300
J. Phys. Chem. 1980, 84, 300-304
Dynamic Behavior of Organic Solvents Involving Nitroxide Radicals by '@F,'H, and 31P Pulsed NMR Method Kazunaka Endo. Department of Physics, Faculty of Science, Kyoto Universlty, Kyoto, Japan (Received November 8, 1978; Revised Manuscript Recelved August 13, 1979)
The pulsed NMR method has been used to investigate 19F,'H, and 31Pnuclear spin relaxation for fluorineor phosphorus-containing organic solutions containing the di-tert-butyl nitroxide (DTBN) radical. The present study on the frequency dependence of nuclear relaxation times afforded information on the dynamic aspects of the solvent-radical interaction. The observed nuclear relaxation rates were analyzed on the basis of the Hubbard and Solomon-Bloembergen theories. The nuclear relaxation of fluorine- or phosphorus-containing organic molecules (Ca9F6, C6H5C'9F3,(C'H30)3P0,and (C1H30)3P)in the presence of DTBN is explained by Hubbard's diffusion model. The 31Prelaxation of phosphorus-containingorganic molecules ((CH30)331P0and (C4Hg):lP) involving DTBN may be also governed by translational dipolar diffusion. The fluorine- or phosphorus-containingorganic molecule-DTBN interaction system interpreted in terms of the diffusion model may correspond to a weak charge transfer interaction between solvent molecule and DTBN, since a downfield 19For 31Pscalar shift induced by the DTBN radical was observed for the fluorine- or phosphorus-containing organic molecules except for (CH30)3P0. The translational correlation times, the scalar coupling constants for the nucleus, and the closest distances between the nucleus and the odd electron for the fluorine- or phosphorus-containing organic solvent-DTBN interaction system were evaluated.
I. Introduction We have so far reported a number of investigations on the 'H NMR solvent chemical shifhl and relaxation times2 induced by chemical interaction, such as hydrogen bonding with the di-tert-butyl nitroxide (DTBN) radical, with much attention directed toward understanding intermolecular electronic interactions. These studies provided fruitful information on the intrinsic static and dynamic properties of the hydrogen bond between the solvent molecule and the DTBN radical. In this paper, we describe a study of the nuclear relaxation of fluorine- or phosphorus-containing organic solutions involving a nitroxide radical, with an aim to obtain information on molecular distances, scalar coupling constants, and correlation times associated with the DTBN radical-solvent molecular interaction. There have been several NMR relaxation studies performed by others on the interactions between free radicals and solvent molecules. Gutowsky et ala3studied the solvent-radical interaction from measurements of proton and fluorine relaxation times by the pulse method. They demonstrated that 19Frelaxation for the solvent-radical bimolecular system results from a combination of translational and rotational motions and the Fermi contact interaction. Muller-Warmuth et al.4*5and Dwek et aL6 pointed out from dynamic nuclear polarization (DNP) and NMR experiments that the major contribution to relaxation for 19F and electron spin of free radicals in fluorinated organic solutions could be explained by a nucleus-electron dipole-dipole interaction characterized by translational random motions. Muller-Warmuth et al.'y8 studied the nuclear electron Overhauser effect (DNP) of 31Pand I9F in organic solutions involving free radicals. The results were analyzed in terms of the modified diffusion model of molecular collisions. Recently Poindexter et al.9 showed from DNP studies that the l a F of fluorocarbons relaxes by collision with free radicals with a characteristic translational correlation time. Potenza et a1.I0 studied the I9F
* Address correspondence to the author at the Research Laboratory, Kyoto Photographic Factory, Mitsubishi Paper Mills, Ltd. Nagaokakyo-city, Kyoto, 617 Japan. 0022-3654/80/2084-0300$01 .OO/O
