Dec., 1960
ELECTRON PARAMAGNETIC RESONANCE SPECTRA OF GASEOUS FREERADICALS
deals with electrolytes; that is, they neglect the quantity v. Neglect of v also leads them to conclude that they should, in .a standard Hw1A.r graph, approach the reciprocal of the molecular weight of silicotungstic acid with decrease in concentration. Their observations support this. We did not feel that our light scattering measurements were accurate enough to check them in this region but, on the basis of our ultracentrifugation results, such an increase in turbidity should not be observed unless the solutions are so dilute that departures from electroneutrality are occurring in dimensions comparable to the wave length. Their lowest concentration appears to be about 3 X mole/liter. Here v’ computed by Equation 9 is within 10%; of 5. Their results in the low concentration range cannot be understood on this basis. 2. Silicotungstic Acid in Calibration of Photometers.-Two component solutions of silicotungstic acid have possible usefulness as primary light scattering calibration standards for aqueous solutions. The material is inexpensive and obtainable in adequate purity. Although, because of variation in water content of the salt, one would need analysis t o establish concentration, a density or perhaps a refractive index measurement should suffice. Scattering by this solute is sufficient to allow calibration a t concentrations for which the refractive index of the solut,ion would not be greatly different fiom those of usual “unknown” aqueous solutions. The activity coefficients and refractive index increments presented here should be accurate enough to allow computation of excess turbidities to about 3% (based on an uncertainty of 0.15 in a’ and of 0.3% in refractive index increments) in the concentration range 0.01-0.04 nioles/liter. Ludox has been used as a calibration standard but its properties have been criticized. Recently,
187‘3
sucrosez1also has been used for this purpose. At concentrations which scatter desirable levels of light, however, the refractive indices of sucrose solutions are somewhat higher than those of the usual aqueous solutions of interest, and the PCSsibility of differences in apparatus constants is thus introduced. In addition, recent recomputations of the expected turbidity with exact equations by Stigterzzhave not given quite as good agreement between experimenta1 and computed values as previously reported.z3 Further, there is an impurity (or impurities) apparently present in even the best sucrose samples, which must be eliminated by charcoal treatment. Finally, agreementPz~23 of computed with experimental turbidities has been obtained with use of a substantial depolarization correction, the magnitude of which may be questionable with an optically active solute. There thus still seems to be a need for a convenient aqueous standard. Silicotungstate turbidities in the presence of supporting electrolyte could be used for seoondary standards, although the requisite activity coefficient derivatives are not available for Computation of turbidities for a given solution. The scattering would, however, be less sensitive to interference by low molecular weight impurities than with the two component system. Acknowledgment.-We wish to acknowledge indebtedness to Dr. W. R. Busing for advice concerning programming of computations both in this work and, belatedly, in an earlier study.14 We are indebted to Dr. E. W. Anacker for helpful comments on the paper, and to Miss Neva Harrison for technical assistance. (21) K. J . Mysels and L. H. Prinoen, J . Phys. Chem., 68, 1696 (1959). (22) D. Stigter, J. Phys. Chom., 64, 114 (1960). ( 2 3 ) 9. H.Maron and R. L. H. Lou. ibid., 69, 231 (1955).
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SOME OBSERTiATIONS ON THE ELECTROS PARAMAGNETIC RESONANCE SPECTRA OF GASEOUS FREE RADICALS BY C. J. ULTEE* Research Laboratory, Linde Cmnpuny, U i v i s i m of U n i o n Carbide Corporation, Tonawunda, h e u R
1.01 A:
,enred M a y 6 , 1960
The e.p.r. spectra of atomic nitrogen, oxygen and hydrogen have been observed Over a wide pressure range. Relaxation and pressure broadening effects complicate the spectra and have limited the present study to qualitative observations. It seems quite feasible, however, to adopt e.p.r. methods for kinetic studies of free radical reactions in the gas phase. In the case of atomic nitrogen an unexpected effect of oxygen was observed which may have some bearing on the theory of active nitrogen.
