3418
The Journal of Physical Chemistry, Vol. 83, No.
26, 1979
Morton, Preston, and Strach
Free Radicals Trapped in y-Irradiated KPF,‘ J. R. Morton, K. F. Preston,” and S. J. Strach2 Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR9 (Received August 14, 1979) Publication costs assisted by the National Research Councll of Canada
An examination of y-irradiated KPFs at room temperature has revealed the presence of a number of isotropic free-radical EPR spectra in addition to the intense and well-known spectrum of PF5-. Two of the spectra are ascribed to geometrical isomers of PF402-,isoelectronic analogues of PF5-. Other spectra are assigned to PF4, PF30-, FP02-, PO3’-, AsF5-, and AsO?-, the latter two species presumably arising from traces of AsF6-. The validity of an earlier identification of ClF, is discussed in the light of the hyperfine interactions in PFf and related pentacoordinated species. Introduction It has been known for many years that salts of the ion PFs- are “rotator solids” a t room temperature, and that after y irradiation isotropic EPR spectra can be detected in powdered samplesS3Some years ago, there was some uncertainty regarding the identity of the major species present in irradiated KPF, and NH4PF,, which is now known4 to be PF5-rather than PF4as originally thoughta3 Our recent success with EPR spectra in SF6 as a host matrix5 led us to contemplate the possibility of doping KPF, with foreign anions by co-crystaIIization, As a first step in this process we report here the spectra of nine radicals which we have detected after y irradiation in “as received” and recrystallized KPF,. Experimental Section Potassium hexafluorophosphate was purchased from PCR Inc., Gainesville, Fla. Samples (ca. 100 mg) were sealed into quartz tubes and irradiated at 25 “C for periods up to 72 h in a 9000-Ci ,OCo source. Some samples were recrystallized from distilled water before irradiation. EPR spectra were recorded with a Varian E-12 spectrometer equipped with a variable temperature accessory. The microwave frequency of the resonant cavity was measured with a Systron-Donner Model 6054 frequency counter and magnetic fields were determined with a Cyclotron Corp. Model 5300 NMR gaussmeter. Spectral parameters were derived from the field and frequency measurements by exact computerized diagonalization of the full-spin Hamiltonian appropriate to an isotropic spectrum. Wherever possible, a least-squares program was used to obtain the parameters giving the best fit (minimum x2)to all the measured line positions of a spectrum. Results and Discussion Spectra observed at 20 “C in irradiated KPF, were (a) those of PF5-and PF4 native to the pure matrix, (b) those of PF402-,PF30-,FP02-,and P032-derived from partially hydrolyzed PFs-, and (c) those of AsF5- and AsOZ- derived from Ad?,- impurity. EPR spectral parameters for all these species are listed in Table I. Radicals Derived from Pure KPF,. By far the most intense spectrum in y-irradiated KPF6 is that of PF5-,a spectrum characterized by a large 31Phyperfine interaction and hyperfine interactions with four equivalent I9F nuclei. In PF5-, a square pyramidal species, four of the 19Fnuclei are equivalent, and the fifth or apical 19Fnucleus is unique, having a hyperfine interaction of only 3 G. We have now detected a weak spectrum of PF4 in KPFs which had been subjected to prolonged y irradiation. In the new spectrum the four ‘9nuclei are equivalent in pairs 0022-3654/79/2083-34 18$0 1.OO/O
TABLE I: Isotropic Spectral Parameters for Radicals Detected in KPF,) at 20 “C factor
central atom hfi
PF;
2.001Ta
1357b
PF,O,,*PF, ohs * _.
2.0019 2.00 23
1390 1243
PF,
2.0013
1373
PF,O-
2.0018
1163
FP0,Po,*AsF;
2.0013 2.0016 2.0006
675 601 1808
AsO,*-
2.0033
536
radical
P
I9Fhfi 198(4)c 3(1) 212( 4 ) 176(2) 224(1) 292( 2) 60( 2) 307i2j 73(1) 176( 1)
181(4) 5(1)
Errors are less than t 1 i n t h e last digit throughout. Units of hfi are Gauss. Numbers in parentheses are the number of equivalent nuclei having this hfi. a
due to the trigonal bipyramidal geometry of the radical. Both of these radicals (PF,, PF
