J . Phys. Chem. 1990, 94, 4051-4054 sorption ~ t u d i e s have ~ ~ ~provided ~~' conclusive evidence for our interpretation. We believe picosecond transient Raman spectroscopy and femtosecond transient absorption spectroscopy of isomeric C20 aldehyde will resolve the contradiction in the near future.
Acknowledgmenr. We are indebted to Prof. Katsumi Tokumaru
4051
and Dr. Tatsuo Arai at Department of Chemistry, University of Tsukuba, for discussion on the determination of quantum yields. Thanks are also due to Dr. Robert Wilbrandt and Dr. NielsHenrik Jensen for sending ref 45 prior to publication. Forwarding of a preprint (Ganapathy, S.; Trehan, A.; Liu, R. S. H. Photoisomerization of 7-cis-Retinal. The Concentration Effect) by one of the referees is acknowledged.
Absolute Raman Cross Sections for Cyclohexane, Acetonitrile, and Water in the Far-Ultraviolet Region Bulang Li and Anne B. Myers* Department of Chemistry, University of Rochester, Rochester, New York 14627 (Received: October 24, 1989)
Absolute Raman cross sections with excitation at wavelengths between 266 and 200 nm have been determined for the CH and CN stretching modes of acetonitrile, the symmetric OH stretch of water, and the CH stretches of cyclohexane. The measurements have been performed both directly, using an integrating cavity method, and indirectly, by reference to methane gas as an external standard. The two methods agree to within 35% or better in all cases. The cross sections are fit to Albrecht A-term frequency dependences to permit interpolation and modest extrapolation from the measured points. These solvent Raman bands can now be employed as internal standards for the determination of absolute resonance Raman cross sections of solutes in the far-UV spectral region.
Introduction It has long been known that resonance Raman intensities contain information about the structure and dynamics of the resonant molecular excited state. Early resonance Raman intensity studies focused on either Raman spectra (the relative intensities of different vibrations at a given excitation frequency) or excitation profiles (the dependence of the Raman intensity on excitation frequency).' Excitation profiles were usually measured relative to a solvent Raman band or an added nonresonant species such as sulfate or cacodylate as an internal standard. The frequency dependence of the internal standard Raman cross section was often assumed to follow either a pure v4 dependence (totally nonresonant scattering) or a single-state Albrecht A-term preresonance with the lowest allowed electronic state. While these were questionable assumptions, they often did not lead to serious uncertainties in the excitation profile band shapes, particularly when the profiles covered a rather narrow range of frequencies in the visible region of the spectrum. More recently, however, there has been increasing interest in the measurement of excitation profiles in the far-UV region,2" where the internal standard cross sections are expected to be more strongly frequency dependent, as well as in the use of absolute UV resonance Raman cross sections to probe solvation dynamics.' For these purposes it is essential to have direct measurements of the internal standard absolute cross section as a function of excitation frequency in the far-ultraviolet region. Recently several groups have addressed this need through different experimental methods. Dudik et aL8 developed a method based on ratioing the Raman intensity to the laser scattering from a suspension of BaSO, powder which enabled them to measure the frequency dependence of the differential Raman cross section for several substances in aqueous solution well into the UV region. Their absolute cross sections were scaled to a single primary measurement on benzene in the v i ~ i b l e . ~Bischel and Blacklo employed a very direct method, based essentially on measuring the solid angle of collection and the efficiency of their collection optics, to measure absolute differential cross sections for several gases down to 193 nm. The excellent agreement of their results with high-quality calculations on Hz provided confidence in the accuracy of their measurements. Trulson and Mathies" developed
* Author
to whom correspondence should be addressed.
