Molecular cluster structure determination method based on molecular collision cross section closure

Abstract

The invention discloses a molecular cluster structure determination method based on molecular collision cross section closing. The method comprises the following steps: modeling a molecular cluster structure, calculating and solving a self-consistent field equation by using a quantum chemistry method, and performing configuration optimization search to obtain several stable structures of the molecular cluster; carrying out electromobility simulation on the stable structure of the molecular cluster, and solving an electromobility and a collision cross section (CCS); the stable structure of the molecular cluster is subjected to a DMA-MS determination analysis experiment, chemical components and electromobility information of the cluster are synchronously obtained, and the electromobility obtains the cluster CCS through theoretical calculation; by comparing and analyzing simulation and experiment results, accurate molecular cluster structure information can be obtained. According to the method, the problem that the consistency of a simulated cluster structure calculated by a quantum chemistry method and an actual measurement result is difficult to examine is solved, and compared with a molecular cluster optimal simulated structure calculated by using a single quantization method, the reliability of the result is improved.

Description

Technical Field

[0001] The invention belongs to the research field of cluster configuration search, and relates to quantum chemical calculation, electromigration simulation and DMA-MS system measurement and analysis of molecular cluster systems, and in particular to a method for determining molecular cluster structure based on molecular collision cross section closure. Background Art

[0002] Quantum chemistry is a method of studying chemical molecular systems by applying the basic principles and methods of quantum mechanics. This method describes and predicts various chemical properties such as the structure and energy of molecules by solving the Schrödinger equation or approximate methods. Quantum chemistry plays an important role in studying the structure, reactivity and interaction of atoms and molecules. In the past few decades, the density functional theory (DFT) developed by Kohn-Sham has become the most commonly used method for molecular structure calculation. It uses electron density instead of wave functions to describe the physical and chemical properties of molecules such as structure, energy, and electron distribution by solving the Kohn-Sham equation. At present, density functional theory has a wide range of applications in studying the structure and properties of molecules in chemical systems, and plays an important role in fields such as environmental chemistry and materials science. However, although quantum chemical calculations can obtain cluster structures, it is difficult to verify the consistency between the simulated structures and the measured results.

[0003] Mass spectrometry is an important tool for studying molecular clusters. It is used to measure the chemical composition information of molecular clusters, but it cannot obtain the structural information of clusters. The differential mobility analyzer (DMA) is the only known spatially dispersed ion mobility spectrometer that can measure true mobility and has high transmission efficiency and screening resolution. Because different charged clusters have different charge-to-mass ratios, spatial structures and collision cross sections, they have different migration speeds. After being acted on by the electric field, they are separated and reach the collecting electrode one by one to complete the screening process. In this way, DMA can screen out a stable ion flow with a specific mobility, and use the mobility determined by separation to infer and calculate the collision cross section of the charged cluster. Using DMA in series with mass spectrometry (MS) can simultaneously have the ability of electromigration particle size screening and chemical component analysis, thereby realizing real-time synchronous measurement of ion mobility and mass-to-charge ratio.

[0004] The collision cross section (CCS) is a key parameter of molecular cluster structure, which is used to evaluate the interaction between molecular clusters and free gas molecules. The complexity of this interaction is affected by many factors, including the size, mass, relative velocity of gas molecules and charged nanoparticles, and the potential interaction between them. These particles are constantly moving irregularly and colliding with each other. The characteristics of the charged molecular clusters to be measured are closely related to the collision effects between the carrier gas molecules they carry. In order to effectively calculate the CCS of molecular clusters, a variety of algorithms have emerged in the research field, such as projection approximation (PA), elastic / diffuse hard sphere scattering (EHSS / DHSS), and trajectory method (TM). Through quantitative simulations consistent with experimental measurements, researchers can verify the accuracy of the simulated molecular cluster structure and thus determine the reliability of the simulation results. Summary of the invention

[0005] The purpose of the present invention is to address the deficiencies of the prior art and to propose a method for determining the structure of molecular clusters based on the closure of molecular collision cross sections.

[0006] To achieve the above object, the specific technical solution adopted by the present invention is as follows: a method for determining the structure of a molecular cluster based on the closure of a molecular collision cross section is provided, comprising the following steps:

[0007] Model the molecular cluster structure, use quantum chemical methods to calculate and solve the self-consistent field equation, perform configuration optimization search, and obtain multiple stable structures of the molecular cluster as candidates;

[0008] Conducting an electromigration simulation on the stable structure of the molecular cluster to solve the electromigration rate and the cluster CCS;

[0009] Conducting an electric mobility analyzer-mass spectrometry system measurement and analysis experiment on the molecular clusters to obtain the chemical composition and electric mobility information of the clusters, and calculating the cluster CCS based on the electric mobility;

[0010] The cluster CCS obtained by electromigration simulation is compared and verified with the cluster CCS obtained by experiment, and the closest stable structure is screened to finally obtain the accurate structure of the molecular cluster.

