Seung Koo Shin

Rules and trends of metal cation driven hydride-transfer mechanisms in metal amidoboranes.

Group I and II metal amidoboranes have been identified as one of the promising families of materials for efficient H(2) storage. However, the underlying mechanism of the dehydrogenation of these materials is not well understood. Thus, the mechanisms and kinetics of H(2) release in metal amidoboranes are investigated using high level ab initio calculations and kinetic simulations. The metal plays the role of catalyst for the hydride transfer with formation of a metal hydride intermediate towards the dehydrogenation. In this process, with increasing ionic character of the metal hydride bond in the intermediate, the stability of the intermediate decreases, while the dehydrogenation process involving ionic recombination of the hydridic H with the protic H proceeds with a reduced barrier. Such correlations lead directly to a U-shaped relationship between the activation energy barrier for H(2) elimination and the ionicity of metal hydride bond. Oligomerized intermediates are formed by the chain reaction of the size-driven catalytic effects of metals, competing with the non-oligomerization pathway. The kinetic rates at low temperatures are determined by the maximum barrier height in the pathway (a Lambda-shaped relation), while those at moderately high temperatures are determined by most of multiple-barriers. This requires kinetic simulations. At the operating temperatures of proton exchange membrane fuel cells, the metal amidoboranes with lithium and sodium release H(2) along both oligomerization and non-oligomerization paths. The sodium amidoboranes show the most accelerated rates, while others release H(2) at similar rates. In addition, we predict that the novel metal amidoborane-based adducts and mixtures would release H(2) with accelerated rates as well as with enhanced reversibility. This comprehensive study is useful for further developments of active metal-based better hydrogen storage materials.

Photoinitiated H‐ and D‐atom reactions with N2O in the gas phase and in N2O–HI and N2O–DI complexes

Reactions of H atoms with N2O have two product channels yielding NH+NO and OH+N2. Both channels were observed via NH A 3Π←X 3∑ and OH A 2∑←X 2Π laser‐induced fluorescence spectra. Photoinitiated reactions with N2O–HI complexes yield a much lower [NH]/[OH] ratio than under the corresponding bulk conditions at the same photolysis wavelength. For hot D‐atom reactions with N2O, this effect is somewhat more pronounced. These results can be interpreted in terms of entrance channel geometric specificity, namely, biasing hydrogen attack toward the oxygen. Another striking observation is that the OH and OD rotational level distributions (RLD) obtained under bulk conditions differ markedly from those obtained under complexed conditions, while the NH as well as the ND RLD are similar for the two environments. In addition, OH Doppler profiles change considerably in going from bulk to complexed conditions, while such an effect is not observed for NH. The changes observed with the OH RLD are most likely due to OH–halog...

Singlet–triplet energy gaps in fluorine‐substituted methylenes and silylenes

We report singlet and triplet state splittings (ΔE_(ST)) for fluorine‐substituted methylenes and silylenes using dissociation‐consistent configuration interaction (CI) (based on generalized valence bond wave functions). These relatively simple CI calculations emphasize correlation consistency between the singlet and triplet states. Values of ΔE_(ST) for CH_2, CF_2, SiH_2, and SiF_2 are in excellent agreement with available experimental results, and we expect the predictions for the other cases CHF (14.5) and SiHF (41.3) to be equally accurate. This result strongly suggests that the correct choice among the experimental values for ΔE_(ST) of CHF is 14.7±0.2 kcal/mol.

Surface-dependent, ligand-mediated photochemical etching of CdSe nanoplatelets.

