Photolysis Dynamics Laser Chemistry Lab

Competitive bond dissociation mechanisms for bromoacetyl chloride and 2- and 3-bromopropionyl chloride following the 1[n(O) →π*(Cï£O)] transition at 234-235 nm are investigated. Branching ratios for C-Br/C-Cl bond fission are found by using the (2+1) resonance-enhanced multiphoton ionization (REMPI) technique coupled with velocity ion imaging. The fragment branching ratios depend mainly on the dissociation pathways and the distances between the orbitals of Br and the Cï£O chromophore. C-Cl bond fission is anticipated to follow an adiabatic potential surface for a strong diabatic coupling between the n(O)π*(Cï£O) and np(Cl)σ*(C-Cl) bands. In contrast, C-Br bond fission is subject to much weaker coupling between n(O)π*(Cï£O) and np(Br)σ*(C-Br). Thus, a diabatic pathway is preferred for bromoacetyl chloride and 2-bromopropionyl chloride, which leads to excited-state products. For 3-bromopropionyl chloride, the available energy is not high enough to reach the excited-state products such that C-Br bond fission must proceed through an adiabatic pathway with severe suppression by nonadiabatic coupling. The fragment translational energies and anisotropy parameters for the three molecules are also analyzed and appropriately interpreted. Busted open: Insight into the mechanisms causing C-Cl and C-Br bond fission of bromoacetyl chloride and 2- and 3-bromopropionyl chloride by following the 1[n(O) →π*(Cï£O)] transition is obtained. The figure shows the center-of-mass translational energy distributions of ground-state Br formation through a diabatic pathway for the dissociation of 2-bromopropionyl chloride. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The fluorogenic aptamer sensing of interferon-gamma (IFN-γ) was scrutinized using two-dimensional (2D) ReS2 and TiS2 nanosheets (NSs) as a platform. The IFN-γ an important cytokine, functions as a bio-indicator to detect infectious diseases such as tuberculosis and human immunodeficiency virus. This 2D NSs based aptamer sensor was implemented to induce the fluorescence off/on resulting from an aptamer, in the absence or presence of a target to be probed. The fluorescence emitting from the aptamer is quenched by interacting with NSs, while the ensuing fluorescence is recovered upon addition of target. Such a fluorescence off/on mechanism was proposed based on the behavior of fluorescence resonance energy transfer (FRET) between the aptamer and NSs. The fluorescence response exhibits linearity as a function of target, and the detection limit of IFN-γ was evaluated to be 57.6 and 82.7 pM for ReS2 and TiS2 NSs, respectively, being comparable to or even better than those methods adopted for probing IFN-γ. The selectivity property was also characterized with various targets, exhibiting a very specific selectivity for IFN-γ. The findings reveal that the aptamer-transition metal dichalcogenides (TMD) NSs will be a great sensing pair to the development of aptamer-based biosensors. Moreover, the biocompatibility and sensing capability of IFN-γ was implemented in human embryonic kidney 293T (HEK) live cells. This is the first report to emerging fluorogenic sensing of IFN-γ aptamer with 2D TMD, showing a promising trend for future design of biosensors. © 2017 Elsevier B.V.

Atmospheric halogen chemistry has drawn much attention, because the halogen atom (X) playing a catalytic role may cause severe stratospheric ozone depletion. Atomic X elimination from X-containing hydrocarbons is recognized as the major primary dissociation process upon UV-light irradiation, whereas direct elimination of the X2 product has been seldom discussed or remained a controversial issue. This account is intended to review the detection of X2 primary products using cavity ring-down absorption spectroscopy in the photolysis at 248 nm of a variety of X-containing compounds, focusing on bromomethanes (CH2Br2, CF2Br2, CHBr2Cl, and CHBr3), dibromoethanes (1,1-C 2H4Br2 and 1,2-C2H 4Br2) and dibromoethylenes (1,1-C2H 2Br2 and 1,2-C2H2Br2), diiodomethane (CH2I2), thionyl chloride (SOCl 2), and sulfuryl chloride (SO2Cl2), along with a brief discussion on acyl bromides (BrCOCOBr and CH2BrCOBr). The optical spectra, quantum yields, and vibrational population distributions of the X2 fragments have been characterized, especially for Br2 and I2. With the aid of ab initio calculations of potential energies and rate constants, the detailed photodissociation mechanisms may be comprehended. Such studies are fundamentally important to gain insight into the dissociation dynamics and may also practically help to assess the halogen-related environmental variation. This journal is © the Partner Organisations 2014.

