1170
NOTHIS
above the corresponding state of the aliphatic ketones (based on absorption spectra), singlet-singlet energy transfer from the latter to the former in the case of 3130-i irradiation can be ruled out. The mechanism for sensitized photolysis of DCB must hence involve collisional deactivation of the ketone S1 state (80-85 kcal/mol) by DCB whereby the energy released is used to rupture C-C1 bonds (bond strength -80 kcal/mol) and produce the detailed chemistry described earlier.1° Accordingly, the order of relative efficiencies for ketone sensitization of biacetyl f l u ~ r e s c e n c e , ~orJ ~for ketone photocycloaddition to 1,2-dicyanoethylene,l7 should not be expected to carry over exactly to the ketonesensitized photolysis of DCB. Quenching of acetone fluorescence at 405 nm by DCB provided further substantiation of the singlet mechanism beyond the arguments advanced originally.9 Thus, the value of kqrs 0.04 M-l, given by the slope of the Stern-Volmer plot for fluorescence quenching, agrees well with the kqrs 0.03 M-l obtained from the intercept/slope ratio of the acetone sensitization plot in Figure 1. It is noteworthy that these rather low values are of the same order of magnitude as the kqrs 0.08 M-l found for fluorescence quenching of 2-pentanone by 1,3-~entadiene.~ Finally, it should be mentioned that acetophenone and benzophenone (-0.02-0.05 M ) were also found to sensitize the DCB photolysis, with CP values in neat DCB of 0.008 and 0.010, respectively. These yields drop sharply to -0.001 in 5 M DCB in isooctane, no doubt due to competing photoreduction of the aromatic ketones by solvent. I n view of the recent report on delayed fluorescencels in acetophenone and benzophenone, which indicates significant concentrations of SI molecules (Es = 76 and 72 kcal/mol, respectively) in equilibrium with TI molecules (ET = 72 and 69 kcal/ mol, respectively), the assumption is that these compounds likewise serve as singlet sensitizers of the DCB photolysis. That their maximum CP values are about 1/4 to as large as that for acetone (%0.03 at 9 M DCB), lo despite their reportedly very short singlet lifetimes ( ~ 0 . 1n ~ e c ) could , ~ be due to an effectively longer r s as a result of the S1-T1 equilibrium in which T1 serves to repopulate S1.
=
Acknowledgment. The author wishes to thank Dr. John A. Parker for his continued support during this study.
(17) According to N. J. Turro and P. A. Wriede, J . Org. Chem., 34, 3562 (1969), cyclohexanone > 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).
NOTBS
1171
+ 2HF M + CH3NF2* * M + CHsNFz CHsNF2* +HCN
(6)
(7)
The HCN most likely comes from the intermediate CH2NF after loss of another HF. Bumgardner, Lawton, and CarmichaeP (BLC) have also made vibrationally hot methyldifluoramine, with results similar to those of Frazer. The added features of the present work are a tenfold extension of the pressure range to obtain a more precise ratio of rate constants, and the RRK and RRKM treatments to derive the activation energy for H F elimination. I n each experiment, 3.5 Torr of N2F4 and 3.5 Torr of di-tert-butyl peroxide were heated for 15 min with either N2or CF4 as diluent in a monel reaction vessel at 198". At this temperature the NzF4 is almost completely dissociated into 2NF2 and methyl radicals are formed at a convenient rate from the pyrolysis of the peroxide. After quenching, an aliquot was analyzed by gas chromatography for the relative amounts of HCN and CH3NF2using a Poropak Q column and a flame ionization detector. The results are plotted in Figure 1. As expected, CF4 is more effective at stabilizing the hot methyldifluoramine. The slopes of the lines give rate constant ratios k4/ks for N2 and C R . The ratios are in qualitative agreement with those of BLC. No CH3F could be detected, which places an upper limit of about 0.01 on the ratio of rate constants ks/ks. CH3 NF2 +CH3F NF (8)
+
CH3
CH,NF, HCN
PRESSURE
- atm,
198' C
Figure 1. CH3NF2/HCN us. pressure for CHa with Nz and CFa as diluents.
+ NFz at 198'
Figure 2. Reaction coordinate diagram for CHI
+ NFz.
+
+ NF2 +CH3NF2
(9)
A value for kg can be calculated from IC6 = ZX where Z is the number of collisions per second and X is the collision effi~iency.~At 198" for CF4 as 31, 2 is lo7 Torr-' 8ec-I and X is -1, giving k5 (CF,) = lo7 Torr-I sec-'. From Figure 1, k6/k4 (CFS = 0.4 atm-', k4 = 2.5 X 760 X lo7 = 1010.3sec-1. For the RRK treatment, A I = 1013a5sec-l and E = 64 kcal mol-' comprised of 60 kcal mol-I for the CHINF2 bond strength at 25" (ref 7) and 4 kcal mol-' for the thermal energy a t 198" (see Figure 2). The total number of oscillators is 3n - 6, where n is the number of atoms. For methyldifluoramine, n = 7, giving a total of 15 oscillators. The usual procedure3 is to assume that of the oscillators are effective. Taking s = 10, we calculate E1 = 35 kcal mol-'. If s = 9, E1 = 38 kcal mol-l, and if s = 11, El = 33 kcal mol-'. For the RRKM treatments the transition state was assumed to be
I , H--...F
where the dotted lines are bonds. The frequencies for these partial bonds were assumed to be those esti-
CH3 NF, HCN
0
2 4 PRESSURE
6
- atm
8 (198'C)
10
Figure 3. Comparison of theoretical and experimental data for CHI NF2 with CFa as diluent.
+
mated by O'Neal and Benson.' The RRKM calculation predicts the relative yield CH3NF2/HCNas a function of M for different values of El. I n Figure 3, the (6) C. L. Bumgardner, E. Lawton, and H. Carmichael, Chem. Commun., 1079 (1968).
