Calculational Modes And Temperature-related Research On Terahertz Spectra Of Hydroxybenzene Derivatives

Terahertz(THz)Wave refers to electromagnetic radiation in the range of 0.1-10 THz,between microwaves and infrared with the advantages of good coherence,strong penetration,and low energy.As an emerging technology,terahertz technology has a wide range of application prospects,such as food safety,biomedicine,materials science,and communication technology.Terahertz time-domain spectroscopy(THz-TDS),as a key technical application,performs well in investigating the vibration mode,group vibration information,and intermolecular interaction of biomolecules exclusively.In this paper,terahertz time-domain spectroscopy combined with quantum chemistry and first-principles calculation is separately used to study the terahertz absorption spectra of hydroxybenzene derivatives,and consequently,the differences between the two calculation models are further compared and analyzed.Meanwhile,the thermal effect on terahertz spectra is also studied based on first-principles calculation model.The specific research of this paper is as follows:1.A study of THz-TDS spectra and Wide-angle X-ray Diffraction(WA-XRD)spectra for 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde are conducted at room temperature.Through WA-XRD results,it is found that although the two are isomers of each other,they have completely different crystalline forms and space groups.While the measurement of THz-TDS in the range of 0.4-2.9 THz indicates the two have a lot of similarities.For example,they both have four absorption peaks,of which the weakest absorption peak is below 1 THz,while the remaining three absorption peaks are above 1THz.Meanwhile,they both have certain differences,for example,the absorption peak at around 1.5 THz is unique for 4-hydroxybenzaldehyde,while similar peaks cannot be observed in this region for 3-hydroxybenzaldehyde.This feature can be used as a characteristic fingerprint of the two in the low-frequency band and also shows that terahertz time-domain technology has its unique advantages in resolving isomers with high similarity.2.The terahertz spectra of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde are studied based on quantum chemistry calculation model.In this paper,PBE functional with D3 dispersion correction and def2-TZVP basis set is used to compare monomer,trimer,pentamer,and heptamer.Through the comparison results,it is found that monomer and trimer fail to replicate the experimental absorption peaks,and pentamer can restore part of the experimental absorption peaks,while heptamer can reasonably reflect experimental absorption peaks.This shows that compared with the long-range force,the short-range force is far more important for the formation of the terahertz absorption peak,while a too-small cluster size cannot describe the intermolecular interactions,therefore,leading to the absence of absorption peaks in the low-frequency terahertz band.Furthermore,The vibration modes of THz spectroscopy of 3-hydroxybenzaldehyde and4-hydroxybenzaldehyde are studied.For every vibration peak,the vibration mode is decomposed to determine the specific vibration configuration.Vibrational mode automatic relevance determination(VMARD)is utilized to identify the vibration modes of characteristic spectra and the corresponding contribution percentage of each mode and then combined with VMD to plot the vibration mode corresponding to each absorption peak of the cluster model calculated by Gaussian.3.Terahertz spectra of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde are studied based on first-principles calculation model.In this paper,PBE functional with D3 dispersion correction and pob-TZVP-rev2 basis set is utilized to calculate the spectra of3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde.The results show that calculation through a periodic boundary manner is more accurate than a cluster model.Just from the perspective of position,terahertz peaks through the periodic boundary calculation and cluster model calculation do not have distinct discrimination.However,by visualizing the vibration configuration calculated by Crystal software based on periodic structure,it can be found that the result is quite different from the vibration configuration calculated by the cluster structure.This is similar to previously reported article-cluster structures and single unit cell structures cannot reflect the absorption peaks of the terahertz spectra in gas models,and from this paper,we find that they perform equally undesirable in solid-state conditions.This suggests that using cluster models to study terahertz bands,especially low-frequency bands,is relatively unreliable.4.Terahertz spectra of 3-hydroxybenzaldehyde under variable temperature conditions are investigated.The measurement results show that the absorption peak intensity of3-hydroxybenzaldehyde decreases and the peak full-width-at-half-maximum broadens with the increase in temperature,meanwhile,the absorption peak shifts to a lower frequency.This result may be related to the unit cell expansion and the change of weak interaction forces between molecules,for example,a weakened hydrogen bonding.Meanwhile,Quasi-harmonic approximation method for 3-hydroxybenzaldehyde is implemented.This method imposes 0%,2%,and 4% unit cell expansion based on a zero temperature and zero pressure structure.Based on the result,an investigation of the terahertz spectra corresponding to a specific volume size is carried out.The experimental results show that the temperature increase and unit cell expansion impose similar effects to terahertz spectra.At the same time,compared with the volume expansion results of 0% and 4%,the terahertz spectra of 3-hydroxybenzaldehyde in 2% volume expansion results are more consistent with the corresponding spectra measured at room temperature.This shows 0% exhibits in-phase and out-of-phase with 4% and 2%,respectively,leading to a large dipole moment difference in comparison to 0% and 4% with 2%,and affects the strength of the absorption peak.Judging by anharmonic calculation of frequencies of X-H bond stretch method,the difference may come from some covalent bonds affected by van der Waals forces seeking a lower potential energy of the system when vibrating around their equilibrium positions.The above research results provide a theoretical basis for the selection of the terahertz spectral calculation model and give a new idea for the temperature-related study of low-frequency terahertz spectroscopy.