NOTES
Acknowledgment. The author wishes to thank Mr. Chahei Asakawa for taking part in the measurements of light scattering and Mr. Haruhiko Arai for carrying out the purification of the samples.
The Proton Magnetic Resonance Study of the Protonation of Pyrazine
by Hirotake Kamei Physics Division, Electrotechnical Laboratory, Tanashkmachi, Kitatama-gun, Tokyo, Japan (Received M a y 18, 1964)
During the course of an investigation of the electric properties of pyrazine and its derivatives, it became desirable to study the behavior of pyrazine in the presence of a proton donor. While some work has been done on the spectroscopic studies of the protonation of pyrazine in sulfuric little attention has been paid to the behavior of the protonat’ion a t different pyrazine-acid compositions. Nuclear magnetic resonance (n.m.r.) methods5 are suited to such an investigation. In the present investigation, the protonation of pyrazine by trifluoroacetic acid was studied by n.m.r. techniques. One of the advantages of using trifluoroacetic acid as solvent is that an internal reference may be employed. When the internal reference is used, the solvent effects6can be dealt with fairly readily.
Experimental Reagent grade pyrazine was fractionally distilled under dry nitrogen and purified by sublimation in vacuo a t liquid nitrogen temperature just before use. Reagent grade trifluoroacetic acid and cyclohexane were dried over phosphorus pentoxide and purified by fractional distillation under dry argon. Reagent grade concentrated sulfuric acid was used without further purification. Samples consisting of approximately 0.5 ml. of solution in each n.m.r. sample tube were prepared in a glove box under an atmosphere of dry nitrogen. The sample tubes containing the solutions were stoppered with a rubber tube and stopcock and then removed from the glove box. The tubes were immersed in liquid nitrogen, evacuated, and sealed. The compositions of all solutions were determined by weight. The n.m.r. spectra were obtained with a Japan Electron Optics Laboratory JNM-3 instrument operating at 40 Mc./sec. which was equipped with a tempera-
2791
ture control device. Measurements were made a t 23.5 i= 0.2 and -35 i= 1’. The resonance spectrum of aromatic protons in pyrazine consists of a single line with a chemical shift depending on concentration. This indicates that the lifetime of an exchanging species is short. The chemical shifts were determined by linear interpolation between two bracketing side bands generated by modulating the magnetic field. Ten to twelve measurements of the chemical shift were averaged for each sample. The result’ingaverage deviations were approximately 0.004 p.p.m.
Results and Discussion Chemical shift of Protonated Pyrazine. It is apparent from the ultraviolet absorption spectral that pyrazine dissolved in 60% and concentrated sulfuric acid forms monoprotonated and diprotonated species, respectively. The observed proton resonance shifts of pyrazine were measured at low concentrations in 64.4% and concentrated (95%) sulfuric acid. The experimental data are plotted in Figure 1. The chemical shifts of monoprotonated and diprotonated pyrazine were determined from the infinite dilution shifts obtained by graphical extrapolation of curves a and b in Figure 1, respectively. These observed shifts must be corrected for the solvent effects. The bulk susceptibility shift, CTb, is given by z/3 r A x q , where Axv is the difference in the volume susceptJibilities of the solvent and benzene. The values of xq (X106) for benzene, 64.4% sulfuric acid, and concentrated sulfuric acid are taken to be -0.617,’ -0.803,8 and -0.777,s respectively. The magnitude of the perturbation shifts arising from other solvent effects is not directly obtainable from the experimental data of the pyrazine-sulfuric acid system. It may be assumed that the magnitude of the solvent effects shift, uo (except polar effect), of protonated pyrazines is equal to that of benzene. Benzene has no electric moment, and its molecular shape (1) F,Halverson and R. C. Hirt, J. Chem. Phys., 19, 711 (1951). (2) L.F.Wiggins and W. S. Wise, J . Chem. Sac., 4780 (1956). (3) D.A. Keyworth, J . Org. Chem., 24, 1355 (1959). (4) A. S. Chia and R. F. Trimble, Jr., J . Phys. Chem., 65, 863 (1961). (5) J. A. Pople, W. G. Schneider, and H. J. Bernstein, “High Resolution Nuclear Magnetic Resonance,” McGraw-Hill Book Co., Inc., New York, N. Y.,1959. (6) A. D. Buckingham, T. Schaefer, and W. G. Schneider, J. Chem. Phys., 32, 1227 (1960). (7) V. C. G. Trew, Trans. Faraday Sac., 49, 604 (1953). (8) By assuming Wiedemann’s additivity law for the systems, these values were calculated from molar susceptibility, XM. The values of XM. (XlOe) for water and H&Oa are taken to be -12.97 and - 39.8, respectively (“Handbook of Chemistry and Physics,” Chemical Rubber Publishing Co., Cleveland, Ohio, 1963).
