Reports of the Academy of Sciences of the USSR

1. Volume 118, No. 5

PHYSICAL CHEMISTRY

P. A. Akishin and N. G. Rambidi

ELECTRON-DIFFRACTION STUDY OF THE STRUCTURE OF THE LITHIUM OXIDE MOLECULE

(Presented by Academician N. N. Semenov on 2 VIII 1957)

Until now the literature has contained no experimental data on the structure of the lithium oxide molecule; only an approximate calculated estimate is known for the Li—O distance in the lithium oxide molecule, made by Brewer and Mastick \((^{1})\) on the basis of an ionic model. The acquisition of experimental data on the configuration and geometric parameters of the lithium oxide molecule in the gas phase is of interest for the theory of molecular structure, and is also necessary for calculating the thermodynamic functions and equilibrium constants of certain gas reactions by the methods of statistical thermodynamics.

The present work is devoted to the experimental determination of the geometric structure of the \(\mathrm{Li_2O}\) molecule by the method of diffraction of fast electrons by a jet of vapor of the substance under study.

The work was carried out on the Moscow State University electron-diffraction apparatus for studying the structure of molecules of low-volatility compounds, equipped with a sector device. To obtain a jet of vapor of the substance under study, a high-temperature evaporator was used, with the ampoule heated by electron bombardment; lithium oxide was evaporated from a molybdenum ampoule at temperatures of the order of \(1300\)—\(1350^\circ\) C. Electron-diffraction patterns of the vapors were recorded on positive photographic plates (GOST 2817-50, sensitivity 0.7 units) with exposures from 15 sec to 1.5 min. Since, under the experimental conditions, the intense radiation from the incandescent parts of the evaporator causes strong fogging of the photographic plates, completely masking the electron-diffraction pattern, we used special methods of protecting the photoemulsion: a) by means of aluminum foil \(5\)—\(7\,\mu\) thick, stretched over a metal frame and tightly adjoining the emulsion, and b) by means of a thin layer of black India ink, applied by pouring onto the emulsion of the photographic plates and washed off before development of the electron-diffraction patterns. Preliminary control photographs of electron-diffraction patterns of crystalline ZnO and of \(\mathrm{CdCl_2}\) vapors on ordinary photographic plates and with protection established the suitability of the indicated methods*.

In the work a lithium oxide preparation was used (purity 99.62%), obtained by thermal decomposition of lithium nitrate in a silver crucible**.

From the lithium oxide vapors, using an \(s^2\)-sector, seven series of electron-diffraction patterns were obtained, 2—3 photographs in each series, at different wavelengths

* The apparatus and the method of investigation of low-volatility inorganic compounds are described in more detail in \((^{2})\).

** The preparation was synthesized at the Department of Inorganic Chemistry of the Faculty of Chemistry of Moscow State University by I. A. Savich, V. G. Knyaginina, and N. I. Pecherova, to whom we express our gratitude.

electrons lying within the range \(\lambda = 0.0443\text{–}0.0488\ \text{Å}\) (\(\lambda\) was determined from electron diffraction patterns of the crystalline ZnO standard, obtained before and after photographing the electron diffraction patterns of the vapors).

The electron diffraction patterns of the vapors had 3–4 distinct interference rings, with a distribution of the intensity of the scattered electrons in the diffraction pattern differing little from a damped harmonic function (see Fig. 1).

Fig. 1. Theoretical curves of electron-scattering intensity for different models of the Li\(_2\)O molecule and the experimental intensity distribution (from visual evaluation of electron diffraction patterns obtained by the sector method)

It should be noted that although the use of the sector method ensured the production of good electron diffraction patterns of lithium oxide vapors, the use of microphotometry of the electron diffraction patterns did not promise particular advantages in the interpretation, compared with the visual method, because of the simplicity of the structure of the molecule under study (see, for linear triatomic molecules, \((^3)\); for complex molecules, \((^{10})\)).

The electron diffraction patterns of lithium oxide vapors were processed by the method of radial distribution \((^4)\) and by the method of successive approximations \((^5)\). The radial distribution curve was calculated from the equation:

\[ D(r)=\int_{0}^{s_{\max}} s I(s)e^{-a s^{2}}\sin sr\,ds, \tag{1} \]

where \(I(s)\) is the intensity of the extremum, estimated visually;

\[ s=\frac{4\pi}{\lambda}\sin \vartheta/2; \]

\(\lambda\) is the electron wavelength; \(\vartheta\) is the scattering angle; \(a\) is a factor determined from the relation \(\exp[-a s_{\max}^{2}]=0.1\). In constructing the radial distribution curve, the experimental curve \(I(s)\) in the region of small scattering angles \((0 \leq s \leq 4)\) was extrapolated by a segment of the theoretical electron-scattering intensity curve for a model of the Li\(_2\)O molecule with an angle between bonds of \(110^\circ\) (the reasons for choosing this model will be set forth below). On the radial distribution curve (Fig. 2) there are two peaks at values of \(r\) equal to \(1.82\ \text{Å}\) and \(2.90\ \text{Å}\). The peak at \(r=1.82\ \text{Å}\) is assigned by us to the internuclear distance Li—O. The peak at \(r=2.90\ \text{Å}\), from the ratio of the areas bounded by the radial distribution curve (the ratio of the areas was found to be 9.4; theoretical 8.6), should naturally be assigned to the internuclear distance Li...Li; however, in magnitude it is comparable with the so-called diffraction effects, and therefore the indicated interpretation of the peak, while quite convincing, cannot be considered unambiguous. If it is assumed that the internuclear distance Li...Li in the lithium oxide molecule is equal to \(2.90\ \text{Å}\), then the angle between the bonds

Fig. 2. Radial distribution curve for the Li\(_2\)O molecule, constructed according to equation (1)

Li—O will be close to 110°; this agrees with the literature data that the valence angle of oxygen in the molecules of most of the compounds investigated (see the survey \(^{(12)}\)) has a value of 100–120°. It should also be noted that for the lithium oxide molecule there are theoretical grounds \(^{(11)}\) for assuming a structure similar to that of its closest analogue—the water molecule.

