Spectrophotometric and Spectroscopic studies of charge transfer complexes of Benzamide as an electron donor with Picric acid as an electron acceptor in different polar solvents

 

Neeti Singh and Afaq Ahmad*

Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh -202002,  India

*Corresponding Author E-mail: _{afaqahmad212@gmail.com; neetisingh72@gmail.com}

The charge transfer complexes of the donor benzamide (BZ) with π-acceptor picric acid (PA) have been studied spectrophotometrically in various solvents such as carbon tetra chloride, ethanol, and methanol at room temperature using absorption spectrophotometer. The results indicate that formation of CTC in less polar solvent is high. The stoichiometry of the CT complex was found to be 1:1. The physical parameters of CT complex were evaluated by the Benesi–Hildebrand equation. The data are discussed in terms of  formation constant (K_CT), ^{molar extinction coefficient (ε}_CT^{), standard free energy (∆Go), oscillator strength(ƒ), transition} dipole moment (μ_EN), resonance energy (R_N) and ionization potential (I_D). The results indicate that the formation constant (K_CT) for the complex were shown to be dependent upon the nature of electron acceptor, donor and polarity of solvents which were used. The results show that a charge transfer molecular complex between picric acid and benzamide stabilized by hydrogen bonding. The formation of the complex has been confirmed by UV-visible, FT-IR, ¹H–NMR, TGA/DTA spectral data methods. On the basis of the studies, the structure of CT complex is [(BZ)⁺ (PA)^−], _{and a general mechanism for its formation is proposed.}

 

 

1. INTRODUCTION:

Charge transfer complexes (CTCs) play a central role in bioelectrical and biological systems such as  bactericides, fungicides, Insecticides and various light-driven physical and chemical processes [1–5]. Some of these complexes show interesting electrical conductivity properties and have found applications in electronics and  solar cell [6, 7]. CT-interactions between aromatic electron acceptors and electron donors containing nitrogen, oxygen, or sulfur atoms have been reported over the last years [8–10]. The Charge transfer interaction has been  utilized for the estimation  of  different  pharmaceuticals  [11,  12],  micro-emulsion  [13]  and  also  as  organic semiconductors [14].

 

 The CT-complexes act as intermediates in a wide variety of reactions involving  nucleophiles  and  electron  deficient  molecules.  There  exists  a  vast  literature  on theoretical [15, 16] and experimental studies [17–25] in relation to the stoichiometry, structural, spectral, thermal and electronic properties of the CT-complexes.

 

The electron acceptor, picric acid (PA) is known to form stable colored charge transfer complexes   with   many  donors  such as 7,7-bis(piperazino)-8,8-dicyanoquinodimethane, 8-hydroxyquinoline, 2,2-bipyridine and 2,9-dimethyl-1,10-phenanthroline have been studied with the help of spectroscopic techniques  like FTIR, ¹H-NMR, TGA–DTA and UV–vis electronic absorption to obtain the stoichiometry, molecular structure and nature of interaction for the CT- complexes [26–29], single crystal studies of the crystal of  CT-complexes formed between the reactions of picric acid  and acenaphthene, 2-nitor aniline and phenanthrene  have also been reported [30–32]. Interactions and stoichiometry of the proton transfer (charge transfer) reactions of p-phenylenediamine with p-chloranil and chloranil have been studied by the above mentioned _{techniques [33-34].  The  proton  transfer  interaction  between  O-phenylenediamine  (OPD)} tetracyanoethylene (TCNE) and PPD-xylidine were investigated in both solid and liquid states [35, 36].

 

This paper presents studies of the charge-transfer interaction between Benzamide and picric acid in  both liquid and solid states. The aim of the work is to determine the reaction stoichiometry,  nature  of  bonding  between  BZ  and  PA,  and  some  physical  parameters.  In addition, the nature and structure of the reaction product CT complex in both solution and solid can be estimated using the above-mentioned techniques.