Volume 69, Number 8 August 1966
NOTES
2792
-2.7
-5- -
t
Table I: Aromatic Proton Chemical Shift of Pyrazine and Its Protonated Species in P.p.m. Relative to Cyclohexane and Solvent Effects Parameter Species
2.9
Pyrazine Monoprotonated Diprotonated
a
'Jb
0,389 0.334
(To
0.186 0.745
6oar
-7.075' -7.567b - 7. 894b
This value is the infinite dilution shift of pyrazine in carbon tetrachloride relative to the internal cyclohexane reference. These values were obtained by considering the chemical shift of pure benzene relative to cyclohexane, -5.305 p.p.m.
-3.6
m
'
-3.7
-3.8
was measured as a function of pyrazine mole fraction, z. The internal reference was approximately 1 mole
-3.9
% cyclohexane. The results are given in Table 11. 5
0 'io
10
Table 11: Proton Chemical Shift of Pyrazine in Trifluoroacetic Acid in P.p.m. Relative t o Internal Cyclohexane Reference
PYRAZINE BY WEIGHT
Figure 1. The chemical shift of pyrazine (relative to external benzene reference) as a function of concentration in sulfuric acid: a, 64.4%; b, 95%.
is analogous to that of protonated pyrazines. At infinite dilution the resonance shift of benzene in concentrated sulfuric acid (corrected for the bulk susceptibility) has a lower field shift of 0.745 p.p.m. relative to gaseous benzene. This value corresponds to the solvent effects shift of diprotonated pyrazine. When 64.4% sulfuric acid is used as solvent, the signal of benzene cannot be observed because it overlaps with the signal of the acid protons. The stoichiometric mole fraction of H2SO4in 64.4% sulfuric acid is 0.25. The solvent effects shift of monoprotonated pyrazine may be given by 0.745 X 0.25 = 0.186 p.p.m. The polar effect shift, UE, is approximately proportional to the solvent parameter, (e - 1 ) / ( ~ l),where e is the dielectric constant of the solvent. This perturbation shift is also involved in the observed shift which was measured in trifluoroacetic acid using the internal reference. The values of ( E - 1)/(e 1) for sulfuric a,cid and trifluoroacetic acid are 0.97 and 0.95, respec1,ively. Therefore, UE in sulfuric acid has approximately the same value as in trifluoroacetic acid, and the correction for this effect may be unnecessary. The observed chemical shifts corrected for solvent effects, are given in Table I, along with the solvent effects parameters needed for these corrections. Protonation of Pyraxine by TriJluoroacetic Acid. The chemical shift of pyrazine in trifluoroacetic acid
+
+
The Journal of Physical Chemistry
Mole fraction of pyrazine
6, p.p.m.
0.0170 0.0315 0.0554 0.0914 0.1203 0.1557 0.1839
-7.899 -7.911 -7.922 -7.940 -7.953 -7.964 -7.973
Mole fraction of pyrazine
6, p.p.m.
