Alkaline Earth Dihalides Research Articles

Ion model and equilibrium configuration of the alkaline-earth dihalide dimers. The Mg2Cl4 molecule

The polarized ion model has been applied in the study of the magnesium dichloride dimeric molecule (Mg2Cl4). The calculations predict as the equilibrium configuration for this molecule a planar structure with D2h symmetry with the outer Mg–Cl distance equal to 2.215 Å and the ring Mg–Cl′ distance equal to 2.36 Å. The angle Cl′–Mg–Cl′ in the ring is 105°.

Crossed molecular beam metathesis: Alkaline earth dihalides+Cl2 and HCl

Exploratory crossed molecular beam studies of bimolecular reactions of alkaline earth dihalides with halogen containing compounds are reported. The c.m. product distribution is symmetric about ϑ=90° for the BaI2+Cl2 halogen-exchange reaction indicating formation of a collision complex which persists for a least a few rotational periods. The data are consistent with, but do not establish, formation of a similar complex in the SrI2+Cl2 reaction and with a statistical loose transition state energy partitioning in both reactions. The BaI2+HCl→BaICl+HI reaction is direct, favoring backscattering of the metal atom. Relative total reaction cross sections for the MX2+Cl2 halogen-exchange reactions decrease as 1.0:0.45:<0.007 when M is varied from Ba to Sr to Mg. All of these observations support an ion-pair adduct model which was previously advanced to interpret facile four-center exchange reactions of cesium halides with diatomic halogens. This model, which qualitatively correlated observed trends with reactant electronic structure, pictures these present reactions as (MI)+I−+Cl2→ (MI)+(Cl−I−Cl)−→products and (MI)+I−+HCl→ (MI)+(I−H−Cl)−→MICl+HI.

Ion model and equilibrium configuration of the gaseous alkaline-earth dihalides

The polarized ion model has been applied to calculate binding energies, equilibrium configurations, and force constants of the gaseous alkaline-earth dihalides. The capability of the model to predict bent structures has been proved and good agreement between experimental and calculated bond angles as well as binding energies has been achieved.

Ion-ion interaction potentials and their application to the theory of alkali halide and alkaline earth dihalide molecules

Interaction potentials of many ion pairs are calculated using the model presented in our previous paper [J. Chem. Phys. 56, 3122 (1972)]. In the calculation of the interactions of two ions, the electron density of the combined system of the ions is assumed to be equal to the sum of the two separate ionic electron densities. The Coulomb energy contribution to the interaction energy is calculated directly from the charge distribution of the nuclei and the assumed electron density. The non-Coulombic part of the interaction energy is calculated from the electron density using the electron gas energy expression in terms of the local electron density. The ions treated are the alkali, the alkaline earth, and the halide ions. The calculated ion-ion interaction potentials are applied to the theory of alkali halide and alkaline earth dihalide molecules. The predicted molecular bond energies, bond lengths, and vibration frequencies agree quite well with existing experimental data.

Relation between electronic structure and the chemiluminescence arising from collisions between alkaline earth atoms and halogen molecules

The chemiluminescence ascribed by Jonah and Zare to radiative association of Ba and Cl2 is examined in the light of the electronic structure of the ground and excited states of alkaline earth dihalides. A simple discussion is first given in terms of the possible curve crossings and avoided crossings. In addition ab initio self-consistent-field calculations are reported for CaF2 using an extended basis set. The 1 B 2 excited state, from which the molecule may radiate to the ground state, is predicted to have an equilibrium bond angle of 54° and bond distance of 4·06 bohr. The bertical excitation energy to the 1 B 2 state is 7·3 eV and the vertical energy difference (1 A 1-1 B 2) at the 1 B 2 equilibrium geometry is 1·3 eV. These results appear consistent with the model proposed by Jonah and Zare. In addition, a variety of properties (dipole moments, field gradients, etc). of CaF2 are reported.

Geometry and Vibrational Spectra of Alkaline-Earth Dihalides. II. CaF2, SrF2, and BaF2

The infrared spectra of 40CaF2, 44CaF2, 86SrF2, 88SrF2, and BaF2 trapped in solid krypton matrices at 20°K are reported. From precise measurements of the shifts of the vibrational modes on isotopic substitution upper and lower limits for the apex angles of these molecules can be established. The mean values of the apex angle based on these limits are found to be 140° for CaF2, 108° for SrF2, and a reanalysis of the earlier data for MgF2 gives 158°. For BaF2 the apex angle has been estimated at 100°. Using a valence force field a set of force constants have been obtained from the measured fundamental frequencies, as well as approximate values for the anharmonic corrections to these vibrational modes. The statistical entropies calculated from the vibrational frequencies and molecular geometries are in excellent agreement with the thermodynamic measurements.

Geometry of the Alkaline-Earth Dihalides

It has been shown by the deflection of beams of alkaline-earth dihalide molecules by inhomogeneous electric fields that the following compounds have permanent electric dipole moments and are therefore assumed bent: BaF2, BaCl2, BaBr2, BaI2, SrF2, SrCl2, CaF2. No polarity was observed for the following compounds investigated: CaCl2, CaBr2, SrBr2, SrI2. The absence of deflection of beams of BaCl2 by inhomogeneous magnetic fields shows that this molecule has no electronic magnetic moment and hence is in a singlet ground state.