1557
J . Phys. Chem. 1986, 90, 1557-1560
Radical Pair Formation in Phenyl Ring Containing Molecules: Crystal Structure of Ph,P+CH,SCH,CI- and ESR Study of the X-Irradiated Single Crystal M. Geoffrey,* M. V. V. S. Reddy, Department of Physical Chemistry, University of Geneva, 121 1 Geneva, Switzerland
and G . Bernardinelli Laboratoire de Cristallographie aux Rayons X,121 1 Geneva, Switzerland (Received: September 24, 1985)
X-irradiation of Ph3P+CH2SCH3CI-single crystals, at 77 K, gives rise to ESR spectra whose angular variations are characteristic of pairwise trapped radicals. The crystal structure of Ph3P+CH2SCH3CI-is determined and shows that, in the crystal lattice, the proximity and the mutual orientation of two phenyl rings are suitable for the formation of a pair made of a cyclohexadienyl and a phenyl u-radical. The electron-electron dipolar interaction is calculated for this model and is compared with the corresponding tensor. The most probable structure for the cyclohexadienyl radical is deduced from this tensor and agrees with the results obtained from the 'H and 31Phyperfine tensors: the cyclohexadienyl exhibits an appreciable out-of-plane deformation and the phosphonium moiety is located in the equatorial position.
Introduction
TABLE I: Atom Coordinates for Ph3PtCHzSCH3CIX Y Z 0.03975 (8) 0.1 18 75 (9) 0.15945 (5) 0.25751 (7) 0.67440 (8) 0.52308 (5) 0.21267 (10) 0.388 06 (1 1) 0.447 97 (7) 0.1403 (3) 0.443 19 (21) 0.748 5 (4) 0.1733 (3) 0.377 70 (24) 0.7903 (4) 0.0907 (4) 0.3184 (3) 0.863 0 (5) 0.323 4 (3) 0.8903 (5) -0.023 2 (5) 0.387 9 (3) 0.845 4 (5) -0.0565 (4) 0.448 3 (3) 0.7760 (4) 0.0242 (4) 0.60431 (20) 0.6339 (4) 0.2005 (3) 0.633 80 (24) 0.498 9 (4) 0.2045 (4) 0.6992 (3) 0.4765 (6) 0.1637 (4) 0.7339 (3) 0.5865 (6) 0.1206 (4) 0.7040 (3) 0.7191 (6) 0.1164 (4) 0.6403 (3) 0.1571 (4) 0.7433 ( 5 ) 0.557 95 (21) 0.3724 (3) 0.805 4 (4) 0.6085 (3) 0.4828 (4) 0.7653 (4) 0.641 5 (3) 0.569 2 (4) 0.868 5 (5) 0.6244 (3) 0.543 0 (4) 1.007 5 (4) 0.573 73 (25) 1.0475 (4) 0.4347 (4) 0.54026 (22) 0.9476 (4) 0.348 6 (3) 0.491 92 (20) 0.522 2 (3) 0.323 6 (3) 0.3448 (3) 0.418 1 (5) 0.188 7 (4)
Formation of radical pairs in irradiated organic compounds has been intensively investigated during the past ten years1-6 and it has been suggested that this mechanism can play an important role in radiation chemistry and, even, in some biological processes.' Particular attention has been given to the pairwise trapping in benzenic compoundss-I2 and the concerted formation of a cyclohexadienyl radical and a phenyl u-radical has frequently been However, no description of the structure of this pair in relation with the crystallographic data of the undamaged molecules has ever been obtained. In fact, as far as we know, no full single-crystal ESR study has ever been performed on this system. Two conditions are expected to be decisive for the trapping and the ESR analysis of a pair made up of a cyclohexadienyl radical and a phenyl o-radical: (i) the crystal packing must favor hydrogen transfer between two phenyl rings; (ii) a nucleus giving rise to a quite large coupling must be present in order to allow the hyperfine structure to be resolved. We report here the crystal structure of a triphenylphosphonium derivative, Ph3P+CH2SCH3C1-, which shows that, in the crystal lattice, two neighboring phenyl rings are parallel and are suitable for the stabilization of a radical pair. The angular variations of the ESR signals obtained after X-irradiation a t 77 K, indeed, lead to ESR tensors, g, electron-electron dipolar interaction, 'H and 31Phyperfine interactions, which are consistent with the trapping of a cyclohexadienyl-phenyl radical pair. Furthermore, these spectroscopic parameters give interesting information about the structure of the cyclohexadienyl radical which has recently been the object of several
Results Crystal Structure.
