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Concerning the barrier to internal rotation in isopropylbenzene in solution
JOURNAL
OF MAGNETIC
RESONANCE
25,167-l
70 (1977)
Concerningthe Barrier to Internal Rotation in Isopropylbenzenein Solution T.SCHAEFER,WILLIAMJ. E.PARR,* ANDWERNERDANCHURA Department
of Chemistry,
University
of Manitoba,
Winnipeg,
Canada,
R3T 2N2
Received June 1,1976 The barrier to rotation about the sp*C-sp3C bond in 3,5-dibromoisopropylbenzene in CSZ solution is derived as 2.0 rfr 0.2 kcal/mole from the magnitude of the spin-spin coupling constant between the methine proton and the ring proton in the para position. It is assumed that the coupling, equal to -0.25 + 0.02 Hz, depends on the rotational angle in a simple manner and that the angle dependence can be predicted from a solution of the hindered-rotor problem using a twofold hindering potential. The derived value of the barrier is compared with values obtained from electron spin resonance data, from spin-lattice relaxation times, and from molecular orbital calculations for some related radicals and molecules. INTRODUCTION
Various values are available for the barrier to internal rotation in isopropylbenzene derivatives. It is usually assumed that 1 and 2 are low- and high-energy conformations, respectively, the barrier therefore being predominantly twofold in nature.
---;b__-
,CH,
---H-C<--
rrplane
CH'\CH 3 3
CH3
2
1
/H
---&C-C,
--CHs
3
For the&,$‘-triisopropyltrityl radical in toluene solution (I), the hyperfine coupling to the a proton (methine) depends (2) on (sin’ 0),,. Here 8 is the angle by which the methine C-H bond is twisted out of the aromatic plane and the average of sin2 8 is determined by the hindered internal rotation. The twofold barrier, V,, derived from a bindered-rotor treatment is 2.13 kcal/mole. For the anion radical ofp-nitroisopropylbenzene in polar solvents a similar treatment (3) of the hyperfine coupling data (4) yields V, as 1.7 kcal/mole. * Postdoctoral Fellow, 1974-1976. Copyright 0 1977 by Academic Press, Inc. All rights of reproduction in any form reserved. hinted in Great Britain
167
168
SCHAEFER,
PARR AND
DANCHURA
On the other hand, the hyperfine coupling data for the anion radical of p,p’-diisopropylbiphenyl in dimethoxyethane (5) have been interpreted to mean that 3 is 1.2 kcal/mole higher in energy than 1 and that 2 is only 0.5 kcal/mole less stable than 1. Calculations (5) based on a 6-12 potential are in reasonable agreement with this interpretation, although it may be noted that no allowance is made for bond bending in these calculations. A recent study (6) of the hyperfine coupling to the P-carbon nuclei (methyl carbons) in p-isopropylaniline nickel acetylacetonate complexes suggests 1.5 kcal/mole for I’,, while the temperature dependence of the methyl carbon spin-lattice relaxation time for p-isopropylaniline in chloroform solution indicates V, as 2.8 + 0.1 kcal/mole. In the same work (6) an INDO-MO calculation finds 2 as 3.5 kcal/mole less stable than 1. Now, the spin-spin coupling constant over six bonds, “J, between methyl protons and the ring proton in the para position in toluene displays a sin2 8 dependence (7), so that one may write 6J= A sin’ 8. It has been shown (8) that substitution to give ethylbenzene does not noticeably alter the magnitude of A. The invariance of A to substitution by a methyl group may be attributed to the small difference in electronegativity between a hydrogen atom and the methyl group which replaces it. When the hydrogen atom is substituted by the highly