COMMUNICATIONS TO THE EDITOR
763 and water are considerably higher. The above suggests that at least near the melting point BiCls may be an ionic fluid in which the mobilities of the ions are less than those of KCl, possibly due to the higher charged Bia+ or to a bulky complex such as BiClz+. Clearly, other types of studies such as Raman spectroscopy will be needed to resolve this question. (9) D. Cubicciotti, H. Eding, F . J. Keneshea, and J. W. Johnson, J. Phy.9. Chem., 70, 2389 (1966).
ATOMICB INTERNATIONAL DIVISION L. F. GRANTHAM NORTHAMERICAN ROCKWELL CORPORATION S. J. YOSIM CANOGA PARK,CALIFORNIA91304 RECEIVED NOVEMBER 30, 1967
High Internal Excitation Energies of Products
1
’O-2600
Following Energetic Substitution of Tritium I
I
300
400
I
I
600 700 TEMPERATURE (‘C)
500
I
800
I
900
llO-3 1000
Figure 1. Molar conductance of liquid and saturated vapor of BiCla and HgCla to their critical temperatures.
the curves are dotted in Figure 1. Also, since the molar conductance of the supercritical fluid is a function of density, these portions of the curves are also dotted. Although at their critical temperatures the conductivity of BiCl3 is still orders of magnitude greater than HgC12,the former is about two orders of magnitude less than that of KCl at its melting point. This relatively low conductivity would therefore be consistent with Johnson and Cubicciotti’s conclusion that BiCls is essentially a molecular fluid in the vicinity of the critical temperature. The small conductivity found for the BiCla system could be attributed to the ionization of only a small fraction of the molecules. In addition, it should be pointed out that the conductivity of BiC4 near the critical temperature may be too high since a small amount of impurities (especially BiC1) may increase the conductivity appre~iably.~ The nature of the species of BiCL at its melting point is still open to question. As Cubicciotti, et aZ.,9pointed out, on the basis of the thermodynamics of its vaporpoint) and the ization low existence curve, BiC&acts like a molecular fluid. However, the molar conductance of BiC13 (28 cmZ/ohm mole at its melting point) is the same order of magnias that Of KC’ (‘lo cm2/ohm In the “activation energy” for if One conduction at the melting point from a log of the molar conductance vs. 1/T plot, one obtains values of 3.8, 3.1, 7.0, and 8.4 for -KCl, BiC13, HgCl2, and water, respectively. Thus the “activation energies” for BiCla and KC1 are between 3 and 4 while those for HgClz
for Hydrogen in Methyl Isocyanide
Sir: The survival without further chemical reaction of the excited molecule formed by the substitution of an energetic tritium atom for a hydrogen atom, as in (l), depends upon the relative rate constants for secondary reaction for collisional deexcitation. Experiments in several systems have already demonstrated that the excitation of RT* (from RH) is sufficiently great to permit extensive secondary rea~tion.l-~These earlier experiments are consistent with a broad range of excitation energies in the electron volt region following the T-for-H substitution, and exhibited such varied rates of secondary reaction that some molecules are collisionally stabilized under all conditions while others decompose even at the high collision frequencies of liquid-phase reactions. We have sought additional information about the lower range of such excitation energies through the study of energetic tritium reactions with methyl isocyanide. This molecule was chosen as the target molecule because of its low activation energy, 38.4 kcal/mole, for secondary reaction-in this case, for its isomerization to methyl cyanide, as in (2).6-* For comparison, an excitation energy of 5 eV, the (1) E, K. C, Lee and F. S. Rowland, J. Am. Chem. sot., 85, 897 (1963). (2) Y . -N. Tang, E. K. C. Lee, and F . 8. Rowland, ibid., 86, 1280 (1964). (3) R. Wolfgang, P r o p . Reaction Kinetica, 3 , 97 (1966). (4) E. K. C. Lee and F. S. Rowland, J. Chem. Phys., 36,664 (1962). (5) F. W. Schneider and B. 8. Rabinovitch, J . Am. c h m , Sot,, 84, 4216 (1962). (6) F. W. Schneider and B. 8. Rabinovitch, ibid., 85, 2366 (1963). (7) B. 5. Rabinovitch, P. W. Gilderson, and F. W. Schneider, ibid., 87, 168 (1966). J. Fletcher, B, s. Rabinovitch, K. W. Watkins, and D. J. Locker, J. ~ h y a Chem., . 70, 2823 (1966).
Volume 78, Number 8 February 1068
COMMUNICATIONS TO THE EDITOR
764 median energy inferred for the T-for-H reaction in cyclobutane,’ corresponds to a rate constant for isomerization of methyl isocyanide of sec-’ and involves a long extrapolation from the regions of actual measurement of unimolecular rate constants in thermal systems. CH2TNC* 4CHzTCN
(2)
Typical distributions of radioactive products from recoil tritium reactions with methyl isocyanide are summarized in Table I for several sets of experimental conditions. While we have consistently observed the substitution reaction (1) in the liquid phase,g no detectable yield of CHzTNC has been found in the gas-phase experiments. The isomerization product, CH2TCN, is found in both gaseous and liquid phases. The most important other products include H T and molecules indicating the presence of CHzT radicals in the gaseous systems; the activation energy for the decomposition of CH2TCN* or CN2TNC* to form CHzT CN radicals is approximately 100 kcal/mole,lOJ1and the reactions are much more readily suppressed by collisional stabilization than is the isomerization reaction with much lower activation energy.
+
Table I: Relative Yields of Radioactive Products from the Reaction of Recoil Tritium with CHsNC (Yield of HT = 100) Product
CHsT
LiquidCHaNC Liquid CHaNC I2
+
7.2
C0.05
9.6 3.7
CH2TNC
CHzTCN
