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J. Phys. Chem. B 2001, 105, 118-122
Photoinduced Charge Separation of N-Alkylphenothiazines in X Zeolites Koodali T. Ranjit and Larry Kevan* Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641 ReceiVed: May 24, 2000; In Final Form: October 10, 2000
Transition metal ion containing X and A zeolite microporous materials were examined for the photoionization of N-alkylphenothiazine (PCn) with 320 nm irradiation at room temperature. N-Alkylphenothiazine cation radicals were produced in these zeolitic materials and are characterized by electron spin resonance. Copper containing X zeolite serves as an efficient host for the formation and stabilization of methylphenothiazine cation radicals. For methylphenothiazine (PC1), the photoionization efficiency was found to decrease in the order Cu-X-PC1 > Ni-X-PC1 > Co-X-PC1 > H-X-PC1 > Cr-X-PC1 ∼ Na-X-PC1 ∼ Mn-X-PC1. The photoionization efficiency was found to depend on the nature and concentration of the metal ion.
Introduction Photochemical energy conversion and storage of solar energy is an active area of research.1-4 Considerable effort is being spent on developing molecular systems in which each part of the energy conversion process can be incorporated into one material. The fundamental limitation in such a process is reverse electron transfer, which is thermodynamically favorable and very rapid. Recombination results in the loss of the converted energy into heat. Hence, the challenge lies in designing a system in which long-lived charge separation can be realized. In this respect, many systems have been examined as hosts to improve the efficiency of the energy storage by preventing the rapid back electron-transfer reaction.5,6 Heterogeneous systems such as silica gels, vesicles, micelles, and molecular sieves can provide appropriate spatial organization of both the donor and acceptor molecules to retard back electron transfer. Thus, appropriate tuning of the electronic and spatial properties of the host system7-10 can minimize undesirable back electron transfer. Methylphenothiazine and other N-alkylphenothiazines with longer alkyl chains have excellent electron donating capacity, which leads to the formation of charge-transfer complexes with a number of electron acceptors. They can be easily oxidized by ultraviolet irradiation, to form phenothiazine cation radicals, which have been characterized by electron spin resonance (ESR) and optical spectroscopic techniques.11-14 A photosensitizer molecule such as methylphenothiazine can be incorporated inside any microheterogeneous host system, which has pore dimensions greater than its molecular dimension; such a host system can in principle stabilize the photoinduced radical ions with relative ease.15,16 Aluminosilicate zeolites have shown considerable promise for stabilizing photochemically generated redox species.17 The arrangement of channels and cages in these materials allows the incorporation of molecules in well defined and unique spatial arrangements. The photooxidation of phenothiazine and methylphenothiazine in micelles with metal ions as acceptors have been studied by Gra¨tzel et al.18,19 In the present study we have examined the photoionization of N-alkylphenothiazine incorporated into X and A zeolites. The idea that the metal ions act as suitable electron acceptors prompted us to incorporate the metal ions in the host material by ion exchange and then examine the photooxidation of methylphenothiazine after ultraviolet irradiation. The photo-
yield was found to depend on the nature and concentration of the metal ion. Relatively high photoyield and excellent stability were observed in Cu-X zeolite, which suggests that such systems can act as potential candidates for photochemical conversion and storage devices. Experimental Section Preparation of M-X Zeolite. Na-X (Grace Division Chemicals, Maryland) and Na-A (Union Carbide) zeolite were commercial samples. The zeolite samples were exchanged with 0.1 M sodium acetate solutions to decrease any Fe3+ impurities. The metal ions were incorporated into extraframework positions by liquid-state ion exchange. Typical liquid-state ion exchange was performed by adding 10 mL of 1 × 10-1 M Cr(NO3)3, Mn(NO3)2, Co(NO3)2, Ni(NO3)2, or Cu(NO3)2 and 40 mL water to 2 g of Na-X zeolite and the mixture was stirred