Spectroscopic Characterization, Molecular Modeling and DFT/TD-DFT/PCM Calculations of Novel Hydrogen-Bonded Charge Transfer Complex between Chloranilic Acid and 2-Amino-4,6-Dimethylpyridine

A charge transfer hydrogen bonded complex between the electron donor (proton acceptor) 2-amino-4,6-dimethylpyridine with the electron acceptor (proton donor) chloranilic acid has been synthesized and studied experimentally and theoretically. The stability constant recorded high values indicating the high stability of the formed complex. In chloroform, ethanol, methanol and acetonitrile were found the stoichiometric ratio 1:1. The solid complex was prepared and characterized by different spectroscopy techniques. FTIR, 1H and 13C NMR studies supported the presence of proton and charge transfers in the formed complex. Complemented with experimental results, molecular modelling using the density functional theory (DFT) calculations was carried out in the gas, chloroform and methanol phases where the existence of charge and hydrogen transfers. Finally, a good consistency between experimental and theoretical calculations was found confirming that the applied basis set is the suitable one for the system under investigation.


Introduction
The charge transfer is an interaction between the electronic donors (low ioniza-tion potential) and the electronic receptors (high electronic affinity), resulting in a complex formation in solution and shows the absorption bands in the visible region of electromagnetic radiations [1] [2].
Pauling suggested that charge transfer interaction (CT) is possible when there is a hydrogen bonding between two molecules. H-bonds can occur intermolecularly as well as intramolecularly and can exist in a non-polar environment.
Therefore, H-bonds are especially important in macromolecular and biological structures, such as proteins, and nucleic acids, where they are responsible for the structure of DNA molecules [3]- [9]. Zhao et al. confirmed that the hydrogen-bonding dynamics in electronically excited states plays a leading role in various phenomena, such as photoinduced electron transfer and fluorescence [10] [11] [12].
Aminopyridine a heterogeneous aromatic ring plays an important role in the production of medical drugs and in natural and industrial pigments and plastics [13], also in the preparation of solutions regulating the measurement of pH of seawater [14], used in technology as optical inhibitors [15], in the preparation of known polymers [16], in the biological properties of sugars [17] and as inhibitors in biological systems [18].
Because of the importance of the aminopyridine derivatives, the aim of this work is to develop new hydrogen-linked compounds that bind the pyridine derivative 2-amino-4,6-dimethylpyridine as an electron donor with chloranilic acid as an electron acceptor in solution and in solid states. The formed complexes study uses various physicochemical techniques such as elemental analysis, UV-vis, 1 H, 13 C NMR and FT-IR spectra. In order to confirm the experimental results, a theoretical computations using the density functional theory (DFT) at the basis set B3LYP/6-31 G(d,p) will be carried out to study the ground-state properties as optimization the structure, geometrical parameters, reactivity parameters, Fukui functions and molecular electrostatic potential maps (MEP).
The origin of electronic spectra and the composition of the frontier molecular orbitals will be studied using TD-DFT through the continuum polarizable solvation model PCM. The consistency between the measured and calculated results is an important aim of this work.
confirming the formation of stable charge transfer complex between the e-donor  and the e-acceptor (CLA). A comparative study UV-Vis spectrum between the reactants and the formed complex was estimated using the same concentration of CLA as a reference to prevent absorption overlap of the e-acceptor (CLA) with the absorption of the formed complex. Figure 1 shows the electronic absorption spectrum of CLA, 2-ADMeP and a mixture of CLA + 2-ADMeP in CHL, EtOH, MeOH and AN at the region 250 -700 nm. New absorption bands are found at 535.5, 530.5, 529.5 and 519.0 nm in CHL, EtOH, MeOH and AN, respectively. This attributed to the n-π * transition, which provides assurance over the complex formation. It is worthy to mention that the reactants do not show any absorption in the region of study, which confirms the production of a new compound with absorption maxima over 500 nm. Meanwhile, the donor and acceptor do not influence on the spectral characteristics of the formed complex and this means neither appear absorption maximum of the donor alone and acceptor alone in this range (500.0 -550.0 nm), which assert the complex formation. One observes in (Figure 1) a red shift on moving from acetonitrile to chloroform, confirming the sensitivity of the formed complex to the investigated solvents polarity.
In order to study the stability of the formed complex the effect of time, temperature and e-donor concentration were investigated. In the effect of time, the absorbance was recorded for a mixture of 2-ADMeP and CLA at different times. It has been found that the absorbance was constant within 2 hours, indicating that no side chemical reaction occurs. Also, the effect of temperature on the stability of the formed complex was measured by following the absorbance of 2-ADMeP-CLA complex between of 2-ADMeP with CLA at different temperatures (20˚C, 25˚C, 30˚C, 35˚C and 40˚C). Figure 2 appears that the absorbance values of the complex formed were nearly constant (slight decrease) with temperature change in the various solvents, the absorption of 2-ADMeP-CLA complex increases with increasing 2-ADMeP concentration as mentioned in effect of donor concentration. Hence, from ( Figure 2) room temperature the optimum temperature degree to form the charge transfer reaction.

