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The pK, values thus determined show remarkable strengths for the cyanocarbon acids. Pentacyanopropene and the first ionization of hexacyanoisobutylene are comparable with the stronger mineral acids. Hydrochloric acid, for example, is thought to have a pk’, in the neighborhood of - i . 7 Al high degree of resonance stabilization in the anions (which is not possible in protonated form) appears to be responsible for the high acidity.l Smoothed values of the acidity function Hcalculated according to
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t h a t ethane formation could be inhibited effectively by radical scavengers in the gas phase photolysis, but not in the liquid, and t h a t the ratio of CH4 2 CaH6/X2was nearly 2. We believe that since the cage effect is an iinportant concept in solution kinetics, i t is of value to confirm its existence directly and unequivocally. We wish in this paper to report such a confirmat ion. l y e have photolyzed mixtures of azomethane and de-azomethane in the gas phase and in isooctane solution using a procedure essentially similar t o 11.. = pk‘a log CA-/CB.~ was ~ t h a t of Herk, Feld, and S ~ w a r c . Azomethane are listed in Table I1 along with values of fic for prepared by oxidation of sym-dimethylhydrazine comparison.? The H- function presented here is with mercuric oxide, while d6-azomethane was based entirely on p - (tricyanovinyl)- phenyldicyano- purchased from lferck of Canada. Both materials methane, methyl dicyanoacetate and his-(tricyano- were proven to be chemically pure by gas chromatography. The isotopic purity of the de-azomethane vinyl) -amine. I-1- behaves in a maimer surprisingly similar to was checked by mass spectra. 3Yc CD3N&D2H H,. although there are quantitative differences. and no other impurities were found. The isooctane Xt lower concentrations ( < 5 M ) these appear to was spectroscopic grade, further purified by passing be of the kind expected on the basis of the differ- through silica gel. Its purity and the cleanliness ences in charge types. -4t higher concentrations of our handling procedure were checked by ultraH- roughly parallels Ho. The similarity of these violet spectra. The reaction products were analyzed b y mass two acidity functions is further evidence that these indicator acidity functions depend primarily on spectroscopy. The cracking patterns of Nz, CD3the properties of the hydrogen ion in solution N S C D 3 , CH3NNCH3,and C2H6were determined relatively unencumbered by activity coefficient for our mass spectrometer, while the cracking patterns of C2D6: CaD5H, and CD3CH3 were effects of the indicators t h e m ~ e l v e s . ~ kindly provided by l f r . J. Bell of Harvard Univer(7) R. P . Bell, “ T h e Proton in Chemistry,” Chaps. V I , V I I , ~ i t y .C ~ z D ~ ,C P D ~ H ,CD3CH3, and C2H6 were Cornell University Press, Ithaca, N. Y . , 1869. determined from the mass 36, 35, 33, and 27 peaks, COSTRIBUTION S o . il4 respectively. From these peak heights, the reCENTRAL RESEARCH DEPARTMENT STATIOS RICHARD H. BOYD maining peaks in the 36 t o 24 range could be EXPERIMENTAL E. I. DU PONTDE KEMOURS ASD COMPANY predicted within 4%. \ ~ T I L h f I S G T O N98, DELAWARE I n the gas phase photolysis of azomethane one RECEIVED AKGUST29, 1961 would expect ethane to be formed by random recombination of methyl radical. For such a random process, the ethane is formed in proportions such DEMONSTRATION OF THE (‘CAGE’’ EFFECT = 4. The obthat (CH3CD3)*”(CzH~)(C2D6) Sir: In 1934, Franck and Kabinowitchl pointed out served ratio of parent peak heights is 4.1 f 0.8. However in the isooctane solution photolysis, that collision times of reactive molecules should be much longer in solution than in the gas phase. the inass 33 peak is less than 27, of the 36 peak. Further, in solution, after the colliding molecules This peak can be accounted for satisfactorily by the contributions of isotopic carbon and CDsH. The have diffused apart by a distance small compared authors estimate that if any CD3CH, is formed, it t o the mean distance between reactive molecules, there remains a finite probability of re-encounter must be less than 0.3%, of the total ethane. before diffusion to the mean distance. Effectively (1) W e have recently becr,me aware of a n unpublished s t u d y by the solvent holds the reactive molecules toget her, .4usluos and co-workers a t the Sational Bureau of Standards, whose somewhat parallel our results. forming a “cage.” .Imore sophisticated treatment results ( 3 ) J. Bell, private communication. using the theory of random walks has been given RICHARD K. Lrox Esso RESEARCH & ENGINEERIXG Co. by Noyes.? DONALD H. LEVY LINDES,SEW JERSEY Numerous observatioiis have been explained by RECEIVED SEPTEMBER 11, 1901 invoking the cage effect with varying degrees of certainty. Probably the best is a recent study by Herk, Feld, and SzwarcJ3in which they reaffirm ROTATORY-DISPERSION CHANGES DURING THE THERMAL DENATURATION OF Szwarc’s previous conclusion that in the photolysis CHYMOTRYPSINOGEN AND CHYMOTRYPSIN of azomethane in liquid isooctane, ethane is formed chiefly by recombination of methyl radicals inside S i r : the solvent cage. This conclusion was inferred The effects oi denaturing reagents such as urea from three observations : that the CHI ,‘I\j2 ratio in producing drastic unfolding of proteins have was independent of azomethane concentration, become so well known as to direct our attention ( I ) Franck and Rabinowitch, 7 ‘ i . a ~ ~P .t r u n i i i i y .Cor., 30, 120, 9 away from the search for other interpretations of (1934). major denaturatiori reactions. I n particular, the (2) IilrciJ . A m Chew S o , ’ , 83, 2 Y W ( I 9 t i l )
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