M. J. Dick et al- High resolution laser excitation spectroscopy of the B^2-E-X^2-A1 transitions of calcium and strontium monoborohydride

8/2/2019 M. J. Dick et al- High resolution laser excitation spectroscopy of the B^2-E-X^2-A1 transitions of calcium and stronti

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High resolution laser excitation spectroscopy of the B 2E -X 2A 1 transitionsof calcium and strontium monoborohydride

M. J. Dick Department of Physics, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

P. M. Sheridan a and J.-G. Wang b Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

P. F. Bernathb ,c

Department of Physics, University of Waterloo, Waterloo, Ontario N2L 3G1, Canadaand Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

Received 22 January 2007; accepted 14 March 2007; published online 27 April 2007

High resolution spectra of theB 2 E - X 2 A1 transitions of CaBH4 and SrBH4 have been recorded usinglaser excitation spectroscopy in a laser ablation/molecular jet source. Because of rotational coolingin the molecular jet and nuclear spin statistics, transitions arising from only theK =1 K =0,K =2 K =1, andK =0 K =1 subbands have been observed. For each molecule, an analysis of the data using2 E and 2 A1 symmetric top Hamiltonians yielded rotational, spin-orbit, andspin-rotation parameters for the observed states. For both molecules the rotational constantscompare well with those calculated for a tridentate borohydride structure. A large reduction in the

spin-orbit splitting and in the metal-ligand separation for each molecule indicates an increase in theamount of d atomic orbital character in the rst excited2 E states of the monoborohydrides ascompared to the monomethyl derivatives. For each molecule no evidence of internal rotation of theBH4 ligand was found.A change in the magnitude and sign of the spin-rotation constant1 conrmsan energy reordering of the rst excited2 E and 2 A1 states in both CaBH4 and SrBH4 as comparedto CaCH3 and SrCH3. The data also suggest that theB

2 E 1/2 rotational energy levels of CaBH4 maybe perturbed by a vibronic component of theA 2 A1 state. 2007 American Institute of Physics .DOI:10.1063/1.2723097

I. INTRODUCTION

CaBH4 and SrBH4 are the only members of the isoelec-tronic 2 p ligand family of molecules CaF/SrF,CaOH/SrOH, CaNH2 /SrNH2, and CaCH3 /SrCH3 16 thathave not yet been observed using high resolution spectros-copy. From the previous investigations of the molecules inthis series, the metal-ligand bonding has been found to belargely ionic and to occur between the metal cation and theheaviest atom of the negatively charged ligand. In contrast,the borohydride anionBH4 , which possesses a tetrahedralstructure, cannot form a bond directly between the centralboron and metal atoms.7,8 In this case, the metal-ligandbonding in the alkaline-earth borohydrides must occurthrough bridging hydrogens. As a result, three structures arepossible, monodentateone bridging hydrogen,C 3v symme-try , bidentate two bridging hydrogens,C 2v symmetry, andtridentate three bridging hydrogens,C 3v symmetry struc-tures can be found in Ref.9 . In addition, the electronic

structure of the molecules in the 2 p ligand series has beenwell described in terms of a perturbation of the metal atomorbitals by the ligand. The unique geometric conguratipossible for the alkaline-earth borohydrides may disrupt tperturbation description and may have an affect on the cited state electronic structure, which makes them of partilar interest for spectroscopic investigation.

One group of metal borohydrides that have been the su ject of several studies, both theoretical and experimentalthe alkali metal borohydrides. The experimental work918 hasconsisted primarily of microwave spectroscopic investigtions in the gas phase. In these studies, the rotational ahyperne parameters have been determined for the grouelectronic states of NaBH4,912 NaBD4,10,13 LiBH4,10,14,15LiBD4,13 and KBH4.11,13 Based on isotopic data, lithium andsodium borohydrides were found to possess the tridentstructure in the ground state. Theoretical studies1928 havealso predicted this geometric conguration for the groustate. Additionally, several of these theoretical studies hacalculated a low-energy barrier to the internal rotation of ligand,19,2225,27,28 in which the BH4 cycles from one triden-tate minimum to another through a bidentate intermedistructure. This phenomenon will manifest itself in theob-served rotational spectra as a doubling of spectral lines.29 Inthe rotational spectroscopic studies of the alkali metal bo

a Present address: Department of Chemistry and Biochemistry, Canisius Col-lege, Buffalo, New York, 14208-1098.

b Present address: Department of Chemistry, University of York, Heslington,York, YO10 5DD, UK.

c Author to whom correspondence should be addressed. Tel:44-1904-434526; Fax: 44-1904-432516; Electronic mail: [email protected]

THE JOURNAL OF CHEMICAL PHYSICS126 , 164311 2007

0021-9606/2007/126 16 /164311/10/$23.00 2007 American Institute of Physics126 , 164311-1

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hydrides, this doubling of spectral lines was not resolved,indicating that the barrier to the internal motion of the BH4group is larger than that calculated.

Studies involving the alkaline-earth borohydrides havebeen far less extensive. Pianaltoet al. 30 have used laser ex-citation and dispersed uorescence experiments in a Broidaoven to examine theA 2 A1- X 2 A1 and B 2 E - X 2 A1 electronictransitions of CaBH4 and SrBH4 at low resolution. Fromtheir spectra, they suggested that these molecules possessedC 3v symmetry and a tridentate structure similar to the struc-ture found later for the alkali borohydrides. Most interest-ingly, they found that the rst excited2 A1 and 2 E states ap-peared to be reversed in energy relativeto the correspondingstates in the metalmonomethyls, SrCH35,6 and CaCH3.6,31,32Subsequently, Ortiz33 used electron propagator methods tocalculate the energy, vibrational frequencies, geometricstructure, and orbital character of several electronic states of CaBH4. From this work a tridentate structure was predictedfor CaBH4. However, the rst excited2 E and 2 A1 electronicstates were found to be ordered in energy the same as inCaCH3, contrary to the experimental work. In addition, Chanand Hamilton34 have used density functional theory to calcu-late ground state geometries and vibrational frequencies forthe monoborohydrides of calcium and strontium. In theirwork they also found that the tridentate structure was thelowest in energy in the ground electronic state.

