L. D. KISPERT,J. S. HYDE,C. DE BOER,D. LAFOLLETTE, AND R. BRESLOW
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Acknowledgment. We are indebted to Professor N. L. Bauld and J. D. McDermed of the University of Texas at Austin, Austin, Texas, for the gift of the tris(p-toly1)chloromethane. This work was sponsored in part by Research Grant AM-05895, U. S. Public Health
Service. We are also indebted to Professor A. Ehrenberg, University of Stockholm. His interest in this work is gratefully acknowledged. A grant from the Swedish Technical Research Council made it possible for L. E. G. E. to visit Palo Alto, Calif,
Combined Endor and Electron Paramagnetic Resonance Techniques in a Study of Some Low-Symmetry Triphenylmethyl Derivatives by Lowell D. Kispert,l&VbJames S. Hyde,l*,cCharles de Boer,2&lb Douglas LaF~llette,~a~c and Ronald Breslow2&J and the Analytical Instrument Division, V a r i a n Associates, Palo Alto, California 9@0S Department of Chemistry, Columbia University, N e w York, N e w York 10087 (Received J u n e 12, 1966)
Free radicals have been prepared in which triphenylmethyl carries a pendent o-CHzSCH3, o-CHz0CH3,oCH~CHZCH~, or p-CHzSCHa on one of the rings; in some cases these chains were also prepared with deuterium substitution. Electron-nuclear double resonance (endor) and electron paramagnetic resonance (epr) were combined in a novel method by which electron-proton coupling constants were determined from the endor spectrum, and the precise assignments were confirmed by computer simulation of the very complex epr spectrum. Endor spectra a t different temperatures showed that all of these radicals undergo a conformational change which we ascribe to a flipping of the triphenylmethyl propeller from one chirality to the mirror image. Values of the -CH3 hyperfine coupling constants support the idea of weak solvation of the radical by the sulfur atom of an o-CHzSCH3group. Molecular orbital calculations have been carried out. Although many of the features of these systems could be accommodated, unresolved difficulties were encountered in calculating the spin densities of the ortho-substituted phenyl rings.
Introduction The point of departure for the present work was to search for effects in the epr and endor (electron-nuclear double resonance) spectra of free radicals in solution which could be attributed to intramolecular solvation. During the course of the investigation, interesting steric interactions and conformational changes were encountered, and new methods of analysis were developed. These by-products of the experiment probably are of greater significance than the intramolecular solvation effects, which turned out to be rather weak. A considerable amount of evidence has accumulated that free radicals can be stabilized by neighboring-group interaction with a halogen3 or sulfur4 atom. Furthermore, kinetic effects suggest5 that free radicals in solution may be stabilized by interaction with some kinds of solvent molecules. However, to date almost no direct physical observation of a radicala has indicated the kind of neighboring-group or solvent coordination which these kinetic studies suggest. The Journal of Physical Chemistry
We have recently p r e ~ a r e d a~ ,series ~ of triphenylmethyl cations with potential intramolecular solvation by neighboring groups. Cation Ia7 exists completely8 as a cyclic sulfonium ion; the strong interaction be(1) (a) Varian Associates, Palo Alto, Calif. 94303; (b) Varian Postdoctoral Fellow, 1967; (c) requests for reprints should be addressed to this author. (2) (a) Columbia University, New York, N. Y. 10027; (b) Postdoctoral Fellow of the National Institutes of Health, 1965-1966; (c) Predoctoral Fellow of the National Institute of Health, 19661967; (d) Partial support of this work by a grant from HoffmannLaRoche Research Foundation is gratefully acknowledged. (3) P. Skell and R. Pavlis, J . Amer. Chem. Soc., 86, 2956 (1964), and references therein. (4) T. H. Fisher and J. C. Martin, ibid., 88, 3382 (1966). (5) C. Walling and M. Mayahi, ibid., 81, 1485 (1959); G. A. Russell, ibid., 79,2977 (1957). (6) P. I. Abell and L. H. Piette, ibid., 84, 916 (1962), have epr evidence in favor of bromine-bridged radicals a t 77"K, but as they point out, the data could also be accommodated by rapidly equilibrating unbridged structures. (7) R. Breslow, 5. Garrett, L. Kaplan, and D. LaFollette, ibid., 90, 4051 (1968). (8) R. Breslow, L. Kaplan, and D. LaFollette, ibid., 90, 4056 (1968).
