2016 Vol. 29, No. 1

Content
Content
2016, 29(1): 0-0.
Chinese abstract
Chinese Abstracts
2016, 29(1): 0-0.
Article
Polarization dependent time-resolved infrared (TRIR) spectroscopy has proven to be a useful technique to study the structural dynamics in a photochemical process. The angular information of transient species is obtainable in this measurement, which makes it a valuable technique for the investigation of electron distribution, molecular structure, and conformational dynamics. In this review, we briefly introduce the principles and applications of polarization dependent TRIR spectroscopy. We mainly focused on the following topics: (i) an overview of TRIR spectroscopy, (ii) principles of TRIR spectroscopy and its advantages compared to the other ultrafast techniques, (iii) examples that use polarization dependent TRIR spectroscopy to probe a variety of chemical and dynamical phenomena including protein conformational dynamics, excited state electron localization, and photoisomerization, (iv) the limitations and prospects of TRIR spectroscopy.
The reaction of laser-ablated vanadium, niobium and tantalum atoms with hydrogen sulfide has been investigated using matrix isolation FTIR and theoretical calculations. The metal atoms inserted into the H-S bond of H2S to form the HMSH molecules (M=V, Nb, Ta), which rearranged to H2MS molecules on annealing for Nb and Ta. The HMSH molecule can also further react with another H2S to form the H2M(SH)2 molecules. These new molecules were identified on the basis of the D2S and H234S isotopic substitutions. DFT (B3LYP and BPW91) theoretical calculations are used to predict energies, geometries, and vibrational frequencies for these novel metal dihydrido complexes and molecules. Reaction mechanism for formation of group V dihydrido complex was investigated by DFT internal reaction coordinate calculations. The dissociation of HVSH gave VS+H2 on broad band irradiation and reverse reaction happened on annealing. Based on B3LYP calculation releasing hydrogen from HVSH is endothermic only by 13.5 kcal/mol with lower energy barrier of 16.9 kcal/mol.
The excited state structural dynamics of phenyl azide (PhN3) after excitation to the light absorbing S2(A'), S3(A'), and S6(A') states were studied using the resonance Raman spectroscopy and complete active space self-consistent field calculations. The vibrational spectra and the UV absorption bands were assigned on the basis of the Fourier transform (FT)-Raman, FT-infrared measurements, the density-functional theory computations and the normal mode analysis. The A-, B-, and C-bands resonance Raman spectra in cyclohexane, acetonitrile, and methanol solvents were, respectively, obtained at 273.9, 252.7, 245.9, 228.7, 223.1, and 208.8 nm excitation wavelengths to probe the corresponding structural dynamics of PhN3. The results indicated that the structural dynamics in the S2(A'), S3(A'), and S6(A') states were significantly different. The crossing points of the potential energy surfaces, S2S1(1) and S2S1(2), were predicted to play a key role in the low-lying excited state decay dynamics, in accordance with Kasha's rule, and N7=N8 dissociation. Two decay channels initiated from the Franck-Condon region of the S2(A') state were predicted: the radiative S2,min→S0 radiative decay and the S2→S1 internal conversion through the crossing points S2S1(1)/S2S1(2).
The effect of solvation on the conformation of acetylene has been studied by adding one water molecule at a time. Quantum chemical calculations of the H+(C2H2)(H2O)n (n=1-5) clusters indicate that the H2O molecules prefer to form the OH…π interaction rather than the CH…O interaction. This solvation motif is different from that of neutral (C2H2)(H2O)n (n=1-4) clusters, in which the H2O molecules prefer to form the CH…O and OH…C H-bonds. For the H+(C2H2)(H2O)n cationic clusters, the first solvation shell consists of one ring structure with two OH…π H-bonds and three water molecules, which is completed at n=4. Simulated infrared spectra reveal that vibrational frequencies of OH…π H-bonded O-H stretching afford a sensitive probe for exploring the solvation of acetylene by protonated water molecules. Infrared spectra of the H+(C2H2)(H2O)n(n=1-5) clusters could be readily measured by the infrared photodissociation technique and thus provide useful information for the understanding of solvation processes.
