3141
NOTES [LiPFb], M 4.01
hi2t]=1~10"3
-0.01
0.025 en
-1.5
[LiPFd, M E
E
2.0
-
Ir '
-OD4
-a06
-ao
-0.08
-a01
-0.12
- 014
Y O
9
Figure 2. Dependence of the rate parameter on the O H P potential. Current measured at -0.8 V(sce).
-E, Volts
VI.
SCE.
Figure 1. Dependence of prewave on t h e concentration of lithium hexafluorophosphate. [OPDA] = 2 x 10-4 M , PH 7.0.
reaction was first order with respect to this ion. The variation of the rate parameter as a function of the OPDA concentration showed that the reaction was first order with respect to this species also. The same results were obtained in the studies with other electrolytes; it was therefore thought that the overall reaction mechanism was the same for LiPF6 as previously demonstrated for other electroIytes.ldtflg In order to determine the value of x , it is simply necessary to maintain a constant bulk concentration of OPDA and nickel(I1) and vary the potential of the OHP, q0,by varying the concentration of LiPFe.ld84 Under these conditions eq 1 reduces to log X
=
xF RT
constant - --!Po
(2)
As the value of qobecomes more negative (a decrease in LiPFe concentration cathodic to the electrocapillary maximum),1dNrt8ey 2 predicts that the rate parameter and, hence, the limiting current of the prewave will increase as is observed in Figure 1. The rate parameter, A, can be determined experimentally from the polarographic data of Figure 1 according to the following relationship i/id = F(h) (3) where i is the limiting current of the prewave and id is the diff usion-limited current of nickel(I1). Typical plots of log h vs. qol 3 are shown in Figure 2. The slope
of the line is in agreement with a value of x = + 2 as one would expect for the reaction of the hexaaquonickel(I1) ion with the neutral OPDA. This result also lends some support to the conclusion that the coadsorbed anion does act as a partner in the surface reaction as suggested previously for the other anion^.^ This would account for the "anomalous" value for x of +1 found in the studies with other anions as the electrolyte. (13) Actual to values for Pli'6- have not been determined. Anson, however, has found that the adsorption characteristics of PFa- are essentially identical with F-10. Thus to values for F- as given by Russell's tables" were used here. (14) C. D.Russell, J . Electroanal. Chem., 6,486 (1963).
The Empirical Shielding Parameter Q and Trisubstituted Benzenes
by G. Socrat'es Department of Polymer Science, Brunel University, London, England (Received October 1.6,1969)
Anomalous ortho proton and fluorine chemical shifts have been observed for chloro-, bromo-, and iodo-substituted ben~enes.l-~These anomalies cannot be explained by the magnetic anisotropy of the halogens.6 (1) H. Spiesecke and W. G. Schneider, J . Chem. Phys., 35, 731 (1961). (2) H.S. Gutowsky, D. W. McCall, B. R. McGarvey, and L. H. Mayer, J . Amer. Chem. Soc., 74, 4809 (1952). (3) A. J. R. Bourns, D. G. Giles, and E. W. Randall, Proc. Chem. Soc., 200 (1963). (4) N. Boden, 3. W. Emsley, J. Feeney, and L. H. Sutcliffe, Mol. Phys., 8, 133 (1964). (5) 3. S. Martin and B. P. Dailey, J . Chem. Phys., 39, 2722 (1963).
The Journal of Physical Chemistry, Vol. 74,No. 16,1970
NOTES
3142 Table I: Observed Chemical Shifts,
1 2 3 4 5 6 7 8 9 10
H CHS CHIO OH CN c1 Br
I NO2 NHz
T
2.28 1.17 0.6 0.38" 3.5 2.55 3.16 3.98 4.0-6.3* 0.11
3.13 3.18 3.18 3.20 2.94 3.07 3.05 3.05 2.94 3.20
3.15 3.27 3.21 3.28 2.99 3.18 3.23 3.40 2.94 3: 25
a T h e value employed here was obtained from monosubstituted benzenes. given.
