1316
Communications to the Editor
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intensity under the 982 cm-1 995 cm-1 envelope in every MgS04 solution. The concentration of MgS04 contact ion pairs was calculated using the following relationship
where C is the stoichiometric concentration of MgS04, determined as magnesium using atomic absorption spectroscopy. In Table I the concentrations of contact ion-paired MgS04 found by the Raman technique are compared with those calculated from ultrasonic result^.^ The agreement between the methods is good, and the results support our assignment of the bands a t 982 and 995 cm-l. Our Raman results indicate that it is not possible to distinguish the two types of solvent separated ion pairs in eq 1 from each other or, the sulfate in solvent separated ion pairs from the solvated sulfate ion. The band at 982 cm-1 seems to originate from sulfate in all three of these forms, [S042-(aq)], [MgWWS04], and [MgWS04], whereas the band at 995 cm-1 originates only from sulfate in the contact ion pair [MgS04]. Water Quality Research Division Department of the Environment Ottawa, Ontario
Anthony R. Davis* Barry G. Oliver
Received January 11, 1973
Thermal Decomposition of Cyclobutanone Publication costs assisted by the Research Council of Alberta
Sir: A recent article on the thermolysis of cyclobutanonel contains a confirmation of the Arrhenius parameters of , ~ the production of ethylene and ketene, Das, et ~ l . for but compares rather poorly with the original data of Blades3 for the minor products, cyclopropane and carbon monoxide. An activation energy for the latter reaction 4.4 kcal mol-1 higher than the former is to be compared with the previous value of 6.0 f 0.5 kcal mol-I, an intrinsically more reliable figure since it was derived from relative rate measurements. It is noteworthy, however, that both sets of data indicate relative rates within 5%. The computational procedure of O’Neal and Benson4 which has been used in evaluating the rate parameters for these two modes of decomposition depends on heats of formation of diradicals determined from the independent removal of two H atoms from linear molecules and on the activation energies of geometric and structural isomerizations and decompositions. The heat of formation of the simplest such diradical, trimethylene, has been questioned on interpretational grounds5 and, since the publication of Hoffmann’se theoretical study of trimethylene employing extended Huckel molecular orbital theory, several articles?-11 dealing with the generation of potential energy surfaces based on semiempirical and ab initio quantum mechanical methods for small ring compounds have appeared. Also the correlations of these surfaces with the kinetic behavior of these systems under thermal conditions have been discussed. These calculations predict a small barrier for ring closing, -1 kcal mol-1, of trimethylene diradical to cyclopropane,7,9JO in disagreement with Benson’s estimate of 9.3 kcal mol-1. Hence, any definitive The Journal of Physical Chemistry, Vol. 77, No.
IO, 7973
pronouncements on the involvement of a particular diradical and/or concerted pathways in more complex systems, such as cyclobutanone, seems premature. These considerations are of particular significance in the possibility of the diradical COCH2CH2cH2 undergoing a concerted rearrangement to cyclopropane and CO, which was ruled out on the grounds that the activation energy is “likely to be much higher” than “the normal C4 ring-closing activation energy, 6.6 kcal mol-1.” McGee and Schleifer also carry out a calculation designed to show that the thermal rearrangement of cyclopropane should be negligible under reaction conditions. An alternate source of propylene would be cyclopropane formed in the reaction with energy in excess of the activation energy for its rearrangement (65.1 kcal mol-1).12 Using 58.0 kcal mol-1 as the activation energy for cyclopropane f ~ r m a t i o nthe , ~ excitation energy of cyclopropane and CO is 51.4 kcal mol-1 plus an amount determined by the “energy distribution of the products” function which 51.41 where s is the numis a maximum at [(s - 1)RT iber of oscillators taking part in intramolecular energy transfer. With s N 15, this suggests a most probable product energy of 70 kcal mol-I which must be partitioned between cyclopropane and CO. In view of the pressure of the system, p l < 40 Torr,l in relation to the effect of pressure on the cyclopropane rearrangement,lz some propylene should be expected, and, indeed, has been reported.3 It is interesting to note that the heat of formation of the C3H6 entity initially formed will determine the energy partitioned between it and CO, and hence the formation of propylene is, theoretically at least, a key to the preferred mechanism. It is perhaps worthy of note with regard to reaction mechanisms that activation energies define only the minimum energy path for reaction, but in no way exclude formation of the same products via a quite distinct mechanism whose activation energy, if measurable, would be higher. This point is well illustrated by cyclobutanone in the formation of cyclopropane and CO via the ground singlet in thermolysis3 and the triplet in photolysis.l3 Clearly additional thermolysis studies, particularly of substituted cyclobutanones,~4will be of considerable value in the understanding of reaction mechanisms. (1) T. H. McGee and A. Schleifer, J. Phys. Chem., 76, 963 (1972). (2) M. N. Das, F. Kern, T. D. Coyie, and W. D. Walters, J. Amer. Chem. SOC.,76,6271 (1954). (3) A . T. Blades, Can. J. Chem., 47, 615 (1969). (4) H. E. O’Neai and S. W. Benson, J. Phys. Chem., 72, 1866 (1968): S. W. Benson, “Thermochemical Kinetics,” Wiley, New York, N. Y . , 1968. (5) G. R. Freeman, Can. J. Chem., 44, 245 (1966). (6) R. Hoffmann, J. Amer. Chem. Soc., 90, 1475 (1968). (7) R. Hoffmann, S. Swaminathan, B. G. Odell, and R. Gleiter, J. Amef. Chem. SOC.,92, 7091 (1970). (8) R. Hoffmann, C. C. Wan, and V. Neagn, Mol. Phys., 19, 113 (1970). (9) P. J. Hay, W. J. Hunt, and W, A. Goddard, I i i , J. Amer. Chem. Soc.. 94. 637 (1972). (10) J. A. Horsley, Y . Jean, C. Moser, L. Salem, R. M. Stevens, and J. S . Wright, J . Amer. Chem. SOC..94, 279 (1972). (11) J. S. Wrightand L. Salem, J. Amer, Chem. SOC.,94, 322 (1972). (12) E. W. Schiag and B. S. Rabinovitch, J. Amer. Chem. SOC.,82, 5996 (1960). (13) . . H. 0. Denschlaa and E. K. C. Lee, J. Amer. Chem. SOC., 90, 3628 (1968). (14) H. A. J. Cariess and E. K. C. Lee, J. Amer. Chem. Soc., 92, 4482, 6682 (1970); 94, 1 (1972).
Research Council of Alberta Edmonton, Canada Received May 25, 1972
A. T. Blades* H. S. Sandhu
