Time-dependent fluorescence (TDF) measurements are frequently used to probe solvation dynamics in nanoconfined solvents. It was previously found [J. Chem. Phys. 2002, 117, 6618] in simple model systems that long time scales can arise in the TDF signal due to diffusion of the solute chromophore. Here, the effects of solute diffusion after electronic excitation on the TDF signal are studied using a generalized Smoluchowski equation. In this approach, the probability density in terms of the solute radial position and a collective solvent coordinate is determined as a function of time after excitation. Model one-dimensional free energy curves with a single barrier in the solute coordinate—appropriate for relatively small, e.g., ∼2 nm diameter, cavities or pores—are used to construct the full two-dimensional free energy surface and probe different physical models. The effects of position-dependence of diffusion coefficients and solvent coordinate force constants are also considered. The results indicate that the inclusion of radial diffusion leads to rich phenomena in the TDF. The position dependence of the diffusion coefficients is also found to have significant effects on the solute diffusion and solvation dynamics relevant to TDF. The results allow the comparison of the characteristics of the TDF signal predicted by different physical models that have been proposed to account for the long-time decays found for measurements in nanoconfined solvents.

Empirical maps are presented for the OH stretching vibrations in neat alcohols in which the relevant spectroscopic quantities are expressed in terms of the electric field exerted on the hydrogen atom by the surrounding liquid. It is found, by examination of the four lowest linear alcohols, methanol, ethanol, n-propanol, and n-butanol, that a single map can be used for alcohols with different alkyl groups. This “universal” map is in very good agreement with maps optimized for the individual alcohols but differs from those previously developed for water. This suggests that one map can be used for all alcohols, perhaps even those not examined in the present study. The universal map gives IR lineshapes in good agreement with measured spectra for isotopically dilute methanol and ethanol, while the two-dimensional IR photon echo spectra give results that differ from experiments. The role of non-Condon effects, reorientation dynamics, hydrogen bonding, and spectral diffusion is discussed.

Hydroxyacetone (CH3C(O)CH2OH) is observed as a stable end product from reactions of the (CH3)2COO Criegee intermediate, acetone oxide, in a flow tube coupled with multiplexed photoionization mass spectrometer detection. In the experiment, the isomers at m/z = 74 are distinguished by their different photoionization spectra and reaction times. Hydroxyacetone is observed as a persistent signal at longer reaction times at a higher photoionization threshold of ca. 9.7 eV than Criegee intermediate and definitively identified by comparison with the known photoionization spectrum. Complementary electronic structure calculations reveal multiple possible reaction pathways for hydroxyacetone formation, including unimolecular isomerization via hydrogen atom transfer and −OH group migration as well as self-reaction of Criegee intermediates. Varying the concentration of Criegee intermediates suggests contributions from both unimolecular and self-reaction pathways to hydroxyacetone. The hydroxyacetone end product can provide an effective, stable marker for the production of transient Criegee intermediates in future studies of alkene ozonolysis.

Molecular dynamics simulations are used to investigate OH reorientation in the four isomeric butanols in their bulk liquid state to examine the influence of the arrangement of the steric bulk on the alcohol reorientational and hydrogen-bond (H-bond) dynamics. The results are interpreted within the extended jump model in which the OH reorientation is decomposed into contributions due to “jumps” between H-bond partners and “frame” reorientation of the intact H-bonded pair. Reorientation is fastest in iso-butanol and slowest in tert-butanol, while sec- and n-butanol have similar reorientation times. This latter result is a fortuitous cancellation between the jump and frame reorientation in the two alcohols. The extended jump model is shown to provide a quantitative description of the OH reorientation times. A detailed analysis of the jump times shows that a combination of entropic, enthalpic, and dynamical factors, including transition state recrossing effects, all play a role. A simple model based on the liquid structure is proposed to estimate the energetic and entropic contributions to the jump time. This represents the groundwork for a predictive model of OH reorientation in alcohols, but additional studies are required to better understand the frame reorientation and transition state recrossing effects.