Crystallographic data:
C( 1)-P-C( 7) C(l)-P-C(13) C( 1)-P-C( 19) C(7)-P-C( 13) C(7)-P-C( 19) C(13)-P-C( 19) C( 19)-S-C(20) C(19)-P-C( 1)-C(2) C ( 19)-P-C( 7)-C (8) C(19)-P-C(13)-C(14) C( 1)-P-C( 19)-s C(7)-P-C( 19)-s C( 13)-P-C( 19)-S C(20)-S-C( 19)-P
(1) G.Nilson, A. Lund, and P. 0. Samskog, J . Phys. Chem., 86, 4144 ( 1982).
(2) R. Franzi, M. Geoffroy, and G. Bernardinelli, Mol. Phys., 52, 947 (1984). (3) R. Knopp and A . Muller, Mol. Phys., 42, 1245 (1981). (4) R. Knopp and A. Muller, Mol. Phys., 50, 369 (1983). (5) G.Nilsson and A. Lund, J . Phys. Chem., 83, 3292 (1984). (6) H. C. Box, Radiation Effects, ESR and ENDOR Analysis, Academic Press, New York, 1977. (7) T. Matsuyama and H. Yamaoka, J . Phys. SOC.Jpn., 45, 717 (1978). (8) T. Matsuyama and H. Yamaoka, Chem. Phys. Lett., 60,468 (1978). (9) S. Konishi, M. Hoshino, and H. Imamura, J . Phys. Chem., 85, 1701 (1981). (10) T. Matsuyama and H. Yamaoka, J . Chern. Phys., 68, 331 (1978). (1 1) T. Matsuyama and H. Yamaoka, Chem. Phys. Lett., 57,269 (1978). (12) A. B. Jaffe and R. W. Kreilick, Chem. Phys. Lett., 32, 572 (1975). (13) M. Kira and H. Sakurai, J . Am. Chem. SOC.,99, 3892 (1977). (14) H. Sakurai, I. Nozue, and A. Hosomi, J . A m . Chem. SOC.,99,8279 (1977). I
50.8 (3) 35.8 (3) 63.0 (4) 42.0 (1 2) 54.3 (14) 65.9 (17) 74.4 (18) 74.0 (19) 55.9 (15) 41.9 (11) 56.5 (15) 71.9 (19) 71.9 (20) 71.6 (20) 61.3 (16) 40.0 (11) 56.1 (14) 66.8 (17) 60.5 (16) 56.2 (15) 49.0 (13) 42.4 (12) 69.2 (17)
TABLE 11: Relevant Interatomic Distances (A), Bond Angles (deg), and Torsional Andes (ded for Ph,PtCH,SCH, P-C(1) 1.796 (3) P-C(7) 1.795 (4) P-C( 13) 1.793 (3) P-C(19) 1.799 (4) S-C( 19) 1.815 (3) S-C(20) 1.798 (5)
C20H20PSC1,
0022-3654 ,186,12090- 1557SO 1S O I O
U
110.9 (2) 108.0 (2) 111.1 (2) 106.0 (2) 111.1 (2) 109.5 (2) 102.0 (2) -58.5 (3) -2.8 (3) -46.1 (4) -56.1 (2) 67.9 (2) -175.5 (2) 99.7 (2)
monoclinic, P 2 , / n , a = 11.501 ( l ) , 6 = 9.536 ( l ) , c = 17.799 (5) = 106.88 ( 2 ) O , 2 = 4, d , = 1.276 g - ~ m - F?,, ~ , = 752, p = 3.915 cm-'. The structure was solved by direct methods (MULTAN 8016) and refined by full matrix least-squares (XRAY
A, /3
b 1986 American Chemical Societv -
1558 The Journal of Physical Chemistry, Vol. 90, No. 8, 1986
Geoffroy et al. = 2 0021
50 G y c 1 9
c1
q R
d
"
Figure 1. Cation Ph3P+CH,SCH3in the crystal of Ph,P+CH,SCH,CIshowing the numbering scheme.
