Microwave spectrum and molecular structure of aminoborane, BH2NH2

JOURNAL OF MOLECULAR SPECTROSCOPY

123,286-292(1987)

Microwave Spectrum and Molecular Structure of Aminoborane, BH2NH2 MASAAKISLJGIE,HARUTOSHI

TAKEO, AND CHI MATSUMURA

NationatChemical Laboratoryfor Zndustry,Yatabe, Tsukuba, Zbaraki,305 Japan The microwave spectra of five isotopic species of aminoborane have been observed. The rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants of “B and “N were determined from the spectra. The planarity and symmetry of the molecule were confnmed by the measurements of inertia defects, dipole components, and statistical spin weight due to the four hydrogen nuclei. The complete r, structure determined from the rotational constants is: r(BN) = 1.391(2) A, r(BH) = 1.195(4) A, r(NH) = 1.004(2) A, LHBH = 122.2(2)“, and LHNH

= 114.2(2)‘. 0 1987 AcademicPress,Inc. INTRODUCTION The molecular structure and physical properties of aminoborane, BH2NHp, have attracted much interest since it is isoelectronic with ethylene. The microwave spectrum of BHzNHz was observed among the reaction products of diborane with ammonia in our previous investigation (I), and its high-resolution infrared spectrum was measured by Gerry et al. with a fairly pure sample produced by controlled thermal decomposition of borane monoammoniate, BH3NH3 (2). However, the molecular structure remains undetermined because only the rotational constants of normal and 1°B species have been determined. The precise molecular structure is necessary in order to shed light on the property of the B-N bond which is of considerable interest for many experimental and theoretical chemists. As for borane monoammoniate, BHJNH,, which is the isoelectronic counterpart of ethane, Thorne et al. (3) observed the microwave spectra of nine isotopic species and determined the complete r, structure. The comparison of molecular structure between BHzNHz and BHsNH3 would also be of great interest. In the present paper we report the microwave spectra of five isotopic species of aminoborane, their rotational and centrifugal distortion constants, quadrupole coupling constants of B and N nuclei, and the derived substituted molecular geometry. EXPERIMENTALDETAILS The experimental setup used for the observation of the spectrum of BH2NH2 is the same as described in the previous paper (I). Briefly, a mixture of diborane and ammonia was passed through a quartz tube heated to 500°C and was introduced into a waveguide cell. The quartz tube was 30 cm long and 4 mm in inner diameter, and the cell was a 1.5-m X-band stainless steel waveguide. The partial pressure of each reactant gas was kept to 5-10 mTorr. The flow-through method was adopted, since the half-life of CO22-2852/87$3.00 Copyright Q 1987 by Academic F’ress,Inc. All rightsof reproductionin any form reserved.

286

MICROWAVE

SPECTRUM

OF BH2NH2

287

aminoborane was about 5 min in the cell. The spectrum of naturally occurring l”B species was observed along with that of the normal species. For the deuterium-labeled species (BDzNH2 and BHzNDz), fulIy deuterated diborane BzD6 or ammonia ND3 and the normal counterpart reactant were used. The “N-labeled species was prepared by 15NH3. The deuterated diborane BzD6 was prepared by the reaction of NaBD4 with D2S04 and the isotope-labeled ammonia ND3 and 15NH3were purchased from Merck Sharp & Dohme. The spectra were observed by a conventional lOO-kHz Stark modulation spectrometer which was controlled by an NEC PC9801 microcomputer through an HP-IB interface. The microwave sources were an HP 8672A synthesized signal generator for 8-l 8 GHz, the signal generator with a frequency doubler for 18-27 GHz, an HP 8690B sweep oscillator with two plug-in RF units phase-locked to the synthesized signal generator for 27-50 GHz, and an OKI 5OVlO klystron for the frequency region higher than 50 GHz. The measured frequency range was 8-52 GHz. The spectra were observed with a cell at room temperature keeping the pressure of the sample to lo20 mTorr in the cell. ANALYSIS

