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Solvent effects on the internal proton chemical shifts of nitrobenzene, aniline and halobenzenes
JOURNAL
OF MAGNETIC
RESONANCE
3, 269-217
(1970)
Solvent Effects on the Internal Proton Chemical Shifts of Nitrobenzene, Aniline and Halobenzenes LARRY Department
of Chemistry
G. ROBINSON AND GEORGE B. SAVITSKY and Geology,
Clemson
University,
Clemson,
South
Carolina
29631
Received October 13, 1969; accepted January 16, 1970 The internal chemical shifts of six aromatic molecules (CeH5X, X=N02, NH2,F,Cl,Br,I) were determined in ten different solvents, by using mixtures of these molecules which were specifically labeled with deuterium. The internal chemical shifts showed a good linear relationship with the dielectric function formulated by the reaction field theory over a wide range of dielectric constant. Although the results obtained were generally in poor quantitative agreement with the reaction field theory, indicating that the effects of other specific solute solvent interactions are not entirely cancelled out by internal referencing technique, they appear to support the basic validity of the reaction field theory. INTRODUCTION
The contribution of a solvent to the screening constant of a proton can effectively be separatedinto five different effects due to (1) bulk susceptibility (I), (2) (2) magnetic anisotropy of solvent m o lecules (2) (3) van der W a a ls or dispersion interactions (I), (2) (3), (4), (4) reaction field (5) and (5) complex formation, or in general,any specific solute-solvent interactions. This paper will be concerned with the contribution of the reaction field to the proton chemical shifts of six aromatic m o lecules,C,H,X(X=NO,, NH,,F, Cl, Br, I). The reaction field created by a polar solute in a polarizable solvent, according to Onsager (6), can be representedby the equation E=r W-1) E---l 3a E+l/2nZp where /.L,n, and CIare, respectively, the dipole moment, refractive index, and polarizability of the solute and c is the dielectric constant of the m e d ium. Buckingham (5) adopting the above expression for E, derived the expression CT,== -kEcosB-k’E2
PI
to representthe reaction field contribution to the screeningconstant. In Eq. [2] E cos 8 is the component of E in the bond direction, k = 2 x lo-“, and k’ = lOdIE. Onsager’sexpressionfor E given in Eq. [l] was later m o d ified by Diehl and Freeman (7), who, in order to take into account the m o lecular shape of the solute, extendedthe simple theory of considering the solute as a polarizable dipole in a spherical cavity to treating the solute as a polarizable dipole in an ellipsoidal cavity. According to Diehl 269
270
ROBINSON
AND
SAVITSKY
and Freeman (7), E is given by E=
(
$
>
3 5, [l
+(n2- 1) &I w
PI
where a, b, and c are the semi-axes of the cavity ellipsoid, 2 (E) is given by
and <, is the shape parameter, which is a function of a, b, and c. It has been shown that the E* term in Eq. [2] negligible (8); therefore, a plot of chemical shift versus Z (E) should result in a straight line. Linear relationships have been observed in the correlation of chemical shifts versus Z (E) for solutes such as (1) substituted ethylenes (9), Uo), (II), (2) paraldehyde (7), (3) acetonitrile (2), and (4) meta-dinitrobenzene (12). On the other hand, some solutes (1) cis-2,6-dibromo-4,4-diphenylcyclohexanone and 2-bromo-4,4-diphenylcyclohexanone (8), (2) alkyl ketones (13), and (3) substituted ethylenes (14), exhibited nonlinear correlations. EXPERIMENTAL
In order to observe the proton chemical shifts of the monosubstituted benzenes a deuteration technique, described previously (15), was used. The preparation of aniline was accomplished by the hydrogenation of the mixture of isotopomers of nitrobenzene in the presence of a palladium on carbon catalyst. This preparation was used to avoid hydrogen-deuterium exchange which occurs in the normal reduction of nitrobenzene with Sn and HCl. Fluorobenzene and chlorobenzene were prepared by diazotization of the isotopomers of aniline. Bromobenzene was prepared by the direct bromination of pentadeuterobenzene using AICI, as a catalyst. Iodobenzene was prepared by the iodine decomposition of phenyl magnesium bromide which was prepared from the bromobenzene isotopomer mixture. Purity, chemical and isotopic, of the compounds was evidenced by the absence of any extraneous peaks in their nmr spectra. Proton chemical shifts were calibrated by the normal side band technique and were recorded on a Varian A-60 nmr spectrometer. The shifts were measured in Hz relative to TMS as an internal standard. Each measured chemical shift was an average of two forward and two reverse sweeps. To eliminate line broadening due to deuterium coupling the NMR Specialties Inc., HD-60 Heteronuclear Spin Decoupler was used which increased the probe temperature to approximately 52°C. Infinite dilution values were determined by measuring the chemical shifts at three different concentrations (approximately 15, 7, and 3 per cent weight-volume) and by extrapolating to infinite dilution a plot of chemical shift versus concentration. Over the concentration range studied all plots were linear, within experimental error. The error of the extrapolated value is estimated to be in the range of 0.2-0.5 Hz. RESULTS
AND
DISCUSSION
The proton chemical shifts of each monosubstituted benzene were determined in ten different solvents which were chosen to cover a wide range of dielectric constant. Table 1 gives the ortho, meta, and para chemical shifts at infinite dilution in each of the solvents as well as the dielectric constants of the solvents at 52°C.
