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An experimental and theoretical study on the interaction of PHBA ions and H2O molecules
Spectrochimica Acta Part A 65 (2006) 930–934
An experimental and theoretical study on the interaction of PHBA ions and H2O molecules Xinfeng Zhao, Yan Fang ∗ Beijing Key Lab of Nanophotonics and Nanostructure, Capital Normal University, Beijing 100037, PR China Received 18 November 2005; received in revised form 16 January 2006; accepted 25 January 2006
Abstract We studied the dependence of the Raman spectra on the concentration of PHBA aqueous solution under UV laser excitation. Through analyzing the spectra, we conclude that the interaction between PHBA ions and H2 O molecules is weak. To further explore the problem, we studied the interaction between PHBA ions and H2 O molecules by virtue of theoretical calculations, DFT-B3PW91/6-31+g* was employed. We draw a coincident conclusion with the experiments and dug out the reasonable interaction model that reflects the actual interaction configuration between PHBA ions and H2 O molecules. We supply a new thinking for studying interactions between solute molecules and solvent molecules, which also can be applied to interactions between solute molecules and other solute molecules in solutions. © 2006 Elsevier B.V. All rights reserved. Keywords: p-Hydroxybenzoic acid (PHBA); Density functional theory (DFT); Calculated Raman frequencies; Interaction model
1. Introduction Employing photons as probes, Raman spectroscopy that has no damage to samples has been widely used for studying chemical structure and physical interface phenomena [1]. Conventional Raman spectroscopy usually was obtained and worked well at NIR or visible excitations [2,3]. However, an obvious weakness of Raman spectroscopy is its low sensitivity, which results in much more difficulties in obtaining Raman spectrum of low solubility materials in solution. UV Raman spectroscopy has been well developed in these years. Due to its high energy, UV line could lead to resonant Raman transition or near resonant Raman transition among the electron energy levels [4], increasing greatly sensitivity and acquiring more information about molecule vibration and structure. Moreover, UV Raman could avoid fluorescence [5], and this reduces influences on Raman spectrum. p-Hydroxybenzoic acid (PHBA) is an organic substance, which has many meaningful applications. First, in biologic reactions, PHBA plays an important role and is a naturally occurring molecule with significant importance [6,7]. Second, PHBA is also used in synthesizing LC polymers [8,9]. Besides, the struc-
∗
Corresponding author. Tel.: +86 1068902965. E-mail address: [email protected] (X. Zhao).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2006.01.034
ture of PHBA molecule is simple, which is usually used as probe molecule for studying SERS and adsorption behaviors in metal aqueous colloid [10]. A high quality Raman spectrum of PHBA in aqueous solution under ultraviolet (UV) excitation had been obtained [11], which could not be observed with visible and near infrared (NIR) excitations due to its low concentration. Ref. [11] has proved that PHBA exists as ion state in aqueous solution by means of theoretical calculation combining with experimental instruments. We studied the dependence of the UV Raman spectra on the concentration of PHBA aqueous solution under UV laser excitation. Through analyzing the spectra, we conclude that the interaction between PHBA ions and H2 O molecules is weak. To further explore the problem, we study the interaction between PHBA ions and H2 O molecules by virtue of theoretical calculation, DFT [12,13]-B3PW91 [14]/6-31+g* is applied. Theoretically, the difficulty lies in the establishment of a simple model, which should describe the interaction of molecules properly, and at the same time can be used to calculate the vibrational spectra easily. The theoretical results were compared with the experimental values, and the reliability of the models and the calculation methods were analyzed. It was found that the models lead to different vibrational spectra, which correspond to the experimental observation. The possible models were established and Raman calculations were applied to these models. Through comparing the results of theoretical calculations with experi-
X. Zhao, Y. Fang / Spectrochimica Acta Part A 65 (2006) 930–934
ments, we draw a coincident conclusion with the experiments and dug out the reasonable interaction model that reflects the actual interaction configuration between PHBA ions and H2 O molecules. There are many papers about theoretical studies combining with vibrational spectra: studies of ion adsorption on metal surface could be found in Refs. [15–17] and molecule structure found in Refs. [18–20]. In our work, we study the interaction between PHBA ions and H2 O molecules by means of the experimental instruments combining with the theoretical calculations. From the results of the experiments, we conclude that the interaction between PHBA ions and H2 O molecules is weak. On the other hand, the weak interaction is coming from O atoms of PHBA ion and H2 O molecules, and the reasonable interaction model between PHBA ions and H2 O molecules is dug out by virtue of the theoretical calculations. The traditional methods studying the interaction are difficult on account of PHBA’s very low solubility. We supply a new thinking for studying interactions between solute molecules and solvent molecules, which also can be applied to interactions between solute molecules and other solute molecules in solutions.
