2779
NOTES
Radiative Neutron Capture Organic Yields As an Indication of the State of Aggregation of IC1 and Iz in Cc-Hydrocarbon Matrices at 770R1 by R. A I . Lambrecht, H. K. J. Hahn, and, E. P. Rack
clusters and give the same or constant organic yields a t comparable halogen concentrations. For example, in polycrystalline n-hexane, the soBrm(IT)80Brorganic and 1 X mf yields are about 5% between 5 X Brz, whereas 1271(n,y)1281 organic yields are about 10% between 1 X 10-6 and 2 X lov4mf 1 2 .
Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68608 (Received January 8,1969)
50
i-\
' 3 8 ~ 1from ICI ' 1281 from ICI 0
Organic yields induced by nuclear transformations can give useful information about the state of aggregation of a solute and its tendency toward complexing.2 Constant organic yields over an appreciable halogen concentration have been ascribed to the formation of preferred (XJ. (where X = Br and I) cluster sizes and/or similar numbers of halogen molecules a t sites in polycrystalline organic mat rice^.^^^ This note represents an extension of the nuclear transformation technique to deduce information on clustering and segregation during crystallization of IC1 and 1 2 in polycrystalline n-hexane and 2,2-dimethylbutane, and glassy 3-methylpentane (3AIP) a t 77°K. The study is unique in that 1281 yields from IC1 and I 2 may be compared, and the state of aggregation of IC1 may be traced by both 38Cl and Iz8I formed by the radiative neutron capture processes. An interesting test for the nuclear transformation technique as a valid indicator of the state of aggregation would be whether lZ8I and 38Clfrom IC1 gave similar results. The organic yields resulting from the 1z71(n,y)1281 and the 37Cl(n,r)3sC1reactions as a function of IC14t6and I 2 concentration in polycrystalline n-hexane are depicted in Figure 1. The lZsIdata for It in polycrystalline n-hexane have been reported previously3 but are presented again for purposes of comparison to the organic yields from ICl, The lZsIand 38Clorganic yields from IC1 are identical, within experimental error, with those of lZsIfrom I2 over the concentration range studied. Below mf (mole fraction) IC1 no 38Cl organic yields are reported because of lack of 38Cldetection sensitivity. Under the conditions of monomolecular dispersion, the organic yields are about 52%. Since the organic yields are nearly constant at 10% between 1 x loW6 and 2 X mf halogen, we conclude that the IC1 and IZapparently form similar (XZ). cluster sizes and/or are composed of similar numbers of halogen molecules a t sites in the polycrystalline organic matrices. Different halogens would not necessarily be expected to form (XZ)@
1281from 12
W
-Vz
30
a
20 0
IO
10-6
10-5 (LOG
10-4
10-3
SCALE)
Figure 1. T h e effect of IC1 and 1 2 concentration (mole fraction) on W l and lZ*Iorganic yields in polycrystalline n-hexane at 77°K.
The 38Cland lZ8I organic yields in polycrystalline 2,2dimethylbutane were similar in trend and in magnitude to those observed in polycrystalline n-hexane. This result is not surprising since Iyer and Willard2 found the mf Br2in poly8oBrm(IT)soBr organic yields a t 5 X crystalline n-hexane and n-pentane to be similar (;.e., 4.7 and 4.2 ==I 1%, respectively). The low organic yields (
