Steric Effects on Electronically Excited Product Channels in Reactions between Ca(1D2) and CH3X(JKM) (X = Cl, Br)

Stereoselectivity in Autoionization Reactions of Hydrogenated Molecules by Metastable Noble Gas Atoms: The Role of Electronic Couplings

Focus in the present paper is on the analysis of total and partial ionization cross sections, measured in absolute value as a function of the collision energy, representative of the probability of ionic product formation in selected electronic states in Ne*-H2 O, H2 S, and NH3 collisions. In order to characterize the imaginary part of the optical potential, related to electronic couplings, we generalize a methodology to obtain direct information on the opacity function of these reactions. Such a methodology has been recently exploited to test the real part of the optical potential (S. Falcinelli et al., Chem. Eur. J., 2016, 22, 764-771). Depending on the balance of noncovalent contributions, the real part controls the approach of neutral reactants, the removal of ionic products, and the structure of the transition state. Strength, range, and stereoselectivity of electronic couplings, triggering these and many other reactions, are directly obtained from the present investigation.

Multidimensional steric effect for the XeF* (B, C) formation in the oriented Xe* ((3)P(2), M(J) = 2) + oriented NF(3) reaction

Steric effect for the XeF* (B, C) formations in the oriented Xe* ((3)P(2), M(J) = 2) + oriented NF(3) reaction has been observed as a function of the mutual configuration between the molecular orientation and the atomic orientation in the collision frame. Molecular steric opacity function has been determined as a function of the atomic orbital alignment (L(Z)') in the collision frame. The larger reactivity at the side with the smaller reactivity at the molecular axis direction is observed for the XeF* (B, C) channels at each atomic orbital alignment. A good correlation between the shape of the molecular steric opacity function and the molecular geometry of NF(3) is recognized. The L(Z)' selectivity in the molecular steric opacity function is different between the XeF* (B, C) channels; in the sideways direction, the XeF* (B) channel is favorable at L(Z)' = 0, while the XeF* (C) channel is favorable at |L(Z)'| = 1. In contrast, at the molecular axis direction, the XeF* (B) channel is favorable at |L(Z)'| = 1, while the XeF* (C) channel is favorable at L(Z)' = 0. We propose the collision-induced harpoon mechanism for the XeF* (B, C) formation in the Xe* ((3)P(2)) + NF(3) reaction.

Multidimensional molecular steric opacity function for XeCl*(B, C) formation in the oriented Xe* (³P₂, MJ = 2) + oriented CCl₃F reaction

The steric effect for the XeCl*(B, C) formations in the oriented Xe* (³P₂, MJ = 2) + oriented CCl₃F reaction has been observed as a function of the mutual configuration between the molecular orientation and the atomic orbital alignment in the collision frame. Molecular steric opacity functions have been determined as a function of the atomic orbital alignment (M(L)') in the collision frame. The XeCl*(B, C) channels show similar molecular steric opacity functions at M(L)' = 0 but not at |M(L)'| = 1. The large molecular alignment dependence (i.e., the reactivity of the Cl₃ end and the F end is comparable, but a very poor reactivity for the sideway) is recognized for the XeCl*(B, C) channels except for the XeCl*(C) channel at |M(L)'| = 1, which shows an almost isotropic molecular orientation dependence. The M(L)' selectivity is different between the XeCl*(B, C) channels. At the molecular axis direction, the XeCl*(B) channel has little M(L)' selectivity whereas the XeCl*(C) channel is significantly favorable at M(L)' = 0. On the other hand, |M(L)'| = 1 is favorable at the sideway for the XeCl*(B, C) channels.

