The Journal of Physical Chemistry, Vol. 83, No. 5, 1979 633
Ring Inversion of Tetralin-l-yl
(23) W. R. Moomaw and M. F. Anton, J. Phys. Chem, 80, 2243 (1976). (24) NOTEADDEDIN PROOF:Professor J. W. Verhoven and Professor E. M. Kosower have drawn our attention to the "multiple accepting orbital" explanation offered by them for double (CT) bands in methiodides of some pyridine derivatives (J. W. Verhoven et al., Tetrahedron, 25, 3395 (1969); R. A. Mackay et al., J . Am. Chem. Soc., 93, 5026 (1971); E. M. Kosower et al., ibid, 94, 986 (1972)). Further work to test the applicability of their mechanlsm to our compounds is in progress.
(16) S. Sakanoue, Y. Kai, N. Yasuoka, N. Kasai, and M. Kakudo, Bull. Chem. SOC.Jpn., 43, 1306 (1970). (17) C. K. Jlrgensen, frog. Inorg. Chem., 12 (1970). (18) L. E. Orgel, "Reports to the Tenth Solvay Council Stoops", Brussels, 1956. (19) H. Yamatera, J . Inorg. Nucl. Chem., 15, 50 (1960). (20) R. Grinter and E. Heilbronner, Helv. Chim. Acta, 43, 2496 (1962). (21) R. S. Muliiken, J. Am. Chem. Soc., 64, 811 (1952). (22) M. Pileni, F'. Walrant, and R. Santus, J . Phys. Chem., 80, 1804 (1976).
ESR and Ring Inversion of Tetralin-l-yl and ESR of Related Benzyl Radicalst Mark S. Conradi,"' Henry Zeldes, and Ralph Livingston Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 (Received October 13, 1978) Publication costs assisted by Oak Ridge National Laboratory
The tetralin-l-yl radical 1 was generated in solution by hydrogen abstraction from tetralin by photoexcited benzophenone. The proton hyperfine splittings and the g value are reported from -69 to 146 "C. An alternating line width effect occurs and is ascribed to ring inversion in the saturated portion of the radical. Line widths and amplitudes were analyzed at low and high inversion rates (near -50 and 130 "C, respectively) to yield values of the ring inversion rate. Arrhenius parameters for the inversion are E = 30 kJ/molf12% and log k = 13.1 f 0.7. Two azimuthal angles which indicate the extent of ring puckering are determined from the proton splittings at -69 "C. The room temperature hyperfine constants and g values of m- and o-xylyl and a-methylbenzyl are reported and support the assignment of proton splittings in tetralin-l-yl. The methyl group splittings in mand o-xylyl are smaller than expected from comparison with related radicals.
Introduction During a study of substituted benzyl radicals of importance in coal chemistry we observed the ESR spectrum of the tetralin-l-yl radical 1 in solution. The absence of
65 H2
H H2 Y P
17 I
certain lines from the room temperature ESR spectrum indicated the occurrence of an alternating line width effect. This effect results from ring inversion at an intermediate rate in the saturated portion of the radical. Subsequently, ESR spectra of 1 were obtained in the slow and fast ring inversion regimes. The line widths at low and high temperatures were analyzed to yield values of the inversion rate and the Arrhenius parameters of the inversion. The rate parameters of this unimolecular reaction reflect the detailed skeletal and electronic structure of the radical. Detailed discussions of the effect of ring inversion upon ESR spectra have appeared.2B Consequently, some details of the theory will not be repeated here. The proton hyperfine splittings (hfs) of tetralin-l-yl are reported from -69 to 146 "C. Some of these couplings determine values of certain azimuthal angles in the radical. The proton hfs and g values at 30 "C of m- and o-xylyl and a-methylbenzyl are reported and confirm the assignment of couplings in tetralin-l-yl. The methyl hfs of m- and o-xylyl are somewhat unusual and are discussed. The radicals reported here were usually generated by UV photolysis of benzophenone in the appropriate soluResearch supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract W-7405-eng-26 with the Union Carbide Corporation. 0022-3654/79/2083-0633$0 1.OO/O
tion: tetralin (1,2,3,44etrahydronaphthalene), m- or oxylene, or ethylbenzene. Photoexcited benzophenone is known to readily abstract benzylic h y d r ~ g e nhowever, ;~ it does not appear to be widely used in ESR to generate benzyl radicals. Benzophenone strongly absorbs light in a wavelength region near 340 nm where many benzene derivatives are transparent and where many UV sources are brighte5 This technique also produces very intense signals from the hydroxydiphenylmethyl radical (Ph,COH) because this radical is relatively long-lived.6 These signals obscure a 28-G region near the center of the s p e ~ t r u m . ~ This required that two of the radicals (the xylyls) be generated by other means; photolysis with 20% (v/v) di-tert-butyl peroxide was used. This produced somewhat weaker signals from the xylyl radicals, but