(7) This value is based on our observation (manuscript in preparation) that a tertiary C-NFz bond is 56 kcal/mol. We estimate about a 4 koal/mol increase in strength in proceeding from a tertiary to a primary bond. In addition, we have calculated the heat of the reaction CHsNFz CHa NFz t o be 60.4 kcal/mol (from data in S. W. Benson, "Thermochemical Kinetics," Wiley, New Y o r k , N. Y . , 1968). (8) The program was generously supplied by Dr. G. Haugen of the Institute's Thermochemistry and Kinetics Department.
-
+
The Journal of Physical Chemistry, Vol. 76, No. 8,1971
NOTES
1172 experimentally observed yields are compared with yields calculated using RRKM for different values of E1 between 38 and 44 kcal mol-'. From Figure 3, u7e conclude that El = 42 kcal mol-'. Taking methyldifluoramine as the model compound, R2HCNF2, the rate parameters for H F elimination are then log (ICl/ sec-l) = 13.5 - 42/0, where 0 is 2.3RT kcal mol-', and those for C-N bond breaking2 are log (k2/sec-l) = 17.5 - 60/0. The isokinetic temperature where both rates are equal is in the region of 1OOOK. For other difluoraminoalkanes, R f; H, the C-N bond strength is significantly lower,2 which reduces the isokinetic temperature.
Acknowledgment. This work was supported by the Office of Naval Research on Contract No. Nonr 3760 (00)
-
Mass Spectrometric Study of the Reaction of Nitrogen Atoms with Nitrosyl Chloride by M. R. D u m , C. G. Freeman, M. J. RilcEwan, and L. F. Phillips Chemistry Department, University of Canterbury, Christchurch, New Zealand (Received December 9, 1970) Publication costs borne completely by The Journal of Physical Chemistry
The reaction of N with ONCI might reasonably be expected to resemble that of Tu' with NOz. The latter' is a very fast reaction ( k = 1.8 X lo-" cm3 molecule-' sec-l) which is remarkable chiefly for the variety of decomposition channels that are available to the N-N02 transition state. Thus 43% of primary reactions yield N20 0, and 33% yield NO NO, with about 10% each of Nz 0 0 and N2 0 2 . It is therefore of interest to determine whether the reaction of N with ONCl can take a similar variety of paths. Also, in view of the recent suggestion2 that the reaction with ONCl could serve as a gas-phase titration for estimating N-atom concentrations, it is useful to reexamine the stoichiometry of the reaction and to establish whether the primary reaction is sufficiently rapid to give an accurate end point in the presence of wall and homogeneous recombination of N atoms. We have studied the reaction in a fast-flow system by a combination of mass spectrometric and photometric techniques. Our conclusions concerning the stoichiometry of the reaction differ significantly from those of Biordi;2 this difference is attributable to the differing importance of wall recombination of C1 atoms in the two systems. We find that the primary rate constant is about three orders of magnitude smaller than that of the usual NO titration reaction, so that the
+
+ +
+
+
The Journal of Physical Chemistry, Vol. 76, No. 8,1971
usefulness of the ONCl titration must be restricted to systems with large N-atom concentrations and relatively slow flow speeds, preferably with a large tube diameter or an effective wall poison to minimize heterogeneous recombination. I n contrast to the reaction with NOZ,the reaction of N with ONCl does not produce detectable amounts of N20, and it appears that the observations can be satisfactorily accounted for by a mechanism based on a single primary step yielding NO KCl.
+
Experimental Section The apparatus, procedures (including poisoning the walls of the flow tube with phosphoric acid), and materials were as previously d e ~ c r i b e d . ~A) ~sample of 016NC1was prepared from 99% enriched 16N0by reaction with an excess of chlorine at 180°K and purified by repeated fractional distillation in a LeRoy still. Photometric observations were made downstream from the mass spectrometer sampling leak using a 1P21 photomultiplier in conjunction with either a Spectrolab Ptype interference filter (1.5 nm half-width a t 625 nm) to detect the nitrogen afterglow, or a Corning 7-39 filter to detect the NO p bands. The consumption of ONCl could be monitored from the height of either the NO+ peak a t mass 30 or the NCl+ peak at mass 49; the results obtained in the presence of excess N were independent of which peak was used. Both NO and NC1 are products of the primary reaction and their parent peaks might therefore contribute appreciably to the peak heights measured at masses 30 and 49. However, in the presence of excess N atoms the steady-state concentration of each of these species is expected to be low enough to cause negligible error in measurements of the ONCl concentration. Also, in a previous study of the reaction of N atoms with C120,6where there was no interference from the mass spectrum of the reactants, we were not able to detect any peak at mass 49 from NC1 radicals even though they were certainly present in the reaction system.
Results and Discussion The products of the reaction at long times (ea. 300 msec) were N2, C12, and 02. The production of N2 could not normally be observed because of the large amount of Nz already present, but was readily apparent in experiments with 016NC1. From the lack of a detectable increase in the mass 45 peak due to 'WNO in these experiments we conclude that the yield of N2O in the primary reaction is less than 0.1%. A smaller value for this limit might have been obtained but for (1) L.F. Phillips and H. I. Schiff, J. Chem. Phys., 42, 3171 (1965). (2) J. C.Biordi, J. Phys. Chem., 73, 3163 (1969). (3) C.G.Freeman and L. F. Phillips, ibid., 72, 3025 (1968). (4) M. R. Dunn, M. M. Button, C. G. Freeman, M. J. McEwan, and L. F. Phillips, ibid., in press. (5) C. G. Freeman and L. F. Phillips, ibid., 72, 3028 (1968).
