Calorimetric, 1H nuclear magnetic resonance, and molecular orbital studies of hydrogen bonding between peroxy acids and oxygen bases. Implications for mono-oxygen donation potential of peroxy acids

Abstract

The enthalpies of formation of hydrogen-bonded complexes between peroxybenzoic acids and various oxygen bases, ΔH°, were obtained by calorimetry. The ¹H n.m.r. shifts of the OOH absorptions, under conditions of complete peroxy acid complexation, were measured. ΔH° and ‘hydrogen-bond shifts’, ΔδOOH =δOOH(base)–δOOH(CCl₄), were found to be sensitive to the acidity of the peroxy acid as well as the basicity of the oxygen base. A satisfactory linear correlation between ΔH° and ΔδOOH, expressed by the equation –ΔH°(kJ mol^–1)=(7.59 ± 0.92)ΔδOOH +(7.35 ± 1.61), was found. Ab initio MO SCF calculations (STO-4G, 4–31 G*) were carried out for the peroxyformic acid–dimethyl ether complex and peroxyformate ion. The peroxy acid isotope effect for epoxidation of cyclohexene with 4-t-butylperoxybenzoic acid (OH/OD) in NN-dimethylacetamide (k_H/k_D1.09), together with the results of kinetic studies in other oxygen bases as solvents, suggest that oxygen base is most probably a part of an ‘early’, loose charge-transfer-type transition state for these reactions (‘direct attack mechanism’), and that reduced reactivity of peroxy acids in basic solvents most probably reflects lowered electrophilicity of the peroxy acid due to the forcing of the enlarged negative charge on the terminal oxygen atom after complexation.