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Quantum chemical study of tautomerism in 2-[(4-phenylthiazol-2-yl)hydrazonomethyl]phenol
Computational and Theoretical Chemistry 1086 (2016) 12–17
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Quantum chemical study of tautomerism in 2-[(4-phenylthiazol-2-yl)hydrazonomethyl]phenol Namık Özdemir Department of Secondary School Science and Mathematics Education, Faculty of Education, Ondokuz Mayıs University, 55139 Samsun, Turkey
a r t i c l e
i n f o
Article history: Received 28 January 2016 Received in revised form 10 April 2016 Accepted 15 April 2016 Available online 16 April 2016 Keywords: Tautomerism DFT MP2 Proton transfer Solvent effect
a b s t r a c t The relative stabilities of five tautomers of 2-[(4-phenylthiazol-2-yl)hydrazonomethyl]phenol were calculated at the B3LYP/6–311+G(d,p) and MP2/6–311+G(d,p) levels of theory. The possible tautomeric transformations were also analyzed at the same level taking into account the solvent effect with the integral equation formalism polarizable continuum model (IEF-PCM) using three different solvents. The enolhydrazo-thiazole (T1) form has been found to be the predominant tautomer, and the tautomers follow the stability pattern: enol-hydrazo-thiazole (T1) > enol-imine-thiazoline (T2) > keto-hydrazo-thiazoline (T4) > keto-amine-thiazole (T5) > enol-azo-thiazole (T3). The tautomeric barrier heights for T1 T2, T1 T3 and T4 T5 reactions are very high in both the forward and reverse directions. In the case of T1 T5 and T2 T4 tautomerizations, the forward proton transfer is not possible because of large barrier height. According to the B3LYP results of the reverse reactions, only the proton transfer in the gas phase and in chloroform is allowed for T1 T5 while it is found to be barrierless for T2 T4. However, the MP2 computations show that the reverse T1 T5 reaction impossible for all cases while the reverse T2 T4 reaction needs very low energy. These results are also corroborated by the thermodynamic parameters obtained at the B3LYP/6–311+G(d,p) level. The barrier energy height increases with the increasing polarity of the solvent in general, however, the trend is not observed for all cases. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction It has been reported in literature that compounds having nitrogen and sulfur atoms are mainly used in medical applications for treatment of different kinds of fungal and bacterial infections as well as treatment of gastric ulcer, cancer etc. [1]. This type of organic moieties results in higher efficiency against several diseases since sulfur is capable to interact with receptors [2,3]. Thiazoles are important structural units in the field of medicinal chemistry and have been confirmed for a wide spectrum of biological processes [4,5]. The thiazole core often exists in the structure of several natural products and biologically active compounds, like thiamine (vitamin-B), also in some antibiotics drugs like penicillin, micrococcin [6] and many metabolic products of fungi and primitive marine animal etc. [7]. Thiazoles continue to attract interest because of their utility such as antibacterial, antifungal, antiinflammatory, anthelmintic, analgesic, antitubercular, central nervous system (CNS) stimulate, anti-HIV, and algicidol [8–15]. Moreover, recent studies suggest that 2-(2-hydrazinyl)thiazole derivatives including 2-aminothiazole scaffold have shown good
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activity against Mycobacterium tuberculosis (Mtb) strain, H37Rv [16–18]. Tautomerism is an interconversion between isomeric forms that involves proton transportation and a double-bond (pelectron) shift within a ring or within a chain, or both. Keto-enol, thione-thiol, enamine-imine, acinitro-nitro, nitroso-oxime, and amide-iminole transformations can be considered the most common types of tautomerism [19], and there are a lot of papers in the literature studying tautomerism computationally [20–25]. Understanding tautomeric equilibria is of central importance in synthetic chemistry and especially in medicinal chemistry since it has a fundamental function in many organic and biochemical reactions, in structural assignments and in the biochemical activity of amino acids, sugars and nucleic acids [26–29]. Among a number of physical and chemical factors which are responsible for the tautomeric equilibrium, solvation plays an important role, because most biochemical reactions of interest occur in solution phase [30]. A variety of theoretical methods have been used to study tautomeric equilibria and the DFT (B3LYP) method has been shown to be reliable for studying such processes with low cost in computation time [31,32]. Schiff bases derived from salicylaldimines have been a subject of intense study probably as a result of the close proximity of the
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hydroxyl and imine groups [33,34]. Aromatic Schiff bases containing a hydroxyl group in the ortho position may have two tautomeric forms, namely enol-imine [35] and the keto-amine [36] structures. These compounds exhibit interesting chromic behavior (thermochromism and photochromism) due to the interconversion between the keto-amine and enol-imine forms [37–39]. Previous studies revealed that compounds showing thermochromism are planar, while those showing photochromism are nonplanar, and both events associated with H-atom migration from the hydroxyl oxygen atom to the imine nitrogen atom [40–42]. Furthermore, azo compounds are very important molecules due to their applications in dyes, pigments, and functional materials [43,44]. Azo dyes with ortho and para hydroxy substituents to the azo linker generally exhibit azo-hydrazone tautomerism, which involves transfer of the hydroxyl hydrogen to one of the nitrogens in the azo group. The color and properties of one tautomer may be completely different from the other [45,46]. Such tautomerizations are very important not only in fundamental research but also for some applications [47]. In this paper, we present the results of density functional theory (DFT) and second-order Møller-Plesset perturbation theory (MP2) calculations of the energy difference between tautomers of 2-[(4-phenylthiazol-2-yl)hydrazonomethyl]phenol, and of the barrier height for the possible tautomeric conversions. In order to investigate the impact of the solvent with different polarity on the tautomerism, we choose three solvents [e = 4.90, chloroform (CHCl3); e = 32.63, methanol (CH4O); e = 78.39, water (H2O)] by performing SCRF calculations combined with the integral equation formalism polarizable continuum model (IEF-PCM). Details of computations and the results obtained are presented below. 