TABLES OF THE HEATS OF FORMATION AND IONIZATION POTENTIALS OF HYDROCARBON RADICALS BY M. SZWARC Received 20th July, 1951 The values of bond dissociation energies and heats of formation of radicals have been collected recently in two publications.*t The latter contains all the experimental data published up to the end of 1949, thus enabling one to assess the uncertainties involved. The work done during the last two years provides some new data and requires a slight revision of the old. The present meeting provides an opportunity to supplement these Tables t and thus to bring the data on hydrocarbon radicals up to date. The following equation enables one to compute the desired R-R, bond dissociation energy : The heats of formation used in this equation should be computed for the relevant species in the gaseous state and at oo K.However, the extra- po!ations to oo K are somehow uncertain, and it seems that the corrections which should be introduced are of the same magnitude as, or perhaps even smaller than, the experimental errors involved in the currently available data on bond dissociation energies. Consequently, the values AHf(R . R,) corresponding to 25" C have been used for calculating the heats of formation of radicals listed in Table I. Our knowledge of ionization poten tia?s of hydrocarbon radicals is very scanty. The ionization potentials of CH,, C,H5, sec.-C,H,, and tert.-C,H, have been determined by electron impact method, and the relevant data are summarized in the paper by Stevenson. There are, however, data which enable us to calculate ionization potentials of few more hydrocarbon radicals.(i) PROPYL RADICAL.-The appearance potential of C,H,+ ion pro- duced in the mass-spectrum of n-butane is given by Stevenson and Hipple 8 at 11-14 eV/molecule. D(CH, . CH, . CH,-CH,) is calculated as 3-57 eV/ molecule on the basis of D(CH, . CH, . CH,-H) = 4-3 eV/molecule (see ref. (2) and (3)). (ii) ALLYL RADICAL.-The appearance potential of C,H,+ ion pro- duced in the mass-spectrum of butene-1 is given by Stevenson s as 11-65 eV/molecule.$ D(CH, : CH . CH,-CH,) has been determined 1 0 as 2.6 eV/molecule. Therefore, Iz(CH, : CH . CH,) = 9.0, eV/molecule. This value seems to be reasonable, the ionization potential of ally1 radical is expected to be only slightly lower than that of methyl radical.This has been the writers' intention in preparing Table I. D(R-R,) =z AH,(R) + AHf(R1) - AH,(R. R1). Therefore, Iz(CH,. CH, . CH,) = 7.5, eV/molecule. * Roberts and Skinner, Trans. Faraday SOC., 1949, 45, 339. t Szwarc, Chem. Rev., 1950. 47, 75. 8 Stevenson and Hipple, J . Amer. Chem. SOC., 1942, 64, 1588. 9 Stevenson, J . Amer. Chem. SOC., 1943, 65, 299. $ The objections which Stevenson raises in his paper seems to be irrelevant. I t appears that electron impact dissociates the molecule of propylene into CH, : C . CH, ion and not into CH, : CH . CH,+ ion. Indeed, this assumption is consistent with the results of electron impact studies of iso-butene. This probkm is discussed fully by Evans and Szwarc, J . Clwn. Physics (in press). 10 Sehon and Szwarc, Proc.Roy. SOC. A , 1950, 202, 263. 336M. SZWARC 337 TABLE I.-TABLE OF C-H BOND DISSOCIATION ENERGIES AND HEATS OF FORMATION OF HYDROCARBON RADICALS Radical CH, . C*H5 * n-C,H, iso-C,H, . tert.-C4H, . C2H3 * CH, : CH . CH,' . C H , : C . CH,' . HC: C - YL-C~H, CH,' Ph- . Ph.CH,* . P-CH, . C6H4. CH,' WZ-CH,. C6H4. CH,' o-CH,. C G H d , CH,* Ph.CH- . dH3 CH, CH, P h . $-CH,. C,H, . CH* CH, '">,He . Ph ph\ ;v= -CH; m D (R-H) kcal. /mole 101-102 96-97 - 9 9 ? - 94 N I01 ? - 88 92-104 77-78 - 76 < I 2 1 104 ? 77'5 75 77'5 - 74 - 74 - 71 - 74 - 72 75 - 76 - 76 AH, f W kcal./mole 31-32 24-25 - 22 ? - 17 N 1 9 . - 4'5 52-64 29. j-30'5 - 20 - I20 ? - 72 ? 37'5 27 29.5 - 26.5 - 29 - 20 - 2 1 - 53 Experimental Data published since 1949 Pyrolysis CH, . NO, { Ph PZgi?(?2H5 Electron impact Pyrolysis Ph .CH, . C,H, Electron impact Electron impact Electron impact Pyrolysis of dibenzyl Pyrolysis PhCH (CH 3) Pyrolysis PhC(CIX,) Pyrolysis P-CH3. CeHdCH(CH3)Z Pyrolysis Ph, . CH2 Ref. N 0. I 2 3 2 4 3 4 Cottrell, Graham and Reid, Trans. Faraday SOC., 1951, 47, 584. Leigh and Szwarc (in course of publication). Stevenson, this Discussion. 3 Stevenson (private communication). 5 Horrex and Miles, this Discussion. 6 Leigh and Szwarc (in course of publication). 7 Horrex and McCrae (communication to this Discussion).338 (ii;) VINYL RADIcAL-The appearance potentials of C,H,+ and C2H,+ ions in the mass-spectrum of butene-1 make i t possible to calculate D(CH, : CH-H) and ionization potential of vinyl radical. However, these calculations lead to D(CH, : CH-CH) = g z kcal./mole, a value which seems to be too low. The data on ionization potentiah of the six hydrocarbon radicals dis- cussed above are listed in Table 11. TABLES OF HEATS OF FORMATION TABLE 11.-IONIZATION POTENTIALS OF HYDROCARBON RADICALS Radical CH, . n-C,H, . CH,:CH.CH,' : tert.-C,H, . C2H6 - ZSO-C~H, Ionization Potential eV /molecule 1 k cal. /mole 10'1 8'7 7.57 7'4s 9-05 6.9 232 I74 172 208 I59 2 00 I wish to express my appreciation to Dr. Stevenson for allowing me to use his unpublished results. Chemistry Department, University of Manchester, Manchester, 13.