c. r. brazier et al- laser spectroscopy of alkaline earth monoalkoxide free radicals

8/2/2019 C. R. Brazier et al- Laser Spectroscopy of Alkaline Earth Monoalkoxide Free Radicals

1/7

2126 J . Am. Chem. SOC.1986, 108, 2126-2132as VC+, VCH+, and VCH 2+which are not seen with zinc. Similarreactivity is observed for reactions of vanadium ions with otheralkanes.27 Conversely, reaction 2 is not observed for th e reactionof vanadium ions with ethane and propane. For the isobutaneand neopentane systems, this process is observed although it issmall relative to the competing V CH 3+ channel.There ar e many dissimilarities between V+ and Zn' which mayaccou nt for their different reactivity. At first glance, one of thestriking differences is that van adium has a mu ch lower ionizationpotential (6.74 eV) than zinc (9.458 eV). Considering only thereactant molecular orbitals analogous to those shown in Figure5 , the difference in the orbital energy (IP) of the me tal ions doesnot change the relative ordering of th e orbitals. Thus , the lowerionization p otential of vanadium should not affect the qualitativecorrelations.The most im portant consideration is electronic structure. Th egrou nd sta te of Zn' is *S 3dI04s') while for V + it is SD (3d4).If we were to consider only the 4s orbital from the vanadium ion,the orbital correlations would be th e sam e as in the zinc system.Mixing of the filled uRM nd th e now emp ty 4s of V+ would occur.T he diabatic path would sti ll be the formation of R+ , for boththe vanadium and th e zinc system, in this simplistic M O approach.But the d orbitals must be considered in the vanadium case, andthis complicates the molecular correlation picture. Dependingon the symmetry, some of the d orbitals will remain unperturbedand nonbonding, while others will be involved in avoided crossing sin the MO corre lat ion diagram. The adiabat ic pathway wil linvolve many more crossings for the vanadium system comparedwith zinc.The reactions of vanadium ions with various hydrocarbons havebeen interpreted based on the assumption tha t the reaction pro-

ceeds by metal-ion insertion into C- C or C-H bonds.26 Formationof such an intermediate complex would be facilitated by theavailability of empty d orbitals and would allow electronic re-organization to occur. This is consistent with the dominance ofthe adia batic processes in the reactions of V+ with alkanes. Sim ilarcons iderat ions pres uma bly hold for Sc', Ti+ , Fe', Co', and Ni'.Preliminary results for the reactions of two other S I ions, Ca'(4sI) and M n+ (3d5 4s'), with ethan e have been examined for slowrising threshold behavior and for the presence of reaction 2.Neith er is observed in the Ca+ system. Although not a transitionmetal, i t is possible tha t the empty d orbitals on Ca+ contributeto its bonding, much like the vanadium system. For M n+, reaction2 is not observed. Reactio n 1 is observed but the existence of asmall amoun t (


2/7

Laser Spectroscopy of Alkaline Earth MonoalkoxidesTh e key discovery in the alkaline earth alkoxide work was thatsubstantial quantities of cool (500K) M O H could be producedin a flow reactor (Broida ovenI2) by the reaction of alktlin e tar thmetal vapors with H20.'3*14otational analyses of the A2H-X22+and B2Z+ -X2Z + electronic transitions by laser techniques providedbond lengths and vibrational Worm sbecher andSuenram*l prepared the next member of the alkoxide series ina Broida oven by the reaction of Ca and Sr metal vapors withDuring an unsuccessful a t tempt to make a lkal ine ear th ace-tylides in a Broida oven, we discovered tha t acetone reacted with

Sr vapor to produce two new free radicals.] Furth er experime ntsproved that the major product was strontium isopropoxide, SrO-CH (CH ,)2. Th e init ial strontium isopropoxide observations en-couraged us to explore the gas-phase chemical reactions of alkalineearth metals with alcohols. Our chemical and spectroscopic ob-servations for the reactions of Ca a nd Sr vapors with methanol,ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, and 2-methyl-2-propanol as well as acetone and acetaldehyde are re-ported here. Som e preliminary work on Ba monoalkoxides is alsopresented.Experimental Methods

