Aromatization as an Impetus to Harness Ketones for Metallaphotoredox-Catalyzed Benzoylation/Benzylation of (Hetero)arenes.

Herein we report ketones as feedstock materials in radical cross-coupling reactions under Ni/photoredox dual catalysis. In this approach, simple condensation first converts ketones into prearomatic intermediates that then act as activated radical sources for cross-coupling with aryl halides. Our strategy enables the direct benzylation/benzoylation of (hetero)arenes under mild reaction conditions with high functional group tolerance.

Time-Resolved Pair-Correlated Imaging of the Photodissociation of Acetaldehyde at 267 nm: Pathway Partitioning.

Photodissociation of acetaldehyde (CH3CHO) by UV excitation involves interwoven multiple reaction pathways, including nonradiative decay, isomerization, transition-state pathway, roaming, and other dissociation mechanisms. Recently, we employed picosecond time-resolved, pair-correlated product imaging in a study of acetaldehyde photodissociation at 267 nm to disentangle those competing mechanisms and to elucidate the possible roaming pathways (Yang, C. H.; Chem. Sci. 2020, 11, 6423-6430). Here, we complement the pair-correlated product speed distribution of CO(v = 0) at the high-j side of the CO rotational state distribution in the CO + CH4 channel and detail the two-dimensional data analysis of the time-resolved images. As a result, extensive comparisons with other studies can be made and the branching fractions of the previously assigned TScc(S0), non-TScc(S0), and CI(S1/S0) pathways for the CO(v = 0) + CH4 molecular channel are evaluated to be 0.74 ± 0.08, 0.15 ± 0.02, and 0.11 ± 0.02, respectively. Together with the macroscopic branching ratio between the molecular (CO + CH4) and radical (CH3 + HCO) channels at 267 nm from the literature, a global view of the microscopic pathways can then be delineated, which provides invaluable insights and should pave the way for further studies of this interesting system.

Kinetics of Unimolecular Decay of Methyl Vinyl Ketone Oxide, an Isoprene-Derived Criegee Intermediate, under Atmospherically Relevant Conditions.

Isoprene is the most abundant unsaturated hydrocarbon in the atmosphere. Ozonolysis of isoprene produces methyl vinyl ketone oxide (MVKO), which may react with atmospheric SO2, formic acid, and other important species at substantial levels. In this study, we utilized ultraviolet absorption to monitor the unimolecular decay kinetics of syn-MVKO in real time at 278-319 K and 100-503 Torr. After removing the contributions of radical reactions and wall loss, the unimolecular decay rate coefficient of syn-MVKO was measured to be kuni = 70 ± 15 s-1 (1σ uncertainty) at 298 K with negligible pressure dependence. In addition, kuni increases from ca. 30 s-1 at 278 K to ca. 175 s-1 at 319 K with an effective Arrhenius activation energy of 8.3 ± 2.5 kcal mol-1, kuni(T) = (9.3 × 107)exp(-4200/T) s-1. Our results indicate that unimolecular decay is the major sink of MVKO in the troposphere. The data would improve the estimation for the steady-state concentrations of MVKO and thus its oxidizing ability.

Real-time tracking of the entangled pathways in the multichannel photodissociation of acetaldehyde.

The roaming mechanism, an unconventional reaction path, was discovered more than a decade ago in the studies of formaldehyde photodissociation, H2CO → H2 + CO. Since then, observations of roaming have been claimed in numerous photochemical processes. A closer examination of the presented data, however, revealed that evidence for roaming is not always unequivocal, and some of the conclusions could be misleading. We report here an in-depth, joint experimental and theoretical study of the title reaction. By tracking the time-evolution of the pair-correlated product state distributions, we decipher the competing, interwoven reaction pathways that lead to the radical (CH3 + HCO) and molecular (CH4 + CO) products. Possible roaming pathways are then elucidated and a more precise descriptor of the phenomenon is delineated.

Imaging pair-correlated reaction cross sections in F + CH3D(νb = 0, 1) → CH2D(ν4 = 1) + HF(ν).

