c.i. frum et al- fourier transform emission spectroscopy of bef2 at 6.5-mu-m

8/2/2019 C.I. Frum et al- Fourier transform emission spectroscopy of BeF2 at 6.5-mu-m

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Fourier transform emission spectroscopy of BeF2 at 6.5 pmC. 1. Frum, FL Engl eman, Jr., and P. F. Bernatha)pb)Department of Chemistry, The Universityof Arizona, Tucson,Arizona 85721(Received 14 January 1991; accepted 16 April 1991)The high resolution infrared emission spectrum of BeF, was observed. The fundamentalantisymmetric stretching mode v3 and numerous hot bands involving y1 and v2 were foundnear 1550 cm - by Fourier transform spectroscopy. Eight vibration-rotation bands wererotationally analyzed and the spectroscopic constants are reported. The equilibrium berylliumfluorine distance (r, ) was found to be 1.372 9710( 95) A in BeF,.

I. INTRODUCTIONA considerable body of work exists on the subject of the

geometry of triatomic metal dihalide molecules. The avail-able experimental data includes electron diffraction, photo-electron spectra, matrix isolation studies, and low resolutiongas-phase spectroscopy. In spite of this work, the question oflinear versus bent geometries remains to be definitively an-swered for many molecules.* Recently, there has been a revi-val in the theoretical interest in this area.3-9 The problem isthat no high resolution, rotationally resolved spectra areavailable. Some progress has been made by jet-cooling metaldihalides such as NiCl, and recording electronic transitionsby dye laser spectroscopy. lo We report here the first com-plete rotational analysis of the spectrum of a metal dihalidemolecule.The structure and bonding in metal halide moleculeshas been of great interest to both experimentalists and theor-eticians. The experimental work on alkaline earth dihalidemolecules started in the 1950s with the observation of elec-tron diffraction by Akishin, Spiridonov and co-workers.They concluded that all of the alkaline earth dihalide mole-cules have linear geometries although the determination ofthe X-M-X angle was subject to a large error ( f. 3v-40).Klemperer and co-workers12-4 studied the moleculargeometry of high temperature molecules by the deflection ofmolecular beams by electric fields. The refocusing of a beamof polar molecules by an electric quadrupole was detectedwith a surface ionization detector or a mass spectrom-eter. 2- Surprisingly, all of the barium dihalides, SrF,,SrCl,, and CaF,, were found to have a dipole moment asexpected for bent molecules of C,, symmetry. It was notedthat the symmetrical alkaline earth dihalides become bent asthe central metal atom becomes heavier and as the halogenatoms become more electronegative.The observation of infrared and Raman spectra of ma-trix isolated alkaline earth dihalides30 confirmed the de-flection results of Klemperer and co-workers, althoughMgFz was briefly controversial.30 The more recent electrondiffraction data3-34 are consistent with conclusions ofKlemperer and co-workers. Photoelectron spectra35v36arealso available for some of the alkaline earth dihalides. Camille and Henry Dreyfus Teacher-Scholar.h Also: Department of Chemistry, University of Waterloo, Waterloo, On-tario N2L 3G1, Canada.

Bi.ichler and Klemperer27 observed the low resolutiongas-phase infrared absorption spectrum of hot BeF,. Theyfound the vibrational frequencies, v2 and v3 to be 825 and1520 cm - * , respectively. The definitive low resolution anal-ysis, however, was the matrix isolation work of Snelson.28Snelson determined v2 and v3 to be 345 and 1555 cm- forBeF,, empirically correcting for matrix shifts. Remarkably,Snelsons value for Yeagrees exactly with the gas-phase valuedetermined in our work. The v2 value of Snelson is also un-doubtedly correct and, indeed, we find no evidence for v2above 500 cm - in our high resolution spectra.

The alkaline earth dihalides are mainly monomeric inthe gas phase, although dimers and higher aggregates arepresent. 37-42Mass spectrometric studies38d are generallyconsistent with dimer concentrations of 1s-2% and matrixisolation work on the dimers is available. 19,42The alkaline earth dihalides have been the subject ofnumerous theoretical papers,67Y42-62partly because theyviolate simple bonding rules. For example, both the va-lence shell electron pair repulsion (VSEPR) theory andWalshs rules predict linear geometries for the alkaline earthdihalides.Two general physical models are used to rationalizebent structures. The participation of d orbitals on the alka-line earth toms7,43-45 favors nonlinear geometries as doesstrong polarization of the alkaline earth ion by anions.457Although these simple models are physically appealing, theyboth have been heavily criticized. The Rittner-type63 ionicmodels are particularly vulnerable because they use arbi-trary values of the polarizabilities.5-7

