importance in treatment of tumors,

monitoring hypoxia [19] and safer imaging contrast agents to monitor the

therapeutic progress. 5-nitroimidazole derivatives have been tested in cell-based

assays and in enzyme assays against HIV-1 recombinant reverse transcriptase [20,

21]. They also constitute a major class of naturally occurring compounds, such as

128

theophylline molecules, which is extracted from tea leaves and coffee beans, which

stimulates the central nervous system [22]. N-alkyl derivatives of imidazole were

identified as potential candidates for the antibacterial agents and also possess

inhibitory effect on microsomal oxidation and cytotoxic activity [23]. Imidazole

derivatives also display potent biological applications as glucagon receptors and

biomimetic applications [24] CBI cannabinoid receptor antagonists [25], anti-filarial

agent [26], analgesic , cardiovascular , anthelmintic and anti-ulcer activities [27].

In addition, imidazoles have been used extensively as a corrosion inhibitors,

fire retardant, powerful explosives, photography, dyes and agricultural chemicals

[22, 28]. They also find applications in biochemical processes, catalysis, electron

transport systems, and nanoparticle electron synthesis [29]. Furthermore, imidazole

shows several commercial applications such as epoxy curing agents, adhesives,

plastic modifiers and optical data storage [30]. The imidazole ring can be easily

tailored to accommodate functional groups which allow the NLO chromophores into

NLO side chain polymers [27]. Imidazole-based ionic liquids and ionic liquid

monomers utilized in emerging technologies [31]. Recently etal investigated

protonated imidazole dimers are the basic components for the fuel cells [32].

Koji Hasegawa et al. [33] have reported vibrational spectra and ab initio

DFT calculations of 4-methylimidazole and its different protonated forms. Literature

is enriched with lot of work on synthesis of potent substituted imidazole derivatives

with diverse pharmacological activities [34, 35]. Most of the research works were

focused on the determination of state of protonation on the imidazole nitrogen atoms

[36-38] or on the type of metal ion binding [39-41] and only a few reports on

vibrational spectroscopic studies. Detailed knowledge on the structure and spectral

129

behavior of imidazoles is a necessary prerequisite for understanding its chemical and

biological properties. Herein we describe our results on 5-chloro-1-methyl-4-nitro-

1H-imidazole (CMNI) concerning the structural, vibrational, electronic, NMR, NLO

and NBO analyses through spectral measurements. Theoretical calculations were

carried out by HF and DFT (B3LYP) with the use of 6-311G+(d,p) basis sets. The

calculated results were compared with the experimental and the observed spectra

were analyzed in detail.

5.2. Materials and methods

5-chloro-1-methyl-4-nitro-1H-imidazole (CMNI) was purchased from Sigma

Aldrich Chemical Company with a stated purity of 99 %. IR measurements of

CMNI were carried out using KBr pellets on a Varian 7000 series FT-IR-Raman

spectrometer equipped with a Varian UMA600 microscope (Stingray system) which

can be controlled by resolutions Pro™ software. The spectral resolution was 2 cm-1

over a spectral range of 3500 - 500 cm-1 using a high sensitivity MCT detector. FT-

Raman spectrum was recorded in the region 100 - 3500 cm-1with Nd:YAG laser

(1064 nm excitation) operating at a scanning speed of 90 spectra/second at 8 per cm

data point resolution. The ultraviolet absorption spectrum of CMNI was examined in

the range 200 – 800 nm using Shimadzu UV-2401 PC UV–VIS recording

spectrometer. The UV pattern is taken from a 10-5 M solution of CMNI in water.

Data was analyzed by UV PC personal spectroscopy software, version 3.91. 1H

NMR and 13C NMR spectra were recorded in DMSO-d6 using TMS as an internal

standard on a Bruker high-resolution NMR spectrometer at 400 MHz and 100 MHz

respectively.

