1168 that the La atoms locate at site I’ and site 11‘positions and are joined by an oxygen atom. Since the epr lines were broadened upon addition of 150 Torr of oxygen, we proposed that the Sc atom which is involved in the adsorption site of the 02-ion must be located a t site 11’,which is available to molecules in the supercage of the zeolite. Conclusions based on the spectrum of 02-on L a y are more tentative because of the poor resolution and the greater overlap of the hyperfine structure. It appears that the interactions of 02-on L a y are much the same as those found in ScY; that is, the 0 2 - ion is experiencing coupling with 139La(I = 7/2) ions at site
11‘. Acknowledgment. The authors wish to acknowledge the contributions of Drs. B. O’Brien and J. B. Natowitz in obtaining an analysis of the zeolite. This work was supported by the National Science Foundation under Grants GP-8319 and GP-19875 (6) J. V. Smith, J. M . Bennett, and E. M. Flanigen, Nature, 215, 241 (1967).
NOTEIS spectroquality acetone from J. T. Baker Chemical Co.; research grade acetone-d6, 3-pentanone, 2-hexanone, 3-hexanone, and 2,4-dimethyl-3-pentanone from Aldrich Chemicals Co. ; chromatoquality 2-butanone and cyclohexanone from Matheson Coleman and Bell; chromatoquality cyclopentanone from Analabs, Inc. ; and 2-pentanone, 2-heptanone, 3-heptanone, and 4heptanone from Chem Service Inc. The last four ketones were distilled prior to use. A purified sample of 1,4-dichlorobutane (Aldrich Chemicals Co.) comparable to that in the previous was used in the present study. Acetophenone and benzophenone (from J. T. Baker photosensitizer kit) were used without further purification. PTocedure. Irradiations of degassed DCB-isooctane-ketone solutions in Pyrex tubes at 3130 A, as well as analysis for HC1 evolution, followed the procedures described in ref 9 and 10. I n addition, quenching of fluorescence of acetone (0.068 M in isooctane) at 405 nm by DCB was measured with an Aminco-Bowman spectrofluorometer, using excitation at 313 nm.
Results and Discussion Photolysis of 1,4-Dichlorobutane Sensitized
by Various Aliphatic Ketones
by Morton A. Golub’ A m e s Research Center, National Aeronautics and Space Administration, Moffett Field, California 94086 (Received August 18, 1970) Publication costs assisted by the A m e s Research Center, N A S A
While photosensitization by the n,a* triplet state of ketones has been known for some timeJ2only recently has the involvement of the corresponding singlet state in organic photochemistry become generally appreciated. Thus, photocycloaddition of ketones to olefins to form oxetanesJa ketone sensitization of biacetyl flu~rescence,~ quenching of alkyl ketone fluorescence by high concentrations of 1,3-~entadiene,~ Norrish type I1 photoelimination,2j6and a-cleavage (or type I) reactions from cyclic7 and acyclic* ketones, are all processes in which [n,n*]I states of ketones have been implicated for part or all of the respective chemistry. Recently we disclosed still another process attributable to [n,a*I1 sensitization, viz., the acetone-sensitized ~ of photolyses of several d i c h l o r o b ~ t a n e s . ~I n~ ~view increasing interest in the role of excited singlet states,I1 we extended the dichlorobutane photolysis by examining the relative efficiencies of a number of aliphatic ketones in photosensitizing the evolution of HC1, the main product, from 1,4-dichlorobutane (DCB) in solution.
Experimental Section Materials. The following ketones were employed: The Journal of Physical Chemistry, Vol. 76,N o . 8, 1971
Figure 1 presents a Stern-Volmer analysis of HCl formation from DCB (-2-8 M in isooctane) photosensitized by a number of aliphatic ketones (0.540.56 M ) . 1 2 From the intercept/slope ratios of the linear W1 us. [DCBI-l plots, relative sensitization efficiencies for various ketones were determined and presented in Table I. A common intercept (which corresponds to the reciprocal of the probability that quenching of singlet ketone by DCB leads to product, analogous to the treatment used, e.g., in photocycloaddition studiesa) is expected since there was negligible loss of ketone in any run.
