Semiclassical and Quantum Mechanical Calculations of Isotopic Kinetic Branching Ratios for the Reactionof O(³P) with HD

Abstract

S e m ic la s s ic a l a n d Q u a n tu m M e c h a n ic a l C a lc u la tio n s o f Is o to p ic K in e tic B ra n c h in g R a tio s f o r th e R e a c tio n o f 0 ( 3P ) w ith H D Dedicated to Professor Jacob Bigeleisen on the occasion of his 70th birthday Tunneling probabilities for the reactions O + HD ->OH + D and O + DH ->OD + H have been calculated by semiclassical dynamical methods and compared to accurate quantal calculations for the same potential energy surface. The results are used to test the reliability of variational transition state theory with the least-action semiclassical method for tunneling probabilities for the prediction of intramolecular kinetic isotope effects.

Tunneling in enzymatic and nonenzymatic hydrogen transfer reactions

“…The above argument justifies the approximation in a low‐ T limit, but because the approximation builds in the correct limits for both low‐ and high‐ T , it is useful over the entire range of T 31, 32. Further evidence that the approximation is reasonable is provided by the observation that low‐energy flux is concentrated (funneled) through the quantized transition states,18–21, 33 and the line shape and peak width in the density of reactive states is similar for states differing from the ground state by low‐frequency excitations 18–21…”

Current Status of Transition-State Theory

“…The O + H 2 reaction has also provided a prototype test case for VTST/MT. VTST/MT calculations have been tested successfully against accurate quantal dynamics, ,, and they have been very successful at interpreting experimental kinetic isotope effects. , O + H 2 ( v =1) → OH + H and OH( v =0,1) + H 2 ( v =0,1) → H 2 O + H, where v denotes a vibrational quantum number, have been used to test the extension of VTST concepts to predict state-selected rates for high-frequency mode excitation. ,− The first analysis of quantized transition-state structure in a cumulative reaction probability based on accurate quantal calculations 128 was reported for the O + H 2 system by Bowman, who noted the existence of structure due to a bend excited transition state. Most recently, this system provided the most fertile ground to date for analyzing the detailed state-to-state dynamics of a chemical reaction in terms of variational and supernumerary transition states observed in accurate quantum dynamics calculations …”

Ensemble‐averaged variational transition state theory with optimized multidimensional tunneling for enzyme kinetics and other condensed‐phase reactions

“…The largest number of coordinates that we have explicitly coupled in tunneling calculations is 171 [74]. Vibrationally and rotationally nonadiabatic processes are fully consistent with quantized transition states having good quantum numbers [75][76][77].…”