Ind. Eng. Chem. Res. 1998, 37, 3685-3690
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SEPARATIONS Selective Adsorption of Methoxyphenol Positional Isomers Amy Jo Glemza, Jeffrey A. Koehler, Brian J. Brune, and Gregory F. Payne* Department of Chemical and Biochemical Engineering and Center for Agricultural Biotechnology, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250
Disubstituted aromatics are commonly synthesized by electrophilic substitution of the ring. However, these reactions typically yield a mixture of ortho and para isomers. In this study we examined whether adsorption from a nonpolar solvent onto a polar sorbent could offer regioselectivity for separating such positional isomers. Specifically, we examined adsorption of substituted phenols onto an acrylic ester sorbent that has previously been observed to adsorb phenolic compounds through a hydrogen-bonding mechanism. The isotherms for the individual isomers indicated small regioselectivities for the adsorption of the alkylphenols cresol and propylphenol. Considerably larger regioselectivities were observed for methoxyphenol adsorption, with the para isomer being preferentially adsorbed compared to o-methoxyphenol. In mixture studies, the separation factor for p-/o-methoxyphenol separations was observed to exceed 15. Infrared measurements suggest that these high separation factors were attained because o-methoxyphenol can form an intramolecular hydrogen bond, and this intramolecular hydrogenbonding mechanism competes with the hydrogen-bonding adsorption mechanism. These results demonstrate that when hydrogen bonding is the primary adsorption mechanism, solution-phase intramolecular hydrogen bonding suppresses adsorption and this suppression can be exploited to enhance adsorptive separation factors. Introduction Adsorption is gaining increased attention for the industrial-scale separations of lower-valued chemicals.1-3 Key to adsorption-based processes is the sorbent, which must be capable of preferentially adsorbing one component from a mixture.4,5 For instance, the development of molecular sieve zeolites permitted the commercialization of adsorption processes for separating the isomers of xylene and linear from branched-chain aliphatics. In addition to achieving separations by imposing steric constraints, there is a growing interest in exploiting specific molecular interactions to confer selectivity to adsorption. The industrial-scale separation of glucose and fructose is achieved by exploiting specific interactions between sugars and calcium.6-8 More recently, there have been efforts to exploit interactions between metals (e.g., silver) and the π electrons of olefins to develop adsorption-based approaches to separate olefins and aliphatics.9-11 To more effectively exploit sorbents to confer selectivity to adsorption, it is necessary to control the molecular interactions responsible for sorbate-sorbent binding. Unfortunately, the common industrial adsorbents (e.g., activated carbon, alumina, and silica) have heterogeneous surfaces that allow sorbates to adsorb at differing sorption sites through numerous binding mechanisms. Because surface heterogeneity reduces the discriminating capability of the sorbent, we have been studying polymer-based sorbents in the hope that polymeric * Corresponding author: Phone 410-455-3413; FAX 410455-6500; E-mail
[email protected].
