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G. Orlandi, G . Poggi, F. Barigeletti,and A. Breccia
There is no prior evidence for either reaction. They cannot be the dominant chain termination reactions in our system because then the rate expression would be independent of [CO]. The dependence of our data on [CO] excludes the possibility that these reactions occur to any significant extent. With the known values of k8 and kg, and the initial rates a t 60 and 120 mTorr of NO, the value of k l z may be computed with eq IV. The result is 5.6 X 10-12 and 6.5 X 10-12 cm3/sec a t 120 and 60 mTorr, respectively. The average value is 6.0 X 10-12 cms/sec. The values of k9 = 7 X 10-15 and k g = 1.35 X 10-13 cm3/sec used in the calculation are those obtained by Stuhl and Niki.14 These values for k8 and k9 are in very good agreement with other determinations15 and are probably good to 20%. Recently two measurements of kl2 have been reported. One of these is by Stuhl and Nikil4 who found reaction 12 to be in the fall off region a t -80 Torr of He. They calculated a limiting high-pressure rate constant kl2m = 2 x 10-'2 cm3/ sec. The other report was by Morley and Smith16 who, contrary to Stuhl and Niki, found the reaction to be entirely in the third-order region a t similar pressures with klzo = 9.4 x 10-31 cmG/sec for H2 as the third body. Using RRKM theory, Morley and Smith computed the high-pressure limiting rate constant, k12m, to be 1.7 X 10-10 cm3/sec. With this value for k12m and the value for k12'3, it may be computed that a t 1 atm of Ha, reaction 12 should still be almost entirely in the third-order regime,
with a pseudo-second-order rate constant of 2 X 10-11 cm3/sec. Thus the value of k12 = 6.0 X 10-12 cm3/sec obtained in this work a t 1 atm of H2 is midway between the values of Morley and Smith and of Stuhl and Niki. Since reaction 11 is not important in this system, a value of k2/k111/2 cannot be determined. However, a lower limit can be found since [HOz] (Ia/k11)1/2.Then
-
CP i{NOz] = Lz[NOIIHO~I/I,< ~ z [ N O I / ( ~ I J , ) ' Our data led to the result that k ~ / k 1 1 ~ /> 2 0.6 X 10-7 ( ~ m ~ / s e c )With ~ / ~ the . average value of k l l = 6 X 1O-I2 cm3/sec found by Paukert and Johnston9 and Hochanadel, et a1.,10 kz > 1.5 x 10-13 cm3/sec. Acknowledgment. The authors wish to thank Professors Marcel Nicolet and Eduardo Lissi for useful discussions. This work was supported by the Atmosphere Sciences Section of the National Science Foundation through Grant No. GA-12385 and the National Aeronautics and Space Administration through Grant No. NGL-009-003 for which we are grateful. F. Stuhl and H. Nlki, Presented at the 10th inlormal Conference on Photochemistry, Oklahoma State University, 1972. D. L. Baulch, D. D. Drysdale, and A. C. Lloyed, "High Temperature Reaction Rate Data," No. 1 and 2, Department of Physical Chemistry, The University of Leads, 1968. C. Morle and I W M. Smith, J. Chem. Soc., Faraday Trans. 2, I016 (1972). ' '
An Electron Spin Resonance Study of the Radicals Produced by the y-Irradiation of'xanthene G. Orlandi,* G. Poggi. F. Barigelletti, Laboratorio di Fotochimica e Radiazioni Alfa Energia, de/ C.N.R., 40126 Bologna, itaiy
and A. Breccia Cattedra di Chimica Generale ed lnorganica, Universita di Bologna, 40126 Bologna, ltaiy (Received September 21, 1972) Publication costs assisted by Consiglio Nazionale delle Ricerche, Laboratorio di Fotochimica e fiadiazioni d'Alta Energia
Xanthene powders were y-irradiated a t 77°K and the radical species forme'd were investigated by esr technique. The spectra obtained are attributed to the overlapping of a singlet and a triplet. The triplet spectrum disappears upon heating to 105°K. On the basis of theoretical spin density calculations the triplet is attributed to xanthene radical ions while the singlet is tentatively assigned to xanthyl radical.
Introduction In many y-irradiated solid or frozen systems radical and ionic species are trapped and can be detected by esr. The purpose of this work is to clarify the nature of the radicals produced in xanthene microcrystals. Experimental Section Xanthene was obtained from Fluka and recrystallized from benzene. The Journal of Physical Chemistry, Vol. 77, No. 9, 1973
Irradiations were performed with a Gammacell 6oCo source, with doses ranging from 0.5 to 3 Mrads. The shape of the spectra was found to be independent of the dose. The samples were irradiated at 77°K and the esr spectra were recorded a t the same temperature (a) immediately after irradiation and (b) after a 15 min of annealing a t 105°K. The spectrometer used was a Varian 4502 operating at a modulation frequency of 100 kHz. Second-derivative presentation, obtained by means of audio range modulation, was used throughout.