•Mechanism of Nb-doped silica-catalyzed C2H4 epoxidation by H2O2 has been examined.•Rate-limiting barrier (11.6 kcal/mol) is lower than reported for other catalysts.•This work will assist metal-doped mesoporous epoxidation catalyst development.A mechanistic study of ethylene epoxidation by hydrogen peroxide catalyzed by niobium doped in a silica mesopore is reported. Density functional theory calculations at the M06-L/aug-cc-pVDZ level were used to investigate the catalytic pathway. A five-step cycle is proposed. The initial steps are the adsorption of H2O2 to the Nb center followed by coordination of ethylene to the hydrogen peroxide. The rate-limiting step is the subsequent epoxidation of ethylene via transfer of an oxygen atom, which has a calculated enthalpic barrier of 11.6 kcal/mol relative to the preceding intermediate. This is followed by desorption of the ethylene oxide product and dehydration to regenerate the catalyst. The reaction barrier is lower than reported for other catalysts in the literature, consistent with recent experimental reports of the efficacy of Nb-doped mesoporous silica catalysts [Catal. Sci. Technol.,2014, 4, 4433–4439]. The mechanistic details elucidated in the present calculations may aid in the rational design of new epoxidation catalysts and thus the factors influencing the reaction, e.g., geometry and charge changes, are discussed.Schematic illustration of the catalytic cycle for ethylene epoxidation by Nb-doped silica

•Study of one of the most selective hydroformylations of butadiene to adipaldehyde, which is industrially useful.•The first experimental observation of phosphine rhodium-catalyzed isomerizing hydroformylation to form adipaldehyde.•The selective formation of adipaldehyde from butadiene is due to standard hydroformylation and isomerizing hydroformylation.•DFT calculations show the intermediates, transition states and energetics of the key alkene isomerization.•Energy barriers for the formation of isomeric alkenes are similar.The (DIOP)rhodium-catalyzed hydroformylation of butadiene has been shown to give among the highest selectivities for formation of adipaldehyde, which is useful for the synthesis of nylon. Herein, isomerizing hydroformylation is shown to be a mechanism that is partially responsible for this selectivity and density functional theory studies are used to reveal the detailed pathway for the requisite alkene isomerization.

Using grand canonical Monte Carlo (GCMC) and molecular dynamics simulation, we examine the phase equilibrium and transport of a gas-expanded liquid under confinement. The system chosen is ethylene-expanded methanol confined in model silica mesopores, but in equilibrium with the bulk mixture—a system that has received recent interest as a reaction medium, e.g., for epoxidation of ethylene. This system was studied at 20 °C and pressures ranging from 5 to 55 bar. In addition, two different pore surface chemistries were examined: a hydrophilic pore, in which the silica dangling bonds were terminated by −OH groups, and a model “hydrophobic” pore, in which the charges on the pore atoms (including the −OH groups) were turned off. The chemical potentials for the mixture necessary to perform the GCMC simulations were obtained using a novel Gibbs–Duhem integration method along a previously calculated binary vapor–liquid equilibrium curve. We find that the pressure significantly affects the ethylene mole fraction in the confined mixture. The pore surface chemistry has a significant effect on the composition and transport properties of the confined ethylene–methanol mixture, relative to the bulk. In addition, there are significant qualitative differences between the hydrophilic and hydrophobic pores with regard to the spatial distributions of the confined ethylene and methanol.

Traditional descriptions of vibrational energy transfer consider a quantum oscillator interacting with a classical environment. However, a major limitation of this simplified description is the neglect of quantum decoherence induced by the different interactions between two distinct quantum states and their environment, which can strongly affect the predicted energy-transfer rate and vibrational spectra. Here, we use quantum–classical molecular dynamics simulations to determine the vibrational quantum decoherence time for an OH stretch vibration in liquid heavy water. We show that coherence is lost on a sub-100 fs time scale due to the different responses of the first shell neighbors to the ground and excited OH vibrational states. This ultrafast decoherence induces a strong homogeneous contribution to the linear infrared spectrum and suggests that resonant vibrational energy transfer in H2O may be more incoherent than previously thought.

