The Theoretical Research On C₃ And Rare Gas Atom Interaction Potential And Vibration Bound States

During the last decades, C₃ has attracted a lot of experimental and theoretical attention. It is accepted that C₃ is a typical non-rigid molecule and the singlet ground state has a linear geometry. In recent years, the study of the weak intermolecular interaction of rare gas and linear molecule has attracted considerable attention. In terms of experiment, Hsu's group recorded lots of significative C3-Rg complex spectra with LIF excitation and emission techniques.Ab initio intermolecular potential energy surfaces (PES) calculation on the C₃-Rg complexes with C₃ at its single ground state is reported using a supramolecular approach. A large basis set containing midpoint bond function (332) is employed and the full counterpoise procedure (FCP) of Boys and Bernardi is used to correct for basis set superposition error (BSSE). It can be seen from our study that the major features of the MP4 potential are in qualitative agreement with those of the CCSD(T) potential. However, there are several quantitative differences. Their global minima all correspond to a slightly distorted T-shaped configuration. After we adopted many other methods to verify the characteristic of the PES, the results are almost the same. In order to ensure the integrity of the research system and ease into regularity, the intermolecular potential energy surface for C3-Xe has also been calculated by supermolecular CCSD(T) with the recently developed small-core pseudopotential and augmented correlation-consistent polarized valence pVTZ-zeta (aug-cc-pVTZ-PP ) basis set for the Xe and Dunning's augmented correlation- consistent polarized valence triple-zeta (aug-cc-pVTZ) basis set for C₃. Thus, the C₃-Xe complex accurate ab initio potential energy surface and corresponding vibration levels and wave functions are obtained. The C₃-Rg intermolecular PES mentioned above is not considered the C₃ bending coordinates. For a full understanding of the observed spectra of the C₃–He complex, we made a preliminary calculation to model the C₃–He PES involving the C₃ bending coordinate by supramolecular CCSD(T) method with aug-cc-pVTZ basis sets supplemented with an additional set of bond functions (3s3p2d). There are two local minima in the PES, corresponding to a"Y-shaped"and"arrow-shaped"structure, respectively. Between the two minima, a barrier of 14.08 cm-1 is located at R=5.13 ? andθm =83°. Compared with the frozen nuclear approximation C₃ linear molecule, considering C3 bending coordinate's potential energy surface will have more accurate analysis of electronic spectra and more comprehensive study of its dynamics.In the chapter 4, we focus on the theoretical study of the C₂–Xe complex. The recent reports show that the C2–Rg(Rg = He,Ne,Ar)complexes adopt linear geometry at equilibrium, in contrast to the well-known T-shaped equilibrium structure for N₂–Rg complexes. From our calculated results, the C₂–Xe complex favors a nearly linear geometry at equilibrium. This unusual behavior results from the fact that the high electron density along the C–C bond acts to repel the Xe atom more strongly in the T-shaped than the linear configuration. The resulting C₂–Xe linear equilibrium configuration should therefore be independent of the nature of the rare gas atom. Although the C₂–Xe complex has a linear geometry at equilibrium, the relatively floppy angular motion within each of the intermolecular states will distort the vibrationally averaged geometry away from linearity.There are currently no experimental data with which to compare the binding energies and vdW vibrational energy levels, However, the vibrational energy level predictions should serve as a useful guide to any future spectroscopic studies of C₃–Rg and C₂–Xe complexes.