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STUDYOF SOMELOW-SYMMETRY TRIPHENYLMETHYL DERIVATIVES
being isosteric with IIa but unlikely to furnish neighboring-group interaction with the radical. IIf was
@my '
IO
Q I I O
Figure 1. Reaction scheme for the o-CHzSCHs derivative.
tween the sulfur atom and the central carbon is promoted by the usual entropy advantage of neighboringgroup participation and by the steric effect which twists the ortho-substituted phenyl ring of a triphenylmethyl cation so that the side chain normally liess on the face of the central carbon, in a position to coordinate. See Figure 1. Reduction of I a to the radical I I a was effected in a standard fashion (see Experimental Section) , and this material was examined by epr spectroscopy. Interaction of the electron spin with the sulfur would not be directly observable, but if the odd electron were strongly localized between the central carbon and the sulfur atom, one should observe a decrease in the protonelectron coupling constants for the aromatic ring protons. Even with a weak solvating interaction one might expect to see an abnormally high coupling of the electron spin wiLh the S-methyl group. Of course, the magnitude of such an increase is only indirectly related to the spin density at the sulfur, and no reliable basis exists for predicting the fraction of spin density on the sulfur (presumably in a 3d orbital) which would be transmitted to the hydrogen nuclei of the neighboring methyl group. Compound I l a has a low symmetry and a large number of prot,ons. Accordingly, the epr spectrum proved too complex for straightforward analysis, and therefore it seemed desirable to apply the endor technique.$ As has been described elsewhere,IOendor simplifies situations of this kind and permits the direct determination OF hyperfine constants in quite complex molecules. Endor studies also were performed on triphenylmethyl radicals bearing an o-CD2SCH3group (IIb), an o-CH2OCH3 group (IIc), an o-CH20CD3group (IId), an o-CH2CH2CH3 group (IIe), a p-CH2SCH3 group (IIf), and a p-CD2SCH3 group (IIg). Endor spectra were obtained at -80 and -20" for each of these compounds. IIc and IIe were selected as
IIa, X IIb, X IIc, X IId, X IIe, X
IIf, X
= = = = = =
IIg, X
=
x
CHZSCH3, Y = H CDzSCH3, Y = H CH20CH3, Y = H CH20CD8, Y = H CH2CH2CH3,Y = H H, Y CHzSCHa H, Y = CDZSCH,
selected to examine the extent to which spin density could be transmitted along a -CHzSCHa chain into the methyl group. The deuterated compounds were used to assist in the assignment of coupling constants. Assignment of the Coupling Constants
Determination of the Number of Protons Assigned to Each Coupling Constant. Although the endor technique simplifies the determination of coupling constants, it does not directly indicate the number of protons to be assigned to each coupling constant. In an earlier publication by one of us,11 the usefulness of endor intensities to determine the number of protons which should be assigned to each line of an endor spectrum was investigated. It was concluded that this intensity information could be used as a rough guide in assignment but that some auxiliary technique, in that case selective deuteration, should be used to verify the assignment. There are two reasons for the difficulty: the sensitivity of the instrument varies as the nuclear radiofrequency is swept, and the relative endor signal heights depend critically on the details of the relaxation processes of the spin system. With improved instrumentation and better knowledge of relaxation processes, the intensity information should become more useful. We describe here an alternative technique for determining the number of protons which should be assigned to each endor line. The basic idea is to vary the number of protons assigned to each coupling in integer steps, to compute a simulated epr spectrum, and to compare it with an actual epr spectrum-trying to find that assignment which yields the best fit. Usually, of course, one selects those combinations of integers which are reasonable from a chemical point of view, rather than compute for all possible combinations. (9) J. S. Hyds and A. H. Maki, J . Chem. Phys., 40, 3117 (1964); J. S. Hyde, {bid., 43, 1806 (1965). (10) For a preliminary report, see J. S. Hyde, R. Breslow, and C. de Boer, J . Amer. Chem. Soc., 88, 4763 (1966). (11) J. S. Hyde, J. Phys. Chem., 71, 68 (1967). Volume '72*Number 12 November 1968
4278
L. D. KISPERT,J. S. HYDE,C. DE BOER,D. LAFOLLETTE, AND R. BRESLOW EXPERIMENTRL SPECTRUM
Figure 2. Experimental and computed spectra of compound IIb. The epr spectra and the endor couplings were obtained at -80” and the Simplex routine was used.