Figure 3. Example of an ESR spectrum obtained at 77 K with an X-irradiated single crystal of Ph3P+CH,SCH3CI-. Signals marked R are attributed to the pair. 3. CG.
5c. SO. 123. ;53.
:eo.
Ring A
Q4 i
L
Figure 2. Mutual orientation of the two cations related through the inversion center at I / , , I , ' / ?in the undamaged crystal.
76"). Atomic scattering factors (for neutral atoms) and anomalous dispersion terms (for P, S, and C1 atoms) are taken from ref 18. All the coordinates of the hydrogen atoms have been calculated. The final R factor, based on 1343 observed reflections ( F , > 3 (F,) and F, > 8), is 0.042 ( w R = 0.072 with w = exp(18(sin O/A)2). An O R T E P illustration of the phosphonium cation is presented in Figure 1; the atomic coordinates are given in Table I and the relevant bond lengths and bond angles are shown in Table 11. As expected for a phosphonium moiety the intramolecular C-P bonds make angles close to 109'. An interesting feature of the crystal packing is shown in Figure 2; the planes of the phenyl rings C( 13)-C( 18) (ring A) of molecule XYZ and C( 13')-C( 18') (ring B) of molecule l-X,Z-Y,I-Z are almost parallel and the distance between the center of these rings is only 4 A. Electron Spin Resonance. An example of an ESR spectrum obtained at 77 K with a single crystal of Ph3P+CH2SCH3C1-which has been X-irradiated in liquid nitrogen is shown in Figure 3. The angular dependences of the signals marked R a r e reported in (15) J. Lusztyk and K. U. Ingold, J . Phys. Chew., 89, 1865 (1985). (16) P. Main, S. J. Fiske, S. E. Hull, L. Lessinger, G . Germain. J. P. Declercq, and M. M. Woolfson. A System of Computer Programs for the Automatic Solution of Crystal Structures from X-Ray Diffraction Data, Universities of York, England, and Louvain-la-Neuve, Belgium, 1980. (17) J. M. Stewart, P. A. Machin, C. W. Dickinson, H. L. Ammon, H. Heck, and H. Flack, Technical Report TR 447, Computer Sciences Center, University of Maryland, College Park, MD, 1976. (18) International Tables f o r X-Ray Crysrallography, Vol. IV. Kynoch Press. Birmingham, 1974
3500 G
3050
Figure 4. Angular dependence of the ESR signals due to the radical pair. TABLE 111: ESR Tensors for the Radical Pair tensor eigenvalues eigenvectors
e;
2.0080 2.0041 1.9993 MHz
266 100
-366 T I , MHz
107
92 73 T2. M H r
151 105 97
0.578 -0.182 0.795
0.401 0.912 -0.082
-0.711
0.561 0.831 0.156
-0.388 0.092 0.917
0.731 -0.576 0.367
0.429 0.888 0.165
-0.650 0.431 -0.626
-0.627 0.161 0.762
0.045 0.907 0.418
-0.549 -0.327 0.769
0.835 -0.263 0.483
0.367 0.600
Figure 4. The large anisotropy of the main splitting is characteristic of the presence of a radical pair and the additional structure is clearly due to hyperfine coupling with two spin nuclei. The angular variations of these ESR lines has been analyzed by using a second-order perturbation and the following Hamiltonian:
The Journal of Physical Chemistry, Vol. 90, No. 8, 1986 1559
ESR Study of Triphenylphosphonium 7f = PS-gh
+ S.TI.1 + S-TyI + S*d-S
TI and ‘iare ’* the hyperfine coupling tensors and b is the electron-electron dipolar interaction tensor. The experimental eigenvalues and eigenvectors are given in Table 111. The B tensor leads to the zero-field splitting parameters: ID1 = 549 MHz, IEl = 82 MHz. The half-field transition ( A M 3 = 2) has not been observed; this is not surprising due to the broad line width of the allowed transitions.
