Electron Paramagnetic Resonance Spectra of Halogen Atoms

9 Electron Paramagnetic Resonance Spectra of Halogen Atoms N. VANDERKOOI, Jr., and J. S. MacKENZIE

Downloaded by UNIV LAVAL on September 18, 2015 | http://pubs.acs.org Publication Date: January 1, 1962 | doi: 10.1021/ba-1962-0036.ch009

General Chemical Research Laboratory, Allied Chemical Corp., Morristown, N. J.

The electron paramagnetic resonance spectra of fluorine, chlorine, and bromine atoms have been detected in the gas phase with a conventional X-band spectrometer. The atomic species were generated from the parent molecules in a flow system by microwave discharge and thermal dis sociation techniques. An atomic g-ffactor of 4/3 was observed for all species (F , Cl , Cl , Br , and Br ) as required by Russell-Saunders coupling. In a general way, the hyperfine structure of the spectra could be predicted from a consideration of orbital and nuclear magnetic effects. Halogen atom concentrations were estimated using nitro gen dioxide as a reference compound. Virtually 100% dissociation of chlorine and bromine into the atomic species could be achieved by microwave methods. Attainable fluorine atom concentra tions were relatively much lower, because of reaction with the vessel walls. 19

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pecent studies have shown the feasibility of generating appreciable halogen atom concentrations in a vacuum flow system by the use of microwave discharge equipment (5, 6). Experimental success depends almost entirely upon treatment of the reaction vessel walls with suitable "poisons," found effective in preventing atom recombination. Oxy acids, such as sulfuric acid and phosphoric acid, provide a convenient and effective way of treating the walls of glass equipment. The atom concentrations attainable with this technique make possible a num ber of interesting kinetic investigations, if convenient methods can be found to fol low concentration changes. Chlorine and bromine atom concentrations can be de termined calorimetrically and by chemical titration (5), both of which might serve to study reaction rates involving the atomic species. Rather strangely, however, the simple technique of measuring halogen atom concentration by means of their electron paramagnetic resonance (EPR) spectra seems to have been virtually ignored. 98 In FREE RADICALS in Inorganic Chemistry; COLBURN, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

VANDERKOOI AND MacKENZIE

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EPR Spectra of Halogen Atoms

Indications of the feasibility of this approach are to be found in the work of Bowers et al. (2, 3) on the photolysis of iodine vapor in the cavity of an EPR spectrometer, and in the theoretical investigations of Beltran-Lopez et al. on the microwave Zeeman spectra of atomic fluorine and chlorine (I, 8). However, these studies were mainly concerned with the precise determination of atomic g faotors to verify the Zeeman theory and involved specialized spectral equipment. The present investigation demonstrates that the spectra of atomicfluorine,chlorine, and bromine are readily observable in commercially available EPR equipment and that reasonable estimates may be made of their concentrations.


Experimental All spectra were observed with a Varian 4500-10A EPR spectrometer using 100-kc. magnetic field modulation. Atomic species were generated from the appropriate halogen by either microwave or thermal dissociation techniques. In the former case the resonant cavity of a 125-watt Raytheon microwave power generator was placed around the quartz tube leading to the EPR cavity and as close as possible to the point of detection. When thermal dissociation was em ployed, a Nichrome heater was wrapped around the quartz tube leading to the EPR cavity. The distance between the heater and detection point was kept at a minimum. A fast-flow vacuum system (constructed of quartz in those parts exposed to the halogen atoms) operating in the range of 0.1 to 5 mm. of Hg pressure provided residence times of the order of a few milliseconds from time of generation to time of observation of the atomic EPR spectra. The walls of the quartz tube leading from the generation source through the EPR cavity had to be washed with concentrated sulfuric acid or phosphoric acid, to obtain detectable quantities of atomic chlorine or bromine. No sucn treatment was necessary for fluorine. Results and Discussion The magnetic field values of the EPR spectra of atomicfluorine,chlorine, and bromine are recorded in Table 1. For the sake of completeness, the data of Bowers et al. {2,3) on atomic iodine have also been included. Accuracy of these data is probably within ± 2 gauss, as estimated from the atomic oxygen Une in the fluorine spectra. A graphical representation of the spectral data is shown in Figure 1. The species giving rise to the various spectra was easily identified by determination of the atomic g factor and measurement of the number of lines observed. For atomicfluorine,chlorine, and bromine the observed g factor was / , in exact agreement with the value predicted by Russell-Saunders coupling. The atomic iodine g factor is uncertain, since the complete spectrum was not investigated. 4

3

Table I.

Magnetic Field Data for EPR Spectra of F, CI, Br, and I Atoms

Species Resonant frequency, mc. Magnetic field values, gauss

1

(2,

3).

