Theoretical Study Of SERS Spectra Of P-fluoroaniline And Its Derivatives On Silver Surface

Recently,surface-enhanced Raman scattering（SERS）research has rapidly developed,and it is widely used in the fields of physics,chemistry,biomedicine,and mineral surveying.SERS can enhance the Raman signal by more than 10⁶orders of magnitude.Further,it can accept many inorganic and organic molecules as Raman analytes,and the analytes can be solid,liquid,or gas,and the reactants are not damaged during the experiment.During the experiment,solvent water,fluorescent agent,etc.,do not influence the Raman scattering result.In addition to these advantages,the characteristic peaks of SERS can be used to analyze the type of functional group of the substance in the experiment,the composition of the derivative and the change of the experimental environment,etc.,SERS technology has played a pivotal role in the process of material analysis.Aniline derivatives are widely used in the fields of medicine and industry.They are very important to people’s production and life.Therefore,the development of SERS spectroscopy studies of aniline derivatives is of practical significance to further clarify the structure-activity relationship of the derivatives.In this paper,p-fluoroaniline（PFAN）is selected as the research object,and the experimental SERS spectral information is combined with the calculation results of density functional theory（DFT）.Through theoretical simulation and comparative analysis,the relationship between the characteristic peak information of the Raman spectrum of PFAN and the interaction with silver nanoparticles（Ag NPs）under SERS conditions was explored.This article mainly introduces two aspects.In the first part of the work,we tested the Raman and SERS spectra of PFAN and PFAN-Ag with a 532 nm excitation light source,and the results showed that PFAN-Ag has a distinct characteristic peak at 1433 cm^-1;the DFT method is used to calculate and simulate their ground state geometric structure and Raman vibration spectrum and the analysis of the ground state geometric structure shows that the vibration of the benzene ring has nothing to do with the characteristic peak.This characteristic peak is mainly derived from the vibration of the nitrogen-nitrogen double bond generated by the coupling of PFAN to 4,4’-difluoroazobenzene（DFAB）under SERS conditions;furthermore,the frontier molecular orbital,transition density matrix and charge density difference map of PFAN and DFAB are simulated by calculation,the analysis shows that the charge distribution of the amino group changes most obviously under the action of silver sol,which is the active site for the coupling of PFAN to produce DFAB.In the second part of the work,we found from the SERS experimental spectra of a mixture of PFAN and p-mercaptoaniline（PATP）on silver sol that the interaction between PATP and Ag NPs is stronger than that of PFAN and Ag NPs.Therefore,we speculate that in this experiment,substances with sulfhydryl groups are more likely to interact with Ag NPs than those with fluorine;we considered two theoretical simulations:PATP and PFAN,4,4’-dimercaptoazobenzene（DMAB）and DFAB,which are formed by coupling the nitrogen-nitrogen double bond of PATP and PFAN;the DFT method is used to calculate and simulate the ground state geometric configuration,frontline molecular orbital,adsorption energy,excited state transition density matrix and charge density difference map of the two sets of contrasts.Theoretical analysis shows that the substances with sulfhydryl groups in the two sets of comparisons have stronger interaction with silver than those with fluorine.