Vibrational Quantum-State-Controlled Reactivity in the O2+ + C3H4 Reaction

TL;DR

This study demonstrates vibrational-state–controlled reactivity in the ion–molecule O2+ + C3H4 system across allene and propyne. Using a Ca+ Coulomb-crystal trap to achieve single-collision, ultra-clean conditions, O2+ is prepared in either the ground state or excited states ($v=2,3$) and reacts with neutral C3H4, with product branching tracked by time-of-flight mass spectrometry and indirect observation methods. A key finding is that a new product channel, C2O+, forms exclusively when O2+ is vibrationally excited, with isotopically purified Ca+ experiments confirming the $m/z = 40$ signal as C2O+. KinBot-based potential-energy-surface analysis reveals a barrierless C2O+ pathway and suggests rapid IVR is unlikely, yielding a calculated complex lifetime of $ ilde{1}$ ps; collectively, the results illustrate dynamical, quantum-state–driven selectivity and mark a significant advance toward quantum-state-controlled chemistry in polyatomic systems.

Abstract

Quantum-state-controlled reactivity is a long-standing goal in the field of physical chemistry. In this work, we explore the vibrational-state-dependent behavior of the ion-molecule reaction between O2+ in distinct vibrational states and two isomers of C3H4, allene (H2C3H2) and propyne (H3C3H). While most products are formed regardless of the vibrational state of O2+, the branching ratios are influenced by vibrational excitation, and a new product, C2O+, appears exclusively in the excited-state reactions. This selective formation of C2O+ demonstrates that vibrational excitation can effectively activate a reaction pathway, providing direct evidence of quantum-state control in reactivity. These results represent an important step towards the goal of quantum-state-controlled chemistry in molecular systems.

Figures (4)

• Figure 1: Reaction curves for (a) O2+(v =2,3) + H2C3H2 (allene) and (b) H3C3H (propyne) reaction series. As the O2+ signal decreases (green), immediate growth is observed in the $39$m/z (teal) and $40$m/z (orange) mass channels. The product signal in the $40$m/z channel was determined from conservation of charge because of the mass overlap of this product with the trapped Ca+. Delayed growth is observed in the $77$ and $79$m/z secondary product channels (combined into one curve displayed in purple). The appearance times, mass-to-charge ratios, and stability of these channels identifies these products as c-C3H3+ ($39$m/z), C2O+/C3H4+ ($40$m/z), C6H5+ ($77$m/z) and C6H7+ ($79$m/z). c-C3H3+, C2O+, and C3H4+ are primary products and the C6H_y+ ($y=5,7$) ions are secondary products originating from the reaction: C3H4++C3H4. Ion signals have been normalized to the fitted initial number of O2+ ions. Error bars correspond to 1$\sigma$ standard error. Reproduced from Ref. lewandowski2024 with permission from the Royal Society of Chemistry.
• Figure 2: Reaction curves for (a) O2+(v = 0) + H2C3H2 (allene) and (b) H3C3H (propyne) reaction series. As the O2+ signal decreases (green), immediate growth is observed in a non-reacting $39$m/z (teal) channel. Because of the mass overlap of a $40$m/z product with the most abundant isotope of Ca+, monitoring of this channel had to be carried out by detection of secondary products produced from the reaction: C3H4+ + C3H4 (combined into one curve displayed in purple). A non-reacting product signal in the $40$m/z channel (orange) was estimated from conservation of charge and revealed to be absent in these ground-state reactions. The appearance times, mass-to-charge ratios, and stability of these channels identifies the products formed in the ground-vibrational state O2+ + C3H4 reactions as c-C3H3+ ($39$m/z), C3H4+, C6H5+ ($77$m/z) and C6H7+ ($79$m/z). c-C3H3+ and C3H4+ are the primary products and the C6H_y+ ($y=5,7$) ions are secondary products. Ion signals have been normalized to the fitted initial number of O2+ ions. Error bars correspond to 1$\sigma$ standard error.
• Figure 3: TOF-MS traces depicting the reaction of O2+(v = 2,3) + C3D4 within an isotopically pure ^44Ca+ Coulomb crystal. These data correspond to a reaction using d4-propyne (D3C3D). (a) Example TOF-MS trace at t = 0 in which O2+ ions are observed at $32$m/z and ^44Ca+ at $44$m/z. The two smaller peaks present correspond to ^42Ca+ and ^40CaOH+ ($57$m/z). We observe no ions in the $40$m/z location with single ion sensitivity. (b) Example of a TOF-MS trace at t = 300 s in which O2+ has fully reacted with C3D4 to form: C2O+ at $40$m/z, C3D3+ at $42$m/z, and C3D4+ at $44$m/z. Additional peaks are also observed at $41$m/z that corresponds to C3HD2+, which has formed from H-D swapping between C3D4+ and ambient water molecules within the chamber, and at $57$m/z, $61$m/z and $62$m/z corresponding to CaOH+ and CaOD+ ions, which are formed from background reactions of Ca+ with ambient water.
• Figure 4: Potential energy surface for the production of C2O+ in the the reaction between O2+(v = 0) and propyne (H3C3H). All structures were optimized at the MP2/aug-cc-pVTZ level of theory. Single-point energies of these structures were calculated at the CCSD(T)-F12/cc-pVDZ-F12 and have been zero-point energy corrected at the MP2/aug-cc-pVTZ level. All energies are shown relative to reactants energy at infinite separation, with INT$1$ corresponding to the first stationary point following complex formation. Note that two values are reported for the production of C2O+, one from CCSD(T)-F12 and the asterisked value computed using values from the ATcT database.Ruscic2004Ruscic2005 See Computational Methods for more explanation.