Boron, nitrogen, and fluorine nuclear quadrupole coupling and the electronic structure of the boron-nitrogen single bond

JOURNAL

OF MAGNETIC

RESONANCE

69,302-3 10 (1986)

Boron, Nitrogen, and Fluorine Nuclear Quadrupole Coupling and the Electronic Structure of the Boron-Nitrogen Single Bond J. OLLIGES,

A. LCITZ, AND J. VOITLANDER

Institut fir Physikalische Chemie der Universitiit MUnchen, Sophienstrasse II, D-8000 Miinchen 2, West Germany ANTI H. BARFUSS,

G. B~HNLEIN, F. GUBITZ, W. ITIUER, G. LANZENDORFER, W. KREISCHE, AND B. ROSELER

Physikalisches Institut der Universitiit Erlangen-Niirnberg, Erwin-Rommel-Strasse D-8520 Erlangen, West Germany

1,

Received March 20, 1986 The “B and 19F nuclear quadrupole couplings in F,BNH#ZHs),, compounds were measured in the solid state by quadrupole perturbed nuclear magnetic resonance of boron and time-differential, perturbed angular distribution of the y radiation of excited fluorine nuclei. The coupling constants are independent of the number of methyl substituents within experimental error both in the case of boron (e2qQ/h = 80 + 8 kHz) and in the case offluorine (e*qe/h = 30.0 + 0.5 MHz). Similarly, there is no change of the asymmetry parameter of the electric field gradient of fluorine (0.20-0.24) within the series. For F,BNH(CHs)r, a boron asymmetry parameter of 0.30 f 0.05 was found. Earlier results on the nitrogen quadrupole couplings in F~NH~Rr,, and the nitrogen and boron couplings in HsBNH&-, compounds, extended by the present findings, provide a clear picture of the chemical bonding in these compounds, especially the donor-acceptor character and the ionicity of the boron-nitrogen bond. Q 1986 AC&U& Rtq Inc.

In a short communication (I), we have recently reported the boron and fluorine quadrupole coupling constants in F3BNHJCH&-, compounds. The purpose of the present paper is to give a more detailed account of the experimental work and discussion of the results which are in close relationship to our earlier measurements of the nitrogen coupling constants in the same series (2), and the boron and nitrogen coupling constants in H3BNHxR3, compounds (3, 4). The new results especially help to explain the earlier surprising finding of similar nuclear quadrupole coupling constants of nitrogen in HsBN and FsBN compounds in spite of the large electronegativity difference between H and F, when these compounds possess an equal number of organic substituents at N. EXPERIMENTAL “B. The “B quadrupole-perturbed

NMR spectra were recorded at room temperature

in the solid state (5) on a Varian DP 60 CW-NMR 0022-2364186 $3.00 Copyright 0 1986 by Academic Press, Inc. AU ri$kts of r’qmduction in my form irsumd.

302

spectrometer at 19.24 MHz and

QUADRUPOLE

COUPLING

AND BORON-NITROGEN

303

BOND

1.409 T. The field sweep was 25 mT per 150 s. 500 sweeps were averaged with a Princeton Applied Research 4202 signal averager. Magnet field inhomogeneity was kO.08 mT. The insert with the receiver coil was made from quartz glass to avoid boron resonances which usual glass inserts give rise to. The experimental spectra were evaluated with the method of Ebert (6) which computes the transition fields and the transition probabilities in the field domain applying perturbation theory and the triangular integration method. The spectra were broadened with a single Gaussian broadening function for both central line and satellites. The theoretical spectra were displayed on a screen and interactively fitted to the experimental spectra. Figure 1 shows the theoretical powder pattern of the unbroadened, quadrupoleperturbed “B NMR line in first order (5). The central peak is not drawn. The singularities I+, u2 coincide for 4 = 0. The experimental spectra (Figs. 2a-c) are the derivatives of the absorption lines. Only one satellite on each side of the central transition was detectable in the spectra of F3BNH3 and F3BN(CH&. From the crystal structure of the two compounds (7, 8) 1 should be zero for F3BN(CH& and close to zero for FjBNH3 so that both spectra were evaluated with n fixed to zero. Since no difference in the boron quadrupole coupling constants of F3BNH3 and F3BN(CH& was found within experimental error, the coupling constant of F3BNH(CH3)2 should be very similar to that of the former two compounds. This led us to assign one of the two experimentally found peaks in the spectrum of F3BNH(CH,)2 to u3, whereas the second peak represents the shoulders u1 and u2 which are not resolved. Nevertheless, it proved possible to derive an asymmetry parameter by the fitting procedure (Fig. 2c) whose magnitude is in the expected range (cf. the boron asymmetry parameter of 3 1% in H3BNH2tBu (4)). 19F. Bombardment of a sample containing ‘v nuclei with a beam of accelerated protons excites them to a state 197 keV above the ground state with Z = s, and leads to a partial alignment of the fluorine nuclei with respect to the beam axis. The y radiation from the aligned ensemble of excited nuclei is anisotropic. The coupling of the nuclear quadrupole moments with a field gradient causes a precession of the nuclei and thus a radiation which oscillates in space and time according to