overnight at room temperature. The samples were then filtered, washed with hot distilled water to remove any excess metal ions on the surface of the sample, and then dried in air to form M-X materials. In addition, three Cu-X zeolites containing different amounts of Cu2+ were prepared. These samples were prepared by varying the initial concentration of Cu(NO3)2. H-X zeolite was prepared by exchanging Na+ with NH4+ four times followed by calcination as reported in the literature.20 Cu-A zeolite was prepared in a similar manner as described for Cu-X zeolite. Methylphenothiazine Incorporation. Methylphenothiazine (Aldrich) is designated PC1 and was incorporated by immersing 0.1 g of M-X zeolite in 1 mL of 1 × 10-2 M methylphenothiazine in benzene solution for about 7 h in the dark. The benzene was removed by flowing nitrogen gas over the sample for 1 h. For electron spin resonance (ESR) measurements, 0.1 g of the sample was transferred into Suprasil quartz tubes (2 mm i.d. × 3 mm o.d.), which were sealed at one end and evacuated for 4 h at room temperature. The filled sample tubes were then flame sealed at the other end. N-Alkylphenothiazines (PCn where n ) 4, 6, and 10) were synthesized as reported in the literature.21 The ratio of transition metal ion to methylphenothiazine was approximately 10 for all samples for ESR studies. Characterization. XRD powder patterns were recorded on a Siemens 5000 X-ray diffractometer using Cu KR radiation of wavelength 1.541 Å in the range 10° < 2θ < 50°. Chemical
10.1021/jp001918f CCC: $20.00 © 2001 American Chemical Society Published on Web 12/07/2000
Photoionization of N-Alkylphenothiazines
J. Phys. Chem. B, Vol. 105, No. 1, 2001 119
Figure 1. (a) The basic structural unit of X zeolite. (b) The basic structural unit of A zeolite.
analysis was performed by electron microprobe analysis on a JEOL JXA-8600 spectrometer. The composition of the zeolite material was determined by calibration with known standards and by averaging over several defocused areas to give the bulk composition. ESR spectra were recorded at room temperature at 9.5 GHz using a Bruker ESP 300 spectrometer with 100 kHz field modulation and low microwave power to avoid power saturation. Photoproduced phenothiazine radical cation (PC1+) yields were determined by double integration of the ESR spectra using the ESP 300 software. Each photoyield is an average of three determinations. Thermal gravimetric analysis (TGA) of the samples were performed using a TGA 2050 analyzer from TA instruments in oxygen atmosphere at a heating rate of 10 °C/min. For TGA experiments, samples were impregnated with a higher amount of methylphenothiazine. The ratio of transition metal ion to methylphenothiazine was five. Photoirradiation. The methylphenothiazine containing zeolite materials were irradiated using a 300 W Cermax Xenon lamp (ILC-LX 300 UV) at room temperature. The incoming light was passed through a 10 cm water filter to prevent infrared radiation and through a Corning No. 7-54 filter with 90% transparency from 240 to 400 nm and a maximum at 320 nm. The samples were placed in a quartz Dewar and rotated at a speed of 4 rpm to ensure even irradiation. The photoproduced methylphenothiazine cation radicals were identified by ESR. Results The basic structural unit of X and A zeolites are shown in Figure 1. The cage structure of X zeolite can be described as assembled from sodalite cages linked by double six rings of oxygen bridged tetrahedra thus producing large 13 Å super cages connected by 12-ring windows with 7.5 Å diameter. From Figure 1 it is evident that the pore size of X is bigger than the molecular dimension of the methylphenothiazine molecule of 6.5 Å. Thus it is possible to incorporate this photosensitive molecule into X zeolite. The X-ray diffraction patterns of Na-X and Na-A and metal ion exchanged zeolites reveal them to be highly crystalline and agree with the literature data. Preliminary photoionization experiments were carried out with H-X zeolitic materials. Samples of H-X zeolite impregnated with methylphenothiazine molecules show very weak ESR signals of PC1+ based on g ) 1.9993 with incompletely resolved hyperfine lines before irradiation21,22 as shown in Figure 2. This
Figure 2. ESR spectra of H-X-PC1 at room temperature after 320 nm irradiation for (a) 0 min, (b) 5 min, (c) 30 min, and (d) 60 min.