Multi-dimensional steric effect for XeI* (B) formation in the oriented Xe* ((3)P(2), M(J) = 2) + oriented CH(3)I reaction

Multi-dimensional steric effect for the XeI* (B) formation in the oriented Xe* ((3)P(2), M(J) = 2) + oriented CH(3)I reaction has been observed as a function of the mutual configuration between the molecular orientation and the atomic alignment in the collision frame. The molecular steric opacity function has been determined as a function of the atomic orbital alignment. The large molecular orientation dependence (i.e., the largest reactivity at the I-end and the large difference in the reaction probability between the I-end and the CH(3)-end) and the large molecular alignment dependence (the poor reactivity at the sideway) is recognized for each atomic orbital alignment. In addition, a clear correlation between the molecular orientation and the atomic orbital alignment is recognized (i.e., the L(Z)' = 0 atomic orbital alignment is favorable for the molecular axis direction, while the |L(Z)'| = 1 atomic orbital alignment is favorable for the sideway direction).

Multidimensional steric effect for the XeBr* (B, C) formation in the oriented Xe*((3)P2, M(J) = 2) + oriented CF3Br reaction

Steric effect for the XeBr(*) (B, C) formation in the oriented Xe(*)((3)P(2), M(J) = 2) + oriented CF(3)Br reaction has been observed as a function of the mutual configuration between the molecular orientation and the atomic orientation in the collision frame. Molecular steric opacity function has been determined as a function of the atomic orbital alignment (L(Z)(')) in the collision frame. The L(Z)(') selectivity in the molecular steric opacity function is different between the XeBr(*) (B, C) channels: For the XeBr(*) (C) channel, the L(Z)(') = 0 alignment is favorable at the molecular axis direction and the absolute value(L(Z)(')) = 1 alignment is favorable at the sideway direction, whereas for the XeBr(*) (B) channel, the L(Z)(') = 0 alignment is favorable at the sideway direction and the absolute value(L(Z)(')) = 1 alignment is favorable at the molecular axis direction. However, the shape of the steric opacity function for the XeBr(*) (B) channel at the L(Z)(') = 0 (and absolute value(L(Z)(')) = 1) alignment is similar to that for the XeBr(*) (C) channel at the absolute value(L(Z)(')) = 1 (and L(Z)(') = 0) alignment, respectively: A large molecular orientation dependence (i.e., the largest reactivity at the Br-end with the small molecular alignment dependence) is recognized for the XeBr(*) (B) channel at the L(Z)(') = 0 alignment and for the XeBr(*) (C) channel at the absolute value(L(Z)(')) = 1 alignment, whereas a large molecular alignment dependence (i.e., the largest reactivity at the Br-end with the poor reactivity at the sideway) is recognized for the XeBr(*) (B) channel at the absolute value(L(Z)(')) = 1 alignment and for the XeBr(*) (C) channel at the L(Z)(') = 0 alignment. We propose the indirect mechanism for the dark channels (Xe + Br + CF(3)) via the back-electron transfer from the CF(3) segment (or dissociating CF(3)...Br(-)) to Xe(+) as the origin of the significant molecular alignment dependence in the molecular steric opacity function.

Conformational identification of tryptamine embedded in superfluid helium droplets using electronic polarization spectroscopy

We report electronic polarization spectroscopy of tryptamine embedded in superfluid helium droplets. In a dc electric field, dependence of laser induced fluorescence from tryptamine on the polarization direction of the excitation laser is measured. Among the three observed major conformers A, D, and E, conformers D and E display preference for perpendicular excitation relative to the orientation field, while conformer A is insensitive to the polarization direction of the excitation laser. We attribute the behavior of conformer A to the fact that the angle between the permanent dipole and the transition dipole is close to the magic angle. Using a linear variation method, we can reproduce the polarization preference of the three conformers and determine the angle between the transition dipole and the permanent dipole. Since the side chain exerts small effect on the direction of the transition dipole in the frame of the indole chromophore, all three conformers have a common transition dipole more or less in the indole plane at an angle of approximately 60 degrees relative to the long axis of the chromophore. The orientation of the side chain, on the other hand, determines the size and direction of the permanent dipole, thereby affecting the angle between the permanent dipole and the transition dipole. For conformer D in the droplet, our results agree with the Anti(ph) structure, rather than the Anti(py) structure. Our work demonstrates that polarization spectroscopy is effective in conformational identification for molecules that contain a known chromophore. Although coupling of the electronic transition with the helium matrix is not negligible, it does not affect the direction of the transition dipole.

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.