The Theoretical Simulation Of Sum Frequency Generation Spectrum

Recently, sum frequency generation(SFG) spectrum has more and more widely application due to the continuous improvement of the laser technique. SFG vibrational spectroscopy has been developed into a powerful probe to real-time and in situ study the surface and interfacial structures at the molecular level. This paper with a series of amino acids as model, the geometries of all amino acids are fully optimized by the density functional theory at the B3LYP/6-31+(d)) level and ab initio method at the RHF/6-311++(d,p) level, then the related data gained by the method of configuration interaction singles (CIS/6-311++G(d,p)) quantum chemistry calculations. We considered the first120excited states in this letter. SFG spectra of a series of amino acid molecules are simulated using sum over states (SOS) theory. The results show that ab initio method can obtain more accurate simulated SFG spectra.Optical active sum frequency generation(OA-SFG) spectra is simulated in the dipole length and velocity formalisms. Our calculation results showed that the intensity order of OA-SFG for different amino acids in dipole velocity formulation agrees with the experimental results reported by Shen, but fails in length formulations, which demonstrate the two formalisms aren’t equivalent in approximate calculation. The theoretical calculation of SFG spectra demonstrates the origin-sensitivity of dipole length formulations, which is similar to the calculations of circular dichroism. Velocity formalism provides a better simulation results in obtaining theoretical SFG spectra. According to the chiral induction perturbation theory further research the SFG spectra of molecular origin.Calculation D(m,n) versus to the excited state, D(m,n)=μgn·(μnm×μmg), microscopic description the chiral induction perturbation interaction, the result shows D(m,n) to the every excited state is zero as expected for achiral amino acid Gly, and chiral amino acid not be all vanished, which demonstratethe chiral induction perturbation interactions with the chromophore lower its symmetry so that the three excited state dipole moment must not coplanar. The first time prove chiral induction perturbation theory by the quantum chemical calculation. At the same time, discussion the chiral side chain group(R) size relation to D(m, n). When the chiral side chain group is bigger, value of D(m, n) becomes larger and also the intensity of SFG is stronger. The OA-SFG intensity of amino acids in the order of isoleucine(Ile R=CH(CH3)CH2CH3)>valine(Val R=CH(CH3)2)>alanine(Ala R=CH3)>glycine(Gly R=H) is in good agreement with experimental results and expectation of dynamic coupling model. Obviously, SFG spectra is very sensitive to a molecular structure.This thesis designed the other three amino acid molecules, F-Val, Cl-Val, and Br-Val respectively. The amino acids are fully optimized by ab initio method at the RHF/6-311++(d,p) level, then the related data gained by CIS/6-311++G(d,p) quantum chemistry calculations. SFG electronic spectra of amino acid molecules are simulated in the dipole length and velocity formalisms. Comparing with valine, the intensity order of SFG for four amino acids is F-Val> Valine>Cl-Val> Br-Val, which demonstrates a bond with large polarizability is a more effective perturber. The effective perturber cause more large intensity of SFG. Similarly, intensity order of SFG has not regularity in the dipole length formalisms, which prove velocity formalism provides a better simulation results in obtaining theoretical SFG spectra again. SFG electronic spectra of three binaphthalene derivatives molecules are simulated in the dipole length and velocity formalisms. The results show that the electronic conjugate and strong push electron group largely contribute to SFG signal, at the same time also contribute to binaphthalene spectrum resonance peak split more obvious.