n-Hexane Cyclohexane Carbon Tetrachloride Ethyl Ether Ethyl Acetate Methyl Acetate Methylene Chloride Paraldehyde Cyclohexanone Acetone
1.84” 1.97” 2.18” 3.79” 5.65” 6.62” 7.96” 12.10” 17.45” 18.22”
489.5 488.9 492.2 492.4 494.2 495.8 492.2 493.3 493.5 494.3
443.9 443.6 450.7 452.7 457.8 459.7 453.0 459.5 459.6 460.2
451.2 451.2 458.0 460.7 466.1 468.2 462.1 468.4 468.3 469.2
388.6” 388.8 391.2 392.5 395.1 396.0 398.8 398.2 396.0 398.2
ortho
CHEMICAL
Nitrobenzene -__~ ortho meta para
VALUES OF PROTON
(52”)
DILUTION
419.1h 419.0 420.8 418.6 419.4 420.0 426.4 421.7 419.0 421.0
meta
Aniline
396.0” 395.7 396.6 391.8 392.3 392.8 401.6 394.6 390.7 393.2
para
SHIFTS IN Hz
420.0 434.7
436.2 437.4 441.2 442.2 440.2
423.6 425.1 428.6 429.3 427.7
437.3 435.4 434.3 438.4 441.0 439.4 441.9 440.6 440.4 438.0
429.0
429.2
423.6 441.2 428.4 440.3 440.0 424.4 442.2 429.0 441.2 440.9 426.0 443.6 430.6 442.7 442.2
419.3 420.6 424.0 424.8 423.0
BENZENES
446.7 448.0 450.3 451.3 449.7
444.0
444.6
430.4 431.5 435.8 437.2 434.2
425.5
425.7
417.6
meta
433.2 434.5 438.6 440.0 437.8
459.2 460.7 463.0 463.8 462.4
422.7 423.7 427.6 428.6 426.3
428.8 456.8 417.6
428.9 451.5
ortho
Iodobcnrene
TMS
441.8 441.8 443.2
435.4 437.0 440.6 441.7 439.8
430.9
431.1
para
at this concentration
436.6 449.7 436.4 439.6 461.9 428.6 437.1 450.7 437.9 440.3 462.9 429.2 438.5 452.0 438.7 441.4 464.3 430.3
430.4 431.8 435.6 436.8 434.9
425.6
TO INTEKNAL
Bromobenzene - --~ ortho meta para
RELATIVE
425.9
Chlorobenzene ____~~~~__ ortho meta para
415.1 431.6 419.4 434.1
415.6 432.i
Fluorobenzene -__ ortho meta para
OP THE MONOSUBSTITUTED
1
““Handbook of Chemistry and Physics,” (R. C. Weast, ed.), 48th Ed., The Chemical Rubber Company, Cleveland (1967). SDue to low solubility only one sample (3.21% weight-volume) could be run; therefore, the chemical shifts values obtained were taken as infinite dilution. “‘1nternationa1 Critical Tables,” (E. W. Washburn, ed.), Vol. 6, p. 91, McGraw-Hill Book Company, Inc., New York (1929).
