KetoâEnol Tautomeric Equilibrium of Acetylacetone Solution Confined

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Keto-Enol Tautomeric Equilibrium of Acetylacetone Solution Confined in Extended Nanospaces Takehiko Tsukahara, Kyosuke Nagaoka, Kyojiro Morikawa, Kazuma Mawatari, and Takehiko Kitamori J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b08020 • Publication Date (Web): 26 Oct 2015 Downloaded from http://pubs.acs.org on November 1, 2015

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The Journal of Physical Chemistry

Submitted as an article to Journal of Physical Chemistry B

Keto-Enol Tautomeric Equilibrium of Acetylacetone Solution Confined in Extended Nanospaces Takehiko Tsukahara,† Kyosuke Nagaoka,‡ Kyojiro Morikawa,† Kazuma Mawatari,‡ and Takehiko Kitamori‡,* †Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1-N1-6, O-Okayama, Meguro-ku, Tokyo 152-8550 Japan, ‡

Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-31, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

*To whom all correspondence should be addressed: T. Kitamori; E-mail: [email protected]; Tel: +81-3-5841-7231; Fax: +81-3-5841-6039

Keywords: Keto-enol tautomerization, Proton mobility, Nanofluidics, Extended nanospace, NMR

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ABSTRACT We aim to clarify the effects of size-confinement, solvent, and deuterium-substitution on keto-enol tautomerization of acetylacetone (AcAc) in solutions confined in 10 – 100 nm spaces (i.e., extended nanospaces) using 1H-NMR spectroscopy. The keto-enol equilibrium constants of AcAc (KEQ=[keto]/[enol]) in various solvents confined in extended nanospaces of 200 – 3,000 nm were examined using the area ratios of –CH3 peaks in keto to enol forms. The results showed that the keto form of AcAc in hydrogen-bonded solvents such as water and ethanol increased drastically with decreasing space sizes below about 500 nm, but the size-confinement didn’t induce equilibrium shifts in aprotic solvents such as DMSO. The magnitudes of KEQ enhancement were well correlated with solvent proton donicity. It followed from the determination of thermodynamic parameters that the stabilization of intermolecular interactions between protons in water and carbonyl oxygen (C=O) in the keto form of AcAc were promoted by size-confinement, and that the keto form could be energetically and structurally favored in extended nanospaces vis-à-vis the bulk space. Furthermore, the measurements of deuterium-dependence of the KEQ values verified that the nanoconfinement-induced shifts of keto-enol tautomerization of AcAc are attributable to high proton mobility via a proton hopping mechanism of the confined water.


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INTRODUCTION Nanoconfinement environments have attracted much attention as unique reaction fields, because molecular interactions at liquid-solid interfaces are expected to affect not only the behavior of liquids but also the selectivity and activity of chemical reactions. It has been reported that liquids confined to single nanometer-scale spaces such as porous materials and carbon nanotubes have specific properties differing from those of the bulk material, and that product- and shape-selective reactions are generated through large surface-to-volume ratios, catalytic effects, and surface adsorption.1-5 Single nanometer-scale space handling a small number of molecules at liquid-solid interfaces are quite powerful tools for the field of chemical syntheses, but are inadequate to handle molecular and ionic clusters in liquid phases for chemical analyses involving extraction and mixing. A space on the order of 10 to 100 nm, referred to as an extended nanospace, has been recognized as a potential experimental space for establishing innovative reactions and analyses in liquid phases, because the extended nanospace is a transitional region for molecular behavior from single molecules to condensed liquids, and is comparable to the range of the electric double layer (EDL) in an aqueous solution.6-9 Many researchers have reported that fluids in extended nanospaces show unique behaviors due to the effect of EDL overlap associated with surface charge density, e.g., increase in liquid viscosity,10-12 decrease in dielectric constant10, 13 , 14 , selective transport of ions, 15 , 16 enhancement of liquid conductivity17,18 and decrease in pH.19,20 Recently, we examined basic liquid properties in extended nanospaces using spectroscopic analyses, and clarified that water molecules confined in such spaces have slower motions, higher proton mobility, and more localized proton charge distributions along hydrogen bonding chains compared with bulk water.21,22



