Isotope Effect in the Hydrogen Atom‐Formaldehyde Reaction

The isotope effect in the hydrogen abstraction from formaldehyde by hydrogen atoms has been measured. The activation energy difference derived from relative rate measurements of the pair of reactions $$\mathrm{H}+{\mathrm{H}}_2\mathrm{CO}\to {\mathrm{H}}_2+\mathrm{HCO}$$ $$\mathrm{H}+{\mathrm{D}}_2\mathrm{CO}\to \mathrm{HD}+\mathrm{DCO}$$ is E₅ — E₂ = 1.0 kcal. The value for the corresponding pair $$\mathrm{D}+{\mathrm{H}}_2\mathrm{CO}\to \mathrm{HD}+\mathrm{HCO}$$ $$\mathrm{D}+{\mathrm{D}}_2\mathrm{CO}\to {\mathrm{D}}_2+\mathrm{DCO}$$ E₇ — E₆ = 0.9 kcal, is calculated from (5), (2), and the variation of the hydrogen‐deuterium equilibrium constant with temperature. Application of the Bigeleisen theory of the isotope effect to these reactions suggests a loosely bound activated complex.