In an attempt to further understand the geometric andexcited state electronic properties of CaBH4 and SrBH4 andto reconcile the discrepancy between theory and experimentfor CaBH4, we have recorded high resolution laser excitationspectra of theB 2 E - X 2 A1 transitions of both calcium andstrontium monoborohydride. For each molecule the rota-tional and ne structure parameters have been determined for

the ground and excited electronic states. These constantshave conclusively established that the rst two excited elec-tronic states have reversed their order in energy relative totheir MCH3 analogs. A discussion of the geometry, energyordering, and orbital character of the observed states will bepresented. In addition, a comparison of the observed spectro-scopic parameters with those of SrCH3 and CaCH3 will bedescribed.

II. EXPERIMENT

A laser ablation/molecular jet source was used to pro-duce the borohydrides of calcium and strontium in the gas

phase.35

First, a gas mixture was introduced into a reactionnozzle via a pulsed valve. For this synthesis a mixture of 5%diborane in argonPraxair, at a backing pressure of 100 psi,was used as the reactant gas. Next, the third harmonic355 nm of a pulsed 10 Hz neodymium-doped yttrium

aluminum garnet laser10 mJ/ pulsewas used to vaporize ametal target rodcalcium or strontiumlocated in the nozzle.Inside the reaction region a high-energy plasma was formed.Here the diborane reacted with the metal vapor, producingthe metal borohydrides. The molecules then exited into thelow pressure chamber 1 107 Torr , forming a molecu-lar jet with a low rotational temperatureT rot 46 K. Aprobe laser was then sent perpendicularly through the expan-

sion 15 cm downstream. The resulting molecular uorecence was collected by a photomultiplier tube, sent througpreamplier100 current, and processed by a boxcar inte-grator. Bandpass lters20 nm were used to help attenuatestray radiation from the ablation laser and plasma emissi

Initially, low resolution survey scans were obtained uing an argon-ion-pumped linear dye laserlinewidth

30 GHz scanned at a speed of 50 cm1 /min. DCM andpyridine 2 laser dyes were used to observe the spectral regof interest 13 50015 500 cm1 with a typical maximumoutput power of 1 W pump power 5 W. A LABVIEWdata acquisition program was used to plot the output signfrom the boxcar integrator versus the wave number of laser, which was obtained using a wavemeterBurleigh WA-2500 Wavemeter Jr.interfaced with the program.

For the B 2 E - X 2 A1 transitions of both CaBH4 andSrBH4, an argon-ion-pumped Coherent Autoscan 699-ring dye laser system, operating with DCM laser dye, wused to obtain high resolution spectralinewidth 10 MHz,

output power 1 W pump power 8 W . Generally, spec-tra were obtained in 5 cm1 portions at a scan rate of 180 s /cm1 and a data sampling interval of 10 MHz. Tachieve an adequate signal to noise ratio, several24 por-tions were usually averaged together. The nal spectra wthen calibrated using the line positions of I2 obtained fromsimultaneously recording its laser excitation spectra.36

III. RESULTS

Figure 1 shows low resolution survey spectra of th A 2 A1- X

2 A1 and B 2 E - X 2 A1 transitions of CaBH4 and

SrBH4. In each spectrum, three peaks are clearly present. F

FIG. 1. Low resolution spectra recorded for theA 2 A1- X 2 A1 and B

2 E - X 2 A1transitions of SrBH4 upper paneland CaBH4 lower panel. For each mol-ecule theA 2 A1- X 2 A1 transition was weaker in intensity and located at lower energy than the correspondingB 2 E - X 2 A1 transitions. This energyordering is in contrast to that of CaCH3 and SrCH3, but is consistent withthe previous low resolution observation of calcium and strontiborohydrides.

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CaBH4 the separation between the rst and second peaks is600 cm1, while the second and third peaks are spaced by70 cm1. The 70 cm1 separation is in the range of the

spin-orbit splitting previously measured for theA states of CaF, CaOH, and CaCH3.1,2,6,31,32 This suggests that the sec-ond and third peaks are two spin-orbit components of an

orbitally degenerate electronic state. This scenario is onlypossible if the molecule possesses a geometric congurationwith C 3v symmetry. For a molecule havingC 2v symmetry,such as CaNH2,37 no two of the rst three excited statesshould be spaced by this70 cm1 separation exhibited bycalcium-containing moleculesC 2v symmetry eliminates rstorder spin-orbit coupling. Therefore, the wave number sepa-rations exhibited by the rst three peaks for CaBH4 suggestthat C 2v symmetry is unlikely. A similar conclusion can bearrived at for SrBH4, where the second and third peaks arespaced by 200 cm1. This separation is consistent with thepreviously observed spin-orbit splitting in theA states of SrF,SrOH, and SrCH3.13,5 As a result, for the borohydrides therst excited2 A1 state was found to lie lower in energy thanthe rst excited2 E state, consistent with the previous obser-vations of the metal borohydrides.30 It is also of interest tonote the decreased intensity of theA 2 A1- X

2 A1 transitionsas compared to theB 2 E - X 2 A1 transitions in the spectra inFig. 1. For each molecule the decreased signal strength wasobserved previously30 and does not seem to be simply aneffect of the lower laser power. The decrease in intensity of the A 2 A1- X

