Glycine Canonical and Zwitterionic Isomers within Zeolites - The

Jul 2, 2009 - The glycine zwitterion (GlyA) in the gas phase is not a local energy minimum and transforms to the canonical isomer (GlyB) via a barrier...
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J. Phys. Chem. B 2009, 113, 10399–10402

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Glycine Canonical and Zwitterionic Isomers within Zeolites Gang Yang,*,†,‡ Lijun Zhou,‡ and Chengbu Liu*,† Institute of Theoretical Chemistry, Shandong UniVersity, Jinan 250100, P. R. China, and Key Laboratory of Forest Plant Ecology, Ministry of Education, Northeast Forestry UniVersity, Harbin, 150040 P. R. China ReceiVed: March 16, 2009

The glycine zwitterion (GlyA) in the gas phase is not a local energy minimum and transforms to the canonical isomer (GlyB) via a barrierless process. Within ZSM-5 zeolite, it is rendered geometrically stable and even has a lower energy of 7.57 kcal mol-1 than GlyC, the most stable isomer of glycine in the gas phase; GlyB represents the lowest energy minimum, which is facile to transform into the zwitterion with low-energy barrier (4.46 kcal mol-1). In addition, the zwitterion can be efficiently obtained by adsorption of glycine in the deprotonated form at the acidic site of HZSM-5 zeolite. The relative stability of glycine isomers in silicalite-1 increases in the order GlyA < GlyB < GlyC, the same as that in the gas phase. Silicalite-1 stabilizes GlyA somewhat, whereas it destabilizes GlyB greatly. The negative charges of ZSM-5 zeolite created by Al doping are indispensable to the stabilizations of the zwitterion; however, the lattices also play an important role and approximate 74.1% of the contributions of the negative charges. 1. Introduction Zwitterions create strong electric fields, being the driving forces that determine the structures, functions, and activities of amino acids, peptides, and proteins.1,2 The theoretical calculations at various levels3-8 indicated that the glycine zwitterion in the gas phase is not a local minimum on the potential energy surface (PES), consistent with the experimental observations.9 Through the interactions with oxalic and malonic dianions, the glycine zwitterion is rendered geometrically stable; however, the oxalic and malonic dianions can not exist independently but instead spontaneously lose one electron.10,11 Very recently, many self-stable dianions and two monoanions have been explored by our group,12 which can be directly and conveniently applied in the stabilizations of zwitterions. Zeolites are an important type of solid-state material widely used in adsorption, separation, and catalysis. With the doping of Al ions, negative charges are created in the lattices of zeolites and may potentially stabilize the zwitterions. Presently, the adsorption and formation of various glycine isomers, including the zwitterion (see Scheme 1) were explored in ZSM-5 zeolite, with the aid of the two-layer ONIOM scheme. In addition, the respective roles of negative charges and lattices played during the stabilizations of zwitterions were assessed by performing parallel calculations on silicalite-1. 2. Computational Details 2.1. Cluster Models of ZSM-5 Zeolite and Silicalite-1. In this work, cluster models with 33-T sites were used to study the interactions of glycine structures with ZSM-5 zeolite or silicalte-1. Two intersecting 10-membered ring windows of the straight and zigzag channels are included. The present clusters are larger than our previous ones with 17-T and 25-T sites.13,14 As the calculated results indicate, all the glycine structures presently studied fall completely within the channels and have no observable interactions with the boundary atoms. * Corresponding authors. E-mail: [email protected] (G.Y.), cbliu@ sdu.edu.cn (C.L.). † Shandong University. ‡ Northeast Forestry University.

SCHEME 1: Several Isomers of Glycine to be Studied in this Worka

a GlyA is the zwitterion, GlyB is the canonical isomer that can transform into GlyA through the intramolecular proton transfer, and GlyC is the most stable isomer in gas phase.

