Effect of solvents on tautomeric equilibrium of acetyl acetone: A Theoretical study

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The proton transfer between interconversion tautomers is of importance in synthetic chemistry, such as: keto-enol, imine-enamine, oxime-nitroso [1, 2]…. The keto-enol tautomerization, especially in the -diketone compounds is a common one. Acetyl acetone, one of the -diketone compound, was studied experimentally early and thoroughly [1, 2]. This compound usually exists an equilibrium mixture of enol and keto tautomers with equilibrium

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Nội dung Text: Effect of solvents on tautomeric equilibrium of acetyl acetone: A Theoretical study

Journal of Chemistry, Vol. 44 (2), P. 249 - 254, 2006 effect of solvents on tautomeric equilibrium of acetyl acetone: A Theoretical stUdy Received 22 June 2005 Nguyen Thanh Cuong, Le Kim Long and Dang Ung Van Center for computational chemistry, Faculty of Chemistry College of Natural Sciences, Vietnam National University, Hanoi summary The tautomeric equilibrium of acetyl acetone in gas and different solvents were studied by electronic structure calculations. The mechanism of reaction was proposed. In gaseous phase, the results of B3LYP level are more agreement with experiments than that of HF level. The cis-enolic form was found to be the most thermodynamic stable structure. The solvent effects on tautomeric equilibrium are estimated by performing self-consistent reaction field (SCRF)-Onsager and PCM models at B3LYP level. The latter is shown of a better model for solvation. The solvents are shown to be not effecting on the thermodynamic stabilities of enol or keto form, but have more influence on the transition state. In solvents, the activation energy decreases ca. 20 kcal.mol-1. The enol/keto concentration ratio in different solvents were calculated and compared with experimental data. In more polar solvent, the more content of keto form was found. I - Introduction constant Ke ( K e = [enol ] ). The proton transfer between interconversion [keto] tautomers is of importance in synthetic Enol form of acetyl acetone exists mainly in chemistry, such as: keto-enol, imine-enamine, gaseous phase, however, the enol/keto ratio oxime-nitroso [1, 2]…. The keto-enol depends on strongly the polarization of solvents. tautomerization, especially in the -diketone The equilibrium constant measured by 1H-NMR compounds is a common one. Acetyl acetone, spectroscopy indicates a higher enol content in one of the -diketone compound, was studied apolar aprotic than in dipolar protic or dipolar experimentally early and thoroughly [1, 2]. This aprotic solvents [2]. On the other hand, enol compound usually exists an equilibrium mixture form of acetyl acetone exists two confor- of enol and keto tautomers with equilibrium mations: cis–enolic form and trans-enolic form. H O CH3 TS1 O O TS2 O O C C C C C C CH3 CH OH CH3 CH2 CH3 CH3 CH CH3 trans-enolic tautomer Keto tautomer cis-enolic tautomer The purpose of the present study is to re- of acetyl acetone by electronic structure examine the keto - enol tautomeric equilibrium calculations in gas phase and the effects of 249 solvent on tautomeric equilibrium. solute is placed in a uniform electric field of solvent with a dielectric constant . The solute is II - Calculation Methods assumed to occupy a spherical cavity of radius a0 in the medium. A dipole in the molecule is All calculations were carried out using induced by a dipole of the medium and vice Gaussian 98. A3 [4]. All geometries of versa. The electric field applied to the solute by structures were fully optimized and frequencies the solvent dipole in turn