Herein we have employed high-level multi-reference CASSCF and MS-CASPT2 electronic structure methods to systematically study the photochemical mechanism of intramolecularly hydrogen-bonded 2-(2'-hydroxyphenyl)-4-methyloxazole. At the CASSCF level, we have optimized minima, conical intersections, minimum-energy reaction paths relevant to the excited-state intramolecular proton transfer (ESIPT), rotation, photoisomerization, and the excited-state deactivation pathways. The energies of all structures and paths are refined by the MS-CASPT2 method. On the basis of the present results, we found that the ESIPT process in a conformer with the OH…N hydrogen bond is essentially barrierless process; whereas, the ESIPT process is inhibited in the other conformer with the OH…O hydrogen bond. The central single-bond rotation of the S1 enol species is energetically unfavorable due to a large barrier. In addition, the excited-state deactivation of the S1 keto species, as a result of the ultrafast ESIPT, is very efficient because of the existence of two easily-approached keto S1/S0 conical intersections. In stark contrast to the S1 keto species, the decay of the S1 enol species is almostly blocked. The present theoretical study contributes valuable knowledge to the understanding of photochemistry of similar intramolecularly hydrogen-bonded molecular and biological systems.
The boron carbonyl cation complexes B(CO)3+, B(CO)4+ and B2(CO)4+ are studied by infrared photodissociation spectroscopy and theoretical calculations. The B(CO)4+ ions are characterized to be very weakly bound complexes involving a B(CO)3+ core ion, which is predicted to have a planar DD3h structure with the central boron retaining the most favorable 8-electron configuration. The B2(CO)4+ cation is determined to have a planar D2h structure involving a B-B one and half bond. The analysis of the B-CO interactions with the EDA-NOCV method indicates that the OC→B σ donation is stronger than the B→CO π back donation in both ions.
The photodissociation dynamics of 2-iodotoluene following excitation at 266 nm have been investigated employing femtosecond time-resolved mass spectrometry. The photofragments are detected by multiphoton ionization using an intense laser field centered at 800 nm. A dissociation time of 38±50 fs was measured from the rising time of the co-fragments of toluene radical (C7H7) and iodine atom (I), which is attributed to the averaged time needed for the C-I bond breaking for the simultaneously excited nσ* and ππ* states by 266 nm pump light. In addition, a probe light centered at 298.23 nm corresponding to resonance wavelength of ground-state iodine atom is used to selectively ionize ground-state iodine atoms generated from the dissociation of initially populated nσ* and ππ* states. And a rise time of 40±50 fs is extracted from the fitting of time-dependent I+ transient, which is in agreement with the dissociation time obtained by multiphoton ionization with 800 nm, suggesting that the main dissociative products are ground-state iodine atoms.
We have obtained the high-resolution threshold photoelectron (TPE) spectra of chlorobenzene C6H5Cl (X1A1), propargyl radical C3H3 (X2B1), and allyl radical C3H5 (X2A1) by employing the vacuum ultraviolet (VUV) laser velocity-map-imaging-TPE (VUV-VMI-TPE) method. The photoelectron energy resolution of 1-2 cm-1 observed for the VUV-VMI-TPE method is comparable to that achieved in VUV laser pulsed-field ionization-photoelectron (VUV-PFI-PE) measurements. Similar to VUV-PFI-PE measurements, the energy resolutions for VUV-VMI-photoelectron (VUV-VMI-PE) and VUV-VMI-TPE measurements are found to depend on the dc electric field F in V/cm used at the photoionization region for electron extraction. The decrease of the ionization thresholds of C6H5Cl and C3H3 observed as a function of F shows that the Stark shift correction for VUV-VMI-TPE measurements is governed by the formula -3.1√F in cm-1, which is half of the classical prediction of -6.1√F in cm-1.} We have also measured the VUV-VMI-PE spectra of C6H5Cl and C3H5 at VUV energies near their ionization thresholds. The cationic vibrational bands observed in the VUV-VMI-PE measurements were assigned to be the vibrational progression, nv7+ (n=0-3), for C3H5+. The higher experimental sensitivity and similar energy resolutions achieved in VUV-VMI-TPE compared to VUV-PFI-PE measurements make the VUV-VMI-TPE method an excellent alternative for high-resolution VUV-PFI-PE measurements.
