Computational Study of the Stability of Tautomers and equilibrium constants of Cyanuric acid ( CA ) in Different solvents

3485 | P a g e J a n u a r y 2 2 , 2 0 1 5 Computational Study of the Stability of Tautomers and equilibrium constants of Cyanuric acid (CA) in Different solvents. Numbury Surendra Babu* and #Didugu Jayaprakash *Department of Chemistry, Hawassa University, Hawassa, Post Box No: 5, Ethiopia. Email:nsbabusk@gmail.com #Department of Chemistry, Acharya Nagarjuna University, Nagarjunanagar, Guntur, A.P, INDIA Email: didugujayaprakash@gmail.com ABSTRACT


INTRODUCTION
Studies of tautomerism phenomena have been valuable in many areas of chemistry as demonstrated by several reviews of experimental and theoretical studies in chemistry and biochemistry [1][2][3][4][5] .The experimental studies on tautomerism are still a challenging problem in chemistry and molecular biology. Most tautomers are not observed in the experimental studies because of their low concentration. A detailed analysis of the structure and changes in geometrical and energetic parameters caused by the migration of hydrogen atom would enable us to understand the different properties of tautomers. Knowledge of the relative stabilities of tautomeric forms of heterocycles as well as the conversion from one tautomeric form to another is important from the point of view of structural chemistry.
In this research article we calculates various parameters to determine, the theoretical possible tautomers of Cyanuric acid. Tautomeric equlibria are important and it is impossible to correctly interpret the detail mechanism of reactions of tautomeric heterocycles without knowing the dominant tautomeric structure. Equally, it is impossible to correctly interpret the biological activity and function of potentially tautomeric heterocycles without representing them in correct structure. Also, understanding of the relative stabilities of heterocyclic tautomers and any subsequent conversions between tautomeric forms is very vital for both structural chemists and biologists [12] . Along this line, relative stabilities of various tautomeric structures of five-, six-and seven-membered heterocyclic rings were investigated using both theoretical and experimental tools [13] .Tautomerism interconversion has been investigated by chemists during last decades. Recently, study of tautomerism received renewed attention due to its importance on determination of compounds' properties and their area of applications. The importance of tautomerism is revealed more since in recent years the investigation about tautomerism has been the major topic in theoretical chemistry.

COMPUTATIONAL SECTION
Molecular geometries of tautomeric forms of Cyanuricacid were fully optimized by using the Gaussian quantum chemistry software package Gaussian 09w [14] . Programs at the HF level of theory, using the 6-311++G(d,p) basis sets [14] . Initial geometry generated from standard geometrical parameters minimized without any constraint in the potential energy surface at HF level, adopting the standard 6-311++G(d,p) basis set. Following the geometry optimizations, analytical frequency calculations were proceeded at the HF/6-311++G (d,p) level, following standard procedures, to obtain the thermochemical properties. In addition the effects of solvents on the tautomeric structure properties were studied by means of the self-consistent reaction-field (SCRF) method based on PCM developed by Tomasi and coworkers [15] . It is one of the most widely used approaches. In this model, a solute is considered inside a cavity and the solvent as a structureless medium characterized by some parameters such as its dielectric constant, molar volume and Polarizability. This consideration can substantially improve the simulation results for the electronic or vibrational spectroscopy of real molecular systems [16,17] .The solvents chose for this studies are polar protic solvents namely water (ε = 74.80) andethanol (ε = 24.55) and polar aprotic solvents liketetrahydrofuran(THF) (ε = 7.50) and dimethylformamide (ε = 38.00).
In the present paper, we report the relative stability, Gibbs free energy, enthalpy and entropy, tautomeric equilibrium constants and dipole moments for the Cyanuric acid 298.15 K. After having predicted the relative stability of tautomers, we have found the tautomeric equilibrium constants with respect to the more stabletautomer in the gas phase and indifferent solvents.

