March, 1959
NOTES
443
tem crossing, That this appears to be the mechanism for triplet population in the case of retinene is substantiated by the fact that in the flash illumination of the corresponding alcohol, vitamin A, where no n-n states are possible, no long-lived states are evident. Results and Discussion Retinene readily forms protonated Schiff base A series of flash spectrograms of solutions of complexes with aminesgshowing a marked red shift trans-retinene in methylcyclohexane was taken a t in the absorption spectrum due to the introduction various time intervals following flash illumination. of charge resonance into the conjugated system. A spectrogram taken ten microseconds after flash- The visual pigment, rhodopsin, is believed to be ing showed pronounced bleaching of the ground just such a complex between neo-retinene b and the state absorption peak at 3700 A. accompanied by protein, opsin. a marked increase in absorption in the region of The p-toluidine complex of trans-retinene has an 4400 A. At 100 microseconds after flashing t,he absorption spectrum very similar to rhodopsin spectrum was identical to that of the ground state. (Fig. 3). When oxygen free solutions of this comApparently flash illumination of trans;retinene plex in tetrahydrofuran (0.25 M in HC1) were flash produces a new species absorbing a t 4400 A. which illuminated, neither transient nor irreversible specrapidly reverts to the ground state in times of the tral changes could be detected. This was not an order of ten microseconds. In air saturated solu- unexpected result since no n-n transition is postions these effects are barely detectable indicating sible in the protonated complex and hence no longthat oxygen markedly quenches the new species. lived triplet state is likely to be populated. The decay kinetics of this new species were folTo the extent that the protonated p-toluidine lowed on the flash kinetic apparatus and a typical complex is representative of rhodopsin the above oscillogram is shown in Fig. 1. Rate data obtained results are consistent with two of the mechanisms from the oscillograms are shown in Fig. 2. The proposed for the primary photochemical process in linearity of the log Do/Dvs. time plot shows that the the bleaching of rhodopsin : namely, photoisomeridecay is first order up to ground state retinene con- zation” and photoionization. l 2 centrations as high as 4.0 X M. Acknowledgments.-We wish to thank ProReasonable consideration of the structure of reti- fessor Henry Linschitz for the use of his flash nene, the decay kinetics and the excitation energies photolysis apparatus’* and his helpful suggestions involved essentially rule out ionized or free radical and interest in this work. We also acknowledge the fragments as the new species. The fact that it has a assistance of Drs. K. V. Sarkanen, Moshe Levy and rather long lifetime (hip = 9.9 X second) and Sonja Gross in various phases of the investigation. is markedly quenched by oxygen suggest that the S. Ball, F. P. Collins. P. D. Dalvi and R. A. Morton, Biochem. new species is the lowest triplet state of retinene J . ,(9) 46, 304 (1949). showing a triplet-triplet absorption peak a t 4400 A. (10) R. A. Morton and G. A. J. Pitt, ibid.. 59, 128 (1955). (11) R. Hubbard and C. C. St. George, .I. Gen. Phvsiol., 41, 524 This behavior is in essential agreement with the results obtained in the flash illumination of anthra- (1958). (12) C. Reid, ”Excited States in Chemistry and Biology,” Academic cene6 and chlorophyll5 although the first-order rate Press, New York, N. Y., 1957, p. 161. constants for the t8ripletdecay in these cases are (13) Developed and constructed with the help of AEC support (Syracuse University Contract No. AT(30-1)-820). considerably smaller. If the new species is indeed a triplet it is not likely that it is populated by direct internal conversion (intersystem crossing) from the n-n excited DIFFUSION EFFECTS I N T H E TRANSPIRATION METHOD OF VAPOR PRESSURE singlet state as is the case with anthracene,b since reasonable estimates of the total yield of actinic MEASUREMENT’ light from the flash and the extent of population of BY ULRICK MERTEN the long-lived state are inconsistent with the relaJay Hopkina Laboratory for Pure and Applied Science, CeneraE tively short maximum (radiative) lifetime (-2 x John Atomic Division of Ueneral Dynamics Corporation, San Diego, California second) of the n-a excited singlet state and Received September 8 4 , 1968 the lower limit of lo-’ second imposed on the raAn important technique for the determination diationless transition between states of different of vapor pressures is the one variously known as the multiplicity in hydrocarbons.7 I n retinene where the conj’ugation includes an transpiration or transportation method. In esoxygen atom there is the possibility that an n-a sence, the measurement consists of passing R stream singlet state lies at somewhat lower energies than of an inert gas over the ssmple of interest a t a the n-n singlet.8 Under these conditions internal known rate which is slow enough to achieve satconversion can occur rapidly (-10-l2 second) from uration of this “carrier” gas with the vapor. The the n-n t o the n-n singlet. The radiative life- vapor from the sample is then condensed a t some time of an n-n singlet is sufficiently long (10-5 to point downstream and the vapor pressure is callo-’ second) to permit appreciable population of a culated from the amount of the sample material lower lying n-n triplet state through intersys- collected in a known time period. The application of the method to studies of vapor pressures of (6) G. Porter and M. Windsor, Disc. Faradav SOC.,ll, 183 (1954). inorganic compounds a t elevated temperature has (7) One of us (EWA) is indebted to Professor Michael Kasha for an scattered time profile of the flash (Fig. 1, uppermost trace). The shutter then was opened and a single sweep taken without flashing t o obtain the 100% transmission line (lowest trace). The solvent cell then was replaced by an identical cell containing solution and the solution flashed with the shutter to the scanning arc open.