[0011] Furthermore, the quantum chemical method is a density functional theory method wB97XD / 6-311G+(2df,p).

[0012] Furthermore, the molecular cluster carries at least one charge.

[0013] Furthermore, the electromobility simulation to solve the electromobility is specifically:

[0014] The trajectory method is used to simulate the trajectories of different gas molecules. tThe total time τ required for gas molecules to collide with cluster particles t , calculate the total resistance F D :

[0015]

[0016] Where m is the mass of the gas, M is the mass of the molecular cluster, and g f and g i are the initial and final relative velocities of the gas molecules, respectively;

[0017] Solve for the electrical mobility K:

[0018]

[0019] where ze is the charge of the ion, F D is the total resistance, v d When the electric field force and the total resistance F D The final drift velocity reached by the cluster velocity when equilibrium is reached;

[0020] The calculation is repeated for three perpendicular directions and the average value is taken to obtain the final electrical mobility.

[0021] Furthermore, the electromigration simulation to solve CCS is specifically as follows:

[0022] By substituting the final electrical mobility into the calculation, we get:

[0023]

[0024] Where K1 is the electrical mobility, ze is the charge of the ion; n is the gas density; μ = mM / (m+M) is the reduced mass, where m is the mass of the gas and M is the mass of the molecular cluster; k b is the Boltzmann constant; T eff is the effective temperature, calculated as: It is the cluster CCS.

[0025] Furthermore, the trajectory method uses LJ theory to calculate the interaction potential energy between molecules to simulate the trajectories of different gas molecules:

[0026]

[0027] Among them, r i =(x i, y i ,z i ) is the relative distance between each atom or charge and the gas molecule; α is the polarizability of the buffer gas; ε and σ are the Lennard-Jones gas-atom parameters, ε represents the depth of potential energy, σ represents the zero potential energy crossing point; ze is the charge of the ion, and n is the gas density.

[0028] Furthermore, before the molecular clusters are subjected to an electric mobility analyzer-mass spectrometer system measurement and analysis experiment, the method further includes: using electrospray to ionize and introducing the clusters into a flow tube.

[0029] Furthermore, the electric mobility analyzer-mass spectrometer system uses nitrogen as sheath gas.

[0030] In the above-mentioned method for determining the structure of molecular clusters based on the closure of molecular collision cross sections, step 3 involves a DMA-MS coupling system. DMA can screen out a stable ion flow with specific mobility, and only aerosols with specific electrical mobility can pass through the slits of the electrode plates into the downstream, thereby obtaining an approximately monodisperse aerosol beam. Based on the high-resolution selective screening and counting of ion electrical mobility, it can be efficiently coupled with a mass spectrometer (MS) to realize real-time synchronous measurement of ion electrical mobility and mass-to-charge ratio.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] High-level quantum chemical calculations are used to obtain the optimal simulation structure of molecular clusters. The CCS obtained by theoretical calculation of the simulation structure is compared with the experimental measurement value. Compared with the optimal simulation structure of molecular clusters obtained by using a single quantitative method, the reliability of the results is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a schematic diagram of the structural process of determining molecular clusters based on the CCS closing method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0034] The present invention provides a method for determining the structure of a molecular cluster based on the closure of a molecular collision cross section. The method involves quantitative simulation of charged molecular clusters, and the optimized stable cluster structure is subjected to an electric mobility simulation calculation to obtain its collision cross section (CCS). At the same time, a DMA-MS system is used to perform a synchronous determination and analysis experiment of chemical composition and electric mobility on the molecular cluster to obtain the collision cross section (CCS) obtained by experimental measurement. By comparing and analyzing the simulation and experimental results, accurate molecular cluster structure information can be obtained.

[0035] In order to make the implementation of the method of the present invention more obvious and easy to understand, the technical scheme in the embodiment of the present invention will be clearly and completely described below in combination with the flow chart in the present invention. Obviously, the embodiment described below is only a part of the embodiment of the present invention, not all of the embodiments. Based on the embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] Please refer to Figure 1 , Figure 1 A schematic diagram of a process for determining a molecular cluster structure based on a CCS closure method provided in an embodiment of the present application, the method comprising the following steps:

[0037] Step 1: Model the molecular cluster structure, and use Guassian software to calculate and solve the self-consistent field equation using quantum chemical methods, perform configuration optimization search, and finally obtain several most stable candidate structures of the molecular cluster;

[0038] In a specific embodiment, a suitable initial cluster configuration is selected as the starting point of the search, which may be a randomly generated structure or a structure derived from previous experiments or theoretical studies.