2 A1 transitions impeded the measurement of their high resolution spectra.

Figure2 shows the overall high resolution spectra ob-tained for both spin-orbit components of theB 2 E - X 2 A1 tran-

sitions of CaBH4 and SrBH4. TheB 2 E - X 2 A1 transitions cor-

respond to ap s promotion on the calcium or strontiumion. Transitions of this type should have the general appeance of a Hunds casea 2 Hunds caseb 2 + perpen-dicular transition of a corresponding linear molecule suchCaCCH38,39 and SrCCH.40 In this case two spin-orbit com-ponents, each with a well dened origin and1 B and 3 Bspaced branches, are present. For SrBH4, the B

2 E 3/2- X 2 A1componentlower left panelindeed exhibits the appearanceof a perpendicular transition with1 B and 3 B spacedbranches. TheB 2 E 1/2- X

2 A1 component upper left panel,on the other hand, exhibits a pattern that closely resembleparallel transition with only2 B spaced branches present.For theB 2 E - X 2 A1 transition of CaBH4, neither spin-orbitcomponent top or bottom right panelhas the anticipatedperpendicular-like appearance nor a clear origin.

The energy level structure of both theB 2 E and X 2 A1states of the metal monoborohydrides closely resembles thatof the corresponding monomethyls.5,31,32 These energy levelscan be described by considering two factors. First, the bohydrides are prolate symmetric top molecules belongingthe C 3v symmetry group. Therefore, their rotational energlevel structure is split into various sublevels, labeled K =K R+ e , whereK R and e are the projections of the rota-tional angular momentum of the nucleiR and the elec-tronic orbital angular momentumL onto the symmetryaxis, respectively. Second, the spin angular momentum asciated with the unpaired electronS must also be taken intoaccount. In the ground2 A1 state, the spin of the unpairedelectron interacts with the rotational angular momentumthe molecule to split each rotational level into two sprotation components, labeled byJ , the total angular momen-

tum. These two components are identied byF 1 e parityand F 2 f parity. In the2 E state, the spin of the unpairedelectron interacts with the electronic orbital angular momtum and gives rise to the2 E 1/2 F 1 and 2 E 3/2 F 2 spin-orbitcomponents. Each rotational level in each spin-orbit comnent is further split intoe and f parity levels by the spin-rotation and Jahn-Teller interactions. This splitting is similarto -type doubling in a linear molecule41 and, as noted byboth Hougen41 and Brown,42 should be most predominant inthe K =1 levels and may be negligible in the other observK levels. A more detailed description of the energy levstructure along with energy level diagrams for2 E and 2 A1states can be found in Refs.5, 31, and 32

The allowed rotational transitions between theB 2 E and X 2 A1 states are determined by the selection rules for perpendicular-type transition, K =1 and J =0,1. In-cluding parity dependence,43 this results in six branchesper K subband within each spin-orbit component. Eacindividual branch is labeled by the notationK J F i F ji =1,2; j=1,2 , where K is represented by a lower case

p K =1 or r K = +1 .42 Because the rotation of theborohydrides about the symmetry axis exchanges the protof the BH4 ligand, the effect of nuclear spin statistics mualso be considered. In this case, this phenomenon gives rto two nuclear spin states: orthoK =3 N , whereN is an in-

FIG. 2. High resolution spectra of the two spin-orbit components of the B 2 E - X 2 A1 transition of SrBH4 and CaBH4 are plotted on a relative wave-number axis scale for comparison. TheB 2 E 3/2- X 2 A1 spin-orbit componentof SrBH4 is shown in the bottom left panel. This component has an expectedappearance similar to a Hunds casea 2 Hunds caseb 2 + transitionwith 1 B and 3 B spaced branches. TheB 2 E 1/2- X

2 A1 spin-orbit compo-

nent of SrBH4 is shown in the top left panel. This component has a veryunique structure with no clear origin, and all of the lines of the observedbranches appear to be spaced by2 B. Neither component of theB 2 E - X 2 A1transition of CaBH4 right side has the appearance of a perpendicular tran-sition. Each component is highly congested and lacks any clear origin,which hampered the rotational assignment of the lines in these spectra.

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teger and para K 3 N .43 Molecules in each of thesenuclear spin states behave like separate species and cannotrotationally cool to one another in the free jet expansion. Asa result, bothK =0 andK =1 levels are populated in the jet.Therefore, according to the selection ruleK = 1, three K subbands should be predominantly observed in our spectra,K =1 K =0, K =0 K =1, and K =2 K =1, whichgive rise to 36 branches in total.

Rotational assignments of theB 2 E - X 2 A1 transition of SrBH4 were completed by rst examining theB

2 E 3/2- X 2 A1spin-orbit component. The similar appearance of this compo-nent with theA 2 E 3/2- X

2 A1 transition of SrCH35 facilitatedbranch assignments. Because no previous ground state com-bination differences were available, rotational assignmentswere made within eachK subband using rst lines and thengenerated lower state combination differences. Subsequently,this set of combination differences was used to make rota-tional assignments in theB 2 E 1/2- X

2 A1 component, whichlacked any similarity with the corresponding component of SrCH3. In this case, a series of branches on each side of the

origin were rst identied and then grouped together utiing the combination differences. Using this method, 17 of 18 possible branches from all three observedK subbandswere identied in this spin-orbit component. Figure3 showsa portion of the spectra with line assignmentsK J F i F ji =1,2; j=1,2 , for each spin-orbit component of th

B 2 E - X 2 A1 transition of SrBH4. The unusual structure of the B 2 E 1/2- X 2 A1 component, as compared to theB 2 E 3/2- X 2 A1spin-orbit component, can clearly be observed in the fobranches r P 11, r Q12, p R12, and pQ11 that have convergednear the origin and have adopted an appearance similar tstrongQ branch.