Here we report the theory formulation and the experiment realization of sum-frequency generation vibrational spectroscopy (SFG-VS) in the cross-propagation (XP) geometry or configuration. In the XP-SFG-VS, the visible and the infrared (IR) beams in the SFG experiment are delivered to the same location on the surface from visible and IR incident planes perpendicular to each other, avoiding the requirement to have windows or optics to be transparent to both the visible and IR frequencies. Therefore, the XP geometry is applicable to study surfaces in the enclosed vacuum or high pressure chambers with far infrared (FIR) frequencies that can directly access the metal oxide and other lower frequency surface modes, with much broader selection of visible and IR transparent window materials. The potential applications include surface science, material science, fundamental catalytic sciences, as well as low temperature molecular sciences, etc.
Vibrational and structural dynamics of two transition metal carbonyl complexes, Mn(CO)5Br and Re(CO)5Br were examined in DMSO, using ultrafast infrared pump-probe spectroscopy, steady-state linear infrared spectroscopy and quantum chemistry computations. Two carbonyl stretching vibrational modes (a low-frequency A1 mode and two high-frequency degenerate E modes) were used as vibrational probes. Central metal effect on the CO bond order and force constant was responsible for a larger E-A1 frequency separation and a generally more red-shifted E and A1 peaks in the Re complex than in the Mn complex. A generally broader spectral width for the A1 mode than the E mode is believed to be partially due to vibrational lifetime effect. Vibrational mode-dependent diagonal anharmonicity was observed in transient infrared spectra, with a generally smaller anharmonicity found for the E mode in both the Mn and Re complexes.
Electrochemical (EC) reactions play vital roles in many disciplines, and its molecular-level understanding is highly desired, in particular under reactions. The vibration spectroscopy is a powerful in situ technique for chemical analysis, yet its application to EC reactions is hindered by the strong attenuation of infrared (IR) light in both electrodes and electrolytes. Here we demonstrate that by incorporating appropriate sub-wavelength plasmonic structures at the metal electrode, the IR field at the EC interface can be greatly enhanced via the excitation of surface plasmon. This scheme facilitates in situ vibrational spectroscopic studies, especially using the surface-specific sum-frequency generation technique.
The benzoin group caged compound has received strong interests due to its excellent photo-deprotection properties and wide use in chemical and biological studies. We used time-resolved infrared spectroscopy to investigate the photochemical reaction of the benzoin caged compound, o-(2-methylbenzoyl)-DL-benzoin under 266 nm laser irradiation. Taking advantage of the specific vibrational marker bands and the IR discerning capability, we have detected and identified the uncaging product 2-methylbenzoic acid, and two intermediate radicals of benzoyl and 2-methylbenzoate benzyl in the transient infrared spectra. Our results provide spectral evidence to support the homolytic cleavage reaction of C-C=O bond in competition with the deprotection reaction. Moreover, the product yields of 2-methylbenzoic} acid and benzoyl radical were observed to be affected by solvents and a largely water containing solvent can be in favor of the deprotection reaction.
