2007 Vol. 20, No. 4

Review
Article
The reaction of C-H stretch-excited CHD3 with Cl atom was studied in a crossed-beam experiment by imaging of the ground vibrational state of CD3 products. he methyl product was interrogated by (2+1) resonance-enhanced multiphoton ionization via the Q-head of the origin band. We found that the observed image appearances depend sensitively on the precise frequency of the probe laser. Weattributed the effects to the slight differences in the subset of N-levels of CD3(v=0) being sampled. The implication of collision dynamics is that, ompared to the ground-state reaction, the CH stretch-excited reaction preferentially yields rotationally warmer CD3(v=0) products. And a negative correlation between the CD3 rotational excitation and the vibrational excitation of the coincidently formed HCl coproducts was demonstrated, which enabled us to uncover a hidden, stereodynamical aspect of the title reaction.
The K2 electronic states calculated theoretically and/or observed experimentally have been relabeled according to their dominant hydrogenic nl? characters of the Rydberg orbitals. These states can be divided into core-penetrating and core-nonpenetrating states. This classiˉcation provides physical insights for interpreting and predicting experimental observations. The application of this method to K2 is compared with that to Na2 and Li2.
The H+N3 channel in the ultraviolet photodissociation of HN3 has been investigated from 190 nm to 248 nm using the high-n Rydberg H-atom time-of-°ight technique. Product translational energy distributions as well as product angular anisotropy parameters were determined for the H+N3 channel at di?erent photolysis wavelengths. N3 vibrational state distribution has also been derived from the product translational energy distribution at these wavelengths. Above photolysis wavelength 225 nm, HN3 predominantly dissociatethrough the repulsive state. Below 225 nm, a new slow channel starts to appear at 220 nm in addition to the existing channel. This channel is attributed to a ring closure dissociation channel to produce the cyclic N3 product. As photolysis energy increases, this new channel becomes more important.
We have obtained a rotationally resolved vacuum ultraviolet pulsed ˉeld ionization-photoelectron (VUV-PFI-PE) spectrum of H2 in the energy range of 15.30-18.09 eV, covering the ionization transitions H2+(X2§+g ,v+=0-18, N+=0-5)?H2(X1§+g , v00=0, J00=0-4). The assignment of the rotational transitions resolved inthe VUV-PFI-PE vibrational bands for H2+(X2§+g , v+=0-18) and their simulation using the Buckingham-Orr-Sichel (BOS) model are presented. Only the ¢N=N+?J00=0 and §2 rotational branches are observed in the VUV-PFI-PE spectrum of H2. However, the vibrational band is increasingly dominated by the 4N=0 rotational branch as v+ is increased. The BOS simulation reveals that the perturbation of VUV-PFI-PE rotational line intensities by near-resonance autoionizing Rydberg states is minor at v+?6 and decreases as v+ is increased. Thus, the rotationally resolved PFI-PE bands for H2+(v+?6) presented here providereliable estimates of state-to-state cross sections for direct photoionization of H2, while the rotationally resolved PFI-PE bands for H2+(v+·5) are useful data for fundamental understanding of the near resonance autoionizing mechanism. On the basis of the rovibrational assignment of the VUV-PFI-PE spectrum of H2, the ionization energies for the formation of H2+(X2§+g , v+=0-18, N+=0-5) from H2+(X1§+g , v00=0,J00=0-4), the vibrational constants (!e, !e?e, !eye, and !eze), the rotational constants (Bv+, Dv+, Be,and ?e), and the vibrational energy spacings ¢G(v++1/2) for H2+(X2§+g , v+=0-18) are determined. With a signiˉcantly higher photoelectron energy resolution achieved in the present study, the precisions of these spectroscopic values are higher than those obtained in the previous photoelectron studies. As expected, the spectroscopic results for H2+(X2§+g , v+=0-18) derived from this VUV-PFI-PE study are in excellent agreement with high-level theoretical predictions.