without interference. Experimental Section The ESR spectra were recorded with a spectrometer operating at 9.5 GHz with 100-kHz field modulation. The spectrometer was recently upgraded by use of a silicon Schottky detector in place of a back diode and by installation of a low noise preamplifier employing ten field-effect transistors in parallel. Light from a Philips SP 500-W high-pressure mercury lamp was focused by lenses onto a flat cell located in the ESR cavity. The solutions were freed of dissolved oxygen by purging with helium gas during recirculation. Flow rates of about 3 cm3/min were provided by a helium gas lift pump (bubble lift) and were sufficient for room temperature operation. For other temperatures the solution was passed through a thermostatically regulated heat exchanger and then immediately into the ESR cell. The heat exchanger was cooled by liquid nitrogen and/or heated electrically. Flow rates from 30 to 60 cm3/min were provided by a gear pump for this technique. Temperatures were measured by a thermocouple in the fluid stream, about 3.5 cm above the illumination zone. An earlier but similar version of 0 1979 American Chemical Society
634
The Journal of Physical Chemistry, Vol. 83, No. 5, 1979
TABLE I : Hfs and g Values of Tetralin-1-yl: Intermediate Inversion Rates proton assignt Y 597 8 6 CY
171 t q 2 p l + p2 g value a Equal
apprnt no. of protonsa 2 2
1 1 1 1 1
M. S. Conradi, H. Zeldes, and R. Livingston
TABLE 11: Hfs and g Values of Methylbenzyls at 30 O C
hfs, G
hfs, G - 33 " C
30 " C
78 " C
0.66 1.68 4.97 6.07 15.74 9.48 45.29 2.00262
0.64 1.69 4.97 6.05 15.71 9.47 44.91 2.00263
0.61 1.71 4.96 6.04 15.71 9.47 44.79 2.00259
to no. of lines - 1.
this equipment has been d e ~ c r i b e d . ~ The magnetic field was swept linearly (f0.2%) by use of an induction coil flux stabilizer built in this laboratory. Proton NMR field readings and ESR klystron frequencies were recorded about every 10 G and interpolated in between. Some of the room temperature spectra were measured during a second run by stopping on the center of many of the lines and recording the proton NMR and klystron frequencies. Either procedure allowed the measurement of splittings to f0.03 G and measurement of g to f0.00004, unless stated otherwise. All g values reported here have been corrected for second-order effects as described by Fessenden.8 All of the chemicals in this work were used as received. Tetralin was obtained from Aldrich; benzophenone, mxylene, and methylcyclohexane were from Eastman; oxylene was from J. T. Baker; and ethylbenzene from Matheson Coleman and Bell.
Results and Discussion 1. Tetrulin-1-yl a t Intermediate Temperatures and Related Radicals. The ESR spectrum of tetralin-1-yl was obtained at 30 and 78 "C from solutions of 0.2 M benzophenone in neat tetralin. Measurements at -33 "C were performed with a 0.15 M solution of benzophenone in tetralin with 40% (v/v) acetone to avoid freezing of the tetralin. These spectra were analyzed in terms of the couplings and g values listed in Table I. The spectrum of a-methylbenzyl at 30 "C was obtained with a solution of 0.4 M benzophenone in neat ethylbenzene. Measurements on m- and 0-xylyl were performed at 30 "C using 20% (v/v) solutions of di-tert-butyl peroxide in the appropriate xylene. The hyperfine splittings and g values of these related radicals are reported in Table I1 along with previously reported values for the benzyl r a d i ~ a l The . ~ line widths in all of these spectra were about 0.1 G peak-topeak. Dynamic polarization was observed in all cases, with the low field portions of the spectra appearing in diminished absorption or in emission. The proton couplings in the xylyls and in a-methylbenzyl (Table 11) were assigned by comparison with the couplings in the benzyl radical. The meta proton hfs values in 0-xylyl were not found to be equal, but they could not be directly resolved. The difference between these couplings was found from computer simulation to be 0.09 f 0.05 G. The average of the two couplings is accurate to f0.03 G. In m-xylyl the a protons have equal couplings to within 0.16 G; the average value is reported in Table I1 and is accurate to f0.03 G. The difference of the hfs of the ortho protons in m-xylyl is 0.16 f 0.09 G and was measured by computer simulation. Again, the average of the values is accurate to f0.03 G. Because the meta proton and meta methyl group in m-xylyl have nearly equal couplings, these values (Table 11) are accurate only to
benzyla
m-xylyl
0-xylyl
benzyl
meta
1.77
1.77
1.69
ortho
5.13
5.05, 5.21 6.15 16.18
1.73, 1.82 5.22
assignt
para CY
methyl substituent g value CI
6.17 16.34
1.66 2.00260
2.00258
6.27 15.66, 15.93 3.98 2.00265
4.81, 5.08 5.86 16.34 17.63 2.00262
From ref 9.
