Calculation of Ionization Energy and Electron Affinity of Molecule of Hydro- and Fluorinefullerenes C 60 H(F) n (0 n 60)

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1 FULLERENES, NANOTUBES, AND CARBON NANOSTRUCTURES Vol. 12, No. 1, pp , 2004 Calculation of Ionization Energy and Electron Affinity of Molecule of Hydro- and Fluorinefullerenes C 60 H(F) n (0 n 60) S. K. Nasibullaev, 1, * G. D. Davletbaeva, 1 Y. V. Vasil ev, 2,3 and I. S. Nasibullayev 2 1 Bashkir State University, Ufa, Russia 2 Institute of Physics of Molecules and Crystals, Ufa Research Center of RAS, Ufa, Russia 3 Bashkir State Agriculture University, Ufa, Russia ABSTRACT Vertical and adiabatic electron affinities (EA) and ionization energies (IE) of C 60 H(F) n (0 n 60) have been determined using semiempirical calculations with AM1 Hamiltonian. Comparison of the thermo-chemical characteristics of the fluorinated and hydrogenated fullerene positive and negative ions has been carried out on the basis of these calculations. Key Words: Electron affinity; Ionization energy; C 60 H(F) n ; Semiempirical calculation; Hydrofullerene; Fluorine fullerene. *Correspondence: S. K. Nasibullaev, Bashkir State University, Ufa, Russia; Fax: (3472) ; nsk@anrb.ru. DOI: /FST Copyright # 2004 by Marcel Dekker, Inc X (Print); (Online) Q1 Q2

2 514 Nasibullaev et al INTRODUCTION Hydrogenated and fluorinated fullerenes have been a subject of extensive investigation over the last decade with a particular emphasis on the search of different isomeric structures with either the same [1,2] or different [3] hydrogen and fluorine content. Although charged species of these compounds are the most experimentally studied fullerene derivatives, particularly by mass spectrometry, their theoretical examination is rather rare. Indeed, along with a great number of possible isomers that needed to be included, this study is even more time consuming than that of the corresponding neutral molecules because of the open-shell structure of anion- and cation-radicals. Precise quantitative calculations of the charged species can be achieved by applying sophisticated ab initio or density functional methods using a basis set with the necessary polarization and diffusion functions that take into account effects of electron correlation and are known to be particularly important in open-shell systems like anions or cations. The accuracy of semiempirical calculations of open-shell systems is certainly worse because initially these methods are parameterized using characteristics of neutral molecules. Nevertheless, semiempirical calculations are much less time consuming and normally give reasonably good qualitative predictions while considering relative changes within a set of similar molecular systems. The main aim of the present work is the theoretical consideration of positive and negative ions of hydrogenated and fluorinated fullerenes C 60 H(F) n with n ranging from 2 to 60 on the base of semiempirical calculations with AM1 Hamiltonian. The parallel between fluorination and hydrogenation of fullerenes was already manifested in the literature [4] and here the comparative analysis of their positive and negative ions characteristics will be given. RESULTS OF CALCULATIONS AND DISCUSSION At the present, it is unrealistic to calculate all possible isomeric structures of C 60 H(F) n even using semiempirical methods because of their enormous number. For this reason, the calculations were restricted to the isomeric species containing only three-fold axes and lower. This restriction appears reasonable since up to now the vast majority of experimentally isolated C 60 H(F) n species belong to this class of molecules. The only exceptions were C 60 H(F) 60 molecules that were chosen of I h structure. It should be emphasized, however, that different hydrogenation/fluorination reactions can result in production of different isomers of the same elemental composition and what is the major isomer for a particular reaction is often an unresolved problem. To make the comparison of analogous charged species of F- and H-substituted fullerenes

