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7900 J . Am. Chem. SOC. 990, 112, 7900-7903Gas-Phase Inorganic Chemistry: Laser Spectroscopy ofCalcium and Strontium MonoborohydridesF. S. Pianalto,+ A. M . R . P. Bopegedera,t W . T. M. L. Fernando, R . Hailey,L. C. O'Brien,s C. R. Brazie r,l P . C. Keller, and P. F. Bernath*s"Contributionfrom the Department of Chemistry, The U niversity of Arizona,Tucson, Arizona 85721. Received April 13 . I990

Abstract: The CaBH4-and SrBH, free radicals were synthesized by the reaction of Ca or Sr vapor with diborane, Th eA 2 A l -X 2 A I and B2E-X2AI electronic transitions were detected by laser-induced fluorescence. Th e spectra ar e consis tent withtridentate molecular s tructures with three bridging hydrogens and C3" y m m e t r y .

IntroductionTh e t e t r a h y d r o b o r a t e anion (BH,-) forms a large number ofinteresting ionic and covalent complexes w i t h metals.'S2 Thecoordination between the metal and the BH, ion invariably occursthrough bridging hydrogens, and rapid interconversion betweenthe various coordination modes is k n o w n in th e liquid phase.'q2BH4- is a commonly used reducing agent. Nevertheless, verylittle is k n o w n a b o u t the spectroscopicproperties of the metal-BH,complexes, p a r t i c u la r ly in th e ga s phase. Of th e alkaline eart hborohydrides, only Be(BH4)2has been analyzed in th e gas phase,bu t the molecular s t r u c t u r e remains uncertain. Gundersen,Hedberg, an d Hedberg3 studied the Be(BH4)2 structure usinge lec tron diffraction methods, while Nib le8 examined the infraredand Raman spectra. The gas-phase N M R spectrum shows onlya single set of equivalent protons, consistent with fluxional be-h a ~ i o r . ~ recent theoretical calculation by S t a n t o n , Lipscomb,and BartletP has provided ne w insight into this long-standingproblem.I n contrast to th e covalent Be(BH4)2,he alkali borohydridesan d t h e heavier a l k a l i n e earth b o ro h y d r id e s are a l l ionic solidswith low vapor pressures.'s2 We report here on the discovery ofth e gas-phase calcium and strontium monoborohydride freeradicals, CaBH, an d SrBH,. These molecules are made by th ereaction of Ca and Sr vapors with diborane , B2H6 The observationof CaBH, comple tes the isoelectronic pblock family of molecules ,CaBH,, C a C H 3 , ' CaNH2,8CaOH? an d CaF.IO We have pre-viously studied many monova len t derivatives of Ca an d Sr in ou rlaboratory, including c a rb o x y la t e s , ' I azides,12 ~oc ya na t e s , ' ~~ l o p e n t a d i e n i d e s , ' ~ c e t y l i d e ~ , ' ~ormamidates,16 pyrrolates,a lk a n a mid e s , ' * an d a l k o ~ i d e s . ' ~CaBH, an d SrBH, are the first examples of gas-phase m e t a lmonoborohydrides to be experimentally characterized. However,there has been considerable theoretical activity in this area. A binitio predictions for th e properties of LiBH,,2"25 NaBH4,23-26KBH424 an d CUBH, ~ ,are a v a i l a b le in t h e l i t e ra t u r e. T h es t r u c t u r e s a r e all p re d ic t e d to b e f lu x io n a l w i th e i th e r t r id e n ta t eor bidentate ( f o r CuBH,) coordination. There is the tantalizingpossibility of observ ing some ev idence of fluxional behavior in th espectra of CaBH, an d SrBH,.Experimental MethodsThe calcium and strontium borohydrides were produced in a Broidaovenz7 by the reaction of C a or Sr with diborane (B2H6). similar toprevious work7-I9 in this area. Th e Ca or Sr m etal was vaporized froma resistively heated crucible, carried to the reaction region by argoncarrier gas, and reacted with diborane. The diborane was stored as asolid i n a liquid nitrogen ba th. During the experiment, the diborane wasmelted with a pentane/liquid N 2 bath (-1 30 "C) and added as a gas to

'Current Address: U.S.D.A., Western Human Nutrition Research Center,'Current Address: NOAA, ERL, R/E/A L2, 325 Broadway, Boulder, COPost Office Box 29997, Presidio of San Francisco, CA 94129.80303.'Curr ent Address: Department of Chemistry Southern Illinois University

Current Address: A stronautics Lab./LSX , Edwards AFB, CA 93523.Alfred P. Sloan Fellow; Camille and Henry Dreyfus Teacher-Scholar.at Edwardsville, Edwardsville, IL 62026.

