Interaction potentials for Br−–Rg (Rg=He–Rn): Spectroscopy and transport coefficients

Review: gas-phase ion chemistry of the noble gases: recent advances and future perspectives

This review article surveys recent experimental and theoretical advances in the gas-phase ion chemistry of the noble gases. Covered issues include the interaction of the noble gases with metal and non-metal cations, the conceivable existence of covalent noble-gas anions, the occurrence of ion-molecule reactions involving singly-charged xenon cations, and the occurrence of bond-forming reactions involving doubly-charged cations. Research themes are also highlighted, that are expected to attract further interest in the future.

Radii of atomic ions determined from diatomic ion-He bond lengths

We propose a new definition of the effective radius of an atomic ion: the bond distance (R(e)) of the ion/He diatomic complex minus the van der Waals radius of the helium atom. Our rationale is that He is the most chemically inert and least polarizable atom, so that its interaction with the outer portions of the electron cloud causes the smallest perturbation of it. We show that such radii, which we denote R(XHe), make good qualitative sense. We also compare our R(XHe) values to more traditional ionic radii from solid crystal X-ray measurements, as well as estimates of such radii from "ionic" gas-phase MF, MOM, MF(+), and MO molecules, where M is a metal atom. Such comparisons lead to interesting conclusions about bonding in ionic crystals and in simple gas-phase oxide and fluoride molecules. The definition is shown to be reasonable for -1, +1, and even for many of the larger +2 atomic ions. Another advantage of the R(XHe) definition is that it is also consistently valid for ground states and excited states of both neutral atoms and atomic ions, even for open-shell np and nd cases where the electron clouds of the ions are not spherically symmetric and R(XHe) thus depends on the "approach" direction of the He atom. Finally, we note that when there is a contribution from covalent bonding with the He atom, and/or in cases where the ion is small and has a very high charge, so that there is distortion even of the He 1s electrons, R(XHe) is not expected to be representative of the size of the ion. We then suggest that in these cases small, and sometimes unphysical, values of R(XHe) are diagnostic of the fact that simple "physical" interactions have been supplemented by a "chemical" component.

Theoretical and experimental studies of collision-induced electronic energy transfer from v=0-3 of the E(0g +) ion-pair state of Br2: collisions with He and Ar

Collisions of Br(2), prepared in the E(0(g)+) ion-pair (IP) electronic state, with He or Ar result in electronic energy transfer to the D, D', and beta IP states. These events have been examined in experimental and theoretical investigations. Experimentally, analysis of the wavelength resolved emission spectra reveals the distribution of population in the vibrational levels of the final electronic states and the relative efficiencies of He and Ar collisions in promoting a specific electronic energy transfer channel. Theoretically, semiempirical rare gas-Br(2) potential energy surfaces and diabatic couplings are used in quantum scattering calculations of the state-to-state rate constants for electronic energy transfer and distributions of population in the final electronic state vibrational levels. Agreement between theory and experiment is excellent. Comparison of the results with those obtained for similar processes in the IP excited I(2) molecule points to the general importance of Franck-Condon effects in determining vibrational populations, although this effect is more important for He collisions than for Ar collisions.

Noble gas anions: a theoretical investigation of FNgBN- (Ng = He-Xe)

Noble gas anions of general formula FNgBN- (Ng = He-Xe) have been investigated by MP2, coupled-cluster, and multireference-CI calculations with correlation-consistent basis sets. These species reside in deep wells on the singlet potential energy surface and are thermodynamically stable with respect to the loss of F, F-, BN, and BN-. They are unstable with respect to Ng + FBN-, but at least for Ng = Ar, Kr, and Xe, the involved energy barriers are high enough to suggest their conceivable existence as metastable species. The stability of FNgBN- arises from the strong F--stabilization of the elusive NgBN. The character of the boron-noble gas bond passes from purely ionic for FHeBN- and FNeBN- to covalent for FXeBN-.

Accurate potential energy curves for F−–Rg (Rg = He–Rn): Spectroscopy and transport coefficients

High-quality ab initio potential energy curves are presented for the F(-)-Rg series (Rg = He-Rn). Calculations are performed at the CCSD(T) level of theory, employing d-aug-cc-pV5Z quality basis sets, with "small core" relativistic effective core potentials being used for Kr-Rn. The quality of the curves is judged by agreement with recent high-level calculations in the case of F(-)-He and F(-)-Ne and by excellent agreement with mobility data for the systems F(-)-Rg (Rg = He-Xe). Except for these recent high-level calculations on the two lightest systems, we are able to deduce that all other previous potentials for the whole set of these systems are inadequate. We also present spectroscopic information for the titular species, derived from our potential energy curves.