Both single-laser and two-laser experiments were conducted to look into the ion-imaging of Br*(2P1/2) and Br(2P3/2) photofragmented from 1-bromo-2-methylbutane in the range 232-240 nm via a detection scheme of (2+1) resonance-enhanced multiphoton ionization. The angular analysis of these photofragment distributions yields the anisotropy parameter β = 1.88 ± 0.06 for the Br∗ excited state which arises from a parallel transition, while β = 0.63 ± 0.09 for the Br ground state indicates the contribution from both a perpendicular transition and a non-adiabatic transition. When a hexapole coupled with an orienting field was implemented, the parent molecules are spatially oriented to yield an orientation efficiency |«cos θ »| of 0.15. Besides the χ angle between the recoil velocity v and the transition dipole moment μ, orienting molecules allows for the evaluation of the angle α between v and the permanent molecular dipole moment d. The angular analysis of Br∗ photofragment distribution yields χ = 11.5° and α in the range from 160° to 180° with weak dependency. In the two-laser experiments, the angular anisotropy of Br photofragment distribution was found to be smaller (0.38 ± 0.10) when the photolysis wavelength was red-shifted to 240 nm, suggesting the increasing contributions from perpendicular transitions. © 2019 American Chemical Society.

This perspective focuses on the mechanistic insights and complexity, which are difficult to acquire from pure experimental techniques, of the computational studies of Pd-catalyzed Suzuki, Heck, and Sonogashira carbon-carbon bond-forming reactions. These reactions consist of three fundamental steps including oxidative addition (OA), transmetalation (TM), and reductive elimination (RE) for the generation of carbon-carbon bonds from the bond-forming reactions of aryl halides (R1X) and organometallic species (R2M). Computational studies of these coupling reactions allow us to understand specific reaction pathways in the analysis of OA (resolving the linkage between coordination number and selectivity in Suzuki reaction), TM (the function of the base in the Suzuki reaction and various mechanistic options in the Sonogashira reaction), and RE (way of efficient β-hydride elimination in the Heck reaction). In addition, the reaction pathways and complexities in the full catalytic cycle of each reaction along with the future perspective are also discussed. © 2017 American Chemical Society.

Ion imaging coupled with (2 + 1) resonance-enhanced multiphoton ionization (REMPI) technique is employed to probe CO(v″ = 0) fragments at different rotational levels following photodissociation of methyl formate (HCOOCH 3) at 234 nm. When the rotational level, J″CO, is larger than 24, only a broad translational energy distribution extending beyond 70 kcal mol-1 with an average energy of about 23 kcal mol -1 appears. The dissociation process is initiated on the energetic ground state HCOOCH3 that surpasses a tight transition state along the reaction coordinate prior to breaking into CO + CH3OH. This molecular dissociation pathway accounts for the CO fragment with larger rotational energy and large translational energy. As J″CO decreases, a bimodal distribution arises with one broad component and the other sharp component carrying the average energy of only 1-2 kcal mol-1. The branching ratio of the sharp component increases with a decrease of J″CO; (7.3 ± 0.6)% is reached as the image is probed at J″CO = 10. The production of a sharp component is ascribed to a roaming mechanism that has the following features: a small total translational energy, a low rotational energy partitioning in CO, but a large internal energy in the CH3OH co-product. The internal energy deposition in the fragments shows distinct difference from those via the conventional transition state. © the Owner Societies 2011.

The present study involves the synthesis, characterization, and catalytic application of ruthenium nanoparticles (Ru NPs) supported on plastic-derived carbons (PDCs) synthesized from plastic wastes (soft drink bottles) as an alternative carbon source. PDCs have been further activated with CO2 and characterized by various analytical techniques. The catalytic activity of Ru@PDC for the reduction of potassium hexacyanoferrate(III), (K3[Fe(CN)6]), and new fuchsin (NF) dye by NaBH4 was performed under mild conditions. The PDCs had spherical morphology with an average size of 0.5 μm, and the Ru NP (5 ± 0.2 nm) loading (4.01 wt %) into the PDC provided high catalytic performance for catalytic reduction of ferrocyanate(III) and NF dye. This catalyst can be recycled more than six times with only a minor loss of its catalytic activity. In addition, the stability and reusability of the Ru@PDC catalyst are also discussed. Copyright © 2018 American Chemical Society.

Recent experimental and theoretical advances in the study of the dissociation of excited molecules are revealing unexpected mechanisms, when their outcomes are tackled by combining (i) space-time ion imaging of translational features, with (ii) spectroscopic probing of rotational and vibrational distributions; crucial is the assistance of (iii) the quantum chemistry of structural investigations of rearrangements of chemical bonds, and of (iv) the simulations of molecular dynamics to follow the evolution of selective bond stretching and breaking. Here we present results of such an integrated approach to methyl formate, HCOOCH3, the simplest of esters; the main focus is on the rotovibrationally excited CO (v = 1) product and in general on the energy distribution in the fragments. Previous laser studies of dissociation into CO and CH3OH at a sequence of various wavelengths discovered signatures of a roaming mechanism by the late arrival of CO (v = 0) products in time-of-flight ion imaging. Subsequent detailed investigations as a function of excitation energy provided the assessment of the threshold, which opens for triple breakdown into CO and further fragments H and CH3O, as spectroscopically characterized by ion imaging and FTIR respectively. Accompanying quantum mechanical electronic structure calculations and classical molecular dynamics simulations clarify the origin of these fragments through "roaming" pathways involving incipient radical intermediates at energies below the triple fragmentation threshold: a specific role is played by nonadiabatic transitions at a conical intersection between ground and excited states; alternative pathways focalize our attention to regions of the potential energy surfaces other than those in the neighbourhoods of saddle points along minimum energy paths: eventually this leads us to look for avenues in reaction kinetics beyond those of venerable transition state theories. This journal is © The Royal Society of Chemistry.