f0.05 G. However, the methyl splitting is definitely the smaller of the two. The assignment in Table I of the a, 5 , 6 , 7 , and 8 protons of tetralin-1-yl was made by comparison with the couplings of benzyl, a-methylbenzyl, and the xylyls. The 5 and 7 protons, not equivalent by symmetry, nevertheless have couplings equal to within 0.05 G. The y protons were assigned to the smallest splittings in tetralin-1-yl after comparison with typical y splittings of alkyl radicals.1° A dynamic process must be involved to explain the remaining two splittings in tetralin-1-yl. These each appear only as doublets while there are four remaining protons: 01, p2, 71, and 72. All of these are expected to have sizeable hyperfine couplings. Furthermore, the approximately 45-G splitting is too large to be due to a single p proton when compared with the methyl group hfs in a-methylbenzyl (Table 11). Evidently, an alternating line width effect occurs and makes many of the expected lines unobservableell This effect likely occurs through a process which interchanges the hfs of the two 0 protons as well as the hfs of the two 7 protons. For tetralin-1-yl, ring inversion is such a process. A simple model of the radical has the plane of the sp2 a carbon in the plane of the conjugated ring, maximizing T electron delocalization into the conjugated ring. This model indicates that the saturated portion of the ring may pucker the y carbon above or below the plane of the remaining nine carbon atoms. In each of these energetically equivalent conformations the two p hfs (and the two 7 hfs) are expected to be different. Inversion of the ring exchanges the hfs of the two protons as well as the two 17 protons. This process is equivalent to chemical exchange, in terms of the resulting ESR signals. As is usual, spin orientations are assumed to be unchanged after the inversion process. The effect of chemical exchange (or inversion) upon magnetic resonance lines has been calculated.12 At slow exchange rates the observed spectrum is a weighted sum of the spectra characteristic of each of the configurations. For the inversion studied here the conformations are equally probable and have identical spectra. Hence, the observed spectrum is characteristic of either conformation. At intermediate exchange rates, only the invariant lines (those whose positions are the same in both conformations) remain sharp; the other lines are broadened and are often unobservably weak. At high exchange rates the observed spectrum displays hfs constants equal to the average of the hfs constants in the two conformations. This corresponds to complete motional narrowing. For the ring inversion studied here, the fast exchange rate spectrum
The Journal of Physical Chemistry, Vol. 83, No. 5, 1979 635
Ring Inversion of Tetralin-1-yl
Figure 1. Comparison of spectra of tetralin-1-yi at (a) 78 and (b) 146 "C,showing effects of rapid ring inversion. The length of both traces is 14.7 G.