relaxation mechanism for C6F6 interacting with a free radical in a variety of hydrocarbon solvents from DNP experiments. They analyzed the C6F6scalar spectra in terms of a modified diffusion model with two frequency components. These studies, however, dealt with the weak molecular interaction without taking account of the specific chemical interaction. In this report, investigations of nuclear relaxation times have been carried out over a wide frequency range by the pulse method to substantiate the specific chemical interaction between an organic solvent molecule and DTBN, and to obtain information on molecular motions involved in a radical-solvent interaction system. The observed nuclear relaxation rates were well explained in terms of the diffusion model."J2 The I9Frelaxation of fluorinated molecules in the presence of DTBN is well interpreted by the diffusion model12 in which both dipolar and scalar interactions between the electrons and nuclei are considered. The 'H and 31Prelaxation of phosphorus-containing organic solvents in the presence of DTBN may be dominated by translational dipolar diffusion. The fluorine- or phosphorus-containing molecule-DTBN interaction system explained in terms of the diffusion model may correspond to a weak charge transfer interaction between solvent and DTBN, since the downfield 19For 31Pscalar shift induced by DTBN radical was observed for fluorineor phosphorus-containing molecules except for (CH30)3P0. We have evaluated translational correlation times, scalar coupling constants for the nucleus, and the closest distances between the nucleus and the odd electron for the fluorine- or phosphorus-containingorganic solvent-DTBN interaction system. 11. Theoretical Background
It was pointed out from our NMR experiments2that 'H relaxation of aprotic solvent molecules in the presence of DTBN can be explained in terms of a pure dipolar interaction between solvent protons and free-radical odd electrons governed by translational random motions. Thus, we called this solvent-radical interaction a "no chemical interaction". However, scalar hyperfine inter@ 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 3, 1980 301
Interaction of Organlc Solvents with Nitroxide Radicals
actions for fluorine or phosphorus are generally larger than for hydrogen, so that we consider both dipolar and scalar interactions upon 19For 31Prelaxation of organic solvent molecules. In the present study, the mode of this interaction between a solvent molecule and a free radical may be classified into the following limited cases: (a) a weak charge-transfer interaction, or “no chemical interaction” and (b) formation of a labile molecular complex. In case a, nuclear relaxation may be explained in terms of the translational diffusion model12in which both dipolar and scalar interactions between the electrons and nuclei are considered. We feel that the concept of Hubbard’s diffusion model may describe the chemical interaction, “a weak charge-transfer interaction”. In the diffusion model the scalar interaction is assumed to have a very short range with a local dependence proportional to exp[-X(R - d ) ] where Ad >> 1. X is a constant, R is the electron-nuclear separation, and d is the distance of closest approach of the nucleus and electron. In case b, when a solvent molecule forms a labile complex with a free radical, the solventnuclear relaxation may be viewed by use of the SolomonBloembergen theory.13 If the major contributions to nuclear relaxation for solvent-radical interaction is explained by the nucleuselectron dipole-dipole interaction, the equations for relaxation rates are simplified2 to (1/NTJtheo (l/N)(l/TINE)dip B[3jd(O) + 7jd(%)l (1) for case a, and (No/NTT)theo ( N O / N ( l / T l N E ) d i p = B73Jd(O) + 7Jd(%)l ( 2 ) for case b, where B and Ware constants including yN,Y ~ , etc, and where j d ( u J = (15/2)1(~) (3) I(u) = d [ u 2 - 2
+ exp(-u)((u2 U
=
-
2) sin u
+ (u2+ 4u +
2 ) cos 41 (4)
1aBTd11/2
(5)
(6) Here, the meaning of each symbol is defined by using the conventional n o t a t i o n ~ . ~ JIn~ Jeach ~ interaction case, the nuclear relaxation rates may have different frequency dependences. ’Hence, by studying the frequency dependence of nuclear relaxation rates, the dominant interaction between solvents and free radicals can be made clear, Jd(WB)
=
Tc/(l
-k
W:T>)
111. Experimtantal Section 19F,lH, and :I1Prelaxation experiments for DTBN solutions of fluorinated and phosphorus-containing organic molecules were performed with a Bruker pulsed NMR spectrometer (SXP 4-100) at Osaka University. The spin-lattice relaxation time TI was measured by the 18Oo-~-9O0pulse sequence and spin-pin relaxation time T2 by the Carr--Purcell-Meiboom-Gill (CPMG) method. DTBN was prepared by the method of Briere et al.14 The samples were redistilled and the NMR samples were sealed under vacuum by the freeze-pump-thaw technique in order to eliminate dissolved oxygen.