Stoichiometry of the Formed HBCT Complex
The molecular composition of the formed complex (2-ADMeP-CLA) in the investigated solvents was estimated using the conventional method of continuous variations by Job's method [22] and photometric titration method [23]. Figure 3 represents the continuous variation plot where the maximum absorbance was recorded at 0.5-mole fraction indicating the formation of a 1:1 CT complex. Figure 4 represents the photometric titration plot where two straight lines were produced intercepting at a 1:1 ratio in the system [2-ADMeP-CLA]. The same molecular composition was obtained in all investigated solvents, meaning that the solvent has no effect on the complex composition. The constant molecular composition in all solvents means also, that the same HOMO of donor interacts with same LUMO of acceptor in all investigated media.     ( )

Formation Constant of the Formed Complex
where: K CT : is the complex formation constant (L·mol −1 ), max ε : is the molar extinction (L·mol − 1 ·cm − 1 ), A max : is maximum absorbance of the formed complex, A min : is minimum absorbance of the formed complex, A comp : is the complex absorption values between A max and A min , and C 2-ADMeP : is the concentration of 2-ADMeP added (mol·L −1 ).
The results of the formation constants of 2-ADMeP-CLA complex in the studied solvents (average of the formation constants) are provided in Table 1 L·mol −1 ). These results may be due to using 10.0% AN in the preparation of the e-acceptor (CLA) and indicate that the non-polar solvent is suitable for the stability of the formed complex.

Spectroscopic Physical Data for HBCT Complex
The stability of the formed complex can be evident from calculating the spectroscopic physical parameters. Six physical parameters were selected to express the charge transfer probability of 2-ADMeP-CLA complex. These parameters included the charge transfer energy (E CT ), are a measure the ease of charge transfer from the donor to the acceptor and include the transitions, n-π * and π-π * . The charge transfer energy can be calculated from Equation (  The ionization potential (I P ) [27]: is the energy desired to eject an electron of the donor molecular orbital participating in charge transfer interaction. The ionization potential of the donor can be calculated from Equation (4) where: (ν CT ): is the wavenumber in cm −1 of 2-ADMeP-CLA complex band. The dissociation energy of the formed charge transfer complex (W) was calculated by using the following relationship (5) [28]: where (E A ): is the electron affinity of the acceptor, (E A of CLA is 1.1 eV). The resonance energy (R N ) [29]: is a factor contributing to the formed complex stability (a ground state characteristic), which can be calculated using Equation (6) [ ] where ε CT : is the coefficient of the molar extinction of the complex at the CT absorption maximum.
The oscillator strength (f) [30]: is a measurement of the probability of electron transmission to form a charge transfer band and is a dimensionless quantity. Transition dipole moment (µ) [31]: is a measure that asserts the presence of proton transfer interaction in the formed complex. The oscillator strength and the transition dipole moment can be calculated by Equations (7) and (8)  : is the half of absorbance bandwidth, ε max : is the coefficient of the molar extinction of the complex at the CT absorption maximum and max ν : is the wavenumber of the formed complex at the maximum absorption. Using the previous equations, the calculated values of the spectral physical parameters of the formed 2-ADMeP-CLA complex are listed in Table 2. The values of the charge transfer energy increased slightly and gradually on moving from chloroform to acetonitrile as shown in Table 2. A further parameter can be discussed is the potential ionization values (I P ) of 2-ADMeP-CLA complex. The ionization potential recorded small values in all solvent, asserting the formation of a stable complex in all media, due to the ease of charge transfer from donor to acceptor. The ionization potential values for 2-ADMeP with CLA in all solvents are nearly the same. Thus, one can conclude that the same donor molecular orbital interacts with CLA in all the studied solvents to produce the charge transfer complex. The obtained ionization potential values recorded small values due to the high basicity of 2-ADMeP (nitrogen ring and amino group). This behaviour suggests that the electrons responsible for the basic strength of 2-ADMeP Open Journal of Physical Chemistry (n-electrons) are the same involved in the CT interaction of 2-ADMeP with CLA in all solvents. Hence, the investigated 2-ADMeP behaves as n-donor towards π-acceptor (CLA), which means that the highest occupied molecular orbital (HOMO) is the non-bonding molecular orbital and the H-bond complexing sites of 2-ADMeP is a pyridinic nitrogen atom by its lone electron pair. Therefore, the CT interaction is attributed to the promotion of non-bonding electrons of the donor 2-ADMeP to the lowest unoccupied π-molecular orbital of the acceptor CLA (LUMO). Consequently, one can deduce that the formed charge transfer complex is mainly n-π * type in all solvents. It seems that the high donating power of 2-ADMeP from the presence of two methyl groups and one amino group is presumably responsible for this situation. Also, the dissociation energy (W) of the 2-ADMeP-CLA complex was recorded high values in all studied solvents, confirming its high stability due to the existence of both charge and proton transfer. Thus, the same donor molecular orbital interacts with CLA in the studied solvents.
From Table 2, it is clear that the highest value of the oscillator strengths was in acetonitrile, confirming the high probability of charge transfer with higher polarity solvent as acetonitrile in consisting with the stability constant values. On the other hand, the value of the oscillator strengths in CHL recorded higher value compared with MeOH and EtOH, due to the presence of the hydroxyl group which leads to a hydrogen bond formation with the nitrogen atom of the pyridinic ring retarding the charge migration from the donor to the acceptor in a protic solvent.
The calculated transition dipole moment recorded high values in all solvents, confirming the formation of stable charge transfer complexation besides the proton transfer interaction. Furthermore, the transition dipole moment (μ, Debye) of the formed complex, as well as the resonance energy (R N ), follows the same trend as the oscillator strength corresponding with the values of the formation constants (K CT ) of 2-ADMeP-CLA complex formed in all the solvents ( Table 1). The values increase as follows: MeOH < EtOH < CHL < AN. One can deduct from this, the stability of the formed complex is attributed to the presence of two interactions, the charge transfer (CT) and proton transfer (PT). The highest probability of the charge transfer interaction and proton transfer was in the highest polarity and aprotic solvent acetonitrile compared with other solvents.

