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Some physicochemical and electronic properties of C60H2 and Be@C60H2 systems—a theoretical study
Journal of Molecular Structure (Theochem) 593 (2002) 175–178 www.elsevier.com/locate/theochem
Some physicochemical and electronic properties of C60H2 and Be@C60H2 systems—a theoretical study Lemi Tu¨rker* Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey Received 15 March 2002; accepted 10 May 2002
Abstract Regio and stereoisomers of vicinal C60H2 and Be@C60H2 systems are considered within the framework of AM1 (SCF) type semiempirical calculations. Some of their physicochemical and electronic properties are reported. In-isomers of Be@C60H2 structure after the geometry optimization yields quasi Be@C60H2 structure. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrofullerenes; Hydrogenation; C60H2; Be@C60H2; BeH2@C60
1. Introduction Partially hydrogenated fullerenes on the way to the completely hydrogenated C60H60 (fulleranes) constitute a class of interesting C60 derivatives which are very problematic because of the identification problems. For example, the dihydrofullerene, C60H2 possesses 23 different regioisomers possible at least in principle [1]. Although, the number of regioisomers reaches to huge numbers for up to a certain number of hydrogen molecules added, generally the polyhydrofullerenes are unstable [1]. However, in the case of stoichiometrically controlled additions to only one or a few reactive 6 – 6 double bonds of C60 then the number of energetically favorable isomers is limited [2 – 6]. The calculations at the level of MNDO and ab initio for different isomers of C60H2 have been reported [5,7 –9]. The theoretical predictions and the experimental evidence are in favor of vicinal addition * Tel.: þ90-312-210-3244; fax: þ90-312-210-1280. E-mail address: [email protected] (L. Tu¨rker).
(1,2-addition) of hydrogen. However, it was theoretically predicted (MNDO level) that as the number of hydrogens, n . 6 then 1,4-addition mode becomes more favorable than 1,2-additions [10]. In the case of C60H60, molecular mechanics type calculations (MM3) show that one hydrogen inside the cage (Inisomer) is stable and should form as a result of an exothermic process [11]. Thus, for each regioisomer, the stereoisomers (In- and Out-forms) should also be considered in terms of their energetics to investigate how likely their formations are. In the present study, vicinally hydrogenated beryllium doped C 60H2 (Be@C60H2) regio and stereoisomers have been the subject of research. Endohedrally doped fullerenes are interesting class of structures [12]. For instance, a theoretical study at the level of AM1 type calculations revealed that endohedrally Be doped C60 structure (Be@C60) should have the ability to accommodate hydrogen molecules inside the cage. However, after a certain number of hydrogens inside, a hydridic interaction grows between Be atom and the hydrogens [13].
0166-1280/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 1 2 8 0 ( 0 2 ) 0 0 3 0 9 - 3
L. Tu¨rker / Journal of Molecular Structure (Theochem) 593 (2002) 175–178
176
Fig. 1. The geometry optimized structures of In-56 and In-66 types Be@C60H2 isomers.
2. Method In the present treatise, the initial structure of C60 [(5,6)-fullerene-60-I h] was excerpted from Hyperchem Library [14]. The geometry optimizations of all the structures leading to energy minima were achieved by using AM1 self-consistent fields molecular orbital (SCF MO) [15] method at the restricted Hartree –Fock (RHF) level [16]. The optimizations were obtained by the application of the steepestdescent method followed by conjugate gradient methods, Fletcher-Rieves and Polak-Ribiere, consecutively (convergence limit of 4.18 £ 1024 kJ/mol (0.0001 kcal/mol) and RMS gradient of 4.18 £ 107 kJ/(mmol) (0.001 kcal/(A mol))). All these computations were performed by using the Hyperchem (release 5.1) and ChemPlus (2.0) package programs [14].