IV. Results and Data Analysis (a)Fluorinated Organic Solutions. The observed values of TI, T2,and the ratio Tl/T, for the fluorine signals of solvent molecules (CGFGand C6H5CF3)are summarized in Table I. The ratio Tl/T2falls in the range 1.0-1.7 over
TABLE I: Frequency Dependences of Fluorine and T ,/ T , of Fluorocarbons Containing the DTBN Radical* at 19.0 “ C
T,,a
freq, MHz quantity
10
5
T I , ms T,, ms T,/T,
24.4 23.5 1.04
T,,ms T,,ms TIIT,
86.8
40
60
Solvent 33.7 36.6 27.0 26.9 1.25 1.36
20
41.0 28.5 1.43
51.3 30.0 1.70
C,H,CF, Solvent 98.1 111 1 2 3 97.2 98.1 1 0 2 1.01 1.14 1.20
137 110 1.24
146 111 1.31
C,F, 28.5 25.7 1.11
30
The probable error of T I is 13% and that of T , is * 5%. The concentration of DTBN, N, is 2.8 x l o z 0molecules c m F 3for fluorocarbon solutions. a
t~
- (T,FE)d,p
(theory o f Hubbad) ~
L
J
2
5
10
20
1 0 60
WF MHz
Flgure 1. Log-log plots of the frequency dependence of the fluorline T,s observed In CeF, and CeHBCF, solutions of DTBN at 19.0 OC. The abscissa is the fluorine resonance frequency. (a) The solid line is the theoretical curve of eq 1. The point on the theoretical curve which corresponds to W , T ~ = 1 is shown by an arrow. (b) The solid line Is the theoretical curve of eq 2. The point on the theoretlcal curve which corresponds to w , ~ , = 1 is shown by an arrow.
the wide frequency ranges examined here. The TI/”, ratios are larger than unity, indicating both dipolar and scalar hyperfine interactions between the fluorine nucleus of fluorocarbons and the odd electron of DTBN. In the presence of both dipolar and scalar hyperfine coupling between solvent fluorine and free-radical electron spins the analysis of data is more complex than in the simple proton case. However, the major contribution to fluorine relaxation may also be explained by the nucleus-electron dipole-dipole interaction. Thus, experimental values d spin-lattice relaxation rates would be interpreted in terms of (l/Tl)th, by using either eq 1or 2. Fluorine spin-lattice relaxation rates are presented with two types of frequency dependency in Figure 1 for aromatic and aliphatic fluorocarbons, C6F6and C6H6CF3. Figure 1 shows that the relaxation of fluorocarbons in the presence of DTBN may be dominated by translational diffusion. If we consider that the diffusion model is a correct approximation to the behavior in these solution, the observed values of l/Tl and 1/T2 can be fit with the frequencydependent values obtained from following equations. We
302
The Journal of Physical Chemistty, Vol. 84, No. 3, 1980
Endo
TABLE 11: Frequency Dependences of Proton and Phosphorus Tl,a T,,a and T , / T , of Phosphorus Molecules Involving the DTBN Radicalb at 15.0 “ C freq, MHz solvent
quantity
( CH 30)3 31P0
5
TI, ms T.. ms
10
16
13.0 12.0 1.08 23.0 22.5 1.02 128 126 1.02
13.9
20
23
50
16.7 14.6 1.14
17.9
60
28.1
36.2 34.0 1.07
134 130 1.03
143 133 1.08 160
101 94.5 1.07
109 98.0
113 99.8 1.13
1.11
The probable error of T,is r 3 % and that of T , is *5%. The concentration of DTBN, N , is 2.4 X 4.5 x l o z omolecules c m - , for (CH,O),P, (CH,O),PO, and (C,H,),P solutions, respectively. a
can, then, decompose the total relaxation rates T1 and T2. For the diffusion model, the equations are given as 1/T1 = B[3jd(O) + 7jd(%)] + c u e ( w s ) ] (7) 1/TZ = (B/2)[7jd(O) + 13jd(us)l
lozo,3.6 X I
radls
I
I
’ CgF6
TI
‘CgHgCF3
Ti
“6F6
T2
‘‘6H5CF3
T2
- T! & Tz
+ (c/2)Pe(o) + j e ( u a ) ]
,
l o z o ,and
(theory of Hubbard)
(8) L N
where je(uJ = (1/2u){1 + exp(-u)(sin u - cos u ) )
(9)
B = (!hNY~~y:h~Td/75d~)
(10)
c = (TNAN2dTd/A2)
(11)