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
INFINITE
TABLE
2 E
N
2
m
5 3 2 E +I =i 5 “a
z”
2
L2 cl n 7
r.0 0
272
ROBINSON
AND
SAVITSKY
AS is well known, in order to test the reaction field theory, it is necessaryto separate the reaction field from the other contributions to the screeningconstant. Severalinvestigators have apparently done this successfully by measuring the chemical shifts relative to an internal standard (generally TMS or cyclohexane) (8), (9), (IO), (II), (12) (14). If the individual ortho, meta, and para chemical shifts of nitrobenzene and aniline relative to TMS are plotted versus Z (E), however, neither a straight line nor a smooth curve is obtained. Other than a general trend there appears to be no correlation. This lack of correlation may be attributed to solvent effects upon TMS and/or to the less understood magnetic anisotropy of the solvent molecules. To circumvent this problem, following the suggestion of Laszlo (16), only internal shifts such as (6,--6,) and (6,, - 8,) were correlated with Z (8). It should be noted that any effect which results in unequal shielding of the different protons will not be cancelled even by using internal shifts. The internal chemical shifts of the aromatic molecules studied show a good linear correlation with Z (E) for all of the solvents studied except methylene chloride and carbon tetrachloride, as seen in Figs. 1-6. Similar anomalous behavior of halogenated solvents have been reported by other investigators (7), (9), (14). The value of k in Eq. [2] can be calculated from the slopes of the linear plots of internal chemical shift versus Z (8). Such calculations rarely agree with the approximate theoretical value of 2 x IO-“, but the best results generally fall in the range of 2 x lo-” to 4 x lo-” (7), (17). The necessarydata for the calculation of k are given in Table 2, and the calculated values of k are given in Table 3.
FIG. 1. Plot of the Internal Chemical Shifts (6,-&)(u)
and (6,-&J(6)
of Nitrobenzene VerwsZ(&).
SOLVENT
EFFECTS
201 0.2
FIG.
I 0.3
ON MONOSUBSTITUTED
I 0.4
I 0.5
I 0.6
2. Plot of the Internal Chemical Shifts (6,-6,)(a)
0.2
0.3
0.4
0.5
0.6
I 0.7
I 0.8
BENZENES
I 0.9
and (&-6,)(b)
0.7
0.8
273
0.9
of Aniline Versus Z(E).
1.0
2 (E) FIG.
3. Plot of the Internal Chemical Shifts (6,--6,)(a) and (&-6,)(b)
of Fluorobenzene VersusZ(E).
274
FIG.
ROBINSON AND SAVITSKY
4. Plot of the Internal Chemical Shifts (S,-&J(Q)
and (8,-&,)(b)
of Chlorobenzene Versus
ad.
-!a Y 4 ”Ei 2
0.2
0.3
0.4
0.5
0.6
0.7
3.8
0.9
1.0
2 (E) FIG.
m.
5. Plot of the Internal Chemical Shifts (&,-&,)(a)
and (&-6,)(b)
of Bromobenzene Versus
275
SOLVENT EFFECTS ON MONOSUBSTITUTED BENZENES
I
I
I
I
I
” (E I FIG. 6. Plot of the Internal Chemical Shifts (&-&)(a)
TABLE
and (&-6,)(b)
of Iodobenzene Versus Z(E).
2
DATA USED IN CALCULATION OF k
Solute Nitrobenzene Aniline Fluorobenzene Chlorobenzene Bromobenzene Iodobenzene
4.21
0.523’ 1.58 1.70 1.70 1.70
1.5529 1.5863 1.4617 1.5248 1.5598 1.6197
7.86 7.70 7.81 8.40 8.64 8.98
6.46 6.46 6.46 6.46 6.46 6.46
2.88 2.88 2.88 3.14 3.30 3.54
0.236d 0.184” 0.179e 0.178” 0.179” 0.179’
a “Handbook of Chemistry and Physics,” (R. C. Weast, ed.) 48th Ed., The Chemical Rubber Company, Cleveland (1967). b Component of p along the C-N bond in the direction of the nitrogen atom. If free rotation around the C-N bond is assumed the other components of p will average to 0 in a complete rotation of the group. Angle of the dipole with the C-N bond was taken as 70” given by W. D. Kumler and 1. F. Halverstadt, J. Am. Chenz. Sot. 63, 2182 (1941). c Calculated using interatomic distances and Van der Waals radii given, respectively, by “Interatomic Distances,” (L. E. Sutton, ed.), The Chemical Society Burlington House, London (1958) and A. L. G. Rees, J. Chem. Phys. 16, 995 (1948). d Given by I. G. Ross and R. A. Sack, Proc. Phys. Sot. Lond. B 63, 893 (1950). e Calculated using method described by I. G. Ross and R. A. Sack, Proc. Phys. Sot. Lond. B 63, 893 (1950).
