JAMESL. ROARKAND WILLIAMB. SMITH
1046 The change at Hs on rotating the nitro group out of the plane of the ring is not due to magnetic anisotropy as might be supposed.16 The change a t He on altering solvents still follows urn (Figure 4) for o-nitrophenol. There is a similar change at Hi. The slight change at Hs is uninformative since HS is on the symmetry axis of the nitro group. If the alterration in chemical shifts at Ha in o-nitrophenol on changing solvents were
due to the magnetic anisotropy of the nitro group, one could reasonably expect the effect to appear also at H4 and Ha.@
Acknowledgment. We wish to acknowledge the generous support of The Robert A. Welch Foundation, (16) I. Yamaguchi, iw0i.mys..6, 106 (1963).
The Nuclear Magnetic Resonance Spectra of Six ortho-Substituted Fluorobenzenes by James L. Roark and William B. Smith Department of Chemistry, Texas Christian University, Fort Worth, Texas 76189
(Receioed September 1 8 , 1968)
The proton and fluorine nuclear magnetic resonance parameters have been determined for six ortho-substituted fluorobenzenes. The chemical shifts for the protons adjacent to the ortho substituents were found to follow the Q parameter correlation quite well, and there was also a fair correlation of Q with the fluorine chemical shifts. The nitro group provides an exception in both series which is taken to indicate that the nitro group is somewhat out of the plane of the aromatic ring on the average of the nmr time scale for o-nitrofluorobenxene. During the course of our studies of substituent effects on proton chemical shifts in aromatic systems, we have found that the chemical shifts of protons adjacent to the substituents on the ring are well correlated by the parameter &.Ie3 This parameter, originis presumably ally defined by Schaefer and coivorker~,~ a measure of the paramagnetic shielding induced by the mixing of excited states of the substituent with ground states of the electrons in the ortho C-€I bo11da4 While the definition of Q is not restated here, it is true that the parameter may be calculated only for hydrogen and the halogens. Experimental methods for determining Q for other functional. groups have been developedI2J and it has been noted that groups without cylindrical symmetry along the C-X axis may show multiple Q values depending on the average orientation of X with respect to the ring.3 T‘alues of Q for groups coplanar with the ring are characterized by Q ( l ) while “out of the plane’’ values are called Q(2). Several sets of these have been given in the preceding paper,3 and an argument was presented against magnetic anisotropy as an explanation of the effect. We wish to report here the parameters for a series of ortho-substituted fluorobenzenes. This series was attractive in that it completed our earlier work with the halobenzeneslJ and it has been noted that ortho fluorines for the ortho fluorohalobenzenes also follow the Q relation.4a We wished to know if this was true for other substituent groups. The Journal of Physical @hemistr#
Experimental Section The fluorobenzenes were all commercially available compounds. They were run in 10% v/v solutions and degassed as before. The general technique has been des~ribed.‘-~>jSpectra were determined on a Varian A-60h instrument. The fluorine decoupled spectra were taken with the aid of a Nuclear Magnetic Resonance Specialties SD-60B heteronuclea,r decoupler operating through a modified A-60 probe. Fluorine spectra were taken on a Varian HA-100 operating at 94.1 Mcps. Chemical shifts were taken with reference to 1,1,2,2-tetrach1oro-3,3,4,4-tetrafluorocyc1obutane (TCTSB) as an internal standard. All spectra were calibrated by the usual audio-side-band technique. The fluorine decoupled spectra were used to obtain a preliminary set of parameters for the ring protons. These were then used to fit the normal spectra. The fluorine decoupled, normal, and calculated spectra for o-cyanofluorobenzene are shown in Figure 1. The various chemical shifts are given in Table I and coupling constants in Table 11. The values for 2-bromoand 2-iodofluorobenzene have been determined in (1) W. B. Smith and G . M. Cole, J. Phys. Chem., 69, 4413 (1905). J. Amer. Chem. Soc., 89, 5018 (1967). (3) J. L. Roark and W.B . Smith, J. Phys. Chem., 7 3 , 1043 (1969). (4) (a) F. Hruska, H. M. Hutton, and T.Schaefer, Can. J . Chem., 43, 2392 (1965); (b) T. Schaefer, F. Hruska, H. M. Hutton, i b i d . , 54, 3143 (1967). (5) W. B . Smith and J. L. Roark, J . Chem. Eng. Data, 12, 587 (1967). (2) W. B. Smith and J. L. Roark,
1047
NMRSPECTRAOF OT~~O-SUBBTITUTED FLUOROBENZENEB
L I
I
ria
L
I
E.P.S.
442 E.P.B.
452 C.P.S.
43s c.,...
Figure 1. Fluorine decoupled (upper), normal (middle), and calculated spectra (lower) of o-cyanofluorobeneene.
Table I: Chemical Shifts for 2-Substituted Flurorbenzenes 2-XQ
F C1
Br I CN NO2
78
7 4
n
7 6
2.93 3.01 2.66 2.50 2.56 2.30 2.34 2.38 1.97
2.93 3.01 2.99 3.03 3.12 3.16 3.24 2.71 2.67
2.93 3.01 2.84 2.78 2.87 2.75 2.83 2.37 2.34
2.93 3.01 2.93 2.94 3.03 3.00 3.08 2.78 2.69
Rmsb
6F0
...