The F+HCl and F+DCl reactions are studied by the time-dependent quantum wave packet method, using the most recent potential energy surface reported by Deskevich et al.. Total reaction probabilities for a number of initial ro-vibrational states of HCl and DCl diatomic moiety are presented in the case of total angular momentum J=0. It is found that for both reactions the initial rotational excitation of the diatomic moiety enhances greatly the reaction probabilities but this e?ect is more signiˉcant for F+HCl system. This is mainly due to larger rotational constant of the HCl reagent. The initial vibrational excitation of the diatomic moiety has little e?ect on the reactivity for both systems except shifting down the collision energy threshold. The results indicate that the reaction coordinates for these two systems are e?ectively along rotational freedom degree. More quantum phenomena, such as tunneling and resonance, are observed in F+HCl reaction than F+DCl reaction, and for the initial states studied, the reactivity of the later is lower. Di?erent skewing angles of these two systems account for these isotopic di?erences.
The (2+1) resonance-enhanced multiphoton ionization (REMPI) spectrum of SF has been obtained in the single-photon wavelength region of 307-321 nm. Five vibronic bands were observed and assigned to the two-photon transitions from the ground state to a 2§ Rydberg state. The term value Te, vibrational frequency, and the rotational constant of the 2§ Rydberg state were determined. Another 2P state was observed near 312 nm.
Photodissociation dynamics of jet-cooled thiomethoxy radical (CH3S) via the eA2A1? eX2E transition is investigated near 352 nm. The H-atom product channel is observed directly for the ˉrst time by H-atom product yield spectrum and photofragment translational spectroscopy. The 2132 vibrational level of the eA2A1 state dissociates to the H+H2CS products. The H+H2CS product translational energy release is modest and peaks around 33 kJ/mol; the H-atom angular distribution is isotropic. The dissociation mechanism is consistent with internal conversion of the excited eA2A1 state to the eX2E ground state and subsequent unimolecular dissociation on the ground state to the H+H2CS products.
The elementary reaction of CH2Cl+O2 in gas phase was investigated by time-resolved FTIR emission spectroscopy. Vibrationally excited products CO (v·4), and CO2 (o3, v·7) were observed. The yield ratio of CO/CO2(o3) was 72.2§7. The reaction pathways were studied theoretically at QCISD//UB3LYP/6-311++G(d,p) level. In the beginning of the reaction, CH2Cl radical associated with O2 to form CH2ClOO, followed by removal of the Cl atom to yield another intermediate dioxirane CH2OO. Subsequently, a series of isomerization and decomposition of the CH2OO took place, yielding the ˉnal products of CO and CO2. The calculated result was in consistent with the experimental observation.
The photodissociation of isocyanic acid (HNCO) on the ˉrst excited singlet state following the excitation at 210 nm was investigated with an ion velocity slice imaging technique by probing the CO fragment. It was found from the (2+1) resonance-enhanced multi-photon ionization (REMPI) spectrum that the CO fragments are rotationally hot with population up to Jmax=50. The velocity imagings of the CO fragments at JCO=30 and 35 indicate that formation of NH(a1¢)+CO(X1§+, v=0) is the predominant dissociation channel at 210 nm. From analysis of the CO fragment translational energy distributions, the NH(a1¢) fragment was observed to be rotationally cold, about half of the available energy was partitioned into the translational motion of fragments after dissociation, and the NH(a1¢)+CO(X1§+) dissociation threshold was determined at 42738§30 cm?1. From analysis of the CO fragment angular distributions, the dissociationanisotropy parameter ˉ was found to be negative, and increasing with the rotational quantum number of the NH fragment, i.e., from ?0.75 at JNH=2-4 to ?0.17 at JNH=11. Impulsive direct and vertical dissociation process of HNCO on the singlet state at 210 nm was conˉrmed experimentally. A classical impact dissociation model was employed to explain the dependence of the ˉ value on the rotational excitation of the NH fragment.
The property of the lowest excited triplet states of xanthone in acetonitrile was investigated using time-resolved laser °ash photolysis at 355 nm. The transient absorption spectra and the quenching rate constants(kq) of the excited xanthone with several amines were determined. Good correlation between lgkq and the driving force of the reactions suggests the electron transfer mechanism, except aniline and 3-nitroaniline (3-NO2-A) which showed energy transfer mechanism. With the appearance of ketyl radical, hydrogen atom transfer also happened between xanthone and dimethyl-p-toluidine, 3,5,N,N-tetramethylaniline, N,N-dimethylaniline, and triethylamine. Therefore, both electron transfer and H-atom transfer occured in these systems. Great discrepancies of kq values were discovered in H-atom abstraction reactions for alcohols and phenols, which can be explained by di?erent abstraction mechanisms. The quenching rate constants between xanthone and alcohols correlate well with the ?-C?H bonding energy of alcohols.
