PCM analysis

Journal of Molecular Structure 1196 (2019) 462e477

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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc

Donor-acceptor complex of 1-benzoylpiperazine with p-chloranil: Synthesis, spectroscopic, thermodynamic and computational DFT gas phase/PCM analysis Nampally Venkatesh, Baindla Naveen, Abbu Venugopal, Gangadhari Suresh, Varukolu Mahipal, Palnati Manojkumar, Tigulla Parthasarathy* Department of Chemistry, Osmania University, Hyderabad, 500007, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 March 2019 Received in revised form 13 June 2019 Accepted 25 June 2019 Available online 28 June 2019

A new charge transfer (CT) complex is formed between the n- and p-donor 1-Benzoylpiperazine (1BNZP) with p- acceptor p-chloranil (CHL). The CT-interaction is characterized in the defined polar solvent (acetonitile: ACN) at different temperatures. The 1:1 composition of the CT complex is proved. Benesi-Hildebrand method is used to find the stability constant (KCT) and molar extinction coefficient (ε). The more stability of CT complex is suggested from observed results. The thermodynamic parameters (DH
and DS
) were calculated by using Van't Hoff plot. From these values, CT complex formation process is found to be endothermic and spontaneous. FT-IR study gives information about stretching frequencies of mono substituted product of CT complex. TGA-DTA study is used for thermal stability of product. Morphology study is used to estimate the size and structure of the product. EDX spectrum is used for elemental analysis in the stoichiometric ratio. Theoretical investigation of the CT complex using DFT and TD-DFT also supports the experimental work. The C¼O bond length of CHL increases on complexation with 1-BNZP along with significant amount of charge transfer from donor to acceptor. The HOMO-LUMO computation for protonated form of complex suggests a mono substituted product is formed via inner sigma (s) complex. © 2019 Elsevier B.V. All rights reserved.

Keywords: 1-Benzoylpiperazine p-Chloranil DFT study Polarizable continuum model Molecular electrostatic potential Mulliken charges

1. Introduction Charge transfer complexes are formed from the electronic interaction between the acceptor and donor molecules, general fact in chemistry [1]. This CT interaction was first introduced by Mulliken and Foster [2]. Mulliken surveyed such complexes to arise from a Lewis acid-base type of interaction [3]. The CT interaction process generates weak bonds between donor and acceptor. The molecular interactions of these molecules are associated with the formation of colored complexes which absorb visible radiation in the correspond region [4]. The CTCs play supreme role in daily life due to their various biological and physical properties [5]. These are used as potential high efficiency non-linear optical materials [6], Solar cells [7], photo-catalysts [8], electrical conductors and organic semi-conductors [9]. These can be used in redox processes as catalysts [10]. It has a great significance in the field of DNA binding,

* Corresponding author. E-mail address: [email protected] (T. Parthasarathy). https://doi.org/10.1016/j.molstruc.2019.06.083 0022-2860/© 2019 Elsevier B.V. All rights reserved.

drug receptor, anti microbial and anti bacterial studies [11]. The acceptor p-chloranil (CHL: 2, 3, 5, 6-tetrachloro p-benzoquinone) is an important quinone type electron p-acceptor. It is widely used as dye intermediate, agriculture fungicide. Even though, it's potential symptoms of overexposure such as watery diarrhea, central nervous system depression and coma [12], it helps us to understand many biological and chemical processes. on electron transfer from donor to quinone molecule it forms radical ion pair and also inner s-complex [13]. 1-Benzoylpiperazine (1-BNZP) is rarely used as chemical compound in pharmaceutical industry. The piperazine derivatives are classified as privileged structures generally found in bio-active compounds. Therefore, these derivatives are used intensively in many therapeutic areas such as antifungal, antidepressants, antiviral, and serotonin receptor (C5-HT) antagoinists/agonisgts binding [14e16]. 1-BNZP contains the antitumor, inotropic, nootropic activities and has cardiovascular properties [17e19]. It acts as a good fascinate electron donor for many of these reasons. The main objective of the present article is to comprehend the

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spectral, thermodynamic and computational DFT (gas phase/PCM) investigations and also find the mechanism of product comes from 1-BNZP / CHL CT complex.

463

ACN. Dry conditions were maintained for whole experimental work. The stock solutions were protected from light. 2.4. General methods for the stoichiometry

