The role of different solvent environments in determining the behavior of molecules in solution is a fundamental aspect of chemical reactivity. We present an approach for exploring the influence of solvent properties on condensed-phase dynamics using ultrafast transient absorption spectroscopy in supercritical CO2. Using supercritical CO2 permits adjustment of the density, by varying the temperature and pressure, whereas varying the concentration or identity of a second solvent, the cosolvent, in a binary mixture allows for adjustments of the degree of interaction between the solute and the solvent. Salicylidene aniline, a prototypical excited-state intramolecular proton-transfer system, is the subject of this study. In this system, the decay rate of the transient absorption signal decreases as the fraction of the cosolvent (for both 1-propanol and cyclohexane) increases. The decay rate also decreases with an increase in the viscosity of the mixture, but the effect is much larger for the 1-propanol cosolvent than for cyclohexane. These observations illustrate that the decay rate of the photoexcited salicylidene aniline depends on more than just the solvent viscosity, suggesting that properties such as polarity also play a role in the dynamics.

A Cl atom can react with 2,3-dimethylbutane (DMB), 2,3-dimethyl-2-butene (DMBE), and 2,5-dimethyl-2,4-hexadiene (DMHD) in solution via a hydrogen-abstraction reaction. The large exoergicity of the reaction between a Cl atom and alkenes (DMBE and DMHD) makes vibrational excitation of the HCl product possible, and we observe the formation of vibrationally excited HCl (v = 1) for both reactions. In CCl4, the branching fractions of HCl (v = 1), Γ (v = 1), for the Cl-atom reactions with DMBE and DMHD are 0.14 and 0.23, respectively, reflecting an increased amount of vibrational excitation in the products of the more exoergic reaction. In addition, Γ (v = 1) for both reactions is larger in the solvent CDCl3, being 0.23 and 0.40, as the less viscous solvent apparently dampens the vibrational excitation of the nascent HCl less effectively. The bimolecular reaction rates for the Cl reactions with DMB, DMBE, and DMHD in CCl4 are diffusion limited (having rate constants of 1.5 × 1010, 3.6 × 1010, and 17.5 × 1010 M–1 s–1, respectively). In fact, the bimolecular reaction rate for Cl + DMHD exceeds a typical diffusion-limited reaction rate, implying that the attractive intermolecular forces between a Cl atom and a C═C bond increase the rate of favorable encounters. The 2-fold increase in the reaction rate of the Cl + DMBE reaction from that of the Cl + DMB reaction likely reflects the effect of the C═C bond, while both the number of C═C bonds and the molecular geometry likely play a role in the large reaction rate of the Cl + DMHD reaction.

Transient infrared absorption spectroscopy monitors condensed-phase photodissociation dynamics of 30 mM CHBr3 and 50 mM CHI3 in liquid CCl4. The experiments have picosecond time resolution and monitor the C–H stretch region of both the parent polyhalomethanes and their photolytically generated isomers. The C–H stretching transitions of these isomers, in which the emergent halogen atom returns to form a C–X–X bonding motif, appear about 9 ps after photolysis for iso-CHBr2–Br and in about 46 ps for iso-CHI2–I. These time scales are consistent with, but differ from, the time evolution of the transient electronic absorption spectra of the same samples, highlighting the subtle differences between monitoring the vibrational and electronic chromophores. The specificity of using vibrational transitions to track condensed-phase reaction dynamics permits reassessment of the transient electronic spectrum of photolysis in neat CHBr3, which has an additional prompt feature near 400 nm. Calculations show that this feature, which arises from a precursor to the isomer, is a charge-transfer transition of a contact pair between the nascent Br fragment and a nearby CHBr3 molecule. Dilution and solvent studies show that transition is independent of the solvent. The iso-CHBr2–Br transition wavelength, however, shifts over the range of 400 to 510 nm depending on the solvent. Time-dependent density functional calculations faithfully reproduce these trends.