shows that some methylphenothiazine cation radicals are produced during the sample preparation. After being irradiated by 320 nm light at room temperature for up to 60 min, the signal increased substantially in intensity. Thus, there is a significant increase in the production of stable methylphenothiazine radicals. The spectral widths of the ESR spectra are the same as that of the methylphenothiazine cation radical in homogeneous solution and in micelles at room temperature.22,23 The weak ESR signals before irradiation have the same line shape as those observed after irradiation. The background signals before irradiation were subtracted from the signals after irradiation to estimate the net photoyield. The partially resolved ESR spectra of PC1+ at room temperature show that the radicals have some mobility in the zeolite framework at room temperature. In addition a visual change in the color of the M-X-PC1+ is easily discerned; the samples are light pink in color prior to irradiation but turn dark pink after irradiation. This is characteristic of PC1+ cation radicals.24,25 This further confirms the photoionization of PC1 into PC1+ cation radicals. Figure 3 shows the changes in the intensity of the ESR signal due to PC1+ cation radicals in H-X, Na-X, and Mn-X zeolites. Figure 3 shows that the highest yield is obtained for H-XPC1. Also one can see that the ESR signals rapidly increase during the first 10 min of irradiation and then reach a plateau in about 60 min. An irradiation time of 60 min was selected for comparative photoyield and stability studies. Figure 4 shows the variation in the intensity of the ESR signal due to PC1+ cation radicals in Cu-X, Ni-X, Co-X, and Cr-X zeolites. The presence of Cu increases the photoyield of PC1+ cation radicals when compared with Ni, Co or Cr ions. Thus, the nature of the transition metal ion in X zeolite plays an important role in stabilizing the photoproduced PC1+ cation radicals. The rates of formation of the PC1+ cation radicals can be evaluated from the initial slopes in Figures 3 and 4 assuming
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Ranjit and Kevan
Figure 5. Room-temperature photoinduced methylphenothiazine radical cation yield versus the Cu/Si ratio for Cu-X-PC1 materials. Figure 3. Room-temperature photoinduced methylphenothiazine radical cation yield measured by ESR versus irradiation time for M-XPC1 (M ) H, Na, Mn).
Figure 6. Room-temperature photoyield of Cu-X-PCn (n ) 1, 4, 6, 10) measured by ESR after 60 min photoirradiation versus the alkyl chain length of the alkylphenothiazine. Figure 4. Room-temperature photoinduced methylphenothiazine radical cation yield measured by ESR versus irradiation time for M-XPC1 (M ) Cu, Ni, Co, Cr).
first-order kinetics. The calculated rate constants for the formation of the PC1+ cation radicals on this basis are k ) 4.1 × 10-3 s-1 for Cu-X; k ) 3.0 × 10-3 s-1 for Ni-X; k ) 2.8 × 10-3 s-1 for Co-X; k ) 1.6 × 10-3 s-1 for H-X; k ) 1.2 × 10-3 s-1 for Cr-X, and k ) 1.0 × 10-3 s-1 for Na-X. This order of the rate constants is consistent with the order of the photoyields. The stability of the photoproduced PC1+ radical is also an important factor in the design of efficient artificial photoredox systems. The stability of the PC1+ radicals was monitored by ESR spectroscopy. Among the transition metal ion exchanged X zeolites studied, Cu-X was found to exhibit the best stability. The rates of decay of PC1+ cation radicals for M-X zeolites were evaluated from the initial slopes in the decay curves assuming first-order kinetics. In the cases of Cu-X, Co-X, and Ni-X the ESR intensity was essentially the same for several days. Hence for these samples the intensity of the ESR signal was monitored every 24 h. The half-lives (t1/2) of the PC1+ cation radicals were determined to be 30 days for Cu-X, 25 days for Ni-X, and 20 days for Co-X. The half-life (t1/2) of the PC1+ cation radicals for H-X, Cr-X, Na-X, and Mn-X were determined to be 10 days, 6 days, 3 days, and 3 days, respectively. We have observed that Cu-X zeolite exhibits the maximum photoyield. To understand the influence of the Cu content on the photoyield and the stability, X zeolites containing varying
