Electron spin resonance spectra of certain fluorohydride radicals of

1855

ESR Spectra of P, As, and Sb Fluorohydride Radicals

Electron Spin Resonance Spectra of Certain Fluorohydride Radicals of Phosphorus, Arsenic, and Antimonyla A. J. Colussi,'b J. R. Morton;

and K. F. Preston

Division of Chemistry, National Research Council of Canada, Ottawa K1A OR9, Canada (Received December 16, 1974) Publication costs assisted by the National Research Council of Canada

Isotropic ESR spectra of a series of phosphorus fluorohydride radicals PH,F4-,, where n = 0-4, have been observed in y-irradiated solid solutions of PF3, PF1H, and PH:' in neopentane or SF6. The spectra of AsH3F and SbHRF are also reported. The hyperfine interactions of these radicals are consistent with a totally symmetric semioccupied orbital in a framework possessing either CaUor C, symmetry. The relative merits of Huckel and INDO molecular orbital calculations in predicting the various observed trends are discussed.

Introduction The existence of free radicals derived from tetravalent phosphorus (phosphoranyl radicals) is now well establi~hed.~ These - ~ radicals have attracted considerable interest among both ESR spectroscopists and theoreticians because of their unusual structure. Thus, the spectra of both PF44 and PH45 show, in addition to a large 31Phyperfine interaction, interactions with two pairs of equivalent ligand nuclei, indicating trigonal bipyramidal geometries for both species. Since all phosphoranyl radicals (and a number of isoelectronic radicals) may be regarded as derivatives of one or other of these species, there is good reason to believe that all radicals of tetracoordinated phosphorus have trigonal bipyramidal structures. This is borne out by ESR hyperfine interaction measurements for radicals such as R O P H X ,PC14,6 ~ ~ P O C ~ S - and , ~ SOF:+8 While there remains little doubt concerning the molecular geometry of these radicals, their electronic structure is still a subject of debate. Higuchi, in his theoretical studyg of PF4, was able to account for the experimental data with either the valence-bond or molecular orbital approach by assuming phosphorus 3d orbital participation. Huckel and INDO MO calculations for PF4 and PHI, on the other hand, reproduce the experimental hyperfine measurements with a minimal basis set of s and p orbitals only.3 Furthermore, a number of additional features brought out by recent studies of derivatives of PHdZaand PFd3s7 are more readily explained in terms of MO theory. For example, the greater stability of apically substituted derivatives of PH4 and PF4 is consistent with the charge-density distributions predicted by INDO MO calculations. Moreover, the effect of increasing electronegativity of RO in ROPF3 radicals3 upon the various hyperfine interactions is readily accounted for in terms of a semioccupied orbital which is an antibonding, three-center (Rundle'()) MO involving the phosphorus atom and the two apical ligands. In this paper we present hyperfine interaction measurements for some simple phosphoranyl radicals PH,F4-, (where n = 0-4) and for the related species AsH:'F and SbH:jF. The data show trends consistent with a simple MO description of the semioccupied orbital, in which d orbital participation is ignored. '

Experimental Section Samples of SF6, neopentane, or neopentane-dl2 were

doped with approximately 5% PF3, PH3, or PF2H by mixing appropriate proportions of the two gases, followed by condensation of the mixture into an ESR sample tube. The samples were irradiated a t -196' with y rays from a 9000Ci source, and their ESR spectra examined at -170' with a Varian E-12 spectrometer.3 PF3 and SF6 were obtained from Ozark-Mahoning Inc., Tulsa, Okla., the PH3 was a gift from the Defence Research Board Laboratories, Ottawa; AsH3 was obtained from Matheson Gas Products Inc., East Rutherford, N.J.; PF2H was prepared by the method of Rudolph and Schiller;" a sample of antimony metal enriched to 99% in the isotope 123Sb was obtained from Oak Ridge National Laboratory, Oak Ridge, Tenn., and was used to prepare 12%SbH3. l 2

