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J . Phys. Chem. 1987, 91, 2779-2781Laser Spectroscopy of Strontlum and Calcium Monoalkylamides

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A. M. R. P. Bopegedera, C. R. Brazier, and P. F. Bernath*Department of Chemistry, University of Arizona, Tucson, Arizona 85721 (Received: D ecember 19 , 1986)

The reaction of stron tium and calcium vapors with primary aminzs was studjed in the g_as phase. These reactions_producethe metal monoalkylamides Ca NH R and S rN HR . The C2Al(2A)-X2AI(2A), 2Bl(2A)-X2Al(2A),nd A2B2(2A)-X2Al(2A)electronic transitions were observed by laser excitation spectroscopy. Som e Ca-N and Sr-N stretching frequencies wereassigned from the electronic transitions.

IntroductionRecently, we have explored th e reactivity of Ca , Sr, and Bavapors with a variety of o rganic m olecules including alcohols,s2aldehydes and ketones,2 carboxylic acids, th i~ et he rs ,~N C 0 , 4and C5 Hs.5 The reaction products are a variety of novel gas-phasefree radicals containing one metal atom and one ligand. In thispaper we report our work on the reactions of Ca and Sr withalkylamines to produce the monoalkylamides MNHR (R = H ,CH3, CH2CH3, CH(CH J2, C(CH3)3).The only previous work on gas-phase metal amides was carriedou t by D. 9.Har ris and co-workers. They rotationally analyzedthe C2Al-X 2AI transition of CaNH: and observed chemilumi-nescent emission f r o p SrNH 2_and C_aNH2.7 W e have recentlyanalyzed the B2Bl-X2Al and A2B2-X2Al transitions of S T N H ~ . ~Gas-phase metal am ide molecules have been explored throughquantum chemical calculations of the structure of LiNH2.9 Theproduction of Li NH 2 was suspected in t he photoinduced reactionof Li with NH 3 n a rare gas matrix.I0 Finally, the reactions ofmetal ions with amines were studied by Babinec and Allison byion cyclotron resonance.There is a well-developed inorganic chemistry of solid-statemetal amides.12 Crystals of compounds such as LiN H2 ,Mg(N-H 2)2 r nd C ~ ( N - B U ~ ) ~ave been synthesized.I2

Experimental SectionThe monoalkylamides were prepared in a Broida-type ovenI3by the reaction of Sr and Ca metal vapor with the appropriateprimary amine (ammonia, monomethylamine, monoethylamine,isopropylamine, and tert-butylamine). The metal was vaporizedfrom a n electrically heated alumina crucible and entrained in argoncarrier gas. The total pressure was approximately 1 .5 Torr, witha partial pressure of a few milliTorr of the amine.(1) Brazier, C. R. ; Bernath, P. F.; Kinsey-Nielsen,S . ; Ellingboe, L. C. J .(2) Brazier, C. R. ; Ellingboe, L. C.; Kinsey-Nielsen,S . ; Bernath, P. F. J .(3) Ram, R. S.; Bernath, P. F. , unpublished results.(4) Ellingboe, L. C.; Bopegedera, A . M . R . P.; Brazier, C. R.; Bernath,(5) OBrien, L. C.; Bernath, P. F. J. Am . Chem. SOC. 986, 108, 5017.(6) Wormsbecher, R. F. ; Penn, R. E. ; Harris, D . 0. . Mol. Spectrosc.1983, 97 , 65 .(7) Wormsbecher, R. F. ;Trkula, M .; Martner, C .; Penn, R. E.; Harris, D .

