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AM1 Treatment of (Be+nH2)@C70 systems
Journal of Molecular Structure (Theochem) 668 (2004) 225–228 www.elsevier.com/locate/theochem
AM1 Treatment of (Be þ n H2)@C70 systems Lemi Tu¨rker Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey Received 28 July 2003; accepted 28 October 2003
Abstract AM1 type semiempirical quantum chemical calculations at the level of restricted Hartree – Fock approach have been performed on endohedrally Be and various numbers of hydrogen doped C70 system. The results indicate that (Be þ n H2)@C70 ð0 # n # 10Þ structures are stable and highly endothermic in nature. Moreover, interactions between the endohedral substituents and the cage are weak. q 2004 Elsevier B.V. All rights reserved. Keywords: C70; Endohedral doping; Endohedral substitution; Be; Fullerenes; Hydrogen
1. Introduction As a clean and sustainable energy source in the future, hydrogen is a promising alternative to fossil based fuels. The combustion product of hydrogen gas is water instead of greenhouse gases of fossil fuels. However, it is highly inflammable and creates storage and safety problems in use. Four main technologies for storage of hydrogen are currently used, which include compressed gas, liquefaction, metal hydrides and physorption [1 –17]. Carbon materials (nanofibers, nanotubes and activated carbon) for hydrogen storage purposes are being continually developed [1 –9, 11 – 15]. The discovery of new type materials having nanometer and subnanometer size has encouraged researchers because these materials have more excellent hydrogen storage capacity than the traditional hydrogen storage systems [3]. Fullerenes having endohedrally doped atoms (including certain gases and solids) have been synthesized in the past [18]. The semiempirical quantum chemical studies (at the level of AM1 type calculations) on endohedrally hydrogen doped C58H4 vesicles and endohedrally hydroand gen and Be doped [(Be þ n H 2)@C60 (Li þ n H2)@C60 systems forecast potential hydrogen storage systems [19,20]. In these systems certain interactions grow between the fullerene cage, Be and some of the hydrogens such that quasi-metal hydride E-mail address: [email protected] (L. Tu¨rker). 0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2003.10.063
formation is predicted theoretically [20,21]. The calculations have revealed that in the case of C60, Be atom is more effective than Li atom to polarize hydrogen molecules to form quasi-metal hydride [20,21]. In the present study, endohedrally Be doped C70 structure is considered for hydrogen storage purpose. C70 is a rugby ball shaped molecule [18] and it is the next higher stable and the isolated pentagon rule (IPR) satisfying fullerene [21], whose systematic name is [5,6]-fullerene-70-D5h. The rugby ball shape of C70 [22] can be envisioned either by adding a ring of ten carbon atoms or equivalently, adding a belt of five hexagons around the equatorial plane of the C60 molecule which is normal to one of the fivefold axes and suitably rotating the two hemispheres of C60 by 368 so that they fit continuously on to the belt of hexagons. In contrast to C60, which has only one unique carbon site, the C70 molecule has five inequivalent sites.
2. Method In the present treatise, the geometry optimizations of all the structures leading to energy minima were achieved by using AM1 self-consistent fields molecular orbital (SCF MO) [23] method at the restricted Hartree –Fock (RHF) level [24]. The optimizations were obtained by the application of the steepest-descent method followed by conjugate gradient methods, Fletcher-Rieves and Polak-
L. Tu¨rker / Journal of Molecular Structure (Theochem) 668 (2004) 225–228
226
Table 1 Some calculated energies of the systems presently considered Energy
Total Binding Isolated atomic Electronic Core–core interaction Heat of formation
n 0
5
6
7
8
9
10
2863730 245390 2818340 212664650 11800922
2876885 247550 2829335 213652024 12775139
2879427 247893 2831534 213851145 12971717
2881902 248169 2833733 214045115 13163236
2884365 248432 2835933 214235252 13350887
2886737 248605 2838132 214425491 13538753
2889069 248738 2840331 214614474 13725405
4982.17
5002.19
5095.03
5255.45
5428.03
5691.27
5994.10
Energies in kJ/mol.
3. Results and discussion
Fig. 1. The geometry optimized structures of the (Be þ n H2)@C70 systems.
Ribiere, consecutively (convergence limit of 4.18 £ 1024 kJ/mol (0.0001 kcal/mol) and RMS gradient ˚ .mol))). All of 4.18 £ 107 (kJ/m.mol) (0.001 kcal/(A these computations were performed by using the Hyperchem (release 5.1) and ChemPlus (2.0) package programs [25].
