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on the 12 correlations shown in Table I1 is only 5~0.016 v for this system. It appears that these SCF approaches give a small, but real, improvement over the HMO method used to date. The latter method, however, is definitely less complex and the question of whether the small improvement is worth the extra effort is a moot one. We feel that the real advantage of the calculations described here will be in treating nonalternant and alternant systems within the same correlation. Mention must also be made to the fact that the theoretically more sound idea of correlation with an energy difference rather than a ground-state property does not particularly improve results. It appears that in all of the systems treated here, a fortuitous correlation exists between this quantity and the energy of the highest occupied orbital. If this is truly general, future investigations should be greatly simplified.
Acknowledgment. We wish to extend our thanks to the Computer Center of Oregon State University for generously supplying the facilities necessary for carrying out these calculations.
NOTES
N-alkylpyridinium ions should not be reduced by chromous or vanadous ion. Table 16-" summarizes the results of experiments in which salts of several organic cations were allowed to react with chromous perchlorate. The cations are listed in the order of their half-wave reduction potentials, the pyridinium ions being the most difficult to reduce. The chromous-chromic potential, relative to sce, is included, and those ions listed above this potential should not accept an electron from chromous ion.
Table I: Reduction of Organic Cations with Chromous Perchlorate
Cation
1,2,4,6-Tetramet hylpyridinium N-Methylpyridinium
- E'/z, v us. ace
Anion
1.477
NO3
32 (5 hr) 47 ( 5 hr) (b) 0
1.06'
C1 NOa clod
34 (24 hr) 0 (24 hr) 53*
0 . 855,8
c104 c1
Anomalous Reductive Dimerizations
0.66 ( E " ) Cr(I1) 2,4,6-Triphenylpyryli~1m 0 . 309 0 . 271° Tropylium
of Alkylpyridinium Ions
reductionYield, %
c104 c1 I
(1.8)O
N-Benz ylpyridinium 1-Benzyl-3-carbamidopyridinium 2,4,6-Trimethylpyrylium
-Cr(C10&
c1 clo4 C1
0 (12 hr) 0 (12 hr) (e) 100 (rapid) 100 (rapid)
Estimate based on charge-transfer spectra of iodide salt. See ref 6. b The dimer has not been isolated from this reaction or from the sodium amalgam reduction, but its characteristic behavior, including its unusual color changes, serves to establish its formation.11 c As described in ref 4; actual yield not reported. Reduced by VCl, as well as CrC12. Q
by William T. Bowiel and Martin Feldman Department of Chemistry, Howard University, Washington, D. C. (Received M a y 5 , 1907)
The reductive dimerization of N-alkylpyridinium ions in aqueous solution, which yields 1,l'-dialkyl1,1',4,4'-tetrahydrodipyridyls, may be brought about by sodium amalgam,Z electroly~is,~ or chromous or vanadous ion.4
We have begun a study of the mechanism of reduction of stable organic cations with metal ions in order to compare the rates of electron transfer with the relative electron affinities of the cation^.^ We have noted an apparent anomaly in the reduction of X-alkylpyridinium ions: according to their one-electron reversible polarographic reduction potentials in aqueous solution, T h e Journal of Physical Chemistry
As shown in Table I, we have found no evidence for the reduction of 2,4,6-trimethylpyrylium ion by chromous (1) NASA Predoctoral Trainee, 1966-1967. (2) A. R. Hofmann, Ber., 14, 1503 (1881). (3) B. Emmert, ibid., 42, 1997 (1909). (4) J. B. Conant and A. W. Sloan, J . Am. Chem. Soc., 45, 2466 (1923). (5) Ill.Feldman and S. Winstein, Tetrahedron Letters, 853 (1962). (6) hl. Feldman, Ph.D. Thesis, University of California a t Los Angeles, 1963. (7) P . C. Tompkins and C. L. A. Schmidt, Univ. California Publ. Physiol., 8, 237 (1944). (8) K. Wallenfels and M. Gellrich, Chem. Ber., 92, 1406 (1959). Coupling occurs at the 6 position for this ion. (9) E. Gird and A. T. Balaban, J . Electroanal. Chem., 4, 48 (1962). (10) P. Zuman and J. Chodkowsky, Collection Czech. Chem. Commun., 27, 759 (1962). (11) B. Emmert and R. Buchert, Ber., 54, 204 (1921).