an integrating cavity technique that allowed them to measure directly the total Raman cross sections for cyclohexane, benzene, and aqueous cacodylate from 647 to 240 nm. Schomacker et a1.12 also employed a direct method, based on first measuring the total Rayleigh plus Raman cross section and then ratioing the Raman to the Rayleigh, to determine absolute cross sections for benzene in the visible to near-UV region. Finally, very recently Fodor et aL6 applied the relative method of ref 8 to obtain cross sections for aqueous sulfate and for water down to 193 nm. At the time we began this work, no direct measurements on liquids had been reported at wavelengths below 240 nm, and the relative measurements of Dudik et al. extended only to 220 nm. We set out to provide ourselves and other workers with reliable solution-phase standards down to 200 nm by applying the integrating cavity method of Trulson and Mathies" to cyclohexane, acetonitrile, and water, with comparison to methane gas as an external standardlo used to verify our results. The data are fit to a single-state A-term frequency dependence, providing a convenient fitting function with which to interpolate and, if necessary, slightly extrapolate to wavelengths other than those used in this study. It is now comparatively straightforward to employ these (1) For early reviews, see: (a) Johnson, B. B.; Peticolas, W. L. Annu. Reu. Phys. Chem. 1976, 27,465. (b) Warshel, A. Annu. Reo. Biophys. Bioeng. 1977, 6 , 273. (2) (a) Blazej, D. C.; Peticolas, W. L. Proc. Nail. Acad. Sri. USA 1977, 74, 2639. (b) Blazej, D. C.; Peticolas, W. L. J . Chem. Phys. 1980, 72, 3134. (3) (a) Myers, A. B.; Trulson, M. 0.; Pardoen, J . A,; Heeremans, C.; Lugtenburg, J.; Mathies, R. A. J . Chem. Phys. 1986,84, 633. (b) Trulson, M. 0.; Dollinger, G. D.; Mathies, R. A. J . Chem. Phys. 1989, 90, 4274. (4) Cable, J. R.; Albrecht, A. C. J . Chem. Phys. 1986, 84, 1969. ( 5 ) Asher, S. A.; Ludwig, M.; Johnson, C. R. J . Am. Chem. Sor. 1986, 108, 3186. (6) Fodor, S. P. A.; Copeland, R. A,; Grygon, C. A,; Spiro, T. G.J . Am. Chem. SOC.1989, 111, 5509. (7) (a) Myers, A. B.; Li, B.; Ci, X . J . Chem. Phys. 1988,89, 1876. (b) Myers, A. B.; Li, B. J . Chem. Phys., in press. (8) Dudik, J. M.; Johnson, C. R.; Asher, S. A. J . Chem. Phys. 1985,82, 1132. (9) Abe, N.; Wakayama, M.; Ito, M. J . Raman Spectrosc. 1977, 6, 38. (10) Bischel, W. K.; Black, G. In Excimer Lasers-1983; Rhodes, C. K.; Egger, H.; Pummer, H., Eds.; American Institute of Physics: New York, 1983; p 181. ( 1 1 ) Trulson, M. 0.; Mathies, R. A. J . Chem. Phys. 1986, 84, 2068. (12) Schomacker, K. T.; Delaney, J. K.; Champion, P. M. J . Chem. Phys. 1986, 85, 4240.
0022-3654/90/2094-405 1%02.50/0 0 1990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. IO, 1990
liquids either as internal standards in resonance Raman cross section measurements or as external standards for other solvents which may then be used as internal standards themselves.
Experimental Section Integrating Cuoity Method. The integrating cavity method, described in detail by Trulson and Mathies," measures absolute Raman scattering cross sections by comparison to the elastic scattering from a reference scatterer of unit yield. Light scattered from a sample placed inside the cavity has its direction and polarization randomized by multiple diffuse reflections from the coated white cavity walls. A small fraction of the scattered light escapes through a small hole into the entrance slit of a spectrograph and is detected. Each measurement involves two separate experiments. First the Raman scattering of the liquid sample is recorded. Then, under the same conditions, the incident laser light is dumped into the cavity by diffuse reflection from a nonabsorbing beam stop (which replaces the sample cell) and collected as the reference signal. The total absolute Raman cross section for the m to n transition is then given by"
