Quantum Dynamics Study Of The OH+HBr/DBr And CH₃+HBr Reactions

Till now,there have been no quantum dynamics studies on the OH+HBr/DBr and CH₃+HBr reaction.In addition,for the kinetic isotope effect of k?OH+HBr?/k?OH+DBr?,the experimental measurements and quasi-classical trajectory calculations are inconsistent.Thus,we carry out the first,quantum dynamics approach to investigate the energy requirement on reactivity for the three reactions and the kinetic isotope effect of k?OH+HBr?/k?OH+DBr?.We also study the O+CD₄ reaction by using the mixed quantum-classical reaction dynamics.A time-dependent,quantum reaction dynamics approach in full dimensional,six degrees of freedom?6DOF?was carried out to study the energy requirement on reactivity for the HBr+OH reaction with an early,negative energy barrier.The calculation shows both the HBr and OH vibrational excitations enhance the reactivity,whereas the rotational excitations of both the HBr and OH hinder the reactivity.On the basis of equal amount of total energy,the vibrational energies of both the HBr and OH are more effective in enhancing the reactivity than the translational energy.This also indicates,even this reaction has a negative energy barrier,the calculation shows not all forms of energy are equally effective in promoting the reactivity.The rate constants were also calculated for the temperature range between 5 to 500 K.The quantal rate constants have a better slope agreement with the experimental data than quasi-classical trajectory results.The experimental measurements of the kinetic isotope effect of k?OH+HBr?/k?OH+DBr?,found that the primary kinetic isotope effects are temperature-independent.Previous quasi-classical trajectory calculations on an accurate,ab initio potential energy surface show that the primary kinetic isotope effect is temperature-dependent.In contrast,the present full-dimensional,time-dependent quantum dynamics calculations on the same potential energy surface find the primary kinetic isotope effect is temperature-independent,agreeing well with the former two experimental studies both qualitatively and quantitatively.Furthermore,the rate constants from both quantum dynamics and quasi-classical trajectory calculations have a peak around 15 K whereas the experimental data is not available in this low temperature range?T<53 K?.The good agreement of the temperature-dependence of primary kinetic isotope effects between the present quantum dynamics calculations and experimental measurements indicates that the primary kinetic isotope effect of k?OH+HBr?/k?OH+DBr?should be temperature-independent and the peak of the rate constants from the theoretical calculations call for experimental measurements at a very low temperature range.For the CH₃+HBr?Br+CH₄ reaction,a time-dependent,reduced six-degrees-of-freedom,quantum reaction dynamics wavepacket approach is employed to investigate the impacts of the translational,vibrational,and rotational motion using the Czakópublised a high-quality,full dimensional ab initio potential energy surface.The initial state selected integral cross sections show that the vibrational excitations of HBr greatly enhance the reactivity with the reaction probabilities.However,the vibrational excitations of CH₃ hinder the reactivity.And,rotational excitations of both the HBr and CH₃ hinder the reactivity.We also calculated the rate constants using the reduced 6DOF model.The reslut shows our quantum results are disagreement with the experimental data and quasi-classical trajectory results,when the temperature is higher 350 K.For the current unsatisfactory temperature rate constants using the6DOF model,we believe that the errors caused by the frequency of the harmonic oscillator in the transition state are used to calculate the vibration partition function in the process of converting the 6DOF to the 12DOF cumulative reaction probability.We also developed a mixed quantum-classical reaction dynamics,and tested the O+CD₄reaction.The results show that the reaction probability obtained by the mixed quantum-classical reaction dynamics is similar with the quantum results.However,the mixed quantum-classical reaction dynamics's reaction probability is larger than the quantum's.In addition,we find that the mixed quantum-classical reaction dynamics's threshold is lower than the quantum calculation,and this is unreasonable.Due to the existence of tunneling effect,the quantum result shows a reaction threshold of about 0.4 eV,which is lower than the ground-state adiabatic height 0.51 eV.However,it is impossible that the mixed quantum-classical reaction dynamics method can reduce reaction threshold than the quantum method.We speculate that the errors caused by the current 6DOF model.