Many alkyl-substituted Criegee intermediates are predicted to undergo an intramolecular 1,4-hydrogen transfer to form isomeric vinyl hydroperoxide species (CCOOH moiety), which break apart to release OH and vinoxy radicals. We report direct detection of stabilized vinyl hydroperoxides formed via carboxylic acid-catalyzed tautomerization of Criegee intermediates. A doubly hydrogen-bonded interaction between the Criegee intermediate and carboxylic acid facilitates efficient hydrogen transfer through a double hydrogen shift. Deuteration of formic or acetic acid permits migration of a D atom to yield partially deuterated vinyl hydroperoxides, which are distinguished from the CH3CHOO, (CH3)2COO, and CH3CH2CHOO Criegee intermediates by mass. Using 10.5 eV photoionization, three prototypical vinyl hydroperoxides, CH2CHOOD, CH2C(CH3)OOD, and CH3CHCHOOD, are detected directly. Complementary electronic structure calculations reveal several reaction pathways, including the barrierless acid-catalyzed tautomerization reaction predicted previously and a barrierless addition reaction that yields hydroperoxy alkyl formate.

Replica exchange molecular dynamics simulations are used to investigate the position-dependent densities of three small molecules dissolved in acetonitrile confined in nanoscale hydrophilic silica pores. The solutes, methanol, acetone, and carbon dioxide, differ in polarity and hydrogen-bonding properties. All three molecules are found preferentially near the pore interface at room temperature, but the surface affinity differs with the solute interactions. Methanol, in particular, exists in two distinct conformations that differ in the hydrogen-bonding state. Free energy profiles as a function of distance from the pore surface are decomposed into internal energy and entropic contributions. These reveal that entropy as well as hydrogen bonding can play important roles in determining the solute location and orientation. These and other relevant factors are examined to elucidate the origins of the solute density profiles within the pore.

The results of replica exchange molecular dynamics simulations of a coumarin 153 (C153) dye molecule dissolved in ethanol confined within a 2.4 nm hydrophilic amorphous silica pore are presented. The C153 dye position and orientation distributions provide insight into time-dependent fluorescence measurements in nanoconfined solvents as well as general features of chemistry in mesoporous materials. In addition to the distributions themselves, the free energy, internal energy, and entropic contributions have been calculated to explore the factors determining the distributions. The most likely location of C153 is found to be near the pore surface, but two possible hydrogen-bonding structures lead to differing orientations. Internal energy and entropy are found to be competing forces within the pore, with entropy playing a significant role with unexpected consequences. These results represent a crucial step in determining how the nanoconfining framework can affect measurements of solvation dynamics.

The tautomerization of Criegee intermediates via a 1,4 β-hydrogen atom transfer to yield a vinyl hydroperoxide has been examined in the absence and presence of carboxylic acids. Electronic structure calculations indicate that the organic acids catalyze the tautomerization reaction to such an extent that it becomes a barrierless process. In contrast, water produces only a nominal catalytic effect. Since organic acids are present in parts-per-billion concentrations in the troposphere, the present results suggest that the acid-catalyzed tautomerization, which can also result in formation of hydroxyl radicals, may be a significant pathway for Criegee intermediates.

