Theoretical Studies Of The Ro-vibrational Spectra For OCS-Hydrogen Dimer,Trimer,Cluster And CO-N₂ Complex

Since its discovery in 1938,superfluidity has been the subject of much investigation because it provides a unique example of a macroscopic manifestation of quantum mechanics.As we known,⁴He and ³He are the only substances that have been proven to exhibit superfluidity.Therefore,considerable interests concentrate on the discovery of new superfluid substances.Similar to helium,para-H₂ is the most popular candidate for superfluidity.However,it is difficult to obtain superfluid states for para-H₂ because of its high triple point,and the main research approach of para-H₂ superfluidity studies is to immerse hydrogen into helium droplets.Follow the study of helium superfluidity,the spectra studies of a chromophore doped in the para-H₂ droplets are used to study hydrogen superfluidity.However,the spectra of the cluster increase rapidly with the increasing hydrogen molecular which makes it difficult to assign.Therefore,theoretical simulations are necessary.Until now,there are two aspects of work to detect superfluidity:One is to identify the spectral lines of the system by accurately calculating the spectra of doped molecules in different hydrogen clusters.The other is to directly simulate the rotational constants of the chromophore in hydrogen clusters and explore the starting position of superfluidity.A highly accurate potential energy surface is the essential starting point for no matter simulations of cluster spectra and simulations of quantum dynamics.In order to generate a high-precision potential energy surface,fist we must calculate the interaction forces accurately.The key is to select a method that is accurate enough and computationally reasonable.The next step is to fit the high-precision ab initio energies into a specific analytical form.Here,we use Morse/Long-range potential model for the fitness of interaction forces because MLR form explicitly incorporates the theoretically known inverse-power long-range behavior and its anisotropy dependence on the intermolecular coordinates.In addition,in order to simulate the infrared spectra theoretically,the intramolecular vibrational coordinates of the center molecule must be taken into consideration to generate the high-precision potential energy surface.The most popular chromophore OCS is select here to the studies of hydrogen surperfluidity.We generate the high-precision potential energy surface of OCS-H₂ dimer which is used to calculated the ro-vibrational energy levels.Then we extend our spectral researches into OCS-（H₂）₂ trimer.Finally,we simulate the OCS-（H₂）_N N=1-20 cluster.The mean results are:1.An effective six-dimensional ab initio potential energy surface for H₂–OCS which explicitly includes the intramolecular stretch normal modes of carbonyl sulfide is presented.The electronic structure computations are carried out using the explicitly correlated coupled cluster method with the augmented correlation-consistent aug-cc-pVTZ basis set.Analytic four-dimensional PESs are obtained by least-squares fitting vibrationally averaged interaction energies to the Morse/long-range potential model.The predicted transition frequencies and intensities based on the resulting vibrationally averaged PESs are in good agreement with the available experimental values which validate our high quality of our PES.2.The calculated rovibrational energy levels and infrared spectra for OCS-（pH₂）₂,OCS-（oD₂）₂,OCS-（HD）₂ and mixed OCS-pH₂-He trimers are obtained by performing the exact basis-set calculations for the first time based on our developed potential energy surfaces.The“Adiabatic-hindered rotor”method is used for reduced-dimension treatment of the hydrogen rotation.The predicted band origin shifts and the infrared spectra are in good agreement with the available experimental values.We extend the assignments to the unrecorded infrared transitions for OCS-（pH₂）₂ and OCS-（HD）₂ complexes,and identify the infrared spectra for OCS-（oD₂）₂ and OCS-pH₂-He for the first time.3.With our V_MLR potentials,the path integral Monte Carlo algorithm and a first order perturbation theory estimate are used to simulate the v₁ vibrational band origin frequency shifts of OCS in（para H₂）_N-OCS clusters for N=1-20.The predicted vibrational frequency shifts are all redshifts,for N=1-7,the predicted vibrational frequency shifts are in good agreement of the experimental values due to the high quality of these potentialsVan der Walls force is important for chemistry,physics and the life sciences.The collision CO-N₂ complex is of long-standing interest as they are widely present in the atmosphere.Although a number of spectra have already been recorded for the CO-N₂ dimer,a group of lines forming an apparent series still remains unassigned.Therefore,reliable theoretical predictions are expected to further extend the spectroscopic studies.The fairly flat intermolecular potentials with multiple minima differing slightly in energy are the reasons why highly accurate PESs are difficult to obtain.This work is on this subject:4.A new five-dimensional potential energy surface for the van der Waals complex of CO-N₂ which explicitly incorporates the dependence on the stretch coordinate of the CO monomer is generated.Energy decomposition analysis is carried out,and it reveals that the dominant factor in controlling intermolecular configurations is quadrupole–quadrupole electrostatic interactions.Moreover,the rovibrational levels and wave functions are obtained for the first time.The agreement with experimental values validates the high quality of the PESs and enhances our confidence to explain the observed mystery lines around 2163 cm^-1.