l.c. ellingboe et al- laser spectroscopy of calcium and strontium monocyanates

8/2/2019 L.C. Ellingboe et al- Laser Spectroscopy of Calcium and Strontium Monocyanates

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Volume 126, number 3,4 CHEMICAL PHYSICS LETTERS 9 May 1986

LASER SPECTRO SCOPY OF CALCIUM AND STRO NTIUM MO NOCYANATESL.C. ELLINGBOE, A.M.R.P. BOPEGEDERA, C.R. BRAZIER and P.F. BERNATHDepartment of Chemutt y, University of Arizona, Tucson, AZ 85721, USA

Received 12 November 1985

The Ca and Sr monocyanate species were observed by laser-induced fluorescence. The spectra are interpreted as arising froma molecule with a linear, ionic M+-- OCN structure.

1. IntroductionWe have recently discovered that Ca, Sr and Ba va-

pors react with a wide variety of organic compounds[l] including alcohols, aldehydes and ketones [2],thioethers [3], and carboxylic acids [4]. The productsare a variety of gas-phase, free radical organometallicscontaining one alkaline earth atom and a single ligand.The organometallics have visible electronic spectraand are detected by laser-induced fluorescence. In thispaper we report the observation of calcium and stron-tium monocyanate from the reaction of the metalatoms with hydrogen isocyanate vapor.There has been no previous work on gas-phase mono-cyanates except the mass spectrometric detection ofBOCN [5] and AlOCN [6] by Gingerich. However, themost likely isomers for these molecules are O=B-CNand O=Al-CN (61 rather than the (iso)cyanate or(iso)fulminate structures [MOCN, cyanate; MCNO,

fulminate; MONC, isofulminate].The calcium and strontium monocyanate mole-cules have a linear, probably oxygen-bonded struc-ture, MOCN. These cyanate observations are in con-trast to work on the alkaline earth monocyanides.Calcium and strontium monocyanldes are predictedto have linear isocyanide MNC structures [7]. Wehave made calcium monocyanide by reaction ofCa(3P1) with CH3CN [3]. The laser-induced fluores-cence spectrum of CaCN consisted of a relaxed, fea-tureless blob near 6300 A with no hint of vibrationalstructure or of resonant fluorescence. This suggeststhat CaCN may have very little barrier to the motion

of Ca+ around CN- although the equilibrium struc-ture may be T-shaped like NaCN [8] and KCN [9] orlinear like LiNC [lo]. Our CaCN observations agreewith the work of Pasternack and Dagdigian [ 1 ] whomade calcium, strontium and barium cyanides by thebeam-gas reactions of metal vapor with BrCN. Morerecently, Furio and Dagdigian [12] observed chemi-luminescence attributable to CaCN (and CaBr) fromthe Ca(lD) plus BrCN reaction.

2. Experimental methodsMOCN (M = Ca, Sr) molecules were produced in aBroida oven [ 131 by the reaction of the alkalineearth vapor with isocyanic acid, HNCO. The alkalineearth metal was resistively heated in an alumina cruci-ble and entrained in Ar carrier gas; pressures were ap-

proximately 1.5 Torr argon and a few mTorr acid.The preparation of isocyanic acid has been describedpreviously [ 141. Briefly, concentrated aqueous KNCOwas added slowly to chilled (0C) concentrated phos-phoric acid under vacuum. The reaction productswere trapped at liquid nitrogen temperatures, and dis-tilled once at -30C. DNCO was similarly preparedby the reaction of D3P04 (prepared from P,O, t D,O)with a deuterated aqueous solution of KNCO. TheDNCO was used to demonstrate that the product mol-ecule contained no hydrogen: the HNCO and DNCOoxidants gave identical spectra.A broadband (1 cm-l) dye laser was used to excitethe 3P1-1S,, transition of the alkaline earth metal

0 009-2614/86/$ 03.50 0 Elsevier Science Publishers B.V.(North-Holland Physics Publishing Division) 285


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Volume 126, number 3,4 CHEMICAL PHYSICS LETTERS 9 May 1986atoms. This resulted in the production of a large con-centration of MOCN. Without the excitation of theatomic line essentially no product was formed. A sec-ond broadband dye laser was tuned to the x 211-% 2Zc+MOCN transition. The resulting fluorescence was dis-persed through a 0.64 m monochromator, and de-tected with a cooled photomultiplier tube (RCAC3 1034) with photon counting electronics.