Results A. Phosphorus-Centered Radicals. An irradiated sample of SF6 containing a few percent PF3 exhibits the powerful ESR spectrum attributed by Fessenden and Schuler4 to the radical PF4. The g factor and hyperfine data (Table I) were obtained by exact diagonalization of the appropriate spin matrix under the assumption that all hyperfine interactions had the same (positive) ,sign. As Fessenden has shown,':' this combination leads to a much smaller standard deviation in the spectral parameters than any, other. Our data for PF4-SF6 are in virtually exact agreement with those of Fessenden,'" but differ substantially from those of Nelson, Jackel, and Gordy.14 These authors generated an anisotropic spectrum of PF4 in a PF:j matrix at 4'K. The discrepancy between their data and ours could be a matrix or a temperature effect, but is probably an artifact resulting from the inherent difficulties of interpreting anisotropic spectra. Irradiation of a matrix consisting of 5% PH:] in neopentane enabled us to detect the spectrum of PH4 a t -170°.5 Its four protons are equivalent in pairs-apical and equatorial. Exact analyses of the data show unequivocally that the signs of the :jlP hyperfine interaction and that of the two apical protons are the same (positive). The results of these calculations appear in Table I. In addition to the two prototype radicals PF4 and PH4 we were also able to generate and detect (Table I) certain other radicals in the series PH,F4-,. For example, after irradiation of a matrix of PH3 in SF6 we detected a spectrum attributable to PHBF. A very interesting result was the deThe Journal of phvsical Chemistry, Vol. 79. No, 17, 1975

1856

A. J. Colussi, J. I?. Morton, and K. F. Preston

TABLE I: ESR Dataa for Certain PH,Fd-, Radicals Radical g v a l u e PF, 2.0009 PF3HmC2.0016 PF,H,

2.0020

hfs,* Apical hfs,b Equatorial G hfs, G

G

1322.0 1030.8 1027.3

293.5 225.2(F)

59.5 38.5 (F) 38.5 (H) 5.9 (D) 35.0 (F) 35.0 (HI

226.8 (F)

5.1 (D) 347.2 (F) 12.6 (H) 130.1 (H) PHdC 2.0030 519.3 198.7 (H) 6.0 (H) 30.2 (D) Not r e s (D) a Errors are i 2 in last significant figure given. b Shown to have the same sign by best-fit procedures.*3 c In neopentane; others in PH,F,,

2.0032

TABLEII: ESR Parameters of PH3F, A s H ~ F and , SbH3F Radicals" Central atom

Apical hfs, G

Radical

gvalue

hfs, G

3iPH3F

2.0032

721.3

75AsH,F '%bH,F

2.0093 2.0158

860.7 1048.9

721.3

a

347.2 130.1 300.7 167.5 233.8 178.7

Equatorial hfs, G

(F)

12.6 (H)

(H)

(F)

7.7 (H)

(H) (F)

Not r e s (H)

(H)

Errors are f 2 in last significant figure given.

SF6.

I

1 2500

I I

I 2750 G

I

1

I

PF3H

Figure 1. The w / 3 1 ) = 0.5 transition of PF3 observed in 5% PF2H in SFe at approximately 9.1 GHr. The hype ine interactions of the 'H and lQFequatorial ligands are accidentally equal in this radical.

77

-

6.2

'

tection in both PF3-neopentane and PF2H-SF6 of a spectrum which appeared to be that of PFpHz (i.e., the hyperfine pattern indicated two pairs of apparently equivalent ligands (Figure 1)). However, deuteration studies with both PF3-neopentane-d 12 and PFzD-SFs revealed that the radical possessed only one equatorial proton, whose hyperfine interaction happened to be the same as that of the equatorial fluorine nucleus. The species was thus identified as PFsH,,; its parameters in both neopentane and SF6 are given in Table I. Unfortunately, we have not been able as yet to obtain an ESR spectrum of the radical PF2H2. R. Radicals AsHsF and SbH3F. Irradiation of a matrix consisting of -5% AsH:3 in SF6 a t -196' resulted in the formation of the radical AsHsF. Its ESR spectrum a t -170' consisted of 16 7 . 7 4 , 1:2:1 triplets, of which four (the Jvf(Z*,) = -0.5 transition) were obscured by the radicals SF5 and SFs-. The spectral parameters (g, hyperfine interactions) were determined by computerized diagonalization of the appropriate spin matrix, a process which clearly demonstrated that the signs of the 15As, 19F, and apical proton hyperfine interactions were all positive. Table I1 shows the average values of several independent determinations of these parameters. Irradiation of an SF6 matrix containing dissolved SbHs enabled us to detect the ESR spectrum of SbH:3F at -170'. In this case, however, analysis of the spectrum was complicated by the following factors: (a) the existence of two magThe Journal of Physical Chemistry. Vol. 79, No. 17, 1975