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This reason is due to that long-range ordered hydrogen bond networks of water in a direction perpendicular to the charged surfaces invoking ionizable silanol (SiOH) groups, where excess protons transfer quickly along a hydrogen-bonded chain between adjacent water molecules according to a proton hopping pathway (Grotthuss mechanism); SiO-···H+···H2O + H2O → SiO- + H3O+ + H2O → SiO- + H2O + H3O+ (Figure S1).23,24 Such protonic diffusion phenomena in extended nanospaces have been supported by a fluorescence microscope observation. 25 Considering that liquids confined in extended nanospaces has lower dielectric constant compared to the bulk, the extended nanospace environments would make it possible to induce unusual chemical reactions that are fundamentally different than those of the bulk and 1 nm-scale spaces. However, the mechanisms by which space sizeconfinement affects the dynamic behaviors of chemical reactions such as equilibrium and kinetics are still quite unknown. Acetylacetone (AcAc) is a prototype -diketone where the chemical equilibrium between the keto and enol forms is shifted depending on various factors such as solvent properties or phase conditions as depicted in Scheme 1. It has been broadly reported that the enol form is more stable than the keto from in non-polar solvents such as hexane and in gas-like phases due to intramolecular hydrogen bonding, and the equilibrium shifts toward the keto form as the solvent polarity increases.26-30 Considering that liquids confined in extended nanospaces have unique physicochemical properties differing from bulk, the equilibrium shifts of AcAc can be expected, while there have been no insights into the chemical equilibrium in extended nanospaces. Thus, in this report, we aim to characterize confinement-induced effects on keto-enol tautomerization of AcAc in solutions confined in extended nanospaces using NMR


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spectroscopy, which is recognized as a useful technique for characterizing keto-enol tautomerization.

EXPERIMENTAL Detailed information about the fabrication procedures of an NMR chip equipped with extended nanospaces has been given elsewhere.21,22 In brief, the extended nanospaces having rectangular shapes (width w = 250 – 4,500 nm, depth d = 170 – 3,000 nm, length; 42 mm) were fabricated on a fused-silica glass substrate by electron beam lithography and plasma etching. The rectangular nanospace sizes can be assigned an equivalent diameter (R = 200 – 3,000 nm) according to R = 2wd/(w+d) (see Table 1). The extended nanospaces were bridged between microchannels (200 m wide and 8 m deep), which were used to introduce sample solutions, using photolithographic patterning and plasma etching methods. The fabricated substrate was then thermally laminated with a cover glass in a vacuum furnace at 1,080 C. A schematic diagram of the fabrication steps and a picture of the NMR chip are given in Figure S2. Aqueous solutions containing ultra-pure water (18.0 M·cm) and AcAc (1.0 M; M = mol dm-3) were degassed by means of a number of freeze-pump-thaw cycles and then introduced into extended nanospaces under an argon atmosphere by pressurizing with a syringe. 1H-NMR spectra of AcAc in water confined in the size range from 200 to 3,000 nm were measured using a 500 MHz spectrometer without spinning and locking. The temperatures were controlled by a temperature control unit with a dry N2 stream.