0002-7863 /90/ 15 12-7900$ 02.50/0

the Broida oven. Th e pressures were approximately 1 Torr of argon and0.035 Torr of diborane.The diborane was prepared by slowly adding sodium borohydride(N aBH I) to heated polyphosphoric acid under vacuum. The diboranegas produced was collected with a liquid N , bath.Two types of low-resolution spectr a were recorded. Laser excitatio nspectra were obtained by scanning a broad-band ( 1 cm-I) C W dye laserthrough a spectral region where Ca BH 4or SrBH, absorb and detectingtotal fluorescence through red pass filters with a photom ultiplier tube.Resolved fluorescence spectra were obtained by fixing the dye laser at

( I ) James, B. D.; Wallbridge, M. G. H . Prog. Inorg. Chem. 1970, I / ,(2 ) Marks, T. J.; Kolb, J. R. Chem. Reu. 1977, 77, 263-293.(3 ) Gundersen, G.; edberg, L.; Hedberg, K. J . Chem. Phys. 1973, 59,(4 ) Nibler, J. W. J . Am. Chem. SOC. 972, 94, 3349-3359.( 5 ) Gaines, D. F.; Walsh, J. L.; Morris, J. H.; Hillenbrand, D. F. Inorg.

99-23 1.

3777-3785.

Chem. 1978. 17 . 1516-1522.(6 ) Stant on, J. F.; Lipscomb, W. N.; Bartlett, R. J. J . Chem. Phys. 1988,( 7 ) Brazier, C. R.; Bernath, P.F. J . Chem. Phys. 1987 ,86, 5918-5922;88, 5726-5734.1989. 91,4548-4554.(8 ) Wormsbecher, R. F.; Penn, R. E.; Harris, D. 0. J . Mol. Specfrosc.1983. 97.65-72.--- . , - -(9 ) Bernath, P.; Kinsey-Nielsen, S. Chem. Phys. Lett. 1984, 105, 663-666.Bernath, P. F.; Brazier, C. R. Asfrophys. J . 1985, 288, 373-376.(IO) Bernath, P.; Field, R. W. J . Mol. Spectrosc. 1980, 82, 339-347.Dulick, M.; Bernath, P.; ield, R. W. Can. J . Phys. 1980, 58, 703-712.(1 ) OB rie n, L. C.; Brazier, C. R. ; Kinsey-Nielsen, S.; Bernath, P. F. J .Phys. Chem. 1990, 94, 3543-3547.(12) Brazier, C. R.: Bernath, P. F. J . Chem. Phys. 1988,88, 21 12-21 16.(13) Ellingboe, L. C.; B opegedera, A. M . R. P.; Brazier, C. R.; Bernath,P. F. Chem. Phys. Let t . 1986,126, 285-289. OBrien, L. C.; Bernath, P. F.J . Chem. Phys. 1988 ,88, 2117-2120.(14) O'Brien, L. C.; Bernath, P. F. J . Am. Chem. SOC.1986, 108,( 1 5 ) Bopegedera, A. M. R. P.; Brazier, C. R.; Bernath, P. F. Chem. Phys.Lett. 1987, 136, 97-100, J . Mol. Spectrosc. 1988, 129, 268-275.(16) Bopegedera, A. M. R. P.; Fernando, W . T.M. L.; Bernath, P. F. J .Phys. Chem. 1990, 94, 3547-3549.( 1 7 ) Bopegedera, A. M. . P.; ernando, W . T . M. L.; Bernath, P. F. J .Phys. Chem. 1990, 94,4476-4479.(18 ) Bopegedera, A. M. R. P.; Brazier, C. R.; Bernath, P. F. J . Phys.Chem. 1987, 91, 2779-2781.(19) Brazier, C. R.; Ellingboe, L. C.; Kinsey-Nielsen, S.; Bernath, P. F.J . Am. Chem.Soc . 1986, 108, 2126-2132.(20) Dill, J . D.; Schleyer, P. v. R.; Binkley, J. S.;Pople, J. A. J . Am. Chem.SOC. 977, 99, 6159-6173. DeFrees, D. J . ; Raghavachari, K.; Schlegel, H .B. ; Pople, J . A,; Schleyer, P. v. R. J . Phys. Chem. 1987, 91. 1857-1864.(21) Boldyrev, A. I. ; Charkin, 0.P.; Rambidi, N. G.; vdeev, V. . Chem.Phys. Lett. 1976, 44. 20-24; Zhur. Sfruk t . Khim. 1977, 18, 13-22.(22) Kello, V. ; Urban, M.; Boldyrev, A . 1. Chem. Phys. Left . 1984, 106,455-459.(23) Baranov, L. Ya.; B oldyrev,A. 1. Chem. Phys. Left. 1983,96,218-222.(24) Charkin, 0.P.; Musaev, D. G. ; Klimenko, M. N . Kw r d . Khi m. 1985,(25) Bonaccorsi, R.; Scrocco, E.; Tomasi, J. Theor. Chim. Acta 1979,52,(26) Barone, V. ; Dolcetti, G.; Lelj, F.; Russo, N. Inorg. Chem. 1981, 20.(27) West, J. B.; Bradford, R. S.; Eversole, J . D.; Jones C. R. Reo. Sci.

