Isotope Exchange vs Abstraction for H + DCl

Rates of isotope exchange reactions in deuterium chloride—hydrogen mixtures photolyzed at 1849 Å have been measured by mass spectroscopy. Under conditions where reactions of product molecules were negligible quantum yields of HD and D₂ were calculated using DCl for actinometry. Large HD quantum yields indicate that chain reactions occur. The ratio of HD to D₂ formed in these chain reactions is shown to be equal to the ratio k₂/k₃ of rate coefficients for the abstraction (2), $\mathrm{H}+\mathrm{DCl}\to \mathrm{HD}+\mathrm{Cl}$ , and the exchange (3), $\mathrm{H}+\mathrm{DCl}\to \mathrm{HCl}+\mathrm{D}$ . From quantum yields at 27–225°C this ratio is calculated to be $k_2{\mathrm{/k}}_3\text{=(850 ± 200)\hspace{0.17em}}\exp \text{[ − (2220 ± 180) / RT]}$ . A quantum yield of D₂ independent of chain lengths and with little temperature dependence is attributed to the reaction ${\mathrm{D}}^{*}+\mathrm{DCl}\to {\mathrm{D}}_2+\mathrm{Cl}$ of hot atoms from DCl photodissociation.