where C is the detected signal in photon counts, E is the wavelength-dependent efficiency of the collection and detection system, T is the signal accumulation time, and is the incident laser power. The subscripts Ra and Rf specify the Raman experiment and the reference experiment, respectively. Nand L are the density of scatterers (molecules ~ m - and ~ ) the path length of the sample cell in the Raman experiment (1 cm in our case). A is the absorbance of the attenuating solution used in the reference experiment as described below. A'is the absorbance of the sample at the laser wavelength. The second factor emerges as a correction for the attenuation of the incident laser light as it passes through the sample at wavelengths where absorption is not negligible. Absorption of the scattered Raman light by the sample is not significant at any of the excitation wavelengths employed. Our integrating cavity was constructed similarly to that of Trulson and Mathies." Ours is a cubic box (1 3 X 13 X 13 cm) with 8-mm entrance and exit ports for the laser and 2-mm holes through which the exiting scattered light passes through the attenuator, a 1 cm path length cuvette. Instead of using the etched quartz side of the sample cell to scatter the laser light in the reference experiment, we used a piece of plastic coated with the same material used to coat the inside of the cavity (Eastman Kodak Analytical Standard White Reflectance Coating). This change was made because the etched quartz face of our cell was found to give too much absorption of the laser light when working in the far-UV region. An additional 1.5 in. focal length lens was used to focus the light leaving the attenuator cell into the entrance slit of the spectrograph (Spex 1702), which was equipped with either a 1200 or a 2400 groove/" grating, used in first order, and an intensified diode array detector (Princeton Instruments IRY-700). The four wavelengths we employed were 266, 240, 21 8, and 200 nm, generated from the fourth harmonic of a 20-Hz Q-switched Nd:YAG laser (Quantel 660) together with Raman shifting in hydrogen. Typical average laser powers were 2-10 mW with a beam diameter at the sample of -2 mm; this appeared to be well below the threshold for stimulated Raman scattering in these liquids. The incident laser power in both the Raman and reference experiments was monitored by using a quartz plate to split a small portion of the beam into a solar blind photomultiplier whose output was processed with a boxcar averager. The attenuator cell was filled with water for the Raman measurements. During measurement of the reference signal, the cell was filled with aqueous solutions of stable, nonfluorescent inorganic ions: potassium dichromate (266 and 240 nm), potassium thiocyanate (21 8 nm), and potassium nitrate (200 nm). The optical densities of these solutions were too high to be measured directly to the accuracy required. Instead, the optical density of each solution was measured in a 1 mm path length cell, and the actual path length ratio of the nominally I-cm and 1-mm
Li and Myers cells was obtained from absorbance measurements on a series of solutions in both cells. The spectrophotometer (Cary 118) was spectrally calibrated from 240 to 640 nm with a holmium(II1) solution (absorption spectrum taken from refs 13 and 14) prepared by dissolving 0.5 g of holmium oxide in 10 mL of 17% perchloric acid. Calibration below 240 nm was extrapolated from a quadratic polynomial fit. The Cary was calibrated photometrically with the NBS recommended standard,I5potassium dichromate, in the optical density range 0.1-1 .O. It is desirable to keep the optical density of the attenuator as low as possible in order to minimize the errors in the fraction of light transmitted through the cell. The optical densities we employed were in the range of 2-3, causing the reference signal to be 100 times larger than the Raman signal of the weakest vibration studied. The linearity of the detector in this range was checked with a deuterium lamp and calibrated neutral-density filters. Spectral grade acetonitrile was used as purchased. Distilled water was obtained from a tap in the laboratory and used without further purification. Both the acetonitrile and the water had absorbances of less than 0.03 in a 1 cm path length down to 200 nm. Spectral grade cyclohexane was used as received at 266 and 240 nm. For the 2 18-nm experiments the cyclohexane was degassed by bubbling with N, to reduce its optical density (due to dissolved 0,) to below 0.1. No experiments were performed on cyclohexane below 218 rtm due to the difficulty in maintaining a sufficiently low optical density at these wavelengths throughout the experiment. The wavelength dependence of the collection and detection efficiency was measured by directing a calibrated standard UV lamp (Optronics Model UV-40) into the integrating cavity through the laser entrance port and recording the signal levels at the laser and Raman scattered wavelengths. The integrating cavity contained the sample cell