3. Results and discussion The present study is confined to C60H2 isomers
which possess hydrogens at vicinal positions only. Thus, C60 structure allows just two regioisomers that is hydrogens at the fusion sites of a five and sixmembered rings or at the respective sites of two sixmembered rings (56-and 66-types). As a further limitation, only cis-configuration of hydrogens in C60H2 and Be doped C60H2 has been dealt with. Because of that each regioisomer possesses two stereoisomers, presently they are named as In- and Out-forms. Hence, only four structures arise, In- and Out-forms of 56- and 66-types. In the present study, all the structures containing Be atom are nonexistent yet. However, it has been reported that Ca@C60 was produced by laser vaporization technique [12]. Fig. 1 shows the geometry optimized structures of In-56 and In-66 type Be@C60H2 As seen in the figure, the extremely elongated (3.85, 3.18 and 3.96, 3.07 10210 m, respectively) C – H bonds show that as compared to In-56 and In-66 forms of C60H2 systems, In-Be@C60H2 systems should be actually BeH2@C60 system where a quasi beryllium hydride formation should exist. Tables 1 and 2 show various calculated properties
Table 1 Some properties of the presently considered C60H2 structures Structure
Area Volume Hydration energy log P Refractability Polarizability Molecular point group
In-56
In-66
Out-56
Out-66
543.09 1206.29 21.1296 9.44 232.46 112.86 CS
542.48 1204.48 21.1296 9.44 232.46 112.86 C2V
555.44 1218.86 21.0460 9.44 232.46 112.86 CS
551.23 1215.85 21.0460 9.44 232.46 112.86 C2V
Area, volume, refractability, polarizability and dipole moment values are in the order of 10220 m2, 10230 m3, 10230 m3, 10230 m3, 10 C m, respectively. Hydration energy is in kJ/mol. 230
L. Tu¨rker / Journal of Molecular Structure (Theochem) 593 (2002) 175–178
177
Table 2 Some properties of the presently considered Be@C60H2 structures Structure
Area Volume Polarizability Dipole moment Molecular point group
In-56
In-66
Out-56
Out-66
544.30 1205.19 112.87 0 D3d
538.43 1203.55 112.87 0 D3d
552.27 1220.37 112.87 8.7525 CS
553.11 1220.36 112.87 9.6330 C2V
Area, volume, polarizability and dipole moment values are in the order of 10220 m2, 10230 m3, 10230 m3, 10230 C m, respectively.
Table 3 The HOMO, LUMO energies and the interfrontier energy gaps for C60H2 isomers Structure Energy
In-56
In-66
Out-56
Out-66
LUMO HOMO DE
25.1319 A 214.1687 A00 9.0368
24.8129 B1 214.6498 B2 9.8369
24.8708 A0 214.3017 A00 9.4309
24.5416 A1 214.8364 B2 10.2948
Energies in kJ/mol; DE ¼ 1LUMO 2 e HOMO :
of the system of present interest. A striking fact is the comparatively very high dipole moments for Outforms of either undoped or Be doped systems. The direction of the dipole moment vectors in each of the Out-forms is from the hydrogenated site to the center of the axes of inertia. Tables 3 and 4 show the HOMO and LUMO energies and the interfrontier energy gaps for C60H2 and Be@C60H2 systems (actually BeH2@C60 system). As seen in Table 3, the LUMO energies of In-forms of C60H2 structures are lower than the respective energies of the related Out-forms. Whereas, the HOMO energies
for In-forms are higher than the respective energies of the related Out-forms. On the other hand, the LUMO energy of In-56 form is lower than the LUMO energy of In-66 form, but the HOMO energy for In-56 is higher than the respective energy for In-66 form. A similar trend holds for Out-56 and Out-66 pair. As for the Be doped species, in Table 4 the energies for In-56 and In-66 forms are identical because due to the formation of quasi BeH2 structure, the two forms, initially different, eventually correspond to the same structure, BeH2@C60 as the result of geometry optimization. The LUMO energy of this specie lies
Table 4 The HOMO, LUMO energies and the interfrontier energy gaps for Be@C60H2 isomers Structure Energy
In-56
In-66
Out-56
Out-66
LUMO HOMO DE
24.7282 A2U 215.3440 EU 10.6158
24.7282 A2U 215.3440 EU 10.6158
24.8491 A0 213.1878 A0 8.3387
24.5172 A1 213.1766 A1 8.6594
Energies in kJ/mol; DE ¼ 1LUMO 2 1HOMO :
178
L. Tu¨rker / Journal of Molecular Structure (Theochem) 593 (2002) 175–178
Fig. 2. The HOMO and LUMO of some BeH2@C60H2 systems.
in between the respective energies of Out-56 and Out66 forms, whereas the HOMO is much lower than the HOMO energies of the Out-forms. As compared to the undoped C60H2 systems, the presence of Be atom raises the LUMO but lowers the HOMO energies while correlating to the HOMO and LUMO of BeH2@C60 system. Fig. 2 shows the HOMO and LUMO of BeH2@C60 system.