130

5.3. Computational details

The geometry of CMNI was fully optimized without any constraint in the

potential energy surface at Hartree-Fock level, adopting the standard 6-311+G(d,p)

basis set. This geometry was then re-optimized again at B3LYP level, using 6-

311+G(d,p) basis set for better description. The optimized structural parameters

were used in the vibrational frequency calculations at the HF and DFT levels to

characterize all stationary points as minima. Then vibrationally averaged nuclear

positions of this compound were used for harmonic vibrational frequency

calculations resulting in IR & Raman frequency together with intensities. The

stability of the optimized geometries was confirmed by frequency calculations,

which gave positive values for all the obtained frequencies. The entire set of

calculations was performed on a Pentium IV/3.06 GHz personal computer using the

Gaussian 03 program package [42]. Assignment of the calculated wavenumbers was

aided by the animation option of the Guass -View 3.0TM graphical interface [43].

However, the vibrational frequency values computed at these levels contain known

systematic errors [44]. Therefore, it is customary to scale down the calculated

harmonic frequencies in order to improve the agreement with the experimental. UV

absorption energies of this compound were calculated by TD-DFT method in water

solvent. A computed (Guassian TD-DFT output data) transition was plotted on the

experimental spectra using Chemission software. The natural bonding orbitals

(NBO) calculations [45] were performed using NBO 3.1 program as implemented in

the Gaussian 03W package at the DFT/B3LYP level. The second order Fock matrix

was carried out to evaluate the donor-acceptor interactions in the NBO basis [46].

131

For each donor (i) and acceptor (j), the stabilization energy E(2) associated with the

delocalization i→j is estimated as

(5.1)

To investigate the reactive sites of the title compound, the molecular

electrostatic potentials for the 0.02 a.u. isosurfaces of electron density was evaluated

using the B3LYP/6-311+G(d,p) method. The nuclear magnetic resonance (NMR)

chemical shifts calculations were performed using Gauge-Included Atomic Orbital

(GIAO) method [47,48] at B3LYP/6-311+G(d,p) level and the 1H and

13C isotropic

chemical shifts were referenced to the corresponding values for TMS, which was

calculated at the same level of theory. The effect of solvent on the theoretical NMR

parameters was included using default IEF-PCM model provided by GAUSSIAN

03. Dimethylsulfoxide (DMSO), which has a dielectric constant (ε) of 46.7, was

used as the solvent.

5.4. Results and discussion

5.4.1. Molecular Geometry

The molecular structure of CMNI and its atom numbering scheme is shown

in Fig. 5.1. The global energy minimum obtained by DFT of the structure

optimization was -929.77573863 a.u. and showed that the molecule belongs to Cs

symmetry point group. The optimized structural parameters of CMNI calculated at

the HF and B3LYP level of theory using 6-311+G(d,p) basis sets are listed in Table

5.1 and compared with the crystallographic geometrical data reported for 5-chloro-

1,2-dimethyl-4-nitro-1H-imidazole [49] and 4-methyl imidazole [33] respectively.

The results revealed that the calculated values were slightly deviated with the

132

experimental. These deviations may be attributed to the solid state intermolecular

interactions related to crystal packing effects.

The imidazole ring was predicted to have planar structure and all the

substituents were coplanar with the ring. This result was supported by the fact that

the computed torsional angles for N5-C1-N2-C3, C1-N2-C3-C4, N2-C3-C4-N5, C3-

C4-N5-C1,N2-C1-N5-C4, N2-C3-C4-Cl10, C1-N5-C4-Cl10, C4-C3-N6-O8, C4-

N5-C9-H14, N2-C3-N6-O7 and N6-C3-C4-Cl10 were either 0⁰ or 180⁰ and further,

in good agreement with the crystallographic data [49]. The C4-Cl10 bond length

1.709 Å calculated by DFT method was in accordance with the literature value [50].

It is interesting to note that the bond lengths of N6-O7 and N6-O8 (nitro group)

were 1.237 Å and 1.227 Å respectively, and the difference in bond length of 0.01 Å

was due to the repulsion between the lone pair of electron on the oxygen and the

nitrogen atoms [51].

5.4.2. Vibrational assignments

The CMNI under consideration belongs to Cs point group symmetry and has

36 normal modes of fundamental vibrations. The normal vibrations are active both

in IR and Raman and are distributed as 24 in-plane vibrations of A′ species and 12

out-of-plane vibrations of A″ species, i.e. Гvib=24A′+12A″. The assignments of

vibrational frequencies of this compound based on the normal mode analysis are

presented in Table 5.2. The experimental and theoretically predicted FT-IR and FT-

Raman spectra for CMNI are shown in Figs. 5.2-5.3.

