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1. Chiral π-Conjugated Double Helical Aminyl Diradical with the Triplet Ground State

   https://doi.org/10.1055/a-2379-9406  

   Guo, Haoxin; Rajca, Andrzej (February 2025, Synlett)

   Abstract We describe effective development of the highly diastereoselective synthesis of double helical tetraamine 2-H2-C2 and propose a mechanism for its formation. The resolution of 2-H2-C2 is facilitated by a high racemization barrier of 43 kcal mol–1 and it is implemented via either a chiral auxiliary or preparative supercritical fluid chromatography. This enables preparation of the first high-spin neutral diradical, with spin density delocalized within an enantiomeric double helical π-system. The presence of two effective 3-electron C–N bonds in the diradical leads to: (1) the triplet (S = 1) high-spin ground state with a singlet-triplet energy gap of 0.4 kcal mol–1 and (2) the long half-life of up to 6 days in 2-MeTHF at room temperature. The diradical possesses a racemization barrier of at least 26 kcal mol–1 in 2-MeTHF at 293 K and chiroptical properties, with an absorption anisotropy factor |g| ≈ 0.005 at 548 nm. These unique magnetic and optical properties of our diradical form the basis for the development of next-generation spintronic devices. 1 Introduction 2 Synthesis and Resolution of the C 2-Symmetric Double Helical Tetraamine 2-H2-C 2 3 Synthesis and Characterization of Neutral High-Spin Aminyl Diradical 22• -C 2 4 Conclusion 

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2. Perfluoroolefin complexes versus perfluorometallacycles and perfluorocarbene complexes in cyclopentadienylcobalt chemistry

   https://doi.org/10.1039/C9CP06685C  

   Wen, Limei; Li, Guoliang; Xie, Yaoming; King, R. Bruce; Schaefer, Henry F. (April 2020, Physical Chemistry Chemical Physics)

   Fluorocarbons have been shown experimentally by Baker and coworkers to combine with the cyclopentadienylcobalt (CpCo) moiety to form fluoroolefin and fluorocarbene complexes as well as fluorinated cobaltacyclic rings. In this connection density functional theory (DFT) studies on the cyclopentadienylcobalt fluorocarbon complexes CpCo(L)(C n F 2n ) (L = CO, PMe 3 ; n = 3 and 4) indicate structures with perfluoroolefin ligands to be the lowest energy structures followed by perfluorometallacycle structures and finally by structures with perfluorocarbene ligands. Thus, for the CpCo(L)(C 3 F 6 ) (L = CO, PMe 3 ) complexes, the perfluoropropene structure has the lowest energy, followed by the perfluorocobaltacyclobutane structure and the perfluoroisopropylidene structure less stable by 8 to 11 kcal mol −1 , and the highest energy perfluoropropylidene structure less stable by more than 12 kcal mol −1 . For the two metal carbene structures Cp(L)CoC(CF 3 ) 2 and Cp(L)CoCF(C 2 F 5 ), the former is more stable than the latter, even though the latter has Fischer carbene character. For the CpCo(L)(C 4 F 8 ) (L = CO, PMe 3 ) complexes, the perfluoroolefin complex structures have the lowest energies, followed by the perfluorometallacycle structures at 10 to 20 kcal mol −1 , and the structures with perfluorocarbene ligands at yet higher energies more than 20 kcal mol −1 above the lowest energy structure. This is consistent with the experimentally observed isomerization of the perfluorinated cobaltacyclobutane complexes CpCo(PPh 2 Me)(–CFR–CF 2 –CF 2 –) (R = F, CF 3 ) to the perfluoroolefin complexes CpCo(PPh 2 Me)(RCFCF 2 ) in the presence of catalytic quantities of HN(SO 2 CF 3 ) 2 . Further refinement of the relative energies by the state-of-the-art DLPNO-CCSD(T) method gives results essentially consistent with the DFT results summarized above. 

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3. Formation and detection of metastable formic acid in a supersonic expansion: High resolution infrared spectroscopy of the jet-cooled cis -HCOOH conformer

   https://doi.org/10.1063/5.0093401  

   Doney, Kirstin D.; Kortyna, Andrew; Chan, Ya-Chu; Nesbitt, David J. (May 2022, The Journal of Chemical Physics)

   High-resolution direct absorption infrared spectra of metastable cis-formic acid (HCOOH) trapped in a cis-well resonance behind a 15 kcal/mol barrier are reported for the first time, with the energetically unstable conformer produced in a supersonic slit plasma expansion of trans-formic acid/H 2 mixtures. We present a detailed high-resolution rovibrational analysis for cis-formic acid species in the OH stretch ( ν 1 ) fundamental, providing first precision vibrational band origin, rotational constants, and term values, which in conjunction with ab initio calculations at the couple-cluster with single, double, and perturbative triple [CCSD(T)]/ANOn (n = 0, 1, 2) level support the experimental assignments and establish critical points on the potential energy surface for internal rotor trans-to-cis isomerization. Relative intensities for a- and b-type transitions observed in the spectra permit the transition dipole moment components to be determined in the body fixed frame and prove to be in good agreement with ab initio CCSD(T) theoretical estimates but in poor agreement with simple bond-dipole predictions. The observed signal dependence on H 2 in the discharge suggests the presence of a novel H atom radical chemical mechanism for strongly endothermic “up-hill” internal rotor isomerization between trans- and cis-formic acid conformers. 