TABLE 111: 'Hfs and g Values of Tetralin-1-yl at - 69 and 146 C O
hfs, G proton assignt Y 597 8 6 a rll
112
Pl P2
P I -t P2 g value
-69
O
C
0.67 1.67 4.99 6.06 15.73 1.56 7.90 12.18 33.3
146 " C 0.58 1.73 4.96 6.05 15.70 4.73 4.73 44.61 2.00259
should display new, sharp lines of doubled intensity centered between invariant lines observed at intermediate rates. The tetralin-1-yl radical is in the intermediate exchange rate regime at -33,30, and 78 "C, as discussed above. Consequently, the couplings of approximately 9.47 and 45 G are assigned in Table I to the hfs value sums aVl aV2and agl + ap2,respectively. 2. Tetralin-1-yl ut High and Low Temperatures. The ESR spectrum of tetralin-1-yl at 146 "C was obtained from a 0.2 M solution of benzophenone in tetralin. Lines appear in this spectrum which are not present below about 100 "C. A comparison of the 78 and 146 "C spectra appears in Figure 1; the arrows point to new features at 146 "C. At the high temperature a hfs of 4.73 G is observed with an approximately 1:l:l intensity pattern. This hfs value is the average of awland aV2and is almost exactly half the value assigned to aVl + uV2at 78 "C (Table I). The abnormal intensity pattern indicates incomplete narrowing. A computer siinulation of the 146 "C spectrum agrees well with the experimental recording and indicates that the center lines from the p protons are not present. This too indicates incomplete narrowing. The couplings and g value at this temperature appear in Table 111. The spectrum of tetralin-1-yl at -69 "C was obtained in a low-freezing ,solution of 0.021 M benzophenone and 21% (v/v) tetralin in methylcyclohexane. This spectrum exhibits lines not present at and above -33 "C, as shown in Figure 2. Hence, this is the slow exchange limit with the observed spectrum characterizing the individual conformations. New couplings are observed and are assigned to be a,, = 1.56 G and a,? = 7.90 G. The sum of these values, 9.46 G, agrees well with the 9.48-G value assigned to aVl + uq2 at -33 "C (Table I). Another new coupling was observed, 12.113 G, and was assigned to upl. The dynamic polarization effect made the low field portion of this spectrum unusably weak and the spectral center was obscured by lines from Ph2COH. As a result neither agz nor agl ag2could be measured. Extrapolation of the weakly temperature dependent value of a + am from -33 "C yields a value of 45.5 f 0.2 G at -69 By subtraction am is 33.3 f 0.2 G. Because the new lines at -69 "C appear inside of the invariant lines, it may be concluded that a,, and aV2are of the same sign. Similarly, apl and ap2have the same sign.
+
+
, "d
Flgure 2. Comparison of spectra of tetralin-1-yl at (a) 30 and (b) -69 "C, showing effects of slow ring inversion. The length of both traces is 10.2 G.
The hyperfine couplings of tetralin-1-yl at -69 "C appear in Table 111. These data were obtained quickly while the cavity frequency was still drifting due to the lamp's heat input. This was done to minimize an opaque deposit on the face of the ESR cell which formed during photolysis of the solution containing methylcyclohexane. Consequently, the hfs reported at -69 "C are only accurate to about kO.8% or k0.03 G, whichever is larger. For tetralin-1-yl at these inversion rates second-order corrections must be performed by treating the ql,q2,Pl, and p2 protons separately as discussed by Fraenkel.13 The g values reported in Tables I and I11 have been corrected for second-order effects in this manner, using the hfs values of the q and p protons from -69 "C. The slight temperature dependence of these couplings causes negligible error. 3. Ring Inversion Rate. As the temperature was raised from -69 "C or lowered from 146 "C the new lines (not the invariant lines) broadened and correspondingly decreased in peak amplitude. A 10-G portion of the highest field end of the spectrum was recorded at seven temperatures between 100 and 136 "C. This region of the spectrum produced relatively strong signals without too many accidental overlaps of lines. The solutions had the same composition as that used at 146 "C. Similarly, a 10-G field interval was recorded at six temperatures between -69 and -42 "C. These recordings were used to determine the ring inversion rate. Following a published treatment of chemical exchange12 the ring conformations are labeled A and B. The unimolecular reaction rate for A B (and for B A, by symmetry) is 7A-l. Any particular ESR line in conformation A changes angular frequency after inversion to conformation B by an amount wAB, taken to be positive. The natural line width (half-width at half-amplitude of the assumed Lorentzian absorption curve) in angular frequency units is T2,0-1. This natural line width is the observed width of the invariant lines (urn= 0). The other lines broaden because of the inversion and have widths
-
-
provided @AB >> T2,0-1and WAB >> 7A-l.12 Hence, the excess width is 7A-l in the slow exchange regime. In the fast exchange regime (7A-l >> WAB) center lines appear between the invariant lines. These lines have widths12
The value of w a is given by the magnetogyric ratio of the radical times H A B , the magnitude of the algebraic difference of the two couplings aVland un2(or agl and aB2). For the q protons H a = 6.34 G and for the f? protons HAB = 21.1 G. Hence, at a given inversion rate, the center lines from the f? protons are expected to have 11times the excess