Tntroduction Electron paramagnetic resonance (e.p.r.) studies of gases have been limited mainly to the determination of spectroscopic parameters for a number of paramagnetic species. There seems t o have been no application of e.p.r. spectroscopy bo the study of the chemical properties of gaseous free radicals, although this technique would seem ideally suited to enlarge the present knowledge and understanding * .4VCO Research and Admnced Developmeat Division, 201 Lowell Street, WilminL+on, Mws.
of gas-phase free-radical reactions. This study was undertaken to explore the use of e.p.r. spectroscopy in the investigation oi gas-phase kinetics, using a commercially available instrument without extensive modifications. A survey of our observations on atomic nitrogen, oxygen and hydrogen is given below. Since the spectra of these species have been reported the discussion is limited t o (1) E. R. Rawson and R. Beringer, Phus. Rei,., 88,677 (1952). ( 2 ) M.A. Heald and R. Beringer. z b d . , 96,645 (1954). (3) R. Belinger and M. A. Heald, tbzd., 96, 1474 (1954).
C.J. ULTEE
1874
/ I
t
T
I
TO VACUUM C*U.ES
Fig. 1.-Flow system for activation of the gases: A, gas inlet; B, flowmeters; C, needle valves; D, exitation cavity; E, spectrometer cavity.
0
Fig. ?.-The
cyclotron resonance signal in active nitrogen.
specific esperimental details and to the effects of some variables riot previously reported. Experimental The general principles and instrumental techniques of e.p.r. spectroscopy have been the subject of recent reviews4.b and will not be discussed here. The e.p.r. spectrometer used in this work is a Varian Associates Model V-4500 e.p.r. spectrometer operating at a nominal frequency of 9.4 khic./sec. A rectangular cavity operating in the T E o Imode ~ is used, with sample openings in the center of the narrow face such that the sample is positioned in a region of maximum microwave magnetic field. The only modification made in the cavity was an incresse in the diameter of the sample openings to l l mm. Modulation of the magnetic field is provided by two small coils attached to the side of the cavity. The coils are driven by a 400 (../see. audiooscillntor. Resonance absorption in the cavity appesrs as audio modulation at a R1.4 408A crvstnl used aa a detector. The signal is amplified, demoduiated :md recorded by conventional techniques. With low modulation amplitudes the recorder tracing approximatcs the derivative of the resonance absorption curve. Magnetic field strengths were determined by measuring corresponding proton resonance frequencies. Microwave frequencies were measured with a heterodyne frequency meter. The accuracy of the frequency measurements is estimated at 0.00570. Modulating amplitudes were generally adjusted for optimum signal prcsentation,e i . e . H , = 2 A H i / , , except ( 4 ) J. E. Wertr. Chem. Revs.. 65, 829 (1955). ( 5 ) G. Feher. Bell St/stcm Techn. J . , 84, 449 (1967).
Vol. 64
during line width measurements for which the lowest feasible amplitudes were used. The vacuum line and flow system are shown in Fig. 1. The electrodeless discharge is excited in B quartz section of the line which passes through a slot in a resonant cavity. A 2450 Mc./sec., 125-watt Raytheon diathermy unit is used to supply the discharge power. Commercial high purity cylinder gases were used mthout further purification.