111

IV@, t) = 1 + &G22(t)P2(cos 0)

-1

“3

-‘/

“2

0

2

+‘/

2

1

w X

“I

FIG. I. Theoretical powder pattern of the unbroadened, quadmpole-perturbed (central peak omitted).

’ ‘B NMR line in first order

304

OLLIGES

L

1

ET

I

AL.

I1

l.LO2

l.LOL

I

I

I

I

GO6

u.08

lit10

v.412

l.LlL

V.16 BIT]

LLO2

l.LOL

UO6

~08

IL10

l.Ll2

I.LlL

IL16 BfTJ

F3BNHMe2 e*qO/h -8OkHz,q=0.31

!

l.LO2 FIG. 2. Experimental

l.LOL and fitted

I.406 “B

NMR

1

UO8

l.Llo

spectra

of (a) F&G&

1412

l.LlL

(b) F3BN(CH,h

U16

Bl I

(c) F3BNH(CH&.

QUADRUPOLE

COUPLING

AND

BORON-NITROGEN

BOND

305

where 9 is the angle of observation with respect to tbe beam axis, & the modulation amplitude, and &(cos lls)a Legendre function. The time dependence of the oscillation is described by the perturbation factor G22(0 = s20(711)+ i

szn(~l)exp(-G~w,t)cos[wds)tl

PI

n=l

with w, representing all transitions in the nuclear quadrupole level system (three for Z = 3). The coefficients s and w are functions of TJ,the asymmetry parameter of the electric field gradient (9), and 8L represents the Lorentzian spread of the nuclear quadrupole transitions. The second excited level of 19F was populated via a (p, p’) reaction with a 5 MeV pulsed proton beam of the Erlangen tandem accelerator. The pulse repetition period was sufficiently long to observe more than 5 lifetimes T = 128.8 ns of the excited level. The 197 keV y rays of the decaying level were detected by NaI( Tl) scintillation counters under angles of 90 and 180” relative to the proton beam axis. The counting rates N(b, t) were reduced according to R(t) = 2(N(180”, t) - iV(90”, t))/(ZV(l80”, t) + 2iV(90”, t)).

131

This ratio is equal to the amplitude A22 times the perturbation factor Gz2(t). Figure 3 shows R(t) obtained in F3BNI-13. The time spectra of the crystalline samples could only be fitted under the assumption of two or three different quadrupole interactions (Table 1). All time spectra contain frequencies corresponding to coupling constants of about 30 MHz, 7 = 0.20-0.24, and about 40 MHz, r] = 0. The latter is close to the value observed in solid HF (IO), and can therefore be attributed to the formation of HF molecules by the recoiling nuclei. In the presence of more than one methyl group in the molecule, a coupling constant of about 60 MHz can be observed. This frequency is close to those found in CF,, FIFE, and some fluoromethanes (ZZ), and indicates the formation of a C-F bond. The coupling constant of 30 MHz represents the regular B-F bond in the molecule and is, within experimental error, equal to the coupling constant in BF3 (12). The quadrupole couplings of fluorine in the FXBN compounds and BF3 differ in their asymmetry parameter which is 0.20-0.24 for the former and 0.56 for the latter. ANALYSIS