Measurements of the nascent OH product from photodissociation of gaseous nitromethane and nitroethane at 266 nm were performed using the single-photon laser induced °uorescence technique. The OH fragment is found to be vibrationally cold for both systems. The rotational state distribution of nitromethane are Boltzmann, with rotational temperature of Trot=2045§150 and 1923§150 K for both 2|3=2 and 2|1=2 states, respectively. For nitroethane, the rotational state distribution shows none Boltzmann and cannot be well characterized by a rotational temperature, which indicates the di?erent mechanisms in producing OH radicals from photodissociation of nitromethane and nitroethane. The rotational energy is calculated as 14.36§0.8 and 4.98§0.8 kJ/mol for nitromethane and nitroethane, respectively. A preferential population of the low spin-orbit component (2|3=2) is observed for both nitromethane and nitroethane. The dominant population of |+ state in two ¤-doublet states is also observed for both nitromethane and nitroethane,which indicates that the unpaired ? lobe of the OH fragment is parallel to the plane of rotation.
The free radical reaction of C2Cl3 with NO2 was investigated by step-scan time-resolved FTIR (TR-FTIR) emission spectroscopy. Due to the vibrationally excited products of Cl2CO, NO, and CO, strong IR emission bands were observed with high resolution TR-FTIR spectra. Four reaction channels forming C2Cl3O+NO, CCl3CO+NO, CO+NO+CCl3, and ClCNO+Cl2CO were elucidated, respectively. Spectralˉtting showed that the product CO was highly vibrationally excited with the nascent average vibrational energy of 60.2 kJ/mol. Possible reaction mechanism via intermediates C2Cl3NO2 and C2Cl3ONO was proposed.
A time of °ight mass spectrometer coupled with a cluster formation and reaction source is employed to study the reactivity of cationic vanadium oxide clusters (VmOn+) toward ethylene (C2H4) in the gas phase. The cationic vanadium oxide clusters with m=1-10 and n=1-26 (depending on m) are generated by reaction of laser ablation created vanadium plasma with O2 in a supersonic expansion and then reacted with the ethylene in a °ow tube reactor. Hydrogen atoms are attached in most of the oxygen saturated clusters(2n?5m) in our experimental condition. The reactivity of VmOn+ toward C2H4 is usually higher than that of hydrogen containing clusters, VmOnH2x+. Larger clusters show less reactivity than smaller ones. Most of the observed products are in the forms of VmOnC2H4+ and VmOnH2xC2H4+ due to direct association. C2H4 clustering products ((C2H4)n+, n=2-6) are also observed.
The predissociation dynamics of B Rydberg state of methyl iodide is studied with femtosecond two-color pump-probe time-of-°ight spectra at pump pulse of 400 nm and probe pulse of 800 nm. The dominant product channels are the CH3I+ and CH3+ formation. The time-dependent signals for CH3I+ and CH3+ ions are obtained. Both of the signal curves can be ˉtted by biexponential decays with time constants of ?1 and ?2, ?1 is assigned to the lifetimes of high Rydberg states, which can be accessed by absorbing three 400 nm pump pulses and ?2 re°ects the dynamics of B Rydberg state, which is accessed with two pump pulses. The lifetime of B Rydberg state is determined to be about 1.57 ps, which is incredibly consistent with the previous studies. The results were interpreted as a multiphoton dissociative ionization processes.
The triplet state phenylnitrene (PhN) species generated from the low-pressure (4.0 kPa) premixed laminar pyridine/oxygen/argon °ame was detected and identiˉed using tunable synchrotron vacuum ultraviolet photoionization and molecular-beam mass spectrometry techniques. The ionization energies of PhN were determined experimentally by photoionization e±ciency spectra and theoretically by calculations. The results indicate that PhN has a 3A2 ground state and its ˉrst and second adiabatic ionization energies are 8.04 and9.15§0.05 eV, respectively. Furthermore, the formation and consumption pathways of PhN are proposed according to the species detected in the present work. PhN is the ˉrst nitrogen-containing diradical detected in combustion chemistry, and so it should be added to the kinetic model of pyridine °ames.