electronegative fluorine atom the magnitude of A is decreased by about 15% (9). On the assumption that substitution by two methyl groups to give isopropylbenzene does not alter A, a measurement of 6J should yield a value for I’,. A decrease in A of as much as 5 % would imply a barrier well within the estimated error of 10 % in V, below. Unfortunately, the proton magnetic resonance spectrum at 100 MHz of isopropylbenzene is too complex for a precise analysis. However, the spectrum of 3,5-dibromoisopropylbenzene is amenable to an accurate analysis. EXPERIMENTAL
The preparation of 3,5-dibromoisopropylbenzene by bromination of p-aminocumene followed the literature procedure exactly (10). The proton magnetic resonance spectrum was consistent only with a compound containing identical substituents at the positions meta to an isopropyl group. A 10 mole % solution in CS2, containing a small amount of tetramethylsilane, was degassed by the freeze-pump-thaw technique. The TABLE PROTON
VCH3 VCH V&6
V4
1
CHEMICAL SHIFTS'AND COUPLING CONSTANT@ SOLUTIONOF~,~-DIBROMOISOPROPYLBENZENEINCS~
121.0 280.0 721.230 (3) 738.088 (2)
3JH,CH3 “JE,” 4JlpH
6JpH*CH
FOR A 10 MOLE%
7.0 1.769 (3) -0.568 (7) -0.253 (5)
n In hertz at 100 MHz to low field of internal tetramethylsilane at 305 K. h In hertz, numbers in brackets giving the standard deviation in the last place, as found from the LAME analysis. The rms deviation between calculated and observed transition frequencies is 0.005 Hz. All peaks in the ring proton region were assigned.
INTERNAL
ROTATION
IN
ISOPROPYLBENZENE
169
proton magnetic resonance spectrum was calibrated on an HA-100 spectrometer in the frequency sweep mode at a probe temperature of 305 K. Linewidths of the peaks from H4 were typically about 0.12 Hz. The ring proton and methine proton resonance spectra were analyzed by means of the computer program LAME (II, 12). Coupling between the methyl protons and the ring protons was very small, manifesting itself as perhaps a slight broadening of the peaks from the protons ortho to the isopropyl group. The spectral parameters are given in Table 1. The important parameter is 6JF,CH and its standard deviation was 0.005 Hz. This value leads us to believe that 6JFyCHis known to an accuracy of 0.02 Hz so that the coupling becomes -0.25 I 0.02 Hz. RESULTS
AND
DISCUSSION
In toluene 6JF,CH is -0.62 Hz (13), so that A above is -1.24 Hz. In a variety of toluene derivatives (14,1.5) there is no evidence that 6Jis altered in the presence of 3,5substitution. For example, 6Jis -0.60 +_0.02 Hz in 3,5-dichlorotoluene. Furthermore, in 3,5-dibromo- and in 2,6-dichloroethylbenzene the magnitudes of 6J are consistent with an A of -1.24 Hz (8,15). On the assumption that A remains unchanged by substitution of a second methyl group, it follows that (sin’ 0) = 0.25/l .24 = 0.20. A treatment (9) based on the twofold hindered-rotor problem (26), using an unperturbed basis set of the eleven lowest free rotor functions, yields a plot of sin2 0 versus barrier at 305 K. For a reduced moment of inertia of 1.1 x 1O-38 g cm2 and for a lowenergy form 1, the twofold barrier is then found as 2.0 &-0.2 kcal/mole. The error is that arising from uncertainties in the measurement of the 6Jvalues only. The barrier is relatively insensitive to the reduced moment of inertia. The derived barrier value also assumes the absence of an angle independent contribution to 6J. In the 2,6-dichloro derivatives of benzal chloride and of benzal iodide, the twofold barriers to rotation are 15 and 21 kcal/mole, respectively (17, 18), so