3. Results and discussionThe alkaline earth monocyanate spectra closely re-

semble the alkaline earth hydroxide [ 15-201 and al-

Table 1The band origins of Ca and Sr cyanate in cmr ,The uncer-tainties are approximately *5 cm- for the A-X transitionsof CaOCN and i30 cm- for the other transitionsMoleculeCaOCN!WCN

& =h,2 x2 =?/2 5 =x+16230 16298 1718014944 15249 16016

l

koxide spectra [2] ~The observed band origins for thecalcium and strontium cyanate, MOCN, g 2E+-% 2Z+and x %I-% 2Z transitions are reported in table 1.These data were obtained from monochromator scansof the laser-induced fluorescence, for example figs. 1and 2. The emission from these molecules was partlyresonant and partly relaxed. This is particularly clearin the spectrum of SrOCN (fig. 2) since the sequenceband structure is more open for SrOCN than CaOCN.In fig. 2 the laser is exciting a sequence band of SrOCNand produces some resonance fluorescence but mostof the emission comes from vibrationally relaxedlevels. Although only the x 23,2 spin component isexcited there is relaxation to the other spin compo-nent (i2II1,2) as well as to the B Z+ state at ourpressure of el.5 Torr. The O-O band appears to lie onthe red side of the x-2 features displayed in figs. 1and 2 on the blue side of the 5-2 feature (like thecorresponding CaOH and SrOH trans$ions~lS-191).The calcium and strontium cyanate A %I-X ?Z+spectra exhibit spin-orbit splittings of 68 + 7 cm-land 305 + 40 cm-l, respectively, for the i state(table 1). These spin-orbit coupling constants areidentical (within experimental error) with the corre-

Ca-OCN

Fig. 1. Laser-induced fluorescence from the x-8 transition of CaOCN. The splitting between the two strong eatures is due to thespin-orbit coupling in the i =TJtate. The asterisk marks scattered light from the dye laser exciting the x %I state from an excitedvibrational level in the ground state (i.e. the laser is not resonant with the O-O band). The lower trace has the sensitivity reducedby 10 in order to put the O-O band on scale.286


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Volume 126, number 3,4 CHEMICAL PHYSICS LETTERS

Sr-OCN

9 May 1986

..

do0 sdoo do0y-x [

6600 6000 aFig. 2. Laser-induced fluorescence from the x-g transition of SrOCN. The splitting between the two strong features is due to thespin-orbit splitting in the i:m state. The asterisks mark the scattered light from the two dye lasers exciting the atomic 3P1-iSotransition (6892 A) and the A2 21712-Z 2Z+ transition. Note that the emission is partly resonant and partly relaxed.

sponding values for the linear CaOH (67 cm-l [ 181)and SrOH (264 cm-l [ 191) molecules as well as thecorresponding Ca and Sr alkoxide values [2]. Thisconstitutes strong evidence for a linear oxygen-bonded structure. If MOCN were bent then the sym-metry would be lowered from C,, tc C, and no spin-orbit interaction could exist for the A state. Insteadthe x %I state would correlate with two electronicstates of A and A symmetry [20]. The bent CaSHand SrSH molecules [3] have C, symmetry and com-pletely different (more complicated) spectra than thecorresponding linear CaOH and SrOH molecules.

The size of the spin-orbit coupling constant of thei state suggests M-O bonding rather than M-Nbonding for the cyanate group. The electronic struc-ture of the alkaline earth monohalides and mono-alkoxides is well described by an ionic M+--X- struc-ture. The remaining valence electron on the Ca+, Sr+and Ba+ is perturbed by the X- ligan_d.Hence, thespin-orbit coupling constant of the A state is deter-mined by properties of M+ but modulated by thenearest neighbor, X-, atom or group. For example,Ca+ perturbed by -OR (R = alkyl) has A = 70 cm-l[2] while A = 72.5,70.5,64.3 and 45.7 cm-l forCaF [21],CaCl [22],CaBr [23] andCa1 [24],re-spectively. The observed spin-orbit coupling constantsare consistent with the expected M+---OR (M = Sr, Ca;

R = CN) values rather than the probably differentM+--NR (M = Sr, Ca; R = CO) values.The known affinity of the alkali and alkaline earthmetals for oxygen also favors the M-O bonding struc-ture. This is in contrast to the transition metal isocy-anate complexes which are almost always N-bonded[25]. However, the early transition elements such asTi, Zr and Hf, which also have a strong affinity foroxygen, can form O-bonded cyanate complexes [26].Many main group elements such as Si, Ge, Sn and Gahave N-bonded structures (Si(NCO)4, Ge(NCO),,(CH3)$nNC0, (CH&GaNCO [27,28]). The NCO-ligand also probably bonds through the N atom to sur-face Si atoms of silica [29].The laser-induced fluorescence spectra of Ca+--0CNelectronic transitions contain vibrational information.Since the electronic transitions involve a non-bonding,unpaired, metal-centered electron all of the electronicstates have similar geometries and vibrational frequen-cies. The Franck-Condon factors for non-diagonaltransitions are small and only the vibrational modesassociated with the metal center (such as the M-Ostretch) are expected to occur strongly.The MOCN molecules have five vibrational modesv1 (C-N stretch), v2 (C-O stretch), v3 (M-O stretch),v4 (O-C-N bend) and v5 (M-O-C bend). The vl, v2and v3 modes have C symmetry while v4 and v5 have