2. Computational details The calculations were carried out using the GaussView [48] program and Gaussian 03 W package [49] at the B3LYP [50,51] and MP2 [52] levels of theory with 6–311+G(d,p) [53,54] basis set. The default parameters and criteria of the programs without any symmetry restrictions were used to find the structures at the local minimum or the transition state (TS). Frequency calculations at the same level of theory were carried out for all the structures reported in the study to determine whether the optimized structures are local minima or TSs. The stable structures exhibited all positive frequencies, whereas the TSs possessed one imaginary frequency. To estimate the effect of the medium on the tautomerism of the studied compound, we applied the IE-PCM method [55] at the same level. The thermodynamic parameters were taken from the frequency analyses of the optimized structures, and were computed using the thermodynamic equations and DG = DH – TDS [56,57]. 3. Results and discussion 3.1. Experimental vs. theoretical structure The X-ray structure of the title compound, which has been reported previously (Fig. 1a) [58], was optimized by DFT method with the 6–311+G(d,p) basis set (Fig. 1b) in the gas phase. Some important experimental and theoretical geometric parameters are given in Table S1 for comparison (see Supplementary materials). As can be seen in Table S1, the pertinent bond lengths and angles are generally in good agreement with the computed values. The biggest deviation of the bond lengths is 0.18 Å at N2AH2 and the biggest deviation of the bond angles is 4.7° at C1AN2AN3. In the solid state, the thiazole ring makes dihedral angles of 14.9 (2)° and 19.65(14)° with the planes of benzene and phenol rings,
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Fig. 1. (a) Experimental structure of the title compound with the atom-numbering scheme [35]. (b) Theoretical structure of the title compound. (c) Superimposition of the experimental (black) and calculated (red) structures. Hydrogen atoms are omitted for clarity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
respectively. These angles are theoretically determined as 7.57° and 2.18°, respectively. Furthermore, the experimental dihedral angle between the benzene and phenol rings is 34.07(13)°, while its corresponding theoretical value is 7.63°. The discrepancy between the experimental and calculated values for the atomic coordinates might result from the different environments of a molecule in the experimental crystalline state and in the theoretical gas phase. When the optimized and experimental structures are globally compared by overlaying them using a least-squares algorithm that minimizes the distances between the corresponding non-hydrogen atoms as shown in Fig. 1c, the obtained RMSE is 0.325 Å. In spite of these differences, the optimized geometry represents a good approximation with the crystal structure and the modeling technique can be successfully applied for investigating the tautomerism in the title compound.
3.2. Tautomerism The title compound can exist in five possible tautomeric forms, namely enol-hydrazo-thiazole (T1), enol-imine-thiazoline (T2), enol-azo-thiazole (T3), keto-hydrazo-thiazoline (T4) and keto-aminethiazole (T5) as shown in Fig. 2. The energetic and thermodynamic parameters related to the possible tautomeric transformations at the B3LYP/6–311+G(d,p) level are given in Table S2 (see Supplementary materials). As can be seen from data in Table S2, T1 is the most stable form and the stability trend is found as T1 > T2 > T4 > T5 > T3. When the tautomeric conversions between the tautomers are considered, there are apparently ten pathways for intramolecular proton transfer reactions. However, the paths taken by protons are too long for T2 T3 and T3 T4 tautomerizations. Concerning the transition structure (TS) for T1 T4 and T2 T5 reactions, we were not able to locate a true TS using full geometry optimization. Despite many attempts, every calculations for both reactions resulted in a TS structure which is the same as that in T1 T2
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Fig. 2. Schematic representation of all tautomers and possible tautomeric conversions of the title compound.
reaction. In addition, attempting to locate a TS for T3 T5 reaction gave a rotamer of T1 formed by rotation about N2AC1 bond. Consequently, only five remaining tautomeric conversions are discussed below. In the discussion of these proton transfer reactions, we choose the forward reaction as the proton transfer from the more stable tautomer to the less stable one, and the reverse is the reaction in the opposite direction. In the T1 T2 tautomerization, the imaginary frequency at the transition state is found as 1885i cm1 for the gas phase, 1913i cm1 for chloroform, 1923i cm1 for methanol and 1925i cm1 for water. The T1 ? TS ? T2 proton transfer process occurs in a concerted way with the breaking of the N2AH2 bond and the formation of the N1AH2 bond (1.009 Å). The N2 H2 and N1 H2 distances for TS structure are determined as 1.399 and 1.348 Å, respectively. Due to the migration of a hydrogen atom from atom N2 to atom N1, the geometries show important structural changes. When going from T1 to T2 form, the N2AC1 bond length is reduced from 1.372 to 1.290 Å, which shows corresponding NAC single bond is transformed into N@C double bond after the hydrogen transfer. Furthermore, the N1AC1 distance increases from 1.297 to 1.373 Å indicating that the p bond in this N@C bond is broken down and the N@C double bond are transformed into an NAC single bond. A lengthening in the S1AC1, S1AC2 and N2AN3 bonds and a shortening in the C2AC3 bond are also observed. In the bond angles, the C1AN2AN3, C10AN3AN2, N1AC1AS1 and C2AC3AN1 angles contract as the C1AS1AC2, C1AN1AC3 and N2AC1AS1 angles expand. Although the obtained structure for T1 is planar, the dihedral angle between the thiazoline and benzene rings increases to 32.52° in T2, while the dihedral angle between the thiazoline and phenol rings remains almost unchanged with an angle of 1.98°. Fig. 3a shows the potential energy diagram for T1 T2 isomerization. The energy difference between the two tautomers is calculated to be 4.2, 3.3, 3.7 and 3.8 kJ mol1 for B3LYP, and 4.8, 5.4, 6.4 and 6.5 kJ mol1 for MP2 in going from the gas phase to water, respectively. Although the difference is small, the reaction barrier is very high and in the range 201.2–210.0 kJ mol1 for the forward reaction and in the range 197.0–206.2 kJ mol1 for the reverse reaction. The MP2 results predict larger barrier
