Cheng-Liang Huang

Vibrational levels of the transition state and rate of dissociation of triplet acetaldehyde

Fluorescence decay of the S1 state of d4-acetaldehyde is measured. Below the dissociation threshold for formation of fragments of formyl and methyl radicals, single exponential decays of fluorescence are observed. Biexponential decay is observed when the excitation energies are near and above the dissociation threshold. Hence, in this region the mechanism for intramolecular relaxation of energy reaches the “intermediate case.” Strong coupling between S1 and T1 states or reversible reaction for S1↔T1 results in this biexponential behavior in the fluorescence decay of S1. Rates of appearance of DCO from dissociation of acetaldehyde are measured. The stepwise increases in plots of both rate of appearance of formyl radicals and rate of decay of excited acetaldehyde versus excitation energy for both isotopic variants of acetaldehyde are observed. According to fits to Rice–Ramsperger–Kassel–Marcus (RRKM) theory, these sharp increases correspond to vibrational levels of transition state in the triplet surface. F...

Ring opening dissociation of d6-benzene

The photodissociation of jet-cooled d6-benzene at 193 nm was studied. Photofragment translational energy distributions were measured by multimass ion imaging techniques. In addition to the major channel of D atom elimination from one-photon excitation, elimination of two D atoms and two ring opening dissociation channels, C6D6→CD3+C5D3 and C6D6→C2D3+C4D3, resulting from two-photon dissociation was observed. These ring opening channels were confirmed by the momentum match between the two fragments in each channel.

Photodissociation of ethylbenzene and n-propylbenzene in a molecular beam

The photodissociation of jet-cooled ethylbenzene and n-propylbenzene at both 193 and 248 nm was studied using vacuum ultraviolet photoionization/multimass ion imaging techniques. The photofragment translational energy distributions from both the molecules obtained at 193 nm show that the probability of portioning energy to product translational energy decreases monotonically with increasing translational energy. They indicate that the dissociation occurs from the ground electronic state. However, the photofragment translational energy distributions from both molecules obtained at 248 nm contain a fast and a slow component. 75% of ethylbenzene and 80% of n-propylbenzene following the 248 nm photoexcitation dissociate from electronic excited state, resulting in the fast component. The remaining 25% of ethylbenzene and 20% of n-propylbenzene dissociate through the ground electronic state, giving rise to the slow component. A comparison with an ab initio calculation suggests that the dissociation from the first triplet state corresponds to the fast component in translational energy distribution.

Time‐Dependent Surface Plasmon Resonance Spectroscopy of Silver Nanoprisms in the Presence of Halide Ions

Herein, we find that the surface plasmon resonance (SPR) spectra of silver nanoprisms in the presence of halide ions change gradually with reaction time. The changes in the spectra correspond to the shape transformation of silver nanoprisms. There are threshold concentrations of halide ions that initiate the shape-transformation reaction. The threshold concentrations for Cl(-), Br(-), and I(-) are about 3x10(-4) M, 1x10(-6) M, and 1.5x10(-6) M, respectively. Any concentrations of the added halide ions above these thresholds can eventually etch the silver nanoprisms into nanodisks if the reaction time is long enough. The higher the concentration of the halide ions, the higher the etching rate will be. The kinetics of the shape transformation of the silver nanoprisms can be studied by recording their time-dependent surface plasmon resonance (SPR) spectra on a commercial UV/Vis-NIR spectrometer. The peak positions of in-plane dipole SPR bands of silver colloids in the presence of chloride and bromide ions can be fitted very well with the biexponential functions. We propose that the fast components of the biexponential behaviors should correlate to the truncating effect on the corners of silver nanoprisms, and the slow component should correlate to the redeposition of the truncated residues onto the basal plane of the nanoplates.

Photodissociation of ethylbenzene at 248 nm

Photodissociation of jet-cooled ethylbenzene at 248 nm was studied using VUV photoionization/multimass ion imaging techniques. The photofragment translational energy distribution obtained at 248 nm showed that after the excitation 75% of the ethylbenzene molecules dissociate from electronic excited state, and the rest 25% of the molecules dissociate through a hot molecule mechanism. This is the first experimental evidence which proves that the dissociation of alkyl-substituted benzenes can occur not only from hot molecule mechanism in this UV region.