Results and Discussions Nitrogen.-Nitrogen, after passing through the discharge, showed the characteristic yellow afterglow. The e.p.r. spectrum of this active nitrogen consists of a group of three narrow, evenly spaced lines of equal intensity which are superimposed on a very broad and intense line. Both signals are centered about g = 2.0. However, under experimental conditions that are optimum for the observation of the spectrum of atomic nitrogen, i.e., low modulation amplitude, low microwave power and slow scanning over a narrow magnetic field region, the broad line is not observed. The width of this broad line varies with pressure and microwave power. At 15 mm. pressure its width (between maximum and minimum points on the derivative curve) is about 200 gauss. The line shows considerable structure (Fig. 2) which is, however, not readily resolved. When oxygen is added to the nitrogen prior to its passage through the discharge, this signal increases sharply in amplitude, reaches a maximum a t 0.2 volume % (total pressure 10 mm.), then slowly decreases, reaching zero a t 2.5y0oxygen. Several investigators have demonstrated the presence of free electrons in active nitrogen by probe and microwave meth~ds.~-lO The present data may be correlated closely with the microwave electron density measurements in active nitrogen reported by Kunkel and Gardner'O who observed a maximum a t about 0.2 volume gi; oxygen. The broad line can therefore be explained as a cyclotron resonance signal from free electrons in active nitlogen. The transition probability for cyclotron resonance is proportional to the square of the electric dipole moment; for fields of the same order of magnitude it is about lo1*times the transition probability for spin resonance. The higher probability accounts for the intensity of the line, which is too high to interpret it as an e.p.r. line of an impurity in the nitrogen, A similar line observed in gas discharges has been assigned to cyclotron resonance by Ingram and Taplcy.I1 If it is assumed that the electrons have their origin in the ionization of NO by the reaction N N NO + Nz KO+ e-,l0 than the effect of oxygen is readily explained. The addition of oxygen leads to the formation of nitric oxide. As long as nitrogen atoms are present in excess, the ionization step mill take place. However, a t high oxygen concentrations the reaction of oxygen with atomic nitrogen will decrease the concentration of the latter. Consequently the concentration of free electrons will also decrease. The three narrow lines (Fig. 3) form the spectrum of 4S3/2 (ground state) nitrogen atoms. At
+ +
+
(6) R. Beringer and J. G. Caatle, Jr., Phys. Rev., 78, 581 (1950). (7) Rayleigh, Ptoc. R o y . SOC.(London), 8180, 140 (1952). ( 8 ) J. M. Benaon, J . A p p l . Phys., 23, 757 (1952). (9) A. L. Gardner, Phys. Re%, 98, 263 9 (1955). (10) W. B. Kunkel and A. L. Gardner, ibid., 98, 558 A (1955). (11) D.J. E. Ingram and J. A. Tapley, ibrd., 97, 238 (1955).
+
Dec., 1960
ELECTRON PARAMAGNETIC RESONANCE SPECTRA OF GASEOUS FREERADICALS
20 mm. total pressure, the observed spacing of 3.8 gauss, line width of 0.09 gauss, and g-factor of 2.0021 are identical with those reported by Heald and Beringer . The atomic nitrogen spectrum was observed over the range of 0.5 t o 80 mm. total pressure. Atomic nitrogen concentrations as determined by “titration” with N012 varied from 1% (volume) at 20 mm. to 4oj, at 1 mm. From 5 to 20 mm. the partial pressure of atomic nitrogen remained essentially constant. If we use the Van Vleck-Weisskopf theoryla of pressure broadening and identify the lifetime r of a spin state with the time t, between collisions of atomic species, the half-width a t half height on a 1/27r~. Using a frequency scale is given by Av kinetic theory cross section and making the appropriate conversions to magnetic field units, the calculated line width for atomic nitrogen at a partial pressure of 0.2 mm. (total pressure 10 mm.) is 0.1 gauss, in good agreement with the observed width under these conditions. A study of the reaction between atomic nitrogen and molecular oxygen showed that addition of oxygen affects the atomic nitrogen signal to a considerable extent. Figure 4 shows the effect of oxygen on the signal amplitude at various power levels. The initial parts of the curves are difficult to explain on the basis of any plausible reaction mechanism. The curve at lower power levels show a less prominent maximum, suggesting that the phenomenon is related to saturation at high power levels. Figure 5 shows the effect of microwave power on nitrogen containing varying amounts of oxygen. These results were obtained by decreasing the power level in the cavity with a variable attenuater, keeping bias and signal amplification constant. Under these conditions, the signal amplitude varies as the square root of the power. The signal amplitudes are therefore normalized by dividing by
