Laboratory IR Spectra of the Ionic Oxidized Fullerenes C60O+ and C60OH

Abstract

We present the first experimental vibrational spectra of gaseous oxidized derivatives of C₆₀ in protonated and radical cation forms, obtained through infrared multiple-photon dissociation spectroscopy using the FELIX free-electron laser. Neutral C₆₀O has two nearly iso-energetic isomers: the epoxide isomer in which the O atom bridges a CC bond that connects two six-membered rings and the annulene isomer in which the O atom inserts into a CC bond connecting a five- and a six-membered ring. To determine the isomer formed for C₆₀O⁺ in our experiment─a question that cannot be confidently answered on the basis of the DFT-computed stabilities alone─we compare our experimental IR spectra to vibrational spectra predicted by DFT calculations. We conclude that the annulene-like isomer is formed in our experiment. For C₆₀OH⁺, a strong OH stretch vibration observed in the 3 μm range of the spectrum immediately reveals its structure as C₆₀ with a hydroxyl group attached, which is further confirmed by the spectrum in the 400-1600 cm^-1 range. We compare the experimental spectra of C₆₀O⁺ and C₆₀OH⁺ to the astronomical IR emission spectrum of a fullerene-rich planetary nebula and discuss their astrophysical relevance.

Figure 1

[5,6] and [6,6] isomers of C₆₀O.

Figure 2

High-resolution mass spectrum of the fullerene sample as generated using the APCI source and recorded with a Fourier-transform ion cyclotron resonance spectrometer. On the top left figure, the m/z 736 peak corresponds to ¹²C₆₀O⁺. On the top right, the m/z 737 peak has two components: the minor ¹³C¹²C₅₉O⁺ and the major ¹²C₆₀OH⁺ peak. The IRMPD spectra are recorded in a QIT MS, which is unable to resolve the two ions. For comparison, the high-resolution mass spectrum (black) is overlapped with the mass spectrum recorded in the QIT MS (red) on the bottom figure.

Figure 3

IRMPD spectrum of C₆₀O⁺ (black line) compared to theoretical spectra of the two isomers, the [5,6] (red) and [6,6] (blue) configurations. The computed IR spectra are calculated at the B3LYP/6-311+G(d,p) level, and a frequency scaling factor of 0.967 is used.

Figure 4

IRMPD spectrum of C₆₀OH⁺ (top) compared with the theoretical spectrum computed at the B3LYP/6-311+G(d,p) level. Frequency scaling factors used are 0.955 for the hydrogen stretch range and 0.967 for the 400–1600 cm^–1 range. The relative intensities in the fingerprint region and in the 3 μm part of the experimental spectrum are unrelated; they have both been normalized to 1.

Figure 5

Comparison of the IR spectra of C₆₀O⁺, protonated C₆₀O, protonated C₆₀, and neutral C₆₀O. Spectra of ionized species are measured by IRMPD spectroscopy. The spectrum of protonated C₆₀ is reproduced with permission from ref (19). Copyright 2019, Springer Nature Limited. The FTIR absorption spectrum of a thin film of neutral C₆₀O is reproduced with permission from ref (31). Copyright 1994, American Chemical Society. Dashed lines indicate the vibrational frequencies of neutral C₆₀. The experimental spectra are compared to the emission spectrum of the SMP SMC16 nebula, reproduced with permission from ref (47). Copyright 2012, American Astronomical Society.