 

2. EXPERIMENTAL:

2.1   Materials

Analytical grade chemicals were used throughout. Benzamide (BZ) and picric acid (PA) was obtained from, CDH. Methanol (Merck), Ethanol (Merck) and Carbon tetra chloride (Merck) were all analytical grade (AR) and used without further purification.

                           

2.2   Preparation of standard solutions

Solutions of donor of different concentrations, 0.01M, 0.015M, 0.02M, and 0.03M, 0.1M, 0.2M, 0.3M and 0.5M were prepared in different volumetric flask by dissolving benzamide accurately weighed in different solvents such as carbon tetra chloride, ethanol and methanol.  A standard solution  of acceptor,  picric acid  (0.01M) concentration  was  prepared  by dissolving accurate weight of acceptor in above solvents in different volumetric flask

 

2.3   Synthesis of solid CT complex

Analar R grade samples of benzamide and picric acid were employed for the synthesis of the title compound.  Equimolar solutions of the two reactants were separately prepared in methanol and mixed together. The resulting solution was stirred well for about thirty minutes.  The precipitated adduct was filtered off at the pump and repeatedly recrystallised from methanol to enhance the degree of purity of the synthesized compound.

 

2.4 Spectrophotometric analyses

When 3ml solution each of the acceptor and the donor were mixed, a charge transfer complex was  formed. The wavelength of maximum absorption of the resulting solution was determined. The CT complex of the 1:1 reaction mixture was kept overnight at room temperature to form stable complexes, were analyzed. The maximum wavelength of the charge transfer complex was determined by spectrophotometer to be 370 nm for carbon tetra chloride, 380nm for ethanol and 385nm for methanol.

 

2.5 Analyses

The electronic absorption spectra of the donor benzamide, acceptor picric acid and the resulting complex in carbon tetra chloride, ethanol and methanol were recorded in the visible  range  200nm-600nm  using  a  spectrophotometer  ELICO  SL  177  scanning  mini spectrophotometer with a 1cm quartz cell path length. The FTIR spectra of the reactants and the resulting CT complex were recorded with the help of FTIR spectrometer INTERSPEC–2020 (spectra lab U.K.) measured in KBr pellets. The nuclear magnetic resonance, ¹H NMR spectrum  of  CT  complex  is  measured  in  CDCl₃    using Bruker  Advance  II  400  NMR spectrometer  and the thermal analysis (TGA and DTA) were carried out under nitrogen atmosphere with flow rate of 30mL min^-1 and a heating rate of 25⁰C min^-1 in the temperature range 20-400⁰C for TGA and DTA using Shimadzu model DTG-60H thermal analyzers.

 

3.    RESULTS AND DISCUSSION:

3.1    Observation of CT bands

A  3ml  volume  of  donor  and  acceptor  were  scanned separately through  a spectrophotometric titration [37] at room temperature with their wavelength of maximum absorption 285nm  for picric acid, 240nm for benzamide in methanol, 265nm for blank solvent (methanol) and 385nm for CTC of .01M PA and .01M BZ in methanol shown in Fig 1. The reaction mixture of donor (10ml) and acceptor (10ml) in different solvents viz, carbon  tetra  chloride,  ethanol  and  methanol  formed  a  yellow  colored  charge  transfer complex.  The  complex  for  each  of  the  reaction  mixture  standing  overnight  at  room temperature to form stable complex before analysis at the maximum absorbance 370nm for carbon tetra chloride, 380nm for ethanol and 385nm for methanol. The concentration of the donor in the reaction  mixture was kept greater than acceptor, [D₀]>> [A_o] [38, 39] and changed over a wide range of concentration  from 0.01M to 0.5M while concentration of π- acceptor (picric acid) was kept fixed[38] at 0.01M in each solvents, these produced solutions with donor: acceptor molar ratios varying from 1:1 to 50:1, these concentrations ratios were used to straight line diagram for determination of the formation constants  of CTC were shown in Table 3.

