L. C. O'Brien and P. F. Bernath- Laser Spectroscopy of Calcium and Strontium Monoc y...
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J. Am. Chem. SOC. 1986, 108, 5017-5018 5017 Acknowledgment. The research of H.J.S. and J.A.P. was supported by the National Science Foundation (Grant CHE- 841 7548), the Charles and Johanna Busch Fund-Rutgers Univ- ersity, and a crystallographic instrumentation grant from the National Institutes of Health (Grant 1510 R R O 1486 01A1). The research of K.K.-J. was supported by th e donors of the Petroleum Research Fund, administered by the American Chemical Society, and the National Institutes of Health (Grant G M 341 11). Supplementary Material Available: Tables of crystal and re- finement data and lists of ato mic coordinat es, anisotropic thermal parameters, and observed and calculated structure factors for 1 (10 pages). Ordering information is given on any current masthead page. (23) Profe ssor Robert Holwerda, Texas Tech University. Laser Spectroscopy o f Calcium and Strontium Monoc y clopentadienide L. C. O'Brien and P. F. Bernath* Department o f Chemistry, University o f Arizona Tucson, Arizona 85721 Received April 22, 1986 The gas-phase reaction of Ca or Sr vapors with cyclopentadiene has resulted in the laser spectroscopic observation of the metal monocyclopentadienide free radicals, CaCp and SrCp (Cp = C,H,). The spectra are interpreted as arising from "open-faced sandwich" complexes of C , , ymmetry. Ou r experimental methods have general utility for the synthesis and characterization of inorganic free radicals. The cyclopentadienyl ligand is one of the popular and fasci- nating ligands in inorganic chemistry.' Th e most celebrated molecule containing C p is ferrocene, Fe(Cp),, which has a "sandwich" structure.' The alkali metals and the heavier alka- line-earth metals form ionic complexes with C5H5.2-5 nCp6 and TICp' have C , , symmetry in the gas phase. InCp and TlC p are the covalent closed-shell analogues of the ionic free radicals, Ca'Cp- an d Sr'Cp-, th at we hav e discovered. We have recently observed a large number of Ca-, Sr-, and Ba-containing free radicals with only one ligand, M L (L = OH,"" OCH3, OCH2CH3, OCH(CH3)2, O(CH2)&H3, OC(CH3)3,1Z OCN,I3 CHO,, CH3C0214,15). The bonding and electronic structure in these molecules is well described by a n M + ion perturbed by the L- ligand. Th e M' ions are isoelectronic with the alkali atoms so the molecular states ca n be described in terms (1) Cotton, F. A ,; Wil kinso n, G . Advanced Inorganic Chemistry, 4th 4.; (2 ) Zerger, R.; Stucky, G. J . Organomet. Chem. 1974,80, 7-17. (3 ) Gowenlock, B. G.; Lindsell, W. E.; Singh, B. J. Chem. Soc., Dalton (4 ) Faegri , K.; Almlof, J. ; Luthi, H. P. J . Organomet. Chem. 1983, 249, (5) Alexandros, S.; Streitwieser, A, , Jr.; Schaefer, H. F ., I11 J . Am. Chem. (6) Shibata, S .; Bartell, L. S.; avin, R. M., J r. J . Chem. Phys. 1964, 4 1 , (7 ) Tyler, J. K.; Cox, A. P.; Sheridan, J . Nature (London) 1959, 183, ( 8 ) Bernath, P. F. ; Kinsey-Nielsen, S. Chem. Phys. Lett. 1984, 105, (9) Bernath, P. F.; Brazier, C. R. Astrophys. J . 1985, 288, 313-316. (10) Brazier , C. R.; Remath, P. F. J . Mol. Spectrosc. 1985,114, 163-173. (1 1) Kinsey-Nielsen, S.; Brazi er, C . R.; Bernath, P. F. J . Chem. Phys. (12) Brazier, C . R.; Bernat h, P. F.; Kinsey-Nielsen, S.; Ellingboe, L. C. (13) Brazier, C. R.; Ellingboe, L. C.; Kinsey-Nielsen, S.; Bernath, P. F. (14) Ellingboe. L. C.; Bopegedera , A. M. R. P. ; Brazier, C. R.; Bernat h, (15) Brazi er, C. R.; Ellingboe, L. C.; Kinscy-Nielsen, S.; Bernath, P. F., Wiley: New York, 1980; pp 1163-1167. Trans. 