Fengyan Wang

Steric Control of the Reaction of CH Stretch–Excited CHD3 with Chlorine Atom

Spectroscopy elucidates the complex interplay between orientational and vibrational effects in a simple chemical reaction. Exciting the CH-stretching mode of CHD3 (where D is deuterium) is known to promote the C-H bond’s reactivity toward chlorine (Cl) atom. Conventional wisdom ascribes the vibrational-rate enhancement to a widening of the cone of acceptance (i.e., the collective Cl approach trajectories that lead to reaction). A previous study of this reaction indicated an intriguing alignment effect by infrared laser–excited reagents, which on intuitive grounds is not fully compatible with the above interpretation. We report here an in-depth experimental study of reagent alignment effects in this reaction. Pronounced impacts are evident not only in total reactivity but also in product state and angular distributions. By contrasting the data with previously reported stereodynamics in reactions of unpolarized, excited CHD3 with fluorine (F) and O(3P), we elucidate the decisive role of long-range anisotropic interactions in steric control of this chemical reaction.

Vibrational Enhancement Factor of the Cl + CHD3(v1 = 1) Reaction: Rotational-Probe Effects.

The vibrational enhancement factor in the Cl + CHD3(v1 = 1) reaction is revisited over the collisional energy range of 2-5.9 kcal mol(-1). Contrary to the previous results obtained by probing the low-|N, K⟩ states of CD3(v = 0) products, CH stretching excitation becomes more efficacious than the same amount of translational energy in promoting the HCl(v) + CD3(v = 0) product pairs when all-|N, K⟩ states are probed. Whereas the new vibrational enhancement factors, which are three to four times larger than the previous report, agree reasonably well with a recent reduced-dimensionality quantum dynamics calculation, a cautious note is made on the different initial |J,K⟩ rotational selections of the CHD3 reactants in the present theory-experiment comparison.

Rotational mode specificity in the Cl + CHD3 -> HCl + CD3 reaction

By exciting the rotational modes of vibrationally excited CHD3(v1 = 1, JK), the reactivity for the Cl + CHD3 → HCl + CD3 reaction is observed enhanced by as much as a factor of two relative to the rotationless reactant. To understand the mode specificity, the reaction dynamics was studied using both a reduced-dimensional quantum dynamical model and the conventional quasi-classical trajectory method, both of which reproduced qualitatively the measured enhancements. The mechanism of enhancement was analyzed using a Franck-Condon model and by inspecting trajectories. It is shown that the higher reactivity for higher J states of CHD3 with K = 0 can be attributed to the enlargement of the cone of acceptance. On the other hand, the less pronounced enhancement for the higher J = K states is apparently due to the fact that the rotation along the C-H bond is less effective in opening up the cone of acceptance.

Enlarging the reactive cone of acceptance by exciting the C–H bond in the O(3P) + CHD3 reaction

The effects of the CH stretching vibration of methane on the O(3P) + CHD3 reaction are elucidated in a crossed-beam imaging experiment. For the H-atom abstraction channel, the ground state reaction leads mainly to the vibrational ground states of products, OH(v = 0) + CD3(v = 0). When the reactant CHD3 is prepared with one-quantum excitation of CH stretching vibration (v1 = 1), more than 90% of OH coproducts that formed concomitantly with the CD3(v = 0) product are in v = 1. Significant vibrational enhancement in the reaction rate is observed, and the product angular distribution shifts from a backward-dominance in O(3P) + CHD3(v = 0) to a sideways-peaking upon CH stretching excitation. As to the D-atom abstraction channel, the C–H bond excitation, despite being conserved in the CHD2 product, unexpectedly hinders the overall reactivity of the unexcited C–D bond. We interpret the experimental findings as the result of enlarging the reactive cone of acceptance in abstracting the H atom at the expense of the cone for the D-atom transfer channel.

Photofragment slice imaging studies of pyrrole and the Xe..pyrrole cluster

The photolysis of pyrrole has been studied in a molecular beam at wavelengths of 250, 240, and 193.3 nm, using two different carrier gases, He and Xe. A broad bimodal distribution of H-atom fragment velocities has been observed at all wavelengths. Near threshold at both 240 and 250 nm, sharp features have been observed in the fast part of the H-atom distribution. Under appropriate molecular beam conditions, the entire H-atom loss signal from the photolysis of pyrrole at both 240 and 250 nm (including the sharp features) disappear when using Xe as opposed to He as the carrier gas. We attribute this phenomenon to cluster formation between Xe and pyrrole, and this assumption is supported by the observation of resonance enhanced multiphoton ionization spectra for the (Xe...pyrrole) cluster followed by photofragmentation of the nascent cation cluster. Ab initio calculations are presented for the ground states of the neutral and cationic (Xe...pyrrole) clusters as a means of understanding their structural and energetic properties.

