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J . Phys. Chem. 1990, 94, 3543-3541 3543

0 MHz/ -2eqyy = ( 4 / 5 ) e ( F 3 ) , , nd if it is the only electron in the valenceshell, the quadrupole coupling tensor in the form of eq 1 wouldbe as follows.

50 MHz

7 7 -Figure 5. ESR powder patterns of AuOl simula ted based on th e g andAu hyperfine coupling tensors given in the text and increasingly largerquadrupole tensors sym metric about the z axis. The assumed P,value( i n MH z ) is given a t left.

tensor of AuC2H4were redetermined corrected for the shift ofthe perpendicular signals by 3 P I l 2 / ( 2 A ) 6.5 G .

The electric field gradient eq at the nucleus due to an electronin a valence p z orbital, for example, is given by eqrz = -2eq,, =

Substituting the known quadrupole moment of the 1 9 7 A ~ucleus(0.59 X cm2) for Q an d 5.19 au-3 for (r-3),,u,6p the valueestimated from the fine structure interval of Au atomsI5) oneobtains P: = 96 MHz.

Neglect ing the shielding effect of the co re electrons, the observed

quadrupole tensors then indica te the electron population of - .O

in the Au p, orbit al of AuC, H4 and -0.5 in the Au pz orbitalin the case of AuO,. Analysis of the hyperfine coupling tensorof AuC2H4 eported earlier showed the unpaired electron densityof -0.5 in the Au px orbita1.l This is significantly less than th atgiven from the analysis of the quadrupole tensor. Th e differencema y be construed as a n evidence for the dative interaction betweenthe bonding 7, rbital and the vacant sp, orbital of the Au atom .

In A u 0 2 he unpaired electron is in the antibonding rY * rbitaland does not contribute to the electric field gradient at the Aunucleus. Th e dative interaction of the electrons in the bondingA, orbital is more likely to involve the A u 6s orbital only as t hela tte r is to ta lly vacan t in the A u + 0 ~ituation. Back migrationtha t would ge nerate electric field gradient of consequence at the

Au nucleus is that from th e doubly occupied r X * rbital of oxygeninto the Au pz orbital. Th e electron population of -0.5 in theAu pz orbital concluded in the anlysis of the quad rupole tensorindicates the extent of such migration.

Registry No. Au(C2H4), 1943-23-5; A u ( 0 2 ) , 60294-90-8.

(15) Moore, C. E. Natl . Stand. Ref Data Ser. (US. at l . Bur. Stand.)1971, 35.

Gas-Phase Inorganic Chemistry: Laser Spectroscopy of Calcium and StrontiumMonocarboxylates

L. C. O’Brien,’ C. R. Brazier,$S. Kinsey-Nielsen, and P. F. Bernath*.s

Department of Chemistry, University of Arizona, Tucson, Arizona 85721 (Received: October IO, 1989)

The calcium and strontium monocarboxylate_ rze iad_icalsyere_made by the gas-phase reaction of the metal vapors withcarboxylic acids. Three electronic transitions,A-X, B-X, nd C-X, ere detected by laser-induced fluorescence. M etal-ligandstretching frequ encies were derived from these low-resolution spec tra. The carboxyla te ligand binds in a bidentate mannerto the metal.

Introduction

As part of a continuing study of gas-phase alkaline-earth-containing free radicals, we report here on the metal mono-carboxylate derivatives. The laser-induced fluorescence spectraof calcium and s trontium monoalkoxides, ’ mo no al k~ la mi de s ,~ -~monoalkylth i0 lates,4~~ onomethyl,6 m on oa ~ et y l id e, ~ ~~onoiso-~ y a n a t e , ~ J ~onoazide,” monocyclopentadienide,12 ono-

pyrro late,13 and m~noformamidatel~ere all recorded in thislaboratory. These new molecules were Produced by the W-P ha se

( I ) Brazier, C. R.; Ellingboe, L. C.; Kinsey-Nielsen,s.; Bernath, P.F. J .Am. Chem. SOC.1986. 108, 2126. OBrien, L. C.; Brazier, C. R.; Bernath,P. F. J . Mol. Spectrosc. 1988, 130, 33.

