J. Phys. Chem. 1996, 100, 12135-12137
12135
Infrared Spectroscopic Studies of Carbon Clusters Trapped in Solid Parahydrogen Masaaki Miki, Tomonari Wakabayashi, Takamasa Momose, and Tadamasa Shida* DiVision of Chemistry, Graduate School of Science, Kyoto UniVersity, Kyoto 606-01, Japan ReceiVed: April 30, 1996X
Small carbon clusters produced by laser ablation of a carbon rod are trapped in solid parahydrogen at 4.8 K. Infrared spectra show the presence of C3, C5, C9, and a few new clusters. The observed vibrational spectra with multiplet structures are tentatively associated with hindered rotation of the clusters. Temperature dependence of the IR spectra reveals the diffusion of C3 and C5 clusters in the crystal at around 8 K, while no diffusion of C9 and the larger clusters is noticed. Any hydrocarbons which might be produced by reactions between the carbon clusters and the substrate hydrogen molecules are not observed both during the deposition and after the thermal annealing.
Introduction Carbon clusters attract attention in various fields such as fullerene chemistry, material sciences, combustion chemistry, and astrophysics.1 Even if we restrict ourselves to citing references dealing with only the infrared-active vibration of the linear clusters in the electronic ground state, numerous pieces of work are in the literature.2-24 So far, guided by the infrared frequencies observed in the matrix isolation spectroscopy,12-14 high-resolution diode laser spectroscopic studies have unambiguously revealed that odd-numbered clusters possess linear singlet ground states while even-numbered clusters possess a triplet linear form.2-11 The bending dynamics in the linear clusters have become accessible by far-infrared laser spectroscopy3 or by the observation of hot bands.2,5,10 Also, the search for the infrared transition of predicted singlet cyclic isomers of larger clusters remains a challenging problem.1,14,24 In this paper we report the observation of small carbon clusters trapped in a solid parahydrogen matrix. Solid parahydrogen has recently been shown to be a promising medium for matrix-isolation spectroscopy.25,26 The matrix has several advantageous features for high-resolution spectroscopy of doped molecules.25 For example, infrared spectra of methane in solid parahydrogen exhibit rotational structures with spectral linewidths as narrow as 100 MHz (fwhm).27 Such a highresolution spectrum provides information on the molecular structure as well as the electronic structure including the spin state of doped molecules. Another prominent feature of the parahydrogen matrix is that it may react chemically with doped species under certain conditions which contrasts with the conventional monoatomic rare-gas matrixes.26 Such a reaction taking place at low temperatures may provide information on less explored tunneling reactions. With these salient features of the new matrix in mind, we have initiated use of the parahydrogen matrix for the study of molecules of chemical interest.26 This paper is to communicate preliminarily the infrared observation of carbon clusters in solid parahydrogen. Experiments Normal hydrogen gas was converted to parahydrogen using a ferric oxide catalyst. A solid parahydrogen crystal with 0.1% residual orthohydrogen was grown by continuously flowing the converted gas into a copper optical cell equipped with BaF2 windows. The optical length of the cell was 3 cm. A detailed X
Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)01214-2 CCC: $12.00
description of the crystal growth was given in a previous paper.26 Carbon clusters were produced by vaporization of a carbon rod placed inside the optical cell with the use of the second harmonic of a Q-switched Nd:YLF laser (Spectra Physics TFR 200 µJ/ pulse). The rod contained 13C to its natural abundance. Laser pulses with a repetition rate of 200 Hz were focused onto the rod through a 10 cm focal length lens during the growth of the crystal. The temperature of the sample during the deposition was kept at about 8 K. Infrared spectra were recorded on a Nicolet FTIR spectrometer (Magna 750) with a CaF2 beam splitter and a liquid N2 cooled MCT detector. The resolution of the spectrometer was 0.25 cm-1. All the measurements were performed at 4.8 K. To see the effect of thermal annealing the sample was kept for several minutes at temperatures between 4.8 and 12.4 K and then recooled to about 5 K for the optical recording. The temperature of the sample measured by a silicon diode was regulated by an autotuning temperature controller (LakeShore Model 330). Results Figure 1 shows the FTIR spectra obtained in the 2000-2200 cm-1 region. No absorption was observed at any temperature in other spectral regions of the typical C-H stretching at around 3000 cm-1. Spectrum I was observed immediately after the deposition of the ablated carbon clusters. The downward arrows indicate the absorptions appearing upon the irradiation of laser pulses. Spectra II and III are the same as spectrum I after successive elevation of temperature to 9.5 and 12.6 K and recooling to about 5 K. Spectral measurements were also made at other temperatures, but the essential features are similar to spectra II and III. All the absorptions disappeared at around 14 K which is just above the melting point of the parahydrogen crystal. Table 1 summarizes the observed vibrational frequencies of the ablated species with tentative assignments. The absorption bands at 2034.6-2036.4 cm-1 and 2043.6-2044.0 cm-1 are assigned to C3 because the frequencies are reasonably close to the gaseous phase value of 2040.0198 cm-1,2 and in the raregas matrices at 2038.9 cm-1 (Ar),12-15 2036.4 cm-1 (Ne),21 and 2034.7 cm-1 (Kr).15 The absorption band at 2163.9-2165.8 cm-1 is attributable to one of the antisymmetric stretching modes of the linear C5 cluster because of the proximity to the previously observed frequencies of 2169.4410 cm-1 in the gas phase,5,6 of 2166.4 cm-1 in Ne,21 and of 2164.3 cm-1 in Ar matrix.17 © 1996 American Chemical Society
12136 J. Phys. Chem., Vol. 100, No. 30, 1996
Letters
Figure 1. Infrared absorption spectra of carbon clusters trapped in solid parahydrogen. The spectra II and III were recorded after thermal annealing of the crystal (see text).
