2015 Vol. 28, No. 4

2015, 28(4): 0-0.
As an emerging label-free imaging modality with chemical selectivity and millimeter-depth resolvability, vibrational photoacoustic imaging provides a new avenue to map chemical content in biological tissue. It heralds the potential for detection of white matter loss and regeneration, assessment of breast tumor margin, and clinical diagnosis of vulnerable plaques in atherosclerosis. In this work, we provide an overview of the development of bond-selective photoacoustic imaging and various biomedical applications enabled by this new technology.
The reactivity of specific sites on rutile TiO2(110)-(1×1) surface and anatase TiO2(001)-(1×4) surface has been comparably studied by means of high resolution scanning tunneling microscopy. At the rutile TiO2(110)-(1×1) surface, we find the defects of oxygen vacancy provide distinct reactivity for O2 and CO2 adsorption, while the terminal fivefold-coordinated Ti sites dominate the photocatalytic reactivity for H2O and CH3OH dissociation. At the anatase TiO2(001)-(1×4) surface, the sixfold-coordinated terminal Ti sites at the oxidized surface seem to be inert in both O2 and H2O reactions, but the Ti-rich defects which introduce the Ti3+ state into the reduced surface are found to provide high reactivity for the reactions of O2 and H2O. By comparing the reactions on both rutile and anatase surfaces under similar experimental conditions, we find the reactivity of anatase TiO2(001) is actually lower than rutile TiO2(110), which challenges the conventional knowledge that the anatase (001) is the most reactive TiO2 surface. Our findings could provide atomic level insights into the mechanisms of TiO2 based catalytic and photocatalytic chemical reactions.
The state-to-state photodissociassion dynamics for the B band of D2O have been explored from quantum dynamical calculations including the electronic ~X and ~B states. The calculations were carried out using a Chebyshev real wave packet method. The calculated absorption spectra, product state distributions, and branching ratios from different initial vibrational states show di?erent dynamic features, due to the different shapes of the vibrational wavefunctions. The initial bending mode (0,1,0) generates two lobes with a shallow minimum on the absorption spectrum and a slight inverted vibrational population of OD(~X )product at high total energies. The rotational state distributions of OD(~X , v=0) product are highly inverted and depend weakly on the initial state and total energy. On the other hand, the ro-vibrational distributions of OD(A~) product strongly oscillate with the total energy, which are dominated by the long-living resonances and depend sensitively on the potential surfaces. The antisymmetric stretching mode (0,0,1) has large OD( ~ A)/OD(~X ) branching ratios at high total energies, which indicates that the B band dissociation proceeds mainly via the adiabatic pathway in some cases.
Extensive quasiclassical trajectory calculations for the O(1D)+CD4 multichannel reaction were carried out on a new global potential energy surface fit by permutationally invariant polynomials. The product branching ratios, translational energy distributions, and angular distributions of OD+CD3, D+CD2OD/CD3O, and D2+DCOD/D2CO product channels were calculated and compared with the available experimental results. Good agreement between theory and experiment has been achieved, indicating small isotope effects for the title reaction. The O(1D)+CD4 reaction mainly proceeds through the CD3OD intermediate via the trapped abstraction mechanism, with initial abstraction of the D atom rather than the direct insertion, followed by decomposition of CD3OD into various products.
This work aims at a priori accuracy controlled truncation to the dissipaton equation of motion formalism for non-perturbative quantum dissipative dynamics. A new truncation scheme is proposed by adopting the Markovian and high-temperature approximation similar to the Caldeira-Leggett master equation made at the termination level. An accuracy criterion to determine the truncation level is put forward via a Markovianicity analysis. Performances of both the new truncation scheme and the control criterion are illustrated via dynamics simulation of electron transfer systems.
