Strongly bound excited states of heteronuclear diatomics: BeC

The theoretical study of strongly bound doubly excited electronic states, discussed by the authors for a series of homonuclear diatomic molecules, is extended to the case of heteronuclear diatomics. Using BeC as a prototype, it is shown that the significant bond shortening and strengthening shown in the isoelectronic B2 also persists in BeC, in spite of the much greater configuration mixing in BeC due to the lower symmetry. In particular, excitations of the form σ→π cause a predictable increase in (local) bond strength. Potential curves and spectroscopic constants are given for the low-lying states correlating up to the sixth dissociation channel. Compared to the ground state X 3Σ- with R e=3.20 bohr and ωe=905 cm-1, the doubly excited state 3 1Σ+ has a local minimum at R e=2.87 bohr where ωe=1294 cm-1, a significant bond strengthening comparable to the homonuclear diatomics. The contribution to the total (CI) wave function for each state is analyzed in terms of the dominant contributions from the important configurations. SCF potential curves for the pure configurations and MO contour maps are used to understand the relationship with the homonuclear case. Vertical transition moments for the most important transitions are given to help in identification of the (as yet unobserved) electronic absorption spectrum

Thermodynamically stable diatomic dications: MgN2+, MgO2+, MgF2+ and MgNe2+

Thermodynamic stability of diatomic dications is an unusual phenomenon, which is mostly restricted to noble gas compounds. Potential energy curves are reported for the four MgX2+ diatomic dications listed in the title. Of the four, MgN2+, MgF2+ and MgNe2+ are thermodynamically stable, i.e. lie below their lowest dissociation limits, by 0.59, 0.82 and 0.48 eV, respectively. MgO2+ has the largest kinetic stability, with an effective dissociation barrier of 1.33 eV. Possible modes of formation of these dications are discussed

Thermodynamically stable dications: AlF2+ and SiF2+

The potential energy curves of the low-lying electronic states of two diatomic (+2) ions AlF2+ and SiF2+ have been calculated by using a multireference CI method with an extended Gaussian basis set. The X1Σ+ state of AlF2+ is predicted to be thermodynamically stable by 1.89 eV, with a barrier to dissociation of 3.29 eV. SiF2+ has two thermodynamically stable electronic states, X2Σ+ bound by 1.89 eV and 12Π bound by 0.68 eV. These dications can therefore be added to a growing list of such dications which are thermodynamically stable in their ground electronic states. The nature of the bond in these species is described and the spectroscopic parameters are reported. Some possible ways of observing the ions are discussed

Multireference configuration-interaction potential surfaces for the collinear F+H2 reaction

Potential-energy surfaces are reported for the collinear FHH system, using a multireference configuration interaction calculation combined with the rotated Morse curve-cubic spline technique to fit the ab initio points. Basis A contains only nuclear-centered atomic orbitals, while basis B adds bond functions at the midpoints of the FH and HH bonds. Results using basis B show significant improvement in the diatomic asymptotes, including the spectroscopic constants for H2 and HF, and the reaction exoergicity. The FHH surface data are fitted along each ray in the (θ,l) coordinate system using either three-parameter (GMF3) or five-parameter (GMF5) generalized Morse functions. The five-parameter fits are more accurate over a wider range, but the three-parameter fits are good enough for most purposes. Based on the five-parameter fits, surfaces A5 and B5 show the following properties (experimental values in parentheses): exoergicity 28.9 and 31.6 kcal/mol (31.7), barrier height 4.0 and 1.8 kcal/mol (1.5-2.3 kcal/mol), barrier location R FH, RHH=(1.47 and 0.77 Å) and (1.48 and 0.77 Å), and vibration frequencies (3550 and 720i cm-1) and (3500 and 830i cm-1). In addition, surface B5 contains reactant and product wells of depth 0.24 and 0.16 kcal/mol, respectively. The surface features obtained using basis B are consistent with the best current estimates obtained from either experimental or theoretical values, except for the location of the barrier which occurs somewhat later along the reaction coordinate

NEW ROVIBRATIONAL DATA FOR MgOH AND MgOD, AND THE INTERNUCLEAR POTENTIAL FUNCTION OF THE GROUND ELECTRONIC STATE

$^{1}$ P. Jensen, J. Mol. Spectrosc. 128, 478-501 (1988); J. Chem. Soc. Faraday Trans. 2 84, 1315-1340 (1988); in: ""Methods in Computational Molecular Physics'' (S. Wilson and G.H.F. Diercksen, Eds.), Plenum, New York, 1992.Author Institution: National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6; Bergische Universität, Gesamthochschule Wuppertal, D-42097 Wuppertal, Germany.; Arizona State University, Tempe, AZ 85287-1604, U.S.A.; The University of California, Santa Barbara, CA 93106, U.S.A.Using millimeter wave absorption spectroscopy we have recorded pure rotation spectra of MgOH for the ground vibrational state and for six excited vibrational states. We have also determined a large number of bending vibrational energies, and the MgO stretching frequency, using dispersed fluorescence studies of both MgOH and MgOD. By a weighted least squares optimized fitting to this data using the variational approach $MORBID^{1}$ Morse Oscillator Rigid Bender Internal Dynamics), with a basis set of Morse oscillator stretching functions and rigid bender functions, we have derived the full ground state potential surface in an analytical form. Although the equilibrium structure is determined to be linear, the bending part of the potential is rather flat and symptoms of quasilinearity are present; for example in the $\nu_{2} = 2$ state the l = 2 component is below the l = 0 component