Photolysis Dynamics Laser Chemistry Lab

Upon one-photon excitation at 248 nm, gaseous CH3C(O)SH is dissociated following three pathways with the products of (1) OCS + CH 4, (2) CH3SH + CO, and (3) CH2CO + H 2S that are detected using time-resolved Fourier-transform infrared emission spectroscopy. The excited state 1(nO, π *CO) has a radiative lifetime of 249 ± 11 ns long enough to allow for Ar collisions that induce internal conversion and enhance the fragment yields. The rate constant of collision-induced internal conversion is estimated to be 1.1 × 10-10 cm3 molecule -1 s-1. Among the primary dissociation products, a fraction of the CH2CO moiety may undergo further decomposition to CH2 + CO, of which CH2 is confirmed by reaction with O2 producing CO2, CO, OH, and H2CO. Such a secondary decomposition was not observed previously in the Ar matrix-isolated experiments. The high-resolution spectra of CO are analyzed to determine the ro-vibrational energy deposition of 8.7 ± 0.7 kcal/mol, while the remaining primary products with smaller rotational constants are recognized but cannot be spectrally resolved. The CO fragment detected is mainly ascribed to the primary production. A prior distribution method is applied to predict the vibrational distribution of CO that is consistent with the experimental findings. © 2013 American Institute of Physics.

A new reaction scheme is proposed to account for roaming and chaotic behaviors in collisional and photo-initiated molecular-beam reactions, where nonadiabatic dynamics plays a key role and the collapse of superposition of wave functions is considered to be important in the beginning of the present scheme. Since the feature of molecular orbitals of reagents is crucial in reaction, we showed how to map out the spatial distribution of the relevant HOMO molecular orbitals of CH3Cl in the impact of fast electrons. We identified by experiment that the multiple overlap of nearby molecular orbitals affects even the vibrational motion of adjacent molecule DCl of the transient [ClDCl] chemical species. We also showed dynamical steric effects in the HBr + OH four-atom reaction as a manifestation of the nonadiabatic dynamics in complex systems. The roaming mechanism in the photo-initiated reaction of methyl formate is clarified in detail by experiment as well as the QCT trajectory calculation, where the conical intersection region plays an essential role. We suggest that two types of roaming trajectories coexist, i.e., deterministic and chaotic roaming trajectories based on classical trajectory calculations. To clarify the nonadiabatic dynamics in the roaming mechanism for non-collinear three-dimensional (3D) collisions, a new model of the 3D Polanyi rule is proposed as the extension of the well-established 2D Polanyi rule. In the 3D Polanyi rule, it is expected that the curvature and torsion of Frenet–Serret formulas in three-dimensional space would provide us key concepts in understanding reaction dynamics. © 2018, Accademia Nazionale dei Lincei.

This perspective article aims at accounting for the versatility of some current experimental investigations for exploring novel paths in chemical reactions. It updates a previous one [Phys. Chem. Chem. Phys., 2005, 5, 291] and is limited to work by the authors. The use of advanced molecular beam techniques together with a combination of modern tools for specific preparation, selection and detection permits us to discover new trends in reactivity in the gas phase as well as at interfaces. We specifically discuss new facets of stereodynamics, namely the effects of molecular orientation and alignment on reactive and photodissociation processes. Further topics involve roaming paths and triple fragmentation in photodissociation probed by imaging techniques, chirality effects in collisions and deviations from Arrhenius behavior in the temperature dependence of chemical reactions. © the Partner Organisations 2014.

Palladium-copper nanoparticles were placed on activated carbon to give a nanocomposite for electrochemical sensing of riboflavin (vitamin B 2 ). The activated carbon was produced by pyrolysis of natural waste of pistachio nutshells after KOH activation and under a nitrogen atmosphere. The carbons possess a large surface area and micro/meso-porosity. The nanocomposite was characterized by a variety of techniques to confirm structures and morphology. A screen-printed electrode modified with the composite was examined by EIS, CV, DPV, and amperometry. The effects of pH value, scan rate, and stability of the modified electrode were studied. Under optimized conditions, vitamin B 2 displays a well-expressed oxidation peak at −0.15 V (vs. Ag/AgCl) in solutions with a pH value of 7.0. The voltammetric signal increases linearly in the 0.02 to 9 μM concentrations range and a lower detection limit of 7.6 pM. The sensor was successfully applied to the determination of vitamin B 2 even in the presence of other common vitamins and in (spiked) raw milk samples. [Figure not available: see fulltext.]. © 2019, Springer-Verlag GmbH Austria, part of Springer Nature.

