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Concerning the long-range proton-proton spin-spin coupling constants and the internal rotation barrier in styrene
JOURNAL
OF MOLECULAR
SPECTROSCOPY
61,479-480
(1976)
Concerning The Long-Range Proton-Proton Constants and the Internal Rotation
Spin-Spin Coupling Barrier in Styrene
A thermodynamic value for the internal rotational barrier in styrene is 2.2 kcal/mole (I), while a far-infrared study implied an upper limit of 0.5 kcal/mole (2). A Raman investigation of a torsional overtone in the gas phase yields a predominantly twofold barrier of 1.78f 0.02 kcal/mole (3), the planar form being stable, in agreement with photoelectron data (4). We have shown (5) that twofold barriers between 0.2 and ‘2 kcal/mole can be deduced for benzene derivatives in solution from the magnitudes of long-range nuclear spin-spin coupling constants, J, between nuclei in the side chain and ring protons situated para to the suhstituent. In 1, the rou~>lin~: over six bonds may be written as 6J = Jgo sin%9 for various X and
if 1
8 = 0’ when the X-H bond lies in the plane of the ring. The average value of sin% is determined h! the barrier and can be matched to a calculated value obtained from a hindered rotor treatment (5). In 3-bromostyrene (6), OJ is -0.25 f 0.04 Hz at ambient temperatures. The coupling is insensitive to intrinsic (nonsteric) substituent effects, as witnessed by its value of -0.24 Hz in 2-vinylpyridine (7). In 3-vinyl-6-methylpyridine 7JCH3,CH= h as a magnitude of 0.25 f 0.03 Hz, confirming the c--?r mechanism for 6J and its sin% dependence (7). INDO-MO-FPT calculations (6) of 6J yield -0.24 Hz for 0 = O”, consistent with delocalization through the extended conjugated ?r system. However, such calculations (8) also predict -0.29 Hz for planar benzaldehyde in which the internal barrier is near 5 kcal/mole (Y, IO). Such a barrier implies a very small 6J according to a hindered rotor treatment and the experimental value is indeed unobservably small (II), indicating that the calculations overestimate “J sub stantially in benzaldehyde and, by analogy, in styrene when 0 = 0”. An alternative explanation of 6J in styrene assumes that 6J = 0 at 0 = O’, that the barrier is small, and therefore that (sir?@ is nonzero at room temperature. A value of 6J at e = 90’ is given by our INDO calculations using Pitzer’s geometry (1) as - 1.00 Hz, while a standard geometry (6) yields - 1.05 Hz. For toluene, similar calculations (12) agree exactly with experiment and, for toluene and styrene, the non-sin’% component of the calculated 9r vanishes at 0 = 90’. Then (sin*@ is 0.24 & 0.04 and a reduced moment of inertia of 0.1997 X 1O-3s g cm2 (1) yields 1.6 f 0.3 kcal/mole as the internal barrier in solution. The fitting procedure assumes a temperature of 300 K. The result agrees with the gas-phase Raman deduction and with an ab initio MO value of 1.9 kcal/mole (13). Because the dipole moment of styrene is negligible and because the r-electron distribution is uniform (IS), a large solvent influence on the internal barrier is unlikely. ACKNOWLEDGMENT We thank
the National
Research
Council
of Canada 470
Copyright hll
rights
0
1Y76 by Academic
of reproduction
Press.
in any form
Inc. reserved.
for financial
support.
480
NOTES REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
K. S. PITZER, L. GUTTMAN, AND E. F. WESTRUM, JR., J. Amer. Ckem. Sot. 68, 2209 (1946). W. G. FATELEY, G. L. CARLSON, AND F. E. DICKSON, Appl. Spectrosc. 22, 650 (1968). L. A. CARREIRA AND T. G. TOWNS, J. Chem. Phys. 63, 5283 (1975). J. P. MAIER ANJJD. W. TURNER, J. Chem. Sot. Faraday ZZ 196 (1973). T. SCHAEFER, J. B. ROWBOTHAM,W. J. E. PARR, K. MARAT, AND A. F. JANZEN, Can. J. Chem. 54 (1976). M. BARFIELD, C. J. MACDONALD, I. R. PEAT, AND W. F. REYNOLDS, J. Amer. Chem. Sot. 93,4195 (1971). J. B. ROWBOTHAMAND T, SCHAEFER,Can. J. Chem. 52, 136 (1974). R. WAYLISHEN AND T. SCHAEFER,Can. J. Chem. 49,3216 (1971). R. K. KAKAR, E. A. RINEHART, C. R. QUADE, AND T. KOJIMA, J. Chem. Phys. 52, 3803 (1970). W. G. FATELEY, R. K. HARRIS, F. A. MILLER, AND R. E. WITKOWSKI, Spectrochim. Ada 21, 231 (1965). R. J. KOSTELNIK, M. P. WILLIAMSON, D. E. WISNOSKY, AND S. M. CASTELLANO,Can. J. Chem. 47, 3313 (1969). R. E. WASYLISHEN AND T. SCHAEFER,Can. J. Chem. 50, 1852 (1972). W. J. HEHRE, L. RADOY, AND J. A. POPLE, J. Amer. Chem. Sot. 94, 1496 (1972). TED SCHAEFERAND WILLIAM J. E. PARRI
Department of Chemistry University of Manitoba Winnipeg, Manitoba Canada, R3T 2NZ Received March 12, 1976 1 Postdoctoral Fellow, 1974-76.
