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. P h y s . , 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
WILLIAMB. SMITHAND JAMEBL. ROAM
1050 Table I: Parameters for 2-Substituted Pyridines0 2-x "2
CH&3 CH3 B
c1 Br I CN NO? a
JSt
8.37 8.35 7.71 8.23 8-08 8.03 7.75 7.80 8.26
JSS
1.01 0.98 1.17 0.91 0.98 0.94 1.25 1.19 1.05
Jsa
J4a
0.98 0&81 0.93 0.78 0.80 0.79 0.82 0.96 0.86
J46
7.17 7.09 7.50 7.23 7.41 7.39 7.45 7.85 7.38
1.97 2.06 1.89 2.10 2.06 2.09 2.06 1.74 1.86
Dilute solutions in carbon tetrachloride. Coupling constants in cps.
JaI
78
7 4
76
7 6
Rmsb
5.05 5.08 4.88 4.95 4.86 4.81 4.82 4.84 4.62
3.65 3.38 2.99 3.12 2.76 2.60 2.35 2.30 1.84
2.74 2.57 2.58 2.23 2.42 2.54 2.73 2.12 1.94
3.53 3.27 3.04 2.85 2.85 2.83 2.80 2.45 2.32
2.06 1.94 1.63 1.81 1.69 1.69 1.72 1.31 1.41
0.020 0.032 0.048 0.030 0.029 0,013 0.031 0.041 0.029
Root-mean-squareerror in line positions.
cal shifts are referenced to these protons. These ciently soluble in carbon tetrachloride and were studied spectra were usually nearly first order, and chemical in mixtures of carbon tetrachloride and acetone. shifts could be readily ascertained by comparison with These results were extrapolated to pure carbon tetrathe spectra of the free bases. It is estimated that these chloride. Studies on several soluble pyridines indicated shifts are good to 0.01 ppm. (See Table XI.) results obtained in this fashion to be quite good. In general, techniques and analyses were carried out Discussion as previously described.' The exact analysis of H6 in The proton chemical shifts in the 2-substituted these compounds was facilitated by the use of nitrogen pyridines show some of the same patterns observed in decoupling provided by a Nuclear Magnetic Resonance the monosubstituted benzenes. The shifts at Hh Specialities Model SD-60 B heteronuclear decoupler. (Table I) follow up quite well as do paru-proton The spectrum of 2-methylpyridine was obtained at chemical shifts in the benzenes.' Since it has been 100 ?c/Icps (Varian HA-100) with the methyl group recently shown that carbon-13 shifts a t (3-5 follow up decoupled. Couplings to the methyl group were taken just as the para carbons in the monosubstituted fiom a first-order analysis of the Varian A-60A spectrum as JBCHI = f0.50 cps, J I C ~=( f 0 . 2 5 cps, J ~ C = H ~ benzenes, it would seem that T-electron density effects produced by substituents a t C-2 are reflected by both f0.60 cps, and JGCH2 = f 0 . 2 0 cps, respectively. The types of chemical shifts.8 While not shown, the spectrum of 2-fluoropyridine was analyzed in toto. correlation of Hsin the protonated pyridines with up The values for the couplings to fluorine were JSF= is fairly good though the points for the cyano and -2.85 cps, J 4 s = 8.18 cps, and J ~ = F 2.49 cpsS6 The nitro group are unaccountably off the plot. coupling Jepcould not be resolved and was assigned as The behaviors of Hq and HBin both series are quite zero in the computer analysis. All other spectral different though both are meta to the substituent. parameters for the pyridines are given in Table I. of Hq correlate well with metaThe chemical shifts The various cations were studied in dilute solutions proton shifts in the monosubstituted benzenes. I n the (lo(% v/v) of 1:l trifluoroacetic and acetic acids. protonated series, the plot is somewhat cruder. Since Spectra were recorded on a Varian €€A-100 using the substituent effects a t this position are not well underacetic acid methyl protons as the internal lock. Chemistood, these observations are passed without further comment, I n contrast, the chemical shifts of He in the pyridines Table 11: Chemical Shifts for Protonated follow urnquite well (Figure l ) , the methyl and cyano 2-Substituted Pyridinesa groups being exceptions in this case. In the protonated pyridines, also shown in Figure 1, there appears to be at best only an extremely rough correlation. Indeed, 4.77 5.79 5.72 4.94 "2 6.20 5.39 6.38 CHiO 5.38 comparison with the meta-proton chemical shifts us. urn 6.49 5.67 6.39 5.76 CH, for the monosubstituted benzenes shows about the 6.37 5.55 6.26 5.35 F same quality of fit. 6.64 5.89 6.45 5.92 c1 Br I CN NOz
6.03 6.32 6.14 6.29
6.30 6.13 6.44 6.16
5.89 5.92 6.05 5.76
6.69 6.69 6.8 6.67
5 Solutions (10% v/v) in 1 :1 trifluoroacetic acid-acetic acid. Chemical shifts are in ppm from the acetic acid methyl protons which eerved as an internal reference.
