SULFUR-CONTAINING RADICALS TRAPPED IN SINGLE CRYSTALS
89
Identification of Some Sulfur-Containing Radicals Trapped in Single Crystals1
by J. R. Morton Division of Applied Chemistry, National Research Council, Ottawa, Canada
(Received September 27, 1966)
The esr spectral parameters of radicals identified as SZ-,' SZO-, S202-, Sz03-, and SO4are presented and discussed.
Introduction Electron spin resonance spectroscopy has been the means whereby a large number of organic and inorganic radicals have been discovered and identified. Among the inorganic radicals, most involve first-row elements only, and the present paper will attempt to correct this imbalance by presenting data on some recently identified sulfur-containing radicals. Electron Resonance of S2- in KBr When potassium bromide crystals are grown from the melt in an atmosphere of oxygen or sulfur and their fluorescent emission is afterward examined a t 77"K, it is f o ~ n d that ~ ; ~the latter consists of a series of bands the separations between which appear to correspond to vibrational intervals in the ground state of a diatomic molecule. It has been suggested that 0 2 - and S2- are trapped at halide ion vacancies of the oxygen- and sulfur-doped crystals, respectively. The presence of 02ions in oxygen-containing alkali halide crystals had already been established4 by electron spin resonance experiments at 20°K. The 02-ions are oriented so that the 0-0 bonds are parallel to the face diagonals of the cubic unit cell. Electron resonance experiments have now confirmed that S I - ions are present in luminescent, sulfur-doped KBr crystals.6 Figure 1 shows the resonance at 1950 gauss in a crystal doped with sulfur enriched to 60% in the isotope S33( I = 3/z). Three isotopic combinations are apparent: (S33)2-gives rise to a manifold of seven lines having relative intensities 1:2:3:4:3:2:1; (S33S32)- appears as a quartet of equally intense lines; and contributes to the intensity of the central line. The S33hyperfine splitting is 29 gauss in this spectrum, for which Ho is parallel to the S-S bond. In Figure 2
the variation in the position of the Sz- resonances as
Ho explores the (001) plane of a KBr crystal is shown. It will be noted that the anisotropy is very much more pronounced than that of 02-in the same matrixa4 The positions of the Sz- resonances are described by the equation
H~ = hvp-l(gI12 cos2 e
+ gL2 sin2 e) -'I2
(1)
where e is the angle between Ho and the internuclear axes { 1101, and the spectral parameters 911 and g1 are 3.5010 f 0.0002 and 1.05 f 0.05, respectively. The large errors in gL is due to the highly anisotropic width of the resonances, which renders the line undetectable at 0 = 90" and so prevents a direct determination of g.l The maximum-slope widths for 8 = 0, 45,60, and 75" were 3.5, 10, 15, and -45 gauss, respectively; the reason for this behavior has not yet been established. The g factors of a molecule in a 2 ~ , / 2 state depend on the relative magnitudes of A, the spin-orbit interaction constant, and A, the crystal-field splitting of the rTg energy levels
+ 2X(X2 + A2)-'" geA(X2 + A2)-'"
911 = ge
91
=
(2)
(3)
If A >> A, orbital angular momentum is quenched and both 911 and g1 approach ge, the free-spin g factor 2.0023. However, if X >> A, 911 approaches 4, and (1) NRC No. 9332. (2) J. Rolfe, F. R. Lipsett, and W. J. King, Phys. Rev., 123, 447 (1961). (3) J. H.Schulman and R. D. Kirk, Solid State Commun., 2, 105 (1964). (4) W.Khnsig, J . Phys. Chem. Solids. 23,479 (1962). (5) J. R.Morton, J . Chem. Phys., 43, 3418 (1965).
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sider the possibility of doping the crystals with SO-. Unfortunately, SO is not a stable gas, and attempts t o convert SZ- into SO- by heating the sulfur-doped crystals in air or oxygen have so far proved unsuccessful. Schneider, et u Z . , ~ have used this technique to convert S3- into SO2-.
