A. M. R. P. Bopegedera et al- Gas-Phase Inorganic Chemlstry: Laser Spectroscopy of Calcium and Strontium Monopyrrolate Molecules

8/2/2019 A. M. R. P. Bopegedera et al- Gas-Phase Inorganic Chemlstry: Laser Spectroscopy of Calcium and Strontium Monop

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4476 J . Phys. Chem. 1990, 94, 4416-4419should not only be able to provide a guide as to the ra nge of validityfor the classical continuum theory of solvation relaxation butshould also clarify which aspects of solvent molecularity are im-portant in the solvation response and need to be included inmicroscopic theories of nonequ ilibrium solvation. From the presentwork and other simulation^^-^" it is apparent that the solute shapeand charge distribution will have a large effect on the particularrelaxation mechanism that governs the solvent response to anelectronic excitation. For a large polyatomic solute where changesin the atomic charges are small and/or shielded from the solventby bulky solute groups, the classical cavity model should providean ad equa te description of the solvent response to the excitation.When th e solute is small and/ or there are large surface chargesgenerated, the solvation shell structure and dynamics will dominatethe relaxation. For this situation, the forces on the solventmolecules will be large and rapidly varying and the relaxation willoccur via nondiffusional translation and rotation. In this regime,the dynam ics of the solvation shell speeds up rather than slowsdown the solvent response compared with th e bulk response. Also,we expect i n this regime that linear response theory will notadequately describe the relaxation because of the highly anha r-mo nk, impulsive nature of the forces exerted on the solvation shellfollowing the excitation. In this context we note the very surprisingresults of Bader and Chan dler in their recent simulations ofphotoinduced electron transfer in the aqueous ferrous-ferricsystem.3s They found that linear response theory predicted thenonequilibrium response to a surprising degree of acc uracy despitethe fact that the reorganization energies associated with theperturbation were very large for their model system. Calculationsof the corresponding equilibrium correlation functions of thesolvation dynamics of forma ldehyde in water are being completedby us for comparison with the nonequilibrium simulations reportedin the present paper.

~

(38) Bader, J.; Chandler, D. Chem. Phys. Lett. 1989, 157, 501.

The present study of the time dependence of the fluorescenceshift for formaldehyde in water complements ou r recent study ofhydration effects on the absorption line shape of this molecule.'6The choice of formaldehyde was dictated in part by the methodswe used to parame terize the exc ited-state potential surface .1s-"In the context of time-resolved simulations, we regard form-aldehyde as repre sentative of a class of small solute molecules tha thave a sufficiently large change in the dipole moment upon ex-citation to couple the solvent response to the fluorescence. Theprobes of choice for experimental time-resolved-fluorescencestudies such as the coumarin dyes39q40or bis(C(dimethy1-amino)phe nyl) sulfone (DMA PS),I tend to have much largerdipole moment changes upon excitation than does formaldehyde,but they are also considerably larger polyatomics. Since the detailsof the solute shape and a tomic charges control which of severalpossible solvent relaxation me chanisms is domin ant, it is of con-siderable interest to carry out simulations on correspondingpolyatomic dyes for which nonequilibrium solvation measurementsare available . Work along these lines is i n progress in our lab-oratory.

Acknowledgment. This work has been supported in part byNational Institutes of Health Grants GM-30580 (to R.M.L.) andGM-34111 to K.K.J.) and by the donors of the Petroleum Re-search Fund, administered by the American Chemical Society.D.B.K. and J.T.B. have been supported in part by Sup ercomputerPostdoctoral Fellowships from the New Jersey Commission onScience and Technology. D.B.K. is the recipient of a NationalInstitutes of Hea lth N RS A fellowship. W e also acknowledge agrant of supercomputer time from the John Von Neumann Center.

Registry No. HC OH, 50-00-0.(39) Maroncelli, M.; Fleming, G. R . J . Chem. Phys. 1987, 86 , 6221.(40) Kahlow, M. A,; Kang, T. J . ; Barbara, P. F. J . Chem. Phys. 1988,88,(41) Su, S. G. ; Simon, J . D. J . Phys. Chem. 1987, 91 , 2693.2312.

