ISOTOPE EFFECTS I N Hg(63P1)-PHOTOSENSITIZED REACTIONS
Nov., 1960
of the potassium salt, on account of the markedly greater viscosity of the former. However, a naive attempt to account for the difference in ionic conductance on the basis of macroscopic viscosity grossly over-corrects. For example, the conductance-relative viscosity productsll for chloride ion a t 1.0 N are 57.00 and 58.65 for KC1 and NaC1, respectively, while a t 2.0 N they are 54.25 and 56.60. (11) Ref. 5. Vol. 5, p. 15,17.
1753
It is possible that similar measurements of the conductance of other ions might suggest a qualitative pattern, but a t the moment any quantitative interpretation of transport phenomena under these conditions must await further theoretical development. I n conclusion, we wish to express our thanks to the National Research Council of Canada for a grant in aid of this research.
ISOTOPE EFFECTS I N Hg(63P,)-PHOTOSENSITIZED REACTIONS: AND H 2 + KO’ BY MORTONZ. HOFFMAN~ AND RICHARD B. BERNSTEIN
H 2
+ N,O
Department of Chemistry, The University of Michigan, Ann Arbor, Michigan Received June 10,1869
The N14/”5 and O16/O18 isotope effects in the Hg(63P1)-photosensitized (MPS) reaction of hydrogen with nitrous oxide and with nitric oxide have been measured. In the MPS reaction of N 2 0 Ha, then N14/N15 isotope effect was independent of the ratio (H2)/(Nz0) over the range 1.0 to 12, with an average value of So = 1.018 f 0.001 compared with So = 1.017 f 0.001 for the MPS decomposition of NzO. The O16/OlS isotope effect was 1.017 f 0.002 compared with 1.019 i 0.001 for the decomposition. These observations imply that the rate of H-atom attack on NzO is a small fraction of the rate of the Hz, the N14/N16 isotope effect was found to be “inverted,” L e . , the N 2 0 decomposition. In the MPS reaction of NO first fraction of N2 produced was enriched in N’S relative to the original NO. At a ratio (H2)/(NO) = 1.0, the fractionation factor was 1.008, dminishing to the null value of 1.000 a t a ratio of 29. The enrichment is interpreted on the basis of a postulated isotope exchange equilibrium between NO and HNO.
+
+
+
+
+
Hg hv +Hg‘ Hg’+Hz+H + H +Hg Hg’ Nz0 +Nz 0 Hg Ha 0 +H OH
+
+
+ + +
+ NzO +Nz + OH + 0 chain steps +H 2 0
H
Introduction The use of the kinetic isotope effect technique as a means of studying mechanisms of gas phase thermal reactions is well known.3 Measurements of isotope effects in photolytic reactions have been reported4 and recently isotopic fractionation in the Hg(63P1)-photosensitixed (MPS) decomposition of K20 has been observed6 and related to differences in the quenching cross section of the isotopic molecules. It was of interest, therefore, to extend the use of kinetic isotope effects in order to elucidate the mechanisms of two MPS reactions. A detailed kinetic study of the MPS reaction Hz WZO+ N2 H20 was carried out by Taylor and Zwiebels who found that the reaction rate was affected only slightly by PH,but significantly . postulated a mechanism involvby P N ~ They ing a sensitized decomposition of NzO a t a rate roughly 1000 times that of H-atoms with N20 (Ish.)
(1) (2)
H
(4)
( 5 ) , etc.
Since the MPS decomposition could be studied separately,s a test of this mechanism, particularly the relative importance of steps 2 and 4, could be achieved using the isotope effect technique. Taylor and Tanford’ investigated the MPS reaction HZ NO * ‘/z N2 HzO. They found that: (1) the rate of reaction was proportional to PR,a t low pressures (with the dependence diminishing a t higher pressures); (2) with excess Hz, the rate was essentially independent of P N O ; (3) with excess NO, the rate was retarded and the stoichiometry was not simple. They suggested the following mechanism to account qualitatively for their kinetic observations
+
+
+
Hg hv Hg’ Hg’+Hz+H + H +Hg Hg‘+NO+NO+Hg H+NO+HNO H HNO +H n NO HNO + decomposition
+
(3)
+
Gab.)