 

Fig.1 Abs  orption spectra of (A) 0.01M Benzamide (B) Blank solvent  (Methanol) (C) 0.01 M Picric acid (D) CTC of 0.01M PA and 0.01M  BZ in methanol

The spectrum of solution of 0.01M PA, and 0.01M BZ in different solvents were recorded with solvents used as a reference, the longest wavelength peak was considered as CT peak [40]. The change of the absorption intensity to higher for all Complexes in this study when adding the donor was detected and investigated are shown in Table 3. These measurements were based on the CT absorption bands exhibited by the spectra of the systems which were above mentioned and given in Figs. [2, 3 and 4]. In all system studied the absorption spectra are of similar nature except for the position of absorption maxima (λ_CT) of the complex. The CTC absorption spectra were analyzed by fitting to the Gaussian function

 

y ₌  y₀  + [A/ w√ (π/2))] exp [−2 (x- x_c)²/w²]

Where, x and y denote wavelength and absorbance, respectively. The results of the Gaussian analysis for all systems under study are shown in Table 1. The wavelengths at these new absorption maxima (λ_CT = x_c) and the corresponding transition energies (hν) are summarized in Table 2.

 

Table 1

Gaussian curve analysis for the CTC in spectrum of PA with BZ in different polar solvents

Systems	Solvent	A	W	Xc	Y0
PA+ BZ         PA+ BZ     PA+ BZ	Carbon tetra chloride   Ethanol     Methanol	111.17±7.20     233.78±12.73   188.22 ±12.07	52.94 ± 3.57     114.68±4.88   99.28  ±5.19	362.20±1.51     370.57±1.86   372.99±2.02	0.00137 ± 0.0313     0.02933 ± 0.0318   0.00531 ± 0.0351

 

Table 2: CTC absorption maxima (λ_CT), transition energies (hν_CT), of the PA+BZ complexes, experimentally determined values of ionization potentials (I_D), oscillator strength (ƒ), dipole moments (μ_EN), and resonance energies (R_N) of complexes

Systems	Solvent	λCT  (nm)	hνCT (eV)	ID(ev)	ƒ × 105	μEN  (Debye)	[RN] (eV)
PA+BZ     PA+BZ   PA+BZ	Carbon tetra chloride   Ethanol   Methanol	362.20   370.57   372.99	3.43   3.35   3.33	9.98   9.87   9.86	2.12   4.79   4.18	0.917   0.939   0.939	0.0082   0.0084   0.0083

 

Table 3: Data for spectrophotometric determination of stoichometry, absorption maxima (λ_CT), and association constants (K_CT), molar absorptivities (ε_CT), of CTC of PA and BZ in carbon tetra chloride, ethanol, and methanol at 298 K

Systems	Solvent	Temperature (K)	Donor concentration in  M	[A]­0 in M	Absorbance at λCT(nm)	λCT (nm)	KCT (lmol-1)	εCT                                   (1mol-1 cm-1)
PA+BZ                   PA+BZ                   PA+BZ	Carbon tetra chloride               Ethanol                   Methanol	298                    298                     298	0.01 0.015 0.02 0.03 0.05 0.1 0.2 0.3 0.5       0.01 0.015 0.02 0.03 0.05 0.1 0.2 0.3 0.5   0.01 0.015 0.02 0.03 0.05 0.1 0.2 0.3 0.5	0.01                   0.01                    0.01	1.677 1.733  1.757 1.788 1.814 1.838 1.855 1.862 1.869     1.718 1.779 1.814 1.855 1.890 1.919 1.941 1.949 1.953    1.626 1.724 1.769 1.828 1.872      1.912                    1.934 1.941 1.945	370                   380                   385	865                   697                    488	186                   195                     195

 

 

Fig 2.  Absorption spectra of   picric acid (1 × 10^-2M) in carbon tetra chloride with addition of Benzamide concentrations ranging 0.1M to   0.5M are shown with increasing concentrations bottom to top.

Fig 3.   Absorption spectra of   picric acid (1 × 10^-2M) in ethanol with  addition of Benzamide concentrations ranging 0.1M to 0.5M are  shown with increasing concentrations bottom to top.

 

Fig 4.   Absorption spectra of   picric acid (1 × 10^-2M) in methanol with addition of Benzamide concentrations ranging 0.1M to 0.5M are shown with increasing concentrations bottom to top.

