NOTES
1162 ceptor the value of Av increases with the acidity of the donor, except in the case of ortho-substituted amines. In Figure 4, log K is plotted as a function of Av for amine-DMSO We obtain a linear relationship between these two factors. The points off of the straight line correspond to the ortho compounds. The values of Av are plotted in Figure 5 as a function of the Hammett u parameter for the substituted anilineHMPT systems. The correlation between these two factors is good because of the relationships previously
established. Similar correlations have been found in the systems phenol-pyridine and phen0l--aniline.~3
Acknowledgments. We wish to thank Dr. G. Turre11 (Bordeaux) for valuable comments on the manuscript, and Dr. P. Dizabo (Paris) for the gift of aminopyrimidines. (29) A Fermi resonance is observed between n ( N H 2 ) u and the first overtone of the in-plane bending vibration at about 3200 cm-1. This resonance is weaker in the case of 1-1 complexes but can slightly perturb the value of the [vi(NHz) 11-1 freq~ency.'~
NOTES
The Far-Ultraviolet Spectrum of Ice
by Allen P. Minton' Polymer Department, Weizmann Institute of Science, Rehovoth, Israel (Received October 16, 1970)
The following procedure was employed to prepare the sample for measurement. The end plate not containing the quartz window wm removed and a small aliquot (0.1-0.3 cc) of previously degassed doubledistilled water was introduced into the prerefrigerated
Publication costs borne completely by The Journal of Physical Chemistry
During a study of the relationship between optical properties and intermolecular interactions in the water substance, the absorption spectrum of ice in the fas-uv (180-190 mp) was considered to be of interest, especially when compared to those of liquid water and water vapor. The only previous quantitatively reported measurements of uv absorption in ice2 were limited to wavelengths shorter than 170 mp, and it was not clear whether these data could be safely extrapolated into the region of interest. The molar extinction coefficient B of liquid water increases exponentially with decreasing wavelength throughout this region and shifts to the blue with decreasing t e m p e r a t ~ r e . ~It , ~was therefore assumed that the absorption of ice would be somewhat less than that of water at the same wavelength, and that correspondingly longer transmission path lengths would be required to bring the absorptivity of the sample into the region capable of accurate measurement. Experimental Section The sample cell illustrated in Figure 1 was constructed of copper to ensure effective thermal connection between the sample and the thermostat (a refrigerated bath of ethylene glycol solution). The length of the path was determined by the thickness of the central spacer, and several of these were prepared with thicknesses ranging from 1to 10 mm. The Journal of Phyekal Chemistry, Vol. 76, No. 8,1971
A
6
A'
/
0
-4
cm ----+
Figure 1. Sample cell for the memurement of the far-uv spectrum of ice: A,A', end plates with cooling coils; B, central spacer; C, quartz window. (1) Chaim Weizmann Junior Postdoctoral Fellow; present address: Laboratory of Biophysical Chemistry, National Institute of Arthritis and Metabolic Diseases, NIH, Bethesda, Md. 20014. (2) K. Dressler and 0. Schnepp, J . Chem. Phys., 33, 270 (1960). (3) M. Halmann and I. Platzner, J . Phys. Chem., 70, 580 (1966). (4) D. P. Stevenson, ;bid., 69, 2145 (1965).
1163
NOTUS
/
a /
d//
/I' 5
184
182
WAVELENGTH (mA)
Figure 2. a, Far-uv absorption of ice; b, far-uv absorption of water, 0"; c, far-uv absorption of water, 36'; d, far-uv absorption of water, 36", data of ref 4.
I
1
180
190
cell. Because of thermal nonuniformity the water froze unevenly leaving an uneven and cloudy ice-air interface. This interface was smoothed and polished with a clean tissue and slight application of pressure. Another aliquot was added, and the procedure was repeated until the central spacer was completely filled with ice, with a small quantity extending beyond the surface of the spacer. Subsequent polishing leveled off the interface and set it parallel to and contiguous with the surface of the spacer. In this way a clear sample of polycrystalline ice containing two parallel faces of known separation was obtained. Any residual deviations from parallelicity between the faces would serve to scatter transmitted light and increase the observed absorptivity. The same may be said of any impurities which may have gotten into the sample during the preparation procedure. Thus the measured extinction coefficients may be regarded as reliable upper limits t o the true extinction coefficients. The absorption measurements were carried out on a Cary 15 spectrophotometer purged with dry nitrogen. As a check of the reproducibility of the results, several samples were prepared and measured at each of several path lengths. As a check of the instrumentation and technique the absorption spectrum of liquid water in this wavelength region was measured in a conventional thermostated quartz cell at several temperatures in the range 1-50'. The results so obtained were found to agree, where comparable, with the results of Stevenson4to within 5% in e .