2. Experimental section 2.1. Materials and safety The materials were purchased from chemical sources. 1Benzoylpiperazine (97% Sigma-aldrich) and p-chloranil (99% Sigma-aldrich) were commercially available. The 1-BNZP and CHL are soluble in organic polar solvents like ACN and have acute toxic nature (oral, dermal, inhalation). CHL is hazardous to the aquatic environment. The analytical grade of acetonitrile (99.9% Merck) solvent is used for the preparation of stock solutions. Gloves, dustmasks, eyeshields were used throughout all experiments. 2.2. Synthesis of solid mono substituted product of CT complex The CT complex of 1-Benzoylpiperazine (1-BNZP) with pchloranil (CHL) was synthesized by mixing saturated solution of one equivalent of 1-BNZP in ACN with saturated solution of one equivalent of CHL in ACN and purple color solution appeared upon mixing. The solution mixture was stirred continuously for 4 h and left standing at room temperature for 3-4 days. Purple color solid crystalline mono substituted product of CT complex was formed. It was washed with small amounts of hexane for 5 min. After that it was dried over anhydrous calcium chloride. Mono substituted product of CT complex was: C17H13N2Cl3O3 (M.W. 399.6 g, m.p. ¼ 215-220
C). 2.2. Instrumentation and analytical measurements Electronic absorption spectra was recorded in the nanometer region (780e250) using SHIMADZU UV-2600, UV-VIS Spectrophotometer. The cell holder temperature was controlled with a SHIMADZU CORP.01929 (±0.2
C). FT-IR spectra of the compounds were recorded in the range 4000-250 cm1 on Perkin-Elmer Infrared model 337 using KBr pellets. 1H NMR of the compound was recorded on Bruker 400 MHz NMR instrument using DMSO as internal reference. An ESI mass spectrum of product was recorded on VG AUTOSPEC mass spectrometer. Melting point was determined on a Polmon instrument (model No: MP-96). The thermo gravimetric analysis (TGA) was carried out in a dynamic nitrogen atmosphere with a heating rate of 10
Cmin1 using a Shimadzu TGA-50H in the temperature range of 20e1000
C. The morphology of CT complex was identified by SEM (scanning electron microscopy on Zeiss evo18). The chemical composition of product was analyzed via an energy dispersive X-ray (EDX) spectrometer attached to an SEM instrument. 2. 3. Standard stock solutions 2.3.1. 1-BNZP stock solutions The 102 mol.L1 stock solution of 1-BNZP was prepared by dissolving 0.019 g in a 10 ml volumetric flask using ACN solvent. The solutions of different concentrations 1  103 mol.L1 to 5.0  103 mol.L1 were prepared in different volumetric flasks by diluting 102 mol.L1 solution of 1-BNZP with the same solvent. 2.3.2. CHL The stock solution of the CHL 102 mol.L1 was prepared by dissolving 0.024 g in a 10 ml volumetric flask in the same solvent. The 1  103 mol.L1 standard solution was prepared in a 25 ml volumetric flask by diluting102 mol.L1 solution with the same

2.4.1. Job's continuous variation method The Job's method of continuous variation [20] was employed for determination of the stoichiometry of the reaction between 1-BNZP and CHL. The maximum absorbance 0.5 mol fraction indicates that the formation of 1:1 [(1-BNZP)(CHL)] CT complex when a graph plotted of absorbance against mole fraction of CHL. 2.4.2. Spectrophotometric titration method The photometric titrations [21] were carried out at 549.5 nm 1.00, 1.50, 2.00, 2.50, 3.00, 3.50, 4.00, 4.50, 5.00, 5.50 and 6.00 mL aliquots of a standard solution (1.0  103 mol L1) of the 1-BNZP in ACN were added to 1.0 mL of acceptor (1.0  103 mol.L1). The concentration of CHL (Ca) in the reaction mixture was fixed at 1.0  103 mol.L1 while the concentration of the 1-BNZP (Cd) was changing over wide range concentrations between 0.67  103 and 4.0  103 mol.L1, to give ratios of donor to acceptor varying from 1:4 to 3:2. The molecular composition 1:1 of CT-complex was determined by the application of conventional photometric molar ratio corresponding to methods. 2.4.3. Conductometric titration method The molecular composition of the formed CT-complex was also determined by conductometric titration [22]. The conductivity values were measured by titrating 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10 mL aliquots of a standard solution of the donor (1.0  103 mol.L1) with 5.0 ml of acceptor (1.0  103 mol.L1) in ACN solvent in each step. The maximum conductance was obtained when 5.0 ml of donor titrated with 5.0 ml of acceptor. The two lines intersect at 1:1 [(1-BNZP)(CHL)] CT complex which is observed in the graph plotted in Fig. 4. 2.5. DFT computational analysis The calculations of density functional theory (DFT) of the acceptor, donor and CT complex were carried out using Gaussian 09 w program [23] in gas phase as well as in solvation using Polarizable continuum model (PCM). The protonated form of CT complex HOMO-LUMO gap is also calculated with the same PCM solvation model. The basis set 6-31G was used for the geometry optimization and employing the Becke3eLeeeYangeParr hybrid exchangeecorrelation three-parameter functional (B3LYP). Gauss View 5.0.8 [24] was used to create input file by drawing molecular structures. The geometrical parameters bond lengths, bond angles, molecular electrostatic potential map values and characterization of the occupied and unoccupied molecular orbital surfaces, Mulliken atomic charges of CT complex were computed in gas phase reported after optimization. 3. Results and discussion 3.1. Monitoring the charge transfer band Electronic absorption spectra of donor (1-BNZP), acceptor (CHL) and CT complex are presented in Fig. 1. The computational UV spectrum was also confirmed the charge transfer complex recorded in Fig. S6 by using TD DFT (6e31g) gas phase method which correlates the output result of experimental electronic spectra [25,26]. The experimentally recorded spectrum shows charge transfer band after adding both free donor and acceptor with particular concentrations in ACN at lmax 549.5 nm, which do not exist individual

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Fig. 1. UV absorption spectrum belongs to Donor, Acceptor and CT complex in ACN solvent.

Fig. 2. Job's continuous variation plot of 1:1 [(1-BNZP)(CHL)] CT complex in ACN solvent.

spectra. The occurrence of CT band indicates the transfer of electrons from donor to acceptor [27]. These transition corresponds to the electronic transition which is closely located from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) [28,29]. Further, it was clearly monitored as purple color by mixing of acceptor to donor at low concentrated solutions in ACN, which is primary and best indication for the complex formation. The lone pair containing N-atoms and ring p 

electrons of 1-BNZP is responsible for donating nature, but the Natom which is nearer to the carbonyl group cannot easily participate in the electron transfer of the CT complex. The negative charge on amine nitrogen is more than on the amide nitrogen because the electrons on the latter are resonating with the adjacent carbonyl group [30]. So the free amine group is easily participating in the donation of electrons to the acceptor. The donating nature of 1BNZP from the presence of nitrogen center as well as CHL is for

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465

Fig. 3. Photometric titration plot of 1:1 [(1-BNZP)(CHL)] charge transfer complex.

