Matrix isolation spectroscopy and spectral simulations of isotopically substituted C-60 molecules

摘要

Isotopically enriched (3.5% C-13) and depleted (0.5% C-13) fullerene C-60 molecules are isolated in parahydrogen (pH(2)) solids at cryogenic temperatures and studied by high resolution (0.01-0.1 cm(-1)) infrared (IR) absorption measurements. Spectra of natural isotopic abundance (1.1% C-13) C-60 molecules isolated in solid pH(2), orthodeuterium (oD(2)), and Ne matrix hosts serve to identify the relatively minor spectral perturbations due to the trapping environments. Spectral features observed for the four IR-active Tiu modes of threefold degeneracy in I-h symmetry, namely, T-1u(1) at 529.77 cm(-1), T-1u(2) at 578.24 cm(-1), T-1u(3) at 1184.7 cm(-1), and T-1u(4) at 1432 cm(-1), are assigned to the superpositions of matrix perturbed vibrational-mode spectra of a number of (CnC60-n)-C-13-C-12, isotopologues. New molecular orbital calculations show the symmetry lowering effects of C-13 substitution, namely, split vibrational frequencies and modified IR intensities. IR spectral patterns calculated for the 328 distinct isotopomers of (CnC60-n)-C-13-C-12 up to n = 3 are used to satisfactorily simulate most of the observed absorption features. For the T-1u(4) mode at 1432 cm(-1), the observed splitting is insensitive to the C-13 abundance, indicating spectral perturbations due to Fermi resonance. Weak absorption features at 1545 cm(-1) are assigned to a combination of lower frequency modes. We discuss relative and absolute band strengths for the astrophysical application of estimating C-60 abundances in planetary nebulae. Published under license by AIP Publishing.