A. M. R. P. Bopegedera et al- Gas-Phase Inorganic Chemistry: Laser Spectroscopy of Calcium and Strontium Monoformamidates

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J . Phys. Chem. 1990, 94, 3541-3549 are not related. We do believe, however, that these two new transitions are to the 4d-5p strontium atopic orbitals, now of bl and b2 symmetry, which correlate to the A211state of the p e t a l monoalkoxides. The assignment of the symmetry of the B-and C states is somewhat dtbious, although we prefer B2Bl and C2B2 (rather than B2B2and C2BI). From crystal field arguments, the p orbital in-plane (b,) should be higher in energy than the p orbital out-of-plane (b,)
  J. Phys. Chem. 1990, 94, 3541-3549 3547 are not related. We do believe, however, that these two newtransitions are to the 4d-5p strontium atopic orbitals, now of bland b2 symmetry, which correlate to the A211state of the petalmonoalkoxides. The assignment of the symmetry of the B-andC states is somewhat dtbious, although we prefer B2Bl and C2B2(rather than B2B2 nd C2BI). From crystal field arguments, thep orbital in-plane (b,) should be higher in energy than the p orbitalout-of-plane (b,) due to the repulsion of the negative ch_arge 0; the oxygen atoms. The observed splitting between the B and Cstates is, however, so small (e200 cm-I from the strontium car-boxylates) that other interactions may be more important. Thecorresponding splitting between the B and C states is unresolvedfor the calcium monocarboxylates.This ordering of the in-plane and ?ut-of-plan_eexcited p orbitalsof the alkaline-earth carboxylates (B2BI and C2B2) s in contrastwithjhat observed for the corresponding states of SrNH2 A2B2and B2Bl)where the symmetIy is kn_ow_n rom a high-resolutionrotational analysis of the A-X and B-X transition^.^ Note thatfor the carboxylates the negatively charged oxygen atoms pointdirectly at the metal, while the partially positive hydrogens in theamides point away from the metal.A definitive high-resolution analysis was attempted to determinethe symmetry and molecular geometry of the metal carboxylatestates. However, the molecules proved to be too relaxed for anyresonant laser-induced fluorescence to be observed, so a high-resolution analysis was impossible. This means that our assign-ments are based more on supposition than fact. Perhaps someab initio calculations would help to clarify the problem. Gas-Phase Chemistry of Alkaline-Earth Compounds Little is known about the gas-phase chemistry of the largerpolyatomic free radicals. Several studies performed by matrixisolation techniques provide some insight into the gas-phase re-actions of these molecules. For example, in an argon matrix thereaction of an alkaline-earth atom with a water atom first producesthe M-OH2 complex.36 Upon photolysis, the metal atom insertsbetween an oxygen-hydrogen bond to form H-M-OH. On UV irradiation, H-M-OH dissociates to form MOH.36The reaction between excited strontium (or calcium) canprobably proceed directly in a single step: Sr* + HOOCR - rOOCR + H (1) However, ground-state Sr (or Ca) atoms are also found to react,although reaction 1 is probably endothermic in this case.Another possible mechanism for the formation of alkaline-earthmonocarboxylates is 0 0 II (2) Ar Sr + H-0-C--R 4 H-Sr-0-C--R m- 0 0- II \\ '0 ;/- + HZ (4) SrH + H-0-C--R - r' This mechanism accounts for our observation of substantialamounts of SrH in the oven. Surface and metal cluster reactionsare also possible. It is also not clear whether the observed SrOH(and CaOH) comes from H20 mpurity or from a chemical re-action with the carboxylic acid. The study of the reactions ofalkaline-earth vapors with carboxylic acids under molecular beamconditions