The Journal of Physical Chemistry
(6) These values are in reasonably good agreement with the earlier reported values of BrugeL88 The sign of J g p was assumed to be negative on the basis of the reported work of R . H . Cox and A . A. Bothner-By, J. Phys. Chem., 72, 1646 (1968), and J. C . Deck, Thesis, University of Illinois, 1966. (7) H . Spiesecke and W. G. Schneider, J. Chem. Phys., 35, 731
(1961). (8) H . L. (1968).
Retcofsky and R . A. Friedel, J. Phys. Chem.. 7 2 , 2619
1051
P M R SPECTRA OF %SUBSTITUTED PYRIDINES
t
-
0.987
~ N H , I
I
0
I
'
I
I
0.4
0.2
I
I
0.6
3 Figure 1. The relationship of the chemical shifts of H6 in the %substituted pyridines (upper) and pyridinium cations (lower) with urn.
These results tend to confirm the speculation that substituent effects on proton chemical shift nzeta to the substituent consist, in part, of a field effectmeasured by urnwhich may be relayed by an intervening polarizable substituent. From our various studies, the chemical shift change in going from a methoxyl group to a nitro group is as follows with the indicated intervening groups: methyl, 0.18 ppm; chloro, 0.28 ppm; bromo, 0.27 ppm; iodo, 0.30 ppni; and the nitrogen lone pair. 0.53ppm. The chemical shifts for H3us. the parameter Q for the free bases and the pyridinium ions are shown in Figure 2. The definition and presumed meanings of Q have been given a number of times.lrD Suffice it to say here we have found that two values of Q are required for groups such as the nitro group which may be either or out of the plane normally in the plane [ & N O , ( l ) ] [ Q N o , ( ~ ) ] when flanked by a large group.1° I n some situations where the nitro group is only slightly out of the ring plane on the average of the nmr time scale, intermediate values of Q are required.lond For the pyridines, it is evident that the NO^(^) value of 6.33 was needed; i.e., the nitro group is in the plane of the ring just as in nitrobenzene itself. However, when protonated, the electronic interaction between the nitro group and the ring must be reduced, and the nitro appears to be out of the ring plane [the Q N O ~ ( ~ ) value is required]. A rather similar situation occurs in
Q
Figure 2. The correlation of Hg in the 2-substituted pyridines (upper) and their respective cations (lower) with Q. The least-squares correlation coefficient for each is given.
the vinyl compounds. The cis vinyl hydrogens follow the Q relation.1° For nitroethylene the &oz(2) value is found to fit best. Here, too, electronic interactions must be sufficiently small as to produce no appreciable barrier to rotation about the C-N bond. Finally, it may be noted that the normal Q N H ~ ( ~ ) value of -O.67lc was used for both the free-base plot and for the protonated pyridine plot in Figure 2. Clearly, the 2-amino group is not protonated in the acid mixture used here. No doubt a much different value of Q would be needed for the ammonium group.
Acknowledgmenl. The generous financial support of The Robert A. Welch Foundation is hereby gratefully acknowledged. The Varian HA-100 used in this research was made available by virtue of a grant from the National Science Foundation. (9) (a) F.
Hruska, H. M . Hutton, and T. Schaefer, Can. J. Chem., Schaefer, F. Hurska, and H. M . Hutton,
43, 2392 (1965);(b) T.
i b i d . , 45, 3143 (1967). (10) See ref lb, Figure 4.
Volume Y9, Number 4 April 1969