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4. Direct observation of the intermediate in an ultrafast isomerization

   https://doi.org/10.1039/c8sc03258k  

   Porter, Tyler M.; Wang, Jiaxi; Li, Yingmin; Xiang, Bo; Salsman, Catherine; Miller, Joel S.; Xiong, Wei; Kubiak, Clifford P. (January 2019, Chemical Science)

   Using a combination of two-dimensional infrared (2D IR) and variable temperature Fourier transform infrared (FTIR) spectroscopies the rapid structural isomerization of a five-coordinate ruthenium complex is investigated. In methylene chloride, three exchanging isomers were observed: (1) square pyramidal equatorial, ( 1 ); (2) trigonal bipyramidal, ( 0 ); and (3) square pyramidal apical, ( 2 ). Exchange between 1 and 0 was found to be an endergonic process (Δ H = 0.84 (0.08) kcal mol −1 , Δ S = 0.6 (0.4) eu) with an isomerization time constant of 4.3 (1.5) picoseconds (ps, 10 −12 s). Exchange between 0 and 2 however was found to be exergonic (Δ H = −2.18 (0.06) kcal mol −1 , Δ S = −5.3 (0.3) eu) and rate limiting with an isomerization time constant of 6.3 (1.6) ps. The trigonal bipyramidal complex was found to be an intermediate, with an activation barrier of 2.2 (0.2) kcal mol −1 and 2.4 (0.2) kcal mol −1 relative to the equatorial and apical square pyramidal isomers respectively. This study provides direct validation of the mechanism of Berry pseudorotation – the pairwise exchange of ligands in a five-coordinate complex – a process that was first described over fifty years ago. This study also clearly demonstrates that the rate of pseudorotation approaches the frequency of molecular vibrations. 

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5. Reaction mechanism, energetics, and kinetics of the water-assisted thioformaldehyde + ˙OH reaction and the fate of its product radical under tropospheric conditions

   https://doi.org/10.1039/D0CP00570C  

   Arathala, Parandaman; Katz, Mark; Musah, Rabi A. (May 2020, Physical Chemistry Chemical Physics)

   The reactions of thioformaldehyde (H 2 CS) with OH radicals and assisted by a single water molecule have been investigated using high level ab initio quantum chemistry calculations. The H 2 CS + ˙OH reaction can in principle proceed through: (1) abstraction, and (2) addition pathways. The barrier height for the addition reaction in the absence of a catalyst was found to be −0.8 kcal mol −1 , relative to the separated reactants, which has a ∼1.0 kcal mol −1 lower barrier than the abstraction channel. The H 2 CS + ˙OH reaction assisted by a single water molecule reduces the barrier heights significantly for both the addition and abstraction channels, to −5.5 and −6.7 kcal mol −1 respectively, compared to the un-catalyzed H 2 CS + ˙OH reaction. These values suggest that water lowers the barriers by ∼6.0 kcal mol −1 for both reaction paths. The rate constants for the H 2 CS⋯H 2 O + ˙OH and OH⋯H 2 O + H 2 CS bimolecular reaction channels were calculated using Canonical Variational Transition state theory (CVT) in conjunction with the Small Curvature Tunneling (SCT) method over the atmospherically relevant temperatures between 200 and 400 K. Rate constants for the H 2 CS + ˙OH reaction paths for comparison with the H 2 CS + ˙OH + H 2 O reaction in the same temperature range were also computed. The results suggest that the rate of the H 2 CS + ˙OH + H 2 O reaction is slower than that of the H 2 CS + ˙OH reaction by ∼1–4 orders of magnitude in the temperatures between 200 and 400 K. For example, at 300 K, the rates of the H 2 CS + ˙OH + H 2 O and H 2 CS + ˙OH reactions were found to be 2.2 × 10 −8 s −1 and 6.4 × 10 −6 s −1 , respectively, calculated using [OH] = 1.0 × 10 6 molecules cm −3 , and [H 2 O] = 8.2 × 10 17 molecules cm −3 (300 K, RH 100%) atmospheric conditions. Electronic structure calculations on the H 2 C(OH)S˙ product in the presence of 3 O 2 were also performed. The results show that H 2 CS is removed from the atmosphere primarily by reacting with ˙OH and O 2 to form thioformic acid, HO 2 , formaldehyde, and SO 2 as the main end products. 

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