DIPOLE MOMENT
As we can see from Table 2, the calculated dipole moments are changed on moving from thegas phase (ε = 1) to solvents [THF(ε = 7.80);ethanol (ε = 24.55); DMF (ε = 38.00) and water (ε = 78.4) and the dipole moments are sensitive to the polarity of the medium. The calculated dipole moments are substantially higher in a medium of high relative permittivity, mainly due to major charge redistribution in the molecule, and also by changes in the distances between the charge separations. The magnitude of the influence of the solvent reaction field on electronic structure is different in different tautomers. This may also explain the great variation of the calculated dipole moments of the tautomers.
The CA8 tautomer has the largest dipole moment and the CA11 has the smallest dipole moment in all phases. Having the largest dipole moment the CA8 tautomer is expected to have the strongest interaction with polar molecules of water in biological environment. The structure of a tautomer obviously affects the magnitude and orientation of a dipole moment. For example, a transition from an keto-to a enol-form usually leads to an increase of the dipole moment, and upon a transition from one enol form of CA11 and tautomer to the keto form of CA1 form we observe 0.0002 and 0.0013D dipole moment change respectively. However, the position of hydrogen attached to O in the ring of cyanuric acid has much greater influence on the dipole moment. Not only does it affect the magnitude of the dipole moment but it even changes its orientation. This is indicated by the fact that the orientation of dipole moments in the CA2 and CA3, tautomers of cynuric acid with H attached to the O atom is almost opposite, and the similar effect to absevered in all cynuric acid tautomers.The calculated dipole moments for the studied tautomers at the HF/6-311++G (d,p) level in solution are given in Table 2. The order of the dipole moment of tautomers in gas phase is CA11 > CA1 > CA10 > CA4 > CA3 > CA6 > CA2 > CA5 > CA7 > CA9 > CA8. The figure 3shows the effect of solvents on dipole movements for tautomers of cynuric acid. The two tautomers CA8 and CA9have large magnitude in total energy differences in gas phase and large dipole moments ( Table  2). This leads to the inversion of their order of stability in polar solvent. J a n u a r y 2 2 , 2 0 1 5 (1) J a n u a r y 2 2 , 2 0 1 5 When a molecule is placed in a liquid environment, it is polarized, i.e., its electrostatic moments are changed giving rise to induced multipole moments. The amount of polarization will depend on several factors including, of course, the Polarizability of the solute and the electric field provided by the environment. As seen from the Figure 4. inthe gas phase the polarizabilities are low and in all the solvents are high values, the order of polarizabilities is Gas > THF > ethanol > DMF > water, due to increase of dielectric constant of solvents. The largest Polarizability was observed for CA8 tautomer in all phases.CA8 tautomer is more polarizable than other tautomers because of its dipole moment is high value in all solvents.

TAUTOMERIC EQUILIBRIA IN THE GAS PHASE AND SOLUTION
Cyanuric acid exists mainly in eleven tautomeric forms (CA1-CA11) that are in equilibrium. CA1 has a keto form and can be converted to CA2 and CA3 tautomers through a proton transfer and rotation around the C-O bond, simultaneously. The CA4 to CA9 isomers give the dihydroxy form by two protons transfer and considering the orientation of the hydroxyl J a n u a r y 2 2 , 2 0 1 5 hydrogen's. CA10 and CA11 are converted try hydroxyl isomers through three proton transfer.Therefore, only eleven isomers were considered in the present study.
The HF method calculated tautomeric equilibrium constants with respect to the most stable tautomer CA1 of Cyanuric acid both in the gas and in different solution are listed in Table 3. The tautomeric equilibrium between tautomers a and b is described as Equilibrium constants for each species were calculated by using the following equation Where KT is the tautomeric equilibrium constant between the tautomers, the gas constant R is 1.987x10 -3 kcal/mol; and the temperature T is 298.15 K. The T pK values of the studied molecules were calculated by means of the following The equilibrium between the tautomers 1 and 3 of cyanuric acid values for show that the CA2 is more dominant than the CA3, both for the gas and different solvents with a pKT value of 13.0,(gas) and 11.93(THF) , 11.66 (ethanol),11.64 DMF and 13.01(water) respectively. This is confirm to thermodynamic data of CA1↔CA3, see the table 3. The remaining tautomers are not detectable amounts because of the T pK values are very high.
From the Table 4 the T pK values are decreasing with increase in polarity of solvents. Solvent effects have been ascribed to two major components [18] , electrostatic solvent-solute interaction and hydrogen bonding. The hydrogen bonding effects cannot be estimated in a quantitative manner from the salvation model, and needs further large-scale calculations. The electrostatic solvent-solute effects, however, are readily estimated by the reaction field continuum model [19] using the dipole moments and molecular polarizabilities. Application of the reaction field continuum model leads to an explanation of the change in order of tautomeric stability on going from gas phase to solution. However, such a treatment lacks explicit consideration of base-water hydrogen bonding effects [20] .
From the interconversiondiagram, to evaluate in a more realistic way theconstants of equilibriums as represented in Figure. 5. Table 5 contained the T pK of the equilibrium constants calculated starting from the general outline of interconversion. Some T pK were positive and the other negative; that determinate the privileged direction of equilibrium.
If the T pK was positive, equilibrium moved from right towards the left and when it was negative, equilibrium moved from left towards the right. By taking account of all these directions of displacement, we obtained.

CONCLUSION
Tautomers of Cyanuric acid acid have been studied by using HF methods in the gas phase and in different solvents (IPCM). The following conclusions have been drawn from the present study:All the optimized tautomers present at the stationary points are corresponding to local minima in the potential energy surface. The keto form CA1 was found to be the most stable form over than all enol forms in the gas phase and different solvents. The influence of the polar environment substantially enhanced the dipole moment for all the tautomers in going from the gaseous to aqueous phase, which indicates that there is an increase in stability of the molecular system due to the solvent-molecule interaction and redistribution of the charge. The T pK values are decreasing to increase the polarity of solvents. Solvent effects have been ascribed to two major components, electrostatic solvent-solute interaction and hydrogen bonding.