for measurements of the efficiency at Raman wavelength and the Basorcoated beam stop for measurements of the efficiency at the laser (reference) wavelength. Finally, the absolute Raman cross sections were calculated from eq 1. Methane Comparison Method. For the methane comparison experiments, the integrated Raman intensity of the liquid sample was compared with the intensity of the 2917-cm-I Q-branch of methane gas, for which absolute differential cross sections have been reported.I0 The liquid sample was held in a 1 cm path length fluorescence cuvette and the methane at -1 atm pressure (measured with a capacitance manometer, MKS Instruments) was contained in a 500-mL bulb fused to a 1-cm fluorescence cuvette. The incident laser light was lightly focused by using a 1 m focal length lens and the Raman scattering was collected in a 90' scattering geometry with anfll.1 fused silica condenser lens and polarization scrambler as the collection optics. Care was taken to ensure that the image of the spectrograph slit fell within the illuminated area of the sample to satisfy the conditions for validity of the n2 correction due to the different internal collection angles in media with different refractive indices.16 The laser power was attenuated with calibrated neutral-density filters in collecting the solvent Raman scattering, which was 2-3 orders of magnitude stronger than that of methane gas due largely to the higher concentration of scatterers in the liquid. The laser power was monitored as described above. The 2400 groovelmm grating was used in these experiments to provide adequate resolution (- 10-20 cm-I) to isolate the methane Q-branch transition. The absolute differential cross sections for the liquids were obtained from eq 2, where C is the detected signal in photon
-
(13) Standards in Absorption Spectrometry; Burgess, C., Knowles, A., Eds.; Chapman and Hall: London, 1989; p 133. (14) Weidner, V. R.; Mavrodineanu, R.; Mielenz, K. D.; Velapoldi, R. A,; Eckerle, K. L.; A d a m , B. NBS Special Publication 260-102, 1986. (15) Burke, R . W.; Mavrodineanu, R. NBS Special Publication 260-102, Appendix A-2.4, 1986. (16) Ediger, M. D.; Moog, R. S.; Boxer, S . G.; Fayer, M. D. Chem. Phys. Left. 1982. 88, 123.
Absolute Raman Cross Sections for C6HI2,CH3CN, and H 2 0 The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4053 TABLE I: Absolute Raman Cross Sections for Acetonitrile, Water, and Cyclohexane
excitation wavelength, nm 266 240 218 200
K: mb u , , ~ IO3 cm-'
acetonitrile CH str CN str
water OH str
cyclohexane CH str
3.61 (4.49)' 8.71 (10.1) 15.5 (20.3) 31.7 (33.8)
3.09 (3.31) 5.84 (5.84) 8.70 (10.6) 25.4 (22.9)
25.2 (27.4) 61.3 (74.5) 104 (1 20)
1.59 (1.14) 2.99 (2.60) 5.63 (6.10) 13.2 (8.77)
96 96
22 92
44 91
87d
ison experiment. counts, is the incident laser power, N is the particle density, n is the refractive index, and A is the optical density of the neutral-density filters used in collecting the liquid signal. The subscripts liq and gas indicate the solvent and methane measurements, respectively. A ' is the absorbance of the liquid in a I cm path length; the Raman scattering was collected from a point approximately halfway through the cell, where the transmittance is 10-A'12. (daR/dR),,, is the absolute differential cross section for methane, for which values of 0.103, 0.192, 0.363, and 0.687 cm2) were calculated at 266, 240, 218, and 200 mb ( 1 mb = nm, respectively, from the A-term-fitting parameters given in ref IO. Finally, the data were converted from differential cross section to total cross section by using eq 3. Depolarization ratios of 0.23, =
-)( 3)
i
314d
'Numbers outside parentheses are from the integrating cavity method while the values within parentheses are methane comparison experiment results. Cross sections are in units of millibarns (1 mb = cm2). b K is the coupling strength in the Albrecht A-term expression [eq 41. is the resonant-state energy in the A-term expression [eq 41. dParameters from fitting the data of ref 11 and the average of our data in integrating cavity experiment and methane compar-
OR
9.00
' 37,'OOO ' 41,'OOO
' 45,'OOO ' 49,'OOO ' 53,'OOO '
Excitation Wavenumber (cm-') Figure 1. Absolute cross sections for the CH stretching (upper curve) and the CN stretching (lower curve) bands of acetonitrile. Crosses represent integrating cavity data while circles represent methane comparison experiment data. The solid lines are the A-term fits to the average of the two data sets.