It is important to mention that the theoretical results refer to the isolated

molecule in the gas phase, while comparisons were made with FT-IR and FT-Raman

spectra of the solid sample. In general, the IR gas-phase spectra were found to be

133

advantageous over the solution or solid phase because the symmetry of most

vibrational coordinates is easily determined from the rotational profiles of the IR

bands, and hydrogen bond formation and other inter-molecular interactions are

mostly avoided. Comparison of the frequencies calculated at HF and B3LYP with

the experimental (Table 2) reveals the overestimation of the calculated vibrational

modes due to neglect of anharmonicity in real system. Calculated wavenumbers

were corrected by linear scaling method. Vibrational frequencies calculated at

B3LYP and HF levels were scaled by 0.96 [52] and 0.89 [53] respectively.

Comparison of the wavenumbers calculated by HF and DFT methods using 6-

311+G(d,p) basis set with experimental values reveals both the methods show a very

good agreement.

Fig. 5.1. The optimized structure of CMNI.

134

Fig. 5.2. Comparision of the calculated and experimental

FT-IR spectra of CMNI

135

D :\sridev i\k frh\pa pe r w ork \da ta fo r tab le s\imidazole raman 3\im idazole ram an 4 .ti f

Fig. 5.3. Comparision of the calculated and experimental FT-Raman spectra of CMNI

136

Table 5.1. Optimized and experimental geometric data for CMNI in the ground state

Parameters

Calculated

Experimental [49,33]

HF/6311+G(d,p) B3LYP/6311G+(d,p)

Bond lengths(A⁰) C1-N2 1.283 1.310 1.313 C1-N5 1.357 1.375 1.364 N2-C3 1.351 1.361 1.381 C3-C4 1.358 1.387 1.372 C3-N6 1.438 1.442 - C4-N5 1.358 1.374 1.383 C4-Cl10 1.700 1.709 - N5-C9 1.454 1.459 - N6-O7 1.192 1.237 - N6-O8 1.180 1.227 - Bond angles(⁰) N2-C1-N5 112.7 112.5 113.7(18) C1-N2-C3 104.9 105.0 102.7(2) N2-C3-C4 111.0 111.0 - N2-C3-N6 121.3 121.5 121.5(2) C4-C3-N6 127.6 127.3 125.6(2) C3-C4-N5 105.1 105.1 106.4(2) C4-C3-Cl10 133.7 133.7 - N5-C4-Cl10 121.1 121.1 - C1-N5-C9 126.6 126.8 - C1-N5-C4 106.1 106.2 - C4-N5-C9 127.2 126.8 - C3-N6-O7 116.6 116.6 119.0(2) C3-N6-O8 117.7 118.0 117.0(2) O7-N6-O8 125.6 125.3 - Torsion angles(⁰) N5-C1-N2-C3 0.0 0.0 - N2-C1-N5-C4 0 0.0 - N2-C1-N5-C9 180.0 179.9 - C1-N2-C3-C4 0.0 0.0 - C1-N2-C3-N6 180.0 180.0 - N2-C3-C4-N5 0.0 0.0 - N2-C3-C4-Cl10 180.0 180.0 - N6-C3-C4-N5 180.0 180.0 - N6-C3-C4-Cl10 0.0 0.0 - N2-C3-N6-O7 180.0 180.0 180.0 N2-C3-N6-O8 0.0 0.0 0.0 C4-C3-N6-O7 0.0 0.0 - C4-C3-N6-O8 180.0 180.0 - C3-C4-N5-C1 0.0 0.0 - C3-C4-N5-C9 180.0 180.0 - Cl10-C4-N5-C1 180.0 180.0 - Cl10-C4-N5-C9 0.0 0.0 -

137

Table 5.2. The observed FT-IR,FT-Raman and calculated frequencies using HF/6-311+G(d,p) andB3LYP/6-311+G(d,p) for CMNI

Symmetry

Species

Cs

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

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

Experimental

Vibrational

Assignments Scaled

Freq (cm-1)

IIR

(Km

mol-1)

IRa

(Å4

/amu-1)

Scaled

Freq (cm-1)