A metallofullerene or endofullerene is obtained by doping an atom or ion into the interior hallow space of the fullerene molecule [18]. These type of structures having 1– 3 metal species inside the cage is common [18]. Beryllium atom possesses 1s2,2s2 electronic configuration. Endohedrally Be doped C70 structure, Be@C70 has not been synthesized yet. In the present study, AM1 (RHF) type semiempirical quantum chemical calculations have been carried out for (Be þ n H2)@C70, where n : 0; 5 – 10. Table 1 shows some energies of these composite systems. As seen in the table, all these structures are stable (the total and binding energies are negative) but endothermic (the heat of formation values) in nature. As the number of hydrogen molecules inserted increases, all the energies except the core – core interaction and the heat of formation values become more and more negative whereas the lastly mentioned energies tend to be more positive. It was reported that in the case of Be@C60 system, the results of AM1 (RHF) type calculations infer that hydrogens in the system undergo some interaction with the endohedrally doped Be atom resulting in quasi-hydride formation
Table 2 Some geometrical and physicochemical properties of the systems presently concerned n
Area Volume Polarizability Dipole moment Molecular point group
0
5
6
7
8
9
10
590.76 1369.98 132.73 0.00 C5h
593.39 1376.14 136.60 7.26 C1
593.79 1376.63 137.37 7.00 C2
594.20 1378.97 138.15 9.53 C1
596.53 1383.25 138.92 7.49 C2
595.62 1386.53 139.69 7.28 C1
599.26 1391.10 140.47 7.52 C2
Area, volume, polarizability and dipole moment values are in 10220 m2, 10230 m3, 10230 m3 and 10230 C m, respectively.
L. Tu¨rker / Journal of Molecular Structure (Theochem) 668 (2004) 225–228
Fig. 2. Three-dimensional electrostatic maps of some of the presently considered structures.
Fig. 3. The direction of dipole moment vector in some of the structures.
227
L. Tu¨rker / Journal of Molecular Structure (Theochem) 668 (2004) 225–228
228
Table 3 The HOMO, LUMO energies and the interfrontier molecular orbital energy gaps ðDEÞ presently considered structures (Be þ n H2)@C70 n
HOMO
LUMO
DE
0 5 6 7 8 9 10
213.68 212.80 212.70 212.78 212.71 212.66 212.68
25.23 25.83 25.84 25.94 25.89 25.95 25.92
8.45 6.97 6.86 6.84 6.82 6.71 6.76
Energies in 10219 J.
[19]. In contrast to that, Be@C70 system does not exhibit such kind of interaction up to n # 10: Fig. 1 shows the geometry optimized structures of the presently considered systems whereas Table 2 shows some calculated geometrical and physicochemical properties of (Be þ n H2)@C70 structure as n varies. Note that whenever n is an even number the molecular point group is C2 otherwise C1. Fig. 2 shows the 3-dimensional electrostatic potential field maps of some of the presently considered structures. In Fig. 3 the directions of dipole moment for some of the structures are shown whereas the magnitudes of them are included in Table 2. Note that the vector in each case originates from the cage, somewhere away from the Be site and aims at the intersection of axes of inertia. Table 3 includes the HOMO and LUMO (frontier molecular orbitals, FMO) energies and the interfrontier molecular orbital energy gaps for the systems presently considered. The results are indicative of the fact that presence of hydrogen molecules in system does not cause any drastic changes on the FMO energies of (Be þ n H2)@C70 systems.
4. Conclusion The results of AM1 (RHF) type calculations on (Be þ n H2)@C70 type systems give clues about the differences with analogous structures of C60. Thus one can
better visualize the behavioral differences between these stable fullerenes C60 and C70. Although, structurally they are quite akin to each other being the later one somewhat oblong in shape, their behavior on endohedral substituents might be quite different as it is shown in the present study.