276
ROBINSON
AND
TABLE
SAVITSKY
3
k
VALUES OBTAINED FROM THE LINEAR CORRELATIONS OF ortho,meta and meta,para INTERNAL SHIFTS VERSUS Z(E) k(lO’y
Compound
Nitrobenzene Aniline Fluorobenzene Chlorobenzene Bromobenzene Iodobenzene
___ ortho,meta
meta,para
1.6 11.1 0.7 2.8 3.6
0.4 14.9 -1.1 -0.5 -0.8
3.8
-0.7
As can be seen from these results, only in few instances is the value of k in quantitative agreement with the reaction field theory, and in the case of halo-benzenes the negative values of k obtained from the meta, paru internal shifts actually point to a qualitative disagreement with the theory. A similar qualitative disagreement with the reaction field theory was evidenced in the study of para-nitroanisole by Hutton and Schaefer (18), and two bromocyclohexanones by Laszlo and Musher (8). It is apparent that the internal shift treatment of the data does not cancel out some other specific solute solvent interactions. In the case of aniline hydrogen bonding with polar solvents may dominate over the reaction field effect. Charge transfer complexes and any type of association complexes between the aromatic and solvent molecules may affect the shieldings on the three aromatic protons in a selectively specific way. Schmidt, Butler, and Goldstein (20) treated successfully the nonlinear dependence of the solvent shifts in cc-chloroacrylonitrile on 2 (E) in terms of a model involving collision complexes. In the case of the aromatic molecules studied, however, there is a surprisingly good linear relationship between the internal chemical shifts and 2 (E) extending well above the value of E = 9. This seems to indicate that whatever the nature of effects other than the reaction field may be, these effects must be also linearly related to 2 (E). It would be highly fortuitous if both the reaction field effect and other effects would have separately nonlinear dependence on 2 (E) and yet would be linearly dependent on 2 (E)when acting together. It is felt, therefore, that the results presented in this work strongly support the basic validity of the reaction field theory. REFERENCES J. Mol. Spectros. 5, 52 (1960).
1. A. A. BOTHNER-BY, 2. A. D. BUCKINGHAM, T. SCHAEFER AND W. G. SCHNEIDER, J. Chem. Phys. 32, 1227 3. W. T. RAYNES, A. D. BUCKINGHAM, AND H. J. BERNSTEIN, J. Chem. Phys. 36,348l 4. B. B. HOWARD, B. LINDER AND M. T. EMERSON, J. Chem. Phys. 36, 485 (1962). 5. A. D. BUCKINGHAM, Can. J. Chem. 38, 300 (1960). 6. L. ONSAGER, J. Amer. Chem. Sot. 58, 1486 (1936). 7. P. DIEHL AND R. FREEMAN, Mol. Phys. 4, 39 (1961). 8. P. LASZLO AND J. I. MUSHER, J. Chem. Phys. 41, 3906 (1964). 9. F. HRUSHKA, E. BOCK AND T. SCHAEFER, Cnn. J. Chem. 41, 3034 (1963).
(1960). (1962).
SOLVENT EFFECTS ON MONOSUBSTITUTED
BENZENES
10. V. S. WATTS AND J. H. GOLDSTEIN, J. Clw~. Phys. 42, 228 (1965). Il. V. S. WATTS AND J. H. GOLDSTEIN, J. Mol. Spectros. 21, 260 (1966). 12. P. DIEHL, J. Chim. Phys. Physicochim. Biol. 61, 199 (1964). 13. B. FONTAINE, M. CHENON, AND N. LUMBROSO-BADER, J. Chim. Phys.
277
Physicochim. Biol. 62,
1075 (196% 14. H. M. HUTTON AND T. SCHAEFER, Cufz. J. Chem. 45, 1111 (1967). IS. G. B. SAVITSKY, L. G. ROBINSON, W. A. TALLON, AND L. R. WOMBLE,
J. Mug. Res. 1,139 (1969). 16. P. LASZLO, “Progress inN.M.R. Spectroscopy,” (J. W. Emsley, J. Feeney and L. H. Sutcliffe, Eds.), Vol. 3, Chapter 6, Pergamon, London/New York, 1967. 17. G. KOTO~YCZ AND T. SCHAEFER, Can. J. Chem. 45, 1093 (1967). 18. H. M. HUTTON AND T. SCHAEFER, Cux. J. Chem. 43, 3116 (1965). 19. R. L. SCHMIDT, R. S. BUTLER AND J. H. GOLDSTEIN, J. Phys. Chem. 73, 1117 (1969).