The orientation dependence for the Br atom formation in the reaction of the oriented OH radicals with HBr molecules at 0.26 eV collision energy has been observed for the first time using the hexapole electric field, and we found that the reaction cross-section for O-end attack is more favorable than that for H-end attack by a factor of 3.4 ± 2.3. © the Owner Societies.

Following photodissociation of acetaldehyde (CH3CHO) at 308 nm, the CO(v = 1-4) fragment is acquired using time-resolved Fourier-transform infrared emission spectroscopy. The CO(v = 1) rotational distribution shows a bimodal feature; the low- and high-J components result from H-roaming around CH3CO core and CH3-roaming around CHO radical, respectively, in consistency with a recent assignment by Kable and co-workers (Lee et al., Chem. Sci. 5, 4633 (2014)). The H-roaming pathway disappears at the CO(v 2) states, because of insufficient available energy following bond-breaking of H + CH3CO. By analyzing the CH4 emission spectrum, we obtained a bimodal vibrational distribution; the low-energy component is ascribed to the transition state (TS) pathway, consistent with prediction by quasiclassical trajectory calculations, while the high-energy component results from H- and CH3-roamings. A branching fraction of H-roaming/CH3-roaming/TS contribution is evaluated to be (8% ± 3%)/(68% ± 10%)/(25% ± 5%), in which the TS pathway was observed for the first time. The three pathways proceed concomitantly along the electronic ground state surface. © 2015 AIP Publishing LLC.

Molecular orientation techniques are becoming available in the study of elementary chemical processes, in order to highlight those structural and dynamical properties that would be concealed by random rotational motions. Recently successful orientation was achieved for asymmetric-top and chiral molecules of much larger complexity than hitherto. In this work, we report and discuss the correlation between the vectors' photofragment recoil velocity v, transition dipole moment μ, and permanent dipole moment d in a dissociation experiment on hexapole oriented 2-bromobutane, photoinitiated by a linearly polarized laser. The sliced ion images of the Br∗(2P1/2) and Br(2P3/2) photofragments were acquired at 234.0 and 254.1 nm, respectively, by a (2 + 1) resonance-enhanced multiphoton ionization technique. A detailed analysis of the sliced ion images obtained at a tilting angle 45° of laser polarization provides information on the correlation of the three vectors, which are confined by two polar angles α and χ and one azimuthal angle φμd in the recoil frame. The sliced ion images of Br fragments eliminated individually from the enantiomers at 254.1 nm yield an asymmetric factor close to zero; for this reason the photofragment angular distributions do not show significant differences. The elimination of the Br∗ fragment at 234.0 nm is mainly correlated with a parallel transition, giving rise to a large anisotropy parameter of 1.85, and thus can be considered as a single state excitation. The resulting recoil frame angles are optimized to 163° ± 8° and 164° ± 1° for α and χ, respectively, whereas φμd is approaching 0° for the best fit. Since for the present molecule, the three vectors have an only slight spatial arrangement, the photofragment angular distributions of the two enantiomers do not show appreciable differences. Theoretical and computational simulations provide us the basis to state that oriented enantiomers can be discriminated on-the-fly in photodissociation processes even initiated by non-circularly polarized light, provided that the three vectors encountered above have specific three-dimensional arrangements. The fact that Br fragment elimination involves a multi-potential dissociation carries uncertainties in theoretical estimates of the vector direction. Therefore, this work represents a preliminary but significant step on the road to chiral discrimination on-the-fly, which is shown to be best propitiated in molecules where vectors are far from having degenerate mutual angular directions. © 2019 the Owner Societies.

Steady-state and time-resolved fluorescence spectroscopy techniques were used to probe multifluorescence resulting from citric-acid-derived carbon dots (C-dots). Commonly, both carboxyl-/amine-functionalized C-dots exhibit three distinct emissive states corresponding to the carbon-core and surface domain. The shorter-wavelength fluorescence (below 400 nm) originates from the carbon-core absorption band at ∼290 nm, whereas the fluorescence (above 400 nm) is caused by two surface states at ∼350 and 385 nm. In addition to three emissive states, a molecular state was also found in amine-functionalized C-dots. Time-resolved emission spectra (TRES) and time-resolved area normalized emission spectra (TRANES) were analyzed to confirm the origin of excitation wavelength-dependent fluorescence of C-dots. The surface functional groups on the C-dots are capable of regulating the electron transfer to affect the multifluorescence behavior. The electron transfer takes place from the carbon-core to surface domain by the presence of -COOH on the surface and vice versa for the case of -NH2 present on the surface. To the best of our knowledge, this is the first report that the multiemissive states are probed in C-dots systems using TRES and TRANES analyses, and related fluorescence mechanisms are verified clearly. © 2015 American Chemical Society.