Cu/Si ratios were prepared. The results obtained from the photoionization of methylphenothiazine in Cu-X samples are shown in Figure 5. It is evident that the photoyield initially increases as the concentration of cupric ion increases, but above Cu/Si ) 7 × 10-4 the photoyield drops. Thus, there is an optimum concentration near Cu/Si ) 7 × 10-4 at which the photoyield is maximum. The stability of the photoproduced PC1+ radical cations was monitored and it was observed that Cu-X-PC1 (Cu/Si ) 7 × 10-4) exhibited the best stability. The half-life of the PC1+ cation radicals was found to be about 60 days. The effects of the alkyl chain length on the photoyield of alkylphenothiazine cation radicals (PCn+) from 1 to 10 carbons is shown in Figure 6. As one can observe from Figure 6, the yield of the PCn+ cation radicals decreases monotonically from methylphenothiazine to decylphenothiazine. The molecular dimension of the methylphenothiazine molecule is 6.5 Å. Zeolite A is a small pore zeolite having a pore opening of 4.1 Å. Thus, zeolite A may not be able to accommodate methylphenothiazine molecule. Consequently, the photoyield of methylphenothiazine in A zeolite is expected to be zero. The results obtained from the photoionization are shown in Figure 7. The photoyield in Cu-A zeolite is found to be eight times lower compared to Cu-X zeolite. The adsorption of methylphenothiazine on the surface of Cu-A zeolite may explain this small photoyield. Figure 8 shows TGA results obtained from incorporation of methylphenothiazine into X and A zeolites. The curves show
Photoionization of N-Alkylphenothiazines
Figure 7. Room-temperature photoinduced methylphenothiazine radical cation yield measured by ESR versus irradiation time for Cu-XPC1 and Cu-A-PC1.
J. Phys. Chem. B, Vol. 105, No. 1, 2001 121 increase in the intensity of the ESR signal due to PC1+ with irradiation time for M-X, with transition metal ions in ionexchange sites. As one can observe from Figure 4, the presence of Cu in X zeolite enhances the photoyield compared to other metal ions such as Ni, Co, or Cr. Additional experiments to detect any paramagnetic species such as Ni1+, the probable photoproduct of the photoreaction, were not successful due to overlap with the PC1+ spectrum. The photoionization efficiency was found to be highest for Cu-X zeolite. The relative increase in the yield of PC1+ after 60 min irradiation with respect to the dark reaction was found to be 200% for Cu-X-PC1, whereas it was 80% for Ni-X-PC1 and 60% for Co-X-PC1. The photoyields of PC1 for Cr-X, Na-X and Mn-X were similar and small. The photoefficiency for PC1 in M-X zeolites decreases in the order, Cu-X-PC1 > Ni-XPC1 > Co-X-PC1 > H-X-PC1 > Cr-X-PC1 ∼ Na-X-PC1 ∼MnX-PC1. The trend obtained for the photoyield of PC1 in M-X zeolites seems to be correlated wth the reduction potentials of the metal ion incorporated into the zeolites. The reduction potentials of Cr3+ (E°Cr3+/Cr2+ ) -0.407 V), Na1+ (Eo Na1+/Na0 ) -2.17 V), and Mn2+ (Eo Mn2+/Mn1+ ) -3.0 V)26 seem too negative to much enhance the photoyield by reaction 1.
PC1 + Mn+ f PC1+• + M(n-1)+
Figure 8. Differential thermal analysis of methylphenothiazine impregnated into Na-X and Na-A zeolites.