0.2158 0.2444 0.2734 0.3011 0.3446 0.3957
-7.981 -7.963 -7.934 -7.905 -7.844 -7.779
In the presence of aromatics, the signal position of the internal reference compound is markedly altered. For this reason, correction for the shift of cyclohexane must be applied to the observed shift of pyrazine. Since the precise volume susceptibility of the solutions is not known, the internal reference shift cannot be determined wit,h respect to external reference. Fortunately, as the temperature is lowered a new resonance signal of cyclohexane appears a t a lower field. The chemical shift of the new peak is a few tenths p.p.m. relative to the signal which appears a t room temperature. Under this condition the Tyndall phenomenon was observed, corresponding to the formation of a colloidal solution. Spherical particles are dispersed in the medium which has approximately the same composition of pyrazine and trifluoroacetic acid as that a t room temperature. Cyclohexane is the main component of the dispersed particle, and the concentration of pyrazine in it is very low. Solvent effects on the resonance shift of dispersed cyclohexane are negligible. The particle corresponds to a spherical
NOTES
2793
0.5
-5
dilution, -7.889 p.p.m., is in good agreement with that of diprotonated pyrazine, -7.894 p.p.m. There is a hump in the cwve at a concentration x = 0.22. The chemical shift at this concentration, -7.58 p.p.m., is in good agreement with the shift of monoprotonated species, -7.567 p.p.m. These results indicate that monoprotonation is incomplete at concentrations z > 0.22 and that this reaction approaches completion at x = 0.22. Diprotonation occurs at x < 0.22, at which concentratlions unprotonated species do not exist. At very low concentration pyrazine exists largely as diprotonated species. That the two protonation reactions do not overlap is attributed to the stabilization of monoprotonated pyrazine.
1
0.4
,"0.3 Lo P 0.2 0.1
0
0
0.I
0.2
0.3
0.4
MOLE FRACTION PYRAZINE Figure 2. Plot of A8 us. concentration of pyrazine a t -35".
The Behavior of the Silversilver Iodide Electrode in the Presence of
-7.3
Tetraalkylammonium Ions
- 7.4
by R. FernBndez-Prini and J. E. Prue
-
-7.5
E
Department of Chemistry, University of Reading, Reading, Berkshire, England (Received August 6 , 1964)
a P
-7.6 Lo
- 7.7 - 7.8 -7.9 0
0.1
0.2
0.3
0.4
MOLE FRACTION PYRAZI'NE Figure 3. The chemical shift of pyrazine as a function of concentration in trifluoroacetic acid. Chemical shifts are corrected for reference shift and referred to the internal cyclohexane signal.
external cyclohexane reference and gives a low-field signal. The separation, As, of this signal from the high-field signal may be the internal reference shift. The results obtained at -35" are shown in Figure 2 as a function of pyrazine concentration. The shift of cyclohexane was determined a t low temperature only and is assumed to be the same for room temperature. The chemical shifts, corrected for the reference shift, are plotted in Figure 3. The chemical shift at i&nite
Measurements of e.m.f. of the cell AgIAgIINRJ(c) lKCl(satd.) IKI(c) IAgIIAg were recently rep0rted.l Mean ionic activity coefficients for tetraalkylammonium iodides were calculated from the results, but it was soon pointed out2 that the values were absurdly high and in conflict with those already reported in' the literature. New measurements by the isopiestic technique3g4 have confirmed Stokes' conclusion. Like other w ~ r k e r s we , ~ ~can ~ confirm that the e.m.f. values reported in the original work' are approximately correct. Frank6 has maintained that meaningful values of the ratio of the activity coefficients of the iodide ion in the two solutions can be obtained from the results (e.g., he calculates ~I-(NE~,I)/~I-(KI) = 14.7 when c = 0.3 M ) , but by inserting experimental mean ionic activity coefficients and reasonable estimates of transport numbers in an exact formula' we have satis(1) M. A. V. Devanathan and M. J. Fernando, Trans. Faraday SOC., 784 (1962). R. H. Stokes, ibkl., 59, 761 (1963).
58, (2) (3) (4) (5) (6)
V. E. Bower and R. A. Robinson, ibid., 59, 1717 (1963). S. Lindenbaum and G. E. Boyd, J. Phys. Chem., 68, 911 (1964). J. C. Rassiah, quoted in ref. 6. H. S. Frank, J. Phys. Chem., 67, 1554 (1963).
volume 69, Number 8 August 1966