Using the reactant coordinate based time-dependent wave packet method, on the APW potential energy surface, the differential and integral cross sections of the Li+DF/HF(v=0, j=0, 1) reactions were calculated over the collision energy range from the threshold to 0.25 eV. The initial state-specified reaction rate constants of the title reaction were also calculated. The results indicate that, compared with the Li+DF reaction, the product LiF of Li+HF reaction is a little more rotationally excited but essentially similar. The initial rotational excitation from j=0 to 1 has little effect on the Li+DF reaction. However, the rotational excitation of DF does result in a little more rotationally excited product LiF. The different cross section of both reactions is forward biased in the studied collision energy range, especially at relatively high collision energy. The resonances in the Li+HF reaction may be identifiable as the oscillations in the product ro-vibrational state-resolved integral cross sections and backward scattering as a function of collusion energy. For the Li+HF reaction, the rate constant is not sensitive to the temperature and almost has no change in the temperature range considered. For the Li+DF reaction, the rate constant increase by a factor of about 10 in the temperature range of 100?300 K. Brief comparison for the total reaction probabilities and integral cross section of the Li+HF reaction has been carried out between ours and the values reported previously. The agreement is good, and the difference should come from the better convergence of our present calculations.
The manipulation of two 2nd-order quantum pathways has been successfully demonstrated experimentally in atomic Rubidium. This work would explore the coherent control of path-ways in the quantum system with N intermediate states connecting the initial and target states. A 4N-block scheme is proposed to control the N 2nd-order pathways which are driven by a weak broadband pulse. The model with N=3 is given as an example to demonstrate our strategy. In this scheme, boundaries of the spectral blocks are functions of the resonant frequencies, and the resonant and non-resonant contributions interfere mutually to achieve the quantum pathway control e ectively. The simple strategy may provide a practical choice for pathway manipulation experiments since there are only 2N phase variables to adjust.
We employ the numerically exact hierarchical equations of motion (HEOM) method to perform benchmark tests for the popular modified Redfield method in calculating linear and nonlinear spectroscopic signals of molecular aggregates in photosynthetic light harvesting complexes. It is currently well known that the perturbative and Markovian approximations involved in the modified Redfield equation may give inappropriate description of the excitation energy transfer processes in the intermediate coupling regime. An interesting topic is thus to test the validity of the modified Redfield method in calculating various types of spectroscopic signals. By using model dimers with different sets of parameters and a model of the Fenna-Matthews-Olson complex, we calculate and compare the absorption, emission, and 2D spectra using the modified Redfield and HEOM methods. It is found that results from the modified Redfield method agree well with the HEOM ones in a wide range of parameter regimes. The comparison also helps to understand the quantum beating signals in the 2D spectra of the photosynthetic light harvesting complexes.
Quantitative measurement of water vapor is essential in many fields including semiconduc-tor industry, combustion diagnosis, meteorology, and atmospheric studies. We present an optical hygrometer based on cavity ring-down spectroscopy. The instrument is high-vacuum compatible, self-calibrated by using the free-spectral-range of the ring-down cavity made of low-thermal-expansion Invar. Using a single tunable diode laser working at 1.39 μm, detection of trace water vapor in vacuum and in high-purity helium gas, and also determination of humidity at ambient conditions, have been demonstrated. It indicates that the instrument can be used to detect the partial pressure of water vapor in a very broad range from 10-7 Pa to 103 Pa. Such an optical hygrometer can be potentially applied as a primary moisture standard to determine the vapor pressures of water (ice) at low temperatures.
Because of their unique chemical and physical properties, long-lived rare krypton radioisotopes, 85Kr and 81Kr, are ideal tracers for environmental samples, including air, groundwater and ice. Atom trap trace analysis (ATTA) is a new laser-based method for counting both 85Kr and 81Kr atoms with the abundance as low as 10-14 with micro-liters (STP) krypton gas. The entire system for rare radio-krypton measurement built at Hefei is presented, including the atom trap trace analysis instrument and sampling apparatus of gas extraction from water and krypton purification. Atmospheric85Kr concentrations at different places in China were measured, showing a range of 1.3-1.6 Bq/m3, consistent with the northern hemispheric baseline. As a demonstration of the system, some shallow and deep groundwater samples in north and south China were sampled and dated.
