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Modeling complexes of H2 molecules in fullerenes
Chemical Physics Letters 410 (2005) 39–41 www.elsevier.com/locate/cplett
Modeling complexes of H2 molecules in fullerenes Helena Dodziuk
*
Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Kasprzaka 44/52, Poland Received 11 May 2005 Available online 9 June 2005
Abstract Molecular mechanics calculations indicate that the closure, with the preservation of two H2 guests inside C70 host, of the recently synthesized fullerene complex having the cage with a hole, can prove very difficult. C76 of D2 symmetry seems to be the smallest fullerene for which the endohedral complex with two guest molecules can be obtained while that with three guests must be larger than C80. These results and the analysis of spatial relationships for the recently published semiempirical quantum calculations for 24H2@C60 cast doubts in the reliability of low level quantum calculations for supramolecular systems. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction On the basis of molecular mechanics, MM [1], calculations, we have recently shown [2] that the space inside C60, C70, and few other higher fullerenes is very limited. For instance, only H2 and H2O have been found to be stabilized in the C60 cage. Therefore, we have postulated, that to obtain endohedral fullerene complexes with molecules of practical significance, chemistry of higher fullerenes should be mastered to get access to larger quantities of them on the one hand. On the other, the insertion should not proceed during the process of the fullerenes manufacturing, as is the case with metal cationic guests [3], but rather require intricate synthetic chemistry involving an opening of the cage, insertion of a molecule and the subsequent cage closure. First synthesis of an opened C60 with H2 and He inserted followed soon [4,5] with the NMR study of the former complex [6]. The full synthesis of H2@C60 involving all three stages (the cage opening, insertion of H2 and the closure) was published this year by Komatsu et al. [7,8]. In addition, during a recent conference, the same *
Fax: +48 22 632 5276. E-mail address: [email protected]. URL: http://ichf.edu.pl/person/dodziuk.htm.
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.05.038
group reported an observation of two H2 molecules inside an opened C70 cage [7]. The latter report and surprising works by the Erkoc group [9,10] claiming, on the basis of semiempirical quantum calculations, that up to 24H2 or 6NH3 molecules can be stored inside C60 prompted us to study endohedral complexes of fullerenes with hydrogen molecules in more detail.
2. Method As earlier [2] MM method was chosen to study endohedral fullerenes complexes with hydrogen molecules since we strongly believe that low-level quantum calculations that could be carried out for such systems do not describe properly non-bonded interactions in supramolecular systems. The results of semiempirical quantum calculations reported in [9,10] which will be discussed in some detail later, strongly support this opinion. Fullerenes C60 1, C70 2, two isomers of C76 3 and 4, 5 isomers of C78 5–9, and 7 isomers of C80 10–16 were built on the basis of the diagrams given in [11] and the isomers numbering followed that introduced in this reference. The calculations of the steric energy Es of the complexes were carried out using HYPERCHEM program with MM+ force field. Stabilization energy DEs was defined
40
H. Dodziuk / Chemical Physics Letters 410 (2005) 39–41
as the steric energy difference between the energy of the complex consisting of the fullerene C2k (k = 30, 35, 38– 40) with nH2 molecules inside the cage and the corresponding energy of its constituent parts. Taking into account that in the MM+ force field the steric energy of H2 is equal to zero, the stabilization energy of the complex DEs(nH2@C2k) was calculated as Es(nH2@C2k) Es(C2k). The negative values of the difference corresponded to stabilization of the complex while the positive ones denoted destabilization.
