The Computed Microwave Spectrum of the Protonated Fullerene C₆₀H

The largest known molecule in the interstellar medium of galaxies, C₆₀, has been detected in its neutral and cationic form through its vibrational, UV-driven fluorescence emission spectrum and its electronic absorption spectrum, respectively. The detection of several polycyclic aromatic hydrocarbon molecules through their pure rotation spectrum in cold, dense, molecular cloud cores suggests that C₆₀ might be present in these environments as well. The low flux of UV pumping photons in molecular cloud cores and the absence of suitably bright background stars, make detection of C₆₀ and its cation through the commonly used methods impractical. As C₆₀ has no permanent dipole moment, its pure rotational transitions are forbidden, and its presence must be inferred from the rotational transitions of C₆₀ derivatives with permanent dipole moments. Here, we present a study of the predicted rotational spectrum of protonated C₆₀ that has a sizable permanent dipole moment. Protonation of C₆₀ reduces the icosahedral symmetry to C_s and results in a dipole moment of about 3.8 D. The resulting C₆₀H⁺ is a closed shell system with no electron spin. The ground electronic symmetry is A'. The goal of the present calculations is to simulate rotational spectra in radio astronomy frequency ranges. The simulations are based on geometry and electric dipole moment values from harmonic and anharmonic density functional theory (DFT) calculations at the B3LYP level. Using the PGOPHER spectral simulation program, the rotational structure of the spectra at excitation temperatures of 5 and 10 K were computed to guide future laboratory studies and facilitate radio astronomy searches for protonated C₆₀ in cold dark molecular clouds.