5017-5018.

1 1 , 409-414.1 13-1 27.1687-169 I .Instrum. 1975, 46, 164-168.

0 1990 American Chemical S o c ie ty

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Laser Spectroscopy of Ca and Sr MonoborohydridesCaBH4 Conf igurat ions

'a' BidentateTridentate MonodentateFigure 1. Three possible structures of CaBH, with Ca+ bonded to a BH4-tetrahedron are likely: tridentate (th ree bridging hydrogens), bidentate(two bridging hydrogens), and monodentate (one bridging hydrogen).Figure 1 is very similar to the diagram in the work of Boldyrev etBy analogy with the ionic alkali monoborohydrides, the monodentatestructure is expected to lie quite high in energy. The tridentate andbidenta te structures should lie close in energy with the tridentate struc-ture the global minimum on the potential surface. If the molecule isfluxional the bidentate structures may be transition states between thefour tridentate minima.

0 -0BYE-R'A,h

63 0 6 7 0 nmFigure 2. This is the laser excitation sp ct ru m of CaBH,. The 0-0vibrational bands of the B2E-R2A, and A 2 A l - ~ 2 A lransitions are la-beled. Only the Ca-BH, stretching modes have Franck-Condon activity.The relative intensities of the two trans itions are distorted by the effectsof variation in laser power with frequency and the use of a red-pass filterfor detection. The small features at 6550 and 6350 A are the 1 -0 and2- 0 bands of the A2A,-R2A, transition, and the small features at 6290and 6675 A are the 1-0 and 0-1 bands of the 8*E-RZA, transition.the frequency of a CaBH, or SrBH, molecular transition and scanningthe wavelength of a monochromator to detect laser-induced fluorescence.I n similar Broida oven experiments,a second dye laser tuned to theCa or Sr 'P,-'S0atomic line was required to promote the reaction be-tween the alkaline earth vapor and the various oxidan ts. The reactionbetween Ca or Sr with diborane, however, was vigorous enough to bedetected without the use of the second dye laser to excite the metal atom.Results and DiscussionThe spectra of the calcium and strontium borohydrides wereassigned by comparison to the spectra of other alkaline earthTransition s to_two exsited st ates were observed,Cnd these were ass igned A2AI-X2Al, f i 2 E l ~ 2 -~ 2 A , ,ndB2E3/,-k2A l. Th e presence of a spin-orbit sp itting (and itsmagnitude) in the 2E state suggests a symm etric top structure.The stru ctural assignment of C3,symmetry is consistent with thepredicted lowest energy tridentate structure (see Figure 1) of theab initio*26 calculations on the ionic LiBH4, NaB H4 , and K BH 4molecules.Figure 2 is a Ert io n Of a CaBH, laser excitation scan. Th e0-0 bands of the A2A,-X2AI and the fi2E=X2AL transitions r elabeled. The 1-0 and 2- 0 bands of the; A2A1-X2A, transitionappear as t he small features at approximate1 655-0 nd 6350A,respectively. The 1- 0 an d 0 - 1 bands of the d2E-X2A, transitionappear a t approximately 6290 nd 6675A, respe ctive ly. Th e 'E312spin component of the B2E state a t higher energy is slightly moreintense than the 2E, spin component, and the spin-orbit couplingconstant of the B2 i state is approximately 59 cm-' for Ca BH 4