heights: in the range 265.2–276.4 kJ mol1 for the forward reaction and in the range 260.4–269.9 kJ mol1 for the reverse reaction. Consequently, it seems that a very high energy is necessary for the proton transfer in both directions to occur. The calculated standard enthalpy and free energy changes related to T1 T2 tautomerism also show that the single proton transfer reaction in both directions is strongly endothermic with large positive standard enthalpy and free energy changes. In the T1 T3 tautomerization, the imaginary frequency at the transition state is found as 2064i cm1 for the gas phase, 2073i cm1 for chloroform, 2075i cm1 for methanol and 2068i cm1 for water. During the T1 ? TS ? T3 proton transfer reaction, the N2AH2 bond breaks and the C10AH2 bond (1.093 Å) forms simultaneously. The N2 H2 and C10 H2 distances for TS structure are determined as 1.396 and 1.482 Å, respectively. When going from T1 to T3 form, the N2AN3 bond length is reduced from 1.352 to 1.243 Å, which shows corresponding NAN single bond is transformed into N@N double bond after the hydrogen transfer. In addition, the N3AC10 distance increases from 1.291 to 1.469 Å indicating that the p bond in this N@C bond is broken down and the N@C double bond are transformed into an NAC single bond. A lengthening in the N2AC1 and C10AC11 bonds and a shortening in the S1AC2 bond are also noticed. In the bond angles, the C1AN2AN3, C10AN3AN2 and N3AC10AC11 angles contract as the N2AC1AS1 angle expands. Unlike the planar structure of T1, the dihedral angle between the thiazole and phenol rings reaches to 48.51° in T3, while the dihedral angle between the thiazole and benzene rings is found to be almost unchanged with an angle of 8.06°. Fig. 3b shows the potential energy diagram for T1 T3 isomerization. The energy difference between the two tautomers is calculated to be 63.6, 65.9, 66.7 and 66.8 kJ mol1 for B3YP, and 54.2, 58.2, 59.9 and 60.1 kJ mol1 for MP2 in going from the gas phase to water, respectively. The reaction barrier is very high and in the range 302.5–311.1 kJ mol1 for the forward reaction and in the range 235.8–244.3 kJ mol1 for the reverse reaction. The MP2 barrier heights are found to be higher than those of B3LYP: in the range 412.7–420.1 kJ mol1 for the forward reaction and in the range 358.5–360.0 kJ mol1 for the reverse reaction. So,
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Fig. 3. Potential energy diagrams for T1 T2 (a), T1 T3 (b), T1 T5 (c) and T4 T5 (d) tautomerizations of the title compound in the gas phase and various solvents at the B3LYP/6–311+G(d,p) level.
it is inferred that a very high energy is necessary for the proton transfer in both directions to occur. The calculated standard enthalpy and free energy changes corresponding to T1 T3 tautomerism also indicate that the single proton transfer reaction in both directions is strongly endothermic with large positive standard enthalpy and free energy changes. In the T1 T5 tautomerization, the imaginary frequency at the transition state is found as 975i cm1 for the gas phase, 1043i cm1 for chloroform, 1055i cm1 for methanol and 1056i cm1 for water. During the T1 ? TS ? T5 proton transfer reaction, the O1AH1 bond breaks and the N3AH1 bond (1.042 Å) forms simultaneously. The O1 H1 and N3 H1 distances for TS structure are determined as 1.345 and 1.166 Å, respectively. On going from T1 to T5 form, the O1AC12 bond length is reduced from 1.349 to 1.259 Å, while the N3AC10 distance increases from 1.291 to 1.333 Å. This is consistent with the breaking of the C@N double bond and corresponding formation of a C@O double bond. A lengthening in the N2AC1 and N2AN3 bonds and a shortening in the C10AC11 bond are also remarkable. In the bond angles, the C1AN2AN3, N3AN2AH2 and C1AN2AH2 angles contract as the C10AN3AN2 and O1AC12AC13 angles expands. When compared to T1 structure, it is seen that the (C11AC16) ring is highly folded with respect to the thiazole ring by 80.67° in T5. However, the dihedral angle between the thiazole and benzene rings remains almost unchanged with an angle of 1.91°. Fig. 3c shows the potential energy diagram for T1 T5 isomerization. The energy difference between the two tautomers is calculated to be 51.0, 45.2, 42.7 and 42.4 kJ mol1 for B3LYP, and 49.7, 40.8, 36.9 and 36.2 kJ mol1 for MP2 in going from the gas phase to water, respectively. The relative energies of the TS
with respect to T1 are obtained as 56.5, 53.1, 51.7 and 51.5 kJ mol1 in the gas phase, in chloroform, in methanol, and in water, respectively. These energies as well as the positive standard enthalpy and free energy changes make the forward reaction a disfavored process or not a spontaneous process. In the case of the reverse reaction, very low energy barriers are found as 5.5, 7.9, 8.9 and 9.1 kJ mol1 in going from the gas phase to water, respectively. In the case of MP2 calculations, large barrier heights are obtained for both directions, in the range 70.8–76.3 kJ mol1 for the forward reaction and in the range 26.6–34.6 kJ mol1 for the reverse reaction. The values of the standard enthalpies for the reverse reaction show that all the proton transfers are enthalpically favored (exothermic). However, the proton transfer reaction in methanol and water needs larger entropy change rather than energy change to occur (|TDS298|>|DH298|). Therefore, these reactions are thermodynamically disfavored (DG298 > 0) [59]. In the T2 T4 tautomerization, the imaginary frequency at the transition state is found as 166i cm1 for the gas phase, 762i cm1 for chloroform, 909i cm1 for methanol and 928i cm1 for water. During the T2 ? TS ? T4 proton transfer reaction, the O1AH1 bond breaks and the N3AH1 bond (1.088 Å) forms simultaneously. The O1 H1 and N3 H1 distances for TS structure are determined as 1.464 and 1.107 Å, respectively. On going from T2 to T4 form, the O1AC12 bond length is reduced from 1.347 to 1.278 Å, while the N3AC10 distance increases from 1.293 to 1.317 Å. This corresponds to the breaking of the C@N double bond and the formation of a C@O double bond. A shortening in the C10AC11 bond is also observed. In the bond angles, the N3AC10AC11 angle contracts as the C1AN2AN3, C10AN3AN2 and O1AC12AC13 angles