would be very fruitful. Acknowledgment. This work was supported by the NationalScience Foundation (Grants CHE-8306504 and CHE-8608630). (36) Kauffman, J. M.; auge, R. H.; Margrave, J. L. High Temp. Sci. 1984, 8, 97. Gas-Phase Inorganic Chemistry: Laser Spectroscopy of Calcium and StrontiumMonoformamidates A. M. R. P. Bopegedera,+ W. T. M. L. Fernando, and P. F. Bernath*,* Department of Chemistry. University of Arizona, Tucson, Arizona 85721 (Received: October IO. 1989) The reaction products of calcium and strontium metal vapors with formamide were studied by using laser spectroscopic techniques.Three electronic transitions were observed for the resulting metal monoformamidates,MNHCOH. The formamidate ligand is probably bonding to the metal in a bidentate manner. The metal-ligand stretching vibrational frequencies were assignedfrom the low-resolution spectra. Introduction In our laboratory, we have investigated the spectra of alkalineearth metal containing free radicals including metal monoal-koxides,lP2 mon~thiolates,~socyanates: cy~lopentadienides,~monoalkylamides,6 monomethides,' acetylides,* a~ides,~oro-hydrides,I0 and carboxylates. J2 All these free radicals have asingle metal-ligand bond (monodentate bonding) except for themetal borohydrides and carboxylates. The borohydride ligandbonds to the metal in a tridentate fashionlo while the carboxylateligand bonds in a bidentate fashion.I2Although the formate anion (HCOO-) is a commonly en-countered ligand in transition-metal chemistry, the chemistry ofthe isoelectronic formamidate anion (HCONH-) has hardly been 'Current address: NOAA, ERL, R/E/AL2, 325 Broadway, Boulder, CO'Alfred P. Sloan Fellow; Camille and Henry Dreyfus Teacher-Scholar. *To whom correspondence should be addressed. 80303. 0022-3654/90/2094-3547$02.50/0 exp10red.I~ A few workers have explored the substitution of formate ligands by amidato ligands in, for example, Rh2(0NH- (1) Brazier, C. R.; Bernath, P. F.; Kinsey-Nielsen, S.; llingboe, L. C. J. Chem. Phvs. 1985. 82. 1043. (2) Brazier, C. R.; Ellingboe, L. C.; Kinsey-Nielsen, S.; Bernath, P. F. J. (3) Fernando, W. T. M. L.; Ram, R. S.; Bernath, P. F. Work in progress. (4) Ellingboe, L. C.; Bopegedera, A. M. R. P.; Brazier, C. R.; Bernath, P. F. Chem. Phsy. Left. 1986, 26, 285. O'Brien, L. C.; Bernath, P. F. J. Am. Chem. SOC. 986, 08, 2126. Chem. Phys. 1988, 8, 1 17.(5) 'Brien, L. C.; Bernath, P. F. J. Am. Chem. SOC. 1986, 108, 5017. (6) Bownedera. A. M. R. P.; Brazier, C. R.; Bernath, P. F. J.Phys. Chem. 1987, 1,'2i79.(7) Brazier, C. .; Bernath, P. F. J. Chem. Phys. 1987,86,5918; J. Chem.Phys. 1989, 1, 4548.(8) Bopegedera, A. M. R. P.; Brazier, C. R.; Bernath, P. F. Chem. Phys.Lett. 1987, 36, 91; . Mol. Spectrosc. 1988, 29, 268. (9) Brazier,C. R.; ernath, P. F. J. Chem. Phys. 1988,88, 112. (10) Pianalto, F. S.; Bopegedera, A. M. R. P.; Brazier, C. R.; Fernando, W T. M. L.; Hailey. R.; Bernath, P. F. Work in progress. 0 990 American Chemical Society  3548 The Journal of Physical Chemistry, Vol. 94, No. 9, I990 Bopegedera et al. I I 050 680 nm Figure 1. Resolved fluorescence spectrum of the b2A'-i(2A' transitionof calcium monoformamidate. The asterisk marks the position of the laser, which is tuned to the 0-0 band. The strong feature to the blue ofthe asterisk is the 'P,-'S0 tomic transition of Ca. Features at ==680 nd 2645 nm are assigned to-the 071 and 1-0 bands, respectively. Although not shown here, strong A2A'-X2A' emission was observed in this spec- trum at =706 nm (see text).CCF3)4.14 We report here on the gas-phase calcium and strontiummonoformamidates synthesized by the direct reaction betweenthe metal vapor and formamide. Experimental Section The gas-phase alkaline-earth monoformamidates