II. EXPERIMENTALThe high resolution infrared emission spectrum of BeF,was observed with the National Solar Observatory Fouriertransform spectrometer at Kitt Peak. The unapodized reso-lution was 0.0055 cm - with liquid helium cooled As:Sidetectors and a KC1 beam splitter. The spectral bandpasswas limited to 500-2900 cm - I with a wedged InAs filterplaced at the entrance aperture of the instrument. The upperwave number limit was set by this filter while the lower limitwas determined by the transmission of the KC1 beam splitterand by the detector response.Gas-phase BeF, was produced in an alumina tube fur-nace by heating solid BeF, to a maximum temperature of

J. Chem. Phys. 95 (3), 1 August 1991 .0021-9606/91/l 51435-06$03.00 @ 1991 American Institute of Physics 1435


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1436 Frum, Engleman, Jr., and Bernath: Spectroscopy of BeF, at 6.5 pm

about 1000 C. The apparatus used was described previouslyin our observation of emission spectrum of SiS.64The depo-sition of solid material onto the windows was avoided bypressurizing the system with 5 Torr of Ar. The temperatureof the furnace, as measured by a chromel-alumel thermocou-ple placed between the heating elements and the ceramictube, was increased at a steady rate of about 2 C/min. Aseries of spectra were recorded as the cell heated up and thencooled down. Initially we placed a glower behind the cell andfocused its image on the 8 mm aperture of the instrument tolook for absorption spectra. No absorption was observed,but when the glower was shut off, strong emission spectra ofBeF, were recorded. As the furnace cooled down from1000 C!, he intensity of the emission signal decreased rapid-ly and disappeared at about 500 C!.The best spectrum (withthe least amount of congestion) was obtained at about700 C. Spectra recorded at lower temperatures were tooweak, while higher temperature spectra exhibit line intensityanomalies as well as stronger hot bands. The intensity anom-alies seen, or example, near the band heads shown in Fig. 1,seem to be associated with the changing temperature andpressure during the scan integration. As the temperaturedropped, a series of spectra were recorded by co-addingthree scans in fifteen minute integration intervals.

tion-enhanced linewidth was 0.002 cm - . The precision ofstrong, unblended lines is better than 0.000 1 cm - , but thepresence of numerous overlapping lines (Fig. 2) degradesthis. The absolute calibration ( f 0.0002 cm- ) was pro-vided by impurity H2065 vapor absorption inside the cell.In the antisymmetric stretch regi on we were able toidentify 24 infrared bands and we assigned eight: the funda-mental transition and seven hot bands. The bands werepicked out using an interactive color Loomis-Wood pro-gram which runs on a 386/25 microcomputer. An energylevel diagram indicating the assigned transitions is shown inFig. 3.The antisymmetric stretching fundamental, v3, OOI-000,X: -2:) was easy to identify because it was the stron-gest band in this region. In addition, the high-JR branch ofthis band was free of overlap. The hot bands, due to vibra-tional anharmonicity, are shifted to the red. Because of thehigh temperature all of the bands display prominent R heads(Fig. 1).

Ill. RESULTS AND ANALYSISPC-DECOMP, a spectral analysis program developed by J.W. Brault of the National Solar Observatory, was used fordata analysis. The rotational profiles were fitted to Voigt lineshape functions. Although the spectrum shows considerablecongestion due to the overlap of numerous hot bands, theline density is much lower at the blue end of the region wherethe fundamental band makes an R head (Fig. 1). Thesestrong sharp lines show ringing caused by the sin x/x in-strument line shape function of the Fourier transformspectrometer. The ringing was eliminated by using the fil-ter fitting routine available in PC-DECOMP. The filter fitoption also provided some degree of resolution enhance-ment. For the strong lines of BeF, (Figs. 1 and 2)) the signal-to-noise ratio was better than 120 and the resulting resolu-

More than 200 rotational lines of the fundamental v3transition, 128 n the Pbranch and 91 in the R branch, weremeasured. The weak R(0) and P( I) lines were also found,but they were heavily blended with other lines (Fig. 2). Theabsolute rotational assignment of each band was difficultbecause of the high density of overlapping lines in the originregions. Nuclear spin statistics caused by the equivalent flu-orine nuclei (I = f) produces a 3:l intensity alternation(Fig. 1) and constrains the rotational assignment. For eachband, various absolute rotational assignments were consid-ered and the final assignments were made by comparisonwith the analogous spectrum of the isoelectronic molecule,co2.66v67Fortunately, the key assignment for the fundamen-tal band could be made on the basis of the missing line atthe band origin (Fig. 2).The absolute rotational assignment of the hot bands wasconsiderably harder than for the fundamental band. The fol-lowing hot bands were assigned:

011-01 0, I& - II,,0221-0220, A, - A,,0331-0330, @g a,,

R(J) 20 31 33 35 37 SD 41 43 45 4, 49 5, 53 55 a a, wIllI

I! ll11?J;v I,fkFIG. 1. Infrared emission suectrum of BeF, inthe region of the antisymmetric stretchingmode v,. All of the vibrational bands make Rheads at high J values. The intensity altema-tion due to fluorine nuclear spin statistics canbe clearly seen n the fundamental band.

