Photolysis Dynamics Laser Chemistry Lab

This Note aims to clarify the source of CO in photodissociation of acetyl cyanide (CH3COCN) at 308 nm. From the theoretical aspects, a new pathway via isomerization transition state (TS) at 391 ± 8 kJ/mol is found leading to the CO + CH3NC products. An amount of 60% reactant molecules at 300 K is estimated to successfully surpass the average TS barrier lying above the excitation energy by 3.5 kJ/mol. Further, a prior distribution method is conducted to characterize the vibrational energy distribution of CO on a statistical basis. The pathway to CH3NC + CO yields a vibrational branching ratio (v = 0:v = 1:v = 2:v = 3∼0.63:0.25:0.093:0.032) in excellent agreement with the observation (0.62:0.25:0.09:0.05). © 2013 AIP Publishing LLC.

Without the need to construct complicated potential energy surfaces, a multicenter impulsive model is developed to characterize the dynamical feature of conventional transition state (TS) and roaming pathways in the photodissociation of formaldehyde, H2CO → CO + H2. The photofragment energy distributions (PED) resulting from the roaming mechanism are found to closely correlate to a particular configuration that lies close to the edge of the plateau-like intrinsic reaction coordinate, whereas such a PED is associated with the configuration at the saddle point when the conventional TS pathway is followed. The evaluated PED results are consistent with those by experimental findings and quasi-classical trajectory calculations. Following impulsive analysis, the roaming pathway can be viewed as a consequence of energy transfer events between several vibrational modes. For H2CO, the available energy initially accumulated at the C-H bond is transferred to other transitional mode(s) via stretching-bending coupling, and finally to the HH stretching. (Chemical Presented). © 2014 American Chemical Society.

In one-photon dissociation of propionyl chloride at 248 nm, time-resolved Fourier-transform infrared emission spectroscopy is used to detect the fragments of HCl and CO in the presence of Ar. The inert gas Ar plays a role to enhance the internal conversion. The time-dependence of high-resolution HCl spectra yields a bimodal rotational distribution in the early stage. The total rotational and vibrational energy partitioned in HCl are evaluated to be 1.7 ± 0.3 and 8.8 ± 1.9 kcal/mol, respectively. The CO appearance indicates that HCl may be eliminated through a five-center mechanism accompanied with three-body dissociation of C2H2, HCl, and CO. A four-center mechanism forming HCl and CH3CHCO also contributes to the HCl fragment with a feature of rotational bimodality. However, the probability for the HCl contribution from the hot Cl reaction is negligible. The reaction with CH4 is carried out to evaluate the HCl and Cl elimination rate constants. © 2010 Elsevier B.V. All rights reserved.

The behavior of λ-doublet resolved rotational energy transfer (RET) by Ar collisions within the SH(X 2Π, v′=0) state is characterized. The matrix elements of terms in the interaction potential responsible for interference effects are calculated to explain the propensity rules for collision-induced transitions within and between spin-orbit manifolds. In this manner, the physical mechanisms responsible for the F 1-F 1, F 2-F 2, and F 1-F 2 transitions may be reasonably identified. As collision energy increases, the propensity for collisional population of the final e or f level is replaced by the e/f-conserving propensity. Such a change in propensity rule can be predicted in terms of energy sudden approximation at high J limit for the pure Hund's case scheme. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The exploration of alternative roads that open to molecules with sufficient energy to yield different products permits prediction and eventually control of the outcomes of chemical reactions. Advanced imaging techniques for monitoring laser-induced photodissociation are here combined with dynamical simulations, involving ample sets of classical trajectories generated on a quantum chemical potential energy surface. Methyl formate, HCOOCH3, is photodissociated at energies near the triple fragmentation threshold into H, CO and OCH3. Images of velocity and rotational distributions of CO exhibit signatures of alternative routes, such as those recently designated as transition-state vs. roaming-mediated. Furthermore, a demonstration of the triple fragmentation route is given, and also confirmed by H-atom product imaging and FTIR time-resolved spectra of the intermediate HCO radical. In addition, the relevance of nonadiabatic transitions promoted by a conical intersection is clarified by simulations as the privileged "reactivity funnel" of organic photochemistry, whereby the outcomes of molecular photoexcitation are delivered to electronic ground states. This journal is © the Owner Societies 2014.

A multi-center impulsive model has been recently developed to characterize the dynamic feature of product energy distribution in photodissociation of formaldehyde, H2CO → CO + H2. (J. Phys. Chem. A, 2015, 119, 29) The model is extended to predict the vector correlations among transition dipole moment μ of the parent molecule, recoil velocity v and rotational angular momentum j of the fragments produced via the transition state (TS) and roaming path. The correlation results of μ-j, j-j and μ-v vectors of the fragments are consistent with those reported using quasi-classical trajectory simulation on the global potential energy surface. In contrast to the TS route, the vector properties via the roaming path are loosely correlated. This work offers an alternative method to study stereodynamics of the photodissociation process, and is conducive to clarifying the origin of photofragment vector correlation especially for the roaming pathway. This journal is © the Owner Societies.

The orientation dependence for the Br atom formation in the reaction of the oriented OH radicals with HBr molecules at 0.26 eV collision energy has been observed for the first time using the hexapole electric field, and we found that the reaction cross-section for O-end attack is more favorable than that for H-end attack by a factor of 3.4 ± 2.3. © the Owner Societies.