3.2   Determination of Ionization potentials of the donor

The ionization potentials of the donor (I_D) in the charge transfer complexes are calculated using empirical equation derived by Aloisi and Piganatro [41]

 

 

I_D (eV) = 5.76 + 1.53 ×10^-4    ν_CT                      ……….(1)

Where, ν_CT is the wave number in cm^-1 of the complex were determined in different solvents,      

viz, carbon tetra chloride, ethanol and methanol.

 

3.3 Determination of oscillator strength, (ƒ), and transition dipole moment,  (μ_EN)

From the CT absorption spectra, one can extract oscillator strength. The oscillator strength ƒ is estimated using the formula

 

ƒ = 4.32 × 10^-9 ∫ ε_CT dν                                      ……….(2)

 

Where,   ∫ε_CTdν is the area under the curve of the extinction coefficient of the absorption band in question vs. frequency. To a  first  approximation

 

 

ƒ = 4.32 × 10^-9 ε_CT ∆ν_1/2                                   ……….(3)

 

Where, ε_CT is the maximum extinction coefficient of the band and ∆ν_1/2 is the half- width, i.e., the width of the band at half the maximum extinction. The observed oscillator strengths of the CT bands are summarized in Table 2.

 

The extinction coefficient is related to the transition dipole by

 

μ_EN = 0.0952 [ε_CT ∆ν_1/2/∆ν]^1/2                        ……….(4)

 

Where,   ∆υ  ≈ υ  at  ε_CT and   μ_EN   is defined as –e ∫  ψ_ex∑_ir_iψ_g   dτ.  μ_EN  for the complexes of  PA with  BZ are given in Table 2.

 

3.4   Determination of resonance energy (R_N)

Briegleb and Czekalla [42] theoretically derived the relation

 

ε_CT = 7.7 × 10^-4 / [hν_CT/ [R_N] − 3.5 ]                ……….(5)

 

Where, ε_CT  is the molar extinction coefficient of the complex at the maximum of the CT absorption, ν_CT   is the frequency of the CT peak and R_N   is the resonance energy of the complex  in  the  ground  state,  which,  obviously  is  a  contributing  factor  to  the  stability constant of the complex (a ground state property). The values of R_{N for} the complexes under study have been given in Table 2.

 

 

3.5   Determination of Standard free energy changes (∆G^o) and energy (E_CT) of  the π-π* interaction between donor and acceptor

The standard free energy changes of complexation (∆G^o) were calculated from the association   constants by the following  equation  derived  by  Martin,  Swarbrick  and Cammarata [43].

 

      ∆G^{o =} − 2.303 RT log K_CT                            ……….(6)                                                                                                              

Where ∆G^o   is the free energy change of the complexes (KJ mol^-1), R is the gas constant (8.314 J mol^-1 K), T is the temperature in Kelvin degrees (273 + ⁰C) and K_CT is the association constant of the complexes (l mol-¹) in different solvents at room temperature.

 

The energy (E_CT) of the π –π* interaction between donor (BZ), and acceptor (PA), is calculated using the following equation derived by G. Briegleb and Z. Angew  [44].

 

The calculated values of E_CT given in Table 5.

 

               1243.667

 E_CT =————                                                    ……….(7)

                ^λCT

Where,   λ_CT   is the wavelength of the CT band.

 

3.6   Spectrophotometric study of formation constants of the charge transfer complexes of PA+BZ in different polar solvents

Stoichiometries  and  the  formation  constants  of  the  charge  transfer  complex  of benzamide with picric acid have been determined in different polar solvents viz- carbon tetra chloride, ethanol and methanol at room temperature using Benesi–Hildebrand  equation [45,46].  The  spectrophotometric  data  were  employed  to  calculate  the  values  of  formation constants, K_CT  of  the complex. The changes in the absorbance upon addition of BZ to a solution of PA of fixed concentration follow the Benesi- Hildebrand [45, 46] equation in the form.