636 The Journal of Physical Chemistry, Vol. 83, No. 5, 1979
width of the center lines from the 77 protons. This explains I ' l l I l l the experimental absence of the 0proton center lines from the 146 "C spectrum. It is interesting that the y proton hfs are equal to within 0.04 G at -69 "C as~indicatedby the very nearly 1:2:1 intensity pattern. At this temperature there is very little excess broadening of any lines, so 7A-l < T2,0-1(see eq 1). Hence, the ring inversion rate cannot make the y protons appear equivalent. No other plausible process appears capable of exchanging these protons or their hfs. It must be that the y protons are equivalent by accident at -69 "C. This is surprising, given that most y hfs are sensitive to details of geometry.14 407 The excess width of the broadened lines was determined \ by computer simulation of the high field end of the spectrum, for both slow and fast inversion rates. The invariant lines were simulated with width T2,01chosen to match the experimental recordings and with the peak heights proportional to the nuclear spin multiplicity of each line. The broadened (or not fully narrowed) lines were simulated with widths from 1 to 3.6 times T2,0-1 in seven steps of 20%. Each of the simulations included both 30 40 50 ~ o ~ / TK,- I broadened and invariant lines. For small and fixed Figure 3. Arrhenius plot of the ring inversion rate 7A-1in the tetralin-1-yl modulation amplitudes in a first derivative recording, the radical. peak amplitude varies as the inverse square of the line width. The actual variation with the modulation field 6' is about 60" for the 71 and (31 protons. Because the employed was determined numerically from published function cos2 d is essentially equal to unity and indestudies and is slower than the inverse square depenpendent of angle, for angles near 180°, the hyperfine dence.15J6 These studies were also used to compute the couplings us2and ap2should be good estimates of p Q in eq effect of modulation on the peak-to-peak line widths. 3 for the 7 and p protons, respectively. Hence, the lines were simulated as derivatives of LorenSubstantial evidence for this last claim may be taken tizian lines with peak amplitudes and widths corrected for from the hfs values of 0-xylyl and a-methylbenzyl (Table modulation effects. The modulation field amplitude had 11). The rapidly rotating methyl groups (as shown by the been previously calibrated for the above procedures both equivalence of the three protons in each group) have time by direct measurement with an inductive pick-up coil and average values of cos2d equal to 112. If substitution effects by observation of the increase in width of an ESR line by do not change the spin density in the conjugated ring and overmodulation. The two methods agreed well. No if the 172 and (32 protons actually do reside near 0 = 180", corrections were made for partial saturation of the lines. then as2should be twice the methyl coupling in 0-xylyl and A low enough microwave power was used to avoid diffiap2should be twice the methyl coupling in a-methylbenzyl. culties with saturation. Indeed, this occurs in both cases to within 6%. The extent The experimental spectra were compared with the of substitution effects may be judged from the ring proton computer simulations. A best match was identified at each and a-proton splittings in the three radicals: tetralin-1-yl, temperature, yielding values of T2-l. In turn, 7 A - l was 0-xylyl, and a-methylbenzyl. The largest difference is determined through eq 1 or 2 , using Hm = 6.34 G. Figure about 8%. Hence, aV2and ap2are good choices for p Q in 3 is an Arrhenius plot of the inversion rate 7A-l. The eq 3. Arrhenius parameters for T A - ~= k exp(-E/RT) are E = The angles dsl and dol can be computed then directly 30 kJ/mol f 12% and log h = 13.1 f 0.7. The errors on from the ratios aq1/aq2 and apl/ap2.The values are 8,; = the parameters resulted in part from a f 5 "C uncertainty 64" and 4 , = 53'. Both of these values are in accord with in the fluid temperatures due to the heat input from the the assumption that dV2 and dp2 are about 180". The angles lamp and the flow distance to the thermocouple (3.5 cm). so determined are only as accurate as the cos2d dependence Further, the measurements of 7A-l at low temperatures are of hyperconjugative coupling and the absence of any other uncertain within +40%, -30% while 7 ~ - lmeasurements coupling mechanisms. The angular dependence of a t high temperatures are accurate to +60%, -38%. couplings has also been taken to bel9 4. Geometry of Tetralin-1-yl. The dependence of /3 proton couplings upon the azimuthal angle fl has been ~ ( 0= ) pQ(A + B cos2 d) (4) widely used to determine the geometries of radical species. with A N O.lB, instead of the dependence expressed in eq This azimuthal angle is the angle of rotation about the 3. These questions cannot be decided from the results C,-C, bond which brings the Co-H, bond into eclipse with reported here. the axis of the T orbital at C,. The -69 "C hfs values of 5, Couplings in Xylyl Radicals. Neta and Schuler the 71, 772, 81, and p2 protons may be employed to defound that p-methyl substitution of benzyl radical has little termine the geometry of tetralin-l-yl. For these purposes, effect on the spin density in the ring.g This is indicated C, functions as a "p' carbon and the ring carbon to which by the small changes (