The alkaline earth monoalkoxides were made in a Broida-type ovent2by the reaction of a metal vapor (Ca, Sr, or Ba) with a suitable oxidant(alcohol, acetone, or acetaldehyde). The metal was evaporated from aresistively heated alumina crucible and entrained in a flow of argoncarrier gas. The argon and m etal vapor flowed through a hole (0.6 cmin diameter) in the center of an oxidant ring approximately 6 cm fromthe crucible. Th e organic oxidant was added as a vapor through smallholes in the oxidant ring to produce a "flame" (there may be no chem-iluminescent emission) typically 1 cm wide and 10 cm long. The totalpressure was adjusted to about 8 torr by varying the pumping speed andargon flow rate in order to optimize the laser-induced emission signal.Th e butanols and 2-methyl-2-propanol were heated in order t o increasethe vapor pressures. Th e production of metal alkoxide was not a strongfunction of oxidant partial pressure bu t, in general, additional oxidantimproved the signal.For our initial strontium plus acetone experiments, it was found thatthe Sr 3 P I - 1 S ~tomic line overlapped the molecular transition (c f. Figure2) and that excitation of the atomic line increased the signal by 3 ordersof magnitude. Although the ground-state atoms react at pressures of8-10 torr, the 3P , excited atoms produce more product molecules. Thu sfor most of the experiments two continuous wave dye lasers were used:a single-mode (1-MHz) Coherent 699-29 excited the 3P,-1S,atomictransition and a broad band ( 1 - c d ) Coherent 599-01 excited the mo-lecular transition. The dye lasers were operated with D CM or Pyridine2 dye and were pumped by Coherent Innova 20 and/or Coherent Innova90 argon ion lasers. Th e dye laser output beams (200-1000 mW ) werespatially overlapped and focussed into the weakly chemiluminescentmetal flames.Resolved fluorescence detection was accomplished by imaging thelaser-induced fluorescence onto the slits of a 0.64-m monochromatorequipped with an RCA C 3 1 0 3 4 photomultiplier and photon-countingdetection electronics. Th e monochromator (not the laser) was scannedto provide a dispersed fluorescence signal. Since the molecular emissionwas almost entirely relaxed, the laser-induced fluorescence pattern didnot depend on the wavelength of the laser. Th e initial location of themain spectral features was usually established by observation of chem-iluminescence produced by a single laser resonant with the atomic line.

CH3ONO.

J . Am . Chem. SOC.,Vol. 108, No. 9, 1986 2127Table I. Band Origins of Calcium Monoalkoxides (cm-I)

molecule AI2n1/2 A2,113/2 B2z+CaOH' 1 5 9 6 5 1 6 0 3 2 1 8 0 2 2CaOCH3 1 5 9 1 7 1 5 9 8 2 1 7 6 7 6CaOCH2CH3 1 5 8 5 0 1 5 9 1 3CaOCH2CH2CH3 1 5 8 6 0 1 5 9 3 4CaOCH(CH,), 1 5 8 2 3 1 5 8 9 6CaO(CH2),CH3 15853 15931CaOCH(CH3)CH2CH3 15 8 13 15 896CaOC(CH3)3 1 5 8 1 8 1 5 8 8 6

'Reference 14 .Table 11. Band Origins of Strontium Monoalkoxides (cm-I)

molecule A,2n,,, A?r l3 / , B22+SrOH" 1 4 5 4 3 1 4 8 0 6 1 6 3 7 7SrOCH, 1 4 5 2 0 1 4 8 0 4 1 6 0 9 8SrOCH2CH3 1 4 5 0 7 1 4 7 6 7 1 6 0 9 0SrOCH2CH2CH3 1 4 5 1 2 1 4 7 8 4 1 6 1 7 6SrOCH(CH3), 1 4 4 8 5 1 4 7 5 6 1 6 1 0 0SrO(CH2)$H3 1 4 5 0 8 1 4 7 7 8 1 6 1 7 3SrOCH(CH3)CH2CH3 1 4 4 7 2 14 745 1609 8SrOC(CH3)3 1 4 4 8 0 1 4 7 4 9 1 6 0 4 1

a References 12 and 15.Table 111. Band Origins of Barium Monoalkoxides (cm-I)

molecule AI2n1/* A,2n?i, B2Z+BaOH' 11 57 2 12207 13 200BaOCH3 11 448 12 108 1 2 9 2 3BaOCH,CH, 11 395 12 047 12 942BaOCH(CH,), 11 363 12 003 1 2 9 2 2

