[ lG]Annulene Anion Radical
2037
was a recipient of a NDEA Title IV Predoctoral Fellowship and this paper is based largely upon his thesis, submitted as partial fulfillment for the Ph.D. a t the University of Utah.
References and Notes (1) A. Abragam, "The Principles of Nuclear Magnetism", Oxford University Press, London, 1961, Chapter 8. (2) J. S. Waugh in "Molecular Relaxation Processes", Academic Press, London, 1966. (3) D. K. Green and J. G. Powles, Proc. Phys. SOC.(London), 85,87 (1965). (4) (a) K. F. Kuhlmann and D. M. Grant, J. Chem. Phys., 55, 2998 (1971); (b) T. D. Alger. D. M. Grant, and R. K. Harris, J. Phys. Chem., 76, 281 (1972): (c) J. R. Lyerla, Jr., and D. M. Grant, ibid., 76, 3213 (1972). (5) J. R. Lyerla, Jr.. and D. M. Grant, MTP lnt. Rev. Sci., Ser. One, 4, 155 (1972). (6) (a) c. H. Wang, J. R. Lyerla, Jr., and D. M. Grant, J. Chem. Phys., 55, 4674 (1971); (b) J. R. Lyerla, Jr., D. M. Grant, and C. H. Wang, ibid., 55, 4676 (1971). (7) K. F. Kuhlmann, D.M. Grant, and R. K. Harris, J. Chem. Phys., 52, 3439 (1970). (8) T. D. Alger, S. C. Collins, and D. M. Grant, J. Chem. Phys., 54, 2820 (197 1). (9) T. D.Alger and D. M. Grant, J. Phys. Chem., 75, 2538 (1971).
(IO) D.D. Woessner, J. Chem. Phys., 37, 647 (1962). (11) D E. Woessner, B. S. Snowden, Jr., and G. H. Meyer, J. Chem. Phys., 50, 719 (1969). (12) (a) W. T. Huntress, Jr., J. Chem. Phys., 48, 3524 (1968); (b) D. Wallach and W.T. Huntress, Jr., ibid., 50, 1219 (1969). (13) K. T. Gillenand J. H. Noggle, J. Chem. Phys., 53, 801 (1970). (14) P. S. Hubbard, Phys. Rev., 131, 1155(1963). (15) A. A. Maryott, T. C. Farrar, and M. S. Malmberg, J. Chem. Phys., 54,64 (197 1 ). (16) W. H. Flygare. J. Chem. Phys., 41,793 (1964). (17) I. Ozier, L. M. Crapo, and N. F. Ramsey, J. Chem. Phys., 49, 2314 (1968). (18) P. Rigney and J. Viriet, J. Chem. Phys., 47,4645 (1967). (19) One avoids the difficult problems of gage and the separdtion of the firstand second-order perturbation terms (normally characterized as the diamagnetic paramagnetic shielding terms) by selecting the coordinate system at the Mh carbon atom. All of the nonlocalized diamagnetic terms already have been incorporated along with the nuclear terms into the last term of eq 15. See ref 16 for details. It is the mutual cancellation of remote electron and nuclear terms which reduces the significance of this particular component of eq 15. (20) A. B. Strong, D. Ikenberry, and D. M. Grant, J. Mag. Resonance, 9, 145 (1973). (21) R. A. Assink and J. Jonas, J. Chem. Phys., 53, 1710 (1970). (22) D. R. Linde, J. Chem. Phys., 35, 1372 (1961). (23) D.R. Linde and D. E. Mann, J. Chem. Phys., 27, 866 (1957). (24) D. R. Herschbach and L. C. Krisher, J. Chem. Phys., 28, 728 (1958). (25) J. E. Kitpatrick and K. S.Pitrer, J. Res. Nat. Bur. Stand., 37, 136 (1946).
Electron Spin Resonance of the [ 16]Annulene Anion Radical. ion Association in Hexamethyiphosphoramide Jesus Gilbert0 Concepclon and Gershon Vlncow * Department of Chemistry, Syracuse University, Syracuse, New York 132 10 (Received September 27, 1974, Revised Manuscript Received April 18, 1975) Publication costs assisted by Syracuse University
+
The syn (2 2) dimer of cyclooctatetraene reacts with alkali metal producing the [16]annulene anion radical, which is in disproportionation equilibrium with its dianion and the neutral hydrocarbon. We have exploited this new method of preparing the [16]annulene anion to study ion association in hexamethylphosphoramide. The thermodynamic parameters controlling this disproportionation equilibrium have been obtained for three counterions (Li, Na, K) and the variation has been compared with previous results on the cyclooctatetraene anion and related systems. The variation in the thermodynamic parameters is ascribed to metal association with the [16]annulene dianion. This interpretation is based in part on ESR evidence that the [16]annulene anion occurs in hexamethylphosphoramide as a free ion. The g value and proton splittings are independent of counterion. The g value and line width are independent of temperature.