References [1] J.S. Noh, R.K. Agarwal, J.A. Schwarz, Int. J. Hydrogen Energy 12 (1987) 693. [2] K.A.G. Amankwah, J.S. Noh, J.A. Schwarz, Int. J. Hydrogen Energy 14 (1989) 437. [3] A. Chambers, C. Park, RTK Baker, N. Rodrigez, J. Phys. Chem. B 102 (1998) 4253. [4] C. Park, P.E. Anderson, A. Chambers, C.D. Tan, R. Hidalgo, N.M. Rodrigez, J. Phys. Chem. B 103 (1999) 10572. [5] Y.Y. Fan, B. Liao, M. Liu, Y.L. Wei, M.Q. Lu, H.M. Cheng, Carbon 37 (1999) 1649. [6] C.C. Ahn, Y. Ye, B.V. Ratnakumar, C. Witham, R.C. Bowman Jr., B. Fultz, Appl. Phys. Lett. 73 (1998) 3378. [7] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [8] Y. Ye, C.C. Ahn, C. Witham, B. Fultz, A.G. Rinzler, D. Colbert, K.A. Smith, R.E. Smalley, Appl. Phys. Lett. 74 (16) (1999) 2307. [9] P. Chen, X. Wu, J. Lin, K.L. Tan, Science 285 (1999) 91. [10] R. Chahine, T.K. Int, J. Hydrogen Energy 19 (1994) 161. [11] S. Hynek, W. Fuller, J. Bentley, Int. J. Hydrogen Energy 22 (1997) 601. [12] E. Poirier, R. Chahine, T.K. Bose, Int. J. Hydrogen Energy 26 (2001) 831. [13] H.M. Cheng, Q.H. Yang, C. Liu, Carbon 39 (2001) 1447. [14] R. Strobel, L. Jorissen, T. Schliermann, V. Trapp, W. Schutz, K. Bohmhammel, G. Wolf, J. Garche, J. Power Sources 84 (1999) 221. [15] P. Benard, R. Chahine, Int. J. Hydrogen Energy 26 (2001) 849. [16] V. Guther, A. Otto, J. Alloy Compd 925 (1999) 889. [17] K. Nakatsuka, M. Yoshino, H. Yukawa, M. Morinage, J. Alloy Compd 295 (1999) 222. [18] M.S. Dresselhause, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, New York, 1996, p. 132, 228. [19] L. Tu¨rker, J. Mol. Struct. (Theochem) 577 (2000) 205. [20] L. Tu¨rker, Int. J. Hydrogen Energy 28 (2003) 223. [21] A. Hirsch, The Chemistry of Fullerenes, Georg Thieme, Stuugart, 1994. [22] F. Diederich, R.L. Whetten, Acc. Chem. Res. 25 (1992) 119. [23] M.J.S. Dewar, E.G. Zoebish, E.F. Healey, J.J.P. Stewart, J. Am. Chem. Soc. 107 (1985) 3902. [24] A.R. Leach, Molecular Modelling, Longman, Essex, 1997. [25] Hyperchem, Hypercube Inc., 1996, Gainesville, Florida, USA.
AM1 Treatment of (Be þ n H2)@C70 systems Lemi Tu¨rker Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey Received 28 July 2003; accepted 28 October 2003
Abstract AM1 type semiempirical quantum chemical calculations at the level of restricted Hartree – Fock approach have been performed on endohedrally Be and various numbers of hydrogen doped C70 system. The results indicate that (Be þ n H2)@C70 ð0 # n # 10Þ structures are stable and highly endothermic in nature. Moreover, interactions between the endohedral substituents and the cage are weak. q 2004 Elsevier B.V. All rights reserved. Keywords: C70; Endohedral doping; Endohedral substitution; Be; Fullerenes; Hydrogen
1. Introduction As a clean and sustainable energy source in the future, hydrogen is a promising alternative to fossil based fuels. The combustion product of hydrogen gas is water instead of greenhouse gases of fossil fuels. However, it is highly inflammable and creates storage and safety problems in use. Four main technologies for storage of hydrogen are currently used, which include compressed gas, liquefaction, metal hydrides and physorption [1 –17]. Carbon materials (nanofibers, nanotubes and activated carbon) for hydrogen storage purposes are being continually developed [1 –9, 11 – 15]. The discovery of new type materials having nanometer and subnanometer size has encouraged researchers because these materials have more excellent hydrogen storage capacity than the traditional hydrogen storage systems [3]. Fullerenes having endohedrally doped atoms (including certain gases and solids) have been synthesized in the past [18]. The semiempirical quantum chemical studies (at the level of AM1 type calculations) on endohedrally hydrogen doped C58H4 vesicles and endohedrally hydroand gen and Be doped [(Be þ n H 2)@C60 (Li þ n H2)@C60 systems forecast potential hydrogen storage systems [19,20]. In these systems certain interactions grow between the fullerene cage, Be and some of the hydrogens such that quasi-metal hydride E-mail address: [email protected] (L. Tu¨rker). 0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2003.10.063
formation is predicted theoretically [20,21]. The calculations have revealed that in the case of C60, Be atom is more effective than Li atom to polarize hydrogen molecules to form quasi-metal hydride [20,21]. In the present study, endohedrally Be doped C70 structure is considered for hydrogen storage purpose. C70 is a rugby ball shaped molecule [18] and it is the next higher stable and the isolated pentagon rule (IPR) satisfying fullerene [21], whose systematic name is [5,6]-fullerene-70-D5h. The rugby ball shape of C70 [22] can be envisioned either by adding a ring of ten carbon atoms or equivalently, adding a belt of five hexagons around the equatorial plane of the C60 molecule which is normal to one of the fivefold axes and suitably rotating the two hemispheres of C60 by 368 so that they fit continuously on to the belt of hexagons. In contrast to C60, which has only one unique carbon site, the C70 molecule has five inequivalent sites.