The assignment of theB 2 E - X 2 A1 transition of CaBH4was not as straightforward. Unfortunately, neither spin-orcomponent exhibited the expected appearance of a perpdicular transition, and both were exceptionally congestedaddition, each component lacked a clear origin, makingdifcult to identify the starting position of any branch. A

FIG. 3. Subsections of the high resolution spectra of theB 2 E 1/2- X 2 A1 toppanel and the B 2 E 3/2- X

2 A1 bottom panel spin-orbit components of SrBH4. For each component, 15 of the 18 possible branches are shown. Ineach spectrum, branches assigned to theK =1 K =0 subband are labeledon the bottom in italics and those belonging to theK =2 K =1 andK =0 K =1 subbands are labeled on the top. Individual rotational linesare labeled by the notationK J F

iF

j

i =1,2; j=1,2 . In theB 2 E 1/2- X 2 A1spin-orbit componenttop panel the rotational lines of 11 of the 15 ob-served branches are spaced by2 B and the r P 11, r Q12, p R12, and pQ11branches have almost converged near the origin.

FIG. 4. A portion of the high resolution spectra, including rotational assments, for each spin-orbit component of theB 2 E 1/2- X 2 A1 transition of CaBH4. Again, theK =1 K =0 on the bottom in italics, K =2 K =1, andK =0 K =1 on the top subbands are shown, and individualrotational lines are labeled byK J F i F j i =1,2; j=1,2 . For the B 2 E 3/2- X 2 A1 spin-orbit componentbottom panel, the pQ22 and pP 21branches begin at the blue of ther Q21 and r R22 branches, resulting in a veryoverlapped and congested structure for this spin-orbit component. T B 2 E 1/2- X 2 A1 spin-orbit componenttop panel has a clear origin; however,the pP 11 branch can only be ambiguously assigned, suggesting a perturbtion see text.




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result, a different approach was utilized in order to make therotational assignments. Again, as with SrBH4, a set of lowerstate combination differences was not available from previ-ous spectroscopic work. However, in the analysis of the B 2 E - X 2 A1 transition of SrBH4, the lower state combination

differences were found to be consistently1.02 timessmaller then the analogous combination differences forSrCH3.44 Therefore, initially, this factor wasapplied to thelower state combination differences of CaCH345 in order togenerate an approximate set of combination differences forCaBH4, which were then used to make rotational assign-ments for all of the possibleR and P branches in the B 2 E 3/2- X 2 A1 spin-orbit component. As was the case forSrBH4, the lower state combination differences from thiscomponent were then applied to theB 2 E 1/2- X

2 A1 spin-orbitcomponent of CaBH4. Using this method, 10 of the 12possibleR and P branches for this spin-orbit componentwere identied. The two branches that were not observed

r P 12 and p R11 form combination difference pairs with ther R12 and pP 11 branches, respectively. As a result theJ assign-

ments of ther

R12 andp

P 11 branches were made by identify-ing their rst lines. All 12 possibleQ branches, from bothspin-orbit components, were also assigned. However, themajority of these branches were not completely resolvedfrom their associatedP or R branch, suggesting that theground state spin-rotation interaction is quite small. Figure4shows a portion of the high resolution spectra with rotationalassignments K J F i F j i =1,2; j=1,2 for each spin-orbitcomponent of this electronic transition. In the top spectrum,the starting positions of the branches of theB 2 E 3/2- X

2 A1component can be observed to vary greatly from those of SrBH4. For example, thepP 21 branch begins at the blue of the r R22 branch, which obscures the origin of the B 2 E 3/2- X 2 A1 spin-orbit component. This irregular patterncontributed to the difculty in identifying the starting posi-tions for each series of lines.

IV. ANALYSIS

The B 2 E - X 2 A1 transitions of SrBH4 and CaBH4 weremodeled using a symmetric top effective Hamiltonian41,42,46that incorporated the rotation of the molecule, along wcentrifugal distortion corrections, and the spin-rotation, sporbit, Jahn-Teller, and Coriolis interactions. Using the matrixelements of Endoet al. ,46 in a Hunds casea basis, this

Hamiltonian was incorporated into a computer program, aa least squares t of the measured data was performed.For SrBH4, 181 lines from 35 of the 36 possibl

branches from all three observedK subbands were includedin the t. Clearly resolved lines were weighted accordingthe experimental uncertainty0.004 cm1 , while overlappedlines were deweighted slightly0.008 cm1 . The measuredlines can be found in Ref.47. In the ground state only onerotation B and one spin-rotation bc = bb + cc /2 con-stant were allowed to vary. TheA rotational constant in theground state could not be determined from the current dset because no K =0 transitions were observed. As a resulit was xed to the theoretical value given by Chan anHamilton.34 Additionally, no centrifugal distortion termwere found to be necessary due to the lowJ values J 9.5 observed in the spectra. For the excited state, a sof molecular constants similar to those used to describe A 2 E state of SrCH35 were employed. These included rota

tion A, B , spin-orbita ed ,a D ed , Coriolis A t , and spin-rotation aa , 1 terms. The spectroscopic parameters of th B 2 E andX 2 A1 states of SrBH4 derived from the nal t arelisted in TableI along with those of theA 2 E andX 2 A1 statesof SrCH3 for comparison.

The tting of theB 2 E - X 2 A1 transition data of CaBH4was not as straightforward as it was for SrBH4. In the rstattempt at tting the rotational transitions of the three oserved K subbands, residuals for thepP 11 branch werefound to be unusually large. As a result, a second t w

TABLE I. Spectroscopic constantsin cm1 for theA 2 E and X 2 A1 states of SrCH3 and theB 2 E and X 2 A1states of SrBH4.