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expands. When the two structures are compared, it is seen that the (C11AC16) and thiazoline rings are almost planar in both structures with dihedral angles of 1.98° and 2.67° for T2 and T4, respectively. In addition, although the dihedral angle between the thiazoline and benzene rings is nearly equal (32.57° for T2 and 34.17° for T4), the orientation of the benzene ring is opposite in the two structures. The energy difference between the two tautomers is calculated to be 44.5, 36.2, 32.5 and 32.0 kJ mol1 for B3LYP, and 72.3, 56.3, 50.4 and 49.8 kJ mol1 for MP2 in going from the gas phase to water, respectively. The relative energies of the TS with respect to T2 are obtained as 44.5, 30.2, 23.9 and 22.5 kJ mol1 for B3LYP, and 75.1, 69.8, 67.0 and 66.7 kJ mol1 for MP2 in the gas phase, in chloroform, in methanol, and in water, respectively. These energies as well as the B3LYP positive standard enthalpy and free energy changes make the forward reaction a disfavored process or not a spontaneous process. In the case of the reverse reaction, the B3LYP energy of TS is found to be almost equal to that of T4 in the gas phase and smaller than that of T4 in solvent media, indicating no barrier. As a result, the reverse reaction can be considered as barrierless. The MP2 results predict a favored process for the reverse reaction with a barrier height changing from 2.8 to 16.9 kJ mol1. In the T4 T5 tautomerization, the imaginary frequency at the transition state is found as 1888i cm1 for the gas phase, 1917i cm1 for chloroform, 1930i cm1 for methanol and 1931i cm1 for water. The T4 ? TS ? T5 proton transfer process occurs in a concerted way with the breaking of the N1AH2 bond (1.009 Å) and the formation of the N2AH2 bond (1.016 Å). The N1 H2 and N2 H2 distances for TS structure are determined as 1.333 and 1.416 Å, respectively. Because of the migration of a hydrogen atom from atom N1 to atom N2, the geometries show important structural changes. When going from T4 to T5 form, the N1AC1 bond length is reduced from 1.376 to 1.292 Å, which shows corresponding NAC single bond is transformed into N@C double bond after the hydrogen transfer. Furthermore, the N2AC1 distance increases from 1.290 to 1.395 Å indicating that the p bond in this N@C bond is broken down and the N@C double bond are transformed into an NAC single bond. A lengthening in the N2AN3, N3AC10 and C2AC3 bonds and a shortening in the S1AC1, S1AC2, O1AC12 and C10AC11 bonds are also noticed. Among the bond angles, the C1AS1AC2, C1AN1AC3, N2AC1AS1 and N2AN3AH1 angles contract as the N1AC1AS1, C2AC3AN1, N3AC10AC11 and C10AN3AH1 angles expand. In T4, the (C11AC16) and thiazoline rings share almost the same plane with a dihedral angle of 2.67°, while the thiazole and benzene rings lie on the same plane in T5 with a dihedral angle of 1.91°. The dihedral angle between the thiazoline and benzene rings is 34.17° in T4, as the thiazole ring is almost perpendicular to the (C11AC16) ring in T5 with a dihedral angle of 80.67°. Fig. 3d shows the potential energy diagram for T4 T5 isomerization. The energy difference between the two tautomers is calculated to be 2.3, 5.6, 6.5 and 6.6 kJ mol1 for B3LYP, and 27.4, 20.9, 19.9 and 20.1 kJ mol1 for MP2 in going from the gas phase to water, respectively. Although the difference is small, the reaction barrier is very high and in the range 206.5– 213.5 kJ mol1 for the forward reaction and in the range 204.2– 206.9 kJ mol1 for the reverse reaction. According to the MP2 results, these barriers range from 257.7 to 266.1 kJ mol1 for the forward reaction, and range from 284.5 to 286.2 kJ mol1 for the reverse reaction. Consequently, it seems that a very high energy is necessary for the proton transfer in both directions to occur. The calculated standard enthalpy and free energy changes related to T4 T5 tautomerism also prove that the single proton transfer reaction in both directions is strongly endothermic with large positive standard enthalpy and free energy changes.
4. Conclusions The paper studies the tautomerism and possible tautomeric conversions for the title compound at the B3LYP/6–311+G(d,p) and MP2/6–311+G(d,p) levels of theory. The effect of solvent is introduced at the same level through a continuum description (IEF-PCM) using chloroform, methanol and water as solvents. The following conclusions can be inferred from the results: 1. The match between the experimental and calculated structural parameters is good, as evident from the RMSE value of 0.325 Å between them. 2. In consonance with the theoretically predicted relative energies of the five tautomers; enol-hydrazo-thiazole (T1), enol-iminethiazoline (T2), enol-azo-thiazole (T3), keto-hydrazothiazoline (T4) and keto-amine-thiazole (T5)]; T1 tautomer is found to be the most stable one in the gas phase and in solution, and the order of stability of the tautomers is T1 > T2 > T4 > T5 > T3. 3. Among the possible tautomeric conversions, very high barrier heights are found for T1 T2, T1 T3 and T4 T5 reactions in both the forward and reverse directions making these tautomerization processes unfavorable. These findings are also supported by large positive standard enthalpy and free energy changes obtained at the B3LYP/6–311+G(d,p) level. 4. In T1 T5 reaction, the forward proton transfer is not feasible due to the relatively large barrier height as well as positive standard enthalpy and free energy changes. However, the reverse proton transfer has very low energy barrier but only the transfer process in the gas phase and in chloroform is allowed by a negative value in enthalpy and free energy changes. MP2 energy value of this reaction are higher those of B3LYP for both directions, indicating a disfavored process. 5. In T2 T4 reaction, the high barrier height together with the positive standard enthalpy and free energy changes makes the forward proton transfer reaction unfavorable. In the case of the reverse proton transfer, the energy of the TS is very close or smaller than T4, and this proton transfer is barrierless in the gas phase and in solution. The MP2 computations support the B3LYP results for the forward proton transfer reaction. However, it predicts a proton transfer reaction with small barrier height. 6. The barrier height mostly increases with the increasing polarity of the solvent but it cannot be generalized to all cases.