3. Results and discussion The calculated values of steric energies for fullerenes under study and the (de)stabilization energies of their complexes with nH2 molecules inside (n = 1–3 or 1–4) are collected in Table 1. The results obtained can be summarized as follows: 1. As stated previously [2], there was place for only one H2 molecule inside the C60 cage. This fact was corroborated by the recent synthesis of H2@C60 complex [8]. Table 1 Steric energy of the fullerenes under study and the stabilization energy a of their complexes with H2 molecules (in kcal/mol) C2k/Es C60 1 619.8 C70 2 669.7 C76D2_1 3 703.9 C76Td_2 4 688.8 C78D3_1 5 724.0 C78C2v_2 6 722.3 C78 C2v0 _3 7 704.9 C78D3h_4 8 745.0 C78 D3h0 _5 9 692.9 C80D5d_1 10 736.0 C80D2_2 11 734.8 C80C2v_3 12 719.8 C80D3_4 13 727.7 C80 C2v0 5 14 707.1 C80D5h_6 15 691.9 C80Ih_7 16 682.2
DEs (H2@C2k)
DEs (2H2@C2k)
DEs (3H2@C2k)
DEs (4H2@C2k)
4.1
+14.5
–
–
4.8
3.0
+14.8
–
4.6
5.6
+2.2
–
4.7
2.4
+3.4
–
4.6
6.6
+3.4
–
4.6
6.3
+2.2
–
4.7
5.7
+0.4
–
4.6
6.4
+4.0
–
4.6
2.4
+2.1
–
4.5
7.4
+2.0
–
4.5
7.0
0.4
10.5
4.6
6.0
2.7
11.1
4.4
5.5
3.8
10.3
4.5
5.3
1.6
8.0
4.5
4.0
+0.4
–
5.2
4.2
+0.2
–
2. Interestingly, C70 complexes with one or two H2 molecules were more stable than the empty fullerene. However, the second complex with two guest molecules seemed to be considerably less stable than that with only one hydrogen molecule. This could mean that in spite of the observation of two H2 guests in the ÔopenedÕ fullerene cage [7], it would be very difficult to obtain the 2H2@C70 complex. 3. The calculated values of DEs for the corresponding complexes of two isomers of C76 exhibited interesting differences. The situation with the isomer with Td symmetry was practically the same as that found for C70. On the other hand, for the D2 isomer the most stable was the complex with two hydrogen molecules inside. Thus, the 2H2@C76_1 (of the D2 symmetry) complex seemed to be the smallest fullerene capable of hosting more than one molecular guest. 4. Understandably, for most isomers of the C78 cage the complexes with two guest hydrogen molecules were found to be the most stable. However, similarly to the situation found for C70 and C76_2 (of Td symmetry) for C78 (of D3h0 5 symmetry) the complex with one H2 guest was the most stable. 5. Somewhat surprisingly, also for C80 there are two isomers (C80 D5h_6 and C80 Ih_7) for which the complexes with one H2 molecule were the most stable. The high symmetry of these isomers precluding the spread of guest molecules inside the host cage resulted also in small but positive, thus exhibiting destabilization, values of the stabilization energy. Interestingly, the complex of C80 isomer of high symmetry, D5d_1 with two H2 molecules exhibited the largest stabilization of all complexes under study on the one hand but showed the destabilization with three guests while for four remaining C80 isomers (D2_2, C2v_3, D3_4 and C2v0 5) the most stable complexes had two H2 guests inside the cage with a smaller stabilization of the complexes involving one or three guests. As mentioned in the introduction, our former and present results are in contradiction with the recently published AM1 semiempirical quantum calculations claiming that up to 24H2 guests can be hosted in the C60 cage [10]. They are also incompatible with the results of PM3 calculations by the same group stating that several ammonia molecules can form the endohedral complex with the same fullerene [9]. To analyse the significance of such statements, 24H2 molecules were put into C60. The subsequent MM+ minimization yielded a system with several non-bonded distances between hydrogen ˚ with the lowest one equal to 1.56 atoms of about 1.6 A ˚ ˚ distances A. Moreover, non-physical smaller than 2 A were found between some hydrogen atoms and the cage carbon ones. We believe that this example, and the analogous one with several ammonia guests inside C60, shows that quantum calculations, unless carried out at high
H. Dodziuk / Chemical Physics Letters 410 (2005) 39–41
level of theory with large basis sets, are not suitable for the analysis of supramolecular systems in which nonbonded interactions play decisive role.