OF

THE

EXPERIMENTAL

DATA

“F. The quadrupole couplings of boron, fluorine, and nitrogen can be used to obtain a picture of the electronic distribution in F3BN compounds by means of the theory of Townes and Dailey (13). In analogy to the established procedure for chlorine compounds, we assume that the z axis of the field gradient tensor of fluorine lies in the F-B bond direction. This bond will be a linear combination of a sphybrid orbital of fluorine and a $-hybrid orbital of boron. The actual FBN angles deviate somewhat from the tetrahedral angle, and, in addition, are different within one and the same compound (7, 8, Z4). Yet, in view of the approximate character of this discussion and the fact that the spectra do not resolve different fluorine couplings, we assume the fluorines to be equivalent and that there is tetrahedral sp3 hybridization at boron. The population of the nonbonding

306

OLLIGES

0

100

200

ET AL.

300

400

500

600

timehs FIG.

3. TDPAD spectrum of F3BNHj.

sp-hybrid orbital of fluorine is set equal to 2 as is the population of one of the p orbitals in the plane perpendicular to the F-B bond (Fig. 4). To account for the nonzero asymmetry parameter, the population of the other fluorine p orbital must be smaller than 2. We then have e2qQ/e2q& = (1 - Cx2)Ch- iph 141 (e2sQ/e2q0Qh = h

PI

in which the populations of the fluorine sphybrid orbital in the F-B bond (c) and of the p orbital perpendicular to it (p) have been replaced by the number of equivalent holes (ur, , ph). The s character of the sp hybrid (cx2)is set equal to 0.15 (13). The atomic coupling constant e2q0/h of r9Fr is not known. The coupling constant in molecular fluorine (F2), which should be a good approximation for the atomic coupling constant, was determined to be 127.2 MHz (II). In chlorine, the molecular coupling constant amounts to 108.5 MHz (15), the atomic coupling constant to 109.7 MHz (16). A

QUADRUPOLE

COUPLING

AND

BORON-NITROGEN

307

BOND

TABLE 1 Fluorine Nuclear Quadrupole Coupling in F3BNHJCH&

TW F,BNHx

23 80

FlBNH2Me

23 80

F,BNHMe2

23

80 FSBNMe,

23

80

Compounds

(%I

&L@)

10.5 8.5 5.5 5.0

(6) (9) (4) (9)

3.6 (5) 6.4 (9) 1.4 (6) 8.2 (2.1)

30.3 42.2 30.1 40.8

(3) (4) (3) (4)

0.20 0.00 0.20 0.00

(5) (5) (5) (5)

I II I II

4.3 4.1 3.5 5.3

(7) (6) (5) (4)

6.6 5.6 2.2 4.8

(1.9) (1.4) (1.2) (8)

29.4 39.9 29.5 40.6

(3) (4) (3) (4)

0.24 0.00 0.24 0.00

(5) (5) (5) (5)

I II I II

6.4 9.3 2.3 2.2 5.1

(4) (3) (5) (2) (4)

2.1 (6) 1.9 (3) 4.0 (1.7) 0 2.7 (7)

30.0 42.7 59.1 29.7 40.4

(3) (4) (6) (3) (4)

0.24 0.00 0.00 0.24 0.00

(5) (5) (5) (5) (5)

I II III I II

13.4 (4.2) 4.2 (1.9) 1.2 (2.9) 4.4 (2.5) 9.2 (2.2) 0.9 (1.2)

29.6 39.5 55.9 29.8 40.4 60.1

(5) (4) (9) (4) (5) (6)

0.24 0.00 0.00 0.24 0.00 0.00

(5) (5) (5) (5) (5) (5)

I II III I II III

A22

7.5 (3) 3.2 (8) 0.6 (3) 3.6 (9) 6.0 (1.0) 1.6 (3)

e2qQ/hW-M

7

reasonable approximation for the atomic coupling constant of fluorine, therefore, seems to be 130 MHz. This leads to the population numbers at fluorine shown in Fig. 4. “B and 141V.Since the electrons in a bond add up to two, the population of the boron orbitals directed toward the fluorines results immediately. For s$-hybridized atoms whose orbitals point into the comers of a tetrahedron, and with presence of a threefold symmetry axis with respect to the orbital populations, Townes-Dailey theory gives le2qQ/e2qoQl = :
308

OLLIGES

FIG. 4. Electron distribution quadrupole coupling.