We report a newly constructed laser ablation crossed molecular beam apparatus, equipped with time-sliced velocity map imaging technique, to study state-to-state metal atom reaction dynamics. Supersonic metal atomic beam is generated by laser vaporization of metal rod, and free expansion design without gas flow channel has been employed to obtain a good quality of metal atomic beam. We have chosen the crossed-beam reaction Al+O2 to test the performance of the new apparatus. Two-rotational-states selected AlO(X2+, v=0, N and N+14) products can be imaged via P(N) and R(N+14) branches of the Δv=1 band at the same wavelength, during (1+1) resonance-enhanced multi-photon ionization through the AlO(D2+) intermediate state. In our experiment at 244.145 nm for simultaneous transitions of P(15) and R(29) branch, two rings in slice image were clearly distinguishable, corresponding to the AlO(v=0, N=15) and AlO(v=0, N=29) states respectively. The energy difference between the two rotational levels is 403 cm-1. The success of two states resolved in our apparatus suggests a better collisional energy resolution compared with the recent research study [J. Chem. Phys. 140, 214304 (2014)].
The crystal phase, morphology and facet significantly influence the catalytic and photocatalytic activity of TiO2. In view of optimizing the performance of catalysts, extensive efforts have been devoted to designing new sophisticate TiO2 structures with desired facet exposure, necessitating the understanding of chemical properties of individual surface. In this work, we have examined the photooxidation of methanol on TiO2(011)-(2×1) and TiO2(110)-(1×1) by two-photon photoemission spectroscopy (2PPE). An excited state at 2.5 eV above the Fermi level (EF) on methanol covered (011) and (110) interface has been detected. The excited state is an indicator of reduction of TiO2 interface. Irradiation dependence of the excited resonance signal during the photochemistry of methanol on TiO2(011)-(2×1) and TiO2(110)-(1×1) is ascribed to the interface reduction by producing surface hydroxyls. The reaction rate of photooxidation of methanol on TiO2(110)-(1×1) is about 11.4 times faster than that on TiO2(011)-(2×1), which is tentatively explained by the difference in the surface atomic configuration. This work not only provides a detailed characterization of the electronic structure of methanol/TiO2 interface by 2PPE, but also shows the importance of the surface structure in the photoreactivity on TiO2.
In quantum calculations a transformed Hamiltonian is often used to avoid singularities in a certain basis set or to reduce computation time. We demonstrate for the Fourier basis set that the Hamiltonian can not be arbitrarily transformed. Otherwise, the Hamiltonian matrix becomes non-hermitian, which may lead to numerical problems. Methods for correctly constructing the Hamiltonian operators are discussed. Specific examples involving the Fourier basis functions for a triatomic molecular Hamiltonian (J=0) in bond-bond angle and Radau coordinates are presented. For illustration, absorption spectra are calculated for the OClO molecule using the time-dependent wavepacket method. Numerical results indicate that the non-hermiticity of the Hamiltonian matrix may also result from integration errors. The conclusion drawn here is generally useful for quantum calculation using basis expansion method using quadrature scheme.
The growth pattern and electronic properties of TiGen- (n=7-12) clusters were investigated using anion photoelectron spectroscopy and density functional theory calculations. For both anionic and neutral TiGen clusters, a half-encapsulated boat-shaped structure appears at n=8, and the boat-shaped structure is gradually covered by the additional Ge atoms to form Gen cage at n=9-11. TiGe12- cluster has a distorted hexagonal prism cage structure. According to the natural population analysis, the electron transfers from the Gen framework to the Ti atom for TiGen-/0 clusters at n=8-12, implying that the electron transfer pattern is related to the structural evolution.
The amide A band of protein is sensitive to the hydrogen bands of amide groups of proteins. However, it is hard to distinguish the amide A band of aqueous protein in situ directly, since it overlaps with O-H stretching vibration of water. In this work, we presented a new analytical method of Raman ratio spectrum, which can extract the amide A band of proteins in water. To obtain the Raman ratio spectrum, the Raman spectrum of aqueous protein was divided by that of pure water. A mathematical simulation was employed to examine whether Raman ratio spectrum is effective. Two kinds of protein, lysozyme and α-chymotrypsin were employed. The amide A bands of them in water were extracted from Raman ratio spectra. Additionally, the process of thermal denaturation of lysozyme was detected from Raman ratio spectrum. These results demonstrated the Raman ratio spectra could be employed to study the amide A modes of proteins in water.