OF MAGNETIC
RESONANCE
3, 269-217
(1970)
Solvent Effects on the Internal Proton Chemical Shifts of Nitrobenzene, Aniline and Halobenzenes LARRY Department
of Chemistry
G. ROBINSON AND GEORGE B. SAVITSKY and Geology,
Clemson
University,
Clemson,
South
Carolina
29631
Received October 13, 1969; accepted January 16, 1970 The internal chemical shifts of six aromatic molecules (CeH5X, X=N02, NH2,F,Cl,Br,I) were determined in ten different solvents, by using mixtures of these molecules which were specifically labeled with deuterium. The internal chemical shifts showed a good linear relationship with the dielectric function formulated by the reaction field theory over a wide range of dielectric constant. Although the results obtained were generally in poor quantitative agreement with the reaction field theory, indicating that the effects of other specific solute solvent interactions are not entirely cancelled out by internal referencing technique, they appear to support the basic validity of the reaction field theory. INTRODUCTION
The contribution of a solvent to the screening constant of a proton can effectively be separatedinto five different effects due to (1) bulk susceptibility (I), (2) (2) magnetic anisotropy of solvent m o lecules (2) (3) van der W a a ls or dispersion interactions (I), (2) (3), (4), (4) reaction field (5) and (5) complex formation, or in general,any specific solute-solvent interactions. This paper will be concerned with the contribution of the reaction field to the proton chemical shifts of six aromatic m o lecules,C,H,X(X=NO,, NH,,F, Cl, Br, I). The reaction field created by a polar solute in a polarizable solvent, according to Onsager (6), can be representedby the equation E=r W-1) E---l 3a E+l/2nZp where /.L,n, and CIare, respectively, the dipole moment, refractive index, and polarizability of the solute and c is the dielectric constant of the m e d ium. Buckingham (5) adopting the above expression for E, derived the expression CT,== -kEcosB-k’E2
PI
to representthe reaction field contribution to the screeningconstant. In Eq. [2] E cos 8 is the component of E in the bond direction, k = 2 x lo-“, and k’ = lOdIE. Onsager’sexpressionfor E given in Eq. [l] was later m o d ified by Diehl and Freeman (7), who, in order to take into account the m o lecular shape of the solute, extendedthe simple theory of considering the solute as a polarizable dipole in a spherical cavity to treating the solute as a polarizable dipole in an ellipsoidal cavity. According to Diehl 269
270
ROBINSON
AND
SAVITSKY
and Freeman (7), E is given by E=
(
$
>
3 5, [l
+(n2- 1) &I w
PI
where a, b, and c are the semi-axes of the cavity ellipsoid, 2 (E) is given by
and <, is the shape parameter, which is a function of a, b, and c. It has been shown that the E* term in Eq. [2] negligible (8); therefore, a plot of chemical shift versus Z (E) should result in a straight line. Linear relationships have been observed in the correlation of chemical shifts versus Z (E) for solutes such as (1) substituted ethylenes (9), Uo), (II), (2) paraldehyde (7), (3) acetonitrile (2), and (4) meta-dinitrobenzene (12). On the other hand, some solutes (1) cis-2,6-dibromo-4,4-diphenylcyclohexanone and 2-bromo-4,4-diphenylcyclohexanone (8), (2) alkyl ketones (13), and (3) substituted ethylenes (14), exhibited nonlinear correlations. EXPERIMENTAL
In order to observe the proton chemical shifts of the monosubstituted benzenes a deuteration technique, described previously (15), was used. The preparation of aniline was accomplished by the hydrogenation of the mixture of isotopomers of nitrobenzene in the presence of a palladium on carbon catalyst. This preparation was used to avoid hydrogen-deuterium exchange which occurs in the normal reduction of nitrobenzene with Sn and HCl. Fluorobenzene and chlorobenzene were prepared by diazotization of the isotopomers of aniline. Bromobenzene was prepared by the direct bromination of pentadeuterobenzene using AICI, as a catalyst. Iodobenzene was prepared by the iodine decomposition of phenyl magnesium bromide which was prepared from the bromobenzene isotopomer mixture. Purity, chemical and isotopic, of the compounds was evidenced by the absence of any extraneous peaks in their nmr spectra. Proton chemical shifts were calibrated by the normal side band technique and were recorded on a Varian A-60 nmr spectrometer. The shifts were measured in Hz relative to TMS as an internal standard. Each measured chemical shift was an average of two forward and two reverse sweeps. To eliminate line broadening due to deuterium coupling the NMR Specialties Inc., HD-60 Heteronuclear Spin Decoupler was used which increased the probe temperature to approximately 52°C. Infinite dilution values were determined by measuring the chemical shifts at three different concentrations (approximately 15, 7, and 3 per cent weight-volume) and by extrapolating to infinite dilution a plot of chemical shift versus concentration. Over the concentration range studied all plots were linear, within experimental error. The error of the extrapolated value is estimated to be in the range of 0.2-0.5 Hz. RESULTS
AND
DISCUSSION
The proton chemical shifts of each monosubstituted benzene were determined in ten different solvents which were chosen to cover a wide range of dielectric constant. Table 1 gives the ortho, meta, and para chemical shifts at infinite dilution in each of the solvents as well as the dielectric constants of the solvents at 52°C.