The transition frequencies were predicted on the basis of the rotational constants estimated from the calculated molecular geometry (4). Since all the five isotopic species observed in the present study have only pLadipole moment, the 10~-000lines were first searched for in the neighborhood of the predicted frequency regions and were readily found from their characteristic Stark patterns. Other lines observable by our spectrometer were due to the transitions between K doublets. In the earlier stage of analysis, transition lines having small J and K_, could be found by use of the predicted rotational constants. Transitions with larger K-i were assigned with the bootstrap method on the basis of the rotational constants which were updated continually as transitions with smaller Kl were assigned. Transitions having small J showed partially resolved hyperfine structure due to the nuclear quadrupole coupling of the boron and nitrogen atoms. The nuclear quadrupole coupling constants and the hypothetical unsplit frequencies were determined from such transitions. On the other hand, some spectral lines exhibited only broadened features. The central positions of such unresolved hype&e components were regarded as the hypothetical transition frequencies without detailed analysis. The transition frequencies measured for five isotopic species are listed in Table I. The rotational and centrifugal distortion constants were derived by least-squares fitting of these frequencies as shown in Table II. In this calculation the sixth-order centrifugal distortion constant, hi, was added because the fitting was slightly poor for the transition frequencies of large K-i without this constant. The electric dipole moment C(~of normal isotopic species was determined to be 1.844( 15) D in the previous work. Further measurement of the dipole moment for other isotopic species was not performed. MOLECULAR

GEOMETRY

The ab initio MO calculation carried out by Dill et al. (4) shows that aminoborane has planar CZ, symmetry in the equilibrium state. The small positive inertia defects

288

SUGIE, TAKEO, AND MATSUMURA TABLE I Transition Frequencies of Aminobanes’

BH2NH2

(in MHz)

