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1. Rotation‐Inversion Isomerization of Tertiary Carbamates: Potential Energy Surface Analysis of Multi‐Paths Isomerization Using Boltzmann Statistics

   https://doi.org/10.1002/cphc.202200893  

   Jameson, Brian; Glaser, Rainer (January 2023, ChemPhysChem)

   Abstract The front cover artwork is provided by Prof. Rainer Glaser's group at the Missouri University of Science and Technology. The image shows one of four potential energy surfaces generated from our rotation‐inversion study of tertiary carbamates and highlights two of the eight possible transition state pathways between two ensembles of E‐ and Z‐minima. In the context of synthetic studies of fluorinated carbamates R₁O−CO−N(R₂)CH₂CF₃, we unexpectedly observed two sets of ¹³C NMR quartets for the CF₃ group and we needed to understand their origin. Read the full text of the Research Article at 10.1002/cphc.2022005442. 

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2. Isomerization-induced enhancement of luminescence in Au ₂₈ (SR) ₂₀ nanoclusters

   https://doi.org/10.1039/d0sc01270j  

   Chen, Yuxiang; Zhou, Meng; Li, Qi; Gronlund, Harrison; Jin, Rongchao (August 2020, Chemical Science)

   null (Ed.)

   Understanding the origin and structural basis of the photoluminescence (PL) phenomenon in thiolate-protected metal nanoclusters is of paramount importance for both fundamental science and practical applications. It remains a major challenge to correlate the PL properties with the atomic-level structure due to the complex interplay of the metal core ( i.e. the inner kernel) and the exterior shell ( i.e. surface Au( i )-thiolate staple motifs). Decoupling these two intertwined structural factors is critical in order to understand the PL origin. Herein, we utilize two Au 28 (SR) 20 nanoclusters with different –R groups, which possess the same core but different shell structures and thus provide an ideal system for the PL study. We discover that the Au 28 (CHT) 20 (CHT: cyclohexanethiolate) nanocluster exhibits a more than 15-fold higher PL quantum yield than the Au 28 (TBBT) 20 nanocluster (TBBT: p-tert -butylbenzenethiolate). Such an enhancement is found to originate from the different structural arrangement of the staple motifs in the shell, which modifies the electron relaxation dynamics in the inner core to different extents for the two nanoclusters. The emergence of a long PL lifetime component in the more emissive Au 28 (CHT) 20 nanocluster reveals that its PL is enhanced by suppressing the nonradiative pathway. The presence of long, interlocked staple motifs is further identified as a key structural parameter that favors the luminescence. Overall, this work offers structural insights into the PL origin in Au 28 (SR) 20 nanoclusters and provides some guidelines for designing luminescent metal nanoclusters for sensing and optoelectronic applications. 

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3. Rotation‐Inversion Isomerization of Tertiary Carbamates: Potential Energy Surface Analysis of Multi‐Paths Isomerization Using Boltzmann Statistics

   https://doi.org/10.1002/cphc.202200442  

   Jameson, Brian; Glaser, Rainer (October 2022, ChemPhysChem)