Fig. 4. Conductometric titration plot of [(1-BNZP)(CHL)] in ACN solvent at 298 K.

generating the radical anion which is the symbol of the formation of radical ion pair resulting from the electron transfer of 1-BNZP towards CHL and go on with time from solution state to solid state the formation of stable mono substituted product takes place (Scheme 1). It clearly indicates the intermediate stage is inner

scomplex [31]. The solvent ACN (polar) is used to determine the charge transfer reaction between 1-BNZP and CHL. Appearance of new absorption band of the complex was stable for more than 100 min at different temperatures from 293 K to 313 K in ACN medium.

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Scheme 1. Proposal reaction pathway between the donor and the acceptor.

3.2. Stoichiometry relationship of the CT complex

DA 4 Dþ þ A

We have used three major methods to determine the composition of the formed CT complex. In the Job's continuous variation plot method, sum of concentrations of acceptor and donor solutions were kept fixed. Using these solutions, we have plotted absorption values of formed CT complex verses mole fraction corresponding to CHL shown in Fig. 2. From the figure, it can be observed that the absorption is maximum at 0.5 mol fraction which indicates 1:1 CT complex [(1-BNZP)(CHL)] formed. This was done at lmax 549.5 nm wavelength. In Spectrophometric method, produced two lines were intersecting at 1:1 donor to acceptor ratio (Fig. 3), therefore 1:1 composition at lmax 549.5 nm was confirmed. Similarly in Conductometric titration method, the maximum conductance in ms cm1 was obtained when 5.0 ml of donor titrated with 5.0 ml of acceptor plotted in Fig. 4 so that the 1:1 stoichiometry at 298 K has been confirmed. The charge transfer probability enhances with enhancing dielectric constant of the medium. In ACN, a medium of sufficiently high dielectric constant (~36.64) and the resulting [(1-BNZP)(CHL)] complex may dissociate into ions giving rise to appreciable conductivity according to the general scheme:

D þ A4DA

(1)

(2)

where DA represents the [(1-BNZP)(CHL)] complex. 3.3. Absorbance versus time study of CT complex The CT spectra of the complexes solution are time dependent. For this, 1.0  103 moL.L1 of CHL is added to 1.0  103 moL.L1 of 1-BNZP with 1:1 M ratio at room temperature (~298 K) for observation of UV absorbance spectra. In this case, the absorption strength of this band increases slowly with time (Fig. 5). This can be interpreted on the basis that CTC is strong. The wavelength of the CT complex (lmax ~549.5 nm) is not changed but the formation of CT complex increases with absorbance and also with time [32]. 3.4. Stability constant (KCT) and energy (ECT) of the CT complex The UV absorption spectra of 1.0  103 moL.L1 CHL with different concentrations of 1-BNZP ranging from 1.0  103 moL.L1 to 5.0  103 moL.L1 in ACN solvent is displayed in Fig. 6. The concentration is proportional to the absorbance from LambertBeer law. It is expected that if the donor concentration changes, then the absorbance of charge transfer complex also changes. By reducing the concentration from 5.0  103 moL.L1 to 1.0  103 moL.L1 at 549.5 nm the wavelength remains same, the absorbance correspondingly reduces. This is compatible with the visibility of

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467

Fig. 5. (a) Changing the absorbance of [(1-BNZP)(CHL)] system with time. (b) Variation of absorbance of [(1-BNZP)(CHL)] with time at [1-BNZP] ¼ 103mol.L1, [CHL] ¼ 103 mol.L1.

Fig. 6. Absorption spectra of CHL (1.0  103 mol.L1) containing various concentrations of 1-BNZP at 298 K.

stable purple color throughout all concentrations of 1-BNZP compound. The BenesieHildebrand method is a good approximation method that has been used many times and gives better acceptable results. The formation constant (KCT) and molar extinction coefficient (εCT) were calculated by using the BenesieHildebrand equation [33] at 293-313 K.

Ca Cd 1 ðCa þ Cd Þ ¼ þ KCT ε ε A

(1)

In equation (1), Ca and Cd are the initial concentrations of the acceptor (CHL) and the donor (1-BNZP) respectively; A is the

absorbance of the CT-band at lmax 549.5 nm. Plotting CaCd/A vs (CdþCa), straight line is obtained which supports the formation of 1:1 [(1-BNZP)(CHL)]complex. The slope and intercept are equal 1/ε and 1/KCT ε respectively. The BenesieHildebrand plots were shown in Fig. 7 and the values of Ca, Cd, A, (Ca þ Cd) and CaCd/A are listed in Table 1. The higher values of KCT and ε are attributed to the more donating power of 1-BNZP, high electron affinity of CHL (~1.34 eV) and the high electric permittivity of ACN solvent. The energy of 1-BNZP, CHL interactions (ECT) [34] is calculated using the following relation

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Fig. 7. BenesieHildebrand plot of charge transfer complex (in ACN) at different temperatures.