The presence of time-dependent electric fields arising from the substituent may account for the ortho effect observed in halogen compounds but this concept fails badly when the substituent is a p r ~ t o n . ~ ,An ~ Jincrease in the paramagnetic term of the Ramsey Shielding equationa should account for the ortho effecta9 A very good linear correlation is found between ortho shifts and the quantity Q = P/Irawhere P is the polarizability of the C-X bond, r is the length of the bond, and I is the first ionization potential of the substituent atom X.'0-l6
Experimental Section The spectra were observed on a Varian 100-Ha highresolution nuclear magnetic spectrometer. All samples were thoroughly degassed before recording the spectra. Most of the compounds studied were available commercially. All the samples were purified by distillation under reduced pressure. 4-Chloro- 1,3-dimethylbenzene was prepared from xylidine by forming the diazonium salt and then adding cuprous chloride and hydrochloric acid, boiling point 187". 4-Bromo-l,3-dimethylbenzenewas prepared by the direct bromination of m-xylene, boiling point 203". 4-Iodo-1,3-dimethylbenzene was prepared from xylidine by forming the diazonium salt and then adding excess potassium iodide, boiling point 109" a t 10 mm pressure.
Results and Discussion The proton magnetic resonance spectra of the series of compounds, l-X,2,4-dimethylbenzene,studied were observed in dilute carbon tetrachloride solution (5% v/v), tetramethylsilane being employed as an internal reference. The ring proton resonances were analyzed using a computer assuming an ABC system. A simple noniterative program was employed which from the input paramet,ers set up and diagonalized the Hamiltonian matrix by successive approximations, hence calculating transition energies and line intensities. Preliminary spectral parameters were determined by employing the empirical substituent additivity rules5,17and where The Jownal of Physical Chemistry, Vol. 74, No. 16, 1970
2.95 3.17 3.46 3.53 2.61 2.88 2.68 2.45 2.19 3.65 The
7.73 7.83 7.90 7.86 7.58 7.75 7.72 7.69 7.53 8.00
Q value for the nitro
7.73 7.73 7.83 7.82 7.67 7.79 7.81 7.81 7.68 7.89
3.15 7.73 6.32 4.61
6.89
group varies over the range
possible, homonuclear spin decoupling was also used, By the comparison of the observed and analyzed spectra, the chemical shifts of the aromatic protons could be estimated to be better than 0.01 ppm. The chemical shifts for the l-X,2,4-dimethylbenzenes are given in Table I. The shielding parameters & for the various substituents, X, are also given in this table. I n Figure 1 the chemical shift of the aromatic proton H(6) adjacent to the substituent is plotted against the parameter &. An extremely good linear correlation is observed. The equation of the best straight line obtained by the method of least squares is 7H(6) =
-0.30&
+ 3.66
The maximum deviation observed from this line is 0.02 ppm. I n Figure 2 the chemical shift of the methyl group adjacent to the substituent is plotted against the parameter Q. A definite linear correlation exists although not as good as that obtained above. A similar correlation for the para methyl group chemical shift is not found to exist. Therefore it would appear that the same mechanism determines the shielding of both the aromatic proton and the methyl group adjacent to the substituent. I n (6) T.Schaefer, W. F. Reynolds, and T. Yonemoto, Can. J . Chem., 41, 2969 (1963). (7) V. M.S.Gill and W. A. Gibbons, Mol. Phys., 8, 199 (1964). (8) E. Pitcher, A . D. Buokingham, and F. G. A. Stone, J . Chem. Phys., 36, 124 (1962). (9) N. F.Ramsey, Phya. Rev., 78, 609 (1951). (10) F. Hruska, H.W. Hutton, and T. Schaefer, Can. J . Chem., 43, 2395 (1965). (11) T. Schaefer, F. Hruska, and H. M. Hutton, ibid., 45, 3143 (1967). (12) W.B.Smith and G. M. Cole, J . Phys. Chem., 69, 4413 (1965). (13) W.B. Smith and J. L. Roark, J . Amer. Chsm. Hoc., 89, 5018 (1967). (14) J. L. Roark and W. B. Smith, J . Phys. Chem., 73, 1043 (1969). (15) J. L.Roark and W. B. Smith, ibid., 73, 1046 (1969). (16) W.B.Smith and J. L. Roark, ibid., 73, 1049 (1969). (17).'I Diehl, Helv.Chim. Acta, 44, 829 (1961).