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Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
Quantum chemical study of tautomerism in 2-[(4-phenylthiazol-2-yl)hydrazonomethyl]phenol Namık Özdemir Department of Secondary School Science and Mathematics Education, Faculty of Education, Ondokuz Mayıs University, 55139 Samsun, Turkey
a r t i c l e
i n f o
Article history: Received 28 January 2016 Received in revised form 10 April 2016 Accepted 15 April 2016 Available online 16 April 2016 Keywords: Tautomerism DFT MP2 Proton transfer Solvent effect
a b s t r a c t The relative stabilities of five tautomers of 2-[(4-phenylthiazol-2-yl)hydrazonomethyl]phenol were calculated at the B3LYP/6–311+G(d,p) and MP2/6–311+G(d,p) levels of theory. The possible tautomeric transformations were also analyzed at the same level taking into account the solvent effect with the integral equation formalism polarizable continuum model (IEF-PCM) using three different solvents. The enolhydrazo-thiazole (T1) form has been found to be the predominant tautomer, and the tautomers follow the stability pattern: enol-hydrazo-thiazole (T1) > enol-imine-thiazoline (T2) > keto-hydrazo-thiazoline (T4) > keto-amine-thiazole (T5) > enol-azo-thiazole (T3). The tautomeric barrier heights for T1 T2, T1 T3 and T4 T5 reactions are very high in both the forward and reverse directions. In the case of T1 T5 and T2 T4 tautomerizations, the forward proton transfer is not possible because of large barrier height. According to the B3LYP results of the reverse reactions, only the proton transfer in the gas phase and in chloroform is allowed for T1 T5 while it is found to be barrierless for T2 T4. However, the MP2 computations show that the reverse T1 T5 reaction impossible for all cases while the reverse T2 T4 reaction needs very low energy. These results are also corroborated by the thermodynamic parameters obtained at the B3LYP/6–311+G(d,p) level. The barrier energy height increases with the increasing polarity of the solvent in general, however, the trend is not observed for all cases. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction It has been reported in literature that compounds having nitrogen and sulfur atoms are mainly used in medical applications for treatment of different kinds of fungal and bacterial infections as well as treatment of gastric ulcer, cancer etc. [1]. This type of organic moieties results in higher efficiency against several diseases since sulfur is capable to interact with receptors [2,3]. Thiazoles are important structural units in the field of medicinal chemistry and have been confirmed for a wide spectrum of biological processes [4,5]. The thiazole core often exists in the structure of several natural products and biologically active compounds, like thiamine (vitamin-B), also in some antibiotics drugs like penicillin, micrococcin [6] and many metabolic products of fungi and primitive marine animal etc. [7]. Thiazoles continue to attract interest because of their utility such as antibacterial, antifungal, antiinflammatory, anthelmintic, analgesic, antitubercular, central nervous system (CNS) stimulate, anti-HIV, and algicidol [8–15]. Moreover, recent studies suggest that 2-(2-hydrazinyl)thiazole derivatives including 2-aminothiazole scaffold have shown good
E-mail address: [email protected] http://dx.doi.org/10.1016/j.comptc.2016.04.013 2210-271X/Ó 2016 Elsevier B.V. All rights reserved.
activity against Mycobacterium tuberculosis (Mtb) strain, H37Rv [16–18]. Tautomerism is an interconversion between isomeric forms that involves proton transportation and a double-bond (pelectron) shift within a ring or within a chain, or both. Keto-enol, thione-thiol, enamine-imine, acinitro-nitro, nitroso-oxime, and amide-iminole transformations can be considered the most common types of tautomerism [19], and there are a lot of papers in the literature studying tautomerism computationally [20–25]. Understanding tautomeric equilibria is of central importance in synthetic chemistry and especially in medicinal chemistry since it has a fundamental function in many organic and biochemical reactions, in structural assignments and in the biochemical activity of amino acids, sugars and nucleic acids [26–29]. Among a number of physical and chemical factors which are responsible for the tautomeric equilibrium, solvation plays an important role, because most biochemical reactions of interest occur in solution phase [30]. A variety of theoretical methods have been used to study tautomeric equilibria and the DFT (B3LYP) method has been shown to be reliable for studying such processes with low cost in computation time [31,32]. Schiff bases derived from salicylaldimines have been a subject of intense study probably as a result of the close proximity of the
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hydroxyl and imine groups [33,34]. Aromatic Schiff bases containing a hydroxyl group in the ortho position may have two tautomeric forms, namely enol-imine [35] and the keto-amine [36] structures. These compounds exhibit interesting chromic behavior (thermochromism and photochromism) due to the interconversion between the keto-amine and enol-imine forms [37–39]. Previous studies revealed that compounds showing thermochromism are planar, while those showing photochromism are nonplanar, and both events associated with H-atom migration from the hydroxyl oxygen atom to the imine nitrogen atom [40–42]. Furthermore, azo compounds are very important molecules due to their applications in dyes, pigments, and functional materials [43,44]. Azo dyes with ortho and para hydroxy substituents to the azo linker generally exhibit azo-hydrazone tautomerism, which involves transfer of the hydroxyl hydrogen to one of the nitrogens in the azo group. The color and properties of one tautomer may be completely different from the other [45,46]. Such tautomerizations are very important not only in fundamental research but also for some applications [47]. In this paper, we present the results of density functional theory (DFT) and second-order Møller-Plesset perturbation theory (MP2) calculations of the energy difference between tautomers of 2-[(4-phenylthiazol-2-yl)hydrazonomethyl]phenol, and of the barrier height for the possible tautomeric conversions. In order to investigate the impact of the solvent with different polarity on the tautomerism, we choose three solvents [e = 4.90, chloroform (CHCl3); e = 32.63, methanol (CH4O); e = 78.39, water (H2O)] by performing SCRF calculations combined with the integral equation formalism polarizable continuum model (IEF-PCM). Details of computations and the results obtained are presented below. 