Observation of Selectively Populated Monohalide Excited States from the Reactions of Group 3 Metal (Sc, Y, and La) Monomers and Dimers with Halogen-Containing Molecules

Abstract

The chemiluminescent reactions of the group 3 metals Sc and Y with F 2 , Cl 2 , Br 2 , ClF, ICl (Sc), IBr (Y), and SF 6 and La with F 2 , SF 6 , Cl 2 , and ClF have been studied at low pressures (6 × 10 −6 to 4 × 10 −4 Torr) using a beam-gas arrangement and extended to the 10 −3 Torr multiple collision pressure range. Contrary to previous reports, the observed chemiluminescent spectra are primarily attributed to emission from the metal monohalides. Extensive pressure and temperature dependence studies and high-level correlated molecular orbital theory calculations of the bond dissociation energies support this conclusion and the attribution of the chemiluminescence. Evidence for the "selective" production of a monohalide excited electronic state is obtained for several of the Sc and Y reactions. All reactions producing the metal monofluorides are first order with respect to the oxidant, while reactions producing the monochlorides and monobromides are found to be "faster than first order" with respect to the oxidant. This difference is associated with the metal halide bond dissociation energies and the metal halide product internal density of states. Analysis of the temperature dependence for six representative reactions indicates that the "selective" excited-state formation of the metal monohalides proceeds via a direct mechanism with negligible activation energy. We compare and contrast the present results with previous experiments and interpretations which have assigned the selective emission from these systems to the group 3 dihalides produced in a two-step reaction sequence analogous to an electron jump process. The current results suggest a distinctly different interpretation of the observed processes in these systems. The observed selectivity observed in these studies is remarkable given the significant number of known and potential excited states in the scandium and yttrium halides as well as their different electronic configurations.

Excited Electronic State Cross Sections for Group 3 Halide and Oxide Production: Evaluating Relative Excited-State Quantum Yields

“…Representative scans of the chemiluminescence spectra resulting from the reactions of Sc, Y, and La with F 2 are presented in the previous paper. 1 Scans of the chemiluminescence from reactions with NO 2 have been presented previously. 2,3 All spectra resulting from the reactions of each metal with the various oxidants employed have been discussed elsewhere.…”

“…The reactions of group 3 metals with fluorine result in the formation of the diatomic halides in a moderate to highly selective population of excited electronic states. 1 Reaction exothermicity plays an important role in these systems but unlike the formation of the metal oxides, reaction exoergicity appears to have a distinctly different role as the distribution of excited states is quite selective. The requirement is that the reaction process is of sufficient exothermicity to access the selective state energetically.…”

“…Further, "heavier" halogen (Cl 2 , Br 2 ) reactions have been demonstrated to involve metal dimer-halogen-molecule four-center reactions. 1 The selective formation of excited states observed in these systems demonstrates a remarkable propensity to form these excitedstate emitters based simply on available energy.…”

“…Oven temperatures were measured with a calibrated Leeds and Northrup disappearing filament optical pyrometer focused on the back center of the crucible from which the metal is vaporized, and also by a calibrated tungsten5% rhenium versus tungsten26% rhenium thermocouple. 1 Photon emission rates were determined with an RCA 6256S photomultiplier calibrated using a standardizing arrangement in which the geometric configuration of the apparatus used to measure chemiluminescence was precisely duplicated. A standard lamp (EG&G #B115A), in its holder, was placed 50 cm from a 0.00381 × 0.1 cm slit (single slit diffraction).…”

“…In the current experiments, a capacitance manometer was placed 3 cm from the metalhalogen reaction zone and an ionization gauge, 25 cm from this zone; the observed pressure gradient is less than 15%. Oven temperatures were measured with a calibrated Leeds and Northrup disappearing filament optical pyrometer focused on the back center of the crucible from which the metal is vaporized, and also by a calibrated tungsten5% rhenium versus tungsten26% rhenium thermocouple 1. Photon emission rates were determined with an RCA 6256S photomultiplier calibrated using a standardizing arrangement in which the geometric configuration of the apparatus used to measure chemiluminescence was precisely duplicated.…”