J . Am. Chem. Soc.. Vol. 112, No. 22,1990 7901Sr + B2H8*E,"-~*A,nO-O

690 720 nmFigure 3. This is a laser excitation ye ct rum of SrBH,. The 0-0 vi-brational bands of the 82E-A2Al and A2A,-f(2A, transitions are labeled.Like CaBH,, only th e Sr-BH4 stretching mode displays Franck-Condona_ctivity, The feature at 7075 A is a blend of the 1 -0 band of theA2AI-X2A, transition and th e 0-1 vibration of the i)2E-ft2A, transition.The asterisk marks the Sr 3PI-'Sotomic line.

0 -0B'E - R k ,

Ca + B2H61

6 4 0 7 0 0 nmFigure4. The frequency of the d e l a y was held fixed on the 2E l/ 2s pi ncomponent of the 0-0 band &E-X2AI transition in this resolvedfluorescence spectrum of CaBH,. The scattered laser light is marked bythe asterisk. By coincidence, the laser is also exciting the 1-0 band ofthe A2AI-k2AI transition. The features at higher wavelength (the threeexpanded peaks) res$ from a mixture of emission to ex ci tg vibrationallevels of th e ground X 2 A ,state_from he v = 0 level of the B2E sta te andfrom vibrat ional levels of the A2AI state.an d 200 cm-' for SrBH,. The se values ar e somewhat lower thanwhat is typically observed for calcium (65-75 cm-I) and strontium(260-290 m-I) bonding with other ligands (Table 111). Th e lowvalues might be du e to the three bridging hydrogens involved inthe metal-ligand bond. They could act to diminish the spin-ofbitinteraction by increasing the amount of d-character in the B2Estate or by Jahn-Teller quenchin g of the orbital angular mo-mentum about the top axis.Th e laser excitation spec irum of SrB H4 s shown i,n Fig ule 3.The0 ands of the B2E-X2A, transitions and the A2A,-X 2A,:ransition ar e labeled. Th e large un kbe led peak is a blend of theB2E-X2A, 0-1 band and the A2AI-X2AI1-0 band. The asteriskmarks the Sr atomic line at 6892A.A portion of a CaBH,-resolved fluorescence spectrum is shownin Figure 4. To record this spectrum, the dye laser was fixedat the frequency of the 2E1 /2pin component of the B 2E state whilethe monochromato r was scanned. By coincidence, the jas er is_alsopositioned at the frequency of the 1-0 band of the A2AI-XZAltransition. The asterisk mark s the scattered light from the laser.The blue (higher energy) side of the first expanded feature a tapproximately 6550 A consis_ts mostly of fluorescence to th evibrational level v = 1 of the X2A, state from the v = 0 level ofthe B2E!,2 state (0-1 band). Th e red (lower energy) side of thefeature ISmostly du e to fluorescenc_e rom th e v = 0 level of th eA2AI tate to the v = 0 level of the X 2A, state (0-0 band at 6750

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7902 J. Am. Chem. SOC.,Vol. 112. No. 22 , 1990 Pianalto et al.Table I . Band Origins for the CaBH , and Sr BH 4 VibronicTransitions (cm-')

band CaBH, SrBH,2-0 15 347'1-0 15934 14 9390-0 15 458 145290- I 15 006 I 4 134

B2E1p-%Al2-0 15 1441-0 1586 5 I4 7390-0 15 394 143310-1 14948 13937

82E,p- i (2Al

A 2 ~ , - j I 2 ~ ,2-0 157411 - 0 15212 I4 1270-0 14804 137230-1 14 349 I3 3330-2 13 895 129 53

"Errors are approximately 1 1 0 cm-l.Table 11. Vibrational Frequencies for CaB H, and SrBH , (cm-I)

s ta te CaBH, SrBHdB2E 472' 408A2A 469 404g2A 1 455 388" Errors are approximately f O cm".

