Effect of changing reagent energy on reaction dynamics. XI. Dependence of reaction rate on vibrational excitation in endothermic reactions HX(vreag)+Na→H+NaX(X≡F,Cl)

Abstract

The chemiluminescence depletion (CD) method has been applied to two endothermic reactions to obtain approximate relative rates of reaction k_endo(v_reag) out of specified reagent vibrational levels v_reag for a range of vibrational energies V_reag extending to well above the energy barrier Q. The reactions were (1) HCl(v_reag = 1–4)+Na→H+NaCl(Q = −4.2 kcal mole⁻¹, Q_c = −10 kcal mole⁻¹) and (2) HF(v_reag = 1–5)+Na→H+NaF (Q = −12 kcal mole ⁻¹, Q_c = −18 kcal mole ⁻¹). The major finding is that for high V_reag, with over 90% of the reagent energy present as vibration, the collision‐efficiency for both reactions is approximately unity. From this we conjecture that the barrier crest is ’’late,’’ with little extension into the entry valley (type IIS). The reactive cross sections exceed by an order of magnitude the (large) cross section of several Ų obtained in part VI for endothermic reactions HX+Y(e.g., HF+Cl). The salient difference is likely to be the fact that the present reactions involve a switch from covalent to ionic bonding. To within experimental error the same absolute vibrational energy in either HCl or HF gives rise to the same absolute cross section for reaction with Na.