A Crossed Molecular Beams Investigation on the Dynamics and Energetics of Elementary Boron Reactions with Unsaturated Hydrocarbons Ralf I. Kaiser Department of Chemistry, University of Hawai’i at Manoa, Honolulu, HI Introduction The energetics and dynamics of reactions of ground state boron atoms, B( 2 P), are of paramount importance in understanding boron combustion processes and in the manufacture of boron-bearing semiconductors and nanotubes. Electron-deficient bonding is a common feature in boron-rich solids which would make it a logical choice for use in semiconductors. One set of semiconductors which is currently of interest are the boron carbides. They are very stable, possess high electrical as well as low thermal conductivities, and have been considered as the most promising materials for realizing high-efficiency thermoelectric energy conversion given n-type boron carbides. These materials also have potential applications as high-temperature semiconductors. In particular, elemental boron is appealing for use as an enhancing agent in combustion applications due to its high theoretical energy density, both on a volumetric and gravimetric basis. This, coupled with a high energy of combustion process and low-molecular-weight products, explains why boron is considered an attractive material for use in rocket propellants and explosives. But despite the key role of atomic boron reactions to form organo-boron molecules and their hydrogen-deficient precursors in combustion processes, the fundamental question ‘How are these molecules and their precursors actually formed?’ has not been conclusively resolved. Our aim is to shed light on this question and to elucidate the energetics and the dynamics of reactions of ground state boron atoms, B( 2 P j ), with unsaturated hydrocarbons acetylene (C 2 H 2 (X 1  g + )), ethylene (C 2 H 4 (X 1 A g )), methylacetylene (CH 3 CCH(X 1 A 1 )), allene (H 2 CCCH 2 (X 1 A 1 )), and benzene (C 6 H 6 (X 1 A 1g )). The closed shell hydrocarbons serve as prototype reaction partners with triple (acetylene), double (ethylene), and aromatic (benzene) bonds; methylacetylene and allene are chosen as the simplest representatives of closed shell hydrocarbon species to investigate how the reaction dynamics change from one structural isomer to the other. The experiments are pooled together with electronic structure calculations to verify the elucidated reaction mechanisms theoretically. Here, we present new data on the crossed beams reactions of boron atoms with allene and methylacetylene. Experimental  universal crossed molecular beams machine  angular resolved time-of-flight spectra  hydrocarbon free operation of the main chamber (10 -9 torr) and source chambers (10 -8 torr) maglev, scroll, roots, EcoDry, and cryo pumps  electron impact ionization of reactively scattered species  hydrocarbon free boron-based laser ablation source (266 nm; 30 Hz; 5~10 mJ pulse -1 )  tunable electron energy (7 eV – 200 eV) with emission currents up to 2 mA  hydrocarbon free operation of triply differentially pumped detector (2  torr with liquid nitrogen cooling; < torr with 1.5 W cold head)  home-made Brink ionizer, QMS (Extrel; 1.2 MHz, 2.1 MHz, 2.9 MHz), and Daly detector with air-cooled voltage divider chain B( 2 P)+CH 3 CCH(X 1 A 1 )(E C =21.4 kJmol -1 ) B( 2 P)+H 2 CCCH 2 (X 1 A 1 )(E C =21.4 kJmol -1 ) Results  R G= -128 (-134) kJmol -1  R G= -58 (-64) kJmol -1  R G= -59 (-65) kJmol -1  R G= -87 (-93) kJmol -1  R G= +17 (+11) kJmol -1