Emergence of biochemical homochirality is an intriguing topic, and none of the proposed scenarios has encountered a unanimous consensus. Candidates for naturally occurring processes, which may originate chiral selection, involve interaction of matter with light and molecular collisions. We performed and report here: (1) simulations of photodissociation of an oriented chiral molecule by linearly polarized (achiral) light observing that the angular distribution of the photofragments is characteristic of each enantiomer and both differ from the racemic mixture; and (2) molecular dynamics simulations (elastic collisions of oriented hydrogen peroxide, one of the most simple chiral molecules, with Ne atom) demonstrating that the scattering and the recoil angles are specific of the enantiomeric form. The efficacy of non-chiral light (in the case of photodissociation) and of non-chiral projectile (in the case of collisions) is due to the molecular orientation, as an essential requirement to observe chiral effects. The results of the simulations, that we report in this article, provide the background for the perspective realization of experiments which go beyond the well-documented ones involving interaction of circularly polarized laser (chiral light) with the matter, specifically by making use of non-chiral, i.e. linearly polarized or unpolarized light sources, and also by obtaining chiral effects with no use at all of light, but simply inducing them by molecular collisions. The case of vortices is discussed in a companion paper. © 2013 Accademia Nazionale dei Lincei.

In this study, Liquidambar formosana tree leaves have been used as a renewable biomass precursor for preparing porous carbons (PCs). The PCs were produced by pyrolysis of natural waste of leaves after 10% KOH activation under a nitrogen atmosphere and characterized by a variety of state-of-the-art techniques. The PCs possess a large surface area, micro-/mesoporosity, and functional groups on its surface. A glassy carbon electrode modified with high PCs was explored as an efficient binder-free electrocatalyst material for the voltammetric determination of nitro isomers such as 3-nitroaniline (3-NA) and 4-nitroaniline (4-NA). Under optimal experimental conditions, the electrochemical detection of 3-NA and 4-NA was found to have a wide linear range of 0.2-115.6 and 0.5-120 μM and a low detection limit of 0.0551 and 0.0326 μM, respectively, with appreciable selectivity. This route not only enhanced the benefit from biomass wastes but also reduced the cost of producing electrode materials for electrochemical sensors. Additionally, the sensor was successfully applied in the determination of nitro isomers even in the presence of other common electroactive interference and real samples analysis (beverage and pineapple jam solutions). Therefore, the proposed method is simple, rapid, stable, sensitive, specific, reproducible, and cost-effective and can be applicable for real sample detection. © 2019 American Chemical Society.

Amine-functionalized graphitic carbon nitride (NH2/GCN) nanosheets photoluminescence, catalytic properties and excellent water dispersion stability were prepared and characterized by a variety of different analytical and spectroscopic techniques. The well-dispersed NH2/GCN nanosheets were found to exhibit remarkable pH sensing sensitivity at an ambient temperature with desirable broad detection range (1 ≤ pH ≤ 12). Moreover, upon incorporating silver nanoparticles (Ag NPs), the Ag-NH2/GCN nanocomposites showed excellent performances for catalytic reduction of 4-nitrophenol (4-NP) in NaBH4 with a superior rate constant (k) of 0.1594 s-1 within as short as 30 s. The NH2/GCN and Ag-NH2/GCN nanocomposites reported herein therefore render prospective applications as sensitive pH and practical catalytic applications. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

We utilize single molecule spectroscopy combined with time-correlated single-photon counting to probe electron transfer (ET) kinetics from CdSe/ZnS (core/shell) quantum dots (QDs) to TiO2 through various lengths of linker molecules. The QD-linker-TiO2 complexes with varied linker length, linker structure, and QD size are fabricated by a surface-based stepwise method to show control of the rate and of the magnitude of fluctuations of photo-induced ET at the single molecule level. The ET rate constants are determined to be 2.8×107, 1.9×107, and 3.5×106s-1 for the chain length of 1.5, 6.2 and 13.8Å, respectively. The electronic coupling strengths between QDs and TiO2 are further calculated to be 3.68, 3.60, and 1.59cm-1 for the three different chain lengths by using the Marcus ET model. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this work, we describe a facile fabrication of silver nanoparticles decorated on porous ultrathin two dimensional (2D) graphitic carbon nitride nanosheets (AgNPs@g-CN) via chemical approach, which was characterized by various analytical techniques including cyclic voltammetry (CV), differential pulse voltammetry (DPV) and chronoamperometry. As expected, the AgNPs@g-CN modified glassy carbon electrode (AgNPs@g-CN/GCE) exhibited remarkable electrocatalytic activity towards the detection of quercetin (QCR) with a wide linear range from 1.0 × 10−8 to 1.2 × 10−4 mol L−1 and a lower detection limit of 6.0 × 10−9 mol L−1. Besides, the amperometric results revealed that the peak current for QCR could not affect upon the sequential additions of electroactive interfering species such as metal ions (300 μM), biomolecules (100 μM), and other flavonoids (50 μM) indicating the selectivity of the proposed sensor. Moreover, the AgNPs@g-CN modified electrode displayed higher stability and reproducibility towards the detection of QCR. The AgNPs@g-CN/GCE could also be used to detect QCR in green apple (GA) samples with satisfactory recoveries for practical applications. The concepts behind the novel architecture to modify electrodes can be potentially harnessed in other electrochemical sensors and photocatalysis applications. © 2018 Elsevier B.V.

