EPR Spectra of hexafluoride radicals

JOURNAL

OF MAGNETIC

29,243-249

RESONANCE

(1978)

EPR Spectra of Hexafluoride Radicals* A. R. BOATE,~ J. R. MORTON, AND K. F. PRESTON+ Division

PRESENTED

of Chemistry, National Research Council Ottawa, Canada KIA OR9

AT THE SIXTH

INTERNATIONAL

BANFF,

The

preparation

The

compared. state

0,

in

symmetry.

semioccupied atomic

of various central-atom

orbital,

contribution

and

hexafluoride

radicals

six

are correlated

interactions

is confirmed

consists the

CANADA,

hype&e

This

orbital

of

an

by the atomic

electronegativity

MAY

0~ MAGNETIC

is described,

their

of

spectra

of an *A,,

19F hype&e

combination changes

and

indicative

anisotropic orbitals.

RESONANCE,

1977

are large,

antibonding

2p

fluorine

with

SYMPOSJUM

ALBERTA,

of Canada,

the

Changes in the central

are

ground

structure.

The

central-atom

ns

in the

central-atom

atom.

INTRODUCTION

The first reported EPR spectrum of a main-group hexafluoride radical was that of SF,-, discovered in 1966 by Fessenden and Schuler (1). This was followed, 9 years later, by the discovery of its analogs SeF,- and TeF,- (2), and shortly thereafter by the detection of the EPR spectra of the halogen hexafluorides ClF,, BrF,, and IF, (3-6). We have now extended this set of isoelectronic radicals to include the dianions of Group V (AsF,*-, SbF,*- and BiFe2-), a trianion of Group IV (PbFG3-) and a tetraanion of Group III (TlFe4-). The hexafluorides constitute, therefore, one of the largest sets of isoelectronic radicals known; it is the purpose of the present article to review their preparation, the characteristics of their EPR spectra, and to compare their EPR parameters (g factors and hyperfine interactions). PREPARATION

OF

HEXAFLUORIDE

RADICALS

The radical SF,- was originally prepared by continuous electron bombardment at 100 K of SF, inside the cavity of an EPR spectrometer (1). SeF,- and TeF,- were prepared by y irradiation at 77 K of solid solutions of SeF, or TeF,, respectively, in SF,. This technique (y irradiation in an SF, matrix) was also used to prepare ClF, and BrF, from the respective pentafluorides, while IF, was prepared by the uv photolysis of IF, dissolved in an SF, matrix. The hexafluoride anions of Groups III to V were generated by the y irradidtion of certain polycrystalline salts. AsFG2- and SbFe2-, for example, were detected in 7. irradiated CsAsF, and CsSbF,, respectively. The best spectrum of BiFe2- was obtained in an irradiated sample of CsAsF, doped (approximately 5% w/w) with BiF,-. The * Issued I’ Research $ Author

as

NRCC Associate,

to whom

No.

16382. 1975-1977.

correspondence

should

be addressed.

243

0022-2364!78/0292Jl243%02.00/0 Copyright 0 1978 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain

244

BOATE,

MORTON,

AND

PRESTON

radicals PbFe3- and T1Fe4- were prepared by the y irradiation of BaPbF, and Cs,TlF,, respectively. EPR SPECTRA

OF HEXAFLUORIDE

RADICALS

The most abundant isotopes of sulfur, selenium, and tellurium possess zero spin, so that the EPR spectra of SF,-, SeF,-, and TeF,- are dominated by a hyperfine manifold of six equivalent 19F nuclei. At 100 K the isotropic 19F hyperflne interactions of SF,-, SeF,-, and TeF,- in an SF, matrix are 549, 487, and 596 MHz, respectively (I, 2). To date, no analysis of the anisotropic 19F hyperfine structure (observed below 94 K) has *05TlF;-

NMR ESR-‘/2

2 (I,1

(I,0

I,O) -

1,-I)

I

\

‘25TeF;

NYRI

(I,-b-0,0)

FIG. I. Positions of various transitions for S = 4, I = 4 system. The ordinate is the hyperfine interaction (MHz) divided by microwave frequency; the abscissa is the resonant field for the transition converted to dimensionless form by multiplying by gp/o.