The title reactions were studied in a crossed-beam experiment at collisional energies (Ec) from 0.5 to 4.7 kcal mol-1. The νb (ν4) vibrational mode denotes the bending (umbrella) motion of the CH3D reactant (CH2D product). Using a time-sliced, velocity-map imaging technique, we extracted the state-specific, pair-correlated integral and differential cross sections. As with other isotopically analogous ground-state reactions, an inverted vibrational population of the HF coproduct was observed. Both the step-like excitation function near the threshold and the oscillatory forward-backward peakings in the Ec-evolution of the two dominant pair-correlated angular distributions at lower Ec suggest a resonance-mediated, time-delay mechanism. As Ec increases, the angular distribution of the HF(ν = 2) product evolves into a smooth and broad swath in the backward hemisphere, indicative of a direct rebound mechanism. One quantum excitation of the bending modes of CH3D(νb = 1) promotes the reaction rate by two- to three-fold up to Ec = 2.1 kcal mol-1. Broadly speaking, all major findings are qualitatively in line with previous results in the reactions of the F atom with other isotopologues. However, the rainbow feature recently observed in the CH2D(00) + HF(ν = 3) product channel is entirely absent. A possible rationale is put forward, which reinforces the previous reactive rainbow conjecture and calls for future theoretical investigations.

Imaging characterization of the rapid adiabatic passage in a source-rotatable, crossed-beam scattering experiment.

In order to achieve a more efficient preparation of a specific ro-vibrationally excited reactant state for reactive scattering experiments, we implemented the rapid adiabatic passage (RAP) scheme to our pulsed crossed-beam machine, using a single-mode, continuous-wave mid-infrared laser. The challenge for this source-rotatable apparatus lies in the non-orthogonal geometry between the molecular beam and the laser propagation directions. As such, the velocity spread of the supersonic beam results in a significantly broader Doppler distribution that needs to be activated for RAP to occur than the conventional orthogonal configuration. In this report, we detail our approach to shifting, locking, and stabilizing the absolute mid-infrared frequency. We exploited the imaging detection technique to characterize the RAP process and to quantify the excitation efficiency. We showed that with appropriate focusing of the IR laser, a nearly complete population transfer can still be achieved in favorable cases. Compared to our previous setup-a pulsed optical parametric oscillator/amplifier in combination with a multipass ring reflector for saturated absorption, the present RAP scheme with a single-pass, continuous-wave laser yields noticeably higher population-transfer efficiency.

Collision energy dependence of state-to-state differential cross sections for rotationally inelastic scattering of H2O by He.

The inelastic scattering of H2O by He as a function of collision energy in the range 381 cm-1 to 763 cm-1 at an energy interval of approximately 100 cm-1 has been investigated in a crossed beam experiment using velocity map imaging. Change in collision energy was achieved by varying the collision angle between the H2O and He beam. We measured the state-to-state differential cross section (DCS) of scattered H2O products for the final rotational states JKaKc = 110, 111, 221 and 414. Rotational excitation of H2O is probed by (2 + 1) resonance enhanced multiphoton ionization (REMPI) spectroscopy. DCS measurements over a wide range of collision energies allowed us to probe the H2O-He potential energy surface (PES) with greater detail than in previous work. We found that a classical approximation of rotational rainbows can predict the collision energy dependence of the DCS. Close-coupling quantum mechanical calculations were used to produce DCS and partial cross sections. The forward-backward ratio (FBR), is introduced here to compare the experimental and theoretical DCS. Both theory and experiments suggest that an increase in the collision energy is accompanied with more forward scattering.

Interference stabilization of autoionizing states in molecular N2 studied by time- and angular-resolved photoelectron spectroscopy.

