The Influence of Hydrogen Bonds on the Roaming Reaction

Roaming bypasses the conventional transition state and is a significant reaction pathway due to the unusual energy distributions of its products; however, its reaction pathway under external environmental interactions remains unclear. Herein, we report for the first time the roaming process of nitrobenzene, which is influenced by the hydrogen bonds (H-bonds) between nitro- and phenyl radicals and water molecules in the gas phase. Notably, despite the fact that the single water structure produces a higher but narrower barrier, whereas the double water structure leads to a lower but wider barrier, the roaming reaction still occurs. The underlying mechanism responsible for these influences of H-bonds is ascribed to the dramatically changed polarization and correlation interactions between the roaming radicals. The reaction rates and thermal perturbation probabilities are also remarkably influenced due to the presence of the H-bonds, by approximately 2 orders of magnitude. It is anticipated that this work will encourage the promising feasibility of introducing environmental molecules to modulate the roaming reaction.

Influence of second-order saddles on reaction mechanisms

The transition state, a first-order saddle point on the potential energy surface, plays a central role in understanding the mechanism, dynamics, and rate of chemical reactions. However, we recently identified...

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 TS_cc(S₀), non-TS_cc(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.

Roaming Dynamics and Conformational Memory in Photolysis of Formic Acid at 193 nm Using Time-resolved Fourier-transform Infrared Emission Spectroscopy

In photodissociation of trans-formic acid (HCOOH) at 193 nm, we have observed two molecular channels of CO + H₂O and CO₂ + H₂ by using 1 μs-resolved Fourier-transform infrared emission spectroscopy. With the aid of spectral simulation, the CO spectra are rotationally resolved for each vibrational state (v = 1-8). Each of the resulting vibrational and rotational population distributions is characteristic of two Boltzmann profiles with different temperatures, originating from either transition state pathway or OH-roaming to form the same CO + H₂O products. The H₂O roaming co-product is also spectrally simulated to understand the interplay with the CO product in the internal energy partitioning. Accordingly, this work has evaluated the internal energy disposal for the CO and H₂O roaming products; especially the vibrational-state dependence of the roaming signature is reported for the first time. Further, given a 1 μs resolution, the temporal dependence of the CO/CO₂ product ratio at v ≥ 1 rises from 3 to 10 of study, thereby characterizing the effect of conformational memory and well reconciling with the disputed results reported previously between absorption and emission methods.

How important is roaming in the photodegradation of nitrobenzene?

Three pathways have been found for the formation of NO from nitrobenzene photodegradation that lead to either low or high translational energy, with a roaming mechanism involved at high excitation energies.

Discovery of conical intersection mediated photochemistry with growing string methods

Conical intersections (CIs) are important features of photochemistry that determine yields and selectivity. Traditional CI optimizers require significant human effort and chemical intuition, which typically restricts searching to only a small region of the CI space. Herein, a systematic approach utilizing the growing string method is introduced to locate multiple CIs. Unintuitive MECI are found using driving coordinates that can be generated using a combinatorial search, and subsequent optimization allows reaction pathways, transition states, products, and seam-space pathways to be located. These capabilities are demonstrated by application to two prototypical photoisomerization reactions and the dimerization of butadiene. In total, many reaction pathways were uncovered, including the elusive stilbene hula-twist mechanism, and a previously unidentified product in butadiene dimerization. Overall, these results suggest that growing string methods provide a predictive strategy for exploring photochemistry.

Liberation of H2 from (o-C6H4Me)3P-H(+) + (-)H-B(p-C6F4H)3 ion-pair: A transition-state in the minimum energy path versus the transient species in Born-Oppenheimer molecular dynamics