The Al(OSiH3)4 fragments in ZSM-5 zeolite and Si(OSiH3)4 fragments in silicate-1 were designated as the active sites, whereas the other atoms were designated as the environment. The boundary Si and O atoms were saturated with H atoms, which are oriented in the directions of what would normally be the next framework atoms. The corresponding Si-H and O-H distances were altered to 1.500 and 1.000 Å, respectively. 2.2. Theoretical Methods. All the calculations were performed within the Gaussian 03 suite of programs.15 The twolayer ONIOM scheme16 was used to study the zeolite and glycine interacting systems, using B3LYP density functional17,18 for both high and low layers. The active sites (high layer) defined in the above section were described with the 6-31+G** basis set as well as the adsorbed glycine structures. The environmental atoms (low layer) were treated with the economic 3-21G basis set. As to the gas-phase glycine isomers (Scheme 1), they were also optimized at the MP2/6-311++G** level of theory. The geometries and relative energies were found to be comparable with those obtained by the default B3LYP/6-31+G** methods. It is consistent with the literature that the B3LYP density functional is a powerful tool to study amino acid related

10.1021/jp903835j CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009

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TABLE 1: Relative Energies (RE) of Several Glycine Isomers under Different Conditions as Well as Their Interaction Energies (IE) in the Complexes Formed with ZSM-5 Zeolite or Silicalite-1a silicalite-1c

ZSM-5 RE

IEb

RE

IEb

gas phase:c,d RE

GlyA -7.57 -40.27 (-44.94) 15.78 -20.83 (-23.70) 18.03 (19.39) GlyB -10.28 -25.38 (-30.04) 4.51 -14.49 (-17.60) 0.42 (0.53) GlyC 0.00 -14.67 (-20.30) 0.00 -18.58 (-19.26) 0.00 (0.00) a All energy units are in kcal mol-1. b The data in parentheses were obtained at the ONIOM(MP2/6-311++G**:B3LYP/3-21G)// ONIOM(B3LYP/ 6-31+G**:B3LYP/3-21G) level of theory. c The N-H1 distances in the GlyA-related geometries were fixed at 1.061 Å as in GlyA-ZSM. d The data in parentheses were obtained at the MP2/6-311++G** level of theory.

Figure 2. The transition state structure (TS-ZSM) of the intramolecular proton transfer between GlyB and GlyA in ZSM-5 zeolite.

Figure 3. Energy profile of the proton transfer between GlyB and GlyA within ZSM-5 zeolite.

Figure 1. Optimized structures of GlyA (top), GlyB (middle), and GlyC (bottom) within ZSM-5 zeolite.

systems.19-29 As suggested by the reviewers, the interaction energies between glycine isomers and zeolites (IE) were also reported at the ONIOM(MP2/6-311++G**:B3LYP/3-21G)// ONIOM(B3LYP/6-31+G**:B3LYP/3-21G) level of theory. The data in Table 1 showed that the sequences of ZSM-5 zeolite and silicalite-1 are the same as those of the default computational methods, i.e., the ONIOM(B3LYP/6-31+G**:B3LYP/3-21G) scheme. 3. Results and Discussion 3.1. Glycine Isomers in ZSM-5 Zeolite. In contrast to the barrierless collapse to the canonical isomer in gas phase,3-8 the glycine zwitterion (GlyA; see Figure 1) is geometrically stable when embedded into ZSM-5 zeolite. The interaction energy (IE) between GlyA and ZSM-5 zeolite was calculated to be as large

