The molecular structure of HBF+ by microwave spectroscopy

JOURNAL OF MOLECULAR SPEC~‘ROSCOPY 121,278-282 (1987)

The Molecular Structure of HBF+ by Microwave Spectroscopy’ G. CAZZOLI,* C. DEGLI ESFQSTI,* L. DORE,~ AND P. G. FAVERO* Tentro di Studio di Spetttoscopiaa Microonde, Universitddi Bologna, Via Selmi 2, Bologna, Italy; and thtituto di SpettroscopiaMolecolare de1 CNR, Via de’ Castgnoli1, 40126 Bologna, Italy The millimeter- and submillimeter-wave spectra of the H”BF, H’%F, D”BF, and D’%F+ isotopomers have been observed and analyzed. The derived moments of inertia have been used to determine an r, structure of this molecular ion. The average bond distances obtained are: rABH)= 1.1736A

r,(BF)= 1.2102A.

To our knowledge this is the first experimental determination of the HBF structure. @ 1987 AcademicPress,Inc.

INTRODUCTION

Microwave spectroscopy is one of the most accurate methods for determining molecular structure. Among the different molecular structures that can be obtained by this technique, namely effective, average, substitution, and equilibrium structures, the last one is the most satisfactory. At the moment, however, this method cannot be applied to HBFf because of the lack of experimental data. The closest approximation to the equilibrium structure is the substitution structure (rJ which is obtained by using the effective moments of inertia in Kraitchman’s equations (I). Costain (2) has shown that, using this procedure, the zero-point vibrational effects tend to cancel out and so more consistent structural parameters can be derived. In this paper we report the first experimental determination of the HBP molecular structure which turns out to be a partial r, structure since fluorine has only one isotope in natural abundance, and thus its coordinate can be determined using the fnst moment equation. The first observation of an absorption spectrum of HBFf has been performed by Kawaguchi and Hirota (3) in the IR region. They have detected the v3band of H”BF+ in a dc discharge plasma of a BF3/H2 mixture and they confirmed that HBFe is a linear molecular ion. Haese and Woods (4) have computed ab initio SCF and CI electric dipole moments of HBF+ obtaining values of 4.20 and 3.98 D, respectively. Because of its large dipole moment, HBFf has been proposed by these authors as a good case of a molecular ion to be studied by microwave spectroscopy. Recently, we have observed the millimeter-wave spectrum of H”BF, and preliminary results have been published (5). The search for this spectrum has been made easier by using the rotational constants from Ref. (3). ’ This work was supportedby MPI and CNR. 0022-2852187 $3.00 Cq+&: Q 1987 by Aademic

PK.s, Inc. AU right.3of nmoduclioo in my fom rexwed.

278

MOLECULAR

STRUCTURE

OF HBF+

279

In order to obtain the molecular structure of HBF+, we decided to investigate the microwave spectrum of the H”‘BFt, D”BP, and DLeBF+ isotopomers. EXPERIMENTAL

DETAILS AND ANALYSIS

The microwave spectrometer employed has been described in previous papers (5, 6). The microwave radiation is produced by a crystal harmonic generator driven by an OKI klystron (3OV12, 35V12,4OV12) phase locked to a computer-controlled synthesizer. The computer also performs the averaging and the smoothing of the digitized spectrum and, in addition, the suppression of the background due to the source modulation. The radiation was detected using a homemade InSb photodetector which operates at 1.6 K. HBF was produced in a glow discharge inside a Pyrex cell 3 m long and 9 cm in diameter. Around this tube a solenoid which provides a magnetic field up to 200 G has been wound in order to enhance the concentration of positive molecular ions in the glow discharge as pointed out by De Lucia and co-workers (7). A 20: 1 mixture of BF:, and HZ (or 4) was pumped through the cell at a temperature just below the melting point of BF3 (146 K). The total pressure at the pump end of the cell was regulated at 10 mTorr and the discharge current at 30/40 mA. The best signal-to-noise ratio was obtained when, by decreasing the temperature, the blue glow discharge turns into red-violet. By comparing the intensity of the J = 2-1 transition of HCN and HBP, we can estimate the concentration of HBF+, on the above-mentioned conditions, to be 2 to 5 X 10” ions cm-3. The observed spectra correspond to rotational transitions of H”BF+, H”BF+, D”BF, D”BF+ in their ground vibrational states. The measured frequencies and quantum assignments are listed in Table I. Under sufficiently good conditions of signal-to-noise ratio the observed spectral lines would be split by the boron quadrupolar coupling constant. These conditions have been achieved only in the case of the 2-l transition of H”BP and D”BFe. The hyperhne structure is @tially resolved and the unsplit frequency and the “B quadrupole coupling constant were derived from the observed data by fitting the experimental profile to the second derivative of a Lorentzian shape function frequency modulated by a sine wave. The half-width, the modulation depth, the quadrupole coupling constant, the unsplit frequency, and the scale intensity are the parameters of the nonlinear least-squares fitting while the relative intensities were fixed to the calculated values. Figure 1 shows the experimental and the calculated profile of the J = 2- 1 transition of H”BF+. The hyperfine structure of the ‘Q isotope has not been observed since this spectrum is weaker than the “B spectrum. This difference is not only due to different isotopic natural abundances (80.39 and 19.6 1% for ’ 'B and “‘B, respectively), but also to different values of the nuclear spins (3/2 and 3 for “B and “‘B, respectively) and of the quadrupole coupling constant (see Table II). All the measured transitions, except the J = 2- 1 of H”BF+ and D’ ‘BP, correspond to the strongest hyperfme components and have been corrected to include the unre-

280

CAZZOLI ET AL. TABLE I Observed Rotational Transitions of H”BF+, H’%F, Isotopomer

J'-J

H"BFt

2-l

H'%F+

3/Z-1/2;5/2-512

145 293.45

7/Z-5/2;5/2-312 l/2-1/2;3/2-5/Z

145 294.78

3/Z-3/2

145 295.84

(in MHz)

Corrected=

Cow.-Calc.