TABLE 1: Observed Wavenumbers of Ablated Carbon Clusters in Solid Parahydrogen and Their Assignments obs (cm-1)
assignment
obs (cm-1)
assignment
2007.3 2010.1 2034.6 2035.4 2036.0 2036.4 2043.6 2044.0
C9 C9 C3 C3 C3 C3 C3 C3
2063.5 2076.8 2078.7 2080.3 2163.9 2164.5 2165.2 2165.8
C5 C5 C5 C5
TABLE 2: Observed Vibrational Frequencies of Asymmetric Stretching Motion of Small Carbon Clusters (in cm-1) species assign. C3 C4 C5 C6 C7 C8 C9 C10 C13
ν3 ν3 ν3 ν4 ν4 ν5 ν4 ν5 ν6 ν7
gas
Ne
2040.0198(8)a 1548.9368(21)b 2169.4410(2)c
2036.4i 1547.2i 2166.4i 1444.3i 1959.85852(18)d 1958.7i 1199.4i 2138.3152(5)e 2134.6i 1898.3758(8)f 1897.5i 2067.8i 2014.3383(10)g 2010.0i
Ar 2038.9k 1543.4l 2164.3m 1446.6n 1952.5o 1197.3o 2127.9i,p 1894.3i 1998p 1998q 1601.0q
H2 (present work)r 2036.0 2164.5
2007.3
2094.8j 1808.96399(7)h
a Reference 2. b Reference 4. c Reference 6. d Reference 7. e Reference 8. f Reference 9. g Reference 10. h Reference 11. i Reference 21. j Reference 22. k Reference 15. l Reference 16. m Reference 17. n Reference 18. o Reference 19. p Reference 14. q Reference 20. r Only the typical peaks listed in Table 1 are representatively reproduced.
Another σu stretching band of the C5 cluster observed at 1446.6 cm-1 n Ar18 was not detected because the intensity is reported 20 times weaker than that of 2164 cm-1 band.21,24 The doublet at around 2010 cm-1 is assigned to C9 by referring to the band origin of the ν6 fundamental transition at 2014.3383 cm-1 observed in the gas phase,10 and the absorptions at 2010.0 cm-1 in the Ne matrix.21 However, a strong absorption appearing at 2076.8 cm-1 with small satellites on the higher energy side and a weak singlet at 2063.5 cm-1 remain unassigned. Since no absorption was observed in the C-H stretching region, the unassigned absorptions are also attributed to some carbon clusters. Table 2 compiles the vibrational frequencies of small carbon clusters up to C13 so far observed in the gas-phase and in rare-gas matrixes. Since there are no literature values of frequency in Table 2 which correspond to the four unassigned
Figure 2. Fine structures observed in the infrared spectra of (a) C3 and (b) C5.
