1147
NOTES r
Efhonol 7 : 3 Cyclohexonot IO
0
L
F F cyclohexunol
Ternperafure, "C Figure 2. Dielectric relaxation of ethanol-cyclohexanol a t 1000 Mcps. C indicates the mole fraction of ethanol in the mixture.
mixtures
two associative liquids or of two nonassociative liquids. Consequently, Schallamach's rule seemed not to hold. For the purpose of confirming clearly these contradictory results, the dielectric dispersion of mixtures of two associative polar liquids was observed both in a supercooled state and in a true liquid state. A mixture of ethanol and cyclohexanol was chosen in this experiment because they have a tendency to supercool easily and partly because they have a dielectric dispersion in a convenient frequency range. Hypersonics of 50 kcps, 100 W was applied to the solutions so as to make mixing as homogeneous as possible. Figure 1 shows the dielectric dispersion observed a t 2 Mcps (capacitance bridge) of pure ethanol, pure cyclohexanol, and mixtures of them a t various concentrations. The concentration of ethanol in the specimen is indicated by the mole fraction C, in the figure. These experimental results show that two individual dispersions are no longer distinguishable in the mixtures. Figure 2 shows the dielectric dispersion of the ethanol-cyclohexanol mixtures observed a t 1000 Mcps (coaxial line) a t temperatures above 0". Two regions of dielectric dispersion appear clearly in this temperature range. The experimental curve may be considered to be the superposition of the two dispersions as shown by the dotted curve in the figure. The results also indicate that the dispersion corresponding to ethanol in the mixture shifts to the higher temperature, and that corresponding to cyclohexanol shifts to the lower temperature with the increase in the concentration of
m
-
40 *C IGc
03Gc
ethanol. Figure 3 shows the Cole-Cole plot of the dispersion observed a t 40" on the ethanol-cyclohexanol mixture whose concentration of ethanol is 0.7 mole fraction. The curve clearly contains two regions of dielectric dispersion. The experimental curve may be considered to be the superposition of the two semicircles as shown by the dotted circles in the figure, but Smyth, et a1.,8 suggest a new method concerning the separation of two superposed dispersions. It is seen from these experimental results that two individual dielectric dispersions of the ethanol-cyclohexanol mixture are clearly distinguishable in the liquid &ate, whereas they are no longer distinguishable in the supercooled state.
Hyperfine and g=Tensor Anisotropy
of the Tropenyl Radical1 by Walter V. Volland2 and Gershon Vincow3 Department of Chemistry, University of Washington, Seattle, Washington 98105 (Received October 9 , 1968)
Theories have been proposed for the hyperfine and g-tensor allisotropy of polycentric ?r-electron radical^.^ These quantities also enter into the theory of esr line widths.6 Comparison of theory with experiment has been very limited due to a paucity of measurements of these molecular parameters.6 I n this note we wish to report anisotropy parameters for the tropenyl radical, (1) Supported by the U. S.Army Research Offlce (Durham). (2) Department of Chemistry, Cornel1 Cniversity, Ithaca, N. Y. (3) Alfred P.Sloan Research Fellow. (4) (a) H. M. McConnell and J. Strathdee, Mol. P h y s . , 2 , 129 (1959): (b) W. Derbyshire, ibid., 5 , 225 (1962): (c) A. J. Stone, ibid., 7 , 311 (1964); (d) H. J. Silverstone, D . E. Wood, and H. 111. McConnell, J . Chem. P h y s , 41, 2311 (1964). (5) G. K. Fraenkel, J . Phys. Chem., 71, 139 (1967). (6) (a) 0. Heller and T. Cole, J . Chem. Phys., 3 7 , 243 (1962); (b) H. C.Heller and T. Cole, J . Amer. Chem. Soc., 84, 4448 (1962): (c) R. J. Cook, J. R. Rowlands, and D. H. Whiffen, Mol. P h y s . , 7 , 57 (1963); (d) 9. H. Glarum and J. H. Marshall, J . Chem. Phys., 44, 2884 (1966). Volume YS, Number 4 April 1069
NOTES
1148
a.