Constanta

SrCH3b SrBH4

X 2 A1 A 2 E X 2 A1 B 2 E

T 0.0 13 800.37629 0.0 14 422.556618a ed 279.165117 198.499436

a D ed 0.029715 0.033715 A t 5.248 3383 4.012311 A 5.390c 5.334 9475 4.213c 4.177 0876 B 0.193 833 33616 0.195 06821 0.188 36750 0.194 29044

D N 2.148 9311 107 D NK 1.613 4951 105H NK 2.92021 1010 H KN 3.18 43 109

aa 0.000 88334 0.369233 0.079244bc 0.004 128 5243 0.002 62391 0.094 1512 0.185 0916

aValues in parentheses are 1 standard deviations, in units of the last signicant digits.bFrom Ref.5cFixed to the theoretical valueRef.34 .




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conducted without this branch. A subsequent prediction, us-ing the parameters of this t, indicated that theJ values of the pP 11 branch should be decreased by 1 from the originalassignment. Initially, this result was not surprising as thisbranch was originally assigned by the identication of a sup-posed rst line. However, these newJ assignments of the pP 11 branch resulted in one lineidentied with an asterisk* in Fig.4 being unassigned. Subsequent attempts to as-

certain the identity of this line in the context of this t wereunsuccessful. The iodine calibration spectrum and etalontrace signal were examined closely to ensure that this linewas a real feature and not the result of a laser mode hop. Inaddition, attempts at reassigning the observed branches of allthree K subbands in theB 2 E 1/2- X

2 A1 spin-orbit componentto include this line were unsuccessful. The line positions of the pP 11 branch were also calculated using the lower statecombination differences from the 2-1 subband to determinethe K =1 rotational level energies, and the upper stateK =0 rotational level energies were calculated from upper

state combination differences of thep

R12 andp

P 12 branchesassuming that thee and f parity splitting in theK =0 levelswere negligible as they were for CaCH3,31,32 SrCH3,5 andSrBH4 . This calculation showed that the newJ assignmentsfor thepP 11 branch with one line left unassigned were plau-sible.

Using these rotational line assignments, 169 lines from34 of the 36 possible branches were included in the t andare listed in Ref.47. As with SrBH4, clearly resolved0.004 cm1 and overlapped lines 0.008 cm1 were

weighted differently. In the ground state only the rotationalconstant,B, was allowed to vary. As with SrBH4, no K =0 transitions were observed; therefore, theA rotational con-

stant in the ground state could not be determined and wxed to the theoretical value.34 As mentioned previously, thespin-rotation splitting in theX 2 A1 state appeared unresolved;thus, bc was xed to zero. In the upper state, the rotatio A, B , spin-orbit a ed , Coriolis A t , and spin-rotation

aa , 1 terms were included in the t. Unlike for SrBH4, itwas found that thea D ed term could not be determined forCaBH4. The spectroscopic parameters of theB 2 E andX 2 A1states of CaBH4 determined from this t are listed in TableIIalong with those of theA 2 E and X 2 A1 states of CaCH3 forcomparison.

One likely explanation for the identity of the unassignline in theB 2 E 1/2- X

2 A1 spin-orbit component is that it isindeed the rst line of thepP 11 branch. This would suggestthat the rotational energy levels of theK =0 state of the B 2 E 1/2 spin-orbit component are being perturbed by som

nearby electronic state. For CaCH3, the K =1 level of the B 2 A1 state was found to be perturbed by a vibrational lev

of theA 2

E state. A similar scenario is possible for CaBH4, inwhich a vibronic level of theA 2 A1 state interacts with theK =0 state of theB 2 E 1/2 spin-orbit component. As a resultan alternative t of theB 2 E - X 2 A1 transition data of CaBH4,in which the data from the 0-1 subband of theB 2 E 1/2- X 2 A1spin-orbit component were removed, was performed. Talternative t included both spin-orbit components of the and 2-1 subbands and only the 0-1 subband of t B 2 B3/2- X

2 A1 component. In this t, the rotation A, B , spin-orbit a ed , Coriolis A t , and spin-rotation 1, aa param-eters were determined and are also included in TableII forcomparison.

TABLE II. Spectroscopic constantsin cm1 for theA 2 E and X 2 A1 states of CaCH3 and theB 2 E and X 2 A1

states of CaBH4.

Constanta

CaCH3b CaBH4c CaBH4d

X 2 A1 A 2 E X 2 A1 B 2 E X 2 A1 B 2 E

T 0.0 14 743.382211 0.0 15 438.393262 0.0 15 438.393619a ed 72.709218 62.965411 62.964634

a D ed 8.86 104 A t 5.36003 4.126 6949 4.127212 A 5.448 31 5.38556 4.230e 4.185 9644 4.230e 4.186 3765 B 0.252 384 7 0.254 2707 0.247 35730 0.255 10626 0.247 36731 0.255 12730

DK 7.03 105 6.437 105 D N 3.544 852 107 3.6203 107

D NK 1.995 314 105 1.2980 105

e t 3.522 105

aa 0.012917 0.048011 0.047730bc 0.001 851 05 0.0196171 0.02511 0.086 04581 0.086 1810

aValues in parentheses are 1 standard deviations, in units of the last signicant digits.bFrom Ref.32.cFit included 34 of the 36 possible branches for theB 2 E - X 2 A1 transition. ThepP 11 branch is labeled such thatone line remains unassigned.dFit included all of the possible branches for theB 2 E 3/2- X 2 A1 spin-orbit component and all of the observedbranches for the 1-0 and 2-1 subbands of theB 2 E 1/2- X 2 A1 spin-orbit component. No lines from the 0-1 subbandof theB 2 E 1/2- X 2 A1 spin-orbit component were included.eFixed to the theoretical valueRef.34 .