that the angle 0 should be very near zero. In fact, 6J is zero to within experimental error, suggesting the absence of an angle-independent contribution to 6J. Comparison with Other Data The closest agreement between the present value of 2.0 + 0.2 kcal/mole and that based on other measurements occurs for the neutral trityl radical (I), for which the twofold barrier to rotation is quoted as 2.13 kcal/mole. The assumption of a double minimum in the rotational barrier for p,p’-diisopropylbiphenyl anion (5), apparently supported by the calculations based on the 6-12 potential, is questionable. In similar cl,a-disubstituted toluene derivatives (19), double minima appear in the calculated potential curves if bond angles are not permitted to relax during the rotation; but they disappear (18,20) if bond angle relaxation is allowed in the energy minimization procedure. It is interesting to note that the value for (sin2 0> in the biphenyl radical is 0.21, very similar to the number found here for 3,5-dibromoisopropylbenzene (5). Finally, it is of interest that for p-alkylphenylthallium trifluoroacetates (21) in solution 6JH,T1for the methyl, ethyl, and isopropyl derivatives has values whose ratios give sir?‘0 numbers in reasonable agreement with the numbers found from 6JH*CHin 3,5-
170
SCHAEFER,
PARR
AND
DANCHURA
ethylbenzene (8) and 3,5-dibromoisopropylbenzene. 6JH*T1is apparently two orders of magnitude larger than the corresponding 6JH,CH. Th e accurate determination in appropriate derivatives of JHjT1 as a function of temperature should allow a further test of the present method of determining relatively small barriers to internal rotation. The 6JH-CH are somewhat too small to be employed in this manner because nonuniform temperature changes of sample and probe inevitably would produce some broadening of the resonance peaks. ACKNOWLEDGMENT We
are grateful to the National Research Council of Canada for financial assistance. REFERENCES
1. N. L. BAULD, C. E. HUDSON, AND J. S. HYDE, J. Chem. Whys. 54,1834 (1971). 2. C. HELLER AND H. M. MCCONNELL, J. Chem. P/zys 32,1535 (1960). 3. L. M. STOCK AND P. E. YOUNG, J. Amer. Chem. Sot. 94,7686 (1972). 4. T. M. MCKINNEY AND D. H. GESKE, J. Amer. Chem. Sot. 89,2806 (1967). 5. F. NEMOTO, F. SHIMODA, AND K. ISHIZU, Bull. Chem. Sot. Japan 48,2627 (1975). 6. C. CHACHATY, A. FORCHIONI, AND J. C. RONFARD-HARET, Mol. Whys. 31,325 (1976). 7. R. WASYLISHEN AND T. SCHAEFER, Canad. J. Chem. 50,1852 (1972), and references therein. 8. T. SCHAEFER, L. KRUCZYNSKI, AND W. NIEMCZURA, Chem. Phys. Lett. 38,498 (1976). 9. T. SCHAEFER, J. B. ROWBOTHAM, W. J. E. PARR, K. MARAT, AND A. F. JANZEN, Canad. J. Chem. 54, 1322 (1976). 10. R. A. BENKESER, R. A. HICKNER, D. I. HOKE, AND 0. H. THOMAS, J. Amer. Chem. Sot. SO,5289 (1958). II. S. CASTELLANO AND A. A. BOTHNER-BY, J. Chem. Phys. 41,3863 (1964). 12. C. W. HAIGH AND J. M. WILLIAMS, J. Mol. Spectrosc. 32,398 (1969). 13. M. P. WILLIAMSON, R. J. KOSTELNIK, AND S. M. CASTELLANO, J. Chem. Phys. 49,2218 (1968). 14. D. G. GEHRING AND G. S. REDDY, Anal. Chem. 40,792 (1968). 1.5. A. F. JANZEN AND T. SCHAEFER, Canad. J. Chem. 49,181s (1971). 16. P. B. AYSCOUGH, M. C. PRICE, AND R. E. D. MCCLUNG, Mol. Phys. 20,41(1971). 17. T. SCHAEFER, R. SCHWENK, AND C. J. MACDONALD, Canad. J. Chem. 46,2187 (1968). 18. J. PEELING, J. B. ROWBOTHAM, L. ERNST, AND T. SCHAEFER, Canad. J. Chem. 52,2414 (1974). 19. B. H. BARBER AND T. SCHAEFER, Canad. J. Chem. 49,789 (1971). 20. A. MANNSCHRECK AND L. ERNST, Chem. Ber. 104,228 (1971). 21. L. ERNST, J. Organometal. Chem. 82,319 (1974).