I
I
170 160 WAVELENGTH (mN)
150
Figure 3. Comparison of data, far-uv absorption: -, water vapor, ref 5; -O-O-O-, liquid water, 2 4 O , ref 3; -0- -0- -0. -, ice, this study; , ice, ref 2. Crosshatched area corresponds to the estimated uncertainty.
-
-.-.-.-
Results I n Figure 2 the logarithm (base 10) of the extinction coefficient of ice (in units of liters/(mole-cm)) is plotted against the wavelength, showing the average values obtained and the spread of measured values. For comparison the corresponding results for water at 1 and 36" are also shown. In Figure 3 the same ice data are reproduced on a graph of smaller scale so as to include for additional comparison the results of Halmann and Platzner3 for liquid water, Watanabe and Zelikoff for water vapor, and Dressler and Schnepp2for ice. The results for ice reported were obtained at a cell temperature of - 10". Test measurements yielded negligible change in the spectrum upon lowering the temperature to -40".
Discussion One point worth remarking upon is that the data of Dressler and Schnepp,2 if extrapolated to the wavelengths at which the present measurements were made, would provide apparent extinction coefficients significantly greater than those reported here. The differ(5) K. Watanabe and M. Zelikoff, J . Opt. SOC.Amer., 43,763 (1953).
The Journal of Physical Chemistry, Vol. 76,No. 8,1871
1164 ence in temperature at which the two studies were performed (- 10" vs. - 100" to -200") would not seem to account for the discrepancy since in both studies variations in temperature did not significantly affect the spectrum. However, Dressler and Schnepp have pointed out that there exist large possible errors in their measurement of E due to the uncertainty in the determination of path length; when this uncertainty is taken into account (see crosshatching in Figure 3) the two sets of data are easily seen to be compatible. The results further indicate that the uv spectrum of liquid water is less like that of ice than might have previously been supposed. The spectrum of ice is presumed not to vary substantially between 0" and the temperature of measurement. If we assume that the difference between the spectra of ice and water at 0" is due primarily to a shift in frequency of the lowest energy uv continuum (maximum at -170 mp in the vapor5), then the electronic transition responsible for this continuum increases in energy by approximately 5 kcal/mol upon freezing of liquid water. A structural interpretation of this result would require knowledge of the extent to which the interactions between a molecule and its neighbors differ in the ground and excited states.
NOT~S
N2
/1
PULSES Figure 1.
-'f
14
I
I
I
I
I
I
I
1
'
04
The Formation of Ethane, Ethylene, and Acetylene from Methane on Radiolysis with High-Intensity Electron Pulses
by R. W. Hummel* and J. A. Hearne U.K . Atomic Energy Authority, Wantage Research Laboratory ( A E R E ) , Wantage, Berks, England (Received October 2, 1070) Publication costs assisted by the Atomic Energy Research Establishment
The radiolysis of CHI with electrons from a Febetron-pulsed electron generator has been reported1 to yield significant quantities of C2H2 and C2He (G = 0.5 and 0.7, respectively, for a dose of 1.8 X lo6 rads). However, the C2Heyield (G = 0.7) was very low compared to that usually obtained at much lower dose rates (G = 2.l).2-4 Because knowledge of the dose rate dependence of the yields of these three primary products assists in understanding the radiation chemistry of the system, it was considered worthwhile to confirm, or disprove, the results reported. As shown below, we can confirm the relatively high unsaturate yields but find G(C2Hs)unchanged from the low dose rate value. Methane, purified as described p r e v i ~ u s l y , ~ was *~ irradiated in 220-ml Pyrex vessels fitted with a 0.002-in. A1 window compressed onto an indium wire to make The Journal of Physical Chemistry, Vol. 76,N o . 8,1071
PULSES Figure 2.
a vacuum-tight seal. Analyses were by gas chromatography. Nitrous oxide, purified by distillation into a continuously pumped Torr) trap held at -195", was used for dosimetry, taking G(N2) = 12.4 as reporteds for Febetron irradiations. The dose rate in CHI was calculated on the basis of the ratio of electron densities (1.25). Nitrogen analyses were by gas chromatography. (1) R. W. Cahill, A. K. Seeler, and R. A. Glass, J . Phys. Chem., 71, 4564 (1967). (2) F.W. Lampe, J . Amer. Chem. Soc., 79, 1055 (1957). (3) K.Yang and P. J. Manno, {bid., 81, 3507 (1959). (4) R.W. Hummel, Trans. Faraday Soc., 62, 59 (1966). (5) R. W. Hummel and J, A. Hearne, Chem. Ind. (London), 1827 (1961). (6) C.Willis, A. W. Boyd, and D. A. Armatrong, Can. J . Chem., 47, 3783 (1969).