Reference 16.Ca-O-CHZCH,

-76200 6250 6300 6350Figure 1. Laser-induced fluorescence from the A-R transition of CaO-CH2_CH3.The asterisk marks scattered light from the dye laser excitingthe A$I13/2-R2Zt molecular transition . Th e A12111/2-%2Z+ransitionis near 6300 A. An additional laser excited the Ca 3P1-'S0 ransition at6572 A. Th e horizontal s cale is in A .

(12) West, J. B.; Bradford, R. S . ; Eversole, J . D.; Jones, C. R. Rev. c i.(13) Benard, D. J.; Safer, W. D.; Hecht, J . J . Chem. Phys. 1977, 66,(14) Wormsbecher, R. F.; Trkula, M.; Martner, C.; Penn, R. E.; Harris,(15) Hilborn, R. C.;Qingshi, Z. ; Harris, D. 0.J . Mol. Spectrosc. 1983,(16) Nakagawa, J. ; Wormsbecher, R. F.; Harris, D. 0. J . Mol . Spectrosc.(17) Bernath, P. F.; Kinsey-Nielsen, S . M. Chem. Phys. Lett. 1984, 105,(18) Bernath, P. F.; Brazier, C. R. Astrophys. J . 1985, 228, 373-376.(19) Brazier, C. R.; Bemath, P. F. J . Mol. Spectrosc. 1985, 114, 163-173.(20) Kinsey-Nielsen, S . M.; Brazier, C. R. ; Bernath, P. F. J . Chem. Phys.(21) Wormsbecher, R. F.; Suenram, R. D. J . Mol . Specrrosc. 1982, 95,

Instrum. 1975, 46 , 164-168.1012-101 6.D. 0. J . Mol. Spectrosc. 1983, 97, 29-36.97,73-91.1983, 97 , 37-64.663-666.

1986, 84 , 698-708.39 1-404.

6750 6800 6 8 i O 6900 aFigure 2. Laser-induced fluorescence from the A-R ransition of SrO-CH(CH3),. Th e laser is simultaneously exciting the atomic line at 6892A and the Al2IIII2-R2Z+molecular transition. The strong A?I13i2-R2Z'transition is near 6770 A.A second laser, resonant with a molecular transition, was then used toprovide more intense emission.ResultsThe alkaline earth alkoxide spectra all resemble the linear M O Hspectra (which, in turn, resemble the M F diatomic spectra) sothe observed electronic states will be_named- by analog y. Th e twoobserved electronic transitions are B22+-X2Z+ and A2H-X2Z+


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2128 J . A m . Chem. Soc . , Vof .108, No. 9, 1986

P.