4. Conclusions Molecular mechanics calculations indicate that the closure of recently synthesized 2H2@C70 with the preservation of two guests inside the host cage can prove very difficult. C76 of D2 symmetry seems to be the smallest fullerene for which the endohedral complex with two guest molecules can be obtained. The results of the calculations also suggest that to get an endohedral fullerene complex with three guests, the cage must be larger than that of C80. These results and the analysis of spatial relationships for the recently published AM1 modeling of up to 24H2 molecules in C60 [10], as well as the PM3 calculations for several NH3 molecules inside the same fullerene [9], cast doubts in the reliability of quantum calculations for supramolecular systems unless they are high level ones with large basis sets.
41
References [1] H. Dodziuk, in: A.P. Marchand (Ed.), Methods in Stereochemical Analysis, VCH Publishers, Inc., New York, 1995 (Chapter 3.3). [2] H. Dodziuk, G. Dolgonos, O. Lukin, Carbon 39 (2001) 1907. [3] L. Dunsch, P. Georgi, M. Krause, C. Wang, Synth. Met. 135 (2003) 761. [4] Y. Murata, M. Murata, K. Komatsu, J. Am. Chem. Soc. 125 (2003) 7152. [5] Y. Rubin, T. Jarrosson, G. Wang, M. Bartberger, K. Houk, G. Schick, M. Saunders, R. Cross, T. Jarrosson, G. Wang, M. Bartberger, K. Houk, G. Schick, M. Saunders, R. Cross, Angew. Chem., Int. Ed. Engl. 40 (2001) 1543. [6] M. Carravetta, Y. Murata, M. Murata, I. Heinmaa, R. Stern, A. Tontcheva, A. Samoson, Y. Rubin, K. Komatsu, M. Levitt, J. Am. Chem. Soc. 126 (2004) 4092. [7] K. Komatsu, M. Murata, Y. Murata, in: XIX International Winterschool on Electronic Properties of Novel Materials, Kirchberg, Tirol, 2005. [8] K. Komatsu, M. Murata, Y. Murata, Science 307 (2005) 238. [9] S. Erkoc, L. Turker, J. Mol. Struct. (Theochem) 640 (2003) 57. [10] L. Turker, S. Erkoc, J. Mol. Struct. (Theochem) 638 (2003) 37. [11] P. Fowler, D. Manolopoulos, An atlas of fullerenes. The International Series of Monographs on Chemistry, Clarandon Press, Oxford, 1995.
Modeling complexes of H2 molecules in fullerenes Helena Dodziuk
*
Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Kasprzaka 44/52, Poland Received 11 May 2005 Available online 9 June 2005
Abstract Molecular mechanics calculations indicate that the closure, with the preservation of two H2 guests inside C70 host, of the recently synthesized fullerene complex having the cage with a hole, can prove very difficult. C76 of D2 symmetry seems to be the smallest fullerene for which the endohedral complex with two guest molecules can be obtained while that with three guests must be larger than C80. These results and the analysis of spatial relationships for the recently published semiempirical quantum calculations for 24H2@C60 cast doubts in the reliability of low level quantum calculations for supramolecular systems. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction On the basis of molecular mechanics, MM [1], calculations, we have recently shown [2] that the space inside C60, C70, and few other higher fullerenes is very limited. For instance, only H2 and H2O have been found to be stabilized in the C60 cage. Therefore, we have postulated, that to obtain endohedral fullerene complexes with molecules of practical significance, chemistry of higher fullerenes should be mastered to get access to larger quantities of them on the one hand. On the other, the insertion should not proceed during the process of the fullerenes manufacturing, as is the case with metal cationic guests [3], but rather require intricate synthetic chemistry involving an opening of the cage, insertion of a molecule and the subsequent cage closure. First synthesis of an opened C60 with H2 and He inserted followed soon [4,5] with the NMR study of the former complex [6]. The full synthesis of H2@C60 involving all three stages (the cage opening, insertion of H2 and the closure) was published this year by Komatsu et al. [7,8]. In addition, during a recent conference, the same *
Fax: +48 22 632 5276. E-mail address: [email protected]. URL: http://ichf.edu.pl/person/dodziuk.htm.
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.05.038
group reported an observation of two H2 molecules inside an opened C70 cage [7]. The latter report and surprising works by the Erkoc group [9,10] claiming, on the basis of semiempirical quantum calculations, that up to 24H2 or 6NH3 molecules can be stored inside C60 prompted us to study endohedral complexes of fullerenes with hydrogen molecules in more detail.