been reported. The central-atom hypertke interaction in SF,-, SeF,-, and TeF,- has also been detected (2). The isotopes 33S, “Se, and lzsTe have nonzero spin and the spectra of their hexafluoride anion radicals were observed as satellite transitions. The halogen nuclei have nonzero spin and the appearance of their spectra is dominated by the large hyperfine interactions of their central atoms. The central-atom hypertine interaction in the halogen hexafluorides is so large, indeed, that only in the case of ClF, is a “normal” EPR spectrum observed (6). In both BrF, and IF, (4-6) certain EPR transitions are inaccessible (7); formally forbidden “NMR” transitions are observed and used to determine the spectral parameters. The hyperfine interactions of 15As, ?Sb, iz3Sb, 205T1,lo7Pb, and 209Biare also very large, and necessitated the use of both “EPR” and “NMR” transitions in the analysis of the spectra of their hexafluoride anions. Since their central-atom hyperfme interactions are unusually large, the spectra of the hexafluoride radicals are most readily analyzed with the aid of the Breit-Rabi (d),

EPR

SPECTRA

OF

HEXAFLUORIDE

RADICALS

245

NafeNelson (9) equations. The low-field quantum numbers F, mF can be used to construct a diagram such as that shown in Fig. 1, which is a plot for I = + of the “reduced” central-atom hyperfine interaction a/u, against magnetic field (I?,) expressed as a multiple of v/g/3 (gauss). This diagram enables one to identify the observed transitions and obtain an estimate of the hyperiine interaction. Similar diagrams can be constructed for any nuclear spin (7). The Breit-Rabi (d), Nafe-Nelson (9) equations can then be used to obtain accurate values of the central-atom hyperfine interaction and the g-factor from the measured line positions and microwave frequencies. The analysis is only possible because the hexafluoride spectra are “pseudoisotropic.” That is, the 19F ligands are equivalent in pairs for all orientations of the magnetic field, generating strong central features which can be analyzed for the central-atom hyperfine interaction and the g factor as an isotropic spectrum. The principal values of the anisotropic fluorine hyperfine interaction matrix were obtained by the methods we have described elsewhere (6). This procedure was confirmed where possible by a computer simulation of the transition (10). Experimental data for the hexafluoride spectra obtained to date are summarized in Table 1. TABLE EPR

1

PAMMETERS OF HEXAFLUORIDE RADICALS Hyperfine interactions (MHz)

Radical “CIF, ‘9BrF, ‘2’IFg %F,“SeF,lz5TeF,“AsF,~‘*%bF,**09BiF62207PbF63‘05TlF,‘~

Temp. (K) 21 21 21

100 100 100 30 30 30 15 30

g 2.0154 2.0147 2.0105 2.0078 2.0098 2.0070 2.0030 1.9994 2.0134 2.0023

1.9903

Central atom

-

Fluorine nuclei --Parallel Perpendicular

2,211

11,112 17,550 1,807 10,222 -28,318 9,403 21,370 36,020 47,868

125,010

824 835 962

-34 -51 146 549" 487" 596"

1,013 1,006 741 701 517

533 542 251 314 312

a Isotropic hypefine interactions. THE SEMIOCCUPIED

ORBITAL

OF THE HEXAFLUORIDE

RADICALS

Two features of the EPR spectra of the hexafluoride radicals give a key to a description of their semioccupied orbital. These are (a) the large central-atom hyperfine interactions, and (b) the 19F hyperfine structure. Taking these features in turn, the large central-atom hyperfine interactions tell us that the semioccupied orbital contains considerable central-atom (M) valence s character, and hence that its representation is totally symmetric. The fluorine hyperfine structure of the anisotropic spectrum indicates that the fluorine nuclei are equivalent in pairs for all orientations of the magnetic field, generating a strong central feature composed of the superposition of many mICF)= 0