' 8d00 I 8400 00'00 ' ssb0Figure 3. Laser-induced fluorescence from the A-2ransition of BaO-CH(C H&.-T he dye laser excited the B2Z+-%2Z+transition at 7738 8,and the-A-X tra_nsition appears by cascade and relaxed emission. Thestrong A2ZI13,z-X2Z+ransition lies near 8330 A, while the equally in-tense A12111/2omponent at 8800 8, appears weak because of a sharpdecline in photomultiplier tube sensitivity. The horizontal scale is in 8,.with the A211 state split into two spin components A2113 an dA211,/, by spin-orbit coupling. Since the symmetry of the aikalineearth alkoxides is lower than linear this is not strictly correct.However, the M - O bond is very ionic so the electronic transitionsinvolve nonbonding M + orbitals perturbed by an R O- ligand. Th eM -0 -C bond angle is 1SOo. Hence the focal symmetry near themetal is linear, so fo r instant:, there is always a "spin-orbit"splitting of -65 cm-' for the A stat e of the calcium-containingspecies. T he orbitals on the m etal ar e only slightly affected bythe natu re of R on the ligand, so it is useful to utilize th e effectivelinear symmetry in labeling the electronic states. A more completedescription of the electronic struc ture is presented in the D iscussionsection.The observed band origins for calcium, strontium, and bariumalkoxides obtained from resolved fluorescence scans ar e reportedin Tables 1-111, respectively. Figu res 1-3 a re typical examplesof data recorded by monochromator scans of the laser-inducedfluorescence. Th e hydroxides and methoxides produced resonancefluorescence while emission from the larger m olecules was relaxed.T h e jw o strong , broad (-40 cm-!) features of Figures 1-3 areth e A,z lI l j2-X2Z+ n d A22113iL-X22+ransitions. Vibrationalrelaxation, relaxatio? between A , and A2 spin components, andrelaxation between B and A states were all prominent (but notcomplete) in the spectra. The peak positions were determinedto an estimated accuracy of f2-5 cm-' .The reactions with acetone and acetaldehyde produced strongfeatures identical with those observed with 2-propanol and ethanol,respectively. Formaldehyde did not appe ar to react, while vinyla lcohol (C H2 =C HC H 20 H) and propargyl a lcohol (H-CC-H20H)-se_emed to produce only oxides (CaO , S rO) .The A-X emission spec tra (Fig ures 1-3) show evidence of weakvibrational structu re. Examples are features near 6250 and 6340A in Figure 1, near 6810 an d 6860 A in Figure 2, and 8160 and8620 A ic Figure 3. Th e peak near 8800 A in Figure 3 is a strongA2111i2~-X28+-0 band fea ture that only appears weak becauseof rapidly decreasing photomultiplier sensitivity. Since theelectronic transition involves a me tal-cen tered , nonbondingelectron, the Franck-Condon factors for Av # 0 transitions areexpected to be small . Fur therm ore, any observed vibrationalactivity will be associated w ith the metal cen ter.In some spectra the detector amplifier gain was such tha t thestrong features w ere off scale but the vibrational structure wasclearer (e.g., Figures 4 an d 5 ) . In these two scans the zero oft_he wavpumber scale is the (off-scale) 0-0 band of theAZIII,,-XZZ+ ransition of CaO R. The structure to the right (tohigher wavenumbe., to the red in wavelength) represents emijsionfrom v = 0 of the A state to excited vibrational levels of the X2Z+state. The observed transitions could be assigned to activity infour normal modes: vB, the M-0-R bend; vs , th e M-O strength;vb, h e 0-C st re tch; and us, the 0-C-C bend. T he vibra tionalfrequencies ar e l isted in Tables IV-VI. The accuracy of thevibra tional data i s f 3 m-I a t best and is worse (f10 -20 cm-I)for blended and weak features. Details of the vibrational as-signments ar e presented in the Discussion section.

Brazier et al.

B0 20 0 40 0 600 Cm.1

Figure 4. (A ) Laser-induced fluorescence spectra of the CaO CH 2CH ,,CaOCH(CH3)2,and CaO C(C H,), molecules (the 0-0 &.%bandemission is off scale ). Each vibraJional featu re is doubled by the spin-orbit splitting ( 7 6 5 cm-') of the A state. The x-axis scale is set to zerofor the Ai211y-X2Z+ 0-0 and so the scale refers to the higher frequencycomponent o any doublet. Each pair of features corresponds to emissionfrom u = 0 of the A tate into the vibrationally excited levels of the g 2 Z +state. The symbol X marks uB, he M-0-R bend, the filled circles areUS, the M-O stretch; and the open circles mark the 0-C-C bends ( us ) .Pairs of spin-orbit components are connected by horizontal lines. (B )Laser-induced fluorescence spectra of the butoxide isomers CaOC (C-H3)j, CaOCH (CH3)CH2 CH3, nd CaO(CH 2)3CH,. The bottom twotraces are on the same vertical scale while the top trace is a factor of 3less sensitive. The two sharp features on the bottom trace a re artifacts,probably from CaOH.