interacts with the were calculated at the RHF/ 6-31+G(d) and molecular dipole to lead to net stabilization. B3LYP/6-31+G(d,p) level of theory. Zero-point This model has a major drawback that the vibrational and thermal corrections were molecule is a sphere that is usually far away calculated at the same level and scaled by 0.9 at from the realistic picture. In the second model the HF level [3, 4] and 0.98 at the B3LYP level (polarized continuum models-PCM), solvent is [3, 4] to account for the overestimation of assumed to be a continuous medium with a vibrational frequencies at these levels. The dielectric constant that surrounds a cavity scaled ZPE corrections were included in the containing the solute and the shape molecule to relative energy values (RE). The transition afford more accurate solvation energies. We states were found by Synchronous Transit have considered various solvents with dielectric Guided Newton (STQN) algorithm (Opt = constants 20.7 (acetonitrile), and 78.39 (water), QST2) at two levels. The transition states were and 2.247 (benzene) to understand the solvent characterized by frequency calculation and effects on tautomeric equilibrium. The radius of instrinsic reaction coordinate (IRC). the spherical cavity for the Onsager’s model was The effects of solvent on the structure were calculated by performing single-point studied by using the self-consistent reaction calculations at the optimized geometry of the field (SCRF) method with two models: HF level (gas phase) by specifying the keyword Onsager’s reaction field theory [4, 5] and VOLUME as in the Gaussian 98 packages. The polarized continue model (PCM) proposed by thermodynamic parameters were calculated Tomasi and co-workers [4, 6]. In the former, the from obtained frequence results. 2-cis 1 2-trans TS1 TS2 Figure 1: The geometry and transition states at B3LYP/6-31+G(d,p) level 250 Table 1: Geometrical parameters of structures at B3LYP/6-31+G(d,p) level Bond/Angle 2-cis 2-trans 1 TS1 TS2 H8-O11 (Å) 0.963 0.965 2.98 1.234 1.27 H8-C7 (Å) 2.670 2.40 1.06 1.54 1.50 O6C5C7C10 0.04 -2.135 -60.00 5.02 -6.85 O11C10C7C5 -0.01 -178.93 -140.28 -86.7 139.69 H8O11C10C7 -0.090 0.73 -32.61 -5.01 -8.29 III - Results and Discussion structures TS1, TS2 (figure 1) have one negative eigenvalue in the Hessian matrix and 1. Structure and enol-keto equilibrium in one imaginary vibrational frequency each. gas phase The geometrical parameters of structures at B3LYP/6-31+G(d,p) level was given in table 1. The structures 1 (keto form), 2-cis (cis- The energies, relative energies, dipole moment enolic form), 2-trans (trans-enolic form) (figure of the optimized structures and transition 1) are minima at all levels with all eigenvalues structures TS1, TS2 calculated at the HF, in the Hessian matrix and the vibrational B3LYP levels of theory are given in table 2. The frequencies being positive. The transition PES of two levels is showed on figure 2. Table 2: Total, Zero Point and Relative Energies, Dipole moments* of all structures HF/6-31+G(d) level B3LYP/6-31+G(d,p) level E E + ZPE RE DM DMexp E E + ZPE RE DM 1 -343.7374906-343.6311974 -3.08 2.0086 -345.8228370 -345.703533 5.47 1.6330 2-cis -343.7338400-343.6311974 0 3.3948 2.78 -345.8323928 -345.7122503 0 3.3699 2- -345.8140774 -343.7187243 8.98 3.5044 11.32 3.4494 trans -343.6311974 -345.6942058 TS1 -343.6485367-343.6311974 51.73 4.9054 -345.7302872 -345.616663259.98 4.0400 TS2 -343.6177696-343.6311974 68.30 3.1064 -345.7333186 -345.619801958.01 2.5984 * E, E+ZPE: hartree; RE: Kcal.mol ; DM: Debye; 1 hartree=627.5 kcal.mol-1 -1 Energies and activation energies The electronic energies of all structures at the B3LYP level are lower than ones at HF level (table 1). At the HF level, the RE values show a thermodynamics preference for keto form more than enolic form. Conversely, at B3LYP level, cis-enolic form is the most stable. According to the measured 1H- NMR spectroscopy, the cis-enolic Figure 2: Schematic potential energy of geometries form occupies 92% context in at HF and B3LYP levels 251 gaseous phase [2]. So the energies at B3LYP are 2. Structure and enol-keto equilibrium in in more agreement with the experiments. These solvent phase results were also obtained from previous From the above results and the fact that theoretical calculations [9, 10]. It also shows trans-enolic form was observed experimentally that it is difficult to determine what the main in rare cases [2], the trans-enolic form is tautomeric equilibrium exits based on the excluded in solvent calculations. The solvent activation energies. Both two levels, the calculations are carried out at the B3LYP/6- activation energies are approximate value. 31+G(d,p) level with two models: Onsager and Dipole Moments PCM, in three solvents: water and acetonitrile and benzene. The calculated dipole moments of all The energies and GSol, dipole moments structures (table 2) also are quite different at the values of optimized structures and TS1 in three two levels. Compared with the experimental solvents with two models are given in table 3a, dipole moment of 2-cis (2,78 D) [7], both two b, c. In Onsager model, the GSol value is levels have the higher value. calculated by formula GSol = (E+ZPE)solvent - So the calculations at B3LYP/6-31+G(d,p) (E+ZPE)gas. GSol of PCM model is given level give a good agreement with the directly from the output file. As ZPE values in experiments and theoritical calculations and this PCM model are not given in the output file, RE level is used in solvent calculations. is calculated by formula RE= Esolvent – Egas. Table 3a: Electronic, total and relative, Gibbs free Energies, Dipole moments* of all structures based on Onsager’s Model and PCM Model in benzene solvent at 6-31+G(d,p) at B3LYP level Onsager Model PCM Model E E+ZPE RE GSolv DM E RE GSolv DM 1 -345.8228380 -345.703738 5.74 -0.13 1.6762 -345.822838024 6.55 -0.17 1.7744 2-cis -345.8332809 -345.712897 0 -0.41 3.5925 -345.833280999 0 -0.60 3.6363 TS1 -345.7650346 -345.647525 41.02 -19.37 3.9192 -345.765034677 42.82 -0.79 3.9543 * -1 -1 E, E+ ZPE: hartree; RE, GSolv: kcal.mol ; DM: Debye; 1 hartree = 627.5 kcal.mol Table 3b: Electronic, total and relative, Gibbs free Energies, Dipole moments of all structures based on Onsager’s Model and PCM model in acetonitrile solvent at 6-31+G(d,p) at B3LYP level Onsager Model PCM Model E E+ZPE RE GSolv DM E RE GSolv* DM 1 -345.8230801 -345.703998 6.47 -0.29 1.7800 -345.8222619 5.50 0.24 2.0548 2-cis -345.8346714 -345.71431 0 -1.29 3.9588 -345.831023 0 0.76 4.0692 TS1 -345.7667263 -345.649301 40.79 -20.48 4.3828 -345.7661197 40.73 -1.46 4.4857 Relative energies and solvation free energy values of cis-enolic form and keto form in two Compared with those of the structures in gas models change negligible, meanwhile the RE phase, these results show that the medium does value of TS1 changes strongly,e.g. decreases not affect on the thermodynamic stability of the from ca. 60 kcal.mol-1 to ca. 40 kcal.mol-1. So enol and keto forms, but on TS1 and therefore TS1 may be solvated by solvents and it on the tautomeric activation barrier. The RE decreases the energy of TS1. 