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RESULTS AND DISCUSSION The most special feature of water molecules confined in extended nanospaces is the formation of well-ordered hydrogen bonding networks, which are accompanied by fast transfer of protons along a hydrogen-bonded chain between adjacent water via a proton hopping mechanism H3O+ + H2O → H2O + H3O+. In order to confirm whether the properties of the confined water are maintained irrespective of the addition of AcAc, 1H-NMR spinlattice relaxation rate (1/T1) values of water containing AcAc confined in the extended nanospaces were measured and compared with those of pure water as given in previous reports.21,22 Plots of the 1/T1 values vs. the space sizes are shown in Figure 1. Similar 1/T1 enhancement phenomena were observed for both water containing AcAc and pure water in the extended nanospaces, though there were slight differences in size-dependence behavior. The 1/T1 values of water at 3,000 nm were increased from 0.30 sec-1 to 0.41 sec-1 by adding AcAc, because the molecular motions of water were inhibited due to the hydration surrounding AcAc compared with pure water. The 1/T1 values of water containing AcAc decreased gradually with decreasing space sizes from 0.41 s-1 at 3,000 nm to 0.33 s-1 at 520 nm, but increased inversely with a further decrease in space sizes below 500 nm; from 0.35 s-1 at 360 nm to 0.71 s-1 at 210 nm. Such size-confinement effects on 1/T1 values indicated the slowing down of molecular motions of water, and could be related to the rearrangement of hydrogen bonding networks of water by space size-confinement, i.e., the cleavage process of the hydration above 500 nm was shifted to the formation process of well-ordered water below 500 nm. The results are evidence that water containing AcAc (1.0 M) also exhibit unique properties in extended nanospaces, similar to the pure water case.


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Therefore, the size-dependence of 1H-NMR spectra of AcAc in water confined in extended nanospaces (200 – 3,000 nm) was examined at 20 C. The proton signals were observed at around 2.2 and 5.8 ppm, 2.4 and 4.2 ppm, and 5.2 ppm, and they were assigned respectively as –CH3 and –CH groups in the enol form, –CH3 and –CH2 groups in the keto form, and water. As shown in Figure 2, the peak positions of the spectra and the peak area ratio (e.g., in keto –CH3 : –CH2 = 3 : 1) were almost constant regardless of space size, suggesting that the AcAc structure in the extended nanospaces was the same as that in bulk. Lineshape fitting analyses based on Voigt curves were performed for the peaks of –CH3 groups in the keto and enol forms. Figure 3 shows the experimental and fitted spectra of these –CH3 groups. From the area ratios of the –CH3 peak in keto form to that of the –CH3 peak in enol form, space size-dependence of the keto-enol equilibrium constant for AcAc was determined through KEQ=[keto]/[enol]. The results showed that the KEQ values could be changed with decreasing space sizes. The KEQ value at 3000 nm and 20 C was determined to be 3.5, which corresponds to the formation of AcAc consisting of about 80 % keto form in bulk water.29-31 When the KEQ value of the bulk (KEQbulk) was normalized to unity, the KEQ values in extended nanospaces (KEQnano) could be relatively plotted against the space sizes as shown in Figure 4. The relative changes of KEQ values (KEQnano/KEQbulk) were almost constant over the size of 500 – 3,000 nm, whereas they began to increase drastically with decreasing space sizes below about 500 nm and the value at 200 nm became 1.4 times as large as that in bulk. The enhancement of KEQ values means that the percentage of keto form of AcAc will increase due sorely to space size-confinement. Moreover, the KEQnano/KEQbulk values of AcAc in ethanol and in DMSO confined in extended nanospaces were also measured from the peak area ratios of –CH3 groups in the