3.5. Thermodynamic study of donor-acceptor complex

ECT ¼

1243:667

(2)

lCT

where lCT is the wavelength of the band of the CT complex. The values of KCT, εCT and ECT support for the stability of the DonorAcceptor complex. The experimental oscillator strength (f ), which is dimensionless quantity, used to express the transition probability of the CT-band and transition dipole moment (m in Debye) of the formed complex [35,36] were calculated through the subsequent equations:

2

3

9 6

7 4εmax , Dw1 2 5 =

f ¼ 4:32  10

(3)

where εmax is the maximum molar extinction coefficient of the band and Dw1 2 is the band width at half the maximum absorption. =

31 2

2

=

6εmax :Dw1 2 7 7 5 w =

m ¼ 0:09586 4

(4)

max

where △w1 2 is the band width at half absorbance value, εmax and wmax are the molar extinction coefficient and wave number at the maximum absorption of the complex band, respectively. The results have been presented in Table 3.

The absorbance of CT complex increases by mixing of 1.0  103 mol.L1 CHL with different concentrations of 1-BNZP from 1.0  103 mol.L1 to 5  103 mol.L1 on increasing temperature (Table 2). The effect of temperature of the CT complex [37] is shown in (Fig. 8). BenesieHildebrand equation is applied to estimate the formation constant and molar extinction coefficient at different temperatures from 293 K to 313 K (Table 1). The CT complex shows highest formation constant and lowest molar extinction coefficient values at 313 K temperature (Table 3) compared with other. The physical parameters like DH
and DS
are calculated by using Van't Hoff plot of the following equation [38].

lnKCT ¼

DH
DS
þ RT R

(5)

where DSo and DHo are entropy and enthalpy change during formation of the CT-complex. The graph ln KCT vs 1000/T produces straight line as it is shown in Fig. 9. From the linear fitted line of the graph, the values of DHo and DSo were measured in Table 3. The negative slope indicates that the CT complex formation process is an endothermic process and enthalpy is driven with a positive entropic contribution. The relatively high negative value of the enthalpy change of the Donor-Acceptor complex formation increases the strength of the bond between the donor (1-BNZP) and acceptor (CHL) compounds. Therefore, the Donor-Acceptor complex formation more as temperature increases. It strongly supports

=

N. Venkatesh et al. / Journal of Molecular Structure 1196 (2019) 462e477 Table 1 Benesi-Hildebrand data for 1:1 CT complex from 293 K to 313 K. Ca 293 K 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 298 K 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 303 K 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 308 K 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 313 K 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001

Cd

A

CaCd/A

CaþCd

(CaCd/A)x106

(CaþCd)  103

0.005 0.0045 0.004 0.0035 0.0030 0.0025 0.002 0.001

0.486 0.451 0.413 0.375 0.334 0.281 0.236 0.129

10.28  106 9.97  106 9.68  106 9.33  106 8.98  106 8.89  106 8.47  106 7.75  106

0.006 0.0055 0.005 0.0045 0.004 0.0035 0.003 0.002

10.28 9.97 9.68 9.33 8.98 8.89 8.47 7.75

6 5.5 5 4.5 4 3.5 3 2

0.005 0.0045 0.004 0.0035 0.0030 0.0025 0.002 0.001

0.518 0.484 0.446 0.41 0.367 0.316 0.261 0.142

9.65  106 9.29  106 8.96  106 8.53  106 8.17  106 7.91  106 7.66  106 7.04  106

0.006 0.0055 0.005 0.0045 0.004 0.0035 0.003 0.002

9.65 9.29 8.96 8.53 8.17 7.91 7.66 7.04

6 5.5 5 4.5 4 3.5 3 2

0.005 0.0045 0.004 0.0035 0.0030 0.0025 0.002 0.001

0.545 0.504 0.471 0.423 0.389 0.335 0.281 0.154

9.17  106 8.92  106 8.49  106 8.27  106 7.71  106 7.46  106 7.11  106 6.49  106

0.006 0.0055 0.005 0.0045 0.004 0.0035 0.003 0.002

9.17 8.92 8.49 8.27 7.71 7.46 7.11 6.49

6 5.5 5 4.5 4 3.5 3 2

0.005 0.0045 0.004 0.0035 0.0030 0.0025 0.002 0.001

0.581 0.546 0.501 0.457 0.411 0.368 0.312 0.178

8.60  106 8.24  106 7.98  106 7.65  106 7.29  106 6.79  106 6.41  106 5.61  106

0.006 0.0055 0.005 0.0045 0.004 0.0035 0.003 0.002

8.60 8.24 7.98 7.65 7.29 6.79 6.41 5.61

6 5.5 5 4.5 4 3.5 3 2

0.629 0.591 0.546 0.501 0.462 0.412 0.354 0.211

6

0.006 0.0055 0.005 0.0045 0.004 0.0035 0.003 0.002

7.94 7.61 7.32 6.98 6.49 6.06 5.64 4.73

6 5.5 5 4.5 4 3.5 3 2

0.005 0.0045 0.004 0.0035 0.0030 0.0025 0.002 0.001

7.94  10 7.61  106 7.32  106 6.98  106 6.49  106 6.06  106 5.64  106 4.73  106

469

to formation of radical ion pairs of donor (1-BNZP) and acceptor (CHL) and inner sigma (s) complex with high electrostatic interaction. The highly increase in CT complex formation indicates spontaneous in nature (Table 3). It reflects the simple composition of the Donor-Acceptor complex with the solvent interference. The standard free energy change values of the complex [(1BNZP)(CHL)] (DG
in kJ.mol1) were calculated from Gibbs equation.