IIR

(Km

mol-1)

IRa

(Å4/ amu-1)

FT-

IR (cm-1)

FT-

Raman (cm-1)

A″ 38 0 1.0 41 0 0.7 η(NO2) A″ 101 0.5 0.1 99 0.4 0.2 t(CH3) A″ 126 0.1 0 125 0.1 0.1 t(CH3) A′ 172 0.4 0.2 167 0.1 0.3 150 ρ( NO2)β(C-Cl) A′ 228 1.0 0.9 226 0.9 1.0 β(C-Cl) A″ 243 0.4 0.9 237 0.1 0.5 t(CH3)π(ring) A″ 258 0.6 0.9 253 0.7 0.8 254 t(CH3)π(C-Cl) A′ 361 0.3 2.6 354 0.6 2.6 β(CH3)ρ( NO2) A′ 393 4.1 2.5 386 1.1 2.9 374 δ (NO2) A′ 502 3.5 6.6 500 3.8 5.9 501 516 β(CH3)νs(C-Cl) A′ 573 1.4 1.6 566 0.6 3.6 576 β (NO2) A″ 635 10.3 0.2 609 0.5 0.1 605 η(ring) A″ 651 16.5 0.9 636 17.6 0.3 648 η(ring) A′ 704 3.8 12.2 702 3.6 12.5 β (CN ) A″ 773 7.8 3.0 732 2.5 1.0 727 717 ω( NO2) A″ 847 100.1 3.5 788 12.6 1.0 756 π (C-H) A′ 869 8.9 0.9 812 73.2 4.8 871 825 δ (NO2) A′ 1001 17.8 1.6 980 19.4 2.9 935 β (ring) β (C-H) A′ 1036 0.3 8.8 1028 1.4 10.4 1018 1014 ρ (CH3) A′ 1108 37.7 1.4 1098 37.3 2.2 1103 1056 ρ (CH3) A″ 1115 0.7 0.9 1100 0 0.6 1126 1134 π (CH3) A′ 1216 27.1 6.9 1198 8.5 3.5 1269 1226 β (C-H) A′ 1286 65.7 23.2 1271 62.0 10.0 ν s (C-N) A′ 1316 36.2 49.9 1299 76.1 121.2 ν s (C-N)β (CH3)

A′ 1380 88.0 15.9 1318 232.0 88.9 1319 ν s (NO2) ν s (C-N)

A′ 1419 4.8 13.5 1360 7.7 59.2 1357 β(CH3)ν s (C-N) A′ 1433 11.4 9.4 1405 10.4 12.6 1409 1381 β(CH3) A″ 1450 161.9 82.1 1425 13.1 9.7 π (CH3) A′ 1457 7.0 12.2 1442 32.5 36.3 β (CH3) ν s (C=C) A′ 1512 166.0 91.1 1463 53.4 33.0 1463 1473 β (CH3)ν s (C=C) A′ 1530 47.3 10.2 1475 42.9 5.1 1506 β(CH3)ν s(C=N)

A′

1624

701.5

19.8

1530

337.2

28.9

1541

1539 ν as(NO2) ν s

(C3=C4) A′ 2864 26.8 139.5 2934 18.7 174.0 2900 2962 ν s (CH3) A″ 2932 11.0 51.5 3001 5.6 58.7 2980 ν as (CH3) A′ 2945 9.3 51.1 3023 4.2 54.5 3066 ν s(CH3) A′ 3043 0.3 75.2 3115 0.5 96.5 3116 ν s(C-H)

138

5.4.2.1. Methyl group vibrations

The bands observed at 2980 and 3066 cm-1

in FT-IR was assigned to methyl

stretching mode which was also supported by the literature [54]. The in-plane

bending mode (β- CH3) was observed at 1409 and 1381 cm-1 in FT-IR and FT-

Raman respectively. Peaks at 1463 & 1473 cm-1 in FT-IR and FT-Raman spectrum

was attributed to out-of-plane bending vibration. This was also in agreement with

the literature data [55]. The rocking vibrations of CH3 group are generally observed

in the region 1070 -1010 cm-1 [56]. This mode appears at 1018 and 1014 cm-1 in FT-

IR and FT-Raman spectrum which is in agreement with the calculated value 1036

cm-1 and 1028 cm-1. The twisting modes was not observed in the FT-IR because they

appear at very low frequency. The scaling procedure predicts that these vibrations

could appear around 99, 125, 237 and 253 cm-1

. The FT-Raman band observed at

254 cm-1 showed an excellent agreement with the computed value.

