Solvation enthalpies of free radicals: O-O bond strength in di-tert-butylperoxide.

The photolysis reaction of di-tert-butylperoxide was studied in various solvents by photoacoustic calorimetry (PAC). This technique allows the determination of the enthalpy of this homolysis reaction, which by definition corresponds to the O-O bond dissociation enthalpy of the peroxide in solution, DHsin(degrees)(O-O). The derived value from these experiments in benzene, 156.7 +/- 9.9 kJ mol(-1), is very similar to a widely accepted value for the gas-phase bond dissociation enthalpy, DH(degrees)(O-O) = 159.0 +/- 2.1 kJ mol(-1). However, when the PAC-based value is used together with auxiliary experimental data and Drago's ECW model to estimate the required solvation terms, it leads to 172.3 +/- 10.2 kJ mol(-1) for the gas-phase bond dissociation enthalpy. This result, significantly higher than the early literature value, is however in excellent agreement with a recent gas-phase determination of 172.5 +/- 6.6 kJ mol(-1). The procedure to derive the gas-phase DH(degrees)(O-O) was tested by repeating the PAC experiments in carbon tetrachloride and acetonitrile. The average of the values thus obtained was DH(degrees)(O-O) = 179.6 +/- 4.5 kJ mol(-1), confirming that the early gas-phase result is a lower limit. More importantly, the present study questions the usual assumption that the solvation terms of homolysis reactions producing free radicals in solution should cancel, and suggests a methodology to estimate solvation enthalpies of free radicals.

Energetics of C-Cl, C-Br, and C-I bonds in haloacetic acids: enthalpies of formation of XCH2COOH (X=CI, Br, I) compounds and the carboxymethyl radical.

The standard molar enthalpies of formation of chloro-, bromo-, and iodoacetic acids in the crystalline state, at 298.15 K, were determined as deltafH(o)m(C2H3O2Cl, cr alpha)=-(509.74+/- 0.49) kJ x mol(-1), deltafH(o)m(C2H3O2Br, cr I)-(466.98 +/- 1.08) kJ x mol(-1), and deltafH(o)m (C2H3O2I, cr)=-(415.44 +/- 1.53) kJ x mol(-1), respectively, by rotating-bomb combustion calorimetry. Vapor pressure versus temperature measurements by the Knudsen effusion method led to deltasubH(o)m(C2H3O2Cl)=(82.19 +/- 0.92) kJ x mol(-1), deltasubH(o)m(C2H3O2Br)=(83.50 +/- 2.95) kJ x mol(-1), and deltasubH(o)m-(C2H3O2I) = (86.47 +/- 1.02) kJ x mol(-1), at 298.15 K. From the obtained deltafH(o)m(cr) and deltasubH(o)m values it was possible to derive deltafH(o)m(C2H3O2Cl, g)=-(427.55 +/- 1.04) kJ x mol(-1), deltafH(o)m (C2H3O2Br, g)=-(383.48 +/- 3.14) kJ x mol(-1), and deltafH(o)m(C2H3O2I, g)=-(328.97 +/- 1.84) kJ x mol(-1). These data, taken with a published value of the enthalpy of formation of acetic acid, and the enthalpy of formation of the carboxymethyl radical, deltafH(o)m(CH2COOH, g)=-(238 +/- 2) kJ x mol(-1), obtained from density functional theory calculations, led to DHo(H-CH2COOH)=(412.8 +/- 3.2) kJ x mol(-1), DHo(Cl-CH2COOH)=(310.9 +/- 2.2) kJ x mol(-1), DHo(Br-CH2COOH)=(257.4 +/- 3.7) kJ x mol(-1), and DHo(I-CH2COOH)=(197.8 +/- 2.7) kJ x mol(-1). A discussion of the C-X bonding energetics in XCH2COOH, CH3X, C2H5X, C2H3X, and C6H5X (X=H, Cl, Br, I) compounds is presented.