3980
J. Phys. Chem. 1996, 100, 3980-3982
Electronic and Vibrational Structures of Countercation of Tetrakis(dimethylamino)ethylene (TDAE)-C60. Neutral and Cationic States of TDAE Kazuyoshi Tanaka,* Tohru Sato, and Tokio Yamabe DiVision of Molecular Engineering, Faculty of Engineering, Kyoto UniVersity, Sakyo-ku, Kyoto 606-01, Japan ReceiVed: October 9, 1995X
The electronic and the vibrational structures of TDAEn+ (n ) 0-2), where TDAE is tetrakis(dimethylamino)ethylene, have been first studied by the ab initio molecular orbital (MO) method with the 3-21G basis set. This TDAE molecule is of interest since it forms a ferromagnetic salt TDAE-C60 but its electronic structure has never been reported. Comparison of the present vibrational analysis with the Raman spectral observation has confirmed that there is only the TDAE+ species in the solid phase of TDAE-C60 and TDAE-C70.
Tetrakis(dimethylamino)ethylene (abbreviated as TDAE in this paper; see Figure 1) is one of the strongest organic electron donors.1 It has a half-wave potential of -0.75 V vs aqueous SCE.2 This signifies that TDAE can easily form many kinds of charge transfer (CT) complexes with various electron acceptors. In particular, TDAE can make a fully ionic complex with C60, TDAE-C60,3 which has been found to undergo a “soft” ferromagnetic transition at TC ) 16.1 K.4 Since this discovery, there have been many studies5-9 of the structure and the solid-state properties of TDAE-C60. It has also been reported that the ionic complexes of larger fullerenes such as C70, C84, C90, and C96 with TDAE do not show any ferromagnetic transition down to 4.5 K.6-10 The lattice structure of TDAE-C60 has been studied by X-ray powder diffractometry at both room temperature and 11 K,5 in which the composition of TDAE-C60 is stoichiometric with an almost 1:1 ratio of TDAE to C60. The crystal is c-centered monoclinic and there are two formula units in a cell. The magnetization measurements by a Faraday balance6 have confirmed a ferromagnetic-type transition at 16.7-17.5 K and provided a small value of the saturation magnetization (0.84 emu G/g). Recently, moreover, weak magnetic hysteresis has been observed.11 The ESR study of TDAE-C60 has observed a simple Lorentzian spectrum at room temperature.6 The g-value of TDAEC60 is in good agreement with that of electrochemically prepared C60-,12 which implies that only the unpaired spin located on C60- has been observed in the solid-state TDAE-C60 by the ESR measurement. On the other hand, the ESR study of TDAEC60 in benzonitrile solution13 has confirmed the existence of the TDAE+ radical as well as the C60- radical. Moreover, the triplet-state C602- has also been observed in the liquid phase. Although, under such circumstances, information on the electronic structures of cationic species of TDAE would be of interest, no such studies have ever been performed. Hence, in the present paper, we report ab initio molecular orbital (MO) calculations of the electronic structures of neutral and cationic species of TDAE as well as the vibrational frequencies of the energetically optimized structures. Furthermore, we extend the discussion to the electronic structure of cationic TDAE in TDAE-C60 and TDAE-C70 in the solid phase based on the vibrational analyses performed. All of the geometrical parameters (108 internal degrees of freedom) for TDAE, TDAE+, and the singlet-state TDAE2+ were optimized by the ab initio MO method with the 3-21G * Corresponding author. X Abstract published in AdVance ACS Abstracts, February 1, 1996.
0022-3654/96/20100-3980$12.00/0
Figure 1. Molecular structure of TDAE and the atomic numberings.