1978, 657-664. 303-31 3. Sac. 1976, 98. 7959. 717-722. 1182-1 183. 663-666. 1986, 84, 698-708. J . Chem. Phys. 1985.82, 1043-1045. J . Am. Chem. SOC. 986, 108, 2126-2132. P. F. Chem. Phys. Lett. 1986, 126, 285-289. manuscript in preparation. h 67b0 a & Ssbo Figure 1. Resolved Cuorescence spectrum showing laser-induced fluorescence from the A-X transition of CaCp. The selitting between the strong features is due to spi n-orbit coupling in the A2El state. The asterisk marks scattered light from the dye laser exciting the molecular transition. Table I. A-% an d 8-R Vibronic Transition Frequencies for CaCp and SrCD in cm-' 3-0 15495 2-0 15170 1-0 14848 0-0 14518 0-1 14237 0-2 13955 0-3 13690 0-4 13458" " Blended. 15 548 14014" 14300 15 226 13763" 14039 14897 17072 13510" 13782 15106 14575 16772 13268 13523 14846 14282 16470 13040 13289" 14616 1 4 002 12822 13053" 13739 13 505" of one-electron metal-centered atomic orbitals. For the Cp- ligand the ground 4s (Ca') or 5 s (Sr') atomic orbital gives rise to the X2Al ground state of MCp. The excited 3d (Ca ') and 4d (Sr') atomic o rbitals result in d2 2 (2Al); d,,, dy, (2El); an d d d L (,E2) states. The expected electronic tran- forbidden.) n fact, the B2Al and A2E, states are 3d2r4p2 Ca') and 4dZz-5p , (Sr') mix tures becaus e of d-p mixing induced by th e C , , igand field. MCp (M = Ca, r) molecules were produced in a B roida ove n16 by the reaction of t he alkalin e earth vapor with cyclopentadiene (CsH6). Pressures were approximately 1.5 torr of argon and 3 mtorr of cyclopentadiene. The cyclopentadiene was prepared from the Diels-Alder dimer by a simple distillation. The direct reaction of the dimer with Ca vapor was not successful. A broad-band (1 cm-I) CW dye laser beam was focused into the Broida oven in order to excite the molecular emission. A second broad-band (1 cm-I) dye laser beam was always used to excite the alkaline earth 3P1-'S0 tomic transition (6892 8, or Sr, 6573 8, for Ca). The reaction was greatly enhanced by electroni c excitation of the metal. Two types of experiments were used to record the spectra. Laser excitati on spectra were obtained by scanning th e dye laser probing the molecular transitions, and recording the fluorescence through red-pass filters selected to block the scattered laser light. The second dye laser, resonant with the metal atomic line, was chopped to m odulate the molecular fluorescence for lock-in de- tection. Spectra were also recorded by exciting the molecular emission and record ing the resolved fluorescence with a monochromator. For these experiments the molecular emission was imaged onto the entrance slit of a 0.64-m monochromator. Scanning the monochromator (not the laser) provided resolved laser-induced fluorescence spectra. Figure 1 is a portion of the A2E1-g2AI spectrum of CaCp. This spe ctr um was rec orded by resolving t he fluoresc_ence prod_uced by a laser (marked with an asterisk) exciting the A2E1(3/2)-X2AI sitions are BZAi-X I I an A2El-x2AI. (2E2-ZAl s electric-dipole (16) West, J. B.; Bradford, R. S.; Eversole, J. D. ; Jones, C. R. Reu. Sci. Imtrum. 1975,46, 164-168. 0002-7863/86/1508-5017$01.50/0 0 1986 American Chemical Society
L. C. O'Brien and P. F. Bernath- Laser Spectroscopy of Calcium and Strontium Monoc y clopentadienide