(2 ) Bopegedera, A. M. R. P.; Brazier, C. R.; Bernath, P. F. J. Phys. Chem.1987, 91 , 2179.

(3 ) Brazier, C. R.; Bernath, P. F. Work i n progress.(4 ) Fernando, W . T. M. L.; Ram, R. S.;Bernath, P. F. Work in progress.(5) O’Brien, L. C. ; Ram, R. S.; ernath, P. F. Work i n progress.(6) Brazier, C. R. ; Bernath, P. F. J . Chem. Phys. 1987,86,5918; J . Chrm.

(7 ) Bopegedera,A. M. R. P.; Brazier, C. R.; Bernath, P. F. Chcm. Phys.

‘Current address: Food and Drug Administration, Division of Drug

‘Current address: Astronautics Laboratory/LSX, Edwards Ai r Force

Alfred P. Sloan Fellow; Camille an d Henry Dreyfus Teacher-Scholar.

Analysis, 1 1 14 Market Street, St. Louis, MO 63101.

Base, CA 93523. Phys. 1989, 9 1 . 4 5 4 8 .

Lert. 1987, 136, 97.To hom correspondence should be addressed.

0022-3654/90/2094-3543$02.50/0 0 1990 American Chemical Society

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3544

reaction of calcium or strontium vapor with the correspondingorganic oxidant (e.g., Ca + H N , - aN , + H ). In an earlierletter we reported our preliminary results on the observation ofcalcium and strontium monoformate and monoa~etate.’~ res-ented here a re our complete results of this earlier study, plus ourresults for two larger metal monocarboxylates, calcium andstrontium monopropanoate and m onobutanoate.

The carboxylate ion, RCOO-, is a well-known ligand in tran-sition-metal chemistry. It can bond in a monodentate (M OC OR ),bidentate (M OOC R), or bridging (MO CR OM ) geometry.I6 Thenature of the bonding is often determined by X-ray crystallog raphyor by infrared spectrosc opy of the two carbon-oxyge n stretch ingvibrations. The metal-oxygen stret ching vibrations ar e usuallyquite low in frequency (e.g., 100-480 cm-’ for the rare-earthformates”), but scarcely any data are available for metal car-boxylate comp ounds in the far-infrared region. Most previousinfrared studies have focused on modes associated with the twocarbon-xygen b o n d ~ ’ ~ J ~o determine whether C= O double-bondcharacter is present (relatively high vibrational frequency), whichwould indicate m onodentate type bonding. Several far-infraredstudies of metal formate crystals are available, but these a re notisolated metal formates ( e g , anhydrous Ca(O OCH )2crystal hasseven oxygen atom s less than 2.6 A from a calcium atom20), andthe metal-oxygen frequencies occur i n the sam e spectral regionas crystal lattice vibrations.21

Metal carboxylates are also well-known i n surface chemistry,since formic and acetic acid strongly adsorb on many surfaces.22The surface metal atoms can cleave the 0-H bond of the car -boxylic acid, and the resulting carboxylate anion can bind to thesurface i n a monodentate, bidentate, or bridging structure.23 I n

fact, the bonding geometry on a given surface can change de-pending on the surface temperat~re.~,Electron energy lossspectroscopy (EELS) can be used to record the vibrationalspectrum of the surface carboxylate, and the mode of bondingcan usually be determined from the two C - 0 frequencies.22 Themetal-xygen stretching frequencies of adsorbed formates on metalsurfaces range from 280 cm-’ for Ag( 1 1 1 ) to 440 cm-’ for Ni-( I The symmetric and asymmetric M- O frequencies werenot resolved in these experiments.

The Journal of Physical Chemistry, Vol. 94 , N o . 9, 1990 O’Brien et al.

(8) Bopegedera, A. M. R. P.; Brazier, C. R.; Bernath, P. F. J. Mol.

(9) Ellingboe, L. C.; Bopegedera, A . M. R. P.; Brazier, C. R. ; Bernath,

( I O ) O B r i e n , L. C.; Bernath, P. F. J . Chem. Phys. 1988, 88, 2117.