 

 [A]_o / A = (1 / K_CTε_CT) × 1 / [D] _{0 +} 1/ε_CT      ……….(8)

 

Where [D]_{o and} [A]_o are the concentrations of the benzamide donor, and picric acid acceptor, respectively,  A is the absorbance of the donor-acceptor mixture at λ_CT, against the solvents as reference, K_CT is the formation constant and     ε_CT is the molar extinction coefficient, is not quite that of complex eq.(8) [45, 46] is valid under the condition [D]_o  >> [A]_o  [38, 39] for 1:1 donor- acceptor complexes. The concentration of the donor (BZ) was changed over a wide range from 0.01M to 0.5M while concentration of π acceptor PA was kept fixed at 0.01M in each reaction mixture. These produced solution with donor: acceptor molar ratio varying from 1:1 to 50:1, experimental data are given in Table 3.

 

The Benesi – Hildebrand [45, 46] method is an approximation that has been used many times and gives decent results. But the extinction coefficient is really a different one between the complex and free species that absorbs at the same wavelength. The intensity in the visible region of the absorption bands, measured against the solvent as reference, increases with increased in the polarity and addition of BZ. The typical absorbance data for charge transfer complexes of BZ with PA in different polar solvents at room temperature are reported in Table  1 and 3. In all systems very good linear plots according to eq. (7) [45, 46] are obtained, shown in Figs 5, 6 and7. Formation constants for the complex in different polar solvents at room temperature determined from the BH plots are summarized in Table 3. The correlation coefficients of all such plots were above than 0.99. Plots of [A]₀/A against 1/[D]_o  were found to be linear in all systems in Figs. 5, 6 and 7 showing 1:1 charge transfer complex, i.e. the straight lines are obtained with the slopes 1/K_CTε_CT, these results prove the formation of the 1:1 CTC. From slope 1/K_CTε_CT and intercept, ^1/ε_CT^{, K}_CT ^{and ε}_CT ^{of the complex were calculated.}

 

Fig5.    Relation between [A]₀/A and 1/[D]₀ of PA+BZ in carbon tetrachloride.

 

Fig 6.  Relation between [A] ₀/A and 1/[D] ₀ of PA+BZ in ethanol.

 

Fig 7.  Relation between [A] ₀/A and 1/[D] ₀ of PA+BZ in methanol.

 

3.7          Effect of solvents on the formation of CT- complexes

The experimental results of the CT interaction between PA with BZ in different polar solvents show  the values of association constants K_CT, 865(1mol^-1) in carbon tetra chloride, 697(1mol^-1)  in  ethanol,  and  488(1mol^-1)  in  methanol  and  the  values  of  molar  extinction coefficient  ε_CT,   186(1mol^-1cm^-1)  in  carbon  tetra  chloride,  195(1mol^-1cm^-1)  in  ethanol,  and  195(1mol^-1cm^-1)  in  methanol  and  spectroscopic  properties  were  markedly  affected  by  the variation in solvent polarity in which measurements were carried out. In the present investigation the K_{CT values} increases significantly from methanol to carbon tetra chloride with decreasing solvents polarity. Moreover, the increase in K_{CT values} with decreasing solvents polarity, May also be due to the fact that, CTC should be stabilized in less polar solvent [47]. Dissociation of the complexes into D⁺——A^-   radicals have been found to occur in the ground state [48]. It means the CTC should be strong in less polar solvent than polar solvent. The red shift occurred in CTC complex caused by polarity change on going from carbon tetra chloride to methanol.