0 . . Mol. Spectrosc. 1983, 97 , 29 .(8) Brazier, C. R. ; Bernath, P. F., in preparation.(9 ) Dill, J . D.; chleyer, P. v. R.; Binkley, J. S.;Pople, J . A. J. Am. Chem.SOC. 977, 99 , 6159. Hinchliffe, A .; Dobson, J. C. Theor. Chim. Acta 1975,39 , 17 . Hinchliffe, A. Chem. Phys. Lett . 1977, 45 , 88. Wurthwein, E.-U.;Sen,K. D.; Pople, J. A.; Schleyer, P. v. R. Inorg. Chem. 1983, 22 , 496. Sapse,A.-M .; Kaufmann, E. ; Schleyer, P. v. R. ; Gleiter, R. Inorg. Chem. 1984, 23 ,1569.(IO) Meier, P. F.; Hauge, R. H.; Margrave, J. L. J . Am. Chem.SOC. 978,100, 2108.( 1 1 ) Babinec, S. J .; Allison, J . J. Am. Chem. SOC. 984, 106, 7718.(12) Lappert, M . F.; Power, P. P .; Sanger, A. R.; Srivastava, R . C . Metal( 1 3 ) West, J . B.; Bradford, R. S.;Eversole, J . D.; Jones, C . R . Reu. Sci.

Chem. Phys. 1985, 8 2 , 1043.Am . Chem. SO C. 986, 108, 2126.

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

and Metalloid Amides; Ellis Horwood: Chichester, U. K., 1980.Instrum. 1975, 46, 164.

T A B L E I: Band Centers of the Strontium Monoalkvlamides (in cm)molecule A2B2(,A) B2B1(2A ) e2A 1(2A )

SrNH, 14274 14 724 15862SrNHCH3 I 4 1 7 0 1 4 6 8 8 aSrNHC,H, I 4 I 6 6 14 641 1586 7SrNHCH(CH3), 1 4 1 3 5 - 1 4 6 2 3 - 1 5 8 8 5SrNHC(CH,), - 1 4 1 3 0 -14600 -15895Obscured by SrNH2. b f l O O cm-l , very wide peak.

TA BLE 11: Band Centers of the Calcium Monoalkvlamides (in cm-)molecule A 2 ~ , ( 2 ~ f ) B 2 ~ 1 ( 2 ~ / ) 2 ~ , ( 2 ~ i )

CaNH, 1 5 6 0 5 1580 2 17 364CaNHCH3 15 338 - 5 690 bCaNHC2H5 15320 -155625 bCaNHCH(CH,), 15 298 - 5 590 - 7 499CaNHC(CH& 1 5 2 4 2 - 1 5 5 5 0 - 1 7 4 9 7Obscured by CaH. b ob scu red b y CaNH,Two CW broad-band (1 cm-I) dye lasers pumped by CoherentInnova 20 and Coherent Innova 90 argon ion lasers were usedfor the experiment. The dye lasers were operated with DCM andR6 G dyes. On e dye laser was used to excite the 3Pl-1So tomictransition of strontium (6892 A) or calcium (6573 A), while the

wavelength of th e second dye laser was scanned to record the laserexcitation spe ctra of the metal monoalkylamides. For the exci-tation spectra, only the fluorescence to the red of both lasers wasdetected by a filter-photomultiplier comb ination. The laser re-sonant with the molecular electronic transition was chopped forlock-in detection.The laser-induced fluorescence was also dispersed with a smallmonochromator, but these experiments were not very informative.Collisional and, possibly, intramolecular relaxation is sufficientlyfast for these molecules that almost all of the fluorescence comesfrom the lowest excited state.Results and Discussion

Portions of the laser excitation spectra of the SrNHR andCa NH R free radicals ( R = H, C H3, C2H5,CC 3H7 , -C4H,) areprovided in Figures 1 and- 2, re_spectiv_ely. Three elec_tronic$ansitions were observed, A2B2-X2Al,B2Bl-R2Al , nd C2A,-X2Al, nd the band centers are recorded in Tables I and 11. Th etransitions are labeled with t he irreducible representations of th eC2, point group, although th e actual point group for the m etalmonoalkylamides is C,.There is such a strong correspondencebetween the spectra of CaNH, (SrNH,) and CaNHR (SrNHR)tha t this is useful. A similar correspondence was previouslyobserved between the spectra of CaOH (SrOH) and CaOR(SrOR ), so that the C,, point group was retained for the alkalineearth monoalkoxide molecules.2The alkaline ear th m onoamide spe_ctra resemble the corre-sponding mon?hydro_xides. The B2Z+-X2 8 transition of Ca_OHI4becomes the C 2A1-X 2A, ransition in C aN H 2, while the A211-(14) Bernath, P. F.; Kinsey-Nielsen, S. Chem. Phys. Lett . 1984, 105, 6 6 3 .