3 Ionization Energy and Electron Affinity of Hydro- and Fluorinefullerenes 515 more meaningful, the most stable isomers have been selected for each particular n to determine common trends. This comparison will thus reflect a general trend even if some isomeric species of fluorinated or hydrogenated fullerenes with a particular n will slightly deviate. At the first step of the calculations, the more stable isomers of neutral C 60 H(F) n have thus been selected (Table 1) and then corresponding negative and positive ions have been calculated (Table 2). In both cases, the full geometry optimization has been carried out. The quality of calculations of the neutral species was checked by comparison with available literature data. [2,3] The agreement has been found to be very good for the same isomers in Refs. [2,3] and in the present work. To facilitate rationalization of the results of calculations in Tables 1 and 2, a Schlegel diagram is depicted in Fig. 1, where each carbon atoms on the Table 1. Point group symmetries, heat of formations (HOF), and energies of highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) of C 60 H(F) n neutral molecules. n Symmetry HOF (kcal/mol) C 60 H n HOMO LUMO HOF (kcal/mol) C 60 F n HOMO LUMO 0 I h C 2v C C s C s C C s C C s C 3v C s C s C C s C C C C S D I h T1 T2 F1

4 516 Nasibullaev et al Table 2. Heat of formations (HOF), election affinities (EA, corresponds to neutral molecule), Ionization energies (IE, corresponds to neutral molecule) of C 60 H(F) n negative and positive ions. Negative ions Positive ions C60Hn C60Fn C60Hn C60Fn HOF (kcal/mol) EA HOF (kcal/mol) EA HOF (kcal/mol) IE HOF (kcal/mol) IE n (2.65) (7.64) (2.68) (2.91) (7.28) (7.57) (2.69) (2.85) (6.90) (7.46) (2.58) (2.88) (6.77) (7.87) (2.38) (3.08) (6.95) (7.79) (2.26) (3.12) (6.83) (7.80) (1.86) (3.19) (6.72) (8.00) (1.72) (2.95) (6.54) (8.20) (1.70) (3.05) (6.53) (8.20) (0.85) (2.82) (6.23) (8.87) (1.43) (2.82) (6.35) (8.27) (1.18) (3.21) (6.22) (8.32) (0.90) (3.14) (6.10) (8.56) (0.54) (3.01) (6.24) (8.93) (0.12) (2.83) (6.49) (9.41) (20.02) (2.85) (6.51) (9.50) (20.19) (2.84) (6.48) (9.69) (20.32) (2.41) (6.44) (9.27) (21.02) (2.49) (6.89) (9.8) (23.02) (3.17) (6.63) (13.26) (23.56) (1.92) (8.23) (11.86)

5 Ionization Energy and Electron Affinity of Hydro- and Fluorinefullerenes Figure 1. Schlegel diagram of C 60. The numbers correspond to the positions of carbon atoms where addition of H- or F-atom occurs. fullerene cage has been assigned to a particular number. Addition of hydrogen or fluorine atoms to the numbered carbon atoms results in different isomers of C 60 H(F) n, which are the following: n ¼ 2: 1,2;4: 1, 2, 30, 36; 6: 1, 4, 14, 17, 18, 23; 8: 14, 23, 26, 32, 37, 40, 46, 49; 10: 14, 23, 26, 32, 37, 40, 45, 46, 49, 54; 12: 14, 23, 26, 32, 37, 39, 40, 45, 46, 48, 49, 54; 14: 14, 23, 26, 29, 32, 35, 37, 39, 40, 45, 46, 48, 49, 54; 16: 14, 15, 23, 24, 26, 29, 32, 35, 37, 39, 40, 45, 46, 48, 49, 54; 18: 1, 2, 5, 15, 16, 21, 22, 28, 29, 34, 37, 40, 41, 50, 52, 53, 57, 60; 20: 1, 2, 3, 4, 7, 10, 13, 15, 16, 19, 22, 24, 29, 30, 35, 36, 39, 41, 48, 50; 22: 1, 2, 3, 4, 7, 10, 13, 15, 16, 19, 22, 24, 29, 30, 35, 36, 39, 41, 42, 48, 50, 51; 24: 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 45, 54, 55, 56, 57, 60; 26: 1, 2, 3, 4, 7, 10, 13, 15, 16, 19, 22, 24, 29, 30, 35, 36, 37, 38, 39, 41, 42, 46, 47, 48, 50, 51; 28: 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 41, 43, 45, 50, 52, 54, 55, 56, 57, 60; 30: 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 26, 32, 41, 43, 45, 50, 52, 54, 55, 56, 57, 60; 32: 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 26, 30,

