NOTES
1018
the OH groups in hydrated ketones are perfectly normal alcohols. The strong absorption doublet in the 1100 cm.-’ region due the C-0 stretch shows much greater splitting for triquinoyl than for the other hydrates. This may be due to the presence of two kinds of OH groups (polar and equatorial). I n conclusion, the infrared spectra of leuconic acid and triquinoyl definitely indicate that all the carbonyls are completely hydrated, for both compounds. This is indicated not only by the absence of the characteristic strong C=O absorption in the region from 1500 to 1800 crn.-l, but also by the agreement between the spectra of these molecules with that of the known hydrate, chloral hydrate. The spectra of all three molecules are entirely consistent with the characteristic frequencies expected for normal OH groups. It is a pleasure to acknowledge the many helpful discussions with Dr. Norman C. Baenziger. SPECTROSCOPIC SPLITTING FACTORS I N AROMATIC RADICALS BY H. M. MCCONNELL AND R. E. ROBERTSON Contribution No. 9194, Gate8 and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena 4 , California Received March 22, 1967
This note presents a few qualitative remarks on the relation between molecular structure and spectroscopic splitting factors (g-factors) observed for aromatic radicals with paramagnetic resonance. A tabulation of experimental -factors for solutions is given by Wertz’; -factors are averages of the elements of the diagonal molecular g-tensor, g(m). Practically all observed ’s are -0.00024.004 greater than the “free-spin” factor, gfs = 2.0023. Anisotropic g-tensors for single crystals, g(sc) , have been measured by Kikuchi and Cohen,2 and by van Roggen, van Roggen and Gordye3 For example, for diphenylpicrylhydrazyl the g(sc) tensor has an axis of symmetry, and gll(sc) = 2.0095 and gl(sc) = 2.0035. Unfortunately, the g(sc) tensors give little direct information on g(m) because (a) the crystal structures are not known and (b) the observed g(sc) tensors are very probably electron exchange averaged g(m) tensors of non-equivalent molecules in the crystalline lattice. The single crystal g-factors do however show the molecular g-factors to be anisotropic, and probably give the correct order of magnitude for this anisotropy. These observations can be explained quaIitatively in terms of spin-orbit interaction which mixes u and a configurationally excited states with the ground state. For brevity consider an axially symmetric planar aromatic radical where gli and gl refer to field directions perpendicular and parallel U* exto the plane of the aromatic ring. The a citations with average energy AE1 involve excitation of the odd a-electron into an antibonding u orbital, u*, and reduce gl by 2p/AE1 where { = 28 (1) J. E. Wertz, Chem. Reus., 66, 829 (1955). +-
C. Kikuchi and V. Cohen, Phys. Rev., 98, 394 (1954). (3) A. van Roggen, L. van Roggen and W. Gordy, ibid., 105, 50 (19571, (2)
Vol. 61
cm.-l is the 2p spin-orbit coupling parameter of the carbon atom.4 The u + ?r excitations increase g l by 2 {/AE2 and involve excitation of a-bonding electrons to a-orbital states, with average energy AEz. 911 must be close to gfs because only highly energetic (and hence relatively unimportant) u (bonding) -+ u* (antibonding) transitions, or very weak multicenter spin-orbit interactions involving ?r + a* transition^,^ can contribute to gll. Thus = 1/3 (sis 2 g i ) and > grs providing AE1 > AEz. This inequality corresponds to a qualitatively reasonable conclusion that the u* states are energetically more antibonding than are the CT states bonding. If (A&-’ - AE1-l) is in the reasonable range, 1 - 0.1 e.v: l, = 2.007 - 2.003 and gL = 2.009 - 2.003. This rough estimate is in accord with the experimental observati0ns.l Most hetero atom substitutions in aromatics are predicted t o increase further and g l because (a) typical substituents have relatively large 5 values4: 0 (152 em.-’); F(272); Cl(587); Br(2460); I(5060) and (b) several hetero groups (e.g., -N=O; C=O) have non-bonding (n) uelectrons which show low energy n + a transitions6 and thus reduce the effective AE2. Effects (a) and (b) are proportional to the odd electron density on the hetero atom. These ideas are supported by the observation that the largest ’s, ~ 2 . 0 0 6 , are observed for compounds (diphenylnitric OXide,1,6 semiquinone ions1) which are expected t o a transitions.5 Similarly, the show low energy n observed effect of halogen substitution in increasing paramagnetic resonance line widths in several polycrystalline aromatic radicals’ may be due t o the expected enhancement of the g-factor anisotropy, (91 -. &I)’ In conclusion we note that, on the basis of the present discussion, all the elements of g(m) and ~ ( s c can ) be significantly greater than gfsfor a nonplanar aromatic radical.
+
t
*
+-
(4) D. 9. McClure, J . Chem. Phys., 20, 682 (1952); 17, 905 (1949). (5) For a summary of data and additional references, see H. hi. hlcConnel1, ibid.,20, 700 (1952). (6) R. Hoskins, ibid., 26, 788 (1956). (7) R. I. Walter, R. D. Codrington, A. F. D’Adains, Jr., and H. C. Torrey, ibid., 25, 319 (1956).
POTENTIAL OF T H E Ru(VII)-R1I(VIII) COUPLE BY ROBERTE. CONNICK AND C. ROBERT HUREEY Contribution from the Department of Chemistry. University of Calzfornia, Berkeley, Cahf. Received A p r d 1 , 1967
Silverman and Levy’ have reported
ti
value of
- 1.00 volt for the Ru(VI1)-Ru(VII1)couple from polarographic measurements. We have checked their value by means of an equilibrium involving the IV, VI1 a,nd VI11 oxidation states of ruthenium. When RuOois dissolved in aqueous solution there is a slow reduction to lower oxidation states and simultaneous liberation of oxygen. The rate de( 1 ) M. D. Silverman and H. A. Levy, J . A m . Chem. Sac., 7 6 , 3319 (1954).
r