0022-3654/87/2091-2779.$01.50/0 0 1987 American Chemical Society

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2780 The Journal of Physical Chemistry, Vol. 91, No. 11 , 1987A%*- f'A1

Bopegedera et al.

sr",-'soI

/

/kP"-Ido0 mbo ' I

Figure 1. Excitation spectra of strontium monoalkylamides. The A-Atransition is on the right-hand side, and the 8-3 is on the left-hand side.The strontium 3PI-1Sotomic transition (6892 A) is-qrk ed. The centralpeak is assigned to the 1-0 vibronic band of the A-X transition.x 2 Z + s corre la tes to the AZ B2 -~ ZA 1nd B2BI- g2Al transitionsof CaN H 2 (Figure 3) .

Th e observed electronic states for the alkaline eart h monoam idescan be explained with the aid of t he correlation diagrams in Figures3 and 4. The alkaline ea rth monoalkylamides are ionic moleculeswell represented by the charge distributions Ca+ NH R- andS r + NH R-.The valence ns, (n- l)d , and np atomic orbitals of the M+ iongive rise to the electronic states shown in F igure 3 for the C*, pointgroup (when the ligand is OH-). The ligand also mixes the p r e n tatomic orbitals of the states so that for the hydroxides the B2Z+an d A211 states are po-dc and pn - d r mixtures.16 The exactlocation of th e *A state and the higher lying 2Z+an d 211statesis unknown. When the symmetry is lowered to C (ligand isNH,), the in-plane p/d orbitals and the outs f-pla ne p/d orbitalsa re co longer degenerate. Therefore, the AZI Istate splits intothe B2B, (out-of-plane) and A2B2 in-plane) states. In addition,one component of the forbidden 2A-zZ+ (Cmo oint group) tran -sition becomes allowed (2A1-zA1 n the C point group) bu t wasnot experimentally observed. When the symmetry is loweredfurther from C,, (-NH2) to C, -NHR), the number of statesremains unchanged but now all the states can be connected byelectric dipole allowed transitions (F igure 4) .Experimentally, only three electronic transitions are-known forC aN F , and-SrNH,. The planarity of Sr NH 2and the B2B,-g2Aland A2B2-X2Al assignments wsre proven by a high-resolutionrotational anal ysk 8 The C2A I-X2Al assignment for Ca NH 2was(15) Hilbrn , R. C.;Quingshi,Z.;Harris, D. 0. J . Mol. Spectrosc. 1983,9 7 , 73 . Bernath, P. F.; Brazier, C. R. Astrophys. J . 1985, 228, 373.(16) or the CaX (X = F,C1,Br, I) molecule this mixing is discussed in:Dagdigian, P. J.; Cruse, H. W. ; Zare, R.N . J . Chem. Phys. 1974, 60, 2330.Bernath P:F. Ph.D. Thesis, MI T, 1980. Rice, S. F.; Martin, H.; Field, R .W . J. Chem. Phys. 1985.82, 5023.

rjoo d o 0 0,'oo 1Figure 2. Excitation spectra of calcium monoalkylamides. Th e asterisksmark the B2Z+-XZZ+ transition of the CaH molecule. Note that forCaNH2, CaNHCH2CH3, nd CaNHC(CH3)3Ca H is not observed. Th ecalcium 3PI-1S0tomic transition (6573 A ) is marked.

C o r r e l a t i o n D i a g r a m

2f3*0,TT---c,--

.2np

-2

ns --'s -_________A1

M+-- /HN\"M+ Mf- 0-- H

c m " c2vFigure 3. Correlation diagram fo r th e M+ ion (M = Sr, Ca) perturbedby a linear OH- ligand (C-" symmetry) an d NH,- ligand (C, symmetry).made by Wormsbecher, Penn, and Harris6 from their rotationalanalysis. All other assignments are made by analogy. The ,A,an d 2A2(2A ' nd 2A'') s tates which correlate t o th e ,A state ofth e MO H molecule were not observed.-