We study the photodissociation dynamics of nitrous oxide using the time-sliced ion velocity imaging technique at three photolysis wavelengths of 134.20, 135.30, and 136.43 nm. The O(1SJ=0)+N2(X1g+) product channels were investigated by measuring images of the O(1SJ=0) products. Vibrational states of N2(X1g+) products were fully resolved in the images. Product total kinetic energy releases (TKER) and the branching ratios of vibrational states of N2 products were determined. It is found that the most populated vibrational states of N2 products are v=2 and v=3. The angular anisotropy parameters (β values) were also derived. The β values are very close to 2 at low vibrational states of the correlated N2(X1g+) products at all three photolysis wavelengths, and gradually decrease to about 1.4 at v=7. This indicates the dissociation is mainly through a parallel transition state to form products at lower vibrational states, and the highly vibrational exited products are from a more bent configuration. This is consistent with the observed shift of the most intense rotational structure in the TKER as the vibrational quantum number increases.
The photochemical reaction process of anthraquinone-2-sodium sulfonate (AQS) in the mixture of water (H2O) and N-butylpyridinium tetrafluoroborate ([BPy][BF4]) was studied using the laser flash photolysis technique. Experimental results show that the excited triplet of AQS (3AQS*) could react rapidly with H2O and the transient absorption spectra greatly changed by increasing the volume fraction of the ionic liquid (VIL) in [BPy][BF4]/H2O mixtures. The absorbance at 510 nm increased gradually with increasing VIL when 0< VIL< 0.1. By contrast, the absorbance decreased gradually when VIL>0.1. Otherwise, the absorbance of the band near 380 nm steadily increased. The apparent kinetic parameters of transient species B and 3AQS* are obtained approximately. 3AQS* abstracting hydrogen from [BPy]+ was also explored. It was deduced that the 350-420 nm band was the superposition of the peaks of 3AQS* and AQSH·. The two reactions of 3AQS* with [BPy][BF4] and H2O are a pair of competitive reactions. We also concluded that the entire reaction processes slow down in the case of high [BPy][BF4] concentrations.
Femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy can extract the curve of spectral gain from its parametric superfluorescence. This unique spectrum correction method enables fluorescence non-collinear optical parametric amplification spectroscopy acquiring the genuine transient fluorescence spectrum of the studied system. In this work we employ fluorescence non-collinear optical parametric amplification spectroscopy technique to study the solvation dynamics of DCM dye in ethanol solution, and confirm that genuine solvation correlation function and shift of peak frequency can be derived from transient fluorescence spectra after the spectral gain correction. It demonstrates that fluorescence non-collinear optical parametric amplification spectroscopy can benefit the research fields, which focuses on both fluorescence intensity dynamics and fluorescence spectral shape evolution.
Radioactive noble-gas isotopes, 85Kr (half-life t1/2=10.8 y), 39Ar (t1/2=269 y), and 81Kr (t1/2=229,000 y), are ideal tracers and can be detected by atom trap trace analysis (ATTA), a laser-based technique, from environmental samples like air and groundwater. Prior to ATTA measurements, it is necessary to efficiently extract krypton and argon gases from samples. Using a combination of cryogenic distillation, titanium chemical reaction and gas chromatography, we demonstrate that we can recover both krypton and argon gases from 1-10 L "air-like" samples with yields in excess of 90% and 98%, respectively, which meet well the requirements for ATTA measurements. A group of testing samples are analyzed to verify the performance of the system, including two groundwater samples obtained from north China plain.