Using Born-Oppenheimer molecular dynamics (BOMD) with density functional theory, transition-state (TS) calculations, and the quantitative energy decomposition analysis (EDA), we examined the mechanism of H₂-liberation from LB-H⁽⁺⁾ + ^(-)H-LA ion-pair, 1, in which the Lewis base (LB) is (o-C₆H₄Me)₃P and the Lewis acid (LA) is B(p-C₆F₄H)₃. BOMD simulations indicate that the path of H₂ liberation from the ion-pair 1 goes via the short-lived transient species, LB⋯H₂⋯LA, which are structurally reminiscent of the TS-structure in the minimum-energy-path describing the reversible reaction between H₂ and (o-C₆H₄Me)₃P/B(p-C₆F₄H)₃ frustrated Lewis pair (FLP). With electronic structure calculations performed on graphics processing units, our BOMD data-set covers more than 1 ns of evolution of the ion-pair 1 at temperature T ≈ 400 K. BOMD simulations produced H₂-recombination events with various durations of H₂ remaining fully recombined as a molecule within a LB/LA attractive "pocket"-from very short vibrational-time scale to time scales in the range of a few hundred femtoseconds. With the help of perturbational approach to trajectory-propagation over a saddle-area, we directly examined dynamics of H₂-liberation. Using EDA, we elucidated interactions between the cationic and anionic fragments in the ion-pair 1 and between the molecular fragments in the TS-structure. We have also considered a model that qualitatively takes into account the potential energy characteristics of H-H recombination and H₂-release plus inertia of molecular motion of the (o-C₆H₄Me)₃P/B(p-C₆F₄H)₃ FLP.

Excited-state dynamics and electron transfer process of 1,3,5-triamino-2,4,6-trinitrobenzene

Insights into the excited-state dynamics and electron transfer processes of nitro explosives offer an efficient tool for unravelling ultrafast and complex detonation physics.

Nonstatistical dynamics in the thermal Garratt-Braverman/[1,5]-H shift of one ene-diallene: an experimental and computational study

A mechanistic study of the thermal Garratt-Braverman/[1,5]-H shift of ene-diallene 6 leading to alkenes (E)-8 and (Z)-8 is reported. The product ratio was found to be temperature-independent and does not agree with the computed energy barriers (i.e., TS2E(⧧) and TS2Z(⧧)). On the basis of experimental and DFT computational results, we propose a mechanism that is strongly controlled by nonstatistical dynamic effects.

Uncovering the role of intra- and intermolecular motion in frustrated Lewis acid/base chemistry: ab initio molecular dynamics study of CO2 binding by phosphorus/boron frustrated Lewis pair [tBu3P/B(C6F5)3]

The role of the intra- and intermolecular motion, i.e., molecular vibrations and the relative motion of reactants, remains largely unexplored in the frustrated Lewis acid/base chemistry. Here, we address the issue with the ab initio molecular dynamics (AIMD) study of CO2 binding by a Lewis acid (LA) and a Lewis base (LB), i.e., tBu3P + CO2 + B(C6F5)3 → tBu3P-C(O)O-B(C6F5)3 ([1]). Reasonably large ensemble of AIMD trajectories propagated at 300 K from structures in the saddle region as well as trajectories propagated directly from the reactants region revealed an effect arising from significant recrossing of the saddle area. The effect is that transient complexes composed of weakly interacting reactants nearly cease to progress along the segment of the minimum energy pathway (MEP) at the saddle region for a (subpicosecond) period of time during which the dominant factor is the light-to-heavy type of relative motion of the vibrating reactants, i.e., the "bouncing"-like movement of CO2 with respect to much heavier phosphine and borane as main contributor to the mode that is perpendicular to the MEP-direction. In terms of how P···C and B···O distances change with time, the roaming-like patterns of typical AIMD trajectories, reactive and nonreactive alike, extend far beyond the saddle region. In addition to the dynamical portrayal of [1], we provide the energy-landscape perspective that takes into account the hierarchy of time scales. The verifiable implication of the effect found here is that the isotopically substituted (heavier) LB/LA "pair" should be less reactive that the "normal" and thus lighter counterpart.

Automated Exploration of Photolytic Channels of HCOOH: Conformational Memory via Excited-State Roaming

To elucidate the photodissociation mechanism of HCOOH, we systematically explored reaction pathways starting from the first excited singlet state (S1) by using automated reaction path search methods. All critical points, that is, minima, transition states, minimum energy conical intersections, and minima on seam of crossing, for the S0, T1, and S1 potential energy surfaces (PESs) obtained in the present search were optimized at the CASPT2 level. The structure list obtained by the search explained all experimentally reported photolytic channels. A new mechanism for the previously suggested but unexplained conformational memory in the 193 nm photolysis is proposed, which involves two steps: partial dissociation and succeeding roaming of one of H atoms on the S1 PES, followed by intramolecular recombination on the S0 PES after radiationless transition through a conical intersection at a partially dissociated geometry. This is partially similar to the excited-state roaming recently discovered for the NO3 radical.