2. Computational details The calculations were carried out using the GaussView [48] program and Gaussian 03 W package [49] at the B3LYP [50,51] and MP2 [52] levels of theory with 6–311+G(d,p) [53,54] basis set. The default parameters and criteria of the programs without any symmetry restrictions were used to find the structures at the local minimum or the transition state (TS). Frequency calculations at the same level of theory were carried out for all the structures reported in the study to determine whether the optimized structures are local minima or TSs. The stable structures exhibited all positive frequencies, whereas the TSs possessed one imaginary frequency. To estimate the effect of the medium on the tautomerism of the studied compound, we applied the IE-PCM method [55] at the same level. The thermodynamic parameters were taken from the frequency analyses of the optimized structures, and were computed using the thermodynamic equations and DG = DH – TDS [56,57]. 3. Results and discussion 3.1. Experimental vs. theoretical structure The X-ray structure of the title compound, which has been reported previously (Fig. 1a) [58], was optimized by DFT method with the 6–311+G(d,p) basis set (Fig. 1b) in the gas phase. Some important experimental and theoretical geometric parameters are given in Table S1 for comparison (see Supplementary materials). As can be seen in Table S1, the pertinent bond lengths and angles are generally in good agreement with the computed values. The biggest deviation of the bond lengths is 0.18 Å at N2AH2 and the biggest deviation of the bond angles is 4.7° at C1AN2AN3. In the solid state, the thiazole ring makes dihedral angles of 14.9 (2)° and 19.65(14)° with the planes of benzene and phenol rings,
13
Fig. 1. (a) Experimental structure of the title compound with the atom-numbering scheme [35]. (b) Theoretical structure of the title compound. (c) Superimposition of the experimental (black) and calculated (red) structures. Hydrogen atoms are omitted for clarity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
respectively. These angles are theoretically determined as 7.57° and 2.18°, respectively. Furthermore, the experimental dihedral angle between the benzene and phenol rings is 34.07(13)°, while its corresponding theoretical value is 7.63°. The discrepancy between the experimental and calculated values for the atomic coordinates might result from the different environments of a molecule in the experimental crystalline state and in the theoretical gas phase. When the optimized and experimental structures are globally compared by overlaying them using a least-squares algorithm that minimizes the distances between the corresponding non-hydrogen atoms as shown in Fig. 1c, the obtained RMSE is 0.325 Å. In spite of these differences, the optimized geometry represents a good approximation with the crystal structure and the modeling technique can be successfully applied for investigating the tautomerism in the title compound.
3.2. Tautomerism The title compound can exist in five possible tautomeric forms, namely enol-hydrazo-thiazole (T1), enol-imine-thiazoline (T2), enol-azo-thiazole (T3), keto-hydrazo-thiazoline (T4) and keto-aminethiazole (T5) as shown in Fig. 2. The energetic and thermodynamic parameters related to the possible tautomeric transformations at the B3LYP/6–311+G(d,p) level are given in Table S2 (see Supplementary materials). As can be seen from data in Table S2, T1 is the most stable form and the stability trend is found as T1 > T2 > T4 > T5 > T3. When the tautomeric conversions between the tautomers are considered, there are apparently ten pathways for intramolecular proton transfer reactions. However, the paths taken by protons are too long for T2 T3 and T3 T4 tautomerizations. Concerning the transition structure (TS) for T1 T4 and T2 T5 reactions, we were not able to locate a true TS using full geometry optimization. Despite many attempts, every calculations for both reactions resulted in a TS structure which is the same as that in T1 T2
14
N. Özdemir / Computational and Theoretical Chemistry 1086 (2016) 12–17
Fig. 2. Schematic representation of all tautomers and possible tautomeric conversions of the title compound.
reaction. In addition, attempting to locate a TS for T3 T5 reaction gave a rotamer of T1 formed by rotation about N2AC1 bond. Consequently, only five remaining tautomeric conversions are discussed below. In the discussion of these proton transfer reactions, we choose the forward reaction as the proton transfer from the more stable tautomer to the less stable one, and the reverse is the reaction in the opposite direction. In the T1 T2 tautomerization, the imaginary frequency at the transition state is found as 1885i cm1 for the gas phase, 1913i cm1 for chloroform, 1923i cm1 for methanol and 1925i cm1 for water. The T1 ? TS ? T2 proton transfer process occurs in a concerted way with the breaking of the N2AH2 bond and the formation of the N1AH2 bond (1.009 Å). The N2 H2 and N1 H2 distances for TS structure are determined as 1.399 and 1.348 Å, respectively. Due to the migration of a hydrogen atom from atom N2 to atom N1, the geometries show important structural changes. When going from T1 to T2 form, the N2AC1 bond length is reduced from 1.372 to 1.290 Å, which shows corresponding NAC single bond is transformed into N@C double bond after the hydrogen transfer. Furthermore, the N1AC1 distance increases from 1.297 to 1.373 Å indicating that the p bond in this N@C bond is broken down and the N@C double bond are transformed into an NAC single bond. A lengthening in the S1AC1, S1AC2 and N2AN3 bonds and a shortening in the C2AC3 bond are also observed. In the bond angles, the C1AN2AN3, C10AN3AN2, N1AC1AS1 and C2AC3AN1 angles contract as the C1AS1AC2, C1AN1AC3 and N2AC1AS1 angles expand. Although the obtained structure for T1 is planar, the dihedral angle between the thiazoline and benzene rings increases to 32.52° in T2, while the dihedral angle between the thiazoline and phenol rings remains almost unchanged with an angle of 1.98°. Fig. 3a shows the potential energy diagram for T1 T2 isomerization. The energy difference between the two tautomers is calculated to be 4.2, 3.3, 3.7 and 3.8 kJ mol1 for B3LYP, and 4.8, 5.4, 6.4 and 6.5 kJ mol1 for MP2 in going from the gas phase to water, respectively. Although the difference is small, the reaction barrier is very high and in the range 201.2–210.0 kJ mol1 for the forward reaction and in the range 197.0–206.2 kJ mol1 for the reverse reaction. The MP2 results predict larger barrier
