a.m.r.p. bopegedera et al- laser spectroscopy of calcium and strontium monoacetylides

8/2/2019 A.M.R.P. Bopegedera et al- Laser Spectroscopy of Calcium and Strontium Monoacetylides

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Volume 136, number 1 CHEMICAL PHYSICS LETTERS 24 April 1987

LASER SPECTRO SCOPY OF CALCIUM AND STRO NTIUM MO NOACElYLIDESA.M.R.P. BOPEGEDERA, C.R. BRAZIER and P.F. BERNATHDepartment of Chemistry, University ofArizona, Tucson, AZ 85721, USAReceived 18 December 1986; in final form 6 February 1987

The gas-phase free radicals CaCCH and SrCCH were synthesized by the reaction of Ca or Sr vapor with HCCH. The electronicand vibrational structure of these molecules were investigated by laser excitation spectroscopy and laser-induced fluorescence.These ionic metal acetylides proved to be linear in geometry.

1. IntroductionIn our laboratory we are studying the reactions ofmetal vapors (particularly calcium and strontium)

with organic molecules. To date we have detectedpolyatomic free radicals containing calcium andstrontium bonded to oxygen, sulfur, nitrogen andcarbon. The metal-oxygen derivatives includehydroxides [ 11, alkoxides [ 21 and carboxylates [ 31while some of the corresponding metal-sulfur radi-cals MSH, MSR and MSCN have also been found[ 4 1 .The metal-nitrogen containing compounds aremetal amides [ 5 1, alkylamides [ 61, azides [ 71 andprobably, isocyanates [ 81. The first free radicals witha metal-carbon bond that we have discovered are theopen-faced sandwich molecules CaCSHS andSrCSHs [ 9 1. More recently we have found the metalalkyls CaCH3 and SrCHS [ lo]. In this paper we reporton the CaCCH and SrCCH free radicals.

Solid state calcium, strontium and barium diace-tylides have been previously synthesized. The directreaction of the metal with alkynes in liquid ammoniaproduces these dialkynyl compounds [ 111.2RC=CH +M- (RC-C )2M+Hz ,hqNH ,

M=Ca, Sr, Ba, R=H [ 121, Ph [ 131.Since the time ref. [81 was published, we have decided thatCaNCO and SrNCO are the correct structures rather thanCaOCN and SrOCN. The comparison with the CaNNN andSrNNN spectra is very suggestive of M-N rather than M-Obonding.

The M( C=CH )* compounds are quite unstableand decompose to provide the metal carbides, MC2.Alkynylcalcium iodides RCCCaI were synthesized bythe reaction of alkynes with PhCaI [ 141. The coor-dination chemistry of metal acetylides was reviewedby Nast [ 151.

Veillard [ 161 and Streitweiser et al. [ 171 carriedout ab initio calculations to determine the structureof lithium acetylide. A linear geometry was assumedin these calculations. They concluded that the lith-ium acetylide molecule is highly ionic in character.The molecule is described as an ionic associationbetween the Li+ ion and the - (CCH) ligand, withthe first carbon atom bearing most of the negativecharge. A neutron powder-diffraction study ofmonosodium acetylide by Atoji [ 18] also concludesthat the molecule is ionic in nature and has a lineargeometry.

Acetylides can form on metal surfaces. For exam-ple Madix [ 191 has applied a battery of surface tech-niques (UPS, XPS, LEED and EELS) to thecharacterization of the CCH fragment bonded to theAg( I 10) surface.

Our work on CaCCH and SrCCH represents thefirst gas-phase spectroscopic observation of a metalmonoacetylide. These radicals were synthesized byreacting the metal vapors with HCCH. Their elec-tronic and vibrational structures were detected bylaser-induced fluorescence. The assignments weremade by analogy to the corresponding CaOH andSIGH spectra. A high-resolution rotational analysisof the A 2H-g 2E+ transition of CaCCH is cur-rently in progress.

0 009-26141871%03.50 0 Elsevier Science Publishers B.V.(North-Holland Physics Publishing Division) 97


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Volume 136, number 1 CHEMICAL PHYSICS LETTERS 24 April 19872. Experimental

The metal acetylides were generated in a Broida-type oven [ 201 by the reaction of metal vapor withacetylene. The metal was vaporized by resistive heat-ing in an alumina crucible and entrained in argoncarrier gas. The total pressure was approximately 1.5Torr with about 10 mTorr of purified acetylene. Thesignal-to-noise ratio increased when the pressure inthe reaction chamber was increased from 1.5 to 10Torr by partially closing the valve to the vacuumpump.Two years ago in our initial acetylene experimentsunpurified acetylene was used. Welding-grade acety-lene contains acetone, which reacts with calcium orstrontium to produce mainly the CaOCH(CH3)2 orSrGCH( CH3)2 molecules [ 21. The alkaline earthvapors react more easily with water and acetone thanwith acetylene so a purification step is vital. In thecurrent experiments, welding-grade acetylene waspassed through a trap cooled by a mixture of dry iceand acetone, bubbled through concentrated sulfuricacid and then cleaned with solid NaOH and anhy-drous CaCl*.

