The pressure dependence of the rate of isomerization of cyclopropane at 897 K

Abstract

HIROSHI FURUE and PHILIP D. PACEY. Can. J. Chem. 60,916 (1982). The isomerization of cyclopropane has been studied in a flow reactor at 897 K and from 4 to 406 Tom. The pressure dependence of the rate constants from this work and from the literature have been compared with predictions obtained from two models based on the theory of Yau and Pritchard and from three RRKM models. The collisional deactivation efficiency was found to generally decline with increasing temperature. The average energy removed per collision was calculated by the method of Tardy and Rabinovitch and was found to have values from 15 to 23 kJ mol-I at 897 K. Combining the results of this work with data from the literature from 690 to 1038 K, the limiting high pressure rate constants follow the relation log k , = 15.50 k 0.14 -(276.0 k 2.1 kJ mol-')/2.3RT. HIROSHI FURUE Introductionselves, reliable, limiting high pressure values, k,, The isomerization of cyclopropane has played an based on extrapolation of data from a number of important part in the study of unimolecular reac-Pressures, have been obtained between 718 K and tions. Because it is a reasonably clean reaction 773K 2, 59 7). There have been six recent system, it was the subject of early, successful studies of the pressure dependence of the rate at experimental investigations (l,2). In turn, because higher temperatures (17-221, but the pressure of the availability of the experimental data, it was ranges were narrow and the authors did not extrapone of the first reactions used to test RRKM theory olate their data to determine k,. In the studies of (3, 4). It has now been the subject of at least 20 the collision efficiency at high temperatures (16, 18, experimental studies (1, 2, 5-22) and at least three 20, 221, km was calculated starting with the above RRKM models (3,4, 23).Arrhenius parameters determined at lower temperDespite this impressive body of work, some atures. Even these relatively precise Arrhenius questions remain. RRKM models which were de-Parameters can lead to an uncertainty in k, at veloped to reproduce the fall-off of the rate con-1mOK of50%-stant at low pressures at one temperature haveThe Purpose of the Present work is to study the been found to be unsuccessful at other tempera-reaction at high Pressures and high temperatures tures (4). It has been suggested (16,17) that this is using a flow system. Such a system can provide caused by a decline in P, the efficiency of collisions activation energies (24) and reaction orders (251, in removing excess vibrational energy, as the d 1n kld In P , with a precision of 2 or 3%. This temperature increases. The evidence for this is precision is not better than that of the activation strong for collisions of cyclopropane with small energy quoted above, soexperiments were restrict-

Rate constant and RRKM product study for the reaction between CH3 and C2H3 at T = 298 K

“…Specific Arrhenius parameters (log 10 A and E a (kcal mol Ϫ1 )) were: C 3 H 6 : CH 3 ϩ C 2 H 3 (16.9, 99 kcal mol Ϫ1 ) [53], C 3 H 6 : H ϩ C 3 H 5 (15.3, 86 kcal mol Ϫ1 ) [54], and c-C 3 H 6 : C 3 H 6 (15.5, 66 kcal mol Ϫ1 ) [55]; ⌬E o o ϭ 8 kcal mol Ϫ1 [56,57]. For the helium weak collider model an exponential model with ⌬E d ϭ 400 cm Ϫ1 was used [29,47,48].…”

Structural isomerization of cyclopropane: a new mechanism through propylidene

“…Even though cyclopropane (1) and propene (2) have comparable thermodynamic stabilities (8 kcal mol 21 in favor of 2), 1 a remarkably high activation energy of approximately 65 kcal mol 21 hampers their interconversion. [2][3][4][5][6][7][8][9] Both concerted and stepwise mechanisms have been considered for this isomerization. 3,5,[10][11][12][13][14] The stepwise pathway (A in Scheme 1) involving trimethylene (3) has generally been favored since Benson 11 presented arguments against a concerted reaction: his estimated barrier for such a process was significantly higher than the activation energy determined experimentally.…”

“…Our more extensive computations suggest that the carbene mechanism for the rearrangement of 1 to 2 has a zero-point corrected barrier of 66.6 kcal mol 21 which agrees with the barrier deduced experimentally (64-66 kcal mol 21 ). [2][3][4][5][6][7][8][9] Hence, the carbene pathway might compete energetically with the trimethylene mechanism (64.2 kcal mol 21 ). As experimental data have been discussed traditionally in terms of the latter mechanism, 5,11 additional investigations of the rearrangement of appropriately labeled cyclopropanes to propenes are desirable.…”

Gas-phase thermal dissociation of uranium hexafluoride: Investigation by the technique of laser-powered homogeneous pyrolysis

“…Since the data reported by Schug and Wagner are for rates measured in a bath comprised of a "weak" collision partner, modeling via classical RRK theory requires parameterization of both Seff and X , with additional "leverage" from the selected high-pressure Arrhenius parameters [see Furue and Pacey (1982)l. As shown in Figs.…”