( 1 1 ) Brazier, C. R.; Bernath, P. F. J . Chem. Phys. 1988,88, 2112.

( 12) OBr i en , L. C.; Bernath, P. F. J . Am. Chem. Soc. 1986, 108, 5017.

(13) Bopegedera. A. M . R. P.; Bernath, P. F. J . Phys. Chem., in press.

(14) Bopegedera, A. M. R . P. ; Fernando, W. T. M . L.; Bernath, P. F. J .

Spectrosc. 1988, 129, 268.

P. F. Chem. Phys. Lett . 1986, 126, 285.

Phys. Chem., following paper i n this issue.

( 1 5 ) Brazier, C. R.; Bernath, P . F. ; Kinsey-Nielsen, S.; Ellingboe, L. C .J . Chem. Phys. 1985,82, 1043.

(16) Cotton, F. A,; Wilkinson, G. dvanced Inorganic Chemistry, 5th ed.;

(17) Alcock, N. W.; Tracy, V . M.; Waddington, T . C. J . Chem. Soc.,

(18) Edwards, D . A.; Hayward, R. N . Can. J . Chem. 1968, 46, 3443.

Wiley-Interscience: Ne w York , 1988; pp 483-484.

Dalton Trans. 1976, 2243.

(19) Grigor’ev, A . I.; Sipachev, V . A, ; Pogodilova, E. G. J . Struct. Chem.( E ng l . Trunsl.) 1970. 11, 420.

(20) Watanabe, T . ; Matsui , M. Acta Crystallogr. 1978, B34, 2731.

(21) Maas, J . P . M.; Kellendonk, F. Spectrochim. Acta 1978, 3 5 A, 87 .

(22) Canning, N. D. S.; Madix, R. J . J . Phys. Chem. 1984, 88, 2437.

(23) Madix. R. J . Adu. Catal. 1980, 29, I.(24) Deacon, G. B.; Phillips, R. J . Coord. Chem. Reu. 1980, 33 , 227.

(25) Shapley, J. R.; St. George, G. M.; Churchill, M . R.; Hollander, F.

(26) Sexton, B. A. Surf. Sci. 1979, 88, 319.

(27) Avery, N . R. Appl . Surf . Sci . 1982, 14 , 149.

J . Inorg. Chem. 1982, 2 1 , 3295.

(28) Avery, N. R.: Toby, B. .; Anton, A. B.; Weinberg, W. J.Surf. Sci.1982, 122, L574.

(29) Madix, R. J. ; Gland, J. L. ; Mitchell, G. E.; Sexton, B. A. Surf. Sci .1983, 125, 48 .

Dye Laser

Chopper

I0.64 meter

ChartRecorder

Figure 1. E xper imen ta l d i ag r a m for low-resolution laser spectroscopy.

Experimental Section

The alkaline-earth monocarboxylates were produced in aBroida-type oven30 by the gas-phase reaction of the m etal vapor(C a or Sr ) with the appropriate carboxylic acid. The metal wasvaporized in a resistively heated alumina crucible, and the metalvapor was entrained in an argon flow. The acid vapor was addedthrough an oxidant ring.

The laser beams a re introduced into the oven through the top

window and directed into the alumina crucible through the oxidantring. Th e total pressure inside the Broida oven was typically 2Torr, with a few milliTorr of oxidant. Since the vapor pressureof propionic and butanoic acid is quite low, a flow of argon wasbubbled through the acid and injected into the Broida oven at theoxidant ring.

Two type s of low-resolution ( 1 -cm-I) spectra were recorded:laser excitation spectra and resolved fluorescence spectra. Laserexcitation spectra were normally recorded first to observe themolecular transitions. Resolved fluorescence spectra were thenrecorded to determine which transitions connect to the sameexcited state and to determine vibrational frequencies.

I n some of our initial experiments on the reaction of strontiumand ace tone (in 19 84), it was found tha t excitation of the strontiumatoms greatly enhanc ed the production of the gas-phase product.