In interpreting the electron diffraction patterns of lithium oxide vapors by the method of successive approximations \(^{(5)}\), theoretical curves \(I(s)\) were constructed for a number of models of the \(\mathrm{Li_2O}\) molecule with angles between the bonds varying from 180° to 90° in steps of 10°. As an example, Fig. 1 gives theoretical curves for models with angles equal to 180°, 110°, and 90°; from these one sees the low sensitivity of the curves \(I(s)\) to variation of the valence angle, which is connected with the insignificant contribution of the distance Li...Li to the total scattering by the \(\mathrm{Li_2O}\) molecule. Indeed, the ratio of the scattering powers of the atomic nuclei determining the interatomic distances Li—O and Li...Li is

\[ \frac{2z_{\mathrm{Li}}z_{\mathrm{O}}r(\mathrm{Li}-\mathrm{Li})}{z_{\mathrm{Li}}^{2}r(\mathrm{Li}-\mathrm{O})}=8.6, \]

therefore the character of the curves \(I(s)\), both theoretical and experimental, is determined mainly by scattering from the nuclei of the atoms forming the Li—O bond. An analogous situation is observed in electron diffraction studies of molecules of vaporous fluorides of the elements of the second group of the periodic system \(^{(3)}\), for which the curves \(I(s)\) are likewise characterized by a low sensitivity to changes in the valence angle F—Me—F, determined in this case from the radial distribution curves. Proceeding from the most probable, triangular model of the lithium oxide molecule, with the angle between the Li—O bonds equal to 110°, we calculated the interatomic distance Li—O by the method of successive approximations (see Table 1); the value obtained was \(r(\mathrm{Li}-\mathrm{O})=1.82\pm0.02\) Å.

When the interatomic distance \(r(\mathrm{Li}-\mathrm{O})=1.82\) Å obtained in the present work for the \(\mathrm{Li_2O}\) molecule in the gas phase is compared with the distance between atoms in the crystal lattice of lithium oxide, equal to 2.00 Å (calculated from the lattice constant \(a=4.619\) Å \(^{(6)}\)), a characteristic difference of approximately 10% is found, known in the literature for gaseous molecules and crystalline structures of a number of inorganic compounds.

Table 1

Molecule \(\mathrm{Li_2O}\); \(r_{\mathrm{theor}}(\mathrm{Li}-\mathrm{O})=2.00\) Å

It is interesting to compare the experimental value of the interatomic distance Li—O with its approximate estimates by various methods. Brewer and Mastick \(^{(1)}\), as a result of calculation of an ionic model of the lithium oxide molecule, obtained \(r(\mathrm{Li}-\mathrm{O})=1.52\) Å. An estimate by the empirical relation of Shoemaker—Stevenson \(^{(7)}\)

\[ r(\mathrm{Li}-\mathrm{O})=r_{\mathrm{Li}}+r_{\mathrm{O}}-\beta |x_{\mathrm{O}}-x_{\mathrm{Li}}|, \]

where \(r_{\mathrm{Li}}\) and \(r_{\mathrm{O}}\) are the covalent radii of lithium and oxygen atoms, \(x_{\mathrm{Li}}\) and \(x_{\mathrm{O}}\) are the electronegativities of the atoms; \(\beta\) is an empirical constant. Using the values \(r_{\mathrm{Li}}=1.34\) Å, \(r_{\mathrm{O}}=0.73\) Å \(^{(8)}\), \(x_{\mathrm{Li}}=0.95\), \(x_{\mathrm{O}}=3.50\) \(^{(9)}\), and \(\beta=0.09\) gives a value equal to 1.85 Å (using the same values and \(\beta=0.06\) \(^{(8)}\), we obtain \(r(\mathrm{Li}-\mathrm{O})=1.92\) Å).

We believe that the closeness of the experimentally found value of \(r(\mathrm{Li}—\mathrm{O})\) to that estimated from the Schomaker–Stevenson relation, and its large difference from the calculated value for the ionic model of \(\mathrm{Li}_2\mathrm{O}\), provide grounds for concluding that the bond in the lithium oxide molecule has a covalent character. This is consistent with the interesting data for crystalline lithium oxide recently published by Weinstein and Dvoryankin \((^{13})\), who, by comparing experimental curves \(f_{\mathrm{el}}(s)\) at small electron-scattering angles with theoretical curves calculated for structures with covalent and ionic bonds, concluded that the bond in \(\mathrm{Li}_2\mathrm{O}\) is covalent, with a small fraction of ionic type.

Moscow State University
named after M. V. Lomonosov

Received
30 VII 1957

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