In order to perform a quantitative analysis of the experimental relaxation rates in Table I, we substituted B and C as parameters into eq 7 and 8 and performed the numerical calculations; the experimental results could fit with the theoretical curves. The best fits of theoretical values are shown in Figure 2, together with experimental points for cJ?6and C6H6CF3solvents interacting with the DTBN radical, The translational correlation times, Td, distances of closest approach, d , and scalar hyperfine coupling constants, AF,are listed in Table 111. The values of Td and d for the C6F6 solution agree well with results for C6Fs involving the phenoxy1 radical obtained by DNP.‘ The results, summarized in Table 111, show scalar contributions to the total spin-lattice relaxation rates of 41 and 15% for C6F6 and C6H5CF3,respectively. This scalar interaction was also confirmed by our studied5 on the NMR chemical shift. Fluorine nuclei for fluorocarbons exhibited downfield scalar shifts. Thus, we can examine suitability of the diffusion model from chemical shift measurements, For the diffusion model, the relation of the shift to the scalar coupling is approximated by the equationa -A W-N - -yJV’(4ad3) ~ A , s ( s+ 1) (12) WN YNXd 3kT assuming that Ad >> 1. We observed ‘V shifts of -2.10 and -0.96 ppm for cJ?F, and C6H&F3,respectively. The radical concentration of solvent molecules, N’, is 7.9 X lozomolecules for C6F6and C6H6CF3solutions. We, thus, individually obtained the positive scalar hyperfine coupling constants, AFs,of 15 and 3.0 MHz for c($6 and CBH5CF3 for Ad = 10. The resulting values of A F are in good accordance with those values from relaxation rates. (b) Phosphorus-Containing Organic Solutions. The proton and phosphorus Tl/T2 ratios for the phosphoruscontaining molecule-DTBN interaction system in Table
i 2
5
10
20
40 60
w, MHz Figure 2. A log-log plot of the frequency dependence of the fluorlne T,s and T@ observed in C,F, and C,H,CF3 solutions of DTBN at 19.0 ‘C. The abscissa is the fluorlne resonance frequency. The solid llnes of I / T I and I/ T2 are theoretlcal curves of eq 7 and 8. The point on the theoretlcal curve which corresponds to ward = 1 IS shown by an arrow. Best fits of theoretical values in eq 7 and 8 are 6 = 0.10,c = 0.90 for C,F, and 19= 0.30. C = 0.70 for C,H&F3.
I1 are close to unity in frequency ranges examined here. The lH and slP relaxation of phosphoruscontaining molecules ((CH,O),PO, (CH,O),P, and (C,H,),P) in the presence of DTBN would be dominated by the nucleuselectron dipole-dipole interaction. Hence, experimental values of nuclear spin-lattice relaxation rates may be ex. plained in terms of (l/Tl),heoby using either eq 1 or 2. First, we tried to examine frequency dependence of spin-lattice relaxation times. We could not obtain the data of 31Prelaxation times at low fields because of the weak intensity of the 31Psignals. We, thus, studied the frequency dependence of the ‘Hrelaxation time for (CH,O)3P0 and (CH,O),P involving the DTBN radical. In Figure 3 proton relaxation rates for (CH30)3PO- and (CH,O),P-DTBN interaction systems are presented with a Hubbard type of frequency dependence. The result shows that the protons of these molecules are relaxed dominantly by translational dipolar random motions. The values of 7 d and d for these compounds obtained from Figure 3 by use of eq 1 are listed in Table 111. If we assume that dipolar coupling to the electron spins is the same for lH and 31Pnuclei, 7d from lH NMR data may be equal to that from 31PNMR. We, thus, evaluated closest distances, d, that the phosphorus nucleus approaches the odd electron in Table 111.
