Recombination, Solvation and Reaction of CN Radicals Following Ultraviolet Photolysis of ICN in Organic Solvents

Exploring the impact of vibrational cavity coupling strength on ultrafast CN + c-C6H12 reaction dynamics

Molecular polaritons, hybrid light-matter states resulting from strong cavity coupling of optical transitions, may provide a new route to guide chemical reactions. However, demonstrations of cavity-modified reactivity in clean benchmark systems are still needed to clarify the mechanisms and scope of polariton chemistry. Here, we use transient absorption to observe the ultrafast dynamics of CN radicals interacting with a cyclohexane (c-C6H12) and chloroform (CHCl3) solvent mixture under vibrational strong coupling of a C-H stretching mode of c-C6H12. By modulating the c-C6H12:CHCl3 ratio, we explore how solvent complexation and hydrogen (H)-abstraction processes proceed under collective cavity coupling strengths ranging from 55 to 85 cm-1. Reaction rates remain unchanged for all extracavity, on-resonance, and off-resonance cavity coupling conditions, regardless of coupling strength. These results suggest that insufficient vibrational cavity coupling strength may not be the determining factor for the negligible cavity effects observed previously in H-abstraction reactions of CN with CHCl3.

Ultrafast dynamics of CN radical reactions with chloroform solvent under vibrational strong coupling

Polariton chemistry may provide a new means to control molecular reactivity, permitting remote, reversible modification of reaction energetics, kinetics, and product yields. A considerable body of experimental and theoretical work has already demonstrated that strong coupling between a molecular vibrational mode and the confined electromagnetic field of an optical cavity can alter chemical reactivity without external illumination. However, the mechanisms underlying cavity-altered chemistry remain unclear in large part because the experimental systems examined previously are too complex for detailed analysis of their reaction dynamics. Here, we experimentally investigate photolysis-induced reactions of cyanide radicals with strongly-coupled chloroform (CHCl3) solvent molecules and examine the intracavity rates of photofragment recombination, solvent complexation, and hydrogen abstraction. We use a microfluidic optical cavity fitted with dichroic mirrors to facilitate vibrational strong coupling (VSC) of the C-H stretching mode of CHCl3 while simultaneously permitting optical access at visible wavelengths. Ultrafast transient absorption experiments performed with cavities tuned on- and off-resonance reveal that VSC of the CHCl3 C-H stretching transition does not significantly modify any measured rate constants, including those associated with the hydrogen abstraction reaction. This work represents, to the best of our knowledge, the first experimental study of an elementary bimolecular reaction under VSC. We discuss how the conspicuous absence of cavity-altered effects in this system may provide insights into the mechanisms of modified ground state reactivity under VSC and help bridge the divide between experimental results and theoretical predictions in vibrational polariton chemistry.

Filming ultrafast roaming-mediated isomerization of bismuth triiodide in solution

Roaming reaction, defined as a reaction yielding products via reorientational motion in the long-range region (3 - 8 Å) of the potential, is a relatively recently proposed reaction pathway and is now regarded as a universal mechanism that can explain the unimolecular dissociation and isomerization of various molecules. The structural movements of the partially dissociated fragments originating from the frustrated bond fission at the onset of roaming, however, have been explored mostly via theoretical simulations and rarely observed experimentally. Here, we report an investigation of the structural dynamics during a roaming-mediated isomerization reaction of bismuth triiodide (BiI₃) in acetonitrile solution using femtosecond time-resolved x-ray liquidography. Structural analysis of the data visualizes the atomic movements during the roaming-mediated isomerization process including the opening of the Bi-I_b-I_c angle and the closing of I_a-Bi-I_b-I_c dihedral angle, each by ~40°, as well as the shortening of the I_b···I_c distance, following the frustrated bond fission.