A ). The other two expanded features at approximately 6900 and7300 A also result from mixtures of emission to excited vibrationallevels of th e ground X2 Al sta te fEom the u = 0 level of the B2Estate and the u = 0 level of the A2 AI state.Band origins determined for CaB H, and SrB H, are presentedin Table I . For each band, the origin was determined by averagingvalues from both laser excitation and resolved fluorescence scans.Finally, vibrational frequencies for the t hree observed states weredetermined from the band origins and are presented in Tab le 11.Because the electronic transitions ar e centered on the metal atom ,only one mode, the M-BH4 stretch, displays Franck-Condonactivity.Ab initio calculations are not available for CaBH , or SrBH,,but the predictions2e26 for LiBH,, NaBH,, and KBH, a re veryhelpful. Th e presence of an extra nonbonding electron in thecorresponding alkaline ear th monoborohydrides should not greatlyaffect the geometric structure. In Figure I , the three most likelystructures are shown. The tridentate configuration is best de-scribed as Ca+ bonding to one face of the BH4- tetrahedronthrough three bridging hydrogens. Th e bidentate structu re hasthe Ca+ ion bonding to the edge of the BH4- tetrahedron withtwo bridging hydrogens. Finally, the monodentate structure hasthe Ca + bonding to a vertex of the BH4- tetrahe dron th rough asingle hydrogen atom. Th e point groups are C3,,C an d C3, forthe tridentate, bidentate, and monodentate bonding, respectively.The ab initio calculations2*26 consistently predict that themonodentate stru cture lies about 20 kcal/m ole higher in energythan the bidentate and tridentate structures. Also, monodentatecoordination has never been observed in a metalso is not likely to be important for CaBH, or SrBH,.Our assignment of C3, symmetry for CaBH, and SrBH, restson the similarity between the spectra of these molecules and thespectra of, for example, CaC H3 ,5CaN3,I2 nd CaCSH5 ,l4 hichall have axial symmetry (i.e., an axis of rotational symmetry, C,,,with n > 2) . I f the CaBH, molecule has C symmetry withbidentate coordination (Figure l ) , then the excited 2E state wouldsplit into two electronic sta tes of 2B l and 2B2symmetry similarto CaN H2 aJ8 nd Ca02C H." If the CaBH, molecule has C,,symmetry (Figure I ) , then the 2E state will split into two spincomponents, ZEl/2. nd tE312, like CaCH,.' For comparisonpurposes, other sp in-orb it splittings for the 2 E and 211states ofmonovalent alkaline earth derivatives are provided in Table 111.On the whole, the CaBH , and SrBH, 2E state splittings are more

Table 111. Observed Spin-Orbit Splittings (cm-I) for the Lowest 211or 2E States of Some Alkaline Earth C ontaining Free Radicals, ML( M = Ca o r S r and L = Ligand)molecule CaL Sr L molecule Ca L SrLM 1 73 281 MN CO ' 68 293MOCH, ' 65 268 MCSHjh 57 255MN3' 76 296

References I O and 29. *Referen ces 9 and 30. References 19 and28. dRefer ence 15. eRefere nce 12. 'Reference 13. 'Reference 7.hRefe rence 14. 'Th is work.

M O H b 61 264 MCH,' 73 309MC CH d 70 275 MBH, 59 200

CORRELATION DIAGRAM

Mt

Figure 5. This correlation diag ram shows the effects of various ligandsperturbing the metal ion (Sr+ and Ca') atomic orbitals. Th e orderingof the first excited states, 2E and 2 Al , or MB H, radicals is reversed whencompared with the MC H, radicals. The difference in ordering is at-tributed to differences in ionic bonding. For the MCH , mo lecules,bonding occurs on the metal-ligand axis, between the alkaline eart hcation M+ and the negatively charged carbon. For the MBH, molecules,however, bonding also occ urs off the metal-ligand axis, between thecation M + and thr ee bridging hydrogens that bear a partial neg ativecharge. This partial negative charge destabilizes the off-axis 2E staterelative to the 2 Ai state. The atomic orbital character of 2E and 2A Istates and the natu re of the metal-ligand bond determine the relativeordering of the states.suggestive of a C3,spin-orbit splitting than a 2BI-2B2electronicsplitting.Calcium borohydride is currently being analyzed using high-resolution laser techniques in an attemp t to m ake a definite as-signment of the states observed, and to determ ine t h e molecularrotational constants. Several su bb an ds of the B2E-X2A1 ransitionhave been recorded but the fits to determine th e molecular con-stants were not very reasonable, presumably because of theperturbations. However, there was no sign of the expectedsplitting2, of the rotational lines due to the fluxional behavior ofCaBH,. Either the fluxional motion is so fast that very largesplittings result or so slow that the small splittings were notresolved.Th e expected size of these splittings is hard to predict becausethey depend critically on the unknown height of the barrier forthe BH4- internal rotation. For example, Baranov and B ~ l d y r e v ~ ~calculate a ground s tate splitting of 6 X cm-' for LiBH, ifthe barrier is about 1000 cm-I, but if the barrier drops to about600 cm-' (e.g. in LiCH,+), then th e tunnelling splitting is cal-culated2, t o be 0.001 cm-l. As is often the case for fluxionalbehavior, the stud y of deuterated derivatives may also prove tobe useful.The ordering of the excited states of the alkaline ear th boro-hydrides is surprising. Previously analyzed symmetric top mol-ecules (CaCH,,' SrCH 3' and SrOCH 328 have 2E states lower