 

However the data given in Table 3, shows that PA interacts more strongly with BZ in carbon tetra chloride  among the other two solvents. The experimentally determined values of oscillator strength,(ƒ),  2.12× 10^-5 in carbon tetra chloride, 4.79 × 10^-5 in ethanol, and  4.18  × 10^-5 in methanol, and the values of transition dipole  moment,(μ_EN),  0.917(Debyes) in carbon tetra chloride, 0.939 (Debyes) in ethanol, and 0.939 (Debyes)in methanol,  and values of resonance energy(R_N),(eV) 0.0082 in carbon tetra chloride, 0.0084 in ethanol, and 0.00835 methanol, given in Table 2, indicate that complex   should be stable in less polar solvent (carbon tetra chloride)  than other two solvents (ethanol and methanol). The very low values of ƒ, indicate that CT complex studied here have _{almost neutral character in their ground state.}

 

The  parameters  thus  obtained  are  represented  in  Table  4,  and  these  values  show  that complexation  is  thermodynamically favored.  The free energy change of the complexation also reveals that the CTC formation between used donor (BZ) and acceptor (PA) is of exothermic in nature. The values of ∆G^o  -16.718 (KJmol^-1) in carbon tetra chloride, -16.204(KJmol^-1) in ethanol, and  -15.291(KJmol^-1) in  methanol  given  in  Table  4,  generally  become  more  negative  as  the association constants for  molecular  complex  increases.  As  the  bond  between  the  components becomes  stronger and thus the components are subjected to more physical strain or loss of freedom, the values of ∆G^o  more negative.

 

The ionization potentials I_D, (eV) of the donor can be calculated using the experimentally determined λmax of the CTC from eq. (1), [41]. The calculated values of I_{D 9.89}(ev) in carbon tetra chloride, 9.78(ev) in ethanol, and 9.72(ev) in methanol of PA/BZ system are shown in Table 5. The approximate constancy of I_{D values}, indicates that the ionization potential show a negligibly small effect on K_CT values.

 

Table 4: Association constant (K_CT), correlation coefficients (r) and standard free energy changes (∆G^o) of PA+BZ complexes obtained from Benesi-Hildebrand plots

 

Systems	Solvent	KCT (lmol-1)	-∆Go(298K)       (kJmol-1)	r
PA+BZ         PA+BZ     PA+BZ	Carbon tetra chloride   Ethanol   Methanol	865       697   488	16.718       16.204   15.291	0.996        0.997             0.999

 

Table5

The CTC transition energies (E_CT), CTC absorption maxima (λ_max), and Ionization potential (I_D) of donor of in different polar solvents 

Systems	Solvent	ECT (eV)	λCT (nm)	ID(eV)
PA+BZ         PA+BZ     PA+BZ	Carbon tetra chloride   Ethanol   Methanol	3.36             3.27   3.23	370             380       385	9.89           9.78   9.72

 

3.8 FTIR spectra of CT complex and reactants

FTIR spectra of the benzamide (donor), picric acid (acceptor) and their CT complex are shown in Fig. 8, while their assignments of the characteristic FTIR spectral bands are reported in Table 6. The formation of the charge transfer complex during the reaction of BZ with PA is strongly evidenced by the presence of the main characteristic infrared bands of the donor and _{acceptor in the spectrum of the product. However, the bands of the donor are shifted to lower} frequencies while that of the acceptor are shifted to the higher frequencies. There are changes in their intensities compared with that of the free donor and acceptor. This shift has been attributed to the charge transfer from donor to acceptor upon the complexation. These changes may be attributed to the expected symmetry and electronic structure modifications in both donor and acceptor units in the product thus formed relative to the free molecules. 

 