Determination of Thermodynamic Parameters of the HBCT Complex
After studying the effect of temperature on the formed 2-AMDeP-CLA complex in all used solvents, minimum-maximum absorbance Equation (1) was applied to calculate the formation constant of the complex formed at different temperatures grades. In addition, Van't Hoff equation was applied to calculate the thermodynamic parameters (ΔH˚, ΔS˚) through the following relationship [33]: where: ΔH˚: is enthalpy of the formed CT complex, ΔS˚: is the entropy of the formed CT complex, T: is the absolute temperature in Kelvin and R: is the gas Plotting the values of lnK CT versus 1000/T, a straight line was obtained, Figure   7. The slope and intercept were equal to −ΔH˚/R and ΔS˚/R, respectively. The values of K CT and the thermodynamic parameters in different solvents were collected in (Table 3).
Furthermore, standard Gibbs free energy change of the complexation process (ΔG˚) for 2-ADMeP-CLA complex was determined from the K CT value at room temperature using the Equation (10) [34]: where ΔG˚: is the free energy change of the formed complex (k·J·mol −1 ) and K CT is the formed CT complex formation constant at 25˚C.
From Figure 6, the positive slope indicates that the CT complex formation is exothermic, reflecting the decrease in K CT with increasing the temperature in acetonitrile (AN). From the results in Table 3 (Table 3) indicate the spontaneous and exothermic nature of the reaction between the donor and the acceptor with strong interaction. Also, the values of ΔG˚ correspond with the formation constants values of the complex (K CT ), where the values increase as follows: MeOH < EtOH < CHL < AN, which make the donor and acceptor undergo to lose a degree of freedom or more physical strain [38].