ET AL.

in F,BNH, based on the Townes-Dailey analysis of the 19F, “B, and 14N

and e2q,-,Q/Iz(N) = - 10 MHz we end up with the electronic distribution in that compound as shown in Fig. 4. The analysis performed for FjBNH3 can also be applied to H3BNH3, except that no experimentally derived value for the electron distribution within the H-B bond is available. Gordy’s formula (13) i = f IXB - XHI [71 could be applied (with the eleCtrOnegatidieS xa being 2.0 and xu being 2.2) which results in an s-orbital population of 1.1 e at hydrogen. A SCF calculation (18) yields 1.2 e. We use the latter value and obtain the populations listed in Table 2 which are based on the experimental coupling constants of boron and nitrogen in H,BNH3 (3). For the compounds with other substituents than H at nitrogen, no differences in the fluorine coupling constants were found. Similarly, for the HJBN compounds (3, TABLE 2 Orbital Populations (pu) and Atomic Charges (41) in FsBNHx and HjBNHS X

X=F X=H

N

B

H

4x

Pxs

wx

al

PEm

Pm

h

Pm

PHN

-0.68 -0.20

1.71 1.20

0.29 0.80

+1.86 +0.17

0.27 0.43

1.73 1.56

-1.35 -0.73

1.54 1.39

0.46 0.61

4H

+0.54 +0.39

QUADRUPOLE

COUPLING

AND BORON-NITROGEN

BOND

309

4,29) as well as for the F3BN compounds, no significant changes in the boron coupling constants within each series could be detected. The asymmetry parameters reflect only the different asymmetry at nitrogen in an attenuated manner. Thus, the picture of the electron distribution in H3BN and FaN compounds given in Table 2 is approximately valid for all substituents attached to nitrogen, the only actual change being the electron distribution within the N-H or N-C bonds. CONCLUSIONS

In Table 2 the charges residing on the various atoms of H3BNH3 and F3BNH3 have been calculated from the populations of their associated orbitals and their nuclear charge. A remarkable fact is the increase of negative charge on nitrogen in F3BN compared to H3BN compounds. This is not only a result‘of a stronger polarization of the N-H bond toward nitrogen in the former compounds but a stronger polarisation of the B-N bond toward nitrogen as well. A comparable result is obtained from both SCF (I 7, 18) and CNDO/2 (20) calculations according to which the increase in the charge on nitrogen in FJBNI-13 compared to HJBNH~ is somewhat higher than the corresponding decrease on the hydrogens. In FJBN compounds, nitrogen should therefore be a less efficient donor than in H3BN compounds, in contrast to expectation according to which F3B should be a better electron acceptor than HjB on account of the high electronegativity of fluorine. It must be mentioned here that no difference in the B-N bond lengths in HjBNH3 and F3BNI-13 was found within experimental error, the B-N distance being 1.60 +- 0.02 A in F3BNH3 (7) and 1.56 f 0.05 A (21) or 1.6 A (22) in H3BNH3. There are undoubtedly major approximations in Townes-Dailey theory, and we would not like to put too much stress on the numbers obtained here. Looking for major sources of error in the Townes-Dailey analysis, one may state that in view of the smallness of the coupling constant of boron a wrong assignment of the sign of this coupling constant is certainly not decisive. One important point might be the s character of the sp hybrid at fluorine ((u’ in Eq. [4]) which was assumed to be 0.15. Yet, to obtain a charge distribution within the B-N bond of FsBNH~ comparable to that of H3BNH3, an s character of 0.5 would have to be used. This seems unreasonably high (13) so that at least the qualitative features of the findings for the B-N bond do not change. Another point is the approximate nature of Townes-Dailey analysis itself. As was pointed out previously (23) the experimentally determined coupling constant and asymmetry parameter of bromine in BBr3 may be used to calculate the coupling constant of boron by Townes-Dailey analysis. There is a large deviation between the predicted and experimentally found boron coupling constant. A similar situation holds for BF3. Yet, at least the qualitative features of the electronic structure as obtained by a Townes-Dailey analysis are expected to be correct. This means that the equality of the coupling constant of nitrogen in corresponding members of the two series of compounds, HjBN and F,BN, is due to two counterbalancing effects: the larger polar&ion of the N-H(C) bonds toward nitrogen in F3BN compounds, and decreased donation of charge from nitrogen to boron. We think that the higher negative charge on nitrogen in F,BN compounds can be explained by energetic considerations. The high polarity