Gas-Phase Inorganic Chemlstry: Laser Spectroscopy of Calcium and StrontiumMonopyrrolate Molecules

A . M . R . P. Bopegedera,+W. T. M . L. Fernando, and P. F. Bernath*v*Department of Chemistry, University of Arizona, Tucson, Arizona 85721 (Received: November 8, 1989;I n Final Form: January 15, 1990)

The gas-phase calcium and strontium monopyrrolate molecules were synthesized by the direct reaction between the metalvapor and pyrrole. The electronic and vibrational structures of these molecules were probed by low-resolution laser techniques.The s pc tr a are _consistentwith a, ring-bonding, ionic, M+(C 4H4 N)- tructure of pseudo-C5, symmetry. The assignmentsof the A2El(l/2,-XZA1, 2E l(3/2)-X2Al, nd B2AI-X2Al electronic transitions were made by analogy to the isoelectronic metalmonocyclopentadienide molecules, CaCSH5 nd SrC5H5.

IntroductionAlthough cyclopentadienyl compounds a re very common, th eisoelectronic pyrrolyl derivatives ar e rare.' Only a few moleculessuch as (CSHS)Fe(C4H 4N)*nd (C4H4N)Mn(C0):v4 have beencharacterize d. Th e neutral pyrrole molecule can also serve as aligand, for example, [C4H4NH]Cr(C0)3 .5Very recently, the repla cemen t of hydrogen atoms by m ethylgroups was found to increase the stability of pyrrolyl-metalcomplexes analogous to the effect in cyclopentadienyl-metalcomplexes.6 This strategy allowed the preparation of a derivativeof 1, l '-diazaferrocene,' [C,(CH3),N],Fe[C4(CH,),NH],.'Current address: NO AA, ERL, R/E /AL 2,3 25 Broadway, Boulder, CO*Alfred P. Sloan Fellow; Camille and Henry Dreyfus Teacher-Scholar.80303.

0022-3654/90/2094-4416$02.50/0

We report the gas-phase synthesis and laser spectroscopiccharacterizat ion of Ca(C,H,N ) an d Sr(C,H,N). These freeradicals are isoelectronic with the CaC 5H 5 nd SrC 5H 5molecules(1 ) Cotton, F. A. ; Wilkinson, G . Advanced Inorganic Chemistry, 5th ed.;Wiley-Interscience: New York, 1977; p 1180.(2) Joshi, K. K.; Paulson, P. .; azi, A. R.; Stubbs, W . H . J . Organo-metal. Chem. 1964, I , 471-475.(3) King, R . B.; Efraty, A. J . Organometa/. Chem. 1969, 20 , 264-268.(4 ) (a ) Ji , L.-N;ershner, D. L.; Rerek, M. E.; Basolo, F. J . Organomera/.Chem. 1985, 296. 83-94 . (b) Kershner, D. L.; Rheingold, A. L.; Basolo, F.Organometallics 1967, 6 , 196-198.( 5 ) dfele, K.; Dotzauer, E. J . Organomeral. Chem. 1971, 30 , 21 1.(6) Kuhn, N. ; Horn, E.-M.;auder, E.; BIBser, D.; Boese, R . Angew.Chem. , n t. Ed. Engl. 1988, 27, 579-580.(7 ) Kuhn, N .; Horn, E.-M.; Boese, R .; Angart, N . Angew. Chem., Int. Ed.Engl . 1988, 27 , 1368-1 369.

0 99 0 American Chemical Society


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Gas-Phase Inorganic Chemistry The Journal of Physical Chemistry, Vol. 94, No. 1 I , 1990 4411

I I680 720 nmFigure 1. The resolved fluorescence spectrum of the A2E1-%2A1 ran-sitio! of calcium monopyrrolate. The laser is tuned to the-0-0 band ofthe A2El(l12)-X2A,pin component. Colli_sionsconnect the A2EI(,/,) pincomponent to the other spin component, A2El(3/2), hich is =76 cm-l tothe blue of the laser. The wavelength of the laser is marked with anasterisk. The broad features which are =300 cm-l to the red and to theblue of the laser are assigned to progressions in the metal-ligandstretching vibration.previously discovered i n ou r laboratory.8Experimental Details