This was discovered accidentally because a s tronti um atom ic linewas also coincident with a molecular transition. It was determinedthat excitation of the metal in this manner increased the molecularsignal by as much as 3 orders of magnitude. Excitation of the3Pl-’Soalkaline-earth atomic transition was used in many of therecent experiments on the larger polyatomic free

I n our carboxylate experiment, two dye laser systems wereusually used, one to drive the chemistry by providing excited me talatoms and a second to excite the molecular emission. The 5- Wall-lines output of a Coherent Innova 90-4 argon ion laser wasused to pum p a Coherent 599-01 dye laser operated with D CMdye, which lases from 6150 to 7200 A . The output from this dyelaser was tuned to th e 3P,-’So Ca ato mic transition at 6573 Aor to the corresponding Sr atomic transition a t 6892 A. A singlethin etalon (from the intracavity assembly of a Coherent dye laser)was used inside the laser cavity for excitation of the Ca atomic

line to narrow the line width and stabilize the excitation at 6 5 7 3A.

A second low-resolution dye laser (Coherent 599-01) was usedto obtain the laser-induced fluorescence spectra. This dye laserwas operated with DCM or Pyridine 2 dyes. The two dye laserbeams were spatially overlapped and focused into the Broida oven.Figure 1 is a block diagram of the experimental arrangement forthe low-resolution laser spectra.

Low-resolution laser excitation spectra were recorded byscanning the broad-band laser ( - 1 cm-I) while detecting thelaser-induced fluorescence with the photomultiplier tube attach edto the Broida oven. Laser excitation spe ctra are analogous to

(30) West, J. B.; Bradford, R .S. ; Eversole, J . D.; Jones, C . R . Rev. Sci .Instrum. 1975, 4 6 , 164.

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Gas-Phase Inorganic Chemistry The Journal of Physical Chemistry, VOI.94, No. 9, I990 3545

Figure 2. Laser excitation spectrum of the strontium plus formic acidreaction products. SrH, SrOH, an d SrOOCH are observed in this re-action.

classical absorption spectra. As the laser is scanned through afrequency resonant with a molecular transition, the moleculeabsorbs the radiation. The excited molecule fluoresces, and asignal is detec ted with the photomultiplier tube . A red pass filter(Schott glass filter RG9 for calcium carboxylates and RG780 forstrontium carboxylates) was inserted before the photomultiplier

tube to eliminate sc attered laser light and the strong fluorescenceof the alkaline-earth atom ic line. Th e scanning laser was me-chanically chopped, and lock-in detection was employed for re-cording the laser excitation spectra. This eliminates any chem-iluminescent signal and cuts out th e fluorescence from the atom ictransitions (which is quite often coincident with a molecular metalcarboxylate transition).

Low-resolution resolved fluorescence spectra were recorded bytuning th e laser to a previously observed molecular transition a ndfocusing the emission onto the slits of a 0.64-m monochromator.The intensity of the laser-induced fluorescence was recorded asa function of wavelength by scanning the monochromator. Theemission was detected by a cooled photomultiplier tube (RCAC 31034) operated i n photon counting mode. Resolved fluorescencespectra yield inform ation about vibrational levels in the ground

state . The laser excites molecules to a particular vibrational levelof an excited electronic sta te, and the excited molecules em it toa variety of ground-state vibrational levels depending on the se-lection rules and the Franck-Condon factors. Unfortunately, thereis usually collisional relaxation in the excited state s so considerablenonresonant emission is also present in the spectrum.

Results

The alkaline-earth monocarboxylates were produced by thegas-phase reaction of the metal vapor with formic, acetic,-pr?-piocic, a n i b_utanoic acids. Three electronic transitions (A-X,B-X, a nd C-X) were found for these new molecules. This reactionproduces the alkaline-earth monohydride and monohydroxidemolecules as well as the alkaline-earth monocarboxylate, so th espec tra were very congested.I5 Figure 2 shows a portion of th elaser excitation spectrum of the strontium plus formic acid re-

action. No filter or modulation was used to record this spectrum.The features due to S r H an d SrOH are labeled, but three newpeaks at 6785, 6710, and 7285 A (not shown in Figure 2) wereobserved and assigned to the strontium monoformate radical,S r O O C H .