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Fig. 1. Raman spectra for PHBA aqueous solution at different concentrations: 1 × 10−2 (a), 5 × 10−3 (b), 2.5 × 10−3 (c), 1 × 10−3 (d), 5 × 10−4 M (e) and for deionized water (f).
2. Experimental The UV Raman spectrum was obtained through a RENISHAW H13325 spectrophotomer, and the UV laser line was at 325 nm. The output laser power, which could not induce changes to the sample, was about 12 mW. A high quality Raman spectrum of PHBA in aqueous solution under ultraviolet (UV) excitation had been obtained, which could not be observed with visible and near infrared (NIR) excitations due to its low concentration [11]. Ref. [11] has proved that PHBA exists as ion state in aqueous solution by means of the theoretical calculations combining with experiments. PHBA exists as ion state in aqueous solution, then, how do PHBA ions exist? The interaction between PHBA ions and H2 O molecules is strong or weak? How do H2 O molecules interact with PHBA ions? We will answer these questions. We studied the dependence of UV Raman spectra on the concentration of PHBA aqueous solution under UV laser excitation, the results are shown in the Fig. 1. From the Fig. 1, we can find that the Raman frequencies are mainly divided into three segments: near 850, 1100–1300, and 1600 cm−1 . It is clear that the lower the concentration of PHBA aqueous solution, the weaker intensity of Raman frequencies in the UV Raman spectra. When the solution is diluted to 10−3 M, only several strong bands at 1610, 1277, 1172, and 847 cm−1 could be observed. When the solution is diluted to 5 × 10−4 M, the spectrum is similar to that of deionized water, just shown in Fig. 1. This is because the quantity of PHBA ions became less and less in unit volume. However, the Raman frequencies do not shift evidently, which demonstrates that the transformation of its structure is not appreciable, but tiny. That further proved the interaction between PHBA ions and H2 O molecules is weak. In comparison with the UV Raman spectrum of PHBA solid powder [11], we find that an additional band appears at 1688 cm−1 , which is due to the C O stretching mode in Fig. 2b.
Fig. 2. Interaction models of PHBA ion and H2 O molecule. (a) The structure of PHBA ion. (b) Interaction model of PHBA ion and one H2 O molecule. The H2 O molecule interact with the O ion on carboxyl group of PHBA ion. (c) Interaction model of PHBA ion and two H2 O molecules. The H2 O molecules interact with the O atom and the O ion on carboxyl group of PHBA ion. (d) Interaction model of PHBA ion and two H2 O molecules. The H2 O molecules interact with the O ion on carboxyl group and the O atom on hydroxyl group of PHBA ion. (e) Interaction model of PHBA ion and three H2 O molecules. The H2 O molecules interact with the O atom and the O ion on carboxyl group and the O atom on hydroxyl group of PHBA ion.
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And Fig. 2c does not yield the weak band at the 1388 cm−1 that appears in the solution. The 1288 cm−1 band in Fig. 2c downshift to 1277 cm−1 in Fig. 2b. Another new shoulder band appears at 822 cm−1 in the spectrum of PHBA in aqueous solution. In the UV Raman spectrum of PHBA in aqueous solution, a band at 1688 cm−1 attributed to (C O) stretching can be detected, which also can be detected in the NIR FT Raman spectrum of PHBA in ethanol but not observed in that of PHBA solid powder [21]. In crystal, pairs of PHBA molecule are linked through hydrogen bonds (2.635A in pure acid) between carboxyl groups to form cyclic dimmers [22]. However, in aqueous solution, the hydrogen bonds between carboxyl groups were disconnected. The weak band at 1388 cm−1 in Fig. 2b, due to the carboxyl symmetric stretching mode, also confirm the dissociation of dimmers [21]. In addition, the band at 1388 cm−1 could not be observed in ethanol at low intensity, which also shows the high sensitivity of UV Raman spectroscopy. The 1525 cm−1 band appears in the spectrum of PHBA in aqueous solution, while the 1444 cm−1 band emerges from that of solid powder, which are, respectively, due to 19a and 19b of C C stretching modes [23]. The downshift of the 1277 cm−1 band indicates the breakdown of hydrogen bonds between dimmers. The dimmers in PHBA are linked by hydrogen bonds (2.897A) between phenolic groups. The hydrogen-bonded molecules spiral around the twofold screw axes to constitute layers of dimmers parallel to (401) [22]. For the PHBA in aqueous solution, hydrogen bonds established between phenolic groups are destroyed and the downshift undergone by the band is due to C OH stretching. A new shoulder band arises at 822 cm−1 in the solution spectrum, which is possibly associated with ionization [24]. Through comparing the Raman spectra of PHBA in solid with in aqueous solution, we find that the state of PHBA in aqueous solution is completely different from that in solid. What is the state of PHBA in aqueous solution? The results of theoretical calculation give us approving response to that. 3. Theoretical calculations All calculations were carried out with Gaussian03 program [25]. All interaction models were optimized and the Raman frequencies for the models were calculated. We studied the interaction between PHBA ions and H2 O molecules in theoretical calculation, DFT [12,13]-B3PW91 [14]/6-31+g* is applied. B3PW91 functionals have proven to be superior to the traditional functionals defined so far [26]. As PHBA ion, H2 O molecules interact easily with it from three sites, are those: the O atom and the O ion on carboxyl group and the O atom on hydroxyl group. Because the effect of H atom on carboxyl group, the O ion more easily interacts with H2 O molecules. The following results prove the correctness. According to this thinking, we established possible models by means of approaching the actual situation gradually. Till set completely PHBA ions in aqueous environment, this can be accomplished by Onsager solution model [27]. The possible interaction models we established are shown in Fig. 2.