The gas-phase reaction between the alkaline-earth metal (Ca,Sr) and pyrrole was used to synthesize the calcium and strontiummonopyrrolate molecules. The experimental arrangement wassimilar to the configuration in our previous work in this area.8-16The metal was vaporized in a Broida oveni7 by resistive heatingand carried to the reaction region entrained in argon carrier gas.Ground-state metal atoms were found not to react with pyrrole.Ca and Sr atoms were, therefore, excited to th e 3P I tate by usinga Coherent 599-01 broad band dye laser tuned to the 3PI-'Soatomic transition (6573 8, or Ca , 6892 8, or Sr). Pyrrole hasa low vapor pressure at room temperature (boiling point = 13 1"C). I n order t o obtain a sufficient flow of pyrrole into the Broidaoven, it was necessary to bubble argon through the glass cellcontaining pyrrole. The total pressure in the oven was about 4Tor r of mainly argon carrier gas. Th e direct reaction betweenthe excited m etal vapor a nd pyrrole vapor produced th e metalmonopyrrolates. Analytical grade pyrrole (Alfa Chem ical) wasused without further purification.A Coherent 599-01 broad band dye laser operated with DCMor Pyridine 2 laser dyes was used to probe the molecular tran-sitions. Laser excitation s pectra were recorded by scannin g thewavelength of this laser and recording the total fluorescence fromthe electronically excited metal monopyrrolate molecules usinga photomultiplier-filter combination. Sch ott RG 9 and RG 780red-pass filters were used to block the scattered laser radiation.The laser resonant with the molecular transition was chopped andthe modulated signal was detected with a lock-in amplifier.Once the frequencies of the two electronic transitions were

(8)O'Brien, L. C.;Bernath, P. F. J . Am. Chem. SOC.1986, 108,(9)Brazier, C. .; Bernath, P. F.; Kinsey-Nielsen, S.; llingboe, L. C. J .Am . Chem. Soc. 1986, 108,2126-2132.(IO) Ellingboe, L.C.; Bopegedera, A . M. R. P.; Brazier, C. R.; Bernath,P.F.Chem. Phys. Lett. 1986, 26, 285-289.(I I ) Brazier, C. R.; Bernath,P. F. J . Chem. Phys. 1987,86,5918-5922;also, J . Chem. Phys. 1989, 1, 4548-4554.(12) opegedera, A. M. . P.; razier, C. R.; Bernath, P. F. Chem. Phys.Lett. 1987, 36, 97-100 J . Mol. Spectrosc. 1988, 129, 268-275.(13)Bopegedera, A. M. R. P.; Brazier, C. R.; Bernath, P. F. J . Phys.Chem. 1987, 1, 2719-2781.( I 4)OBrien,L.C.; razier, C. R.; Bernath, P. F. J . Mol. Spectrosc. 1988,( 1 5 ) Brazier, C. R.; Bernath, P. . J . Chem. Phys. 1988,88, 112-2116.(16)OBrien, L.C.; ernath, P. F. J . Chem. Phys. 1988,88,2117-2120.(17)West , J. B.; Bradford, R. S; versole, J. D.; Jones, C. R. Reu. S ci .(18)Herzberg, G. lectronic Spectra of Polyatomic Molecules; Va n

5017-501 8.

130. 33-45.

Instrum. 1975,46, 64-168.Nostrand Reinhold: New York, 1966.

0;o

C a f - N 3B2~,-iZA,It-.--

I I680 710 nmFigure 2. The resolved fluorescence spectrum of th e fiZAl-%2Alran-sition of calcium monopyrrolate. The asterisk marks the position of thelaser tuned to the 0-0 band. A progression of vibronic bands in themetal-ligand stretch are clearly seen in the spectrum. The sharp featurejust to the left of the 0-1 band is the Sr 3P1-1S tomic line. Sr presentas an impurity in the Ca metal and is, presumably, excited by energytransfer from the excited calcium monopyrrolate.TABLE I: Band Origins for the Observed Vibronic Transitions ofCalcium and Strontium Monopyrrolate Molecules (in cm-')

band Ca(C4H4N) Sr(CdH4N). . . . . . .A2El(lp)-%2A,2-0 14830 13 6861-0 I4 584 13 449