The rotational energy transfer (RET) by Ar collisions within the SH X 2Π (v′′ = 0, J′′ = 0.5-10.5) state is characterized. The integral cross sections as a function of collision energy for each rotational transition are calculated using a quantum scattering method in which the constructed potential energy functions are based on a ground state potential energy surface (PES) reported previously. On the other hand, a laser-induced excitation fluorescence technique is employed to monitor the relaxation of the rotational population as a function of photolysis-probe delay time following the photodissociation of H2S at 248 nm. The rotational population evolution is comparable to its theoretical counterpart based on calculated Λ-resolved RET rate constants. The propensity in Λ-resolved RET transitions is found to approximately resemble the case of OH(X 2Π, v′′ = 0) + Ar. The Λ-averaged RET collisions are also analyzed and result in several propensity rules in the transitions. Most propensity rules are similar to those observed in the collisions of SH(A 2Σ+) by Ar. However, the behavior of the conserving ratio, defined as rate constants for spin-orbit conserving transition divided by those for spin-orbit changing transition, shows distinct difference from those described by Hund’s case (b). © the Owner Societies.

Time-resolved Fourier transform infrared emission spectroscopy is employed in the photolysis of propionaldehyde (CH3CH2CHO) at 248 nm to characterize the role of the roaming pathway. High-resolution spectra of CO are analyzed to yield a single Boltzmann rotational distribution for each vibrational level (ν = 1-4) with small rotational and large vibrational energy disposals. A roaming saddle point is found containing two far separated moieties of HCO and CH3CH2 with a weak interaction between them. Quasiclassical trajectory calculations on this configuration yield the CO energy flow behavior, consistent with the findings. The rate constant along the roaming pathway is evaluated to be larger by >1-2 orders of magnitude than those along tight transition state or three-body dissociation pathways. This work implies that the roaming mechanism plays an increasingly important role in aliphatic aldehydes as the molecular size becomes larger. © 2013 American Chemical Society.

Photodissociation of isobutyraldehyde (C3H7CHO) at 248 nm is investigated using time-resolved Fourier-transform infrared emission spectroscopy to demonstrate the growing importance of the roaming pathway with increasing molecular size of aliphatic aldehydes. Each acquired CO rotational distribution from v = 1 to 4 is well characterized by a single Boltzmann rotational temperature from 637 to 750 K, corresponding to an average rotational energy of 5.9 ± 0.6 kJ mol-1. The roaming signature that shows a small fraction of CO rotational energy disposal accompanied by a vibrationally hot C3H8 co-fragment is supported by theoretical prediction. The energy difference between the tight transition state (TS) and the roaming saddle point (SP) is found to be -27, 4, 15, 22, and 30 kJ mol-1 for formaldehyde, acetaldehyde, propionaldehyde, isobutyraldehyde, and 2,2-dimethyl propanal, respectively. The roaming SP is stabilized by a larger alkyl moiety. It is suggested that the roaming photodissociation rate of aldehydes increasingly exceeds those via the tight TS, resulting in the dominance of the CO + alkane products, as the size of aldehydes becomes larger. Along with formaldehyde, acetaldehyde, and propionaldehyde, in this work isobutyraldehyde is further demonstrated that this aldehyde family with special functional group is the first case in the organic compound to follow predominantly a roaming dissociation pathway, as the molecular size becomes larger. © the Owner Societies 2015.

Rotational energy transfer (RET) by Ar collisions within the v′ = 0 level of the SH A2Σ+ state is probed using a laser-induced dispersed fluorescence technique, following photodissociation of H2S at 248 nm. The Ar pressure is adjusted appropriately to allow for significant observation of the single-collision induced RET process. The spin-resolved and spin-averaged rate constants are then evaluated with the aid of a kinetic model under single-collision conditions. The theoretical counterparts are calculated using a quantum scattering method, in which a newly fitted potential energy function is based on ab initio potential energy surface reported previously. The experimental and theoretical kinetic data are essentially consistent in the trend of N and ΔN dependence. Several propensity rules are found in the RET collisions. For instance, for ΔN = 1, 2, and 3, the rate constants decrease with increasing N or ΔN. Given a fixed ΔN, the rate constants of the same initial N in the downward transition appear to be larger than those in the upward transitions. In ΔN = 0, the F2 → F1 transitions prevail over the F 1 → F2 transitions (F1 = N + 1/2, F 2 = N - 1/2), whereas in ΔN ≠ 0, the fine-structure- conserving collisions are more favored than the fine-structure-changing collisions. The principle of microscopic reversibility is also examined for both experimental and theoretical kinetic data, showing that translational energies of the RET collisions are close to thermal equilibrium at room temperature. The propensity rules may be rationalized according to this principle. © 2010 the Owner Societies.

The orientation dependence of Br-atom formation in the reaction of the oriented OH radical with the HBr molecule using the hexapole electrostatic field was studied. Experimental results for the orientation dependence in the reaction were analyzed using a Legendre polynomial fit. The results show two reactive sites. It was found that O-end attack is most favored for this reaction, and that H-end attack also shows a pronounced reactivity. The reactivity of the side-ways attack was found to be small. By comparing the results of the orientation dependence in the reaction with studies of inelastic collisions and theoretical calculations, two reaction pathways are proposed. Reaction by O-end attack is followed by a direct abstraction of the H-atom from the HBr molecule. The mechanism for H-end attack may have H-atom migration from HBr to form the water molecule. © 2011 the Owner Societies.

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. © 2020, The Author(s).