were preparedin a Broida type ovenI5 by the reaction of the metal (Ca, Sr) vaporwith formamide (HCONH2). The metal was resistively heatedin an alumina crucible and the vapor entrained in 1.5 Torr of argoncarrier gas.Formamide has a very low vapor pressure at room temperature (1 Torr at 70 C).I6 Therefore, to provide a sufficient partialpressure of formamide vapor inside the oven, argon gas wasbubbled through the glass cell containing formamide. The totalpressure inside the oven was maintained at approximately 3 Torr.Since analytical grade formamide contains undesirable impuritiessuch as NH3, spectrometric grade (99+%) formamide (Aldrich)was used for our experiments.Two CW broad-band (I-cm-l) dye lasers pumped by a 5.5-Wall-lines output of a Coherent Innova 70 argon ion laser were usedin this experiment. One dye laser was tuned to excite the 3P -'So atomic transition of the metal (6573 A for Ca and 6892 A or Sr). The second dye laser was used to excite the moleculartransitions of the calcium and strontium monoformamidates.Several laser dyes (DCM, Pyridine 2, and Rhodamine 6G) wererequired to cover the desired spectral region.Two types of spectra were recorded. Laser excitation spectrawere recorded by scanning the wavelength of the laser that wasexciting the molecular transition. The beam from this laser waschopped and the signal demodulated with a lock-in amplifier. Redpass filters (Schott RG9, RG780, and RG830), were used to blockthe scattered laser light. A photomultiplier-filter combinationwas used to detect the total fluorescence from the excited electronicstates.Resolved fluorescence spectra were recorded by tuning thewavelength of the second dye laser to a molecular transition and (1 1) Brazier. C. R.; ernath, P. F.; Kinsey-Nielsen, S.; Ellingboe, L. . (12) 'Brien, L. C.;Brazier. C. R.; Kinsey-Nielsen, S.; Bernath. P. F. J. (I 3) Cotton. F. A.; Wilkinson, G. Advanced Inorganic Chemistry. 5th ed.: (14) Denis, A. M.; Korp, J. D.; Bernal, I.; Howard, R. A,; Bear, J. L. (15) West,J. B.; Bradford, R. S.; Eversole, J. D.; Jones, C.R. Rec. Sci. (16) he Merck Index. 10th ed.; Merck & Co.: Rahway, NJ, 1983; J. Chem. Phys. 1985, 82, 1043. Phys. Chem.. preceding paper in this issue. Wiley-Interscience: New York, 1988; p 376. Inorg. Chem. 1983, 22, 1522. Insrrum. 1975. 46, 164.4127. 7lb i~onm I Figure 2. Resolved fluorescence spectrum of the B2A'-RzA' transitionof strontium monoformamide. The asterisk ?arks t_he position of thelaser, which is tuned to the 0-0 band. Strong A2A'-X2A' emission canalso be seen to the red (see text). The features at ~705,735, nd ~745 nm are assigned to the 1-0, 0-1, and 0-2 vibronic bands, respectively.The feature at =780 nm is the 0-1 band of the A2A'-i?A' transition. TABLE I: Band Origins of the Calcium and StrontiumMonoformamidate Vibronic Transitions (in cm-') band CaNHCOH SrNHCOH 2-0 1-0 0-0 0- 0-2 0-3 1-0 0-0 0- 1 0-2 2-0 1-0 0-0 0- 1 A2Af-i(2Al 1485914 50914 I541380313457 I3 IO8 fi2At-R(2Af 15 44015083147271660116248 15 89613624 13351 13 077I2 7891250112 2221420113917 I3 630 13 389 15 205148951458014 296 dispersing the fluorescence through a small monochromatorequipped with photon counting detection electronics. Results and Discussion Three electronic transitions A2A'-W2A', B2A'-R2A' andC2A -R2Af were observed in the excitation spectra of the metalmonoformamidates. Figures 1 an 2 show parts of the resolvedfluorescence spectra of the B2A'-X2A' transition of calcium andstrontium monoformamidate molecules, respectively. To obtainthese spectra, the dye laser exciticg the-molecular transition wastuned to the 0-0 band of the B2A'-X2A' transition, and thefluorescence dispersed with the monochromator. Emission fromthe excited electronic