8.00
T
t t
/
&( 1 + 2 p 3
l + p
(3)
0.25, and 0.19 were measured as described in ref 7a at 218 nm for the CH stretches of cyclohexane and acetonitrile and at 218 and 240 nm for the OH stretch of water, respectively, and were assumed independent of wavelength. The C N stretch of acetonitrile was assumed to be totally polarized (p = 0) based on Dudik et al.'s upper limit of p = 0.05.8 The refractive indices of water and cyclohexane as a function of wavelength were obtained from refs 17 and 18, respectively. Refractive indices of acetonitrile in the U V were extrapolated from the value at the sodium D lineIg by assuming the same wavelength dependence [eq 1 in ref 171 as that of water. The increase in refractive index from the sodium D line to the wavelengths used in our experiments is 5-8%. Integrated areas of all the vibrational transitions studied in both the integrating cavity and methane comparison experiments were obtained by simple computer integration (rectangle rule). The CH stretches of cyclohexane and acetonitrile were integrated from 2600 to 3100 and 2700 to 3200 cm-l, respectively. The CN stretch of acetonitrile and the OH stretch of water were integrated from 2160 to 2360 and 2800 to 3900 cm-I, respectively. Results The absolute Raman cross sections are listed in Table I. For the purpose of data analysis, the measured absolute cross sections were divided by the prefactor of vo(uo - v ) ~where , vo is the laser wavenumber in I O 3 cm-I and v the wavenumber in lo3 cm-' of the Raman transition. This reveals the true frequency dependence (17) Thormahlen, I.; Straub, J.; Grigull, U. J . Phys. Chem. ReJ Dara 1985, 14, 933. (18) Sowers. B. L.:Arakawa. E. T.: Birkhoff, R. D. J . Chem. Phvs. 1971, 54, 23'19. (19) C R C Handbook of Chemistry and Physics, 67th ed.; CRC Press: Boca Raton, FL, 1986: p C-52.
' 37,'OOO ' 41,000
' 45,'OOO ' 49,'OOO ' 53,'OOO '
Excitation Wavenumber (cm-') Figure 2. Absolute cross sections for the OH stretching vibration of water. Crosses and circles represent integrating cavity and methane comparison data, respectively. The solid line is the A-term fit to the average of the two data sets.
of the Raman polarizability and tends to prevent overweighting the data points at shorter wavelengths. The data were then fit by using a nonlinear least-squares algorithm to an Albrecht A-term dependence
where u, is the average wavenumber in lo3 cm-' of the resonant excited state and K is the coupling strength. Table I gives the parameters obtained by fitting to the average of the integrating cavity and methane comparison data for acetonitrile and water, and to our data together with those of Trulson and Mathies for cyclohexane. Figures 1 and 2 show plots of the fitted curves along with our measured cross sections for acetonitrile and water, respectively. Our cross sections for cyclohexane along with the data of Trulson and Mathies at longer wavelengths are plotted with the A-term fit in Figure 3. Discussion The integrating cavity data and those from the methane comparison experiments agree to within 35% or better for all Raman lines at all excitation wavelengths as shown in Table I. This is encouraging considering that they are results from two completely different experiments, although it does appear that the methane
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The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 -r
45.0
candidate to enhance the C H stretching vibration. In water vapor the two lowest allowed electronic transitions are X at -60000 cm-' and the B X at -78 000 cm-I. the A Raman spectra obtained on resonance with the A state exhibit strong activity in the symmetric stretch.21 However, this state does not appear to dominate the preresonance enhancement of the O H stretch in liquid water. The resonant frequency obtained from our A-term fit (u, = 91 000 cm-') falls in a region of multiple strong absorptions considered to be mainly Rydberg in character with the ( 1 b,, a,.*) valence configuration mixed in.22 Rydberg transitions around 90 000 cm-' correspond to the 1b, 3d and 1b, 4p promotions, where lb, is the uppermost lone-pair orbital and 3d and 4p are the Rydberg orbitals. These transitions are not likely to make contributions to the Raman scattering of the O H stretch mode since no valence electrons are involved. On the other hand, the lb, a,u* valence component should play a part in the preresonance enhancement. The vacuum-UV absorption spectrum of acetonitrile vapor shows strong absorption throughout the region from 70 000 to 90000 c ~ - I . * ~ This absorption is attributed to a variety of Rydberg transitions involving promotion of an electron out of the highest lying slightly ~ ~ ,transitions, ~ ~ occupied MO, the C N ~ ( e ) . These below the 92 000 cm-' energy found by our A-term fit, are good candidates to enhance the C N stretching vibration. It is not clear that these Rydberg transitions should be coupled to the C H stretches, although the strong absorption having its origin at 77 370 cm-' does appear to show a weak vibrational progression in the C H stretch as well as a stronger one in the CN stretch.25 There is relatively little absorption in the region near 96000 cm-', the resonant energy found by our A-term fit for the CH stretches. This fitting parameter may represent an average of contributions from the Rydberg states near 80000 cm-l and other higher lying states. This study provides three commonly used solvents as intensity standards for resonance Raman cross section measurements. The resonant state energies obtained from our A-term fit should not be interpreted too literally in view of the relatively small excitation frequency range employed in this study and the likelihood that multiple electronic states contribute to the preresonant enhancement. The single-state A term should be considered mainly as a convenient fitting function which allows us to interpolate and slightly extrapolate to wavelengths other than those used here.