Fig. 3. (A) Experimental spectrum of PHBA aqueous solution; (B) calculated Raman frequencies of model “a” in Fig. 2; (C) calculated Raman frequencies of model “b” in Fig. 2; (D) calculated Raman frequencies of model “c” in Fig. 2; (E) calculated Raman frequencies of model “d” in Fig. 2; (F) calculated Raman frequencies of model “e” in Fig. 2; (G) calculated Raman frequencies of PHBA ion in solvation model.
Although practically PHBA ions interact with many H2 O molecules at the same time, it is impossible to calculate the vibrational spectra for such a large number of atoms; therefore we used just one H2 O molecule instead of the whole H2 O “cluster”. In aqueous solution, H2 O molecules to be linked with the PHBA ion may not be fixed so strongly with the other H2 O molecules on account of large distance. This is also favorable to the model adopted in this paper. We minimize the energy of all above models at first, and then calculate Raman frequencies on them. The calculated frequencies are shown in Fig. 3. From the Fig. 3, we can find that near 800 cm−1 , B, C, D, E, F, G graphs all are in agreement with the experimental values. Among 1100–1300 cm−1 , B, E graphs are in agreement with the experimental values. Near 1600 cm−1 , C, D, E graphs all are in agreement with the experimental values. To further analyze the results: B graph is in good agreement with the experimental values, except that three frequencies near 1600 cm−1 . B graph is the calculated spectrum of PHBA ion in gas, which is not considered interaction with H2 O molecules. This tells us that the actual structure of PHBA in aqueous solution is similar to that in gas and demonstrates indirectly the interaction between PHBA ion and H2 O molecules is weak. Near 1600 cm−1 is not in agreement with experimental values, this also shows that PHBA exists not as pure ion, but do have interaction with H2 O molecules. C, D, E graphs all are in good agreement with experimental values, which all are considered interaction with H2 O molecules. So, we believe that PHBA ions have interaction with H2 O molecules. How do H2 O molecules interact with PHBA ions? E graph answers it. Among all calculated spectra, E graph is in the best agreement with experimental values in three segments. So we believe that H2 O molecules
X. Zhao, Y. Fang / Spectrochimica Acta Part A 65 (2006) 930–934 Table 1 The comparison of experimental frequencies and theoretical Raman values of E graph Experimental (cm−1 )
Calculated frequencies (cm−1 )
V (cm−1 )
641 770 847 1131 1172 1277 1317 1388 1458 1525 1610 1688
641 750 859 1147 1179 1255 1310 1408 1465 1540 1610 1671
0 20 12 16 7 22 7 20 7 15 0 16
interact with the O ion on carboxyl group and the O atom on hydroxyl group, the O atom on the carboxyl group do not interact with H2 O molecules. F graph is the results of considering H2 O molecules interacting with three sites: the O atom and the O ion on carboxyl group and the O atom on hydroxyl group. The difference of models corresponding to E and F graphs is only the interaction of H2 O molecules and the O atom on carboxyl group, the corresponding models are shown as “d” and “e” in Fig. 2. The discrepancy of F graph and experimental spectrum implies that the O atom on carboxyl group do not interact with H2 O molecules. This because that the effect of the lost H atom. This demonstrates indirectly that the corresponding interaction model of E graph reflects the actual interaction situation. The comparison of experimental and theoretical results is shown in Table 1. From the results shown in Table 1, we see results above, except for 770, 1277 and 1388 cm−1 , other values all have a difference less than 20 cm−1 , the theoretical values are in good agreement with experimental values, and the result is our very expectation. So, we conjecture that the model “d” is the experimental configuration. G graph is the calculated spectrum of considering setting completely PHBA ion in an aqueous environment, Onsager solution model [17] is applied. The result is unexpected, only among 800–1000 cm−1 is in good agreement with experimental values, other values all are discrepant with the experimental values. This tells us that the interaction between PHBA ions and H2 O molecules is considered to strong is not reasonable. In all, we conclude that the interaction between PHBA ions and H2 O molecules is weak, and the reasonable interaction model that reflects the actual interaction configuration is that H2 O molecules interact with the O ion on the carboxyl group and the O atom on hydroxyl group, the O atom on the carboxyl group do not interact with H2 O molecules. The model is shown as “d” in Fig. 2. 4. Conclusion We studied the dependence of the UV Raman spectra on the concentration of PHBA aqueous solution under UV laser