(28) OBrien, L. C.; Brazier, C. R.; Bernath. P. F. J . Mol. Spectrosc. 1988,(29) Steimle, T. C.; Domaille, P. J.; Harris, D. 0. J . Mol . Spectrosc. 1978,7 3 , 441-443. Nakagawa, J.; Domaille, P. J. ; Steimle, T. C. ; Harris, D. 0.J . M o l . Spectrosc. 1978, 70,374-385. Field, R. W.; Harris, D. 0.; anaka,T. J . Mol . Spectrosc. 1975, 57, 107-117.

130. 33-45.

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J . Am. Chem. SOC.1990, 112, 7903-7908 7903in energy than ZA i tates, whereas in the borohydrides the orderingis reversed (Figure 5 ) . In this respect the CaBH, and SrBH,molecules resemble the formate derivatives,' I C a 0 2 C H a ndSrO zC H (as well as the formamidateI6 derivatives). Th e 2Al statesof the formates" lie lower in energy than t he 2B l,2B2pair of state swhich correlate to the ZEor 211states of a high symmetry axialligand.Th e alkaline earth borohydride and formate molecules shar eone striking similarity. In each one bonding occurs off themetal-ligand axis with bridging ligand atoms (three hydrogensfor the borohydrides an d two oxygens for the formates). Th ebridging atoms have a partial negative charge. When they pointtowards the off-axis Ca + and Sr+p nd d-orbitals containing theexcited electron, the d i k e orbitals ar e destabilized relative to theon-axis d i k e orbitals.In the cases of the methyl and mcthoxy derivatives, bondingoccurs on he metal-ligand axis so the negative charge of the ligandpoints to the on-axis orbitals of the cation. When containing the

excited extra electron, the resulting d i k e molecular orbitals aredestabilized relative to the d i k e orbitals.Figure 5 is a correlation diagram of the energy levels resultingfrom ligand and alkaline earth cation interactions. Th e BH4-ligand only partially lifts the p- and d-orbital degeneracy of theC a+ or Sr+ atom. In addition, the BH4- lig_and mixes the p a ndd character of the atsm ic orbitals so that the AZAl tate is a po -dumixture while the BZE state is a p r - d r mixture. The locationof the zA tat e is uncertain, bu t w,e suspect (con trary to Figure5 ) that the *A state lies above the A an d B states for CaBH, andSrBH,.ConclusionWe have discovered the CaBH, and SrBH, molecules by thereaction of Ca an d Sr vapors with dib orane. Th e low-resolutionspectra are consistent with a tridentate molecular structure of C,,symmetry with three bridging hydrogens. N o evidence of fluxionalbehavior has been found yet, but th e possibility cannot be ruledout. Some ab initio calculations on the alkaline earth mono-borohydrides would be most welcome,

Acknowledgment. This research w as supported by th e NationalScience Foundation (CHE-8608630, CHE-8913785) and theAstronautics Laboratory, Edwards Air Force Base, CA.

(30 ) Brazier, C. R.; ernath, P. F. J . Mol. Specrrosc. 1985, 114, 163-173.Hilborn, R.C.: Zhu, Q. : Harris, D. 0.J . Mol. Specrrosc. 1983, 97 , 73-91.Nakagawa, J. ; Wormsbecher, R.F.; Harris, D. 0.J . Mol. Specfrosc. 1983,97 , 37-64.