n-Hexane Cyclohexane Carbon Tetrachloride Ethyl Ether Ethyl Acetate Methyl Acetate Methylene Chloride Paraldehyde Cyclohexanone Acetone
1.84” 1.97” 2.18” 3.79” 5.65” 6.62” 7.96” 12.10” 17.45” 18.22”
489.5 488.9 492.2 492.4 494.2 495.8 492.2 493.3 493.5 494.3
443.9 443.6 450.7 452.7 457.8 459.7 453.0 459.5 459.6 460.2
451.2 451.2 458.0 460.7 466.1 468.2 462.1 468.4 468.3 469.2
388.6” 388.8 391.2 392.5 395.1 396.0 398.8 398.2 396.0 398.2
ortho
CHEMICAL
Nitrobenzene -__~ ortho meta para
VALUES OF PROTON
(52”)
DILUTION
419.1h 419.0 420.8 418.6 419.4 420.0 426.4 421.7 419.0 421.0
meta
Aniline
396.0” 395.7 396.6 391.8 392.3 392.8 401.6 394.6 390.7 393.2
para
SHIFTS IN Hz
420.0 434.7
436.2 437.4 441.2 442.2 440.2
423.6 425.1 428.6 429.3 427.7
437.3 435.4 434.3 438.4 441.0 439.4 441.9 440.6 440.4 438.0
429.0
429.2
423.6 441.2 428.4 440.3 440.0 424.4 442.2 429.0 441.2 440.9 426.0 443.6 430.6 442.7 442.2
419.3 420.6 424.0 424.8 423.0
BENZENES
446.7 448.0 450.3 451.3 449.7
444.0
444.6
430.4 431.5 435.8 437.2 434.2
425.5
425.7
417.6
meta
433.2 434.5 438.6 440.0 437.8
459.2 460.7 463.0 463.8 462.4
422.7 423.7 427.6 428.6 426.3
428.8 456.8 417.6
428.9 451.5
ortho
Iodobcnrene
TMS
441.8 441.8 443.2
435.4 437.0 440.6 441.7 439.8
430.9
431.1
para
at this concentration
436.6 449.7 436.4 439.6 461.9 428.6 437.1 450.7 437.9 440.3 462.9 429.2 438.5 452.0 438.7 441.4 464.3 430.3
430.4 431.8 435.6 436.8 434.9
425.6
TO INTEKNAL
Bromobenzene - --~ ortho meta para
RELATIVE
425.9
Chlorobenzene ____~~~~__ ortho meta para
415.1 431.6 419.4 434.1
415.6 432.i
Fluorobenzene -__ ortho meta para
OP THE MONOSUBSTITUTED
1
““Handbook of Chemistry and Physics,” (R. C. Weast, ed.), 48th Ed., The Chemical Rubber Company, Cleveland (1967). SDue to low solubility only one sample (3.21% weight-volume) could be run; therefore, the chemical shifts values obtained were taken as infinite dilution. “‘1nternationa1 Critical Tables,” (E. W. Washburn, ed.), Vol. 6, p. 91, McGraw-Hill Book Company, Inc., New York (1929).