Direct observation of o-benzyne formation in photochemical hexadehydro-Diels-Alder (hν-HDDA) reactions

Reactive ortho-benzyne derivatives are believed to be the initial products of liquid-phase [4 + 2]-cycloadditions between a 1,3-diyne and an alkyne via what is known as a hexadehydro-Diels–Alder (HDDA) reaction. The UV/VIS spectroscopic observation of o-benzyne derivatives and their photochemical dynamics in solution, however, have not been reported previously. Herein, we report direct UV/VIS spectroscopic evidence for the existence of an o-benzyne in solution, and establish the dynamics of its formation in a photoinduced reaction. For this purpose, we investigated a bis-diyne compound using femtosecond transient absorption spectroscopy in the ultraviolet/visible region. In the first step, we observe excited-state isomerization on a sub-10 ps time scale. For identification of the o-benzyne species formed within 50–70 ps, and the corresponding photochemical hexadehydro-Diels–Alder (hν-HDDA) reactions, we employed two intermolecular trapping strategies. In the first case, the o-benzyne was trapped by a second bis-diyne, i.e., self-trapping. The self-trapping products were then identified in the transient absorption experiments by comparing their spectral features to those of the isolated products. In the second case, we used perylene for trapping and reconstructed the spectrum of the trapping product by removing the contribution of irrelevant species from the experimentally observed spectra. Taken together, the UV/VIS spectroscopic data provide a consistent picture for o-benzyne derivatives in solution as the products of photo-initiated HDDA reactions, and we deduce the time scales for their formation.

We report the transient ultraviolet/visible absorption spectrum of an o-benzyne species in solution for the first time.

Importance of the Parallel Component of the Transition Moments to the 1Π1 (5A') and 3Π1 (3A') Excited States of ICN in the Ã-Band Photodissociation

ICN is one of the few simple triatomic molecules whose photodissociation mechanisms have been thoroughly investigated. Since it has a linear structure in the electronic ground state, the dissociation follows a photoexcitation at a linear or slightly bent structure. It is generally believed that the Ã-band consists of the dominant excitation to ³Π₀₊ (4A') with the transition dipole moment (TDM) parallel to the molecular axis ( z), a slightly weaker transition to ¹Π₁ (5A', 4A″), and a much weaker transition to ³Π₁ (3A', 2A″), both of the latter two having perpendicular TDMs. In the present work, we have theoretically studied the geometry dependence of these TDMs and found a pronounced θ (bending angle) dependence in the parallel ( z) component of the TDMs to ¹Π₁ (5A') and ³Π₁ (3A'), both of which should be zero at a linear geometry by symmetry and thus have been previously ignored. We estimated that the z component TDM to ¹Π₁ (5A') has a contribution of 15-20% to the total absorption cross-section at 249 nm at room temperature. Interestingly, the TDM to ³Π₀₊ (4A') does not exhibit such θ dependency and thus has only the z component. We compare the TDMs of ICN and CH₃I molecules having similar excited states. The fact that all the TDMs to 3A', 4A', and 5A' have nonnegligible z components implies the importance of the coherent excitation contributions to various observables of CN fragment, such as the anisotropy parameter, the orientation parameter, and the rotational level distribution as well as the rotational fine structure level distribution.

Alkyl hydrogen atom abstraction reactions of the CN radical with ethanol

We present a study of the abstraction of alkyl hydrogen atoms from the β and α positions of ethanol by the CN radical in solution using the Empirical Valence Bond (EVB) method. We have built separate 2 × 2 EVB models for the H_β and H_α reactions, where the atom transfer is parameterized using ab initio calculations. The intra- and intermolecular potentials of the reactant and product molecules were modelled with the General AMBER Force Field, with some modifications. We have carried out the dynamics in water and chloroform, which are solvents of contrasting polarity. We have computed the potential of mean force for both abstractions in each of the solvents. They are found to have a small and early barrier along the reaction coordinate with a large energy release. Analyzing the solvent structure around the reaction system, we have found two solvents to have little effect on either reaction. Simulating the dynamics from the transition state, we also study the fate of the energies in the HCN vibrational modes. The HCN molecule is born vibrationally hot in the CH stretch in both reactions and additionally in the HCN bends for the H_α abstraction reaction. In the early stage of the dynamics, we find that the CN stretch mode gains energy at the expense of the energy in CH stretch mode.