sevenfold axis (but not about axes in the aromatic plane). Because of this motion the 7 protons are equivalent and C7H7. is effectively axially symmetric; i e . , the in-plane components of the g and dipolar hyperfine tensors are averaged. Principal values of the g and hyperfine tensors are designated 911,gr, and g r and a 2b, a - b, and a - b, respectively. Corresponding principal axes are the sevenfold axis ( z axis) and two mutually perpendicular axes in the molecular plane. Since (9. - 911) and b are relatively small, their effects are masked by the large line width, and the spectrum is similar to that of the radical in liquid solution. We have prepared C7H7. in polycrystalline perdeuterionaphthalene, CloDs, in order to reduce the magnitude of the intermolecular dipolar interactions and hence the line width. The spectrum (Figure 1) is strikingly diferent from that of C7H1. in CloHs: hyperfine components are not overlapped, peak-height ratios differ markedly from binomial, and several components are quite asymmetric (Figure l b ) . The theory of polycrystalline line shapes, for the case of a radical with axial symmetry and infinitesimal line width, has been employed to indicate which resonance fields and spacings should be measured (see Figure lb).7cJj We have also measured 91" from the mean of H*-1/2,~=x,2 and H*+llz,~=n/2.The stars denote that these are approximate values of the parameters which must be corrected for the effect of finit,e line width and the nuclear Zeeman interaction. Results of our measurements (room temperature) are: 1 ( a - b)" I = 3.69 f 0.01 C, Xlp = 1.19 f 0.02 G, X 3 / 2= 1.74 f 0.04 G, gr* = 2.00294 =t 0.00001, and AH = 0.55 f 0.03, 0.50 f 0.02, 0.57 f 0.02, 0.57 f 0.01, 0.54 f 0.01, and 0.54 f 0.03 G for M I = -5, --i, , . . +$,respectively. Computer calculations of the polycrystalline line shape for the case of finite line width have been performed.9 The following values lead to excellent agreement between calculated and experimental spectra: a = -3.87 G, b = -0.18 G, g r = 2.00286, 911 = 2.00236, and the Gaussian line width equals 0.48 G.lo The z component of the dipolar hyperfine tensor is 2b = -0.36 G ( p " F% 3 ) . Corresponding values for CHa.ll (p' M 1) and .CH(COOH)2l2 ( p l F% 1) are 0.5
+
I 3
b.
'S
I 3
'S
I
.$
+g +;
l
l
ti
Figure 1. a. Tracing of the esr spectrum of C7H7. in polycrystalline CLOD* a t room temperature. The outermost lines are obscured by noise. Components are labeled according to the value of M I , the z component of the total nuclear spin angular momentum (units of ti) ; i t is assumed that a < 0. b. Tracing of the M I = and +$components showing the various spacings which are measured. The quantity XM,= [(SA* - ~ I I * ) / ~ ~ I-*3b*Mris ] H M ~ameasure ,~* of the effect of anisotropy on component shape. H,ur,e* designates the resonance field for radicals with angular momentum MI and angle 6 between the z axis and the applied field.
+%
C ~ H ~ In S .addition, ~ we report the first measurement of the temperature coefficient of a dipolar proton hyperfine splitting for a ?r radical. The esr spectrum of C7H7- in solid polycrystalline naphthalene (CloHs) a t room temperature consists of eight equally spaced, overlapped components with AH,, M 1.7 G and relative intensities close to the binomial ratios.4d It is inferred from the spectral shape that the tropenyl radical is reorienting rapidly about its The Journal of Physical chemistry
(7) For previous work on tropenyl radical, see: (a) C. Vincow, M. L. Morrell, W. V. Volland, H. J. Dauben, Jr., and F. R. Hunter, J. Amer. Chem. Soc., 87, 3527 (1965); (b) M. L. Morrell, Ph.D. Thesis, University of Washington, Seattle, Wash., 1966; (c) W. V. Volland, Ph.D. Thesis, University of Washington. Seattle, Wash.. 1967. (8) R. Neiman and D. Kivelson, J. Chem. Phys., 35, 156 (1961). (9) R. Lefebvre and J. Maruani, $ b i d . , 42, 1480 (1965). We wish to thank Drs. Lefebvre and Maruani for sending us a copy of their
program. (10) These values are in good agreement with the isotropic splitting and g value (ref 7). (11) M. T.Rogers and L. D. Kispert, J. Chem. P h y s . , 46, 221 (1967). (12) T. Cole, T. Kushida, and H. C.Heller. i b i d . , 38, 2915 (1963).