sorbents will offer more homogeneous surfaces for enhanced adsorptive selectivities. To exploit specific, polar interactions for separations, we have been studying an acrylic ester sorbent (XAD-7, Rohm and Haas) that offers a polar carbonyl site for adsorption. Previous studies12-16 indicate that hydrogen bonding is a dominant mechanism for the adsorption of phenolic solutes from nonpolar solvents (e.g., hexane) onto this sorbent as suggested by
Ar-OH + OdC-sorbent T Ar-OH‚‚‚OdC-sorbent In this investigation, we examined the selectivity for adsorbing positional isomers (i.e., ortho, meta, and para isomers) of substituted phenols. Commonly, substituted aromatics are produced through electrophilic substitution reactions that are not entirely regiospecific. Separations of individual positional isomers by traditional distillation approaches can be problematic if the isomers have similar properties or if the substituents have polar functionalities which reduce volatilities. Despite the difficulties, separations of positional isomers can be important since individual isomers are commonly used as intermediates in the synthesis of pharmaceutical, food, and agricultural chemicals. The increasing concerns for the safety and quality of these chemical products is driving the development of improved approaches for separating individual isomers. The specific problem for our study is the separation of o- and p-methoxyphenols. As illustrated in Scheme 1, an initial step in methoxyphenol production is the oxidation of phenol to a mixture of dihydroxy speciess
S0888-5885(98)00155-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/11/1998
3686 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 Scheme 1
the ortho (i.e., catechol) and para (i.e., hydroquinone) isomers. Addition of methanol yields methoxyphenols, which are then used as intermediates in the synthesis of various specialty chemicals. p-Methoxyphenol is used for the synthesis of butylated hydroxyanisole, which is an antioxidant used in foods. The ortho isomer, also known as guaiacol, is used for the synthesis of vanillin, dihydroxyphenylalanine, and the expectorant guaifenesin. Since the initial oxidation is not entirely regioselective, both the o- and p-dihydroxy isomers are formed.17 Further, since dihydroxy compounds are less stable than the methoxyphenols, it is useful to consider separating the isomers after the methylation reaction.18 Because of the relatively high boiling points of these isomers (205 and 243 °C for o- and p-methoxyphenol, respectively), distillation-based separations require high temperatures and/or low pressures. Thus, it would be desirable to explore alternative techniques that can separate these isomers under milder conditions. Materials and Methods The sorbents used in this study were Rohm and Haas XAD-7 (acrylic ester) and XAD-16 (styrenic) with specific surface areas reported by the supplier (Sigma Chemical Co.) to be approximately 450 and 800 m2/g for XAD-7 and XAD-16, respectively. All sorbents were sequentially washed with water, methanol, acetone, and hexane and vacuum-dried prior to use. All chemicals used in this study were obtained from Sigma (St. Louis, MO) or Aldrich (Milwaukee, WI). Adsorption studies were conducted by equilibrating known amounts of sorbent with hexane solutions containing either a single solute or a mixture of solutes. For the single-solute studies, the equilibrated concentration (C) of solute was determined by UV-visible spectrophotometry (Spectronics Genesys II). The adsorbed amount of solute (q) was calculated from the difference between the initial (C0) and equilibrated concentrations of the solute in hexane by
q)
(C0 - C)V A
(1)
where V is the volume of hexane and A is the mass of sorbent. The adsorption affinity was determined as the ratio of adsorbed to dissolved solute concentrations (q/ C). Since our studies were confined to the linear region of the isotherm, the affinity is also the slope of the adsorption isotherm. When mixtures of o- and p-methoxyphenol were studied, spectrophotometry was used to determine
Figure 1. Adsorption of p-methoxyphenol from hexane onto acrylic ester and styrenic sorbents. The adsorption data is normalized in terms of the specific surface areas and the normalized adsorption affinities (q′/C) are reported in units of (millimoles adsorbed per square meter)/(millimoles dissolved per liter of solution). Adsorption temperature was 25 °C.