0-0 I4333 132120- 14021 129580-2 13714 127060-3 124550-4 12207

2- 01-00-00-0-20-0-42-01-00-00-0-20-30-4

146501440914 09713786

1473214419I4 IO813 799

13770135121325713 0041214912 497

14 0691384913 62013367I3 11612 86312612

obtained from laser excitation spectroscopy, the laser probing themolecular transitions was tuned to the frequency of the electronictransitio n. Resolved fluorescence spectr a were reporte d by dis-persing the laser-induced fluorescence with a 2/3-m monochro-mator equipped with a GaAs photomulitiplier tube and photon-counting electronics.Results and AnalysisSome sample low-resolution laser excitation spectra an d resolvedfluorescence spectra recorded for the C a(C 4H4 N)and Sr(C,H4N)TolecuJes are givsn in tigu res 1-3. Two electronic transitions,A2EI-X2Al and B2A I-XZAI,were observed for the metal mon-opyrrolates. No te that these transitions are labeled by analogywith the CaC5H5 nd SrC5H5molecules, using the orb ital sym-metries of the C Su oint group, as will be discussed below. Th eband origins of the observed vibronic transitions are given in Tab leI. Figure 1 shows the A2EI-W2AI ransition of Ca( C4H 4N ). Thisresolved fluorescence spectrum was recorded by tuning the dyelaser probing the molecular transition to the 0-0 band of theA2E l(l,2)-X2A , spin component. Th e position of the laser ismarked with an asterisk. The strong feature 76 cm-I to the blue


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4478 The Journal of Physical Chemistry, Vol. 94 , No. 11 . 1990 Bopegedera et al.

I I I I I7200 7400 76b0Figure 3. The laser excitation spectrum of th e strontium-monopyrrolate molecule. Two electronic transitions, A 2 E I - g 2 A 1 nd B2A 1-g2A 1 ,were observedand assigned. The two spin components of the A 2El -X 2A l transition are separated by 30 0 cm-I. The assignments of the vibronic bands were aidedby resolved fluorescence spectra recorded for each vibronic tran sition.TABLE 11: Vibrational Frequencies of the Metal-Ligand Stretch ofthe Calcium and Strontium Monopyrrolate Molecules (i n cm-')

Alkaline Earth Monopymlatesstate Ca(C4H4N) Sr(C4H4N)

ZZA, 31 1 253A2E(I,Z, 249 237A2EI(,,Z, 24 1 258B2A 225of th_e laser is th e A 2E1(3 /2)-% zAIpin component. Emission fromthe A2E l(3, 2) pin compone nt occur_s because collisions tran sferpopulation from the laser-excited A2E,(l/2)pin com_pone$Figure 2 is a resolved fluorescence spectrum of the B2A I-X2 A1transition of calcium m onopyrrolate. Th e asterjsk marks theposition of the laser, tuned to the 0-0 band of the B-X transit ion.In both Figures 1 an d 2, several broad features ar e seen to thered of the laser. When th e laser excites the 0-0 band of a givenelectronic transition, emission to excited vibrational levels of theground electronic state occurs. Only the metal-ligand vibrationalmodes were found to be Franck-Condon active in the electronicspectrum. Therefore, the observed progression, separated by ~ 3 0 0cm-I, is assigned to the metal-ligand stretching vibration. Thesespectra were very useful in obtaining the vibrational frequenciesfor the ground an d excited electronic states, which a re reportedin Table 11.

Figure 3 is a laser excitation spectrum of the strontium mon-opyrrolate molecule, recorded by scanning the wavelength of th edye laser probing the molecular transition. Th e assignmen tsprovided on this figure were made by using the laser excitationspectrum as well as several resolved fluorescence spectra recordedb_y individual vibronic-transitions. Two electronic trans ition s,A2E I-X2A I nd f% 2Ai-X2A1, ere observed and assigned as shownon the figure. The band origins of these trcnsitions ar_egiven inTab le I. The two spin components A2El ( l /2 , -X2 A1 ndA2EI(3/2!-%2Al ar e separated by 300 cm-I. Long vibrationalprogressions in the metal-ligand stre tch were observed for thestrontium monopyrrolate molecule (see Table I ) . The metal-ligand stretching frequencies of Sr(C4H4N)molecule-were ex-Lracted from the low-resolution spectra for the X ZA I,AZEl , ndB2Al elect_ronic states. These are rep prte djn Table 11. TheA 2 E l ( ~ ~ 2 ) - X Z A lransition and the B2A l-X2 A1ransition ofstrontium monopyrrolate are separated by only = I O 0 cm-'. Asa result, there are several vibronic bands of these two electronictransitions tha t are close to one another. Therefore, it is notpossible to selectively excite one of these bands with a dy e laser,without simultaneously exciting others which ar e close by. Thismade the assignment of spectra more difficult for the strontiummonopyrrolate molecule than for the calcium monopyrrolatemolecule.Discussion