2. Method As earlier [2] MM method was chosen to study endohedral fullerenes complexes with hydrogen molecules since we strongly believe that low-level quantum calculations that could be carried out for such systems do not describe properly non-bonded interactions in supramolecular systems. The results of semiempirical quantum calculations reported in [9,10] which will be discussed in some detail later, strongly support this opinion. Fullerenes C60 1, C70 2, two isomers of C76 3 and 4, 5 isomers of C78 5–9, and 7 isomers of C80 10–16 were built on the basis of the diagrams given in [11] and the isomers numbering followed that introduced in this reference. The calculations of the steric energy Es of the complexes were carried out using HYPERCHEM program with MM+ force field. Stabilization energy DEs was defined
40
H. Dodziuk / Chemical Physics Letters 410 (2005) 39–41
as the steric energy difference between the energy of the complex consisting of the fullerene C2k (k = 30, 35, 38– 40) with nH2 molecules inside the cage and the corresponding energy of its constituent parts. Taking into account that in the MM+ force field the steric energy of H2 is equal to zero, the stabilization energy of the complex DEs(nH2@C2k) was calculated as Es(nH2@C2k) Es(C2k). The negative values of the difference corresponded to stabilization of the complex while the positive ones denoted destabilization.
3. Results and discussion The calculated values of steric energies for fullerenes under study and the (de)stabilization energies of their complexes with nH2 molecules inside (n = 1–3 or 1–4) are collected in Table 1. The results obtained can be summarized as follows: 1. As stated previously [2], there was place for only one H2 molecule inside the C60 cage. This fact was corroborated by the recent synthesis of H2@C60 complex [8]. Table 1 Steric energy of the fullerenes under study and the stabilization energy a of their complexes with H2 molecules (in kcal/mol) C2k/Es C60 1 619.8 C70 2 669.7 C76D2_1 3 703.9 C76Td_2 4 688.8 C78D3_1 5 724.0 C78C2v_2 6 722.3 C78 C2v0 _3 7 704.9 C78D3h_4 8 745.0 C78 D3h0 _5 9 692.9 C80D5d_1 10 736.0 C80D2_2 11 734.8 C80C2v_3 12 719.8 C80D3_4 13 727.7 C80 C2v0 5 14 707.1 C80D5h_6 15 691.9 C80Ih_7 16 682.2
DEs (H2@C2k)
DEs (2H2@C2k)
DEs (3H2@C2k)
DEs (4H2@C2k)
4.1
+14.5
–
–
4.8
3.0
+14.8
–
4.6
5.6
+2.2
–
4.7
2.4
+3.4
–
4.6
6.6
+3.4
–
4.6
6.3
+2.2
–
4.7
5.7
+0.4
–
4.6
6.4
+4.0
–
4.6
2.4
+2.1
–
4.5
7.4
+2.0
–
4.5
7.0
0.4
10.5
4.6
6.0
2.7
11.1
4.4
5.5
3.8
10.3
4.5
5.3
1.6
8.0
4.5
4.0
+0.4
–
5.2
4.2
+0.2
–
2. Interestingly, C70 complexes with one or two H2 molecules were more stable than the empty fullerene. However, the second complex with two guest molecules seemed to be considerably less stable than that with only one hydrogen molecule. This could mean that in spite of the observation of two H2 guests in the ÔopenedÕ fullerene cage [7], it would be very difficult to obtain the 2H2@C70 complex. 3. The calculated values of DEs for the corresponding complexes of two isomers of C76 exhibited interesting differences. The situation with the isomer with Td symmetry was practically the same as that found for C70. On the other hand, for the D2 isomer the most stable was the complex with two hydrogen molecules inside. Thus, the 2H2@C76_1 (of the D2 symmetry) complex seemed to be the smallest fullerene capable of hosting more than one molecular guest. 4. Understandably, for most isomers of the C78 cage the complexes with two guest hydrogen molecules were found to be the most stable. However, similarly to the situation found for C70 and C76_2 (of Td symmetry) for C78 (of D3h0 5 symmetry) the complex with one H2 guest was the most stable. 