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The Journal of Physical Chemistry, Val. 83, No. 12, 1979
Comrnunicatlons to the Editor
COMMUNICATIONS TO THE EDITOR Detection of Translent Species by Measurement of Dlelectrlc Loss’ Publication costs asslsted by the DepaHment of Energy
Sir: The absorption of microwave energy has been used to detect ions in gas,2 l i q ~ i dand , ~ solid4 phases and to measure certain of their properties and reaction kinetics. In this communication we wish to point out that such methods are not limited to the detection of ions and that other chemical changes such as those giving rise to changes in molecular dipole moment will also affect the absorption of microwaves. Thus, it should be possible to measure the corresponding properties and to follow reaction kinetics by these means. Several kinds of experiments are possible. The one to be described involves photochemical generation of uncharged radicals in the liquid phase by modulated light and their detection by what will be termed dielectric absorption. Every ESR spectroscopist knows of the strong effect of polar compounds on the electrical Q of a microwave cavity as a result of absorption of energy from the electric field. It is just this effect which can be exploited. The absorption of energy by a dilute solution of a lossy solute is proportional to the integral of the loss, E”, times the electric field strength over the sample v ~ l u m e .The ~ value of eft for a dilute solution of a polar solute in a nonpolar solvent is given6 by
where eo is the dielectric constant of the solvent, N is the number of solute molecules per cm3, p is the dipole moment of the solute, w is the angular frequency of the sensing electric field, T is the relaxation time, h is Boltzmann’s constant, and T is the absolute temperature. To produce a loss, w of the electric field must match the relaxation time T but the function is not a sharply peaked one so that a loss will exist over a range of frequencies. The relaxation times of typical molecules such as cyclohexanone or benzophenone in low viscosity solvents are such that the maximum values of t” will occur in the range 5-50 GHz.’ The microwave circuit of an ESR spectrometer is designed to detect small changes in energy absorption and can be used for such an experiment. However, the sample cell must allow interaction of the sample with the microwave electric field. This requirement restricts the technique to nearly nonpolar solvents. The reader should not be confused by the use of portions of an ESR spectrometer for this initial demonstration as no magnetic field is used and no magnetic resonance is involved in the measurement of dielectric loss. A radical system was chosen for this study because its kinetic behavior is well known and because a parallel ESR experiment with the same apparatus could be used to verify that the intermediate being detected in the dielectric loss experiment is indeed a radical. A typical microwave circuit with a cavity critically coupled to the waveguide is equivalent to a bridge so that only changes in microwave power are reflected to the detector? If a second arm in the circuit is used to bias the 0022-3654/79/2083- 1676$0 1.0010
microwave detector, it will be sensitive to the change in microwave electric field. The relative change A V can be expressed by i
where the A[&] are changes in concentration of solutes and the Ci are coefficients describing the effects of the solutes. Both loss of reactants and formation of products must be included. These coefficients depend on the configuration of the sample cell, the cavity Q, the molecular dipole moment, and the dielectric relaxation time. Values of C for model compounds can be determined from static measurements of reflected power on solutions of the solute by using the same cavity and cell. The experiment described here used a sample cell of 8-mm i.d. silica tubing in a TEIMdual cavity. The rest of the apparatus consisted of portions of a Varian V4502 ESR spectrometer. To provide high sensitivity, the photolyzing light was sinusoidally modulated and the output of the low-frequency preamplifier fed to a synchronous detector. Much of the apparatus was as described by Paul.Q Both the dielectric absorption and parallel ESR experiments made use of the variation of signal amplitude with light modulation frequency to measure the lifetime of the intermediatesag The system studied was 2 mM duroquinone (DQ) and 1 mM of the corresponding hydroquinone (H2DQ) in cyclohexane. A filterlowhich passed only the wavelength band from about 330 to 380 nm was used. The expected chemistry is DQ 2DQ* DQ* + HZDQ 2HDQ.
-+
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2HDQ.
DQ + H2DQ
producing the neutral durosemiquinone which will disproportionate back to the starting materials. This nondestructive scheme must be realized because photolysis of a sample up to a total of 30 min caused only a slight decrease in ESR intensity. The ESR signal detected was that of the neutral semiquinone.ll At low light modulation frequencies, a signal corresponding to the dielectric absorption was detected at about 50:l signal-to-noise ratio. The phase corresponded to increased loss upon irradiation. This signal disappeared if all light was cut off or if a filter passing only the light above 400 nm was used. Clearly photochemistryand not heating of the cavity is responsible for the signal. Figure 1shows how the amplitude of the dielectric and ESR signal amplitudes depend on the frequency of light modulation. The ratios of slope to intercept give a lifetime of 0.74 ms for the dielectric experiment and 0.67 ms for the ESR experiment. The close agreement shows that the dielectric absorption must result from the radicals. The ESR spectrum lends no support to the alternative suggestion that the species observed is a more polar species such as an ion pair. The radical concentration was calibrated by comparison of the ESR signal with that of a solution of galvinoxyl in the same sample cell. The concentration found was 2.7 X lo-’ Mal2 The absolute sensitivity for change in mi@ 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 12, 1979
Communications to the Editor I
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Figure 1. Plots of (signal against the square of the angular velocity corresponding to the light modubtion frequency for the dielectric loss (0)and ESR (0)signals. The square root of the ratio of slope to intercept gives the lifetime.g The solid line is a least-squares curve for the dielectric signal and the dashed curve is for the ESR signal.