Introduction 0 t h et a1.l have prepared the radical anion and dianion of [16]annulene and have studied ESR, NMR, and electronic spectra in an effort to elucidate the geometrical and electronic configurations of these species. In this paper we report a new and convenient approach for generating these species. We have investigated the ESR of the radical anion in hexamethylphosphoramide (HMPA) solvent using a number of alkali metal counterions. Thermodynamic parameters have been obtained for the disproportionation reaction P
where
ir, P-
and
ir2-
+ x2-
=== 2r.-
(1)
denote the neutral molecule, anion
radical, and dianion respectively.2 The results are discussed in terms of the extent of ion association in HMPA.
Experimental Section A sample of the syn (2 2) dimer of cyclooctatetraene, syn-tricyclo[8.6.0.0~g]hexadeca-3,5,7,ll,l3,l5-hexaene (see Figure 1; abbreviation to be used is DCOT), was kindly supplied by Dr. A. Anastassiou. Its melting paint and NMR spectrum were consistent with those reported for this dimer (mp 53O; NMR singlets at 6.0, 5.65,and 3.25 ~ p m ) . ~ Hexamethylphosphoramide (HMPA) was purchased from the Aldrich Chemical Go. It was dried over calcium hydride, distilled under reduced pressure, and stored over Linde 4A molecular sieves. Tetrahydrofuran (THF) and
+
The Journal of Physical Chemistry, Vol. 79, No. 19, 1975
Jesus Gilbert0 Concepcibn and Gershon Vincow
2038 a
&
16
ii
[a*-], and [a2-]are the concentrations of [16]annulene neutral molecule, anion radical, and dianion, respectively, and [a2-]= ${[MI - [a*-])
[DCOT] = [R] 7
8
11
I Z
Figure 1. Structural formula for the syn (2 4- 2) dimer of cyclooctatetraene.
1,2-dimethoxyethane (DME) were purchased from Eastman Chemicals. They were dried with lithium aluminum hydride, distilled under a nitrogen atmosphere, and stored under vacuum in a flask containing sodium pyrenide plus an excess of sodium metal. Anion radical samples in HMPA were prepared under reduced pressure Torr) as previously explained in ref 7 . The solution formed was allowed to warm up to 5 O and was stirred until all of the alkali metal dissolved. The time required for the metal to dissolve ranged from 4 hr for potassium to 1 2 hr for lithium. An aliquot of the resultant brownish solution was transferred to a side arm tube (3-mm o.d., Pyrex). These solutions of the [16]annulene anion radical are very stable. Over a period of 2 weeks a sample stored a t room temperature showed only a very small decrease in the intensity of the ESR signal. Preparation of samples with T H F or DME as solvent followed along well-known lines.* The reaction with the alkali metal mirror was conducted a t -70'. At higher temperatures the mirror became covered with a brown precipitate preventing further reaction. Samples in these solvents typically yielded a large amount of a precipitate, presumably the [16]annulene dianion salt. In order to eliminate the occurrence of this precipitate the solution had to be dilute M in DCOT) and only a very weak ESR signal was obtained. Thus only an order of magnitude of the disproportionation equilibrium constant could be obtained using these solvents. X-Band ESR spectra were recorded using a Varian Associates E-9 spectrometer. The temperature in the ESR cavity was controlled using a constant flow of nitrogen gas and was measured with a copper-constantan thermocouple. Errors in relative values of temperature are 4 ~ 0 . 5To ~ . obtain hyperfine coupling constants, the magnetic field sweep was calibrated using a dual cavity with a sample of potassium naphthalenide in DME as ~ t a n d a r d The . ~ g values were also determined using dual-cavity techniques. Potassium pyrenide in DME6 was the standard for the system Cl6H16--HMPA-Na which was then used as the standard for C~~HI~--HMPA-K and C1&16--HMPA--Li. The determination of thermodynamic parameters followed along previously described line^^-^ and is only briefly outlined here. The [16]annulene anion radical concentration was determined a t room temperature by comparison of an overmodulated single-component spectrum to that of a standard sample of cyclooctatetraene (COT)-potassium in HMPA.l0 The equilibrium constant for the disproportionation reaction of [16]annulene (eq 1) can be expressed in terms of readily measured quantities as in
where [MI is the total concentration of metal added, [DCOT] is the concentration of the dimer of COT, [R], The Journal of Physical Chemistry, Vol. 79,No. 19, 1975
+ [P-] +
[a*-]