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V. DISCUSSION

A. Geometry

Because rotational constants for only one isotopolog of both CaBH

4and SrBH

4were determined in this study, it is

difcult to ascertain the structural parameters of these mol-ecules. One approach that can be employed to extract struc-tural information from the observed rotational constants is tox the geometry of the BH4 ligand to that calculated for theground state by Chan and Hamilton34 and assume that theligand structure is invariant upon promotion from theX 2 A1to B 2 E state. Using this method, the rotational constant,B,can be used to derive the metal-boron separation. The calcu-lated distancesare listed in TableIII along with the theoret-ical values33,34 for comparison. For the ground states, there isa very good agreementless than 1% differencebetween themetal-boron separations obtained in this work and those cal-culated by Chan and Hamilton.34 The agreement is slightlyworse for the calculations of Ortiz.33 For the excited statesno theoretical predictions of the rotational constants exist;therefore, no direct comparison of the experimental metal-boron separations to the theoretical values can be made.

Using these experimental values of the metal-ligandseparation, the change in this separation from theX 2 A1 to B 2 E state can be examined. For CaBH4 and SrBH4, themetal-ligand separations decrease by factors of 0.98340.040 and 0.98370.042 , respectively. This is in con-

trast to the metal-ligand separation in CaCH3 and SrCH3,where from theX 2 A1 to A 2 E states the metal-carbon dis-tances Table III decreased by factors of only 0.99630.006 and 0.99680.006 , respectively. The larger de-crease in the metal-ligand separation for the metal borohy-drides, as compared to the metal monomethyls, is consistentwith the theoretical calculations of Ortiz33 for CaBH4. In thiswork, the metal-boron separation in the rst excited2 E statewas found to decrease by a factor of 0.97940.05 ascompared to the ground state. This larger change in themetal-ligand separation was attributed to an increasedamount of d atomic orbital character in the rst excited2 E state of CaBH4 as compared to CaCH3. For calcium andstrontium-containing molecules, the rst two excited statesare derived predominantly fromp and d atomic orbitals lo-

cated on the metal atom. For example, the orbital characof theA 2 state of SrOH48 is 92% comprised of contribu-tions from the 5 p and 4d atomic orbitals on the strontiumion. The molecular parameters in these states are oftenreection of the ratio of p andd atomic orbital characters inthe state. In the case of CaBH4, Ortiz suggested that theincreasedd orbital character would result in the unpaireelectron residing in a more diffuse orbital, which, in tuallows the BH4 ligand to more closely approach the Ca+cation. Similar calculations for SrBH4 do not exist, but, asmentioned previously, a similar change between the grouand excited state rotational constants was observed. Tmay imply that an increased amount of d character is alsopresent in theB 2 E state of SrBH4 as compared to SrCH3.

Finally, uxional behavior, in which the BH4 ligand un-dergoes internal rotation, was predicted to have a low-enebarrier in the alkali metal borohydrides. However, a splittof rotational lines inherent with the internal rotation of tBH4 ligand

29 was not resolved in theB 2 E - X 2 A1 transitionsof CaBH4 and SrBH4. This indicates that the barrier to thi

internal motion is high in the borohydrides of calcium andstrontium, as it is in NaBH49 and LiBH4.14

B. Jahn-Teller and ne structure interactions

For both CaBH4 and SrBH4 the orbitally degenerate B 2 E state may be subject to Jahn-Teller coupling. Howevelectronic states that exhibit a large spin-orbit interaction areexpected to exhibit a small Jahn-Teller splitting.49 Addition-ally, electronic states arising predominantly from atomic bitals located on the metal atom, such as theB 2 E states of the metal borohydrides, should have reduced Jahn-Telcoupling. This is a result of the hydrogen atoms of the lig

being located far from the metal atom, which causes tcoupling of degenerate vibrational and electronic wave functions to be small.50

Assuming that the Jahn-Teller coupling is small in t B 2 E states of CaBH4 and SrBH4, the projections of the elec-

tronic e and vibronic t angular momenta on the symme-try axis can be calculated for these states. First, t can becalculated from the ratio of A t to A for the B

2 E state of each moleculeCaBH4 t =0.9859; SrBH4 t =0.9606. Next,these valuesof t can be used to calculate e via the follow-ing equation:51

t =

ed +

1 d

2 2, 1

in whichd is the Jahn-Teller quenching parameter1 d 0 and 2 is the vibrational Coriolis coupling coefcien

for the Jahn-Teller active degenerate vibrational mode. If Jahn-Teller effect is neglectedd =1 , then according to Eq.1 , e = t and hence e =0.9859 and 0.9606 for CaBH4 and

SrBH4, respectively. For an electronic state derived primarfrom ap atomic orbital, e should be close to 1. Any reduc-tion in this value can be attributed to an increase in tamount of atomic orbital character other thenp. Althoughthese values of e indicate that theB

2 E states of CaBH4 andSrBH4 arise predominantly fromp orbitals located on the

TABLE III. Metalligand separations for the borohydrides and monometh-yls of calcium and strontium.

State M-L separation CaBH4a CaBH4 CaCH3b

X 2 A1 r CaL 2.415 2.414,c 2.430d 2.348

2 E r CaL 2.376 2.342

SrBH4a SrBH4 SrCH3e

X 2

A1 r SrL 2.574 2.596c

2.4872 E r SrL 2.532 2.481aCalculated from the rotational constant,B, from the current investigationand xing the BH4 ligand geometry to that of Ref.34.bExperimental data from Ref.6.cTheoretical data from Ref.34.dTheoretical data from Ref.33.eExperimental data from Ref.5.