DG
¼  RT ln KCT

(6)

where DG
is the free energy of the CT-complex, R is the gas constant (8.314 J mol1 K), T is the temperature in Kelvin, KCT is the formation constant of Donor-Acceptor complex (L.mol1). The DG
values for the Donor-Acceptor complex are given in Table 3. The signs of DH
, DG
reveal that the complex formation is endothermic and spontaneous. The values of DG
become high negative when the bonds between the components become stronger. Thus, the components are subjected to less physical strain or gain of freedom so that forward reaction proceeds in order to achieve equilibrium.

3.6. Determination of physical parameters from UV absorption spectra Ionization potential of donor (1-BNZP) in the Donor-Acceptor complex was calculated by using an equation which was derived by Aloisi and Piganataro [39].

ID ¼ 5:76 þ 1:53  104 vCT

(7)

where ID is the ionization potential of the 1-BNZP molecule and nCT is the wave number in cm1 equivalent to the CT band. The electron donating nature of a donor is measured by its ionization potential which is the energy required to remove an electron from the highest occupied molecular orbital. Briegleb and Czekalla theoretically derived the relation for the calculation of change of resonance energy (RN) in the formed CT complex [40].

. εCT ¼ 7:7  104 ½hvCT =jRN j  3:5 Table 2 Effect of 1-BNZP concentration with 1.0  103 mol.L1 CHL at different temperatures on the absorbance of its CT complex at lmax 549.5 nm Ca

Cd

293 K

298 K

303 K

308 K

313 K

0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001

0.005 0.0045 0.004 0.0035 0.0030 0.0025 0.002 0.001

0.486 0.451 0.413 0.375 0.334 0.281 0.236 0.129

0.518 0.484 0.446 0.41 0.367 0.316 0.261 0.142

0.545 0.504 0.471 0.423 0.389 0.335 0.281 0.154

0.581 0.546 0.501 0.457 0.411 0.368 0.312 0.178

0.629 0.591 0.546 0.501 0.462 0.412 0.354 0.211

(8)

where, nCT is the frequency of the CT band and RN is the resonance energy of the CT complex in the ground state, which is contributing factor to the formation constant of the CT complex. εCT is the molar extinction coefficient of the CT complex at maximum absorption of CT peak. The dissociation energy (W) of formed CTC was calculated with the use of energy (ECT), ionization potential of the donor (ID) and electron affinity (EA) of the acceptor.

W ¼ ID  EA  ECT

(9)

Table 3 Physical parameters of the formed CT complex. Temp (K) KCTx103 (L.mol1) ε x 103 (L.mol1cm1) DH
(kJ.mol1) DS
x102 (J.K1mol1) DG
(kJ.mol1) ECT (eV) ID (eV) W (eV) RNx102 (eV) f

m(Debye)

293 298 303 308 313

10.07 9.84 9.78 9.44 9.10

0.094 0.115 0.135 0.179 0.251

1.618 1.526 1.451 1.333 1.239

37.34

1.647

11.06 11.77 12.37 13.28 14.38

2.263

8.544

4.94

4.43 4.19 4.00 3.69 3.45

0.87 0.83 0.82 0.77 0.72

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Fig. 8. Effect of Donor concentration on the absorbance of CT complex with 1.0  103 mol.L1 CHL at different temperatures in ACN solvent.

Fig. 9. Van't Hoff plot for [(1-BNZP)(CHL)] CT complex.

3.7. FT-infrared spectral study The IR absorption spectra of the 1-BNZP, CHL and the CT complex were recorded. The complete IR data are compiled in Table 4, and the spectra are shown in Fig. S1. The electron donation process from the 1-BNZP to the p-acceptor can occur either from the lone pair of the nitrogen atom of -NH group or from the aromatic ring

[41,42]. The nitrogen atoms were identified as the donation source in several cases studied. The IR spectra of reactants and product had been used to distinguish with two possibilities. In first case, the y(NeH) and d(NeH) bands of 1-BNZP donor were shifted to lower frequencies on complex formation with decreased intensities. y(NeH) is very weak compared to donor peak and it is strongly supports the mono substituted product of CT complex. Hydrogen

N. Venkatesh et al. / Journal of Molecular Structure 1196 (2019) 462e477 Table 4 Characteristic FT-IR frequencies (cm1) and tentative band assignments. 1-BNZP

CHL

[(1-BNZP)(CHL)]

Assignments

1690s

1686s 3356w 2999w 1257mw 749m 1556m 1110m 1647mw 2938w 466w 1016m 811w

y(C ¼ O) y(NeH)

3465s 1276s 1578s 1137m 1622s 3026m 471w 995w 788m

753m 1569m

Hydrogen bonding y(C-N) y(C-Cl) y(C ¼ C) y(C - C) d N-H y(CeH) d (CH) OOP bending d (CH) deformation d (CH) OOP wag

Abbreviations: 1-BNZP, 1-benzoylpiperazine; CHL, p-chloronil; s, strong; m, medium; w, weak; y, stretching; d, bending.

bonding(Nþ-H-O-) peak 2999 cm1 with very weak intensity is also confirmed the product formation via inner sigma (s) complex. Further in second case the y(CeCl) of the free acceptor is also shifted to lower frequency. From the IR spectrum of the product, it has been observed that the y(C ¼ O) of the free acceptor molecule that exhibited at 1690 cm1 is shifted to a lower wave number value of 1686 cm1, while the y(C ¼ C) absorption band at 1569 cm1 is shifted to a lower value at 1556 cm1. Since CHL is derived from any acidic centers, we may conclude that the molecular complexes are formed through pep* and/or nep* charge migration from HOMO of the donor to the LUMO of the acceptor.

3.8.

