A. M. R. P. Bopegedera, C. R. Brazier and P. F. Bernath- Laser Spectroscopy of Strontlum and Calcium Monoalkylamides

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J . Phys. Chem. 1987, 91, 2779-2781 2779 Laser Spectroscopy of Strontlum and Calcium Monoalkylamides A. M. R. P. Bopegedera, C. R. Brazier, and P. F. Bernath* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 (Received: December 19, 1986) The reaction of strontium and calcium vapors with primary aminzs was studjed in the g_as phase. These reactions_produce the metal monoalkylamides CaNHR and SrNHR. The C2Al(2A’)-X2AI(2A’), B2Bl(2A”)-X2Al(2A’), A2B2(2A’)-X2Al(2A’) and elect
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  J. Phys. Chem. 1987, 91, 2779-2781 Laser Spectroscopy of Strontlum and Calcium Monoalkylamides 2779 A. M. R. P. Bopegedera, C. R. Brazier, and P. F. Bernath* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 (Received: December 19, 1986) The reaction of strontium and calcium vapors with primary aminzs was studjed in the g_as phase. These reactions_producethe metal monoalkylamides CaNHR and SrNHR. The C2Al(2A’)-X2AI(2A’), 2Bl(2A”)-X2Al(2A’),nd A2B2(2A’)-X2Al(2A’)electronic transitions were observed by laser excitation spectroscopy. Some Ca-N and Sr-N stretching frequencies wereassigned from the electronic transitions. Introduction Recently, we have explored the reactivity of Ca, Sr, and Bavapors with a variety of organic molecules including alcohols,’s2aldehydes and ketones,2 carboxylic acids,’ thi~ethers,~NC0,4and C5Hs.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(CHJ2, C(CH3)3).The only previous work on gas-phase metal amides was carriedout by D. 9. Harris and co-workers. They rotationally analyzedthe C2Al-X2AI transition of CaNH: and observed chemilumi-nescent emission frop SrNH2_and C_aNH2.7 We have recentlyanalyzed the B2Bl-X2Al and A2B2-X2Al transitions of STNH~.~Gas-phase metal amide molecules have been explored throughquantum chemical calculations of the structure of LiNH2.9 Theproduction of LiNH2 was suspected in the photoinduced reactionof Li with NH3 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 LiNH2, Mg(N- H2)2r nd C~(N-BU~)~ave been synthesized.I2 Experimental Section The 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 an 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. (11) 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 (13) West, J. B.; Bradford, R. S.; Eversole, J. D.; Jones, C. R. Reu. Sci. Chem. Phys. 1985, 82, 1043. Am. Chem. SOC. 986, 108, 2126. P. F. Chem. Phys. Lett. 1986, 126, 285. and Metalloid Amides; Ellis Horwood: Chichester, U.K., 1980. Instrum. 1975, 46, 164.TABLE I: Band Centers of the Strontium Monoalkvlamides (in cm“) molecule A2B2(,A”)B2B1(2A’) e2A1(2A’) SrNH, 14274 14 724 15862 SrNHCH3 I4 170 14688 a SrNHC,H, I4 I6614 641 15867 SrNHCH(CH3), 14135 -14623 -15885 SrNHC(CH,), -14130-14600’ -15895 ‘Obscured by SrNH2. bflOO cm-l, very wide peak. TABLE 11: Band Centers of the Calcium Monoalkvlamides (in cm-’) molecule A2~,(2~’f) B2~1(2~/) 2~,(2~i) CaNH, 1560515802 17 364 CaNHCH3 15 338 - 5 690“ b CaNHC2H5 15320 -155625“ b CaNHCH(CH,), 15 298 - 5 590 - 7 499 CaNHC(CH& 15242 -15550-17497“Obscured by CaH. bobscured by 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 andR6G dyes. One dye laser was used to excite the 3Pl-1So tomictransition of strontium (6892 A) or calcium (6573 A), while thewavelength of the second dye laser was scanned to record the laserexcitation spectra of the metal monoalkylamides. For the exci-tation spectra, only the fluorescence to the red of both lasers wasdetected by a filter-photomultiplier combination. 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 andCaNHR free radicals (R = H, CH3, C2H5,CC3H7, -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. Thetransitions are labeled with the irreducible representations of the C2, point group, although the actual point group for the metalmonoalkylamides is C,. There is such a strong correspondencebetween the spectra of CaNH, (SrNH,) and CaNHR (SrNHR)that 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 earth monoamide spe_ctra resemble the corre-sponding mon?hydro_xides. The B2Z+-X28’ transition of Ca_OHI4becomes the C2A1-X2A, ransition in CaNH2, while the A211- (14) Bernath, P. F.; Kinsey-Nielsen, S. Chem. Phys. Lett. 1984, 105, 663. 0022-3654/87/2091-2779.