XOTES
perchlorate or chloride, which is in agreement with their relative reduction potentials. After 12 hr, zinc dust was added to the reaction mixture and the unreacted pyrylium ion was reductively dimerized to the known bipyranss12 (goyoyield). Bipyran which was added to the chromous solutions and allowed to stand for 24 hr could be recovered, and we conclude that chromous ion does not reduce the trimethylpyrylium ion. Similar experiments indicate that vanadous ion (E" 0.45 v us. sce) is likewise ineffective in the reduction of the trimethylpyrylium ion. In contrast to the expected behavior of the trimethylpyrylium ion, the pyridinium ions are reduced by chromous ion. The presence of halide ion seems to be required for the reduction of S-methyl and X-benzylpyridinium ions, but not for 1,2,4,6-tetramethylpyridinium i0n.I3 The apparent anomaly is not limited to chromous ion; zinc14 (E" 1.0 v us. sce) reductively dimerizes S-benzylpyridinium chloride (99% yield), K-benzylpyridinium nitrate (38Yc), and K-methylpyridinium iodideI5in neutral aqueous solutions. The reduction potentials of N-alkylpyridinium ions which have no ring substituents have been reported by several workers and are in the range of 1.3-1.5 v.7,16-lS These values appear reasonable, since the high reduction potentials of the pyridinium ions parallel the large charge-transfer transition energies of pyridinium iodides, relative to pyrylium or tropylium i0dides.j Thus, we cannot attribute the surprising behavior of the alkylpyridinium ions to inaccurate measurement of the reduction potentials, but we decline to offer an alternative explanation for these facile reductions.1g
Experimental Section All salts and dimeric products have been previously prepared, and their physical properties and chemical behavior agree with those reported in the literature. I n a typical reduction with Cr(C104)2,10 mequiv of the organic salt was dissolved in 30 ml of water, which was acidified with HClOl to prevent hydrolysis of the tropylium and pyrylium salts. The solution was purged with oxygen-free nitrogen for 1 hr, after which 20 ml of 1 N Cr(C104)2was added. The solution was allowed to stand for at least 5 hr under nitrogen and, for salts of low solubility, gently warmed. A t the end of the reaction period, the aqueous solution was extracted with five 40-ml portions of ether (for the pyridinium salts, the solutions were made basic before extraction). Evaporation of the dried ether extract yielded the dimer. In some reactions KCl was added to the reaction solution, and the pyridinium halides were con-
3697
verted to nitrates with AgSOs. Zinc reductions were carried out as described by Balaban.'2 (12) A. R. Balaban, C. Bratu, and C. N. Rentea, Tetrahedron, 20, 265 (1964). (13) E . M. Kosower and E. J. Poziomek ( J . Am. Chem. SOC.,86, 5515 (1964)) note the need for iodide ion in the reduction of l-ethyl4-carbomethoxypyridinium ion to the stable pyridinyl radical, using magnesium in acetonitrile. (14) E . Weitz, et al. (Ber., 57, 153 (1924)), have claimed that zinc does not reduce N-alkylpyridinium ions, although zinc will reduce N-phenylpyridinium ion. In view of the virtually quantitative yield obtained in our reduction of N-benzylpyridinium chloride, we are puzzled by Weitz's unsuccessful reactions. (15) See footnote b, Table I. (16) F. Sorm and 2 . Sormova, Chem. Listy, 42, 82 (1948). (17) E. L. Colichman and P. A . O'Donovan, J . Am. Chem. Soc., 76, 3588 (1954). (18) H. Yasuda and S. Kitagawa, Yakugaku K e n k y u , 27, 779 (1955); Chem. Abstr., 51, 13246f (1957). (19) Complex formation between the pyridinium ion and the reducing agent, followed either by dissociation of the complex subsequent to or simultaneous with electron transfer, or by a bimolecular reaction of the complex to form the dimer, might be suggested as alternatives which could resolve the anomaly. Such explanations, however, must take into account the variety of active reducing agents (zinc metal, chromous, and vanadous ions), as well as the "normal" behavior of the trimethylpyrylium ion. If the complex involved bonding between the metal and a particular carbon atom, the pyrylium ion should be much more reactive than the pyridinium ions, by analogy to their reaction with nucleophiles. I t is difficult to assess the relative stabilities of n-bonded metal complexes of pyridinium and pyrylium ions, but if the frequencies of chargetransfer bands of their iodide salts are relevant here, again the pyrylium ion would be predicted to form the more stable complex.
Chlorine Nuclear Quadrupole Resonance in the Symmetrical Hydrogen Dichloride Ion
by J. C. Evans and G. Y-S. Lo Chemical Physics Research Laboratory, T h e Dow Chemical Company, Midland, Michigan 48640 (Received M a y 15, 1967)
Previous attempts' to record the C13jnuclear quadrupole resonance (nqr) of several hydrogen dichloride salts yielded only one resonance, that of tetramethylammonium hydrogen dichloride near 20 NHz. This frequency indicated an unsymmetrical structure for the ClHCi ion in this salt, which agreed with the conclusion reached earlier2 from vibrational data. Repeated attempts with different preparations have now yielded another C13j resonance, near 12 MHz, for tetraethylammonium hydrogen dichloride. The large frequency shift shows that markedly different electric field gradi(1) J. C. Evans and G. Y - S . Lo, J . P h y s . Chem., 70, 2702 (1966). (2) J. C. Evans and G. Y-S. Lo, ibid., 70, 11 (1966).
Volume 71, Number 11 October 1967
