Vibrational spectra of cyclic C8 in solid argon

doi:10.1016/S0009-2614(98)01409-2

Chemical Physics Letters

Volume 300, Issues 5–6, 12 February 1999, Pages 719-726

https://doi.org/10.1016/S0009-2614(98)01409-2 Get rights and content

Abstract

Vapors from solid and powdered carbon, emitted from an oven cycled between 2000 and 3000 K, were co-condensed with argon onto a CsI substrate maintained at 10 K. The cycling process produced a multilayered matrix with regions of high carbon density alternating with layers of argon. FTIR measurements including $^{13} C$ isotopic data, supported by ab initio calculations, allow the assignment of a band observed at 1817.8 cm⁻¹ to the ν₁₂(e_u) fundamental of cyclic C₈.

Introduction

The US Air Force High Energy Density Matter (HEDM) Program is examining the feasibility of using cryogenic solids seeded with energetic species as rocket propellants (see e.g., Ref. [1]). In the course of this work, attempts to deposit carbon atoms in solid argon resulted in the first conclusive identification of cyclic C₆[2], which appeared as the most abundant condensation product. This observation suggested that other small cyclic neutral polycarbons also may have been present, as quantum–chemical calculations predict that each of the C_2n (n=2–9) clusters have cyclic isomers that are more stable than their linear counterparts 3, 4, 5, 6.

There have been several computational studies of the structures, energetics, and infrared spectra of C₈ isomers 3, 4, 5, 6. The extensive electron-correlation calculations of Martin and Taylor [6], employing single- and double-excitation coupled-cluster wave functions with a non-iterative treatment of connected triple excitations [CCSD(T)], predict that a cyclic polyacetylenic isomer (C_4h symmetry, $^{1} A_{g}$ electronic state) is 8±2 kcal/mol lower in energy than the linear chain (D_∞h, $^{3} Σ_{g}^{−}$ ). This ordering of isomer energies agrees with earlier predictions by Raghavachari and Binkley [3].

Linear C₈ was first identified by Van Zee et al. [7] in electron-spin-resonance studies of laser-vaporized graphite trapped in solid neon. Szczepanski et al. [8] subsequently used theoretical results to interpret their own infrared spectroscopic measurements of carbon clusters trapped in argon matrices, concluding that absorptions at 2071.5 and 1710.5 cm⁻¹ could originate from linear C₈. In our recent Letter on cyclic C₆[2], we confirmed the assignment of the 1710.5 cm⁻¹ band to the ν₉(σ_u) mode of linear C₈, which is consistent with recent measurements of linear C₈ vibrational frequencies in neon matrices [9]. Additionally, we measured a constant intensity ratio of 2.7:1 between the two bands throughout several experiments, in accord with the observation of Szczepanski et al. [8]. Electronic transitions of linear C₈ and C₈⁻ have also been studied 10, 11, 12.

By contrast, experimental evidence for cyclic C₈ is scarce. Based on ab initio calculations, Martin and Taylor [6] suggested that an absorption at 1818 cm⁻¹, first observed but unassigned by Thompson et al. [13], could belong to this species. Recently, however, Wang et al. [14] assigned an absorption at 1844.2 cm⁻¹, observed in the spectra of matrices produced by trapping laser-ablated carbon in solid argon, to the ν₁₂(e_u) fundamental of cyclic C₈. In the present work, we conclusively assign a band at 1817.8 cm⁻¹ to the ν₁₂(e_u) fundamental of cyclic C₈, and we reassign the 1844.2 cm⁻¹ band seen by Wang et al. to the ν₁₃(σ_u) vibration of linear C₁₃. Cyclic C₈ was observed as the second most abundant condensation product in the same argon matrices that contained cyclic C₆[2].

Section snippets

Computational methods

Although calculated harmonic frequencies and, in some cases, infrared intensities for unsubstituted cyclic C₈ have been reported 3, 4, 5, 6, the spectral shifts of $^{13} C_{n}^{12} C_{8−n}$ (n=1–8) isotopomers relative to $^{12} C_{8}$ are generally not available. To obtain these values, which are essential for definitive identification of matrix-isolated species, we performed density-functional (B3LYP) calculations [15] in cc-pVDZ [16], cc-pVTZ [16], and 6-311G(2d) [17] atomic basis sets, as well as CCSD(T)

Experimental methods

Mixtures of carbon-12 and carbon-13 powders (∼100 mg total), in varying ratios, were resistively heated in a tantalum oven lined with an approximately equal mass of solid graphite. The vapors escaping through a small orifice were co-deposited with continuously flowing argon onto a CsI substrate maintained at 10 K. The procedures followed in the initial heating of the oven were similar to those used in our matrix study of BC₂[19], whereas other aspects of the experiment are described elsewhere

Results and discussion

Fig. 1a–c shows spectra between 1740 and 1830 cm⁻¹ of matrices annealed to 27.5 K. The 1817.8 cm⁻¹ absorption, the dominant feature of Fig. 1a, is the same band seen by Krätschmer and Nachtigall [20], tentatively assigned to cyclic C₈ by Martin and Taylor [6] based on CCSD(T)/cc-pVDZ calculations. The harmonic frequencies and infrared intensities for cyclic $^{12} C_{8}$ that resulted from our isotopic shift calculations are listed in Table 1. These show that the computed vibrational spectrum is

Conclusions

Notwithstanding the heavily congested spectra of Fig. 1b,c, we have definitively identified cyclic C₈ through direct comparisons of the measured and calculated isotopic shifts and by identifying other carbon clusters contributing to these spectra. Fourier transform infrared measurements including correlation of isotopomer bands throughout repeated annealings, together with the predictions of ab initio calculations, have resulted in assignment of the ν₁₂(e_u) fundamental at 1817.8 cm⁻¹.

Acknowledgements

JDP-M expresses appreciation for partial support from the Air Force Office of Scientific Research, the National Research Council through its Associateship Programs Office, and the AFRL Propulsion Directorate. JAS acknowledges computational resources provided by the Department of Defense Aeronautical Systems Center. The authors thank Dr. Jeffrey Mills and Dr. Peter Taylor for advice and helpful discussions.

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¹: Presented in part at the 52nd Ohio State University International Symposium on Molecular Spectroscopy, Columbus, OH, 16–20 June 1997.

²: Present address: Schafer Corporation, 26565 W. Agoura Road, Suite 202, Calabasas, CA 91302, USA.

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Vibrational spectra of cyclic C8 in solid argon1

Abstract

Introduction

Section snippets

Computational methods

Experimental methods

Results and discussion

Conclusions

Acknowledgements

Chem. Phys. Lett.

Chem. Phys.

Chem. Phys. Lett.

Chem. Phys Lett.

Chem. Phys. Lett.

J. Chem. Phys.

J. Am. Chem. Soc.

Chem. Phys. Lett.

J. Phys. Chem.

J. Chem. Phys.

Vibrational spectra of cyclic C₈ in solid argon¹