The infrared spectrum of protonated buckminsterfullerene C60H+

Abstract

Although fullerenes have long been hypothesized to occur in interstellar environments, their actual unambiguous spectroscopic identification is of more recent date. 1-4 C 60 , C 70 and C + 60 now constitute the largest molecular species individually identified in the interstellar medium (ISM). Fullerenes have significant proton affinities and it was suggested that C 60 H + is likely the most abundant interstellar analogue of C 60 . 5 We present here the first laboratory infrared (IR) spectrum of gaseous C 60 H + . Symmetry breaking relative to C 60 produces an IR spectrum that is much richer than that of C 60 . The experimental spectrum is used to benchmark theoretical spectra indicating that the B3LYP density functional with the 6-311+G(d,p) basis set accurately reproduces the spectrum. Comparison with IR emission spectra from two planetary nebulae, SMP LMC56 and SMC16, that have been associated with high C 60 abundances, indicate that C 60 H + is a plausible contributor to their IR emission.Buckminsterfullerene C 60 is undoubtedly one of the most iconic molecules of our time. Since its discovery in 1985, 6 its physico-chemical properties have been extensively characterized, including its ion chemistry and spectroscopic properties. IR spectra have been reported in condensed and gas phases, 7-10 and spectra for ionized forms are available as well. 4,8,11,12 The high cosmic abundance of carbon combined with the high stability of fullerenes 13 initiated a quest for their detection in inter-and circumstellar environments. [14][15][16][17] This search culminated in the identifications of neutral C 60 and C 70 in a young planetary nebula (Tc1) 1 based on diagnostic IR features. Accurate gas-phase laboratory spectra in the near-IR range led to the identification of C + 60 as carrier of two of the diffuse interstellar bands near 9600Å. 4 The question of whether or not fullerenes can form in H-rich regions of the interstellar medium (ISM) has been under debate. 1,3 Hydrogenation produces stable fullerene derivatives and partially hydrogenated fullerenes (fulleranes) have been suggested to occur in circumstellar envelopes and in the ISM 3,18,19 . On the other hand, hydrogenation and the concomitant change in orbital hybridization from sp 2 to sp 3 reduces the stability of the fullerene cage, which under the conditions of the ISM would likely lead to dehydrogenation and restoration of the original fullerene 20 or to breakdown of the carbon cage. 5 However, in this latter paper, Kroto also noted that protonation does not compromise cage stability and hypothesized that "protonated C 60 is likely to be the most abundant fullerene analogue," analogous to high abundances of protonated carbon monoxide, HCO + .Ion chemistry studies 21 have determined the proton affinity (PA) of C 60 at 860 kJ/mol. This relatively high value, just above the PA of ammonia, makes C 60 H + (Figure 1a) one of the most relevant stable fullerene derivatives and underpins Kroto's statement above. However, a) Corresponding author: j.oomens@science.ru.nl t...

Synthesis and Spectroscopy of Buckminsterfullerene Cation C60+ in a Cryogenic Ion Trapping Instrument

“…In addition, C 60 was detected earlier [19,20] in various nebulae (see [21] and references therein) by emission of infrared radiation. Recently, laboratory infrared spectra of C + 60 , C 60 H + , and C 70 H + were also reported and compared with observations [22][23][24].…”

Spectroscopy of astrophysically relevant ions in traps

“…The cooling of ions not only simplifies spectra but also provides data that is appropriate for comparison with astronomical observations. Action spectroscopy in traps has been used to observe rotational [56] as well as vibrational [57] transitions of molecular ions of astrochemical interest. In the following sections of this article the focus is on electronic spectroscopy, particularly studies involving the author, and the interested reader is referred to several excellent reviews covering other spectral regions and applications of these methods with similar instruments (Refs.…”

“…Accurate electronic spectroscopy of both C 60 and C − 60 is desired for evaluation of the charge distrubution, and subsequent modelling of conditions in the ISM, as discussed in Ref [54]. Recently, gas-phase infrared spectra of C + 60 − He [89] and C 60 H + [57] were reported and compared with emission data from astrophysical objects. Unlike C 60 , which has only four infrared active modes, the situation for these ions is less favourable due to their more complicated spectra and the broad, overlapping appearance of the UIBs.…”

Methodological Investigation for Hydrogen Addition to Small Cage Carbon Fullerenes

“…However, the formation mechanisms of fullerenes in such extreme environments are still a debated question. Interestingly, while intuitively one might expect hydrogen to stabilise dangling bonds at edge sites and hence promote the formation of polyaromatic hydrocarbons rather than fullerenes, astrochemists have recently found that C60 and C70 are abundant in hydrogen-containing stars [8,9,12,13]. This suggests that hydrogen may play a role in fullerene growth.…”

“…This suggests that relative stability of smaller fullerenes may be modified in the presence of hydrogen. While there has been extensive experimental [13,[18][19][20][21] and theoretical [13,18,22] studies of hydrogenated C60, less attention has been given to smaller hydrogenated fullerenes. It was theoretically proposed that Td-C28H4, a tetrahedral fullerene with triply fused pentagons on each corner of the tetrahedron, might be stable [15], and indeed has since been experimentally identified [23].…”