Y

9146 MHz

6.4

6.6 kG

Flgure 2. The w / 1 2 3 ) = -3.5 transkion of SbH3F enriched to 99% in the isotope lZ3Sb(/= 3.5).

netic isotopes of antimony (lzlSb, 57.25%, Z = 2.5 and 12:'Sb, 42.75%, I = 3.5) and (b) the anticipation of exceedingly large hyperfine interactions for both isotopes. The combined effect of these two factors led us to anticipate that for "%bHsF, only the M(Z121) = -2.5 transition would be accessible but that for lZ3SbH3Fboth the M(Z123) = -3.5 and -2.5 transitions would be accessible. We therefore prepared SbH3 from a sample of antimony enriched to 99% in the isotope lZ3Sb. The M(1123) = -3.5 (Figure 2) transition was analyzed to yield for the apical I9F and lH hyperfine interactions the values 234 and 178 G, respectively. The equation a =

(2H - 2)/(21

+ 1 - H)

(1)

enables one to calculate the hyperfine interaction ( a ) of a nucleus (spin I) from the transition a t highest field (H), if both a and H are expressed in units of vlgp, where u is the microwave frequency and = 1.399611 MHz/G. At 9146 MHz, this transition occurred a t 6402 G, yielding (assuming g = 2.0000 in eq 1) a provisional value for a123 of 1038 G. Unfortunately, the M(Zlz:{) = -2.5 transition was obscured by the powerful spectra of SFF,and SF6- in the g = 2 region, and so accurate values of the g factor and hence the antimony hyperfine interactions could only be obtained

ESR Spectra of P, As, and Sb Fluorohydride Radicals

1857

-

with the aid of the “forbidden” transition 14, -4) 13, -3) (low-field, IF, M ( F ) ) quantum numbers) of lZ3SbH3F.This transition, for which a = (2H 2)/(8 H) in units of u/g& occurred a t 1138 G and, combined with the M ( Z ) = -3.5 transition, yielded g = 2.0158, a123 = 1049 G. Using this g value we calculated a value for a121 of 1929 G from the M(Zlzl) = -2.5 transition observed a t 6961 G in natural SbH3. The ratio of the observed hyperfine interactions, a121/a123 = 1.839, was in close agreement with the ratio of the magnetogyric ratios (1.847). The proton and fluoron hyperfine interactions in SbH3F were assigned (Table 11) on the basis of the line-width variation apparent (Figure 2) in the spectrum. The larger hyperfine interaction is clearly associated with considerable anisotropy, and was therefore assigned to the fluorine nucleus. Assuming this interaction to be positive (as in ASHSF),a greater (or lesser) width is expected for the lowfield (M,(Fap) = 0.5) pair of lines if the inner product of the anisotropic g and 19F hyperfine tensors is positive (or negative).15 This product is likely to be larger for the apical fluorine atom than for the apical proton, because of the expected large contribution of F(2p) orbitals to the semioccupied orbital.