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Volume 126, number 3,4 CHEMICAL PHYSICS LETTERS 9 May 1986Table 2Vibrational frequencies of Ca and Sr monoxcyanates in cm-.The uncertainties are *5 cm- for CaOCN and i30 cm- forSrOCNModeChOCNy3 390 395 39323 718 119 714Vl 2200 - -SrOCN a)v3 291 - 320a) va = 314 cm- for g 2.Z+ state.

II symmetry. v1 and v2 are often described as pseudo-asymmetric stretch and pseudosymmetric stretch, re-spectively, because of the similarities between the iso-electronic NCO- and CO, molecules.The observed MOCN vibrational frequencies arelisted in table 2. The M-O stretch (v3) occurs moststrongly in the spectrum. For example both spin com-ponents of the 00000-00100 X%I-% 2Z+ transitionof CaOCN occur near 6300 A in fig. 1.2~~ and v1(C-N stretch) modes were found for CaOCN but notfor SrOCN. The very compact CaOCN sequence struc-ture allowed very weak modes such as v1 to be locatedby enhancing the effective signal-to-noise ratio forCaOCN.In addition to the stretching modes in table 1,there are very weak features in the CaOCN spectrawhich suggest that v4 (OCN bend) =640 cm-l (650cm-l in the i state) and v5 (Ca-0-C_befd) a50cm-l may occur. The observation of A-X00000-00010 and 00000-00001 transitions are for-mally forbidden because v4 and v5 have II symmetry[20] (transitions involving the allowed modes 2v4and 2v, have very small Franck-Condon factors).However, spin-orbit vibronic coupling caused by abreakdown of the Born-Oppenheimer approximationcan allow these modes to occur [ 191. For SrOH [ 191and BaOH [30] (but not CaOH) we have observedthe analogous forbidden 000-010 vibronic transi-tions. The v5 assignment is not very secure. For in-stance, it is possible that 2v5 rather than v5 occurs inthe spectrum which would Imply a v5 of 25 cm-l.These very low frequency bends are unusual but the288

ionic structure provides a very flat bending potential.The corresponding Ca-O-R bend is 90 cm-l for thealkoxides [2] and 20 cm-l for the quasisymmetrictop SiH3NC0 [3 11.

The expected OCN- vibrational frequencies are631 cm-l for the bend, 2182 cm-l for the pseudo-asymmetric stretch and 1211 cm-l for the pseudo-symmetric stretch, based on solid state spectra ofKOCN in a KC1matrix [32]. The 12 11 cm-l modewas not found but modes at 2200 cm-l and a veryweak overlapped mode =640 cm-l (va) were found.The M-O stretch, 390 cm-l for CaOCN and 297cm-l for SrOCN, are just the frequencies expectedby comparison with the Ca and Sr monoalkoxides.For the a&oxides the M-O stretch was found to cor-relate linearly with ~.(-l/~, where p is a pseudodi-atomic reduced mass computed by assuming that allof the ligand mass is concentrated at a single point.This sort of an empirical correlation suggests that allof the M-O force constants are similar. The M-Ofrequency for MOCN agrees with the observed trendin the corresponding MOH, MOCH3, MOC,H,,MOC3H7 and MOC,H, molecules (M = Ca, Sr). Thus,the value of the M-O stretching frequency is also sug-gestive of M-O rather than M-N bonding.

4. SummaryCa and Sr vapors react with HNCO to form the lin-ear and probably O-bonded CaOCN and SrOCN mol-ecules, The laser-induced fluorescence spectra closelyresemble the corresponding vibronic spectra of the al-kaline earth monoalkoxides. The CaOCN and SrOCNfree radicals are the first gas phase, metal monocy-anates to be detected.

AcknowledgementThis research was supported by the National

Science Foundation (NSF-8306504) and the Re-search Corporation. Acknowledgement is made to thedonors of the Petroleum Research Fund, administeredby the ACS, for partial support of this work. Partialsupport was also provided by the Office of Naval Re-search (ONR N 00014-84-K-0122).


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Volume 126, number 3,4 CHEMICAL PHYSICS LETTERS 9 May 1986References

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l.c. ellingboe et al- laser spectroscopy of calcium and strontium monocyanates

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