Fig 8.  FTIR spectrum of (A) Complex of PA and BZ, (B) Acceptor PA (C) Donor BZ

_3445cm⁻¹  _{(weak, broad).  The disappearance of  the OH peak  in  the  CT  complex  has  been} attributed to the protons transfer from the acceptor to the donor or transfer of lone pair electrons the donor to  acceptor leading to a intermolecular hydrogen bonding as reported  previously [29,49], so as to the new band observed at 3487cm⁻¹(strong) and is due to the υ (N⁺–H· · ·O⁻) while the N–H stretching vibration is  observed  at 3368cm⁻¹  (strong) and 3177cm⁻¹  (strong) which is normally observed at 3300cm⁻¹ (medium) and 3199cm⁻¹(medium weak). The band at 3102cm⁻¹ (medium) is due to the aromatic C–H stretching vibration. The –NH₂ deformation mode is observed by the absorption at 1624cm⁻¹ (very strong) in the CT complex whereas in free BZ this was observed at 1620cm⁻¹ (weak). This band overlaps with the aromatic C=C stretching vibrations. The asymmetric and symmetric stretching vibration of the –NO₂ group are observed at 1537cm⁻¹ (very strong) and 1314cm⁻¹ (very strong) respectively. Normally the asymmetric stretching vibration of the –NO₂   group is sensitive to the polar influences and the electronic states of the species. Therefore, it has been realized that the shift to the lower frequency of as (NO₂) vibration (1537cm⁻¹ very strong) in the spectrum of the CT complex whereas in free picric acid this was observed at 1608cm⁻¹ (very strong) is due to the increased electron density on the picric acid moiety as result of the proton transfer (charge transfer) from acceptor to donor as described elsewhere [28]. The absorption at 1624 and 1449cm⁻¹ are due to the aromatic C=C stretching vibrations.  The  C–H  in  and  out  plane  bending  vibration  and  C–NO₂    stretching vibration  are  observed  at  1088,  766  and  917cm⁻¹   as  medium,  medium  and  weak  band, respectively.

 

3.9   ¹H NMR Spectrum of CT Complex

¹H NMR spectrum of the CT complex is recorded in CDCl₃  and shown in Fig. 9 In the nuclear  magnetic resonance ¹HNMR spectrum of CT complex the protons of picric acid and benzamide ring system were assigned in the region δ=7.43-7.83ppm for benzamide [50] while the  phenolic proton is assigned at δ = 11.940 ppm [51]. All the observed peaks in the spectrum of the individual reactants are also observed in the spectrum of CT complex except –OH peak suggesting that the disappearance  of  –OH peak in the spectrum of CT complex is due to the formation of  ⁺N-H₃  ion through hydrogen bonding between  –OH group of picric acid and ^–CONH₂  ^{group of benzamide [52]. The phenolic –OH proton is assigned at δ=11.94ppm in free} picric acid which has been merged in the bunch of aromatic protons of benzamide moiety in the region δ=7.43-7.83ppm. This up field shift in frequency has been attributed to an increase in electrons density on picric acid part of CT complex due to transfer of protons from picric acid to benzamide. The singlet `peak at δ = 9.193 ppm is assigned to the two protons of the same kind in the picric acid moiety in the CT complex [51]. The formation of hydrogen bond Bridge between NH  proton  of  Benzamide  and  -OH  proton  of  picric  acid  resolved  in  chemical  shift,  non- equivalence of –NH₂  protons of benzamide (marked NHa and NH_b) .The significant downfield shift  of  the  NH  peaks  relative  to  pure  compounds  provide  convincing  evidence  for  the association of molecules [53]. Mechanism and structure of the CT complex of acceptor and _{donor is given in scheme 1.}

 

Fig. 9 ¹HNMR Spectrum of complex

 

3.10    Comparative study of Thermo grams for benzamide, picric acid and their CT complex

The Comparative  study  of  Thermograms  for  picric  acid,  benzamide  and  their  CT complex were carried out as also TGA/DTA analysis of the complex and reactant from DTG- 60H thermal analysis in nitrogen atmosphere with flow rate of 30mL min^-1 and heating rate 25⁰C min^-1  in the temperature range 20-400⁰C showing in Figs 10(a), 10(b) and 10(c). The reference was 10mg alumina powder.  Thermo gravimetric (TGA) and differential Thermo gravimetric (DTA) analysis were carried out using in the temperature range of 20-400⁰C using 7.223mg for picric acid, 6.435mg of benzamide and 4.048mg for complex, respectively in order to confirm the charge transfer interaction between donor and acceptor and thermal stability of the CT complex. The Thermogram of  picric acid acceptor exhibits its decomposes in two step at 133⁰C and 272⁰C with weight losses of about 4.403% and 93.53% respectively with a complete loss in each case, and donor benzamide decomposes in  one step at 224⁰C with weight loss 99.099% complete loss. The CT complex shows decomposition in two steps one 109⁰C and other 220⁰C with weight loss 97.43%.  The thermal decomposition at 1090C is due to   acceptor and decomposition at 220⁰C with weight loss 97.43% is due to donor benzamide. This has been attributed to the formation of CT complex and its stability compared to its constituents.

