Knowledge Bank

The lithium halide dimer is known to have a diamond structure and is believed to be planar. The frequency of the out-of-plane vibration of this ring has been the subject of a number of widely varying theoretical predictions. This situation is caused by the present lack of understanding of the bending force constants in molecules with a high degree of ionic binding. In the present work the out-of-place vibrational frequencies of three lithium halide dimers have been measured and unambiguously characterized by isotopic substitution techniques. The molecular species were isolated in inert matrices at low temperature. The frequencies for the $\mathrm{<mrow\; data-mjx-texclass="ORD">7</mrow>}$Li isotope in solid krypton are as follows: $Li2F2-268cm-1;Li2Cl2-174cm-1;Li2Br2-150cm-1$. For lithium fluoride two bands have been observed at $242cm-1$ and $253cm-1$ (for $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ Li in krypton) which exhibit more complex isotopic substitution patterns. The spectra are consistent with the assignment of these frequencies to a trimer containing three equivalent lithium ions, and exclude assignment to any dimer species. This assignment requires a regular hexagonal cyclic structure for $Li2F3$. In addition to the dimer bands of lithium chloride and bromide an absorption has been measured in these samples which exhibits a monomer isotopic pattern. The frequencies for $\mathrm{<mrow\; data-mjx-texclass="ORD">7</mrow>}$Li in krypton are $293cm-1$ and $253cm-1$ respectively. These absorptions are assigned as the hindered rotation or libration of the monomer molecule in the solid solution cage. This assignment is based on the isotopic frequency shift for $\mathrm{<mrow\; data-mjx-texclass="ORD">7</mrow>}Li-6Li$ and the correlation between the frequencies and the moments of inertia of LiCl and LiBr.