state to higher vibrational levels of the groundelectronic state was observed in all of the resolved fluorescencespectra. The band srcins of these vibronic transitions are givenin Table I. When the B2A'-R2A' transition of the m_etal monoformamidateswas excited by the laser, rela_xation_to he A2A' state was observed(Figure 2). In fact, strong A2A'-X2A' emissio_nwas _observed in:hespectra of both molecules when either the B2A'-X2A' or theC2A -X2A' transition was excited by the laser.The formamidate anion (HCONH-) is isoelectronic with theformate (HCOO-) anion. The low-resolution spectra of calciumand strontium monoformates have been reported previously.12Similar to the formate anion, the formamidate anion could bondto the metal ion in a monodentate (to give molecule I or 11)orbidentate (to give molecule 111) manner. Although both types H 111  Gas-Phase inorganic Chemistry Correlation Diagram 2 A- /?f - _._..._ Al - ______ 2a - 01- -.----A'- np-:: __._ 0,- 2. -. ?n - .-. -..? The Journal of Physical Chemistry,Vol. 94, No. 9, 1990 3549 ns ________ !,+-E _______ A - ...... __._._ A - M+-NH2 M+-NHR M+ 0.c~ HN' + M+-CCH c-, C2 c, c, Figure 3. Correlation diagram for Ca' and Sr' with CCH- (C- ), NHC (Cb), HR- (Cs, monodentate) and HCONH- (C8, bidentate) ligands. Note that in our previous paper on calcium and strontium monoalkyl- amides (ref 6) the corresponding figure (Figure 4) is in error. Theout-of-plane B, states correlate with A states while the in-plane B, states correlate with A'. of bonding result in a molecule in the C, oint group, the natureof bonding will have a strong effect on the electronic spectra ofthe metal monoformamidates. If the bonding is monodentate,then the spectra of the M-NH-COH or M-O-CHNH (M = Ca, Sr) molecules will resemble those of the monoalkylamides6 or themonoalkoxides.2 If the bonding is bidentate, the spectra ofmolecule 111 will resemble those of the isoelectronic metal mono-formates.I2 A close look at the spectra of the metal monoform-amidates reveals considerable similarity with those of the metalmonoformates, suggesting that the formamidate anion is a bi-dentate ligand.The metal monoformamidates are ionic molecules representedby the structure M+HNCOH-, where the HCONH- ligand isclosed shell. Therefore, the molecular orbitals of the metal mo-noformamides can be described as the orbitals of the M+ ionperturbed by the HCONH- ligand in the C, point group. Thecorrelation diagram in Figure 3 is helpful to describe these mo-lecular orbitals and the electronic structure of CaNHCOH andSrNHCOH.The valence ns (n = 4 or Ca, n = 5 for Sr) orbital of the M+ion contains one unpaired electron. This results in a 2Z+ groundstate for the linear (C,,) MCCH molecule. In the C, point group,this transforms to a 2A' state (Figure 3). The 5-fold degeneracyof the (n - I)d orbitals is lifted in the C,, point group, giving riseto 6 (dxLyyl,dxy), a (d,,, dyr), and u (d22) orbitals. Similarly, the3-fold degenerate np orbitals split into a (px, py) and u (p,) orbitals in the C,, point group. In addition, the presence of the linearligand mixes da with pa and du with pa so that these orbitals arenow da-p?r and do-pu mixtures. Transition to the 2A state (ds-9,d,orbitals) from the 2Z+ ground state is forbidden. Consequently, in the C,,point group the first allowed electronic transition isfrom the X2Z+ ground state to the A2H state.When the symmetry is reduced to C2, (as in metal mono-amides), the degeneracy of the 6 nd a orbitals is lifted, givingrise to a,, a2 (dXLyz. ,)and b2, b, (mixture of d,, and p,, d, andp,) orbitals, respectively. The corresponding electronic states aregiven in Figure 3. -Note that the x-axis is out of plane. The relativeordering of the