I n Figure 3 th e B-R an d e-8 laser excitation spectra of th estrontium monocarboxylates are illustrated for comparison pur-poses. The spectra appear clearer compared with Figure 2, becausea red pass filter (Schott glass RG 780) was used in recording thesespectra . The red pass filter discriminate s against fluorescencefrom Z rH and SrOH because only fluorescence from the lowerlying A state in the metal csrboxylates is observed. This happensbecause the lower lying A state i n the met21 carkoxylates ispopula ted by collisional tran sfe r from-th_e B-X a-nd-C-X sta tes .As the alkyl group lengthens, the B-X and C-X electronic

1 5 h

Figure 3. Laser excitation spectra of the 8-,% nd e-i< transitionsofstrontium monocarboxylates. From bottom to the top, the spectra areof strontium monoformate, strontium monoacetate, strontium mono-propanoate,an d strontium monobutanoate.

TABLE I: Observed Positions of the A-g,B-2, and e-%ElectronicTransitions of the Calcium and Strontium Monocarboxylates (incm-')"

molecule A ~ A ~ - % ~ A ,~ B ~ - % ~ A , B,-PA,

14715 15913' 15913'aOOCHCaOOCCH3 14573 I 5 850b 15 850b

CaOOCCH2CH3 14 545 I5 835' 15 835'

CaOOCCH2CH2CH3 14 524 15 795b 15 795'

SrOOCH 13624 14 749 14 903

SrOOCCH, 13 500 I4 643 14832SrOOCCH2CH3 I3 500 14 620 14857

Sr00CCH2CH2CH3 13 459 14 609 14841

OUncertainty -20 cm-I. '&% an d e-% peaks were unresolved.

transition energies a re slightly s_hifLed to lower energy. The ap-pare nt relative intensity of the C-X transi tion decreases as thealkyl group increases, perhaps because the ra te of nonradiativeprocesses is increasing. This effect has been observed in thealkaline-ear th alkoxide s,I alkyla_mi_des; aJd alky!th:olate~.~ Th eelectronic band origins of the A-X, B-X, and C-X trans itionsof the calcium and strontium monocarboxylates are given in T able1.

The smaller peaks to higher energy shown in Figure 3 arevibronic transitio ns involving one or two quanta of the symmetricmetal-ligand stretching vibration in the excited state. Since the

electronic transition is metal-centered, only vibrations involvingthe metal atom have significant vibronic intensity. The metal-oxygen bending modes ar e probably so low in frequency (50-100cm-I) that they ar e hidden by the broad peaks in the spectra. Asexpected, the metal-ligand stretching frequency decreases as thealkyl group increase s. Low-resolution resolved fluoresce nce spectrawere recorded for these molecules by exciting an electronictransition and dispersing the emission with the monochromator(see Figures 1 and 4). Vibrational frequencies of the ground andexcited states were obtained in this manner and are provided inTable 11 .

Discussion

The geometry of the calcium and strontium carboxylates is notimmediately obvious from the low-resolution spectra. The car-

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3546 The Journal of Physical Chemistry, Vol. 9 4 , N o. 9 , 1990 O'Brien et al.

0700 E800 6900 7000

t

Figure 4 . Resolved fluorescence spectrum of the_ ATR transition ofcalcium monoacetate. The laser is exciting the A-X 0-0 transition.

TABLE 11: Vibrational Frequencies of the Symmetric Metal-OxygenStretching Vibrations of the Calcium and StrontiumMonocarboxvlates (in em-')'

molecule R 2 A , A2 AI B2B, @B,

CaOOCHCaOOCCH3CaOOCCH2CH3CaOOCCH2CH2CH3

SrOOCHSrOOCCHlSrOOCCH2CH3SrOOCCH2CH2CHl

'Uncertainty -10 cm-I.

349 356 354 354332 347 341 341315 360279 308

275 307 291 285253 284 28 2 292247 269 258219 251 271

boxylate ligand can bind either in a bidentate fashion throughboth oxygen atoms equally or in a monodentate fashion wherethe bonding occurs through one oxygen. The alkaline-earthmonoformate would thus have C, symmetry for a bidentate ligandand would have C, symmetry if the ligand was binding in amonodentate fashion, similar to the calcium and strontium mo-noalkoxides studied earlier. Because of the many differences in

the spectra of the alkaline-earth monocarboxylates and the al-kaline-earth monoalkoxides, we believe the carboxylates arebonding i n a bidenta te fashion. Although the re are no direct abinitio calculations to support this geometry, the simplest ionicmonocarboxylate, LiO OC H, is predicted3' to have C, symmetry.