Figure 4 hows the two possible ways in which the pyrrolateligand (C 4H4 N)- can bond to the alkaline-earth metal. Thebonding can be through the N atom, i n which case the metalvi-monopyrrolates will have C symmetry, or the pyrrolate anioncan ring-bond to the metal to produce metal qs-monopyrrolates

C 2 Vq -nitrogen bonding

HHM-N:

q5-ring m i n gFigure 4. Possible ways in which the pyrrolate anion (C4H4N)- an bondto the alkaline-earth-metalcation an d the subsequent symmetries of themetal monopyrrolate molecule.of pseudo-C5, symmetry. If the bonding is through the N atom,the resulting metal monopyrrolates should be similar to the metalmonoamides (MN H2 ) and the metal monoalkylamides (MN HR ,R = alkyl group) which have been-studitd prevjouslyt3 In thiscase, three electronic tran sitions, A2B2-X2Al, BZB1-XZAi, ndC2Al-X2Al,will be observed for the metal monopyrrolates similarto the metal monoamides. If, however, the pyrrolate ligand ringbonds to the metal, the metal monopyrrolates will closely resemblethe isoelectronic metal cyclopentadienides, M( CS HS ),which areof C,, symmetry.* A close examination of the spectra indicatestha t of the metal monopyrrolates are similar to those of the metalcyclopentadienides. Like the cyclopentadien ide derivatives,calcium and stron$m monopyrrolates seem to have two excitedelectronic states A ZEl an! B2AI, with- the A 2E l state split byspin-orbit interaction into A2El(i/2)nd A2EIQ ) spin components.Therefore, it was concluded that the (C4H4N/) igand ring bondsto the metal and the geometry of the M+ (C4H 4N)-molecule ispseudo-C,,.The C 4H4 N- on is a closed-shell ligand. The molecular orbitalsof the metal monopyrrolates can be treated as the orbitals of theM+ ion perturbed by the C4 H4N - ligand. The C a+/ Sr+ ioncontains a single unpaired electron in the ns ( n = 4 or Ca, n =5 for Sr) valence orbital. In the C,, point group, the ns orbitalis of a l symmetry resulting in a 2A l ground electronic sta te forthe M(C 4H4N )molecules, as shown in Figure 5. Although theM(C4 H,N) molecule is of C, symmetry, the molecular orbitalsare described by using the symmetry symbols of the C,, pointgroup. There is such a strong correspondence between the spectraof M ( C 4 H 4 N )and M(C,H ,) molecules th at this is useful.The degeneracy of the metal d orbitals is lifted in th e C,, pointgrou p to produce o rbita ls with s ymm etrie s e2(dxz-yz,dxy). l(dx,,d,,), and al(dz z). The degenera te p orbitals of the metal provideel(p,,p,) and al(p,) m olecular orbitals. Th e presence of the ligandalso mixes the metal p and d orbitals so these molecular orbitals


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Gas-Phase Inorganic Chemistry The Journal of Physical Chemistry, Vol. 94, N o. 11. 1990 4479~ .........2A1

E l. ._ __.__

-ns .____________c+-X--- _ _ _ _?A-M+ M+WH M + - N ~1Cv Pseudo csvFigure 5. Correlation diagram for the orbitals of the M+ ion perturbedby the (C4 H4N )- ligand. Although the (C4H 4N)- ligand is of C sym-metry, the notation given here is from the C point group (see text).

are p-d mixtues. Th e above molecular orbitals give rise to the2E2,2 E l ,2Al , ZEl , nd 2AI electronic states (in the order of in-creasing energy) for the M( C4 H4 N) molecule as illustrated inFigure 5 .Electronic transitions from the R 2A I ground state to the 2E2sta te ar e forbidden by the electric dipole selection rules. No tethat the lower symmetry (C,) of the M(C4H4N )molecule couldmake the 2E2-2Al transition allowed but no evidence of thistransition was found. Therefore, the first observed-electrpnictransition of M(C4H4N)molecules is assigned as th e A2E1-X 2A1transition. The spin ang ular mom_entum and th e unquenchedorbital angular mom entum in the A 2E l $ate couple together toprodu ce the two spin-orbit comp onen ts A2E1(112)nd A2E1(3/2).These two spin components ar e separ ated by the spin-orbit cou-pling constant A . For the calcium monopyrrolate molecule A is76 cm-I, whereas for the stro ntium analogue it is 300 cm-l. Thisis slightly higher than the values reported for m etal monocyclo-pentadienides ( A = 57 cm-l for Ca(C,H,) and 255 cm-I forSr(C,H,)2) and the metal monohydroxides ( A = 67 cm-I forCaOH and 264 cm-I for SrOH19*20)ut not unreasonable for theCa' (Sr') ion perturbed by ligands of C,,, C3,,or C,, symmetry.These observations are closely related to the often successfulsemiempirical estimation of diatom ic spin-orbit coupling constantsfrom the corresponding ato mic values (for exam ple, ref 23). TheA sta te spin-orbit splittings of several akaline earth me tal con-taining free radicals are reported in Table 111 for co-mparisonpurposes. Th e observation of spin-orbit sp litting in the A2E l stat eof the m etal mon opyrrolates confirmed t ha t these free radicalseffectively have high symmetry, most probably pseudo-C,,.

