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UA PHYS 131 - The Fourier transform microwave spectrum

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The Fourier transform microwave spectrum of the arsenic dicarbideradical „CCAs: X˜2⌸1/2… and its13C isotopologuesM. Sun,1D. J. Clouthier,2and L. M. Ziurys1,a兲1Department of Chemistry and Department of Astronomy, Arizona Radio Observatory, and StewardObservatory, University of Arizona, Tucson, Arizona 85721, USA2Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, USA共Received 28 September 2009; accepted 4 November 2009; published online 11 December 2009兲The pure rotational spectrum of the CCAs radical in its ground electronic and spin state, X˜2⌸1/2, hasbeen measured using Fourier transform microwave techniques in the frequency range of12–40 GHz. This species was created in a supersonic expansion from a reaction mixture of AsCl3and C2H2or CH4diluted in high pressure argon, using a pulsed nozzle containing a dc dischargesource. Three rotational transitions were measured for the main isotopologue,12C12CAs, in the ⍀=12ladder; both lambda-doubling and arsenic 共I=3/ 2兲 hyperfine interactions were observed in thesespectra. In addition, two to four rotational transitions were recorded for the13C13CAs,13C12CAs,and12C13CAs species. In these three isotopologues, hyperfine splittings were also resolved arisingfrom the13C nuclei共I=12兲, creating complex spectral patterns. The CCAs spectra were analyzed witha case 共a兲 Hamiltonian, and effective rotational, lambda-doubling, and arsenic and carbon-13hyperfine constants were determined for the ⍀ =12ladder. From the effective rotational constants ofthe four isotopologues, an rm共1兲structure has been derived with rC–C=1.287 Å and rC–As=1.745 Å. These bond lengths indicate that the predominant structure for arsenic dicarbide isCv Cv As·, with some contributing CwC and Cw As triple bond characters. The hyperfineconstants established in this work indicate that about 2/ 3 of the unpaired electron density lies on thearsenic atom, with the remaining percentage on the terminal carbon. The value of the arsenicquadrupole coupling constant 共eqQ=−202 MHz兲 suggests that the As–C bond has a mixture ofcovalent and ionic characters, consistent with theoretical predictions that both␲backbonding andelectron transfer play a role in creating a linear, as opposed to a cyclic, structure for certainheteroatom dicarbides. © 2009 American Institute of Physics. 关doi:10.1063/1.3267483兴I. INTRODUCTIONCompared to nitrogen and phosphorus-containing mol-ecules, arsenic-bearing species have not attracted as muchattention from spectroscopists.1In the field of pure rotationalspectroscopy, for example, only a limited number of mol-ecules have been characterized, such as AsF3,2–4AsCl3,5AsBr3,6AsH,7AsH2,8,9AsH3,10,11CH3CAs,12and AsP,13us-ing far-infrared, millimeter-wave, or microwave techniques.However, due to a rising interest in functional materialsmade of As-doped carbon clusters, as well as the reactivity ofarsenic ylides in organic synthesis, examining the basic prop-erties of As-bearing compounds has acquired a renewed im-portance. For example, calculations on numerous arsenic-containing organic species have been carried out at variouslevels of theory to achieve an understanding of As–C bond-ing, and RAsv CF2-type molecules have been synthesizedand their reactivity experimentally investigated.14–17Very recently, a novel arsenic-containing molecule hasbeen produced in the gas phase and studied using electronicspectroscopy: arsenic dicarbide 共CCAs兲, the smallest As-doped carbon cluster. The2⌬r−X˜2⌸rband system of thisfree radical was investigated by Wei et al.,18who were ableto establish estimates of rotational constants for both the12Cand13C isotopologues in the ⍀ =12ladder of the ground elec-tronic state, X˜2⌸r. These authors also determined that themolecule is linear, as predicted theoretically.18CCAs is only the tenth main group dicarbide that hasbeen studied by gas-phase spectroscopy and certainly war-rants additional investigation. In the present paper, wepresent the first pure rotational study of this free radical us-ing Fourier transform microwave 共FTMW兲 techniques. Spec-tra of four isotopologues of arsenic dicarbide, CCAs,13C2As,13CCAs, and C13CAs, have been recorded in their X˜2⌸1/2electronic states; from the resulting rotational constants, theground state geometry has been refined. In addition, hyper-fine structures arising from As and13C nuclear spins werealso resolved in the spectra, providing insight into the bond-ing in this radical. Here we present our data and analysis anda comparison of these results with the properties of othergroup V dicarbides.II. EXPERIMENTALMeasurements of the pure rotational spectra of the fourCCAs isotopologues were conducted in the 12–40 GHzrange using the FTMW spectrometer of the Ziurys group.This Balle–Flygare-type narrow-band spectrometer consistsa兲Electronic mail: [email protected] JOURNAL OF CHEMICAL PHYSICS 131, 224317 共2009兲0021-9606/2009/131共22兲/224317/10/$25.00 © 2009 American Institute of Physics131, 224317-1of a vacuum chamber 共background pressure of ⬃10−8torrmaintained by a cryopump兲 which contains a Fabry–Pérot-type cavity constructed from two spherical aluminum mirrorsin a near-confocal arrangement. Antennas are embedded ineach mirror for injecting and detecting microwave radiation.A supersonic jet expansion is used to introduce the samplegas, produced by a pulsed-valve nozzle 共General Valve兲 con-taining a dc discharge source. In contrast to other FTMWinstruments of this type, the supersonic expansion is injectedinto the chamber at a 40° angle relative to the mirror axis.More details regarding the instrumentation can be found inRef. 19.The12C12CAs radical was generated in the gas phaseusing the precursors AsCl3and unpurified acetylene. Argonat a pressure of 20 psi, seeded with 0.3% acetylene, waspassed over liquid AsCl3共Aldrich, 99%兲 contained in aPyrex U-tube,18and the resultant gas mixture deliveredthrough the pulsed discharge nozzle 共0.8 mm orifice兲 at arepetition rate of 12 Hz. The gas pulse duration was set to500␮s, which resulted in a 20–30 SCCM 共SCCM denotescubic centimeter per minute at STP兲 mass flow. CCAs pro-duction was maximized with a discharge of 1000 V at50 mA. To produce13C13CAs, 0.3% H13C13CH 共CambridgeIsotopes, 99% enrichment兲 in argon was used under the samesample conditions, while a mixture of 0.2% CH4and 0.2%13CH4共Cambridge

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