The MFCC-downhill simplex method is presented to study the binding structure of small ligands in large molecular complex systems. This method employs the Molecular Fractionation with Conjugated Caps (MFCC) approach to compute the interaction energy-structure relation of the system and implements the downhill simplex algorithm for structural optimization. The method is tested on a molecular system of cyclo-AAGAGG¢H2O to optimize the binding position of water molecule to the ˉxed cyclohexapeptide. The MFCC-downhill simplex optimization results are in good agreement with the crystal structure. An MFCC-Powell optimization method which uses the Powell's minimization algorithm is also described and tested on the same system. The MFCC-downhill simplex optimization is more e±cient than the MFCC-Powell method.
While the exact theory of chemical reaction rate processes is always non-Markovian, experimental rates do often show practically Markovian that supports kinetics rate constant description. In this work, we propose to use the Kubo's motional narrowing line shape function to characterize the Markovian character of the simplest two-state electron transfer reaction system. On the basis of analytical results, we demonstrate the related Markovianicity parameter as an interplay between the °uctuating solvent environment and the coherent transfer coupling. It is found that a non-Markovian rate process is most likely to occur in a symmetric system in the fast solvent modulation regime, where the resonant tunneling enhancement plays the important role. The e?ect of quantum solvation on electron transfer, which is dominant in the fast modulation regime, will also be highlighted.
The transport of internalized ?1A-adrenergic receptor (?1A-AR) by myosin protein in live cells was studied. The technique of single particle tracking by °uorescence imaging with high temporal and spatial resolution was used. The endosomes of ?1A-AR were transported along actin ˉlaments in a step-by-step mode. The average step-size in di?erent time resolutions is consistent with the step-size of myosin assay in vitro. With the simulation of the stepwise traces in di?erent time resolutions, we found that the kinetic process of each step is in coherence with the single myosin assay in vitro.
There has been emerging needs for the quantitative polarization analysis for the Coherent Anti-stokes Raman Spectroscopy and Coherent Anti-stokes Hyper-raman Spectroscopy, as the experimental studies with coherent anti-stokes raman spectroscopy and coherent anti-stokes hyper-raman spectroscopy for the interface and membrane studies being growing. Recently we have demonstrated that orientational analysis of linear and nonlinear spectroscopy from the ordered molecular system, such as molecular interfaces and ˉlms, can be carried out with the formulation of the orientational function in simple functional forms. Applications of such formulation for the second order spectroscopy, namely, the Second Harmonic Generation and Sum Frequency Generation Vibrational Spectroscopy, have helped to understand spectral and orientational details of the molecular interfaces and ˉlms. In order to employ this formulation for the higher order coherent nonlinear spectroscopy, the detailed expressions of the experimental observables and the macroscopic sus-ceptibility/microscopic polarizability tensors for the third and fourth-order nonlinear spectroscopy for the interface or ˉlm is presented with the rotational symmetry. General expressions for the typical third and fourth order spectroscopy, such as the Third Harmonic Generation, the degenerated coherent anti-stokes raman spectroscopy, the Fourth Harmonic Generation and the degenerated coherent anti-stokes hyper-raman spectroscopy, are presented for their future applications. The advantages and limitations of the third and fourth order spectroscopic techniques are also discussed.
Carboxyl (COO?) vibrational modes of two amino acids histidine and glycine in D2O solution were investigated by temperature-dependent FTIR spectroscopy and temperature-jump nanosecond time-resolved IR di?erence absorbance spectroscopy. The results show that hydrogen bonds are formed between amino acid molecules as well as between the amino acid molecule and the solvent molecules. The asymmetric vibrational frequency of COO? around 1600-1610 cm?1 is blue shifted when raising temperature, indicating that the strength of the hydrogen bonds becomes weaker at higher temperature. Two bleaching peaks at 1604 and 1612 cm?1 were observed for histidine in response to a temperature jump from 10 ±C to 20 ±C. The lower vibrational frequency at 1604 cm?1 is assigned to the chain COO? group which forms the intermolecular hydrogen bond with NH3+ group, while the higher frequency at 1612 cm?1 is assigned to the end COO? group forming hydrogen bonds with the solvent molecules. This is because that the hydrogen bonds in the former are expected to be stronger than the latter. In addition the intensities of these two bleaching peaks are almost the same. In contrast, only the lower frequency at 1604 cm?1 bleaching peak has been observed for glycine. The fact indicates that histidine molecules form a dimer-like intermolecular chain while glycine forms a relatively longer chain in the solution. The rising phase of the IR absorption kinetics in response to the temperature-jump detected at 1604 cm?1 for histidine is about 30§10 ns, within the resolution limit ofour instrument, indicating that breaking or weakening the hydrogen bond is a very fast process.