OF MAGNETIC
RESONANCE
25,167-l
70 (1977)
Concerningthe Barrier to Internal Rotation in Isopropylbenzenein Solution T.SCHAEFER,WILLIAMJ. E.PARR,* ANDWERNERDANCHURA Department
of Chemistry,
University
of Manitoba,
Winnipeg,
Canada,
R3T 2N2
Received June 1,1976 The barrier to rotation about the sp*C-sp3C bond in 3,5-dibromoisopropylbenzene in CSZ solution is derived as 2.0 rfr 0.2 kcal/mole from the magnitude of the spin-spin coupling constant between the methine proton and the ring proton in the para position. It is assumed that the coupling, equal to -0.25 + 0.02 Hz, depends on the rotational angle in a simple manner and that the angle dependence can be predicted from a solution of the hindered-rotor problem using a twofold hindering potential. The derived value of the barrier is compared with values obtained from electron spin resonance data, from spin-lattice relaxation times, and from molecular orbital calculations for some related radicals and molecules. INTRODUCTION
Various values are available for the barrier to internal rotation in isopropylbenzene derivatives. It is usually assumed that 1 and 2 are low- and high-energy conformations, respectively, the barrier therefore being predominantly twofold in nature.
---;b__-
,CH,
---H-C<--
rrplane
CH'\CH 3 3
CH3
2
1
/H
---&C-C,
--CHs
3
For the&,$‘-triisopropyltrityl radical in toluene solution (I), the hyperfine coupling to the a proton (methine) depends (2) on (sin’ 0),,. Here 8 is the angle by which the methine C-H bond is twisted out of the aromatic plane and the average of sin2 8 is determined by the hindered internal rotation. The twofold barrier, V,, derived from a bindered-rotor treatment is 2.13 kcal/mole. For the anion radical ofp-nitroisopropylbenzene in polar solvents a similar treatment (3) of the hyperfine coupling data (4) yields V, as 1.7 kcal/mole. * Postdoctoral Fellow, 1974-1976. Copyright 0 1977 by Academic Press, Inc. All rights of reproduction in any form reserved. hinted in Great Britain
167
168
SCHAEFER,
PARR AND
DANCHURA
On the other hand, the hyperfine coupling data for the anion radical of p,p’-diisopropylbiphenyl in dimethoxyethane (5) have been interpreted to mean that 3 is 1.2 kcal/mole higher in energy than 1 and that 2 is only 0.5 kcal/mole less stable than 1. Calculations (5) based on a 6-12 potential are in reasonable agreement with this interpretation, although it may be noted that no allowance is made for bond bending in these calculations. A recent study (6) of the hyperfine coupling to the P-carbon nuclei (methyl carbons) in p-isopropylaniline nickel acetylacetonate complexes suggests 1.5 kcal/mole for I’,, while the temperature dependence of the methyl carbon spin-lattice relaxation time for p-isopropylaniline in chloroform solution indicates V, as 2.8 + 0.1 kcal/mole. In the same work (6) an INDO-MO calculation finds 2 as 3.5 kcal/mole less stable than 1. Now, the spin-spin coupling constant over six bonds, “J, between methyl protons and the ring proton in the para position in toluene displays a sin2 8 dependence (7), so that one may write 6J= A sin’ 8. It has been shown (8) that substitution to give ethylbenzene does not noticeably alter the magnitude of A. The invariance of A to substitution by a methyl group may be attributed to the small difference in electronegativity between a hydrogen atom and the methyl group which replaces it. When the hydrogen atom is substituted by the highly electronegative fluorine atom the magnitude of A is decreased by about 15% (9). On the assumption that substitution by two methyl groups to give isopropylbenzene does not alter A, a measurement of 6J should yield a value for I’,. A decrease in A of as much as 5 % would imply a barrier well within the estimated error of 10 % in V, below. Unfortunately, the proton magnetic resonance spectrum at 100 MHz of isopropylbenzene is too complex for a precise analysis. However, the spectrum of 3,5-dibromoisopropylbenzene is amenable to an accurate analysis. EXPERIMENTAL