as -40.27 kcal mol-1, which is probably caused by strong hydrogen-bonding and electrostatic interactions.30-33 As the IE data in Table 1 indicate, the interactions of GlyB and GlyC (two most stable isomers of gas-phase glycine; see Figure 1) with ZSM-5 zeolite are obviously weaker. As a result, GlyA in ZSM-5 zeolite is 7.57 kcal mol-1 more stable than GlyC. Note that, in the gas phase, GlyC predominates whereas GlyA does not exist at all. The relative energies (RE) of GlyA vs GlyC were approximated to be 18.03 and 19.39 kcal mol-1 at the B3LYP/ 6-31+G** and MP2/6-311++G** levels of theory, respectively (Table 1). It agrees with the experimental estimation of ca. 20 kcal mol-1.3-8 Surprisingly, GlyB in ZSM-5 zeolite is 10.28 kcal mol-1 more stable than GlyC and represents the lowest energy minimum. The RE value of GlyA vs GlyB in ZSM-5 zeolite equals 2.71 kcal mol-1 and is rather small. In addition, the intramolecular proton transfer from GlyB to GlyA, which is impossible to take place in gas phase, becomes quite facile within ZSM-5 zeolite. The transition state structure (TS-ZSM) of this proton transfer is shown in Figure 2. In TS-ZSM, the N-H1 and O1-H1 distances were optimized at 1.242 and 1.284 Å, respectively. As the energy profile in Figure 3 indicates, the energy barrier of this proton transfer was calculated to be 4.46 kcal mol-1. ZSM-5 zeolite stabilizes the glycine zwitterion through hydrogen bonding and electrostatic interactions, exactly identical to the situations in crystalline state,34 thus providing an ideal prototype for studying the properties and activities of solid-state zwitterions. 3.2. Formation Routes of Glycine Isomers in ZSM-5 Zeolite. The direct adsorptions of glycine isomers on the acidic sites of HZSM-5 zeolite will be protonated, consistent with the results of Limtrakul et al.35 It is because HZM-5 zeolite (HZSM) has a lower proton affinity (PA) than the glycine isomers and will donate the acidic protons to them via a barrierless way.36

Glycine Isomers within Zeolites

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EZSM ) ENZSM + ELZSM ≈ ENZSM + ELSIL ) ENZSM + ESIL (2)

Figure 4. Formation routes of glycine isomers within ZSM-5 zeolite.

However, the adsorption of glycine in the deprotonated form (NH2CH2COO-, Dep) causes the formation of the zwitterion, see route 1 in Figure 4. GlyC may be produced at the same time; see route 2. GlyA and GlyC are switched by the orientations of the Dep anion, whose N-terminus toward the acidic site of HZSM-5 zeolite forms GlyA, whereas the acidic site forms GlyC. The reaction energies of routes 1 and 2 in Figure 4 were calculated to be -56.03 and -48.46 kcal mol-1, respectively. The geometry optimization procedures indicate that either GlyA or GlyC is produced without any barrier; i.e., the reactions of routes 1 and 2 are irreversible. Accordingly, the product selectivity of GlyA vs GlyC (F) was obtained using the theories of parallel reaction kinetics37

F ) k1 /k2 ≈ exp(-∆E/RT)

(1)

where k1, k2, ∆E, R, and T are the rate constants of routes 1 and 2, the reaction-energy difference between routes 1 vs 2, the gas constant, and temperature (298.15 K), respectively. According to eq 1, F is very large (ca. 350 000), an indication that the glycine zwitterion is almost the sole product. As to the other two isomers of GlyB and GlyC, they can be produced in ZSM-5 zeolite via routes 3 and 4 in Figure 4, respectively. 3.3. Glycine Isomers in Silicalite-1. The geometry optimization procedure showed that the glycine zwitterion within silicate-1 is not a local minimum on the PES and will spontaneously transform into the canonical isomer. It is identical to the situations of the gas phase.3-9 It indicates that the negative charges in ZSM-5 zeolite created by Al doping are indispensable to the stabilizations of the glycine zwitterion. The complexes of GlyA, GlyB, and GlyC interacting with silicatite-1 are displayed in Figures S1-S3 of the Supporting Information, where the N-H1 distance of GlyA was fixed at 1.061 Å as in GlyA-ZSM. Nonetheless, the relative stability of the glycine zwitterion is somewhat improved due to the presence of silicalite-1, since the RE value of GlyA vs GlyC is estimated to be 15.78 kcal mol-1 in silicalite-1 instead of 18.03 kcal mol-1 in the gas phase. On the contrary, the GlyB isomer is remarkably destabilized due to the accommodation into silicalite-1, and the RE value of GlyB vs GlyC is elevated from 0.42 kcal mol-1 in the gas phase to 4.09 kcal mol-1 in silicilate-1. Accordingly, the predominance of the GlyC isomer is further reinforced in silicate-1. 3.4. Roles of Negative Charges and Lattices in the Zwitterion Stabilizations. From Figures S4 and S5of the Supporting Information, it was found that the GlyA and GlyB geometries in ZSM-5 zeolite are finely superposed with the corresponding ones in silicalite-1. Accordingly, the respective roles of negative charges (ENZSM) and lattices (ELZSM) in ZSM-5 zeolite played during the stabilizations of glycine zwitterions are assessed using the GlyB isomers as the benchmarks.38 The contribution of lattices in ZSM-5 zeolite to the stabilizations can be approximated to that in silicalite-1 (ELSIL), where no negative charges are observed.