Methyl Chloride/Formic Acid van der Waals Complex: AModel for Carbon as a Hydrogen Bond DonorCharles H. eynoldsContribution fr om the Computer Applications Research Department, Rohm and Hans Com pany,Spring House, Pennsylvania 19477. Received September 22, 1989

Abstract: AMI and MP2/6-31+G*//6-31G** + ZPE calculations are reported for the van der W aals complex of formicacid and methyl chloride. Both theoretical approaches predict the formation of a relatively strong C-H-0 hydrogen bond.Four minim a were located for the formic acid/methy l chloride van der W aals complex, with hydrogen bond strengths rang ingfrom 2.12 to 5.03 kcal/mol. The strength of this interaction argues that carbon may act as a hydrogen bond donor m ore readilythan is generally assumed. Addition ally, the compute d formic acid/methy l ch loride hydrogen bond has significant implicationsfor polymer compatib ility by providing support for the hypothesis that compatibility of PVC /acry late polymer blends is attributableto the formation of C-H-0 hydrogen bonds.

IntroductionInterest in carbon as a hydrogen bond donor stems from therole this type of hydrogen bond is increasingly thought to playin diverse area s of chemistry.' Th e C-He-X hydrogen bond hasbeen implicated as a factor in determining crystal-packingstructures for a variety of molecules,u particularly biomolecules$>in the anaesth etic mode of action6 for certain halogen ated com-pounds such as chloroform; and in polymer miscibility? Th e latteris a topic of special importance due to burgeoning commercialinterest in polymer blends.( I ) Green, R . D. Hydrogen Bonding by C -H Groups; Wiley: New York.(2) Sarma, J . A. R . P.; Desiraju, G . R.Acc. Chem. Res. 1986, 19, 222.(3) Taylor, R.:Kennard, 0.Acc. Chem. Res. 1984, 17, 320.(4) Saenger, W. Angew. Chem., Inr . Ed. Engl. 1973. 12 , 591.(5) Jeffrey, G. A.: Maluszynska, H. Inr . J . Biol. Mucromol. 1982,4, 173.(6) Sandorfy, C.: Buchet, R.: ussier, L. S.;Menassa, P.: Wilson, L. Pure(7) Olabisi, 0.;Robeson, L. M. ; Shaw, M. T. Polymer-Polymer Misci-

1974.

Appl. Chem. 1986, 58 , I 115.Miry ; Academic Press: New York, 1979.

0002-7863/90/ 1 5 12-7903$02.50/0

Carbon as a hydrogen bond donor has been the subject of threeextensive surveys of the crystallographic lit era t~ reP .* *~ll of whichconcluded that carbon can indeed act as a hydrogen bond donorunder certain circumstances. Theoretical'&I6 and experimentall-"studies aimed at determining the magnitude of C-H-X hydrogenbonds are limited, but those that a re available indicate interactionenergies as large as -5.4 kcal/mol for systems such as m alononitrileand water." This certainly constitutes a respectable hydrogen(8) Desiraju, G. R. . Chem.Soc., Chem. Commun. 1989, 179.(9) Taylor, R. : Kennard, 0.J . Am. Chem. Soc. 1982, 104, 5063.(IO) Deakyne, C. A. Ionic Hydrogen Bonds Part 11. Theoretical Calcu-lations. In Molecular Srrucrure und Energerics; Liebman, J . F.: Greenberg,A., Eds.; VCH: Ne w York, 1987; Vol. 4.( I I ) Kumpf, R. .; Damewood, J . R. , Jr . J . Chem.Soc.,Chem. Commun.1988, 621.(12) Tamura. Y.; Yamamoto, G.: Oki, M. Chem. Lett . 1986, 1619.(13) Oi, T.: Sekreta, E.; Ishida, T. J . Phys. Chem. 1983, 87 , 2323.(14) Popowicz, A.; Ishida, T . Chem. Phys. Lerr. 19fl1, 83 , 520.(15) Vishveshwara, S . Chem. Phys. Le t t . 1978, 59 , 26.(16) Berkovitch-Yellin, Z.; Leiserowitz, L. J . Am. Chem.Soc. 1982, 104,(17) Meot-Ner (Mautner), M. Acc. Chem. Res. 1984, 17, 186.4052.

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