Molecular dynamics simulations are used to investigate the reorientation dynamics of liquid acetonitrile confined within a nanoscale, hydrophilic silica pore. The dynamics are strongly modified relative to the bulk liquid—the time scale for reorientation is increased by orders-of-magnitude and the dynamics become nonexponential—and these effects are examined at the molecular level. In particular, commonly invoked two-state (or core–shell) models, with and without consideration of exchange of molecules between the states, are applied and discussed. A rigorous decomposition of the acetonitrile reorientational correlation function is introduced that permits the approximations implicit in the two-state models to be identified and tested systematically. The results show that exchange is an important component of the nanoconfined acetonitrile reorientation dynamics and a two-state model with exchange can accurately describe the correlation. However, the faithfulness of the model is related to the separation of time scales in the two states, which exists for a wide range of definitions of the two states. This suggests that caution should be exercised when inferring molecular-level details from application of two-state models.

Density functional theory and transition state theory rate constant calculations have been performed to gain insight into the bimolecular reaction of the Criegee intermediate (CI) with carbon monoxide (CO) that is proposed to be important in both atmospheric and industrial chemistry. A new mechanism is suggested in which the CI acts as an oxidant by transferring an oxygen atom to the CO, resulting in the formation of a carbonyl compound (aldehyde or ketone depending upon the CI) and carbon dioxide. Fourteen different CIs, including ones resulting from biogenic ozonolysis, are considered. Consistent with previous reports for other CI bimolecular reactions, the anti conformers are found to react faster than the syn conformers. However, this can be attributed to steric effects and not hyperconjugation as generally invoked. The oxidation reaction is slow, with barrier heights between 6.3 and 14.7 kcal/mol and estimated reaction rate constants 6–12 orders-of-magnitude smaller than previously reported literature estimates. The reaction is thus expected to be unimportant in the context of tropospheric oxidation chemistry. However, the reaction mechanism suggests that CO could be exploited in ozonolysis to selectively obtain industrially important carbonyl compounds.

Density functional theory calculations predict that the gas-phase decomposition of carbonic acid, a high-energy, 1,3-hydrogen atom transfer reaction, can be catalyzed by a monocarboxylic acid or a dicarboxylic acid, including carbonic acid itself. Carboxylic acids are found to be more effective catalysts than water. Among the carboxylic acids, the monocarboxylic acids outperform the dicarboxylic ones wherein the presence of an intramolecular hydrogen bond hampers the hydrogen transfer. Further, the calculations reveal a direct correlation between the catalytic activity of a monocarboxylic acid and its pKa, in contrast to prior assumptions about carboxylic-acid-catalyzed hydrogen-transfer reactions. The catalytic efficacy of a dicarboxylic acid, on the other hand, is significantly affected by the strength of an intramolecular hydrogen bond. Transition-state theory estimates indicate that effective rate constants for the acid-catalyzed decomposition are four orders-of-magnitude larger than those for the water-catalyzed reaction. These results offer new insights into the determinants of general acid catalysis with potentially broad implications.

Electronic structure calculations have been used to investigate possible gas-phase decomposition pathways of α-hydroxyalkyl hydroperoxides (HHPs), an important source of tropospheric hydrogen peroxide and carbonyl compounds. The uncatalyzed as well as water- and acid-catalyzed decomposition of multiple HHPs have been examined at the M06-2X/aug-cc-pVTZ level of theory. The calculations indicate that, compared to an uncatalyzed or water-catalyzed reaction, the free-energy barrier of an acid-catalyzed decomposition leading to an aldehyde or ketone and hydrogen peroxide is dramatically lowered. The calculations also find a direct correlation between the catalytic effect of an acid and the distance separating its hydrogen acceptor and donor sites. Interestingly, the catalytic effect of an acid on the HHP decomposition resulting in the formation of carboxylic acid and water is relatively much smaller. Moreover, since the free-energy barrier of the acid-catalyzed aldehyde- or ketone-forming decomposition is ∼25% lower than that required to break the O–OH linkage of the HHP leading to the formation of hydroxyl radical, these results suggest that HHP decomposition is likely not an important source of tropospheric hydroxyl radical. Finally, transition state theory estimates indicate that the effective rate constants for the acid-catalyzed aldehyde- or ketone-forming HHP decomposition pathways are 2–3 orders of magnitude faster than those for the water-catalyzed reaction, indicating that an acid-catalyzed HHP decomposition is kinetically favored as well.