concentrations of the individual isomers. This approach was possible because the absorption spectra for the two isomers were significantly different, and the spectra were additive. Thus, we measured the absorbance at two wavelengths (276 and 301 nm) and calculated concentrations by using extinction coefficients for both solutes at both wavelengths. In the Fourier transform infrared (FTIR) studies, all spectra were obtained on a Perkin-Elmer System 2000 FTIR (resolution of 4 cm-1) and KBr windows with 0.762 mm spacers. The spectrum for ethyl propionate (soluble model of the acrylic ester sorbent) was also obtained so its peaks could be subtracted from the solute spectra. Results and Discussion To demonstrate the importance of sorbent surface chemistry, we studied the adsorption of p-methoxyphenol from hexane onto the acrylic ester and the styrenic sorbents. The adsorbed concentration is normalized in terms of surface area to account for differences in specific surface area between the two sorbents. As shown in Figure 1, p-methoxyphenol adsorbs onto the acrylic ester sorbent with a 24-fold higher affinity compared to its adsorption onto the styrenic sorbent. Similarly, p-cresol was observed to adsorb onto the acrylic ester sorbent with a 17-fold higher affinity compared to its adsorption onto the styrenic sorbent (data not shown). These results demonstrate the importance of the acrylic ester sorbent’s polar surface for achieving high adsorption affinities from a nonpolar solvent. In addition, these results are consistent with a hydrogen-bonding adsorption mechanism in that the carbonyl site of the acrylic ester sorbent is expected to be a more effective hydrogen-bond acceptor than the π electrons of the styrenic sorbent.19 To determine if the acrylic ester sorbent can adsorb solutes regioselectively, we examined the adsorption of the three positional isomers of methoxyphenol and cresol. The adsorption isotherms for p-, m-, and omethoxyphenol are shown in Figure 2a and demonstrate that the meta and para isomers have a much higher affinity for the sorbent than the ortho isomer. The ratio of affinities for the para and ortho isomers was observed to be 19. The isotherms for p-, m-, and o-cresol adsorption are shown in Figure 2b. Although the order of the adsorption affinities for cresol is the same (i.e., meta > para > ortho), the differences between the cresol isomer
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Figure 3. van’t Hoff plot for adsorption of methoxyphenol isomers. The adsorption affinity is the slope of the adsorption isotherms in the linear region and is reported in units of (millimoles adsorbed per gram of resin)/(millimoles dissolved per liter of solution).
Figure 2. Adsorption of individual regioisomers from hexane onto the acrylic ester sorbent for (a) methoxyphenol, (b) cresol, and (c) propylphenol. The adsorption affinities (q/C) are reported in units of (millimoles adsorbed per gram of resin)/(millimoles dissolved per liter of solution). Adsorption temperature was 25 °C.
affinities is smaller than for the methoxyphenol isomers. The ratio of affinities for p- and o-cresol adsorption was observed to be only 1.7. These results indicate that adsorption from hexane onto the acrylic ester sorbent offers some regioselectivity, especially for the methoxyphenols. Figure 2a,b show that adsorption of the ortho isomers is suppressed compared to that of the para and meta isomers, and that this suppression is more significant for methoxyphenol than for cresol. One possible explanation for the lower affinity for o-methoxyphenol adsorption is that the bulkier methoxy group sterically impedes hydrogen-bond formation between the phenolic hydroxyl group and the sorbent’s carbonyl site. To test this possibility, we studied the adsorption of an alkylphenol with a longer substituent, propylphenol. The isotherms for p- and o-propylphenol adsorption are shown in Figure 2c and again show that the para isomer is adsorbed with a greater affinity than the ortho isomer. However, the difference in affinities between p- and
o-propylphenol is small, with the affinity ratio being only 1.3. These results for propylphenol adsorption suggest that the low adsorption affinity for o-methoxyphenol is not likely due to steric effects. To further characterize methoxyphenol isomer adsorption, we determined adsorption enthalpies using the van’t Hoff method. Adsorption isotherms were measured at numerous temperatures, and Figure 3 shows a semilogarithmic plot of adsorption affinity vs 1/T for the three methoxyphenol isomers. The positive slopes in Figure 3 indicate that adsorption is exothermic, and the enthalpies were determined to be -7.7, -8.5, and -5.7 kcal/mol for m-, p-, and o-methoxyphenol, respectively. The magnitudes of these enthalpies are consistent with a hydrogen-bonding mechanism.19,20 Compared to the other isomers, o-methoxyphenol has both a lower adsorption affinity and a less negative adsorption enthalpy, which is consistent with the previously observed correlation between affinity and enthalpy.14 To obtain more direct information about the nature of the interaction responsible for suppressing o-methoxyphenol adsorption, we examined the IR spectra of the methoxyphenol isomers. As shown in Figure 4a, the spectra for m- and p-methoxyphenol have peaks at 3622 and 3627 cm-1, respectively. These peaks are assigned to O-H stretching of the non-hydrogen-bonded species. The spectra for o-methoxyphenol does not have a peak in the 3620 cm-1 region but has a somewhat broadened peak at a lower frequency of 3566 cm-1, consistent with an intramolecular hydrogen bond.21,22 Thus, the spectra of Figure 4a suggests that o-methoxyphenol exists in hexane primarily as an intramolecularly hydrogenbonded species as indicated by the structure in Figure 4a. For comparison, the IR spectra for the cresol isomers in hexane are shown in Figure 4b. As can be seen from their spectra, all three cresol isomers have peaks in the 3620 cm-1 region, indicating that at the concentrations studied they exist in hexane as nonhydrogen-bonded species. To provide information about the adsorption mechanism, we examined interactions between methoxyphenol isomers and a soluble model of the acrylic ester adsorbent. If solute-adsorbent interactions can be experimentally simulated in solution, then IR provides a simple means to probe these interaction mechanisms. Such an approach offers the potential for overcoming the experimental constraints inherent to the study of adsorption mechanisms for insoluble, polymeric beads.