FT-IR Spectra of the Solid HBCT Complex
The formation of 1:1 charge and proton transfer complex between 2-ADMeP with CLA was established from a comparison of the FTIR spectra of the complex with that of the reactants (2-ADMeP and CLA). The most important infrared bands of the e-donor (2-ADMeP), the e-acceptor (CLA) and the HPCT complex (2-ADMeP-CLA) are shown in Figure 7. The infrared band assignments are given in Table 4. Interestingly, the spectra of the reaction product 2-ADMeP-CLA complex contain the main infrared bands for both the reactants. This strongly supports the formation of the charge transfer complex. However, the absorbance of the donor and acceptor in the formed product showed some changes in band intensities and in some cases small shifts in the frequency wave-number values. These changes could be attributed to the expected symmetry and electronic structure modifications in both donor and acceptor units in the formed products relative to the free molecules. The infrared interpretation for the HBCT complex was dealt with as follows: the FTIR spectra of the complex are shown in Figure 7, where the asymmetric and symmetric stretching vibration of the amino group is broadened and appeared at 3299 cm −1 and 3139 cm −1 comparing with 3354 cm −1 and 3158 cm −1 , respectively for 2-ADMeP itself. The free 2-ADMeP showed two absorption bands at 2979 and 2918 cm −1 and are safely assigned to ν (CH) aromatic and aliphatic, respectively. These two bands are disappeared upon complex formation with CLA. On the other hand, the absence of ν (OH) of CLA acid at 3225 cm −1 in the formed complex is due to the proton transfer from CLA to 2-ADMeP. The carbonyl group appeared at 1655 cm −1 in the complex spectrum compared with 1661 cm −1 for free CLA. This result showed that the carbonyl stretching frequency is supporting the involvement of the carbonyl group in H-bond with the neighbouring amino group. In addition, the ν (C-Cl) of chloranilic acid is affected by the complex formation and it is shifted to 826 and 714 cm −1 upon complexation compared with 837 and 750 cm −1 for free CLA, confirming a charge transfer from a donor (2-ADMeP) to an acceptor (CLA). One can also observe ( Figure 7) the presence of abroad and intense absorption in the 1600 -800 cm −1 region in the complex spectrum which confirming the presence of OHN hydrogen bonding between 2-ADMeP and CLA (OH … N) [4]. An important finding from Figure 7, is the appearance of a vibrational band at 2900 cm −1 complex spectrum which could be assigned to be ν (    and δ = 6.19 ppm to δ = 6.58 and δ = 6.59 ppm, this down-field shift is expected due to the change in electronic structure upon complexation. Hence, 1 H NMR spectra revealed the formation of HBCT complex (2-ADMeP-CLA) in agreement with electronic and infrared spectra.     Table 7 where one can deduce for the proton transfer hydrogen bond O21-H20-N6, the increase in OH bond length from 1.555 to 1.671 and 1.713 Å on moving from gas phase to chloroform and methanol while the NH bond length decreases from 1.069 Å in the gas phase to 1.046 Å in chloroform and 1.042 Å in methanol. This result supported the increase in hydrogen bond strength on moving from gas phase to methanol due to increasing the electrostatic interaction between O − and NH + wings of the proton transfer hydrogen bonding. On the other hand, the OHN bond distance lays in the region 2.624 to 2.755 Å of strong hydrogen bonding.     donor to acceptor. Furthermore, C26-O31 bond is increased to 1.244 Å compared with 1.228 Å for CLA alone, while C27-O28 is shortened to 1.317 Å with respect to 1.322 Å for free acid, which is due to their involvement in the hydrogen bonding. Consequently, the optimal bond lengths support the existence of proton and electron transfer in the formed complex.

Molecular Geometry Optimization of the Molecules
Regarding the dihedral angles between the two methyl and the amino groups with the pyridine ring, it has been found that the dihedral angles between the two methyl groups and the pyridine ring (C13-C5-N6-C1 and C9-C3-C2-C1) recorded 178.276˚ and 179.805˚, respectively, which confirms the co-planarity of these groups with the pyridine ring. The same situation was found for the amino group dihedral angle, N17-C1-N6-C5, with the pyridine ring where it recorded 179.165˚. Hence, the electron density increases on donor moiety, leading to strong e-transfer to CLA acceptor and produces a stable complex, in concordance with the measured results.
Based on the electronic reactivity parameters given in Table 8  is e-donor and CLA is the e-acceptor. Furthermore, as one can deduce from Table 8, the lower energy gap of the complex (2.852 eV), suggesting its high reactivity. The lowering is smaller in the gas phase than in chloroform or methanol, resulting in high reactivity in the gas phase with respect to solvents.

