No abstract

This is the first part of a series of articles reporting critically evaluated thermochemical properties of selected free radicals. The present article contains datasheets for 11 radicals: CH, CH 2 (triplet), CH 2 (singlet), CH 3 , CH 2 OH, CH 3 O, CH 3 CO, C 2 H 5 O, C 6 H 5 CH 2 , OH, and NH 2. The thermochemical properties discussed are the enthalpy of formation, as well as the heat capacity, integrated heat capacity, and entropy of the radicals. One distinguishing feature of the present evaluation is the systematic utilization of available kinetic, spectroscopic and ion thermochemical data as well as high-level theoretical results.

A value of the enthalpy of formation of the phenoxy radical in the gas phaae, A H " , l e s~ (@., g) = 11.4 f 2.0 kcal/mol, has been obtained from the kinetic study of the unimolecular decompositions of phenyl ethyl ether, phenyl ally1 ether, and benzyl methyl ether1 a t very low pressures. Bond fission, producing phenoxy or benzyl radicals, respectively, is the only mode of decomposition in each case. The present value leads to a bond dissociation energy BDE(+O-H) = 86.5 f 2 kcal/mol,' in good agreement with recent estimates made on the basis of competitive oxidation steps in the liquid phase. A comparison with bond dissociation energies of aliphatic alcohols, BDE (RO-H) = 104 kcal/mol, reveals that the stabilization energy of the phenoxy radical (17.5 kcal/mol) is considerably greater than the one observed for the isoelectronic benzyl radical (13.2 kcal/mol). Decomposition of phenoxy radicals intci cyclopentadienyl radicals and CO has been observed at temperatures above 1000"K, and a mechanism for this reaction is proposed.

Peroxynitrates are thermally unstable intermediates (at ambient temperatures) in the atmospheric degradation of hydrocarbons. In this work, thermal lifetimes of nine peroxynitrates have been measured as a function of temperature and, for two of them, also, as a function of total pressure. In the presence of excess NO, relative concentrations of the peroxynitrates were followed in a 420 I reaction chamber as a function of time by means of longpath IR absorption using a Fourier transform spectrometer. Original data on the unimolecular decomposition rate constants are presented for the peroxynitrates RO 2 NO 2 with R ϭ C 6 H 11 , CH 3 C(O)CH 2 , C 6 H 5 CH 2 , CH 2 I, CH 3 C(O)OC(H)CH 3 , C 6 H 5 OCH 2 , (CH 3 ) 2 NC(O), C 6 H 5 OC(O), and C 2 H 5 C(O). Thermal lifetimes at room temperature and atmospheric pressure are very short (in the order of seconds) for substituted methyl peroxynitrates (i.e., RЈCH 2 O 2 NO 2 ) but rather long for substituted formyl peroxynitrates (i.e., RЉC(O)O 2 NO 2 ). Kinetic data from this and previous work from our laboratory are used to derive structure-stability relationships which allow an estimate of the thermal lifetimes of peroxynitrates from readily available 13 C n.m.r. shift data.

Kinetic and Theoretical Studies of the Reactions CH3C(0)02 + N02 + M *e CH3C(0)02N02 + M between 248 and 393 K and between 30 and 760 Torr