crowave electric amplitude was calibrated by use of an amplitude modulator so that a comparison with a standard compound could be made by means of eq A. It was assumed that the reactants (DQ and H,DQ) had no dipole moments. Using cyclohexanone (y = 3.0 D)I3as a model and the fact that loss is proportional to y2 (see eq A), we determined the moment for the radical to be 4.2 D. The as use of 2,6-dimethyl- or 2,2,6-trimethylcyclohexanone slightly closer models gave a somewhat lower value of 3.9 D.I4 It seems clear that no large error is associated with the calibration. Such a large value is surprising since the ketone value is 3.0 D. The existence of a value for the dipole moment of this radical provides a test of electronic structure calculations which is in addition to the hyperfine constants. These results show that dielectric loss measurements can be made on transients for both kinetic and structural studies. The signals found are such that with certain improvements it is reasonable to propose observation on the submicrosecond time scale of transients formed by laser phot01ysis.l~Preliminary observations have shown detectable signals in some cases, including the system studied here. It is clear that the dielectric loss measurements will not replace other transient methods such as ESR and optical absorption because the effects of all the changes induced in the sample are superimposed.16 However, this method can be valuable where the nature of the intermediate is not known and a measurement of dipole moment is of value. Transients with significant charge transfer should be of particular interest. Applications can also be imagined where the transients have little optical absorption or where the absorption overlaps that of the initial reactant.
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References and Notes The research described herein was supported by the Office of Basic Energy Science of the Department of Energy. This is Document No. NDRL-1974 of the Notre Dame Radiation Laboratory. See, for example, R. W. Fessenden and J. M. Warman, A&. Chem. Ser., No. 82, 222 (1968). P. P. Infelta, M. P. DeHaas, and J. M. Warman, Radiat. Phys. Chem., 10, 353 (1977). J. 8. Verberne, H. Loman, J. M. Warman, M. P. DeHaas, A. Hummel, and L. Prinsen, Nature (London),272, 343 (1978). J. C. Slater, Rev. Mod. Phys., 18, 659 (1946). See, C. J. F. Bottcher, "Theory of Electric Polarization", Elsevier, Amsterdam, 1952, 10.102. See ref 6, p 391. For a detailed discussion, see C. P. Poole, Jr., "Electron Spin Resonance", Wiley-Interscience, New York, 1967, Chapters 8 and 14. H. Paul, Chem. Phys., 15, 115 (1976). Corning CS 7-60. The ESR parameters were a(6 H) = 5.8 G, a'(6 H) = 1.4 G, and a ( l H) = 0.83 G with the multiplets for the two sets of SIXprotons having the correct intensity ratios. Parameters reported for benzene solution are 5.49, 1.15, and 0.38: T. A. Ciaxton, T. E. Gough, and M. C. R. Symons, Trans. Faraday Soc., 82, 279 (1966). On the basis of these values,' the rate constant (2k) for radical disappearance is 2.6 X lo' M-' s-'. A similar value of 2.9 X log M-' s-' has been reported for dioxane solution by S. K. Wong, W. Sytnyk, and J. K. S. Wan, Can. J. Chem., 50, 3052 (1972). Experiments with 1 mM DQ showed longer lifetimes of 1.2 ms (dielectric amplitude) and 1.0 ms (ESR amplitude) but a lower radical concentration (1.9 X M), probably as a result of a more uniform light absorption. The rate constant is similar at 2.4 X l o 0 M-' s-'. I. J. Borowitz, A. Liberies, K. Megerle, and R. D. Rapp, Tetrahedron, 30,4209 (1974); J. Crossley, W. F. Hassell, and S . Walker, Can. J . Chem., 46,2181 (1968). The value of C for cyclohexanone was 8.4 M-'. The values for 2,6dimethyl- and 2,2,6-trimethylcyclohexanone were 11.2 and 11.4 M-', respectively, probably as a resuk of slightly hlgher dipole moments than for cyclohexanone itself, and longer relaxation times. The dipole moments of these compounds do not seem to have been measured but that for 3,3dlmethylcyclohexanone is 0.17 D larger than for the unsubstituted compound. (C.P. Smyth, "Dielectric Behavior and Structure", McGraw-Hill, New York, 1955, Table 6.2.) I f this difference is assumed to hold for the current compounds and combined with the most recent value for cyclohexanone, a value of p = 3.17 D might be reasonable for the present compounds. In the modulation experiment the signal at the microwave diode is about 5 pV for about 2 X loi4 radicals. The noise for broad band ampllfication (- 1 MHz) corresponds to about 25 pV at the diode so that 1015radicals would be needed to observe a signal in a pulse experiment. At 4 evlradical, 1 mJ energy will produce 1.5 X loi6 radicals. A factor of about 5 increase in the sensitivity can be obtained by placing the sample in the maximum of the microwave electric field. Thus, a pulse experiment with a nitrogen laser (3 mJ per pulse) seems quite practical. I n principle, it would be possible to separate different species through their relaxation times. This would require loss measurements at different frequencies, using a different apparatus for each frequency. In addition, the loss is not a strongly peaked function (see eq A) and so resolution would in any case be rather poor. Radiation Laboratory and Chemistry Department University of Notre Dame Notre Dame, Indiana 46556
Richard W. Fessenden' Paul M. Carton Henning Paul Hiroshl Shimamori
Received March 5, 1979