5.4.2.2. NO2 group vibrations

Aromatic nitro compounds showed strong absorption due to the asymmetric

and symmetric vibrations of NO2 group at 1570-1485 cm-1 and 1370-1320 cm-1

respectively [57]. FT-IR and FT-Raman bands at 1541 and 1539 cm-1

were

designated as asymmetric stretching modes of NO2 group. The symmetric stretching

mode was observed at 1319 cm-1 in FT-Raman and coincides with the theoretically

computed value 1318 cm-1. The deformation vibrations of NO2 group (scissoring,

wagging, rocking and twisting) contribute to several normal modes in the low

frequency region [58]. A strong band at 871 and 825 cm-1

in FT-IR and FT-Raman

was assigned to NO2 scissoring mode [59]. NO2 wagging was observed at 727 and

717 cm-1 in FT-IR and FT-Raman respectively. The theoretically calculated value of

139

732 cm-1 coincides exactly with the experimental. The FT-Raman band observed at

374 cm-1

corresponds to NO2 rocking vibration while theoretically computed value

was 386 cm-1. The predicted frequency 38 cm-1 is assigned to torsion vibration of

NO2 group [60].

5.4.2.3. C-Cl vibrations

The vibrations due to the halogen atom attached to aromatic ring are worth to

discuss here, since mixing of the vibrations is possible due to the lowering of the

molecular symmetry and the presence of heavy atom [61]. Mooney assigned

vibrations of C-X group (X=Cl, Br and I) in the frequency range 1129-480 cm-1 [62].

C-Cl stretching vibration was observed at 501 and 516 cm-1 in FT-IR and FT-Raman

and was in good agreement with the theoretically calculated value. Sundaraganesan

et al. reported C-Cl in-plane deformation and out-of-plane deformation bands at 160

and 250 cm-1 respectively [63]. In our present study, the FT-Raman band observed

at 150 cm-1 was assigned to C-Cl in-plane bending vibration which was in good

agreement with the theoretically calculated value 167 cm-1. C-Cl out-of-plane

bending vibration was observed at 254 cm-1 in FT-Raman, which correlates exactly

with the calculated value.

5.4.2.4. Aromatic C-H vibrations

In aromatic compounds, C-H stretching band appear in the range 3100-3000

cm-1, while C-H in-plane and out-of-plane bending vibrations were observed in the

range 1300-1000 cm-1and 900-675 cm-1 [56] respectively. The FT-Raman band

observed at 3116 cm-1

was due to aromatic C-H symmetric stretching vibrations

which correlated well with the theoretically calculated value 3115 cm-1. C-H in-

plane bending vibrations observed at 1269 and 1226 cm-1 in FT-IR and FT-Raman

140

was in agreement with the literature values [64] while the out-of-plane bending

vibration appeared at 756 cm-1

.

5.4.2.5. Ring C-C and C-N vibrations

The ring C-C and C-N stretching vibrations occur in the wide range from

1640-900 cm-1 [26]. These vibrations often couple with each other and with various

other deformation vibrations generating complex modes. Koji Hasegawa et al. [26]

reported ring C=C stretching vibrations in the range 1635-1532 cm-1

. The highest

frequency mode that occurred at 1541 and 1539 cm-1 in FT-IR and FT-Raman was

assigned to C=C stretching vibrations. The theoretically calculated value at 1530 cm-

1 showed excellent agreement with the experimental. Bands at 1506 and 1319 cm-1

in FT-IR and FT-Raman was assigned to C=N and C-N stretching vibrations

respectively. The computed value of C-N stretching vibration at 1318 cm-1

and C=N

stretching vibration at 1475 cm-1 correlates exactly with the experimental. The pure

ring deformation mode occurs generally at 943-923 cm-1 [64]. In the FT-IR

spectrum, ring deformation mode was observed at 935 cm-1. The signals at 605 cm-

1and 648 cm-1 was assigned to ring torsion modes and was in good agreement with

the literature values [64]. The theoretically calculated values at 609 and 636 cm-1

showed excellent correlation with the experimental.