Photochemical etching of CdSe nanoplatelets was studied to establish a relationship between the nanocrystal surface and the photochemical activity of an exciton. Nanoplatelets were synthesized in a mixture of octylamine and oleylamine for the wurtzite (W) lattice or in octadecene containing oleic acid for the zinc-blende (ZB) lattice. For photochemical etching, nanoplatelets were dispersed in chloroform containing oleylamine and tributylphosphine in the absence or presence of oleic acid and then irradiated with light at the band-edge absorption maxima. Etching phenomena were characterized using UV-vis absorption spectroscopy and transmission electron microscopy. The absorption spectra of both W and ZB CdSe nanoplatelets showed that the exciton was confined in one dimension along the thickness. However, the two nanoplatelets presented different etching kinetics and erosion patterns. The rate of etching for W CdSe nanoplatelets was much faster than that for ZB nanoplatelets. Small holes were uniformly perforated on the planar surface of W nanoplatelets, whereas the corners and edges of ZB nanoplatelets were massively eroded without a significant perforation on the planar surface. This suggests that the amine-passivated surface of trivalent cadmium atoms on CdSe nanoplatelets is photochemically active, but the carboxylate-passivated surface of divalent cadmium atoms is not. Hence, the ligand, which induces the growth of W or ZB CdSe nanoplatelets, mediates the surface-dependent photochemical etching. This result implies that an electron-hole pair can be extracted from the planar surface of amine-passivated W nanoplatelets but from the corners and edges of carboxylate-passivated ZB nanoplatelets.

AB INITIO POTENTIAL ENERGY SURFACE AND ROVIBRATIONAL ENERGIES OF AR...CO

The potential energy surface for the Ar...CO van der Waals complex is calculated by the supermolecular approach using fourth‐order Mo/ller–Plesset perturbation theory (MP4) with a large basis set containing bond functions. The Hartree–Fock potentials are repulsive for all configurations considered. The second‐order correlation energy accounts for most of the dispersion interactions. The MP4 potential energy surface is characterized by a global minimum of −96.3 cm−1 at Re=3.743 A and θe=98° with the argon atom closer to the oxygen end. There are no local minima in the linear configurations. The linear configurations provide shallow barriers at both of the carbon and oxygen ends. The barrier height at the oxygen end is 13.6 cm−1 at R=4.04 A, while that at the carbon end is 28.0 cm−1 at R=4.58 A. The rovibrational energies of Ar...CO are calculated by the discrete variable representation method. The Ar...CO complex undergoes large amplitude hindered rotations in the ground state with a zero‐point energy of 2...

Calculated rotational spectrum of Ar...CO from an ab initio potential energy surface: A very floppy van der Waals molecule

Using the ab initio potential of Shin et al. (to be published), we have calculated the bound states and infrared absorption spectrum of the van der Waals complex Ar...CO. The results show that Ar...CO cannot be treated as a quasirigid rotor, nor as a molecule with a free internal rotor. In particular, a transition to the first excited van der Waals bending level is predicted to be present in the spectrum, and its frequency varies with Ω (the projection quantum number of the total angular momentum onto the intermolecular axis going from the center of mass of CO to the Ar atom). It is also shown that, although the spectrum cannot be analyzed by the use of a rigid rotor model, rotational ‘‘constants’’ can still be defined for each value of Ω. This is consistent with the available experimental data and the predicted bending excitation can account for unassigned transitions in the infrared spectrum of this complex. Finally, a sensitivity analysis of the calculated spectrum with respect to the potential anisotr...

Mass spectrometric studies of alkali metal ion binding on thrombin-binding aptamer DNA.

The binding sites and consecutive binding constants of alkali metal ions, (M+ = Na+, K+, Rb+, and Cs+), to thrombin-binding aptamer (TBA) DNA were studied by Fourier-transform ion cyclotron resonance spectrometry. TBA-metal complexes were produced by electrospray ionization (ESI) and the ions of interest were mass-selected for further characterization. The structural motif of TBA in an ESI solution was checked by circular dichroism. The metal-binding constants and sites were determined by the titration method and infrared multiphoton dissociation (IRMPD), respectively. The binding constant of potassium is 5–8 times greater than those of other alkali metal ions, and the potassium binding site is different from other metal binding sites. In the 1:1 TBA-metal complex, potassium is coordinated between the bottom G-quartet and two adjacent TT loops of TBA. In the 1:2 TBA—metal complex, the second potassium ion binds at the TGT loop of TBA, which is in line with the antiparallel G-quadruplex structure of TBA. On the other hand, other alkali metal ions bind at the lateral TGT loop in both 1:1 and 1:2 complexes, presumably due to the formation of ion-pair adducts. IRMPD studies of the binding sites in combination with measurements of the consecutive binding constants help elucidate the binding modes of alkali metal ions on DNA aptamer at the molecular level.