bands in Table 1, the sizes of these clusters are surmised to be larger than 10 with the exclusion of the reported C13. The band at 2076.8 cm-1 of the present work may be the same as that reported by Szczepanski and Vala in an annealed sample of carbon clusters in solid argon.14 Both the bands assigned to C3 and C5 in the present work exhibit multiplet structures with a splitting of about 0.5-0.8 cm-1 as shown in Figure 2. Such structures have not been reported for these clusters in rare-gas matrixes.12-22 Two possibilities are conceivable for the origin of the structure. First, one may have recourse in the presence of multisites for the clusters. Alternatively, one may attribute to the rotational motion of the clusters. We will examine the latter possibility first. Since both C3 and C5 are known to be linear or quasi-linear in the gas phase,2,3,5,6 only even rotational quantum numbers are allowed in the vibrational ground state while in the vibrationally excited state odd numbers are permissible exclusively. Therefore, if the rotation of the clusters in the parahydrogen crystal are barrier free, only the P and R branches are to be observed. With the use of the gas-phase values of the rotational constants of 0.43 and 0.085 cm-1 for C32 and C5,6 respectively, the thermal population ratios in the vibrational ground state are calculated as 0.25 (J ) 0): 0.57 (J ) 2):0.17 (J ) 4) for C3 and 0.05 (J ) 0):0.22 (J ) 2):0.27 (J ) 4):0.23 (J ) 6):0.14 (J ) 8) for C5. Therefore, the most intense rotational peaks are predicted to be P(2) and R(2) with a separation of approximately 10B ()4.3 cm-1) for C3 and to be P(4) and R(4) separated by about 18B ()1.53 cm-1) for C5. Since the observed spectra in Figure 2 disagree with the prediction above, the free liner rotator model is not compatible with the experiment. As for the possibility of the multisite effect, it cannot be ruled out definitely but is not accepted with ease either, because the structured absorption always diminished homogeneously upon thermal annealing. Furthermore, the infrared absorption of C3
Letters and C5 in rare-gas matrixes so far reported are single peaked, implying the insignificance of multisite effects in these matrices. Since parahydrogen is less polarizable than Ne and tends to form a uniform hcp crystal,28 it is difficult to conceive of particular reasons for having multisites in the solid hydrogen matrix. Being single-peaked, the clusters in rare-gas matrixes are considered to be rotationless or, at most, only librating. Thus, if the motion of C3 and C5 in the parahydrogen matrix also is rotationless and if the possibility of multisites is discarded, we have to search for an explanation for the multiple structure shown in Figure 2. We propose tentatively that the observed structure is associated with a hindered rotation of the clusters. It means that the rotational state is still quantized but the quantum number J is no longer a good quantum number. Since the spectral line width is about 0.25 cm-1, which is equal to our instrumental limit, the intrinsic line width could be narrower. In fact, our recent laser spectroscopic study on the system of CH4/p-H2 solid revealed that the linewidth of a rotationvibration transition is as narrow as 100 MHz (=0.003 cm-1).27 Similar laser spectroscopy is now planned for the present system. In contrast to C3 and C5, no structure is observed for the species assigned to C9 and for the unassigned one at around 2080 cm-1. Moreover, the line width of about 1 cm-1 for these two is roughly 4 times as large as those of C3 and C5. The difference of the band structure and the line width may reflect the difference in the intermolecular interaction between the clusters and the surrounding hydrogen molecules. It is conjectured that rotational motions of the larger clusters are more constrained than C3 and C5. Another difference between the larger clusters and C3 and C5 is seen upon thermal annealing; with both C3 and C5 disappeared already at around 9-10 K (spectrum II in Figure 1), the larger clusters persisted up to the melting point of the crystal. In the annealing study of C3 and C5 in Ar matrix Szczepanski and Vala observed the decay of C3 and C5 with a concomitant increase of bands at 1998, 1700, and 1695 cm-1 which they assigned to C8 and Cn (n > 5).14 In the present work we did