Two cw broad band ( 1 cm- ) dye lasers, pumpedby two argon ion lasers (Coherent Innova 20 andCoherent Innova 90), were used for the experi-ments. One dye laser excited the 3P,-S0 atomictransition (6573 8, for calcium and 6892 A for stron-tium) while the second laser was resonant with theproduct molecule (CaCCH or SrCCH) A 211-ji 2Z + electronic transition. Unexcited metal atomsdo not react with HCCH. The chemical mechanismresponsible for the production of CaCCH and SrCCHis unclear because of the unknown M-C bond ener-gies and the relatively high pressure in the Broidaoven.

Two types of spectra were recorded. Laser excita-tion spectra were obtained by scanning the frequencyof the second dye laser while detecting the fluores-cence with a photomultiplier-filter combination.Schott RG 9 and RG 780 red pass filters blocked thescattered laser light. Lock-in detection was madepossible by chopping the laser resonant with themolecular transition.

Laser-induced fluorescence spectra were obtainedby fixing the second laser on a molecular absorptionand then dispersing the fluorescence with a mono-98

chromator. In order to improve the signal-to-noiseratio the laser exciting the molecular transition waschopped and lock-in detection utilized. DCM and/orpyridine 2 dyes were used in the dye lasers.

Strontium vapor was also allowed to react withperdeuteroacetylene, which was synthesized by thereaction of calcium carbide with deuterium oxide. Aresolved fluorescence spectrum of SrCCD wasrecorded with the same experimental parameters(pressures and laser wavelengths) as SrCCH. TheSrCCD and SrCCH spectra were different contirm-ing that the product molecule contained hydrogen.

An attempt was made to study the spectra of cal-cium and strontium monomethylacetylide MCCCH,by using monomethylacetylene as an oxidant. Cal-cium vapor produced only calcium monoacetylide inthis reaction. However, there was some evidence fromthe excitation and resolved fluorescence spectra thatSrCCCH3 was formed. These spectra were very poorand obscured by the presence of strontium hydrox-ide (which is an impurity) so that no spectroscopicmeasurements were made.

3. Results and discussionThe laser excitation spectrum of theA H-g 2Z + transition of strontium acetylide is

given in fig. 1. The two main features in fig. 1 areseparated by about 260 cm- , which is a character-istic A 211 spin-orbit splitting when Sr+ is perturbedby a linear l&and. The laser-induced fluorescencespectra of CaCCH and SrCCH (figs. 2 and 3) pro-vide more accurate data on the electronic transition

do 7iOllJtlFig. 1. Laser excitation spectrum of SrCCH. The features near692 and 705 nm are the two spin-orbit components of theA 21T-R *X + electronic transition.


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Volume 136, number 1 CHEMICAL PHYSICS LETTERS 24 April 1987

Fig. 2. Laser-induced fluorescence spectrum from the A-2 tran-sition of CaCCH. The asterisk marks the position of the laserexciting the A *II*,*-% *X+ transition. The peak just to the redof the laser is the A 21T,,2-R 2 + transition. Collisions havepopulated the A *II ,,* spin component from the directly excited.&2H*,2 component. The two peaks near 660 nm are assigned tothe 3p vibrational band of the A-% transition.

frequencies. In both these figures an asterisk marksthe laser wavelength. In fig. 2 the laser excites theA 2rI,,,-X *z+ of CaCCH while in fig. 3 theA *I-Ii,*-ji: *z + spin component of SrCCH isexcited. Collisions connect the A *II ,,* and A *IIu2spin components so both components are observedin fig. 2 although only one component is directlyexcited. The band origins and spin-orbit couplingconstants (A) of the A *II-% *I: + transitions ofCaCCH and SrCCH are reported in table 1, with anestimated uncertainty of f 10 cm- .

&C,HQn-fr*

ib-J.~no 720nm

Fig. 3. Laser-induced fluorescence spectrum of the A-2 transi-tion of SrCCH. The asterisk marks the position of the laser excit-ing the A *II ,,*-2 *Z+ transition. No emission from the A *Hu2spin component appears in this spectrum. The weak feature near710 nm is assigned to the 52 band of the A-2 transition whilethe feature near 720 nm is the 3p band of the A-2 transition.

Table 1Band origins of the A *H-z *E+ calcium and strontium mon-oacetylide (in cm - )Molecule X*n,,*-fi*22+CaCCH 15487SrCCH 14176) Spin-orbit constant.

A*rIj,*-a*E+ A =15560 7314451 275

The B *I; + -% *Z + transitions of calcium andstrontium acetylide were not found in the low-reso-lution scans despite an extensive search in theexpected region. It is possible that the B *Z + -2 *Z + transition was obscured by the strong CaOHor SrOH spectra. However, a preliminary analysis ofthe high-resolution spectrum of calcium acetylide isconsistent with a distant or dissociative B *Z + state.The n doubling constant p of the A *II state is verysmall and positive rather than large and negative asexpected from the B *X + -A *lI interaction.