BH$5NH2

10BH2NH2

BD2NH2

BH2ND2

transition vobs

A

A

h

0.00

49427.26

48954.85

0.02

13318.52 26633.53 44373.89

0.00 0.03 -0.01

0.05

10964.28

0.09

0.04 -0.07

48146.83

-0.07

8944.59 15881.51 25960.82 39756.46

0.01 0.00 0.00 -0.03

1 -

00,

0

50366.12

0.00

1 2 3 -

21, 31, 41,

2 3 4

13824.64 27645.40 46058.66

0.00 0.01 0.00

52. 'j2. '2. 82. 92,

3 4 5 6 7

-

52, 62, '2, 82, 92,

4 5 6 7 8

9662.57 27973.64 42761.19

93, 6 103, 7 113, 8 123, 9 133,lO 143,11

-

93, 103,

153,12

- 153,13

154,11 164,12 174,13 184.14 "4,15

-

154.12 164,13 174,14 164,15 194,16

2o4,16 214,17

- 2o4,17 - 214,18

205.15 215,16 225,17 235,18 245,19 255,20 265,21 275,22 285,23

-

2o5,l6 215,17 225,18 235.19 245,20 255,21 265,22 275,23 285,24

256,19 266,20 276,21 286,22 296,23 3o6,24

-

256,20 266,21 276,22 286,23 296,24 306.25

51941.04

9406.58 15396.56 24065.19

0.01 -0.02 0.03

0.02 -0.03

36106.55

0.05

8033.67 12388.99

-0.03 -0.09

10364.89 15936.18

-0.03 -0.08

18571.88

0.01

27114.78 38613.23

0.04 0.07

12741.40 19977.02

0.00 0.02

30090.02 43715.65

10617.74 15549.93 22331.47 31471.06 43544.50

v obs

0.00

lo, 21, 31. 41,

7 8 9 113, 123,lO 133.11 143,12

Vobs

-0.03 -0.01 -0.04 -0.01 -0.01

34594.05 48999.56

9757.14 14537.20 21205.73 30311.03 42479.86

0.02 0.02

-0.02 -0.02 0.00 0.01 -0.03

11347.48 17821.44

-0.01 0.03

26900.22 39183.23

0.00 0.04

10617.12

-0.03

15946.84

-0.02

23337.81 33330.21

-0.03 0.00

8755.76 12849.68

0.01 0.00

26145.84 36299.69

0.03 -0.02

“obs

0.08

8439.26 12335.75

0.05 0.00

vobs

A

0.00

43470.71

0.00

14888.81 29766.52 49555.27

-0.03 -0.02 0.02

14236.64 28465.84 47407.61

0.06 -0.03 0.00

9522.76 18486.31 31889.31 50289.96

0.08 0.00 0.00 0.02

14583.24 25471.82 40767.33

0.02 -0.03 0.00

8673.98 15606.49 26163.67 41233.57

0.03 0.01 -0.03 -0.06

10068.68 17141.36 27547.77 42069.60

0.01 0.07 0.02 -0.02

18957.75 30077.71 45700.27

0.02 0.01 0.04

10114.89 16354.44 25442.16 38171.48

0.00 -0.03 0.00 0.01

31406.47 46817.08

-0.03 0.00

8988.74 14150.94 21643.51 32194.17 46603.81

-0.02 -0.02 -0.05 -0.04 0.05

20440.96

-0.01

11546.11 17475.54 25863.56 37439.21

0.04 0.03 0.02 -0.03

41463.59

30967.50 45640.51

8465.36

h

0.01 0.00

found for all observed isotopic species in the present investigation also indicate the planarity of the molecule. The b and c components of the dipole moment were determined to be zero within their experimental errors and, in fact, no b- or c-type transitions were observed. These results strongly suggest that the molecule is planar and has C, symmetry. Another evidence for the Cz molecular symmetry was given by the intensity measurements of spectral lines. The relative intensity of the transitions 31,2-31,3, 82,6-82,7,and 143,r1-143,r2was determined to be 1.0:2.6:1.9 for the normal species. The experimental uncertainty of measured intensity was estimated to be less than 20%. The relative statistical weight was derived to be 1.O:1.6(3): 1.0(2), after the correction for linestrength was made. This ratio agreed well with the theoretical value 6: 10:6 of the molecule which has Cz symmetry and posseeses two equivalent pairs of hydrogens. In conclusion it was confirmed that the molecule is planar and has CZ symmetry in its ground vibrational state. Since the rotational constants of five independent isotopic species were determined, the complete substituted structure was derived. The CzUsymmetry was taken into

MICROWAVE SPECTRUM

289

OF BHzNH2

TABLE II Molecular Constants of Aminoboranes” (in MHz and u *A2)

%H*NH2

BH2NH2 138218.0(29)

A

BH215NH2

BH2ND2

BD2NH2

86349.5(13)

99371.3(15)

6

27487.736(75)

28420.256(82)

26933.935(42)

23213.682(57)

.24108.520(60)

c

22878.520(80)

23520.925(89)

22493.468(46)

18250.006(59)

19362.292(63)

n

0.04766(12)

0.04738(143

0.06854(14)

0.05276113)

-17.1(35)

T'aaaa

T aabb Tabab hJ

x107

a1

0.04775(B)

-15.8(25)

-16.9(41)

-0.214(24)

'bbbb

138217.4(20)

138209.3(38)

-0.218(16)

-0.228(31)

-6.8(11)

-7.4(15)

-0.179(21)

-0.173(21)

0.91(31)

0.99(39)

0.74(21)

0.61120)

0.57(22)

-1.21(15)

-1.26(20)

-1.12(10)

-0.83(10)

-0.80(11)

0.68(26)

0.99(34)

0.56123)

0.69130)

0.60(22)

Numbers

in

parentheses

represent

3 times

the

standard

deviations.

account for the calculation of the r, coordinates of inequivalent atoms. The resultant r, structural parameters are listed in Table III. NUCLEAR QUADRUPOLE

COUPLING

Some of the observed transitions had complicated hyperfine structures owing to the nuclear quadrupole coupling interaction of boron and nitrogen atoms. First, the nuclear TABLE III Molecular Structures of Aminobane

and Borane Monoammoniate

BH2NH2

ohs. rs

BH3NH3

ca1c. a

'e

b

(in A and degrees)

re

obs. c

re

d

Ts

e

r(BN)

1.391(Z)

1.372

1.391

1.386

r(BH)

1.195(U)

1.160

1.186

1.191

1.2160(17)

r(NH)

1.004(2)

1.019

0.993

0.995

1.0140(20)

LHBH

122.2(Z)

121.1

121.1

121.2

113.80(11)

LHNH

114.2(2)

112.3

113.8

113.4

108.65(14)

a)

Numbers

in

parentheses

represent

3

times

1.6576(16)

the

standard

deviations. b)

Ref.

