Theoretical Study On Absorption Spectrum,Electronic Circular Dichroism And Anisotropy Of Glyceraldehyde And Valine Spectrum

Studies on the chiral molecules have attracted many attentions due to they are important in a lot of research fields as physics,chemistry and biology,etc.With the development of quantum chemistry theory and software,time-dependent density functional theory(TDDFT)has become a popular workhorse in calculating the electronic circular dichroism(ECD)spectra of the chiral molecules and satisfactory results can be obtained.In this paper,TDDFT is used to systematically study the absorption,ECD and anisotropy spectra of the glyceraldehyde molecule and the valine zwitterion.The main contents are as follows:1 For the valine molecule,we first located 12 stable structures through the global conformer search.The absorption and ECD spectra of the each conformer are calculated on the TDDFT level of theory.Spectra from all conformers are weighted by Boltzmann distribution according to the Gibbs free energy.However,the synchrotron radiation experiment measures the absorption and ECD spectra of valine in form of the amorphous film.The valine molecule exists as the zwitterions due to intramolecular proton transfer.Therefore,we mainly focused on the valine zwitterions.We first performed the geometric optimization by freezing three NH bonds in the –NH3-group because the zwitterionic valine is not stable in gas phase.Three different geometries are obtained and compared with those in crystal.Besides,we optimized the equilibrium geometry of the valine zwitterion in solvent by using the polarized continuum model(PCM).The ECD,absorption and anisotropy spectra of the valine zwitterion are calculated on TDDFT with B3 LYP and M06-2X exchange-correlation(XC)functionals.In order to simulate the amorphous film,Boltzmann statistic is applied to average the spectra of the geometries in gas phase and in solvent separately.Our calculation indicated TDDFT with M06-2X/aug-cc-p VTZ produces the plausible result for the first two peaks of the valine zwitterion in the long wavelength region.A red-shift of 15 nm on the theoretical ECD spectra can reproduce three peaks with the wavelengths shorter than 170 nm in the experimental study.Moreover,ether the geometries of the valine zwitterion in gas phase or in solvent can be averaged to simulate the valine film and the reasonable anisotropy spectra can be obtained.2 For glyceraldehyde,the global conformer search first gives 32 stable structures.Then,TDDFT calculations with different basis sets and XC functionals are used to calculate excited states of the lowest energy geometry.It is found that the M06-2X/aug-cc-p VTZ yield the best result comparing to the preliminary experimental spectra.Finally,the absorption and ECD spectra of 32 different geometries are calculated on the TDDFT/M06-2X/aug-cc-p VTZ level of theory.The spectra from all geometries are averaged via Boltzmann weight to simulate the amorphous film.This work should be the first reported theoretical absorption,ECD and anisotropy spectra of glyceraldehyde,which should be useful in future experiments.3 At the end of the article,it summarizes and reviews the works in this paper,gives the main conclusions and innovations,and provides suggestions on the further works.