[0039] The initial structure is optimized for geometry, and the most stable configuration is found by using the density functional theory method in Gaussian software (wB97XD / 6-311G+(2df,p) is used in this example). The optimization process adjusts the position and configuration of atoms to achieve the lowest energy state.

[0040] After each configuration optimization, the energy of the cluster is calculated, and finally several configurations with the lowest energy are selected as the stable configurations of the cluster.

[0041] Step 2: Combine the trajectory method (TM) and Lennard-Jone (LJ) theory to simulate the electromobility of the found stable cluster structure and solve the electromobility and collision cross section (CCS) of the cluster structure;

[0042] According to the first approximation of the Mason-Schamp two-temperature theory, the steps for calculating the electrical mobility and collision cross section (CCS) are as follows:

[0043] First, calculate the electrical mobility K1 and collision cross section The formula is:

[0044]

[0045] Where ze is the charge of the ion, n is the gas density, μ = mM / (m+M) is the reduced mass, where m is the mass of the gas, M is the mass of the cluster particle, and k b is the Boltzmann constant, T eff is the effective temperature, is the collision cross section (CCS). The effective temperature takes into account the influence of strong field effects on the system and is calculated as: In most cases, i.e. when the electric field is low, the second term of Teff accounts for less than 1% of the calculation.

[0046] Electrical mobility and collision cross section The calculation of the momentum exchange or drag, that is, the degree to which the buffer gas restricts the motion of charged cluster particles that are accelerated by the presence of the electric field, is involved.

[0047] When calculating momentum exchange, the trajectory method (TM) is used to simulate the trajectories of different gas molecules and calculate the momentum transfer Δp of each gas molecule. The formula is:

[0048] Δp=m(g f -g i )

[0049] where g f and g i are the initial and final relative velocities of the gas molecules, respectively. Next, by considering the impact of reducing the mass on the algorithm, and according to N t The total time τ required for gas molecules to collide with cluster particles t , calculate the total resistance F D :

[0050]

[0051] The electric field causes the charged cluster particles to move. When the electric field force and the total resistance F D When equilibrium is reached, the cluster velocity reaches the final drift velocity v d , the total resistance F D With drift speed v d By combining the relationship between the electric field E and the charge ze, the electric mobility K can be solved, that is:

[0052]

[0053] Finally, the calculation is repeated for the three perpendicular directions and the average is taken to obtain the final electrical mobility. The CCS calculation can be done by the first approximation of the Mason-Schamp dual temperature theory.

[0054] Specifically, the trajectory method (TM) needs to consider the interaction potential between molecular clusters and gas molecules, which involves using the Lennard-Jone (LJ) theory to calculate the attraction and repulsion between two molecules. In the Lennard-Jones (LJ) theory, the attraction and repulsion between molecules are calculated by the potential energy function Φ(x, y, z), which describes the interaction between molecules in the form of:

[0055]

[0056] Among them, r i =(x i, y i ,z i) represents the relative distance between each atom (or charge) and the gas molecule, α represents the polarizability of the buffer gas, ε and σ are Lennard-Jones (LJ) gas-atom parameters, ε represents the depth of potential energy, describing the strength of attraction, and σ represents the zero potential energy crossing point, indicating the minimum interaction distance between molecules. It should be noted that each different gas requires different LJ parameters.

[0057] The Lennard-Jones potential function is used to calculate the acceleration of the gas molecules and their trajectories until the gas molecules leave the domain, and the deflection angle of each gas molecule is recorded. i , as well as considering parameters such as polarizability α, potential energy depth ε, and zero crossing point σ, the interaction potential energy between molecules can be determined, thereby deriving the collision cross section (CCS) considering the interaction between molecular clusters and gas molecules.

[0058] Step 3: Conduct electrospray / flow tube + DMA-MS analysis experiments on the studied clusters to simultaneously obtain the chemical composition and electrical mobility information of the clusters, where the electrical mobility of the molecular clusters can be obtained by theoretical calculation to obtain the cluster CCS.

[0059] In the experiment, the reactants are first ionized by electrospray, and then these ions are introduced into the flow tube for reaction. After a period of time, the number of charged molecular clusters under study continues to increase until it reaches a detectable level. By controlling the concentration of precursors and temperature, the generation rate of different molecular clusters can be obtained for studying different clusters.

[0060] In this embodiment, when a DMA-MS system is used for experimental determination, nitrogen is used as the sheath gas to ensure that the experimental sample does not contain other impurities.