An autoionizing resonance in molecular N2 is excited by an ultrashort XUV pulse and probed by a subsequent weak IR pulse, which ionizes the contributing Rydberg states. Time- and angular-resolved photoelectron spectra recorded with a velocity map imaging spectrometer reveal two electronic contributions with different angular distributions. One of them has an exponential decay rate of 20 ± 5 fs, while the other one is shorter than 10 fs. This observation is interpreted as a manifestation of interference stabilization involving the two overlapping discrete Rydberg states. A formalism of interference stabilization for molecular ionization is developed and applied to describe the autoionizing resonance. The results of calculations suggest, that the effect of the interference stabilization is facilitated by rotationally-induced couplings of electronic states with different symmetry.

Direct Imaging of Transient Fano Resonances in N_{2} Using Time-, Energy-, and Angular-Resolved Photoelectron Spectroscopy.

Autoionizing Rydberg states of molecular N_{2} are studied using time-, energy-, and angular-resolved photoelectron spectroscopy. A femtosecond extreme ultraviolet pulse with a photon energy of 17.5 eV excites the resonance and a subsequent IR pulse ionizes the molecule before the autoionization takes place. The angular-resolved photoelectron spectra depend on pump-probe time delay and allow for the distinguishing of two electronic states contributing to the resonance. The lifetime of one of the contributions is determined to be 14±1 fs, while the lifetime of the other appears to be significantly shorter than the time resolution of the experiment. These observations suggest that the Rydberg states in this energy region are influenced by the effect of interference stabilization and merge into a complex resonance.

Dynamics of N2 Dissociation upon Inner-Valence Ionization by Wavelength-Selected XUV Pulses.

Ionization of nitrogen by extreme ultraviolet (XUV) light from the Sun has recently been recognized as an important driver of chemical reactions in the atmosphere of Titan. XUV photons with energies of 24 eV and above convert inert nitrogen molecules into reactive neutral and ionic fragments that initiate chemical reactions. Understanding the XUV-induced fragmentation poses significant challenges to modern theory owing to its ultrafast time scales, complex electronic rearrangements, and strong dependence on the XUV photon energy. Here, we apply femtosecond time-resolved photoelectron and photoion spectroscopy to study dissociative ionization of nitrogen, the most abundant molecule in Titan's atmosphere, at selected XUV photon energies using a table-top XUV time-compensating monochromator. We probe the resulting dynamics using a time-delayed infrared (IR) ionization pulse. Coupled with ab initio calculations, the results allow us to assign the major dissociation channels resulting from production of an inner-valence hole, with important implications for models of Titan's XUV-driven atmospheric chemistry.

The Jahn-Teller effect in the presence of partial isotopic substitution: the B̃(1)E'' state of NH2D and NHD2.

Rotationally resolved resonance enhanced multiphoton ionisation spectra of the B̃(1)E'' state of NH2D are presented and analysed. The analysis indicates a small (34.9 cm(-1)) lifting of the vibronic degeneracy of the zero point level, approximately equal in sign but opposite in magnitude to the splitting observed in NHD2 in previous work. This observation is consistent with previous measurements on systems with partial isotopic substitution subject to a mild Jahn-Teller effect. A model is developed to calculate the splitting induced by asymmetric isotopic substitution of a degenerate electronic state, based on a harmonic force field with linear and quadratic Jahn-Teller terms added. The force field is developed in internal co-ordinates to allow the same parameters to be used to calculate the pattern of vibronic levels for all four isotopologues. The lifting of the degeneracy of the zero point level on asymmetric substitution comes from the quadratic Jahn-Teller effect; the linear term does not lift the degeneracy.

State-to-state resolved differential cross sections for rotationally inelastic scattering of ND3 with He.

