Abinitiocalculations on the positive ions of the alkalinehyphen;earth oxides, fluorides, and hydroxides

摘要

Theoretical spectroscopic parameters are presented at the selfhyphen;consistenthyphen;field and singleshyphen;plushyphen;doubles configurationhyphen;interaction (SDCI) levels for the ground states of the positive ions of the alkalinehyphen;earth fluorides, oxides, and hydroxides. The alkalinehyphen;earth fluoride and hydroxide ions are found to haveXthinsp;1Sgr;+ground states corresponding to a M+2Xminus;1structure. The alkalinehyphen;earth oxide positive ions, on the other hand, undergo a change in ground state symmetry between CaO+(Xthinsp;2Pgr;) and SrO+(Xthinsp;2Sgr;+), whereas the analogous alkali oxides undergo the change one row higher between NaO and KO. The greater stability of the2Pgr; states for the alkalinehyphen;earth oxide positive ions is due to the increased electrostaticfield created by the larger metal ion charge and reduced bond lengths. Calculations on the3Pgr; states of BeF+, MgF+, BeOH+, and MgOH+indicate that these states, corresponding to the M+1X structure, lie much higher in energy. The bond distances of the positive ions are uniformly 0.07plusmn;0.02 Aring; shorter than the analogous electronic states of the neutral. By correcting for the relatively small errors in the theoretical bond distances for the neutrals, bond distances accurate to about 0.005 Aring; are obtained for the ions where experimental values are available for the neutral species. Since extensive onehyphen;particle basis sets are employed and differential correlation effects are minimized by dissociating directly to ions, theoretical dissociation energies and ionization potentials (I.P.) accurate to about 0.15 eV can be obtained. The dissociation energies (D0) in eV for the ground states at the SDCI level are found to be: BeO+(4.04), MgO+(2.31), CaO+(3.29), SrO+(3.28), BeF+(6.25), MgF+(4.46), CaF+(5.67), SrF+(5.71), BeOH+(6.00), MgOH+(3.62), CaOH+(4.73), and SrOH+(4.67). For all of the systems studied, the bonding is dominated by electrostatic interactions. However, the molecules containing Be show some covalent character owing to the large I.P. of Be+. We have found that the constrained space orbital variation analysis, which decomposes the bonding into intrafragment polarization and interfragment donations, is an excellent method for delineating the degree of ionic bonding in a molecular system.