1

H NMR spectra

The 1H NMR spectra of the complex was recorded in DMSO and shown in Fig. S2. The 1H NMR data listed in Table 5. Using the 1H NMR study, we have identified the nature of interactions between the donor and the acceptor of the resulted product (shown in Scheme 1). The formation of the complex was confirmed by the appearance of new signals in the spectrum. The broad multiplet (quartets of doublets) is observed between 2.0 and 4.0 ppm in the spectrum is due to the protons present in piperazine ring. There are no peaks present in between 4.0 and 6.0 ppm, which clearly indicates the absence of eNH proton and also confirms the mono substituted product of the Donor-Acceptor complex. The peaks observed between 7 and 8 ppm are due to the aromatic protons of 1-BNZP [43].

3.9. Mass spectroscopy The electron impact mass spectrum of mono substituted product of 1-BNZP / CHL CT complex (Fig. 10) is showed in the molecular ion peak (Mþ at 399 a.m.u.), confirming the formation of mono substituted product of 1:1 donor ~ acceptor CT complex. The molecular ion peak relative to 1-BNZP (m/z ¼ 191.1; R.I. ¼ 100%) was also detected in the complex fragmentation.

Table 5 1 H NMR spectral data of mono substituted product of Donor-Acceptor complex. Compound [(1-BNZP)(CHL)]

Chemical shift, d (ppm)

Assignments

7.461 8.864 2.073e3.757 1.234

(s, 3H, Ar-H of 1-BNZP) (s, 2H, Ar-H of 1-BNZP) (br, m, 8H, piperazine of 1-BNZP) (s, 6H, DMSO solvent)

br- broad; s-singlet; m-multiplet.

471

3.10. Comparative study of thermograms for 1-BNZP, CHL and their CT-complex The comparative study of thermograms for 1-BNZP, CHL and their CT-complex were carried out by DTG- 60H thermal analysis in a nitrogen atmosphere with a flow rate of 50 ml min1 and heating rate 10
C min1 in the temperature range 20e1000
C (shown in Fig. S3). Thermo gravimetric (TGA) and differential thermo gravimetric (DTA) analysis [44] were carried out in the temperature range of 0e1000
C using 6.057 mg, 3.821 mg, and 0.766 mg for 1BNZP, CHL, CT-complex, respectively in order to confirm the charge transfer interaction and thermal stability of the CT-complex. The thermogram of the donor (1-BNZP) exhibits the decomposition in one step at 246
C with weight loss close to 98.58% (DH ¼ 187.18 J/gm) and acceptor also shows one decomposition step at 224
C (DH ¼ 439.40 J/gm) with a weight loss of approximately 98.77% with a complete loss in each case, at higher decomposition temperature for donor and acceptor in mono substituted product of CT complex compared with free donor and acceptor (shown in Fig. S3). TGA of [(1-BNZP)(CHL)] system exhibits two decomposition steps at 184
C and 251
C with weight loss of 66.58% and 22.65% respectively. The decomposition at 251
C with weight losses 22.65% is probably due to the donor (1-BNZP). This has been attributed to the formation of CT-complex and its stability compared to its constituents. 3.11. Morphology-EDX study The surface morphology [45] and elemental analysis of pure mono substituted product of CT complex were studied by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) respectively, and the results are shown in Fig. 11. A clear plate like micro structures of product is visible from the SEM images. The high stoichiometric mono substituted product of CTcomplex formed with donor and acceptor was confirmed by EDX spectra which showed peaks C, N, O and Cl elements (listed in Table 6). By observation, the structural size is approximately in nanometers range. 3.12. Theoretical DFT study The Density functional theory computations were carried as per Becke's three parameter gradient-corrected exchange potential and the LeeeYangeParr Gradient-corrected correlation potential (B3LYP), and important calculations were done by using 6-31G basis set in both gas phase and PCM. These models have been used for optimization of geometry and forecasting some important electronic properties. The optimized values of bond lengths, bond angles, molecular electrostatic potential map values, interpretation of the HOMO-LUMO orbital [46] surfaces and Mulliken atomic charges were computed. The optimized structures of 1-BNZP, CHL and 1:1 [(1-BNZP)(CHL)] CT complex with atomic number are shown in Fig. 12 and Fig. 13. The optimized bond length values compiled (Table S1) in gas phase shows that the carbon oxygen bond length of CHL in CTC C1eO8 and C4eO7 of the CHL moieties of the complex increased to 1.2402 Å and 1.2405 Å compared with 1.2079 Å and 1.2079 Å for free CHL. This result indicates that the bond length of carboneoxygen bond in CHL increases when compared to double bond in free CHL, and thus confirming the electron transfer from the labile lone pair of electron of the 1-Benzoylpiperazine (-NH) nitrogen towards the carboneoxygen bond of CHL. It is important to report the orientation of the 1-BNZP nitrogen was found to be oriented towards the CHL and this orientation produced the resonating structure from the electron transfer to the CHL of the CT

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Fig. 10. Mass spectrum of mono substituted product of the CT-complex obtained in dimethyl sulfoxide.

Fig. 11. SEM images and EDX spectrum for mono substituted product of the Donor-Acceptor complex.

Table 6 Elemental analysis by EDX spectroscopy. Atom

C17H13Cl3N2O3 (Calculated %)

C17H13Cl3N2O3(Observed %)

C N Cl O

51.09 7.01 26.61 12.01

53.82 6.82 24.57 11.10

complex. The bond lengths of CHL; C5-C6 and C2-C3 increased to 1.3477 Å and 1.3471 Å respectively, compared to 1.3221 Å for free CHL. These results indicate that C¼C group double bond nature decrease, conforming the production of resonating outer p-complex, whereas the bond lengths of 1-BNZP decrease in the complex as compared to itself. This indicates the non bonding electron transfer from the HOMO of 1-BNZP to the p*LUMO of CHL moiety. The C7-O14 bond length is decreased to1.2565 Å compared with

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473

Fig. 12. Optimized geometries in gas phase (A) 1-BNZP (B) CHL (C) [(1-BNZP)(CHL)] complex.