The TGL2 Gene of Saccharomyces cerevisiae Encodes an Active Acylglycerol Lipase Located in the Mitochondria

The Saccharomyces cerevisiae Tgl2 protein shows sequence homology to Pseudomonas triacylglycerol (TAG) lipases, but its role in the yeast lipid metabolism is not known. Using hemagglutinin-tagged Tgl2p purified from yeast, we report that this protein carries a significant lipolytic activity toward long-chain TAG. Importantly, mutant hemagglutinin-Tgl2pS144A, which contains alanine 144 in place of serine 144 in the lipase consensus sequence (G/A)XSXG exhibits no such activity. Although cellular TAG hydrolysis is reduced in the tgl2 deletion mutant, overproduction of Tgl2p in this mutant leads to an increase in TAG degradation in the presence of fatty acid synthesis inhibitor cerulenin, but that of Tgl2pS144A does not. This result demonstrates the lipolytic function of Tgl2p in yeast. Although other yeast TAG lipases are localized to lipid particles, Tgl2p is enriched in the mitochondria. The mitochondrial fraction purified from the TGL2-overexpressing yeast shows a strong lipolytic activity, which was absent in the tgl2 deletion mutant. Therefore, we conclude that Tgl2p is a functional lipase of the yeast mitochondria. By analyzing phenotypic effects of TGL2-deficient yeast, we also find that lipolysis-competent Tgl2p is required for the viability of cells treated with antimitotic drug. The addition of oleic acid, the product of Tgl2p-catalyzed lipolysis, fully complements the antimitotic drug sensitivity of the tgl2 null mutation. Thus, we propose that the mitochondrial Tgl2p-dependent lipolysis is crucial for the survival of cells under antimitotic drug treatment.

Entrance Channel Stereospecificity of Photoinitiated H-Atom Reactions in Weakly Bonded Complexes

Hot H-atom reactions photoinitiated in T-shaped C02_HBr and nearly-linear C02-HC1 complexes show remarkably different reaction probabilities. Broadside H-atom approaches in C02~HBr complexes are greatly favoured over the relatively endon approaches of C02~HC1 complexes, a striking steric effect. Photoinitiated hot H-atom reactions with N20 result in a much lower [NH]/[OH] ratio with N20 _HI complexes than under single-collision conditions at the same photolysis wavelength. In addition, OH rotational distributions differ markedly between bulk and complexed conditions, while NH rotational distributions are similar. These results can be interpreted in terms of entrance channel stereospecificity influencing chemically distinct product channels.

Photoionization mass spectrometric studies of the methylsilanes Si(CH3)nH4−n (n=0-3)

Abstract Photoionization efficiency curves for the low energy fragment ions (M - H)+, (M - H2)+, (M - CH3)+, and (M - CH4)+ for the series of methyl substituted silanes Si(CH3)nH4−n (n= 0-3) are reported. The molecular ions were undetectable except SiH+4. (M - H2)+ and (M - CH4)+ ions show sharp appearance onsets compared with (M - H)+ ions, which have distinct threshold curvature. (M - H2)+ ions are ascribed to silylene positive ions, SiR+2. The alternative silaethylene positive ion structure, CH2SiHR+, is unlikely because the fragmentation process yielding CH2SiD+2 with loss of HD is not observed in the photoionization mass spectrum of CH3SiD3. Thresholds are interpreted in terms of the thermochemistry of the various ionic and neutral silicon species and afford accurate calculation of hydride affinities of the silylene positive ions. The calculated hydride affinities for silylene positive ions are 263.4, 244.3 and 230.6 kcal mol−1 for SiH+2, SiMeH+ and SiMe+2, respectively. Within experimental error, the hydride affinities of the silylene positive ions are identical to those of the silicenium ions with the same number of methyl groups. The present results, combined with other available thermochemical data, lead to the critical assessment of the SiH and SiCH3 bond energies of the neutral and ionic fragments of methylsilanes, as well as ionization potentials of they silyl radicals and silylenes.