/ I

C H z \ / CH I1 I6900 7000 K

Figure 5. Laser-induced fluorescen_ce pectca of the products of the Srvapor plus acetone reactio n. The Ai2111/2-X2Z+ ransition is off scalenear 6903 A. The upper trace corresponds to a large amount (severalhundred millitorr) of acetone oxidant, while the bottom trace was re-corded from a flame with less than 100mtorr of acetone. The vibrationalfeatures arise from emission from u = 0 of the A i2II ,/, state into vibra-tionally excited levels of the ground g 2 Z + state (as i n Figure 4) .


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Laser Spectroscopy of Alkaline Earth Monoalkoxides J . Am . Chem. SOC. ,Vol. 108, No. 9, 1986 2129Table IV. Vibrational Frequencies of Calcium Monoalkoxides (em-')

mode m+ A,%,, A,2n,,, B 2 X +U B 142 1662PB 284 331U S 484 487 486 488"6 + "s 625"0 1156 1133 11352"s 973

CaOCH2CH3"6 85 95"6 512 531 533"S 385 402 400

mode m+ AI2n,, A22n312CaO(CH2)3CH3

CaOCH(CH3)CH2CH3330 339CaOC(CH,),302 315

520 530CaOCHCH2CH374-349 375-425-578

-377

CaOCH(CH3)2319 322440 440551 557

92

10131176

340314528365-440-5579631944 1554

Vibrational st ruc ture was difficult to detect for some of theheavier _alk?xides. For the alkoxides th e intensity of th e main peaksof the A-X emission (Tables 1-111) de crease d in app rox ima teproportion t o the vapor pressure of the parent alcohol. How ever,the vibrational features in the spectra decreased in intensity m uchmore rapidly for any molecule containing a chain of three or mo recarbon atom s (Figure 4). For example, the vapor pressure of2-butanol is about a factor of 2 less than 2-methyl-2-propanol atroom temperature, yet the Ca- O stretch (the pair of peaks near300 cm-' in Figure 4) has decreased a factor of 20 in intensity.The reaction of Sr with acetone (and acetaldehyde) producedtwo products. For instance (Fig ure 5) when a large (severalhundred milli torr) amount of acetone was present, the mainproduct w as SrOC H(C H3 )2. At lower acetone concentrations anadditional molecule, tentatively assigned to SrOC (CH 3)= CH 2,was observed. The strontium isopropoxide assignment is securesince a comparison spectrum (obtained from Sr plus 2-propanol)is available. The reaction of Ca w ith acetone (or acetaldehyde)produced almost entirely the isopropoxide (ethoxide) while thecorresponding Ba reactions have not been explored yet.Discussion

Chemistry. The direct reactionM + H O R +M O R + H

is probably endothermic for ground-sta te Ca and S r a toms andslightly exothermic for Ba. Th e RO-H bond energies22 are inthe range 102-106 kcal/mol (except for H-OH, where DH = 119kcal/m ol). Th e bond energies of M-OR are, however, knownonly for Mg-O H (74 kca l /m01 ) ,~~ a -OH (92 kca l /m 01) ,~~r-OH (92 k ~ a l / m o l ) , ~ ~nd Ba-OH (107 kca l /m 01) .~~ ince thebonding is ionic and very similar for all of the molecules, we

(22) McMillen, D. R.; Golden, D. M. Annu. Reu. Phys. Chem. 1982,33,(23) Murad, E. Chem. Phys. Le t t . 1980, 72 , 295-296.(24) Murad, E. J. Chem. Phys. 1981, 7 5 , 4080-4085.

493-532.

Table V. Vibrational Frequencies of Strontium Monoalkoxides(cm-')mode R 2 . P AI2J&/2 A?n3i2SrOCH,408 4238061141

SrOCH2CH378 813454344901154SrOCH2CH2CH,65 - 0279368550

SrOCH(CH,),78 9327442 1 409533

SrO(CH2)3CH3-53-265SrOCH(CH3)CH2CH361125

SrOC(CH3),142247498SrOC(CH,)=CH,253510

SrOCH=CH2306

430

83357496

-702743205429129 1

419539

- 95-280

261497248486273

Table VI. Vibra tional Frequencies of Barium Monoalkoxides (em-')mode m+ A?n, , A,2nw