basis set. The restricted Hartree-Fock (RHF) method was employed for TDAE and TDAE2+. For the open-shell systems such as TDAE+ and the triplet-state TDAE2+ the unrestricted Hartree-Fock (UHF) method was used. The electronic and the vibrational energies were analyzed based on these geometryoptimized molecular structures. All of the present calculations were carried out with the GAUSSIAN 92 program.14 It has been known that the ground state of ethylene dication is not triplet but singlet and has a skewed structure about the CdC bond.15 There is a possibility, however, that the ground state of TDAE2+ is triplet, unlike dicationic ethylene. The present work has revealed that the triplet-state TDAE2+ is less stable than the singlet-state one by 2.0646 eV at each optimized geometry. Hence only the results of the singlet-state TDAE2+ are shown throughout this paper. As the total charge n of TDAEn+ increases from 0 to 2, the skewed angle about the CdC bond corresponding to the dihedral angle N5-C2-C1-N3 changes from the planarity as shown in Figure 2. This is simply interpreted in terms of the decrease in the π bond order since the highest occupied MO (HOMO) of TDAE2+ mainly consists of π-atomic orbitals on the C atoms at the CdC bond. By use of the total energies of TDAE, TDAE+, and TDAE2+, the first and the second adiabatic ionization potentials (IP) of TDAE are estimated to be 4.20 eV and 8.22 eV, respectively. The first vertical IP calculated by Koopmans’ theorem from the orbital energies of TDAE is 6.90 eV and the first vertical IP of TDAE+ 10.53 eV. The former value can be compared © 1996 American Chemical Society
Neutral and Cationic States of TDAE
J. Phys. Chem., Vol. 100, No. 10, 1996 3981 TABLE 2: Calculated Vibrational Frequencies (in cm-1; after the Scaling with Relatively Strong Infrared or Raman Intensitya TDAE
1005 1198 1498 2770 2806
(VS, w) (M, w) (w, M) (S, w) (w, S)
1017 1290 1502 2771 2830
(M, w) (M, w) (w, M) (w, S) (S, M)
1054 1312 1628 2786 2834
(VS, w) (S, w) (M, S) (VS, VS) (w, S)
1078 1468 2767 2805
(M, w) (w, M) (M, M) (S, M)
TDAE+
554 1034 1205 1371 1485 1527 2836 2878
(w, M) (VS, w) (VS, w) (VS, w) (w, VS) (w, VS) (w, VS) (w, VS)
802 1113 1263 1462 1486 2831 2877 2884
(M, w) (VS, w) (w, VS) (w, M) (M, w) (w, VS) (w, M) (w, S)
952 1130 1304 1473 1507 2833 2877 2889
(w, M) (M, w) (VS, w) (M, w) (w, VS) (M, w) (w, M) (w, S)
986 1181 1345 1473 1512 2835 2878 2928
(VS, w) (M, w) (M, VS) (M, w) (VS, w) (M, M) (w, M) (w, M)
TDAE2+
781 1214 1413 1590 1590
(M, w) (M, w) (w, M) (VS, w) (VS, w)
807 1369 1475 2855 2855
(S, w) (M, w) (M, w) (w, VS) (w, VS)
810 1379 1496 2864 2864
(M, w) (VS, w) (M, w) (w, VS) (w, VS)
1004 1398 1563 2923 2967
(M, w) (VS, w) (VS, w) (w, M) (w, M)
Figure 2. Change of the dihedral angles N5-C2-C1-N3 of (a) TDAE, (b) TDAE+, and (c) TDAE2+.
TABLE 1: Atomic Spin Densities of TDAE+ atomsa
spin density
atomsa
spin density
C1 C2 N3 N4 N5 N6 C7 C8 C9 C10 C11 C12 C13 C14 H15 H16 H17 H18 H19
0.2533 0.2524 0.1360 0.1350 0.1354 0.1355 -0.0102 -0.0231 -0.0100 -0.0230 -0.0101 -0.0231 -0.0101 -0.0230 0.0017 -0.0004 0.0078 0.0023 0.0073
H20 H21 H22 H23 H24 H25 H26 H27 H28 H29 H30 H31 H32 H33 H34 H35 H36 H37 H38
0.0027 -0.0005 0.0078 0.0016 0.0023 0.0073 0.0026 0.0017 -0.0005 0.0078 0.0024 0.0072 0.0027 -0.0005 0.0077 0.0017 0.0023 0.0073 0.0026
a
a
The infrared and the Raman intensities are indicated in the parentheses.