not observe similar spectroscopic changes. Thus, the diffusive behavior of C3 and C5 in the hydrogen matrix seems different from that in the Ar matrix. These conjectures await our further investigation on the system of carbon clusters and the molecular hydrogen which is potentially interesting not only from the viewpoint of high-resolution spectroscopy but also from the viewpoint of chemical reactions between the two molecular components. Acknowledgment. The authors would like to thank Professors Eizi Hirota and Takeshi Oka for their encouragement and invaluable concern for the present work. This research was supported by Grants-in Aid for Scientific Research on Priority Areas No. 05237105, Grant A No. 07404034, and the Grant C No. 6640649 of the Ministry of Education, Science, Culture,
J. Phys. Chem., Vol. 100, No. 30, 1996 12137 and Sports. Also, the support of the Shimadzu Science Foundation, the Kurata Foundation, and the Ogasawara Foundation is greatly appreciated. References and Notes (1) See, for example: Weltner, Jr. W.; Van Zee, R. J. Chem. ReV. 1989, 89, 1713. (2) (a) Matsumura, K.; Kanamori, H.; Kawaguchi, K; Hirota, E. J. Chem. Phys. 1988, 89, 3491. (b) Kawaguchi, K.; Matsumura, K.; Kanamori, H.; Hirota, E. J. Chem. Phys. 1989, 91, 1953. (3) Schmuttenmaer, C. A.; Cohen, R. C.; Pugliano, N.; Heath, J. R.; Cooksy, A. L.; Busarow, K. L.; Saykally, R. J. Science 1990, 249, 897. (4) Heath, J. R.; Saykally, R. J. J. Chem. Phys. 1991, 94, 3271. (5) (a) Moazzen-Ahmadi, N.; McKellar, A. R. W.; Amano, T. Chem. Phys. Lett. 1989, 157, 1. (b) Moazzen-Ahmadi, N.; McKellar, A. R. W.; Amano, T. J. Chem. Phys. 1989, 91, 2140. (6) Heath, J. R.; Cooksy, A. L.; Gruebele, M. H. W.; Schmuttenmaer, C. A.; Saykally, R. J. Science 1989, 244, 564. (7) Hwang, H. J.; Van Orden, A.; Tanaka, K.; Kuo, E. W.; Heath, J. R.; Saykally, R. J. Mol. Phys. 1993, 79, 769. (8) Heath, J. R.; Sheeks, R. A.; Cooksy, A. L.; Saykally, R. J. Science 1990, 249, 895. (9) Heath, J. R.; Van Orden, A.; Kuo, E. W.; Saykally, R. J. Chem. Phys. Lett. 1991, 182, 17. (10) (a) Heath, J. R.; Saykally, R. J. J. Chem. Phys. 1990, 93, 8392. (b) Van Orden, A.; Hwang, H. J.; Kuo, E. W.; Saykally, R. J. J. Chem. Phys. Lett. 1993, 98, 6678. (11) Giesen, T. F.; Van Orden, A.; Hwang, H. J.; Fellers, R. S.; Provenc¸ al, R. A.; Saykally, R. J. Science 1994, 265, 756. (12) Weltner, Jr. W.; Walsh, P. N.; Angell, C. L. J. Chem. Phys. 1964, 40, 1299. (13) Kurtz, J.; Huffman, D. R. J. Chem. Phys. 1990, 92, 30. (14) Szczepanski, J.; Vala, M. J. Chem. Phys. 1991, 95, 2792. (15) Szczepanski, J.; Vala, M. J. Chem. Phys. 1993, 99, 7371. (16) Shen, R. H.; Graham, W. R. M. J. Chem. Phys. 1989, 91, 5115. (17) Vala, M.; Chandrasekhar, T. M.; Szczepanski, J.; Van Zee, R.; Weltner, Jr. W. J. Chem. Phys. 1989, 90, 595. (18) Kranze, R. H.; Graham, W. R. M. J. Chem. Phys. 1992, 96, 2517. (19) Kranze, R. H.; Graham, W. R. M. J. Chem. Phys. 1993, 98, 71. (20) Kranze, R. H.; Withey, P. A.; Rittby, C. M. L.; Graham, W. R. M. J. Chem. Phys. 1995, 103, 6841. (21) Smith, A. M.; Agreiter, J.; Ha¨rtle, M.; Engel, C.; Bondybey, V. E. Chem. Phys. 1994, 189, 315. (22) (a) Forney, D.; Fulara, J.; Freivogel, P.; Jakobi, M.; Lessen, D.; Maier, J. P. J. Chem. Phys. 1995, 103, 48. (b) Freivogel, P.; Fulara, J.; Jakobi, M.; Forney, D.; Maier, J. P. J. Chem. Phys. 1995, 103, 54. (23) Martin, J. M. L.; Franc¸ ois, J. P.; Gijbels, R. J. Chem. Phys. 1990, 93, 8850. (24) (a) Hutter, J.; Lu¨thi, H. P.; Diederich, F. J. Am. Chem. Soc. 1994, 116, 750. (b) Martin, J. M. L.; El-Yazal, J.; Franc¸ ois, J.-P. Chem. Phys. Lett. 1995, 242, 570. (25) (a) Oka, T. Annu. ReV. Phys. Chem. 1993, 44, 299. (b) Weliky, D. P.; Byers, T. J.; Kerr, K. E.; Momose, T.; Dickson, R. M.; Oka, T. Appl. Phys. 1994, B59, 265. (26) (a) Momose, T.; Miki, M.; Uchida, M.; Shimizu, T.; Yoshizawa, I.; Shida, T. J. Chem. Phys. 1995, 103, 1400. (b) Momose, T.; Miki, M.; Uchida, M.; Sogoshi, N.; Shida, T. Chem. Phys. Lett. 1995, 246, 583. (c) Sogoshi, N.; Wakabayashi, T.; Momose, T.; Shida, T. J. Phys. Chem., submitted. (27) (a) Chan, M.-C. Ph.D. Thesis, The University of Chicago, 1991. (b) Momose, T.; Miki, M.; Wakabayashi, T.; Shida, T.; Chan, M.-C.; Oka, T., manuscript in preparation. (28) Momose, T.; Weliky, D. P.; Oka, T. J. Mol. Spectrosc. 1992, 153, 760.
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