BaOCH,4 375 342 344

BaOCH2CH3U S 279 279

BaOCH(CH3)2us 249 243 247

2YB 254

assume that the bond energies are approximately the s ame forall M-OR. The direct reaction is thus too endothermic to be themain route for M O R product ion from ground-sta te Ca and S ratoms.If the alkaline earth atoms are excited by laser irradiation onthe 3P,-'S0tomic transition the direct reaction becomes exo-thermic. For Ca, Sr, and Ba the lowest 3P1tates provide 44, 41,and 36 kcal/mol, respectively, of additional energy to the reac-tion.2s Thus , the 3 orders of magnitude enhancement in productmolecule concentration observed with M * reagents is probablydu e to the direct reaction channel becoming available. ExcitedC a * a n d Sr* (Ba* was not tested) atoms react 3 orders of mag-ni tude more readi ly than ground-sta te Ca and Sr .The gas-phase reaction of ground-state Ca, S r, and Ba atomswith H 2 0 and a lcohols i s unexpected and remark able . Two

(25) Moore, C. E. Atomic Energy Leuels; National Bureau of Standards:Washington, DC 1971; Vol. 1-111, NSRDS-NBS 35.


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2130 J . A m . Chem. SOC., ol . 108, No. 9, 1986 Brazier et al.Table VII. Metal-Oxygen Stretching Frequencies ( u s ) for AlkalineEarth Monoalkoxides (cm-I)

Mmolecules Ca Sr Ba

MO H 606' 528b 492cMOCH3 486 408 315MOCH2CHZCH3 -349 219MOC H(CH3)2 319 214 249M0(CH2)3CH3 - 65MOCH(CH,)CH,CH, 330MOC(CH3)3 302 241MOCHeCH2 306MOC(CH+CHI 253MOCHZCH3 385 345

a Reference 11. Reference 15. Reference 16.possible m echanisms for gro und-sta te Ca, S r , and Ba a tom re-actions areMechanism A

M + H - O R 5 -M-ORH-M-OR + M - H + M-OR

M H + H-OR -+ M O R + HzM + H - O R -* M O R + '/2H2

Mechanism BM + H-OR A H-M-OR

H-M-OR + H - O R -+ M ( O R ) 2 + H2M ( O R ) 2 + M - M O R

M + H - O R - O R + Y2H2The first step involves the insertion of the metal atom into theH-O bond. Th e presence of a third body (Ar arrier gas) increasesthe probability of this event an d accounts for ou r observation th athigh pressures (8-10 torr), produced by decreasing the pumpingspeed, increased the metal monoalkoxide concentration. TheH M O R interm edia te was not detected because i t probably hasno visible electronic absorption spectrum. However, Kauffman ,Hau ge, and Margrave" have observed the photolytic insertionof Mg, C a, Sr , and Ba into H-OH. T h e H M O H p r od u ct w a strapped in an argon matrix an d monitored by infrared absorp tionspectroscopy.In mechanism A the second step involves the production of theM H species. The Ca, Sr, and Ba reactions with formic and aceticacids' did produce MH . Since M H was not observed in our metalplus alcohol experiments, mechanism B is favored. M (O R) 2 shouldhave a UV absorption spectrum and would not be observed in ourvisible laser experiments.

The above two mechanisms are simply plausible speculations.It is possible that surface reactions in t he crucible play a role or(mo re likely) that m etal dimers and clusters are involved. Likethe superficially simple Bunsen burner flame, the flame che mistryof a Broida oven system may also be quite complicated.The reaction of Sr with acetone (or acetaldehyde) is unusualbecause a reduction of S r isopropoxide (or ethoxide) dominatesthe chemistry. Th e simplest explanation is that a 2-propanolimpu rity in the aceton e is responsible, b ut it is unlikely tha t thisrepresents the only route to Sr isopropoxide. A more promisingexplanat ion involves the addi t ion of S rH o r HS rO R across thecarbonyl double bond of acetone. Th e additional vibrationalfeatures that are not Sr isopropoxide (Figure 5 ) ar e tentativelyassigned to the S r acetone enolate (Tabl e V) because this is theonly other product l ikely to have a spectrum so similar to thatobserved for Sr isopropoxide.Vibrational Structure. Th e metal-centered character of the A-2electronic transition both helps and hinders th e vibrational analysis.Franck-Co ndon selection of only a few modes associated with themetal center m akes th e vibrational assignmen ts relatively easy.

vicm'l

2I 0 ,22 .26 11%' . 3 0Figure 6. A plot of the C a-O stretching frequency vs. the inverse squareroot of the 'reduced mass" for the series CaOR (R = H, CH,, C2Hs,C3H7, C4H9).

