2052
NOTES
entropy difference. A comparison of epimers, one of which is a meso form, would be interesting.
4, 1
Experimental Section Pmr spectra were obtained with the Varian A-60 spectrometer and the V-6057 variable-temperature accessory. Compound I was prepared by the addition of N-ethyl-a-naphthylamine, with triethylamine as an HC1 scavenger, to a-chloropropionyl chloride in slight excess in methylene chloride in an ice bath. After filtering off the triethylamine hydrochloride, compound I was purified by molecular distillation and then analyzed. Anal. Calcd: C, 68.8; H, 6.1; N, 5.4. Found: C, 69.0; H, 6.2; N, 5.3.
I25'C
1
I
I .o
2.0
0
PPm
On the Radiolytic Isotopic Exchange
Figure 2. Pmr spectrogram of methyl signals a t different temperatures ; solvent, CHC12CHC12.
of Gaseous Nitrogen
Tables I1 and I11 give the results of studies with three solvents over the temperature range 50 to 125". At about 140" the lifetime of epimerization becomes so short that signal resolution is lost (Figure 2). At 160" merging of the separate signals for the epimers is nearly complete in 1-bromonaphthalene. Within experimental error the enthalpy difference between epimers is zero. Apparently most, if not all, of the free energy difference between epimers is in the entropy term (see Table 11).
The Weizmann Inatitute of Science and the Soreq Nuclear Research Center, Rehovoth, Israel (Received November 15, 1966)
Some time ago' we investigated the isotopic exchange of nitrogen between N214?14 and N2l5~l5 in the gas phase under radiolytic conditions. We have found a value for Gexch namely 9.5 0.25 in the pressure range 33-440 mm. A major part of this exchange has been attributed to the dissociation of excited nitrogen molecules formed by processes other than the recombination of Nz+ e-. I n a recent study,2 another value for Gexchwas re0.5, and a different interpretaported, namely, 7.3 tion of the mechanism of radiolysis of nitrogen was offered, based on the intramolecular isotopic exchange of N4+, which is presumably the predominant ionic species under the experimental conditions. These results prompted us to reinvestigate the system experimentally and to try to reconsider critically the mechanism of this exchange process. I n several independent series of experiments, over a dose rate range between 0.2 and 60 Mrads/hr, using X-rays, y-rays, reactor mixed radiations, and K1.85 p-particles as an internal source of radiation, over a total dose range from 10 to 800 Mrads, Gexoh = 9.5 f 0.5 was obtained. Nitrous oxide was used as gaseous dosimeter (G(-NzO) = 12) as well as Fricke's dosim-
+
Table I11 : Thermodynamic Data Equi-
AF,
librium
cal/mole
Solvent
ratio"
at 50°
AS, cal/(deg mole)
1-Bromonaphthalene Nitrobenzene sym-Tetrachloroethane
1 . 4 0 f 0.Ogb
216 i 40
0 . 6 7 =t0 . 1 2
1 . 5 5 & 0.12 281 += 50 1 . 5 6 i 0 .0 l b 286 f 5
0 . 8 7 f 0.14 0.89 += 0 . 0 2
a Averaged over four temperatures: 50, 75, 100, and 125' (average deviation from mean). Typical deviations for duplicate determinations a t same temperature were 0.10.
On the basis of such limited data the origin of the entropy difference between these epimers is highly speculative. However, it seems likely that the difference arises largely from differences in internal rotational states of the epimers. Since both epimers have onefold symmetry axes and exist as d,l pairs, there appears to be no other physical reason for the The Journal of Physical Chemiatry
by M. Anbar and P. Perlstein
*
(1) M. Anbar and P. Perlstein, J . Phys. Chem., 6 8 , 1234 (1964). (2) D . H. Dawes and R. A. Back, ibid., 69, 2385 (1965). (3) M. Anbar and P. Perlstein, Israel AEC Report IA-1048, 1965.