F 9249 5506 5417 5361 4511 4413 4159 19

Cl 9233 5115 5106 5095 5012 4995 4980 4902 4889 4879 4800 4781 4759 36

C/ 9233 5087 5079 5070 5000 4987 4976 4910 4899 4892 4823 4809 4793 37

Br™ + Br 9236 5848, 4750 5792,4730 5666,4662 5608,4628 5549,4512 5490,4472 5233,4255 5222,4173 5147,4159 5099,4152 4982,4124 4960,4116

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In FREE RADICALS in Inorganic Chemistry; COLBURN, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

I™ 9330 6206.3 5471.2 4818.1 4598.4 4268.0 3292.8 a

ADVANCES IN CHEMISTRY SERIES

100

1

111 111 III II III III II I


ELI

II II II I

3400

3700

4000

4300

II 11 II

I

I

4600

4900

M l I II I

5200

5500

5800

6100

MAGNETIC FIELD, GAUSS

Figure 1. EPR spectra of halogen atoms The number of spectral lines observable for an atomic species in a strong magnetic field is given by the expression 2/(2i + 1), where / is the total angular momentum quantum number and ί is the nuclear spin quantum number. All halogen atoms have the same ground state, P / 2 ( / = / 2 ) > but differ in nuclear spin quantum number. The expected number of lines for the spectra of the halo gen isotopes studied in this investigation are listed in Table II and are compared with our experimental observations. The agreement is excellent, except for atomic iodine, where the complete spectrum was not studied. 2

Table II. Halogen F» Cl» Cl Br™, Br" Ι (2, 3). 37

127β β

3

3

Observed Number of Lines in EPR Spectra I 1/2 3/2 3/2 3/2 5/2

2J(2I + 7) 6 12 12 12 18

No. of Lines 6 12 12 12 6

The position, number, and intensity of the spectral lines of atomic fluorine and chlorine observed in our work agree well with the published data of Beltran-Lopez et al. based on the microwave Zeeman spectra of these species (J, 8). Strong atomicfluorinelines were obtained in the microwave discharge of a 1 to 1fluorine-nitrogenmixture at 2 mm. of Hg pressure, where theflowrate was such that the atoms took about 35 msec, to travel the 20 cm. from the dis charge to the cavity. The data reported in Table I for atomicfluorinewere ob tained by using thermal dissociation. The heater temperature was set at about 1000°C. and the pressure adjusted to 0.4 mm. of Hg; less than 2 msec, was re quired for the atoms to travel the 2 cm. from the lower end of the heater and the center of the cavity. By using nitrogen dioxide as a concentration standard, the molecularfluorinewas estimated to be approximately 10% dissociated in the In FREE RADICALS in Inorganic Chemistry; COLBURN, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

VANDERKOOI AND MacKENZIE

BPR Spectra of Halogen Atoms

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cavity. Since a large atomic oxygen peak was also observed, considerable loss of fluorine atoms must have occurred by reaction with the quartz walls. The atomic oxygen line observed in thefluorinespectra occurred at 4404.8 gauss at 9249 mc. per second. This gives a g value of 1.4998. Values of 1.500921 and 1.500905 have been reported by others (7,9). Both the chlorine and the bromine data were obtained with the microwave discharge. High concentrations of chlorine and bromine atoms were observed 20 cm. from the discharge after 90 msec of travel at a pressure of 0.4 mm. of Hg. The chlorine and bromine were found to be almost 100% dissociated when com pared to nitrogen dioxide as a quantitative reference. Thefirstderivatives of the absorption curves of C l plus CI are shown in Figure 2. The spectra of the two isotopes are easily sorted out from the known natural abundances of CI and CI . The task of sorting out the Br and Br spectra was not attempted, since both isotopes are present to about the same extent in bromine and their hyperfine splitting constants are fairly similar. 35

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Figure 2. EPR derivative curve of atomic Cl + CI 35

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Precise determination of hyperfine constants for the halogen atoms was be yond the scope of the present investigation and, indeed, presents a problem o\ some magnitude (1,4,8). However, the major splitting in the spectra of F , CI . CI , Br , and Br roughly parallels the values of the nuclear magnetic moments. 19

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Literature Cited (1) Beltran-Lopez, V., Robinson, H. G., Phys. Rev. 123, 161 (1961). (2) Bowers, K. D., Kamper, R. Α., Knight, R. B. D.,J.Sci.Instr. 34, 49 (1957). (3) Bowers, K. D., Kamper, R. Α., Lustig, G. D., Proc. Phys. Soc. (London) 70B, 1176 (1957). (4) Goodings, D. Α., Phys. Rev. 123, 1706 (1961). (5) Ogryzlo, Ε. Α., Can. J. Chem. 39, 2556 (1961). (6) Ogryzlo, Ε. Α.,J.Phys. Chem. 65, 191 (1961). (7) Radford, Η. E., Hughes, V. W., Phys. Rev. 114, 1274 (1959). (8) Radford, Η. E., Hughes, V. W., Beltran-Lopez, V., Ibid., 123, 153 (1961). (9) Rawson, Ε. B., Beringer, R., Ibid., 88, 677 (1952). RECEIVED May 16, 1962. Work supported by the Advanced Research Projects Agency (ARPA) undei contract DA-30-069-ORD-2638. In FREE RADICALS in Inorganic Chemistry; COLBURN, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

Electron Paramagnetic Resonance Spectra of Halogen Atoms

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