A molecular beam mass spectrometric study of the formation and photolysis of C(lc)lO dimer

A study of the chlorine oxides present at temperatures and pressures typical of the Antarctic stratosphere was carried out. A series of low temperature flow reactors was constructed and used in conjunction with molecular beam mass spectrometric techniques to identify species and characterize their kinetic behavior at temperatures of -20 to -70 C and pressures of from 30 to 130 Torr. It was found that the gas phase chlorine-oxygen system was quite complex at low temperatures. ClO dimer was identified and found to be thermodynamically very stable under stratospheric conditions. It was also found that any system which contained ClO also contained a larger oxide. The oxide was identified as Cl2O3. A survey of possible higher oxides, which have been postulated as possible chlorine sinks in the stratosphere, was also carried out. The rate of formation of ClO dimer was measured as a function of temperature and pressure. Measurements were made of both the decay of ClO and the formation of the dimer. By comparing these rates it was determined that virtually all of the ClO was converted to the dimer under stratospheric conditions, and that the other ClO reactions were not important under these conditions

ARENE PYROLYSIS. RELATIVE STABILITIES OF BENZYLIC RADICALS.

The thermal unimolecular decomposition of ethylbenzene, iso-propylbenzene, t-butylbenzene, n-propylbenzene, isobutylbenzene, neopentylbenzene, 4-ethylstyrene, 1-phenyl-1-butene, 1-1 diphenyl-ethane, and 2-2 diphenylpropane was studied using the very-low-pressure pyrolysis (VLPP) technique. Each reactant decomposed by way of c-c bond fission producing an alkyl radical plus a benzyl or benzylic-type radical. RRKM calculations show that the observed rate constants when combined with thermochemical estimates are consistent with the following high-pressure rate expressions: log(k/s(\u27-1)) = 15.3 - (72.7/(theta)) for ethylbenzene (1053 - 1234K), log(k/s(\u27-1)) = 15.8 - (71.3/(theta)) for isopropylbenzene (971 - 1151K), log(k/s(\u27-1)) = 15.9 - (69.1/(theta)) for t-butylbenzene (929 - 1157K), log(k/s(\u27-1)) = 15.3 - (69.6/(theta)) for n-propylbenzene (989 - 1195K), log(k/s(\u27-1)) = 15.6 - (67.8/(theta)) for isobutylbenzene (922 - 1087K), log(k/s(\u27-1)) = 15.5 - (64.3/(theta)) for neopentylbenzene (918 - 1064K), log(k/s(\u27-1)) = 15.3 - (71.3/(theta)) for 4-ethylstyrene (1096 - 1186K), log(k/s(\u27-1)) = 15.3 - (67.3/(theta)) for 1-phenyl-1-butene (1030 - 1115K), log (k/s(\u27-1)) = 15.5 - (67.4/(theta)) for 1-1 diphenylethane (1000 - 1066K), and log(k/s(\u27-1)) = 15.7 - (65.8/(theta)) for 2-2 diphenylpropane (910 - 1054K) where (theta)/kcal mol(\u27-1) = 2.303 RT. Resulting activation energies combined with heat capacity data led to the following bond dissociation energies at 298K: DH(\u270)(PhCH(CH(,3))--CH(,3)) = 73.8 kcal mol(\u27-1), DH(\u270)(PhC(CH(,3))(,2)--CH(,3)) = 72.9 kcal mol(\u27-1), DH(\u270)(PhCH(,2)--C(CH(,3))(,3)) = 69.8 kcal mol(\u27-1), DH(\u270)(H(,2)CCHPhCH(,2)--CH(,3)) = 72.5 kcal mol(\u27-1), DH(\u270)(PhCHCHCH(,2)--CH(,3)) = 69.2 kcal mol(\u27-1), DH(\u270)(Ph)(,2)CH--CH(,3)) = 69.8 kcal mol(\u27-1), and DH(\u270)((Ph)(,2)CH(CH(,3))--CH(,3)) = 69.4 kcal mol(\u27-1). Activation energies were also used to derive the following heats of formation at 298K: (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI) In the cases of 4-ethylstyrene, 1-phenyl-1-butene, 1-1 diphenylethane, and 2-2 diphenylpropane relative activation energies for bond fission were related to relative resonance stabilization energies (RSE) in the resulting benzylic radical products. Relative to a RSE of 11 kcal mol(\u27-1) for the benzyl radical PhCH(,2)(.) (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI) experimental results were consistent with the following RSE at 298K: RSE(H(,2)CCHPhCH(,2)(.)) = 13.4 kcal mol(\u27-1), RSE(PhCHCHCH(,2)(.)) = 16.7 kcal mol(\u27-1), RSE((Ph)(,2)CH) = 15.0 kcal mol(\u27-1), and RSE((Ph)(,2)CHCH(,3)) = 14.5 kcal mol(\u27-1). These results were found to be in agreement with the predictions of an empirical technique for estimating RSE in benzylic radicals. Where appropriate, kinetic results were compared with previously reported results of carrier and shock tube experiments