I t I, I I I IO 11,,~,,,,,, I I,.,1565 1570 1575 cm

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Frum, Engleman, Jr., and Bernath: Spectroscopy of BeF, at 6.5 pm 1437

FIG. 2. An e xpanded portion of the infraredemission spectrum of BeF, near the origin ofthe fundamental band. The spectrum is verydense because of the overlap of many hotbands.

the Z-8 Fermi dyad,021-0200, q-q,101-100, 2; --LX;,

and the II-II Fermi dyad,031-030, II,-II,111-110, &-II,.The 01 l-01 0, II,-II, band was the strongest hotband. Although the band origin was very congested, we wereable to obtain an absolute rotational assignment because thevariance of the fit showed a clear minimum for the correctassignment. The other two Q-II, transitions of the II-II

Acm -3000 -2800 -

2400 -

2000 -1800 -1600 -1400 -1200 -1000 -

600 _600 -400 -200 -

0 .

001

1000

0 221

011

t0 220

?010

t0330

Fermi dyad also were assigned in this way although the datawere relatively poor for these two bands. The sign of the I-type doubling constant q was determined by comparisonwith the corresponding transitions in CO,.The 021-02*0, A,-A, band shows l-type resonancesplitting only at very high J values. The absolute rotationalassignment for this band was made by using the a, value(Table II) obtained from the 001400 and the 011-010bands. The correct rotational assignment of the A,-A, bandalso showed a minimum in the variance of the fit. The 0331-0330, a,-*, band was assigned in a similar fashion but, asexpected, no l-type splittings were observed.

The most difficult bands to assign were the two Z,t -&?

0200 -1002FERMI DYAD

111031 -

030 - 1 0

I-IFERMI DYAD

FIG. 3. An energ y level diagram forBeF, showing the observed emissionbands.



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1438 Frum, Engleman, Jr., and Bernath: Spectroscop y of BeF, at 6.5 pm

bands (021-0200, 101-100) in the Z-8 Fermi dyad. Thestrong Fermi interaction perturbs the B values so that, forexample, Bozoo Bo220.The comparison with CO, againproved to be indispensable in guiding our assignments.66*67The Boz,,o alue was shifted down by 0.000 943 cm - by themixing of the 020 and 100 vibrational wave functions. Themolecular constants for the observed BeF, bands are pro-vided in Table I. The line positions are available throughPAPS6* or directly from the authors.In the fitting and labeling of our data we have followedthe conventions of electronic spectroscopy. We used the sim-ple rotational energy expression:

F(J) =BJ(J+ 1) -D[J(J+ l)]k J(J; )X [q+q,J(J+ 111,

where q = qD = 0 for Z states; q = 0 for A states and theupper (lower) sign refers to e( f ) parity.69 The I-type dou-bling parameter q is negative in 0 10 vibrational state becausethe f parity level lies above the e parity level for a given J.This arbitrary choice is in agreement with the conventions ofelectronic spectroscopy but differs from the commonly usedinfrared convention of a positive sign for q. The recent workon the corresponding spectra of CO,, however, by Bailly andco-workers66*67uses a negative sign for q.In addition to the bands observed near 1550 cm - somevery weak Q branches were found in the region 1127-1235cm- . These bands are probably associated with the Y, + V~combination band ( 110-000) of II,-2: symmetry. If this

TABLE I. Molecular constants for BeF,.

assignment is correct, then the value for Y, lies between 780and 890 cm - . This value is somewhat higher than the valueof 680 cm - estimated from the v3 band using the valenceforce approximation. A gas-phase Raman measurement ofY, for BeF, is desirable.IV. DISCUSSION

The geometry of BeF, can be extracted from the molec-ular constants provided in Table I. The r, Be-F bond dis-tance of 1.374 040 3 A is found using the B, value. Sincebands involving all three normal modes were found, it ispossible to determine Be as well as a,, aa, and a3:

B I%9 =B, --aI (u, +:I --a,(~, + 1) -a3(v+i).The a2 and a3 values (Table II) were found by using theB 010 9 Boo,, and B, values of Table I. The observed a, valueof - 0.000 148 6( 103) cm- was corrected for the effectsof Fermi resonance by using the observed Boz20-Bo200hift toprovide an a, (corrected) = 0.000 794 3 (62) cm - . TheB,ofTableIIresultsinanr,of1.372971 O(95)Awherethequoted one standard deviation uncertainty is a simple statis-tical error estimate.The electron diffraction measurement for BeF, pro-vide a Be-F bond distance of 1.40 f 0.03 A. The observed r,value is also in good agreement with the r, of 1.380 A recent-ly calculated by Dyke and Wright,3 as well as the previous abinitio calculations.4s~s9-63 or the diatomic BeF molecule r,is 1.3610 A, very close to the 1.3730 A value found in BeF,.The unpaired electron in the X 22 + ground state of BeF isclearly nonbonding.