~~
enlightening discussion on radiationless transitions. (8) J. R. Platt, J . Opt. SOC.Amer., 48, 262 (1953).
(1) This work was supported in pert by the Cornmiasion under Contract AT(04-3) 164.
U. 8. Atomio Energy
NOTES
444 ////
FURNACE
////////A Fig. 1.-Transpiration apparatus.
Vol. 63
assumed that there is no temperature or total pressure gradient along the capillary and that D is independent of concentration. This solution of the differential equation yields a form of the concentration gradient different from that assumed by Lepore and Van Wazer3 in their treatment, the results of which have been widely used in examining data obtained by this technique. We believe that our result will be more useful in discussing the method. Evaluating B for a capillary of length X and assuming, for the present, that the vapor is condensed immediately as it leaves the capillary region (Le., c = 0 a t x = 0), we have
where c1 is the vapor density at the inlet end of the capillary (i.e., a t 2 = -A). Replacing V by the gas flow rate in volume units v, and assuming the vapor behaves as an ideal gas, we have
Y
and
-
V
V
Fig. 2.-Variation
’
of vapor transport with gas flow rate.
been discussed in recent reviews.2 I n the following paragraphs we wish to present a means of examining certain aspects of the method for the purpose of designing experiments and interpreting results. A typical experimental arrangement is shown schematically in Fig. 1. The carrier gas is introduced at A, flows around a heat shield B, into an essentially isothermal region in the iieighborhood of the sample-containing boat C. The vaporladen gas then passes through a capillary constriction D and into the condensing region E. The flow of vapor through the capillary may be thought of as the sum of a slug-flow term and a dFffusion term so that the mass of the vapor passing any point along the tube in unit time is given by k =A
(Vc
-Dg)
where A is the cross-sect,ional area of the capillary, V is the linear velocity of the gas mixture through the capillary, x is the distance along the capillary measured from left to right in Fig. 1 and is taken as zero a t the exit end, c is the vapor density a t any point and D is the interdiffusion coefficient for the carrier gas and the vapor. At steady state, IC must be constant over the length of the capillary if no condensation occurs in this region, so we may solve the differential equation simply to obtain the result whereIB is a constant of integration and we have (2) See, for example, 0. Kubaschewski and E. L. Evans, “Metallurgical Thermochemistry.” 2nd ed., John Wiley and Sons. Inc., New York, 1956, p. 151.
where pI and T are the pressure and absolute temperature of the vapor a t x = -X, R is the gas constant and M is the molecular weight of the vapor. (Corrections for deviations from the ideal gas law have been discussed by Gerry and Gillespie.4) Thus, if we determine k as a function of v with a given experimental arrangement, and T and pl are constant, we will obtain a curve of the form shown in Fig. 2. For large v, diffusion effects are negligible and p1
k RT --
(2) vM This is essentiaIIy the expression which has generally been used to interpret transpiration measurements. At v = 0, only the diffusion flow is observedand ., Ir, P *’ b. ! $ ?
k X RT
Pl=ijxx
(3)
In the derivation of equation 1 we have assumed that the vapor concentration is zero a t the exit end of the capillary and that the capillary is a t a uniform temperature. In an actual experiment, condensation will generally not occur until a point is reached some distance downstream from the capillary, hence, c > 0 a t 5 = 0. This fact can be corrected for by employing an effective length which is obtained by adding to the actual length of the capillary the quantity (A/A‘)X’ where A’ and X’ are the cross-sectional area of the exit tube and the distance from the end of the capillary to the point of condensation, respectively. In many cases, A’ >> A and A’ S X, so that the correction may be neglected. The temperature gradient in the capillary is actually maintained near zero in most instances to prevent condensation in this (3) J. V. Lepore and J. R. Van Wazer, “A Disoussion of the Trenspiration Method for Determining Vapor Pressure,” MDDC-1188, U. S. Atomio Energy Commission. (4) H. T. Gerry and L. J. Gilleapie, Phye. Reo., 40, 269 (1932)_
I
March, 1959 region and plugging of the passage, but there is a temperature gradient over part of the length A'. Since this is generally in a large-diameter region of the tube, the equivalent capillary length of this region is small, and the assumption that the temperature is constant to the point of condensation should not lead to serious errors. If the transpiration experiment is properly designed, it should always be possible to u$e the simple equation 2 to interpret the results. The equations derived above may be used as a guide to proper design, since D may be estimated by methods outlined by Dushmans and the dimensions of the capillary and the flow rates to be studied may then be chosen in such a way that the term exp(-vA/DA) in equation 1 is negligible compared t o unity. It should be noted that the diffusion correction vanishes as the flow rate is increased and that, as a result, the extrapolation of the lcjv line passes through the origin, not through the zero-flow point on the k axis. A t high flow rates, the measured values of k will fall below the curve in Fig. 2 because saturation will not be achieved, and these points must be neglected in analyzing the results. Even if no straight-line relationship can be achieved because of failure to reach saturation a t flow rates where diffusion is negligible, i t is possible to solve for the vapor pressure provided only that data can be obtained at two significantly different flow rates where saturation is achieved. For instance, results from a zero flow rate experiment may be used to calculate the product plD from equation 3, and the answer used, together with one other result satisfying equation 1, to eliminate D and determine PI. When saturation is not achieved, pl will be a function of the sample surface area. This fact may be used as a test for saturation. The calculation of pi from the experimental data requires knowledge of the molecular weight M of the vapor species. In favorable instaiices, the assumed value may be checked in the following way: once the quantity CI has been calculated, equation 1 or 3 may be solved for D, the interdiffusion coefficient for the carrier gas and the vapor. By methods outlined by Dushman,6 one may then calculate rough values of D for various possible species and compare them to the measured value. A selection of the appropriate species will be possible if the candidates diff'er sufficiently in D. Acknowledgments.-The author is indebted to Drs. W. E. Bell, J. L. White and A. W. Searcy for helpful discussions of this problem. (5) S. Dushman. "Scientific Foundations of Vacuum Technique." John Wiley and f3ons, Inc., New York, N. Y.,1949, p. 74.