 

Fig. 10(a) TGA–DTA curves for Benzamide.

 

Fig. 10(b) TGA–DTA curves for picric acid.

 

Fig. 10(c) TGA–DTA curves for charge transfer complex.

 

Table 6: Characteristic infrared frequencies*(cm^-1) and tentative assignments for PA, BZ and their  complex.

PA                                                      BZ                                  Complex                                           Assignments

3445 w, br                                          -                                      3487s                             ν(O-H),PA, hydrogen bonding b/w    –OH-H--NH -                                                           3368s,br                        3368s                             ν (N-H),BZ                                                             3169s,br                        3177s                       3106s                                               -                                         3102m                           νas (C-H),CH3 + CH3 -                                                           3062 w                           -                                      ν(C-H),aromatic ν (+NH)                             -                                                          2772w                                                                            1962w                            1636 vs                                              1660m                           1656m                                                                                  1608vs                                               1620w                            1624vs                                               νas(NO2), PA 1529 br                                              1573vs                           1573ms                                             ν(C=C), aromatic -                                                           1505vs                           1537vs                                               δ def (N-H), +NH2 ring       breathing bands                                        1445s                             1449vs                          C-H  deformation   1438ms                                             -                                       1402ms                                                                                                                                                                                                                                                                                                                                     -                                                           1398vs                           1342br                          ν(C-C), νsNO2, PA 1346vs                                               -                                       1314vs                          νas(C-N) 1314w 1267vs                                               1298vs                           1211br                           ν(C-O) 1152ms                                             1183w                            1179ms                    1084ms                                             1144br                           1088ms                        νs(C-N)  1076sharp                                         1024m                         921ms                                                921sharp                                                               δ (C-H) in plane bending 834w                                                  846sharp                       917w                                                  δrock , +NH2                                                                                                                                                                 CH2 rock skeletal 782sharp                                                                                                                                               vibrations                                                                          790vs                             766m                                                 C-H out of plane                                           734ms                                                683vs                             635ms                                                bending 703w                                                  528ms                            524sh             651w                                                  468ms                            468br                                                 δ(ONO), PiOH 524w                                                  -                                       -                                                           CNC  deformation 452sharp                                           413sharp                       413ms

4.  CONCLUSIONS:

The UV –Vis   spectrophotometric method for the study of CTC of picric acid with benzamide reveals that it forms 1:1 (A: D) complex in all three solvents, viz –carbon tetra chloride, ethanol and methanol. In all systems the stoichiometry is unaltered by changing the solvent. The association constants, K_CT and molar extinction coefficients, ε_CT, of all systems were evaluated by the Benesi - Hildebrand method.  The spectroscopic and Thermodynamic parameters of the complexes were found to be solvents dependent. The values of oscillator strengths, (ƒ) transition dipole moments, (µ_EN) resonance energies, (R_N) and standard free energies, (∆G^o) have been estimated for the PA/BZ systems in different polar solvents. The results show that the investigated complex is stable, exothermic and spontaneous. From the trends in the CT absorption bands, the ionization potentials of the donor molecules have been estimated. The FTIR and ¹H NMR spectrum shows that the charge transfer molecular complex formed between PA and BZ stabilized by hydrogen bonding which is formed between –OH group of picric acid and hydrogen atom of amino group of benzamide.

 

5. ACKNOWLEDGEMENTS:

Author thanks Dr. Zafar A. Siddiqui Chairman of Chemistry Department, Aligarh Muslim University, India, for providing the facilities of instruments of F.T.I.R Spectrometer, UV- Visible spectrophotometer.  Financial assistance by the UGC, New Delhi extended through the Women- PDF fellowship is also gratefully acknowledged. The authors also thank the learned referee for _{making valuable comments.}

 

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Received on 30.04.2013       Modified on 16.05.2013

Accepted on 20.05.2013      © AJRC All right reserved