5.4.3. Electronic properties

The electronic absorption spectrum of the title compound in water solvent

was recorded within the 200 - 800 nm range and the representative spectrum of

computed transitions on the experimental is shown in Fig. 5.4. Electronic absorption

wavelengths calculated by the TD(DFT)/B3LYP/6-311+G(d,p) level and the

experimental value, excitation energy, oscillator strength and composition of the

141

most significant singlet excited states are listed in Table 5.3. The transitions were

found to be monodeteminantal. As can be seen from the figure 4 the calculated TD-

DFT spectrum of CMNI corresponds well with the experimental.

The energy levels of the HOMO and LUMO orbitals computed at the

B3LYP/6-311+G(d,p) level for the title compound is represented in Fig. 5.5.

Numerous applications are available for the use of the HOMO-LUMO gap as a

quantum descriptor in establishing correlation in various chemical and biochemical

systems [65, 66]. The frontier orbitals, HOMO and LUMO helps to characterize the

chemical reactivity and kinetic stability of the molecule [67]. A molecule with a low

frontier orbital gap is more polarizable and will exhibit a significant degree of ICT

from the electron-donor groups to the electron-accepter groups through π conjugated

path [68]. As shown in Figure 5 the LUMO orbitals are distributed over the

imidazole ring and nitro group. In contrast, HOMO is located only on the imidazole

ring, and consequently HOMO→ LUMO transition implies electron density transfer

from the imidazole ring to the nitro group. This explains a significant degree of

intramolecular charge transfer from the electron-donor groups to electron-acceptor

groups through conjugated path. In addition the lower frontier orbital gap (∆E(H-L) =-

4.42eV) of CMNI indicates the eventual charge transfer interaction taking place

within the molecule. Molecular orbital coefficients analysis showed that the frontier

molecular orbital (FMOs) are composed mainly of p-atomic orbitals. Hence, the

electronic spectrum corresponding to electronic transitions are mainly π→π*. The

lower frontier orbital energy gap (-4.42 eV) and high dipole moment (11.00 D)

illustrates the high reactivity of CMNI (Table 3).

142

Fig. 5.4. Experimental Vs.TD-DFT UV spectra of CMNI

LUMO PLOT

(First excited state)

ELUMO = -2.8996 eV

∆E = -4.4208 eV

EHOMO = -7.3204 eV

HOMO PLOT

(Ground state)

Fig. 5.5. Molecular orbital surfaces for the HOMO and LUMO of CMNI

143

Table 5.3. Wavelength λ (nm), excitation energies (eV), oscillator strength (f),

absolute energies (Hartree), frontier orbital energies (eV), dipole moment (D)

and composition of the most significant singlet excited states for CMNI at the

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

Transition

Experimental TD(DFT) Method

Symmetry

MOs

λ(nm) λ(nm)

Excitation energy

E (eV)

Oscillator strength

f

I 313 315 3.92 0.1896 Singlet A 85%(H→L)

EHOMO -7.32 ELUMO -2.90 ∆E(H-L) -4.42 Etotal -929.796645

Dipole

moment 11.00

Table 5.4. Second order perturbation theory analysis of

Fock matrix in NBO basis

Donor (i)

ED

(e)

Acceptor (j)

ED

(e)

E(2)a

(KJ mol-

1)

E(i)-E(j)b

(KJ mol-1

)

F(i,j)c

(KJ mol-

1) ζ(C1-N2) 1.9792 ζ*(C3-N6) 0.10848 24.18 3019.33 204.79 π (C3-C4) 1.7923 π*(C1-N2) 0.34693 65.27 787.65 165.41 π (C3-C4) 1.7923 π*(N6-O8) 0.62553 79.54 498.85 178.53 π (C1-N2) 1.8700 π*(C3-C4) 0.42455 95.19 813.91 212.67 n(LP1N5) 1.5408 π*(C3-C4) 0.42455 166.73 708.89 246.80 n(LP1N5) 1.5408 π*(C1-N2) 0.34693 176.9 735.14 262.55 n(LP2O7) 1.9020 π*(N6-O8) 0.62553 78.16 1916.62 278.30 n(LP2O7) 1.9020 ζ*(C3-N6) 0.10848 47.03 1522.79 189.04 n(LP3O7) 1.4680 π*(N6-O8) 0.62553 642.2 367.57 357.07 n(LP2O8) 1.8913 π*(N6-O7) 0.05739 80.25 1864.11 275.68 n(LP2O8) 1.8913 ζ*(C3-N6) 0.10848 56.61 1496.54 207.41

aE(2) means energy of hyperconjugative interactions; cf. Eq. (1).