Scanning tunneling microscopy (STM) can provide us the special means to characterize the locally physical and chemical properties of individual molecules, and even help us to manipulate the individual molecules for constructing new molecule-scale devices. Here we have adopted two new types of STM techniques to characterize the encapsulated metal atom inside a fullerene cage, and to construct a molecule-device with strong Kondo effect, respectively. The spatially dI/dV mapping spectra were used to unveil the energy-resolved metal-cage hybrid states of individual Dy@C82 molecule, and the important information about the spatial position of Dy atom inside the cage and the Dy-cage interaction was revealed. The high-voltage pulse by STM tip is controlled to induce the dehydrogenation of Co phthalocyanine molecule and change its adsorption configuration on Au(111) surface, so as to recover Kondo effect that disappears in the case of intact adsorbed molecule.
C?H and C=O stretching modes are two among many structural and dynamic probes of proteins and peptides in condensed phases. Anharmonic properties of these two modes in peptide and sugar have been examined using a second-order perturbative vibrational approach. High order force constants were obtained and examined to ˉnd how crucial they are in determining the degree of mode localization and the nature of mode anharmonicity of the two stretching modes. It is found that the C?H mode is highly localized,and its diagonal anharmonicity is mainly determined by the mode itself. However, the C=O mode is largely delocalized, and the diagonal anharmonicity involves contributions from other modes. The o?-diagonal anharmonicity between C?D and C=O modes is found to be negative in deuterated species, di?ering from those of the non-deuterated ones. It is also found that inter-mode interaction between each of the two modes with low-frequency modes contribute signiˉcantly to the o?-diagonal anharmonicity. These low-frequency modes give rise to a network of energy relaxation or intramolecular vibrational energy redistribution pathways which can be used to examine temporal behavior of intramolecular vibration energy °ow, provided a femtosecond broadband two-dimensional infrared spectroscopy is available.
Time-resolved IR spectroscopy was used to detect the photocatalytic reaction of methanol for H2 production on Pt/TiO2 catalysts. There exists an optimal amount of Pt loading in the Pt/TiO2 catalysts for the reaction of the photogenerated long-lived electrons. For a given amount of Pt loading, the reaction rate of the long-lived electrons on Pt/TiO2 catalysts varies greatly with the di?erent reduction temperature of the catalysts. The possible reason is that the Pt particles occupy the surface active sites for methanol adsorption on Pt/TiO2 catalysts reduced at high temperature. This phenomenon is not observed obviously on Pt/TiO2 catalysts reduced at low temperature. The decay rate of the long-lived electrons evaluated by time-resolved IR method qualitatively correlates well with the activity of H2 production under steady-state irradiation conditions.
The electronic and transport properties of the Z-shaped graphene nanoribbons heterojunction are investigated by a fully self-consistent nonequilibrium Green's function method combined with density functional theory. The first-principles calculations show that the robust quantum confinement effect in the junction can be used to design the quantum dot. The electronic states are confined by the topological structure of the junction. This kind of Z-shapedquantum dot can be realized regardless of doping impurity, edge chemical modification, and the length of junction. Moreover, the spatial distribution and the number of confined states are tunable.
An easy and fast microwave-assisted method of tuning the photoluminescence properties of CdTe nanocrystals in aqueous phase is presented. The photoluminescence could be tuned covering almost the whole visual spectral range (537-680 nm), and even partially extending to the near-infrared spectral range. The e?ect is probably related to the formation of core/shell structure and complex nano-assemblies. These results provide a promising means of tuning the photoluminescence of CdTe nanocrystals, leading to potential applications in biomedical labeling, solar cells, lasers, and other ˉelds.
Spectroscopic properties of new hyperbranched conjugated polymers functionalized in periphery with N,N-dimethylaniline and tert-butyl-benzene as terminal groups are investigated by one- and two-photon excitations. The absorption, °uorescence excitation and emission spectra are examined in chloroform and N,N-dimethylformamide. The two-photon excitation measurements show that the new hyperbranched conjugated polymer possesses large two-photon excitation cross section which makes it a very attractive candidate for the potential application as nonlinear optical materials. As an example, the two-photon induced three-dimensional data storage is also demonstrated.