3688 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 Table 1. Comparison of Binding Strengths and Adsorption Enthalpies for Methoxyphenols net adsorption infrared stretching frequency of O-H enthalpy isomer νfreea (cm-1) νboundb (cm-1) ∆νc (cm-1) ∆H° d (kcal/mol) ortho meta para
3566 3622 3627
3469 3454 3470
97 168 157
-5.7 -7.7 -8.5
a O-H stretching frequency for species not bonded to ethyl propionate (Figure 4a). b O-H stretching frequency for species bonded to ethyl propionate (Figure 5). c Difference between O-H stretching frequencies (∆ν ) νfree - νbound). d Enthalpy observed for adsorption of the individual isomers from hexane (Figure 3).
Figure 4. Infrared spectra of (a) methoxyphenol and (b) cresol isomers. Spectra were obtained at room temperature with 12 mM of the individual isomers dissolved in hexane.
Figure 5. Infrared spectra of methoxyphenol isomers with ethyl propionate in hexane. When m- and p-methoxyphenol were measured, the solute and ethyl propionate concentrations were 12 and 34.8 mM, respectively. For studies with the ortho isomer, 11.4 mM o-methoxyphenol and 415.4 mM ethyl propionate were used. Spectra were obtained at room temperature.
Ethyl propionate was chosen as the hexane-soluble model for the sorbent because it has an ester linkage containing a carbonyl site similar to that of the acrylic ester sorbent. As shown in Figure 5, the spectra for hexane solutions containing ethyl propionate and individual methoxyphenol isomers have peaks at the same positions as observed in Figure 4a (i.e., for species not bonded to ethyl propionate). Also, the spectra for these isomers have broadened peaks in the 3460 cm-1 region for the O-H stretching of the methoxyphenol species hydrogen-bonded to ethyl propionate. It should be noted that when the o-methoxyphenol isomer was studied,
higher ethyl propionate concentrations were required to detect an ethyl propionate-bonded peak at 3460 cm-1. The need for higher ethyl propionate levels indicates that binding with o-methoxyphenol is less favorable than binding with either the para or meta isomers. The structures for the putative ethyl propionate-bonded species are shown in Figure 5. Due to the similarities in the frequencies for the ethyl propionate-bonded species, we believe the ethyl propionate-bonded omethoxyphenol no longer has an intramolecular hydrogen bondsas indicated by the structures in Figure 5. Table 1 shows information on binding strengths and supports the choice of ethyl propionate as a soluble model for the acrylic ester sorbent. Specifically, the mand p-methoxyphenol isomers were observed to adsorb onto the acrylic ester sorbent with more negative enthalpies compared to adsorption of the ortho isomer. For IR measurements, hydrogen-bonding strengths are commonly quantified by the frequency shift (∆ν) between the nonbonded and bonded species.21,23-25 With respect to ethyl propionate binding of the methoxyphenols, Table 1 shows the frequency shifts are larger for the meta and para isomers compared to the ortho isomer. In summary, the