See Figure 1 with respect to the atomic numberings.
with the first IP of TDAE 6.5 eV from the mass spectrometry,1 since the electron impact-ionization corresponds to a FranckCondon transition and gives a vertical IP value.16 The relaxation energies from the vertically ionized TDAE+ and TDAE2+ are 2.69 and 2.31 eV, respectively. Such large differences between the vertical and the adiabatic IP’s signify that the geometrical relaxation accompanied with the ionization process is crucial in cationic TDAE. In the UHF calculation of TDAE+ the spin contamination was calculated to be 〈S2〉 - S2 ) 0.7625 - 0.7500 ) 0.0125, which is very small. The spin density of TDAE+ is shown in Table 2. The unpaired spin is confined almost entirely to the ethylenic carbons and the nitrogen atoms. This indicates that the radical spin is delocalized among the π orbitals on ethylenic carbons and the nonbonding orbitals on nitrogen atoms. The ESR spectrum of TDAE+ electrochemically prepared has given the hyperfine coupling constant of nitrogen aN ) 4.85 ( 0.01 G.2 If we take QN ) 31.8 G employed in ref 2 for the relationship
aN ) QNFN (FN: atomic spin density on N)
(1)
the calculated spin densities on nitrogen atoms in Table 1 give the hyperfine coupling constant of nitrogen aN ) 4.31-4.32 G, which agrees fairly well with the experimental data. It has been reported that the vibrational frequencies calculated by the ab initio Hartree-Fock method with the 3-21G basis set tends to be ca. 1.1 times larger than the experimentally observed values.17 Hence a scaling factor 0.882 that can fit the calculated value of 1845 cm-1 to the most intense peak at 1628 cm-1 in the Raman spectrum of the neutral TDAE18 (see Figure 3) was employed throughout the present work. The calculated vibrational frequencies for TDAE, TDAE+, and TDAE2+ after the scaling are listed in Table 2. A number of low vibrational
Figure 3. Raman spectrum of TDAE.18
TABLE 3: Observed Raman Scattering Frequencies (in cm-1) with the Intensities TDAEa
TDAE+ b
124 189 230 255 324 1053 1435 1596
(S) (S) (M) (M) (S) (M) (S) (M)
961 956
(M)c (S)d
148 195 230 276 345 1127 1481 1602
(S) (S) (M) (M) (M) (M) (S) (S)
157 211 238 290 610 1348 1512 1628
(S) (S) (M) (M) (S) (M) (M) (VS)
167 218 248 314 876 1417 1585 1659
(S) (M) (M) (S) (S) (S) (M) (M)
a From ref 18. b From ref 19. c From the data of TDAE-C . d From 60 the data of TDAE-C70.
frequencies with weak intensities abbreviated in Table 2 are attributed to the umbrella-bending modes of amine groups and so on. Vibrational frequencies of the neutral and cationic TDAE based on the Raman scattering measurements18,19 are summarized in Table 3 for comparison. The Raman peaks at 961 cm-1 in the TDAE-C60 solid and at 956 cm-1 in the TDAE-C70 solid can be assigned to the calculated value of 952 cm-1 of TDAE+ in Table 2. This peak comes from a rocking vibration of methyl groups. On the other hand, both TDAE and TDAE2+ have no such peak in the calculated results, which confirms that there is only TDAE+ species in the solid-state TDAE-C60 and TDAE-C70. In conclusion, the electronic and the vibrational structures of TDAEn+ (n ) 0-2) have been studied by the ab initio MO
3982 J. Phys. Chem., Vol. 100, No. 10, 1996 method with the 3-21G basis set. Four major findings through the present study are as follows: (1) As the total charge n of TDAEn+ increases from 0 to 2, the skewed angle about the CdC bond goes away from planarity. As a consequence of these deformations the adiabatic IPs decrease much more than the vertical IPs. (2) The calculated first vertical IP of TDAE is in good agreement with the experimental data. (3) The calculated spin density on nitrogen of TDAE+ is in agreement with the ESR experimental data. (4) The electronic structures of TDAE in the solid-state phase of TDAE-C60 and TDAE-C70 have been confirmed to be monocationic. Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan. Numerical calculations were carried out at the Supercomputer Laboratory of the Institute for Chemical Research of Kyoto University. References and Notes (1) Wiberg, N. Angew. Chem., Int. Ed. Engl. 1968, 7, 766. (2) Kuwata, K.; Geske, D. J. Am. Chem. Soc. 1964, 86, 2101. (3) Saito, G.; Teramoto, T.; Otsuka, A.; Sugita, Y.; Ban, T.; Kusunoki, M.; Sakaguchi, K. Synth. Met. 1994, 64, 359. (4) Allemand, P.-M.; Khemani, K. C.; Koch, A.; Wudl, F.; Holczer, K.; Donovan, S.; Gruner, G.; Thomson, J. D. Science 1991, 253, 301. (5) Stephens, P. W.; Cox, D.; Lauher, J. W.; Mihaly, L.; Wiley, J. B.; Allemand, P.-M.; Hirsch, A.; Holczer, K.; Li, Q.; Thomson, J. D.; Wudl, F. Nature 1992, 355, 331. (6) Tanaka, K.; Zakhidov, A. A.; Yoshizawa, K.; Okahara, K.; Yamabe, T.; Yakushi, K.; Kikuchi, K.; Suzuki, S.; Ikemoto, I.; Achiba, Y. Phys. Lett. 1992, A164, 221; Phys. ReV. 1992, B47, 7554.