Imaging the Effects of Bend-Excitation in the F + CD4(vb=0,1) → DF(v) + CD3(v2=1,2) Reactions

The title reaction was studied over the collisional energy range 0.9-4.0 kcal mol(-1), using a time-sliced velocity-imaging technique in a crossed-beam experiment. Both the integral and differential cross sections were measured in a product pair-correlation manner. Experimental evidence for the dual reaction mechanisms, a direct abstraction and a resonance-mediated pathway, were presented and await future theoretical confirmation.

Nonstatistical spin dynamics in photodissociation of H2O at 157nm

Photodissociation of H2O via the A band at 157nm has been reinvestigated using the high resolution H atom Rydberg tagging technique. The spin-orbit population distributions were found to be highly nonstatistical for the OH(v=0,1,2) product channels, while nearly statistical for the OH(v=4) channel. These results suggest that the dissociation dynamics of H2O at 157nm is remarkably different for different vibrationally excited OH channels. The result presented here is not entirely consistent with the direct dynamical picture of the A band dissociation of H2O.

Cluster-Controlled photofragmentation:The Case of the Xe–Pyrrole Cluster

In this report we describe a process where chemical control can be used to create a molecular switch. Specifically we demonstrate that the fission of the N-H bond in pyrrole can be “controlled”, i.e., turned ON or OFF, by clustering the system to a single noble gas atom such as Xe. Using slice imaging experiments on the H-atom photofragments in combination with high level quantum mechanical electronic structure calculations, the mechanism underlying the process is unravelled. The effect is general and manifests itself in situations were conical intersections, such as the one present in the pyrrole molecule, a prototype of small DNA bases, are involved in the photochemistry of the system. Hence our observation appears to be a first step towards “biological” control at the molecular level. Part of this work was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, US Department of Energy. Battelle operates the Pacific Northwest National Laboratory for the US Department of Energy.

A velocity map ion-imaging study on ketene photodissociation at 208 and 213 nm: Rotational dependence of product angular anisotropy

Photodissociation dynamics of ketene following excitation at 208.59 and 213.24 nm have been investigated using the velocity map ion-imaging method. Both the angular distribution and translational energy distribution of the CO products at different rotational and vibrational states have been obtained. No significant difference in the translational energy distributions for different CO rotational state products has been observed at both excitation wavelengths. The anisotropy parameter beta is, however, noticeably different for different CO rotational state products at both excitation wavelengths. For lower rotational states of the CO product, beta is smaller than zero, while beta is larger than zero for CO at higher rotational states. The observed rotational dependence of angular anisotropy is interpreted as the dynamical influence of a peculiar conical intersection between the (1)B(1) excited state and (1)A(2) state along the C(S)-I coordinate.

Photofragment Imaging of HNCO Decomposition at 210 nm: the Primary NH(a1?)+CO(X1?+) Channel

The photodissociation of isocyanic acid (HNCO) on the first excited singlet state following the excitation at 210 nm was investigated with an ion velocity slice imaging technique by probing the CO fragment. It was found from the (2+1) resonance-enhanced multi-photon ionization (REMPI) spectrum that the CO fragments are rotationally hot with population up to Jmax=50. The velocity imagings of the CO fragments at JCO=30 and 35 indicate that formation of NH(a1Δ)+CO(X1Σ+, v=0) is the predominant dissociation channel at 210 nm. From analysis of the CO fragment translational energy distributions, the NH(a1Δ) fragment was observed to be rotationally cold, about half of the available energy was partitioned into the translational motion of fragments after dissociation, and the NH(a1Δ)+CO(X1Σ+) dissociation threshold was determined at 42738±30 cm−1. From analysis of the CO fragment angular distributions, the dissociation anisotropy parameter β was found to be negative, and increasing with the rotational quantum number of the NH fragment, i.e., from -0.75 at JNH=2-4 to -0.17 at JNH=11. Impulsive direct and vertical dissociation process of HNCO on the singlet state at 210 nm was confirmed experimentally. A classical impact dissociation model was employed to explain the dependence of the β value on the rotational excitation of the NH fragment.