information from the IR supports previous conclusions that adsorption from hexane onto the acrylic ester sorbent is due to the formation of a hydrogen bond.12-16 Further, the IR data suggests that adsorption of o-methoxyphenol is suppressed due to a competing intramolecular hydrogen-bonding mechanism.26 The structures shown in Figure 5 suggest that in order for o-methoxyphenol to adsorb, the intramolecular hydrogen bond must be broken and a new hydrogen bond must be formed with the carbonyl site of the sorbent. In a final experiment, we tested whether p-methoxyphenol would be selectively adsorbed from a mixture containing its ortho isomer. For this, we prepared hexane solutions containing both the para and ortho isomers. Figure 6a shows the isotherms for adsorption of the individual isomers from the mixture at 25 °C. As expected from the single solute studies (Figure 2a), p-methoxyphenol is preferentially adsorbed and the separation factor was calculated to be 17 ( 2. Adsorption studies were also performed at different temperatures, and a semilogarithmic plot of the separation factor (Rpara-ortho) vs 1/T is shown in Figure 6b. The positive slope of the line in Figure 6b indicates that the enthalpy change for adsorption of the para isomer is more negative than that for o-methoxyphenol adsorption. The slope in Figure 6b can be related to the
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adsorbed species. As indicated in Scheme 2, the net enthalpy change for o-methoxyphenol adsorption (∆H°ortho) is the sum of the intrinsic enthalpy change for adsorption (∆H°ads,ortho) and the enthalpy change to break the intramolecular hydrogen bond. If ∆H°IMHB is the enthalpy change for intramolecular hydrogenbond formation, then
∆H°ortho ) ∆H°ads,ortho - ∆H°IMHB
(3)
It should be noted that IR analysis provides no evidence for an o-methoxyphenol species with a free O-H group, and thus the postulated intermediate in Scheme 2 is a hypothetical, thermodynamic intermediate. If the intrinsic enthalpy changes for adsorption of the two isomers are similar (i.e., if ∆H°ads,ortho is similar to ∆H°para), then the enthalpy difference determined in Figure 6b should be due, in large part, to the enthalpy for intramolecular hydrogen-bond formation:
(∆H°para - ∆H°ortho) ) (∆H°para - ∆H°ads,ortho) + ∆H°IMHB ≈ ∆H°IMHB (4) The enthalpy difference of -3.6 kcal/mol determined from Figure 6b suggests that the intramolecular hydrogen bond for o-methoxyphenol is rather weak. This suggestion is consistent with reports by Berthelot et al.22,27 that the intramolecularly hydrogen-bonded species has a strained five-membered ring. Figure 6. Selective adsorption of p-methoxyphenol from mixtures containing the ortho isomer. (a) Isotherms for adsorption of the individual methoxyphenol isomers from hexane solutions containing both p-methoxyphenol (0.25 mM) and o-methoxyphenol (0.15 mM) (separation factor at 25 °C is 17 ( 2). (b) Semilogarithmic plot of the separation factor versus 1/T [from eq 2; the slope indicates that (∆H°para - ∆H°ortho) is -3.6 kcal/mol].