Fig. 13. Optimized geometries in (PCM) solvation model (A) 1-BNZP (B) CHL (C) [(1-BNZP)(CHL)] complex.

1.2582 Å of free 1-BNZP which clearly indicate the increase in the nature of the double bond without affecting the complex formation. Due to the increase in the electron density of CHL in the complex, the expansion of bond lengths takes place. It mainly indicates that the electron density on 1-BNZP moiety of the CT complex decreases, which results in the contraction of bond lengths compared to 1-BNZP. By the use of PCM model (in ACN) (Table S1) similar results were found. The results which are obtained by using PCM model are slightly higher compared to the gas phase. The CT complex was further confirmed from the changes in its bond angles as compared to the reactants as shown in Table S2. The CHL free radical anion (Scheme 1) can further give evidence from the increasing bond angles of C6eC1eO8 and C5eC4eO7 of CT complex (from 121.509
to 122.084
and from 121.509
to 120.919
). Moreover, the bond angle of C2eC1eC6 decreases to 115.68
compared to 116.98
of CHL. Similar results obtained while applying PCM model observed in Table S2. In the case of 1-BNZP, the bond angles between ring carbon atoms decreases in CT complex as compared to 1-BNZP which confirms the pep* transition from HOMO to LUMO s of the characterized CT complex. 3.13. Molecular electrostatic potential surfaces The MEP’S were extrapolated about how attractive or repulsive a

molecule is to a proton placed at any point surrounding the molecule [47]. The MEP surfaces were calculated by the DFT (B3LYP) method by using basis set (6-31G) for optimization in gas phase/PCM, as shown in Fig. 14. The acceptor (CHL) MEP plot is characterized by a positive region (blue) which is located at the centre (surface map value of 0.0592 a. u.). The negative charge region is coming from O (0.0384 a. u.) atoms C¼O group of CHL. The donor (1-BNZP) major negative region (red) is located on the N4 atom (0.0385 a. u.) and donor (1-BNZP) interacts with acceptor (CHL) positive region of acceptor (CHL) consequently. Thus the value is decreased to 0.0394 a. u. and the donor's (1-BNZP) electron density on N4 atom is decreased to get positive region with the surface map value, which approximately is 0.0482 a.u. These results confirm the electron transfer from n-electrons of the N4 atom of 1-BNZP to C¼O groups of CHL. The surface map values of PCM model also gave similar results. 3.14. HOMO-LUMO orbital energies Molecular orbital analysis exhibits the frontier molecular orbitals are mainly composed with p- atomic orbitals [48]. HOMOLUMO energy (in eV) calculation of [(1-BNZP)(CHL)] complex in ground state is obtained by DFT (Gas phase/PCM) and protonated form of Donor-Acceptor complex DFT study obtained with basis set

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Fig. 14. Molecular electrostatic potential (MEP) maps of 1-BNZP, CHL and [(1-BNZP)(CHL)] complex in gasphase/PCM.

(6-31G) at B3LYP (shown in Fig. 15). From the figure, LUMOs are mainly delocalized on the CHL moiety, while HOMOs are localized on 1-BNZP only. The molecular orbital HOMO was localized on 1BNZP part of the complex and particularly on N-atomic orbital. Thus, one concludes that the n-electrons are localized in HOMO molecular orbital of donor. The other molecular orbital HOMO-3 and HOMO-4 are localized on the p-atomic orbitals of the benzene moieties of 1-BNZP in the CT complex, this HOMO can be

considered as p molecular orbitals and LUMO is p* molecular orbital (Fig. S4 Fig. S5). Consequently, the observed transition can be assigned to be nep* or pep* transition. The comparisons of experimental UV spectra (wavelengths and energy) of CT complex in ACN medium to the (Table 7) HOMO / LUMO transition energy differences (eV) in gas phase/PCM and in protonated DFT (PCM/6-31G) are clearly resulted the red shift of wavelength(lmax). The energy values for HOMO's and LUMO's (gas

Fig. 15. HOMO-LUMO energy gaps of CT complex in Gas phase/PCM/Protonated DFT.

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475

Table 7 The Calculated data of CTC by using DFT (B3LYP-6-31G) and experimentally (in ACN). Assignment of electronic transition

HOMO / LUMO

E(eV) (6-31G)

Experimental data

Gas phase

PCM

Protonated DFT

lmax (nm)

E(eV)

2.0296

2.2046

2.256

549.5

2.263

1 eV ¼ 96.485 kJ mol1

phase/PCM) of CHL, 1-BNZP and [(1-BNZP)(CHL)] CT complex were contributed in hatrees (Table S4) and it was interesting to note from this, LUMO energy values of the [(1-BNZP)(CHL)] CT complex (0.16474 Ha and 0.15722 Ha) are close to the LUMO energy of CHL (0.16800 Ha and 0.16921 Ha). While HOMO energy values (0.23933 Ha and 0.23824 Ha) of the CT complex are close to the HOMO energy value of 1-BNZP (0.21474 Ha and 0.21649 Ha), in Figs. S4 and S5, some frontier molecular orbital are exhibited. This reason for localization of frontier molecular orbital of [(1BNZP)(CHL)] complex system is very similar to the electron donor-acceptor composite systems. Hence, orbital interaction energy arises mainly due to the charge transfer from electron occupied to unoccupied molecular orbital.

