A highly accurate ab initio intermolecular potential energy surface for the benzene–argon van der Waals complex is evaluated using the coupled cluster singles and doubles model including connected triple excitations [CCSD(T)] model with an augmented correlation consistent polarized valence double zeta basis set extended with midbond functions. The vibrational energy levels obtained by full three-dimensional dynamical calculations are in excellent agreement with the available experimental data.

The rotational spectra of three benzene–X complexes, where X=20Ne, 129Xe, or 132Xe, and of the benzene-1,3,5-d3–Ar complex have been observed using pulsed nozzle Fourier transform microwave (FTMW) spectroscopy. Rotational transitions assigned in the 8–18 GHz range have been found to match symmetric top spectra. Rotational constants B and centrifugal constants DJ and DJK were determined from the measured frequencies. Intermolecular motions between benzene and the rare gas atom have been modeled with a rovibrational Hamiltonian. The three-dimensional interaction potential has been assumed of a simple form with three adjustable parameters. These parameters, one of which represents the equilibrium distance of the rare gas atom from the plane of benzene, have been adjusted for all benzene–rare gas complexes in a least-squares fit by direct inversion of the observed rotational transition frequencies. From the potential, the force constants and frequencies of the van der Waals vibrations and the binding energies have been deduced for all benzene–rare gas complexes.

The supermolecular Moller–Plesset perturbation theory (MPPT) is applied to calculate and analyze selected portions of the potential-energy surface (PES) of the H2O⋯CO2 complex. Two kinds of minima have been found. The global minimum, which corresponds to the T-shaped structure with the C atom bonded to the O atom, and the local minimum for the H-bonded arrangement OCO⋯HOH. The global minimum was estimated to be 920 cm−1 deep at the fourth order of MPPT combined with the extended spdf-quality basis set supplemented with bond functions. At the same level of theory the optimal H-bonded structure is 357 cm−1 higher in energy, and reveals a small 10° departure from the collinear arrangement OCO⋯H–O. Both the T-shaped and H-bonded forms are primarily bound by the electrostatic term, which is twice as large as the dispersion component. One-dimensional sections of the potential-energy surface were subsequently used to calculate vibrational energy levels for the wagging motion of the water moiety in the T-shaped and H-bonded forms. Two-dimensional cuts of the PES along the intermolecular Jacobi coordinates, r and θ, were employed to simulate the dynamics of the stretch–bend coupling close to the minima.

Highly accurate analytical intermolecular potential energy surfaces (PESs) of the complexes composed of the water molecule and the rare gas (Rg) atom are presented for Rg=He, Ne, Ar, and Kr. These PESs were scanned using the supermolecule coupled cluster singles and doubles including connected triples method [CCSD(T)]. Efficient basis sets including the bond functions (3s3p2d1f1g) enabled the calculation of more than 430 single-point interaction energies for each complex. These energies were utilized to construct the analytical many-body representations of the PESs. They were refined using the interaction energies evaluated at the complete basis set limit in the PES stationary points. In addition, the corrections from the core correlation were calculated for the complexes including He, Ne, and Ar. The many-body PES of XeH(2)O was built using the ab initio energy values reported by Wen and Jager [J. Phys. Chem. A 110, 7560 (2006)]. The clear regularities of the equilibrium structure and the potential barriers were found in the RgH(2)O series. A comparison of the ab initio and experimental PESs of ArH(2)O [R. C. Cohen and R. J. Saykally, J. Chem. Phys. 98, 6007 (1993)] reveals their close similarity, except for the potential barriers corresponding to the planar saddle points. Their energetic order is different in both PESs. This suggests that an alternative PES with the reversed barriers, consistent with the ab initio ones, could be derived from the experimental data.

In this article, we report on a Fourier transform infrared study of absorption bands belonging to small-sized water clusters formed in a continuous slit nozzle expansion of water vapor seeded in argon carrier gas. Clear signatures of free and H-bonded OH vibrations in water aggregates from dimer to pentamer are seen in our spectra. Following an increase in argon backing pressure, the position of the cluster absorption bands varies from those characteristics of isolated water aggregates in the gas phase to those known for clusters trapped in a static argon matrix. These variations can be interpreted in terms of sequential solvation of the water clusters by an increasing number of argon atoms attached to water clusters. Our measured spectra are in good agreement with those obtained previously either for free or Ar coated small-sized water clusters using pulsed slit-jet expansions. Our results are equally in accord with those originating from a variety of tunable laser based techniques using molecular beams or free jets or from the study of water aggregates embedded in rare gas matrices. Distinctions are reported, however, and discussed. Ab initio calculations have made it possible to speculate on the average size of an argon solvation shell around individual clusters as well as on the development of the OH stretch vibrational shifts in mixed (H(2)O)(m)Ar(n) clusters having different compositions and architectures.