The diatomic ZrO- anion has been prepared by laser ablation and studied by photoelectron imaging spectroscopy combined with quantum chemistry calculations. The observed photoelectron spectra can be well assigned on the basis of reported optical spectroscopy and high-level ab initio calculations. The ground state of ZrO- is a 2△ state with spin-orbit splitting of 578±12 cm-. The electron affinity of ZrO is 1.249±0.005 eV. For the first time, the c3Σ- state of ZrO has been experimentally observed at 13316±24 cm- with respect to the X1+ ground state. A comparison between ZrO and the isoelectronic molecule NbN has been made.
Post-irradiation temperature-programmed desorption (TPD) has been used to study the photocatalyzed oxidation of methanol on TiO2(110) surface under the irradiation of 360, 380 and 400 nm light. The photocatalytic process initiated by ultraviolet light of different wavelength are similar. Methanol has been photocatalytically converted into formaldehyde, and the released hydrogen atoms transfer to the neighboring twofold coordinated oxygen to form bridging hydroxyls. The reaction rate, however, is strongly wavelength dependent. The reaction rate under 360 nm light irradiation is 4.8 times of that in the case of 400 nm exposure, consistent with a previous femtosecond time-resolved absorption measurement on TiO2 which shows the faster charge carrier recombination in the near-band-gap than the over-band-gap excitation. So far, the underlying factors which govern the excitation wavelength dependence of photocatalytic activity of TiO2 and other photocatalysts remain unclear, and future studies are needed to address this important issue.
Photocatalytic dissociation of ethanol molecules on the rutile TiO2(110) surface after UV irradiation has been investigated by scanning tunneling microscope at 80 K. Most of the ethanol molecules adsorb molecularly at Ti sites, similar to the case of methanol. After UV irradiation, two different protrusions of products were observed, one of them has been identified by the technique of tip manipulation, which was likely composed of an acetaldehyde in the middle and two bridge-bonded hydroxyls on both neighbored oxygen sites. Multi-time irradiation experiments have also been performed to further understand the relationship between the two protrusions and the process of ethanol photocatalytic dissociation. These results provide detailed insights into the photocatalysis of ethanol on rutile TiO2(110), which would help us to understand how phtotocatalytic reactions of ethnaol proceed at the fundamental level.
The dynamics of the F+H2(v=0,j=0, 1) reactions have been studied at the collision energy of 1.27 kcal/mol using a high-resolution crossed molecular beam apparatus. HF product rotational state resolved differential cross sections have been obtained at the v′=1, 2, 3 levels. The product HF(v′=2) angular distributions are predominantly backward scattered for both H2 (j=0, 1) reagents. However, the distributions of product HF(v′=2) rotational states for theF+H2(v=0,j=0) reaction are signi cantly di erent from those for the F+H2(v=0,j=1) reaction. Experimental results show that the rotational excitation of H2 produces rotationally ‘hotter’ HF(v′=2) product. In addition, the HF(v′=3) product is more likely scattered into the forward direction when the H2 reagent is populated at j=0 state, which could be attributed to a slow-down mechanism.
Analogous to atoms, superatoms can be used as building blocks to compose molecules and materials. To demonstrate this idea, the possibility of using tetrahedral Ag4 cluster to form a series of superatomic molecules Ag4X4 (X=H, Li, Na, K, Cu, Ag, Au and F, Cl, Br) is discussed. Based on the super valence bond model, a tetrahedral Ag4 cluster can be viewed as a 4-electron superatom, which can mimic a sp3 hybridization C atom. By comparison of the representative superatomic molecules Ag4X4 (X=Au, Cl) with the corresponding simple molecules CX4 (X=H, Cl), the similarities in terms of chemical bonding patterns and molecular orbitals (MOs) are conspicuous. Energy calculations predict that the Ag4 superatom can bind with all the involved ligands. Furthermore, the stabilities of superatomic molecules are enhanced by the large gaps of the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO gaps) and high aromaticity. Our studies may find applications in assembling materials with superatoms.