Since high-resolution spectra ar e una_vailabl_e,he sy-mmetriesof the four observed electronic states X , A, B, and C are un-confirm ed. Th e bonding in these molecules is very ionic, an d theone unpaired valence electron is localized in a metal-centeredorbital.

0-

+ \ \

/ IM :C-R

0

Th e electronic transitions a rise from one-electron excitations toexcited metal-centered orbitals. The presence of the carboxylateligand lowers the symmetry of the atomic Ca + and Sr+orbitals.The metal-ligand interac tions ar e well-characterized for the al-kaline-earth monohalides and monohydroxides and o ther ligandswith axial symmetry (Figure 5 ) .

The energy level pattern for Ca + and Sr+perturbed by a low-symmetry ligand such as a carboxylate is more difficult to predict.The strong similarity between the MOOCR ( M = Ca , Sr ; R =H, CH,, CH,CH,, CH2 CH 2CH 3) pectra (Figure 2 ) implies tha tthe effective electronic symmetry is the s ame (C,") for the entirefamily. The alkyl side chains are dist ant from the unpairedelectron on the metal so that although all of the carboxylates otherthan the formates have C, symmetry, the "local" symmetry ex-perienced by the metal orbitals is always approximately C2,,.similar effect was found for the alkaline-earth monoalkoxideswhere the "local" symmetry at the metal center' was alwaysapproximately C-". Th e C,, point grou p symbols will, therefore,

( 3 ) Kaufmann. E. : Sieber, S.; Schleyer, P. v. R. J . Am. Chem.Soc. 1989,111 , 4005.

Figure 5. Correlation diagram of th e valence orbitals of th e alkaline-earth cations, M', the alkaline-earth monoalkoxides, and th e alkaline-ear th monocarboxylates.

be used to discuss the electronic states of the entir e carboxy latefamily.

A correlation diagram (Figu re 5) is helpful i n interpreting th eobserved spectra. A linear or axial ligand only partially lifts thedegeneracy of the M+ (M = Ca or Sr) atomic orbitals while alow-symmetry ligand completely lifts this degeneracy. For ex-ample, in C aO H the 3d C a+ orbital gives the rise to 22 , II, an d2A states, while in Ca OO CH the ,II an d 2A states are further split("in-plane" and "out-of-plane" components) into ,B,, 2B2 nd 2A,,2A 2 tates. In addition, the ligand induces mixing among theCtomic basis orbitals of the sam e symm etry so, for example, theA211 state of C aO H is primarily a p a- d a mixture of C a+ orbitals.

For the C a an d S r derivatives, the location of the ,A state andthe correcesponding ,A, ande2A2state s is unknown. Althoughwe previously as ~i gn ed '~he A state as the ,A, component of the2A state, we have reinterpreted our spectra an d no longer believethis to be true. Only for BaH,32 BaF,j3 BaC1,34and, presumably,other barium derivatives is the simple d-complex (B28+,A2n , and

A',A) patt ern of Fig ure 5 known to be valid._Notice that we believe the ordering of the A and B states of

the a lkaline-e arth monoalkoxides is ''reversed" for the alkaline-earth monocarboxylates, so that _the B2Z+ state of the alkoxidescorrelates to the lowest excited A2 A, state of the carboxylates.This particular ordering of the states was quite unexpected, butit is also observed for the calcium and strontium formamidates(see following paper14) and the calcium and strontium boro-hydrides, CaB H4 and SrBH 4.35

state energylevels is probably c haracteristic of ligands with ch arge distrib utedoff the z axis. This off-axis char ge destabilizes the nonbondingx-orbitals relative to the a orbitals. In the linear case, the exciteda orbital is higher i n energy than th e a orbital since an electronin the a orbital would be pointing directly toward the negatively

charged ligand. For the carboxylate ligand, the negative chargeis primarily located on the two off-axis oxygen atoms, thus s ta-bilizing the excited a , orbital relative to the b, a nd b, pair oforbitals.

