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1. Aluminium( iii ) dialkyl 2,6-bisimino-4 R -dihydropyridinates(−1): selective synthesis, structure and controlled dimerization

   https://doi.org/10.1039/C9DT00847K  

   Gallardo-Villagrán, Manuel ; Vidal, Fernando ; Palma, Pilar ; Álvarez, Eleuterio ; Chen, Eugene Y.-X. ; Cámpora, Juan ; Rodríguez-Delgado, Antonio ( June 2019 , Dalton Transactions)

   A family of stable and otherwise selectively unachievable 2,6-bisimino-4- R -1,4-dihydropyridinate aluminium (III) dialkyl complexes [AlR' 2 (4-R- i PrBIPH)] (R = Bn, Allyl; R′ = Me, Et, i Bu) have been synthesized, taking advantage of a method for the preparation of the corresponding 4- R -1,4-dihydropiridine precursors developed in our group. All the dihydropyrdinate(−1) dialkyl aluminium complexes have been fully characterized by 1 H- 13 C-NMR, elemental analysis and in the case 2′a , also by X-ray diffraction studies. Upon heating in toluene solution at 110 °C, the dimethyl derivatives 2a and 2′a dimerize selectively through a double cycloaddition. This reaction leads to the formation of two new C–C bonds that involve the both meta positions of the two 4- R -1,4-dihydropyridinate fragments, resulting the binuclear aluminium species [Me 2 Al(4-R- i PrHBIP)] 2 (R = Bn ( 3a ); allyl ( 3′a )). Experimental kinetics showed that the dimerization of 2′a obeys second order rate with negative activation entropy, which is consistent with a bimolecular rate-determining step. Controlled methanolysis of both 3a and 3′a release the metal-free dimeric bases, (4-Bn- i PrHBIPH) 2 and (4-allyl- i PrHBIPH) 2 , providing a convenient route to these potentially useful ditopicmore »ligands. When the R′ groups are bulkier than Me ( 2b , 2′b and 2′c ), the dimerization is hindered or fully disabled, favoring the formation of paramagnetic NMR-silent species, which have been identified on the basis of a controlled methanolysis of the final organometallic products. Thus, when a toluene solution of [AlEt 2 (4-Bn- i PrBIPH)] ( 2b ) was heated at 110 °C, followed by the addition of methanol in excess, it yields a mixture of the dimer (4-Bn- i PrHBIPH) 2 and the aromatized base 4-Bn- i PrBIP, in ca . 1 : 2 ratio, indicating that the dimerization of 2b competes with its spontaneous dehydrogenation, yielding a paramagnetic complex containing a AlEt 2 unit and a non-innocent (4-Bn- i PrBIP) ˙− radical-anion ligand. Similar NMR monitoring experiments on the thermal behavior of [AlEt 2 (4-allyl- i PrBIPH)] ( 2′b ) and [Al i Bu 2 (4-allyl-iPrBIPH)] ( 2′c ) showed that these complexes do not dimerize, but afford exclusively NMR silent products. When such thermally treated samples were subjected to methanolysis, they resulted in mixtures of the alkylated 4-allyl- i PrBIP and non-alkylated i PrBIP ligand, suggesting that dehydrogenation and deallylation reactions take place competitively.« less
2. Single-molecule kinetic studies of DNA hybridization under extreme pressures

   https://doi.org/10.1039/D0CP04035E  

   Sung, Hsuan-Lei ; Nesbitt, David J. ( October 2020 , Physical Chemistry Chemical Physics)

   Hydrostatic pressure can perturb biomolecular function by altering equilibrium structures and folding dynamics. Its influences are particularly important to deep sea organisms, as maximum pressures reach ≈1100 bar at the bottom of the ocean as a result of the rapid increase in hydraulic pressure (1 bar every 10 meters) under water. In this work, DNA hybridization kinetics has been studied at the single molecule level with external, tunable pressure control ( P max ≈ 1500 bar), realized by incorporating a mechanical hydraulic capillary sample cell into a confocal fluorescence microscope. We find that the DNA hairpin construct promotes unfolding (“denatures”) with increasing pressure by simultaneously decelerating and accelerating the unimolecular rate constants for folding and unfolding, respectively. The single molecule kinetics is then investigated via pressure dependent van’t Hoff analysis to infer changes in the thermodynamic molar volume, which unambiguously reveals that the effective DNA plus solvent volume increases (Δ V 0 > 0) along the folding coordinate. Cation effects on the pressure dependent kinetics are also explored as a function of monovalent [Na + ]. In addition to stabilizing the overall DNA secondary structure, sodium ions at low concentrations are also found to weaken any pressure dependence for themore »folding kinetics, but with these effects quickly saturating at physiologically relevant levels of [Na + ]. In particular, the magnitudes of the activation volumes for the DNA dehybridization (Δ V ‡unfold) are significantly reduced with increasing [Na + ], suggesting that sodium cations help DNA adopt a more fold-like transition state configuration.« less
3. Intersystem crossing in tunneling regime: T ₁ → S ₀ relaxation in thiophosgene

   https://doi.org/10.1039/C9CP06956A  

   Lykhin, Aleksandr O. ; Varganov, Sergey A. ( March 2020 , Physical Chemistry Chemical Physics)

