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use of its facility for y irradiation. References and Notes (1) (2) (3) (4) (5) (6) (7)
(8)
(9) (10) (1 1)
(12) (13) (14) (15)
2 . Nowak and W. Stachowicz, Nukleonika, 14, 1113 (1969). L. Kevan, Adv. Radiat. Chem., 4, 181 (1974). W. Schlenk, Annales, 565, 204 (1949). The addition of methanol is not always necessary for complex formation with n-alkanes. In the case of polycrystalline n-C8-urea clathrates, the intensity of the central singlet was decreased by 40% on warming to 130 K. The residual disappeared around 165 K. A. Lund, J. Phys. Chem., 76, 1411 (1972). I t is noted that areas under the integrated curves of Figure 19 are larger by about 30-40% than areas under those of the underlying alkyl-radical spectra in Figure 1A and that the ratios of yields between the singlet species (e,-)and the alkyl radicals in Figure 1A was 1:1.52. The integrated curves were obtained using an electronic integrator. These values were obtained from the corresponding integrated curves of Figure 2A. D. W. Skelly, R. G. Hayes, and W. H. Hamill, J. Chem. Phys., 43, 2795 (1965). The formation of e; in polycrystalline nalkanes, however, has been reported (M. Iwasaki, K. Toriyama, and T. Ohmori, J. Phys. Chem., 72, 4347 (1968). n-Butyronitrile and 1,3-diaminopropane gave weak e; signals in comparison with underlying free-radical signals. n-Butyric acid and proplonic acid n-butyl ester (n-butyl propionate) did not yield e,-. The formation of e; could not be ascertained from the spectra of 2butanone and 1,6-hexanediol. 1-Octanethiol gave a broad singlet on the low field side. A. Carrington and A. D. MacLachlan, "Introduction to Magnetic Resonance", Harper and Row, New York, N.Y., 1967. D. Lin and L. Kevan, J. Chem. Phys., 55, 2629 (1971). T. Ohmori, T. Ichikawa, and M. Iwasaki, Bull. Chem. Soc. Jpn., 46, 1383 (1973). T . ichikawa, unpublished results.
Department of Applied Chemistry Hiroshima University Senda-Machi, Hiroshima 730, Japan
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Received May 11, 1977
Communications to the Editor
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Figure 1. Raman spectra of (a) air and (b) 4A molecular sieves in the frequency regions of 14N2and I6O2scattering: Cary 82, slit = 2 cm-', excitation frequency = 514.5 nm, power = 500 mW.
TABLE I: Measured a n d L i t e r a t u r e NJO, Ratios R a m a n measurementsa L i t e r a t u r e
Raman Investigations of 4A Molecular Sieves Publication costs assisted by Merck Sharp and Dohme Research Laboratories
Sir: I. N2 Adsorption. Molecular sieves and especially sieves of the A type provide a unique method for the separation of Nz from other gases.' Nz adsorption capacities of powdered 4A sieves have been reported for a variety of Typically, such studies have relied on thermodynamic measures, e.g., measurement of the volume of gas desorbed, to estimate adsorptivity.lb We now report the use of Raman spectroscopy to estimate relative amounts of adsorbed Nz on 4A sieves. As an example we show Nz adsorptivity data for sieves4under ambient conditions (25 "C, 1atm) which have (a) had no pretreatment, (b) been calcined to 400 "C in a muffle furnace; and (c) been washed in 0.2 N NaOH and then calcined to 400 "C in a muffle furnace. Figure l a displays part of the Raman spectrum of air.5 The observed vibrational bands at 1555 and 2329 cm-' are those characteristic of l6OZand l4N2,respectively.6aFigure l b displays the same portions of the Raman spectrum of 4A molecular sieves in air. Besides the normal scattering bands of air, two additional maxima at 2324 cm-l (band separation = 4.3 f 0.3 cm-l) and 1548.5 cm-' can be observed. Band positions were obtained from repeated scans5 on several samples of sieves. These extra bands are due since they show (a) shifts upon to adsorbed 14N2and 1602 adsorption characteristic of nonpolar molecule^,^^^ (b) band widths less than 2 ~ m - ' and , ~ (c) an experimental depolarization ratio of ca. 0.8, characteristic of randomized reflections from the powdered solid5brather than the true The Journal of Physical Chemistty, Vol. 8 1, No. 22, 1977
3 . 7 4.0 0.3 b
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H e i g h t 2 3 2 4 c m - ' band/height of 1 5 4 7 c m - ' band; f average deviation of four samples. Reference 15. Extrapolated from Figures l a a n d l b of r e f l a .