Ultrafast photolysis of bromoform (CHBr3) with a 267 nm pulse of light followed by broadband transient electronic absorption identifies the photoproducts and follows their evolution in both neat bromoform and cyclohexane solutions. In neat bromoform, a species absorbing at 390 nm appears promptly and decays with a time constant of 13 ps as another species absorbing at 495 nm appears. The wavelength and time evolution of the first absorption is consistent with the formation of iso-bromoform (CHBr2−Br) by recombination of the fragment radicals within the solvent cage. The presence of an isosbestic point in the transient spectra indicates that this isomer is the precursor of the second absorber. The excess internal energy remaining in iso-bromoform permits release of the weakly bound Br atom to form a complex, CHBr3−Br, with other bromoform molecules. The features in the transient spectra are qualitatively similar in cyclohexane solutions of bromoform. The wavelength of the transition of iso-bromoform does not change upon dilution, but that of the CHBr3−Br complex systematically decreases with addition of cyclohexane. This trend agrees with the predicted dependence of the energy of a charge-transfer transition on the dielectric constant of the medium. Vibrational relaxation is likely to be the controlling feature of the evolution of the initially formed iso-bromoform.

Preparing electronically excited trans-stilbene molecules in deuterated chloroform using both one-photon excitation and excitation through an intermediate vibrational state explores the influence of vibrational energy on excited-state isomerization in solution. After infrared excitation of either two quanta of C−H stretch vibration |2νCH⟩ at 5990 cm−1 or the C−H stretch−bend combination |νCH + νbend⟩ at 4650 cm−1 in the ground electronic state, an ultraviolet photon intercepts the vibrationally excited molecules during the course of vibrational energy flow and promotes them to the electronically excited state. The energy of the infrared and ultraviolet photons together is the same as that added in the one-photon excitation. Transient broadband-continuum absorption monitors the lifetime of electronically excited molecules. The lifetime of excited-state trans-stilbene after one-photon electronic excitation with 33 300 cm−1 of energy is (51 ± 6) ps. The excited-state lifetimes of (55 ± 9) ps and (56 ± 7) ps for the cases of excitation through |2νCH⟩ and |νCH + νbend⟩, respectively, are indistinguishable from that for the one-photon excitation. Vibrational relaxation in the electronically excited state prepared by the two-photon excitation scheme is most likely faster than the barrier crossing, making the isomerization insensitive to the method of initial state preparation.

Ultrafast transient absorption experiments monitor the reaction of CN radicals with 16 different alkane, alcohol, and chloroalkane solutes in CH2Cl2 and with a smaller number of representative solutes in CHCl3 and CH3CCl3. In these experiments, 267-nm photolysis generates CN radicals, and transient electronic absorption at 400 nm probes their time evolution. A crucial feature of the reactions of CN radicals is their rapid formation of two different types of complexes with the solvent that have different stabilities and reactivities. The signature of the formation of these complexes is the CN transient absorption disappearing more slowly than the infrared transient absorption of the HCN product appears. Studying both the growth of HCN and the decay of the CN−solvent complexes in the reaction of CN with pentane in CH2Cl2 and CHCl3 solutions provides the information needed to build a kinetic model that accounts for the reaction of both complexes. This model permits analysis of the reaction of each of the 16 different solutes using only the decay of the CN transient absorption. The reaction of CN−solvent complexes with alkanes and chloroalkanes is slower than the corresponding reactions of Cl. However, the reactions of alcohols with both CN and Cl occur at about the same rate, likely reflecting additional complexation of the CN radical or its ICN precursor by the alcohol. The rates for the reactions of CN with the chloroalkanes decrease with increasing Cl content of the solute, in keeping with previous observations for the reactions of Cl in both gases and liquids.