OF MOLECULAR
SPECTROSCOPY
61,479-480
(1976)
Concerning The Long-Range Proton-Proton Constants and the Internal Rotation
Spin-Spin Coupling Barrier in Styrene
A thermodynamic value for the internal rotational barrier in styrene is 2.2 kcal/mole (I), while a far-infrared study implied an upper limit of 0.5 kcal/mole (2). A Raman investigation of a torsional overtone in the gas phase yields a predominantly twofold barrier of 1.78f 0.02 kcal/mole (3), the planar form being stable, in agreement with photoelectron data (4). We have shown (5) that twofold barriers between 0.2 and ‘2 kcal/mole can be deduced for benzene derivatives in solution from the magnitudes of long-range nuclear spin-spin coupling constants, J, between nuclei in the side chain and ring protons situated para to the suhstituent. In 1, the rou~>lin~: over six bonds may be written as 6J = Jgo sin%9 for various X and
if 1
8 = 0’ when the X-H bond lies in the plane of the ring. The average value of sin% is determined h! the barrier and can be matched to a calculated value obtained from a hindered rotor treatment (5). In 3-bromostyrene (6), OJ is -0.25 f 0.04 Hz at ambient temperatures. The coupling is insensitive to intrinsic (nonsteric) substituent effects, as witnessed by its value of -0.24 Hz in 2-vinylpyridine (7). In 3-vinyl-6-methylpyridine 7JCH3,CH= h as a magnitude of 0.25 f 0.03 Hz, confirming the c--?r mechanism for 6J and its sin% dependence (7). INDO-MO-FPT calculations (6) of 6J yield -0.24 Hz for 0 = O”, consistent with delocalization through the extended conjugated ?r system. However, such calculations (8) also predict -0.29 Hz for planar benzaldehyde in which the internal barrier is near 5 kcal/mole (Y, IO). Such a barrier implies a very small 6J according to a hindered rotor treatment and the experimental value is indeed unobservably small (II), indicating that the calculations overestimate “J sub stantially in benzaldehyde and, by analogy, in styrene when 0 = 0”. An alternative explanation of 6J in styrene assumes that 6J = 0 at 0 = O’, that the barrier is small, and therefore that (sir?@ is nonzero at room temperature. A value of 6J at e = 90’ is given by our INDO calculations using Pitzer’s geometry (1) as - 1.00 Hz, while a standard geometry (6) yields - 1.05 Hz. For toluene, similar calculations (12) agree exactly with experiment and, for toluene and styrene, the non-sin’% component of the calculated 9r vanishes at 0 = 90’. Then (sin*@ is 0.24 & 0.04 and a reduced moment of inertia of 0.1997 X 1O-3s g cm2 (1) yields 1.6 f 0.3 kcal/mole as the internal barrier in solution. The fitting procedure assumes a temperature of 300 K. The result agrees with the gas-phase Raman deduction and with an ab initio MO value of 1.9 kcal/mole (13). Because the dipole moment of styrene is negligible and because the r-electron distribution is uniform (IS), a large solvent influence on the internal barrier is unlikely. ACKNOWLEDGMENT We thank
the National
Research
Council
of Canada 470
Copyright hll
rights
0
1Y76 by Academic
of reproduction
Press.
in any form
Inc. reserved.
for financial
support.
480
NOTES REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
K. S. PITZER, L. GUTTMAN, AND E. F. WESTRUM, JR., J. Amer. Ckem. Sot. 68, 2209 (1946). W. G. FATELEY, G. L. CARLSON, AND F. E. DICKSON, Appl. Spectrosc. 22, 650 (1968). L. A. CARREIRA AND T. G. TOWNS, J. Chem. Phys. 63, 5283 (1975). J. P. MAIER ANJJD. W. TURNER, J. Chem. Sot. Faraday ZZ 196 (1973). T. SCHAEFER, J. B. ROWBOTHAM,W. J. E. PARR, K. MARAT, AND A. F. JANZEN, Can. J. Chem. 54 (1976). M. BARFIELD, C. J. MACDONALD, I. R. PEAT, AND W. F. REYNOLDS, J. Amer. Chem. Sot. 93,4195 (1971). J. B. ROWBOTHAMAND T, SCHAEFER,Can. J. Chem. 52, 136 (1974). R. WAYLISHEN AND T. SCHAEFER,Can. J. Chem. 49,3216 (1971). R. K. KAKAR, E. A. RINEHART, C. R. QUADE, AND T. KOJIMA, J. Chem. Phys. 52, 3803 (1970). W. G. FATELEY, R. K. HARRIS, F. A. MILLER, AND R. E. WITKOWSKI, Spectrochim. Ada 21, 231 (1965). R. J. KOSTELNIK, M. P. WILLIAMSON, D. E. WISNOSKY, AND S. M. CASTELLANO,Can. J. Chem. 47, 3313 (1969). R. E. WASYLISHEN AND T. SCHAEFER,Can. J. Chem. 50, 1852 (1972). W. J. HEHRE, L. RADOY, AND J. A. POPLE, J. Amer. Chem. Sot. 94, 1496 (1972). TED SCHAEFERAND WILLIAM J. E. PARRI
Department of Chemistry University of Manitoba Winnipeg, Manitoba Canada, R3T 2NZ Received March 12, 1976 1 Postdoctoral Fellow, 1974-76.