Reaction Dynamics in Extreme Environments

1. Reaction Dynamics in Extreme Environments C. Ennis,1 B. Jones,1 P. Maksyutenko,1 C. Bennett,1 F. Zhang,1 A. Gregusova,2 A. Perera,2 R.J. Bartlett,2 S.J. Sibener,3 A.M. Mebel,4 A.H.H. Chang,5 R.I. Kaiser1 1Department of Chemistry & UH NASA Astrobiology Institute, University of Hawai’i at Manoa, Honolulu, HI. USA 2 Quantum Theory Project, University of Florida, Gainesville, FL, USA 3 James Franck Institute and Department of Chemistry, University of Chicago, Chicago, IL, USA 4 Department of Chemistry and Biochemistry, Florida International University, Miami, FL, USA 5 Department of Chemistry, National Dong Hwa University, Hualien, Taiwan Crossed Beams We have elucidated the energetics and dynamics of elementary reactions of ground state boron atoms (B(2Pj)) with simple nitrogen-bearing molecules: ammonia (NH3) and hydrogen cyanide (HCN). The closed shell molecules serve as prototype reaction partners to access the HxBCyN (x=0,1,2,3; y=0,1) potential energy surfaces which are important in the fields of basic physical chemistry (reaction dynamics), combustion chemistry, material sciences, chemical propulsion systems, physical organic chemistry, and chemical vapor deposition processes (boron-nitride films, ternary BCN compounds). The experiments are pooled together with electronic structure calculations (Bartlett et al.; Mebel et al.; Chang et al.) to verify the elucidated reaction mechanisms theoretically; this ultimately bridges the understanding of reactive scattering processes involving small boron-bearing systems via quantum chemical methods and experiments. The reaction of boron with hydrogen cyanide revealed that the boronisocyanide molecule, [BNC], is formed as the exclusive product under gas phase single collision conditions. We also show that higher energy isomers such as the hitherto unnoticed, ring-strained cyclic BNC structure, which is isoelectronic to the triplet, cyclic tricarbon molecule, [C3], exist as local minima. Our study presents the first ‘clean’ synthesis and observation of gas phase boron­isocyanide providing a door way for further fundamental, spectroscopic studies on one of the simplest triatomic molecules composed solely of group III – V elements. – The data analysis of the boron – ammonia system is in progress. The data suggest that the reaction dynamics proceed via the formation of BNH3 complex(es) before the latter decompose(s) via atomic hydrogen elimination to the BNH2 product(s). The reaction was further found to be exoergic by 280  20 kJmol-1. This indicates the formation of the HBNH isomer plus atomic hydrogen. Organic Polymers This project investigates the effects of ionizing radiation on polymers (Kapton, Teflon, PE, PS, PMMA) utilizing a surface scattering machine. These experiments help to untangle the stability of polymers toward space weathering (cosmic rays, energetic electrons, ions, photons) at temperatures ranging from 10 K to 300 K, thus simulating the extreme conditions experienced in Low Earth Orbit and the surfaces of the moon and Mars. Here, we report on the interaction of energetic electrons as generated in the track of galactic cosmic ray particles with all polymers in a UHV chamber (510-11 Torr). The irradiated samples are analyzed via infrared and UVVIS spectroscopy; the gas phase is monitored via a quadrupole mass spectrometer. Mechanistical results on polymer degradation via, e.g., chain rupture versus chain branching are discussed. Kinetic profiles of the newly formed molecular bonds are also presented. Successive experiments, done in collaboration with the Sibener Group in Chicago will focus on the effects of singly ionized oxygen atoms with spin-coated polymer films (100 – 200 nm). Here, we report first results on the electron-induced degradation of three polymers utilized in aerospace applications (polyethylene (PE), polytetrafluoroethylene (PTFE), and poly-styrene (PS)). These processes simulate the interaction of secondary electrons generated in the track of galactic cosmic ray particles in the near-Earth space environment with polymer material. The chemical alterations at the macromolecular level were monitored on line and in situ by Fourier-transform infrared spectroscopy and mass spectrometry. These data yielded important information on the temperature dependent kinetics on the formation of, for instance, trans-viny­le­ne groups (–CH=CH–) in PE, benzene (C6H6) production in PS, fluorinated trans-vinylene (–CF=CF–) and terminal vinyl (–CF=CF2) groups in PTFE together with molecular hydrogen release in PE and PS. Additional data on the radiation-induced development of unsaturated, conjugated bonds were collected via UVVIS spec-troscopy. Temperature dependent G-values for trans-vinylene formation (G(–CH=CH–) ~ 25–2.5 × 10-4 units/100 eV from 10–300 K) and molecular hydrogen evolution (G(H2) ~ 8–80 × 10-5 molecules /100 eV from 10–300 K) for irradiated PE were calculated to quantify the degree of polymer degradation following electron irradiation. These values are typically two to three orders of magnitude less than previously published G-values for the irradiation of polymers with energetic particles of higher mass. Scattering Machine A new state of the art surface scattering chamber has been designed to investigate the interaction of ionizing radiation with solid surfaces (4–300K). Multiple tunable irra-diation sources are implemented: charged particles (1eV–5keV; H+, He+/++, On+, e-) and UVVUV photons from a continuous discharge source coupled to a monochroma-tor (7-11eV) or from a laser-based pulsed source utilizing four wave mixing (5-15eV). The chemical and physical alteration of the exposed surfaces is monitored on line and in situ utilizing complementary detection schemes covering NIR, MIR, FIR, Raman, UVVIS, and UVVUV spectroscopy. Molecules subliming into the gas phase are photoionized selectively according to their ionization energies and mass separated in a linear time-of-flight spectrometer. A RGA with soft electron impact ionization is also available.