Sr-OR

100

015 .20 25'IJTFigure 7. The variation of the Sr-0 stretching frequency for the seriesSrOR (R = H, CH3, C2Hs, C3H7,C,H9).However, almost all direct information ab out the na ture of thealkyl grou p is suppressed.Th e metal -oxygen st re tching frequency (vs) is usually thestrongest mode (Figu re 4, indicated by solid circles) and w as easilyassigned. T he ground-state vs frequencies are collected in TableVI1 for comparison purposes. The values range from 606 cm-Ifo r CaOH to 247 cm-I for SrOC (C H3 )3, argely because ofchanges in the effective reduced mass. Th e M+ -O- ionic bondingresults in a similar M -0 force constant for all the observedmolecules. By t rea ting the O R uni t as a single mass a dia tom-ic-like "reduced mass" ( w ) can be calculated. A plot of thestretching frequency vs. p- I I2 produces an approxmately l inearcorrelation with Ca-OR and Sr-OR (Ba has only three points)each producing a slightly different l ine (Figures 6 and 7) .


6/7


7/7

2132 J. Am . Chem. SOC. 986, 108, 2132-2139and BaO R are 63-78, 260-280 and 635-670 cm-l, respectively(Tables t-111). This similarity in sein-orbit constants suggeststhat the A states (and probably the B and X states) have similara tomic parentage to t i e f luocides.Th e location of the B and A states relative to each other an d

the strength of the ligand field interaction. Th e location of N H Tis from the work of Wo rmsbecher et al .I4 and ou r SrNH,ob-servat ions.50 Note that SrNH2 has C2" ymmetry so t ha t A 2 nfurther splits into 2Bl and ZB2 tates. Th e NC O- splitting is fromour own recent discovery of the Ca O CN and Sr O C N molecule^.^th eTh e band origins ( 0 = 0) fo r BzZ+,2113/2 and A 2U l 2cm-l ; 14040,& 12262,& and 11630 cm-l for CaF , SrF, a nd B aF,respectively. In ge te ra l, t he l_arger the alkyl group the more redshif ted are the A-X an d B-X-transitLons (Ta ble s 1-111).The split t ing between the A and B states is a l igand fieldsepara tion be tween d a (pa ) and d r ( p r ) o rb it a ls . The va riousligands can be arrange d in a spectrochemical series49F OH->NH, > 0--R >N C O - >Cl- > Br- > I- for the alkaline earths.Th e order is determined by the B-,& energy separation and reflects

