Theoretical Study on Reaction Mechanism of Ground-State Cyano Radical with 1,3-Butadiene: Prospect of Pyridine Formation

The reaction of ground-state cyano radicals, CN­(X²Σ⁺), with the simplest polyene, 1,3-butadiene (C₄H₆(X¹A_g)), is investigated to explore probable routes and feasibility to form pyridine at ultralow temperatures. The isomerization and dissociation channels for each of the seven initial collision complexes are characterized by utilizing the unrestricted B3LYP/cc-pVTZ and the CCSD­(T)/cc-pVTZ calculations. With facilitation of RRKM rate constants, through ab initio paths composed of 7 collision complexes, 331 intermediates, 62 hydrogen atom, 71 hydrogen molecule, and 3 hydrogen cyanide dissociated products, the most probable paths at collision energies up to 10 kcal/mol, and thus the reaction mechanism, are determined. Subsequently, the corresponding rate equations are solved that the concentration evolutions of collision complexes, intermediates, and products versus time are obtained. As a result, the final products and yields are determined. The low-energy routes for the formation of most thermodynamically stable product, pyridine, are identified. This study, however, predicts that seven collision complexes would produce predominately 1-cyano-1,3-butadiene, CH₂CH­CHCH­CN (<b>p2</b>) plus atomic hydrogen via the collision complex <b>c1</b>(CH₂CH­CHC­H₂CN) and intermediate <b>i2</b>(CH₂CH­CH₂CH­CN), with a very minor amount of pyridine. Our scheme also effectively excludes the presence of 2-cyano-1,3-butadiene, which has energy near-degenerate to 1-cyano-1,3-butadiene, as supported by experimental findings

Silicon Quantum Dot-Based Fluorescence Turn-On Metal Ion Sensors in Live Cells

Multiple sensor systems are designed by varying aza-crown ether moiety in silicon quantum dots (SiQDs) for detecting individual Mg²⁺, Ca²⁺, and Mn²⁺ metal ions with significant selectivity and sensitivity. The detection limit of Mg²⁺, Ca²⁺, and Mn²⁺ can reach 1.81, 3.15, and 0.47 μM, respectively. Upon excitation of the SiQDs which are coordinated with aza-crown ethers, the photoinduced electron transfer (PET) takes place from aza-crown ether moiety to the valence band of SiQDs core such that the reduced probability of electron–hole recombination may diminish the subsequent fluorescence. The fluorescence suppression caused by such PET effect will be relieved after selective metal ion is added. The charge-electron binding force between the metal ion and aza-crown ether hinders the PET and thereby restores the fluorescence of SiQDs. The design of sensor system is based on the fluorescence “turn-on” of SiQDs while in search of the appropriate metal ion. For practical application, the sensing capabilities of metal ions in the live cells are performed and the confocal image results reveal their promising applicability as an effective and nontoxic metal ion sensor

Combined Crossed Molecular Beam and ab Initio Investigation of the Multichannel Reaction of Boron Monoxide (BO; X²Σ⁺) with Propylene (CH₃CHCH₂; X¹A′): Competing Atomic Hydrogen and Methyl Loss Pathways

The reaction dynamics of boron monoxide (¹¹BO; X²Σ⁺) with propylene (CH₃CHCH₂; X¹A′) were investigated under single collision conditions at a collision energy of 22.5 ± 1.3 kJ mol^–1. The crossed molecular beam investigation combined with <i>ab initio</i> electronic structure and statistical (RRKM) calculations reveals that the reaction follows indirect scattering dynamics and proceeds via the barrierless addition of boron monoxide radical with its radical center located at the boron atom. This addition takes place to either the terminal carbon atom (C1) and/or the central carbon atom (C2) of propylene reactant forming ¹¹BOC₃H₆ intermediate(s). The long-lived ¹¹BOC₃H₆ doublet intermediate(s) underwent unimolecular decomposition involving at least three competing reaction mechanisms via an atomic hydrogen loss from the vinyl group, an atomic hydrogen loss from the methyl group, and a methyl group elimination to form <i>cis</i>-/<i>trans</i>-1-propenyl-oxo-borane (CH₃CHCH¹¹BO), 3-propenyl-oxo-borane (CH₂CHCH₂¹¹BO), and ethenyl-oxo-borane (CH₂CH¹¹BO), respectively. Utilizing partially deuterated propylene (CD₃CHCH₂ and CH₃CDCD₂), we reveal that the loss of a vinyl hydrogen atom is the dominant hydrogen elimination pathway (85 ± 10%) forming <i>cis</i>-/<i>trans</i>-1-propenyl-oxo-borane, compared to the loss of a methyl hydrogen atom (15 ± 10%) leading to 3-propenyl-oxo-borane. The branching ratios for an atomic hydrogen loss from the vinyl group, an atomic hydrogen loss from the methyl group, and a methyl group loss are experimentally derived to be 26 ± 8%:5 ± 3%:69 ± 15%, respectively; these data correlate nicely with the branching ratios calculated via RRKM theory of 19%:5%:75%, respectively

Combined Crossed Molecular Beam and Ab Initio Investigation of the Reaction of Boron Monoxide (BO; X²Σ⁺) with 1,3-Butadiene (CH₂CHCHCH₂; X¹A_g) and Its Deuterated Counterparts

The reactions of the boron monoxide (¹¹BO; X²Σ⁺) radical with 1,3-butadiene (CH₂CHCHCH₂; X¹A_g) and its partially deuterated counterparts, 1,3-butadiene-<i>d</i>₂ (CH₂CDCDCH₂; X¹A_g) and 1,3-butadiene-<i>d</i>₄ (CD₂CHCHCD₂; X¹A_g), were investigated under single collision conditions exploiting a crossed molecular beams machine. The experimental data were combined with the state-of-the-art ab initio electronic structure calculations and statistical RRKM calculations to investigate the underlying chemical reaction dynamics and reaction mechanisms computationally. Our investigations revealed that the reaction followed indirect scattering dynamics through the formation of ¹¹BOC₄H₆ doublet radical intermediates via the barrierless addition of the ¹¹BO radical to the terminal carbon atom (C1/C4) and/or the central carbon atom (C2/C3) of 1,3-butadiene. The resulting long-lived ¹¹BOC₄H₆ intermediate(s) underwent isomerization and/or unimolecular decomposition involving eventually at least two distinct atomic hydrogen loss pathways to 1,3-butadienyl-1-oxoboranes (CH₂CHCHCH¹¹BO) and 1,3-butadienyl-2-oxoboranes (CH₂C (¹¹BO)­CHCH₂) in overall exoergic reactions via tight exit transition states. Utilizing partially deuterated 1,3-butadiene-<i>d</i>₂ and -<i>d</i>₄, we revealed that the hydrogen loss from the methylene moiety (CH₂) dominated with 70 ± 10% compared to an atomic hydrogen loss from the methylidyne group (CH) of only 30 ± 10%; these data agree nicely with the theoretically predicted branching ratio of 80% versus 19%