Compared with the gas phase, ZSM-5 zeolite and silicalite-1 improve the relative stabilities of GlyA vs GlyB by -14.90 kcal mol-1 (EZSM) and -6.34 kcal mol-1 (ESIL), respectively (Table 1). According to eq 2, the ENZSM and ELZSM values were calculated to be -8.56 and -6.34 kcal mol-1, respectively. Therefore, the lattices of ZSM-5 zeolite also play an important role in the stabilizations of the zwitterions, albeit the negative charges are indispensable and the role may be somewhat larger. 4. Conclusions In this work, the two-layer ONIOM calculations (B3LYP/631+G**:B3LYP/3-21G) were used to study the adsorption and formation of various glycine isomers within zeolites. The main findings are outlined below. The glycine zwitterion (GlyA) is rendered geometrically stable in ZSM-5 zeolite. In ZSM-5 zeolite, GlyA even has a lower energy of 7.57 kcal mol-1 than GlyC, the most stable isomer of glycine in the gas phase. In ZSM-5 zeolite, the second most stable isomer in the gas phase (GlyB) represents the lowest energy minimum, which is lower in energy than GlyC by 10.28 kcal mol-1. In ZSM-5 zeolite, the intramolecular proton transfer from GlyB to GlyA becomes facile, and the energy barrier was calculated to be 4.46 kcal mol-1. Instead, the transformation processes from GlyA to GlyB in the gas phase or silicalite-1 take place via a barrierless way. That is, the glycine zwitterion does not exist in the gas phase or silicalite-1, indicating that the negative charges of ZSM-5 zeolite created by Al doping are indispensable to the stabilizations of the zwitterions. The relative stability of glycine isomers in silicalite-1 increases in the order GlyA < GlyB < GlyC, the same as that in the gas phase. Silicalite-1 stabilizes GlyA somewhat, whereas it destabilizes GlyB greatly. Through the parallel calculations on silicalite-1, it was concluded that the lattices of ZSM-5 zeolite also play an important role during the stabilizations of zwitterions. The lattices were assessed to approximate 74.1% of the negative charges. The formation routes of various glycine isomers in ZSM-5 zeolite were studied as well. It was found that the zwitterion can be efficiently obtained by adsorption of glycine in the deprotonated form at the acidic site of HZSM-5 zeolite. It thus provides an ideal prototype for studying the properties and activities of solid-state zwitterions. Acknowledgment. The authors thank the Major State Basic Research Development Programs (No. 2004CB719902), National Natural Science Foundation (No. 20633060), and the Talented Funds of Northeast Forestry University (No. 220602042) for generous financial support. Supporting Information Available: Structures of glycine isomers in silicalite-1 and superpositions of glycine isomers in ZSM-5 zeolite and silicalite-1. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wu, R. H.; McMahon, T. B. Angew. Chem., Int. Ed. 2007, 46, 3668. (2) Yang, G.; Zu, Y. G.; Fu, Y. J.; Zhou, L. J.; Zhu, R. X.; Liu, C. B. J. Phys. Chem. B 2009, 113, 4899. (3) Jensen, J. H.; Gordon, M. S. J. Am. Chem. Soc. 1991, 113, 7917.