-
-
f 1 .
35.0
Li and Myers
1
-/-
-
5.00
-/1
1
1
1
22,000
14,000
/
1
30,000
1
1
38,000
1
1
,
46,000
Excitation Wavenumber (cm-') Figure 3. Absolute cross sections for the CH stretching vibrations of cyclohexane. Shown are the data of ref 11 (+), our integrating cavity data ( X ) , and our methane comparison data (0). The solid line is the A-term fit to all of the data.
comparison results are systematically low compared with the integrating cavity. The source of this systematic deviation is not known, and we have chosen to weight the results of both experiments equally in fitting the data. Our results for cyclohexane are in good agreement with those of Trulson and Mathies at 240 and 266 nm as shown in Figure 3, and our measured value at 218 nm is very close to the value extrapolated from the fit to their data between 647 and 240 nm. This suggests that we may be justified in extrapolating the A-term fit to slightly shorter wavelengths, although we were not able to make direct measurements on cyclohexane below 218 nm. The absolute Raman cross sections calculated from our fitting parameters for the acetonitrile CN stretch are about a factor of 1.5 higher than reported by Dudik et al. in the 245-220-nm range.* This discrepancy may be due at least in part to the fact that we worked with neat acetonitrile rather than its aqueous solution as Dudik et al. did. While our experiments were in progress, Spiro and co-workers published cross sections for sulfate ion and for water between 266 and 193 These were relative measurements, scaled to the absolute differential cross section reported by Dudik et al. for sulfate at 282 nm,8 We have measured the cross section for the 982-cm-I line of aqueous SO4*-(0.4 M, pH 7) relative to the C H stretches of acetonitrile at 218 nm, and find a total absolute Raman cross section of 4.7 mb for Sod2-assuming p = 0. This total cross section is quite close to the value of 5.6 mb converted from the differential cross section of reported in ref 6. Our absolute cross sections for water are 3-30 times larger than those reported by Spiro's group. This apparent discrepancy can be attributed to the fact that peak heights rather than integrated areas for the broad water band were used to determine the cross sections given in ref 6.26 Our A-term fit for cyclohexane resulted in a resonant state at u, = 87 000 cm-I. Cyclohexane does show an intense maximum near 85 000 cm-' in its gas-phase vacuum-UV absorption spectrum u (CH) transition,20 which is a likely due to the strong u*
-
(20) Raymonda, J . W.; Simpson, W. T. J . Chem. Phys. 1967, 47, 430.
+
-
Acknowledgment. This work was supported by grants from the N I H (GM-39724) and NSF (CHE-8709485 and CHE8957221). Support of the donors of the Petroleum Research Fund, administered by the American Chemical Society, is acknowledged. A.B.M. is the recipient of a Dreyfus Distinguished New Faculty Award, a Packard Fellowship in Science and Engineering, a Sloan Research Fellowship, and an NSF Presidential Young Investigator award. Registry No. H,O, 7732-18-5;cyclohexane, 1 10-82-7; acetonitrile, 75-05-8. (21) Sension, R. J.; Brudzynski, R. J.; Hudson, B. S. Phys. Rev. Lett. 1988, 61, 694. (22) Robin, M. B. Higher Excited States of Polyatomic Molecules; Academic: New York, 1985; Vol. 111, p 165. (23) Ashfold, M. N. R.; Simons, J. P. J. Chem. SOC.,Faraday Trans. 2 1978, 74, 1263. (24) Stradling, R. S.; Loudon, A G J . Chem. SOC.,Faraday Trans. 2 1911, 73, 623. ( 2 5 ) Cutler, J. A. J . Chem. Phvs. 1948, 16, 136. (26) Spiro, T. G., personal communication