Gauss
Figure 1. Electron resonance of S2-in KBr 4t 1950 gauss, obtained using sulfur enriched to 60% in the isotope S**.
I 6000k
I \
2000 '3
I
I
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IO
20
30
40
50
I 60 ANGLE in degrees
70
80
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Figure 2. Position of the X-band electron resonances of SZ- in KBr a t 4°K : circles are observed resonances, curves are obtained from eq 1 with g = 3.5010and gl = 1.0500.
gL approaches zero. I n the case of Sz- in KBr it would appear that A/A is 1.45 f 0.05.
I n the case of 02-in KBr orbital angular momentum was almost quenched, the principal g factors being4 811 = 2.5203 and gL = 1.9268 and 1.9314, A/A being approximately 0.2. It is therefore interesting to conThe Journal of Physical Chemistry
Radicals in Irradiated Thiosulfate There has been considerable discussion recently concerning the radiolysis' of sodium thiosulfate, Na.S203. 5H2.0, and the nature of the paramagnetic species trapped in irradiated thiosulfate crystals.*-" Two species are present: radical A has an almost isotropic g factor of 2.0054; radical B has a highly anisotropic g tensor with an isotropic component of 2.0143. Radicals A and B have been variously identified as: an F center and so^-,* an S atom and so3-,9and SO2- and SO2+,l0respectively. However, radical A has now been shown to possess two sulfur atoms," which, together with its almost isotropic g-tensor, suggests that it is S202-. This radical is similar to SO3- l 2 except that one oxygen is replaced by sulfur. The magnitude of the S33hyperfine interactions are consistent with this identification. The identification of radical B is more difficult since no hyperfine data for this radical have been reported. Previous workers seem to have assumed that all fragments trapped in irradiated thiosulfate will be formed as a result of rupture of the S-S bond. However, the identification of A as S202- shows that this is not the case, and indeed the principal g factors of radical B show that it is neither so$-,S02-, nor SO-. The three different principal g factorslOallof radical B indicate that its symmetry is not higher than CZv. The possibility that B is SSO- will therefore be considered. I n Table I the principal g factors of B are compared with those of SO2- and S3-, both of which have been introduced into KCl crystals.6 The parameters of B are seen to be intermediate between those of Sot- and S3-, an observation which, although not compelling, is suggestive. The radicals Sot- and Ss- are bent, and the smallest principal g factor was found to be per~~
~
~~
~
~
(6) J. Schneider, B. Dischler, and A. Rauber, Phys. Stat. Sol., 13, 141 (1966). (7) R. L. Eager and D. S. Mahadevappa, Can. J. Chem., 41, 2106 (1963); 43, 614 (1965). (8) R. Servant and J. Rocher, J. Phys. (Paris), 24, 285 (1963). (9) N. Goto and 0. Matumura, J. Phys. SOC. Japan, 18, 1702 (1963). (10) J. M. deLisle and R. M. Golding, J. Chem. Phys., 43, 3298 (1965). (11) J. R. Morton, Can. J . Chem., 43, 1948 (1965). (12) G. W. Chantry, A. Horsfield, J. R. Morton, J. R. Rowlands, and D. H. Whiffen, Mol. Phys., 5, 233 (1962).
SULFUR-CONTAINING RADICALS TRAPPED IN SINGLE CRYSTALS
pendicular to the radical plane. This situation would also be expected to obtain in the case of SSO-. Therefore, if B is SSO .- and if the S-S bond in the radical is parallel to that in the undamaged ion, the direction of the smallest (2.0035) principal g factor would be expected to be perpendicular to the S-S bond in the crystal. The directions of the smallest principal g factor of radical B are (0.984, F0.053, 0.172)," in the a&*-axis system, and those of the S-S bonds in the crystal are (0.031, zt0.789, -0.614).13 The angle between these two directions is 83.3 or 88.1", depending on the relative signs of their central components. In either case the result tends to confirm the hypothesis that B is SSO-. Table I : Comparison of Principal g Factors of SOZ- and S3- in KCP with Those of B (Possibly SSO-) in Irradiated N ~ Z S ~ O ~ . ~ H ~ O " Radical
812
OYY
b'se
so2-
2,0025 2.0035 2.0026
2.0110 2.0287 2.0499
2.0071 2.0106 2.0319
B s 3-
' Axis system: z is perpendicular to plane; z is the twofold axis, if present.