Tanaka et al. (7) Venturini, P.; Mihairovic, D.; Blinc, R.; Cevic, P.; Dolinsek, J.; Abramic, D.; Zalar, B.; Oshio, H.; Allemand, P. M.; Hirsch, A.; Wudl, F. Int. J. Mod. Phys. 1992, B6, 3947. (8) Tanaka, K.; Yoshizawa, K.; Sato, T.; Yamabe, T.; Okahara, K.; Zakhidov, A. A. Solid State Commun. 1993, 87, 1055. Tanaka, K.; Sato, T.; Yamabe, T.; Yoshizawa, K.; Okahara, K.; Zakhidov, A. A. Phys. ReV. 1995, B51, 990. (9) Tanaka, K.; Tanaka, T.; Atake, T.; Yoshizawa, K.; Okahara, K.; Sato, T.; Yamabe, T. Chem. Phys. Lett. 1994, 230, 271. (10) Tanaka, K.; Zakhidov, A. A.; Yoshizawa, K.; Okahara, K.; Yamabe, T.; Kikuchi, K.; Suzuki, S.; Ikemoto, I.; Achiba, Y. Solid State Commun. 1993, 85, 69. (11) Suzuki, A.; Suzuki, T.; Whitehead, R. J.; Maruyama, Y. Chem. Phys. Lett. 1994, 223, 517. (12) Allemand, P.-M.; Srdanov, G.; Koch, A.; Khemani, K.; Wudl, F.; Rubin, F.; Diedrich, F.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Am. Chem. Soc. 1991, 113, 2780. (13) Yoshizawa, K.; Sato, T.; Tanaka, K.; Yamabe, T.; Okahara, K. Chem. Phys. Lett. 1993, 213, 498. (14) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J.; Pople, J. A. GAUSSIAN 92; Gaussian Inc.: Pittsburgh, PA, 1992. (15) Frenking, G. J. Am. Chem. Soc. 1991, 113, 2476 and references therein. (16) Streitwieser, Jr. A. Molecular Orbital Theory for Organic Chemists; Wiley: New York, 1961; Chapter 7. (17) Pople, J. A.; Schlegel, H. B.; Krishnan, R.; Defrees, D. J.; Binkley, J. S.; Frisch, M. J.; Whiteside, R. A.; Hout, R. F.; Hehre, W. J. Int. J. Quantum. Chem.: Quantum Chem. Symp. 1981, 15, 269. (18) Denisov, V. N.; Zakhidov, A. A.; Ruani, G.; Zamboni, R.; Taliani, C.; Tanaka, K.; Yoshizawa, K.; Okahara, K.; Yamabe, T., unpublished result. (19) Denisov, V. N.; Zakhidov, A. A.; Ruani, G.; Zamboni, R.; Taliani, C.; Tanaka, K.; Yoshizawa, K.; Okahara, K.; Yamabe, T. Synth. Met. 1993, 55-57, 3050.
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