246

BOATE,

MORTON,

AND

PRESTON

components of the transition. Moreover, there is no indication, within the limits imposed by linewidth, of either g-factor or central-atom hyperfine anisotropy. The anisotropic spectra are, therefore, only consistent with a radical having a 2A,, ground state in 0, symmetry. The semioccupied orbital consists of a central-atom M(ns) atomic orbital overlapped out of phase (antibonding) with six fluorine 2p atomic orbitals pointing toward the central atom. A comparison of the central-atom hyperfine interaction with values of the parameter A, = (87r/3)y,y, t&(O) should enable one to estimate the central-atom ns contribution to the semioccupied orbital. A wavefunction which we have frequently used in the past (II) yields reasonable values for this parameter in the case of first-row elements, but becomes increasingly suspect for elements of higher atomic number. We have therefore resorted to the wavefunction of Herman and Skillman (12, I3), and the semiempirical correction of Mackey and Wood (14). Dividing the central-atom hyperfine interaction by these A, values, one can obtain an estimate of the corresponding valence s contribution to the semioccupied molecular orbital. These calculations indicate that, for the halogen hexafluorides, the central-atom character is -0.4. As might be anticipated from the antibonding nature of the semi-occupied molecular orbital, the central-atom character increases to -0.5 for Group VI hexafluoride anions and to -0.6 for Group V hexafluoride dianions. In the case of *07PbF,3- and 205TlF,4- the Mackey-Wood/ Herman-Skillman values of A, indicate central-atom characters of approximately 0.64 and 0.73, respectively. The anisotropic “F hyperfine structure is also consistent with this model. An estimate of the F(2p) contribution to the semioccupied molecular orbital can be obtained by dividing f(a ,, - a,) by 0.4 yeyF(r3). Hurd and Coodin (12) have estimated the latter as 1760 MHz from Herman and Skillman’s wavefunction (13). For the halogen hexafluorides one obtains F(2p) contributions in the range 0.15 to 0.16, correlating reasonably with central-atom contributions of approximately 0.4 (there may be negative spin density in the central-atom np orbitals (6)). Comparing these figures with those for the Group V dianions, in which the central atom ns character is thought to have increased to -0.6, we observe a concomitant reduction in the F(2p) contribution to approximately 0.09, estimated by the above procedure. Thus, we see that although it is impossible to have an exact accounting of the spin density distribution because of the difficulty in relating hyperfine interactions to spin densities, the overall interpretation is reasonable regarding both magnitudes and trends with changing electronegativity. We have also tried to compare some of our hexafluorides with other species, particularly the so-called impurity metals in alkali-halide crystals. Compare, for example, our data (Table 1) on TlF,4- with those for Tl*+ surrounded by six Cl- ions in KC1 (1.5). The *05Tl hyperfine interaction in KCl:Tl’+ is isotropic at 105.4 GHz, very similar to that of our TlFh4- (125 GHz). Bearing in mind the lower electronegativity of chlorine compared to fluorine, which would explain the smaller *OsTl hyperfine interaction, it would appear that TlCle4- is a valid description of the KC1 : T12+ center. The species has all the prerequisites of a hexahalide radical anion: isotropic g factor, large and isotropic central-atom hyperfine interaction, and anisotropic ligand hyperfine interactions whose principal directions are parallel to the “bond” directions. The six chlorine nuclei in KC1 : Te2+ show equivalent anisotropic hype&e interactions a,, = 72 MHz, and a, = 20 MHz. An estimate of the Cl(2p) contribution to the semioccupied

EPR

SPECTRA

OF

HEXAFLUORIDE

RADICALS

241

orbital may be obtained by dividing $(a,, - a,) by 0.4 yeyc,(r3). The latter has been estimated (12, 13) to be 176 MHz for C1(2p), yielding a value of 0.1 for each Cl(2p) contribution to the semioccupied molecular orbital. This, too, is entirely consistent with a molecular description of the free radical. Very similar conclusions could be drawn concerning the Pb3+ center in KC1 crystals (16): a molecular description such as PbCle3- would appear to be entirely valid. TEMPERATURE

VARIATION

OF

CENTRAL-ATOM

HYPERFINE

INTERACTIONS

The variation with temperature of the central-atom hyperfine interaction of some of the radicals listed in Table 1 has been investigated. Generally speaking, the hyperfine

a=ll961-235T/l33

a,,,=

ll961-235coth

(133/T)

11700-

MHz 7gBrF6/TeF, Il650-

I I6001

I 50

0

I 100

50

“K FIG. 2. Variation

with

temperature

of the 79Br hyperfine

interaction

in BrF,.

interaction is constant over the range 4 to -30 K, and then decreases with increasing slope as the temperature is raised further (Fig. 2). There appear to be three possible mechanisms (17, 18) for such an effect: (1) lattice expansion; (2) coupling of lowfrequency vibrations of the radical to phonon modes of the matrix; (3) localized, lowfrequency vibrations of the radical itself. We have been unable to devise a convenient method for testing the first mechanism. The second mechanism can be evaluated by fitting the data to a Debye function of the form

o(T)=oo[l -c( ;)4~x3,(~-. l)h] . 0

111

248

BOATE,

MORTON,

AND

PRESTON

On the other hand, if we assume the third mechanism to be operative, we should fit the data to an equation such as a(T) = a, - bcoth(U2T).