Discussion We have previously shownI9 that room temperature irradiation of Ph3P+CH2SCH3C1-crystals leads to the formation of two radical species: (i) Ph3P+CHSCH3and (ii) a sulfur-centered radical. Data reported in Table I11 make it very unlikely that either of these two radicals participates in the formation of the observed pair: the g tensor is not consistent with the presence of a sulfur-centered radical; the spin hyperfine tensors are not in accordance with half the values expected for an isolated R-CH radical. Moreover, the large experimental E value indicates that at least one of the components of the pair is largely delocalized on several atoms and suggests that phenyl rings are involved in pair formation. This interpretation also agrees with the broad line width (r E 20 G) which indicates unresolved couplings with several protons. A rough estimation of the interradical distance r can be obtained from the D value: the p i n t dipole approximation leads to r C 5.2 A. The crystal structure indicates that the group located so close to a benzene ring and expected to easily generate a radical is an aromatic ring of the neighboring molecule. This situation corresponds to that described above for rings A and B (related through the inversion a t 1, and it is probable that the mutual orientation of these two benzene moieties favors the formation of a cyclohexadienyl-phenyl radical pair. One of the two observed spin hyperfine couplings is, however, higher than those generally observed for a proton from a cyclohexadienyl or a phenyl radical and this coupling must therefore be attributed to a 31Pnucleus located in the a-position to the cyclohexadienyl sp3 carbon. In order to check the validity of the model composed of a cyclohexadienyl (located on ring R) and a phenyl u-radical (located on ring A) we have calculated the expected values for (i) the line width, (ii) the B tensor, and (iii) the hyperfine tensors. Line Width. The reported coupling constants for the phenyl radical are -47 M H z (two protons in ortho positions from the radical carbon) and -28 M H z for the ortho and para protons in C6H6P(0)(OR).21~22 These coupling constants must be divided by two for a radical pair. We have simulated the resulting ESR line by using a Gaussian line shape (individual line width 4 G); the resulting signal is a broad line whose width is in perfect accord with the experimental signal. Electron-Electron Dipolar Interaction. In order to calculate the tensor we have to know the positions of the cyclohexadienyl radical’s carbon atoms (ring B: carbons C ( 13’)-C( 18’)); these coordinates are obtained by assuming that the C(13’) and P‘ atoms remain unaffected by the formation of the cyclohexadienyl radical and by taking the following modifications into account: (i) a rotation 0 around the common perpendicular to the C ( 13’)-P‘ bond and to the C ( 13’)-p, direction; this rotation is implied by the hybridization change of the C ( 13’) atom; (ii) an increase of the bond lengths C ( 13’)-C( 18’) and C ( 13’)-C( 14’); (iii) an out-ofplane deformation angle 4 which is expected to occur for a cyclohexadienyl radical. A fractional point electron p , / 2 is located in each lobe of the five pn orbitals (carbons C(i4’)-C(l8’)) at a distance r = 0.645 A from the nucleus. An unpaired electron
TABLE IV: Zero-Field Splitting Parameters Calculated for Some Structures of the Cvclohexadienvl Radical
0.01 -12 0.01 -12 0 -5
-45 -45 -30
0.3 0.35 -0.18 0.3 0.36 -0.18 0 0.40 -0.15 -0.10
0.35
0.39 0.35
664 638
77 72a
0.40
599
38
549
82b
0.50
C
Parameters used for drawing Figure 4. * Experimental values. Spin densities reported for the isolated cyclohexadienyl radical.24 TABLE V Experimental Isotropic Coupling Constants for Various Cyclohexadienyl Radicals radical nucleus A. MHz ref
a” ..