As the alkyl group becomes larger, the A -4 and fI-2 transitionsshift to the red (Figures 1 an d 2) , while the C-X transitions shiftslightly t o the blue (Tables I and 11). The signal-to-noise ratioalso decreases as the vapor pressure of the parent amine decreasesand th e product molecules become more difficult to make. Thesha rp features in the SrNH2and CaN H, spect ra are sub-bandheads. For the heavier Sr-containing compounds, th e Sr atomiclines become prominent. When CH ,NH 2 and CH 3C H2 NH 2 ere

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Strontium and Calcium Monoalkylamides The Journa l of Physical Chemistry, Vol. 91 , No. 1 1 , 1987 2781CORRELATION DIAGRAM

C'"M""+ -4

c,Figure 4. Correlation diagram for Ca+ an d Sr+with NHC and NHR-(C, symmetry) ligands.used as oxidants with Ca, very strong spectra of CaH(B22+-X2Z+) appeared (Figure 2, marked with asterisks). Man yof the alkylamine oxidants also produced some Ca N H 2 andSrNH, . The SrOC(CH3)3molecule appeared in the S rN HC -(CH,), spectra, possibly from a tert-butyl alcohol impurity in thetert-butylamine oxidant.The S r-NH 2 vibrational frequency (459 cm-') matches the450-cm-' splitting bztween the B2BI and A 2B2electronic states,so t_he v = 0 of the B2B1state is extensively perturbed by v = 1of A2B2. As the ligand becomcs heavier, the Sr-N stretchingfrequency decreases but the &A electronic separation remainsthe same . There fore, we assign the centra! peak in the scans ofFigure 1 to the 1-0 vibronic band of the A-X transitio n. Thiscentral feature could also be assigned to a n additional electronictransition (for example 2A1-2Al, Figures 3 and 4), but we do notfavor this assignment. No te that relative intensities of the featuresin Figures 1 and 2 may not be reliable because the red pass filterenhances some fea tures (and_m$lecules) in the excitation spectrum.The intensity of the 1-0 A-X vibronic transitio n may also be

TABLE II: Meta4-Nitrogen Stretching Frequencies of Strontiumand Calcium Monoalkylamides (in em-')M Sr M = Ca

molecule % A B t % AMNH2 459 -450 458 524 520MNHCH3 393 387 480 467MNHC(CHj)3 - 92MNHCIH5 -331 -337 -315

enhanced by the 8(v=O)-A(v= 1) intera_ction.For the C aN H R molecules, the B2B1-X2A1 ransition is "_eve_'as clear as for the corresponding S r N H R molecules. Th e B-Ainterval for Ca N H R is about 300 cm-I, compared to the -500-cm-I interval for SrNHR.The Ca-N and Sr-N stretching frequencies, which were mainlyobtained from the laser excitation spectra, were difficult to measurefor the larger molecules (Table 111). The laser-inducedfluorescence was very relaxed, and vibrational bands were notclear. Thi s is in contrast to the corresponding alkoxides,, w hereeven for the SrOC(CH3), radical the Sr-0 stretch appearedclearly in the spectra. The Sr-N and Ca-N frequencies of themetal monoalkylamides ar e all less than th e corresponding Sr-0and C a- O stretching frequencies of the metal alkoxides, indicatingtha t the force constants ar e smaller. The weaker force constantsfor the metal alkylamides suggest that the M -N HR bond dis-

sociation energies are smaller than the corresponding M-ORdissociation energies.An attempt was made to detect the CaN( CH & and SrN(CH,)2free radicals with the HN (C H3 ), oxidant. The reaction was verysluggish, and no alkylamide product was formed. Th e mainreaction products were the SrO CH 3 and CaO CH , impuritymolecules, not the desired dialkylamides.Conclusion

The CaNHR and SrNHR free radicals were produced byvapor-phase chemical Eeactions. -The laser excitation spectra ofthe C2AI-X2A1,B2B1-X2A1, nd A2B2-X2Alelectronic transitionswere observed. The spectra of other metal alkylamides can beobserved with our experimental methods.Acknowledgment. This research was supported by the NationalScience Foundation (Grant CHE-8608630). Acknowledgmentis made to the donors of the Petroleum Research Fund, admin-istered by the A merican Chemical Society, for partial support ofthis research.