Evanescent wave cavity ring-down absorption spectroscopy (EW-CRDS) is employed to study interaction and binding kinetics of DNA strands by using gold nanoparticles (Au NPs) as sensitive reporters. These Au NPs are connected to target DNA of study that hybridizes with the complementary DNA fixed on the silica surface. By the absorbance of Au NPs, the interaction between two DNA strands may be examined to yield an adsorption equilibrium constant of 2.2×1010M-1 using Langmuir fit. The binding efficiency that is affected by ion concentration, buffer pH and temperature is also examined. This approach is then applied to the label-free detection of the DNA mutation diseases using the sandwich hybridization assay. For monitoring a gene associated with sickle-cell anemia, the detection limit and the adsorption equilibrium constant is determined to be 1.2pM and (3.7±0.8)×1010M-1, distinct difference from the perfectly matched DNA sequence that yields the corresponding 0.5pM and (1.1±0.2)×1011M-1. The EW-CRDS method appears to have great potential for the investigation of the kinetics of a wide range of biological reactions. © 2014 Elsevier B.V.

Evanescent-wave cavity ring-down spectroscopy is applied to investigate the adsorption behavior of rhodamine B at three different interfaces. The adsorption equilibrium constant (Kads) and adsorption free energy of rhodamine B at the silica/methanol interface are determined to be (1.5 ± 0.2) × 104 M-1 and -23.8 ± 0.4 kJ/mol by use of a Langmuir isotherm model. A Langmuir-based kinetic model is also developed to determine the corresponding adsorption and desorption rate constants of (1.02 ± 0.03) × 102 M-1 s-1 and (7.1 ± 0.2) × 10-3 s-1, from which Kads is obtained to be (1.45 ± 0.09) × 104 M-1, in agreement with the value determined under equilibrium conditions. Similarly, when rhodamine B is at the chlorotrimethylsilane-immobilized silica/methanol interface, the adsorption and desorption rate constants are determined to be (1.7 ± 0.2) × 102 M-1 s-1 and (5.0 ± 1.0) × 10-3 s-1· The subsequent Kads is (3.6 ± 0.4) × 104 M-1, which is larger than that at the silica/methanol interface. The former adsorption is dominated by hydrophobic interaction, while the latter is subject to electrostatic attraction. When rhodamine B is at the silica/water interface, there exist three chemical forms, including zwitterion (R+B -), cation (RBH+), and lactone (RBL). A combination of double-layer and Langmuir competitive models is used to fit the adsorption isotherm as a function of solution pH, yielding Kads of (2.5 ± 0.2) × 104 M-1 and (1.1 ± 0.2) × 105 M-1 for R+B- and RBH +, respectively. RBL is considered to have the same Kads value as R+B-. © 2010 American Chemical Society.

The rotational energy transfer (RET) by Ar collisions within the SH X 2Π (v′′ = 0, J′′ = 0.5-10.5) state is characterized. The integral cross sections as a function of collision energy for each rotational transition are calculated using a quantum scattering method in which the constructed potential energy functions are based on a ground state potential energy surface (PES) reported previously. On the other hand, a laser-induced excitation fluorescence technique is employed to monitor the relaxation of the rotational population as a function of photolysis-probe delay time following the photodissociation of H2S at 248 nm. The rotational population evolution is comparable to its theoretical counterpart based on calculated Λ-resolved RET rate constants. The propensity in Λ-resolved RET transitions is found to approximately resemble the case of OH(X 2Π, v′′ = 0) + Ar. The Λ-averaged RET collisions are also analyzed and result in several propensity rules in the transitions. Most propensity rules are similar to those observed in the collisions of SH(A 2Σ+) by Ar. However, the behavior of the conserving ratio, defined as rate constants for spin-orbit conserving transition divided by those for spin-orbit changing transition, shows distinct difference from those described by Hund’s case (b). © the Owner Societies.