Crossed beam studies of radical–radical reactions: O(3P) + C3H5(allyl)

The dynamics of the radical-radical reaction O((3)P) + C(3)H(5) has been investigated by means of the crossed molecular beam technique with mass spectrometric detection at a collision energy of 73.0 kJ mol(-1); the reaction mechanism of the H-displacement channel has been elucidated, while experimental evidence of the occurrence of one or more C-C bond-breaking channels at this collision energy has been obtained.

Differential cross sections from quantum calculations on coupled Ab initio potential energy surfaces and scattering experiments for Cl(2P)+H2 reactions

To assess the relative reactivity of the spin-orbit excited state of atomic Cl with molecular hydrogen, we have measured differential cross sections using an atomic Cl beam with a known concentration of the ground and excited spin-orbit states. These are compared with the first determination of the cross sections from quantum mechanical scattering calculations on a set of coupled ab initio potential energy surfaces. The comparison suggests that these surfaces may underestimate the degree of rotational excitation of the HCl products and that the excited spin-orbit state plays a minor role in the reaction.

Crossed beam studies of elementary reactions of N and C atoms and CN radicals of importance in combustion

The dynamics of some elementary reactions of N(2D), C(3P,1D) and CN(X2 sigma +) of importance in combustion have been investigated by using the crossed molecular beam scattering method with mass spectrometric detection. The novel capability of producing intense, continuous beams of the radical reagents by a radio-frequency discharge beam source was exploited. From angular and velocity distribution measurements obtained in the laboratory frame, primary reaction products have been identified and their angular and translational energy distributions in the center-of-mass system, as well as branching ratios, have been derived. The dominant N/H exchange channel has been examined in the reaction N(2D) + CH4, which is found to lead to H + CH2NH (methylenimine) and H + CH3N (methylnitrene); no H2 elimination is observed. In the reaction N(2D) + H2O the N/H exchange channel has been found to occur via two competing pathways leading to HNO + H and HON + H, while formation of NO + H2 is negligible. Formation of H + H2CCCH (propargyl) is the dominant pathway, at low collision energy (Ec), of the C(3P) + C2H4 reaction, while at high Ec formation of the less stable C3H3 isomers (cyclopropenyl and/or propyn-1-yl) also occurs; the H2 elimination channel is negligible. The H elimination channel has also been found to be the dominant pathway in the C(3P,1D) + CH3CCH reaction leading to C4H3 isomers and, again, no H2 elimination has been observed to occur. In contrast, both H and H2 elimination, leading in comparable ratio to C3H + H and C3(X1 sigma g+) + H2(X1 sigma g+), respectively, have been observed in the reaction C(3P) + C2H2(X1 sigma g+). The occurrence of the spin-forbidden molecular pathway in this reaction, never detected before, has been rationalized by invoking the occurrence of intersystem crossing between triplet and singlet manifolds of the C3H2 potential energy surfaces. The reaction CN(X2 sigma +) + C2H2 has been found to lead to internally excited HCCCN (cyanoacetylene) + H. For all the reactions the dynamics have been discussed in the light of recent theoretical calculations on the relevant potential energy surfaces. Previous, lower resolution studies on C and CN reactions carried out using pulsed beams are noted. Finally, throughout the paper the relevance of these results to combustion chemistry is considered.

CROSSED-BEAM STUDIES OF REACTION DYNAMICS

▪ Abstract  This article reviews recent progress in our understanding of gas-phase neutral reaction dynamics as made possible by improvements in the crossed molecular beam scattering technique for measuring reactive differential cross sections. A selection of crossed-beam studies on systems that play a fundamental role in our basic understanding of reaction phenomena are discussed to illustrate the capabilities of the experimental method. The examples include benchmark atom-diatom abstraction and insertion reactions, and four-atom radical reactions for which state-to-state, state-resolved, or state-averaged differential cross sections have recently been measured. The results are discussed in the light of the latest related theoretical developments regarding the treatment of potential energy surfaces and the dynamics of the systems. Recent results on crossed-beam studies of chemically relevant reactions of carbon, nitrogen, and oxygen atoms are also reviewed, and the latest developments in the technique are noted.

Multiple nodular lesions of the chin and oral mucosa in a patient with Sjögren's syndrome

Sjögren's syndrome is an autoimmune disease characterised by generalised lymphoproliferation. Patients have an increased risk of developing lymphomas which are usually derived from mucosa-associated lymphoid tissue (MALT). We report a low grade, non-Hodgkin's lymphoma of the skin and oral mucosa in a patient with Sjögren's syndrome.

The dynamics of the reaction of O(1D) with HBr studied by crossed molecular beams and time-resolved Fourier transform spectroscopy

The reaction O(1D) + HBr has been investigated by the crossed molecular beams and infrared chemiluminescence methods in an effort to characterize the dynamics of both possible reactive channels. The angular and velocity distribution of the BrO product from the O(1D) + HBr → BrO + H pathway have been obtained in crossed beam experiments at collision energies, Ec, of 5.0 and 14.0 kcal/mol. The product center-of-mass angular distribution is found to be almost backward–forward symmetric at both Ec, with backward scattering being slightly favored, from which it is deduced that two processes contribute to this channel: a dominant one occurring via formation of a long-lived complex, following O(1D) insertion, and another one occurring via direct abstraction of the halogen atom and giving rise to a rebound dynamics. The large fraction (≈50%) of available energy released into translation indicates the existence of a potential barrier for H-displacement in the exit channel. From energy and angular momentum conservation arguments, it is inferred that BrO is formed rotationally very hot in the lowest vibrational levels of both 2Π3/2 and 2Π1/2 electronic states. The initial vibrational distribution of the OH product from the O(1D) + HBr → OH + Br channel has been measured using fast time-resolved Fourier transform spectroscopy. The vibrational distribution is strongly inverted, from which it is deduced that the HOBr intermediate dissociates very rapidly, before energy randomization occurs. A lower limit to the branching ratio of the relative cross sections for the BrO + H and OH + Br channels is derived (σ(BrO + H)/σ(OH + Br) ≥ 0.16 ± 0.07) and compared to recent bulk work. The dynamical results for the overall reaction are discussed with reference to the relevant singlet and triplet potential energy surfaces and possible molecular configurations involved. Comparison with the dynamics of the ground state reaction O(3P) + HBr → OH + Br is carried out also, to examine the effect of electronic excitation on the dynamics of the reactions of atomic oxygen with hydrogen halides.