The Journal of Physical Chemistry, Vol. 84, No. 3, 1980 303
,
I
rad's
-
+ -
,
(C'H,O),PO (C'H30)3P
0
I
I
,
T, T
theory
of Hubbard)
I
I
100 -
c
50 30
c, '
9F6
C,H,C19F, (C'H,O),P (C'H,O),PO (CH310)331P (CH30)331P10 (C,H,),31p
I
1
1.9 * 0.3 1.5 * 0.3 9.0 * 1.0 11 t 1.1
19 19 15 15 15 15
I
1
3.1 4.1 3.8 3.8 3.5a 4. la
12 2.6 0.50b
0.14&
20
The value d was estimated by using 7d from 'H NMR data. The value Ap was estimated from the 31Pshift to the scalar coupling at 20 C. a
O
The results from the relaxation experiment display that nuclear relaxation of phosphorus molecules may be governed by the translational dipolar diffusion. However, for the triply connected phosphorus-containing compounds ((CH30)3Pand (C4H9)3P)with the DTBN radical interaction system, the scalar interaction was confirmed by our studies15of the NMR chemical shift. The 31Presonance of phosphorus-containing molecules also shifted downfield. The 31Pwhift t u the scalar coupling is also approximated by eq 12 for the diffusion model. We observed 31Pshifts of -0.69 and -10.20 ppm for (CH3OI3Pand (C4H,),P, respectively. The radical concentration of the solutions, N', is 4.0 X lom molecules ~ m - We ~ . individually obtained the positive scalar hyperfine coupling constants, Aps, of 0.50 and 0.14 MHz for (CH30)3P and (C4H9)3Pfor Ad = 10. We may estiimate the scalar contribution to the total relaxation rate from this resulting value of A,. For the diffusion model, the scalar term to the spin-lattice relaxation rate is given as (1/ TIL = (aNAp2d7a/A2)*.i,(%) (13) We obtain (l/Tl)w= 0.036 rad/s at 20 MHz (31P)for the (CH30)3P-DT13Ninteraction system from eq 13. We may, thus, ignore this scalar contribution to the 31Pspin-lattice relaxation rate, 0.57%, for the (CH,O),P/DTBN system.
V. Discussion The present study proves that the experimental results may be well explained in terms of the diffusion model. The relaxation of fluorine5 in fluorinated molecules is caused by both dipolar and scalar translational diffusion in collisions. The l1H and 31Prelaxation of phosphorus-containing molecules may be governed by translational dipolar diffusion in collisions. The collision is probably inducing positive electron spin on the fluorine and phosphorus atoms by means of some chemical interaction. It will be of
interest to explore when this chemical interaction occuirs. It may be significant that the nucleus-electron distance is smaller than the molecular radii for the cases where the chemical interaction is strong. We, then, estimate the sum of van der Waals radii and of the covalent radii for the electron-nucleus distance. The DTBN radical is calcula1;ed to bear the shape of knobby, somewhat bent cylinder, 9 A in length and 5.3 8, in diameters3 The odd electron density may be concentrated on the nitrogen at the center where the NO bond is shielded by the two bulky tert-butyl group.3 Therefore, the sum of van der Waals radii is 4.05 and 4.6 A, and of the covalent radii is 2.6 and 3.0 A ffor fluorine- and phosphorus-electron distances, respectively. The closest distance (d = 3.1 A) obtained for the (z6' F6-DTBN interaction system was found to lie between the sum of the covalent radii and the sum of van der Waals radii, In the C6H5CF3-DTBN bimolecular system the closest approach (d = 4.1 A) is comparable with the sum of van der Waals radii. This feature allows us to say that d is smaller in relation to the radii for cases where the interaction is strong. For the electron-phosphorus distance in the (CH30),PDTBN bimoleclar system, d was found to be 3.5 A. The value of d is a little shorter than that value (d = 4.1 A) for the (CH30)3PO-DTBN system, and was also found to be