State-resolved reactions of CH3D molecules containing both C−H and C−D stretching excitation with Cl atoms provide new vibrational spectroscopy and probe the consumption and disposal of vibrational energy in the reactions. The vibrational action spectra have three different components, the combination of the C−H symmetric stretch and the C−D stretch (ν1 + ν2), the combination of the C−D stretch and the C−H antisymmetric stretch (ν2 + ν4), and the combination of the C−D stretch and the first overtone of the CH3 bend (ν2 + 2ν5). The simulation for the previously unanalyzed (ν2 + ν4) state yields a band center of ν0 = 5215.3 cm−1, rotational constants of A = 5.223 cm−1 and B = 3.803 cm−1, and a Coriolis coupling constant of ζ = 0.084. The reaction dynamics largely follow a spectator picture in which the surviving bond retains its initial vibrational excitation. In at least 80% of the reactive encounters of vibrationally excited CH3D with Cl, cleavage of the C−H bond produces CH2D radicals with an excited C−D stretch, and cleavage of the C−D bond produces CH3 radicals with an excited C−H stretch. Deviations from the spectator picture seem to reflect mixing in the initially prepared eigenstates and, possibly, collisional coupling during the reaction.

Time-resolved studies using 100 fs laser pulses generate CN radicals photolytically in solution and probe their subsequent reaction with solvent molecules by monitoring both radical loss and product formation. The experiments follow the CN reactants by transient electronic spectroscopy at 400 nm and monitor the HCN products by transient vibrational spectroscopy near 3.07 μm. The observation that CN disappears more slowly than HCN appears shows that the two processes are decoupled kinetically and suggests that the CN radicals rapidly form two different types of complexes that have different reactivities. Electronic structure calculations find two bound complexes between CN and a typical solvent molecule (CH2Cl2) that are consistent with this picture. The more weakly bound complex is linear with CN bound to an H atom through the N atom, and the more strongly bound complex has a structure in which the CN bridges Cl and H atoms of the solvent. Fitting the transient absorption data with a kinetic model containing two uncoupled complexes reproduces the data for seven different chlorinated alkane solvents and yields rate constants for the reaction of each type of complex. Depending on the solvent, the linear complex reacts between 2.5 and 12 times faster than the bridging complex and is the primary source of the HCN reaction product. Increasing the Cl atom content of the solvents decreases the reaction rate for both complexes.

Understanding the motions of the constituent atoms in reacting molecules lies at the heart of chemistry and is the central focus of chemical reaction dynamics. The most detailed questions one can ask are about the evolution of molecules prepared in a single quantum state to products in individual states, and both calculations and experiments are providing such detailed understanding of increasingly complex systems. A central goal of these studies is uncovering the essential details of chemical change by removing the averaging over the initial conditions that occurs in many cases. Such information provides an exquisite test of theory and helps paint pictures of complicated chemical transformations. The goal of this Special Feature is to provide a snapshot of a portion of the field of chemical reaction dynamics. Much of the work presented here emphasizes a close interplay of experiment and theory in ways that sharpen the conclusions of both and animate future studies. The articles do not completely cover the rich field of chemical reaction dynamics but rather provide a glimpse of some of the emerging insights.

Experimental studies of the chemical reaction dynamics of vibrationally excited molecules reveal the ability of different vibrations to control the course of a reaction. This Perspective describes those studies for the prototypical reaction of vibrationally excited methane and its isotopologues in gases and on surfaces and looks to the prospects of similar studies in liquids. The influences of vibrational excitation on the CH bond cleavage in a single collision reaction with Cl and in dissociative adsorption on a Ni surface bear some striking similarities. Both reactions are bond-selective processes in which the initial preparation of a molecular eigenstate containing a large component of CH stretching results in preferential cleavage of that bond. It is possible to cleave either the CH bond or CD bond in the reaction of Cl with CH₃D, CH₂D₂, or CHD₃ and, similarly, to use initial excitation of the CH stretch to promote dissociation of CHD₃ to CD₃ and H on a Ni surface. Different vibrational modes, such as the symmetric and antisymmetric stretches in CH₃D or CH₄, lead to very different reactivities, and molecules with the symmetric stretching vibration excited can be as much as 10 times more reactive than ones with the antisymmetric stretch excited. The origin of this behavior lies in the change in the vibrational motion induced by the interaction with the atomic reaction partner or the surface.