Fluorine diatomic molecule, orbitals

The diatomic molecule of fluorine does not form higher compounds (such as F3, F4, - ) because each fluorine atom has only one partially filled valence orbital. Each nucleus in Fs is close to a number of electrons sufficient to fill the valence orbitals. Under these circumstances, the diatomic molecule behaves like an inert gas atom toward other such molecules. The forces that cause molecular fluorine to condense at 85°K are, then, the same as those that cause the inert gases to condense. These forces are named van der Waals forces, after the Dutch scientist who studied them. 

A second example is the minimal-basis-set (MBS) Hartree-Fock wave function for the diatomic molecule hydrogen fluoride, HF (Ransil 1960). The basis orbitals are six Slater-type (i.e., single exponential) functions, one for each inner and valence shell orbital of the two atoms. They are the Is function on the hydrogen atom, and the Is, 2s, 2per, and two 2pn functions on the fluorine atom. The 2sF function is an exponential function to which a term is added that introduces the radial node, and ensures orthogonality with the Is function on fluorine. To indicate the orthogonality, it is labeled 2s F. The orbital is described by... 

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. 

Oxygen, fluorine, and man. These three molecules can be treated with the same energy diagram that we have been using for other diatomic molecules of the second-row elements. As we shall see shortly, the intervening molecules, B, C-. and N2. require additional considerations, which lead to an alteration in (he relative energies of the molecular orbitals. 

This is a simple example of a heteronuclear diatomic molecule which is found in a stable molecular substance. We must first choose the basis set. The only AOs that need to be seriously considered are the hydrogen Is, fluorine 2s and fluorine 2p, written for brevity as ls(H), 2s(F) and 2p(F). The fluorine Is orbital lies very low in energy (700 eV lower than 2p) and is so compact that its overlap with orbitals on other atoms is quite negligible. The fluorine 2p level lies somewhat lower than ls(H), as indicated by the higher ionisation potential and electronegativity of F. Interaction between 2p(F) and 2s(H) is very small and can be neglected for all practical purposes. One is tempted to discard 2s(F), which lies more than 20 eV below 2p(F) the 2s-2p separation increases... 

In 1992 Dmitriev, Khait, Kozlov, Labzowsky, Mitrushenkov, Shtoff and Titov [151] used shape consistent relativistic effective core potentials (RECP) to compute the spin-dependent parity violating contribution to the effective spin-rotation Hamiltonian of the diatomic molecules PbF and HgF. Their procedure involved five steps (see also [32]) i) an atomic Dirac-Hartree-Fock calculation for the metal cation in order to obtain the valence orbitals of Pb and Hg, ii) a construction of the shape consistent RECP, which is divided in a electron spin-independent part (ARECP) and an effective spin-orbit potential (ESOP), iii) a molecular SCF calculation with the ARECP in the minimal basis set consisting of the valence pseudoorbitals of the metal atom as well as the core and valence orbitals of the fluorine atom in order to obtain the lowest and the lowest H molecular state, iv) a diagonalisation of the total molecular Hamiltonian, which... 

The bond order in N2, O2, and F2 is the same whether or not mixing is taken into account, but the order of the (Tg(2p) and TT (2p) orbitals is different in N2 than in O2 and F2. As stated previously and further described in Section 5.3.1, the energy difference between the 2s and 2p orbitals of the second row main group elements increases with increasing Z, from 5.7 eV in boron to 21.5 eV in fluorine. As this difference increases, the s-p interaction (mixing) decreases, and the normal order of molecular orbitals returns in O2 and F2. The higher a-g(2p) orbital (relative to 7T (2p)) occurs in many heteronuclear diatomic molecules, such as CO, described in Section 5.3.1. 

For diatomic (and linear polyatomic) molecules, the basis set usually consists of several STOs centered on each atom. Thus the MOs are expressed as LCAOs—linear combinations of atomic orbitals. In a mini- ia/-basis-set calculation, only inner-shell and valence-shell STOs are used. Thus a minimal basis calculation of HF would use as basis functions l.sH, 1 F, 2jf, 2/ ctf, 2p7TF, 2pTTF, where 2poF is a fluorine 2p AO along the internuclear (z) axis (i.e., a 2pz AO), and the 2pir and 2pir AOs are... 

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