The epr spectra were digitized by storing the output of the Varian E-3 spectrometer in a Varian C-1024 timeaveraging device and then preparing punched cards utilizing the Varian C-1001 interface between the C-1024 and an IBM 029 card punch. There are several reasons why the endor coupling constants may differ slightly from the information on the coupling constants contained in the digitized epr spectrum. (1) Because of the nonlinear nature of the endor mechanisms, overlapping endor lines may not properly have been decomposed to find the true line centers. (2) There may be temperature dependences of coupling coefficients. It is convenient to digitize the epr spectrum at temperatures sufficiently high that anisotropic interactions are well averaged and all lines are the same width, but endor spectra usually are best obtained near the freezing point of t,he solvent. If The Journal of Physical Chemistry
endor spectra are run at higher temperatures, the signalto-noise ratio often is poor, introducing uncertainty into the measured values. (3) In our digitized spectra the z-axis scale factor may not be known with sufficient precision. (4) There are a variety of potential instrumental sources of error such as drift of the resonant frequency of the epr cavity and nonlinearity in the scanning of the epr magnetic field. For these reasons a computer program was written, which, for each comparison of computed and measured epr spectra, varied the coupling constants slightly about the values obtained from endor in an effort to achieve a better fit. In this calculation, one of the endor lines was selected as the calibration “standard.” This was a line either with no overlap from neighboring lines or, on one occasion, with overlap from equally intense lines equally spaced on either side. This coupling then
STUDY OF SOME LOW-SYMMETRY TRIPHENYLMETHYL DERIVATIVES
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E X P E R I M E N T R L SPECTRUM
I
COMPUTED SPECTRUM I
Figure 3. Experimental and computed spectra of compound IIg. The epr spectra and the endor couplings were obtained a t -20’ and the Simplex routine was used.
supplied a “scale factor” for the epr spectrum; that is, a slight correction to the dc magnetic field sweep calibration. The search for the best fit was carried out over that part of multidimensional space corresponding to each coupling constant f5% of that coupling constant. The computational technique was the “Simplex” method.12 I n spectra as complex as those considered here, one cannot begin with a set of random numbers for the coupling constants and expect the computer to converge to the proper values. There are many minima in the multidimensional space; conversion to wrong answers does in fact occur. Our experience in this particular problem is that one minimum only is found in that volume defined by each coupling constant *5y0 (providing only one radical species is present). If the search volunie gets larger, other minima may be found. These are empirical results; no effort was made to produce a map showing the location of the minima. This program has been carried through for compound
IIb using endor couplings obtained a t -80” and for the para-sulfur compound IIg using -20’ endor data. The results for these two compounds are given in Tables I and 11. It is difficult to establish an objective Table I : Couplings (MHz) of Compound IIa (O-CHZSCHS) Endor Simplex coupling No. of coupling (-SOo) protons ( - S O 0 )
0.30 0.50 2.28 2.52 3.59 4.08 8.21 8.73 9.05 9.62
3 1 1 2 4 2 2
2 1 1
0.282
... ...
2.424 3.507 4.112 8.210 8.729 9.092 9.375
Endor
% Dev
... ... ... -3.9 -2.5 0.7 0
.. .
0.4 -2.5
ooupling Assignment
CHa CH2 (one) meta (C) metu (A, B) ortho, para ( C i ortho (A or B) ortho (B or A) para (A or B) para (B or A)
(-209
0.03
2.53 3.51 4.02 8.28
(12) J. A. Nelder and R. Mead, Computer J . , 7, 308 (1966).
Volume 78,Number 18 November 1968
L. D. KISPERT,J. S. HYDE,C. DE BOER,D. LAFOLLETTE, AND R. BRESLOW
4280
EXPERIMENTRL
SPECTRUM
d
l Figure 4. Experimental and computed spectrum of compound IIg. The epr spectra and the endor couplings were obtained a t -80". The Simplex routine was not used.