Resonant emission to excited vibrational levels ofthe ground electronic state was observed in the A-2laser-induced fluorescence spectra. The MCCH mol-ecule has five vibrational modes v1 (C-H stretch),v2 (C-C stretch), v3 (M-C stretch), v4 (C-C-Hbend) and us (M-C-C bend) with the stretches andbends having o and x symmetry, respectively. TheM-C stretch ( v3) had the largest Franck-Condonfactor so v3 and 2v3 for the % *Z + state wereobserved (table 2). An additional mode at 18 1 cm- for CaCCH and 139 cm- for SrCCH was also found.The most likely assignment is 2v, in the ji: *I + statesince the observation of v5 is forbidden [ 2 11. Anattempt was made to detect v2 (C=C stretch) butwithout success. The vibrational frequencies in theA and % states are similar so that no sequence struc-ture was resolved.Table 2Vibrational frequencies of the calcium and strontium acetylides(incm-)intheg*X+ state

ModevX M-C stretch)28,2~~ (M-C=C bend)vx+2vs

) 354 cm- for the A *II state.

M=Ca399788181

M=Sr343 =684139500

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Volume 136, number 1 CHEMICAL PHYSICS LETTERS 24 April 1,987The attribution of our spectra to CaCCH and

SrCCH rests largely on similarities to the corre-sponding CaOH and SrGH spectra [ 11. Thespin-orbit coupling constant of the A 21T state ofSrCCH (CaCCH) is 275 cm- (73 cm-) comparedto264cm- (66cm-) forSrGH (CaOH).TheM-Cbond, compared to the M-O bond, shifts the x-2band origins to the red and increases the spin-orbitconstant of the A II state slightly. The SrCH,( CaCH3) molecule shows similar changes with, forexample, A = 273 cm- (79 cm-) for the corre-sponding A *E states [ lo]. The observation ofspin-orbit splittings for the ;i states means that thenew molecules have high symmetry, probably linear.The reactions of Ca and Sr with DCCD show that thenew species contain hydrogen. The chemical evi-dence also strongly suggests that we have discoveredthe linear, CaCCH and SrCCH free radicals.

AcknowledgementThis research was supported by the National Sci-

ence Foundation (CHE-8608630). Acknowledge-ment is made to the donors of the Petroleum ResearchFund, administered by the American Chemical Soci-ety, for partial support of this research. We would liketo thank C. Gottlieb for helpful discussions on thepurification of acetylene.

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P.F. Bemath and C.R. Brazier, Astrophys. J. 228 (1985)373;C.R. Brazier and P.F. Bemath, J. Mol. Spectry. 114 (1985)163;S. Kinsey-Nielsen, C.R. Brazier and P.F. Bemath, J. Chem.Phys. 84 (1986) 698.

[2] C.R. Brazier, L.C. Ellingboe, S. Kinsey-Nielsen and P.F.Bemath, J. Am. Chem. Sot. 108 (1986) 2126.[3] CR. Brazier, P.F. Bemath, S. Kinsey-Nielsen and L.C.Ellingboe, J. Chem. Phys. 82 (1985) 1043.

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[8] L.C. Ellingboe, A.M.R.P. Bopegedera, C.R. Brazier and P.F.Bemath, Chem. Phys. Letters 126 (1986) 285.[91 L.C. OBrien and P.F. Bemath, J. Am. Chem. Sot. 108(1986) 5017.[lo] C.R. Brazier and P.F. Bemath, J. Phys. Chem., to bepublished.[ 111 B.G. Gowenlock and W.E. Lindsell, J. Organomet. Chem.Library 3 (1977) 1.[ 121 H. Moissan, Compt. Rend. Acad. Sci. (Paris) 127 (1898)911.[ 131 M.A. Coles and E.A. Hart, J. Organomet. Chem. 32 (1971)279.[ 14] L.L. Ivanov, V.P. Napochatykh and N.A. Smyslova, Zh. Chg.Khim. 7 (1971) 2623.[ 151 R. Nast, Coord. Chem. Rev. 47 (1982) 89.[ 161 A. Veillard, J. Chem. Phys. 48 (1968) 2012.[ 171 A. Streitweiser Jr., J.E. Williams Jr., S. Alexandratos andJ.M. McKelvey, J. Am. Chem. Sot. 98 (1976) 4778.[ 181 M. Atoji, J. Chem. Phys. 56 (1972) 4947.[ 191 R.J. Madix, Appl. Surface Sci. 14 (1982/1983) 41.[201 J.B. West, R.S. Bradford, J.D. Eversole and C.R. Jones, Rev.Sci. Instr. 46 (1975) 164.[21 ] G. Hemberg, Molecular spectra and molecular structure, Vol.3. Electronic spectra and electronic structure of polyatomicmolecules (Van Nostrand Reinhold, New York, 1966).

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a.m.r.p. bopegedera et al- laser spectroscopy of calcium and strontium monoacetylides

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