   The T 1 excited state relaxation in thiophosgene has attracted much attention as a relatively simple model for the intersystem crossing (ISC) transitions in polyatomic molecules. The very short (20–40 ps) T 1 lifetime predicted in several theoretical studies strongly disagrees with the experimental values (∼20 ns) indicating that the kinetics of T 1 → S 0 ISC is not well understood. We use the nonadiabatic transition state theory (NA-TST) with the Zhu–Nakamura transition probability and the multireference perturbation theory (CASPT2) to show that the T 1 → S 0 ISC occurs in the quantum tunneling regime. We also introduce a new zero-point vibrational energy correction scheme that improves the accuracy of the predicted ISC rate constants at low internal energies. The predicted lifetimes of the T 1 vibrational states are between one and two orders of magnitude larger than the experimental values. This overestimation is attributed to the multidimensional nature of quantum tunneling that facilitates ISC transitions along the non-minimum energy path and is not accounted for in the one-dimensional NA-TST.
4. Multi-instrument Comparative Study of Temperature, Number Density, and Emission Measure during the Precursor Phase of a Solar Flare

   https://doi.org/10.3847/1538-4357/ac6425  

   Liu, Nian ; Jing, Ju ; Xu, Yan ; Wang, Haimin ( May 2022 , The Astrophysical Journal)

   We present a multi-instrument study of the two precursor brightenings prior to the M6.5 flare (SOL2015-06-22T18:23) in the NOAA Active Region 12371, with a focus on the temperature (T), electron number density (n), and emission measure (EM). The data used in this study were obtained from four instruments with a variety of wavelengths, i.e., the Solar Dynamics Observatory’s Atmospheric Imaging Assembly (AIA), in six extreme ultraviolet (EUV) passbands; the Expanded Owens Valley Solar Array (EOVSA) in microwave (MW); the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in hard X-rays (HXR); and the Geostationary Operational Environmental Satellite (GOES) in soft X-rays (SXR). We compare the temporal variations ofT,n, and EM derived from the different data sets. Here are the key results. (1) GOES SXR and AIA EUV have almost identical EM variations (1.5–3 × 10⁴⁸cm⁻³) and very similarTvariations, from 8 to 15 million Kelvin (MK). (2) Listed from highest to lowest, EOVSA MW provides the highest temperature variations (15–60 MK), followed by RHESSI HXR (10–24 MK), then GOES SXR and AIA EUV (8–15 MK). (3) The EM variation from the RHESSI HXR measurements is always less than the values from AIA EUV and GOES SXR by at most 20more »times. The number density variation from EOVSA MW is greater than the value from AIA EUV by at most 100 times. The results quantitatively describe the differences in the thermal parameters at the precursor phase, as measured by different instruments operating at different wavelength regimes and for different emission mechanisms.

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5. Ultrahigh evaporative heat transfer measured locally in submicron water films

   https://doi.org/10.1038/s41598-022-26182-2  

   Wang, Xiaoman ; Ghaffarizadeh, S. Arman ; He, Xiao ; McGaughey, Alan J. H. ; Malen, Jonathan A. ( December 2022 , Scientific Reports)

   Thin film evaporation is a widely-used thermal management solution for micro/nano-devices with high energy densities. Local measurements of the evaporation rate at a liquid-vapor interface, however, are limited. We present a continuous profile of the evaporation heat transfer coefficient ($$h_{\text {evap}}$$$h_{\text{evap}}$) in the submicron thin film region of a water meniscus obtained through local measurements interpreted by a machine learned surrogate of the physical system. Frequency domain thermoreflectance (FDTR), a non-contact laser-based method with micrometer lateral resolution, is used to induce and measure the meniscus evaporation. A neural network is then trained using finite element simulations to extract the$$h_{\text {evap}}$$$h_{\text{evap}}$profile from the FDTR data. For a substrate superheat of 20 K, the maximum$$h_{\text {evap}}$$$h_{\text{evap}}$is$$1.0_{-0.3}^{+0.5}$$$1.0_{-0.3}^{+0.5}$ MW/$$\text {m}^2$$${\text{m}}^2$-K at a film thickness of$$15_{-3}^{+29}$$${15}_{-3}^{+29}$ nm. This ultrahigh$$h_{\text {evap}}$$$h_{\text{evap}}$value is two orders of magnitude larger than the heat transfer coefficient for single-phase forced convection or evaporation from a bulk liquid. Under the assumption of constant wall temperature, our profiles of$$h_{\text {evap}}$$$h_{\text{evap}}$and meniscus thickness suggest that 62% of the heat transfer comes from the region lying 0.1–1 μm from the meniscus edge, whereas just 29% comes from the next 100 μm.