Systematic Study of Ion Reactions with Polar Molecules: Rate Coefficients for Ca + + CH 3 X (X = F, Cl, OH, and NH 2 ) at Low Collision Temperatures

Ion-polar molecule reactions play a crucial role in molecular formation in environments such as the interstellar medium and planetary atmospheres due to their large rate coefficients. However, experimental and theoretical investigations of these reactions, particularly under low-temperature conditions, remain scarce. In this study, we measured rate coefficients for reactions of Ca+ (2S1/2 and 2D3/2) with CH3X (X = F, Cl, OH, and NH2) at discrete collision temperatures near 100 K. Notably, reactions with Ca+(2D3/2) exhibited significantly larger rate coefficients than those with Ca+(2S1/2). To elucidate the underlying mechanisms, we performed ab initio quantum chemical calculations for these reaction systems involving both ground and electronically excited Ca+ states. Significant differences in the reactivity of the Ca+ electronic states were observed, and these differences can be qualitatively explained by the calculated intrinsic reaction coordinate (IRC) and the excitation energies of Ca+. The enhancement in rate coefficients for electronically excited Ca+(2D3/2) is attributed to possible nonadiabatic transitions near the transition state, where the potential energy surface (PES) closely approaches a PES of another excited state. The present study highlights the clear difference in reaction dynamics due to the electronic state and functional-group characteristics in ion-polar molecule reactions involving submerged transition-state barriers. This finding provides new insights into the electronic-state dependence of ion-polar molecule reactions and advances our understanding of reaction dynamics in low-temperature environments.