Experimental absolute cross sections for the reaction of isobutane [CH(CH3)3] with [CH2O]•+ isomers

Isomers frequently exhibit different physical and chemical properties, which can make accurate modeling of their reactivity highly challenging but can also be exploited as a method for isomer identification. In both cases, this requires accurate experimental data recorded at an isomer-specific level. The reactions of the DCOH•+ and H2CO•+ ions with isobutane [CH(CH3)3] are investigated experimentally in the 0.07-10 eV collision energy range using a guided-ion beam tandem mass spectrometer coupled to a tunable vacuum ultraviolet source. The reaction of the DCOH•+ ion is dominated by dissociative proton transfer processes, while the reactivity of H2CO•+ is predominantly driven by dissociative charge transfer processes. The kinematics are also significantly different, with the overall reactivity of the DCOH•+ isomer following an approximately Langevin trend, while the reactivity of H2CO•+ is distinctly non-Langevin. In addition, for the H2CO•+ isomer, sharp changes in reactivity are observed at photon energies corresponding to Rydberg autoionization, leading to vibrational excitation of the C-O stretching mode. The findings highlight the inadequacy of modeling molecular ions as point charges and evidence the need for isomer-specific reactivity data.