23.91
0.038 0.032 0.031 0.030 0.047 0.039 0.045
1.18 -7.07
...
...
...
-20.95
... -6.82 4.27
values and the signs of our coupling constants are in good accord. The spectrum for o-difluorobenzene collapsed to one line on irradiation of the fluorine, and no further analysis was attempted. The chemical shifts for the fluorines in t,his series were determined previously on neat samples.* These results agree with ours determined in dilute carbon tetrachloride solutions.
Discussion
a The first entry is for carbon tetrachloride solutions. Where ri second entry is given, the solvent is cyclohexane. Root-mean-square error in fitting proton lines. In ppm from TCTFB. Values for 2-X of methoxyl, hydroxyl, and methyl were 21.12, 26.80, 3.60 ppm, respectively.
The chemical shift data reported in Table I show many of the regularities exhibited by our previous studies of other ortho-substituted halobenzenes.2 While not shown here, the plot of Hg us. urn€or the substituent
carbon tetrachloride recently by Castellano and KostelnikpBand our values agree very well indeed. These two compounds and 2-chlorofluorobenzene have been recently reported in tetramethylsilane solutions.' Our
(6) S. Castellano and R. Kostelnik, Tetrahedron Lett., 51, 5211 (1967). (7) J. E. Loemker, J. M. Read, Jr., 'and J. H. Goldstein, J. Phys. Chem., 72, 991 (1968). (8) H. S. Gutowsky, D. W. McCall. B. R . McGarvey, and L. H. Meyer, J. Amer. Chem. Soc., 74, 4809 (1952).
Volume Y$! Number 4 April I969
JAMEBL, ROARKAND WILLIAMB. SMITH
1048 Table 11: Coupling Constants for 2-Substituted Fluorobenzenes 2-xu
Jar
c1
8-01 7.96 7.96 7.83 7.92 7.82 8.18
Br I
CN Not a
Jaa
1.70 1.66 1.67 1.64 1.66 1.78 1.75
Jl8
0.31 0.26 0.27 0.26 0.28 0.40 0.31
J4s
7.47 7.40 7.38 7.39 7.46 7.61 7.40
J40
J6
1.46 1.47 1.44 1.43 1.45 1.04 1.27
e
8.29 8.21 8.17 8.22 8.27 8.55 8.30
JSF
7.33 6.80 6.83 6.30 6.17 6.43 7.59
J ~ F
-0.87 -0.64 -0.65 -0.24 -0.27 -0.24 -0.93
J6F
4.79 4.79 4.73 5.03 5.00 5.43 4.35
JSF
9.10 8.50 8.41 7.94 7.97 9.01 10.68
The first entry is for carbon tetrachloride solutions. Where a eecond entry is given, the solvent is cyclohexane.
at the 2-position is quite linear. Similarly, the shifts for H5 follow n P of these substituents. Moreover, the substituent chemical shifts at H4 behave in the same fashion as the nzeta-proton values in the monosubstituted benzenes.2 Differences from our previous halobenzene series, however, appear in the Q correlation plot. In Figure 2 are shown the plots of Q for the various ortho substituents against the proton chemical shifts at H3 and the fluorine chemical shifts, respectively. In our pre3.0-
a.o
-
bl
Figure 2. Chemical shifts of Ha (upper) us. Q and fluorine chemical shifts (lower) us. Q for a series of 2-X-fluorobensenes. The dotted lines are the appropriate shifts for the nitro compound over the range of Q N O ~ . The Journal of Physical Chemistry
vious s t u d i e ~ , ~we J have found that one of two Q values for the nitro group is required. When the nitro group is flanked by a large bulky ortho substituent. at C-1, the Hs shift correlation requires the Q(2) value 4.00. If the nitro group is coplanar with the ring (as when flanked on both sides by hydrogens or when constrained by a hydrogen bond as in o-nitrophenol3) the Q ( l ) value of 6.33 is applied. Clearly neither of these extreme values for &oz applies to o-nitrofluorobenzene. Figure 2 shows that an intermediate value would be required, a situation very much like that encountered previously for o-nitrophenol in dimethyl sulfoxide. Xitrobenzene has been shown to be planar by X-ray rneasurement~.~Infrared studies have been used to show o-bromonitrobenzene to have the nitro group out of the ring plane, and the same is most likely true for o-chloronitrobenzene.lo It has been conjectured that o-nitrofluorobenzene is planar.'l However, the results from these nmr measurements suggest that on the nmr time scale the nitro group in the latter compound is slightly out of the ring plane. Conclusions quite similar to these have been derived from solvent effect studies by Rae.I2 The Q plot for the fluorine chemical shifts (Figure 2) shows fairly good agreement for the various calculated and experimental values of Q. However, the point for o-nitrofluorobenzene is well off the plot to the high-field side. Clearly resonance contributing structures of the o-quinoid type involving the fluorine and nitro group are not the explanation here since these would tend to deshield the fluorine. Such a structure seems to be important in o-nitrophenol as shown previously.3 Magnetic anisotropy also does not offer an explanation since the proton and fluorine have roughly the same geometrical relation to the nitro group, and it may be presumed that anisotropy effects should be approximately the same a t both positions. This is not confirmed by the data. (9) J. Trotter, Acta Cryst., 1 2 , 884 (1959). (10) R. J. Francel, J. Amer. Chem. Soc., 74, 1265 (1962). (11) J. Hamer, L. Placek, and M. Ahmad, Tetrahedron, Suppl., 2 0 , 395 (1964). (12) I. D.Rae, Aust. J . Chem., 2 0 , 2381 (1967).