1103
Radicals Produced by the y-Irradiation of Xanthene Spin density calculations were performed using a modified version of QCPE Program No. 76. Previous attempts to apply McLachlan calculations were unsuccessful due to the high apparent symmetry, in this framework, of the radicals investigated.
A
Results The spectrum of xanthene as observed after irradiation at 77°K is reproduced in Figure l a . It consists of three lines characterized by a coupling of 25 G and by a bandwidth of approximately 20 G. The intensity ratio differs from the value 1:2: 1 typical of a spectrum arising from two equivalent spin 1 / 2 nuclei, in that the center line is too intense. We attributed the excess centerline intensity to the superimposition of a singlet line on a triplet spectrum of similar g value. This interpretation is confirmed by the annealing experiment, the result of which is illustrated by the spectrum in Figure l b . This spectrum consists of one line only, characterized by a bandwidth slightly larger than the one observed before, centered on the field corresponding to the center line of the previous triplet. The difference between the two spectra is illustrated in Figure IC. The intensity distribution here closely approximates the usual 1:2: 1triplet value.
Figure 1. Esr spectra of y-irradiated xanthene at 77°K: (a) taken immediately after irradiation; (b) after annealing at 105°K; ( C ) difference between the esr spectra reported in a and b. The traces were obtained by accumulation of the esr signal into :I computer of average transients. T h e power incident on the sample was kept constant in all experiments (to a value of the ordel, of 1 0 mW) and so was the leakage current.
Discussion The above results can be rationalized assuming that two different radicals are formed upon irradiation at 77°K and that one of them, the one corresponding to the triplet pattern, disappears with heating. The triplet spectrum is attributed to xanthene radical ions (cation or anion), its hyperfine structure arising from coupling with the two methylenic hydrogens. In fact they lie outside the molecular plane and can interact strongly with the electron spin and give rise to a splitting of the order of the one obtained. The aromatic hydrogens, on the other hand, are characterized by a relatively small hyperfine coupling which is not resolvable under the line width magnitudes typical of powder spectra. For the same reason, the singlet spectrum was attributed to a radical whose hydrogens all lie in the molecular plane. A prime candidate for this species is the one resulting from the removal of' one of the methylenic hydrogens, Le., xanthyl radical. Theoretical spin density calculations have been carried out on the three species quoted. The method applied was the UHF H MO method of Pople and Nesbetl.2 as the more sophisticated CNDO methods3 do not seem to be reliable for computing spin densities of methylenic hyd r o g e n ~ In .~~ our ~ calculations bond lengths of 1.4, 1.5, and 1.3 A were taken for the C-C, C-CH2, C-0 bonds, respectively. The parameters and repulsion integrals for the C atoms were those proposed by Amos and Snyder.6 For oxygen the parameters are ionization potential W , = -32.9 eV, monocentric repulsion integral yoo = 21.53 eV, and PCO = -3.3 eV, and the CO repulsion integrals were chosen in such a way as to give the same ratio yco/yCc as obtained by use of the Mataga method.' Since UHF spin densities may be uncorrect due to spurious components arising mainly from quartet states, we evaluated also the projected spin densities6 pna and pass by an approximated projection method.8 In fact, correct spin densities are supposed to lie somewhere between P U H F and paa and it has been s u g g e ~ t e d that ~ . ~ pav = %(pas + pass) gives a good estimate of spin densities. Hyperfine splitting constants for the in-plane hydrogens were calculated with the McConnell formula using Q = 23
G. For the methylenic hydrogens we applied the well-tested relationship10-12 QH
= Bp cos2
(1)
where I9 is the dihedral angle between the planes C-C-p, and C-C-H, which, for tetrahedral hybridization, is equal to 30"; B is a constant equal to 58 G; p is the value of' thl. spin density on the vicinal benzene ring C atom. Since in our case there are two equivalent vicinal atoms, the effec/~ tive spin density to be used in relationship 1 is [ ( p ~ ) l i(p13)1'2]2 = 4p1.13 The spin densities and hyperfine coupling constants obtained are represented in Table I and Table 11, respectively. As expected, one sees clearly that in the xanthene radical ions the splitting due to the methylenic hydrogens dominates over the one due to in-plane hydrogens. For the negative ion we obtain a hyperfine coupling constant cQ 28-32 G, while for the positive ion we get results ranging from 40 to 25 G depending on which spin density we USE', whether projected or unprojected. Accepting lh(paa + pass) as the most reliable spin density, for the two radical ions essentially the same hyperfine coupling constant is obtained, i . e . , approximately 29 G. This value is to be (1) J. A. Pople and R . K. Nesbet, J. Chem. Phys., 22, 571 (1954). (2) A. Brickstock and J. A. Pople, Trans. Faraday SOC., 381, 901 (1954). (3)J. A. Pople, P. D. Santry, and G. A. Segal, J. Chem. Phys., 43, J. A. Pople and G. A. Segal, ibid., 44,3289 (1966). SI29 (1965); (4)J. A. Pople, D. L. Beveridge, and P. A. Dobosh, J. Amer. Chem. SOC.,90,4201 (1968). (5) J. A. Pople and D. L. Beveridge, "Approximate Molecular Orbital Theory," McGraw-Hill, New York, N. Y., 1970. (6) A. T. Amos and L. C. Snyder, J. Chem. Phys., 41, 1773 (1964);42, 3670 (1965). (7) N. Mataga and K. Nishimoto, Z. Phys. Chem., (Frankfurt am Main), 13, 140 (1957). (8) G. Orlandi and G. Giacometti, Ric. Sci., 39, (1969). (9) K. M. Sando and J. F. Harriman, J . Chem. Phys., 47,180 (1967). (10) E. W. Stone and A. H. Maki,J. Chem. Phys., 37,1327 (1962). (11) C. Corvaja and G. Giacometti, Theor. Chim. Acta, 14, 353 (1969); C. Corvaja. M. Brustolon, and G. Giacometti, Z. Phys. Chert. (Frankfurt am Main) 66,279 (1970). (12) K. D. Sales, Advan. Free Radical Chem., 3, 139 (1969);P. B. Ay:,cough, "Electron Spin Resonance in Chemistry," Methuen, Londori.