4.

Cl

This

work.

d)

Ref.

9.

e)

Ref.

3.

deviation.

Numbers

in

parentheses

represent

one

standard

290

SUGIE, TAKEO, AND MATSUMURA

quadrupole coupling constants of “B were determined from the hype&e structures ofthetransitions211-2,2, 312-313,413-414, 82~-827,and927-9280fBH~‘5NH2,inwhich the nitrogen atom causes no quadrupole interaction. The quadrupole coupling constants of nitrogen were then determined by the procedure described below. Among the transitions of normal BHzNHl showing complicated hyperfine structures, the most appreciable effect of the quadrupole of the nitrogen atom appeared in the spectral feature of the 927-928transition. It exhibited a triplet, while the corresponding transition in BH215NH2 showed a doublet. The quadrupole coupling constants of 14Nwere determined so as to reproduce the hypertine structure of the 927-928transition of BHzNI-Iz. Since the hyperhne Hamiltonian and intensities for an arbitrary number of nuclei with quadrupole had been given by Thaddeus et al. (8), the hyperfme splittings and the intensities of components were calculated following their formula. The partially resolved spectral patterns were analyzed with the simulated patterns by summing up contributions of all the components. The determined quadrupole coupling constants are listed in Table IV together with the calculated values. Since all the coupling constants could not be determined independently, we determined them with the aid of an ab initio molecular orbital calculation. The constant xna was fixed at the calculated value for boron and nitrogen atoms. The calculations were carried out by using the program HONDOG, which was originally written by Dupuis and Ring (5) and modified at the Institute for Molecular Science, Okazaki. First, the optimized geometry was calculated with the 4-3 lG* basis set. They are given in Table III. Then the electric field gradients were calculated and the quadrupole coupling constants were obtained by use of the nuclear quadrupole moments, Q(“B) = 41.0 mb (6) and Q(i4N) = 19.3 mb (7). DISCUSSION

Theoretical equilibrium geometry of aminoborane was calculated by Dill et al. (4), with the optimization of all geometrical parameters at the Hat-tree-Fock level using the minimal STO-3G basis set. Since the discrepancy between the rs structure determined in the present study and the r, structure calculated by them seemed to be rather large, the equilibrium geometry was recalculated using the split-valence 4-3 lG* basis set. The structure obtained by the recalculation satisfactorily agreed with the experimental one as shown in Table III; calculated bond lengths and bond angles lay within 0.0 1 A and lo from the corresponding experimental values. Recently Ha (9) presented a theoretical study on the B-N bond in aminoborane and other related molecules. He calculated the equilibrium geometry, rotational constants, vibrational frequencies, and some electronic properties with the 4-3 lG* and 6-3 lG** basis sets. His optimized geometrical parameters calculated by the 4-31G* basis set are included in Table III. They are consistent with our calculated parameters. The bond lengths of BH and NH in BH2NH2 are a little shorter than in BH3NI-13 (3). The bond angle HBH is larger than LHNH by 5 and 8“ in both BH$JI& and BH2NH2, respectively. The BN bond lies between the ordinary single (1.55 A) and double (1.33 A) bond lengths which are derived from the covalent radii of B and N atoms (10). This fact clearly shows that the BN bond has double-bond character as predicted from the

MICROWAVE SPECTRUM OF BH2NH2

291

TABLE IV Quadrupole Coupling Constants of Aminoboranea (in MHz) obs.b

ca1c.d

ca1c.c

-

lb

Xaa

-1.6

-1.37

-1.81

x bb

-2.2(Z)

-2.41

-2.29

3.812)

3.98

4.10

Xaa

0.6

0.56

0.44

x bb

1.615)

2.43

x 14N

cc

x cc

The Xaa

constants

calculated Numbers

in

standard

This

work

gradients

were

-2.80

fixed

at

the

values.

the

Calculated

2.35

-2.99

-2.2(5)

parentheses

represent

3 times

deviations. (4-3X’). from given

the in

electric Ref.