Level Bandorigin lo6 Dv,v,v, 103 42 1094DWO 0.234 990 66( 56) 0.101 680(47)1555.047 92(5)C@l01001102%02=10330031loo01001020002Ol110111030031

0.232 544 62( 57) 0.100 878(49)0.236 244 69( 66) 0.106464(76)1547.829 96(7) 0.233 816 87(66) 0.105 732( 78)0.237 492 62(90) 0.111050(84)1540.635 86(7) 0.235 08407(91) 0.110354(87)0.238 732 02(70) 0.115 750(62)1533.465 82(7)0.236 342 15(71) 0.115059(65)Z-Z FERMI DYAD0.235 139 26( 87) 0.083 689(80)1543.272 14(8)0.232 658 82( 88) 0.083 049( 84)0.236 549 71(97) 0.127 895(80)1542.340 80(92) 0.234 182 86(99) 0.127 254( 83)II-Il FERMI DYAD0.236 428 72(91) 0.092 50( 11)1535.782 31(7)0.233 978 23(93) 0.091 51( 12)0.237 693 22( 84) 0.125 122(80)1535.503 97(6) 0.235 331 14(85) 0.124 554(83)

- 0.384 7( 13) 0.70( 15)- 0.377 6( 13) 0.79( 15)-O&2(89)

- 0.226 (91)

- 0.5514( 10)- 0.531 5( 10)- 0.582 47(64)

- 0.575 96( 64)One standard deviation error is enclosed in parentheses.



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TABLE II. Equilibrium molecular constants for BeF, (in cm - ).4 = 0.235 356 8(33)a, (corrected) = O.C#O 94 3(62)a2 = - 0.001 25403(87)a3 = 0.002 446 04( 80)re = 1.372 971 O(95) A

The Il vibrational levels show I-type doubling splittingsand the A vibrational levels display Z-resonance splittings asexpected for transitions associated with doubly degenera temodes. For a symmetric linear triatomic molecule, themagnitude of the splitting is given by the equation7:Av=qJ(J + 1) +qD[J(J + l)l+ ***.

For vibrational states of A symmetry, q = 0 and the lead-ing term is qD. For a linear, symmetric triatomic moleculesuch as BeF,, the value of q is given by,4=--B: 1+( 4w: ) (v,+ 1).w2 co:w:

Using qo,o = - 0.3874( 13) cm-, Boo0= 0.234 99 cm-iand 3tj = 1555 cm - , Eq. (2) predicts y2 = 347 cm - inremarkable agreement with Snelsons value2* of 345 cm- I.The detailed theory of I-resonance in A states was firstderived by Amat and Nielsen73 with later contributions byMaki and Lide .74 The sign of qD indicates that the 020 vi-brational level lies higher in energy than the 020 level. Inaddition the changes in the centrifugal distortion constantsrelative to the ground state are consistent with the 100 vibra-tional level lying above the 020 level.V. CONCLUSION

The high resolution infrared emission spectrum of BeF,vapor at 700C was observed with a Fourier transformspectrometer. The antisymmetric stretching mode V, near1555 cm - i and seven hot bands were rotationally analyzed.From the equilibrium rotational constant, the equilibriumBe-F bond distance of 1.372 917 O( 95) A was calculated forBeF,. Our work represents the first complete rotationalanalysis of a metal dihalide. The rather neglected techniqueof high resolution infrared emission spectroscopy promisesto be a powerful tool for the determination of molecularstructures of high temperature molecules.

ACKNOWLEDGMENTSThe National Solar Observatory is operated by the As-sociation of Universities for Research in Astronomy, Inc.,under the contract with the National Science Foundation.We would like to thank J. Wagner, C. Plymate, and G. Laddfor assistance in recording the spectra. This work was sup-ported by the Astronautics Laboratory, Edwards Air ForceBase, CA. Acknowledgment is made to the Petroleum Re-search Fund, administered by the American Chemical So-

ciety, for partial support of this work. We also thank A.Maki for his helpful comments.

Frum, Engleman, Jr., and Bernath: Spectroscopy of BeF, at 6.5 pm 1439

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c.i. frum et al- fourier transform emission spectroscopy of bef2 at 6.5-mu-m

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