Effects of isotopic substitution and vibrational excitation on reaction rates. Kinetics of OH(ν= 0, 1) and OD(ν= 0, 1) with HCl and DCl

Abstract

A pulsed photolysis, laser-induced fluorescence method has been used to study the kinetics of collisional processes between OH(ν= 0), OH(ν= 1), OD(ν= 0), OD(ν= 1) and HCl, DCl. The following rate constants (cm³ molecule^–1 s^–1) have been measured at 298 ± 4 K: OH(ν= 0)+ HCl: k₁=(6.8 ± 0.25)× 10^–13(1), OH(ν= 0)+ DCl: k₂=(1.1 ± 0.1)× 10^–13(2), OD(ν= 0)+ HCl: k₃=(4.4 ± 0.6)× 10^–13(3), OD(ν= 0)+ DCl: k₄=(1.3 ± 0.2)× 10^–13(4), OH(ν= 1)+ HCl: k^*₁=(9.7 ± 1.0)× 10^–13(1*), OD(ν= 1)+ DCl: k^*₄=(2.9₅± 0.7)× 10^–13(4*). The errors quoted correspond to two standard deviations and the rate constants k^*₁ and k^*₄ are for total removal of the vibrationally excited radicals, i.e. for reaction plus relaxation. Isotopic scrambling between the photochemical precursor of the radical and the molecular chloride (i.e. between HNO₃ or H₂O and DCl, or DNO₃ or D₂O and HCl) has prevented the accurate measurement of rate constants for removal of OH(ν= 1) by DCl and OD(ν= 1) by HCl, although an approximate value of k^*₂=(1.2 ± 0.4)× 10^–13 cm³ molecule^–1 s^–1 has been obtained for the former process.

The magnitude of the primary kinetic isotope effects, i.e.(k₁/k₂)= 6.2 ± 0.6 and (k₃/k₄)= 3.4 ± 0.7 are shown to be larger than might be expected on the basis of standard transition-state theory calculations, and (k₁/k₂) is also greater than has been found in two previous investigations. Reasons for these findings, and for the lack of enhancement of the reaction rate with excitation of the attacking radical, are discussed.