typically three weight losses. The first near 100 °C is attributed to water desorption, the second near 200 °C is attributed to methylphenothiazine desorption from the external surface and the third broad peak centered around 400 °C is assigned to the decomposition of methylphenothiazine in oxygen flow within the pores. The assignment of the 400 °C peak to methylphenothiazine within the pores is supported by its absence when TGA is done in nitrogen flow. For A zeolite we see a weak shoulder around 100 °C attributed to water desorption and a second predominant peak at 200 °C attributed to methylphenothiazine desorption. TG analysis suggests that X zeolite can accommodate methylphenothiaine inside its pores, whereas A zeolite cannot accommodate methylphenothiazine within its smaller pores. Discussion The ESR results clearly confirm the photooxidation of methylphenothiazine molecules into methylphenothiazine cation radicals in X zeolite at room temperature. The increase in the intensity of the ESR signal due to the PC1+ radical cations with time in the case of M-X samples (Figure 4) suggests that the transition metal ions assist in the photoionization of methylphenothiazine. However the photoyield also depends on the nature of the transition metal ion. Experiments with transition metal ions in M-X-PC1 show strong ESR signals at room temperature. Figure 4 shows the
(1)
The higher photoyield and the best stability for Cu-X-PC1 can be explained on the basis of the higher and positive electron affinity of Cu2+ (E°Cu2+/Cu1+ ) 0.153 V). The one electron reduction potentials for Ni2+ and Co2+ seem to be unavailable. The two electron reduction potentials for Cu2+ (E°Cu2+/Cu0 ) 0.342 V), Ni2+ (Eo Ni2+/Ni0 ) 0.257 V), and Co2+ (E°Co2+/ Co0 ) -0.28 V) suggest that the photoyields of PC1 decrease in the order Cu-X-PC1 > Ni-X-PC1 > Co-X-PC1. This trend is indeed observed and so it is suggested that the one and/or two electron reduction potentials of M2+ may be used as a guide for the efficiency of this process. The relative increase in the yield of PC1 for H-X zeolite is about 50%. This seems to suggest that the protons in H-X zeolite may act as electron acceptors. Low-temperature ESR measurements at 77 K were done in order to detect trapped hydrogen atoms but none were seen. The photoionization efficiency was found to depend on the nature and the amount of the metal ion. Although the metal ion serves as the probable acceptor site this is not yet unambiguously established. However, the following points strongly indicate that the metal ion acts as the electron acceptor. A decrease in the spin concentration of Cu2+ is observed after photoionization of methylphenothiazine but the total spin concentration prior to and after photoionization of methylphenothiazine remains the same. This is consistent with the electron transfer from PC1+ to Cu2+ to form paramagnetic PC1 and nonparamagnetic Cu+. Also, the high photostability (t1/2 ) 60 days) at room temperature obtained for Cu-X (Cu/Si ) 7 × 10-4) zeolite, which has the most positive redox potential among the metal ions studied, suggests that the electron is transferred to Cu2+ to form Cu+. The presence of copper ion in an extraframework position increases the photoyield and the stability of the methylphenothiazine cation radical. Cu-X zeolites were prepared with varying ratios of Cu/Si. Figure 5 shows the photoyield of PC1+ cation radical versus the Cu/Si ratio for Cu-X-PC1 samples. It is observed that the photoyield initially increases as the cupric ion amount increases, but after Cu/Si ) 7 × 10-4 the photoyield decreases. The lower activity obtained by the Cu-X zeolites having Cu/Si ) 7 × 10-3 and Cu/Si ) 6 × 10-2 probably
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Ranjit and Kevan studied exhibited the highest photostability. In contrast, the photoyield and photostability of Mn-X, which has a highly negative redox potential for M2+/1+, was found to be the lowest. Thus the redox potential of the transition metal ion can be used as a guide for optimizing photoproduced charge separation. There also seems to be an optimum concentration for the ionexchanged metal ion to maximize the charge separation efficiency. The photoionization efficiency is also dependent on the molecular dimension of the phenothiazine molecule. These results clearly indicate that Cu-X zeolite provides an appropriate steric and electrostatic environment to retard back electron transfer and increase the lifetime of photogenerated radical ions for many days at room temperature.
Figure 9. Structure of N-alkylphenothiazines along with their molecular dimensions.