b Energy difference between donor and acceptor i and j NBO orbitals. c

F(i,j) is the Fock matrix element between i and j NBO orbitals

5.4.4. NBO analysis

Natural bond orbital analysis provides an efficient method for studying intra-

and intermolecular bonding and interaction among bonds, and also provides a

144

convenient basis for investigating charge transfer or conjugative interaction in

molecular systems [69]. The larger the E(2) value, the more intensive is the

interaction between electron donors and electron acceptors, i.e. the more donating

tendency from electron donors to electron acceptors and the greater the extent of

conjugation of the whole system. Delocalization of electron density between

occupied Lewis-type (bond or lone pair) NBO orbitals and formally unoccupied

(antibond or Rydgberg) non-Lewis NBO orbitals correspond to a stabilizing donor-

acceptor interaction. NBO analysis has been performed on the title molecule

(CMNI) at the B3LYP/6-311+G(d,p) level in order to elucidate the delocalization of

electron density within the molecule. The intramolecular interaction are formed by

the orbital overlap between bonding n(LP1N5) and antibond π*(C3-C4), π* (C1-N2)

orbital which results in intramolecular charge transfer (ICT) causing stabilization of

the system. These interactions are observed as increase in electron density (ED) in

C3-C4, C1-N2 anti-bonding orbital that weakens the respective bonds. The second-

order perturbation theory of Fock matrix in the NBO analysis shows strong

intramolecular hyperconjugative interactions, and the results are presented in Table

5.4. The most important interaction energies of n(LP N5)→π*(C1-N2) and n(LP

N5)→π*(C3-C4) are 176.90 and 166.73 KJ/mol respectively. This larger energy

provides the stabilization to the molecular structure and will also enhances the

bioactivity of the molecule (CMNI).

5.4.5. Molecular electrostatic potential (MEP)

The electrostatic potential has been used primarily for predicting sites and

relative reactivities towards electrophilic attack, and in studies of biological

recognition and hydrogen bonding interactions [70, 71]. To predict reactive sites for

145

electrophilic and nucleophilic attack for the investigated molecule, MEP studies was

carried out by B3LYP using 6-311+G(d,p) basis set. The negative (red and yellow)

regions of the MEP are related to electrophilic reactivity and the positive (blue)

regions to nucleophilic reactivity, as shown in Fig. 5.6. The electrostatic potential

ranges from -97.891 to +97.891 Kcal/mol with dark blue denoting extremely

electron-deficient regions (V(r) > 97.891 Kcal/mol) and red denoting electron rich

regions (V(r) < -97.891 Kcal/mol). As can be seen from the figure, negative region

is mainly localized over the N2 atom of the imidazole ring and oxygen atoms of

nitro group. The maximum positive region is localized on the C-H bonds of the

methyl group and C1 of the imidazole ring with a value of +97.891 Kcal/mol,

indicating a possible site for nucleophilic attack. According to these calculated

results, the MEP map shows that the negative potential sites are on electronegative

N2 atom of the imidazole ring and on oxygen atoms of the nitro group and the

positive potential sites are around C1 of the imidazole ring and the hydrogen atoms

of methyl group. These sites give information about the region from where the

compound can have intermolecular interactions. Thus, it would be predicted that the

nitrogen N2 and C1 of imidazole ring will be the most reactive site for both

electrophilic and nucleophilic attack and this is also in support with the literature

[72].