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metal, a comparison with the e values of CaCH3 0.995331and SrCH3 0.98385 shows that they are reduced slightly forthe borohydrides. This suggests that theB 2 E states of CaBH4 and SrBH4 contain lessp orbital character than the A 2 E states of the corresponding monomethyls.

A nal indication of the reduction of the amount of porbital character inB 2 E states of CaBH4 and SrBH4 as com-pared to theA 2 E states of CaCH

3and SrCH

3can be found

by examining the spin-orbit interaction. The energy separa-tion of the two spin-orbit components of theB 2 E states isgiven bya ed , wherea describes the magnitude of the spin-orbit splitting. A value of a may be calculated using theexpressiona ed =a t , assuming that the Jahn-Teller interac-tion may be neglected. For SrBH4 a ed =198.4994 cm1 ,the magnitude of the spin-orbit coupling constant is esti-mated to bea =206.6411 cm1. This value is 27% smaller ascompared to SrCH3 a =283.772 cm1 .5 This reduction maybe rationalized by considering the atomic orbital contribu-tions to the spin-orbit splitting parameter for the rst excited2 E states of either SrCH3 or SrBH4, which are given by the

equationa = c5 p 2 5 p + c4d 2 4d + cn2 n , 2

wherec5 p , c4d , andcn are expansion coefcients and 5 p, 4d , and n are the atomic spin-orbit coupling parameters.52

This equation can be further simplied by removing the con-tributions from atomic orbitals other than 5 p and 4d togive

a = c5 p 2 5 p + c4d 2 4d . 3

This is a relatively safe assumption because, as mentionedearlier, in the case of SrOH48 the 5 p and 4d orbitals com-prise 92% of the orbital character of theA 2 state. Forstrontium, the atomic spin-orbit coupling parameter is largerfor the 5 p orbital 5 p=534 cm1 than it is for the 4d orbital

4d =112 cm1 .53 Thus, if the amount of the 5 p characterdecreases and the amount of 4d character increases, the mo-lecular spin-orbit coupling constant is reduced. This changein the amount of d character most likely accounts for themajority of the 27% reduction in the magnitude of the mo-lecular spin-orbit coupling constant in SrBH4 as compared toSrCH3.

A similar reduction in the molecular spin-orbit couplingparameter,a , in the rst excited2 E state is observed for

CaBH4 a =63.87 cm1

as compared to CaCH3 a=72.7092 cm1 .32 In this case the spin-orbit coupling con-stant may be calculated according to the equation

a = c4 p 2 4 p + c3d 2 3d . 4

For calcium, the atomic spin-orbit coupling parameter isagain larger for thep orbital 4 p=148 cm1 than for thed orbital 3d =24 cm1 ,54 but the difference in magnitude isnot as great as for strontium. The reduction ina can againmost likely be largely attributed to an increase in the amountof d character in theB 2 E state of CaBH4 as compared to the A 2 E state of CaCH3.

Another interesting comparison that can be made btween the monomethyls and borohydrides of strontium acalcium involves the spin-rotation constant1. Using thepure precession relationship and unique perturber approxi-mation, the following expression for1 has been derived:31,44

1 =aB + 1 E 2 E E 2 A1

, 5

where is the atomic orbital angular momentum andE is thestate energy. This equation shows that1 arises from an in-teraction of a2 E state with a neighboring2 A1 state. This isanalogous to the -doubling constant,p, in a2 state, whichaccounts for its interaction with a neighboring2 + state. Ac-cording to Eq.5 , the sign of 1 should be dependent on therelative energy positioning of the2 E and 2 A1 states. For ex-ample, if the2 E state lies higher in energy then the2 A1 state,then 1 will have a positive sign. For both CaBH40.086 18 cm1 and SrBH4 0.185 09 cm1 the sign of 1

has changed in comparisonwith CaCH3 0.0251 cm1 32and SrCH3 0.094 15 cm1 .5 This is further evidence thatthe rst excited2 E and 2 A1 states in the borohydrides havereordered in energy as compared with the monomethyls.25

For both CaBH4 and SrBH4 the magnitude of 1 hasincreased in comparison with the monomethyls. As bothaandB have decreased for CaBH4 and SrBH4, this increase inmagnitude is a reection of the rst excited2 E and2 A1 stateslying closer in energy for the borohydrides. In theB 2 E stateof SrBH4, the magnitude of 1 is nearly equal to the rota-tional constant,B 0.194 290 cm1 . As mentioned previ-ously, 1 is analogous to the-doubling constant,p, and thetwo parameters can be related by the expression1= p /2.41Kopp and Hougen55 have shown that if p equals approxi-mately twice theB value in a2

1/2state, the energy level

structure of this state becomes very similar to that of a2 +state. Applying these arguments to a2 E 1/2 state, if 1 equalsapproximatelyB, then the energy level structure of this statwill appear like a2 A1 state. This is precisely what has beenobserved in theB 2 E 1/2 spin-orbit component of SrBH4 andexplains the unique structure of this component, in wheach branch is spaced by2 B. A similar structure is not seenin the B 2 E 1/2- X 2 A1 spin-orbit component of CaBH4, as 1has increased but is not quite equal in magnitude toB. There-fore, 1 B and 3 B spaced branches are still observed inthis spin-orbit component.