state also sug gests close_simila_ritywith th e M F molecules.18833:' 165 66," and 16 49 3 cm-I; 17271:* 15372: 4 ndtates5091re

~~~~ (47) Dulick, M.; Bernath, P. F.; Field, R. W . Can. J . Phys. 1980, 58 ,(48) Steimle, T. C.;Domaille, P. J. ; Harris, D. 0.J. Mol . Spectrosc. 1977,(49) Cotton, F.A,; Wilkinson, G. Advanced Inorganic Chem istry, 4th 4.;

703-7 12.68, 134-145.Wiley: New York, 1980; p 663.

ConclusionA large number of new Ca-, Sr- , and Ba-containing free radicalshave been discovered and characterized by laser spectroscopy. T healkaline earth monoalkoxides were produced in the gas phase bythe reaction of the metal vapor with the appro priate alcohol. Th evibrational and electronic structure of the akaline earth mo-noalkoxides has been explained in terms of an M + ion perturbedby an 0-R igand.

Acknowledgment. This research was supported by the Nation alScience Foundat ion (NSF -8306504 ) and the Research Corpo-ration. Acknow ledgement is mad e to the donors of the Petro leumResearch Fund, administered by the American Chemical Society,for partial support of this work. Partial sup port was also providedby the Off ice of Naval Research (O N R N 00014-84-K-0122).(50) Brazier, C. R; Bernath, P. F., unpublished results.

The Use of a Stationary Cationic Surfactant as a SelectiveMatrix in 252Cf-PlasmaDesorption Mass SpectrometryCatherine J. McNeal* and Ronald D. MacfarlaneContribution fro m the Department of Chemistry, Texas A & M University,College Station, Texas 77843. Received July 24, 1985

Abstract: Mylar films impregnated with a cationic surfactant have been tested as an anion exchange matrix for 2s2Cf-PDMSstudies. The purpose of this matrix was to reduce the intermo lecular binding forces and the effects of m atrix impurities whichhinder desorption-ionization of biopolymers bearing multiple anionic groups. The matrix selectively adsorbed the anionicmoiety from aqueous solutions of an inorganic salt and several simple nucleic acid fragments. Significant enhancemen ts inthe yields were observed.

In many of the particle induced emission mass spectrometricme thods (252Cf-PDMS,SIMS, LDMS, FABMS) ion iza t ion-desorption occurs from a solid matrix. Th e disadvantage ofionizing molecules from the solid phase rather than the liquid orgas phase is the complexity of interactions that may attenuateor quench molecular ion formation. Impurities in the matri x orstrong interactions between highly polar species have been iden-tified as probable causes.I In FA BM S when a l iquid matr ix isused these problems seem to be less severe. It has been proposedthat the solvent used in these studies may modify the stronginteractions among s olute molecules, making it easier to desorba molecule from the solutesolvent cluster because the bindingenergy between molecules is reduced. This suggests hat molecularion yields in the solid phase ar e attenua ted when the energy bindinga molecule to its surrounding m atrix exceeds the energy that isavailable in the desorption process. ' In an effort to reduce thestrong in teraction s between molecules in the solid phase we firstreported on th e use of Nafion a s an ion-containing polymer matrixthat could selectively bind large o rgan ic cations, effectively re-ducing the interactions.* The selectivity of the sulfonic acid groupsof Nafion for binding cations was demonstrated by incorporatingonly Cs+ and not I- when the fi lm was exposed to an aqueous

(1) Macfarlane, R.D. Acc. Chem. Res. 1982, 15 , 268-275.(2) Jordan, E. A.; Macfarlane, R.D.; Martin, C. R.;McNeal, C. J. I n f .J . Mass Spectrom. Ion Phys. 1983, 53 , 345-348.

solution of CsI. Molecular ions of large organic cations, includingBleomycin-Cu*+ complex, adsorbed onto t he fi lm hav e beenproduced.The advantages of Nafion are that i t is a relatively lowequivalent molecular weight polymer (1 100 ew) tha t ca n bedissolved in an organic solvent system3 but which will not redissolveafter drying if exposed to aqueous solutions. Thin films of Nafion(< l o0 pg /cm) can be cast on a foil backing, either by electrospray4or spin coat ing5 Th e f i lms are then used as a subst ra te toselectively ad sorb desired cations from a solution. The filmscontain a high surface concentration of the adsorbed cations dueto the low equivalent molecular weight of the polymer. Typicalion exchange resins are not useful because the particulates producefilms too thick for 2 s q f ission fragments to penetrate and becausethe equivalent molecular weight is very high, resulting in a lowsurfac e concentration of the ions of interest. Furth ermo re, thesefilms tend to redissolve when exposed to a solution.W e have sought to develop an anion ex change matrix similarto the Nafion that would reduce the intermolecu lar binding forcesand the effects of matrix impurities which were felt to hinderdesorption-ionization in 252Cf-P DM S, articularly in the analysis

(3) Martin, C. R.; Rhoades, T. A,; Ferguson, J. A. Anal. Chem. 1982,54,1639-1 641.(4) McNeal, C. J.; Macfarlane, R. D.; Thurston, E. L. Anal. Chem. 1979,( 5 ) Meyerhofer, D. J. Appl . Phys. 1978, 49, 3993-3997.51 , 2036-2039.

0002-7863/86/1508-2132$01.50/0 0 1986 American Chemical Society

c. r. brazier et al- laser spectroscopy of alkaline earth monoalkoxide free radicals

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