145 294.66

0.01

217 937.24

217 937.24

0.00

4-3

290 574.07

290 574.08

-0.01

5-4

363 203.32

363 203.32

0.00

2-l

150 813.19

150 812.82

-0.02

3-2

226 214.39

226 214.22

0.02

4-3

301 609.55

301 609.48

-0.01

116 541.27

0.03

2-l

’ Corrected

Observed

and D’%F

3-2

D"BF+

D'%F+

F'-F

D”BF,

3/Z-1/2;5/2-5/Z

116539.96

7/Z-Ji2;5/2-312 1/Z-1/2;3/2-S/2

116 541.37

3/Z-3/2

116 542.27

3-2

174 808.92

174 808.93

0.02

4-3

233 072.99

233 073.00

-0.02

5-4

291 332.34

291 332.35

-0.04

6-5

349 585.87

349 585.67

0.02

2-1

119 480.12

119 479.74

0.02

3-2

179 216.57

179 216.40

4-3

238 949.43

230 949.36

to remove

the

-0.03 0.01

quadru@ar contribution (see text).

solved, blended hyperhne structure in the following manner: The profile of the hyperhne structure of each observed transition has been simulated by using the above-mentioned lineshape hmction. The half-width and modulation depth parameters derived from the fitting procedure outlined before have been used in the calculations together with the quadrupole coupling constants of Table II. The evaluated deviation from the center of gravity of the multiplet and the strongest hyperhne component can then be subtracted from the measured frequency to obtain the corrected frequencies listed in Table I. The rotational constants Be and Do of Table II have been derived by a least-squares procedure from the corrected frequencies. The corrections which have been made to

281

MOLECULAR STRUCTURE OF HBF+

F’ (- F 3l2 (- II7

FIG. 1. The J = 2-l transition of H”BF+, showing the “B hype&e structure partklly resolved. The solid line refersto the observed unsmoothed spectrum whereas the dotted line represents the best fit to the observed pro6le.

remove the quadrupolar contributions produce a remarkable improvement in the standard deviation of the fits, especially for the H’~F and D’*BF-’ isotopomers where the u of the fit decreases from 0.09 and 0.12 to 0.03 and 0.04, respectively. STRUCTURES

As mentioned before, from the available collection of isotopomers, only partial r, structures may be evaluated since the isotopic substitution of fluorine is not practicable. Using the moments of inertia derived from the rotational constants collected in Table II, four sets of the hydrogen and boron coordinates in the principal axis system of H”BP, H’OBF’, D”BFt, and D*%F’, mspectively, may be calculated. The fluorine coordinates in these systems can be obtained using the first moment equation. The resulting bond distances and those obtained by recent ab initio calculations (8) are listed in Table III. The consistency of the determined structures is very satisfactory. The average value of the r, structures is in good agreement with the ab initio r, structure, and it is interesting to note that, even though a straight comparison of these TABLE II Spectral Constants of H”BP, H’%P, D”BF+, and D’%P (in MHz)

Q, H"BF+

36 324.303(Z)

0,07943(S)

H'%F+

37 703.884(B)

0.0843(3)

D"BF+

29 135.70btS)

0.04933(9)

Q'%F+

29 870.350(9)

O.O525(4l

-5.19(31 -10.a2a -5.27(9) -10.9a=

Note. Standard errors are given in parentheses in units of the last quoted digit. ’ Calculated using the value of 2.084 for the quadrupole moment ratio e(‘%)/Q(“B).

282

CAZZOLI ET AL. TABLE III Structures of HBF’ (in A) r =3 z

1.8767

1.8177

1.9001

1 m8385

*s

0.7032

0.6441

0.7266

0. bb48

-0.5659

-0.4837

H

b

zF

-0.5070

-0.5453

r

1.1755

1.1736

1.1735

1.1737

1.1736

1.171

r

1.2102

1.2100

1.2103

1.2101

1.2102

1.210

BH

BF

’ Ref. (8). b Calculated using the hrst moment equation.

two structures cannot be made because of their different meanings, the difference between the measured r, and the ab initio r, distances are within 0.004 A, which is the expected deviation between r, and r, experimental structures (9). Note added in proof: Recently, P. Botschwina (IO) has obtained ab initio rotational constants and r, structure of HBF+ that are in very good agreement with the experimental ones obtained in the present work. ACKNOWLEDGMENTS The authors thank Mr. L. Cludi (University of Bologna) for the interfacing of the spectrometer w-itb the computer, Mr. L. Minghetti (FRAE Bologna) for the project and the arr@emetit of the absorption cell, and Mr. G. Tasini (ISM Bologna) for technical assistance. RECEIVED:

June 24, 1986 REFERENCES

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2. 3. 4. 5. 6.

(1985). 7. F. C. DE LUCIA,E. HERBST,ANDG. M. PLUMMER,J. C’hem.Phys. 78,2312-1316 (1983). 8. D. J. DEFREES,J. S. BINKLEY,AND A. D. MCLEAN,J. Chem. Phys. 80,3720-3725 (1984). 9. R. C. WOODS,R. J. SAYKALLY,T. G. ANDERSON,D. A. DIXON, AND P. G. SZANTO,J. Chem. Phys. 75,4256-4260 (198 1). 10. P. B~TXHWINA, .I. Mol. Spectrosc. 118.76-87 (1986).