$01.50/0 0 1987 American Chemical Society  2780 The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 A%*- f'A1 Bopegedera et al. sr ,-'so I / /k P - I do0 mbo 'I Figure 1. Excitation spectra of strontium monoalkylamides. The A-A transition is on the right-hand side, and the 8-3 is on the left-hand side.The strontium 3PI-1So tomic transition (6892 A) is-qrked. The centralpeak is assigned to the 1-0 vibronic band of the A-X transition. x2Z+ s correlates to the AZB2-~ZA1nd B2BI-g2Al transitionsof CaNH2 (Figure 3).The observed electronic states for the alkaline earth monoamides can be explained with the aid of the correlation diagrams in Figures3 and 4. The alkaline earth monoalkylamides are ionic moleculeswell represented by the charge distributions Ca+NHR- andSr+N H R-.The valence ns, (n - l)d, and np atomic orbitals of the M+ iongive rise to the electronic states shown in Figure 3 for the C*, pointgroup (when the ligand is OH-). The ligand also mixes the prentatomic orbitals of the states so that for the hydroxides the B2Z+and A211 states are po-dc and pn-dr mixtures.16 The exactlocation of the *A state and the higher lying 2Z+ and 211 statesis unknown. When the symmetry is lowered to C (ligand isNH,), the in-plane p/d orbitals and the outsf-plane p/d orbitalsare co longer degenerate. Therefore, the AZIIstate 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) but 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 (Figure 4). Experimentally, only three electronic transitions are-known forCaNF, and-SrNH,. The planarity of SrNH2and the B2B,-g2Aland A2B2-X2Al assignments wsre proven by a high-resolutionrotational analysk8 The C2AI-X2Al assignment for CaNH2was (15) Hilbrn, R. C.; Quingshi, Z.; Harris, D. 0. J. Mol. Spectrosc. 1983, 97, 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, MIT, 1980. Rice, S. F.; Martin, H.; Field, R.W. J. Chem. Phys. 1985.82, 5023. rjoo do0 0,'oo 1 Figure 2. Excitation spectra of calcium monoalkylamides. The asterisksmark the B2Z+-XZZ+ transition of the CaH molecule. Note that for CaNH2, CaNHCH2CH3, nd CaNHC(CH3)3CaH is not observed. Thecalcium 3PI-1S0 tomic transition (6573 A) is marked. Correlation Diagram 2f3* 0, TT---c,-- .2 np -2 ns --'s -_________ A1 M+-- /H N\ M+ Mf- 0-- H cm c2v Figure 3. Correlation diagram for the M+ ion (M = Sr, Ca) perturbedby a linear OH- ligand (C- symmetry) and NH,- ligand (C, symmetry). made by Wormsbecher, Penn, and Harris6 from their rotationalanalysis. All other assignments are made by analogy. The ,A,and 2A2(2A' nd 2A'') states which correlate to the ,A state ofthe MOH molecule were not observed.- As the alkyl group becomes larger, the A-4 and fI-2 transitionsshift to the red (Figures 1 and 2), while the C-X transitions shiftslightly to the blue (Tables I and 11). The signal-to-noise ratioalso decreases as the vapor pressure of the parent amine decreasesand the product molecules become more difficult to make. Thesharp features in the SrNH2 and CaNH, spectra are sub-band heads. For the heavier Sr-containing compounds, the Sr atomic lines become prominent. When CH,NH2 and CH3CH2NH2 ere  Strontium and Calcium MonoalkylamidesThe Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2781 CORRELATION DIAGRAM C' M + -4 c, Figure 4. Correlation diagram for Ca+ and 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). Manyof the alkylamine oxidants also produced some CaNH2 andSrNH,. The SrOC(CH3)3molecule appeared in the SrNHC-(CH,), spectra, possibly from a tert-butyl alcohol impurity in thetert-butylamine oxidant.The Sr-NH2 vibrational frequency (459 cm-') matches the450-cm-' splitting bztween the B2BI and A2B2electronic states, so t_he v = 0 of the B2B1state is extensively perturbed by v = 1 of A2B2. As the ligand becomcs heavier, the Sr-N stretchingfrequency decreases but the &A electronic separation remainsthe same. Therefore, we assign the centra! peak in the scans ofFigure 1 to the 1-0 vibronic band of the A-X transition. Thiscentral feature could also be assigned to an additional electronictransition (for example 2A1-2Al, Figures 3 and 4), but we do notfavor this assignment. Note that relative intensities of the featuresin Figures 1 and 2 may not be reliable because the red pass filterenhances some features (and_m$lecules) in the excitation spectrum.The intensity of the 1-0 A-X vibronic transition may also be TABLE II: Meta4-Nitrogen Stretching Frequencies of Strontiumand Calcium Monoalkylamides (in em-') M Sr M = Ca molecule % A B t%A MNH2 459 -450 458 524 520 MNHCH3 393 387 480 467 MNHC(CHj)3 - 92 MNHCIH5 -331 -337 -315 enhanced by the 8(v=O)-A(v= 1) intera_ction. For the CaNHR molecules, the B2B1-X2A1 ransition is _eve_'as clear as for the corresponding SrNHR molecules. The B-Ainterval for CaNHR 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. This is in contrast to the corresponding alkoxides,, whereeven for the SrOC(CH3), radical the Sr-0 stretch appearedclearly in the spectra. The Sr-N and Ca-N frequencies of themetal monoalkylamides are all less than the corresponding Sr-0 and Ca-O stretching frequencies of the metal alkoxides, indicatingthat the force constants are smaller. The weaker force constantsfor the metal alkylamides suggest that the M-NHR 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(CH3), oxidant. The reaction was verysluggish, and no alkylamide product was formed. The mainreaction products were the SrOCH3 and CaOCH, 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 American Chemical Society, for partial support ofthis research.
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