’HI, IH,
133.5
13, 24
21
H2
PlOllOEt I 2
“I
112
”P
560
IHI
117 320
31P
22
HI
“This study. The coupling constants are calculated for a n isolated radical.
-
(19) M. Geoffroy and M. V. V. S. Reddy, Radiat. Phys. Chem., 26, 377 (1985). (20) P. H. Kasai, E. Hedaya, and E. B. Whipple, J . Am. Chem. SOC.,91, 4364 (1969). (21) S. P. Mishra and M. C. R. Symons, Terrahedron Lett., 41, 4061 f 1975). (22) D. Griller, K. Dimroth, T. M. Fyles, and K. U. Ingold, J . A m . Chem. Sot., 91, 5526 (1975).
Figure 5. Structure of the cyclohexadienyl-phenyl radical pair consistent with the experimental tensor.
e, is placed near one of the atoms of the A ring (carbon C,: C(15), C ( 16), or C(17)) at a distance d from the corresponding nucleus along the direction Cj-2-Ci. The ten B, tensors are calculated and summed; the resulting tensor is diagonalized and the zero-field splitting parameters are obtained from the eigenvalues. It turns out to be rather difficult to find calculated D and E values which are similar to the experimental ones. Satisfactory results are given in Table IV together with the corresponding parameters. It appears that the u-unpaired electron is localized on carbon C ( 16) (ring A) and that the cyclohexadienyl ring adopts the conformation which is shown in Figure 5. It can be remarked that the spin densities for the cyclohexadienyl are slightly different from those previously reported for this r a d i ~ a l ;this ~ ~ is , ~certainly ~
\ -
(23) H. Fischer, J . Chem. Phys., 37, 1094 (1962).
J . PhI,s. C'hem. 1986. 90. 1560-1564
1560
due to the method of calculation which replaces each atomic orbital by two point electron^.^ There are so many adjustable parameters that our purpose is only to show that the experimental parameters are consistent with a reasonable geometry for the cyclohexadienyl radical and we did not try to achieve exact agreement between theory and experiment by performing more sophisticated displacements (rotation around the c-P bond, modification of the C P C angle, ...). Hyperfine Coupling. The anisotropy of the hyperfine tensors given in Table 111 is rather low, as would be expected for interaction with nuclei located in the a-position from the sp3 carbon, C ( 13'), of a cyclohexadienyl ring. Since these experimental isotropic coupling constants have been measured on a radical pair they must be multiplied by two before comparing them with the coupling constants observed on isolated radicals, so they correspond to A , = 185 M H z and A , = 230 M H z . Typical A,,, constants found for cyclohexadienyl radicals are given in Table V . It is clear that A 2 = 230 M H z is too large for a proton coupling and that this constant must be attributed to a 31Pnucleus. W e must therefore assign A , to the proton located on the carbon bearing the phosphorus atom. Taking into account the fact that substituted cyclohexadienyl radicals probably present a n out-of-plane deformation (angle $), two structures (Sl and S2) must be con-
SI
s2
sidered for the present radical. The 31Pcouplings given in Table V range from 320 to 560 M H z and are all larger than the value found% the present study. Similarly, all the previously reported 'H couplings lie between 90 and 135 M H z and are weaker than the present isotropic I H constant, The most likely for these differences is that the phosphonium group occupies an unusual position. F~~~ INDO calculations,25 it has been shown that for the unsubstituted cyclohexadienyl radical the proton coupling is (24) R. W. Fessenden and R. H. Schuler, J . Chem. Phys., 38,773 (1963). (25) M. B . Yirn and D. E. Wood, J . A m . Chem. Soc., 97, 1004 (1975).