The preparation of 3,5-dibromoisopropylbenzene by bromination of p-aminocumene followed the literature procedure exactly (10). The proton magnetic resonance spectrum was consistent only with a compound containing identical substituents at the positions meta to an isopropyl group. A 10 mole % solution in CS2, containing a small amount of tetramethylsilane, was degassed by the freeze-pump-thaw technique. The TABLE PROTON
VCH3 VCH V&6
V4
1
CHEMICAL SHIFTS'AND COUPLING CONSTANT@ SOLUTIONOF~,~-DIBROMOISOPROPYLBENZENEINCS~
121.0 280.0 721.230 (3) 738.088 (2)
3JH,CH3 “JE,” 4JlpH
6JpH*CH
FOR A 10 MOLE%
7.0 1.769 (3) -0.568 (7) -0.253 (5)
n In hertz at 100 MHz to low field of internal tetramethylsilane at 305 K. h In hertz, numbers in brackets giving the standard deviation in the last place, as found from the LAME analysis. The rms deviation between calculated and observed transition frequencies is 0.005 Hz. All peaks in the ring proton region were assigned.
INTERNAL
ROTATION
IN
ISOPROPYLBENZENE
169
proton magnetic resonance spectrum was calibrated on an HA-100 spectrometer in the frequency sweep mode at a probe temperature of 305 K. Linewidths of the peaks from H4 were typically about 0.12 Hz. The ring proton and methine proton resonance spectra were analyzed by means of the computer program LAME (II, 12). Coupling between the methyl protons and the ring protons was very small, manifesting itself as perhaps a slight broadening of the peaks from the protons ortho to the isopropyl group. The spectral parameters are given in Table 1. The important parameter is 6JF,CH and its standard deviation was 0.005 Hz. This value leads us to believe that 6JFyCHis known to an accuracy of 0.02 Hz so that the coupling becomes -0.25 I 0.02 Hz. RESULTS
AND
DISCUSSION
In toluene 6JF,CH is -0.62 Hz (13), so that A above is -1.24 Hz. In a variety of toluene derivatives (14,1.5) there is no evidence that 6Jis altered in the presence of 3,5substitution. For example, 6Jis -0.60 +_0.02 Hz in 3,5-dichlorotoluene. Furthermore, in 3,5-dibromo- and in 2,6-dichloroethylbenzene the magnitudes of 6J are consistent with an A of -1.24 Hz (8,15). On the assumption that A remains unchanged by substitution of a second methyl group, it follows that (sin’ 0) = 0.25/l .24 = 0.20. A treatment (9) based on the twofold hindered-rotor problem (26), using an unperturbed basis set of the eleven lowest free rotor functions, yields a plot of sin2 0 versus barrier at 305 K. For a reduced moment of inertia of 1.1 x 1O-38 g cm2 and for a lowenergy form 1, the twofold barrier is then found as 2.0 &-0.2 kcal/mole. The error is that arising from uncertainties in the measurement of the 6Jvalues only. The barrier is relatively insensitive to the reduced moment of inertia. The derived barrier value also assumes the absence of an angle independent contribution to 6J. In the 2,6-dichloro derivatives of benzal chloride and of benzal iodide, the twofold barriers to rotation are 15 and 21 kcal/mole, respectively (17, 18), so that the angle 0 should be very near zero. In