Quantum state-resolved rotational scattering of C5H+ by H2 in the interstellar medium.

The interstellar medium (ISM) is a complex and dynamic environment in which molecular collisions play a crucial role. Among these, protonated carbon chains are of great interest due to the presence of a permanent dipole moment and their relevance in describing astrochemical processes, making their detection possible in cold molecular clouds such as TMC-1. C5H+ (1Σg+) is an important molecule for understanding the formation and evolution of carbon-rich environments. However, to accurately model its abundance and spectroscopic properties, it is essential to account for its collisional interactions with H2, the most abundant molecule in the ISM. In this study, we present a quantum dynamical study for the C5H+-H2 collision, employing high-level CCSD(T)-F12a/aug-cc-pVTZ calculations to construct an accurate potential energy surface (PES). The PES is further augmented using a neural network fitting model, ensuring spectroscopic accuracy. The PES is expanded into radial components using bispherical harmonics. Then, close coupling methods were used to calculate cross sections and rate coefficients for different rotational transitions of C5H+, up to 100 K. Throughout the temperature range, a propensity is observed for even transitions over odd transitions. The rate coefficients for He and H2 collisions are compared for C5H+, C5, and C6H-. For both low and high temperatures, rate coefficients for C5H+ are found to be higher than C5 and C6H- for both the He and H2 collisions.