suggests the formation of secondary radicals. Evidence for this comes from three facts. First, the intensity of the ESR signals of Cu-X samples having Cu/Si ) 7 × 10-3 and Cu/Si ) 6 × 10-2 increased in the dark by about 10% at room temperature after 60 min of photoirradiation. This might be due to some secondary oxidation leading to the formation of radicals other than PC1+. Second, visually there was a difference in the color of these two Cu-X-PC1 samples after 60 min photoirradiation. The samples usually turn dark pink after irradiation but for CuX-PC1 having Cu/Si ) 7 × 10-3 and 6 × 10-2 the color was light brown. Third, the decay for Cu-X-PC1 samples with Cu/ Si ) 7 × 10-4 and Cu/Si ) 2 × 10-4 could be fit by a simple first-order exponential decay. However, for the Cu-X-PC1 samples with Cu/Si ) 7 × 10-3 and 6 × 10-2, a first-order exponential decay did not give an appropriate fit. This suggests that in these two samples some secondary oxidation products are also formed in addition to PC1+. Figure 6 shows that the photoyield decreases monotonically with increase in the alkyl chain length from 1 to 10. Figure 9 shows the structure of PCn along with the molecular dimensions. As can be seen from Figure 9, the molecular dimensions of butylphenothiazine, hexylphenothiazine, and decylphenothiazine exceed the pore dimension of X zeolite. TGA results obtained from the incorporation of butylphenothiazine, hexylphenothiazine, and decylphenothiaizne show the absence of any peak around 400 °C which indicates that PC4, PC6, and PC10 do not penetrate significantly into X zeolite. Thus the photoyield naturally decreases as the alkyl chain length increases from 1 to 10. Conclusions Metal ion-exchanged zeolites act as efficient hosts for stabilizing PC1+ cation radicals at room temperature. The photoyield seems to be dependent on the standard potential for the Mn+/(n-1)+ or Mn+/(n-2)+ redox couple. Cu-X zeolite which has the most positive redox potential among the metal ions
Acknowledgment. This research was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy and by the Texas Advanced Research Program. References and Notes (1) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: New York, 1987. (2) Ramamurthy, V. Photochemistry in Organized and Constrained Media; VCH Publishers: New York, 1991. (3) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141. (4) Scaiano, J. C.; Garcia, H. Acc. Chem. Res. 1999, 32, 783. (5) Vermeulen, L. A.; Thompson, M. E. Nature 1992, 358, 656. (6) Slama-Schwok, A.; Avnir, D.; Ottolenghi, M. Photochem. Photobiol. 1991, 54, 525. (7) Kalyanasundaram, K. Coord. Chem. ReV. 1982, 46, 159. (8) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press: London, 1992. (9) Brigham, E. S.; Snowden, P. T.; Kim, Y. I.; Mallouk, T. E. J. Phys. Chem. 1993, 97, 8650. (10) Kuchi, V.; Oliver, A. M.; Paddon-Row, M. N.; Howe, R. F. Chem. Commun. 1999, 1149. (11) Forbes, W. F.; Sullivan, P. D. J. Am. Chem. Soc. 1966, 88, 2862. (12) Clarke, D.; Gilbert, B. C.; Hanson, P. J. Chem. Soc., Perkin Trans. 2 1975, 1078. (13) Clarke, D.; Gilbert, B. C.; Hanson, P. J. Chem. Soc., Perkin Trans. 2 1978, 1103. (14) Fujihara, H.; Fuke, S.; Yoshihara, M.; Maeshima, T. Chem. Lett. 1981, 1271. (15) Kurshev, V.; Prakash, A. M.; Krishna, R. M.; Kevan, L. Microporous Mesoporous Mater. 2000, 34, 9. (16) Krishna, R. M.; Prakash, A. M.; Kurshev, V.; Kevan, L. Phys. Chem. Chem. Phys. 1999, 1, 4119. (17) Dutta, P. K.; Ledney, M. Prog. Inorg. Chem. 1997, 44, 209. (18) Alkaitis, S. A.; Beck, G.; Graetzel, M. J. Am. Chem. Soc. 1975, 97, 5723. (19) Moroi, Y.; Braun, A. M.; Graetzel, M. J. Am. Chem. Soc. 1979, 101, 567. (20) Baldovi, M. F.; Cozens, F. L.; Fornes, V.; Garcia, H.; Scaiano, J. C. Chem. Mater. 1996, 8, 152. (21) Gozlan, I.; Ladkani, D.; Halpern, M.; Rabinovitz, M.; Anoir, D. J. J. Heterocyl. Chem. 1984, 21, 613. (22) Shine, H. J.; Thompson, D. R.; Venziani, C. J. Heterocyl. Chem. 1967, 4, 517. (23) Hovey, M. C. J. Am. Chem. Soc. 1982, 104, 4196. (24) Xiang, B.; Kevan, L. Langmuir 1994, 10, 2688. (25) Xiang, B.; Kevan, L. J. Phys. Chem. 1994, 98, 5120. (26) Lide, D. R. Ed. CRC Handbook of Chemistry and Physics, 80th ed.; CRC: Boca Raton, FL, 1999; pp 8-22.