heights: in the range 265.2–276.4 kJ mol1 for the forward reaction and in the range 260.4–269.9 kJ mol1 for the reverse reaction. Consequently, it seems that a very high energy is necessary for the proton transfer in both directions to occur. The calculated standard enthalpy and free energy changes related to T1 T2 tautomerism also show that the single proton transfer reaction in both directions is strongly endothermic with large positive standard enthalpy and free energy changes. In the T1 T3 tautomerization, the imaginary frequency at the transition state is found as 2064i cm1 for the gas phase, 2073i cm1 for chloroform, 2075i cm1 for methanol and 2068i cm1 for water. During the T1 ? TS ? T3 proton transfer reaction, the N2AH2 bond breaks and the C10AH2 bond (1.093 Å) forms simultaneously. The N2 H2 and C10 H2 distances for TS structure are determined as 1.396 and 1.482 Å, respectively. When going from T1 to T3 form, the N2AN3 bond length is reduced from 1.352 to 1.243 Å, which shows corresponding NAN single bond is transformed into N@N double bond after the hydrogen transfer. In addition, the N3AC10 distance increases from 1.291 to 1.469 Å indicating that the p bond in this N@C bond is broken down and the N@C double bond are transformed into an NAC single bond. A lengthening in the N2AC1 and C10AC11 bonds and a shortening in the S1AC2 bond are also noticed. In the bond angles, the C1AN2AN3, C10AN3AN2 and N3AC10AC11 angles contract as the N2AC1AS1 angle expands. Unlike the planar structure of T1, the dihedral angle between the thiazole and phenol rings reaches to 48.51° in T3, while the dihedral angle between the thiazole and benzene rings is found to be almost unchanged with an angle of 8.06°. Fig. 3b shows the potential energy diagram for T1 T3 isomerization. The energy difference between the two tautomers is calculated to be 63.6, 65.9, 66.7 and 66.8 kJ mol1 for B3YP, and 54.2, 58.2, 59.9 and 60.1 kJ mol1 for MP2 in going from the gas phase to water, respectively. The reaction barrier is very high and in the range 302.5–311.1 kJ mol1 for the forward reaction and in the range 235.8–244.3 kJ mol1 for the reverse reaction. The MP2 barrier heights are found to be higher than those of B3LYP: in the range 412.7–420.1 kJ mol1 for the forward reaction and in the range 358.5–360.0 kJ mol1 for the reverse reaction. So,
N. Özdemir / Computational and Theoretical Chemistry 1086 (2016) 12–17
15
Fig. 3. Potential energy diagrams for T1 T2 (a), T1 T3 (b), T1 T5 (c) and T4 T5 (d) tautomerizations of the title compound in the gas phase and various solvents at the B3LYP/6–311+G(d,p) level.
it is inferred that a very high energy is necessary for the proton transfer in both directions to occur. The calculated standard enthalpy and free energy changes corresponding to T1 T3 tautomerism also indicate that the single proton transfer reaction in both directions is strongly endothermic with large positive standard enthalpy and free energy changes. In the T1 T5 tautomerization, the imaginary frequency at the transition state is found as 975i cm1 for the gas phase, 1043i cm1 for chloroform, 1055i cm1 for methanol and 1056i cm1 for water. During the T1 ? TS ? T5 proton transfer reaction, the O1AH1 bond breaks and the N3AH1 bond (1.042 Å) forms simultaneously. The O1 H1 and N3 H1 distances for TS structure are determined as 1.345 and 1.166 Å, respectively. On going from T1 to T5 form, the O1AC12 bond length is reduced from 1.349 to 1.259 Å, while the N3AC10 distance increases from 1.291 to 1.333 Å. This is consistent with the breaking of the C@N double bond and corresponding formation of a C@O double bond. A lengthening in the N2AC1 and N2AN3 bonds and a shortening in the C10AC11 bond are also remarkable. In the bond angles, the C1AN2AN3, N3AN2AH2 and C1AN2AH2 angles contract as the C10AN3AN2 and O1AC12AC13 angles expands. When compared to T1 structure, it is seen that the (C11AC16) ring is highly folded with respect to the thiazole ring by 80.67° in T5. However, the dihedral angle between the thiazole and benzene rings remains almost unchanged with an angle of 1.91°. Fig. 3c shows the potential energy diagram for T1 T5 isomerization. The energy difference between the two tautomers is calculated to be 51.0, 45.2, 42.7 and 42.4 kJ mol1 for B3LYP, and 49.7, 40.8, 36.9 and 36.2 kJ mol1 for MP2 in going from the gas phase to water, respectively. The relative energies of the TS
with respect to T1 are obtained as 56.5, 53.1, 51.7 and 51.5 kJ mol1 in the gas phase, in chloroform, in methanol, and in water, respectively. These energies as well as the positive standard enthalpy and free energy changes make the forward reaction a disfavored process or not a spontaneous process. In the case of the reverse reaction, very low energy barriers are found as 5.5, 7.9, 8.9 and 9.1 kJ mol1 in going from the gas phase to water, respectively. In the case of MP2 calculations, large barrier heights are obtained for both directions, in the range 70.8–76.3 kJ mol1 for the forward reaction and in the range 26.6–34.6 kJ mol1 for the reverse reaction. The values of the standard enthalpies for the reverse reaction show that all the proton transfers are enthalpically favored (exothermic). However, the proton transfer reaction in methanol and water needs larger entropy change rather than energy change to occur (|TDS298|>|DH298|). Therefore, these reactions are thermodynamically disfavored (DG298 > 0) [59]. In the T2 T4 tautomerization, the imaginary frequency at the transition state is found as 166i cm1 for the gas phase, 762i cm1 for chloroform, 909i cm1 for methanol and 928i cm1 for water. During the T2 ? TS ? T4 proton transfer reaction, the O1AH1 bond breaks and the N3AH1 bond (1.088 Å) forms simultaneously. The O1 H1 and N3 H1 distances for TS structure are determined as 1.464 and 1.107 Å, respectively. On going from T2 to T4 form, the O1AC12 bond length is reduced from 1.347 to 1.278 Å, while the N3AC10 distance increases from 1.293 to 1.317 Å. This corresponds to the breaking of the C@N double bond and the formation of a C@O double bond. A shortening in the C10AC11 bond is also observed. In the bond angles, the N3AC10AC11 angle contracts as the C1AN2AN3, C10AN3AN2 and O1AC12AC13 angles