252 Table 3c: Electronic, total and relative, Gibbs free Energies, Dipole moments of all structures based on Onsager’s Model and PCM Model in water solvent at 6-31+G(d,p) at B3LYP level Onsager Model PCM Model E E+ZPE RE GSolv DM E RE GSolv* DM 1 -345.823091 -345.7040098 6.51 -0.3 1.7846 -345.8310098 4.17 -5.25 2.3180 2-cis -345.8347365 -345.7143772 0 -1.33 3.9759 -345.8376499 0 -3.40 4.4639 TS1 -345.7668068 -345.6493854 40.78 -20.53 4.4051 -345.7797851 36.31 -10.04 4.9502 There is a different pace of the effect of Gg0(A B) dielectric constants between Onsager and PCM A(g) B(g) model. The solvation free energy of each 0 0 Gsolv(A) Gsolv(B) structure in Onsager model decreases when the 0 dielectric constants increase (from benzene to Gs(A B) water). The GSol value of each structure in PCM A(s) B(s) model changes randomly. The standard free energy of the reaction in solution[2,11] can be written as: The enol/keto ratio The enol/keto ratio or mole fraction of enol G s0 = G g0 + [ 0 Gsolv ( B) 0 Gsolv ] ( A) = (*) tautomer is determined experimentally based on = G g0 + G solv the equilibrium constant Ke ( K e = [enol ] ) [keto] Since, for equilibrium, the logarithm of the equilibrium constant Ke is proportional to the measured by 1H-NMR spectroscopy [1,2] (table standard free energy change, according to 4). Consider the isomerization reaction A B , equation: G 0 = RT ln K e (**) the standard free energies of reaction in the gas It follows from Eqs. (*) and (**) that the 0 phase and in solution are denoted as G g and difference in the molar Gibbs energies of reactant A and product B, Gsolv, determines G s0 , respectively. Quantities associating with the solvent effect on this equilibrium 2, 11]. the reactant A and product B are free energies of Particularly, the value of Ke or mole fraction of 0 solvation denoted as G solv . From the enol tautomer decreases with increasing Gsolv values. thermodynamic cycle given below Table 4: The Gsolv of reaction in solvents with two models Mole fractions of Gsolv Ke(exp) [2] Solvent enol form (%) [2] Onsager Model PCM Model Gas 0 0 11.7 92 Benzene ( = 2.247) -0.28 -0.43 14.7 94 Acetonitrile ( = 20.7) -1.0 0.52 1.6 62 Water ( = 78.93) -1.03 1.85 0.23 19 253 Table 4 shows that the Gsolv reaction References values in Onsager model decrease when the polarity of solvent increases. In contrast, in 1. Donald L. Pavia, Gary M. Lampman, PCM model, the Gsolv reaction value George S. Kriz. Introduction to decreases in benzene solvent, but increase in Spectroscopy - A Guide for Students of more polar solvent. This is also obtained from Organic Chemistry, Second Edition, 1999. experimental Ke results (table 4). So the PCM Harcourt Brace College Publishers. model gives a much better solvent effects than 2. Christian Reichardt. Solvents and Solvent Onsager model. Effests in Organic Chemistry, chapter 4, P. 104 - 113, Wiley-VCH (2003). IV - Conclusions 3. J. B. Foresman, E. Frisch. Exploring Chemistry with Electronic Structure Methods. ab-initio electronic structure calculations Gaussian Inc, Pittsburgh, PA (1996). demonstrate enol/keto tautomerism for acetylacetone in gas and in solvents as well. In 4. M.J. Frisch et al., Gaussian 98 A3 and gas phase, at HF levels, the keto form was found GaussView 2.1, Gaussian, Inc., Pittsburgh, to be the most thermodynamic stable. In PA (1998). contrast, the cis-enolic form was found to be the 5. M. W. Wong, M. J. Frisch, K. B. Wiberg. J. most thermodynamic stable at B3LYP level. The Am. Chem. 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Struct: Theochem, to Vietnam National Council of Natural 713, 59 - 63 (2004). Sciences for supporting this work. We are also 10. Ivana Matanovic, Nadja Doslic, Zlatko grateful to Prof. Dr. Minh Tho Nguyen, Mihalic. Chem. Phys., 306, 201 - 207 Department of Chemistry, Levuen University, (2004). Belgium for supporting the Gaussian 98.A3. 11. Thanh N. Truong et al. J. Chem. Phys., 107, Software. 1881 - 1889 (1997). 254