2053
NOTES
eter, after applying the appropriate corrections. It is hard therefore to account for the discrepancy of about 20% between our results and those of Dawes and Back. The absolute precise value of Gexchis crucial only when a quantitative evaluation of the energetics of the exchange process can decide in favor of a certain mechanism, in comparison with others. At first glance it appears as if Dawes and Back have achieved this goal and have accounted (in Table V of their paper) for the different pathways of exchange. They have used the known number of ions formed in nitrogen per 100 ev (2.88) and the number of nitrogen molecules undergoing dissociative excitation per 100 ev (1.54). A critical review of the arguments presented in their paper reveals, however, serious inconsistencies between their proposed mechanism and their own experimental results. Dawes and Back propose that an isotopic exchange takes place between the nitrogens of the N4+ ion on neutralization by an electron.* Such a reaction would accordingly yield an exchanged nitrogen molecule plus two nitrogen atoms N2**+ N2+
N4+**
(1)
N4+**
+ e- -+N2* + N + N*
(2)
where N2* is an exchanged nitrogen molecule. It is obvious that reaction 2 will yield Nz* only in half of the cases (the other half will yield Nz** 2W and Nz 2N*). This factor of 0.5 exists in addition to the factor of 0.5 which originates from the interactions of isotopically identical nitrogen molecules, which was taken into account by the authors. This additional factor of 0.5 upsets the balance of Gexehreported by Dawes and Back by 1.44, and the contribution of the dissociative excitation has to be increased accordingly from 1.54 to 2.98. We shall now compare the behavior of the system in the presence of additives as determined by the two groups of investigators. Dawes and Back carried out two series of experiments using NO as scavenger. I n N214J4series G(N214J5)was 2.57, whereas the N150 N140 N2 14,14+ N215J6experiment in the Ni50 G(N214J5)was 2.85. We have repeated the NO experiments using a mixture of 98.4% N215J5 1.6% N140 and obtained G(N214,15)= 2.56 f 0.13,5in excellent agreement with Dawes and Back. This agreement in the presence of NO suggests that the discrepancy in Gexchbetween our results and those of Dawes and Back can hardly be attributed to dosimetry. A comparable value for Gexchwas also observed in both studies’pz for nitrogen containing 0.4-0.570 0 2 . This may suggest that the lower value for Gexch in “pure”
+
+
+
+
+
+
nitrogen reported by Dawes and Back is due to a trace impurity. A critical review of our previous communication also reveals certain points which require a revision. First, it should be pointed out that our value of Gexch can hardly be accounted for in view of the high dissociation energy of N2. Second, our conclusion that excited nitrogen molecules do not undergo isotopic exchange with nitrogen molecules at their ground state is open to criticism. This conclusion, which was reached on the basis of the results of Back and is valid only for the two lowest triplet states of Nz which are generated by N atom recombination downstream from the discharge. Other shorter lived Nz (excited) could not be found outside the discharge tube used in that study. In fact, the reaction
x2l4,I4 (excited) + Nz15,15+2x214,15
(3)
is energetically feasible’ even for the lower excitation levels of Nz,down to 6 ev above ground level. It may be concluded that the mechanisms leading to the N214314-N215915 exchange suggested in both previous reports do not fully account for the observed exchange reaction and the system still requires further study. (4) The occurrence of an intramolecular isotope exchange in Na+ prior to neutralization is excluded by the fact that the equilibrium Np+ N*$ N,+ would then lead to a chain reaction which would imply a dose rate dependence of Gexoh.It has been suggested* that N3 is slow compared with the rate of dissociation of Na+ -L N r + the rate of its neutralization. Considering the equilibrium constant of Na+ NZ = Na+, this cannot be true for the lower dose rates studied. As no increase in Gexehwas observed at lower dose rates, it is unlikely that isotopic exchange takes place in the Nz+ i- Nt e Na + reaction. (5) We have also examined the other products of the “ 4 0 Nz15r15system and found G(N150) = 0.1 (by 0 N ) , G(NaL6~160) = 0.04 (by Na O), G(Na14,150) = 0.1 (by N NO, ?), G(On) = 0.8 (by 0 O), and G(N0a) < 0.01. (6) R. A. Back and J. I. Y. Mui, J. Phys. Chem., 66, 1362 (1962). (7) J. D. Hirschfelder, J. Chem. Phys., 9, 645 (1941); K. Otozai, Bull. Chem. SOC.Japan, 24, 218, 257, 262 (1951).
+
+
+
+
+
+
+
+
Solvent Shifts of Electronic Energy Levels
of Acetone and Benzene
by K. Keith Innes va’anderbilt University, Nashville, Tennessee (Received November 16, 1965)
Solvent shifts of electronic transitions seem always to have been discussed as if it were not possible to determine the shifts separately for the two energy levels giving rise to each transition.’,2 Volume 70, Number 6 June 1966