NOTES and 0.8 G, respectively. It is evident that the contribution of nonadjacent carbon atoms in C7H7- is most important and is negative. Silverstone, Wood, and M ~ C o n n e lhave l ~ ~ made an approximate calculation of 2b for C7H7. and obtain -0.7 G, in fairly good agreement with our measurement. Experiments on C7H7. in C I O Dhave ~ also been conducted a t - 145’. The most interesting result obtained is an approximate value of the temperature coefficient of 2b, A ( 2 b ) / A t m -0,5mG/OC. This increase in j2b I with increasing temperature may be associated with the averaging of the dipolar tensor over the out-ofplane CH bending motion. The value of gll is of interest since the trace of the g tensor of C7H7- (in general of C,H,) is anomalous relative to the value predicted using Stone’s correlation for nondegenerate hydrocarbon r a d i c a l ~ . ~ ~ It J 3 has been proposed that the vibronic near degeneracy of C7H7. might lead to an enhanced magnitude of the deviation of gll from the free-electron Stone has estimated that for all nondegenerate radicals gll = 2.00238.13e From our experimental result, 2.00236, it is clear that the effect of the vibronic near degeneracy is negligible. (13) (a) A. J. Stone. M o l . Phys., 6 , 509 (1963): (b) B. G. Segal, M. Kaplan, and G. K. Fraenkel, J. Chem. Phys., 43, 4191 (1965).
Fluorescent Yields of 1,2,3,4-TetrahydronaphthaIene Excited in the 2850-3100-i Region by M. Grossman, G. P. Semeluk, and I. Unger The Department of Chemistry, University of New Brunswdck, Fredericton, New Brunswick, Canada (Received October 2 1 , 1 9 6 8 )
I n recent years ample evidence has been accumulated demonstrating that benzene and benzene derivatives undergo intramolecular rearrangement when irradiated It has been suggested that in the 2500-A benzvalene is first formed when benzene is irradiated in the neighborhood of 2500A in both liquid and gas phases. I n the gas phase the benzvalene rapidly rearomatizes to benzene, whereas in the liquid phase collisional deactivation stabilizes some of the benzvalene molec~les.~~~ 1,2,3,4-Tetrahydronaphthalene(tetralin) , from the photochemical point of view, is very similar to an ortho-substituted benzene. Thus one might expect tetralin to show the same photochemical behavior as o-xylene’ or o-difluorobenzene.8 Should this be the case, then the irradiation of tetralin could in principle lead to meta- and para-bridged compounds. An investigation of this type should begin with an examination of
1149
Figure 1. A, Absorption spectrum of tetralin in cyclohexane taken on a Coleman-Hitachi, Model EPS-3Tspectropgotometer. B, Fluorescence spectrum of tetralin excited by 2900-A radiation.
the relationship between singlet and triplet yields and exciting wavelengths. Presumably, at the exciting wavelength where the quantum deficit [l - (as @T)] is largest, benzvalene formation will be most likely to occur? This note summarizes the results of such an investigation for tetralin excited in the 2850-3100-A region. All emission experiments were carried out in the liquid phase on a modified Aminco-Bowman spectrofluorometer? The solvent used was fluorometric grade cyclohexane obtained from Aminco. The tetralin used in this study was purchased from the Fisher Scientific Co. fractionally distilled, and recrystallized from Freon 12 at -120’. Tetralin treated in this manner was found to contain less than 0.5% impurity by vpc. The biacetyl was obtained from the Aldrich Chemical Co. All solutions were thoroughly outgassed prior to use. The fluorescence yields reported here are based on a recently reported fluorescence quantum yield of benzene in cyclohexane solution.1° When tetralin is excited by light of 2850-31OOA it exhibits a very strong fluorescent emission extending from 2800 A to about 3800 A; no phosphorescent emission is observed. Figure 1shows a typical fluorescence spectrum along with the absorption spectrum. Exciting wavelengths below 2850A also produce a strong fluorescent emission. However, due to interference by scattered light, the quantum yield of fluorescence excited
+
(1) K. E. Wilzbach, J. 9. Ritscher, and L. Kaplan, J . Amer. Chem. SOC., 89, 1031 (1967). (2) I. Haller. ibid., 8 8 , 2070 (1966). (3) H. R. Ward and J. S. Wishnok, ibid.. 90, 1085 (1968). (4) D. Bryce-Smith, Pure A p p l . Chem., 16, 47 (1968). ( 5 ) K. E. Wilzbach, A. L. Harkness, and L. Kaplan, J. Amer. Chem. Soc., 90, 1116 (1968). (6) L. Kaplan and K. E. Wilzbach, ibid., 90, 3291 (1968). (7) W. A. Noyes, Jr., and 0. S. Burton, Ber Bunsenges. Physik. Chem., 7 2 , 146 (1968). (8) J. L. Durham, G. P. Semeluk, and I. Unger, Can. J. Chem.. in press. (9) F. W. Ayer, F. Grein, G. P. Semeluk, and I. Unger. Ber Bunsenges. Physik. Chem., 7 2 , 282 (1968). (10) I. B. Berlman, “Handbook of Fluorescence Spectra of Aromatic Molecules.” Academic Press, N e w York, N. Y., 1965. Volume YB, Number 4 April 1060
