Vibrationally inelastic scattering is a fundamental collision process that converts some of the kinetic energy of the colliding partners into vibrational excitation(,). The conventional wisdom is that collisions with high impact parameters (where the partners only 'graze' each other) are forward scattered and essentially elastic, whereas collisions with low impact parameters transfer a large amount of energy into vibrations and are mainly back scattered. Here we report experimental observations of exactly the opposite behaviour for the simplest and most studied of all neutral-neutral collisions: we find that the inelastic scattering process H + D(2)(v = 0, j = 0, 2) --> H + D(2)(v' = 3, j' = 0, 2, 4, 6, 8) leads dominantly to forward scattering (v and j respectively refer to the vibrational and rotational quantum numbers of the D(2) molecule). Quasi-classical trajectory calculations show that the vibrational excitation is caused by extension, not compression, of the D-D bond through interaction with the passing H atom. However, the H-D interaction never becomes strong enough for capture of the H atom before it departs with diminished kinetic energy; that is, the inelastic scattering process is essentially a frustrated reaction in which the collision typically excites the outward-going half of the H-D-D symmetric stretch before the H-D(2) complex dissociates. We suggest that this 'tug of war' between H and D(2) is a new mechanism for vibrational excitation that should play a role in all neutral-neutral collisions where strong attraction can develop between the collision partners.
We have measured differential cross sections (DCSs) for the HD (v(')=1,j(')=2,6,10) products of the H+D(2) exchange reaction at five different collision energies in the range 1.48< or =E(coll)< or =1.94 eV. The contribution from the less energetic H atoms formed upon spin-orbit excitation of Br in the photolysis of the HBr precursor is taken into account for two collision energies, E(coll)=1.84 and 1.94 eV, allowing us to disentangle the two different channels. The measured DCSs agree well with new time-dependent quantum-mechanical calculations. As the product rotational excitation increases, the DCSs shift from backward to sideward scattering, as expected. We also find that the shapes of the DCSs show only a small overall dependence on the collision energy, with a notable exception occurring for HD (v(')=1,j(')=2), which appears bimodal at high collision energies. We suggest that this feature results from both direct recoil and indirect scattering from the conical intersection.
We describe a new instrument based on a delay-line detector for imaging the complete three-dimensional velocity distribution of photoionized products from photoinitiated reactions. Doppler-free [2+1] resonantly enhanced multiphoton ionization (REMPI) of H and D atoms formed upon photolysis of HBr and DBr in the range 203 nm < or = lambda photolysis < or = 243 nm yields radial speeds measured to be accurate within 1% of those calculated. The relative speed resolution is about 5% and limited by photoionization recoil broadening. A relative speed resolution of 3.4% is obtained for [3+1] REMPI, which minimizes the ionization recoil. We also determine the branching ratio between ground-state and spin-orbit-excited product channels and their associated anisotropies. We find that DBr photolysis dynamics differs slightly from its HBr counterpart.
A major theme in the development of the chemical sciences resides in improving the capability to negotiate issues of selectivity in the organization of matter on increasingly greater-length scales. 1 While molecular synthesis has advanced to an art through the precise control of chemo-, regio-, stereo-, and enantioselectivity, the identification and orchestration of persistent noncovalent binding motifs is extending synthetic technology to the nanoscopic regime by allowing the construction of supramolecular architectures through the self-assembly of instructed molecular components under equilibrium conditions. 2 Along these lines, the H-bond-mediated self-assembly of molecular precursors represents a powerful strategy for the logic-driven retrosynthesis and construction of nanoscopic materials in structurally homogeneous form. 2e Nevertheless, the nominal stability of most noncovalent aggregates detracts from their usefulness. To address this deficiency, template-directed syntheses involving the covalent capture of discrete noncovalent superstructures 3 and the polymerization of organized assemblies 4,5 have been described. Robust nanoscale assemblies may also be obtained through the stabilization of kinetically labile systems by the preorganization of binding sites and accumulation of multiple binding interactions. 6 Macromolecular systems are well-suited to this latter strategy, and recently much attention has been given to the preparation of polymers incorporating H-bonding moieties 7 and the self-assembly of dendritic macromolecules 8 and block copolymers. 5a,9,10 As part of a program involving the development of synthetic methodologies for the synthesis of nanostructured materials via self-assembly of macromolecular precursors, we herewith report preliminary studies on the design and de novo synthesis of synthetic polymer strands capable of duplex formation through the "covalent casting" of 1-dimensional H-bonding motifs. Excluding systems that borrow from naturally occurring superstructural motifs (e.g., homo-DNA 11 ), to the best of our knowledge this report represents the first efforts toward unnatural polymer duplexes assembling through the action of inter-strand H-bonds. 12 Cooperative binding via preorganization of multiple complementary binding sites enhances association through the reduction of entropic terms for the formation of discrete objects through receptor-substrate interactions. 2e,13 For the application of this principle to 1-dimensional superstructures, such as the H-bonded tape I, it was hypothesized that preorganization of the molecules comprising the 1-dimensional superstructure could be achieved by introduction of a covalent linker as shown schematically for the partially cast H-bonded tape II. Introduction of a second linking group fully casts the H-bonded tape to afford a covalentnoncovalent ladder material, polymer duplex III (Scheme 1).In accord with this strategy, I should be comprised of molecules capable of functioning as a platform for subsequent elaboration to III. Trichloro-...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.