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
INFINITE
TABLE
2 E
N
2
m
5 3 2 E +I =i 5 “a
z”
2
L2 cl n 7
r.0 0
272
ROBINSON
AND
SAVITSKY
AS is well known, in order to test the reaction field theory, it is necessaryto separate the reaction field from the other contributions to the screeningconstant. Severalinvestigators have apparently done this successfully by measuring the chemical shifts relative to an internal standard (generally TMS or cyclohexane) (8), (9), (IO), (II), (12) (14). If the individual ortho, meta, and para chemical shifts of nitrobenzene and aniline relative to TMS are plotted versus Z (E), however, neither a straight line nor a smooth curve is obtained. Other than a general trend there appears to be no correlation. This lack of correlation may be attributed to solvent effects upon TMS and/or to the less understood magnetic anisotropy of the solvent molecules. To circumvent this problem, following the suggestion of Laszlo (16), only internal shifts such as (6,--6,) and (6,, - 8,) were correlated with Z (8). It should be noted that any effect which results in unequal shielding of the different protons will not be cancelled even by using internal shifts. The internal chemical shifts of the aromatic molecules studied show a good linear correlation with Z (E) for all of the solvents studied except methylene chloride and carbon tetrachloride, as seen in Figs. 1-6. Similar anomalous behavior of halogenated solvents have been reported by other investigators (7), (9), (14). The value of k in Eq. [2] can be calculated from the slopes of the linear plots of internal chemical shift versus Z (8). Such calculations rarely agree with the approximate theoretical value of 2 x IO-“, but the best results generally fall in the range of 2 x lo-” to 4 x lo-” (7), (17). The necessarydata for the calculation of k are given in Table 2, and the calculated values of k are given in Table 3.
FIG. 1. Plot of the Internal Chemical Shifts (6,-&)(u)
and (6,-&J(6)
of Nitrobenzene VerwsZ(&).
SOLVENT
EFFECTS
201 0.2
FIG.
I 0.3
ON MONOSUBSTITUTED
I 0.4
I 0.5
I 0.6
2. Plot of the Internal Chemical Shifts (6,-6,)(a)
0.2
0.3
0.4
0.5
0.6
I 0.7
I 0.8
BENZENES
I 0.9
and (&-6,)(b)
0.7
0.8
273
0.9
of Aniline Versus Z(E).
1.0
2 (E) FIG.
3. Plot of the Internal Chemical Shifts (6,--6,)(a) and (&-6,)(b)
of Fluorobenzene VersusZ(E).
274
FIG.
ROBINSON AND SAVITSKY
4. Plot of the Internal Chemical Shifts (S,-&J(Q)
and (8,-&,)(b)
of Chlorobenzene Versus
ad.
-!a Y 4 ”Ei 2
0.2
0.3
0.4
0.5
0.6
0.7
3.8
0.9
1.0
2 (E) FIG.
m.
5. Plot of the Internal Chemical Shifts (&,-&,)(a)
and (&-6,)(b)
of Bromobenzene Versus
275
SOLVENT EFFECTS ON MONOSUBSTITUTED BENZENES
I
I
I
I
I
” (E I FIG. 6. Plot of the Internal Chemical Shifts (&-&)(a)
TABLE
and (&-6,)(b)
of Iodobenzene Versus Z(E).
2
DATA USED IN CALCULATION OF k
Solute Nitrobenzene Aniline Fluorobenzene Chlorobenzene Bromobenzene Iodobenzene
4.21
0.523’ 1.58 1.70 1.70 1.70
1.5529 1.5863 1.4617 1.5248 1.5598 1.6197
7.86 7.70 7.81 8.40 8.64 8.98
6.46 6.46 6.46 6.46 6.46 6.46
2.88 2.88 2.88 3.14 3.30 3.54
0.236d 0.184” 0.179e 0.178” 0.179” 0.179’
a “Handbook of Chemistry and Physics,” (R. C. Weast, ed.) 48th Ed., The Chemical Rubber Company, Cleveland (1967). b Component of p along the C-N bond in the direction of the nitrogen atom. If free rotation around the C-N bond is assumed the other components of p will average to 0 in a complete rotation of the group. Angle of the dipole with the C-N bond was taken as 70” given by W. D. Kumler and 1. F. Halverstadt, J. Am. Chenz. Sot. 63, 2182 (1941). c Calculated using interatomic distances and Van der Waals radii given, respectively, by “Interatomic Distances,” (L. E. Sutton, ed.), The Chemical Society Burlington House, London (1958) and A. L. G. Rees, J. Chem. Phys. 16, 995 (1948). d Given by I. G. Ross and R. A. Sack, Proc. Phys. Sot. Lond. B 63, 893 (1950). e Calculated using method described by I. G. Ross and R. A. Sack, Proc. Phys. Sot. Lond. B 63, 893 (1950).
