Molecular Vibrations Steer Chemical Reactions to Create New Compounds

Scientists have long sought to control chemical reactivity through precise manipulation of molecular vibrations. Now, C. Zagorec-Marks, G. S. Kocheril, and T. Kieft, working with colleagues at the University of Southern California, the University of California, Berkeley, and the Hungarian Academy of Sciences, report detailed observations of vibrational quantum-state-controlled reactivity in the reaction between oxygen ions (O2+) and two isomers of C3H4, allene and propyne. Their research reveals that although most reaction products form irrespective of the O2+ vibrational state, the relative abundance of these products shifts with vibrational excitation, and crucially, a new product, carbon monoxide ion (C2O+) , appears only when O2+ is vibrationally excited. This selective product formation provides direct evidence that vibrational excitation can activate previously inaccessible reaction pathways, representing a significant advance towards achieving precise vibrational control over chemical processes in molecular systems.

Scientists have long sought to control the outcomes of chemical reactions at the most fundamental level, by manipulating the quantum states of the reactants. Recent work demonstrates a significant advance in this pursuit, revealing how vibrational excitation can selectively activate reaction pathways in ion-molecule collisions. While most reaction products form regardless of the oxygen ion’s vibrational state, a previously unobserved product, C2O+, appears exclusively when the O2+ is vibrationally excited. This selective formation of C2O+ provides direct evidence that controlling a molecule’s vibrational energy can dictate which reaction pathways are followed, building upon decades of research into ‘Polanyi’s rules’, principles governing how different forms of energy influence reaction rates, and extending these concepts to more complex polyatomic systems. Unlike earlier work reliant on theoretical models, this research presents experimental evidence of quantum-state control, overcoming the challenge of preparing reactants in a pure quantum state without unwanted energy loss. The team employed a sophisticated experimental setup using laser-cooled calcium ions and a quadrupole ion trap to achieve single-collision conditions and extremely low ion temperatures, below 10 Kelvin. This precise control isolated the effects of vibrational excitation on the reaction dynamics, revealing that vibrational energy unlocks a new reaction channel, leading to the formation of C2O+. This finding represents a crucial step towards achieving fully state-controlled chemistry, with potential applications ranging from materials science to atmospheric chemistry. A linear, quadrupole ion trap operating within an ultra-high-vacuum chamber serves as the core of the experimental setup, facilitating single-collision conditions and achieving ion translational temperatures below 10 K. Laser-cooled calcium ions (Ca+) are initially confined within the trap, forming a Coulomb crystal, a pseudo-crystalline structure, which sympathetically cools co-trapped molecular ions through Coulombic interactions. This sympathetic cooling selectively affects translational motion, preserving the initial vibrational state of the reactant ions. Oxygen ions (O + 2) are generated via two separate (2+1) resonance enhanced multi-photon ionization (REMPI) schemes, a technique employing multiple laser photons to ionize neutral species. These schemes produce ions predominantly in either the ground vibrational state, with approximately 91% of ions, or a mixture of excited vibrational states (v = 2 and 3), comprising 68% and 28% of the ions respectively. The separation of ions, approximately 10μm, ensures that sympathetic cooling does not disturb the vibrational populations. Maintaining an ultra-high-vacuum environment, with background gas collision rates below 1Hz, further preserves the vibrational state of O + 2 throughout the reaction. Neutral C3H4 isomers, allene and propyne, are then introduced into the vacuum system at pressures inducing collision rates around 1Hz. Following a controlled reaction time, the trap contents, including reactants and products, are extracted into a time-of-flight mass spectrometer. By repeating this process with varying reaction times, reaction curves are generated, allowing for the identification and quantification of reaction products. This methodology prioritizes internal energy dissipation through bimolecular dissociation or radiative decay, effectively isolating the vibrational effects on reactivity. The research reveals the exclusive formation of the C2O+ product when O2+ is vibrationally excited, demonstrating a clear link between reactant quantum state and reaction outcome. Specifically, C2O+ production was observed only when utilising O2+ in excited vibrational states, whereas it was entirely absent when the ion was in its ground vibrational state. This selective formation represents direct evidence of quantum-state control of reactivity in an ion-molecule reaction. Previous investigations detected a minor, non-reactive species with a mass-to-charge ratio of 40, tentatively assigned as C2O+, alongside other reaction products. The current work confirms this assignment, observing that this 40m/z species is indeed C2O+, and crucially, that its formation is entirely dependent on the vibrational excitation of the O2+ reactant. Despite a barrierless pathway existing for C2O+ formation even in the ground state, no C2O+ was detected under those conditions. Analysis of the reaction dynamics indicates that vibrational excitation effectively activates a previously inaccessible reaction pathway leading to C2O+. The lifetime of the O2+ vibrational states used in this study is estimated to be as long as 5.7x 105 seconds for the v = 30 mode, ensuring sufficient time for the vibrational energy to influence the reaction. This extended lifetime, coupled with the rapid nature of the ion-molecule reaction (approximately 10−9 cm3s−1), prevents significant energy redistribution before the reaction proceeds. The study focused on reactions between O2+ and two isomers of C3H4, allene and propyne, revealing that the branching ratios of products are also influenced by the vibrational state of O2+. While the primary products, c−C3H+3 and C3H+4, are formed regardless of the vibrational state, the relative amounts differ, highlighting the subtle but significant impact of vibrational excitation on reaction pathways.