Temperature-dependent kinetic isotope effects in the gas-phase reaction: OH+HBr

Citation
Vi. Jaramillo et Ma. Smith, Temperature-dependent kinetic isotope effects in the gas-phase reaction: OH+HBr, J PHYS CH A, 105(24), 2001, pp. 5854-5859
Citations number
30
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF PHYSICAL CHEMISTRY A
ISSN journal
1089-5639 → ACNP
Volume
105
Issue
24
Year of publication
2001
Pages
5854 - 5859
Database
ISI
SICI code
1089-5639(20010621)105:24<5854:TKIEIT>2.0.ZU;2-A
Abstract
The temperature dependence of the hydrogen transfer rate coefficients for t he reactions: OH + HBr (Reaction I), OD + HBr (Reaction 2), OH + DBr (React ion 3), and OD + DBr (Reaction 4) have been investigated at temperatures be tween 120 and 224 K using a pulsed uniform supersonic flow monitoring hydro xyl reactive loss. The lack of observed isotopic scrambling indicates the r eaction occurs by H/D atom transfer from HBr/ DBr at all temperatures. The rate coefficients demonstrate little temperature dependence above 200 K, bu t strong inverse temperature behavior below 200 K. The current work provide s unequivocal experimental evidence of temperature dependent and inverse pr imary and secondary kinetic isotope effects (k(H)/k(D) < 1) at low temperat ures. The observed kinetic isotope ratios, k(H)/k(D), at 120 K are for prim ary substitution on HBr; k(1)/k(3) = 1.00 (+/-0.17) and k(2)/k(4) = 0.46 (/-0.08) while for secondary substitution on OH; k(1)/k(2) = 0.94 (+/-0.20) and k(3)/k(4) = 0.43 (+/-0.05). At the lowest temperature employed (120 K), deuterated reactants react as fast or faster than their natural hydrogen i sotopomer and there is no significant difference between the primary and se condary kinetic isotope effect. The results are discussed within the framew ork of recent theoretical models.