Table I1 : Couplings (MHz) of Compound III (p-CH2SCHS) Endor coupling
No. of
(-goo)
protons
Assignment
3.20 4.68 5.20 7.13 7.47 7.80
6
meta (A, B, C) CHt (one)} CHa (one) ortho (A, B) ortho (C) para (A, B)
1 1 4 2 2
Endor coupling
Simplex coupling (-ZOO)
% dev
3.17 5.25
3.174
..,
7.15 7.41 7.65
7.219 7.513 7.695
(-20')
...
...
...
... 0.9 1.4
0.7
criterion for determining whether or not agreement between simulated and actual spectra is good, but the agreement on thwe compounds is the best we have seen. See Figures 2 and 3. Agreement between simulated and actual epr spectra using the endor couplings for compound IIb at -20" The Journal of Physical Chemistry
and compound IIg at -80" without any Simplex calculation also is quite satisfactory. See Figure 4. One notable result is that with -CH2SCH3 at an ortho position six strongly coupled protons with endor lines near 18 MHz are found, whereas eight such protons are found when the group is at a para position. H M O Calculations. Knowing the number of protons assigned to each coupling constant is helpful in assigning the couplings to particular protons of the molecules. Some of the couplings in this series of compounds were assigned on the basis of selective deuteration. The rule of thumb that ortho and para couplings are two to three times greater than meta couplings on pendent phenyl groups is useful in assignment. Finally, Huckelmolecular orbital calculations may be used as aids in assignment. The couplings in this series of compounds have been assigned using various combinations of these approaches. See Tables I and 11. There seems little
STUDYOF SOMELOW-SYMMETRY TRIPHENYLMETHYL DERIVATIVES point in specifically going through all of the logical arguments leading to the assignments, but some of the HMO calculations do seem to be of general interest. We have followed the method of M ~ L a c h l a n ’but ~ have treated the pendent groups -CH2SCH3, -CH,OCH,, and -CH&H2CH3 by the “heteroatom mode1.”14 I n this model the pendent group is visualized as a single electron pair on a single atom, and it is assumed that these electrons are part of the T system. Empirical parameters are introduced into the calculation which are a measure of the difference of the “heteroatom” from the benzene carbon. Thus the resonance integral between the benzene carbon C and the heteroatom X becomes kc-x/?o, where Po is the value for two adjacent ‘benzene carbons, and the coulomb integral for the heteroatom is a0 hxPo. Here a0is the benzene carbon integral and hx is positive for an atom more electronegative than the standard carbon. Streitwieser gives hx = 2.0 and ICC-X = 0.7 for methyl groups. Steelink, et aZ.,15found that t-butyl perturbations were the same as methyl perturbations and obtained values of hx = 2.2 and kc-x = 0.6 for methoxyl groups. The data for compound IIf (p-CH2SCH3) at -20” in Table TI shows that the ortho-carbon spin density on the ring to which the group is attached has been shifted with respect to the ortho coupling on the other two rings. The ratio is 7.513/7.219 = 1.040. (It is possible that the assignment for the coupling to the two para protons on the unperturbed rings A and B has been interchanged with that for the two ortho protons on the perturbed ring C in Table 11. The assignment in the table was based on the observation that the ratio of para-to-ortho couplings in triphenylmethyl is 8.04/7.31 = 1.1OIg which is in satisfactory agreement with the ratio of para-to-ortho couplings of the unperturbed rings given in Table 11, 7.695/7.219 = 1.07.) The ratio of the unperturbed to perturbed ortho spin densities in the HMO calculation equals the experimental ratio of couplings if hx = 1.5 kc-^ has been held at the methyl values of 0.7). (In this calculation all three phenyl groups were assumed to be twisted 20” about the methyl carbon bonds. The choice of 20” was made on the basis of Luckhurst’s calculstion on Coppinger’s radica1.16 Spin densities are not very sensitive to the choice of this angle as long as it is small, since the resonance integral between the methyl carbon and the attached phenyl carbon varies as /? = Po cos e.) This value for methylene perturbations has been used throughout, and it is suggested that it could be used in other calculations involving similar perturbations of aromatic rings. Deuteration has established that the methylene protons have an average coupling of 4.88 MHz in the para-sulfur compound. The McLachlan spin density at the perturbed para carbon is 0,0847. Thus one can calculate an effective “Q” for the methylene protons of the -CH2SCH3 group (in analogy to McConnell’s Q
