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, The 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 . Phys. Chem., 70, 2702 (1966). (2) J. C. Evans and G. Y-S. Lo, ibid., 70, 11 (1966).
Volume 71, Number 11 October 1967
NOTES
3698
ents can exist at the C1 nuclei in ClHfi ions located in different molecular environments. Estimates of the corresponding chlorine electron densities may be derived, and further insights into the nature of the hydrogen bond may thus be achieved.
C1-H Cl
(1)
Ci H-C1 +
(11)
6
(111)
CiH
where the CI-H in the two equivalent structures (I) and (11) is the purely covalent structure, not the hydrogen chloride molecule. The relative contributions of these structures may be determined from the observed C136resonance (the estimated value of 12.3 MHz at 77°K was used) and the value of 109.7 MHz for the coupling constant of covalently bonded C1 in structure I and the value zero for C1-. With the usual assumption of 15% s character for the C1 bonding orbital, the approximate charge distribution is calculated to be
Experimental Section
Tetraethylammonium hydrogen dichloride (Eastman), dried by azeotropic distillation with benzene followed by prolonged exposure to vacuum, was dissolved in dried methylene chloride. Anhydrous HC1 (illatheson) was passed into the solution; cyclohexane was added to induce crystallization of the desired hydrogen dichloride salt. Titration of a known amount with standard NaOH solution, and its infrared spectrum, served to characterize the product. -0.74 0.48 -0.74 The 10-26-MHz range was scanned using a superC1 H ._________. C] regenerative instrument based on a published d e ~ i g n . ~ With the sample held at 26", one resonance (SIN apThe LCAO-MO description of the symmetrical ion proximately 3) was observed at 11.89 f 0.03 MHz. yields the same charge distribution; the relation beThis was assigned to C135;the C13' resonance was below tween chlorine coupling constant and the bonding the noise level. At 77"K, under poorer filling-factor parameters for this model is given in ref 1. conditions, the C135resonance could not be detected; We now have approximate charge distributions, deits position was estimated, assuming a temperature shift rived from nqr data, for hydrogen dichloride ions of two comparable to that previously observed' to be approxidifferent structures. Also available is the charge dismately 12.3 MHz. tribution for solid HCl, which consists of molecular HC1 units bonded through relatively weak hydrogen bonds, Discussion derived in the same manner from the published C135 The earlier discussion' showed that if the two equivaresonance value.5 lently contributing resonance hybrids A and B 0.43 -0.43 C1-H Cl; C1 H-C1 HC1 insolid HC1
,4
B
unsymmetrical where the C1-H bond is the same as that in the HC1 molecule in crystalline HCl, provide an adequate description of the symmetric [ClHClI- ion, then the two C1 nuclei should yield a single C135resonance near 13 MHz. The value near 20 RIHz observed for tetramethylammonium hydrogen dichloride was interpreted in terms of an unsymmetrical structure to which A and B did not contribute equally. The present case of a resonance near 12 MHz indicates the presence of a symmetrical anion; the lack of splitting shows crystallographic as well as chemical equivalence. The vibrational data also indicated high symmetry for the anion in this crystaL2 We shall accept the symmetrical structure and investigate the electron distribution implied by the observed C135resonance. The interpretation follows Townes and Dailey's method^.^ In terms of resonance among several idealized- covalent and ionic structures, the symmetrical ClHCl ion may be described by three main contributing structures The Journal of Physical Chemistry
-0.86 0.43 -0.57 C]------H-C]
+ in [^lle]&. HC12symmetrical
-0.74
0.48 -0.74
+ Cl-H--Cl
in [Et]JY*Hc12
This series shows that the electronic-charge redistribution which occurs as a chloride ion approaches a hydrogen chloride molecule along its symmetry axis to form a hydrogen bond is largely a transfer of charge from the chloride ion to the vicinity of the other chlorine atom. The charge density near the hydrogen atom remains almost unchanged. (3) G. E. Peterson and P. hf. Bridenbaugh, Rev. Sci. Instr., 35, 698 (1964). (4) C.H.Townes and B. P. Dailey, J . Chem. Phys., 17, 782 (1949); 23, 118 (1955). (5) H.C.Allen, J. Phys. C h m . , 57, 501 (1953).