(1) (2)
(3) (4)
(5)
Hartecka had reported that NO reacted with Hatoms (from a discharge tube) to produce a prod(1) T h e authors appreciate financial support from the U. 8. Atomic Energy Commission, Division of Research and the Michigan Memorial uct, (HNO),, solid a t liquid air temperature, Phoenix Project. which decomposed on warming. The over-all (2) Receipt of fellowships from the Michigan Memorial-Phoenix decomposition of HNO in step 5 was expressed’ Project and the National Science Foundation is gratefully acknowlas a first-order reaction, although the mode of edged. (3) (a) J. Bigeleisen, THIS JOURNAL, 66, 823 (1952); (b) S. Z. decomposition was speculated to be dimerization Roginsky, “Theoretical Principles of Isotope Methoda for Investigatof the HNO to hyponitrous acid witch subsequent ing Chemical Reactions,” Academy of Sciences, U.S.S.R. Press. Mosdecomposition to NzO and H20. The NzO could cow, 1956, Chapters I V and VI; O 5 c e of Technical Services, Department of Commerce, Washington 25, D. C. (English translation); not accumulate under these experimental condi(c) D. R. Stranks and R. G. Wilkins, Cham. Reus., 67, 842 (1957). tions and underwent further reaction to produce (4) A, A. Gordus and R. B. Bernstein, J . Cham. Phgs., 30, 973 Nn. They considered that the rate of step 4 was (1959). (5) M. Z. Hoffman and R. B. Bernstein. ibid., 83,526 (1960). (7) H. 4 . Taylor and C. Tanford, ibid., 12,47 (1944). (6) H. A. Taylor and N. Zwiebel, ibid., 14, 539 (1946).
(8)P. Harteck, Ber., 66, 423 (1933).
MORTON 2. HOFFMAK BSD RICHARD B. BERNSTEIN
1754.
Ht and nitrogen oxide, were separated by passage through a trap a t -78" into a trap a t -196' (-210' for the HTKO reaction). The volatile gases were cycled through CuO (350"; regenerated after use by heating in air a t 600' overnight) and the NZremaining was measured and collected for subsequent Nl6 assay. The product HzO, trapped at -78", was collected for 0 ' 8 isotopic assay. By means of the quantitative (f1%)MPS reduction of the nitrogen oxide with exces? Hs, "reference samples" of S?and HsO were obtained for isotopic analysis. The method of isotopic assay for the N14/X16ratio in the X2 and 0 1 6 / 0 1 8 in the H?O samples was conventionall; the usual definition of the fractionation factor S and the small correction to zero extent of reaction, So, have been given previously?
I OIO-
L 1
0
A 2
3
4
5
6
7
8
9
1 IO
I1
12
R A T I O ( H * ! o l l N 2O!,
Fig. l : - - S 1 4 / P isotope effect data in the MPS reaction of nitrous oxide and hydrogen. Point A is the value for the MPS decomposition of nitrous oxide.
0
Fig.
I
I
I
I
I
2
4
6
8
10
2.-N1*/N1b
I
I
I
I
I
12 14 16 18 20 RATIO ( H t ) o / ( N O I o ,
I
I
I
22
24
26
I 28
Vol. 64
I
I 30
isotope effect data in the MPS reaction of nitric oxide and hydrogen.
several orders of magnitude faster than the rate of step 5. The recent interestg-" in the HNO molecule has called attention to this reaction; a reinvestigation of it, to understand more fully the role played by the HSO, appeared to be in order.
Experimental12 Tank Hz (Liquid Carbonic Corp.) was purified by passage through a Deoxo unit, a trap a t -196", and a Pd thimble a t 370'. Mass spectrometric analysis showed the Hs to be free of NZand 0 2 . X20(Matheson Co., stated purity: > 9S.O%), was purified by repeated distillations from -160 to -196"; the middle fraction8 were collected and stored over Hg at room temperature. The vapor pressure a t the triple point and the infrared spectrum agreed well with the literatiire.13J4 ?\latheson Co. tank NO (stated purity: > 99.0%) was purified16 and stored over Hg a t room temperature. The vapor pressure at the triple point and the infrared spectrum xqrred wdl with the literat~re,~61~7 indicating a probable impurity (,ontent 5 0.1%. The vacuum apparatus and lamp assemblv used have hren previously desrribrd.s~12The reactants were metered into the reaction loop and the reaction (carried out a t room temperature) was followed manometrically for %: time (ea. 1 hr.) corresponding l o a small (