4. Conclusion The geometry optimization process based on AM1(SCF) type semiempirical calculations revealed that out forms of (either 56- or 66-type) C60H2 have much higher dipole moments than the respective Informs. On the other hand, in the case of Be@C60H2 isomers, In-forms (either 56- or 66-type) have nil dipole moments. Whereas, Out-forms possess dipole moments comparable to the respective values of outforms of undoped C60H2. In addition to these characteristic variations in dipole moments, the calculations reveal that In-forms of Be@C60H2 are indeed (BeH2)@C60 because of the quasi BeH2 formation in the cage.
References [1] A. Hirsch, The Chemistry of the Fullerenes, Verlag, New York, 1994. [2] C.C. Henderson, P.A. Cahill, Science 259 (1993) 1885. [3] S. Ballenweg, R. Gleiter, W. Kratschmer, Tetrahedron Lett. 34 (1993) 3737. [4] T.F. Guarr, M.S. Meier, V.K. Vance, M. Clayton, J. Am. Chem. Soc. 115 (1993) 9862. [5] A. Hirsch, A. Soi, H.R. Karfunkel, Angew. Chem. 104 (1992) 808. [6] A. Hirsch, T. Gro¨sser, A. Skiebe, A. Soi, Chem. Ber. 126 (1993) 1061. [7] N. Matsuzawa, D.A. Dixon, T. Fukunaga, J. Phys. Chem. 96 (1992) 7594. [8] C.C. Henderson, C.M. Rofling, P.A. Cahill, Chem. Phys. Lett. 213 (1993) 383. [9] H.R. Karfunkel, A. Hirsch, Angew. Chem. 104 (1992) 1529. [10] N. Matsuzawa, T. Fukunaga, D.A. Dixon, J. Phys. Chem. 96 (1992) 10747. [11] M. Saunders, Science 253 (1991) 330. [12] M.S. Dresselhause, G. Dresselhause, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, New York, 1995. [13] L. Tu¨rker, J. Mol. Struct. (Theochem) 577 (2002) 205. [14] Hyperchem, Hypercube Inc., Gainesville, FL, USA. [15] M.J.S. Dewar, E.G. Zoebish, E.F. Healey, J.J.P. Stewart, J. Am. Chem. Soc. 107 (1985) 3902. [16] A.R. Leach, Molecular Modelling, Longman, Essex, 1997.
Some physicochemical and electronic properties of C60H2 and Be@C60H2 systems—a theoretical study Lemi Tu¨rker* Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey Received 15 March 2002; accepted 10 May 2002
Abstract Regio and stereoisomers of vicinal C60H2 and Be@C60H2 systems are considered within the framework of AM1 (SCF) type semiempirical calculations. Some of their physicochemical and electronic properties are reported. In-isomers of Be@C60H2 structure after the geometry optimization yields quasi Be@C60H2 structure. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrofullerenes; Hydrogenation; C60H2; Be@C60H2; BeH2@C60
1. Introduction Partially hydrogenated fullerenes on the way to the completely hydrogenated C60H60 (fulleranes) constitute a class of interesting C60 derivatives which are very problematic because of the identification problems. For example, the dihydrofullerene, C60H2 possesses 23 different regioisomers possible at least in principle [1]. Although, the number of regioisomers reaches to huge numbers for up to a certain number of hydrogen molecules added, generally the polyhydrofullerenes are unstable [1]. However, in the case of stoichiometrically controlled additions to only one or a few reactive 6 – 6 double bonds of C60 then the number of energetically favorable isomers is limited [2 – 6]. The calculations at the level of MNDO and ab initio for different isomers of C60H2 have been reported [5,7 –9]. The theoretical predictions and the experimental evidence are in favor of vicinal addition * Tel.: þ90-312-210-3244; fax: þ90-312-210-1280. E-mail address: [email protected] (L. Tu¨rker).
(1,2-addition) of hydrogen. However, it was theoretically predicted (MNDO level) that as the number of hydrogens, n . 6 then 1,4-addition mode becomes more favorable than 1,2-additions [10]. In the case of C60H60, molecular mechanics type calculations (MM3) show that one hydrogen inside the cage (Inisomer) is stable and should form as a result of an exothermic process [11]. Thus, for each regioisomer, the stereoisomers (In- and Out-forms) should also be considered in terms of their energetics to investigate how likely their formations are. In the present study, vicinally hydrogenated beryllium doped C 60H2 (Be@C60H2) regio and stereoisomers have been the subject of research. Endohedrally doped fullerenes are interesting class of structures [12]. For instance, a theoretical study at the level of AM1 type calculations revealed that endohedrally Be doped C60 structure (Be@C60) should have the ability to accommodate hydrogen molecules inside the cage. However, after a certain number of hydrogens inside, a hydridic interaction grows between Be atom and the hydrogens [13].