5. Somewhat surprisingly, also for C80 there are two isomers (C80 D5h_6 and C80 Ih_7) for which the complexes with one H2 molecule were the most stable. The high symmetry of these isomers precluding the spread of guest molecules inside the host cage resulted also in small but positive, thus exhibiting destabilization, values of the stabilization energy. Interestingly, the complex of C80 isomer of high symmetry, D5d_1 with two H2 molecules exhibited the largest stabilization of all complexes under study on the one hand but showed the destabilization with three guests while for four remaining C80 isomers (D2_2, C2v_3, D3_4 and C2v0 5) the most stable complexes had two H2 guests inside the cage with a smaller stabilization of the complexes involving one or three guests. As mentioned in the introduction, our former and present results are in contradiction with the recently published AM1 semiempirical quantum calculations claiming that up to 24H2 guests can be hosted in the C60 cage [10]. They are also incompatible with the results of PM3 calculations by the same group stating that several ammonia molecules can form the endohedral complex with the same fullerene [9]. To analyse the significance of such statements, 24H2 molecules were put into C60. The subsequent MM+ minimization yielded a system with several non-bonded distances between hydrogen ˚ with the lowest one equal to 1.56 atoms of about 1.6 A ˚ ˚ distances A. Moreover, non-physical smaller than 2 A were found between some hydrogen atoms and the cage carbon ones. We believe that this example, and the analogous one with several ammonia guests inside C60, shows that quantum calculations, unless carried out at high
H. Dodziuk / Chemical Physics Letters 410 (2005) 39–41
level of theory with large basis sets, are not suitable for the analysis of supramolecular systems in which nonbonded interactions play decisive role.
4. Conclusions Molecular mechanics calculations indicate that the closure of recently synthesized 2H2@C70 with the preservation of two guests inside the host cage can prove very difficult. C76 of D2 symmetry seems to be the smallest fullerene for which the endohedral complex with two guest molecules can be obtained. The results of the calculations also suggest that to get an endohedral fullerene complex with three guests, the cage must be larger than that of C80. These results and the analysis of spatial relationships for the recently published AM1 modeling of up to 24H2 molecules in C60 [10], as well as the PM3 calculations for several NH3 molecules inside the same fullerene [9], cast doubts in the reliability of quantum calculations for supramolecular systems unless they are high level ones with large basis sets.
41
References [1] H. Dodziuk, in: A.P. Marchand (Ed.), Methods in Stereochemical Analysis, VCH Publishers, Inc., New York, 1995 (Chapter 3.3). [2] H. Dodziuk, G. Dolgonos, O. Lukin, Carbon 39 (2001) 1907. [3] L. Dunsch, P. Georgi, M. Krause, C. Wang, Synth. Met. 135 (2003) 761. [4] Y. Murata, M. Murata, K. Komatsu, J. Am. Chem. Soc. 125 (2003) 7152. [5] Y. Rubin, T. Jarrosson, G. Wang, M. Bartberger, K. Houk, G. Schick, M. Saunders, R. Cross, T. Jarrosson, G. Wang, M. Bartberger, K. Houk, G. Schick, M. Saunders, R. Cross, Angew. Chem., Int. Ed. Engl. 40 (2001) 1543. [6] M. Carravetta, Y. Murata, M. Murata, I. Heinmaa, R. Stern, A. Tontcheva, A. Samoson, Y. Rubin, K. Komatsu, M. Levitt, J. Am. Chem. Soc. 126 (2004) 4092. [7] K. Komatsu, M. Murata, Y. Murata, in: XIX International Winterschool on Electronic Properties of Novel Materials, Kirchberg, Tirol, 2005. [8] K. Komatsu, M. Murata, Y. Murata, Science 307 (2005) 238. [9] S. Erkoc, L. Turker, J. Mol. Struct. (Theochem) 640 (2003) 57. [10] L. Turker, S. Erkoc, J. Mol. Struct. (Theochem) 638 (2003) 37. [11] P. Fowler, D. Manolopoulos, An atlas of fullerenes. The International Series of Monographs on Chemistry, Clarandon Press, Oxford, 1995.