fact, 6J is zero to within experimental error, suggesting the absence of an angle-independent contribution to 6J. Comparison with Other Data The closest agreement between the present value of 2.0 + 0.2 kcal/mole and that based on other measurements occurs for the neutral trityl radical (I), for which the twofold barrier to rotation is quoted as 2.13 kcal/mole. The assumption of a double minimum in the rotational barrier for p,p’-diisopropylbiphenyl anion (5), apparently supported by the calculations based on the 6-12 potential, is questionable. In similar cl,a-disubstituted toluene derivatives (19), double minima appear in the calculated potential curves if bond angles are not permitted to relax during the rotation; but they disappear (18,20) if bond angle relaxation is allowed in the energy minimization procedure. It is interesting to note that the value for (sin2 0> in the biphenyl radical is 0.21, very similar to the number found here for 3,5-dibromoisopropylbenzene (5). Finally, it is of interest that for p-alkylphenylthallium trifluoroacetates (21) in solution 6JH,T1for the methyl, ethyl, and isopropyl derivatives has values whose ratios give sir?‘0 numbers in reasonable agreement with the numbers found from 6JH*CHin 3,5-
170
SCHAEFER,
PARR
AND
DANCHURA
ethylbenzene (8) and 3,5-dibromoisopropylbenzene. 6JH*T1is apparently two orders of magnitude larger than the corresponding 6JH,CH. Th e accurate determination in appropriate derivatives of JHjT1 as a function of temperature should allow a further test of the present method of determining relatively small barriers to internal rotation. The 6JH-CH are somewhat too small to be employed in this manner because nonuniform temperature changes of sample and probe inevitably would produce some broadening of the resonance peaks. ACKNOWLEDGMENT We
are grateful to the National Research Council of Canada for financial assistance. REFERENCES
1. N. L. BAULD, C. E. HUDSON, AND J. S. HYDE, J. Chem. Whys. 54,1834 (1971). 2. C. HELLER AND H. M. MCCONNELL, J. Chem. P/zys 32,1535 (1960). 3. L. M. STOCK AND P. E. YOUNG, J. Amer. Chem. Sot. 94,7686 (1972). 4. T. M. MCKINNEY AND D. H. GESKE, J. Amer. Chem. Sot. 89,2806 (1967). 5. F. NEMOTO, F. SHIMODA, AND K. ISHIZU, Bull. Chem. Sot. Japan 48,2627 (1975). 6. C. CHACHATY, A. FORCHIONI, AND J. C. RONFARD-HARET, Mol. Whys. 31,325 (1976). 7. R. WASYLISHEN AND T. SCHAEFER, Canad. J. Chem. 50,1852 (1972), and references therein. 8. T. SCHAEFER, L. KRUCZYNSKI, AND W. NIEMCZURA, Chem. Phys. Lett. 38,498 (1976). 9. T. SCHAEFER, J. B. ROWBOTHAM, W. J. E. PARR, K. MARAT, AND A. F. JANZEN, Canad. J. Chem. 54, 1322 (1976). 10. R. A. BENKESER, R. A. HICKNER, D. I. HOKE, AND 0. H. THOMAS, J. Amer. Chem. Sot. SO,5289 (1958). II. S. CASTELLANO AND A. A. BOTHNER-BY, J. Chem. Phys. 41,3863 (1964). 12. C. W. HAIGH AND J. M. WILLIAMS, J. Mol. Spectrosc. 32,398 (1969). 13. M. P. WILLIAMSON, R. J. KOSTELNIK, AND S. M. CASTELLANO, J. Chem. Phys. 49,2218 (1968). 14. D. G. GEHRING AND G. S. REDDY, Anal. Chem. 40,792 (1968). 1.5. A. F. JANZEN AND T. SCHAEFER, Canad. J. Chem. 49,181s (1971). 16. P. B. AYSCOUGH, M. C. PRICE, AND R. E. D. MCCLUNG, Mol. Phys. 20,41(1971). 17. T. SCHAEFER, R. SCHWENK, AND C. J. MACDONALD, Canad. J. Chem. 46,2187 (1968). 18. J. PEELING, J. B. ROWBOTHAM, L. ERNST, AND T. SCHAEFER, Canad. J. Chem. 52,2414 (1974). 19. B. H. BARBER AND T. SCHAEFER, Canad. J. Chem. 49,789 (1971). 20. A. MANNSCHRECK AND L. ERNST, Chem. Ber. 104,228 (1971). 21. L. ERNST, J. Organometal. Chem. 82,319 (1974).