Theoretical Study On The Quantum Effect For The Low-temperature Dynamics

Dynamics process at low temperatures is easily affected by quantum effects such as zero point energy and quantum tunneling effect,and so it often shows different dynamic characteristics from high-temperature dynamics process.It can be helpful to explore the quantum effects in low-temperature dynamics process for understanding the dynamics mechanism of reactions at low temperatures.This thesis focuses on the theoretical investigation of three kinds of low-temperature dynamics processes which are all greatly affected by quantum effects,by using the canonical variational theory(CVT)with smallcurvature tunneling(SCT)contributions.We analyze the possible quantum effects and their effects on these dynamics process detailedly.Previous experimental studies have reported the significantly increasing reaction rate constant between methanol and OH with the decrease of temperature when the temperature is below 200 K,which then tends to be a constant at ultralow temperature.This unusual phenomenon has been believed to be related with the large quantum effect,but the dynamic mechanism is still unclear.In this thesis,we mainly concentrate on the low-temperature mechanism of methanol reaction with OH.We find a significant effect of the large anharmonicity of high-frequency modes of transition states on the lowtemperature rate constants;the quantum tunneling effect is significantly enhanced at low temperatures,and it depends on the stability of the pre-reaction complex which has a lower energy than the reactants.The completely stablized pre-reaction complex will lead to an extremely large tunneling coefficient,resulting in a significant negative temperature effect at low temperature.At ultralow temperatures,the capture rate for the formation of the complex is the dominant dynamical bottleneck,and therefore the reaction shows weak temperature dependence of the rate constants.Kinetic quantum sieving(KQS)is a promising separation technique for isotopic mixtures based on the difference of diffusion rate constants of isotopes caused by the different quantum effects in microporous materials.The detailed mechanism of how quantum effects affect the diffusion process is unclear yet.We propose an accurate and convenient way to calculate the quantum effects on diffusion with the example of H2 diffusion in zeolite RHO here.We reveal the opposite contribution of zero point energy and quantum tunneling effect.Deuterium has a lower ZPE,which leads to a smaller effective barrier for tunneling because the transition state has a larger ZPE than the precursor stable state;this results in an inverse KIE.However,quantum tunneling would lead to a normal kinetic isotopic sieving effect,in which lighter diprotium diffuses faster than dideuterium.Due to the large mass ratio between muonium(Mu)and H(1:9),calculations of kinetic isotope effects for muonium reactions provide challenging tests of quantum effects on reaction rates.There is a large discrepancy between experiment and theory for the reaction of Mu with C3H8,which reveals the unusual quantum effect mechanism in this reaction system.We find that the vibrational anharmonicity of this muonium reaction is significant and depends greatly on the reaction coordinate,which decreases both the height and width of the vibrationally adiabatic potential barrier,with both effects increasing the rate constants.