change was observed on O7, O8 suggesting that the unoccupied molecular orbitals are localized on these atomic centers. It is also observed that the decrease of positive charges on the chlorine atoms of the complex relative to free CHL could be attributed to the electron charging of these centers in radical ion pair and inner sigma complex (Scheme 1). From observation of the 1-BNZP, the decrease of effective negative charges of donor N4, O14 atoms compared to the free donor 1-BNZP supports the charge transfer process from 1-BNZP to CHL, and the slight change in the effective charge on benzene ring carbon atoms of donor 1-BNZP upon complexation. This information completely emphasizes that the Mulliken atomic charges are useful to explain about the formation of CT-complex between 1-BNZP and CHL.

3.15. Mulliken atomic charges The Mulliken atomic charges play a key role in the relevance of quantum mechanical computations to the molecular systems. The optimized values of Mulliken charges are shown in Table S3. The comparison of Mulliken charge distribution [49] of donor (1-BNZP), acceptor (CHL), CT complex [(1-BNZP)(CHL)] are shown in Fig. 16. The increase in the negative charges on the atoms of acceptor moieties states the transfer of charge from 1-BNZP to CHL. A large

3.16. Reactivity descriptors from DFT analysis The reactivity parameters such as ionization potential (I), electron affinity(A), chemical potential(m), hardness(h), and electrophilicity index (u), Softness (s) all derived from the HOMO and LUMO energy values [50]. The following equations provided for reactivity parameters.

Fig. 16. Pictorial representations of mulliken atomic charge distribution of 1-BNZP, CHL and CT complex.

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N. Venkatesh et al. / Journal of Molecular Structure 1196 (2019) 462e477

I ¼  EHOMO

(10)

A ¼  ELUMO

(11)

h ¼ ðI  AÞ=2

(12)

m ¼  ðI þ AÞ=2

(13)

.

u ¼ m 2 2h

(14)

s ¼ 1=h

(15) Acknowledgements

The electronic action of CHL and 1-BNZP molecules is revealed from Table 8. From the HOMO and LUMO energies (in eV), it is concluded that the molecules with higher EHOMO values are best donors and in contrast, those having lower ELUMO values are best acceptors. From this point of view, CHL is recognized by lower ELUMO value than 1-BNZP. On the other hand, 1-BNZP has a higher EHOMO than the CHL, hence contemplated the electron donor in the reaction. Moreover, the chemical potential is a descriptor that gives the direction of electron transfer among the molecules. Electron moves from compound with a higher m value to the compound which has low m value. Thus it is inspected that 1-BNZP has a higher m value than CHL. The electrophilicity index (u) quantifies electron philicity of molecule, as CHL has high u value; the former molecule is preferable electrophile than the latter. The values of ‘s’ and all these results reveal that 1-BNZP acts as electron donor, where as CHL acts electron acceptor.

4. Conclusions The molecular composition formed as 1:1 for the CT interactions between 1-Benzoyl piperazine and 2, 3, 5, 6-tetrachloro p-benzoquinone in ACN medium. The mono substituted product of DonorAcceptor complex was synthesized and characterized extensively using different structural elucidation methods. The BenesiHildebrand method was used for the evaluation of the formation constants (KCT) and molar extinction coefficients (εCT) in the range of 293e313 K. As temperature increases the values of KCT decrease and inversely εCT values increases. This infers to the relative strength of the donor. The Van't Hoff equation was used for finding the nature of CT complex which confirms the reaction was endothermic and it forms spontaneously. FT-IR reveals that the mono substituted product of CT complex is formed from the inner sigma (s) complex that is reflected in the weak stretching mode of eNH group. 1H NMR reveals the absence of proton on nitrogen atom is good agreement for the mechanism of charge transfer between donor and acceptor. Mass spectrum is clearly shows the molecular

Table 8 The reactivity parameters of acceptor, donor and CTC in gas phase/PCM. Parameter

EHOMO (eV) ELUMO (eV) I A

h m u s

ion peak of the mono substituted product. TGA-DTA method is used for estimation of thermal stability of compounds. SEM-EDX study is used for estimation of size (~nm) and structure of complex product. The protonated form of CT complex is analyzed from DFT-B3LYP (PCM/6-31G) and compared to DFT gas phase and PCM theoretical models. TD-DFT method is used for estimation of the absorption maxima correlates the experimental results. HOMO-LUMO orbital energy calculations reveal that there is a conceivable opportunity for charge transfer from 1-BNZP to CHL in the CT complex. MEP maps showed that the center maximum negative value of 1-BNZP decreases to some extent upon the charge transfer interaction.

Gas phase

ACN (PCM)

CHL

1-BNZP

CTC

CHL

1-BNZP

CTC

11.3232 4.5696 11.3232 4.5696 3.3768 7.9464 9.3498 0.2961

5.8433 0.7605 5.8433 0.7605 2.5414 3.3019 2.1449 0.3934

6.5125 4.4828 6.5125 4.4828 1.0148 5.4976 14.8914 0.9854

7.9778 4.6044 7.9778 4.6044 1.6867 6.2911 11.7282 0.5929

5.8909 0.8802 5.8909 0.8802 2.5054 3.3855 2.2873 0.3991

6.4828 4.2781 6.4828 4.2781 1.1024 5.3804 13.1298 0.9071

1 eV ¼ 96.485 kJ mol1

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