T_he splitting between th e peaks at 6785 8, (B-2)and 67108, (C-2) i n S r O O C H is 165 cm-I. Although this splitting isreminiscent of the strontium spin-orbit splitting in the A2n statesof the strontium monoalkoxides ( - 2 6 0 cm-I), these tw o splittings

This unusual ordering of the excited A, , an d

( 32 ) Bernard, A.; Effantin, C.; D'lncan, J. ; Fabre, G.; tringat, R.; Barrow,R . F. Mol . Phys. 1989, 67, I .

(33) Barrow, R. F.; ernard, A, ; Effantin, C.; D'lncan, J. ; Fabre, G .; ElHachimi, A.; Stringat, R.; V e r g b , J . Chem. Phys. Let?. 1989, 147, 535.

(34) Martin, H.; Royen, P. Chem. P h ys . L ef t . 1983, 9 7 , 127.(35) Pianalto, F. .; Bopegedera, A . M . R. P.; Brazier, C . R.; Hailey, R.;

Fernando, W . T. M. L.; OBrie n, L. C.; Bernath, P. F. Manuscript in prep-ara t ion .

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J . Phys. Chem. 1990, 94, 3541-3549 3547

ar e not related. We do believe, however, tha t these two newtransitions a re to the 4d-5p strontiu m ato pi c orbitals, now of bland b2 symmetry, which correlate to the A211state of the pet almonoalkoxides. Th e assignment of the symm etry of the B-andC state s is somewhat d tbio us, although we prefer B2Bl and C2B2(rather than B2B2and C 2B I). From crystal field arguments, thep orbital in-plane (b,) sh ould be higher in energy than the p orbitalout-of-plane (b,) due to the repulsion of the negative ch_arge 0;the oxygen atoms . The observed splitting between the B and Cstates is, however, so small (e200 cm-I from the strontium car-boxylates) that other interactions may be more important. The

corresponding splitting between the B and C states is unresolvedfor the calcium monocarboxylates.

This ordering of the in-plane and ?ut-of-plan_eexcited p orbitalsof the alkaline-earth carboxylates (B2BIand C 2B2 ) s in contrastwithjhat observed for the corresponding states of SrNH2 A2B2and B2B l) where the sy mme tIy is kn_ow_n rom a high-resolutionrotational analysis of the A-X an d B-X transition^.^ Note thatfor the carboxylates the negatively charged oxygen atoms pointdirectly at the metal, while the partially positive hydrogens in theamides point away from the metal.

A definitive high-resolution analysis was attempted to determinethe symmetry and molecular geometry of the metal carboxylatestates. However, the molecules proved to be too relaxed for anyresonant laser-induced fluorescence to be observed, so a high-resolution analysis was impossible. This means tha t our assign-

ments are based more on supposition than fact. Perhaps someab initio calculations would help to clarify the problem.

Gas-Phase Chemistry of Alkaline-Earth Compounds

Little is known about the gas-phase chemistry of the largerpolyatomic free radicals . Several studies performed by matrixisolation techniques provide some insight into the gas-p hase re-actions of these molecules. For example, in an argon matrix thereaction of an alkaline-earth atom with a water atom first produces

the M-O H2 complex.36 Upon photolysis, the metal atom insertsbetween an oxygen-hydrogen bond to form H-M-OH . On U Virradiation, H-M-OH dissociates to form MOH.3 6

The reaction between excited strontium (o r calcium) canprobably proceed directly i n a single step:

Sr* + H O O C R - r O O C R + H (1 )

However, ground-state Sr (or Ca ) atoms ar e also found to react,although reaction 1 is probably endothermic i n this case.

Another possible mechanism for the formation of alkaline-earthmonocarboxylates is

0 0

I I(2)

ArS r + H-0-C--R 4 H-Sr-0-C--R

m-

0 0-II \ \

'0;/- + HZ (4)

Sr H + H-0-C--R - r'

This mechanism accounts for our observation of substantialamounts of Sr H in the oven. Surfa ce and metal cluster reactionsare also possible. It is also not clear whether th e observed Sr O H(and CaO H) comes from H 2 0 mpuri ty or from a chemical re-

action with the carboxylic acid. Th e study of the reactions ofalkaline-earth vapors with carboxylic acids under molecular beamconditions would be very fruitful.