   Abstract Potential energy surface (PES) analyses at the SMD[MP2/6–311++G(d,p)] level and higher‐level energies up to MP4(fc,SDTQ) are reported for the fluorinated tertiary carbamateN‐ethyl‐N‐(2,2,2‐trifluoroethyl) methyl carbamate (VII) and its parent systemN,N‐dimethyl methyl carbamate (VI). Emphasis is placed on the analysis of the rotational barrier about the CN carbamate bond and its interplay with the hybridization of theN‐lone pair (NLP). All rotational transition state (TS) structures were found by computation of 1D relaxed rotational profiles but only 2D PES scans revealed the rotation‐inversion paths in a compelling fashion. We found four unique chiral minima ofVII, one pair each ofE‐andZ‐rotamers, and we determined theeightunique rotational TS structures associated with every possibleE/Z‐isomerization path. It is a significant finding that all TS structures featureN‐pyramidalization whereas the minima essentially contain sp²‐hybridized nitrogen. We will show that the TS stabilities are affected by the synergetic interplay between NLP/CO₂repulsion minimization, NLP→σ^*(CO) negative hyperconjugation, and two modes of intramolecular through‐space electrostatic stabilization. We demonstrate how Boltzmann statistics must be applied to determine the predicted experimental rotational barrier based on the energetics of all eight rotamerization pathways. The computed barrier forVIIis in complete agreement with the experimentally measured barrier of the very similar fluorinated carbamateN‐Boc‐N‐(2,2,2‐trifluoroethyl)‐4‐aminobutan‐1‐olII. NMR properties ofVIIwere calculated with a variety of density functional/basis set combinations and Boltzmann averaging over theE‐andZ‐rotamers at our best theoretical level results in good agreement with experimental chemical shifts δ(¹³C) andJ(¹³C,¹⁹F) coupling constants ofII(within 6 %). 

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4. Steric and electronic control of an ultrafast isomerization

   https://doi.org/10.1039/c9sc02359c  

   Porter, Tyler M.; Ostericher, Andrew L.; Kubiak, Clifford P. (August 2019, Chemical Science)

   Synthetic control of the influence of steric and electronic factors on the ultrafast (picosecond) isomerization of penta-coordinate ruthenium dithietene complexes (Ru((CF 3 ) 2 C 2 S 2 )(CO)(L) 2 , where L = a monodentate phosphine ligand) is reported. Seven new ruthenium dithietene complexes were prepared and characterized by single crystal X-ray diffraction. The complexes are all square pyramidal and differ only in the axial vs. equatorial coordination of the carbonyl ligand. Fourier Transform Infrared (FTIR) spectroscopy was used to study the ν (CO) bandshapes of the complexes in solution, and these reveal rapid exchange between two or three isomers of each complex. Isomerization is proposed to follow a Berry psuedorotation-like mechanism where a metastable, trigonal bipyramidal (TBP) intermediate is observed spectroscopically. Electronic tuning of the phosphine ligands L = PPh 3 , P(( p -Me)Ph) 3 , (( p -Cl)Ph) 3 , at constant cone angle is found to have little effect on the kinetics or thermodynamic stabilities of the axial, equatorial and TBP isomers of the differently substituted complexes. Steric tuning of the phosphine ligands over a range of phosphine cone angles (135 < θ < 165°) has a profound impact on the isomerization process, and in the limit of greatest steric bulk, the axial isomer is not observable. Temperature dependence of the FTIR spectra was used to obtain the relative thermodynamic stabilities of the different isomers of each of the seven ruthenium dithietene complexes. This study details how ligand steric effects can be used to direct the solution state dynamics on the picosecond time scale of discrete isomers energetically separated by <2.2 kcal mol −1 . This work provides the most detailed description to date of ultrafast isomerization in the ground states of transition metal complexes. 

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5. Double-duty isomerases: a case study of isomerization-coupled enzymatic catalysis

   https://doi.org/10.1016/j.tibs.2024.04.007  

   Solano, Yasmeen J; Kiser, Philip D (August 2024, Trends in Biochemical Sciences)

   Enzymes canusually be unambiguously assigned to one of seven classes specifying the basic chemistry of their catalyzed reactions. Less frequently, two or more reaction classes are catalyzed by a single enzyme within one active site. Two examples are an isomerohydrolase and an isomero-oxygenase that catalyze isomerization-coupled reactions crucial for production of visionsupporting 11-cis-retinoids. In these enzymes, isomerization is obligately paired and mechanistically intertwined with a second reaction class. A handful of other enzymes carrying out similarly coupled isomerization reactions have been described, some ofwhich havebeensubjectedtodetailedstructure–function analyses. Herein we reviewtheserarefied enzymes,focusingonthe mechanisticand structural basis of their reaction coupling with the goal of revealing catalytic commonalities. 

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