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keto and enol forms (see Figure 2) and compared with the water case. The size-dependence of KEQnano/KEQbulk values was shown in Figure 4. Similar KEQ enhancement phenomena were observed in ethanol which is a hydrogen-bonded solvent like water, and the KEQ values in ethanol increased from 0.19 at 3,000 nm to 0.25 at 210 nm. In spite of the fact that the enol form dominates in ethanol, the size-dependence of the KEQnano/KEQbulk values for ethanol was in good agreement with that for water. On the other hand, we found that the KEQnano/KEQbulk values in aprotic solvents such as DMSO were almost constant regardless of space sizes, and size-confinement didn’t appear to affect the keto-enol equilibrium shifts. The magnitudes of changes of the KEQnano/KEQbulk values in extended nanospaces were observed to decrease in the order water > ethanol > DMSO. This observed order was correlated not to solvent polarity (dielectric constants at 25C; water (78.5), ethanol (24.3), DMSO (46.7)). In our previous report, 22 we found that the dynamic behavior of hydrogen-bonded, aprotic, and non-polar solvents could be related with acceptor number of each solvent. The results indicated that the proton-donating abilities of solvents were enhanced by nano sizeconfinement, because the acceptor number was corresponded to proton donicity. In addition, previous several studies showed that the keto-enol tautomeric equilibrium depends not on polarity but on proton-donating ability, and that keto-enol tautomerization shifts were generated through water-assisted relay paths of protons.32-34 On the basis of these previous reports, the chemical equilibrium in extended nanospaces is expected to be dominated by not dielectric constant but proton donicity. Actually, changes of the KEQnano/KEQbulk values in the present study were well consistent with the order of acceptor numbers of each solvent; water (54.8), ethanol (37.1), DMSO (19.3)).35 The detailed mechanisms could be explained as follows. Since the well-ordered water molecules in extended nanospaces have high proton


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mobility and localized proton charge distribution along the hydrogen bonding chains, they act as simultaneously as proton donors to a greater extent than in bulk water. This enhanced donor functionality allows the proton and oxygen atoms of the confined water to interact strongly with the carbonyl oxygen (C=O) and the OH group in the enol form of AcAc, respectively. In particular, the intermolecular hydrogen bond between the oxygen in the confined water and the OH proton in the enol form leads to cleavage of the C=O···HO-C intramolecular hydrogen bond in the enol form, and to conversion of the C-O bond in the enol form into a C=O bond. The electron-rich C=CH double bond in the enol form is then donated a proton from the confined water for neutralizing charges, followed by the production of a C-CH2 bond. As a result, the shifts from enol to keto are driven by the assistance of protons from the confined water, and the keto form in extended nanospaces will be stabilized at lower energies than in the bulk case. In order to examine the validity of the suggestion about the stabilization of the keto form, one reasonable approach is to measure the size-confinement effects on the free energies of KEQ values. The differences of Gibbs free energy (G) in keto-enol tautomerization, which is analyzed by decomposing G into the enthalpy (H) and entropy (S) terms, can be determined.30,36 Therefore, we measured the temperature dependence of KEQ values of AcAc in water in the range 0 to 30 °C. The KEQ values increased with increasing temperatures for all space sizes, and the plots of ln(KEQ) versus the reciprocal of temperatures gave linear relationships over the whole set of space sizes (see Figure 5). From the slopes and intercepts in the van't Hoff's plots, the H and S values could be obtained using linear least-square analysis. The results concerning size-dependence of H and S values of AcAc in water are shown in Figure 6. We found that the H and S values were almost the same in spaces



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between 500 and 3,000 nm in size, while the H and S values decreased drastically with decreasing space sizes, i.e., H and S values were respectively 7.23 kJ mol-1 and 35.34 J mol-1 K-1 at 520 nm and decreased 3.49 kJ mol-1 and 24.94 J mol-1 K-1 at 200 nm. The reductions mean that the keto form of AcAc is energetically more favorable and structurally more ordered in extended nanospaces than that in the bulk, because of stable intermolecular interactions between specific protons in the confined water and C=O groups of the keto form. Actually, the G values calculated from H and S values showed increasingly negative values as size decreased, and these changes occurred sharply in the vicinity of 500 nm over the whole temperature range. For example, as the space size decreased from 3000 to 200 nm,