[21

The data have been fitted to both of these equations (Table 2), and from the results thereof we are led to discard the first (Debye-type) interpretation, for the following reasons. The Debye temperature 9 is a property of the matrix, and yet for both SF, and TeF, matrices, 0 changes markedly from radical to radical. Furthermore, preliminary results which we have obtained for the same radical (IF,) in different matrices (SF, and TeFd also indicate an intramolecular mechanism, since the a(T) versus T curves are virtually superposable. This would not be expected if a(T) were matrix-dependent. TABLE

2

LEAST-SQUARES PARAMETERS FOR DEEIYE-FUNCTION AND HYPERCOTANGENT FITS OF THE TEMPERATURE DEPENDENCES OF CENTRAL-ATOM HYPERFINE INTERACTIONS Debye” Radical

Matrix

SF, SF, SF, TeF, TeF, CaAsF, CsSbF, ’ The functions and associated b One of two sites.

a,,(MHz) 2,213 10,215 -28,582 11,727 17,500 9,406 21,270 parameters

Coth”

c

e 03

1.01 0.3 1 0.07 0.22 0.54 0.10 0.04

450 353 113 415 227 180 63

are defined

2,425 10,496 -28,83 1 11,961 17,898 9,509 21,368

b (MHz)

e (K)

212 282 -251 235 400 104 99

292 227 83 266 164 129 47

in the text.

It is felt, therefore, that the temperature dependence of the central-atom hyperfine interaction in these radicals is due to a low-frequency vibration (possibly flu), whose amplitude increases with increasing temperature, bringing about a decrease in the central-atom ns character and hence its hypertine interaction. There is the further possibility of a vibronic interaction with an excited state (possibly 2Tlu), which would be coupled to the ground state by a vibration of appropriate representation (second-order Jahn-Teller effect). The admixture of such an excited state with its lower central-atom ns character would also reduce the central-atom hyperfine interaction. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

R. W. FESSENDEN AND R. H. SCHULER, J. Chem. Phys. 45,1845 (1966). J. R. MORTON, K. F. PRESTON, AND J. C. TAIT, J. Chem. Phys. 62,2029 (1975). K. NISHIKIDA, F. WILLIAMS, G. MAMANTOV, AND N. SMYRL, J. Amer. Chem. Sot. 97,3526 (1975). K. NISHIKIDA, F. WILLIAMS, G. MAMANTOV, AND N. SMYRL, J. Chem. Phys. 63, 1693 (1975). A. R. BOATE, J. R. MORTON, AND K. F. PRESTON, Inorg. Chem. 14,3 127 (1975). A. R. BOATE, J. R. MORTON, AND K. F. PRESTON, J. Phys. Chem. 80,2954 (1976). A. R. BOATE, J. R. MORTON, AND K. F. PRESTON, J. Magn. Resonance 24,259 (1976). G. BREIT AND I. I. RABI,Phys. Rev. 38,2082 (1931). J. E. NAFE AND E. B. NELSON, Phys. Rev. 73,718 (1948).

EPR 10. 11. 12. 13. 14. 15. 16.

17. 18.

SPECTRA

OF

HEXAFLUORIDE

RADICALS

R. C. C. F.

LEFEBVRE FROESE,J.

W. D. W. W.

DREYBRODT AND D. SILBER, Phys. Status Solidi 20,337 (1967). SCHOEMAKER AND J. L. KOLOPUS, Solid State Commun. 8,435 (1970). M. WALSH, J. JEENER, AND N. BLOEMBERGEN, Phys. Rev. 139, Al 338 (1965). DREYBRODT, Phys. Status Solidi 21,99 (1967).

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J. Chem. Phys. 42, 1480 (1965). 45,1417 (1966). AND P. COODIN, J. Phys. Chem. Solids 28,523 (1967). Chem.

Phys.

M. HURD HERMAN AND S. SKILLMAN, N.J., 1963. J. H. MACKEY ANDD. E. WOOD,

“Atomic

Structure

J. Chem.

Phys.

Calculations.”

52,4914

Prentice-Hall,

(1970).

Englewood

Cliffs,