(1) Xational Research Council-National Aeronautics and Space Administration Resident Research Associate, 1968-1970. (2) See P . J. Wagner and G. 5. Hammond, Advan. Photochem., 5, 21 (1968), for a review of ketone triplet photochemistry. (3) J. C. Dalton, P. A. Wriede, and N. J. Turro, J. A m e r . Chem. Soc., 92, 1318 (1970), and references cited therein. (4) See F. Wilkinson, Advan. Photochem., 3 , 241 (1964), for a review of this topic. (5) F. S. Wettack, G. D. Renkes, M . G. Rockley, N . J. Turro, and J. C. Dalton, J. A m e r . Chem. Soc., 92, 1793 (1970). (6) N. C. Yang, S. P. Elliott, and B. Kim, ibid., 91, 7551 (1969), and references cited therein. (7) J. C . Dalton, D. M. Pond, D. S. Weiss, F. D. Lewis, and N. J. Turro, ibid., 9 2 , 2564 (1970). (8)’N. C. Yang and E. D. Feit, ibid., 90,504 (1968). (9) M. A. Golub, ibid., 91, 4925 (1969). (10) M. A . Golub, ibid., 92, 2615 (1970). (11) For examples of aromatic hydrocarbons as singlet sensitizers, see: P. S. Engel, ibid., 91, 6903 (1969); S. Murov and G. S. Hammond, J. P h y s . Chem., 72, 3797 (1968). (12) By working at a ketone concentration of -0.54-0.86 M , and staying away from nearly neat DCB, we avoid the sharp upturn in the @-[DCB] plot observed p r e v i o ~ a l yfor ~ ~ [acetone] ~~ 40.2 M and [DCB] -9 M .
1169
NOTES Table I : Photosensitization Efficiencies for Different Aliphatic Ketone8
Ketone
Relative Relative sensienergy tization transfer efficiency’ efficiencyc
1.00 0.66 0.45 0.35 0.33 0.33 0.28 0.10 0.03 0.03 0 0
78,
nsec
1.00 0.62
4.4d 2.8d
0.40
1.8,d2 . 0 e 2.7f 1.8,62.00 4.2”
0.44
0.730 1.9dtC
0
0
I
I
I
I
I
1
I
,I
.2
.3
.4
.5
.6
[OCBI-’
[Ketone] = 0.54-0.56 M in all DCB-isooctane-ketone SOIUtions. Given by intercept/slope ratio of a-1 us. [DCBI-’ plot relative to that for 2-butanone (0.092 M-1). 0 Based on singlet-singlet transfer constants for the ketone-sensitized fluorescence of biacetyl (see text). d Reference 4; data for 2butanone and 3-pentanone corrected for typographical errors introduced in reviewing the original work.13 8 Reference 5. f Estimated a t 1.4 times that of acetone.14t16 * Reference 6. Reference 14. ’ Reference 7. a
Figure 1. Concentration dependence of the photoinduced formation of hydrogen chloride from 1,4-dichlorobutane sensitized by various aliphatic ketones a t 3130 d: Al 2-butanone; A, 3-pentanone; U, 3-hexanone; 0, acetone; 0, acetone-&; V, 2-pentanone; V, 2,4-dimethyl-3-pentanone.
2-pentanone) cyclopentanone,’ a c e t ~ n e - d ~ and ,~~,~~ 2,4-dimethyl-3-pentanone14 (see Table I). While the efficiencies for 2-butanone, 3-pentanone, The straight-chain ketones, R1R2C0, where RI and/ acetone, and 2-pentanone in sensitizing the photolysis C3, have photosensitization efficiencies which or Rz of DCB can be correlated with their singlet lifetimes, range from 0.8 to 3.0 times that of acetone (or acethe efficiencies of the other ketones in Table I cannot. tone-&) while the branched ketone, 2,4-dimethyl-3That rS is clearly not the only factor can be seen, for pentanone, has an efficiency which is only l/3 of that of example, in the fact that cyclopentanone is not a sensiacetone. On the other hand, those ketones in which tizer for DCB photolysis, yet it sensitizes biacetyl R1 or Rz > C3, and also the cyclic ketones, cyclopenfluorescence with an efficiency close to that of acetone tanone, and cyclohexanone, have little or no efficiency (and hence has nearly the same “kinetic” lifetime as at sensitizing the photolysis of DCB. Interestingly, does acetone). Again, 2,4-dimethyl-3-pentanone1 with the order of efficiency, 2-butanone > 3-pentanone > a T~ over twice that of acetone, has only a third of the acetone, parallels the order of relative efficiency of these latter’s efficiency; also, the efficiency of acetone-& same ketones in sensitizing biacetyl fluorescencea4J3 does not reflect its deuterium-enhanced 7,. FurtherThe quantities given in the third column of the table, more, 2-hexanone with a r s about a third of that of all relative to the efficiency for 2-butanone taken as acetone is not a sensitizer at all. Evidently, steric unity, were based on average values for the singletfactors must