NOTES
3143
LCN LO2
3.0
2.5
2.0
Q
0
what delocalized n-electron systems in which it is considered that the spin densities on the carbon atoms bonded to the nitrogen atom contribute much less to the observed isotropic component of the nitrogen hf coupling constant (UN) than does the spin density on the nitrogen atom itself. On the other hand, in a preceding papers concerning the free-radical intermediates from amino acid derivatives and related compounds with the hydroxyl radical, it was reported that the splitting due to the nitrogen hf coupling was hardly observed in most of the radicals studied. I n these radicals the unpaired electron is more or less localized in the 2p, orbital of the carbon atom directly bonded to the nitrogen. I n order to obtain further esr data with respect to the relation between U N and the spin density distributions, radicals produced by the reaction of the hydroxyl radical with several nitrogen heterocyclic compounds and pyrimidine derivatives have been studied in this work using a continuous-flow method. The hydroxyl radical was generated chemically in a titanous (Ti3+) chloride--hydrogen peroxide (Hz02) system. Nicolau, et uZ.,9 reported the intermediate radicals from pyrimidine derivatives formed in a Ti3+-H202 system, which did not come to the author's attention until the present experiments were finished.
1
2
3
4
5
Figure 1. The chemical shift of H(6) on the against the parameter Q.
6 T
scale is plotted
Experimental Section Figure 2. The chemical shift on the T scale of the methyl group adjacent to the substituent X is plotted against t,he parameter &.
conclusion, a very good correlation of the Q parameter with the chemical shift of the aromatic proton adjacent to the substituent does exist for l-X,2,4-dimethylbenzenes.
Acknowledgment. The author wishes to thank the Department of Chemistry, University College, London, for the use of their Varian 100 Ha high-resolution nuclear magnetic resonance spectrometer.
Free-Radical Intermediates in the Reaction
of the Hydroxyl Radical with Nitrogen Heterocyclic Compounds1
by Hitoshi Taniguchi Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 6'06, Japan (Rcceived February 0, 1970)
Many electron spin resonance (esr) studies have been carried out on the nitrogen heterocyclic radical ions in solution to clarify the origin of nitrogen nuclear hyperfine (hf) c o ~ p l i n g . ~ -These ~ radical ions have some-
The experimental arrangement and procedures for the observation of intermediate radicals in a Ti3+-H202 system were essentially the same as described elsewhere.sr10 Esr spectra of the intermediate radicals were recorded between 5 and 13 msec after the reaction had started." The modulation width used for the recording was varied from 0.5 to 1.6 G. The hf coupling constants were measured using manganous ion as a reference (splitting between the two central peaks, 86.9 G) Uracil and 1,3-dimethyluracil were synthesized from malic acid, urea, and methyl sulfate according to the
-
(1) This work was presented in part a t the 22nd Annual Meeting of the Chemical Society of Japan, Tokyo, April 1969. (2) J. C. M . Henning, J. Chem. Phys., 44,2139 (1966),and references cited therein. (3) P. T. Cottrell and P. H. Rieger, Mol. Phys., 12, 149 (1967). (4) C. L. Talcott and R. J. Myers, {bid., 12, 549 (1967). (5) A. R.Buick, T. J. Kemp, G. T . Neal, and T. J. Stone, J . Chem. SOC.A , 1609 (1969). (6) L. Lunazzi, A. Mangini, G. F. Pedulli, and F. Taddei, ibid., B , 163 (1970). (7) M . D.Sevilla, J . Phys. Chem., 7 4 , 805 (1970). (8) H. Taniguchi, H. Hatano, H. Hasegawa, and T. Maruyama, ibid., 74, 3063 (1970). (9) C . Nicolau, M. McMillan, and R. 0. C. Norman, Biochim. Bkphys. Acta, 174, 413 (1969). (10) H. Taniguchi, K.Fukui, 5 . Ohnishi, H. Hatano, H. Hasegawa, and T. Maruyama, J . Phys. Chem., 72,1926 (1968). (11) The author is grateful to Japan Electron Optics Laboratory Co. for affording an opportunity to use an esr spectrometer. The J O U T ~ofUPhysical ~ Chemistry, Vol. 74, No. 16, 1970