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excitation. Through analyzing the spectra, we know that the transformation of its structure is not appreciable, but tiny. That further proved the interaction between PHBA ions and H2 O molecules is weak. To further explore the problem, we study the interaction between PHBA ions and H2 O molecules by virtue of theoretical calculation. We find that the interaction between PHBA ions and H2 O molecules is considered to strong is not reasonable, which demonstrates an coincident conclusion with the experiments. Integrating the results of the theoretical calculations with the experiments, we conclude that the interaction between PHBA ions and H2 O molecules is weak, and the reasonable interaction model that reflects the actual interaction configuration is dug out. The model is shown as “d” in Fig. 2. Besides, although we used just one H2 O molecule as the representative of H2 O “cluster”, the calculated vibrational spectra are in good agreement with experimental values. So we can conclude the model is reasonably good in describing the interaction configurations. Acknowledgments The authors thank Professor F.H. Wang and Professor X.X. Gu for their beneficial discussion and are grateful for the support of this research by the National Natural Science Foundations of China and Beijing. References [1] L. Can, C.S. Peter, Catal. Today 33 (1997) 353. [2] E. Duval, G. Mariotto, M. Montagna, O. Pilla, G. Viliani, M. Barland, Europhys. Lett. 3 (1987) 333. [3] K. Katrin, K. Harald, I. Irving, R.D. Ramachandra, S.F. Michael, Chem. Phys. 247 (1999) 155. [4] A.W. Joseph, Alaa J. 37 (1999) 1015. [5] Y. Yi, Y. Jihong, X. Guang, L. Can, X. Fengshou, Phys. Chem. Chem. Phys. 3 (2001) 2692. [6] T.K. Van Dyk, L.J. Templeton, K.A. Cantera, P.L. Sharpe, F.S. Sariaslani, J. Bacteriol. 186 (2004) 7196. [7] C. Lemini, G. Silva, C. Timossi, D. Luque, A. Valverde, M. Gonz´alezMart´ınez, A. Hern´andez, C. Rubio-P´oo, B. Ch´avez Lara, F. Valenzuela, Environ. Res. 75 (1997) 130. [8] W.-Y. Chiang, C.-S. Yan, J. Appl. Polymer Sci. 46 (2003) 1279. [9] J.-C. Ho, Y.-S. Lin, K.-H. Wei, Polymer 40 (1999) 3843. [10] D. Wu, Y. Fang, J. Colloid Interface Sci. 265 (2003) 234. [11] X. Zhao, Y. Fang, J. Mol. Struct. 752 (2005) 198. [12] P. Hohenberg, W. Kohn, Phys. Rev. 136 (1964) B864. [13] W. Kohn, L.J. Sham, Phys. Rev. 140 (1965) A1133. [14] P.J. Stephens, F.J. Delvin, C.F. Chablowski, M.J. Frisch, J. Phys. Chem. 98 (1994) 11623. [15] A. Markovits, M. Garcia-Hernandez, J.M. Ricart, F. Illas, J. Phys. Chem. B. 103 (1999) 509. [16] F. Tielens, M. Saeys, E. Tourwe, G.B. Marin, A. Hubin, P. Geerlings, J. Phys. Chem. A 106 (2002) 1450. [17] M.T.M. Koper, R.A. van Santen, S.A. Wasileski, M.J. Weaver, J. Chem. Phys. 113 (2000) 4392. [18] P. Koczon, J. Cz. Dobrowolski, W. Lewandowski, A.P. Mazurek, J. Mol. Struct. 655 (2003) 89. [19] J.V. Rau, S. Nunziante Cesaro, O.V. Boltalina, V. Agafonov, A.A. Popov, L.N. Sidorov, Vibrational Spectra. 34 (2004) 137. [20] Jose M. Orza, M. Victoria Garcia, Ibon Alkorta, Jose Elguero, Spectrochim. Acta Part A 56 (2000) 1469.
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[21] S. Sanchez-Cortes, J.V. Garcia-Ramos, J. Colloid Interface Sci. 231 (2000) 98. [22] E.A. Heath, P. Singh, Y. Ebisuzaki, Acta Crys. Sect. C 48 (1992) 1960. [23] G. Varsanyi, Assignment for Vibrational Spectra of Seven Hundred Benzene Derivations, Wiley, New York, 1974. [24] B. Humbert, M. Alnot, F. Quiles, Spectrochim. Acta Part A 54 (1998) 465. [25] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C.
Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian 03, Revision B.05, Gaussian Inc., Pittsburgh, PA, 2003. [26] James B. Foresman, Eleen Frisch, Exploring Chemistry with Electronic Structure Methods, Gaussian Inc., Pittsburgh, PA, 2003, pp. 119–138. [27] K.B. Wiberg, M.A. Murcko, J. Phys. Chem. 91 (1987) 3616.
An experimental and theoretical study on the interaction of PHBA ions and H2O molecules Xinfeng Zhao, Yan Fang ∗ Beijing Key Lab of Nanophotonics and Nanostructure, Capital Normal University, Beijing 100037, PR China Received 18 November 2005; received in revised form 16 January 2006; accepted 25 January 2006
Abstract We studied the dependence of the Raman spectra on the concentration of PHBA aqueous solution under UV laser excitation. Through analyzing the spectra, we conclude that the interaction between PHBA ions and H2 O molecules is weak. To further explore the problem, we studied the interaction between PHBA ions and H2 O molecules by virtue of theoretical calculations, DFT-B3PW91/6-31+g* was employed. We draw a coincident conclusion with the experiments and dug out the reasonable interaction model that reflects the actual interaction configuration between PHBA ions and H2 O molecules. We supply a new thinking for studying interactions between solute molecules and solvent molecules, which also can be applied to interactions between solute molecules and other solute molecules in solutions. © 2006 Elsevier B.V. All rights reserved. Keywords: p-Hydroxybenzoic acid (PHBA); Density functional theory (DFT); Calculated Raman frequencies; Interaction model
1. Introduction Employing photons as probes, Raman spectroscopy that has no damage to samples has been widely used for studying chemical structure and physical interface phenomena [1]. Conventional Raman spectroscopy usually was obtained and worked well at NIR or visible excitations [2,3]. However, an obvious weakness of Raman spectroscopy is its low sensitivity, which results in much more difficulties in obtaining Raman spectrum of low solubility materials in solution. UV Raman spectroscopy has been well developed in these years. Due to its high energy, UV line could lead to resonant Raman transition or near resonant Raman transition among the electron energy levels [4], increasing greatly sensitivity and acquiring more information about molecule vibration and structure. Moreover, UV Raman could avoid fluorescence [5], and this reduces influences on Raman spectrum. p-Hydroxybenzoic acid (PHBA) is an organic substance, which has many meaningful applications. First, in biologic reactions, PHBA plays an important role and is a naturally occurring molecule with significant importance [6,7]. Second, PHBA is also used in synthesizing LC polymers [8,9]. Besides, the struc-
∗
Corresponding author. Tel.: +86 1068902965. E-mail address: [email protected] (X. Zhao).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2006.01.034
ture of PHBA molecule is simple, which is usually used as probe molecule for studying SERS and adsorption behaviors in metal aqueous colloid [10]. A high quality Raman spectrum of PHBA in aqueous solution under ultraviolet (UV) excitation had been obtained [11], which could not be observed with visible and near infrared (NIR) excitations due to its low concentration. Ref. [11] has proved that PHBA exists as ion state in aqueous solution by means of theoretical calculation combining with experimental instruments. We studied the dependence of the UV Raman spectra on the concentration of PHBA aqueous solution under UV laser excitation. Through analyzing the spectra, we conclude that the interaction between PHBA ions and H2 O molecules is weak. To further explore the problem, we study the interaction between PHBA ions and H2 O molecules by virtue of theoretical calculation, DFT [12,13]-B3PW91 [14]/6-31+g* is applied. Theoretically, the difficulty lies in the establishment of a simple model, which should describe the interaction of molecules properly, and at the same time can be used to calculate the vibrational spectra easily. The theoretical results were compared with the experimental values, and the reliability of the models and the calculation methods were analyzed. It was found that the models lead to different vibrational spectra, which correspond to the experimental observation. The possible models were established and Raman calculations were applied to these models. Through comparing the results of theoretical calculations with experi-
X. Zhao, Y. Fang / Spectrochimica Acta Part A 65 (2006) 930–934
ments, we draw a coincident conclusion with the experiments and dug out the reasonable interaction model that reflects the actual interaction configuration between PHBA ions and H2 O molecules. There are many papers about theoretical studies combining with vibrational spectra: studies of ion adsorption on metal surface could be found in Refs. [15–17] and molecule structure found in Refs. [18–20]. In our work, we study the interaction between PHBA ions and H2 O molecules by means of the experimental instruments combining with the theoretical calculations. From the results of the experiments, we conclude that the interaction between PHBA ions and H2 O molecules is weak. On the other hand, the weak interaction is coming from O atoms of PHBA ion and H2 O molecules, and the reasonable interaction model between PHBA ions and H2 O molecules is dug out by virtue of the theoretical calculations. The traditional methods studying the interaction are difficult on account of PHBA’s very low solubility. We supply a new thinking for studying interactions between solute molecules and solvent molecules, which also can be applied to interactions between solute molecules and other solute molecules in solutions.