Radiolysis experiments have been carried out7 on NazS2O3.5H20in which the ligand and central sulfur atoms were successively labeled with the radioactive isotope S35. It was established that when the irradiated salt was dissolved in water the ligand sulfur appeared as H2S and colloidal sulfur whereas the central sulfur formed sulfite and sulfate. From these observations it was concluded that the initial act of ionization or excitation was followed by scission of the S-S bond, with the formation of colloidal sulfur and sulfite in the crystal. The present experiments indicate that the fragments trapped in the crystal have intact S-S bonds and, hence, that S-S scission must have taken place on dissolution of the irradiated solid in water.
The Radicals SO4- and S203The previous section on the radiolysis of thiosulfate leads us to a discussion of two rather interesting radicals, SO4- and s&-. The esr spectra of these radicals have been observed in irradiated crystals of KZS04 and Na2S203.5H20, respectively. 14,l5 Since both radicals are thermolabile, irradiation and examination were carried out at 77°K. The principal g factors of these radicals are listed in Table 11,together with the principd values of an S3ahyperfine interaction tensor of s&-; no s33 hyperfine structure has as yet
91
been detected in the spectra of
sod-.
The radical
S203- would be expected to have CaVsymmetry, and the spectral parameters suggest that this is the case: both tensors possess axial symmetry, the unique directions being parallel and parallel to the S-S bond of the thiosulfate The large isotropic S33 hyperfine interaction is indicative of an 2A1ground state for s~o3-, and the positive deviaton of g1 from 2.0023 suggests a low-lying 2E state. The experimental results therefore suggest an . . .e4a1electronic configuration for the radical identified as S2O3-. In the case of so,- the radical symmetry is clearly no higher than CZv, since the three principal g factors are unequal. Similar parameters were found for 804- trapped in photolyzed persulfate crystals."j The directions of two of the principal g factors (2.0486 and 2.0037) are parallel to 0 . . -0 directions of the sulfate ions" (the third is parallel to the bisector of an OS0 angle). These observations suggested that the radical was either SO*- or SOZ-; a comparison of the known g factors of Sodand SO2- with those of the radical in KzS04 led to its identification as S04-. Table I1 : The Principal g Factors of sod- Trapped in KzS04 and SzOa- Trapped in Na2S2O3.5Hz0;Also, the Principal Values (Mcps) of an S33Hyperfine Tensor of S203Radiixl
Principal values
so4- ( 9 ) (9) s203- (S33 a
hfs)
Parallel t o
2.0486 2,0232 346.0
2.0082 2,0230 344.9
2.0037 2.0024" 409,l"
S S bond in thiosulfate crystal.
Concomitant with the above experiments, molecular orbital energy level calculations were carried out for the ions S04- and S203-.1s The semiempirical method of Wolfsberg and H e l m h ~ l z was ' ~ used, the basis set being oxygen 2s and 2p orbitals and sulfur 3s) 3p, and 3d orbitals. These atomic orbitals were combined to provide group orbitals belonging to the irreducible representations of T d (SO,-) and C3V(S203-). The (13) P. G. Taylor and C. A. Beevers, Acta C ~ y s t . 5, , 341 (1952). (14) V. V. Gromov and J. R. Morton, Can. J . Chem., 44, 527 (1966). (15) J. R. Morton, D. M. Bishop, and M.Randic, J . Chem. Phys., 4 5 , 1885 (1966). (16) P. W. Atkins, J. A. Brivati, A. Horsfield, M. C. R. Symons, and P. A. Trevalion, Sixth International Symposium on Free Radicals, Cambridge, U. K., July 1963. (17) R. W. G. Wyckoff, "The Structure of Crystals,"2nd ed, Chemical Catalogue Co., Inc., New York, N. Y., 1931, p 291. (18) D. M. Bishop, M. Randic, and J. R. Morton, J . Chem. Phys., 4 5 , 1880 (1966). (19) 11.Wolfsberg and L. Helmhols, ibid., 20, 837 (1952).
