Terahertz Spectroscopy Study On Isomeric Organics And Organics With Similar Molecular Structures

Terahertz(THz)wave is generally referred to the electromagnetic wave ranging from 0.1 THz to 10 THz.Recently,THz time-domain spectroscopy(THz-TDS)has become mature gradually due to the rapid development of the generation and detection of THz radiation.Since the absorption features of materials in the THz region originate from complex intramolecular and intermolecular interactions,THz-TDS has attracted lots of attention in numerous applications,such as detection and identification of materials,quantitative and qualitative analysis of compounds as well as characterization of crystal structures in food,pharmaceutical and chemical industries.However,in recent years,most of THz spectroscopic studies were focused on experiments without further theoretical interpretation of the origins of the observed absorption features,which seriously impedes the further application of THz-TDS in the investigation of microscopic structures and properties of materials.Therefore,by combining THz-TDS with theoretical calculations(ab initio and density functional theory),isomeric organics and organics with similar molecular structures,which are widely used in pharmaceutical,food and chemical fields but identified difficultly,are experimentally and theoretically studied in the present work.The specific research contents are summarized and divided into the following parts:Firstly,THz absorption spectra of nicotinic acid,isonicotinic acid and 2-picolinic acid have been measured by THz-TDS.It is easy to find that the three isomers exhibit distinctively different THz absorption spectra and can be easily recognized,which indicate high sensitive of THz vibrations to crystal structural differences.Besides,the density functional theory(DFT)calculations have been performed to interpret the origins of the measured spectral features.Specifically,the measured absorption features of nicotinic acid and isonicotinic acid are found to be originated from collective translations and rotations of molecules.And the three measured features of 2-picolinic acid can also attributed to intermolecular vibrations,which are reported for the first time.Moreover,the comparison of PBE and PW91 calculations shows that the latter methodology provides better reproduction of molecular structures and THz spectra,due to its accurate description of electronic exchange-correlation energy.These results reveal that differences of crystal structures play an important role in differences of THz spectra,demonstrating that THz-TDS can be applied in distinguishing isomers in medical and food industries.Secondly,THz spectra of two structural analogues,salicylic acid and sodium salicylate,have been studied by combining THz-TDS with quantum chemistry calculations(PW91,B3 LYP and MP2).The measured absorption features of salicylic acid at 1.63 THz and 1.84 THz as well as the THz spectra of sodium salicylate in the range of 0.1-3.6 THz are reported for the first time.And the distinct THz spectral differences between the two analogous organics have also been observed,implying that they can be easily distinguished by using THz-TDS.Compared with molecular structure of salicylic acid,the ionic bond of sodium salicylate changes not only the molecular structure but also the intermolecular interactions in the unit cell,which lead to the THz spectral differences.Furthermore,the DFT simulations of salicylic acid have been carried out to assign the measured absorption features,which mainly originate from intermolecular interactions except the feature at 2.28 THz.While the theoretical calculations of sodium salicylate show that the absorption features mainly come from intramolecular vibrations except the feature at 0.40 THz.Besides,the MP2 calculations show better agreement with the measured absorption features compared to B3 LYP calculations,since the method made second-order energy correction.These results reveal that the change of single atom can also result in the change of the THz characteristic absorption,indicating that THz spectroscopy is an effective technique in recognizing analogues.Furthermore,THz and infrared spectra of two solid-state monosaccharides(glucose and fructose)as well as their liquid states have been studied.The present work reports the origins of the whole absorption features of fructose for the first time.As for spectra of solid-states,the theoretical calculations based on unit cells achieve better reproduction of molecular structures and absorption spectra than those based on isolated-molecules,indicating the effect of intermolecular interactions on the molecular structures and vibrational modes.Besides,it is easy to find that the infrared absorption features of the two isomers are very close while their THz absorption features vary greatly,because the two isomers have very similar molecular structures but different molecular arrangements and intermolecular interactions in the unit cell.These results can help to understand the effect of crystal environment on the molecular structures and vibrational modes,providing significant references for designing new organic molecules in biological and chemical fields.Finally,THz and infrared absorption bands of liquid-state glucose/fructose shift and are broader compared to those of solid-states,as the interactions between solvent and solute molecules change the molecular structure and intermolecular interactions of solute in aqueous solution.Besides,the higher density solutions show lower THz absorption since the intermolecular hydrogen bonds between glucose/fructose and water molecules lead to the decrease of free water molecules.These results can promote the application of THz-TDS in the investigation of structures and properties of liquid-state organics.