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Fig. 1. Raman spectra for PHBA aqueous solution at different concentrations: 1 × 10−2 (a), 5 × 10−3 (b), 2.5 × 10−3 (c), 1 × 10−3 (d), 5 × 10−4 M (e) and for deionized water (f).
2. Experimental The UV Raman spectrum was obtained through a RENISHAW H13325 spectrophotomer, and the UV laser line was at 325 nm. The output laser power, which could not induce changes to the sample, was about 12 mW. A high quality Raman spectrum of PHBA in aqueous solution under ultraviolet (UV) excitation had been obtained, which could not be observed with visible and near infrared (NIR) excitations due to its low concentration [11]. Ref. [11] has proved that PHBA exists as ion state in aqueous solution by means of the theoretical calculations combining with experiments. PHBA exists as ion state in aqueous solution, then, how do PHBA ions exist? The interaction between PHBA ions and H2 O molecules is strong or weak? How do H2 O molecules interact with PHBA ions? We will answer these questions. We studied the dependence of UV Raman spectra on the concentration of PHBA aqueous solution under UV laser excitation, the results are shown in the Fig. 1. From the Fig. 1, we can find that the Raman frequencies are mainly divided into three segments: near 850, 1100–1300, and 1600 cm−1 . It is clear that the lower the concentration of PHBA aqueous solution, the weaker intensity of Raman frequencies in the UV Raman spectra. When the solution is diluted to 10−3 M, only several strong bands at 1610, 1277, 1172, and 847 cm−1 could be observed. When the solution is diluted to 5 × 10−4 M, the spectrum is similar to that of deionized water, just shown in Fig. 1. This is because the quantity of PHBA ions became less and less in unit volume. However, the Raman frequencies do not shift evidently, which demonstrates that the transformation of its structure is not appreciable, but tiny. That further proved the interaction between PHBA ions and H2 O molecules is weak. In comparison with the UV Raman spectrum of PHBA solid powder [11], we find that an additional band appears at 1688 cm−1 , which is due to the C O stretching mode in Fig. 2b.
Fig. 2. Interaction models of PHBA ion and H2 O molecule. (a) The structure of PHBA ion. (b) Interaction model of PHBA ion and one H2 O molecule. The H2 O molecule interact with the O ion on carboxyl group of PHBA ion. (c) Interaction model of PHBA ion and two H2 O molecules. The H2 O molecules interact with the O atom and the O ion on carboxyl group of PHBA ion. (d) Interaction model of PHBA ion and two H2 O molecules. The H2 O molecules interact with the O ion on carboxyl group and the O atom on hydroxyl group of PHBA ion. (e) Interaction model of PHBA ion and three H2 O molecules. The H2 O molecules interact with the O atom and the O ion on carboxyl group and the O atom on hydroxyl group of PHBA ion.
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X. Zhao, Y. Fang / Spectrochimica Acta Part A 65 (2006) 930–934
And Fig. 2c does not yield the weak band at the 1388 cm−1 that appears in the solution. The 1288 cm−1 band in Fig. 2c downshift to 1277 cm−1 in Fig. 2b. Another new shoulder band appears at 822 cm−1 in the spectrum of PHBA in aqueous solution. In the UV Raman spectrum of PHBA in aqueous solution, a band at 1688 cm−1 attributed to (C O) stretching can be detected, which also can be detected in the NIR FT Raman spectrum of PHBA in ethanol but not observed in that of PHBA solid powder [21]. In crystal, pairs of PHBA molecule are linked through hydrogen bonds (2.635A in pure acid) between carboxyl groups to form cyclic dimmers [22]. However, in aqueous solution, the hydrogen bonds between carboxyl groups were disconnected. The weak band at 1388 cm−1 in Fig. 2b, due to the carboxyl symmetric stretching mode, also confirm the dissociation of dimmers [21]. In addition, the band at 1388 cm−1 could not be observed in ethanol at low intensity, which also shows the high sensitivity of UV Raman spectroscopy. The 1525 cm−1 band appears in the spectrum of PHBA in aqueous solution, while the 1444 cm−1 band emerges from that of solid powder, which are, respectively, due to 19a and 19b of C C stretching modes [23]. The downshift of the 1277 cm−1 band indicates the breakdown of hydrogen bonds between dimmers. The dimmers in PHBA are linked by hydrogen bonds (2.897A) between phenolic groups. The hydrogen-bonded molecules spiral around the twofold screw axes to constitute layers of dimmers parallel to (401) [22]. For the PHBA in aqueous solution, hydrogen bonds established between phenolic groups are destroyed and the downshift undergone by the band is due to C OH stretching. A new shoulder band arises at 822 cm−1 in the solution spectrum, which is possibly associated with ionization [24]. Through comparing the Raman spectra of PHBA in solid with in aqueous solution, we find that the state of PHBA in aqueous solution is completely different from that in solid. What is the state of PHBA in aqueous solution? The results of theoretical calculation give us approving response to that. 3. Theoretical calculations All calculations were carried out with Gaussian03 program [25]. All interaction models were optimized and the Raman frequencies for the models were calculated. We studied the interaction between PHBA ions and H2 O molecules in theoretical calculation, DFT [12,13]-B3PW91 [14]/6-31+g* is applied. B3PW91 functionals have proven to be superior to the traditional functionals defined so far [26]. As PHBA ion, H2 O molecules interact easily with it from three sites, are those: the O atom and the O ion on carboxyl group and the O atom on hydroxyl group. Because the effect of H atom on carboxyl group, the O ion more easily interacts with H2 O molecules. The following results prove the correctness. According to this thinking, we established possible models by means of approaching the actual situation gradually. Till set completely PHBA ions in aqueous environment, this can be accomplished by Onsager solution model [27]. The possible interaction models we established are shown in Fig. 2.