NOTES
445
is 3.8" higher than that of ordinary water.2J This difference in the freezing points has been the basis for a number of attempts to separate the two isotopic forms by fractional crystallization. Although many of the early measurements were inconclusive, Eucken and Schafer* were able t o confirm experimentally the theoretically predicted temperature change which occurred during the process of freesing a mixture of light and heavy water. Weston4 rechecked the theoretical calculations which indicate that the heavy isotope should concentrate in the solid phase. Smith and Posey6 recently measured the concentration difference between the solid and liquid phases of an H20-D>Omixture under equilibrium conditions. Their work shows conclusively that fractional crystallization of a mixture of ordinary and of heavy water produces some isotope Separation, with the heavy isotope concentrating in the solid phase. However, the separation factor is much too small, only 1.0211, and the required freezing rate is too slow for a batch process to give useful separation of the isotopic forms of water. Poseye performed some exploratory experiments t o investigate the applicability of the zonerefining technique. The work discussed in the present report was performed in order to determine whether or not the zone-refining technique would be useful for the separation of heavy water from ordinary water, and to obtain a comparison between the observed and theoretically predicted extent of separation. Experimental Apparatus .-The zone-refining apparatus consisted of a spiral of Tygon tubing wound onto a 1.5-inch diameter cylinder of wire mesh. The Tygon tubing was filled with a mixture of HaOand DpO in approximately equimolar amounts and was plugged at each end with a Pyrex stopper. The spiral was mounted with its axis horizontal and with about one-half of each turn submerged in the refrigerated brinebath which was operated at -10'. I n order to provide a constant seed for freezing, the end of the tubing at which the zones originated was folded inside of the spiral and remained frozen throughout a run. The spiral geometry allowed 40 evenly spaced zones to move simultaneously through the sample. At the beginning of a run the entire sample was frozen by packing Dry Ice around it. The bath was covered with a sheet of plywood in which was cut a 1-inch wide slot. The slot was located immediately above and ran the length of the spiral. A heater, placed above the slot, maintained a temperature of 35-40' at the exposed surface of the spiral. IJnder these operating conditions, each frozen zone moved almoAt to the top of a turn before it was completely meltfd. Freezing occurred within the first 0.5 inch after the tubing re-entered the bath. The melted zone occupied a length of about 1.5 inches, or about 25% of each circumference. The cylinder was rotated by a geared-down Telechron motor, which gave an operating speed of 0.75 revolution per hour, or a linear speed of 4.1 inches per. hour. Water Samples.-The heavy water, which was reported (1) Presented m part a t the Meeting-in-Miniature of the American
THE SEPARATION OF MIXTURES OF ORDINARY AND HEAVY WATER BY ZONE REFINING1 BY HILTONA. SMITH AND CARL0. THOMAS Department 01Chemistry, Universilar of Tennessee, KnozvilEe, Tennessee Received October 6 , 1968
The freezing point of pure heavy water (DzO)
Chemical Society, New York, March 195% The work was supported by the U. 9. Atomio Energy Commission. (2) A. Eucken and K. Schafsr, Z . ancpg. allpem. Chenz., 226, 319 (1935). (3) A. Euoken and K. Schafer. Naehr. Gee. V i a s . G'ottinoen, Math. Phyazk KEasse, Fuchprupps III. ( N . S . ) , 1. 109 (1935). (4) Ralph E. Weston, Jr., Gemhim. et Coamochim. Acta, 8 , 281 (1955). (5) John C. Posey and Hilton A. Smith. J . Am. Chern. Soc., 79, 555 (1957). (6) John C. Posey, Doctoral Dissertation, The University of Tennessee, Knoxville, Tennessee. 1955.