Theoretical Electronic Spectra
Based on the fully optimized structure of the formed complex 2-ADMeP-CLA, TD-DFT/B3LYP/6-31 G(d, p) has been used to calculate the electronic spectra in chloroform and methanol through adding polarizable continuum solvation model PCM, PCM-TD-DFT. The measured experimental values of λ max in chloroform and methanol will be compared with those computed from PCM-TD-DFT/B3LYP/6-31 G(d, p) method ( Figure 12 and Figure 13), respectively. According to Franck Condon principle, the maximum absorption peak in UV-Vis is attributed to vertical excitation only. The calculated electronic spectra of HBCT complex in chloroform and methanol is shown in (Figure 14).
The measured electronic spectra of 2-ADMeP-CLA complex in chloroform and methanol exhibited absorption maxima at 535.5 and 529.5 nm, respectively.
To obtain the absorption spectra from the optimized ground state geometry, the energy was calculated for the first six vertical excitation states (S1, S2, S3, S4, S5 and S6) in chloroform and methanol. The calculated wavelengths and oscillator strengths are collected in (Table 9). The calculated absorption spectra of 2-ADMeP-CLA in chloroform consist of two intense transition bands at 550.64 (f = 0.016) and 320.32 nm (f = 0.245). The transition at 550 nm is corresponding to 70% contribution from HOMO to LUMO (n-π * transition, (ε is lower than 1000 L·mol −1 ·cm −1 ) while the second excitation band at 320 nm is corresponding to two contributions, HOMO-2→LUMO (67%), and HOMO→LUMO+5 (11%) (π-π * transition, with ε higher than 1000 L·mol −1 ·cm −1 ). Hence, the vertical excitation energy states are So→S1 and So→S6 is the only allowed transition states with strong oscillator strengths in chloroform. In methanol, the calculated absorption spectra showed also, two intense bands at 317.59 (f = 0.232) and 540.54 nm (f = 0.015), respectively. The first transition at 317 nm is attributed to the vertical transition (S0→S5) and is corresponding to HOMO-2→LUMO (67%), HOMO→LUMO+4 (10%) and HOMO→LUMO +5 (11%), π-π * transition. The second allowed transition at 540 nm (So→S1) is attributed to HOMO→LUMO (70%), n-π * transition. Hence, one deduces from the previous section the near      Figure 12 and Figure 13 represent the pictures of occupied and virtual molecular orbitals of So→S1 and S0→S6 in chloroform and from S0→S1 and S0→S5 in methanol. In chloroform, the major transition is shown from HOMO→LUMO excitations, where the HOMO is localized on C=C, C=O, OH and chlorine atom, while the LUMO is localized on C-C, C=O and OH of CLA, suggesting that the computed transition at 550.64 nm is n-π * , while that at 320.32 nm is π-π * transition. Figure 13 represents the composition and virtual MOs in methanol for the first allowed transition at 317.59 nm, S0→S5, (taking into consideration the main transition (HOMO-2→LUMO). The HOMO and HOMO-2 are localized on carbonyl oxygen, chlorine and hydroxyl oxygen (π-electrons), while the LUMO Open Journal of Physical Chemistry and LUMO+4 are localized on the π * of CLA, supporting that this transition is attributed to ICT, (π-π * ) transition. For the second allowed transition band in methanol at 540.54 nm (So→S1), one can observe that both the HOMO and LUMO are mainly present on the n and π centres of CLA, confirming that the second band at 540.54 nm is attributed to internal n-π * transition through CLA. It is worth reporting that the interference of the different MOs in HOMO and LUMO confirms the presence of hydrogen bonding in the investigated complex.

Molecular Electrostatic Potential (MEP)
MEP maps ( Figure 15) are a good tool that describes the distribution of electrostatic potential over the surface of a molecule that explores the electrophilic and nucleophilic attack regions and hydrogen bonding interaction. The electrostatic potential is labelled by different colours where blue represents the positive region, green represents the neutral region and red represents the negative region. In our system, 2-ADMeP exhibited a red colour on the pyridinic like nitrogen supporting its contribution as hydrogen bond acceptor and n-donor (nucleophile). Orange colour is distributed among the pyridine ring confirming its contribution as π-donor in the studied CT reaction. Regarding CLA, one can see the blue colour on the OH groups supporting its consideration as H-bond donor (electrophile). On the other hand, the blue colour is distributed among the CLA ring confirming its capability as π acceptor. Considering the complex 2-ADMeP-CLA, it has been found that the blue colour covers the pyridine ring with the disappearance of the red colour. On another hand, red and orange colours cover the CLA part with the disappearance of the blue one. This confirms the charge transfer from the e-donor 2-ADMeP towards the e-acceptor CLA.
Hence, the reported MEP maps revealed the existence of charge transfer besides hydrogen bonding in the studied complex, consisting of the previous results.