G changed from 2.28 to -3.28 kJ mol-1 at 0 °C, from -2.70 to -3.50 kJ mol-1 at 10 °C, from 3.13 to -3.84 kJ mol-1 at 20 °C, and from -3.52 to -4.00 kJ mol-1 at 30 °C. It follows from the thermodynamic results that the keto-enol equilibrium of AcAc proceeds entirely in the direction to get the keto tautomer under nanospace confinement. If proton mobility on the confined water is a dominant factor for causing nanoconfinementinduced shifts of keto-enol tautomerization, the KEQ values of AcAc should be reduced by the substitution of a proton (H) with a deuteron (D), because the exchange rate between H and D becomes slow by a factor of more than ten from that between H and H. Therefore, the D concentration dependence of KEQ values of AcAc dissolved in mixtures of H2O and deuterated water (D2O) were measured in the extended nanospaces. The substitution ratios of D for H (D/(D+H)) are plotted against the KEQ values as shown in Figure 7. As expected, the KEQ values decreased with increasing D substitution ratios in 100 nm-scale extended nanospaces, but did not depend on deuteration for spaces greater than 1500 nm. When the molar fraction of D2O approached 0.4, the size-confinement effects for KEQ values disappeared. These results


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verify that the keto-enol tautomerization of AcAc in water can be controlled by changes of proton mobility between water and AcAc under extended nanospace environments.

CONCLUSION In conclusion, we have for the first time successfully characterized nanoconfinementinduced shifts of the keto-enol tautomerization of AcAc. The keto-enol equilibrium constants (KEQ=[keto]/[enol]) of AcAc in various solvents confined in extended nanospaces in the range of 200 to 3,000 nm were measured using 1H-NMR peak area ratios. In hydrogenbonded solvents such as water and ethanol, the keto form of AcAc was drastically increased with decreasing space sizes, while the size-confinement didn’t generate equilibrium shifts in aprotic solvents such as DMSO. Such KEQ enhancement phenomena were observed in nanospaces below about 500 nm in size, and the magnitudes of the KEQ enhancement due to size confinement were consistent with the order of proton donicity of solvents. Moreover, measurements of temperature- and deuterium-dependence of KEQ values verified that the keto form of AcAc in water could be energetically more favorable and structurally more stable rather than bulk spaces, because the high mobility of protons through the water molecules confined in extended nanospaces could induce more stable intermolecular interactions between the confined water and keto form of AcAc than is the case with bulk water. These new measurements will contribute to a deeper understanding of nanospatial reaction properties.

ACKNOWLEDGMENT



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The present work was partially supported by the Grant of Core Research for Evolutional Science and Technology (CREST) from Japan Science and Technology Agency (JST) and Funding Program for Next Generation World-Leading Researchers (NEXT Program) from Japan Society for the Promotion of Science (JSPS).

Supporting Information: Schematic illustration of water molecules in extended nanospaces and illustration of fabrication scheme and pictures of a NMR chip.


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FIGURE CAPTIONS Scheme 1 Keto-enol tautomeric equilibrium of AcAc. Figure 1 Size-dependence of 1/T1 values of water (■) and water containing AcAc (●) in the range of 200 to 3000 nm at 20 C. The 1/T1 values were measured with the inversion recovery pulse sequence. The solid lines are drawn just to guide the eyes.

Figure 2 1H-NMR spectra of –CH2 and –CH3 groups of AcAc in (a) water, (b) ethanol, and (c) DMSO confined in various extended nanospaces at 20 ºC and 500 MHz (JEOL JMNLA300WB spectrometer). The proton atoms in tetramethylsilane (TMS) were adopted as an external reference (0.0 ppm).

Figure 3 Experimental and lineshape fitting 1H-NMR spectra of –CH3 group (keto form) and –CH3 group (enol form) of AcAc in water confined in various extended nanospaces at 20 C and 500 MHz. Tetramethylsilane (TMS) was adopted as the external reference (0.0 ppm). The spectra are normalized to the same intensity scale.

Figure 4 Plots of the relative changes of KEQ values (KEQnano/KEQbulk) of AcAc in water (■), ethanol (▲), and DMSO (●) against space sizes. The KEQ value of the bulk (KEQbulk) was normalized to unity. Error bars represent 2σ uncertainties.

Figure 5 Plots of ln(KEQ) vs. the reciprocal of temperatures (1000/T) for various extended nanospaces at 20 ºC. Error bars represent 2σ uncertainties.