Acknowledgment. This work was supported by the N ationalScience Foundation (G rants CHE-83065 04 and CHE-86086 30).

(36)Kauffman, J. M.; auge, R. H. ; Margrave, J. L. High Temp. Sci .1984, 8, 97.

Gas-Phase Inorganic Chemistry: Laser Spectroscopy of Calcium and Strontium

Monoformamidates

A. M . R . P. Bopegedera,+W . T. M. L. Fernando, and P. F. Bernath*,*

Department of Chemistry. University of Arizona, Tucson, Arizona 85721 (Received: October IO . 1989)

The reaction products of calcium and strontium metal vapors with formamide were studied by using laser spectroscopic techniques.Three electronic transitions were observed for the resulting metal monoformamidates, M NH CO H. The formamidate ligandis probably bonding to the metal i n a bidentate mann er. The m etal-ligand stretching vibrational frequencies were assignedfrom the low-resolution spectra.

Introduction

I n our laboratory, we have investigated the spectra of alkaline

earth metal containing free radicals including metal monoal-koxides,lP2 m on ~th iol a te s,~socyanates: cy~lopentadienides,~monoalkylamides,6 monomethides,' acetylides,* a ~ i d e s , ~oro-hydrides,I0 and carbox ylates." J2 All these free radicals have asingle metal-ligand bond (mon oden tate bonding) except for themetal borohydrides and carboxylates. The borohydride ligandbonds to the m etal in a tr identate fashionlo while the carboxylateligand bonds i n a bidentate fashion.I2

Although the formate anion (HCOO-) is a commonly en-countered ligand in transition-metal chemistry, the chemistry ofthe isoelectronic formamidate anion (HCONH-) has hardly been

'Current address: NO AA, ERL, R/E/AL2 , 325 Broadway, Boulder, C O

'Alfred P. Sloan Fellow; Camille and Henry Dreyfus Teacher-Scholar.*To whom correspondence should be addressed.

80303.

0022-3654/90/2094-3547$02.50/0

exp10red.I~ A few workers have explored the substitution offormate ligands by amid ato ligands in, for example, R h2 (0 N H -

( 1 ) Brazier, C. R.; Bernath, P. F. ; Kinsey-Nielsen, S.; llingboe, L. C. J .Chem. Phvs. 1985.82 . 1043.

(2) Brazier, C. R.; Ellingboe, L. C.; Kinsey-Nielsen,S. ;Bernath, P. F. J .

(3) Fernando, W. T. M. L. ; Ram, R. S.; Bernath, P. F. Work in progress.(4)Ellingboe, L. C.; Bopegedera, A. M . R . P.; Brazier, C . R.; Bernath,

P. F. Chem. Phsy. L e f t .1986, 26 , 285. O'Brien, L. C.; Bernath, P. F. J .

Am . Chem. SOC.986, 08, 2126.

Chem. Phys. 1988, 8, 1 17.(5) 'Brien, L.C.; Bernath, P. F. J . Am . Chem. SOC.1986,108, 5017.(6 )Bownedera. A. M. R. P.; Brazier, C. R.; Bernath, P. F. J . Phys. Chem.

1987, 1,'2i79.(7)Brazier,C. .; Bernath, P. F. J . Chem. Phys. 1987,86,5918; J . Chem.

Phys. 1989, 1, 4548.(8)Bopegedera, A . M. R. P.; Brazier, C. R.; Bernath, P. F . Chem. Phys.

Lett . 1987, 36 , 91; . M o l . Spectrosc. 1988, 29 , 268.(9)Brazier, C . R.; ernath, P. F. J . Chem. Phys. 1988,88, 112.(10) Pianalto, F. S.; Bopegedera, A. M. R. P. ; Brazier, C. R.; Fernando,

W T. M . L.;Hailey. R. ; Bernath, P. F. W ork i n progress.

0 990 American Chemical Society