+

+

Discussion It has been well established by ESR spectroscopy that all of the radicals whose spectra we shall be discussing possess ground states which are totally symmetric in either Czu or C, symmetry. In the former case the four ligands are equivalent in pairs, the pair subtending the smaller angle a t the central atom being called the equatorial ligands, the other pair being called apical. It follows from the totally symmetric ground-state of these radicals that the orbital occupied by the unpaired electron can possess considerable centralatom n s character, a fact which accounts for the dominant feature of all the spectra: the exceedingly large centralatom hyperfine interaction. In addition, of course, the semioccupied orbital contains contributions from various ligand atomic orbitals (ns, or appropriate combinations of np). The data in Table I demonstrate that successive replacement of F atoms by H atoms in PF4 results in a monotonic decrease in the 31Phyperfine interaction, a trend consistent with the antibonding nature of the half-filled orbital in these radicals. The pronounced effect of equatorial substitution, however, is somewhat surprising since it presumably implies that the description of the semioccupied orbital as a Rundle orbita13v7is not entirely adequate. We have suggested elsewhere? that in the radicals ROSF4 and ROPF3 the contribution to the semioccupied orbital by the fluorine atom trans to RO consisted primarily of F(2p) atomic orbitals. The total contribution of F(2s) including polarization effects was demonstrably small. Increasing the electronegativity in R caused the trans 19Fhyperfine interaction to decrease, an effect which we tentatively ascribed to a decrease in the polarizibility factor Q F M ~(Le., operating on the F(2p) contribution) rather than due to a decrease in any direct F(2s) participation. In the present case, however, very different conditions obtain: the apical H ( l s ) contribution to the semioccupied orbital in PH4 is approximately 40%, and there can be no contribution from p orbitals on hydrogen. Therefore, the decrease in the trans proton hyperfine interaction on substitution of a more electronegative fluorine atom or tert- butoxy2a ligand for an apical proton can only be due to a decrease in the contribution of the apical proton 1s orbital to the

semioccupied orbital. Such a direct effect may also be operative in ROSF4 and ROPF3, although it is impossible to determine the relative importance of the two mechanisms. The 75As hyperfine interaction in AsH3F can be compared with those of certain other arsenic-centered radicals. The general trends discussed above for phosphoranyl radicals are apparent. The largest 75As hyperfine interaction occurs for the radical AsF4 (1576 G16), and the smallest for As(CgH5)d (541 G17). The effect of an electronegative ligand in the apical position is apparent from the value for As(CsH5)30C(CH3)3(666 Gls); two apical tert-butoxy ligands increase the 7sAs interaction still further, to 792 G.18 We turn now to a brief discussion of the effects on the spin-density distribution of a change in the central-atom electronegativity along a series such as PHsF, ASHSF, and SbH:lF (the radicals PF4 and AsF4 have been discussed elsewhere16). For this purpose we need a reliable factor to relate the central-atom hyperfine interaction to its valence s spin density. For want of a better method, and with certain reservation^,'^ we identify unit valence s spin density ~(O) on the central atom (M) with the factor ( 8 ~ / 3 ) r ~ y ~ J . as calculated from Froese’s wave function.20 For the nuclei :j1P(3s),75As(4s), and lZ3Sb(5s)the values 3638, 3393, and 3266 G, respectively, were obtained. From these figures and the corresponding hyperfine interactions (Table 11) the contribution of central-atom ns character to the semioccupied orbital may be calculated to be approximately 0.32 in the case of SbHAF, 0.25 for AsH3F, and 0.20 for PH3F. This decrease in central-atom s character with increasing electronegativity a t that atom is due to the antibonding nature of the semioccupied orbital. As is also evident from Table 11, the spin released from the central atom upon increasing its electronegativity reappears as increasing F(2p,) or F(2s) contributions to the semioccupied orbital. This effect (and the concomitant reduction in the Hap hyperfine interaction) is probably due to the weakly HaP-F bonding nature of the semioccupied orbital. Molecular Orbital Calculations We have carried out molecular orbital calculations on the phosphorus-centered radicals listed in Table I. The INDO I ( K = 1) parameterization,21 with a minimal basis set of valence s and p atomic orbitals, was used. As has been mentioned e l ~ e w h e r e ,the ~ minimum energy configuration of both PH4 and PF4 is predicted by the INDO I method to have neither D4h (square planar) nor T d (tetrahedral), but Cz0 symmetry. This, in itself, is gratifying agreement with the experimental observations. The INDO method furthermore predicts that the observed radicals PFsH,, and PH3Fap are more stable by several kilocalories per mole than their conformers PF3Hap and PH3Fap, respectively, On the other hand, however, the INDO approximation reproduces only the grossest features of the spin-density distributions. For the purposes of comparing atomic spin densities with experimental hyperfine interactions we use as conversion factors the parameter ( 8 ~ / 3 ) ~ ~ r ~ J . , , which ~(o), may be calculated from Froese’s wave function20to be 3638 G for 31Pand 17,100 G for 19F.For protons we use the fattor 507 G. In Table I11 the calculated 31P,19F, and lH hvperfine interactions are given for the radicals PF4, PF3Hiq, PHsFaD, and PH4. I t will be seen that the experimental trend in the S1P hyperfine interactions down the series of radicals is not adequately reproduced by the INDO method. However, much better overall agreement with experiment was obtained not The Journal of Physical Chemistiy, Vol. 79, No. 17, 1975