9

field f6-31G”l.

molecular orbital theory (4). The BN bond length in BFzNHz (II) has been determined as 1.402(24) A by microwave spectroscopy. Although this value has large experimental uncertainty, it seems to be nearly equal to the B-N length in BHzNHz. On the other hand, the BN bond of BH(NH& (12) is 1.4 18( 1) A, and it is appreciably longer than those in BHzNHz and BF$II-Iz . The difference may be attributed to the smaller doublebond character in BH(NH& because two amino groups which are electron donors are attached to the boron atom. Ha (9) calculated electric field gradients of aminoborane with the 6-31G** basis set. The nuclear quadrupole coupling constants derived from his field gradients and nuclear quadrupole moments described in the preceding section are also given in Table IV. They are in fair agreement with our calculated values. It is well known that borane monoammoniate is heated to lose hydrogen and give monomeric and polymeric aminoboranes. In fact, Gerry et al. (2) observed the infrared spectrum of BH#Hz by thermal dehydrogenation of BHJNI& . Therefore, we expected that BH~NHJ exists as a precursor of BHzNHz in our reaction system. Even with careful searches the absorption lines due to BH3NH3 had not been found. However, when the temperature of the cell was raised to 50-60°C and the pressure of the sample was increased, the spectrum due to borane monoammoniate was found at the expected frequency region. The J = 1 * 0 transitions were identified for several isotopic species. This observation suggests the following reaction scheme: BzH6 --, 2BH3, BH3 + NH3 + BH3NH3,

292

SUGIE, TAKEO,

AND MATSUMURA

The weak spectrum of BH(NH& was also observed in this reaction system. Since excessive ammonia makes the spectrum stronger, this molecule is supposed to be produced as follows: BH2NH2 + NH3 --t BH(NH2)* + Hz. ACKNOWLEDGMENT The authors are grateful to the referee for valuable comments on the determination of centrifugal distortion and quadrupole coupling constants. RECEIVED:

September 19, 1986 REFERENCES

1. M. SIJGIE,H. TAKEO, AND C. MATSUMURA,Chem. Phys. Lett. 64,573-575 (1979). 2. M. C. L. GERRY, W. LEWIS-BEVAN,A. J. MERER, AND N. P. C. WESTWOOD,J. Mol. Spectrosc. 110, 153-163 (1985). 3. L. R. THORNE, R. D. SUENRAM,AND F. J. LOVAS, J. Chem. Phys. 78,167- 171 (1983). 4. J. D. DILL, P. v. R. SCHLEYER,AND J. A. POPLE,J. Amer. Chem. Sot. 97,3402-3409 (1975). 5. M. DUPUISAND H. F. KING, J. Chem. Phys. 68,3998-4004 (1978). 6. J. E. RODGERSAND T. P. DAS, Phys. Rev. A 12,353-361 (1975). 7. H. WINTERAND H. J. ANDRII, Phys. Rev. A 21,581-587 (1980). 8. P. THADDEUS,L. C. KRISHER,AND J. H. N. LOUBSER,J. Gem. Phys. 40,257-273 (1964). 9. T.-K. HA, J. Mol. Struct. 136, 165-176 (1986). 10. L. PAULING,“The Nature of the Chemical Bond,” Cornell Univ. Press, Ithaca, N.Y. 1960. 11. F. J. LQVASANDD. R. JOHNSON,J. Chem. Phys. 59,2347-2353 (1973). 12. L. R. THORNEAND W. D. GWINN, J. Amer. Chem. Sot. 104,3822-3827 (1982).