Ab initio spin-density-functional calculations have been performed to study the equilibrium structural, electronic, and magnetic properties of Tcn and Tcn@C70 endohedral metalofullerenes. Our results indicate that C70 can encapsulate Tcn clusters with up to n=9 atoms. Except n=2, the formation of Tcn@C70 endohedral metalofullerenes is predicted to be exothermic when n≤5, while the encapsulation process becomes increasingly endothermic beyond n=5. When encapsulating into the C70 cage, the geometries as well as electronic structures of the Tc cluster undergo comparative changes. Especially, compared to the isolated Tcn clusters, the total magnetic moments of Tcn@C70 reduce significantly. The analyses of the orbital population, Hirshfeld population and density of states show that electrons transfer from the Tc cluster to the carbon cage through the Tc-C efficient hybridization, which is responsible for such reduction of the whole magnetism.
It is generally accepted that electron impact of doped helium nanodroplets initially produces a positively charged helium atom, which then ionizes the dopant if the two come into contact. In effect the He+ can initiate ion-molecule reactions. However, the effect of the surrounding helium on ion-molecule reactions remains ambiguous. To explore this, electron-induced chemistry has been investigated for the diatomic molecules O2, CO and N2. The helium is found to significantly suppress dissociative ion product channels.
Photoinduced chemical reaction between thioxanthen-9-one (TX) and diphenylamine (DPA) were investigated by the nanosecond laser flash photolysis. With photolysis at 355 nm, the triplet TX (3TX*) is produced via a Franck-Condon excitation and intersystem crossing. In the transient absorption spectra of the reduction of 3TX* by DPA in pure and aqueous CH3CN, four bands are clearly observed and assigned to absorption of 3TX*, TXH·, TX·- and DPA·+, respectively. With the increase of solvent polarity, the blue-shift was observed for all absorption bands of the intermediates. With the aid of dynamic decay curves, an electron transfer followed by a protonation process is confirmed for the reduction of 3TX* by DPA. The quenching rate constants of 3TX* by DPA very slightly decrease from 9.7×109 L/(mol·s) in pure CH3CN, to 8.7×109 L/(mol·s) in CH3CN:H2O (9:1), 8.0×109 L/(mol·s) in CH3CN:H2O (4:1) and 7.5×109 L/(mol·s) in CH3CN:H2O (1:1). Therefore water plays a minor role in the title reaction, and moreover no obvious medium effect of solvent polarity is observed for the electron transfer between 3TX* and DPA, indicating that the 3* and 3ππ* states of TX have the approximate ability to attract an electron from DPA.
The solvation of protonated methanol by carbon dioxide has been studied via a cluster model. Quantum chemical calculations of the H+(CH3OH)(CO2)n+(n=1-7) clusters indicate that the rst solvation shell of the OH groups is completed at n=3 or 4. Besides hydrogen-bond interaction, the CCO2…OCO2 intermolecular interaction is also responsible for the stabilization of the larger clusters. The transfer of the proton from methanol onto CO2 with the formation of the OCOH+ moiety might be unfavorable in the early stage of solvation process. Simulated IR spectra reveal that vibrational frequencies of free O-H stretching, hydrogen-bonded O-H stretching, and O-C-O stretching of CO2 unit a ord the sensitive probe for exploring the solvation of protonated methanol by carbon dioxide. IR spectra for the H+(CH3OH)(CO2)n+(n=1-7) clusters could be readily measured by the infrared photodissociation technique and thus provide useful information for the understanding of solvation processes.
This work describes the construction of a phase-stable two dimensional electronic spectrometer operating in a photon echo mode with optical heterodyne detection, where the diffractive optics were used to realize the passive phase stabilization. In addition, a high speed and sensitive EMCCD was configured for shot-to-shot measurement which effectively improved signal-to-noise ratio. Consequently, the phase stability between a pulse pair split by the diffractive optics was determined in terms of standard deviation to be λ/200 during an observation period of 30 min, while the phase stability of the photon echo signal measured with IR140 is λ/90 in 19 min. In addition, a method of phase-shift in the pump pulse is also presented, which can effectively remove the interference from scattering light in collection of pump-probe transient absorption spectrum. The phase-shift method can improve the accuracy of phase adjustment in 2D electronic spectrum of scattering samples.