(19) Brazier, C. R.; Bernath, P. F. J. Mol. Specrrosc. 1985. 114, 163-173.(20) Bernath, P. F.; Brazier, C. R. Astrophys. J . 1985, 288, 373-376.(21) Bernath, P. F. ; Field, R. W J . Mol. Spectrosc. 1980, 82, 339-347.( 2 2 ) Steimle,T. C.; Domaille, P. J. ; Harris, D. 0. J. Mol. Specrrosc. 1978,(23) Lefebvre-Brion H.; Field, R. W . Perturbations in rhe Spectra of73,441-443.Diatomic Molecules; Academic Press: Orlando, FL, 1986; p 216.

TABLE 111: Observed Spinarbit Splittings for the A*II or .&*EStates of Some Alkaline Earth Metal Containing Free Radicals, ML(i n cm-I), Where M = Ca or Sr and L Is a Lieand~~ ~molecule CaL Sr L

M Fa 73 28 1M O H b 6 1 2 64MOCH3' 65 268M C C H ~ 70 275MN3' 76 296MNCO' 68 293MCH jz 73 309M C , H , ~ 57 255MC4H4N' 76 300

References 9 and14. dRe feren ce 12. 'Reference 15. /References 10 and 16.ZReference 1 1 . *Reference 8. 'This work.a References 2 1 and 22. References 19 and 20.

The higher energy electronic transition observed is assigned asthe B_2Al-X:Al transitio n. For the metal monopyr rolate molecules,the A and B electronic statEs are relatively close in energy (seeTable I) compared to the A and B states in the metal cyclo-pentadienides.8 This tends to complicate the spectra of the m etalmono pyrro lates, part!cularly those of the _strontiummonopyrrolatemolecule where th e B2A l state an d the A 2El(s,2) pin componentare separated by only -100 cm-I. A similar problem was en-countered when assigning the spectra of strontium monocyclo-pentadienide for which the spin-orbit coupling constant m atchedthe metal-ligand stretching vibratonal frequencya8 The assign-ments provided here were mad e by comparison of the spectra ofthe Sr (C4H4N )and SrC5HSmolecules.Th e electronic transitions of the metal monopyrrolate moleculesoccur on metal-centered orbitals so the Franck-Condon factorsfavor those vibrations that are associated with the metal atom.Therefore, the vibronic bands reported a re all assigned to a singlemetal-ligand stretching vibration. Progressions in this vibrationalmode were observed for both molecules in all of the electronictransitions reported here. Th e observation of a progression in themetal-ligand mode suggests tha t the metal-ligand bond distances(cf. vibrational frequencies, Table 11) ar e different in th e variouselectronic states. Th e reported frequencies have an estimateduncertainty of f 1 O cm-I.ConclusionIn our continuing study of the gas-phase inorganic chemistryof Ca and Sr,we have discovered the Ca(C 4H4 N)and Sr(C4H4N)molecules. These monopy rrolate free radicals were found to bering-bonding (1,) ike the isoelectronic monocyclopentadienylmolecules, rather than nitrogen-bonding ( V I ) like the mono-alkylamide derivatives. Our assignments of the spectra rest largelyon the comparison between the electronic spectra of Ca (C4 H,N )and Sr(C 4H 4N ) with the isoelectronic C aC5H s and SrC,H,molecules.

Acknowledgment. This research was supported by the NationalScience Foundation (CHE-8608630).

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A. M. R. P. Bopegedera et al- Gas-Phase Inorganic Chemlstry: Laser Spectroscopy of Calcium and Strontium Monopyrrolate Molecules