Scheme 2
enthalpy difference by14
∂ ln (Rpara-ortho) ∂ (1/T)
)
-(∆H°para - ∆H°ortho) R
(2)
From the slope of the line in Figure 6b, (∆H°para ∆H°ortho) was calculated to be -3.6 kcal/mol, which is comparable to the enthalpy difference of -2.8 kcal/mol which can be calculated from the adsorption enthalpies determined from the single solute studies (Table 1). The enthalpy difference observed in Figure 6b is consistent with the physical model of adsorption shown in Scheme 2. Specifically, Scheme 2 shows that adsorption of the para isomer involves the conversion of a nonhydrogen-bonded, solution-phase species into a hydrogen bonded, adsorbed species. Thus the net enthalpy change for adsorption of p-methoxyphenol (∆H°para) is simply the intrinsic enthalpy change for adsorption. In contrast, adsorption of the ortho isomer is believed to involve the conversion of an intramolecularly hydrogenbonded solution-phase species into a hydrogen-bonded,
Conclusions Studies with individual ortho and para isomers indicated that adsorption from a nonpolar solvent onto the polar acrylic ester sorbent offers some regioselectivity. Although small for alkylphenols (e.g., cresol and propylphenol), the regioselectivity of adsorption was observed to be quite large for the methoxyphenol isomers. In mixtures of o- and p-methoxyphenol, the para isomer was preferentially adsorbed and the separation factor was observed to exceed 15. This high separation factor results from a suppression of the adsorption of o-methoxyphenol. IR analysis suggests that adsorption of o-methoxyphenol is suppressed because the adsorptive hydrogenbonding mechanism must compete with a solution-phase intramolecular hydrogen-bonding mechanism. Evidence from this study and the literature21,22,27 indicates that the intramolecular hydrogen bond is relatively weak because it leads to the formation of a species with a strained five-membered ring. Nevertheless, this intramolecular hydrogen bond is sufficiently strong that the intramolecularly hydrogen-bonded species predominates in the nonpolar solvent, and the intramolecular hydrogen bond substantially suppresses adsorption of o-methoxyphenol. These results demonstrate that by limiting adsorption to a specific mechanism (i.e., hydrogen bonding), it is possible to exploit competing solution-phase interactions to attain high separation factors. Acknowledgment Financial support was provided by the National Science Foundation through Grant CTS-9531812 and REU supplements to this grant. Literature Cited (1) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley and Sons: New York, 1984.
3690 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 (2) Sircar, S.; Myers, A. L. Liquid Adsorption Operations: Equilibrium, Kinetics, Column Dynamics and Applications. Sep. Sci. Technol. 1986, 21, 535. (3) Ruthven, D. M.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; VCH Publishers: Weinheim, Germany, 1994. (4) Ruthven, D. M. Zeolites as Selective Adsorbents. Chem. Eng. Prog. 1988, 84 (2), 42. (5) Keller, G. E. Technological Maturity of Sorption Processes and Sorbents. In New Directions in Sorption Technology; Keller, G. E., Yang, R. T., Eds.; Butterworth: Boston, 1989; p 43. (6) Ching, C. B.; Ruthven, D. M. Analysis of the Performance of a Simulated Counter-current Chromatographic System for Fructose-Glucose Separation. Can. J. Chem. Eng. 1984, 62, 398. (7) Ho, C.; Ching, C. B.; Ruthven, D. M. A Comparative Study of Zeolite and Resin Adsorbents for the Separation of FructoseGlucose Mixtures. Ind. Eng. Chem. Res. 1987, 26, 1407. (8) Cheng, Y. L.; Lee, T. Y. Separation of Fructose and Glucose Mixture by Zeolite Y. Biotechnol. Bioeng. 