between the sum of the covalent radii and of van der Waals radii. This result may suggest that the triply connected phosphorus of (CH30)3Pinteracts strongly with the odd electron of DTBN radical. In order to get further insight into the intrinsic nature of the interaction between the nitroxide radical and organic solvents, we have observed DTBN-induced 19F,31P,and 13Cshifts for fluorinated and triply connected phosphoirus m01ecules.l~All of these fluorine and phosphorus nuclei exhibited downfield shifts, while upfield shifts were observed for carbons bonded directly to fluorine or phosphorus atom in these molecules. For example, preferential downfield 31Pscalar shift and substantial upfield 13Cscalar shift for the substituted carbon (Cj)in triphenylphosphine appear to be quite similar to the case of fluorobenzene. It is tempting to suggest that the lone pair electrons of the fluorine or phosphorus atom may serve as an electron donor to DTBN which might act as an electron accepto:r.16 The residual positive spin density of fluorine or phosphorus induced by direct interaction between the F and P atom with DTBN may be distributed to the adjacent carbon atom by the spin polarization mechanism.16 It is, therefore, shown that this specific interaction may be responsible for the charge-transfer interaction between these organic solvents and DTBN radical. The results of this investigation show that the DTBNinduced nuclear relaxation study serves to delineate the dynamic behavior of weak molecular interactions in solutions. Acknowledgment. The author is very much indebted to Professor H. Kiriyama of Osaka University for her generous gift of machine time in the use of the Bruker pulsed NMR spectrometer. I also express my appreciation to Professors K. Suzuki, T. Yonezawa, and I. Morishiina for their interest and encouragement in this study, to lh. T. Inubushi of the department of hydrocarbon chemistry, and to Professors T. Hashi and M. Matsuoka and their fellows of the department of physics for their helpful discussions. References and Notes (1) I. Morlshima, K. Endo, and T. Yonezawa, J . Am. Chem. Soc., 93, 2048 (1971); Chem. Phys. Lett., 9, 143, 203 (1971); J. Chem. phys., 58, 3146 (1973); K. Endo, Y. Hazama, K. Okabayashi, I. Tonolke,
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J. Phys. Chem. 1980, 84, 304-306
and K. Suzuki, Chem. Phys. Left., 81, 336 (1979). (2) K. Endo, I.Morishima, and T. Yonezawa, J. Chem. Phys., 87,4760 (1977);K. Endo, 8. Knuettel, I. Morlshima, T. Inubushi, and T. Yonezawa, Chem. Phys. Left., 31, 387 (1975). (3)H. S. Gutowsky and J. C. Tai, J. Chem. Phys., 39, 208 (1963). (4) W. Muller-Warmuth and V. Prink, 2.Naturforsch. A, 21, 1849 (1966). (5) W. Muller-Warmuth, 2. Naturforsch. A, 21, 153 (1966). (6) R. A. Dwek, J. G. Kenworthy, D. F. S. Natusch, R. E. Richards, and D. J. Shields, Proc. R . SOC. London, Ser. A , 291, 487 (1966). (7) W. Muller-Warmuth, H. Grutzediek, and R. Van Steenwinkel, 2. Naturforsch. A , 25, 1696 (1970). (8) W. Muller-Warmuth, R. Van Steenwinkel, arid A. Yalginer, Mal. Phys.,
21, 449 (1971). (9)E. H. Poindexter, P. J. Caplan, B. E. Wagner, and R. D. Bates, Jr., J. Chem. Phys., 61, 3821 (1974). (10)J. A. Potenza, H. Schmttz, and W. Muller-Warmuth, J. Chem. Phys., 86, 3100 (1977). (11) H. Pfeifer, Ann. Phys., 8, l(1961). (12) P. S. Hubbard, Proc. R . SOC. London, Ser. A , 291, 537 (1966). (13) I.Solomon and N. Bloembergen, J . Chem. Phys., 25, 261 (1956); I.Solomon, Phys. Rev., 90, 559 (1955). (14) R. Briere, H. Lemaire, and Rassat, Bull. Soc. Chlm. Fr., 3273 (1965). (15) T. Inubushi, I. Morishima, and K. Endo, to be published; T. Inubushi, Thesls, Kyoto University, 1977.