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Figure 6 Size-dependence of enthalpy (H) and entropy (S) for keto-enol tautomerization of AcAc in water confined in extended nanospaces in the size range of 200 to 3000 nm. Error bars represent 2σ uncertainties.

Figure 7 Deuterium-concentration dependence (D/(H+D)) of KEQ values of AcAc in water confined in the extended nanospaces (; 3000 nm, ●; 1500 nm, ▲; 360 nm, and ■; 230 nm).

TABLE CAPTION Table 1 The sizes of the fabricated extended nanospaces and their equivalent diameters for NMR measurements of AcAc in water.


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tautomerization of acetylacetone: An ab initio approach J. Chem. Phys. 1999, 110, 39383945. 27 Emsley, J.; Freeman, N. J. -diketone interactions: Part 5. Solvent effects on the keto ⇋ enol equilibrium. J. Mol. Struct. 1987, 161, 193-204 28 Wallen, S. L.; Yonker, C. R.; Phelps, C. L.; Wai, C. M. Effect of fluorine substitution, pressure and temperature on the tautomeric equilibria of acetylacetonate β-diketones. J. Chem. Soc. Faraday Trans. 1997, 93, 2391-2394 29 Folkendt, M. M.; Weiss-Lopez, B. E.; Chauvel, J. P.; True, N. S. Gas-phase proton NMR studies of keto-enol tautomerism of acetylacetone, methyl acetoacetate, and ethyl acetoacetate. J. Phys. Chem. 1985, 89, 3347-3352. 30 Spencer, J. N.; Holmboe, E. S.; Kirshenbaum, M. R.; Firth, D. W.; Pinto, P. B. Solvent effects on the tautomeric equilibrium of 2,4-pentanedione. Can. J. Chem. 1982, 60, 11781182. 31 The KEQ value of 1.0 M AcAc in bulk water could be determined as 3.6 from our 1H-NMR measurements. 32 Tamabe, S; Tsuchida, N; Miyajima, K. Reaction paths of keto-enol tautomerization of diketones. J. Phys. Chem. A 2004, 108, 2750-2757 33 Xue, Y.; Kim, C. K.;, Guo, Y.; Xie, D. Q.; Yan, G. S. DFT study and Monte Carlo simulation on proton transfers of 2-amino-2-oxazoline, 2-amino -2-thiazoline, and 2amino-2-imidazoline in the gas phase and in water. J. Comput. Chem. 2005, 26, 994-1005. 34 Ziolek, M.; Kubicki, J.; Maciejewski, A.; Naskrecki, R.; Grabowska, A. Enol-keto tautomerism of aromatic photochromic Schiff base N,N'-bis(salicylidene)-pphenylenediamine: Ground state equilibrium and excited state deactivation studied by solvatochromic measurements on ultrafast time scale. J. Chem. Phys. 2006, 124, 124518. 35 Linert, W.; Fukuda, Y.; Camard, A. Chromotropism of coordination compounds and its applications in solution. Coord. Chem. Rev. 2001, 218, 113-152 36 Bassetti, M.; Cerichelli, G.; Floris, B. Substituent effects in keto-enol tautomerism. Part 3. influence of substitution on the equilibrium composition of β-dicarbonyl compounds. Tetrahedron 1988, 44, 2997-3004.



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FIGURES AND TABLE

Scheme 1 Tsukahara et al.

Figure 1 Tsukahara et al.


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Table 1 Tsukahara et al. Width w / nm

Depth d / nm

4500

2250

Equivalent diameter R / nm 3000

1500

1500

1500

700

440

540

640

440

520

960

250

400

630

250

360

530

250

340

670

220

330

360

250

300

300

230

260

250

220

230

960

120

210

250

170

200



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For TOC


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KetoâEnol Tautomeric Equilibrium of Acetylacetone Solution Confined

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KetoâEnol Tautomeric Equilibrium of Acetylacetone Solution Confined