5.4.6. Nonlinear Optical Effects

NLO is at the forefront of current research because it provides the key

functions of frequency shifting, optical modulation, optical switching, optical logic,

and optical memory for the emerging technologies in areas such as

telecommunications, signal processing, and optical interconnections [73, 74]. In

146

discussing nonlinear optical properties, the polarization of the molecule by an

external radiation field is often approximated as the creation of an induced dipole

moment by an external electric field. The first hyperpolarizability (β0) of this

molecular system is calculated using B3LYP/6-311+G(d,p)method, based on the

finite field approach. The calculated hyperpolarizability values of CMNI are given

in Table 5.5. Urea is one of the prototypical molecules used in the study of the NLO

properties of molecular systems and frequently used as a threshold value for

comparative purposes. The computed first hyperpolarizability, βtot of CMNI

molecule is 33.9985 x 10-31 cm5/esu and is eleven times more than urea (β of urea is

3.7289 × 10-31 cm5/esu). Thus, this molecule might serve as a prospective building

block for nonlinear optical materials.

5.4.7. NMR spectra

The experimental and theoretical 1H and 13C NMR values for the title

compound are represented in Table 5.6. The theoretical calculations was performed

by Gauge-Including atomic orbital (GIAO) method [47] using B3PLYP/ 6-

311+G(d,p) basis set. The experimental 1H NMR spectrum of CMNI in DMSO is

shown in Fig. 5.7. In 13

C NMR spectrum shown in fig. 5.8, four different signals

were observed , which is consistent with the structure on the basis of molecular

symmetry. Signals for aromatic carbons were observed in the range 120 - 141 ppm

as expected. The chloro and nitro groups are highly electronegative and decreases

the electron density at the ring carbon. Therefore, the chemical shifts value of C(3)

bonded to nitro group shows very high value. In the 1H NMR spectrum, methyl and

aromatic protons showed characteristic signals at 3.68 and 7.96 ppm respectively.

147

1H-13C HMBC spectrum is shown in fig. 5.9 for the clear assignment of NMR

signals.

Fig. 5.6. Electrostatic potentials of CMNI (B3LYP/6-311+G(d,p), 0.02 a.u.,

energy values-97.891 to +97.891 Kcal/mol); color coding: red (very negative),

orange (negative), yellow (slightly negative), green (neutral), turquoise (slightly

positive), light blue (positive), dark blue (very positive).

Fig. 5.7. 1H NMR spectrum of CMNI

148

Fig. 5.8. 13C NMR spectrum of CMNI

Fig. 5.9. 1H-13C HMBC spectrum of CMNI

149

Table 5.5. The first hyperpolarizability βijk and

βtot (×10−31 cm5/esu) values for CMNI

Atom Experimental HF B3LYP

C1 136.3 136.4 140.4 C3 141.2 138.1 151.5 C4 120.0 128.9 136.6 C9 32.5 20.8 33.1 H11 7.96 7.86 7.58

H12,13,14 3.68 3.49 3.65

Table 5.6. Experimental and theoretical 1H and

13C

NMR chemical shifts of CMNI

βxxx 44.4554 βxxy 7.5207 βxxz 0.0014 βyyy -9.3170 βyyz -0.0013 βyyx -5.3897 βxyz -0.0005 βxzz -3.2172 βzzz 0.0020

βyzz -0.0012

βtot 33.9985

5.5. Conclusion

Ab initio HF and DFT calculations on the structure, vibrational, electronic

and NMR spectra of the title compound have been discussed. The calculated results

showed that the predicted geometry can well reproduce the structural parameters.

Predicted vibrational frequencies were assigned and compared with the experimental

and they supported each other. The electronic absorption spectrum was calculated by

TD-DFT method. Molecular orbital coefficient analyses suggest that the electronic

spectrum corresponds to π→π*.

150

The NBO analysis reveals that there is efficient intramolecular charge

transfer (ICT) within the molecule. MEP predicts the electronegative nitrogen (sp2)

and C1in the imidazole to be the most reactive site for electrophilic and nucleophilic

attack. The title compound exhibited good NLO property and was much greater than

that of urea. This makes this chromophore, a prospective building block for

nonlinear optical materials. NMR chemical shifts was calculated and compared with

the experimental. Amides can adopt a variety of tautomeric and rotameric structures,

tautomeric stability and intramolecular interactions of N-[acetylamino-(3-

nitrophenyl)methyl]-acetamide (ANPMA) is discussed in the next chapter.

151

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