Lithium diatomic

Ishiguro, E., Kayama, K., Kotani, M., and Mizuno, Y., J. Phys. Soc. Japan 12, 1355, Electronic structure of simple homonuclear diatomic molecules. II. Lithium molecule. ... 

The first column of the periodic table, Group 1, contains elements that are soft, shiny solids. These alkali metals include lithium, sodium, potassium, mbidium, and cesium. At the other end of the table, fluorine, chlorine, bromine, iodine, and astatine appear in the next-to-last column. These are the halogens, or Group 17 elements. These four elements exist as diatomic molecules, so their formulas have the form X2 A sample of chlorine appears in Figure EV. Each alkali metal combines with any of the halogens in a 1 1 ratio to form a white crystalline solid. The general formula of these compounds s, AX, where A represents the alkali metal and X represents the halogen A X = N a C 1, LiBr, CsBr, KI, etc.). 

Although we have dealt with a diatomic molecule consisting of two hydrogen atoms, the procedure is exactly the same if the molecule is Li2, except that the atomic wave functions are 2s wave functions and the energies involved are those appropriate to lithium atoms. The VSIP for lithium is only 513 kj mol-1 rather than 1312kj mol-1 as it is for hydrogen. 

D. M. Silver, K. Ruedenberg, and E. L. Mehler, Electron correlation and separated pair approximation in diatomic molecules. 2. Lithium hydride and boron hydride. 7. Chem. Phys. 52(3), 1181— 1205 (1970). 

Lithium and fluorine form a diatomic molecule that has a large dipole moment in the gas phase it has been measured to be 6.3248 D in the ground vibrational state. The equilibrium intemuclear distance is 1.564 A, and, therefore, the apparent... 

The weakness of the covalent bond in dilithium is understandable in terms of the low effective nuclear charge, which allows the 2s orbital to be very diffuse. The addition of an electron to the lithium atom is exothermic only to the extent of 59.8 kJ mol-1, which indicates the weakness of the attraction for the extra electron. By comparison, the exother-micity of electron attachment to the fluorine atom is 333 kJ mol-1. The diffuseness of the 2s orbital of lithium is indicated by the large bond length (267 pm) in the dilithium molecule. The metal exists in the form of a body-centred cubic lattice in which the radius of the lithium atoms is 152 pm again a very high value, indicative of the low cohesiveness of the metallic structure. The metallic lattice is preferred to the diatomic molecule as the more stable state of lithium. 

Let us consider what happens as two s-valent atoms A and are brought together from infinity to form the AB diatomic molecule as illustrated schematically in Fig. 3.1. The more deeply bound energy level EA could represent, for example, the hydrogenic Is orbital (EA = —13.6 eV), whereas the less deeply bound energy level EB could represent lithium s 2s orbital (EB = — 5.5 eV. cf Fig. 2.16). Each free atomic orbital satisfies its own effective one-electron Schrodinger equation (cf eqn (2.59)), namely... 

Like the dihelium molecule, Be- is not expected to exist. The experimental facts are thal lithium is diatomic in the gas phase but beryllium is monatomic. 

If we combine the splitting schemes for the 2s and 2p orbitals, we can predict bond order in all of the diatomic molecules and ions composed of elements in the first complete row of the periodic table. Remember that only the valence orbitals of the atoms need be considered as we saw in the cases of lithium hydride and dilithium, the inner orbitals remain tightly bound and retain their localized atomic character. 

TIic coupling of atomic orbitals in lithium-row diatomic molecules, and the resultant bond designations (at right). 

If two atoms forming a diatomic molecule are both lithium, there are only two valence electrons, which would be put in the bonding state the qualitative picture of electronic structure and binding of Lij is exactly the same for H2 the... 

Much theoretical work has been carried out on the lithium hydride molecule, which has become the workbench of the theoretical chemist (J ). Browne ( ), and Fraga and Ransil ( 3) have given the binding energy for the LiH ion by ab initio calculation Com-panion(j4) has applied the diatomic-in-molecule theory to the Li H and LiH. molecules and predicted the stabilities of these molecules. We have intensively studied the Li-H system by means of Knudsen effusion mass spectrometry, and identified all predicted molecules and ions as cited above(5), and reported the thermochemical properties of these gaseous species (, 2, ) ... 

The ultraviolet photoelectron spectra of diatomic alkali halide molecules are reviewed and interpreted. Data for lithium halide dimers, 112X2> are presented and it is shown that the dimers have significantly larger ionization thresholds than the corresponding monomers. Some historical controversies regarding the presence of dimers and their ionization energies are clarified. Photoionization mass spectrometry is used to determine the adiabatic ionization potential of lithium chloride trimer, in order to probe the trend of I.P. with cluster size. The predictions of Hartree-Fock, Xa and ionic model calculations on this point are presented. 