310

OLLIGES

ET

AL.

of the F-B bond enforces a high positive charge on the boron atom so that a higher negative charge on nitrogen, i.e., a more ionic character of the B-N bond seems more favourable. In more polar bonds, the loss of covalent bond energy is made up by the incidence of ionic bond energy plus, according to VB theory, energy of ionic-covalent resonance. ACKNOWLEDGMENTS The Erlangen group thanks The Munich group gratefully

the Bundesministerium acknowledges a grant

fiir Forschung from Deutsche

und Technologie for financial Forschun8s8emeinschah.

support.

REFERENCES 1. J. OLLIGES, A. LUTZ, J. VOIT~~NDER, H. BARFUSS, G. B~HNLEIN, F. GUBITZ, W. ITTNER, G. LANZENDORFER, W. KREISCHE, AND B. R~SELER, Z. Nuturfrxb. 41a, 203 (I 986). 2. A. LUTZ AND J. VOI~NDER, J. Mugn. Resort. 58,235 (1984). 3. A. LUTZ AND J. VOIT~~NDER, J. Mugs. Reson. 48, 1 (1982). 4. A. LUTZ, E. PALANG& AND J. VOI~NDER, J. Magn. Reson. 50,417 (1982). 5. M. H. COHEN AND F. REIF, in “Solid State Physics” (F. Seitz and D. Tumbull, Eds.), Vol. 5, p. 321, Academic Press, New York, 1957. 6. H. EBERT, J. ABART, AND J. VO~NDER, J. Chem. Phys. 79,47 19 (1983). 7. J. L. HOARD, S. GELLER, AND W. M. CASHIN, Acta Crystallog. 4, 396 (1951). 8. S. GELLER AND J. L. HOARD, Acta Crystdogr. 3, 121 (1950). 9. H.FRA UENFELDER AND R. M. STEFFEN, in “Alpha-, Bets-, and Gamma-Ray Spectroscopy” (K Sie&hn, Ed.), p. 997, North-Holland, Amsterdam, 1965. 10. H. HOHENSTEIN, Thesis, Universitit Erlangen-Number& 1982. II. H. BARFUSS, G. B~HNLEIN, G. GRADL, H. HOHENSTEIN, W. RRL%CHE, H. NIEDRIG, AND A. REIMER, J. Chem. Phys. 76,5103 (1982). 12. F. GUBITZ, W. HTNER, W. KREISCHE, G. LANZENDORFER, AND B. ROSELER, Forschungsberichte Physikalisches Institut Universitit Erlangen-Ntlmber8, No. 7, 1984. 13. E. A. C. LUCKEN, “Nuclear Quadrupole Coupling Constants,” Academic Press, New York, 1969. 14. S. GELLER AND J. L. HOARD, Actu Crystdogr. 4, 399 (1951). 15. R. LMNGSTON, J. Chem. Phys. 57,496 (1953). 16. V. JACCARINO AND J. G. RING, Phys. Rev. 63,471 (1951). I 7. D. R. ARMSTRONG, “Volume Commemoratif du Colloque International no 19 1 du CNRS, Paris, Dctobre 1969,” p. 47, Editions du Centre National de la Recherche Scientifique, Paris, 1970. 18. A. VEILLARD AND R. DAUDEL, “Volume Commemoratif du Colloque International no 19 1 du CNRS, Paris, Qctobre, 1969,” p. 47, Editions du Centre National de la Recherche Scientifique, Patis, 1970. 19. A. L&z AND J. VoITLj6NDER, unpublished results. 20. M. C. BACH, J. Chim. Phys. 69, 1775 (1972). 21. E. W. HUGHES, J. Am. Chem. Sot. 78, 502 (1956). 22. E. L. LIPPERT AND W. N. LIPSCOMB, J. Am. Chem. Sot. 78, 503 (1956). 23. P. A. CA~ABELLA AND T. OJA, J. Chem. Phys. 50,4814 (1969).