Individual graphene oxide platelets through direct molecular exfoliation with globular amphiphilic hyperbranched polymers

In this study, we used a solution process, involving direct molecular exfoliation of graphite oxide (GO) stacked layers, to obtain individual graphene oxide platelets (GOPs) through amphiphilic hyperbranched polymers (HPs). The HPs, based on poly(amic acid) and polyimide with terminal amino functionalities, were obtained through a facile “A2 + B3” synthesis; gel permeation chromatography of the HPs revealed molecular weights of 5000–8200 g mol−1 with a distribution of polydispersity between 2.0 and 4.2. Next, we inserted these globular HPs into the interlayer spaces of GO to prepare a series of GO/HP nanohybrids possessing intercalated and exfoliated morphologies. The intercalation processes featured a critical transition point at which a dramatic enlargement occurred to the interlayer spacing, depending on the content of the bulky embedded HPs. From transmission electron microscopy images, we observed transparent folding of the GOPs; powder X-ray diffraction analysis revealed individual nanosheets with well-defined diffraction patterns. Incorporating the bulky three-dimensional globular structure into the layered GO significantly influenced the solution exfoliation, allowing us to examine the intercalating behavior of GO intergallery. This solution-phase methodology, through direct HP molecular exfoliation, provides the way toward obtaining individual nanosheets, opening up opportunities for platelet-like nanographene oxide materials in many technological applications.

State-resolved dissociation dynamics of triplet acetaldehyde near the dissociation threshold to form CH3+HCO

We studied the state-resolved dynamics of S1 acetaldehyde to product channels with quantum-beat spectroscopy. Two bands near the threshold of dissociation to radical products CH3+HCO in a supersonic jet, displaying most quantum-beat features, are recorded with resolution 0.025 cm−1. Evaluated on the basis of a simple asymmetric rotor, the origins of these two bands 1402−1501 and another denoted # are 31 275.045(1) and 31 523.263(1) cm−1; effective rotational constants of excited state are A=5.7883(1), 5.0408(3), B=0.33269(2), 0.32320(2) and C=0.31026(2), 0.32091(2) cm−1, respectively; large A value results from lack of consideration of torsional motion. For these two vibrational levels most rotational states (about 70 percent) display quantum-beat features attributed to coherently excited singlet–triplet eigenstates. The linewidth in transformed spectra for level #, ∼125 cm−1 below the dissociation threshold, increases with increasing total angular momentum J whereas level 142−151 that is 375 cm−1 below s...

Photodissociation dynamics of azulene

Photodissociation of azulene at 193 nm was studied in a molecular beam using multimass ion imaging techniques. Most of the azulene molecules excited by 193 nm photon quickly relax to the ground electronic state through internal conversion, then isomerize to naphthalene, and eventually dissociate through the H atom elimination channel with a rate of 5.1×104 s−1. A small amount of azulene entering different isomerization channels was found. The effect of dissociation in the energy transfer experiments using azulene as a vibrationally highly excited molecule and the existence of azulene in an interstellar medium is discussed.

Preparation of carboxylated magnetic particles for the efficient immobilization of C-terminally lysine-tagged Bacillus stearothermophilus aminopeptidase II.

This article reports the synthesis and use of surface-modified iron oxide particles for the simultaneous purification and immobilization of Bacillus stearothermophilus aminopeptidase II (BsAPII) tagged C-terminally with either tri- or nona-lysines (BsAPII-Lys3/9). The carboxylated magnetic particles were prepared by the simple co-precipitation of Fe3+/Fe2+ in aqueous medium and then subsequently modified with adipic acid. Transmission electron microscopy (TEM) micrographs showed that the carboxylated magnetic particles remained discrete and had no significant change in size after binding BsAPIIs. Wild-type enzyme and BsAPII-Lys3 could be purified to near homogeneity by the carboxylated magnetic particles, but it was not easy to elute the adsorbed BsAPII-Lys9 from the matrix. Free BsAPII, BsAPII-Lys3, and BsAPII-Lys9 were active in the temperature range 50–70°C and all had an optimum of 50°C, whereas the optimum temperature and thermal stability of BsAPII-Lys3 and BsAPII-Lys9 were improved as a result of immobilization. The immobilized BsAPII-Lys9 could be recycled ten times without a significant loss of the enzyme activity and had a better stability during storage than BsAPII. Owing to its high efficiency and cost-effectiveness, this magnetic adsorbent may be used as a novel purification-immobilization system for the positively charged enzymes.

H and CH3 eliminations in the photodissociation of chlorotoluene

Photodissociation of o-, m-, and p-chlorotoluene at 193 nm under collision-free conditions has been studied using multimass ion imaging techniques. In addition to the Cl atom elimination, photofragments corresponding to the reactions of C6H4ClCH3→C6H4ClCH2+H and C6H4ClCH3→C6H4Cl+CH3 were observed. Dissociation rates and fragment translational energy distributions were measured. A comparison with RRKM calculation has been made and the possible dissociation mechanism was discussed.