1967. (13) D. H. Whiffen, Mol. Phys., 6,223 (1963). The Journal of Physical Chemistry, Voi. 77, No. 9, 1973
G. Orlandi, G . Poggi, F. Barigeletti,and A. Breccia
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TABLE I: Electron Spin Densities for Xanthene Radical Cation (I), Xanthene Radical Anion(ll), and Xanthyl(1ll)
TABLE 11: Hyperfine Splitting for Radicals I, 11, and Ill (In Gauss) Radical I
Radical I
II
ill
Position
1,13 2,12 3,11 4,lO 5,9 6,8 7(0) 1,13 2,12 3,ll 4,lO 599 63 7(0) 1,13 2,12 3,ll 4,lO 5,9 6,8 7(0) 14
PllHF
Pan
0.232 -0.145 0.248 -0.142 0.234 0.033 0.080 0.185 0.123 -0.066 0.1 14 0.187 -0.041 -0.005 -0.132 0.178 0.059 0.093 0.013 0.108 0.029 0.569
0.14 -0.04 0.16 -0.05 0.17 0.08 0.07 0.16 0.1 1 -0.02 0.11 0.15 -0.01 -0.00 -0.04 0.12 -0.00 0.09 0.07 0.08 0.02 0.42
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Pav
0.16 -0.07 0.18 -0.07 0.19 0.07 0.07 0.17 0.12 0.03 0.1 1 0.16 -0.02 -0.00 -0.06 0.14 -0.02 0.09 0.02 0.08 0.03 0.45
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compared with the experimental finding of about 25 G. If we, now, accept the assumption that the CH2 group is rapidly flipping up and down the molecular plane, the cos2 19 factor averages to S/g,ll bringing the theoretical result closer to the experimental one. The splittings from the in-plane hydrogens are fairly small and are wiped out by the line width typical of microcrystalline samples, which is due to g tensor anisotropy and to hyperfine splitting tensor anisotropy. For the xanthy1 radical, there is considerable spin density on the C-14 atom, to which there would correspond a doublet structure in the spectrum. The experimental result, showing a
The Journal of Physical Chemistry, Vol. 77, No. 9, 1973
Position
2, 12 3,ll 4,lO 599
Methylenic II
2,12 3,11 4,lO 5,9
Methylenic Ill
2,12 3,11 4,lO 5.9 14
PUHF
3.3 5.7 3.3 5.4 40.4 2.8 1.5 2.6 4.3 32.2 4.1 1.6 2.1 0.30 13.1
Paa
0.92 3.7 1.1 3.9 24.4 2.5 0.46 2.5 3.4 27.8 2.8 0.0 2.1 0.46 9.7
Pav
1.6 4.1 1.6 4.4 27.8 (23.2)" 2.8 0.69 2.5 3.7 29.6 (24.6)a 3.2 0.46 2.1 0.46 10.3
a These figures were evaluated assuming cos2 8 = 5/8 as explained inref 11.
singlet broad band, is a t variance with the theoretical prediction. However, when considering that all the other splittings are relatively large and that the g tensor and hyperfine tensor anisotropies are again present, one cannot rule out the attribution of the singlet spectrum to the xanthyl radical. These effects can easily account for a broadening of the original doublet which could completely obscure the doublet structure itself. However, other radicals containing only in-plane hydrogens could be responsible for the singlet spectrum, but esr does not permit their identification since the hyperfine structure originating from the planar hydrogens is lost.
Conclusion From the present study it appears that the products of y-irradiation of xanthene powders are of two kinds. The species characterized by a singlet spectrum and higher thermal stability appears to result from hydrogen abstraction from the -CH2 group. The triplet spectrum can belong to either of the two xanthene radical ions, positive or negative.
