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1. State selective preparation and nondestructive detection of trapped O2+

   https://doi.org/10.1063/5.0244447  

   Singh, Ambesh Pratik; Mitchell, Michael; Henshon, Will; Hartman, Addison; Lunstad, Annika; Kuzhan, Boran; Hanneke, David (February 2025, The Journal of Chemical Physics)

   The ability to prepare molecular ions in selected quantum states enables studies in areas such as chemistry, metrology, spectroscopy, quantum information, and precision measurements. Here, we demonstrate (2 + 1) resonance-enhanced multiphoton ionization (REMPI) of oxygen, both in a molecular beam and in an ion trap. The two-photon transition in the REMPI spectrum is rotationally resolved, allowing ionization from a selected rovibrational state of O2. Fits to this spectrum determine spectroscopic parameters of the O2d1Πg state and resolve a discrepancy in the literature regarding its band origin. The trapped molecular ions are cooled by co-trapped atomic ions. Fluorescence mass spectrometry nondestructively demonstrates the presence of the photoionized O2+. We discuss strategies for maximizing the fraction of ions produced in the ground rovibrational state. For (2 + 1) REMPI through the d1Πg state, we show that the Q(1) transition is preferred for neutral O2 at rotational temperatures below 50 K, while the O(3) transition is more suitable at higher temperatures. The combination of state-selective loading and nondestructive detection of trapped molecular ions has applications in optical clocks, tests of fundamental physics, and control of chemical reactions. 

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2. Large transition state stabilization from a weak hydrogen bond

   https://doi.org/10.1039/D0SC02806A  

   Vik, Erik C.; Li, Ping; Maier, Josef M.; Madukwe, Daniel O.; Rassolov, Vitaly A.; Pellechia, Perry J.; Masson, Eric; Shimizu, Ken D. (July 2020, Chemical Science)

   A series of molecular rotors was designed to study and measure the rate accelerating effects of an intramolecular hydrogen bond. The rotors form a weak neutral O–H⋯OC hydrogen bond in the planar transition state (TS) of the bond rotation process. The rotational barrier of the hydrogen bonding rotors was dramatically lower (9.9 kcal mol −1 ) than control rotors which could not form hydrogen bonds. The magnitude of the stabilization was significantly larger than predicted based on the independently measured strength of a similar O–H⋯OC hydrogen bond (1.5 kcal mol −1 ). The origins of the large transition state stabilization were studied via experimental substituent effect and computational perturbation analyses. Energy decomposition analysis of the hydrogen bonding interaction revealed a significant reduction in the repulsive component of the hydrogen bonding interaction. The rigid framework of the molecular rotors positions and preorganizes the interacting groups in the transition state. This study demonstrates that with proper design a single hydrogen bond can lead to a TS stabilization that is greater than the intrinsic interaction energy, which has applications in catalyst design and in the study of enzyme mechanisms. 

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3. Direct observation of entangled electronic-nuclear wave packets

   https://doi.org/10.1103/PhysRevResearch.6.L022047  

   Moğol, Gönenç; Kaufman, Brian; Weinacht, Thomas; Cheng, Chuan; Ben-Itzhak, Itzik (May 2024, Physical Review Research)

   We present momentum resolved covariance measurements of entangled electronic-nuclear wave packets created and probed with octave spanning phaselocked ultrafast pulses. We launch vibrational wave packets on multiple electronic states via multiphoton absorption, and probe these wave packets via strong field double ionization using a second phaselocked pulse. Momentum resolved covariance mapping of the fragment ions highlights the nuclear motion, while measurements of the yield as a function of the relative phase between pump and probe pulses highlight the electronic coherence. The combined measurements allow us to directly visualize the entanglement between the electronic and nuclear degrees of freedom and follow the evolution of the complete wavefunction. Published by the American Physical Society2024 

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4. Large transition state stabilization from a weak hydrogen bond

   Vik, E. C.; Li, P.; Maier, J. M.; Madukwe, D.O.; Rassolov, V. A.; Pellechia, P. J.; Masson, E.; Shimizu, K. D. (July 2020, Chemical science)

   A series of molecular rotors was designed to study and measure the rate accelerating effects of an intramolecular hydrogen bond. The rotors form a weak neutral O–H/O]C hydrogen bond in the planar transition state (TS) of the bond rotation process. The rotational barrier of the hydrogen bonding rotors was dramatically lower (9.9 kcal mol
   1) than control rotors which could not form hydrogen bonds. The magnitude of the stabilization was significantly larger than predicted based on the independently measured strength of a similar OH/OC hydrogen bond (1.5 kcal mol-1). The origins of the large transition state stabilization were studied via experimental substituent effect and computational perturbation analyses. Energy decomposition analysis of the hydrogen bonding interaction revealed a significant reduction in the repulsive component of the hydrogen bonding interaction. The rigid framework of the molecular rotors positions and preorganizes the interacting groups in the transition state. This study demonstrates that with proper design a single hydrogen bond can lead to a TS stabilization that is greater than the intrinsic interaction energy, which has applications in catalyst design and in the study of enzyme mechanisms. 

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5. Atomic and Electronic Structure Determinants Distinguish between Ethylene Formation and L‑Arginine Hydroxylation Reaction Mechanisms in the Ethylene-Forming Enzyme

   https://doi.org/https://dx.doi.org/10.1021/acscatal.0c03349  

   Chaturvedi, Shobhit S.; Ramanan, Rajeev; Hu, Jian; Hausinger, Robert P.; Christov, Christo Z. (January 2021, ACS catalysis)

   null (Ed.)

   The ethylene-forming enzyme (EFE) is a non-heme Fe(II), 2-oxoglutarate (2OG), and l-arginine (l-Arg)-dependent oxygenase that catalyzes dual reactions: the generation of ethylene from 2OG and the C5 hydroxylation of l-Arg. Using an integrated molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) approach that references previous experimental studies, we tested the hypothesis that synergy between the conformation of l-Arg and the coordination mode of 2OG directs the reaction toward ethylene formation or l-Arg hydroxylation. The dynamics of EFE·Fe(III)·OO•–·2OG·l-Arg show that l-Arg can exist in conformation A (productive for hydroxylation) and conformation B (unproductive for hydroxylation). QM/MM calculations show that when 2OG is bound in an off-line mode and l-Arg is present in conformation A, the Fe(III)-OO•– intermediate undergoes the standard O2 activation mechanism involving ferryl-dependent hydroxylation. With the same off-line 2OG coordination, but with conformation B of l-Arg, a unique pathway produces a half-bond ferric-bicarbonate intermediate that decomposes to ethylene, two CO2, and a ferrous-bicarbonate species. The results demonstrate that when 2OG is coordinated in off-line mode to the Fe center, the l-Arg conformation acts as a switch that directs the reaction toward ethylene formation or hydroxylation. Analysis of the electronic structure shows that the l-Arg conformation defines the precise location of an unpaired β electron in the Fe(III)-OO– complex, either in a π*∥ orbital that triggers ethylene formation or a π*⊥ orbital that cascades to l-Arg hydroxylation. A change in 2OG coordination from off-line to in-line reduces stabilization of the 2OG C1 carboxylate such that neither conformation of l-Arg produces the ethylene-forming half-bond ferric-bicarbonate intermediate. Thus, l-Arg conformation-dependent changes in the electronic structure of the Fe(III)-OO•– orbitals, together with the 2OG binding mode-associated stabilization of the C1-carboxylate, distinguish whether the EFE reaction proceeds via the ethylene-forming pathway or catalyzes a hydroxylation mechanism. 

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