Application of Electrostatic Potential on Molecular Surface

209 molecules of persistent organic pollutant polybrominated diphenyl ethers (PBDEs) were optimized at the HF 6-31G* level, and the molecular electrostatic potential and its derived parameters were calculated on the basis of the optimized structure. The chromatographic retention time (RRT), n-octanol air partition coefficient (lgKOA) and vapor pressure of 298K ultracold fluid (lgpL) of PBDEs were correlated with molecular structure parameters using multiple linear regression methods. The results show that the molecular surface electrostatic potential parameters combined with the number of bromine atoms substituted on the benzene ring combined with the molecular volume can better express the quantitative relationship between the physical and chemical properties of PBDEs and their molecular structure. The three QSPR models established are related to cross-validation. The coefficients (Rcv) are 0.9819, 0.9911, and 0.9963, respectively, and the standard deviation (SD) is 0.0424, 0.1384, and 0.1020, respectively, indicating that the three models have strong robustness and predictive ability. It also proves that the molecular electrostatic potential parameters are in the PBDE The applicability of the QSPR study of similar compounds.

Calculation method

Constructed with VEGA software and used its built-in AM1 semi-empirical quantum chemistry method to optimize the initial geometric parameters of each polybrominated diphenyl ether compound. After that, Gaussian03 was used to further optimize each molecule at the HF6-31G* level, and on the basis of the optimized structure, the structure parameters were extracted and the electrostatic potential was calculated. The calculation of the electrostatic potential adopts the grid point method, and the precision of the cube grid is set to Cube = 100. In this way, for each molecule, nearly 1003 points of electrostatic potential can be obtained. Then, statistical analysis is performed on the electrostatic potential of these points to obtain the relevant structural parameters. Among the calculated parameters related to this study: Vmin is the most negative electrostatic potential in the molecular space.

The dispersion of the molecular surface electrostatic potential, where s represents the molecular surface, n is the number of points calculated for the molecular surface electrostatic potential, V(ri) is the electrostatic potential at the point ri on the molecular surface, and Vs represents the average electrostatic potential on the molecular surface.

In addition, the physical and chemical properties of a molecule are usually closely related to its molecular weight, surface area and volume. The number of substitutions of bromine atoms on the benzene ring directly affects these properties. For this reason, the number of substitutions (NBr) of bromine atoms on the two benzene rings is also used as a structural parameter. Finally, using the multiple linear regression analysis module in the statistical software STATISTIC5.5, the quantitative relationship between the molecular structure and physical and chemical properties of polybrominated diphenyl ether compounds was established.

Surface electrostatic potential parameters and the number of bromine atoms substituted

Regardless of whether the number of bromine atoms substituted is large or small, there is always a relatively negative electrostatic potential at the oxygen atom of the ether bond. When the number of substitutions of bromine atoms in the molecule is small, there is a negative electrostatic distribution on the upper and lower sides of the benzene ring plane, and the closer to the center of the ring, the more negative the electrostatic potential; as the number of substitutions of bromine atoms increases, the static The potential becomes more and more positive. When all the hydrogen atoms on the benzene ring are replaced by bromine atoms (that is, decabromodiphenyl ether, PBDE209), the region already has a relatively positive electrostatic potential. For the entire molecule, the positive electrostatic potential is mainly distributed near the edge hydrogen atoms of the benzene ring plane, and the surface electrostatic potential is distributed axially symmetrically along the C—Br bond, and the negative electrostatic potential is distributed on both sides of the axis, and on the axis However, there is a relatively positive electrostatic potential at the top. This feature becomes more and more obvious with the increase in the number of bromine atoms substituted. It reflects the amphiphilic characteristics of halogen atoms, which means that the bromine atoms here in addition to act as traditional The electron donor in the sense of the body can also act as an electron acceptor and interact with other electron donors. All other types of structural parameters are difficult to consider and clarify this interaction. In recent years, the electrophilicity of halogen atoms has attracted widespread attention. When the number of substitutions of bromine atoms is the same but the positions are different, the distribution of the electrostatic potential has also changed significantly, but because of the isomers with different substitution positions of the bromine atoms, the surface electrostatic potential parameters can still better distinguish these isomers. To distinguish them, such as 12 dibromodiphenyl ethers, the ΣVs+ value spans from 23.029 to 27.567eV. The surface electrostatic potential parameters of each isomer of other PBDEs also have different degrees of difference. Whether the structural parameters can distinguish the compounds under study is an important prerequisite to ensure that they can be correctly applied to QSPR research.

Quantitative relationship between molecular structure parameters

Except for the parameter NBr, the other parameters introduced in the equation are all molecular surface electrostatic potential parameters. This shows to a certain extent that the surface electrostatic potential has a certain applicability in QSPR research. Vmin reflects the contribution of a molecule\'s electrostatic hydrogen bond, and reflects the ability of the molecule to accept protons to form hydrogen bonds (i.e. hydrogen bond basicity). The smaller the value (the greater the absolute value), the stronger the ability to accept protons to form hydrogen bonds. . Theoretically, the oxygen atoms in the PBDEs molecule and the π electrons on the benzene ring can accept protons to form hydrogen bonds, but because hydrogen bonds are weak and require a certain degree of directionality, the formation of hydrogen bonds may affect the closeness between molecules Arrangement, on the contrary, reduces the total intermolecular interaction, making Vmin and lgpL negatively correlated. ∑Vs+ is a recently proposed surface electrostatic potential parameter. Its summation starts from the most positive electrostatic potential, and an empirical threshold value 0.21nm is introduced in the calculation (summation) of ∑Vs+. This empirical value is between Between the van der Waals radius (0.14nm) and the diameter (0.28nm) of the water molecule, when the distance between two points on the surface of the molecule is less than 0.21nm, only the positive ∑Vs+ is added. In QSPR with multiple physical and chemical properties After the introduction of ∑Vs+, the quality of the equations has been significantly improved.

Electrostatic potential distribution on the surface of polybrominated diphenyl ethers

Describe the uniformity of the electrostatic potential distribution on the surface of the molecule, also known as the degree of charge separation. The larger the value, the more uneven the charge distribution on the surface of the molecule. This parameter is representative of a series of molecular surface electrostatic potential parameters proposed by Murray et al., and its superiority and universality have been confirmed by a large number of related QSRP studies. It shows that the degree of charge separation has a certain influence on the physical and chemical properties of PBDEs, and the more uneven the charge distribution of the molecule, the longer its retention time in the chromatographic column.

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