Fig. 3. (A) Experimental spectrum of PHBA aqueous solution; (B) calculated Raman frequencies of model “a” in Fig. 2; (C) calculated Raman frequencies of model “b” in Fig. 2; (D) calculated Raman frequencies of model “c” in Fig. 2; (E) calculated Raman frequencies of model “d” in Fig. 2; (F) calculated Raman frequencies of model “e” in Fig. 2; (G) calculated Raman frequencies of PHBA ion in solvation model.
Although practically PHBA ions interact with many H2 O molecules at the same time, it is impossible to calculate the vibrational spectra for such a large number of atoms; therefore we used just one H2 O molecule instead of the whole H2 O “cluster”. In aqueous solution, H2 O molecules to be linked with the PHBA ion may not be fixed so strongly with the other H2 O molecules on account of large distance. This is also favorable to the model adopted in this paper. We minimize the energy of all above models at first, and then calculate Raman frequencies on them. The calculated frequencies are shown in Fig. 3. From the Fig. 3, we can find that near 800 cm−1 , B, C, D, E, F, G graphs all are in agreement with the experimental values. Among 1100–1300 cm−1 , B, E graphs are in agreement with the experimental values. Near 1600 cm−1 , C, D, E graphs all are in agreement with the experimental values. To further analyze the results: B graph is in good agreement with the experimental values, except that three frequencies near 1600 cm−1 . B graph is the calculated spectrum of PHBA ion in gas, which is not considered interaction with H2 O molecules. This tells us that the actual structure of PHBA in aqueous solution is similar to that in gas and demonstrates indirectly the interaction between PHBA ion and H2 O molecules is weak. Near 1600 cm−1 is not in agreement with experimental values, this also shows that PHBA exists not as pure ion, but do have interaction with H2 O molecules. C, D, E graphs all are in good agreement with experimental values, which all are considered interaction with H2 O molecules. So, we believe that PHBA ions have interaction with H2 O molecules. How do H2 O molecules interact with PHBA ions? E graph answers it. Among all calculated spectra, E graph is in the best agreement with experimental values in three segments. So we believe that H2 O molecules
X. Zhao, Y. Fang / Spectrochimica Acta Part A 65 (2006) 930–934 Table 1 The comparison of experimental frequencies and theoretical Raman values of E graph Experimental (cm−1 )
Calculated frequencies (cm−1 )
V (cm−1 )
641 770 847 1131 1172 1277 1317 1388 1458 1525 1610 1688
641 750 859 1147 1179 1255 1310 1408 1465 1540 1610 1671
0 20 12 16 7 22 7 20 7 15 0 16
interact with the O ion on carboxyl group and the O atom on hydroxyl group, the O atom on the carboxyl group do not interact with H2 O molecules. F graph is the results of considering H2 O molecules interacting with three sites: the O atom and the O ion on carboxyl group and the O atom on hydroxyl group. The difference of models corresponding to E and F graphs is only the interaction of H2 O molecules and the O atom on carboxyl group, the corresponding models are shown as “d” and “e” in Fig. 2. The discrepancy of F graph and experimental spectrum implies that the O atom on carboxyl group do not interact with H2 O molecules. This because that the effect of the lost H atom. This demonstrates indirectly that the corresponding interaction model of E graph reflects the actual interaction situation. The comparison of experimental and theoretical results is shown in Table 1. From the results shown in Table 1, we see results above, except for 770, 1277 and 1388 cm−1 , other values all have a difference less than 20 cm−1 , the theoretical values are in good agreement with experimental values, and the result is our very expectation. So, we conjecture that the model “d” is the experimental configuration. G graph is the calculated spectrum of considering setting completely PHBA ion in an aqueous environment, Onsager