6 518 Nasibullaev et al , 36, 41, 43, 45, 50, 52, 54, 55, 56, 57, 60; 34: 1, 2, 3, 4, 7, 9, 10, 13, 15, 16, 18, 19, 22, 24, 25, 27, 29, 30, 31, 33, 35, 36, 37, 39, 41, 42, 44, 46, 48, 50, 51, 53, 58, 59; 36: 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 25, 27, 29, 32, 34, 36, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54; 48:1,2,3, 4, 5, 6, 7, 9, 11, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 49, 51, 53, 55, 56, 57, 58, 59. In the frame of Koopmans theorem, the energies of HOMOs and LUMOs (Table 1) are related to the vertical IE and to the vertical EA, respectively. They can be compared with corresponding experimental values determined by photoelectron spectroscopy and electron transmission spectroscopy. The IE and EA values that are present in Table 2 have been determined using the following formulas (1) and (2): EA ¼ E (0) E ( ) (1) Figure 2. Electron affinity (a) and IE (b) of hydrogenated (B) and fluorinated (O) fullerenes C 60 H(F) n as functions of the hydrogen/fluorine content, n.

7 Ionization Energy and Electron Affinity of Hydro- and Fluorinefullerenes 519 where E (0) and E (2) are the total energies of neutral molecule and negative ion, respectively IE ¼ E (þ) E (0) (2) where E (þ) is the total energy of positive ion. The EA and IE magnitudes determined from formulas (1) and (2) correspond to the adiabatic EA and to the adiabatic IE, respectively. 0 Normalized to the known C 60 EA¼2.64 ev and IE ¼ 7.64 ev these values for C 60 H(F) n are given in parentheses. The obvious tendencies are clearly seen from the calculations (Fig. 2): the EA in the case of fluorinated fullerenes varies only slightly, whereas that of hydrogenated fullerenes decreases with increasing hydrogen content at the fast rate, reaching near to zero value for n ¼ 28 or 30 and turning to the steady negative magnitudes at higher n. The situation is exactly reverse for ionization energies where they again go in the opposite directions as the number of substitutions increases and the IEs variation of fluorinated fullerenes is clearly much more dramatic. ACKNOWLEDGMENTS The work has been supported by the Russian Foundation for Basic Research (grant ) and INTAS (grant YSF 01/1-188). REFERENCES 1. Nossal, J.; Saini, R.K.; Sadana, A.K.; Bettinger, H.F.; Alemany, L.B.; Scuseria, G.E.; Billups, W.E.; Saunders, M.; Khong, A.; Weisemann, R. Formation, isolation, spectroscopic properties, and calculated properties of some isomers of C 60 H 36. J. Am. Chem. Soc. 2001, 123 (35), Clare, B.W.; Kepert, D.L. The structures of C 60 F 36 and new possible structures for C 60 H 36. J. Mol. Struct. (Theochem). 1999, 466, Clare, B.W.; Kepert, D.L. Early stages in the addition to C 60 to form C 60 X n, X ¼ H, F, Cl, Br, CH 3,C 4 H 9. J. Mol. Struct. (Theochem). 2003, 621, Boltalina, O.V.; Buhl, M.; Khong, A.; Sauders, M.; Street, J.M.; Taylor, R. The 3 He NMR spectra of C 60 F 18 and C 60 F 36 ; the parallel between hydrogenation and fluorination. J. Chem. Soc. Perkin. Trans , 1, F2 Q3

Electronegativity is a very useful concept for the explanation or understanding of chemical reactivity throughout the periodic table.

1.6. Review of Electronegativity (χ) CONCEPT: Electronegativity is a very useful concept for the explanation or understanding of chemical reactivity throughout the periodic table. There are many definitions