Finally, it is of interest to compare the experimentaldetermined values of 1 with those derived from Eq.5 . ForSrBH4 the valuesa =206.6411 cm1, B=0.194 290 cm1, E B 2 E =14 422.5566 cm1, and E A 2 A1 =13 685 cm1were used. Assuming that theB 2 E state arises solely from a p atomic orbital =1 , 1 was estimated to be 0.1089 cm1.For CaBH4 a =63.87 cm1, B=0.255 127 cm1, E B

2 E =15 438.3936 cm1, E A 2 A1 =14 800 cm1, and =1 asimilar calculation estimated1 to be 0.051 cm1. For eachmolecule the calculated value is approximately 40% smalthan the experimentally determined parameter, consistwith 1 and the additionald orbital character. However,some of these differences may be attributed to the appro




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mations made in deriving Eq.5 . Similar calculations of 1in theA 2 E state of CaCH331 and SrCH35 found differencesbetween the calculated and experimentally determined pa-rameters of only about 20%. This better agreement for themonomethyls as compared to the borohydrides is consistentwith morep atomic orbital character closer to 1 in the A 2 E state of the monomethyls as compared to theB 2 E stateof the borohydrides.

The remaining experimentally determined spin-rotationconstant aa can also be estimated using the unique per-turber approximation and pure precession relationship,56

aa = 4a edA t

E B 2 E E Higher lying2 E . 6

According to the above equation,aa should be positive forthe B 2 E states of SrBH4 and CaBH4. A value of 0.0792 cm1 was determined for aa in the B

2 E state of SrBH4. Unfortunately, this parameter cannot be quantita-tively estimated using Eq.6 as no information exists forany of the other excited2 E states of SrBH4. However, quali-tatively, this equation does indicate thataa should be smalland positive as the neighboring2 E states should be higher inenergy than theB 2 E state of SrBH4. Interestingly, a similarqualitative agreement was not found foraa in theB

2 E stateof CaBH4. Here, this constant aa =0.0477 cm1 wasfound to be small and negative in disagreement with Eq.6 .However, aa was only marginally determined in both ts of CaBH4, making its true physical signicance somewhat du-bious.

C. Perturbation in the B 2E 1/2 state of CaBH 4

One possible explanation for the negative value of aafor CaBH4 is that it is a result of a perturbation to theB

2 E 1/2spin-orbit component. For CaCH3,6 the B

2 A1 state wasfound to be perturbed. In this case, theK =1 energy levelswere affected by an interaction with an excitedA 2 E vibra-tional state. This perturbation caused the energy levels of theF 1 spin-rotation component to be higher in energy than thoseof theF 2 component, contrary to theK =0 energy level. Asa result, theK =1 sublevel data could not be included in thenal t. A similar interaction is possible, in reverse, inCaBH4 where theB

2 E state interacts with an excited vibra-tional level of theA 2 A

1state. The ambiguousJ assignments

of the rotational lines of thepP 11 branch in the 01 subbandof the B 2 E 1/2- X 2 A1 spin-orbit component suggest that theK =0 rotational levels of this spin-orbit component are per-turbed. More specically, thepP 11 branch terminates in lev-els withe parity, which suggests that these levels are themost affected by the perturbation. This is further supportedby the assignment of thepP 12 and p R12 branches withoutcomplication, suggesting that thef levels of theK =0 stateof theB 2 E 1/2 spin-orbit component are relatively unaffectedby the perturbation. This scenario where thee and f paritylevels are affected differently by the perturbation is possibleas long as the parity splitting in the perturbing state is large

and thee level interaction is more in resonance than thef level interaction. Fromthe pure precession and unique per-turber approximations,51 it is known that the parity splittingin theA 2 A1 should be similar to that of theB 2 E 1/2 spin-orbitcomponent. In this component the splitting is described

1, and, as outlined previously, the magnitude of this paraeter has increased signicantly in comparison with CaCH3.This increase in magnitude could result in the perturbatidominating thee levels of theK =0 sublevel of theB 2 E 1/2spin-orbit component. However, when theK =0 data wereremoved from the t, a negative value of aa was still found,further suggesting that theK =1 and 2 levels may also beaffected, although less severely. Unfortunately, it is difcto determine which of the vibrational levels of theA 2 A1 stateis causing the perturbation as little information exists on excited states of CaBH4. Ortiz33 has calculated vibrationalfrequencies for the ground electronic state of CaBH4. In thiswork, the frequency of the lateral motion of the borohydrligand relative to the symmetry axis is predicted to 528 cm1. This is within 20% of theA 2 A1- B 2 E 1/2 separation

638 cm1

, making the rst level of this vibration a posible candidate to be the perturbing state.

VI. CONCLUSION

A rotational analysis of theB 2 E - X 2 A1 transitions of CaBH4 and SrBH4 has been completed. This analysis hayielded rotational and ne structure parameters for each served state for the rst time. No evidence for uxional havior due to the internal rotation of the BH4 ligand wasfound for either molecule. The rotational constants agrwell with those calculated for a tridentate structure. A coparison of the molecular parameters of CaBH4 and SrBH4

with those of their monomethyl counterparts shows a lareduction in the spin-orbit separation and the metal-ligaseparation upon electronic excitation. These suggest an creased amount of d -orbital character in the rst excited2 E state of the monoborohydrides as compared to the monoethyls of strontium and calcium, which is consistent wtheoretical calculations. A reordering of the rst excited2 E and 2 A1 states has also been conrmed for CaBH4 andSrBH4 as compared to CaCH3 and SrCH3. Finally, the am-biguousJ assignments of the rotational lines of thepP 11branch in theB 2 E 1/2 spin-orbit component of CaBH4 and thenegative value of aa suggest that this spin-orbit componenis perturbed, most likely by a vibrational level of theA 2 A

1state.

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M. J. Dick et al- High resolution laser excitation spectroscopy of the B^2-E-X^2-A1 transitions of calcium and strontium monoborohydride