Capto-Dative Substituent Effects. 2.' Spin Trapping of Phosphorus-Centered Radicals by a-tert-Butylthioacrylonitrile. An ESR Study Lucien Stella,2 Robert MerQnyi,Zdenek Janousek, Helnz Gunter Viehe, * 3 Unlverslt6 de Louvaln, Laboratoire de Chimie Organlque, Place Louis Pasteur 1, 6- 1348 Louvah-la-Neuve, Belgium
Paul Tordo, and Aurello Munoz4 Unlverslt6 de Provence, Laboratoire de Chimie Organique Physique, Centre de St J%me, 13397 Marsellie Cedex 4, France (Received March 1, 1979; Revised Manuscript Received October 3, 1979)
The radiophilic olefin a-tert-butylthioacrylonitrileis used to trap a series of phosphorus-centered radicals .PL,. The resulting capto-dative substituted radicals A. are easily detected by ESR in the temperature range 40-180 "C. This result is accounted for by the occurrence of the equilibrium 2A. ==A-A. Spin densities at the radical center are derived from Fischer's relation and the preferred conformations of the spin adducts are deduced from ap analysis.
Introduction We recently reported1 that 1,l-capto dative-substituted olefins behave as powerful scavengers toward free radicals (es 1). X
'C=CH, #
xx
X
+ R. -+
Y X = donor Y = acceptor
\
C-CH,R
Yf
A.
I I * RCH,G-C-CH,R I t
(1)
Y Y A- A
TABLE I: ESR Parametersa for Different Spin Adducts* A. = RCH,C.(CN)S-t-Bu R.
aH
aN
( n-Bu ) 3 Sn.
8.5 8.5 8.3 8.3 7.5
2.5 2.6 2.5 2.5 2.5
Me C. OH MeS. Me;CCN C1,C.C
spin adduct
alaC 01
1 2
3 4
26.8
5
*
a The hyperfine splittings are given in gauss. In In carbon tetrachloride a t chlorobenzene a t 140 C. 140 "C. O
The spin-adducts A. are easily detected by ESR in the temperature range 40-180 "C. The signals disappear on cooling below 40 "C and reappear reversibly upon heating. Such behavior has already been reported among many examples for dialkylaminothiyl radicals and has been interpreted as a demonstration of the occurrence of an equilibrium between the free radical and its dimer.5 In our case, dimers of the A-A type have been isolated in good yie1ds.l The largest steady-state concentration of radicals is observed at high temperature (140-160 "C). However, when the unpaired electron of the free radical R. is localized on a spin-zero nucleus, the information regarding the nature and structure of R- is difficult to abstract from the ESR spectrum of As. When X = CN and Y = S-t-Bu, the spectrum of A. always consists of a 1:2:1 triplet of 1:l:l triplets due to the H, and nitrogen hyperfine couplings (Table I). We report here that a-tert-butylthioacrylonitrileis an efficient trap for diffqent phosphorus-centered radicals 0022-3654/80/2084-0304$01.OO/O
and discuss the spin density value at the radical center as well as the configuration and conformation of the resulting radicals.
Experimental Section Materials. a-tert-Butylthioacrylonitrile was prepared by the method of Gundermann and T h o m a d bp 65 "C (12 torr); lH NMR (CDClJ 6 1.42 (9 H, singlet), 6.2 (1 H, singlet), 6.4 (1H, singlet). 1-Cyano-1-ethylthioethane was prepared by the method of Pochat and Levas:' bp 72 "C (12 torr), 'H NMR (CDC13) 6 1.4 (3 H, triplet, J = 7.5 Hz),1.58 (3 H, doublet, J = 7 Hz), 2.8 (2 H, quadruplet, J = 7.5 Hz), 3.7 (1 H, quadruplet, J = 7 Hz). Radical Generation. As previously described," the transient L,P. radicals were generated directly in the cavity of a Varian E-3 ESR spectrometer by hydrogen abstraction from the parent compounds. A mixture of di-tert-butyl 0 1980 American Chemical Society