A2B2and B2B, states was determined experi-mentally for SrNH2. Although transitions are allowed from theground 2A, state to all the other states in the C2, point group,except to the 2Az state, a transition was not observed to thelow-lying 2A, state in any of the Ca- and Sr-containing molecules (17) Brazier, C. R.; Bernath, P. F. Manuscript in preparation TABLE II: Vibrational Frequencies of Calcium and StrontiumMonoformamidates (in cm-I) state CaNHCOH SrNHCOH PA' 35 1 288 A2A' 355 278 FA' 357 284 CZA!I 353 315 that we studied.When a H atom of a metal monoamide isreplaced by an alkyl group to obtain the metal monoalkylamides,the syFmetry is reduce! from C2, to C,. The relative ordering of the A2Af,B2Arf, nd C2A' states of the metal monoalkylamidesis obtained by correlating to the C2, point group.Since the formamidate ligand bonds to the metal in a bidentatefashion, the nitrogen and the oxygen atoms on the anion *- /- H-C.+. I H are partially negatively charged. We believe that these off-axisnegative charges destabilize the metal orbitals that are perpen-dicular to the z axis (a orbitals) relative to the metal orbitals thatare parallel to the z axis (u orbitals). This has pronounced effectson the relative ordering of the electronic states in_ the metalmonoformamidate. As a result of this, the A and B el_ectroni_cstates of the metal monoalkylamides correlate with_ he B and Cstates of the metal monoformamidates while the C state of themonoalkylamides correlates with the A state of the monoform-amidates (Figure 3). Therefore, the three observed electronictransitions of the metal monoformgmidates are assigned asA2A'-X2A', fi2Ar-j(12Ar, nd C2A -X2A'. We have observed asimilar switching of states in two other families of molecules wehave studied previously: the metal borohydridesI0 and the metalmonoformates.I2 The borohydride (BHJ anion bonds in a tri-dentate fashion. In both these cases off-axis negative charges arepresent on the ligands.The low-resolution electronic spectra enabled us to obtain vi- brational frequencies for the metal monoformamidates which arereported in Table 11. Since only metal-centered orbitals areinvolved in the electronic transitions, any vibrational activity thatis observed is also associated with the metal atom. We thereforeassign the single observed Franck-Condon-active mode as ametal-ligand stretching vibration. The observation of progressionsin the metal-ligand stretching mode suggests that there are sig-nificant changes in the metal-ligand bond lengths in the excitedelectronic states.The electronic assignments of the metal monoformamidatespectra were based on qualitative arguments made by comparingthe spectra of metal monoformamidates, monoformates, andmonoborohydrides. This was necessary because definitive high-resolution spectra are not available.Boldyrev and co-workers18 have carried out ab initio calculationsof the molecular properties of LiBH,. They find that the C,, tridendate structure has the lowest energy on the potential surface.Similar ab initio calculations on metal monoformates and mo-noformamidates would provide some additional insight into thestructures of these molecules. Conclusion The gas-phase reaction between alkaline-earth metals andformamide produces the alkaline-earth-monoformapidate_ reeradicals. Three electronic transitions (A2A'-X2Af, B2A'-X2A',and c2A -X2A') were observed at low resolution. Comparisonof the spectra with those of the alkaline-earth monoborohydridesand monoformates enables us toconclude that the formamideligand bonds to the metal in a bidentate manner. (18) Boldyrev, A. 1.; Charkin, 0. .; Rambidi, N. G.; vdeev, V. . Chem. Phys. Letr. 1976, 44, 20.
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