2. Method In the present treatise, the geometry optimizations of all the structures leading to energy minima were achieved by using AM1 self-consistent fields molecular orbital (SCF MO) [23] method at the restricted Hartree –Fock (RHF) level [24]. The optimizations were obtained by the application of the steepest-descent method followed by conjugate gradient methods, Fletcher-Rieves and Polak-
L. Tu¨rker / Journal of Molecular Structure (Theochem) 668 (2004) 225–228
226
Table 1 Some calculated energies of the systems presently considered Energy
Total Binding Isolated atomic Electronic Core–core interaction Heat of formation
n 0
5
6
7
8
9
10
2863730 245390 2818340 212664650 11800922
2876885 247550 2829335 213652024 12775139
2879427 247893 2831534 213851145 12971717
2881902 248169 2833733 214045115 13163236
2884365 248432 2835933 214235252 13350887
2886737 248605 2838132 214425491 13538753
2889069 248738 2840331 214614474 13725405
4982.17
5002.19
5095.03
5255.45
5428.03
5691.27
5994.10
Energies in kJ/mol.
3. Results and discussion
Fig. 1. The geometry optimized structures of the (Be þ n H2)@C70 systems.
Ribiere, consecutively (convergence limit of 4.18 £ 1024 kJ/mol (0.0001 kcal/mol) and RMS gradient ˚ .mol))). All of 4.18 £ 107 (kJ/m.mol) (0.001 kcal/(A these computations were performed by using the Hyperchem (release 5.1) and ChemPlus (2.0) package programs [25].
A metallofullerene or endofullerene is obtained by doping an atom or ion into the interior hallow space of the fullerene molecule [18]. These type of structures having 1– 3 metal species inside the cage is common [18]. Beryllium atom possesses 1s2,2s2 electronic configuration. Endohedrally Be doped C70 structure, Be@C70 has not been synthesized yet. In the present study, AM1 (RHF) type semiempirical quantum chemical calculations have been carried out for (Be þ n H2)@C70, where n : 0; 5 – 10. Table 1 shows some energies of these composite systems. As seen in the table, all these structures are stable (the total and binding energies are negative) but endothermic (the heat of formation values) in nature. As the number of hydrogen molecules inserted increases, all the energies except the core – core interaction and the heat of formation values become more and more negative whereas the lastly mentioned energies tend to be more positive. It was reported that in the case of Be@C60 system, the results of AM1 (RHF) type calculations infer that hydrogens in the system undergo some interaction with the endohedrally doped Be atom resulting in quasi-hydride formation
Table 2 Some geometrical and physicochemical properties of the systems presently concerned n
Area Volume Polarizability Dipole moment Molecular point group
0
5
6
7
8
9
10
590.76 1369.98 132.73 0.00 C5h
593.39 1376.14 136.60 7.26 C1
593.79 1376.63 137.37 7.00 C2
594.20 1378.97 138.15 9.53 C1
596.53 1383.25 138.92 7.49 C2
595.62 1386.53 139.69 7.28 C1
599.26 1391.10 140.47 7.52 C2
Area, volume, polarizability and dipole moment values are in 10220 m2, 10230 m3, 10230 m3 and 10230 C m, respectively.
L. Tu¨rker / Journal of Molecular Structure (Theochem) 668 (2004) 225–228
Fig. 2. Three-dimensional electrostatic maps of some of the presently considered structures.
Fig. 3. The direction of dipole moment vector in some of the structures.
227
L. Tu¨rker / Journal of Molecular Structure (Theochem) 668 (2004) 225–228
228
Table 3 The HOMO, LUMO energies and the interfrontier molecular orbital energy gaps ðDEÞ presently considered structures (Be þ n H2)@C70 n
HOMO
LUMO
DE
0 5 6 7 8 9 10
213.68 212.80 212.70 212.78 212.71 212.66 212.68
25.23 25.83 25.84 25.94 25.89 25.95 25.92
8.45 6.97 6.86 6.84 6.82 6.71 6.76
Energies in 10219 J.