Molecular dynamics simulations are used to examine the diffusion of acetonitrile within ∼2.4 nm diameter amorphous silica pores with a focus on the mechanism. The role of the pore surface chemistry is examined by comparison of a hydrophilic, −OH terminated, silica pore with one that has hydrogen-bonding turned off and with an effectively hydrophobic pore obtained by setting all pore charges to zero. The anisotropy of diffusion, along and perpendicular to the pore axis, is examined through the mean-squared displacements. The origins of the anisotropy are investigated through the dependence on the acetonitrile position within the pore. The effect of hydrogen bonding of acetonitrile molecules to the hydrophilic pore surface is also probed. The simulations show that acetonitrile molecules do not diffuse axially next to the pore surface. Rather, axial diffusion is preceded by radial diffusion away from the pore surface. The same mechanism is observed for molecules independent of their hydrogen-bonding status to surface silanols though hydrogen-bonded molecules diffuse more slowly.

The free energy and electronic fluorescence spectra of a model solute solvated by ethanol in a nanoscale silica pore are examined as a function of the solute position, with the aim of improving our understanding of solvation in nanoconfined environments. The results indicate that the position distribution of the solute depends on its dipole moment as well as on the surface interactions of the silica pore, i.e., hydrophilic or hydrophobic (uncharged). Further, the solute fluorescence spectrum is a function of the solute position in the hydrophilic pore, but is independent of position in the hydrophobic pore. The origins of these results are investigated, including by decomposition of the free energy as a function of solute position into the contributing interactions. The implications for time-dependent fluorescence (TDF) experiments, used commonly to probe solvation dynamics in nanoconfined solvent systems, are considered. The possible role of chromophore diffusion in TDF measurements, and chemistry in nanoconfined liquids more broadly, is given particular emphasis.

An umbrella sampling approach based on the vibrational energy gap is presented and examined for exploring the reaction coordinate for a proton transfer (PT) reaction. The technique exploits the fact that for a PT reaction the energy gap between the vibrational ground and excited states of the transferring proton reaches a minimum at the transition state. Umbrella sampling is used within mixed quantum-classical simulations to identify the transition state configurations and explore the reaction free energy curve and vibrationally nonadiabatic coupling. The method is illustrated by application to a model phenol–amine proton transfer reaction complex in a nanoconfined solvent. The results from this new umbrella sampling approach are consistent with those obtained from previous umbrella sampling calculations based on a collective solvent coordinate. This sampling approach further provides insight into the vibrationally nonadiabatic coupling for the proton transfer reaction and has potential for simulating vibrational spectra of PT reaction complexes in solution.

The mechanism of the OH bond reorientation in liquid methanol and ethanol is examined. It is found that the extended jump model, recently developed for water, describes the OH reorientation in these liquids. The slower reorientational dynamics in these alcohols compared to water can be explained by two key factors. The alkyl groups on the alcohol molecules exclude potential partners for hydrogen bonding exchanges, an effect that grows with the size of the alkyl chain. This increases the importance of the reorientation of intact hydrogen bonds, which also slows with increasing size of the alcohol and becomes the dominant reorientation pathway.

A molecular-level analysis of the origins of the vibrational frequency shifts of the CN stretching mode in neat liquid acetonitrile is presented. The frequency shifts and infrared spectrum are calculated using a perturbation theory approach within a molecular dynamics simulation and are in good agreement with measured values reported in the literature. The resulting instantaneous frequency of each nitrile group is decomposed into the contributions from each molecule in the liquid and by interaction type. This provides a detailed picture of the mechanisms of frequency shifts, including the number of surrounding molecules that contribute to the shift, the relationship between their position and relative contribution, and the roles of electrostatic and van der Waals interactions. These results provide insight into what information is contained in infrared (IR) and Raman spectra about the environment of the probed vibrational mode.