1049
P M R SPECTRA OF 2-SUBSTITUTED PYRIDINES
According to Schaefer, et al.,4 the Q parameter is a measure of paramagnetic shielding effects resulting from the mixing of the ground state of the proton or fluorine with excited states of the substituent under the aegis of the angular momentum operator. This effect should be different in magnitude for fluorine as opposed to hydrogen and, presumably, geometry dependent. The theoretical aspects of the Q parameter have not
yet been completely delineated. The parameter, however, does provide many empirical correlations of nmr data and seems to provide some useful insights into the geometrical arrangements within these molecules.
Acknowledgment. The generous support of The Robert A. Welch Foundation is hereby gratefully acknowledged.
The Proton Magnetic Resonance Spectra of Several 2-Substituted Pyridines by William B. Smith and James L. Roark Departmeni of Chemistry, Texas Christian University, Fort Worth, l e x a s
76129
(Received September 1 I, 1 9 6 8 )
The nmr parameters for nine 2-substituted pyridines are reported both as the free bases in carbon tetrachloride solution and as the cations in 1:1 trifluoroaceticacid-acetic acid. The chemical shifts in these systems arc compared with those previously derived for a series of ortho-disubstituted benzenes. The shifts for Hg in both solvents correlate with the parameter &. H6correlates with un. Behavior at Hq and Ha is more complex.
Recently, we have examined the nmr parameters for a large number of ortho-disubstituted benzenes.l The results of these studies may be summarized as follows for a series of 1-X-2-Y-benzenes in which X is held constant and Y varied through a range of substituents: (1) protons adjacent to Y follow the parameter QY with certain reservation regarding the steric requirements of X and Y ; (2) the chemical shift of Ha follows the up of Y while He follows a,; and (3) the chemical shift; of H4 behaves just as the mela proton in the monosubstituted benzenes. As an extension of this work,l we have now examined the spectra for a series of 2-substituted pyridines and their cations, This series was of interest for several reasons, Dewar and Marchand2 have discussed the effects of the nitrogen in y-picoline on the proton chemical shifts in that substance as those due to a substituent operating by a pure inductive effect. According to this viewpoint, the 2-substituted pyridines might be considered as ortho-disubstituted benzenes in which the one substituent operates solely by the inductive effect. I n our original work,lb the observation that Ha followed urn was interpreted as suggesting that the substituent a t C-1 insulated Ha from the “field effect” of the substituent at C-2. During the course of discussions with colleagues, it was pointed out that all of our C-1 substituents contained p electrons and should be highly polarizable by Y. Thus, the field effect of Y might be actually enhanced by transmission through
X. If so, the unshared pair of electrons on the pyridine nitrogen should behave as LZ polarizable substituent and He should follow urn. The parameters for several 2-substituted pyridines have been reported previously.* Since these were carried out on either neat liquids or on solutions in dimethyl sulfoxide, the chemical shifts were of little utility for our purposes. Here, we have used dilute solutions in carbon tetrachloride. In addition, we have determined the chemical shifts of the related cations.
Experimental Section The pyridines in this study were cornmercially available except for 2-iodopyridine and 2-nitropyridine. The former was prepared by the method of Baker and McEvoy* and the latter by the method of KirpaI and Bohm.6 Solutions (10% v/v) were made up in carbon tetrachloride, and tetramethylsilane was added as an internal standard. Several pyridines were not suffi(1) (a) W. B. Smith and G. M . Oole, J . Phys. Chem., 69, 4413 (1965); (b) W. B . Smith and J. L. Roark, J . Amer. Chem. Soc., 89, 5018 (1967); (c) J. L. Roark and W. B. Smith, J . Phys. Chem., 7 3 , 1043 (1969); (d) J. L. Roark and W. B. Smith, ibid., 7 3 , 1046 (1969). (2) M . J. 9. Dewar and A. P. Marchand, J . Amer. Chem. Soc., 8 8 , 354 (1966). (3) (a) W. Brugel, Z . Eleklrochem.. 6 6 , 259 (1962); (b) V. J. Kowalewski and D. G. de Kowalewski, J. Chem. Phys., 37, 2603 (1962). (4) B . R. Baker and F. J. McEvoy, J. Org. Chem., 3 0 , 128 (1955). (5) A. Kirpal and W. Bohm, Ber., 65, 680 (1932). Volume 73,Number 4 April 1969