State-to-state differential cross sections are reported for rotationally inelastic scattering of fully state-selected ND3 (j(k)(±) = 1(1)(-)) with He. Experimental measurements are compared with full close-coupling quantum-mechanical scattering calculations that used an ab initio potential energy surface. Results are presented for final states up to j'(k')(±) = 7(7)(-) at a mean collision energy of 430 cm(-1). For selected final quantum states, the effect of collision energy on the differential cross sections is also explored in the range 230-720 cm(-1). For the experimental studies, a hexapole electrostatic lens was used for the j(k)(±) state-selection of ND3 molecules in their electronic and vibrational ground states in a molecular beam. This state-selected molecular beam was then crossed with a beam of He atoms. The velocities of inelastically scattered ND3 molecules in single j'(k')(±) states were obtained by velocity map imaging, and converted to differential cross sections in the centre-of-mass frame by density-to-flux transformation. The close-coupling calculations reproduce well the measured angular distributions. For small changes in the rotational angular momentum quantum number (j), the ND3 is predominantly forward scattered, but the scattering shifts to the sideways and backward directions as Δj increases. For scattering into a given j'(k')(±) state, cross-sections for collisions that conserve the ± symmetry associated with the ND3 inversion vibration are larger and generally more forward scattered than the corresponding symmetry-changing processes.

Resonance enhanced multiphoton ionization spectroscopy of NHD2 via the B1E''-state.

Rotational analysis of the (2 + 1) resonance enhanced multiphoton ionization (REMPI) spectrum of the B[combining tilde](1)E'' Rydberg state of the ammonia isotopologue NHD2 is reported. While the electronic degeneracy is lifted in NHD2 the splitting is small enough that interactions between the two states must be considered, particularly to model the intensity of the transitions. A simple model is developed to account for these interactions, relating them to terms present in the symmetric isotopologues. Spectroscopic parameters for the zero point and (ν2' = 1-6) vibrational levels of the B (1)E'' state have been derived using this model and the spectra are accurately simulated for the first time using the pgopher program. The current work provides the basis for on-going velocity map imaging studies of rotational energy transfer in the mixed isotopologues of ammonia.

Rotational excitation of HDO and D2O by H2: experimental and theoretical differential cross-sections.

We present state-to-state differential cross sections (DCSs) for rotationally inelastic scattering of HDO by normal- and para-H(2) at collision energies of 580 cm(-1) and 440 cm(-1). (2+1) resonance enhanced multiphoton ionization is used to detect rotationally cold HDO molecules before collision and as scattering products, which occupy higher rotational states due to collision with H(2). Relative integral cross sections of HDO are obtained by integrating its DCSs measured at the same experimental conditions. Experimental and theoretical DCSs of HDO scattered by normal- and para-H(2) are in good agreement in 30°-180° range of scattering angles. This partial agreement shows the accuracy of the recently tested potential of H(2)O-H(2), but now by using a completely different set of rotational transitions that are (unlike in H(2)O), not forbidden by nuclear spin restrictions. Similar results are presented for D(2)O scattered by normal-H(2) at collision energy of 584 cm(-1). The agreement between experiment and theory is, however, less good for forward scattering of HDO/D(2)O. A critical analysis of this discrepancy is presented.

A large aperture magnification lens for velocity map imaging.

We have designed and implemented a large aperture electrostatic Einzel lens that magnifies the images of low energy ions or electrons in a standard velocity map imaging apparatus by up to a factor of 5 while allowing the normal use of the apparatus (without blocking any part of the detector). The field strength in the interaction region remains reasonably constant with or without magnification, and the lens can be used in the normal "crush" mode or with any of the different variants of the "slicing" mode. We have characterized the performance of the lens by imaging ion recoil due to two-photon resonant three-photon ionization [(2+1) REMPI] of O((3)P(2)) atoms and by imaging slow NO molecules from the near-threshold photodissociation of the NO-Ar van der Waals complex.

Imaging the inelastic scattering of water with helium. Comparison of experiment and theory.

State-to-state differential cross sections for rotationally inelastic He-H(2)O scattering have been measured at 53.3 meV (429 cm(-1)) collision energy, using the crossed molecular beam technique. The inelastic events are detected by velocity map imaging of nascent H(2)O(+) ions, which are formed by state-selective (2 + 1) resonance enhanced multiphoton ionization (REMPI) of the scattered H(2)O molecules. Raw density images are converted to flux images and the extracted differential cross sections are compared with full close-coupling calculations of state-to-state cross sections for rotational excitation based on a previously published ab initio potential. A hard-shell ellipsoid model is also employed to yield a more physical insight useful in interpreting the results. The excellent agreement of fully quantum theory and experiment found here for water collisions with helium at a collision energy relevant to that of the interstellar media place the theoretically determined potential energy surface and the collision cross sections extracted using this surface on a firmer basis.