0166-1280/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 1 2 8 0 ( 0 2 ) 0 0 3 0 9 - 3
L. Tu¨rker / Journal of Molecular Structure (Theochem) 593 (2002) 175–178
176
Fig. 1. The geometry optimized structures of In-56 and In-66 types Be@C60H2 isomers.
2. Method In the present treatise, the initial structure of C60 [(5,6)-fullerene-60-I h] was excerpted from Hyperchem Library [14]. The geometry optimizations of all the structures leading to energy minima were achieved by using AM1 self-consistent fields molecular orbital (SCF MO) [15] method at the restricted Hartree –Fock (RHF) level [16]. The optimizations were obtained by the application of the steepestdescent method followed by conjugate gradient methods, Fletcher-Rieves and Polak-Ribiere, consecutively (convergence limit of 4.18 £ 1024 kJ/mol (0.0001 kcal/mol) and RMS gradient of 4.18 £ 107 kJ/(mmol) (0.001 kcal/(A mol))). All these computations were performed by using the Hyperchem (release 5.1) and ChemPlus (2.0) package programs [14].
3. Results and discussion The present study is confined to C60H2 isomers
which possess hydrogens at vicinal positions only. Thus, C60 structure allows just two regioisomers that is hydrogens at the fusion sites of a five and sixmembered rings or at the respective sites of two sixmembered rings (56-and 66-types). As a further limitation, only cis-configuration of hydrogens in C60H2 and Be doped C60H2 has been dealt with. Because of that each regioisomer possesses two stereoisomers, presently they are named as In- and Out-forms. Hence, only four structures arise, In- and Out-forms of 56- and 66-types. In the present study, all the structures containing Be atom are nonexistent yet. However, it has been reported that Ca@C60 was produced by laser vaporization technique [12]. Fig. 1 shows the geometry optimized structures of In-56 and In-66 type Be@C60H2 As seen in the figure, the extremely elongated (3.85, 3.18 and 3.96, 3.07 10210 m, respectively) C – H bonds show that as compared to In-56 and In-66 forms of C60H2 systems, In-Be@C60H2 systems should be actually BeH2@C60 system where a quasi beryllium hydride formation should exist. Tables 1 and 2 show various calculated properties
Table 1 Some properties of the presently considered C60H2 structures Structure
Area Volume Hydration energy log P Refractability Polarizability Molecular point group
In-56
In-66
Out-56
Out-66
543.09 1206.29 21.1296 9.44 232.46 112.86 CS
542.48 1204.48 21.1296 9.44 232.46 112.86 C2V
555.44 1218.86 21.0460 9.44 232.46 112.86 CS
551.23 1215.85 21.0460 9.44 232.46 112.86 C2V
Area, volume, refractability, polarizability and dipole moment values are in the order of 10220 m2, 10230 m3, 10230 m3, 10230 m3, 10 C m, respectively. Hydration energy is in kJ/mol. 230
L. Tu¨rker / Journal of Molecular Structure (Theochem) 593 (2002) 175–178
177
Table 2 Some properties of the presently considered Be@C60H2 structures Structure
Area Volume Polarizability Dipole moment Molecular point group
In-56
In-66
Out-56
Out-66
544.30 1205.19 112.87 0 D3d
538.43 1203.55 112.87 0 D3d
552.27 1220.37 112.87 8.7525 CS
553.11 1220.36 112.87 9.6330 C2V
Area, volume, polarizability and dipole moment values are in the order of 10220 m2, 10230 m3, 10230 m3, 10230 C m, respectively.