Taking the plunge: chemical reaction dynamics in liquids

The dynamics of chemical reactions in liquid solutions are now amenable to direct study using ultrafast laser spectroscopy techniques and advances in computer simulation methods. The surrounding solvent affects the chemical reaction dynamics in numerous ways, which include: (i) formation of complexes between reactants and solvent molecules; (ii) modifications to transition state energies and structures relative to the reactants and products; (iii) coupling between the motions of the reacting molecules and the solvent modes, and exchange of energy; (iv) solvent caging of reactants and products; and (v) structural changes to the solvation shells in response to the changing chemical identity of the solutes, on timescales which may be slower than the reactive events. This article reviews progress in the study of bimolecular chemical reaction dynamics in solution, concentrating on reactions which occur on ground electronic states. It illustrates this progress with reference to recent experimental and computational studies, and considers how the various ways in which a solvent affects the chemical reaction dynamics can be unravelled. Implications are considered for research in fields such as mechanistic synthetic chemistry.

Photochemical reaction dynamics of 2,2'-dithiobis(benzothiazole): direct observation of the addition product of an aromatic thiyl radical to an alkene with time-resolved vibrational and electronic absorption spectroscopy

The photochemical reaction dynamics of the benzothiazole-2-thiyl (BS) radical, produced by 330 nm ultraviolet photolysis of 2,2'-dithiobis(benzothiazole) (BSSB), are examined on the picosecond time scale. The initial addition product of a thiol-ene reaction between the BS radical and styrene is directly observed by transient vibrational absorption spectroscopy (TVAS). Transient electronic absorption spectroscopy (TEAS) in the ultraviolet and visible spectral regions reveals rapid formation of the ground state BS radical with a time constant of ∼200 fs. The photolytically generated BS radical decays through geminate recombination to the parent molecule BSSB and competitive formation of a BS radical dimer with a rate coefficient of (3.7 ± 0.2) × 10(10) M(-1) s(-1) in methanol, and thereafter (36 ± 1)% of the initially formed BS radicals survive at the longest time delay (1.3 ns). In styrene solution, in contrast to methanol and toluene solutions, kinetic traces of the BS radical show an additional decay with a time constant of 305 ± 13 ps, and a broad band at 345-500 nm grows with the same time constant, suggesting a bimolecular reaction of the BS radical with styrene. The TVAS measurements reveal an absorption band of the ground state BS radical at 1301 cm(-1) in toluene solution, and the band decays with a time constant of 294 ± 32 ps in styrene solution. Two product bands grow at 1239 cm(-1) and 1429 cm(-1) with respective time constants of 312 ± 68 ps and 325 ± 33 ps, and are attributed to the addition product BS-St radical formed from the BS radical and styrene. A bimolecular reaction rate coefficient of kreact = (3.8 ± 0.2) × 10(8) M(-1) s(-1) is deduced and 22 ± 1% of the initially formed BS radicals are converted to the BS-St radical in neat styrene solution.

Charge-transfer and impulsive electronic-to-vibrational energy conversion in ferricyanide: ultrafast photoelectron and transient infrared studies

The photophysics of ferricyanide in H₂O, D₂O and ethylene glycol was studied upon excitation of ligand-to-metal charge transfer (LMCT) transitions by combining ultrafast photoelectron spectroscopy (PES) of liquids and transient vibrational spectroscopy.

Femtosecond to microsecond observation of the photochemical reaction of 1,2-di(quinolin-2-yl)disulfide with methyl methacrylate

Multiple radical reaction steps have been observed in a continuous sequence with sub-picosecond to microsecond transient absorption spectroscopy.