Reversible Equilibria. As noted in Table I, there are many reversible equilibria available in alkali metal vapors to accomodate external stresses and perturbations. The liquid t vapor equilibrium is of fundamental importance in connection with heat pipes (1) and also in many applications, e.g. lithium in fusion (25). Equilibria involving clustering (beginning with atom-atom recombination to the diatom (3, 4)) are just beginning to be understood quantitatively. Equilibria involving ionization are of major importance for the readily ionized alkali metal species (18, 19, 38). A final equilibrium which has apparently been much less studied (1) is the "Mott" metal-insulator transition which occurs for all alkali metal vapors at high density (52, 53). 

In this section we consider homonuclear diatomic molecules (those composed of two identical atoms) formed by elements in Period 2 of the periodic table. The lithium atom has a 1 s22s electron configuration, and from our discussion in the previous section, it would seem logical to use the Li Is and 2s orbitals to form the MOs of the Li2 molecule. However, the Is orbitals on the lithium atoms are much smaller than the 2s orbitals and therefore do not overlap in space to any appreciable extent (see Fig. 14.33). Thus the two electrons... 

Lithium vaporizes to a mixture of monatomic and diatomic gas. The total vapor pressure reaches 1 atm at 1620 K the vapor pressure of the monatomic gas reaches 1 atm at 1638 K and the enthalpy of vaporization to monatomic gas is 35.16 kcal mol. ... 

Tables of electronic wavefunctions have been compiled for the diatomic hydrides AH, where A denotes the elements Li through F, and Na through Cl.195 X-Ray diffraction patterns at high pressure show that LiH retains the low-pressure NaCl structure up to 12.0 GPa (120 kbar).196 The preparation of the first stable complex metal hydride of copper, lithium dihydro-cuprate(i), is reported from the reduction of LiCuMe2 by lithium aluminium hydride in ether at low temperatures. The solid compound, LiCuH2, is solvated by ether and stable under ambient conditions for several days.197...

If we try to apply our semi-empirical LCAO-MO method to metals, we get poor results. Taking lithium metal as an example, and using two Li atoms as our unit (just as for C, Si and Ge), we have only two electrons to fill up the four possible MOs. The cohesive energy is only P instead of 4/ , the same as it is for the diatomic molecule Li2. This gives AEcov — 25 kcal/mol, using Table 5.2. The value of A exp is 77 kcal/mol. A localized bonding model is not useful for the metals. We must exploit the new features that result because a solid is a very large molecule indeed. 

For two lithium atoms, the cohesive energy is 7.64, compared with Ip for an L12 molecule. This can account for the difference found experimentally 25 kcal/mol compared with 77 kcal/mol. One should also consider internuclear separations, 2.67 A in the diatomic molecule and 3.03 A in the metal, so that p in the metal is less than in the molecule. 

The synthesis of 79, 80 and related spiranes has been of interest because of their potential conversion to centrohexacyclic derivatives bearing three additional bridges across the neopentane carbon atoms C-9, C-10 and C-l l,in analogy to some unusual non-benzoannelated analogues such as the Simmons-Paquette molecule [37,81,82]. In fact, triptindanetrione 72 has been used in several cases to construct three additional five-membered rings (Schemes 15 and 56). A surprisingly efficient three-fold bridging with aliphatic diatomic units has been achieved by treatment of 72 with an excess of lithium acetylides. In this way, centrohexacyclic triptindanes 81-83 and related compounds have become accessible, as shown in Scheme 15 [83]. 

It is well known that certain univalent metals (for example, lithium) form diatomic molecules in the vapour in which the interatomic binding is presumably covalent in character. On the valence-bond theory it is assumed that such bonds also operate in the solid state, but since the number of electrons available is inadequate to give rise to covalent bonds between each atom and all its neighbours (eight in lithium) resonance is assumed to take place throughout the solid in a way which may be symbolized, in two dimensions, thus ... 

Kaslin, V.M. 1983a. Tables of Force Constants ke and Vibrations Constants coe of Ground Electronic States of Diatomic Molecules, Composed ofAtoms with Composition of s and p Shells (Atom from the Chemical Groups of Lithium, Beryllium, Boron and Carbon). Preprint 302. Moscow Optics Laboratory, Optics and Spectroscopy Department, Physical Institute. 

The total energy needed for ion formation is even greater than this because metallic lithium and diatomic fluorine must first be converted to separate gaseous atoms, which also requires energy. Despite this, the standard heat of formation (A//f) of solid LiE is —617 kJ/mol that is, 617 kJ is released when 1 mol of LiF(5) forms from its elements. The case of LiF is typical of many reactions between active metals and nonmetals despite the endothermic electron transfer, ionic solids form readily, often vigorously. Figure 9.6 shows another example, the formation of NaBr. 

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