Probing the structure of giant fullerenes by high resolution trapped ion mobility spectrometry

We present high-resolution trapped ion mobility spectrometry (TIMS) measurements for fullerene ions in molecular nitrogen. Three different charge states were studied (monocations, monoanions and dianions) with fullerenes ranging in size from C₆₀ to C₁₅₀. Ions were prepared by either electrospray ionization (ESI, for mono- and dianions) or by atmospheric pressure chemical ionization (APCI, for monocations) of a preformed fullerene soot extract solution. We demonstrate that TIMS allows to identify (and separate) constituent isomers in favorable cases. Using DFT calculations based on known condensed phase structures and trajectory method (TM) calculations we can reproduce the experimental ^TIMSCCS_N2 for fullerenes up to C₁₀₈ to within 0.5%. Using candidate structures based on quantum chemical predictions, we have also obtained structural information for fullerenes C₁₁₀-C₁₅₀- a size range not previously accessed in condensed phase studies. We find that soluble fullerenes in this size have near-spherical rather than tubular structures. While the TM programs presently available for CCS modelling do a remarkably good job at describing the ion mobility of high (and even giant) fullerenes we observe a slight but systematic size-dependent deviation between ^TIMSCCS_N2 values and our best computational fits which may reflect systematic bonding changes as the cage size increases.