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- 60
so;
- 80 ENERGY ( lo3cm-i )
- 100
- le
-120
.
212
40, 3e
2e
Figure 3. Molecular orbital energy levels for SO4- and
s&-.
secular determinant corresponding to each representation was of the form IHtk = 0, where i and k label the group orbitals belonging to the representation. Values of H i i were estimated from valence-state ionization energies,20 and Clementi’s atomic orbitalsz1 were used to evaluate the Sik. Following the suggestion of Wolfsberg and H e l m h ~ l z the , ~ ~ integrals Hi, were obtained from the equation
The calculated electronic configurations of SO4and SZO3- are shownz2in Figure 3, both radicals being predicted to possess 2AI ground states. I n the case of SOa- no correlation with the esr data is possible a t this stage, since the calculation assumed Td symmetry, but the radical in K2S04 exhibited CZv symmetry, possibly due to the proximity of a potassium ion vacancy. Confirmation of the 2A1 ground state would require the investigation of the S33hyperfine interaction, which, so far, has evaded detection. The possibilities of (a) using S33-enriched KzS04 and (b) repeating the calculation assuming CpV symmetry are being considered. The esr data suggest that, if the
The Journal of Physical Chemistry
ground state of SO4- (C20)is 2A1, there is st low-lying 2Bexcited state. The calculation on S203- predicts the 2A1 ground state suggested by the experiments. Spin-orbit interaction between the . . .e4a12A1 ground state and the 2E state provided by the . . .e3a12configuration would cause AgI to be nonzero and positive, as observed. The 5a1 orbital occupied by the unpaired electron of 8 2 0 3 - is primarily composed of ligand sulfur 3s and 3p orbitals. This suggests that the S33hyperfine interaction which has been observed arises from those radicals possessing an S3anucleus in the ligand, as opposed to the central position. It would be interesting to confirm this by using thiosulfate selectively enriched with S33on the ligand sulfur.
Acknowledgments. The author is very grateful to Drs. J. Schneider and J. H. Schulman for providing the sulfur-doped crystals of KBr used to obtain Figures 1 and 2, respectively, and to Dr. L. D. Calvert for determining the directions and dimensions of the unit cells of N1t2S203’5H20 and K2S04 by means of X-ray diffraction photographs.
Discussion M. C. R. SYMONS (Leicester University, Leicester). You make reference to our work on ultraviolet-irradiated potassium persulfate where we originally postulated pair trapping of so4radicals. We now conclude that the peroxyl radical o~soo-is the trapped species, because this gives us the correct pair separations and directions. Also the absence of 3sS hyperfine coupling in the narrow-line spectrum is hard to understand. We have detected 3% satellites for a different species thought to be SOain the ?-irradiated crystal. The splitting of about 3.5 gauss is almost isotropic. R. J. MYERS(University of California, Berkeley). I n the area of regular chemistry it is known that the molecule SZO has an S S - 0 bent structure and some thermodynamic stability. It is quite likely that some SzOwould be produced during the decomposition of S203and so SzO- is a very probable s p e c k from the irradiation of S20a2-. seem to have assumed J. R. MORTON.Previous ~orkersT-~O that the S S bond of the s&*-ion would break.aa a result of the radiolysis. This does not appear to occur to any appreciable extent. (20) H. Basch, A. Viste, and H. B. Gray, J . Chem. Phys., 44,lO (1966). (21) E. Clementi, I B M J . Res. and Develop., Suppl., 9, 2 (1965);
Tables 45-1 and 45-4. (22) By permission of the American Institute of Physics.