Potassium phosphate buffer solution has been widely used in the biological experiments, which represents an important process of the interaction between ions and biomolecules, yet the in fluences of potassium phosphate on biomolecules such as the cell membrane are still poorly understood at the molecular level. In this work, we have applied sum frequency generation vibrational spectroscopy and carried out a detailed study on the interaction between potassium phosphate buffer solution (PBS) and negative 1,2-dimyristoyl-d54-sn-glycero-3-[phospho-rac-(1-glycerol)] (d54-DMPG) lipid bilayer in real time. The PBS-induced dynamic change in the molecular structure of d54-DMPG lipid bilayer was monitored using the spectral features of CD2, CD3, lipid head phosphate, and carbonyl groups for the first time. It is found that K+ can bind to the cell membrane and cause the signal change of CD2, CD3, lipid head phosphate, and carbonyl groups quickly. Potassium PBS interacts with lipid bilayers most likely by formation of toroidal pores inside the bilayer matrix. This result can provide a molecular basis for the interpretation of the effect of PBS on the ion-assisted transport of protein across the membrane.
Pyrolysis of algae from Taihu Lake water blooms for bio-oil production was conducted from 473 K to 773 K by a fractional way in six steps. Palmitic acid, agarose and egg white were used as model compounds to study the origin of bio-oil ingredients and interaction of the intermediates from the algae components. In the first step at 473 K, the bio-oil obtained was composed of n-heptadecane and some small molecule acids. Quantities of carboxylic acids (mainly palmitic acid) and some amides, hydrocarbons, esters etc. were evolved in the second step at 523 K. For the third step at 573 K, except the carboxylic acids (still mainly palmitic acid), amides, nitriles, and phenols also accounted for a large proportion whereas respectable amount of indoles and alcohol ketones were attained. The main products in the later three steps were nitriles and phenols at 623 K, hydrocarbons and phenols at 673 K, and only phenols at 773 K, respectively. A higher heating value (HHV) of 36.0 MJ/kg of the bio-oil was obtained at 673 K. The hydrocarbons, palmitic acid and esters in the bio-oil were derived from lipids. The phenols, indoles, pyrroles, small molecular acids, amides like acetamide and some nitriles like phenyl-acetonitrile were generated from proteins. Amides and nitriles were also dated from the interaction of pyrolytic intermediates of lipids and proteins. Fewer products directly from the direct pyrolysis of saccharides were detected in the algae bio-oil due to the interaction of pyrolytic intermediates of saccharides and proteins in algae, and those interactions resulted in the formation of oligomers in the bio-oil at 473 and 523 K. Whereas very weak interaction was observed between lipids and saccharides. The process of fractional pyrolysis by varying temperature provided an advisable way for improving the selectivity of bio-oil from direct pyrolysis, and made the bio-oil much more applicable in down streaming utilization.
Conversion of sugars from biomass to platform chemicals or fuels is an attracting topic for the utilization of biomass. Pb2+ ion is an efficient catalyst for the degradation of sugar to lactic acid, and it will be better to fix lead on a solid catalyst to reduce the risk of exposure of Pb2+ to environment. Here, a simple method has been developed to prepare a composite catalyst of Pb(OH)2/rGO, where the nanoparticles of Pb(OH)2 in size of 2-5 nm were prepared and fixed over the as-prepared reduced graphene oxide (rGO) nanosheets. The as-obtained catalyst showed an efficient catalytic activity to degrade glucose, fructose, and cellulose in aqueous solution, and the major product is lactic acid. The yield of lactic acid reached 58.7% when fructose was used as the feedstock (433 K and 2.5 MPa N2), and the catalyst can be recycled with high activity. Cellulose can also be directly converted into lactic acid in aqueous solution over the catalyst without extra acid or alkali, and the maximum yield of lactic acid is 31.7%.
Chinese abstract
Chinese Abstracts
2015, 28(4): 534-534.