276
ROBINSON
AND
TABLE
SAVITSKY
3
k
VALUES OBTAINED FROM THE LINEAR CORRELATIONS OF ortho,meta and meta,para INTERNAL SHIFTS VERSUS Z(E) k(lO’y
Compound
Nitrobenzene Aniline Fluorobenzene Chlorobenzene Bromobenzene Iodobenzene
___ ortho,meta
meta,para
1.6 11.1 0.7 2.8 3.6
0.4 14.9 -1.1 -0.5 -0.8
3.8
-0.7
As can be seen from these results, only in few instances is the value of k in quantitative agreement with the reaction field theory, and in the case of halo-benzenes the negative values of k obtained from the meta, paru internal shifts actually point to a qualitative disagreement with the theory. A similar qualitative disagreement with the reaction field theory was evidenced in the study of para-nitroanisole by Hutton and Schaefer (18), and two bromocyclohexanones by Laszlo and Musher (8). It is apparent that the internal shift treatment of the data does not cancel out some other specific solute solvent interactions. In the case of aniline hydrogen bonding with polar solvents may dominate over the reaction field effect. Charge transfer complexes and any type of association complexes between the aromatic and solvent molecules may affect the shieldings on the three aromatic protons in a selectively specific way. Schmidt, Butler, and Goldstein (20) treated successfully the nonlinear dependence of the solvent shifts in cc-chloroacrylonitrile on 2 (E) in terms of a model involving collision complexes. In the case of the aromatic molecules studied, however, there is a surprisingly good linear relationship between the internal chemical shifts and 2 (E) extending well above the value of E = 9. This seems to indicate that whatever the nature of effects other than the reaction field may be, these effects must be also linearly related to 2 (E). It would be highly fortuitous if both the reaction field effect and other effects would have separately nonlinear dependence on 2 (E) and yet would be linearly dependent on 2 (E)when acting together. It is felt, therefore, that the results presented in this work strongly support the basic validity of the reaction field theory. REFERENCES J. Mol. Spectros. 5, 52 (1960).
1. A. A. BOTHNER-BY, 2. A. D. BUCKINGHAM, T. SCHAEFER AND W. G. SCHNEIDER, J. Chem. Phys. 32, 1227 3. W. T. RAYNES, A. D. BUCKINGHAM, AND H. J. BERNSTEIN, J. Chem. Phys. 36,348l 4. B. B. HOWARD, B. LINDER AND M. T. EMERSON, J. Chem. Phys. 36, 485 (1962). 5. A. D. BUCKINGHAM, Can. J. Chem. 38, 300 (1960). 6. L. ONSAGER, J. Amer. Chem. Sot. 58, 1486 (1936). 7. P. DIEHL AND R. FREEMAN, Mol. Phys. 4, 39 (1961). 8. P. LASZLO AND J. I. MUSHER, J. Chem. Phys. 41, 3906 (1964). 9. F. HRUSHKA, E. BOCK AND T. SCHAEFER, Cnn. J. Chem. 41, 3034 (1963).
(1960). (1962).
SOLVENT EFFECTS ON MONOSUBSTITUTED
BENZENES
10. V. S. WATTS AND J. H. GOLDSTEIN, J. Clw~. Phys. 42, 228 (1965). Il. V. S. WATTS AND J. H. GOLDSTEIN, J. Mol. Spectros. 21, 260 (1966). 12. P. DIEHL, J. Chim. Phys. Physicochim. Biol. 61, 199 (1964). 13. B. FONTAINE, M. CHENON, AND N. LUMBROSO-BADER, J. Chim. Phys.
277
Physicochim. Biol. 62,
1075 (196% 14. H. M. HUTTON AND T. SCHAEFER, Cufz. J. Chem. 45, 1111 (1967). IS. G. B. SAVITSKY, L. G. ROBINSON, W. A. TALLON, AND L. R. WOMBLE,
J. Mug. Res. 1,139 (1969). 16. P. LASZLO, “Progress inN.M.R. Spectroscopy,” (J. W. Emsley, J. Feeney and L. H. Sutcliffe, Eds.), Vol. 3, Chapter 6, Pergamon, London/New York, 1967. 17. G. KOTO~YCZ AND T. SCHAEFER, Can. J. Chem. 45, 1093 (1967). 18. H. M. HUTTON AND T. SCHAEFER, Cux. J. Chem. 43, 3116 (1965). 19. R. L. SCHMIDT, R. S. BUTLER AND J. H. GOLDSTEIN, J. Phys. Chem. 73, 1117 (1969).