Nonadiabatic effects on the reaction rate constant of a model phenol−amine proton transfer system in a nanoconfined solvent have been investigated by employing classical mapping in conjunction with a reactive flux approach. It is observed that allowing nonadiabatic transitions makes the transition state more accessible thermodynamically but decreases the reactive flux due to increased transition state recrossing, resulting in an overall reduction in the rate constant by more than a factor of 2. The physical origins of these features are discussed.

The infrared spectrum of acetonitrile confined in hydrophilic silica pores roughly cylindrical and 2.4 nm in diameter has been simulated using molecular dynamics. Hydrogen bonding interactions between acetonitrile and silanol groups on the pore wall involve charge transfer effects that have been incorporated through corrections based on electronic structure calculations on a dimer. The simulated spectrum of confined acetonitrile differs most prominently from that of the bulk liquid by the appearance of a blue-shifted shoulder, in agreement with previous experimental measurements. The dominant peak is little changed in position relative to the bulk liquid case, but broadened by ∼40%. A detailed analysis of the structure and dynamics of the confined liquid acetonitrile is presented, and the spectral features are examined in this context. It is found that packing effects, hydrogen bonding, and electrostatic interactions all play important roles. Finally, the molecular-level information that can be obtained about the dynamics of the confined liquid from the infrared line shape is discussed.

Grand canonical Monte Carlo simulations have been used to determine the equilibrium density of acetonitrile in model amorphous silica pores with varying radius and surface chemistry. Pores of diameter ∼2−4 nm were considered with different ratios of surface −OH moieties to −OC(CH3)3 groups. The calculations found that the acetonitrile density in the interior of all the pores is essentially identical with that of the bulk liquid. On the other hand, a slightly elevated liquid density is observed near the pore surface for pores with only −OH surface moieties. Replacement of surface −OH groups with −OC(CH3)3 units lengthens the liquid/pore interfacial region as acetonitrile molecules can insert themselves between the −OC(CH3)3 units. The results indicate that the major effect of changing the surface functionality comes from the differences in excluded volume rather than hydrogen-bonding effects. Finally, the choice of the acetonitrile potential can qualitatively change the results.

Many alkyl-substituted Criegee intermediates are predicted to undergo an intramolecular 1,4-hydrogen transfer to form isomeric vinyl hydroperoxide species (CCOOH moiety), which break apart to release OH and vinoxy radicals. We report direct detection of stabilized vinyl hydroperoxides formed via carboxylic acid-catalyzed tautomerization of Criegee intermediates. A doubly hydrogen-bonded interaction between the Criegee intermediate and carboxylic acid facilitates efficient hydrogen transfer through a double hydrogen shift. Deuteration of formic or acetic acid permits migration of a D atom to yield partially deuterated vinyl hydroperoxides, which are distinguished from the CH3CHOO, (CH3)2COO, and CH3CH2CHOO Criegee intermediates by mass. Using 10.5 eV photoionization, three prototypical vinyl hydroperoxides, CH2CHOOD, CH2C(CH3)OOD, and CH3CHCHOOD, are detected directly. Complementary electronic structure calculations reveal several reaction pathways, including the barrierless acid-catalyzed tautomerization reaction predicted previously and a barrierless addition reaction that yields hydroperoxy alkyl formate.

The tautomerization of Criegee intermediates via a 1,4 β-hydrogen atom transfer to yield a vinyl hydroperoxide has been examined in the absence and presence of carboxylic acids. Electronic structure calculations indicate that the organic acids catalyze the tautomerization reaction to such an extent that it becomes a barrierless process. In contrast, water produces only a nominal catalytic effect. Since organic acids are present in parts-per-billion concentrations in the troposphere, the present results suggest that the acid-catalyzed tautomerization, which can also result in formation of hydroxyl radicals, may be a significant pathway for Criegee intermediates.