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A. J. Colussi, J. R. Morton, and K. F. Preston

TABLE 111: Calculated Values of

19F,and l H Hyperfine Interactions" in Certain Phosphorus Fluorohydrides

irradiation clearly indicated that our samples of PFzH in SFe contained PF2H and no detectable amounts of other phosphorus fluorides.

~~

Equatorial ligands

Apical ligands

3'P

Radical HUckel INDO HUckel

Acknowledgments. We thank Dr. K. Sogabe for a preprint of the above-mentioned article and Dr. S. K. Brownstein for NMR spectra.

INDO HUckel INDO References a n d Notes

PF, PF,H,,

1464 1300

569 609

243 232

PH,F,,

1045

506

930

674

438 (F) 201 32 (H) 31 155 245

PH,

71 79

41 4 33 (F) 5 30 (H) 11 26 24 2

2

Values (gauss) obtained by multiplying atomic spin densities by 507 G (IH), 17,100G (lBF).,or 3638 G (31P).

only for slP, but also for l H and I9F interactions by using the Huckel-level spin densities corresponding to the

INDO-optimized geometries. NOTE ADDED IN PROOF:Very recently K. Sogabe (J. Sci. Hiroshima Uniu., Ser. A, 39, 225 (1975)) reported an intense spectrum of PFsHap, detected in 7-irradiated PF2H-SFs mixtures. In spite of many attempts to generate this spectrum under a variety of experimental conditions, we have been unable to do so. We wish to emphasize, however, that NMR analyses obtained both before and after y

The Journal of Physical Chemistry, Vol. 79, No. 17, 1975

(a) NRCC No. 14768. (b) N R C C Postdoctorate Fellow 1974. (a) P. J. Krusic. W. Mahler. and J. K. Kochi, J. Am. Chem. SOC.. 04, 6033 (1972): (b) D. Griller and B. P. Roberts, J. Chem. SOC., Perkin Trans. 2, 1339 (1973). A. J. Colussi, J. R. Morton, and K. F. Preston, J. Phys. Chem.. 7 0 , 651 (1975). R . W. Fessenden and R. H. Schuler, J. Chem. Phys., 45, 1845 (1966). A. J. Colussi, J. R. Morton, and K. F. Preston, J. Chem. Phys.. 82, 2004 (1975). G. F. Kokoszka and F. E. Brinkman, J. Am. Chem. SOC., 02, 1199 (1970). 1.Gillbro and F. Williams, J. Am. Chem. SOC., 08, 5032 (1974). J. R. Morton and K. F. Preston, J. Chem. Phys.. 58, 2657 (1973). J. Higuchi, J. Chem. Phys., 50, 1001 (1969). R. E. Rundle, Surv. hog. Chem.. I, 81 (1963). R. W. Rudolph and H. W . Schiller. J. Am. Chem. Soc.. SO, 3581 (1968). W. L. Jolly and J. E. Drake, horg. Synth., 7, 34 (1963). R . W. Fessenden, J. Magn. Resonance, 1, 277 (1969). W. Nelson, G.Jackel, and W. Gordy, J. Chem. Phys., 52, 4572 (1970). A. Hudson and G. R. Luckhurst, Chem. Rev., 80, 191 (1969). A. J. Colussi, J. R . Morton. and K. F. Preston, Chem. Phys. Len., 30, 317 (1975). S . A. Fieldhouse, H. C. Starkie. and M. C. R. Symons. Chem. Phys. Len., 23, 508 (1973). E. Furimsky. J. A . Howard, and J. R. Morton, J. Am. Chem. Soc., 05, 6574 119731. J. H. Mackey and D. E. Wood, J. Chem. Phys., 52,4914 (1970). C.Froese, J. Chem. Phys., 45, 1417 (1966). A. R . Gregory, J. Chem. Phys.. 80, 3713 (1974).