Accurate Representations Of Long-range Interactions Between Ions And Linear Molecules

The study of reaction dynamics is a very important part of physical chemistry.For the reaction between small molecules in the gas phase,a pre-developed potential energy surface(PES)is a platform for dynamic calculations,while the accuracy of the PES is the bottleneck of the dynamic study.Since the attractive long-range electrostatic interactions between ions and molecules,the ion-molecule reactions are largely controlled by the barrierless entrance channel Therefore,special attention should be paid to that region in the construction of high accuracy PES of ion-molecule reaction.Generally,the PES is constructed by fitting a large number of potential single points,which is expensive and time-consuming,and the fitting error is hard to handle in the long-range regions.In order to descript long-range potential more accurate,the analytical representation of the long-range interaction between ions and linear molecules is sorted out and verified in this thesis,which could reduce the amount of calculations and avoids fitting errors in the construction of PES.This thesis is divided into three parts.The first part is Chapter One,which mainly introduces the research background and the used theoretical methods.Chapter Two is the second part,which mainly shows the representations of the long-range interactions between linear molecules and ions and verifies their correctness and applicability.Then in Chapter Three,the third part,the representations are used in two PESs.Chapter One is an introduction,including background of this project,the classification of intermolecular interactions,the theoretical calculation methods,and calculation programs used in this thesis.In Chapter Two,the representations of long-range interaction terms are discussed in details.In the beginning of the ion-molecule reaction,the interaction of reactants can be regarded as a two-body interaction between an ion and a neutral molecule.When the distance of the two moieties is far enough,the charge transfer can be ignored and the electrostatic interactions are dominated.Therefore,the long-range interactions between ions and molecules can be denoted as the electrostatic interaction terms.Fortunately,we have an accurate representations of the electrostatic potential between molecules.The ion can be approximately regarded as a point charge,and the interaction between ion and neural molecule is the interaction between point charge and molecular moment,which includes the permanent multipole moment and the induced multipole moment.Through the multipole expansion of electrostatic interaction between ion and molecule,the terms of long-range interaction are obtained,which include charge-dipole moment,charge-quadrupole moment,and charge-polarizability,and so on.In the other hand,MRCI-F12,icMRCI,UCCSD(T)-F 12b by Molpro program and ucMRCI by Xi’an-CI program are used to calculate the long-range potentials between C+-H2,N+-H2,O+-H2,H+-O2,H+-N2,S+-OH,C+-OH,NH+-H2,CH+-H2,H3+-CO.The ab initio calculation results are compared with those of analytical representations to verify their correctness and test the scope of application.For the ion-nonpolar molecular system,the error of the long-range interaction is less than 2cm-1 when the distance from ion to the center of mass of the molecule R is greater than 9A,and for the ion-polar molecular system,the error of the long-range interaction is less than 7cm(-1),when R is greater than 20A.Chapter Three is the application of the above potential analytical representations in the construction of PES of NH++H2 and H3+-CO reactions.