The potential energy surface and microwave spectra of the Xe–CO 2 complex

Doped rare gas clusters up to completion of first solvation shell, CO2-(Rg)n, n = 3-17, Rg = Ar, Kr, Xe

Spectra of rare gas atom clusters containing a single carbon dioxide molecule are observed using a tunable mid-infrared (4.3 µm) source to probe a pulsed slit jet supersonic expansion. There are relatively few previous detailed experimental results on such clusters. The assigned clusters include CO₂-Ar_n with n = 3, 4, 6, 9, 10, 11, 12, 15, and 17, and CO₂-Kr_n and CO₂-Xe_n with n = 3, 4, and 5. Each spectrum has (at least) a partially resolved rotational structure, and each yields precise values for the shift of the CO₂ vibrational frequency (ν₃) induced by the nearby rare gas atoms, together with one or more rotational constants. These results are compared with theoretical predictions. The more readily assigned CO₂-Ar_n species tend to be those with symmetric structures, and CO₂-Ar₁₇ represents completion of a highly symmetric (D_5h) solvation shell. Those not assigned (e.g., n = 7 and 13) are probably also present in the observed spectra but with band structures that are not well-resolved and, thus, are not recognizable. The spectra of CO₂-Ar₉, CO₂-Ar₁₅, and CO₂-Ar₁₇ suggest the presence of sequences involving very low frequency (≈2 cm^-1) cluster vibrational modes, an interpretation which should be amenable to theoretical confirmation (or rejection).

Theoretical and experimental studies of the isotope effects for He-CO2 and Ne-CO2 complexes

In this work, we have studied the isotope effects for the He-CO₂ and Ne-CO₂ complexes by means of theoretical calculations and experimental measurements, which were carried out using a distributed quantum cascade laser to probe a pulsed supersonic jet expansion. Firstly, infrared spectra have been recorded for the He/Ne-¹²C¹⁸O₂ complexes. Spectroscopic parameters including band origin ν₀, rotational constants A, B, C, and centrifugal distortion constants Δ_JK were obtained by fitting a Watson A-reduced Hamiltonian with 13 assigned rovibrational transitions for He-¹²C¹⁸O₂. For Ne-¹²C¹⁸O₂, the observed spectrum produces a set of spectroscopic parameters including the band origin, rotational constants and all the quartic centrifugal distortion constants with more than 100 rovibrational transitions (40 new transitions). Secondly, we have calculated the rovibrational energy levels, vibrational shifts, and rotational constants for the He/Ne-CO₂ complexes based on potential energy surfaces (PESs) and bound state calculations for ground and vibrationally excited states. The obtained results show that the spectroscopic characteristics (vibrational shifts and rotational constants) for Ne-CO₂ are analogous to those of Ar-CO₂, while those for He-CO₂ show some differences especially for the rotational constants. Finally, according to the available experimental data and our theoretical calculations, infrared spectra were predicted for six isotopologues with C_2v symmetry of Ne-CO₂ complex.

Ab initio relativistic potential energy surfaces of benzene-Xe complex with application to intermolecular vibrations

The benzene-Xe (BXe) complex in its electronic ground state is studied using ab initio methods. Since this complex contains the heavy Xe atom, the relativistic effects cannot be neglected. We test two different approaches that describe the scalar relativistic effects in the framework of the coupled-cluster level of theory with single, double, and perturbative triple excitations, used for the interaction energy calculations. The first one is based on the small core pseudopotential (PP), and the second one is based on the explicit treatment of scalar relativistic effects using the Douglas-Kroll-Hess (DKH) Hamiltonian. A few basis sets are tested with the PP and DKH, and for each one, the analytical potential energy surface (PES) is constructed. It is shown that the difference between PESs determined with PP and DKH methods is small, if the orbitals of the 4d subshell in Xe are correlated. We select the most appropriate approach for the calculation of the potential energy surface of BXe, with respect to accuracy and computational cost. The optimal level of theory includes a small Dunning's basis set for the benzene monomer and a larger PP basis set for Xe supplemented by midbond functions. The PES obtained using such an approach provides a reasonable accuracy when compared to the empirical one derived from the microwave spectra of BXe. The empirical and the theoretical values of intermolecular vibrational energies agree within 0.5 cm^-1 up to second overtones. The vibrational energy level pattern of BXe is characterized by a distinct polyad structure.