+
4281
for protons bonded directly to aromatic radicals) of 57 MHz. The Q for a para proton of triphenylmethyl is about 90 MHz. Stone and Waters1’ studied ethyl and n-propyl substituents on the phenoxy1 radical. The methylene coupling of the ethyl group was about 5% greater than the corresponding proton coupling of the unsubstituted radical, and the methylene coupling of the n-propyl group was about 10% less. The 35% decrease in the methylene couplings of -CH2SCHs therefore is notable. As a phenyl ring is twisted about the methyl bond, the spin densities decrease. A model of the orthosubstituted compound IIa suggested that the ring to which the pendent group was attached (C) was twisted more than the other rings (A and B). A qualitative idea was that the ring was sufficiently twisted that the ortho and para spin densities were lowered to values about equal to the meta densities on the other ringsthus explaining the observation that the ortho-substituted compound IIa has six strongly coupled protons and the para-substituted compound IIf has eight. Spin densities were calculated holding two of the rings at 20” and varying the twist of the ring to which the pendent group was attached. The results for 50” twist are given in Table 111. Using these spin densities and the experimental values for the hyperfine couplings listed in Table I, Q’s were calculated. The criterion for selecting 50” as yielding best agreement was that the Q for the para proton in ring C was about the same as for the para protons in rings A and B and that the Q’s for all ortho protons were also about the same. A clear difficulty is the unreasonably high Q’s for the meta protons of ring C. The methylene Q also is lower than the previously determined vaIue of 57 MHz. Every calculation which was performed showed the ratio of maximum to minimum coupling in the perturbed ring (C) to be about 3:1, whereas the experimental endor spectra show that this ratio is 1.7: 1. The calculations always show the ortho spin densities somewhat higher than the para spin densities, whereas the hyperfine couplings of the para protons are always greater than the hyperfine couplings of the ortho protons. The spin densities in the two 20” rings were shifted up by about 12% over the values obtained if all three rings are at 20”, and this is in very good agreement with the observed endor spectra. The similarity of the endor spectra of the orthooxygen, ortho-sulfur, and ortho-propyl derivatives is marked. We have, however, been unsuccessful in simulating the epr spectrum of the o-CH20CH3 com(13) A. D.McLachlan, Mol. Phys., 3 , 233 (1960). (14) A. Streitwieser, Jr., “Molecular Orbital Theory for Organic Chemists,” John Wiley & Sons, Inc., New York, N. Y., 1961. (15) C. Steelink, J. D. Fitzpatrick, L. D. Kispert, and J. S. Hyde, J. Amer. Chem. Soc., 90,4354 (1968). (16) G. R. Luckhurst, Mol. Phys., 1 1 , 205 (1966). 17) T.J. Stone and W. A. Waters, J . Chem. Soc., 213 (1964). Volume 72, Number 12 November 1968
L. D. KISPERT,J. S. HYDE,C. DE BOER,D. LAFOLLETTE, AND R. BRESLOW
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Table I11 : HMO (McLachlan) Spin Densities for (RingC at 60°, A Compound IIa (o-CH~SCH~) and B at 20°, hx = 1.5, kc-x = 0.7) Position
P
Q
para (A, B) meta (A, B) ortho (A, B) para (C) meta (C), adjacent X meta (C), opposite X ortho (C), X ortho (C), opposite X
0.1056 0.0401 0.1136 0.0464 0.0169 0.0200 0.0453 0.0543
87 85 73 89 142 120 28 76
pound (IIc) using the endor couplings, and the Simplex routine has yielded no better agreement. We suggest that there may be more than one paramagnetic species H~ (IIe) in our sample. The o - C H ~ C H ~ Ccompound appears to have assignments the same as for IIa, but not as many calculations were carried out on this derivat,ive. Coupling data for compounds IIc and IIe are given in Table IV.
Figure 5. Endor spectra of compound IIa.
Table IV : Endor Couplings (MHz) (o-CHa0CHa)--
---I10
- 80'
- 200
...