larger for Haxi,, than for Hequatorial; furthermore, whereas 'Ha,,,, A,,, increases with the out-of-plane deformation angle $, 'Hequatorial Aisoshows an opposite dependence. The cyclohexadienyl systems which bear a substituting group on the sp3 carbon adopt generally the structure S2 and, due to a (T--B conjugation, the heteroatom (19F,'' ?'Sii3) exhibits a large hyperfine interaction while the methylene proton coupling is considerably reduced. In our case, however, both the low "P Aisovalue and the high value of 'H Ais, agree with a phosphorus atom occupying the equatorial position (e.g., SI). Such a structure is totally in accord with our results obtained from the study of the D tensor shown in Figure 5. Many mechanisms have been suggested for the formation of radical pairs.'6 In our case, it is most probable that, after electron capture by a radiolytic aromatic cation (ring A), the resulting excited phenyl undergoes a C( 16)-H homolytic scission. The ejected H is then trapped by the neighbor C ( 13') carbon of the ring B. The structure of the heavy Ph2P+CH2Rgroup is not likely to be affected by hydrogen addition; the cyclohexadienyl moiety adopts therefore the distorsion which requires both, the minimum change in the P-C bond directions and the minimum reorientation of the ring (role of the crystal lattice). Molecular models show that these conditions are most satisfied when the phosphorus atom is located in the equatorial position. Of course, such effects cannot be observed with freely rotating cyclohexadienyl radicals (liquid phase or adamantane host matrix) or with molecules for which a small substituting group (e.g., P ( O ) ( O H ) H ) can easily reorientate even at low temperature. Moreover, the constraints which prevent the cyclohexadienyl ring from placing the phosphonium group in the axial position maintain this radical near the phenyl o-radical and probably participate in the stabilization of the pair.
Acknowledgment. W e thank the Swiss National Research Fund for their support. Registry No. Ph3P'CH2SCH3CI-, 1779-54-0. Supplementary Material Acailable: Lists of structure factors, atomic positional, and thermal parameters for all atoms and other information in the form of the Standard Crystallographic File Structure of Brown27 (17 pages). Ordering information is given On a n y current masthead page. (26) H. C. Box, J . Phys. Chem., 75, 3426 (1971). (27) Brown. Acta Crysfallogr.,Sect. A , 39, 216 (1983).
Structure and Dynamics of Olefin Radical Cation Aggregates. Time-Resolved Fluorescence Detected Magnetic Resonance M. F. Desrosiers and A. D. Trifunac* Chemistry Division, Argonne National Laboratory. Argonne, Illinois 60439 (Received: September 27, 1985)
The time-resolved EPR spectra and thus the structure and dynamics of transient hydrocarbon radical cations are obtained by the pulse radiolysis-fluorescence detected magnetic resonance (FDMR) technique. Here we report the observation of short-lived radical cations from olefins. FDMR-EPR spectra of radical cations from tetramethylethylene and cyclohexadiene are illustrated. The olefin radical cations' FDMR spectra are concentration-dependent, since dimerization with neutral molecules takes place at higher (>lo-* M) olefin concentration. Rate constants for the dimerization reaction are derived and the effect of solvent viscosity on aggregate formation is demonstrated. By monitoring the further reactions of dimer cations, we have obtained EPR evidence for previously unobserved higher-order (multimer) radical cation aggregates of olefins.
Introduction Despite extensive study, the identity of the relevant intermediates involved in the radiation chemistry of hydrocarbons is not well understood. In particular, special emphasis and discussion have been placed on the nature of the cationic charge carriers and 0022-3654/86/2090-1560$01.50/0
In the course of new ideas have recently been advanced., characterizing radical cations produced in radiolysis, we have ( I ) Trifunac, A. D.; Sauer. M. C., Jr., Jonah, C . D. Chem. Phys. Lett. 1985. 1 1 3 316.
0 1986 American Chemical Society