TABLE 11: Relative Scattering Intensitiesa of N, Adsorbed to N a A (4A) Molecular Sieves Relative adsorption Sieve identificaNo 400 " C tion no. pretreatment calcination 034 104
110 144
1.61 0.63 0.48 0.73
0.2 N N a O H wash, 400 " C calcination
0.80
0.49
1.10 1.11
0.46 0.54 0.47
1.03
Relative adsorptions are t a k e n from t h e r a t i o of t h e h e i g h t of t h e 2324-cm-' b a n d (adsorbed I4N,) to t h a t of t h e 2329-cm" b a n d (atmospheric I4N,). a
depolarization of the gas, PN = 0.02. Table I contains Nz/Oz fiaman scattering intensity ratios of free air and air adsorbed to 4A molecular sieves. That such relative ratios are proportional to gas concentrations can be seen by comparing these measured ratios to their respective literature values, also in Table I. Clearly, the Raman method distinguishes the concentration of the free gases (&lo%)from that of the gases adsorbed to the 4A sieve (k1570). As an example of the use of Raman spectroscopy to evaluate Nz adsorptivity, we show relative Nz scattering intensities (Table 11), as defined by the ratio of the band
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well below those which cause complete sieve dehydration. Figure 2 shows Raman spectral3 of the same sieve lot calcined at 400 “C for 1h before (a) and after (b) washing with 0.2 N NaOH. The spectrum of the untreated sieve is identical with that in Figure 2a. Clearly the spectrum of the sieve washed with NaOH has considerably less background fluorescence and, therefore, more pronounced features. For example, the 1550-cm-l 1602band is prominent (S/N 3) in the caustic washed sieves whereas it is obscured by fluorescence (S/N 0.2) in the unwashed sieve. Sieves that are heated to much higher temperatures, e.g., 600 “C for 5 h, also show fluorescence reduction but the improvement is only two-thirds of that achieved by washing. Moreover, such high temperatures alter cation positions in 4A sieves12athus affecting subsequent adsorption. The reduction of fluorescence through washing with NaOH appears quite general14and, therefore, may be a useful technique for improving Raman spectra of 4A sieves and other alumina/silica adsorbents.
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Flgure 2. Raman spectra with associated fluorescence of 4A sieves (a) calcined at 400 O C and (b) washed with NaOH prior to calcining at 400 “ C : Cary 82, slit = 5 cm-’, excitation frequency = 514.5 nm, power = 200 mW.
height at 2324 cm-l to that at 2329 cm-l, for four different samples of sieves. The effect upon relative adsorptivity of calcining and caustic extractinglO the sieves is also included. The untreated sieves show a 3.5-fold range in their N2 capacities, whereas the treated sieves are more uniform in their adsorption capacities. The spread in N2 adsorption capacities of the untreated sieves probably reflects a poisoning effect by adsorbed species (both organic and inorganic) on the sieve surface. Dry sieves4 which are calcined at 400 “C show increased N2 adsorption due to the removal of the organic surface impurities.ll Sieves which are washed in 0.2 N NaOH and then calcined exhibit a somewhat diminished capacity for Nz sorption as a result of “steaming” effects which occur when wet sieves are dehydrated at elevated temperatures.8*12b Thus, Raman scattering can provide a unique in situ measurement of N2 adsorbed on sieves which might be useful in kinetic and other studies. 