also play an important role in singlet singlet transfer constants, K , and K,, for sensitization transfer. l6 of biacetyl fluorescence and quenching of donor fluoresSince the S1 state of DCB is some 25-35 kcal/mol cence, respectively, in each of the ketone-biacetyl systems examined by Dubois and Since K , = K , = krs, where k is the specific rate constant for the (13) J. T . Dubois and M. Cox, J . Chem. Phys., 38, 2536 (1963). (14) M. O’Sullivan and A. C. Testa, J. Amer. Chem. SOC.,92, 5842 singlet-singlet transfer process (assumed to be diffusion (1970) controlled here) and T, is the mean (singlet) lifetime of (16) N. C. Yang, E. D. Feit, M. H. Hui, N. J. Turro, and J. C. the excited donor, the biacetyl fluorescence work proDalton, ibid., 9 2 , 6974 (1970). vided “kinetic” estimates for 7, for the above three (16) This view is in line with the recent observation (F.5. Wettack, Hope Cdlege, private communication) that fluorescence quenching ketones and also for cy~lopentanone.~Likewise, of aliphatic ketones by l,&pentadiene shows a strong structural biacetyl quenching measurementss yielded r s data for effect. I n particular, the finding that 2,4-dimethyl-3-pentanone exhibits an unexpectedly low rate constant for fluorescence quenching 2-pentanone and 2-hexanonel while fluorescence decay may provide an explanation for the relatively low photosensitination or intensity measurements afforded r , data for acetone,6 efficiency observed here for this particular ketone.
acetone > cyclopentanone > 2pentanone > 2-hexanone in order of efficiency of photocycloaddition to 1,2-dicyanoethylene. Although this reaction proceeds through the [n,?r*]l state of alkyl ketones,a the mechanism involves ?r complex formation and is therefore quite different from the ketone-DCB system. (18) J. Saltiel, H. C. Curtis, L. Metts, J. W. Miley, J. Winterle, and M. Wrighton, J. Amer. Chem. Soc., 92, 410 (1970).
The Journal of Physical Chemistry, Vol. 76,No. 8, 1971
The Kinetics of the Unimolecular Dehydrofluorination of Methyldifluoramine
by David S. Ross* and Robert Shaw Stanford Research Institute, Menlo Park, California 04026 (Received November ,926,1070) Publication
cost8
assisted by the Ofice of Naval Research
This work was undertaken to determine the activation energy of the process R2HCNF2 +RzCNF
+ HF
(1)
By analogy with similar 4-center eliminations, the Arrhenius A factor may be estimated to be 1013e5 sec-I. Direct observation of the H F elimination is usually masked2by the competing reaction RzHCNFz +RzHC
+ NF2
(2)
which is a simple bond fission with an A factor of around sec-’. However, an interesting solution to the problem is made possible by reversing the competing reaction to give a vibrationally hot molecule RzHC
+ NF2 +RzHCNFz*
The hot difluoroaminoalkane can redissociate, eliminate HF, or can be collisionally stabilized.
+ NF2 RzHCNFz* -+-RzCNF + H F RZHCNF2* + M RzHCNFz + M RZHCNFz* +RzHC
--t
(3) (4)
(5)
Experimental measurements of the yields of R2CNF and R2HCNF2as a function of M give the ratio of rate constants k4/k5. An estimate of collision frequency and stabilizing efficiency will give IC6, and from the ratio k4/k5,k4 can be obtained. According to RRK t h e ~ r y , ~ k4 = A1[ ( E - E1)/EIs-I where A1 and E1 are the Arrhenius parameters for the decomposition of a thermalized molecule, E is the energy of the vibrationally hot molecule, and where s is the number of “effective” oscillators. From k4 and estimates of A I , E , and s, El can be obtained. A more sophisticated treatment, RRKM, which requires a model for the transition state, can also be used.4 Methyldifluoramine, the simplest R2HCNF2, was chosen as the model compound. Vibrationally hot methyldifluoramine was first prepared by Frazer,6 who found both HCN and stabilized methyldifluoramine. (1) H. E. Q’Ned and S. W. Benson, J. Phys. Chem., 71,2903 (1967). (2) D. 8.Ross, T. Mill, and M. E. Hill, to be published. (3) 5. W. Benson and G. Hangen, J. P h m Chem., 69, 3898 (1965). (4) 8. W. Benson, “The Fundamentals of Chemistry Kinetics,” McGraw-Hill, New York, N. Y., 1960, p 222. (5) J. W. Brazer, J . Inorg. Nucl. Chem., 16, 63 (1960).