1992, 40, 498. (9) Eldridge. Olefin/Paraffin Separation Technology: A Review. Ind. Eng. Chem. Res. 1993, 32, 2208. (10) Yang, R. T.; Kikkinides, E. S. New Sorbents for Olefin/ Paraffin Separations by Adsorption Via π-Complexation. AIChE J. 1995, 41, 509. (11) Wu, Z.; Han, S.-S.; Cho, S.-H.; Kim, J.-N.; Chue, K.-T.; Yang, R. T. Modification of Resin-Type Adsorbents for Ethane/ Ethylene Separation. Ind. Eng. Chem. Res. 1997, 36, 2749. (12) Payne, G. F.; Payne, N. N.; Ninomiya, Y.; Shuler, M. L. Adsorption of Nonpolar Solutes onto Neutral Polymeric Sorbents. Sep. Sci. Technol. 1989, 24, 457. (13) Payne, G. F.; Ninomiya, Y. Selective Adsorption of Solutes Based on Hydrogen Bonding. Sep. Sci. Technol. 1990, 25, 1117. (14) Maity, N.; Payne, G. F.; Chipchosky, J. L. Adsorptive Separations Based on the Differences in Solute-Sorbent HydrogenBonding Strengths. Ind. Eng. Chem. Res. 1991, 30, 2456. (15) Chaubal, M. V.; Payne, G. F. Use of Acrylic Ester Sorbent for the Selective Adsorption of Avermectins. Biotechnol. Prog. 1995, 11, 468. (16) Brune, B. J.; Payne, G. F.; Chaubal, M. V. Linear Solvation Energy Relationships to Explain Interactions Responsible for Solute Adsorption onto a Polar Polymeric Sorbent. Langmuir 1997, 13, 5766. (17) Tendulkar, S. B.; Tambe, S. S.; Chandra, I.; Rao, P. V.; Naik, R. V.; Kulkarni, B. D. Hydroxylation of Phenol to Dihydroxybenzenes: Development of Artificial Neural-Network-Based
Process Identification and Model Predictive Control Strategies for a Pilot Plant Scale Reactor. Ind. Eng. Chem. Res. 1998, 37, 2081. (18) Lee, M.-J.; Chang, Y.-K.; Lin, H.-M.; Chen, C.-H. SolidLiquid Equilibria for 4-Methoxyphenol with Catechol, Ethylenediamine, or Piperazine. J. Chem. Eng. Data 1997, 42, 349. (19) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H. Freeman and Co.: San Francisco, CA, 1960. (20) Letcher, T. M.; Bricknell, B. C. Calorimetric Investigation of the Interactions of Some Hydrogen-Bonded Systems at 298.15 K. J. Chem. Eng. Data 1996, 41, 166. (21) Takasuka, M.; Matsui, Y. Experimental Observations and CNDO/2 Calculations for Hydroxy Stretching Frequency Shifts, Intensities, and Hydrogen Bond Energies of Intramolecular Hydrogen Bonds in ortho-Substituted Phenols. J. Chem Soc., Perkin Trans. 2 1979, 1743. (22) Berthelot, M.; Laurence, C.; Foucher, D.; Taft, R. W. Partition Coefficients and Intramolecular Hydrogen Bonding. 1. The Hydrogen-Bond Basicity of Intramolecular Hydrogen-Bonded Heteroatoms. J. Phys. Org. Chem. 1996, 9, 255. (23) Badger, R. M.; Bauer, S. H. Spectroscopic Studies of the Hydrogen Bond. II. The Shift of the O-H Vibrational Frequency in the Formation of the Hydrogen Bond. J. Chem. Phys. 1937, 5, 839. (24) Badger, R. M. The Relation Between the Energy of a Hydrogen Bond and the Frequencies of the O-H Bands. J. Chem. Phys. 1940, 8, 288. (25) Gorlaski, P.; Berthelot, M.; Rannon, J.; Legoff, D.; Chabanel, M. The Hydrogen Bonding of Alcohols, Cholesterol and Phenols with Cyanide and Azide Ions. J. Chem Soc., Perkin Trans. 2 1994, 2337. (26) Knauth, P.; Sabbah, R. Energetics of Inter- and Intramolecular Bonds in Alkanediols. IV. The Thermochemical Study of 1,2-Alkanediols at 298.15 K. Thermochim. Acta 1990, 164, 145. (27) Berthelot, M.; Laurence, C.; Lucon, M.; Rossignol, C.; Taft, R. W. Partition Coefficients and Intramolecular Hydrogen Bonding. 1. The Influence of Partition Solvents on the Intramolecular Hydrogen-Bond Stability of Salicylic Acid Derivatives. J. Phys. Org. Chem. 1996, 9, 626.
Received for review March 10, 1998 Revised manuscript received June 22, 1998 Accepted June 23, 1998 IE9801554