11. Reduction of Fluorescence. The typical Raman spectrum of 4A molecular sieve^,^ as well as other adsorbents,ll is often obfuscated by low-level fluorescent impurities. When these fluorescent impurities are caused by adsorbed species, e.g., hydrocarbons, rather than by structural impurities such as color centers, the fluorescence can be minimized by (1)removal of the impurity through pretreatment or burning with the laser, and (2) changing the excitation wavelength. The usual pretreatment method for A-type sieves involves heating at 350-500 “C for ca. 24 h, usually under vacuum, to desorb the impurit i e ~ . ” ~ JFor other closely related material, e.g., silica, porous vycor, heating at 500-600 “C under O2 for ca. 1 2 h is recommended for removing impurities.ll While these methods are satisfactory for fluorescence reduction, they are time consuming, and, moreover, if the latter method were used for the A sieve, complete dehydration of the sieve would occur.12a An interesting consequence of washing sieves with NaOHlO is the diminution of the associated fluorescence in a relatively short period of time, 2 h, at temperatures
Acknowledgment. We thank Dr. Seemon Pines for his aid in the preparation of this manuscript. References and Notes (1) (a) A. M.Ahharov, B. P. Bering, I. A. Kalinnikova, and V. V. Serpinskii, Izv. Akad. Nauk. SSSR, Ser. Khim., 6, 1434 (1972); (b) A. M. Arkarov, I. A. Kalinnikova, and V. V. Serpinskii, ibM., 3, 538 (1972). (2) E. I. Borzenko, Zh. PrM. Khim. (Leningrad), 42(4), 891 (1969). (3) M. Nakagaki and T. Fujie, Yakugaku Zasshi, 90(3), 384 (1970). (4) Linde 4A Molecular Sieves, powdered, 600 Mesh, ca. 2-4% hydration. (5) Cary 82 spectrophotometer, Spectra-Physics argon laser Model 165, power = 500 mW, A,, 514.5 nm, slit = 2.0 cm-’, pen period = 10 s, scan speed = 0.1 cm-l/s, sensitivity = 1000 counts/s full scab. (6) S. K. Freeman, “Applications of Laser Raman Spectroscopy”, Wiley-Interscience, New York, N.Y., 1974: (a) p 314; (b) p 30. (7) C. L. Angell, J. Phys. Chem., 77, 222 (1973). (8) N. T. Tam, R. P. Cooney, and G. Curtheys, J. Chem. SOC., Faraday Trans. 1, 72, 2577 (1976). (9) H. Forster and M. Schuldt, J. Chem. Phys., 86, 5237 (1977). (IO) 1 gram of sieves is boiled for 15 min in a 0.2 N NaOH solutlon, washed free of excess base, fikered, pulverized, and calcined at 400 OC for 1 h. (1 1) (a) E. Buechler and J. Turkevich, J. Phys. Chem., 76, 2325 (1972); (b) T. A. Egerton, A. H. Hardin, Y. Kozirovski, and N. Sheppard, Chem. Commun., 887 (1971); (c) R. 0. Kagei, J. Phys. Chem., 74, 4518 (1970). (12) 0.W. Breck, “Zeolite Molecular Sieves”, Wiley, New York, N.Y.: (a) see p 133 and 442 f f ; (b) p 490. (13) Power = 200 mW, A,, = 514.5 nm, slit = 5.0 cm-’, pen period = 5 s, scan speed = 1.0 cm-’/s, sensitivity = 5000 counts/s full scale. (14) Silica gel (J. T. Baker Co.) washed with 0.2 N NaOH’ shows a 50% higher SIN (lower fluorescence) throughout its Raman spectrum (20-2500 cm-I) than does either untreated sllica gel or silica gel calcined at 600 OC in air for 2 h. (15) R. C. Weast, Ed., “Handbook of Chemistry and Physics”, 56th ed, Chemical Rubber Co., Akron, Ohio, 1976. Merck Sharp and Dohme Research Laboratories Division of Merck and Company, Inc. Rahway, New Jersey 07065
Davld D. Sapersteln’ Alan J. Reln’
Received June 20, 1977
High Protonic Conduction of Polybenzlmldazole Films’ Publication costs assisted by the National Science Foundation
Sir: Pressed disks of polybenzimidazole2(PBI) are known to be insulators for electronic cond~ction.~We have confirmed this finding for polybenzimidazolefilm, but find that protonic conduction is high, the conductivity being in the semiconductor range. The Journal of Physical Chemistry, Vol. 8 I, No. 22, 1977