[19]. In contrast to that, Be@C70 system does not exhibit such kind of interaction up to n # 10: Fig. 1 shows the geometry optimized structures of the presently considered systems whereas Table 2 shows some calculated geometrical and physicochemical properties of (Be þ n H2)@C70 structure as n varies. Note that whenever n is an even number the molecular point group is C2 otherwise C1. Fig. 2 shows the 3-dimensional electrostatic potential field maps of some of the presently considered structures. In Fig. 3 the directions of dipole moment for some of the structures are shown whereas the magnitudes of them are included in Table 2. Note that the vector in each case originates from the cage, somewhere away from the Be site and aims at the intersection of axes of inertia. Table 3 includes the HOMO and LUMO (frontier molecular orbitals, FMO) energies and the interfrontier molecular orbital energy gaps for the systems presently considered. The results are indicative of the fact that presence of hydrogen molecules in system does not cause any drastic changes on the FMO energies of (Be þ n H2)@C70 systems.
4. Conclusion The results of AM1 (RHF) type calculations on (Be þ n H2)@C70 type systems give clues about the differences with analogous structures of C60. Thus one can
better visualize the behavioral differences between these stable fullerenes C60 and C70. Although, structurally they are quite akin to each other being the later one somewhat oblong in shape, their behavior on endohedral substituents might be quite different as it is shown in the present study.
References [1] J.S. Noh, R.K. Agarwal, J.A. Schwarz, Int. J. Hydrogen Energy 12 (1987) 693. [2] K.A.G. Amankwah, J.S. Noh, J.A. Schwarz, Int. J. Hydrogen Energy 14 (1989) 437. [3] A. Chambers, C. Park, RTK Baker, N. Rodrigez, J. Phys. Chem. B 102 (1998) 4253. [4] C. Park, P.E. Anderson, A. Chambers, C.D. Tan, R. Hidalgo, N.M. Rodrigez, J. Phys. Chem. B 103 (1999) 10572. [5] Y.Y. Fan, B. Liao, M. Liu, Y.L. Wei, M.Q. Lu, H.M. Cheng, Carbon 37 (1999) 1649. [6] C.C. Ahn, Y. Ye, B.V. Ratnakumar, C. Witham, R.C. Bowman Jr., B. Fultz, Appl. Phys. Lett. 73 (1998) 3378. [7] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [8] Y. Ye, C.C. Ahn, C. Witham, B. Fultz, A.G. Rinzler, D. Colbert, K.A. Smith, R.E. Smalley, Appl. Phys. Lett. 74 (16) (1999) 2307. [9] P. Chen, X. Wu, J. Lin, K.L. Tan, Science 285 (1999) 91. [10] R. Chahine, T.K. Int, J. Hydrogen Energy 19 (1994) 161. [11] S. Hynek, W. Fuller, J. Bentley, Int. J. Hydrogen Energy 22 (1997) 601. [12] E. Poirier, R. Chahine, T.K. Bose, Int. J. Hydrogen Energy 26 (2001) 831. [13] H.M. Cheng, Q.H. Yang, C. Liu, Carbon 39 (2001) 1447. [14] R. Strobel, L. Jorissen, T. Schliermann, V. Trapp, W. Schutz, K. Bohmhammel, G. Wolf, J. Garche, J. Power Sources 84 (1999) 221. [15] P. Benard, R. Chahine, Int. J. Hydrogen Energy 26 (2001) 849. [16] V. Guther, A. Otto, J. Alloy Compd 925 (1999) 889. [17] K. Nakatsuka, M. Yoshino, H. Yukawa, M. Morinage, J. Alloy Compd 295 (1999) 222. [18] M.S. Dresselhause, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, New York, 1996, p. 132, 228. [19] L. Tu¨rker, J. Mol. Struct. (Theochem) 577 (2000) 205. [20] L. Tu¨rker, Int. J. Hydrogen Energy 28 (2003) 223. [21] A. Hirsch, The Chemistry of Fullerenes, Georg Thieme, Stuugart, 1994. [22] F. Diederich, R.L. Whetten, Acc. Chem. Res. 25 (1992) 119. [23] M.J.S. Dewar, E.G. Zoebish, E.F. Healey, J.J.P. Stewart, J. Am. Chem. Soc. 107 (1985) 3902. [24] A.R. Leach, Molecular Modelling, Longman, Essex, 1997. [25] Hyperchem, Hypercube Inc., 1996, Gainesville, Florida, USA.