solution model [17] is applied. The result is unexpected, only among 800–1000 cm−1 is in good agreement with experimental values, other values all are discrepant with the experimental values. This tells us that the interaction between PHBA ions and H2 O molecules is considered to strong is not reasonable. In all, we conclude that the interaction between PHBA ions and H2 O molecules is weak, and the reasonable interaction model that reflects the actual interaction configuration is that H2 O molecules interact with the O ion on the carboxyl group and the O atom on hydroxyl group, the O atom on the carboxyl group do not interact with H2 O molecules. The model is shown as “d” in Fig. 2. 4. Conclusion We studied the dependence of the UV Raman spectra on the concentration of PHBA aqueous solution under UV laser
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excitation. Through analyzing the spectra, we know that the transformation of its structure is not appreciable, but tiny. That further proved the interaction between PHBA ions and H2 O molecules is weak. To further explore the problem, we study the interaction between PHBA ions and H2 O molecules by virtue of theoretical calculation. We find that the interaction between PHBA ions and H2 O molecules is considered to strong is not reasonable, which demonstrates an coincident conclusion with the experiments. Integrating the results of the theoretical calculations with the experiments, we conclude that the interaction between PHBA ions and H2 O molecules is weak, and the reasonable interaction model that reflects the actual interaction configuration is dug out. The model is shown as “d” in Fig. 2. Besides, although we used just one H2 O molecule as the representative of H2 O “cluster”, the calculated vibrational spectra are in good agreement with experimental values. So we can conclude the model is reasonably good in describing the interaction configurations. Acknowledgments The authors thank Professor F.H. Wang and Professor X.X. Gu for their beneficial discussion and are grateful for the support of this research by the National Natural Science Foundations of China and Beijing. References [1] L. Can, C.S. Peter, Catal. Today 33 (1997) 353. [2] E. Duval, G. Mariotto, M. Montagna, O. Pilla, G. Viliani, M. Barland, Europhys. Lett. 3 (1987) 333. [3] K. Katrin, K. Harald, I. Irving, R.D. Ramachandra, S.F. Michael, Chem. Phys. 247 (1999) 155. [4] A.W. Joseph, Alaa J. 37 (1999) 1015. [5] Y. Yi, Y. Jihong, X. Guang, L. Can, X. Fengshou, Phys. Chem. Chem. Phys. 3 (2001) 2692. [6] T.K. Van Dyk, L.J. Templeton, K.A. Cantera, P.L. Sharpe, F.S. Sariaslani, J. Bacteriol. 186 (2004) 7196. [7] C. Lemini, G. Silva, C. Timossi, D. Luque, A. Valverde, M. Gonz´alezMart´ınez, A. Hern´andez, C. Rubio-P´oo, B. Ch´avez Lara, F. Valenzuela, Environ. Res. 75 (1997) 130. [8] W.-Y. Chiang, C.-S. Yan, J. Appl. Polymer Sci. 46 (2003) 1279. [9] J.-C. Ho, Y.-S. Lin, K.-H. Wei, Polymer 40 (1999) 3843. [10] D. Wu, Y. Fang, J. Colloid Interface Sci. 265 (2003) 234. [11] X. Zhao, Y. Fang, J. Mol. Struct. 752 (2005) 198. [12] P. Hohenberg, W. Kohn, Phys. Rev. 136 (1964) B864. [13] W. Kohn, L.J. Sham, Phys. Rev. 140 (1965) A1133. [14] P.J. Stephens, F.J. Delvin, C.F. Chablowski, M.J. Frisch, J. Phys. Chem. 98 (1994) 11623. [15] A. Markovits, M. Garcia-Hernandez, J.M. Ricart, F. Illas, J. Phys. Chem. B. 103 (1999) 509. [16] F. Tielens, M. Saeys, E. Tourwe, G.B. Marin, A. Hubin, P. Geerlings, J. Phys. Chem. A 106 (2002) 1450. [17] M.T.M. Koper, R.A. van Santen, S.A. Wasileski, M.J. Weaver, J. Chem. Phys. 113 (2000) 4392. [18] P. Koczon, J. Cz. Dobrowolski, W. Lewandowski, A.P. Mazurek, J. Mol. Struct. 655 (2003) 89. [19] J.V. Rau, S. Nunziante Cesaro, O.V. Boltalina, V. Agafonov, A.A. Popov, L.N. Sidorov, Vibrational Spectra. 34 (2004) 137. [20] Jose M. Orza, M. Victoria Garcia, Ibon Alkorta, Jose Elguero, Spectrochim. Acta Part A 56 (2000) 1469.
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