Table 3 The HOMO, LUMO energies and the interfrontier energy gaps for C60H2 isomers Structure Energy
In-56
In-66
Out-56
Out-66
LUMO HOMO DE
25.1319 A 214.1687 A00 9.0368
24.8129 B1 214.6498 B2 9.8369
24.8708 A0 214.3017 A00 9.4309
24.5416 A1 214.8364 B2 10.2948
Energies in kJ/mol; DE ¼ 1LUMO 2 e HOMO :
of the system of present interest. A striking fact is the comparatively very high dipole moments for Outforms of either undoped or Be doped systems. The direction of the dipole moment vectors in each of the Out-forms is from the hydrogenated site to the center of the axes of inertia. Tables 3 and 4 show the HOMO and LUMO energies and the interfrontier energy gaps for C60H2 and Be@C60H2 systems (actually BeH2@C60 system). As seen in Table 3, the LUMO energies of In-forms of C60H2 structures are lower than the respective energies of the related Out-forms. Whereas, the HOMO energies
for In-forms are higher than the respective energies of the related Out-forms. On the other hand, the LUMO energy of In-56 form is lower than the LUMO energy of In-66 form, but the HOMO energy for In-56 is higher than the respective energy for In-66 form. A similar trend holds for Out-56 and Out-66 pair. As for the Be doped species, in Table 4 the energies for In-56 and In-66 forms are identical because due to the formation of quasi BeH2 structure, the two forms, initially different, eventually correspond to the same structure, BeH2@C60 as the result of geometry optimization. The LUMO energy of this specie lies
Table 4 The HOMO, LUMO energies and the interfrontier energy gaps for Be@C60H2 isomers Structure Energy
In-56
In-66
Out-56
Out-66
LUMO HOMO DE
24.7282 A2U 215.3440 EU 10.6158
24.7282 A2U 215.3440 EU 10.6158
24.8491 A0 213.1878 A0 8.3387
24.5172 A1 213.1766 A1 8.6594
Energies in kJ/mol; DE ¼ 1LUMO 2 1HOMO :
178
L. Tu¨rker / Journal of Molecular Structure (Theochem) 593 (2002) 175–178
Fig. 2. The HOMO and LUMO of some BeH2@C60H2 systems.
in between the respective energies of Out-56 and Out66 forms, whereas the HOMO is much lower than the HOMO energies of the Out-forms. As compared to the undoped C60H2 systems, the presence of Be atom raises the LUMO but lowers the HOMO energies while correlating to the HOMO and LUMO of BeH2@C60 system. Fig. 2 shows the HOMO and LUMO of BeH2@C60 system.
4. Conclusion The geometry optimization process based on AM1(SCF) type semiempirical calculations revealed that out forms of (either 56- or 66-type) C60H2 have much higher dipole moments than the respective Informs. On the other hand, in the case of Be@C60H2 isomers, In-forms (either 56- or 66-type) have nil dipole moments. Whereas, Out-forms possess dipole moments comparable to the respective values of outforms of undoped C60H2. In addition to these characteristic variations in dipole moments, the calculations reveal that In-forms of Be@C60H2 are indeed (BeH2)@C60 because of the quasi BeH2 formation in the cage.
References [1] A. Hirsch, The Chemistry of the Fullerenes, Verlag, New York, 1994. [2] C.C. Henderson, P.A. Cahill, Science 259 (1993) 1885. [3] S. Ballenweg, R. Gleiter, W. Kratschmer, Tetrahedron Lett. 34 (1993) 3737. [4] T.F. Guarr, M.S. Meier, V.K. Vance, M. Clayton, J. Am. Chem. Soc. 115 (1993) 9862. [5] A. Hirsch, A. Soi, H.R. Karfunkel, Angew. Chem. 104 (1992) 808. [6] A. Hirsch, T. Gro¨sser, A. Skiebe, A. Soi, Chem. Ber. 126 (1993) 1061. [7] N. Matsuzawa, D.A. Dixon, T. Fukunaga, J. Phys. Chem. 96 (1992) 7594. [8] C.C. Henderson, C.M. Rofling, P.A. Cahill, Chem. Phys. Lett. 213 (1993) 383. [9] H.R. Karfunkel, A. Hirsch, Angew. Chem. 104 (1992) 1529. [10] N. Matsuzawa, T. Fukunaga, D.A. Dixon, J. Phys. Chem. 96 (1992) 10747. [11] M. Saunders, Science 253 (1991) 330. [12] M.S. Dresselhause, G. Dresselhause, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, New York, 1995. [13] L. Tu¨rker, J. Mol. Struct. (Theochem) 577 (2002) 205. [14] Hyperchem, Hypercube Inc., Gainesville, FL, USA. [15] M.J.S. Dewar, E.G. Zoebish, E.F. Healey, J.J.P. Stewart, J. Am. Chem. Soc. 107 (1985) 3902. [16] A.R. Leach, Molecular Modelling, Longman, Essex, 1997.