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N. Özdemir / Computational and Theoretical Chemistry 1086 (2016) 12–17
expands. When the two structures are compared, it is seen that the (C11AC16) and thiazoline rings are almost planar in both structures with dihedral angles of 1.98° and 2.67° for T2 and T4, respectively. In addition, although the dihedral angle between the thiazoline and benzene rings is nearly equal (32.57° for T2 and 34.17° for T4), the orientation of the benzene ring is opposite in the two structures. The energy difference between the two tautomers is calculated to be 44.5, 36.2, 32.5 and 32.0 kJ mol1 for B3LYP, and 72.3, 56.3, 50.4 and 49.8 kJ mol1 for MP2 in going from the gas phase to water, respectively. The relative energies of the TS with respect to T2 are obtained as 44.5, 30.2, 23.9 and 22.5 kJ mol1 for B3LYP, and 75.1, 69.8, 67.0 and 66.7 kJ mol1 for MP2 in the gas phase, in chloroform, in methanol, and in water, respectively. These energies as well as the B3LYP positive standard enthalpy and free energy changes make the forward reaction a disfavored process or not a spontaneous process. In the case of the reverse reaction, the B3LYP energy of TS is found to be almost equal to that of T4 in the gas phase and smaller than that of T4 in solvent media, indicating no barrier. As a result, the reverse reaction can be considered as barrierless. The MP2 results predict a favored process for the reverse reaction with a barrier height changing from 2.8 to 16.9 kJ mol1. In the T4 T5 tautomerization, the imaginary frequency at the transition state is found as 1888i cm1 for the gas phase, 1917i cm1 for chloroform, 1930i cm1 for methanol and 1931i cm1 for water. The T4 ? TS ? T5 proton transfer process occurs in a concerted way with the breaking of the N1AH2 bond (1.009 Å) and the formation of the N2AH2 bond (1.016 Å). The N1 H2 and N2 H2 distances for TS structure are determined as 1.333 and 1.416 Å, respectively. Because of the migration of a hydrogen atom from atom N1 to atom N2, the geometries show important structural changes. When going from T4 to T5 form, the N1AC1 bond length is reduced from 1.376 to 1.292 Å, which shows corresponding NAC single bond is transformed into N@C double bond after the hydrogen transfer. Furthermore, the N2AC1 distance increases from 1.290 to 1.395 Å indicating that the p bond in this N@C bond is broken down and the N@C double bond are transformed into an NAC single bond. A lengthening in the N2AN3, N3AC10 and C2AC3 bonds and a shortening in the S1AC1, S1AC2, O1AC12 and C10AC11 bonds are also noticed. Among the bond angles, the C1AS1AC2, C1AN1AC3, N2AC1AS1 and N2AN3AH1 angles contract as the N1AC1AS1, C2AC3AN1, N3AC10AC11 and C10AN3AH1 angles expand. In T4, the (C11AC16) and thiazoline rings share almost the same plane with a dihedral angle of 2.67°, while the thiazole and benzene rings lie on the same plane in T5 with a dihedral angle of 1.91°. The dihedral angle between the thiazoline and benzene rings is 34.17° in T4, as the thiazole ring is almost perpendicular to the (C11AC16) ring in T5 with a dihedral angle of 80.67°. Fig. 3d shows the potential energy diagram for T4 T5 isomerization. The energy difference between the two tautomers is calculated to be 2.3, 5.6, 6.5 and 6.6 kJ mol1 for B3LYP, and 27.4, 20.9, 19.9 and 20.1 kJ mol1 for MP2 in going from the gas phase to water, respectively. Although the difference is small, the reaction barrier is very high and in the range 206.5– 213.5 kJ mol1 for the forward reaction and in the range 204.2– 206.9 kJ mol1 for the reverse reaction. According to the MP2 results, these barriers range from 257.7 to 266.1 kJ mol1 for the forward reaction, and range from 284.5 to 286.2 kJ mol1 for the reverse reaction. Consequently, it seems that a very high energy is necessary for the proton transfer in both directions to occur. The calculated standard enthalpy and free energy changes related to T4 T5 tautomerism also prove that the single proton transfer reaction in both directions is strongly endothermic with large positive standard enthalpy and free energy changes.
4. Conclusions The paper studies the tautomerism and possible tautomeric conversions for the title compound at the B3LYP/6–311+G(d,p) and MP2/6–311+G(d,p) levels of theory. The effect of solvent is introduced at the same level through a continuum description (IEF-PCM) using chloroform, methanol and water as solvents. The following conclusions can be inferred from the results: 1. The match between the experimental and calculated structural parameters is good, as evident from the RMSE value of 0.325 Å between them. 2. In consonance with the theoretically predicted relative energies of the five tautomers; enol-hydrazo-thiazole (T1), enol-iminethiazoline (T2), enol-azo-thiazole (T3), keto-hydrazothiazoline (T4) and keto-amine-thiazole (T5)]; T1 tautomer is found to be the most stable one in the gas phase and in solution, and the order of stability of the tautomers is T1 > T2 > T4 > T5 > T3. 3. Among the possible tautomeric conversions, very high barrier heights are found for T1 T2, T1 T3 and T4 T5 reactions in both the forward and reverse directions making these tautomerization processes unfavorable. These findings are also supported by large positive standard enthalpy and free energy changes obtained at the B3LYP/6–311+G(d,p) level. 4. In T1 T5 reaction, the forward proton transfer is not feasible due to the relatively large barrier height as well as positive standard enthalpy and free energy changes. However, the reverse proton transfer has very low energy barrier but only the transfer process in the gas phase and in chloroform is allowed by a negative value in enthalpy and free energy changes. MP2 energy value of this reaction are higher those of B3LYP for both directions, indicating a disfavored process. 5. In T2 T4 reaction, the high barrier height together with the positive standard enthalpy and free energy changes makes the forward proton transfer reaction unfavorable. In the case of the reverse proton transfer, the energy of the TS is very close or smaller than T4, and this proton transfer is barrierless in the gas phase and in solution. The MP2 computations support the B3LYP results for the forward proton transfer reaction. However, it predicts a proton transfer reaction with small barrier height. 6. The barrier height mostly increases with the increasing polarity of the solvent but it cannot be generalized to all cases.
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