[0061] DMA can be used to select molecular clusters of a specific size. Its working principle is: in DMA, an electric field is applied so that the charged molecular clusters are moved toward the electrode in the gas under the action of the electric field force. At the same time, the charged molecular clusters will be affected by the gas resistance when moving in the gas, and the size of the resistance is related to the particle size. When the electric field force and the gas resistance reach a balance, the particles will remain at a specific speed and fall in a fixed position, which depends on the size of the charged molecular cluster. After molecular clusters of different compositions enter the electric field, only molecular clusters with a specific electrical mobility can enter the downstream through the slits of the electrode plates, thereby achieving the separation of charged molecular clusters. The determined electrical mobility can also be used to determine the collision cross section of a specific molecular cluster by theoretical calculation.

[0062] Finally, these charged molecular clusters are introduced into a mass spectrometer (MS) for analysis to obtain the chemical composition information of the molecular clusters.

[0063] Step 4: Compare and verify the CCS of several stable clusters obtained by quantitative simulation with the cluster CCS measured by experimental analysis to determine the most likely accurate structure of the molecular cluster.

[0064] In summary, the present invention proposes a method for determining the structure of molecular clusters based on the closure of the molecular collision cross section (CCS). Compared with the optimal simulation structure of molecular clusters obtained by calculating using a single quantitative method, the reliability of the results is improved by adding verification of experimental measurement results.

[0065] For those skilled in the art, it is understood that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present disclosure. Therefore, from any point of view, the embodiments should be regarded as instructive and non-restrictive, and all modifications, equivalent substitutions, improvements, etc. within the spirit and principles of the present disclosure should be included in the protection scope of the present disclosure.

Claims

1. A method for determining the structure of molecular clusters based on molecular collision cross section closure, characterized in that: The following steps are involved: Model the molecular cluster structure, use quantum chemical methods to calculate and solve the self-consistent field equation, perform configuration optimization search, and obtain multiple stable structures of the molecular cluster; Conducting an electromigration simulation on the stable structure of the molecular cluster to solve the electromigration rate and the cluster CCS; Conducting an electric mobility analyzer-mass spectrometry system measurement and analysis experiment on the molecular clusters to obtain the chemical composition and electric mobility information of the clusters, and calculating the cluster CCS based on the electric mobility; The cluster CCS obtained by electromigration simulation is compared and verified with the cluster CCS obtained by experiment, and the closest stable structure is screened to obtain the accurate structure of the molecular cluster.

2. The method for determining the molecular cluster structure based on molecular collision cross section closure according to claim 1, characterized in that: The quantum chemical method is a density functional theory method wB97XD / 6-311G+(2df,p).

3. The method for determining the molecular cluster structure based on molecular collision cross section closure according to claim 1, characterized in that: The molecular cluster carries at least one charge.

4. The method for determining the structure of molecular clusters based on molecular collision cross section closure according to claim 1, characterized in that: The electromobility simulation to solve the electromobility is specifically: The trajectory method is used to simulate the trajectories of different gas molecules. t The total time τ required for gas molecules to collide with cluster particles t , calculate the total resistance F D : Where m is the mass of the gas, M is the mass of the molecular cluster, and g f and g i are the initial and final relative velocities of the gas molecules, respectively; Solve for the electrical mobility K: where ze is the charge of the ion, F D is the total resistance, v d When the electric field force and the total resistance F D The final drift velocity reached by the cluster velocity when equilibrium is reached; The calculation is repeated for three perpendicular directions and the average value is taken to obtain the final electrical mobility.

5. The method for determining the molecular cluster structure based on molecular collision cross section closure according to claim 4, characterized in that: The electromigration simulation to solve CCS is specifically as follows: By substituting the final electrical mobility into the calculation, we get: Where K1 is the electrical mobility, ze is the charge of the ion; n is the gas density; μ = mM / (m+M) is the reduced mass, where m is the mass of the gas and M is the mass of the molecular cluster; k b is the Boltzmann constant; T eff is the effective temperature, calculated as: It is the cluster CCS.

6. The method for determining molecular cluster structure based on molecular collision cross section closure according to claim 4, characterized in that: The trajectory method uses LJ theory to calculate the interaction potential energy between molecules to simulate the trajectories of different gas molecules: Among them, r i =(x i, y i ,z i ) is the relative distance between each atom or charge and the gas molecule; α is the polarizability of the buffer gas; ε and σ are the Lennard-Jones gas-atom parameters, ε represents the depth of potential energy, σ represents the zero potential energy crossing point; ze is the charge of the ion, and n is the gas density.

7. The method for determining molecular cluster structure based on molecular collision cross section closure according to claim 1, characterized in that: Before the molecular clusters are subjected to an electric mobility analyzer-mass spectrometer system measurement and analysis experiment, the method further includes: using electrospray to ionize and introducing the clusters into a flow tube.

8. The method for determining molecular cluster structure based on molecular collision cross section closure according to claim 1, characterized in that: The electric mobility analyzer-mass spectrometer system uses nitrogen as sheath gas.

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