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(4) Csaszar, A. G. J. Am. Chem. Soc. 1992, 114, 9568. (5) Ding, Y.; Krogh-Jespersea, K. Chem. Phys. Lett. 1992, 199, 261. (6) Hu, C. H.; Shen, M.; Schaefer, H. F., III J. Am. Chem. Soc. 1993, 115, 2923. (7) Bertran, J.; Rodriguez-Santiago, L.; Sodupe, M. J. Phys. Chem. B 1999, 103, 2310. (8) Gutowski, M.; Skurski, P.; Simons, J. J. Am. Chem. Soc. 2000, 122, 10159. (9) Kassab, E.; Langlet, J.; Evleth, E.; Akacem, Y. J. Mol. Struct. 2000, 531, 267. (10) Kass, S. R. J. Am. Chem. Soc. 2005, 127, 13098. (11) Wang, L. S.; Ding, C. F.; Wang, X. B.; Nicholas, J. B.; Nicholas, B. Phys. ReV. Lett. 1998, 81, 2667. (12) Yang, G.; Zu, Y. G.; Liu, C. B.; Fu, Y. J.; Zhou, L. J. J. Phys. Chem. B 2008, 112, 7104. (13) Yang, G.; Lan, X. J.; Zhuang, J. Q.; Ma, D.; Zhou, L. J.; Liu, X. C.; Han, X. W.; Bao, X. H. Appl. Catal. A 2008, 337, 58. (14) Yang, G.; Guan, J.; Zhou, L. J.; Liu, X. C.; Han, X. W.; Bao, X. H. J. Photochem. Photobiol. A 2009, 202, 122. (15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T., Jr.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; P. Gill, M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (16) Maseras, F.; Morokuma, K. J. Comput. Chem. 1995, 16, 1170. (17) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (18) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.

Yang et al. (19) Barone, V.; Adamo, C.; Lelj, F. J. Chem. Phys. 1995, 102, 364. (20) Kaschner, B.; Hohl, D. J. Phys. Chem. A 1998, 102, 5111. (21) Simon, S.; Sodupe, M.; Bertran, J. J. Phys. Chem. A 2002, 106, 5697. (22) Balta, B.; Aviyente, V. J. Comput. Chem. 2003, 24, 1789. (23) Croft, A. K.; Easton, C. J.; Radom, L. J. Am. Chem. Soc. 2003, 125, 4119. (24) Tian, S. X. J. Phys. Chem. B 2004, 108, 20388. (25) Reddy, A. S.; Sastry, G. N. J. Phys. Chem. A 2005, 109, 8893. (26) Remko, M.; Rode, B. M. J. Phys. Chem. A 2006, 110, 1960. (27) Polfer, N. C.; Oomens, J.; Moore, D. T.; von Helden, G.; Meijer, G.; Dunbar, R. C. J. Am. Chem. Soc. 2006, 128, 517. (28) Corral, I.; Mo´, O.; Y|Ata; nez, M.; Salpin, J. Y.; Tortajada, J.; Moran, D.; Radom, L. Chem.sEur. J. 2006, 12, 6787. (29) Ji, W. F.; Li, Z. L.; Shen, L.; Kong, D. X.; Zhang, H. Y. J. Phys. Chem. B 2007, 111, 485. (30) Rodgers, M. T.; Armentrout, P. B. Acc. Chem. Res. 2004, 37, 989. (31) Tortonda, F. R.; Pascual-Ahuir, J.; Silla, E.; Tunˇo´n, I.; Ramı´rez, F. J. J. Chem. Phys. 1998, 109, 592. (32) Lemoff, A. S.; Bush, M. F.; Williams, E. R. J. Am. Chem. Soc. 2003, 125, 13576. (33) Rimola, A.; Corno, M.; Zicovich-Wilson, C. M.; Ugliengo, P. J. Am. Chem. Soc. 2008, 130, 16181. (34) Jo¨nsson, P. G.; Kvick, Å. Acta Crystallogr. B 1972, 28, 1827. (35) Boekfa, B.; Pantu, P.; Limtrakul, J. J. Mol. Struct. 2008, 889, 81. (36) At the default level of theory (see the details in the Supporting Information), the PA values of HZSM-5 zeolite, GlyA, GlyB, and GlyC were calculated to be 313.56, 329.32, 346.92, and 347.35 kcal mol-1, respectively. Note that the N-H1 distance in GlyA was fixed at 1.061 Å. (37) Larson, R. A.; Weber, E. J. Reaction Mechanisms in EnVironmental Organic Chemistry; CRC Press: Boca Raton, 1994; p178. (38) As Figure S6 in the Supporting Information indicates, the superposition effects of the two GlyC structures in ZSM-5 zeolite and silicate-1 are not satisfactory. Accordingly, the interactions of the lattices with GlyC structures will differ greatly for ZSM-5 zeolite and silicalite-1, which makes it inappropriate to be used as the energy benchmark.

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