Photodissociation imaging of diatomic sulfur (S2).

The photodissociation of diatomic sulfur, S(2), in the region of the first dissociation limit is studied with velocity map imaging. Correlated fine structure distributions P(J1,J2) for the two S((3)P(J)) fragments are determined at selected photolysis wavelengths. Image analysis of the speed distributions of the atomic fragments following product-state-specific detection results in a revision of the bond energy to D(0) = 35636.9 +/- 2.5 cm(-1) with respect to the lowest rovibrational level. This value arises from reinterpretation of previous spectroscopic data showing onset of predissociation in the B(3)Sigma(u)(-) state, as the measurements presented here demonstrate that the long-range correlation of the excited state invoked as causing the dissociation is S((3)P(2)) + S((3)P(2)) rather than S((3)P(2)) + S((3)P(1)). The wavelength dependence of data for the S((3)P(2)) + S((3)P(2)) channel suggests involvement of photoexcitation through the optically forbidden Herzberg continuum bands in addition to dissociation initiated via the optically allowed B(3)Sigma(u)(-)-X(3)Sigma(g)(-) and B''(3)Pi(u)-X(3)Sigma(g)(-) bands. Changes in product recoil velocity angular distributions and atomic angular momentum polarization were also measured as a function of dissociation wavelength. The results are compared with predictions from an adiabatic model for dissociation, which provides a basis for interpretation but does not explain quantitatively the experimental results.

A crossed molecular beam study on the dynamics of F atom reaction with SiH4.

In this report, the dynamics of the F+SiH4 reaction has been studied using the universal crossed molecular beam method. Angular resolved time-of-flight spectra have been measured for all reaction products in a single set of experiments. Two different reaction channels have been observed: HF+SiH3 and SiH3F+H. Product angular distributions as well as energy distributions were determined for these two product channels. Experimental results show that the HF product is forward scattered relative to the F atom beam direction, while the SiH3F product is backward scattered relative the F atom beam direction, suggesting that two reaction channels proceed with distinctive reaction dynamics. The relative branching ratios of the two channels have also been estimated.

Molecular beam studies of the F atom reaction with propyne: site specific reactivity.

The dynamics of the F atom reaction with propyne (CH(3)CCH) has been investigated using a universal crossed molecular beam apparatus. Two reaction channels have been clearly observed: H+C(3)H(3)F and HF+C(3)H(3). The substitution of F for H occurs mainly via a complex formation mechanism, producing reaction products with some contribution from a direct reaction mechanism. The HF product, however, appears to be dominantly forward scattered relative to the F atom beam direction, suggesting that the HF formation occurs via a direct abstraction mechanism. Branching ratios for the two observed reaction channels are also determined. The H formation channel is found to be the major reaction pathway, while the HF formation channel is also significant. From the measurements of DF versus HF products from the F atom reaction with deuterated propyne, the H atom picked up by the F atom in the reaction with normal propyne seems to come mostly from the CH(3) group. In addition, the H atom produced in the H atom formation channel appears to be mostly from the CH(3) group with some contribution from the CCH group.

Dynamics of the F atom reaction with propene.

The F+C2H3CH3 reaction has been investigated using the crossed molecular beam technique. Three reaction channels have been observed in this reaction: H+C3H5F, CH3+C2H3F, and HF+C3H5. Time-of-flight spectra as well as product laboratory angular distributions have been measured for the HF, C2H3F, and C3H5F products from these three channels. Relative branching ratios of the three observed reaction channels have also been estimated. Experimental results indicate that these different channels exhibit significantly different reaction dynamics.